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Sample records for accretion takes place

  1. Does olfactory specific satiety take place in a natural setting?

    PubMed

    Fernandez, P; Bensafi, M; Rouby, C; Giboreau, A

    2013-01-01

    Olfactory-specific satiety (OSS) is characterized by a specific decrease in the odor pleasantness of a food eaten to satiety or smelled without ingestion. The usual protocol for studying OSS takes place in laboratory, a setting rather removed from the real world. Here, we set out to examine OSS in a natural setting: during a meal in a restaurant. We hypothesized that an aroma contained in a food that is eaten at the beginning of a meal decreases the pleasantness of the flavor of a food with the same aroma eaten at the end of the meal. In the first experiment (Experiment 1), a test group received an appetizer flavored with a test aroma (anise) at the beginning of the meal. After the main dish, they received a dessert flavored with the same aroma. A control group received the same aromatized dessert, but after a non-aromatized appetizer. This experiment was replicated (Experiment 2) using verbena as the test aroma. For both experiments, results revealed that aroma pleasantness, but not intensity or familiarity, significantly decreased in the test groups vs. the control groups. These findings extend the concept of OSS to a realistic eating context. PMID:23079143

  2. 49 CFR 40.221 - Where does an alcohol test take place?

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... 49 Transportation 1 2012-10-01 2012-10-01 false Where does an alcohol test take place? 40.221... WORKPLACE DRUG AND ALCOHOL TESTING PROGRAMS Testing Sites, Forms, Equipment and Supplies Used in Alcohol Testing § 40.221 Where does an alcohol test take place? (a) A DOT alcohol test must take place at...

  3. 49 CFR 40.221 - Where does an alcohol test take place?

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... 49 Transportation 1 2013-10-01 2013-10-01 false Where does an alcohol test take place? 40.221... WORKPLACE DRUG AND ALCOHOL TESTING PROGRAMS Testing Sites, Forms, Equipment and Supplies Used in Alcohol Testing § 40.221 Where does an alcohol test take place? (a) A DOT alcohol test must take place at...

  4. 49 CFR 40.221 - Where does an alcohol test take place?

    Code of Federal Regulations, 2011 CFR

    2011-10-01

    ... 49 Transportation 1 2011-10-01 2011-10-01 false Where does an alcohol test take place? 40.221... WORKPLACE DRUG AND ALCOHOL TESTING PROGRAMS Testing Sites, Forms, Equipment and Supplies Used in Alcohol Testing § 40.221 Where does an alcohol test take place? (a) A DOT alcohol test must take place at...

  5. Kiwifruit: taking its place in the global fruit bowl.

    PubMed

    Ward, Carol; Courtney, David

    2013-01-01

    While the world total production of kiwifruit has increased by over 50% during the last decade, the kiwifruit remains a niche fruit, taking up an estimated 0.22% of the global fruit bowl, which is dominated by apples, oranges, and bananas. Even though kiwifruit's share of the global fruit bowl has remained largely unchanged over the past 15 years, the scope for growth in the category is significant, with the nutritional and production characteristics of kiwifruit being on the right side of key global consumer trends around health and sustainability. Taking advantage of these consumer trends is one of two key challenges for the global kiwifruit industry. The second challenge is to harness the diverse natural and cultivated range of kiwifruit varieties (colors, flavors, sizes, and shapes) to stimulate the interest of consumers and grow the share of kiwifruit in the fruit basket through selecting cultivars that can develop meaningful market segments and meet consumer demand.

  6. Kiwifruit: taking its place in the global fruit bowl.

    PubMed

    Ward, Carol; Courtney, David

    2013-01-01

    While the world total production of kiwifruit has increased by over 50% during the last decade, the kiwifruit remains a niche fruit, taking up an estimated 0.22% of the global fruit bowl, which is dominated by apples, oranges, and bananas. Even though kiwifruit's share of the global fruit bowl has remained largely unchanged over the past 15 years, the scope for growth in the category is significant, with the nutritional and production characteristics of kiwifruit being on the right side of key global consumer trends around health and sustainability. Taking advantage of these consumer trends is one of two key challenges for the global kiwifruit industry. The second challenge is to harness the diverse natural and cultivated range of kiwifruit varieties (colors, flavors, sizes, and shapes) to stimulate the interest of consumers and grow the share of kiwifruit in the fruit basket through selecting cultivars that can develop meaningful market segments and meet consumer demand. PMID:23394979

  7. 2012 CCCC Chair's Address: Stories Take Place--A Performance in One Act

    ERIC Educational Resources Information Center

    Powell, Malea

    2012-01-01

    This is a written version of the address that Malea Powell gave at the Conference on College Composition and Communication (CCCC) Convention in St. Louis, Missouri, on Thursday, March 22, 2012. This address is a collection of stories. According to her, stories take place. Stories practice place into space. Stories produce habitable spaces. She…

  8. Studying Activities That Take Place in Speech Interactions: A Theoretical and Methodological Framework

    ERIC Educational Resources Information Center

    Saint-Dizier de Almeida, Valérie; Colletta, Jean-Marc; Auriac-Slusarczyk, Emmanuelle; Specogna, Antonietta; Simon, Jean-Pascal; Fiema, Gabriela; Luxembourger, Christophe

    2016-01-01

    The paper proposes a theoretical and methodological framework based on a pluralistic, concerted approach to the study of activities that take place in and through speech interactions. The framework has a general scope, applying to any collective activity taking form through language interactions. It contributes to a fuller understanding of the…

  9. Questions and Answers Regarding Actions to Take When Ending Shelter-in-Place

    SciTech Connect

    Shumpert, B.

    2003-12-30

    Shelter-in-place has found increasing acceptance as an effective protective action option for communities participating in the Chemical Stockpile Emergency Preparedness Program. Studies have confirmed that it can provide optimum protection under certain accident conditions. However, emergency managers and planners, as well as the public, continue to be troubled by the need to end sheltering when the plume has passed in order to avoid sustained exposure to the small amount of agent that has penetrated the shelter. One of the concerns posed by this necessity is uncertainty regarding what hazards will then be faced in the environment outside the shelter and what actions can be taken to avoid those hazards. This report attempts to address those uncertainties. It recognizes that there is an extremely low probability that the environment outside the shelter will be contaminated with chemical agent residue. However, as people comply with an official recommendation to leave their shelters, they probably can't be certain that the environment is free from contamination. Therefore, this report identifies and explains specific and simple actions they can take to avoid the possibility of exposure to chemical agent hazards outside their shelters. It addresses such issues as the actions people should take upon ending shelter-in-place, what clothing they should wear, how they should handle animals, and what they should do about food in their homes and produce in their gardens.

  10. 23 CFR 636.402 - What types of information exchange may take place after the release of the RFP document?

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 23 Highways 1 2010-04-01 2010-04-01 false What types of information exchange may take place after... What types of information exchange may take place after the release of the RFP document? Certain types.... These communication methods are optional. Type of information exchange When Purpose Parties involved...

  11. 49 CFR 40.41 - Where does a urine collection for a DOT drug test take place?

    Code of Federal Regulations, 2012 CFR

    2012-10-01

    ... 49 Transportation 1 2012-10-01 2012-10-01 false Where does a urine collection for a DOT drug test... in DOT Urine Collections § 40.41 Where does a urine collection for a DOT drug test take place? (a) A urine collection for a DOT drug test must take place in a collection site meeting the requirements...

  12. 49 CFR 40.41 - Where does a urine collection for a DOT drug test take place?

    Code of Federal Regulations, 2013 CFR

    2013-10-01

    ... 49 Transportation 1 2013-10-01 2013-10-01 false Where does a urine collection for a DOT drug test... in DOT Urine Collections § 40.41 Where does a urine collection for a DOT drug test take place? (a) A urine collection for a DOT drug test must take place in a collection site meeting the requirements...

  13. 49 CFR 40.41 - Where does a urine collection for a DOT drug test take place?

    Code of Federal Regulations, 2014 CFR

    2014-10-01

    ... 49 Transportation 1 2014-10-01 2014-10-01 false Where does a urine collection for a DOT drug test... in DOT Urine Collections § 40.41 Where does a urine collection for a DOT drug test take place? (a) A urine collection for a DOT drug test must take place in a collection site meeting the requirements...

  14. 23 CFR 636.402 - What types of information exchange may take place after the release of the RFP document?

    Code of Federal Regulations, 2013 CFR

    2013-04-01

    ... 23 Highways 1 2013-04-01 2013-04-01 false What types of information exchange may take place after... What types of information exchange may take place after the release of the RFP document? Certain types.... The following table summarizes the types of communications that will be discussed in this...

  15. 23 CFR 636.402 - What types of information exchange may take place after the release of the RFP document?

    Code of Federal Regulations, 2011 CFR

    2011-04-01

    ... 23 Highways 1 2011-04-01 2011-04-01 false What types of information exchange may take place after... What types of information exchange may take place after the release of the RFP document? Certain types.... The following table summarizes the types of communications that will be discussed in this...

  16. 23 CFR 636.402 - What types of information exchange may take place after the release of the RFP document?

    Code of Federal Regulations, 2012 CFR

    2012-04-01

    ... 23 Highways 1 2012-04-01 2012-04-01 false What types of information exchange may take place after... What types of information exchange may take place after the release of the RFP document? Certain types.... The following table summarizes the types of communications that will be discussed in this...

  17. [Marketing approval and market surveillance of medical devices in Germany: Where does policy integration take place?].

    PubMed

    Lang, Achim

    2014-01-01

    Since 2011 new regulatory measures regarding medical devices have been set up with the aim to eliminate obstacles to innovations and to find more coordinated ways to marketing authorisation and market surveillance. This essay investigates whether these new and existing coordination mechanisms build up to a Joined-up Government approach. The analysis shows that the regulatory process should be adjusted along several dimensions. First, many organisations lack awareness regarding their stakeholders and focus solely on their immediate organisational activities. Second, the regulatory process (marketing authorisation and market surveillance) is too fragmented for an effective communication to take place. Finally, the underlying strategy process is an ad-hoc approach lacking continuity and continued involvement of, in particular, the responsible federal ministries.

  18. [Marketing approval and market surveillance of medical devices in Germany: Where does policy integration take place?].

    PubMed

    Lang, Achim

    2014-01-01

    Since 2011 new regulatory measures regarding medical devices have been set up with the aim to eliminate obstacles to innovations and to find more coordinated ways to marketing authorisation and market surveillance. This essay investigates whether these new and existing coordination mechanisms build up to a Joined-up Government approach. The analysis shows that the regulatory process should be adjusted along several dimensions. First, many organisations lack awareness regarding their stakeholders and focus solely on their immediate organisational activities. Second, the regulatory process (marketing authorisation and market surveillance) is too fragmented for an effective communication to take place. Finally, the underlying strategy process is an ad-hoc approach lacking continuity and continued involvement of, in particular, the responsible federal ministries. PMID:25066351

  19. The 'taking place' of health and wellbeing: towards non-representational theory.

    PubMed

    Andrews, Gavin J; Chen, Sandra; Myers, Samantha

    2014-05-01

    For the last two decades health geography has focused on the dynamics between health and place. Although the social constructivist perspective of much research has provided many insights into the meanings of health and health care arguably, mirroring progress in the parent discipline of human geography, there could be a far more serious engagement with non-representational theory and the 'taking place' of health and health care. To showcase the importance and potential of this broadly, the idea of wellbeing is re-approached. The paper reflects on the ways wellbeing has been treated in research primarily as a meaningful and relatively prescribed state of life, to the neglect of process. Based on this critique, a qualitative study then illustrates the most immediate and everyday ways wellbeing might arise through 'affect'; the pre-personal mobile energies and intensities that result from physical encounters within assemblages of bodies and objects. Indeed, theoretically the findings support the proposition that, at one level, wellbeing might not be taken from environment but instead might emerge as the affective environment. They certainly raise awareness of how much in health might originate at the surface, prior to meaning, within life's infinite spatial doings, and thus they launch some final thoughts on the wider challenges and opportunities for non-representational health geographies.

  20. What it Takes to Successfully Implement Technology for Aging in Place: Focus Groups With Stakeholders

    PubMed Central

    Wouters, Eveline JM; Luijkx, Katrien G; Vrijhoef, Hubertus JM

    2016-01-01

    Background There is a growing interest in empowering older adults to age in place by deploying various types of technology (ie, eHealth, ambient assisted living technology, smart home technology, and gerontechnology). However, initiatives aimed at implementing these technologies are complicated by the fact that multiple stakeholder groups are involved. Goals and motives of stakeholders may not always be transparent or aligned, yet research on convergent and divergent positions of stakeholders is scarce. Objective To provide insight into the positions of stakeholder groups involved in the implementation of technology for aging in place by answering the following questions: What kind of technology do stakeholders see as relevant? What do stakeholders aim to achieve by implementing technology? What is needed to achieve successful implementations? Methods Mono-disciplinary focus groups were conducted with participants (n=29) representing five groups of stakeholders: older adults (6/29, 21%), care professionals (7/29, 24%), managers within home care or social work organizations (5/29, 17%), technology designers and suppliers (6/29, 21%), and policy makers (5/29, 17%). Transcripts were analyzed using thematic analysis. Results Stakeholders considered 26 different types of technologies to be relevant for enabling independent living. Only 6 out of 26 (23%) types of technology were mentioned by all stakeholder groups. Care professionals mentioned fewer different types of technology than other groups. All stakeholder groups felt that the implementation of technology for aging in place can be considered a success when (1) older adults’ needs and wishes are prioritized during development and deployment of the technology, (2) the technology is accepted by older adults, (3) the technology provides benefits to older adults, and (4) favorable prerequisites for the use of technology by older adults exist. While stakeholders seemed to have identical aims, several underlying

  1. Makro- and micromorphological evidence of processes taking place during Albeluvisol development in S Norway

    NASA Astrophysics Data System (ADS)

    Sauer, Daniela; Schülli-Maurer, Isabelle; Sperstad, Ragnhild; Sørensen, Rolf

    2014-05-01

    We studied two soil chronosequences in S Norway to identify processes involved in Albeluvisol formation. For this purpose, field observation of vertical and horizontal sections of soil profiles, soil chemical and mineralogical analyses were carried out, and in particular, micromorphological analysis was applied. The study area is located at the western and eastern side of the Oslofjord, S Norway, in the counties Vestfold and Østfold. This region is characterized by continuous glacio-isostatic uplift over the entire Holocene. Hence, the age of the land surface continuously increases from the coast towards higher elevations. Twelve soil profiles in loamy marine sediments were studied. Based on macro- and micromorphological observations and analytical data progressive soil formation is characterized as follows: As soon as the land surface is raised above sea level, five major processes are initiated: 1) development of deep desiccation cracks, forming a polygonal pattern; 2) compaction, taking place as soon as the coarse pores have been drained; 3) pyrite oxidation and release of sulfuric acid; 4) carbonate dissolution by acids from pyrite and iron oxidation resulting in rapid decarbonatization of the originally calcareous sediments; 5) precipitation of iron hypocoatings and coatings in the capillary fringe Soon after these very early processes have taken place, limited water permeability of the fine-textured sediments leads to horizon differentiation into Ah, Eg and Btg horizons within less than 2.1 ka. Eg horizons become lighter in colour with time. Also illuvial clay is already observed in the 2.1 ka-old soil. Soil pH in the upper part of the E horizon of this soil is already too low for significant clay mobilization. Clay illuviation is still active in all soils studied, but the upper boundary of the zone where pH favours clay mobilization is at 20-50 cm depth. Progressive clay illuviation over time is recorded in increasing thickness of clay coatings and

  2. OECD Global Science Forum's Astronomy Workshop to take place in Munich

    NASA Astrophysics Data System (ADS)

    2003-11-01

    members of the national astronomical community. The International Astronomical Union (IAU) and the European Southern Observatory (ESO) are explicitly represented. Experts from the world-wide astronomy community have been invited to set the stage and provide input for the discussions. The choice by Germany and the OECD to make Munich the venue of this Global Science Forum Workshop is no coincidence. It is a recognition of the important role played by many institutions in the Munich region in the field of Astronomy and Astrophysics. They include the Ludwig-Maximilians-Universität where the Workshop will take place, the Max-Planck-Institut für Astrophysik, the Max-Planck Institut für Extraterrestrische Physik and the European Southern Observatory. These institutions are all participating in large programmes and projects in astronomy. ESO, for its part, is at the leading edge of world astronomy with its flagship facility, the Very Large Telescope in Paranal (Chile) and the newly started ALMA project at Chajnantor (Chile), being carried out in partnership between Europe and North America. Public Talks (Munich) on December 1, 2003 As a prelude to the Workshop, two public keynote presentations will take place on December 1 at the Deutsches Museum in Munich at 18:00 CET. The speakers are Malcolm Longair, Jacksonian Professor of Natural Philosophy and Head of Laboratory, Cavendish Laboratory, Cambridge (UK) and Martin Harwit, Professor Emeritus of Astronomy, Cornell University, and former Director of the National Air and Space Museum, Washington, DC (USA). The talks will be given in English and the entry to this public event is free. Professor Longair will speak on "Astrophysics and Cosmology in the Twenty-First Century" and Professor Harwit will speak on "The Growth of Understanding of our Universe". You can find more informaton on the Public Talks web page.

  3. "Why Are We Here?" Taking "Place" into Account in UK Outdoor Environmental Education

    ERIC Educational Resources Information Center

    Harrison, Sam

    2010-01-01

    "Place" is an under-researched and poorly documented element of UK outdoor environmental education. In the international literature, North American and Australian researchers and practitioners show considerable attention to "place". Yet UK outdoor environmental educators and researchers seem to have neglected this area despite calls for increased…

  4. An Experimental Investigation of the Process of Isotope Exchange that Takes Place when Heavy Water Is Exposed to the Atmosphere

    ERIC Educational Resources Information Center

    Deeney, F. A.; O'Leary, J. P.

    2009-01-01

    We have used the recently developed method for rapid measurement of maximum density temperature to determine the rate at which hydrogen and deuterium isotope exchange takes place when a sample of heavy water is exposed to the atmosphere. We also provide a simple explanation for the observed linear rate of transition. (Contains 2 figures.)

  5. 23 CFR 636.402 - What types of information exchange may take place after the release of the RFP document?

    Code of Federal Regulations, 2014 CFR

    2014-04-01

    ... 23 Highways 1 2014-04-01 2014-04-01 false What types of information exchange may take place after the release of the RFP document? 636.402 Section 636.402 Highways FEDERAL HIGHWAY ADMINISTRATION, DEPARTMENT OF TRANSPORTATION ENGINEERING AND TRAFFIC OPERATIONS DESIGN-BUILD CONTRACTING Exchanges § 636.402 What types of information exchange may...

  6. Taking the High Ground: Geothermal's Place in the Revolving Energy Market

    SciTech Connect

    Jaros, Richard

    1992-03-24

    It's a genuine privilege for me to be here today. As Dr. Mock mentioned, I have been President of California Energy for not yet three months and have a total tenure in the industry of only one year. As a newcomer to the industry, I am honored to address this group and share my views on ''The Opportunities and Challenges for Expanding Geothermal Energy''. You will see that my outlook for our industry is generally optimistic, shaped in part, perhaps by a newcomer's enthusiasm, but largely I think by my analysis of the opportunities which are open to us as an industry. Many of you and your predecessors over the last 20 years pioneered the geothermal industry in the United States. The risks were great, the results sometimes rewarding, sometimes disappointing. Government and the private sector forged an alliance that moved the industry ahead. Developers, utilities and federal land managers worked together to bring projects on line. Government helped identify geothermal areas, in many cases doing exploration work. The geothermal pioneers had to form entirely new, multi-disciplinary teams to solve problems unique to this resource. From discovery of fields, to environmental mitigation, to management of reservoirs and all of the steps in between, new teams had to be assembled. Geologists, geochemists, hydrologists, reservoir engineers and drilling technologists now apply their skills. Even anthropologists and biologists routinely get into the act in the environmental assessment phase of development. The care that our industry is taking today to do the job right reflects a maturing industry with high standards of performance. To be sure, mistakes were made in the early years, but the industry learned from them. We all know the value of responsible development and resource management to the long-term future of our industry. Improvements in technology and more efficient operations have helped lower our costs and improve our competitiveness. Our industry's progress has also

  7. 23 CFR 636.401 - What types of information exchange may take place prior to the release of the RFP document?

    Code of Federal Regulations, 2010 CFR

    2010-04-01

    ... 23 Highways 1 2010-04-01 2010-04-01 false What types of information exchange may take place prior... What types of information exchange may take place prior to the release of the RFP document? Verbal or written information exchanges (such as in the first-phase of a two-phase selection procedure) must...

  8. LIDAR for remote sensing of contaminations on water and earth surfaces taking place during oil-gas production

    NASA Astrophysics Data System (ADS)

    Pashayev, A.; Tagiyev, B.; Allahverdiyev, K.; Musayev, A.; Sadikhov, I.

    2015-12-01

    Remote sensing of contaminations on water and earth surfaces (oil spills, films) taking place during oil-gas extraction is an interesting and actual problem. This problem may be solved by using different methods of optical spectroscopy, including: •Raman scattering; •light induced breakdown spectroscopy (LIBS); •fluorescence spectroscopy. Fluorescence Light Detection and Ranging (LIDAR) LIDARs are successfully used for remote sensing of chemical and biological substances at atmosphere. A new laser induced fluorescence (LIF) KA-14 LIDAR system for detecting of oil spills on the sea surface was employed at the National Aviation Academy of Azerbaijan. LIDAR's parameters are as follows: •laser CFR 200- type QUANTEL, λ = 355 nm, beam Ø = 5.35 mm, f = 20 Hz, pulse duration τ = 7 ns, pulse power 60 mJ; •diameter of Newtonian- type telescope is 200 mm; •collimator expansion of the laser beam diameter- not less than 3; •angle range of telescope measurements relative to horizon: from -20 to +20 degree; •spectral range of measurements: from 380 to 750 nm, number of spectral channels- 32; •maximum range of measurements- not less than 250 m. This LIDAR is the first performing these kind of research not only on the Azerbaijan beach of Caspian sea, but also on the earth places of Absheron peninsula, where oil-gas production takes place. We hope that the performance of LIDAR will have an International recognition and will make noticeable input on the International Research of Caspian sea surfaces.

  9. Structural and functional changes in the olfactory pathway of adult Drosophila take place at a critical age.

    PubMed

    Devaud, Jean-Marc; Acebes, Angel; Ramaswami, Mani; Ferrús, Alberto

    2003-07-01

    The olfactory system of several holometabolous insect species undergoes anatomical changes after eclosion of the imago, following those occurring during metamorphosis. In parallel, odor experience and learning performance also evolve with age. Here, we analyze the case of adult Drosophila females. Synaptogenesis in the antennal lobe (AL) starts in late pupa and continues during the first days of adult life, at the same time as the behavioral response to odors matures. Individual olfactory glomeruli (DM6, DM2, and V) display specific growth patterns between days 1 and 12 of adult life. Experience can modify the olfactory pathway both structurally and functionally as shown by adaptation experiments. The modifications associated with this form of nonassociative learning seem to take place at a critical age. Exposure to benzaldehyde at days 2-5 of adult life, but not at 8-11, causes behavioral adaptation as well as structural changes in DM2 and V glomeruli. Altered levels in intracellular cAMP, caused by dunce and rutabaga mutants, do not affect the normal changes in glomerular size, at least at day 6 of development, but they prevent those elicited by experience, establishing a molecular difference between glomerular changes of intrinsic versus environmental origin. Taken together, these data demonstrate an imprinting-like phenomenon in the olfactory pathway of young Drosophila adults, and illustrate its glomerulus-specific dynamics.

  10. Geometric figure-ground cues override standard depth from accretion-deletion.

    PubMed

    Tanrikulu, Ömer Daglar; Froyen, Vicky; Feldman, Jacob; Singh, Manish

    2016-01-01

    Accretion-deletion is widely considered a decisive cue to surface depth ordering, with the accreting or deleting surface interpreted as behind an adjoining surface. However, Froyen, Feldman, and Singh (2013) have shown that when accretion-deletion occurs on both sides of a contour, accreting-deleting regions can also be perceived as in front and as self-occluding due to rotation in three dimensions. In this study we ask whether geometric figure-ground cues can override the traditional "depth from accretion-deletion" interpretation even when accretion-deletion takes place only on one side of a contour. We used two tasks: a relative-depth task (front/back), and a motion-classification task (translation/rotation). We conducted two experiments, in which texture in only one set of alternating regions was moving; the other set was static. Contrary to the traditional interpretation of accretion-deletion, the moving convex and symmetric regions were perceived as figural and rotating in three dimensions in roughly half of the trials. In the second experiment, giving different motion directions to the moving regions (thereby weakening motion-based grouping) further weakened the traditional accretion-deletion interpretation. Our results show that the standard "depth from accretion-deletion" interpretation is overridden by static geometric cues to figure-ground. Overall, the results demonstrate a rich interaction between accretion-deletion, figure-ground, and structure from motion that is not captured by existing models of depth from motion.

  11. Neutron star accretion and the neutrino fireball

    SciTech Connect

    Colgate, S.A.; Herant, M.E.; Benz, W.

    1991-11-26

    The mixing necessary to explain the ``Fe`` line widths and possibly the observed red shifts of 1987A is explained in terms of large scale, entropy conserving, up and down flows (calculated with a smooth particle 2-D code) taking place between the neutron star and the explosion shock wave due to the gravity and neutrino deposition. Depending upon conditions of entropy and mass flux further accretion takes place in single events, similar to relaxation oscillator, fed by the downward flows of low entropy matter. The shock, in turn, is driven by the upflow of the buoyant high entropy bubbles. Some accretion events will reach a temperature high enough to create a neutrino ``fireball,`` a region hot enough, 11 Mev, so as to be partially opaque to its own (neutrino) radiation. The continuing neutrino deposition drives the explosion shock until the entropy of matter flowing downwards onto the neutron star is high enough to prevent further accretion. This process should result in a robust supernova explosion.

  12. Young People Take Their Rightful Places as Full and Contributing Members of a World Class Workforce: Philadelphia Youth Network Annual Report 2006

    ERIC Educational Resources Information Center

    Philadelphia Youth Network, 2006

    2006-01-01

    The title of this year's annual report has particular meaning for all of the staff at the Philadelphia Youth Network. The phrase derives from Philadelphia Youth Network's (PYN's) new vision statement, developed as part of its recent strategic planning process, which reads: All of our city's young people take their rightful places as full and…

  13. The asteroids - Accretion, differentiation, fragmentation, and irradiation

    NASA Technical Reports Server (NTRS)

    Wilkening, L. L.

    1979-01-01

    Various types of meteorites have experienced processes of condensation, accretion, metamorphism, differentiation, brecciation, irradiation and fragmentation. A typical view of meteorite formation has been that the processes following accretion take place in a few asteroidal-sized (approximately 100 km) objects. Discovery of decay products of now extinct Al-26 and Pd-107 in meteorites, discovery of isotopic heterogeneity among meteorite types, re-analysis of meteorite cooling rates, and continuing study of meteoritic compositions have led some meteoriticists to conclude that meteorites obtained their chemical, isotopic, and some textural characteristics in objects initially less than 10 km in diameter. Such a scenario, which is described in this paper, raises the possibility that some of these small planetesimals may have been 'condensation nuclei' for the formation of comets as well as the precursors of asteroids.

  14. Placing the power of real options analysis into the hands of natural resource managers - taking the next step.

    PubMed

    Nelson, Rohan; Howden, Mark; Hayman, Peter

    2013-07-30

    This paper explores heuristic methods with potential to place the analytical power of real options analysis into the hands of natural resource managers. The complexity of real options analysis has led to patchy or ephemeral adoption even by corporate managers familiar with the financial-market origins of valuation methods. Intuitively accessible methods for estimating the value of real options have begun to evolve, but their evaluation has mostly been limited to researcher-driven applications. In this paper we work closely with Bush Heritage Australia to evaluate the potential of real options analysis to support the intuitive judgement of conservation estate managers in covenanting land with uncertain future conservation value due to climate change. The results show that modified decision trees have potential to estimate the option value of covenanting individual properties while time and ongoing research resolves their future conservation value. Complementing this, Luehrman's option space has potential to assist managers with limited budgets to increase the portfolio value of multiple properties with different conservation attributes.

  15. Placing the power of real options analysis into the hands of natural resource managers - taking the next step.

    PubMed

    Nelson, Rohan; Howden, Mark; Hayman, Peter

    2013-07-30

    This paper explores heuristic methods with potential to place the analytical power of real options analysis into the hands of natural resource managers. The complexity of real options analysis has led to patchy or ephemeral adoption even by corporate managers familiar with the financial-market origins of valuation methods. Intuitively accessible methods for estimating the value of real options have begun to evolve, but their evaluation has mostly been limited to researcher-driven applications. In this paper we work closely with Bush Heritage Australia to evaluate the potential of real options analysis to support the intuitive judgement of conservation estate managers in covenanting land with uncertain future conservation value due to climate change. The results show that modified decision trees have potential to estimate the option value of covenanting individual properties while time and ongoing research resolves their future conservation value. Complementing this, Luehrman's option space has potential to assist managers with limited budgets to increase the portfolio value of multiple properties with different conservation attributes. PMID:23702285

  16. A WARM MODE OF GAS ACCRETION ON FORMING GALAXIES

    SciTech Connect

    Murante, Giuseppe; Calabrese, Matteo; De Lucia, Gabriella; Monaco, Pierluigi; Borgani, Stefano; Dolag, Klaus E-mail: monaco@oats.inaf.it E-mail: calabrese@oato.inaf.it E-mail: kdolag@mpa-garching.mpg.de

    2012-04-20

    We present results from high-resolution cosmological hydrodynamical simulations of a Milky-Way-sized halo, aimed at studying the effect of feedback on the nature of gas accretion. Simulations include a model of interstellar medium and star formation, in which supernova (SN) explosions provide effective thermal feedback. We distinguish between gas accretion onto the halo, which occurs when gas particles cross the halo virial radius, and gas accretion onto the central galaxy, which takes place when gas particles cross the inner one-tenth of the virial radius. Gas particles can be accreted through three different channels, depending on the maximum temperature value, T{sub max}, reached during the particles' past evolution: a cold channel for T{sub max} < 2.5 Multiplication-Sign 10{sup 5} K, a hot one for T > 10{sup 6} K, and a warm one for intermediate values of T{sub max}. We find that the warm channel is at least as important as the cold one for gas accretion onto the central galaxy. This result is at variance with previous findings that the cold mode dominates gas accretion at high redshift. We ascribe this difference to the different SN feedback scheme implemented in our simulations. While results presented so far in the literature are based on uneffective SN thermal feedback schemes and/or the presence of a kinetic feedback, our simulations include only effective thermal feedback. We argue that observational detections of a warm accretion mode in the high-redshift circumgalactic medium would provide useful constraints on the nature of the feedback that regulates star formation in galaxies.

  17. Geometric figure–ground cues override standard depth from accretion-deletion

    PubMed Central

    Tanrıkulu, Ömer Dağlar; Froyen, Vicky; Feldman, Jacob; Singh, Manish

    2016-01-01

    Accretion-deletion is widely considered a decisive cue to surface depth ordering, with the accreting or deleting surface interpreted as behind an adjoining surface. However, Froyen, Feldman, and Singh (2013) have shown that when accretion-deletion occurs on both sides of a contour, accreting-deleting regions can also be perceived as in front and as self-occluding due to rotation in three dimensions. In this study we ask whether geometric figure–ground cues can override the traditional “depth from accretion-deletion” interpretation even when accretion-deletion takes place only on one side of a contour. We used two tasks: a relative-depth task (front/back), and a motion-classification task (translation/rotation). We conducted two experiments, in which texture in only one set of alternating regions was moving; the other set was static. Contrary to the traditional interpretation of accretion-deletion, the moving convex and symmetric regions were perceived as figural and rotating in three dimensions in roughly half of the trials. In the second experiment, giving different motion directions to the moving regions (thereby weakening motion-based grouping) further weakened the traditional accretion-deletion interpretation. Our results show that the standard “depth from accretion-deletion” interpretation is overridden by static geometric cues to figure–ground. Overall, the results demonstrate a rich interaction between accretion-deletion, figure–ground, and structure from motion that is not captured by existing models of depth from motion. PMID:26982528

  18. Accretion characteristics in intermediate polars

    NASA Astrophysics Data System (ADS)

    Parker, Tracey Louise

    This thesis concerns the class of interacting binaries known as intermediate polars (IPs). These are semi-detached magnetic cataclysmic variable systems in which a red dwarf secondary transfers material via Roche lobe overflow onto a white dwarf (WD). The magnetic field of the white dwarf (~10 6 to 10 7 Gauss) plays an important part in determining the type of accretion flow from the secondary. In chapter 1, I discuss binary systems in general, moving on to a more in depth look at Intermediate polars (IPs), their geometry and characteristics, ending with a brief look at all known IPs to date. In the first part of the thesis I present an analysis of the X-ray lightcurves in 16 IPs in order to examine the possible cause of the orbital modulation. I show that X-ray orbital modulation is widespread amongst IN, but not ubiquitous. The orbital modulation is most likely due to photoelectric absorption in material at the edge of the accretion disk. Assuming a random distribution of inclination angles, the fact that such a modulation is seen in seven systems out of sixteen studied (plus two eclipsing systems) implies that modulations are visible at inclination angles in excess of 60°. It is also apparent that these modulations can appear and disappear on timescales of ~years or months in an individual system, which may be evidence for precessing, tilted accretion disks. In the second half of the thesis I use a particle hydrodynamical code known as HyDisc, to investigate the accretion flows in IPs, as a function of parameter space for two dipole models. One where we assume that the density and size scale of the blobs being accreted are constant which we refer to as the n 6 model, and the other where the size scale and density of the accreted blobs are not constant referred to as the n 3 model. I show that the accretion flow can take the form of an accretion disk, accretion stream, propeller accretion and ring accretion for the n 3 model and stream and disk accretion in the

  19. Self consistent modeling of accretion columns in accretion powered pulsars

    NASA Astrophysics Data System (ADS)

    Falkner, Sebastian; Schwarm, Fritz-Walter; Wolff, Michael Thomas; Becker, Peter A.; Wilms, Joern

    2016-04-01

    We combine three physical models to self-consistently derive the observed flux and pulse profiles of neutron stars' accretion columns. From the thermal and bulk Comptonization model by Becker & Wolff (2006) we obtain seed photon continua produced in the dense inner regions of the accretion column. In a thin outer layer these seed continua are imprinted with cyclotron resonant scattering features calculated using Monte Carlo simulations. The observed phase and energy dependent flux corresponding to these emission profiles is then calculated, taking relativistic light bending into account. We present simulated pulse profiles and the predicted dependency of the observable X-ray spectrum as a function of pulse phase.

  20. Assembly of the ligand-binding conformation of Mr 46,000 mannose 6- phosphate-specific receptor takes place before reaching the Golgi complex

    PubMed Central

    1990-01-01

    The early steps in the biosynthesis of Mr 46,000 mannose 6-phosphate- specific receptor (MPR 46) have been studied by in vivo labeling of transfected BHK cells. The acquisition of phosphomannan-binding activity was compared with changes in protein structure and posttranslational modifications of MPR 46. Intramolecular disulfide bonds were formed before MPR 46 acquired a ligand-binding conformation. A conformational change that resulted in increased trypsin resistance, formation of highly immunogenic epitopes and assembly to noncovalently linked homodimers was observed almost simultaneously with the acquisition of ligand-binding activity. MPR 46 was shown to acquire ligand-binding activity before N-linked oligosaccharides were processed to complex-type forms. Maturation of the ligand-binding conformation was observed under conditions where transport to the Golgi was blocked by lowering the temperature to 16 degrees C, or by addition of brefeldin A or dinitrophenol to the medium at 37 degrees C. This suggests that receptor maturation and assembly take place before reaching the Golgi complex. The affinity towards phosphomannan- containing ligands was shown to be similar for the high-mannose and complex-glycosylated forms of MPR 46. PMID:2157722

  1. Slim accretion disks

    SciTech Connect

    Abramowicz, M.A.; Czerny, B.; Lasota, J.P.; Szuszkiewicz, E.

    1988-09-01

    A new branch of equilibrium solutions for stationary accretion disks around black holes is found. These solutions correspond to moderately super-Eddington accretion rates. The existence of the new branch is a consequence of an additional cooling due to general relativistic Roche lobe overflow and horizontal advection of heat. On an accretion rate versus surface density plane the new branch forms, together with the two standard branches (corresponding to the Shakura-Sunyaev accretion disk models) a characteristically S-shaped curve. This could imply a limit cycle-type behavior for black hole accretion flows with accretion rates close ot the Eddington one. 29 references.

  2. CSI 2264: Accretion process in classical T Tauri stars in the young cluster NGC 2264

    NASA Astrophysics Data System (ADS)

    Sousa, A. P.; Alencar, S. H. P.; Bouvier, J.; Stauffer, J.; Venuti, L.; Hillenbrand, L.; Cody, A. M.; Teixeira, P. S.; Guimarães, M. M.; McGinnis, P. T.; Rebull, L.; Flaccomio, E.; Fürész, G.; Micela, G.; Gameiro, J. F.

    2016-02-01

    Context. NGC 2264 is a young stellar cluster (~3 Myr) with hundreds of low-mass accreting stars that allow a detailed analysis of the accretion process taking place in the pre-main sequence. Aims: Our goal is to relate the photometric and spectroscopic variability of classical T Tauri stars to the physical processes acting in the stellar and circumstellar environment, within a few stellar radii from the star. Methods: NGC 2264 was the target of a multiwavelength observational campaign with CoRoT, MOST, Spitzer, and Chandra satellites and photometric and spectroscopic observations from the ground. We classified the CoRoT light curves of accreting systems according to their morphology and compared our classification to several accretion diagnostics and disk parameters. Results: The morphology of the CoRoT light curve reflects the evolution of the accretion process and of the inner disk region. Accretion burst stars present high mass-accretion rates and optically thick inner disks. AA Tau-like systems, whose light curves are dominated by circumstellar dust obscuration, show intermediate mass-accretion rates and are located in the transition of thick to anemic disks. Classical T Tauri stars with spot-like light curves correspond mostly to systems with a low mass-accretion rate and low mid-IR excess. About 30% of the classical T Tauri stars observed in the 2008 and 2011 CoRoT runs changed their light-curve morphology. Transitions from AA Tau-like and spot-like to aperiodic light curves and vice versa were common. The analysis of the Hα emission line variability of 58 accreting stars showed that 8 presented a periodicity that in a few cases was coincident with the photometric period. The blue and red wings of the Hα line profiles often do not correlate with each other, indicating that they are strongly influenced by different physical processes. Classical T Tauri stars have a dynamic stellar and circumstellar environment that can be explained by magnetospheric

  3. Places for Children - Children's Places

    ERIC Educational Resources Information Center

    Rasmussen, Kim

    2004-01-01

    In their everyday lives, children largely stay within and relate to three settings - their homes, schools and recreational institutions. These environments have been created by adults and designated by them as "places for children". A more differentiated picture of children's spatial culture emerges when children discuss and take photographs of…

  4. Foundations of Black Hole Accretion Disk Theory

    NASA Astrophysics Data System (ADS)

    Abramowicz, Marek A.; Fragile, P. Chris

    2013-12-01

    This review covers the main aspects of black hole accretion disk theory. We begin with the view that one of the main goals of the theory is to better understand the nature of black holes themselves. In this light we discuss how accretion disks might reveal some of the unique signatures of strong gravity: the event horizon, the innermost stable circular orbit, and the ergosphere. We then review, from a first-principles perspective, the physical processes at play in accretion disks. This leads us to the four primary accretion disk models that we review: Polish doughnuts (thick disks), Shakura-Sunyaev (thin) disks, slim disks, and advection-dominated accretion flows (ADAFs). After presenting the models we discuss issues of stability, oscillations, and jets. Following our review of the analytic work, we take a parallel approach in reviewing numerical studies of black hole accretion disks. We finish with a few select applications that highlight particular astrophysical applications: measurements of black hole mass and spin, black hole vs. neutron star accretion disks, black hole accretion disk spectral states, and quasi-periodic oscillations (QPOs).

  5. Fossil magnetic field of accretion disks of young stars

    NASA Astrophysics Data System (ADS)

    Dudorov, A. E.; Khaibrakhmanov, S. A.

    2014-07-01

    We elaborate the model of accretion disks of young stars with the fossil large-scale magnetic field in the frame of Shakura and Sunyaev approximation. Equations of the MHD model include Shakura and Sunyaev equations, induction equation and equations of ionization balance. Magnetic field is determined taking into account ohmic diffusion, magnetic ambipolar diffusion and buoyancy. Ionization fraction is calculated considering ionization by cosmic rays and X-rays, thermal ionization, radiative recombinations and recombinations on the dust grains. Analytical solution and numerical investigations show that the magnetic field is coupled to the gas in the case of radiative recombinations. Magnetic field is quasi-azimuthal close to accretion disk inner boundary and quasi-radial in the outer regions. Magnetic field is quasi-poloidal in the dusty "dead" zones with low ionization degree, where ohmic diffusion is efficient. Magnetic ambipolar diffusion reduces vertical magnetic field in 10 times comparing to the frozen-in field in this region. Magnetic field is quasi-azimuthal close to the outer boundary of accretion disks for standard ionization rates and dust grain size a d=0.1 μm. In the case of large dust grains ( a d>0.1 μm) or enhanced ionization rates, the magnetic field is quasi-radial in the outer regions. It is shown that the inner boundary of dusty "dead" zone is placed at r=(0.1-0.6) AU for accretion disks of stars with M=(0.5-2) M ⊙. Outer boundary of "dead" zone is placed at r=(3-21) AU and it is determined by magnetic ambipolar diffusion. Mass of solid material in the "dead" zone is more than 3 M ⊕ for stars with M≥1 M ⊙.

  6. Clumpy wind accretion in supergiant neutron star high mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Bozzo, E.; Oskinova, L.; Feldmeier, A.; Falanga, M.

    2016-05-01

    The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.

  7. Accretion and ejection in black-hole X-ray transients

    NASA Astrophysics Data System (ADS)

    Kylafis, N. D.; Belloni, T. M.

    2015-02-01

    Context. A rich phenomenology has been accumulated over the years regarding accretion and ejection in black-hole X-ray transients (BHTs) and it needs an interpretation. Aims: Here we summarize the current observational picture of the outbursts of BHTs, based on the evolution traced in a hardness-luminosity diagram (HLD), and we offer a physical interpretation. Methods: The basic ingredient in our interpretation is the Poynting-Robertson cosmic battery (PRCB), which provides locally the poloidal magnetic field needed for the ejection of the jet. In addition, we make two assumptions, easily justifiable. The first is that the mass-accretion rate to the black hole in a BHT outburst has a generic bell-shaped form, whose characteristic time scale is much longer than the dynamical or the cooling ones. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state, i.e., at very low accretion rate, and that state transitions take place over long time scales (hours to days). The second assumption is that at low accretion rates the accretion flow is geometrically thick, ADAF-like, while at high accretion rates it is geometrically thin. Last, but not least, we demonstrate that the previous history of the system is absolutely necessary for the interpretation of the HLD. Results: Both, at the beginning and the end of an outburst, the PRCB establishes a strong poloidal magnetic field in the ADAF-like part of the accretion flow, and this explains naturally why a jet is always present in the right part of the HLD. In the left part of the HLD, the accretion flow is in the form of a thin disk, and such a disk cannot sustain a strong poloidal magnetic filed. Thus, no jet is expected in this part of the HLD. Finally, the counterclockwise traversal of the HLD is explained as follows: all outbursts start from the quiescent state, in which the inner part of the accretion flow is ADAF-like, threaded by a poloidal magnetic field. As the

  8. ALIGNMENTS OF BLACK HOLES WITH THEIR WARPED ACCRETION DISKS AND EPISODIC LIFETIMES OF ACTIVE GALACTIC NUCLEI

    SciTech Connect

    Li, Yan-Rong; Wang, Jian-Min; Qiu, Jie; Cheng, Cheng

    2015-05-01

    Warped accretion disks have attracted intense attention because of their critical role in shaping the spin of supermassive massive black holes (SMBHs) through the Bardeen–Petterson effect, a general relativistic effect that leads to final alignments or anti-alignments between black holes and warped accretion disks. We study such alignment processes by explicitly taking into account the finite sizes of accretion disks and the episodic lifetimes of active galactic nuclei (AGNs) that delineate the duration of gas fueling onto accretion disks. We employ an approximate global model to simulate the evolution of accretion disks, allowing us to determine the gravitomagnetic torque that drives the alignments in a simple way. We then track down the evolutionary paths for mass and spin of black holes both in a single activity episode and over a series of episodes. Given with randomly and isotropically oriented gas fueling over episodes, we calculate the spin evolution with different episodic lifetimes and find that it is quite sensitive to the lifetimes. We therefore propose that the spin distribution of SMBHs can place constraints on the episodic lifetimes of AGNs and vice versa. The applications of our results on the observed spin distributions of SMBHs and the observed episodic lifetimes of AGNs are discussed, although both measurements at present are too ambiguous for us to draw a firm conclusion. Our prescription can be easily incorporated into semi-analytic models for black hole growth and spin evolution.

  9. Suppression of accretion on to low-mass Population III stars

    NASA Astrophysics Data System (ADS)

    Johnson, Jarrett L.; Khochfar, Sadegh

    2011-05-01

    Motivated by recent theoretical work suggesting that a substantial fraction of Population (Pop) III stars may have had masses low enough for them to survive to the present day, we consider the role that the accretion of metal-enriched gas may have had in altering their surface composition, thereby disguising them as Pop II stars. We demonstrate that if weak, solar-like winds are launched from low-mass Pop III stars formed in the progenitors of the dark matter halo of the Galaxy, then such stars are likely to avoid significant enrichment via accretion of material from the interstellar medium. We find that at early times accretion is easily prevented if the stars are ejected from the central regions of the haloes in which they form, either by dynamical interactions with more massive Pop III stars or by violent relaxation during halo mergers. While accretion may still take place during passage through sufficiently dense molecular clouds at later times, we find that the probability of such a passage is generally low (≲0.1), assuming that stars have velocities of the order of the maximum circular velocity of their host haloes and accounting for the orbital decay of merging haloes. In turn, due to the higher gas density required for accretion on to stars with higher velocities, we find an even lower probability of accretion (˜10-2) for the subset of Pop III stars formed at z > 10, which are more quickly incorporated into massive haloes than stars formed at lower redshift. While there is no a priori reason to assume that low-mass Pop III stars do not have solar-like winds, without them surface enrichment via accretion is likely to be inevitable. We briefly discuss the implications that our results hold for stellar archaeology.

  10. Black hole accretion.

    PubMed

    Narayan, Ramesh; Quataert, Eliot

    2005-01-01

    Black holes are most often detected by the radiation produced when they gravitationally pull in surrounding gas, in a process called accretion. The efficiency with which the hot gas radiates its thermal energy strongly influences the geometry and dynamics of the accretion flow. Both radiatively efficient thin disks and radiatively inefficient thick disks are observed. When the accreting gas gets close to the central black hole, the radiation it produces becomes sensitive to the spin of the hole and the presence of an event horizon. Analysis of the luminosities and spectra of accreting black holes has yielded tantalizing evidence for both rotating holes and event horizons. Numerical simulations imply that the relativistic jets often seen from accreting black holes may be powered in part by the spin of the hole. PMID:15637269

  11. Place and Being

    ERIC Educational Resources Information Center

    Cannatella, Howard

    2007-01-01

    Do places matter educationally? When Edward Casey remarks: "The world is, minimally and forever, a place-world", we might take this statement as presupposing without argument that places exist as a given, that we know what a place is, a point that Aristotle would have never taken for granted and in fact neither does Casey. I find Casey's remark…

  12. Numerical Solution of the Radiative Transfer Equation: X-Ray Spectral Formation from Cylindrical Accretion onto a Magnetized Neutron Star

    NASA Technical Reports Server (NTRS)

    Fairnelli, R.; Ceccobello, C.; Romano, P.; Titarchuk, L.

    2011-01-01

    Predicting the emerging X-ray spectra in several astrophysical objects is of great importance, in particular when the observational data are compared with theoretical models. This requires developing numerical routines for the solution of the radiative transfer equation according to the expected physical conditions of the systems under study. Aims. We have developed an algorithm solving the radiative transfer equation in the Fokker-Planck approximation when both thermal and bulk Comptonization take place. The algorithm is essentially a relaxation method, where stable solutions are obtained when the system has reached its steady-state equilibrium. Methods. We obtained the solution of the radiative transfer equation in the two-dimensional domain defined by the photon energy E and optical depth of the system pi using finite-differences for the partial derivatives, and imposing specific boundary conditions for the solutions. We treated the case of cylindrical accretion onto a magnetized neutron star. Results. We considered a blackbody seed spectrum of photons with exponential distribution across the accretion column and for an accretion where the velocity reaches its maximum at the stellar surface and at the top of the accretion column, respectively. In both cases higher values of the electron temperature and of the optical depth pi produce flatter and harder spectra. Other parameters contributing to the spectral formation are the steepness of the vertical velocity profile, the albedo at the star surface, and the radius of the accretion column. The latter parameter modifies the emerging spectra in a specular way for the two assumed accretion profiles. Conclusions. The algorithm has been implemented in the XPEC package for X-ray fitting and is specifically dedicated to the physical framework of accretion at the polar cap of a neutron star with a high magnetic field (approx > 10(exp 12) G). This latter case is expected to be of typical accreting systems such as X

  13. Place-focused physical activity research, human agency, and social justice in public health: taking agency seriously in studies of the built environment.

    PubMed

    Blacksher, Erika; Lovasi, Gina S

    2012-03-01

    Built environment characteristics have been linked to health outcomes and health disparities. However, the effects of an environment on behavior may depend on human perception, interpretation, motivation, and other forms of human agency. We draw on epidemiological and ethical concepts to articulate a critique of research on the built environment and physical activity. We identify problematic assumptions and enumerate both scientific and ethical reasons to incorporate subjective perspectives and public engagement strategies into built environment research and interventions. We maintain that taking agency seriously is essential to the pursuit of health equity and the broader demands of social justice in public health, an important consideration as studies of the built environment and physical activity increasingly focus on socially disadvantaged communities. Attention to how people understand their environment and navigate competing demands can improve the scientific value of ongoing efforts to promote active living and health, while also better fulfilling our ethical obligations to the individuals and communities whose health we strive to protect.

  14. Effects of proton irradiation on a gas phase in which condensation takes place. I Negative Mg-26 anomalies and Al-26. [applied to solar and meteoritic composition

    NASA Technical Reports Server (NTRS)

    Heymann, D.; Dziczkaniec, M.; Walker, A.; Huss, G.; Morgan, J. A.

    1978-01-01

    In the present paper, isotopic effects in magnesium generated in a proton-irradiated gas phase are examined, taking only (p,n), (p,d), and (p, alpha) reactions in magnesium, aluminum, and silicon into consideration. In the presence of proton radiation, the three elements are 'removed' from the gas phase by condensation. It is required that a value of Al-26/Al-27 greater than 6 times 10 to the -5th must be reached, consistent with the value deduced by Lee Papanastassiou, and Wasserburg (1976) from their studies of the Allende meteorite. The calculations show that fast aluminum condensation reduces the required proton fluence substantially, that a significant fraction of aluminum remains uncondensed when the above value of the Al-26/Al-27 ratio is reached, that a detectable MG-24 excess is very likely to occur, that detectable negative MG-28 anomalies can be generated, and that proton fluxes and irradiation times can be varied simultaneously, and over a wide range of values, without significant changes in the required proton fluence.

  15. Leaf processing behaviour in Atta leafcutter ants: 90% of leaf cutting takes place inside the nest, and ants select pieces that require less cutting.

    PubMed

    Garrett, Ryan W; Carlson, Katherine A; Goggans, Matthew Scott; Nesson, Michael H; Shepard, Christopher A; Schofield, Robert M S

    2016-01-01

    Leafcutter ants cut trimmings from plants, carry them to their underground nests and cut them into smaller pieces before inoculating them with a fungus that serves as a primary food source for the colony. Cutting is energetically costly, so the amount of cutting is important in understanding foraging energetics. Estimates of the cutting density, metres of cutting per square metre of leaf, were made from samples of transported leaf cuttings and of fungal substrate from field colonies of Atta cephalotes and Atta colombica. To investigate cutting inside the nest, we made leaf-processing observations of our laboratory colony, A. cephalotes. We did not observe the commonly reported reduction of the leaf fragments into a pulp, which would greatly increase the energy cost of processing. Video clips of processing behaviours, including behaviours that have not previously been described, are linked. An estimated 2.9 (±0.3) km of cutting with mandibles was required to reduce a square metre of leaf to fungal substrate. Only about 12% (±1%) of this cutting took place outside of the nest. The cutting density and energy cost is lower for leaf material with higher ratios of perimeter to area, so we tested for, and found that the laboratory ants had a preference for leaves that were pre-cut into smaller pieces. Estimates suggest that the energy required to transport and cut up the leaf material is comparable to the metabolic energy available from the fungus grown on the leaves, and so conservation of energy is likely to be a particularly strong selective pressure for leafcutter ants. PMID:26909161

  16. Leaf processing behaviour in Atta leafcutter ants: 90% of leaf cutting takes place inside the nest, and ants select pieces that require less cutting

    PubMed Central

    Garrett, Ryan W.; Carlson, Katherine A.; Goggans, Matthew Scott; Nesson, Michael H.; Shepard, Christopher A.; Schofield, Robert M. S.

    2016-01-01

    Leafcutter ants cut trimmings from plants, carry them to their underground nests and cut them into smaller pieces before inoculating them with a fungus that serves as a primary food source for the colony. Cutting is energetically costly, so the amount of cutting is important in understanding foraging energetics. Estimates of the cutting density, metres of cutting per square metre of leaf, were made from samples of transported leaf cuttings and of fungal substrate from field colonies of Atta cephalotes and Atta colombica. To investigate cutting inside the nest, we made leaf-processing observations of our laboratory colony, A. cephalotes. We did not observe the commonly reported reduction of the leaf fragments into a pulp, which would greatly increase the energy cost of processing. Video clips of processing behaviours, including behaviours that have not previously been described, are linked. An estimated 2.9 (±0.3) km of cutting with mandibles was required to reduce a square metre of leaf to fungal substrate. Only about 12% (±1%) of this cutting took place outside of the nest. The cutting density and energy cost is lower for leaf material with higher ratios of perimeter to area, so we tested for, and found that the laboratory ants had a preference for leaves that were pre-cut into smaller pieces. Estimates suggest that the energy required to transport and cut up the leaf material is comparable to the metabolic energy available from the fungus grown on the leaves, and so conservation of energy is likely to be a particularly strong selective pressure for leafcutter ants. PMID:26909161

  17. Rotating Bondi Accretion Flow

    NASA Astrophysics Data System (ADS)

    Park, Myeong-Gu; Han, Du-Hwan

    2016-06-01

    The characteristics of accretion flow onto a black hole are determined by the physical condition of gas at large radius. When the gas has no angular momentum and is polytropic, the accretion flow becomes the classic Bondi flow. The mass accretion rate in such case is an eigenvalue and uniquely determined by the density and the temperature of the surrounding gas for a given black hole mass. When the gas has angular momentum above some critical value, the angular momentum of the gas should be removed by viscosity to reach the black hole horizon. We study, within the slim disk approximation, rotating polytropic accretion flow with alpha viscosity as an an extension of the Bondi flow. The characteristics of the accretion flow are now determined by the temperature, density, and angular momentum of the gas at the outer boundary. We explore the effects of the viscosity parameter and the outer boundary radius on the physical characteristic of the flow, especially on the mass accretion rate, and compare the result with previous works of Park (2009) and Narayan & Fabian (2011).

  18. The memory of the accreting plate boundary and the continuity of fracture zones

    USGS Publications Warehouse

    Schouten, Hans; Klitgord, Kim D.

    1982-01-01

    A detailed aeromagnetic anomaly map of the Mesozoic seafloor-spreading lineations southwest of Bermuda reveals the dominant magnetic grain of the oceanic crust and the character of the accreting boundary at the time of crustal formation. The magnetic anomaly pattern is that of a series of elongate lobes perpendicular to the fracture zone (flowline) trends. The linear sets of magnetic anomaly peaks and troughs have narrow regions of reduced amplitude anomalies associated with the fracture zones. During the period of Mesozoic geomagnetic polarity reversals (when 1200 km of central North Atlantic seafloor formed), the Atlantic accreting boundary consisted of stationary, elongate, spreading center cells that maintained their independence even though sometimes only minor spatial offsets existed between cells. Normal oceanic crustal structure was formed in the spreading center cells, but structural anomalies and discontinuities characteristic of fracture zones were formed at their boundaries, which parallel flowlines of Mesozoic relative plate motion in the central North Atlantic. We suggest that the memory for a stationary pattern of independent spreading center cells resides in the young brittle lithosphere at the accreting boundary where the lithosphere is weakest; here, each spreading center cell independently goes through its cylce of stress buildup, stress release, and crustal accretion, after which its memory is refreshed. The temporal offset between the peaks of the accretionary activity that takes place within each cell may provide the mechanism for maintaining the independence of adjacent spreading center cells through times when no spatial offset between the cells exists.

  19. The accretion column of AE Aqr

    NASA Astrophysics Data System (ADS)

    Rodrigues, Claudia; Costa, D. Joaquim; Luna, Gerardo; Lima, Isabel J.; Silva, Karleyne M. G.; De Araujo, Jose Carlos N.; Coelho, Jaziel

    2016-07-01

    AE Aqr is a magnetic cataclysmic variable, whose white dwarf rotates at the very fast rate of 33 s modulating the flux from high-energies to optical wavelengths. There are many studies of the origin of its emission, which consider emission from a rotating magnetic field or from an accretion column. Recently, MAGIC observations have discarded AE Aqr emission in very high energy gamma-rays discarding non-thermal emission. Furthermore, soft and hard X-ray data from Swift and NuSTAR were fitted using thermal models. Here we present the modelling of AE Aqr X-ray spectra and light curve considering the emission of a magnetic accretion column using the Cyclops code. The model takes into consideration the 3D geometry of the system, allowing to properly represent the white-dwarf auto eclipse, the pre-shock column absorption, and the varying density and temperature of a tall accretion column.

  20. Subhalo Accretion through Filaments

    NASA Astrophysics Data System (ADS)

    González, Roberto E.; Padilla, Nelson D.

    2016-09-01

    We track subhalo orbits of galaxy- and group-sized halos in cosmological simulations. We identify filamentary structures around halos and use these to define a sample of subhalos accreted from filaments, as well as a control sample of subhalos accreted from other directions. We use these samples to study differences in satellite orbits produced by filamentary accretion. Our results depend on host halo mass. We find that for low masses, subhalos accreted from filaments show ∼10% shorter lifetimes compared to the control sample, show a tendency toward more radial orbits, reach halo central regions earlier, and are more likely to merge with the host. For higher-mass halos this lifetime difference dissipates and even reverses for cluster-sized halos. This behavior appears to be connected to the fact that more massive hosts are connected to stronger filaments with higher velocity coherence and density, with slightly more radial subhalo orbits. Because subhalos tend to follow the coherent flow of the filament, it is possible that such thick filaments are enough to shield the subhalo from the effect of dynamical friction at least during their first infall. We also identify subhalo pairs/clumps that merge with one another after accretion. They survive as a clump for only a very short time, which is even shorter for higher subhalo masses, suggesting that the Magellanic Clouds and other Local group satellite associations may have entered the Milky Way virial radius very recently and probably are in their first infall.

  1. Steady accretion of an elastic body on a hard spherical surface and the notion of a four-dimensional reference space

    NASA Astrophysics Data System (ADS)

    Tomassetti, Giuseppe; Cohen, Tal; Abeyaratne, Rohan

    2016-11-01

    Taking the cue from experiments on actin growth on spherical beads, we formulate and solve a model problem describing the accretion of an incompressible elastic solid on a rigid sphere due to attachment of diffusing free particles. One of the peculiar characteristics of this problem is that accretion takes place on the interior surface that separates the body from its support rather than on its exterior surface, and hence is responsible for stress accumulation. Simultaneously, ablation takes place at the outer surface where material is removed from the body. As the body grows, mechanical effects associated with the build-up of stress and strain energy slow down accretion and promote ablation. Eventually, the system reaches a point where internal accretion is balanced by external ablation. The present study is concerned with this stationary regime called "treadmilling". The principal ingredients of our model are: a nonstandard choice of the reference configuration, which allows us to cope with the continually evolving material structure; and a driving force and a kinetic law for accretion/ablation that involves the difference in chemical potential, strain energy and the radial stress. By combining these ingredients we arrive at an algebraic system which governs the stationary treadmilling state. We establish the conditions under which this system has a solution and we show that this solution is unique. Moreover, by an asymptotic analysis we show that for small beads the thickness of the solid is proportional to the radius of the support and is strongly affected by the stiffness of the solid, whereas for large beads the stiffness of the solid is essentially irrelevant, the thickness being proportional to a characteristic length that depends on the parameters that govern diffusion and accretion kinetics.

  2. Formation of redbacks via accretion-induced collapse

    NASA Astrophysics Data System (ADS)

    Smedley, Sarah L.; Tout, Christopher A.; Ferrario, Lilia; Wickramasinghe, Dayal T.

    2015-01-01

    We examine the growing class of binary millisecond pulsars known as redbacks. In these systems, the pulsar's companion has a mass between 0.1 and about 0.5 M⊙ in an orbital period of less than 1.5 d. All show extended radio eclipses associated with circumbinary material. They do not lie on the period-companion mass relation expected from the canonical intermediate-mass X-ray binary evolution in which the companion filled its Roche lobe as a red giant and has now lost its envelope and cooled as a white dwarf. The redbacks lie closer to, but usually at higher period than, the period-companion mass relation followed by cataclysmic variables and low-mass X-ray binaries. In order to turn on as a pulsar mass accretion on to a neutron star must be sufficiently weak, considerably weaker than expected in systems with low-mass main-sequence companions driven together by magnetic braking or gravitational radiation. If a neutron star is formed by accretion-induced collapse of a white dwarf as it approaches the Chandrasekhar limit some baryonic mass is abruptly lost to its binding energy so that its effective gravitational mass falls. We propose that redbacks form when accretion-induced collapse of a white dwarf takes place during cataclysmic variable binary evolution because the loss of gravitational mass makes the orbit expand suddenly so that the companion no longer fills its Roche lobe. Once activated, the pulsar can ablate its companion and so further expand the orbit and also account for the extended eclipses in the radio emission of the pulsar that are characteristic of these systems. The whole period-companion mass space occupied by the redbacks can be populated in this way.

  3. Face-on accretion onto a protoplanetary disc

    NASA Astrophysics Data System (ADS)

    Wijnen, T. P. G.; Pols, O. R.; Pelupessy, F. I.; Portegies Zwart, S.

    2016-10-01

    ) accretion of material with no azimuthal angular momentum. Our study, as well as previous work, suggests that the former, dominant process is mainly caused by numerical, rather than physical effects, while the latter is not. The latter process, as expected theoretically, causes the disc to become more compact and increases the surface density profile considerably at smaller radii. Conclusions: The disc size is determined in the first place by the ram pressure exerted by the flow when it first hits the disc. Further evolution is governed by the decrease in the specific angular momentum of the disc as it accretes material with no azimuthal angular momentum. Even taking into account the uncertainties in our simulations and the result that the loading rate is within a factor two of a simple geometric estimate, the size and lifetime of the disc are probably not sufficient to accrete the amount of mass required in the early disc accretion scenario. An animation of the simulation is available at http://www.aanda.org

  4. Accretion Flows in Magnetic White Dwarf Systems

    NASA Technical Reports Server (NTRS)

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the

  5. Magnetospheric accretion in EX Lupi

    NASA Astrophysics Data System (ADS)

    Abraham, Peter; Kospal, Agnes; Bouvier, Jerome

    2016-08-01

    We propose to observe EX Lup, the prototype of the EXor class of young eruptive stars, in order to understand how the accretion process works in the quiescent system. Here, we request 2.6 hours of telescope time on Spitzer, to carry out a mid-infrared photometric monitoring, which we will supplement with simultaneous ground-based optical and near-infrared data. The multi-wavelength light curves will allow us to reliably separate the effects of fluctuating accretion rate from the rotation of the star. By analyzing the variations of the accretion rate we will determine whether EX Lup accretes through a few stable accretion columns or several short-lived random accretion streams. With this campaign, EX Lup will become one of the T Tauri systems where the accretion process is best understood.

  6. Hot Accretion Disks Revisited

    NASA Astrophysics Data System (ADS)

    Bjoernsson, Gunnlaugur; Abramowicz, Marek A.; Chen, Xingming; Lasota, Jean-Pierre

    1996-08-01

    All previous studies of hot (Tp 1010-1012 K), optically thin accretion disks have neglected either the presence of e+ e- pairs or advective cooling. Thus all hot disk models constructed previously have not been self-consistent. In this paper we calculate local disk models including pair physics, relevant radiative processes in the hot plasma, and the effect of advective cooling. We use a modification of the Björnsson & Svensson mapping method. We find that the role of e+ e- pairs in the structure of hot, optically thin accretion disks is far less significant than was previously thought. The improved description of the radiation-matter interactions provided in the present paper modify the previously obtained values of the critical parameters characterizing advectively dominated flows.

  7. Accretion of southern Alaska

    USGS Publications Warehouse

    Hillhouse, J.W.

    1987-01-01

    Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ?? as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation. ?? 1987.

  8. Massive star formation by accretion. I. Disc accretion

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  9. Coronal Neutrino Emission in Hypercritical Accretion Flows

    NASA Astrophysics Data System (ADS)

    Kawabata, R.; Mineshige, S.; Kawanaka, N.

    2008-03-01

    Hypercritical accretion flows onto stellar mass black holes (BHs) are commonly believed to be as a promising model of central engines of gamma-ray bursts (GRBs). In this model a certain fraction of the gravitational binding energy of accreting matter is deposited to the energy of relativistic jets via neutrino annihilation and/or magnetic fields. However, some recent studies have indicated that the energy deposition rate by neutrino annihilation is somewhat smaller than that needed to power a GRB. To overcome this difficulty, Ramirez-Ruiz and Socrates proposed that high-energy neutrinos from the hot corona above the accretion disk might enhance the efficiency of the energy deposition. We elucidate the disk corona model in the context of hypercritical accretion flows. From the energy balance in the disk and the corona, we can calculate the disk and coronal temperature, Td and Tc, and neutrino spectra, taking into account the neutrino cooling processes by neutrino-electron scatterings and neutrino pair productions. The calculated neutrino spectra consist of two peaks: one by the neutrino emission from the disk and the other by that from the corona. We find that the disk corona can enhance the efficiency of energy release but only by a factor of 1.5 or so, unless the height of the corona is very small, Hll r. This is because the neutrino emission is very sensitive to the temperature of the emitting region, and then the ratio Tc/Td cannot be very large.

  10. Accretion of the Earth.

    PubMed

    Canup, Robin M

    2008-11-28

    The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon. PMID:18826928

  11. Accretion of the Earth.

    PubMed

    Canup, Robin M

    2008-11-28

    The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.

  12. Accretion disk coronae

    NASA Technical Reports Server (NTRS)

    White, N. E.; Holt, S. S.

    1981-01-01

    Recent observations of partial X-ray eclipses from 4U1822-37 have shown that the central X-ray source in this system is diffused by a large Compton-thick accretion disk corona (ADC). Another binary, 4U2129-47, also displays a partial eclipse and contains an ADC. The possible origin of an ADC is discussed and a simple hydrostatic evaporated ADC model is developed which, when applied to 4U1822-37, 4U2129+47 and Cyg X-3, can explain their temporal and spectral properties. The quasi-sinusoidal modulation of all three sources can be reconciled with the partial occultation of the ADC by a bulge at the edge of the accretion disk which is caused by the inflowing material. The height of this bulge is an order of magnitude larger than the hydrostatic disk height and is the result of turbulence in the outer region of the disk. The spectral properties of all three sources can be understood in terms of Compton scattering of the original source spectrum by the ADC. Spectral variations with epoch in Cyg X-3 are probably caused by changes in the optical depth of the corona. A consequence of our model is that any accreting neutron star X-ray source in a semi-detached binary system which is close to its Eddington limit most likely contains an optically thick ADC.

  13. The geochemical constraints on Earth's accretion and core formation (Invited)

    NASA Astrophysics Data System (ADS)

    Rudge, J. F.; Kleine, T.; Bourdon, B.

    2010-12-01

    There are now a wide range of geochemical observations that can be used to place constraints on Earth's first hundred million years. During this time the Earth accreted through collisions between numerous planetary embryos, and these collisions are thought to have caused significant melting and segregation of metal, forming the Earth's core. Information on the pressure, temperature, and oxygen fugacity conditions of core formation can be obtained from the abundances of siderophile elements in Earth's mantle and high pressure partitioning experiments. Timing information can be obtained from isotopic measurements, notably Hf-W and U-Pb. Here we present a simple geochemical box model that can be used to provide constraints on Earth's accretion and core formation. A key parameter in the model is the degree of equilibration during metal-silicate segregation. Existing models have shown that the siderophile element abundances are consistent with full equilibration in a deep magma ocean, with an increase in oxygen fugacity during accretion. Here we show that the siderophile element abundances are equally consistent with scenarios involving partial equilibration. The Hf-W isotopic observations constrain the degree of equilibration to be at least 36%. The timing constraints depend strongly on the degree of equilibration, but nevertheless bounds can be placed on the timing of Earth's accretion. With full equilibration, the Hf-W observations imply a rapid early accretion stage (at least 80% of Earth accreting within 35 Myr), but with partial equilibration accretion may be much more protracted. If Pb partitions into Earth’s core, the U-Pb observations can be used to constrain the late stages of accretion, and are consistent with the final 10% of Earth’s accretion occurring during the Moon-forming giant impact at ~4.45Ga.

  14. Mass Accretion Rate of Rotating Viscous Accretion Flow

    NASA Astrophysics Data System (ADS)

    Park, Myeong-Gu

    2009-11-01

    The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate, which is determined only by the density and the temperature of gas at the outer boundary. A rotating accretion flow has angular momentum, which modifies the flow profile from the spherical Bondi flow, and hence its mass accretion rate, but most work on disc accretion has taken the mass flux to be given with the relation between that parameter and external conditions left uncertain. Within the framework of a slim α disk, we have constructed global solutions of the rotating, viscous, hot accretion flow in the Paczyński-Wiita potential and determined its mass accretion rate as a function of density, temperature, and angular momentum of gas at the outer boundary. We find that the low angular momentum flow resembles the spherical Bondi flow and its mass accretion rate approaches the Bondi accretion rate for the same density and temperature at the outer boundary. The high angular momentum flow on the other hand is the conventional hot accretion disk with advection, but its mass accretion rate can be significantly smaller than the Bondi accretion rate with the same boundary conditions. We also find that solutions exist only within a limited range of dimensionless mass accretion rate \\dot{m} ≡ \\dot{M}/\\dot{M}_B, where \\dot{M} is the mass accretion rate and \\dot{M}_B is the Bondi accretion rate: when the temperature at the outer boundary is equal to the virial temperature, solutions exist only for 0.05 ≲ \\dot{m} ≤ 1 when α = 0.01. We also find that the dimensionless mass accretion rate is roughly independent of the radius of the outer boundary but inversely proportional to the angular momentum at the outer boundary and proportional to the viscosity parameter, \\dot{m} ≃ 9.0 α λ^{-1} when 0.1 ≲ \\dot{m} ≲ 1, where the dimensionless angular momentum measure λ ≡ l out/lB is the specific angular momentum of gas at

  15. MASS ACCRETION RATE OF ROTATING VISCOUS ACCRETION FLOW

    SciTech Connect

    Park, Myeong-Gu

    2009-11-20

    The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate, which is determined only by the density and the temperature of gas at the outer boundary. A rotating accretion flow has angular momentum, which modifies the flow profile from the spherical Bondi flow, and hence its mass accretion rate, but most work on disc accretion has taken the mass flux to be given with the relation between that parameter and external conditions left uncertain. Within the framework of a slim alpha disk, we have constructed global solutions of the rotating, viscous, hot accretion flow in the Paczynski-Wiita potential and determined its mass accretion rate as a function of density, temperature, and angular momentum of gas at the outer boundary. We find that the low angular momentum flow resembles the spherical Bondi flow and its mass accretion rate approaches the Bondi accretion rate for the same density and temperature at the outer boundary. The high angular momentum flow on the other hand is the conventional hot accretion disk with advection, but its mass accretion rate can be significantly smaller than the Bondi accretion rate with the same boundary conditions. We also find that solutions exist only within a limited range of dimensionless mass accretion rate m-dotident toM-dot/M-dot{sub B}, where M-dot is the mass accretion rate and M-dot{sub B} is the Bondi accretion rate: when the temperature at the outer boundary is equal to the virial temperature, solutions exist only for 0.05approxaccretion rate is roughly independent of the radius of the outer boundary but inversely proportional to the angular momentum at the outer boundary and proportional to the viscosity parameter, m-dotapprox =9.0 alphalambda{sup -1} when 0.1 approx

  16. Magnetic dynamos in accreting planetary bodies

    NASA Astrophysics Data System (ADS)

    Golabek, G.; Labrosse, S.; Gerya, T.; Morishima, R.; Tackley, P. J.

    2012-12-01

    Laboratory measurements revealed ancient remanent magnetization in meteorites [1] indicating the activity of magnetic dynamos in the corresponding meteorite parent body. To study under which circumstances dynamo activity is possible, we use a new methodology to simulate the internal evolution of a planetary body during accretion and differentiation. Using the N-body code PKDGRAV [2] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [3]. The thermomechanical model takes recent parametrizations of impact processes [4] and of the magnetic dynamo [5] into account. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [6], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [7]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the magnetic dynamo activity. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration, whereas in early-formed bodies accretion and iron core growth occur almost simultaneously and a highly variable magnetic dynamo can operate in the interior of these bodies.

  17. Launching jets from accretion belts

    NASA Astrophysics Data System (ADS)

    Schreier, Ron; Soker, Noam

    2016-05-01

    We propose that sub-Keplerian accretion belts around stars might launch jets. The sub-Keplerian inflow does not form a rotationally supported accretion disk, but it rather reaches the accreting object from a wide solid angle. The basic ingredients of the flow are a turbulent region where the accretion belt interacts with the accreting object via a shear layer, and two avoidance regions on the poles where the accretion rate is very low. A dynamo that is developed in the shear layer amplifies magnetic fields to high values. It is likely that the amplified magnetic fields form polar outflows from the avoidance regions. Our speculative belt-launched jets model has implications on a rich variety of astrophysical objects, from the removal of common envelopes to the explosion of core collapse supernovae by jittering jets.

  18. Accreting X-ray Pulsars

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2009-01-01

    This presentation describes the behavior of matter in environments with extreme magnetic and gravitational fields, explains the instability/stability of accretion disks in certain systems, and discusses how emergent radiation affects accretion flow. Magnetic field measurements are obtained by measuring the lowest cyclotron absorption line energy, observing the cutoff of accretion due to centrifugal inhibition and measuring the spin-up rate at high luminosity.

  19. Turbulent Distortion of Condensate Accretion

    NASA Technical Reports Server (NTRS)

    Hazoume, R.; Orou Chabi, J.; Johnson, J. A., III

    1997-01-01

    When a simple model for the relationship between the density-temperature fluctuation correlation and mean values is used, we determine that the rate of change of turbulent intensity can influence directly the accretion rate of droplets. Considerable interest exists in the accretion rate for condensates in nonequilibrium flow with icing and the potential role which reactant accretion can play in nonequilibrium exothermic reactant processes. Turbulence is thought to play an important role in such flows. It has already been experimentally determined that turbulence influences the sizes of droplets in the heterogeneous nucleation of supersaturated vapors. This paper addresses the issue of the possible influence of turbulence on the accretion rate of droplets.

  20. Between the Disk and the Star: Boundary Layers in Astrophysical Accretion Disks

    NASA Astrophysics Data System (ADS)

    Rafikov, Roman

    Accretion disks are encountered in a wide variety of astrophysical settings: compact binaries, active galactic nuclei, star and planet formation. Whenever accretion proceeds onto a central object with a surface, a boundary layer forms at the inner edge of the disk. Energy release taking place in this layer is expected to dramatically modify spectra and give rise to variability of accreting objects. To properly interpret observations it is thus imperative to obtain a good working model of the boundary layer phenomenon. However, at the moment reconciliation of observations and theory of the boundary layers in grossly incomplete, and there are long-standing puzzles such as e.g. the missing boundary layer problem in accreting white dwarfs and young stars. The main reason for this unsatisfactory state of affairs is that the current theory is severely hampered by the lack of understanding of the mechanisms by which angular momentum and mass are transported across the boundary layer region. Our group has recently discovered a novel transport mechanism, which naturally emerges under the boundary layer conditions and has proven to be very robust. Using a suite of large-scale hydrodynamic and magnetohydrodynamic simulations we have shown that supersonic shear near the stellar surface is highly conducive to the excitation of a global instability, which relies on launching the acoustic modes in the system. Saturation of this instability results in a regular, non-axisymmetric, global pattern of acoustic waves that persists on very long time scales. These waves dissipate in weak shocks giving rise to global momentum transport, which in turn drives mass accretion. Transport of angular momentum and energy by these waves is intrinsically non-local and cannot be described in the framework of existing local models of effective viscosity. This new transport mechanism is efficient and robust. Its discovery calls for better and urgent understanding of how the boundary layer operate in a

  1. RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

    SciTech Connect

    Cunningham, Andrew J.; Klein, Richard I.; McKee, Christopher F.; Krumholz, Mark R.; Teyssier, Romain

    2012-01-10

    We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of {approx}2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of {approx}0.2 {beta}{sup 1/2} compared to the Bondi value, where {beta} is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

  2. Observational Tests of the Picture of Disk Accretion

    NASA Astrophysics Data System (ADS)

    Maccarone, Thomas J.

    2014-09-01

    In this chapter, I present a summary of observational tests of the basic picture of disk accretion. An emphasis is placed on tests relevant to black holes, but many of the fundamental results are drawn from studies of other classes of systems. Evidence is discussed for the basic structures of accretion flows. The cases of systems with and without accretion disks are discussed, as is the evidence that disks actually form. Also discussed are the hot spots where accretion streams impact the disks, and the boundary layers in the inner parts of systems where the accretors are not black holes. The nature of slow, large amplitude variability is discussed. It is shown that some of the key predictions of the classical thermal-viscous ionization instability model for producing outbursts are in excellent agreement with observational results. It is also show that there are systems whose outbursts are extremely difficult to explain without invoking variations in the rate of mass transfer from the donor star into the outer accretion disk, or tidally induced variations in the mass transfer rates. Finally, I briefly discuss recent quasar microlensing measurements which give truly independent constraints on the inner accretion geometry around black holes.

  3. ACCRETION OUTBURSTS IN CIRCUMPLANETARY DISKS

    SciTech Connect

    Lubow, S. H.; Martin, R. G.

    2012-04-20

    We describe a model for the long-term evolution of a circumplanetary disk that is fed mass from a circumstellar disk and contains regions of low turbulence (dead zones). We show that such disks can be subject to accretion-driven outbursts, analogous to outbursts previously modeled in the context of circumstellar disks to explain FU Ori phenomena. Circumplanetary disks around a proto-Jupiter can undergo outbursts for infall accretion rates onto the disks in the range M-dot{sub infall} approx. 10{sup -9} to 10{sup -7} M{sub Sun} yr{sup -1}, typical of accretion rates in the T Tauri phase. During outbursts, the accretion rate and disk luminosity increases by several orders of magnitude. Most of the planet mass growth during planetary gas accretion may occur via disk outbursts involving gas that is considerably hotter than predicted by steady state models. For low infall accretion rates M-dot{sub infall} {approx}< 10{sup -10} M{sub sun} yr{sup -1} that occur in late stages of disk accretion, disk outbursts are unlikely to occur, even if dead zones are present. Such conditions are favorable for the formation of icy satellites.

  4. Places to Go: Moodle

    ERIC Educational Resources Information Center

    Downes, Stephen

    2006-01-01

    Educators are becoming increasingly interested in alternatives to learning management systems (LMS) Blackboard and WebCT. Stephen Downes's column Places to Go turns to one internationally popular open source LMS--Moodle. Downes takes the reader through Moodle's Web site, which is simultaneously a Web site about its LMS and an example of what its…

  5. Dynamics of core accretion

    NASA Astrophysics Data System (ADS)

    Nelson, Andrew F.; Ruffert, Maximilian

    2013-02-01

    We perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M⊕ embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the `Piecewise Parabolic Method' with as many as six fixed nested grids, providing spatial resolution on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either `locally isothermal' or `locally isentropic') and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling however, as defined by locally isothermal or

  6. Dynamics of core accretion

    DOE PAGES

    Nelson, Andrew F.; Ruffert, Maximilian

    2012-12-21

    In this paper, we perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M⊕ embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the ‘Piecewise Parabolic Method’ with as many as six fixed nested grids, providing spatial resolutionmore » on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either ‘locally isothermal’ or ‘locally isentropic’) and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling however, as

  7. Dynamics of core accretion

    SciTech Connect

    Nelson, Andrew F.; Ruffert, Maximilian

    2012-12-21

    In this paper, we perform three-dimensional hydrodynamic simulations of gas flowing around a planetary core of mass Mpl = 10M embedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behaviour following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate-temperature background disc with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the ‘Piecewise Parabolic Method’ with as many as six fixed nested grids, providing spatial resolution on the finest grid comparable to the present-day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static envelope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disc. It is sensitive to the thermodynamic treatment of the gas, as modelled by prescribed equations of state (either ‘locally isothermal’ or ‘locally isentropic’) and the temperature of the background disc material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influences the magnitude and character of hydrodynamic feedback of the small-scale flow on the background, and we conclude that accurate modelling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling

  8. An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory.

    PubMed

    Miller-Jones, J C A; Sivakoff, G R; Knigge, C; Körding, E G; Templeton, M; Waagen, E O

    2013-05-24

    Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 ± 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects.

  9. An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory.

    PubMed

    Miller-Jones, J C A; Sivakoff, G R; Knigge, C; Körding, E G; Templeton, M; Waagen, E O

    2013-05-24

    Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 ± 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects. PMID:23704566

  10. To accrete or not accrete, that is the question

    USGS Publications Warehouse

    von, Huene R.

    1986-01-01

    Along modern convergent margins tectonic processes span a spectrum from accretion to erosion. The process of accretion is generally recognized because it leaves a geologic record, whereas the process of erosion is generally hypothetical because it produces a geologic hiatus. Major conditions that determine the dominance of accretion or erosion at modern convergent margins are: 1) rate and direction of plate convergence, 2) sediment supply and type in the trench, and 3) topography of the subducting ocean floor. Most change in structure has been ascribed to plate motion, but both erosion and accretion are observed along the same convergence margin. Thus sediment supply and topography are probably of equivalent importance to plate motion because both erosion and accretion are observed under constant conditions of plate convergence. The dominance of accretion or erosion at a margin varies with the thickness of trench sediment. In a sediment flooded trench, the proportions of subducted and accreted sediment are commonly established by the position of a decollement along a weak horizon in the sediment section. Thus, the vertical variation of sediment strength and the distribution of horizontal stress are important factors. Once deformation begins, the original sediment strength is decreased by sediment remolding and where sediment thickens rapidly, increases in pore fluid pressure can be pronounced. In sediment-starved trenches, where the relief of the subducting ocean floor is not smoothed over, the front of the margin must respond to the topography subducted as well as that accreted. The hypothesized erosion by the drag of positive features against the underside of the upper plate (a high stress environment) may alternate with erosion due to the collapse of a margin front into voids such as graben (a low stress environment). ?? 1986 Ferdinand Enke Verlag Stuttgart.

  11. The impact of angular momentum on black hole accretion rates in simulations of galaxy formation

    NASA Astrophysics Data System (ADS)

    Rosas-Guevara, Y. M.; Bower, R. G.; Schaye, J.; Furlong, M.; Frenk, C. S.; Booth, C. M.; Crain, R. A.; Dalla Vecchia, C.; Schaller, M.; Theuns, T.

    2015-11-01

    Feedback from energy liberated by gas accretion on to black holes (BHs) is an attractive mechanism to explain the exponential cut-off at the massive end of the galaxy stellar mass function. Most previous implementations of BH accretion in hydrodynamical simulations of galaxy formation have assumed that BHs grow at an accretion rate that is proportion to the Bondi rate. A major concern is that the Bondi accretion rate is inappropriate when the accreting material has significant angular momentum. We present an improved accretion model that takes into account the circularization and subsequent viscous transport of infalling material, and implemented as a `subgrid' model in hydrodynamic simulations. The resulting accretion rates are generally low in low mass (≲ 1011.5 M⊙) haloes, but show outbursts of Eddington-limited accretion during galaxy mergers. During outbursts these objects strongly resemble quasars. In higher mass haloes, gas accretion peaks at ˜10 per cent of the Eddington rate, which is thought to be conducive to the formation of radio jets. The resulting accretion rate depends strongly on the effective pressure of the gas surrounding the BH, which in turn depends strongly on halo mass. This induces a sharp transition in the importance of BH feedback. In small haloes, the growth of galaxies is regulated by star formation and supernova feedback, but above a halo mass of 1011.5 M⊙, rapid BH growth leads to the suppression of star formation and reduced growth of stellar mass with increasing halo mass.

  12. He-accreting white dwarfs: accretion regimes and final outcomes

    NASA Astrophysics Data System (ADS)

    Piersanti, L.; Tornambé, A.; Yungelson, L. R.

    2014-12-01

    The behaviour of carbon-oxygen (CO) white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyse the thermal response of an accreting WD to mass deposition at different timescales. The analysis has been performed for initial WD masses and accretion rates in the range 0.60-1.02 M⊙ and 10-9-10-5 M⊙ yr-1, respectively. Thermal regimes in the parameter space MWD-dot{M}_He leading to formation of red-giant-like structures, steady burning of He, and mild, strong and dynamical flashes have been identified and the transition between these regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building up of a He-rich layer via H burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for interpreting He novae is discussed.

  13. Dynamics of continental accretion.

    PubMed

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon.

  14. ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

    SciTech Connect

    Zhu, Zhaohuan

    2015-01-20

    I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.

  15. Core Forensics: Earth's Accretion and Differentiation

    NASA Astrophysics Data System (ADS)

    Badro, J.; Brodholt, J. P.; Siebert, J.; Piet, H.; Ryerson, F. J.

    2013-12-01

    Earth's accretion and its primitive differentiation are intimately interlinked processes. One way to constrain accretionary processes is by looking at the major differentiation event that took place during accretion: core formation. Understanding core formation and core composition can certainly shed a new light on early and late accretionary processes. On the other hand, testing certain accretionary models and hypothesis (fluxes, chemistries, timing) allows -short of validating them- at the very least to unambiguously refute them, through the 'filter'' of core formation and composition. Earth's core formed during accretion as a result of melting, phase-separation, and segregation of accretionary building blocks (from meteorites to planetesimals). The bulk composition of the core and mantle depends on the evolution (pressure, temperature, composition) of core extraction during accretion. The entire process left a compositional imprint on both reservoirs: (1) in the silicate Earth, in terms of siderophile trace-element (Ni, Co, V, Cr, among others) concentrations and isotopic fractionation (Si, Cu, among others), a record that is observed in present-day mantle rocks; and (2) on the core, in terms of major element composition and light elements dissolved in the metal, a record that is observed by seismology through the core density-deficit. This imprint constitutes actually a fairly impressive set of evidence (siderophile element concentration and fractionation, volatile and siderophile element isotopic fractionation), can be used today to trace back the primordial processes that occurred 4.5 billion years ago. We are seeking to provide an overhaul of the standard core formation/composition models, by using a new rationale that bridges geophysics and geochemistry. The new ingredients are (1) new laser-heated diamond anvil cell partitioning data, dramatically extending the previous P-T conditions for experimental work, (2) ab initio molecular dynamics calculations to

  16. Lessons from accretion disks in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Horne, Keith

    1998-04-01

    We survey recent progress in the interpretation of observations of cataclysmic variables, whose accretion disks are heated by viscous dissipation rather than irradiation. Many features of standard viscous accretion disk models are confirmed by tomographic imaging studies of dwarf novae. Eclipse maps indicate that steady disk temperature structures are established during outbursts. Doppler maps of double-peaked emission lines suggest disk chromospheres heated by magnetic activity. Gas streams impacting on the disk rim leave expected signatures both in the eclipses and emission lines. Doppler maps of dwarf nova IP Peg at the beginning of an outburst show evidence for tidally-induced spiral shocks. While enjoying these successes, we must still face up to the dreaded ``SW Sex syndrome'' which afflicts most if not all cataclysmic variables in high accretion states. The anomalies include single-peaked emission lines with skewed kinematics, flat temperature-radius profiles, shallow offset line eclipses, and narrow low-ionization absorption lines at phase 0.5. The enigmatic behavior of AE Aqr is now largely understood in terms of a magnetic propeller model in which the rapidly spinning white dwarf magnetosphere expels the gas stream out of the system before an accretion disk can form. A final piece in this puzzle is the realization that an internal shock zone occurs in the exit stream at just the right place to explain the anomalous kinematics and violent flaring of the single-peaked emission lines. Encouraged by this success, we propose that disk-anchored magnetic propellers operate in the high accretion rate systems afflicted by the SW Sex syndrome. Magnetic fields anchored in the Keplerian disk sweep forward and apply a boost that expels gas stream material flowing above the disk plane. This working hypothesis offers a framework on which we can hang all the SW Sex anomalies. The lesson for theorists is that magnetic links appear to be transporting energy and angular

  17. Current Experimental Basis for Modeling Ice Accretions on Swept Wings

    NASA Technical Reports Server (NTRS)

    Vargas, Mario

    2005-01-01

    This work presents a review of the experimental basis for modeling ice accretions on swept wings. Experimental work related to ice accretion physics on swept wings conducted between 1954 and 2004 is reviewed. Proposed models or explanations of scallop formations are singled out and discussed. Special emphasis is placed on reviewing the work done to determine the basic macroscopic mechanisms of scallop formation. The role of feather growth and its connection to scallop growth is discussed. Conceptual steps in modeling scallop formations are presented. Research elements needed for modeling are discussed.

  18. How do accretion discs break?

    NASA Astrophysics Data System (ADS)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  19. Magnetic dynamos in accreting planetary bodies

    NASA Astrophysics Data System (ADS)

    Golabek, Gregor; Labrosse, Stéphane; Gerya, Taras; Morishima, Ryuji; Tackley, Paul

    2013-04-01

    Laboratory measurements revealed ancient remanent magnetization in meteorites [1] indicating the activity of magnetic dynamos in the corresponding meteorite parent body. To study under which circumstances dynamo activity is possible, we use a new methodology to simulate the internal evolution of a planetary body during accretion and differentiation. Using the N-body code PKDGRAV [2] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [3]. The thermomechanical model takes recent parametrizations of impact processes [4] and of the magnetic dynamo [5] into account. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [6], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [7]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the magnetic dynamo activity. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration, whereas in early-formed bodies accretion and iron core growth occur almost simultaneously and a highly variable magnetic dynamo can operate in the interior of these bodies. [1] Weiss, B.P. et al., Science, 322, 713-716, 2008. [2] Richardson, D. C. et al., Icarus, 143, 45-59, 2000. [3] Gerya, T.V and Yuen, D.J., Phys. Earth Planet. Int., 163, 83-105, 2007. [4] Monteux, J. et al., Geophys. Res. Lett., 34, L24201, 2007. [5] Aubert, J. et al

  20. The Chaotic Light Curves of Accreting Black Holes

    NASA Technical Reports Server (NTRS)

    Kazanas, Demosthenes

    2007-01-01

    We present model light curves for accreting Black Hole Candidates (BHC) based on a recently developed model of these sources. According to this model, the observed light curves and aperiodic variability of BHC are due to a series of soft photon injections at random (Poisson) intervals and the stochastic nature of the Comptonization process in converting these soft photons to the observed high energy radiation. The additional assumption of our model is that the Comptonization process takes place in an extended but non-uniform hot plasma corona surrounding the compact object. We compute the corresponding Power Spectral Densities (PSD), autocorrelation functions, time skewness of the light curves and time lags between the light curves of the sources at different photon energies and compare our results to observation. Our model reproduces the observed light curves well, in that it provides good fits to their overall morphology (as manifest by the autocorrelation and time skewness) and also to their PSDs and time lags, by producing most of the variability power at time scales 2 a few seconds, while at the same time allowing for shots of a few msec in duration, in accordance with observation. We suggest that refinement of this type of model along with spectral and phase lag information can be used to probe the structure of this class of high energy sources.

  1. X-ray observations of the accreting Be/X-ray binary pulsar A 0535+26 in outburst

    NASA Astrophysics Data System (ADS)

    Caballero, I.

    2009-04-01

    Neutron stars are compact objects, characterized by R~10-14 km radius, M~1.4Msun and extremely high central densities ~10e15 g/cm^3. If they are part of a binary system, a flow of matter can take place from the companion star onto the neutron star. The accretion of matter onto neutron stars is one of the most powerful sources of energy in the universe. The accretion of matter takes place under extreme physical conditions, with magnetic fields in the range B~10^(8-15)G, which are impossible to reproduce on terrestrial laboratories. Therefore, accreting neutron stars are unique laboratories to study the matter under extreme conditions. In this thesis, X-ray observations of the accreting Be/X-ray binary A 0535+26 during a normal (type I) outburst are presented. In this system, the neutron star orbits around the optical companion HDE 245770 in an eccentric orbit, and sometimes presents X-ray outbursts (giant or normal) associated with the passage of the neutron star through the periastron. After more than eleven years of quiescence, A 0535+26 showed outbursting activity in 2005. The normal outburst analyzed in this work took place in August/September 2005, and reached a maximum X-ray flux of ~400 mCrab in the 5-100 kev range. The outburst, which lasted for ~30 days, was observed with the RXTE and INTEGRAL observatories. We have measured the spectrum of the source. In particular, two absorption-like features, interpreted as fundamental and first harmonic cyclotron resonant scattering features, have been detected at E~46 kev and E~102 kev with INTEGRAL and RXTE. Cyclotron lines are the only direct way to measure the magnetic field of a neutron star. Our observations have allowed to confirm the magnetic field of A 0535+26 at the site of the X-ray emission to be B~5x10^12 G. We studied the luminosity dependence of the cyclotron line in A 0535+26, and contrary to other sources, we found no significant variation of the cyclotron line energy with the luminosity. Changes of

  2. Constraints on r-process nucleosynthesis in accretion disks

    NASA Technical Reports Server (NTRS)

    Jin, Liping

    1991-01-01

    Systems in which accretion drives an outflow from a region near a compact object may enrich the interstellar medium in r-process elements. A detailed assessment of the efficacy of this mechanism for the r-process is presented here, taking into account the constraints imposed by typical accretion-disk conditions. It is concluded that r-process elements are unlikely to have been made in this way, largely because the total production is too low, by a factor of about 100,000, to explain the observed abundances.

  3. Evolution of a steam atmosphere during Earth's accretion.

    PubMed

    Zahnle, K J; Kasting, J F; Pollack, J B

    1988-01-01

    We have modeled the evolution of an impact-generated steam atmosphere surrounding an accreting Earth. The model assumes Safronov accretion; it includes degassing of planetesimals upon impact, thermal blanketing by a steam atmosphere, interchange of water between the surface and the interior, shock heating and convective cooling of Earth's interior, and hydrogen escape, both by a solar extreme ultraviolet (EUV) powered planetary wind and by impact erosion (atmospheric cratering). The model does not include atmophiles other than water, chemical reaction of water with metallic iron, core formation, compression, and spatial and temporal inhomogeneity of accretion. If the incoming planetesimals were too dry or the EUV flux too high, very little water would accumulate at the surface. Essentially all water retained by such a planet would be through rehydration of silicates. If rehydration were inefficient, very little water would be retained in any form. Degassing of wetter planetesimals produces a steam atmosphere over a magma ocean, the energy of accretion being sufficient to maintain a runaway greenhouse atmosphere. The mass of the atmosphere is limited by water's solubility in the (partial) melt. This type of solution is produced for a wide range of model parameters. During accretion, approximately 30 bars of water could have kept the surface at 1500 degrees K. As the accretional energy input declined below the runaway greenhouse threshold, the steam atmosphere rained out. Outgassing of dissolved water at the close of accretion is quantitatively important. These models can leave from approximately 100 to more than 300 bars of water at the surface at the close of accretion. In general, most of the water accreted remains dissolved in the mantle. H2 could have escaped as rapidly as it formed only if the planetesimals were relatively dry. Consequently H2 should have accumulated until it reached chemical equilibrium with water vapor. Impact erosion (escape caused by impact

  4. Long-orbital-period Prepolars Containing Early K-type Donor Stars. Bottleneck Accretion Mechanism in Action

    NASA Astrophysics Data System (ADS)

    Tovmassian, G.; González–Buitrago, D.; Zharikov, S.; Reichart, D. E.; Haislip, J. B.; Ivarsen, K. M.; LaCluyze, A. P.; Moore, J. P.; Miroshnichenko, A. S.

    2016-03-01

    We studied two objects identified as cataclysmic variables (CVs) with periods exceeding the natural boundary for Roche-lobe-filling zero-age main sequence (ZAMS) secondary stars. We present observational results for V1082 Sgr with a 20.82 hr orbital period, an object that shows a low luminosity state when its flux is totally dominated by a chromospherically active K star with no signs of ongoing accretion. Frequent accretion shutoffs, together with characteristics of emission lines in a high state, indicate that this binary system is probably detached, and the accretion of matter on the magnetic white dwarf takes place through stellar wind from the active donor star via coupled magnetic fields. Its observational characteristics are surprisingly similar to V479 And, a 14.5 hr binary system. They both have early K-type stars as donor stars. We argue that, similar to the shorter-period prepolars containing M dwarfs, these are detached binaries with strong magnetic components. Their magnetic fields are coupled, allowing enhanced stellar wind from the K star to be captured and channeled through the bottleneck connecting the two stars onto the white dwarf’s magnetic pole, mimicking a magnetic CV. Hence, they become interactive binaries before they reach contact. This will help to explain an unexpected lack of systems possessing white dwarfs with strong magnetic fields among detached white+red dwarf systems.

  5. FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

    SciTech Connect

    Kirk, Helen; Myers, Philip C.; Bourke, Tyler L.; Gutermuth, Robert A.; Wilson, Grant W.; Hedden, Abigail

    2013-04-01

    One puzzle in understanding how stars form in clusters is the source of mass-is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of {approx}30 M{sub Sun} Myr{sup -1} (inferred from the N{sub 2}H{sup +} velocity gradient along the filament), and radially contracting onto the filament at {approx}130 M{sub Sun} Myr{sup -1} (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

  6. Electromagnetic Spindown of a Transient Accreting Millisecond Pulsar During Quiescence

    NASA Astrophysics Data System (ADS)

    Melatos, A.; Mastrano, A.

    2016-02-01

    The measured spindown rates in quiescence of the transient accreting millisecond pulsars IGR J00291+5934, XTE J1751-305, SAX J1808.4-3658, and Swift J1756.9-2508 have been used to estimate the magnetic moments of these objects assuming standard magnetic dipole braking. It is shown that this approach leads to an overestimate if the amount of residual accretion is enough to distort the magnetosphere away from a force-free configuration through magnetospheric mass loading or crushing, so that the lever arm of the braking torque migrates inside the light cylinder. We derive an alternative spindown formula and calculate the residual accretion rates where the formula is applicable. As a demonstration we apply the alternative spindown formula to produce updated magnetic moment estimates for the four objects above. We note that based on current uncertain observations of quiescent accretion rates, magnetospheric mass loading and crushing are neither firmly indicated nor ruled out in these four objects. Because quiescent accretion rates are not measured directly (only upper limits are placed), without more data it is impossible to be confident about whether the thresholds for magnetospheric mass loading or crushing are reached or not.

  7. Classical Accreting Pulsars with NICER

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2014-01-01

    Soft excesses are very common center dot Lx > 1038 erg/s - reprocessing by optically thick material at the inner edge of the accretion disk center dot Lx < 1036 erg/s - photoionized or collisionally heated diffuse gas or thermal emission from the NS surface center dot Lx 1037 erg/s - either or both types of emission center dot NICER observations of soft excesses in bright X-ray pulsars combined with reflection modeling will constrain the ionization state, metalicity and dynamics of the inner edge of the magnetically truncated accretion disk Reflection models of an accretion disk for a hard power law - Strong soft excess below 3 keV from hot X-ray heated disk - For weakly ionized case: strong recombination lines - Are we seeing changes in the disk ionization in 4U1626-26? 13 years of weekly monitoring with RXTE PCA center dot Revealed an unexpectedly large population of Be/X-ray binaries compared to the Milky Way center dot Plotted luminosities are typical of "normal" outbursts (once per orbit) center dot The SMC provides an excellent opportunity to study a homogenous population of HMXBs with low interstellar absorption for accretion disk studies. Monitoring with NICER will enable studies of accretion disk physics in X-ray pulsars center dot The SMC provides a potential homogeneous low-absorption population for this study center dot NICER monitoring and TOO observations will also provide measurements of spinfrequencies, QPOs, pulsed fluxes, and energy spectra.

  8. Characterizing Accreting White Dwarf Pulsators

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, Anjum

    2014-02-01

    Understanding the population, mass distribution, and evolution of accreting white dwarfs impacts the entire realm of binary interaction, including the creation of Type Ia supernovae. We are concentrating on accreting white dwarf pulsators, as the pulsation properties allow us a view of how the accretion affects the interior of the star. Our ground- based photometry on 11 accreting pulsators with corresponding temperatures from HST UV spectra suggest a broad instability strip in the range of 10500 to 16000K. Additionally, tracking a post-outburst heated white dwarf as it cools and crosses the blue edge and resumes pulsation provides an independent method to locate the empirical instability strip. Determining a post-outburst cooling curve yields an estimate of the amount of heating and the accreted mass during the outburst. We request additional photometry of 2 objects that present unique properties: GW Lib which has not yet returned to its pre-outburst pulsation spectrum after 6 yrs, and EQ Lyn which returned to its pre- outburst pulsation after 3 yrs but is now turning on and off without ongoing outbursts. Following the pulsation spectrum changes over stretches of several nights in a row will provide specific knowledge of the stability of the observed modes.

  9. Black hole accretion disc impacts

    NASA Astrophysics Data System (ADS)

    Pihajoki, P.

    2016-04-01

    We present an analytic model for computing the luminosity and spectral evolution of flares caused by a supermassive black hole impacting the accretion disc of another supermassive black hole. Our model includes photon diffusion, emission from optically thin regions and relativistic corrections to the observed spectrum and time-scales. We test the observability of the impact scenario with a simulated population of quasars hosting supermassive black hole binaries. The results indicate that for a moderate binary mass ratio of 0.3, and impact distances of 100 primary Schwarzschild radii, the accretion disc impacts can be expected to equal or exceed the host quasar in brightness at observed wavelength λ = 510 nm up to z = 0.6. We conclude that accretion disc impacts may function as an independent probe for supermassive black hole binaries. We release the code used for computing the model light curves to the community.

  10. Privileged Girls: The Place of Femininity and Femininity in Place

    ERIC Educational Resources Information Center

    Fahey, Johannah

    2014-01-01

    Constructions of femininity and attendant notions of feminism are being produced in different ways in different places around the world. This is a complicated global process that cannot be reduced to analyses that take place in nation states. This paper seeks to respond to and enhance Angela McRobbie's compelling argument about understandings…

  11. Accretion Disk Dynamo as the Trigger for X-Ray Binary State Transitions

    NASA Astrophysics Data System (ADS)

    Begelman, Mitchell C.; Armitage, Philip J.; Reynolds, Christopher S.

    2015-08-01

    Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a “dead zone” where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, “intermediate” and “hard” accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in “soft” states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the “soft” state show no signs of the thermal/viscous instability predicted by standard α-models.

  12. ACCRETION DISK DYNAMO AS THE TRIGGER FOR X-RAY BINARY STATE TRANSITIONS

    SciTech Connect

    Begelman, Mitchell C.; Armitage, Philip J.; Reynolds, Christopher S.

    2015-08-20

    Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a “dead zone” where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, “intermediate” and “hard” accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in “soft” states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the “soft” state show no signs of the thermal/viscous instability predicted by standard α-models.

  13. Fundamental Ice Crystal Accretion Physics Studies

    NASA Technical Reports Server (NTRS)

    Struk, Peter M.; Broeren, Andy P.; Tsao, Jen-Ching; Vargas, Mario; Wright, William B.; Currie, Tom; Knezevici, Danny; Fuleki, Dan

    2012-01-01

    Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. In November 2010, two weeks of testing occurred at the NRC Research Altitude Facility utilizing a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15 C. Ice and liquid water contents ranged up to 20 and 3 g/m3, respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leading-edge ice growth to a thickness greater than 15 mm in 3 min. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared to be correlated with the wet-bulb temperature, which was estimated to be above 0 C in tests at 93 kPa and below 0 C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic performance of a compressor component

  14. Fundamental Ice Crystal Accretion Physics Studies

    NASA Technical Reports Server (NTRS)

    Currie, Tom; Knezevici, Danny; Fuleki, Dan; Struk, Peter M.; Broeren, Andy P.; Tsao, Jen-ching; Vargas, Mario; Wright, William

    2011-01-01

    Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice-crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. In November 2010, two weeks of testing occurred at the NRC Research Altitude Facility utilizing a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15 C. Ice and liquid water contents ranged up to 20 and 3 grams per cubic meter, respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leading-edge ice growth to a thickness greater than 15 millimeters in 3 minutes. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared to be correlated with the wet-bulb temperature, which was estimated to be above 0 C in tests at 93 kPa and below 0 C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic

  15. Elemental Fractionation During Rapid Accretion of the Moon Triggered by a Giant Impact

    NASA Technical Reports Server (NTRS)

    Abe, Y.; Zahnle, K. J.; Hashimoto, A.

    1998-01-01

    Recently, Ida et al. made an N-body simulation of lunar accretion from a protolunar disk formed by a giant impact. One of their important conclusions is that the accretion time of the Moon is as short as one month. Such rapid accretion is a necessary consequence of the high surface density of a lunar mass disk accreting just beyond the Roche limit (about 3Re); the Safronov accretion time (a few days) is even shorter. The energy of accretion always exceeds the gravitational binding energy of newly arriving matter. Hence, without an energy sink, the accreting body is thermally unstable. For the Earth and other planets, radiation acts as the sink. However, in such a short accretion time, the Moon cannot radiate the accretional energy. Even radiating at a silicate cloudtop temperature of roughly 2000 K, it would take more than 100 yr to radiatively cool the Moon. The plausible alternative heat sinks are heat capacity, latent heat of vaporization, and thermal escape of the gas to space (i.e., hydrodynamic blowoff). The latter becomes plausible for the Moon because the scale height at 2000 K (about 300 km) is a significant fraction of the lunar radius. The early stages of lunar (or "lunatesimal") growth release relatively little energy and can occur simply by heating the material, especially if the accreting material is originally cold. However, the material is unlikely to be cold, because the disk itself is hot and cooling time is long, while the lunar accretion time iss very short. Therefore, the moon is likely to accrete condensed material just after it condenses. Accordingly, the newly accreted material will be on the verge of vaporization and will have very little heat capacity to spare. The immediate heat sink is the latent heat of vaporization. Most of the vapor will escape from the moon, because the thermal energy in the gas can be used to drive escape. However, vaporization is generally incomplete. the latent heat of vaporization exceeds the energy of accretion

  16. Smearing of mass accretion rate variation by viscous processes in accretion disks in compact binary systems

    NASA Astrophysics Data System (ADS)

    Ghosh, A.; Chakrabarti, Sandip K.

    2016-09-01

    Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. Hence, by the time the flow reaches the inner edge, the variation in X-rays need not reflect the true variation of the mass supply rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale t_{{visc}} also to spread around a mean value. In high mass X-ray binaries, which are thought to be primarily wind-fed, the size of the viscous Keplerian disk is smaller and thus such a spread could be lower as compared to the low mass X-ray binaries which are primarily fed by Roche lobe overflow. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a detailed knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales, such as when there is an ellipticity in the orbit. We study a few compact binaries using long term All Sky monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) and all sky survey data (15-50 keV) of Swift satellites by different methods to look for such smearing effects and to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation and found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in high mass X-ray binaries the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in low mass X-ray binaries with larger Keplerian disk component, the peaks are spreaded out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of

  17. HOYLE-LYTTLETON ACCRETION IN THREE DIMENSIONS

    SciTech Connect

    Blondin, John M.; Raymer, Eric

    2012-06-10

    We investigate the stability of gravitational accretion of an ideal gas onto a compact object moving through a uniform medium at Mach 3. Previous three-dimensional simulations have shown that such accretion is not stable, and that strong rotational 'disk-like' flows are generated and accreted on short timescales. We re-address this problem using overset spherical grids that provide a factor of seven improvement in spatial resolution over previous simulations. With our higher spatial resolution we found these three-dimensional accretion flows remained remarkably axisymmetric. We examined two cases of accretion with different sized accretors. The larger accretor produced very steady flow, with the mass accretion rate varying by less than 0.02% over 30 flow times. The smaller accretor exhibited an axisymmetric breathing mode that modulated the mass accretion rate by a constant 20%. Nonetheless, the flow remained highly axisymmetric with only negligible accretion of angular momentum in both cases.

  18. Perturbation growth in accreting filaments

    NASA Astrophysics Data System (ADS)

    Clarke, S. D.; Whitworth, A. P.; Hubber, D. A.

    2016-05-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long filaments as they form and grow by accretion. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length-scale which is roughly four times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multiwavelength density power spectrum, there exists a clear preferred core separation equal to the largest peak in the dispersion relation. Our results allow one to estimate a minimum age for a filament which is breaking up into regularly spaced fragments, as well as an average accretion rate. We apply the model to observations of filaments in Taurus by Tafalla & Hacar and find accretion rates consistent with those estimated by Palmeirim et al.

  19. COULD SXP 1062 BE AN ACCRETING MAGNETAR?

    SciTech Connect

    Fu Lei; Li Xiangdong

    2012-10-01

    In this work we explore the possible evolutionary track of the neutron star in the newly discovered Be/X-ray binary SXP 1062, which is believed to be the first X-ray pulsar associated with a supernova remnant. Although no cyclotron feature has been detected to indicate the strength of the neutron star's magnetic field, we show that it may be {approx}> 10{sup 14} G. If so, SXP 1062 may belong to the accreting magnetars in binary systems. We attempt to reconcile the short age and long spin period of the pulsar taking account of different initial parameters and spin-down mechanisms of the neutron star. Our calculated results show that to spin down to a period {approx}1000 s within 10-40 kyr requires efficient propeller mechanisms. In particular, the model for angular momentum loss under energy conservation seems to be ruled out.

  20. Could SXP 1062 be an Accreting Magnetar?

    NASA Astrophysics Data System (ADS)

    Fu, Lei; Li, Xiang-Dong

    2012-10-01

    In this work we explore the possible evolutionary track of the neutron star in the newly discovered Be/X-ray binary SXP 1062, which is believed to be the first X-ray pulsar associated with a supernova remnant. Although no cyclotron feature has been detected to indicate the strength of the neutron star's magnetic field, we show that it may be >~ 1014 G. If so, SXP 1062 may belong to the accreting magnetars in binary systems. We attempt to reconcile the short age and long spin period of the pulsar taking account of different initial parameters and spin-down mechanisms of the neutron star. Our calculated results show that to spin down to a period ~1000 s within 10-40 kyr requires efficient propeller mechanisms. In particular, the model for angular momentum loss under energy conservation seems to be ruled out.

  1. On Thermohaline Mixing in Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Koester, Detlev

    2015-06-01

    We discuss the recent claim that the thermohaline (“fingering”) instability is important in accreting white dwarfs, increasing the derived accretion fluxes potentially by orders of magnitude. We present an alternative view and conclude that at least in the steady state this is not the case and the current method of estimating accretion fluxes is correct.

  2. Are You There Gas? It's Me, Planet: The Effects of Gas on Growth of Gas Giant Cores through Planetesimal Accretion

    NASA Astrophysics Data System (ADS)

    Wolansky, Natania R.

    2014-04-01

    Before now, models have not been successful in predicting the rapid growth of rocky cores of gas giant planets at large separations from their host stars. Timescales for growth have far outstripped the lifetime of the gaseous disk surrounding the young star, creating a paradox between the need for the core to accrete material and the depleted supply of gas and dust. I present a model for planetary core accretion taking into account the effect of surrounding gas on the dynamics between the core and the accretable material, thus altering the characteristics of the effective cross section of accretion of the planet. By replacing the Hill radius with a wind shearing (WISH) radius, which tracks the point at which a small particle is not sheared away from a core by differential gas drag force, and by imposing additional energy constraints which determine whether a particle will successfully decouple from the gas during its encounter with the core, I recalculate the timescales of growth of a planetary core under a number of varying parameters. I apply the results to the A-type HR8799 star system, including HR8799b, c, and d, roughly 10MJ planets located at a separation of 68, 38, and 24 AU, respectively. Using the model, I reduce the "last doubling" timescales of growth predicted by classical gravitational focusing models by a factor of 1000, from 107 years to 104 years for HR8799b, c, and d, placing timescales of growth in all three cases within acceptable limits to agree with the lifetime of a gaseous disk and the deduced lifetimes of the planets. These results place within the realm of possibility that these 3 planets are formed by core accretion instead of gravitational instability. In exploring the timescales for growth of planetary cores in systems with varying parameters such as star size, disk density, and dust particle size distributions, I provide a model for predicting the possibility of driftless formation of a gas giant given the protoplanetary system

  3. Bolometric luminosity black hole growth time and slim accretion discs in active galactic nuclei

    NASA Astrophysics Data System (ADS)

    Netzer, Hagai; Trakhtenbrot, Benny

    2014-02-01

    We investigate the accretion rate, bolometric luminosity, black hole (BH) growth time and BH spin in a large active galactic nucleus (AGN) sample under the assumption that all such objects are powered via thin or slim accretion discs (ADs). We use direct estimates of the mass accretion rate, dot{M}, to show that many currently used values of Lbol and L/LEdd are either underestimated or overestimated because they are based on bolometric correction factors that are adjusted to the properties of moderately accreting AGNs and do not take into account the correct combination of BH mass, spin and accretion rate. The consistent application of AD physics to our sample of Sloan Digital Sky Survey (SDSS) AGNs leads to the following findings. (1) Even the most conservative assumption about the radiative efficiency of fast-accreting BHs shows that many of these sources must contain slim ADs. We illustrate this by estimating the fraction of such objects at various redshifts. (2) Many previously estimated BH growth times are inconsistent with the AD theory. In particular, the growth times of the fastest accreting BHs were overestimated in the past by large factors with important consequences to AGN evolution. (3) Currently used bolometric correction factors for low accretion rate very massive SDSS BHs are inconsistent with the AD theory. Applying the AD set of assumptions to such objects, combined with standard photoionization calculations of broad emission lines, leads to the conclusion that many such objects must contain fast-spinning BHs.

  4. The structure and appearance of winds from supercritical accretion disks. I - Numerical models

    NASA Technical Reports Server (NTRS)

    Meier, D. L.

    1979-01-01

    Equations for the structure and appearance of supercritical accretion disks and the radiation-driven winds which emanate from them are derived and solved by a steady-state hydrodynamic computer code with a relaxation technique used in stellar structure problems. The present model takes into account the mass of the accreting star, the total accretion rate, a generalization of the disk alpha parameter which accounts for heating by processes in addition to viscosity, and the ratio of the total luminosity to the Eddington luminosity. Solutions indicate that for accretion onto a hard-surfaced star, steady, optically thick winds result for even slightly supercritical accretion, and the object will appear as a supergiant star with a high mass loss rate and a nonblackbody spectrum. Winds from black hole accretion disks are expected to depend on the form of the accretion interior to the critical radius, possibly consisting of no ejection at all, a wind similar to that of a hard-surfaced star, or a column of material ejected from a hole in the accretion disk.

  5. The insanity of place.

    PubMed

    Scull, Andrew

    2004-12-01

    This paper uses English examples to scrutinize the complex interrelations of insanity and place over the past three centuries, taking as its starting point the late Erving Goffman's paper of the same title. From eighteenth-century Bedlam and the emerging trade in lunacy, through the county asylums and licensed madhouses of the nineteenth century to the return of the the mentally ill to the 'community' in the last half of the twentieth century, the place occupied by insanity has varied sharply, symbolically as well as concretely. These various techniques of containment and damage limitation must be understood as a response to the threats, symbolic and practical, that serious mental illness poses to the social order, at both the micro- and macroscopic levels of analysis.

  6. Crustal accretion at fast spreading ridges and implications for hydrothermal circulation

    NASA Astrophysics Data System (ADS)

    Theissen-Krah, S.; Rupke, L.; Hasenclever, J.

    2015-12-01

    Oceanic crust is continuously created at mid-ocean ridges, but the location of lower crust crystallization continues to be debated since the proposal of the gabbro glacier and many sills end-member models. Geophysical and geochemical studies find evidence for either of the models. The crust is cooled by a combination of heat diffusion and advection, and hydrothermal circulation is thought to play a key role in distinguishing between both models. We use our numerical model for joint modeling of crustal accretion and hydrothermal circulation1 to test different accretion and hydrothermal cooling scenarios. The results match the seismic and structural observations from the East Pacific Rise2 and the Oman Ophiolite3, with a shallow melt lens at the correct location overlaying a narrow volume of partially molten rocks. Our results show that no more than 25-50% of the lower crust crystallizes in situ and that deep circulation is likely to occur at fast and intermediate spreading ridges. The occurrence of deep hydrothermal cooling however does not rule out that a major portion of the lower crust is formed in the shallow melt lens; our simulations rather suggest that it is necessary independent of where in the lower crust crystallization takes place. 1 Theissen-Krah, S., Iyer, K., Rupke, L. H. & Morgan, J. P. Coupled mechanical and hydrothermal modeling of crustal accretion at intermediate to fast spreading ridges. Earth and Planetary Science Letters 311, 275-286, doi:10.1016/j.epsl.2011.09.018 (2011). 2 Dunn, R. A., Toomey, D. R. & Solomon, S. C. Three-dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9 degrees 30'N. Journal of Geophysical Research-Solid Earth 105, 23537-23555 (2000). 3 Nicolas, A. & Boudier, F. Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise. Terra Nova 27, 77-96, doi:10.1111/ter.12137 (2015).

  7. Cyclotron Resonance in Accreting Pulsars

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Dipankar

    2016-07-01

    Cyclotron Resonance Absorption/Scattering features provide direct measurement of magnetic field strength in the line forming region. This has enabled the estimation of magnetic field strengths of nearly two dozen neutron stars in accreting high mass binary systems. With improved spectroscopic sensitivity, new X-ray observatories such as NuSTAR, Astrosat and Hitomi are opening the doors to studying detailed features such as the line shape and phase dependence with high significance. Such studies will help understand the nature of matter accumulation in, and outflow from, the magnetically confined accretion column on the neutron star. This talk will describe the results of MHD simulations of the matter flow in such systems, the diagnostics of such flows using cyclotron lines, and comparison with recent observations from NuSTAR and Astrosat.

  8. Obscured accretion from AGN surveys

    NASA Astrophysics Data System (ADS)

    Vignali, Cristian

    2014-07-01

    Recent models of super-massive black hole (SMBH) and host galaxy joint evolution predict the presence of a key phase where accretion, traced by obscured Active Galactic Nuclei (AGN) emission, is coupled with powerful star formation. Then feedback processes likely self-regulate the SMBH growth and quench the star-formation activity. AGN in this important evolutionary phase have been revealed in the last decade via surveys at different wavelengths. On the one hand, moderate-to-deep X-ray surveys have allowed a systematic search for heavily obscured AGN, up to very high redshifts (z~5). On the other hand, infrared/optical surveys have been invaluable in offering complementary methods to select obscured AGN also in cases where the nuclear X-ray emission below 10 keV is largely hidden to our view. In this review I will present my personal perspective of the field of obscured accretion from AGN surveys.

  9. Taking Risks.

    ERIC Educational Resources Information Center

    Merson, Martha, Ed.; Reuys, Steve, Ed.

    1999-01-01

    Following an introduction on "Taking Risks" (Martha Merson), this journal contains 11 articles on taking risks in teaching adult literacy, mostly by educators in the Boston area. The following are included: "My Dreams Are Bigger than My Fears Now" (Sharon Carey); "Making a Pitch for Poetry in ABE [Adult Basic Education]" (Marie Hassett); "Putting…

  10. ACCRETION RATES OF MOONLETS EMBEDDED IN CIRCUMPLANETARY PARTICLE DISKS

    SciTech Connect

    Ohtsuki, Keiji; Yasui, Yuki; Daisaka, Hiroshi

    2013-08-01

    We examine the gravitational capture probability of colliding particles in circumplanetary particle disks and accretion rates of small particles onto an embedded moonlet, using analytic calculation, three-body orbital integrations, and N-body simulations. Expanding our previous work, we take into account the Rayleigh distribution of particles' orbital eccentricities and inclinations in our analytic calculation and orbital integration and confirm agreement between them when the particle velocity dispersion is comparable to or larger than their mutual escape velocity and the ratio of the sum of the physical radii of colliding particles to their mutual Hill radius (r-tilde{sub p}) is much smaller than unity. As shown by our previous work, the capture probability decreases significantly when the velocity dispersion is larger than the escape velocity and/or r-tilde{sub p}{approx}>0.7. Rough surfaces of particles can enhance the capture probability. We compare the results of three-body calculations with N-body simulations for accretion of small particles by an embedded moonlet and find agreement at the initial stage of accretion. However, when particles forming an aggregate on the moonlet surface nearly fill the Hill sphere, the aggregate reaches a quasi-steady state with a nearly constant number of particles covering the moonlet, and the accretion rate is significantly reduced compared to the three-body results.

  11. Theory of Black Hole Accretion Discs

    NASA Astrophysics Data System (ADS)

    Abramowicz, Marek A.; Björnsson, Gunnlaugur; Pringle, James E.

    1999-03-01

    Part I. Observations of Black Holes: 1. Black holes in our Galaxy: observations P. Charles; 2. Black holes in Active Galactic Nuclei: observations G. M. Madejski; Part II. Physics Close to a Black Hole: 3. Physics of black holes I. D. Novikov; 4. Physics of black hole accretion M. A. Abramowicz; Part III. Turbulence, Viscosity: 5. Disc turbulence and viscosity A. Brandenburg; Part IV. Radiative Processes: 6. The role of electron-positron pairs in accretion flows G. Björnsson; 7. Accretion disc-corona models and X/Y-ray spectra of accreting black holes J. Poutanen; 8. Emission lines: signatures of relativistic rotation A. C. Fabian; Part V. Accretion Discs: 9. Spectral tests of models for accretion disks around black holes J. H. Krolik; 10. Advection-dominated accretion around black holes R. Narayan, R. Mahadevan and E. Quataert; 11. Accretion disc instabilities and advection dominated accretion flows J.-P. Lasota; 12. Magnetic field and multi-phase gas in AGN A. Celotti and M. J. Rees; Part V. Discs in Binary Black Holes: 13. Supermassive binary black holes in galaxies P. Artymowicz; Part VI. Stability of Accretion Discs: 14. Large scale perturbation of an accretion disc by a black hole binary companion J. C. B. Papaloizou, C. Terquem and D. N. C. Lin; 15. Stable oscillations of black hole accretion discs M. Nowak and D. Lehr; Part VI. Coherant Structures: 16. Spotted discs A. Bracco, A. Provenzale, E. A. Spiegel and P. Yecko; Self-organized critically in accretion discs P. Wiita and Y. Xiong; Summary: old and new advances in black hole accretion disc theory R. Svensson.

  12. Theory of Black Hole Accretion Discs

    NASA Astrophysics Data System (ADS)

    Abramowicz, Marek A.; Björnsson, Gunnlaugur; Pringle, James E.

    2010-08-01

    Part I. Observations of Black Holes: 1. Black holes in our Galaxy: observations P. Charles; 2. Black holes in Active Galactic Nuclei: observations G. M. Madejski; Part II. Physics Close to a Black Hole: 3. Physics of black holes I. D. Novikov; 4. Physics of black hole accretion M. A. Abramowicz; Part III. Turbulence, Viscosity: 5. Disc turbulence and viscosity A. Brandenburg; Part IV. Radiative Processes: 6. The role of electron-positron pairs in accretion flows G. Björnsson; 7. Accretion disc-corona models and X/Y-ray spectra of accreting black holes J. Poutanen; 8. Emission lines: signatures of relativistic rotation A. C. Fabian; Part V. Accretion Discs: 9. Spectral tests of models for accretion disks around black holes J. H. Krolik; 10. Advection-dominated accretion around black holes R. Narayan, R. Mahadevan and E. Quataert; 11. Accretion disc instabilities and advection dominated accretion flows J.-P. Lasota; 12. Magnetic field and multi-phase gas in AGN A. Celotti and M. J. Rees; Part V. Discs in Binary Black Holes: 13. Supermassive binary black holes in galaxies P. Artymowicz; Part VI. Stability of Accretion Discs: 14. Large scale perturbation of an accretion disc by a black hole binary companion J. C. B. Papaloizou, C. Terquem and D. N. C. Lin; 15. Stable oscillations of black hole accretion discs M. Nowak and D. Lehr; Part VI. Coherant Structures: 16. Spotted discs A. Bracco, A. Provenzale, E. A. Spiegel and P. Yecko; Self-organized critically in accretion discs P. Wiita and Y. Xiong; Summary: old and new advances in black hole accretion disc theory R. Svensson.

  13. Spherical accretion and AGN feedback

    NASA Astrophysics Data System (ADS)

    Nulsen, Paul

    2014-06-01

    For a supermassive black hole accreting from a hot, quasi-spherical atmosphere, it is almost inevitable that the fluid approximation fails inside some point within the Bondi radius, but well outside the black hole event horizon. Within the region where the particle mean free paths exceed the radius, the flow must be modeled in terms of the Fokker-Planck equation. In the absence of magnetic fields, it is analogous to the "loss cone" problem for consumption of stars by a black hole. The accretion rate is suppressed well below the Bondi accretion rate and a significant power must be conveyed outward for the flow to proceed. This situation is complicated significantly by the presence of a magnetic field, but I will argue that the main outcomes are similar. I will also argue that the power emerging from such a flow, although generally far too little to suppress cooling on large scales, is an important ingredient of the AGN feedback cycle on scales comparable to the Bondi radius.

  14. Accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Abramowicz, M. A.

    1994-01-01

    The physics of accretion flow very close to a black hole is dominated by several general relativistic effects. It cannot be described by the standard Shakura Sunyaev model or by its relativistic version developed by Novikov and Thome. The most important of these effects is a dynamical mass loss from the inner edge of the disk (Roche lobe overflow). The relativistic Roche lobe overflow induces a strong advective cooling, which is sufficient to stabilize local, axially symmetric thermal and viscous modes. It also stabilizes the non-axially-symmetric global modes discovered by Papaloizou and Pringle. The Roche lobe overflow, however, destabilizes sufficiently self-gravitating accretion disks with respect to a catastrophic runaway of mass due to minute changes of the gravitational field induced by the changes in the mass and angular momentum of the central black hole. One of the two acoustic modes may become trapped near the inner edge of the disk. All these effects, absent in the standard model, have dramatic implications for time-dependent behavior of the accretion disks around black holes.

  15. Collapsar Accretion, Shockwaves, and the Gamma-ray Burst X-ray Light Curve

    NASA Astrophysics Data System (ADS)

    Lindner, Christopher C.; Milosavljevic, M.

    2010-03-01

    We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a "collapsar,'' onto the central black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB gamma-ray and X-ray light curve, which lends support to the proposal by Kumar et al. 2008 that the luminosity is modulated by the central accretion rate. In the first "prompt'' phase characterized by an approximately constant accretion rate, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of 2 Msun s-1. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate Mdot t-2.8. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low-angular-momentum material in the axial funnel reverses into an outflow from the surface of the thick disk, and the decay of the accretion rate is slowed. While the duration of the "prompt'' phase depends on the resolution in our simulations, we provide an analytical model taking into account neutrino losses that estimates the duration to be 20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 1010 K, such that neutrino cooling shuts off. We also present results from 1D simulations of the accretion powered acceleration of the shockwave formed in collapsar models.

  16. Mountain Hike North of Big Cottonwood Canyon Road, Begining at the S-Turn at Mill B., Near Hidden Falls, and Taking Trail Leading to Mt. Raymond and Other Intersting Places.

    NASA Astrophysics Data System (ADS)

    McDonald, Keith L.

    2004-11-01

    Our first objective is to leave the highway via Mill B North Fork by taking the Big Cottonwood Canyon trail that leads to Maxfield Basin, where 3 trails intersect, just s. of Mount Raymond (Elev. 10,241 ft.) the n. trail takes us down to the Mill Creek Canyon Road, at about 1 mi. (+) east of intersection with Church Park Picnic Ground road. At Maxfield Basin, again, the east trail skirts around Mt. Raymond and has another intersection with a trail running n. thru the area of Gobblers Knob (elev. 10,246 ft.), to White Fir Pass and turns w. at Bowman Fk. until it connects with Porter Fork and then the Mill Creek Road. The remaining trail at Mill A Basin, just e. of Mount Raymond, long before Gobblers Knob is seen, runs east past a spring, and connects to Butler Fork (which begins at 3.775 mi., measured along highway from Mill B, North Fork), which leads directly to Dog Lake. Evidently both Dog Lake and Lake Desolation (changing U.S. Geological Survey maps from Mount Aire, Utah to Park City West, Utah) have connected outlets, at least during certain times of the year. Following the trail s. e. (down) that follows near Summit Co. and Salt Lake County, we pass by the radio transmitters shown on Park City, West, Utah, map and finally enter the Brighton, Utah map with Scott Hill, Scott Pass, the important highway leading to Midway Reservoir, and beyond, Bloods Lake ( 9500 ft.), Clayton Peak (10,721 ft.) and Lake Lackawaxen ( 9980 ft.), our final destination showing through. One may easily walk the distance to lake Lackawaxen from Bloods Lake by staying south of the ridgecrest and by following the hollow down for a while. This completes our destination. Recall that the main roadway here was already passed over about 1/2 mile n. of Bloods Lake; this thoroughfare has its beginning at about 0.4 miles below (or North) of the Brighton Loop, where the road to city of Midway leaves the main Big Cottonwood Highway going n. and runs e., on the average, going past Midway Reservoir

  17. Taking antacids

    MedlinePlus

    ... magnesium may cause diarrhea. Brands with calcium or aluminum may cause constipation. Rarely, brands with calcium may ... you take large amounts of antacids that contain aluminum, you may be at risk for calcium loss, ...

  18. The size of star clusters accreted by the Milky Way

    NASA Astrophysics Data System (ADS)

    Miholics, Meghan; Webb, Jeremy J.; Sills, Alison

    2014-12-01

    We perform N-body simulations of a cluster that forms in a dwarf galaxy and is then accreted by the Milky Way to investigate how a cluster's structure is affected by a galaxy merger. We find that the cluster's half-mass radius will respond quickly to this change in potential. When the cluster is placed on an orbit in the Milky Way with a stronger tidal field the cluster experiences a sharp decrease in size in response to increased tidal forces. Conversely, when placed on an orbit with a weaker tidal field, the cluster expands since tidal forces decrease and no longer limit the expansion due to internal effects. In all cases, we find that the cluster's half-mass radius will eventually be indistinguishable from a cluster that has always lived in the Milky Way on that orbit. These adjustments occur within 1-2 half-mass relaxation times of the cluster in the dwarf galaxy. We also find this effect to be qualitatively independent of the time that the cluster is taken from the dwarf galaxy. In contrast to the half-mass radius, we show the core radius of the cluster is not affected by the potential the cluster lives in. Our work suggests that structural properties of accreted clusters are not distinct from clusters born in the Milky Way. Other cluster properties, such as metallicity and horizontal branch morphology, may be the only way to identify accreted star clusters in the Milky Way.

  19. What is taking place in science classrooms?: A case study analysis of teaching and learning in seventh-grade science of one Alabama school and its impact on African American student learning

    NASA Astrophysics Data System (ADS)

    Norman, Lashaunda Renea

    This qualitative case study investigated the teaching strategies that improve science learning of African American students. This research study further sought the extent the identified teaching strategies that are used to improve African American science learning reflect culturally responsive teaching. Best teaching strategies and culturally responsive teaching have been researched, but there has been minimal research on the impact that both have on science learning, with an emphasis on the African American population. Consequently, the Black-White achievement gap in science persists. The findings revealed the following teaching strategies have a positive impact on African American science learning: (a) lecture-discussion, (b) notetaking, (c) reading strategies, (d) graphic organizers, (e) hands-on activities, (f) laboratory experiences, and (g) cooperative learning. Culturally responsive teaching strategies were evident in the seventh-grade science classrooms observed. Seven themes emerged from this research data: (1) The participating teachers based their research-based teaching strategies used in the classroom on all of the students' learning styles, abilities, attitudes towards science, and motivational levels about learning science, with no emphasis on the African American student population; (2) The participating teachers taught the state content standards simultaneously using the same instructional model daily, incorporating other content areas when possible; (3) The participating African American students believed their seventh-grade science teachers used a variety of teaching strategies to ensure science learning took place, that science learning was fun, and that science learning was engaging; (4) The participating African American students genuinely liked their teacher; (5) The participating African American students revealed high self-efficacy; (6) The African American student participants' parents value education and moved to Success Middle School

  20. Evolution of a steam atmosphere during earth's accretion

    NASA Technical Reports Server (NTRS)

    Zahnle, Kevin J.; Kasting, James F.; Pollack, James B.

    1988-01-01

    The evolution of an impact-generated steam atmosphere around an accreting earth is presently modeled under the assumption of Safronov (1978) accretion, in a scheme that encompasses the degassing of planetesimals on impact, thermal blanketing by the steam atmosphere, surface-to-interior water exchange, the shock heating and convective cooling of the earth's interior, and hydrogen escape due both to solar EUV-powered planetary wind and impact erosion. The model yields four distinct classes of impact-generated atmospheres: the first, on which emphasis is placed, has as its salient feature a molten surface that is maintained by the opacity of a massive water vapor atmosphere; the second occurs when the EUV-limited escape exceeds the impact degassing rate, while the third is dominated by impact erosion and the fourth is characterized by an atmosphere more massive than any thus far encountered.

  1. Formation of massive stars by growing accretion

    NASA Astrophysics Data System (ADS)

    Maeder, Andre

    There are at present three scenarios for the formation of massive star. 1) The classical scenario of constant mass pre-Main Sequence (MS) evolution on the Kelvin-Helmholtz timescale. 2) The coalescence scenario, with merging of intermediate mass protostars. 3) The accretion scenario. The various arguments for and against these scenarios are briefly reviewed. We examine the pre-MS evolution of accreting stars for constant accretion rates and for accretion rates which are growing with the stellar masses. The location of the birthlines in the HRD and the lifetimes support accretion rates growing fastly with the stellar masses. Remarkably the dependence found is similar to that of the mass outflows from UC HII regions observed by Churchwell (1999) and Henning et al. (2000). The accretion scenario also leads to a new concept for the maximum stellar mass.

  2. Magnetized Accretion-Ejection Structures

    NASA Astrophysics Data System (ADS)

    Ferreira, Jonathan

    1994-09-01

    For both active galactic nuclei (AGN) and young stellar objects (YSO), the common belief is growing that there is an interdependency between accretion of mass onto a central object and the highly collimated jets. This thesis deals with the investigation of the physical mechanism that leads to the formation of jets from a magnetized accretion disk. This has been done by solving the set of magnetohydrodynamical (MHD) equations in the case of an isothermal disk, using a self-similar approach. All the dynamical terms are included, so that the main results are independant of the modelling and thus, completely general. Indeed, a different temperature vertical profile only slightly modifies the parameters required for stationarity. A resistive thin accretion disk is thread by open magnetic field lines, sheared by its differential rotation. The field lines brake the disk and extract both angular momentum and mechanical energy from it. Because of the large magnetic "lever arm" acting on the disk, the magnetic braking is always dominant and the viscous torque is negligible. An equipartition magnetic field is enough, without significantly perturbing the Keplerian rotation. Thus, jets carry away all the angular momentum of the underlying accretion disk. Steady state accretion is achieved in the disk due to an anomalous magnetic diffusivity that allows the matter to slip across the field lines. This anomalous transport coefficient should arise from the saturation of a strong magnetic instability triggered in the disk. Ambipolar diffusion, which could have been used without losing the generality of the present results, remains however smaller than this anomalous diffusivity in the inner parts of a circumstellar disk. It has been found that steady state ejection can be achieved only if the magnetic torque changes its sign at the disk surface. From this point on, the field lines accelerate azimuthaly the matter transfering it both angular momentum and energy. This requires a

  3. 1 Hz FLARING IN THE ACCRETING MILLISECOND PULSAR NGC 6440 X-2: DISK TRAPPING AND ACCRETION CYCLES

    SciTech Connect

    Patruno, Alessandro; D'Angelo, Caroline

    2013-07-10

    The dynamics of the plasma in the inner regions of an accretion disk around accreting millisecond X-ray pulsars (AMXPs) is controlled by the magnetic field of the neutron star. The interaction between an accretion disk and a strong magnetic field is not well understood, particularly at low accretion rates (the so-called propeller regime). This is due in part to the lack of clear observational diagnostics to constrain the physics of the disk-field interaction. Here, we associate the strong {approx}1 Hz modulation seen in the AMXP NGC 6440 X-2 with an instability that arises when the inner edge of the accretion disk is close to the corotation radius (where the stellar rotation rate matches the Keplerian speed in the disk). A similar modulation has previously been observed in another AMXP (SAX J1808.4-3658) and we suggest that the two phenomena are related and that this may be a common phenomenon among other magnetized systems. Detailed comparisons with theoretical models suggest that when the instability is observed, the interaction region between the disk and the field is very narrow-of the order of 1 km. Modeling further suggests that there is a transition region ({approx}1-10 km) around the corotation radius where the disk-field torque changes sign from spin-up to spin-down. This is the first time that a direct observational constraint has been placed on the width of the disk-magnetosphere interaction region, in the frame of the trapped-disk instability model.

  4. Magnetic cataclysmic variable accretion flows

    NASA Astrophysics Data System (ADS)

    Norton, A. J.; Butters, O. W.; Parker, T. L.; Wynn, G. A.

    2007-08-01

    We have used a magnetic accretion model to investigate the accretion flows of magnetic cataclysmic variables (mCVs) throughout a range of parameter space. The results of our numerical simulations demonstrate that broadly four types of flow are possible: discs, streams, rings and propellers. We show that the equilibrium spin periods in asynchronous mCVs, for a given orbital period and magnetic moment, occur where the flow changes from a type characterised by spin-up (i.e. disc or stream) to one characterised by spin-down (i.e. propeller or ring). `Triple points' occur in the plane of spin-to-orbital period ratio versus magnetic moment, at which stream-disc-propeller flows or stream-ring-propeller flows can co-exist. The first of these is identified as corresponding to when the corotation radius is equal to the circularisation radius, and the second as where the corotation radius is equal to the distance from white dwarf to the L1 point. If mCVs are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with Pspin/Porb <~ 0.10 will be disc-like, those with 0.10 <~ Pspin/Porb <~ 0.55 will be stream-like, and those with Pspin/Porb ~ 0.55 will be ring-like. In each case, some material is also lost from the binary in order to maintain angular momentum balance. The spin to orbital period ratio at which the systems transition between these flow types decreases as the mass ratio of the stellar components increases, and vice versa.

  5. Formation of massive stars by growing accretion

    NASA Astrophysics Data System (ADS)

    Maeder, André

    We calculate pre-main sequence evolutionary tracks with accretion rates growing with the actual stellar masses. We show that accretion rates growing at least as M1.5 are necessary to fit the constraints on the lifetimes and HR diagram. Most interestingly, such accretion rates growing with the stellar mass well correspond to those derived from observations of mass outflows (Churchwell 2000; Henning et al. 2000). These rates also lie in the permitted region of the dynamical models.

  6. Lithium synthesis in microquasar accretion.

    PubMed

    Iocco, Fabio; Pato, Miguel

    2012-07-13

    We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes 6Li and 7Li can be produced, the exact figures varying with the characteristics of the torus and reaching as much as 10(-2) M⊙ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis. PMID:23030150

  7. Lithium synthesis in microquasar accretion.

    PubMed

    Iocco, Fabio; Pato, Miguel

    2012-07-13

    We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes 6Li and 7Li can be produced, the exact figures varying with the characteristics of the torus and reaching as much as 10(-2) M⊙ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis.

  8. Pulsed Accretion onto Eccentric and Circular Binaries

    NASA Astrophysics Data System (ADS)

    Muñoz, Diego J.; Lai, Dong

    2016-08-01

    We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ˜ 5 times the binary period P b, accretion onto an eccentric binary is predominantly modulated at the period ˜ 1{P}{{b}}. For an equal-mass circular binary, the accretion rates onto individual stars are quite similar to each other, following the same variable pattern in time. By contrast, for eccentric binaries, one of the stars can accrete at a rate 10–20 times larger than its companion. This “symmetry breaking” between the stars, however, alternates over timescales of order 200P b and can be attributed to a slowly precessing, eccentric circumbinary disk. Over longer timescales, the net accretion rates onto individual stars are the same, reaching a quasi-steady state with the circumbinary disk. These results have important implications for the accretion behavior of binary T Tauri stars and supermassive binary black holes.

  9. Pulsed Accretion onto Eccentric and Circular Binaries

    NASA Astrophysics Data System (ADS)

    Muñoz, Diego J.; Lai, Dong

    2016-08-01

    We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ˜ 5 times the binary period P b, accretion onto an eccentric binary is predominantly modulated at the period ˜ 1{P}{{b}}. For an equal-mass circular binary, the accretion rates onto individual stars are quite similar to each other, following the same variable pattern in time. By contrast, for eccentric binaries, one of the stars can accrete at a rate 10-20 times larger than its companion. This “symmetry breaking” between the stars, however, alternates over timescales of order 200P b and can be attributed to a slowly precessing, eccentric circumbinary disk. Over longer timescales, the net accretion rates onto individual stars are the same, reaching a quasi-steady state with the circumbinary disk. These results have important implications for the accretion behavior of binary T Tauri stars and supermassive binary black holes.

  10. Accretion flows govern black hole jet properties

    NASA Astrophysics Data System (ADS)

    Koljonen, K.; Russell, D.; Fernández Ontiveros, J.; Miller-Jones, J.; Russell, T.; Curran, P.; Soria, R.; Markoff, S.; van der Horst, A.; Casella, P.

    2015-07-01

    The process of jet formation in accreting black holes, and the conditions under which it occurs is currently hotly debated, with competing models predicting the jet power to be governed by black hole spin, the magnetic field strength, the location of the jet base, the mass accretion rate and/or the properties of the inner accretion flow. We present new results that show empirical correlations between the accretion flow properties and the spectral energy distribution of the jets launched from accreting black holes. The X-ray power law is directly related to the particle energy distribution in the hot accretion flow. We find that the photon index of this power law correlates with the characteristic break frequency in the jet spectrum emitted near the jet base, and the jet luminosity up to the break frequency. The observed correlations can be explained by the energy distribution of electrons in the hot accretion flow being subsequently channeled into the jet. These correlations represent a new inflow--outflow connection in accreting black holes, and demonstrate that the spectral properties of the jet rely most critically on the conditions in the inner accretion flow, rather than other parameters such as the black hole mass or spin.

  11. Bondi accretion onto cosmological black holes

    NASA Astrophysics Data System (ADS)

    Karkowski, Janusz; Malec, Edward

    2013-02-01

    In this paper we investigate a steady accretion within the Einstein-Straus vacuole, in the presence of the cosmological constant. The dark energy damps the mass accretion rate and—above a certain limit—completely stops the steady accretion onto black holes, which, in particular, is prohibited in the inflation era and after (roughly) 1012 years from the big bang (assuming the presently known value of the cosmological constant). Steady accretion would not exist in the late phases of the Penrose’s scenario—known as the Weyl curvature hypothesis—of the evolution of the Universe.

  12. Taking Turns

    ERIC Educational Resources Information Center

    Hopkins, Brian

    2010-01-01

    Two people take turns selecting from an even number of items. Their relative preferences over the items can be described as a permutation, then tools from algebraic combinatorics can be used to answer various questions. We describe each person's optimal selection strategies including how each could make use of knowing the other's preferences. We…

  13. Double Take

    ERIC Educational Resources Information Center

    Educational Leadership, 2011

    2011-01-01

    This paper begins by discussing the results of two studies recently conducted in Australia. According to the two studies, taking a gap year between high school and college may help students complete a degree once they return to school. The gap year can involve such activities as travel, service learning, or work. Then, the paper presents links to…

  14. RECONSTRUCTING THE ACCRETION HISTORY OF THE GALACTIC STELLAR HALO FROM CHEMICAL ABUNDANCE RATIO DISTRIBUTIONS

    SciTech Connect

    Lee, Duane M.; Johnston, Kathryn V.; Sen, Bodhisattva; Jessop, Will

    2015-03-20

    Observational studies of halo stars during the past two decades have placed some limits on the quantity and nature of accreted dwarf galaxy contributions to the Milky Way (MW) stellar halo by typically utilizing stellar phase-space information to identify the most recent halo accretion events. In this study we tested the prospects of using 2D chemical abundance ratio distributions (CARDs) found in stars of the stellar halo to determine its formation history. First, we used simulated data from 11 “MW-like” halos to generate satellite template sets (STSs) of 2D CARDs of accreted dwarf satellites, which are composed of accreted dwarfs from various mass regimes and epochs of accretion. Next, we randomly drew samples of ∼10{sup 3–4} mock observations of stellar chemical abundance ratios ([α/Fe], [Fe/H]) from those 11 halos to generate samples of the underlying densities for our CARDs to be compared to our templates in our analysis. Finally, we used the expectation-maximization algorithm to derive accretion histories in relation to the STS used and the sample size. For certain STSs used we typically can identify the relative mass contributions of all accreted satellites to within a factor of two. We also find that this method is particularly sensitive to older accretion events involving low-luminosity dwarfs, e.g., ultra-faint dwarfs—precisely those events that are too ancient to be seen by phase-space studies of stars and too faint to be seen by high-z studies of the early universe. Since our results only exploit two chemical dimensions and near-future surveys promise to provide ∼6–9 dimensions, we conclude that these new high-resolution spectroscopic surveys of the stellar halo will allow us to recover its accretion history—and the luminosity function of infalling dwarf galaxies—across cosmic time.

  15. Swept wing ice accretion modeling

    NASA Technical Reports Server (NTRS)

    Potapczuk, Mark G.; Bidwell, Colin S.

    1990-01-01

    An effort to develop a three-dimensional modeling method was initiated. This first step towards creation of a complete aircraft icing simulation code builds on previously developed methods for calculating three-dimensional flow fields and particle trajectories combined with a two-dimensional ice accretion calculation along coordinate locations corresponding to streamlines. This work is a demonstration of the types of calculations necessary to predict a three-dimensional ice accretion. Results of calculations using the 3-D method for a MS-317 swept wing geometry are projected onto a 2-D plane normal to the wing leading edge and compared to 2-D results for the same geometry. It is anticipated that many modifications will be made to this approach, however, this effort will lay the groundwork for future modeling efforts. Results indicate that the flow field over the surface and the particle trajectories differed for the two calculations. This led to lower collection efficiencies, convective heat transfer coefficients, freezing fractions, and ultimately ice accumulation for the 3-D calculation.

  16. RINGED ACCRETION DISKS: EQUILIBRIUM CONFIGURATIONS

    SciTech Connect

    Pugliese, D.; Stuchlík, Z. E-mail: zdenek.stuchlik@physics.cz

    2015-12-15

    We investigate a model of a ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the general relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can then be determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high-energy phenomena demonstrated in active galactic nuclei.

  17. Nonlinear variations in axisymmetric accretion

    NASA Astrophysics Data System (ADS)

    Bose, Soumyajit; Sengupta, Anindya; Ray, Arnab K.

    2014-05-01

    We subject the stationary solutions of inviscid and axially symmetric rotational accretion to a time-dependent radial perturbation, which includes nonlinearity to any arbitrary order. Regardless of the order of nonlinearity, the equation of the perturbation bears a form that is similar to the metric equation of an analogue acoustic black hole. We bring out the time dependence of the perturbation in the form of a Liénard system by requiring the perturbation to be a standing wave under the second order of nonlinearity. We perform a dynamical systems analysis of the Liénard system to reveal a saddle point in real time, whose implication is that instabilities will develop in the accreting system when the perturbation is extended into the nonlinear regime. We also model the perturbation as a high-frequency traveling wave and carry out a Wentzel-Kramers-Brillouin analysis, treating nonlinearity iteratively as a very feeble effect. Under this approach, both the amplitude and the energy flux of the perturbation exhibit growth, with the acoustic horizon segregating the regions of stability and instability.

  18. Brazilian physicists take centre stage

    NASA Astrophysics Data System (ADS)

    Curtis, Susan

    2014-06-01

    With the FIFA World Cup taking place in Brazil this month, Susan Curtis travels to South America's richest nation to find out how its physicists are exploiting recent big increases in science funding.

  19. Pulsed accretion in a variable protostar.

    PubMed

    Muzerolle, James; Furlan, Elise; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-17

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some pre-main-sequence close binary stars as they grow from circumbinary disks. The phenomenon is known as pulsed accretion and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from zero to 50 per cent from orbit to orbit. Here we report that the infrared luminosity of a young protostar (of age about 10(5) years) increases by a factor of ten in roughly one week every 25.34 days. We attribute this to pulsed accretion associated with an unseen binary companion. The strength and regularity of this accretion signal is surprising; it may be related to the very young age of the system, which is a factor of ten younger than the other pulsed accretors previously studied. PMID:23283175

  20. Pulsed accretion in a variable protostar.

    PubMed

    Muzerolle, James; Furlan, Elise; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-17

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some pre-main-sequence close binary stars as they grow from circumbinary disks. The phenomenon is known as pulsed accretion and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from zero to 50 per cent from orbit to orbit. Here we report that the infrared luminosity of a young protostar (of age about 10(5) years) increases by a factor of ten in roughly one week every 25.34 days. We attribute this to pulsed accretion associated with an unseen binary companion. The strength and regularity of this accretion signal is surprising; it may be related to the very young age of the system, which is a factor of ten younger than the other pulsed accretors previously studied.

  1. Accretion in the galactic halo

    NASA Astrophysics Data System (ADS)

    Stephens, Alex Courtney

    2000-10-01

    The Milky Way disk is enveloped in a diffuse, dynamically-hot collection of stars and star clusters collectively known as the ``stellar halo''. Photometric and chemical analyses suggest that these stars are ancient fossils of the galaxy formation epoch. Yet, little is known about the origin of this trace population. Is this system merely a vestige of the initial burst of star formation within the decoupled proto-Galaxy, or is it the detritus of cannibalized satellite galaxies? In an attempt to unravel the history of the Milky Way's stellar halo, I performed a detailed spectroscopic analysis of 55 metal-poor stars possessing ``extreme'' kinematic properties. It is thought that stars on orbits that either penetrate the remote halo or exhibit large retrograde velocities could have been associated with assimilated (or ``accreted'') dwarf galaxies. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the α-elements (O, Mg, Si, Ca, Ti) with respect to iron, a consequence of sporadic bursts of star formation within the diminutive galaxies. Abundances for a select group of light metals (Li, Na, Mg, Si, Ca, Ti), iron-peak nuclides (Cr, Fe, Ni), and neutron-capture elements (Y, Ba) were calculated using line-strengths measured from high-resolution, high signal-to-noise spectral observations collected with the Keck I 10-m and KPNO 4-m telescopes. The abundances extracted from the spectra reveal: (1)The vast majority of outer halo stars possess supersolar [α/Fe] > 0.0) ratios. (2)The [α/Fe] ratio appears to decrease with increasing metallicity. (3)The outer halo stars have lower ratios of [α/Fe] than inner halo stars at a given metallicity. (4)At the largest metallicities, there is a large spread in the observed [α/Fe] ratios. (5)[α/Fe] anti-correlates with RAPO. (6)Only one star (BD+80° 245) exhibits the peculiar abundances expected of an assimilated star. The general conclusion extracted from these data is that the

  2. Simultaneous Spectral and Timing Observations of Accreting Neuron Stars

    NASA Technical Reports Server (NTRS)

    Kaaret, P.; Oliversen, Ronald J. (Technical Monitor)

    2002-01-01

    The goal of this proposal is to perform simultaneous x-ray spectral and millisecond timing observations of accreting neutron stars to further our understanding of their accretion dynamics and in the hope of using these systems as probes of the physics of strong gravitational fields. NAG5-9104 is the successor grant to NAG5-8408. Observations using the Rossi X-Ray Timing Explorer (RXTE) and BeppoSAX were performed of 4U1702-429, 4U1735-44, and Cyg X-2. Unfortunately, only a small fraction of the approved observing time was obtained for the first two targets and the data are of limited scientific value. Data analysis has been completed on the observations of Cyg X-2. We discovered a correlation between the frequency of the horizontal branch oscillations (HBO) and a soft, thermal component of the x-ray spectrum likely associated with emission from the accretion disk. This correlation may place constraints on models of the oscillations. A paper based on these results appeared in the Astrophysical Journal.

  3. Terrestrial accretion under oxidizing conditions.

    PubMed

    Siebert, Julien; Badro, James; Antonangeli, Daniele; Ryerson, Frederick J

    2013-03-01

    The abundance of siderophile elements in the mantle preserves the signature of core formation. On the basis of partitioning experiments at high pressure (35 to 74 gigapascals) and high temperature (3100 to 4400 kelvin), we demonstrate that depletions of slightly siderophile elements (vanadium and chromium), as well as moderately siderophile elements (nickel and cobalt), can be produced by core formation under more oxidizing conditions than previously proposed. Enhanced solubility of oxygen in the metal perturbs the metal-silicate partitioning of vanadium and chromium, precluding extrapolation of previous results. We propose that Earth accreted from materials as oxidized as ordinary or carbonaceous chondrites. Transfer of oxygen from the mantle to the core provides a mechanism to reduce the initial magma ocean redox state to that of the present-day mantle, reconciling the observed mantle vanadium and chromium concentrations with geophysical constraints on light elements in the core. PMID:23306436

  4. Why Do T Tauri Disks Accrete?

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee; D'Alessio, Paola; Calvet, Nuria; Muzerolle, James

    2006-01-01

    Observations of T Tauri stars and young brown dwarfs suggest that the accretion rates of their disks scales roughly with the square of the central stellar mass. No dependence of accretion rate on stellar mass is predicted by the simplest version of the Gammie layered disk model, in which nonthermal ionization of upper disk layers allows accretion to occur via the magnetorotational instability. We show that a minor modification of Gaminie's model to include heating by irradiation from the central star yields a modest dependence of accretion on the mass of the central star. A purely viscous disk model could provide a strong dependence of accretion rate on stellar mass if the initial disk radius (before much viscous evolution has occurred) has a strong dependence on stellar mass. However, it is far from clear that at least the most massive pre-main-sequence disks can be totally magnetically activated by X-rays or cosmic rays. We suggest that a combination of effects are responsible for the observed dependence, with the lowest mass stars having the lowest mass disks, which can be thoroughly magnetically active, while the higher mass stars have higher mass disks that have layered accret,ion and relatively inactive or "dead" central zones at some radii. In such dead zones, we suggest that gravitational instabilities may play a role in allowing accretion to proceed. In this connection, we emphasize the uncertainty in disk masses derived from dust emission and argue that T Tauri disk masses have been systematically underestimated by conventional analyses. Furtlier study of accretion rates, especially in the lowest mass stars, would help to clarify the mechanisms of accretion in T Tauri stars.

  5. A Case Study of How Teaching Practice Process Takes Place

    ERIC Educational Resources Information Center

    Yalin Ucar, Meltem

    2012-01-01

    The process of "learning" carries an important role in the teaching practice which provides teacher candidates with professional development. Being responsible for the learning experiences in that level, cooperating teacher, teacher candidate, mentor and practice school are the important variables which determine the quality of the teaching…

  6. Using Personalized Education to Take the Place of Standardized Education

    ERIC Educational Resources Information Center

    Gao, Pengyu

    2014-01-01

    Economic model has been greatly shifted from labor demanding to innovation demanding, which requires education system has to produce creative people. This paper illustrates how traditional education model accrued and developed based on satisfying the old economic model for labor demanding but did not meet the new social requirement for innovation…

  7. Towards combined modeling of planetary accretion and differentiation

    NASA Astrophysics Data System (ADS)

    Golabek, G. J.; Gerya, T.; Morishima, R.; Tackley, P. J.; Labrosse, S.

    2011-12-01

    Results of current 1D models on planetesimal accretion yield an onion-like thermal structure with very high internal temperatures due to powerful short-lived radiogenic heating in the planetesimals. These lead to extensive silicate melting in the parent bodies. Yet, magma ocean and impact processes are not considered in these models and core formation is, if taken into account, assumed to be instantaneous with no feedback on the mantle evolution. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [1], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [2]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the onset of mantle convection and cannot be described properly in 1D geometry. Here we present a new methodology, which can be used to simulate the internal evolution of a planetary body during accretion and differentiation: Using the N-body code PKDGRAV [3] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [4]. The thermomechanical model takes recent parametrizations of impact processes like impact heating and crater excavation [5] into account. The model also includes both long- and short-lived radiogenic isotopes and a more realistic treatment of largely molten silicates [6]. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration

  8. Towards combined modeling of planetary accretion and differentiation

    NASA Astrophysics Data System (ADS)

    Golabek, G. J.; Gerya, T. V.; Morishima, R.; Tackley, P. J.; Labrosse, S.

    2012-09-01

    accretion yield an onion-like thermal structure with very high internal temperatures due to powerful short-lived radiogenic heating in the planetesimals. These lead to extensive silicate melting in the parent bodies. Yet, magma ocean and impact processes are not considered in these models and core formation is, if taken into account, assumed to be instantaneous with no feedback on the mantle evolution. It was pointed out that impacts can not only deposit heat deep into the target body, which is later buried by ejecta of further impacts [1], but also that impacts expose in the crater region originally deep-seated layers, thus cooling the interior [2]. This combination of impact effects becomes even more important when we consider that planetesimals of all masses contribute to planetary accretion. This leads occasionally to collisions between bodies with large ratios between impactor and target mass. Thus, all these processes can be expected to have a profound effect on the thermal evolution during the epoch of planetary accretion and may have implications for the onset of mantle convection and cannot be described properly in 1D geometry. Here we present a new methodology, which can be used to simulate the internal evolution of a planetary body during accretion and differentiation: Using the N-body code PKDGRAV[3] we simulate the accretion of planetary embryos from an initial annulus of several thousand planetesimals. The growth history of the largest resulting planetary embryo is used as an input for the thermomechanical 2D code I2ELVIS [4]. The thermomechanical model takes recent parametrizations of impact processes like impact heating and crater excavation [5] into account. The model also includes both long- and short-lived radiogenic isotopes and a more realistic treatment of largely molten silicates [6]. Results show that late-formed planetesimals do not experience silicate melting and avoid thermal alteration, whereas in early-formed bodies accretion and iron

  9. Advection of magnetic flux by accretion disks around neutron stars

    NASA Astrophysics Data System (ADS)

    Flores-Tulian, S.; Reisenegger, A.

    The aim of our research is to address why millisecond pulsars have relatively weak surface magnetic fields, of about 10^8 G, with a narrow spread. We propose that the accretion of plasma from the companion star fully screens the original neutron star field, but the accretion disk carries additional magnetic flux from the companion star, or itself can generate field by means of dynamo processes. For a strongly magnetized star, the field prevents the disk from approaching the star. The accretion is along the field lines and deposits the matter on the polar cap. Then, the accreted plasma flows, dragging with itself the magnetic field lines, from the pole to the equator (Payne & Melatos 2004). In a following stage, when the star becomes non-magnetic, because the field has been buried, the disk touches the star. We suggest that some effective mechanism of magnetic flux transport such as that proposed by Spruit & Uzdensky 2005 (or Bisnovatyi-Kogan & Lovelace 2007), operates and necessarily leads to a "strongly magnetized disk''. It becomes laminar because the magneto-rotational instability saturates (it is considered to be responsible for turbulence in the disk), and the magnetic difussivity is negligible. Then, the loss of angular momentum allowing the accretion is only caused by the magneto-centrifugal disk-wind (Blandford & Payne 1982). Meanwhile, the wind-driven transport of the magnetic flux by the disk re-magnetizes the star. This process continues until the Lorentz force due to the star's magnetic field forbids any further accretion of matter and magnetic flux, in the Ideal Magneto-Hydro-Dynamics approach. Additional of material can fall onto the star (but at lower rate) if some instability process sets in, allowing the diffusion of mass through the magnetic field lines (e.g the Interchange Instability, Spruit & Taam 1990). All these processes might lead to an asymptotic magnetic field of 10^8 G,as is inferred from observations. We are developing a self

  10. RELATIVISTIC GLOBAL SOLUTIONS OF NEUTRINO-DOMINATED ACCRETION FLOWS

    SciTech Connect

    Xue Li; Liu Tong; Gu Weimin; Lu Jufu

    2013-08-15

    Neutrino-dominated accretion flows (NDAFs) around rotating stellar-mass black holes are plausible candidates for the central engines of gamma-ray bursts (GRBs). We investigate one-dimensional global solutions of NDAFs, taking into account general relativity in the Kerr metric, neutrino physics, and nucleosynthesis more precisely than previous works. We calculate 16 solutions with different characterized accretion rates and black hole spins to exhibit the radial distributions of various physical properties in NDAFs. We confirm that the electron degeneracy has important effects in NDAFs and we find that the electron fraction is about 0.46 in the outer region for all 16 solutions. From the perspective of the mass fraction, free nucleons, {sup 4}He, and {sup 5}6Fe dominate in the inner, middle, and outer regions, respectively. The influence of neutrino trapping on the annihilation is of importance for the superhigh accretion ( M-dot =10 M{sub sun} s{sup -1}) and most of the 16 solutions have an adequate annihilation luminosity for GRBs.

  11. High-Resolution N-Body Simulation of Lunar Accretion from an Impact-Generated Disk

    NASA Astrophysics Data System (ADS)

    Kokubo, E.; Ida, S.; Makino, J.

    1998-01-01

    mutual collisions of particles. (2) Gravitational instability takes place and particle clumps grow inside the Roche limit. 3) The clumps are elongated by the Kepler shear, which forms spiral arms with m=5-10 modes. (4) Particles are transferred to the outside of the Roche limit through the spiral arms. When a spiral arm is extended beyond the Roche Limit, the tip of the spiral arm is detached from the arm to form a small aggregate. The rapid accretion of these small aggregates form a lunar seed. (5) The seed grows by sweeping up particles transferred over the Roche Limit. (6) When the moon becomes large enough to gravitationally dominate the disk it shepherds the rest of the disk to the earth. The moon is almost completed by stage 5. Our result shows that the timescale up to stage 5 is about 100 TK (about a month). The efficiency of incorporation of disk material into a moon is 20-50% which increases the initial specific angular momentum of the disk. The results of the high-resolution simulations give a larger moon mass and probability of a single moon. The lunar mass is more consistent with the analytical argument.

  12. [Predicting Spectra of Accretion Disks Around Galactic Black Holes

    NASA Technical Reports Server (NTRS)

    Krolik, Julian H.

    2004-01-01

    The purpose of this grant was to construct detailed atmosphere solutions in order to predict the spectra of accretion disks around Galactic black holes. Our plan of action was to take an existing disk atmosphere code (TLUSTY, created by Ivan Hubeny) and introduce those additional physical processes necessary to make it applicable to disks of this variety. These modifications include: treating Comptonization; introducing continuous opacity due to heavy elements; incorporating line opacity due to heavy elements; adopting a disk structure that reflects readjustments due to radiation pressure effects; and injecting heat via a physically-plausible vertical distribution.

  13. Places for Pedagogies, Pedagogies for Places

    ERIC Educational Resources Information Center

    Duhn, Iris

    2012-01-01

    Working with an understanding of assemblage as the ad hoc groupings of vibrant materials and elements, this article argues that conceptualizing place as an assemblage opens possibilities for bridging the gap between subjects and objects that continue to structure pedagogy. Considering "place" as an assemblage of humans and their multiple "others"…

  14. Hot accretion flow with radiative cooling: state transitions in black hole X-ray binaries

    NASA Astrophysics Data System (ADS)

    Wu, Mao-Chun; Xie, Fu-Guo; Yuan, Ye-Fei; Gan, Zhaoming

    2016-06-01

    We investigate state transitions in black hole X-ray binaries through different parameters by using two-dimensional axisymmetric hydrodynamical simulation method. For radiative cooling in hot accretion flow, we take into account the bremsstrahlung, synchrotron and synchrotron self-Comptonization self-consistently in the dynamics. Our main result is that the state transitions occur when the accretion rate reaches a critical value dot{M} ˜ 3α dot{M}_Edd, above which cold and dense clumpy/filamentary structures are formed, embedded within the hot gas. We argued this mode likely corresponds to the proposed two-phase accretion model, which may be responsible for the intermediate state of black hole X-ray binaries. When the accretion rate becomes sufficiently high, the clumpy/filamentary structures gradually merge and settle down on to the mid-plane. Eventually the accretion geometry transforms to a disc-corona configuration. In summary, our results are consistent with the truncated accretion scenario for the state transition.

  15. Effects of ice accretions on aircraft aerodynamics

    NASA Astrophysics Data System (ADS)

    Lynch, Frank T.; Khodadoust, Abdollah

    2001-11-01

    This article is a systematic and comprehensive review, correlation, and assessment of test results available in the public domain which address the aerodynamic performance and control degradations caused by various types of ice accretions on the lifting surfaces of fixed wing aircraft. To help put the various test results in perspective, overviews are provided first of the important factors and limitations involved in computational and experimental icing simulation techniques, as well as key aerodynamic testing simulation variables and governing flow physics issues. Following these are the actual reviews, assessments, and correlations of a large number of experimental measurements of various forms of mostly simulated in-flight and ground ice accretions, augmented where appropriate by similar measurements for other analogous forms of surface contamination and/or disruptions. In-flight icing categories reviewed include the initial and inter-cycle ice accretions inherent in the use of de-icing systems which are of particular concern because of widespread misconceptions about the thickness of such accretions which can be allowed before any serious consequences occur, and the runback/ridge ice accretions typically associated with larger-than-normal water droplet encounters which are of major concern because of the possible potential for catastrophic reductions in aerodynamic effectiveness. The other in-flight ice accretion category considered includes the more familiar large rime and glaze ice accretions, including ice shapes with rather grotesque features, where the concern is that, in spite of all the research conducted to date, the upper limit of penalties possible has probably not been defined. Lastly, the effects of various possible ground frost/ice accretions are considered. The concern with some of these is that for some types of configurations, all of the normally available operating margins to stall at takeoff may be erased if these accretions are not

  16. Electron thermodynamics in GRMHD simulations of low-luminosity black hole accretion

    NASA Astrophysics Data System (ADS)

    Ressler, S. M.; Tchekhovskoy, A.; Quataert, E.; Chandra, M.; Gammie, C. F.

    2015-12-01

    Simple assumptions made regarding electron thermodynamics often limit the extent to which general relativistic magnetohydrodynamic (GRMHD) simulations can be applied to observations of low-luminosity accreting black holes. We present, implement, and test a model that self-consistently evolves an entropy equation for the electrons and takes into account the effects of spatially varying electron heating and relativistic anisotropic thermal conduction along magnetic field lines. We neglect the backreaction of electron pressure on the dynamics of the accretion flow. Our model is appropriate for systems accreting at ≪10-5 of the Eddington accretion rate, so radiative cooling by electrons can be neglected. It can be extended to higher accretion rates in the future by including electron cooling and proton-electron Coulomb collisions. We present a suite of tests showing that our method recovers the correct solution for electron heating under a range of circumstances, including strong shocks and driven turbulence. Our initial applications to axisymmetric simulations of accreting black holes show that (1) physically motivated electron heating rates that depend on the local magnetic field strength yield electron temperature distributions significantly different from the constant electron-to-proton temperature ratios assumed in previous work, with higher electron temperatures concentrated in the coronal region between the disc and the jet; (2) electron thermal conduction significantly modifies the electron temperature in the inner regions of black hole accretion flows if the effective electron mean free path is larger than the local scaleheight of the disc (at least for the initial conditions and magnetic field configurations we study). The methods developed in this work are important for producing more realistic predictions for the emission from accreting black holes such as Sagittarius A* and M87; these applications will be explored in future work.

  17. Accreting pre-main-sequence models and abundance anomalies in globular clusters

    NASA Astrophysics Data System (ADS)

    Tognelli, E.; Prada Moroni, P. G.; Degl'Innocenti, S.

    2015-12-01

    We investigated the possibility of producing helium-enhanced stars in globular clusters by accreting polluted matter during the pre-main-sequence phase. We followed the evolution of two different classes of pre-main-sequence accreting models, one which neglects and the other that takes into account the protostellar evolution. We analysed the dependence of the final central helium abundance, of the tracks position in the HR diagram and of the surface lithium abundance evolution on the age at which the accretion of polluted material begins and on the main physical parameters that govern the protostellar evolution. The later is the beginning of the late accretion and the lower are both the central helium and the surface lithium abundances at the end of the accretion phase and in Zero Age Main Sequence (ZAMS). In order to produce a relevant increase of the central helium content the accretion of polluted matter should start at ages lower than 1 Myr. The inclusion of the protostellar evolution has a strong impact on the ZAMS models too. The adoption of a very low seed mass (i.e. 0.001 M⊙) results in models with the lowest central helium and surface lithium abundances. The higher is the accretion rate and the lower is the final helium content in the core and the residual surface lithium. In the worst case - i.e. seed mass 0.001 M⊙ and accretion rate ≥10-5 M⊙ yr-1 - the central helium is not increased at all and the surface lithium is fully depleted in the first few million years.

  18. Accretion in Saturn's F Ring

    NASA Astrophysics Data System (ADS)

    Meinke, B. K.; Esposito, L. W.; Stewart, G.

    2012-12-01

    Saturn's F ring is the solar system's principal natural laboratory for direct observation of accretion and disruption processes. The ring resides in the Roche zone, where tidal disruption competes with self-gravity, which allows us to observe the lifecycle of moonlets. Just as nearby moons create structure at the B ring edge (Esposito et al. 2012) and the Keeler gap (Murray 2007), the F ring "shepherding" moons Prometheus and Pandora stir up ring material and create observably changing structures on timescales of days to decades. In fact, Beurle et al (2010) show that Prometheus makes it possible for "distended, yet gravitationally coherent clumps" to form in the F ring, and Barbara and Esposito (2002) predicted a population of ~1 km bodies in the ring. In addition to the observations over the last three decades, the Cassini Ultraviolet Imaging Spectrograph (UVIS) has detected 27 statistically significant features in 101 occultations by Saturn's F ring since July 2004. Seventeen of those 27 features are associated with clumps of ring material. Two features are opaque in occultation, which makes them candidates for solid objects, which we refer to as Moonlets. The 15 other features partially block stellar signal for 22 m to just over 3.7 km along the radial expanse of the occultation. Upon visual inspection of the occultation profile, these features resemble Icicles, thus we will refer to them as such here. The density enhancements responsible for such signal attenuations are likely due to transient clumping of material, evidence that aggregations of material are ubiquitous in the F ring. Our lengthy observing campaign reveals that Icicles are likely transient clumps, while Moonlets are possible solid objects. Optical depth is an indicator of clumping because more-densely aggregated material blocks more light; therefore, it is natural to imagine moonlets as later evolutionary stage of icicle, when looser clumps of material compact to form a feature that appears

  19. Outflows from Accretion Disks around Compact Objects

    NASA Astrophysics Data System (ADS)

    Jiao, Cheng-Liang; Wu, Xue-Bing

    2013-02-01

    We solve the set of hydrodynamic equations for accretion disks in the spherical coordinates (rθφ) to obtain the explicit structure along the θ direction. The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev Disks (SSDs) and thicker, sub-Keplerian disks for Advection Dominated Accretion Flows (ADAFs) and slim disks, which are consistent with previous popular analytical models, while an inflow region and an outflow region always exist, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and stronger in slim disks and ADAFs.

  20. The beaming of subhalo accretion

    NASA Astrophysics Data System (ADS)

    Libeskind, Noam I.

    2016-10-01

    We examine the infall pattern of subhaloes onto hosts in the context of the large-scale structure. We find that the infall pattern is essentially driven by the shear tensor of the ambient velocity field. Dark matter subhaloes are preferentially accreted along the principal axis of the shear tensor which corresponds to the direction of weakest collapse. We examine the dependence of this preferential infall on subhalo mass, host halo mass and redshift. Although strongest for the most massive hosts and the most massive subhaloes at high redshift, the preferential infall of subhaloes is effectively universal in the sense that its always aligned with the axis of weakest collapse of the velocity shear tensor. It is the same shear tensor that dictates the structure of the cosmic web and hence the shear field emerges as the key factor that governs the local anisotropic pattern of structure formation. Since the small (sub-Mpc) scale is strongly correlated with the mid-range (~ 10 Mpc) scale - a scale accessible by current surveys of peculiar velocities - it follows that findings presented here open a new window into the relation between the observed large scale structure unveiled by current surveys of peculiar velocities and the preferential infall direction of the Local Group. This may shed light on the unexpected alignments of dwarf galaxies seen in the Local Group.

  1. Palaeoclimate, Sedimentation and Continental Accretion

    NASA Astrophysics Data System (ADS)

    Ziegler, A. M.; Barrett, S. F.; Scotese, C. R.

    1981-05-01

    Climate has a pervasive effect on sedimentation today, and the same climatic patterns are reflected in the distribution of lithofacies through the Palaeozoic, as the continents migrate beneath the climatic zones. The low-latitude hot wet zone is represented by thick clastics, coals and carbonates and is best developed along east coasts where prevailing winds bring moisture and heated surface waters toward the continent. The desert zones occur on the west sides of continents centred at 20 degrees north and south, and these dry belts are represented in the geological record by evaporites. Tillites, thick clastics and coals occur in the temperate rainy belts, especially on the windward, west sides of continents above 40 degrees latitude. Continental accretion occurs where subduction zones coincide with rainy zones, such that the products of erosion are transported to the trench, and thus thrust back, extending the margin of the continent. The opposite process of `tectonic erosion', wherein the descending oceanic slab continually `rasps' away the margin of the continental crust, may occur in areas where rainfall and surface run-off is insufficient to provide trench sediments. This process has been operating adjacent to the Atacama Desert in South America during the past 200 Ma. To judge by the eastward migration of the calc-alkaline intrusive foci, about 250 km of the margin of South America have been transported down the subduction zone during this period.

  2. Accretion Timescales from Kepler AGN

    NASA Astrophysics Data System (ADS)

    Kasliwal, Vishal P.; Vogeley, Michael S.; Richards, Gordon T.

    2015-01-01

    We constrain AGN accretion disk variability mechanisms using the optical light curves of AGN observed by Kepler. AGN optical fluxes are known to exhibit stochastic variations on timescales of hours, days, months and years. The excellent sampling properties of the original Kepler mission - high S/N ratio (105), short sampling interval (30 minutes), and long sampling duration (~ 3.5 years) - allow for a detailed examination of the differences between the variability processes present in various sub-types of AGN such as Type I and II Seyferts, QSOs, and Blazars. We model the flux data using the Auto-Regressive Moving Average (ARMA) representation from the field of time series analysis. We use the Kalman filter to determine optimal mode parameters and use the Akaike Information Criteria (AIC) to select the optimal model. We find that optical light curves from Kepler AGN cannot be fit by low order statistical models such as the popular AR(1) process or damped random walk. Kepler light curves exhibit complicated power spectra and are better modeled by higher order ARMA processes. We find that Kepler AGN typically exhibit power spectra that change from a bending power law (PSD ~ 1/fa) to a flat power spectrum on timescales in the range of ~ 5 - 100 days consistent with the orbital and thermal timescales of a typical 107 solar mass black hole.

  3. Heat transfer on accreting ice surfaces

    NASA Technical Reports Server (NTRS)

    Yamaguchi, Keiko; Hansman, R. John, Jr.

    1993-01-01

    Based on previous observations of glaze ice accretion on aircraft surfaces, a multizone model with distinct zones of different surface roughness is demonstrated. The use of surface roughness in the LEWICE ice accretion prediction code is examined. It was found that roughness is used in two ways: (1) to determine the laminar to turbulent boundary-layer transition location; and (2) to calculate the convective turbulent heat-transfer coefficient. A two-zone version of the multizone model is implemented in the LEWICE code, and compared with experimental convective heat-transfer coefficient and ice accretion results. The analysis of the boundary-layer transition, surface roughness, and viscous flowfield effects significantly increased the accuracy in predicting heat-transfer coefficients. The multizone model was found to significantly improve the ice accretion prediction for the cases compared.

  4. Accretion of Ghost Condensate by Black Holes

    SciTech Connect

    Frolov, A

    2004-06-02

    The intent of this letter is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

  5. Review of gravitomagnetic acceleration from accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2015-11-01

    We review the development of the equations of gravitoelectromagnetism and summarize how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near the accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism to produce collimated jets, it is a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  6. Gravitomagnetic acceleration from black hole accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2016-05-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  7. Planetary science: Iron fog of accretion

    SciTech Connect

    Anderson, William W.

    2015-03-02

    Here, pinpointing when Earth's core formed depends on the extent of metal–silicate equilibration in the mantle. Vaporization and recondensation of impacting planetesimal cores during accretion may reconcile disparate lines of evidence.

  8. ASYMMETRIC ACCRETION FLOWS WITHIN A COMMON ENVELOPE

    SciTech Connect

    MacLeod, Morgan; Ramirez-Ruiz, Enrico

    2015-04-10

    This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). Flow in the vicinity of the embedded object is gravitationally focused, leading to drag and potentially to gas accretion. This process has been studied numerically and analytically in the context of Hoyle–Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of three-dimensional HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by one to two orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically derived rates of drag and accretion as a function of the density gradient. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.

  9. Accretion on to Magnetic White Dwarfs

    NASA Astrophysics Data System (ADS)

    Wickramasinghe, Dayal

    2014-01-01

    The Magnetic Cataclysmic Variables (MCVs) are close interacting binaries where mass is transferred from a late type secondary star to a magnetic white dwarf. Two modes of accretion can be identified depending on the strength of the magnetic field, the mass transfer rate and orbital parameters. (a) Disced Accretion: In the Intermediate polars (IPs), the mass transfer stream circularises and forms an accretion disc. Material couples on to field lines in a narrow inner transition region where the velocity in the orbital plane changes from Keplerian to co-rotation (b) Discless Accretion: In the polars, the accretion stream is disrupted by the magnetic field before it can circularise. Material couples on to field lines via an inner transition region where the velocity changes from essentially free fall to co-rotation. The polars have no counterparts in neutron star systems and their study provides unique insights into the complex nature of the magnetospheric boundary. The observed properties of accretion shocks at the white dwarf surface such as the anomalous soft-X-ray excess and its time variability provide strong support for the hypothesis that under certain circumstances the field channelled funnel flow is "blobby". This has been attributed to interchange instabilities such as the Magnetic Rayleigh-Taylor instability in the shocked gas at the stream-magnetosphere boundary where the stream fragments into discrete clumps of gas. As the clumps penetrate into the magnetosphere, they are shredded into smaller mass blobs via the Kelvin-Helmholtz instability that then couple on to field lines over an extended inner transition region in the orbital plane. The more massive blobs penetrate deep into the photosphere of the white dwarf releasing their energy as a reprocessed soft-X-ray black body component. Although similar instabilities are expected in the inner transition region in disced accretion albeit on a different scale there has been no direct observational evidence

  10. Re(Place) Your Typical Writing Assignment: An Argument for Place-Based Writing

    ERIC Educational Resources Information Center

    Jacobs, Elliot

    2011-01-01

    Place-based writing affords students an opportunity to write meaningfully about themselves, grounded in a place that they know. Place-based writing is versatile and can be additive--taking just a week or two within a semester of different projects--or transformative, if positioned as the theme for an entire course. If students can learn to write…

  11. Stability of MRI Turbulent Accretion Disks

    NASA Astrophysics Data System (ADS)

    Takahashi, H. R.; Masada, Y.

    2010-12-01

    We study the stability of geometrically thin accretion disks with non-standard α parameter, which characterizes the efficiency of the angular momentum transport. Following recent results of numerical simulations of the Magnetorotational instability (MRI) driven turbulence, we assume that α increases with the magnetic Prandtl number. By adopting Spitzer's microscopic diffusivities, we obtain local structures of geometrically thin accretion disks consistently including effects of MRI-driven turbulence. Since the magnetic Prandtl number increases with the temperature, the efficiency of the angular momentum transport and thus viscous heating rate are smaller for a larger radius when δ > 0. We find that such disks can be unstable to gravitational, thermal, and secular instabilities. It is most remarkable feature that the thermal and secular instabilities can grow in the middle part of accretion disks even when the radiation pressure is negligible, while the standard Shakura & Sunyaev's accretion disk (constant α) is stable to these instabilities. We conclude that it would be difficult to maintain the steady mass accretion state unless the Pm-dependence of the MRI-driven turbulence is weak. Consideration of Pm dependence of α due to the MRI-driven turbulence may make the phase transition of accretion disks less mysterious.

  12. Heating and Cooling in Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Cumming, Andrew

    2015-10-01

    Neutron stars in low mass X-ray binaries accrete enough mass over their lifetimes to replace their entire crust. The accreted matter undergoes a series of nuclear reactions in the crust as it is compressed by continued accretion to higher density. These reactions, which include electron captures, neutron emissions, and pycnonuclear reactions, heat the crust and core of the neutron star. In this talk I will discuss what we can learn from observations of transiently accreting neutron stars in quiescence, when accretion has turned off and we can see emission from the neutron star directly. The quiescent luminosity of these neutron stars constrains the neutrino emissivity in the neutron star core. In systems with long accretion outbursts, observations of thermal relaxation of the crust in quiescence enable, for the first time, constraints on the thermal conductivity and heat capacity of the crust. In this way, low mass X-ray binary neutron stars offer a remarkable chance to constrain the properties of dense neutron-rich matter, such as neutron superfluidity and pasta phases in the inner crust of neutron stars.

  13. Bondi accretion in early-type galaxies

    NASA Astrophysics Data System (ADS)

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-08-01

    Accretion on to central massive black holes in galaxies is often modelled with the Bondi solution. In this paper, we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of a Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore, allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  14. Formation of Jupiter's Core and Early Stages of Envelope Accretion

    NASA Astrophysics Data System (ADS)

    D'Angelo, G.; Weidenschilling, S.; Lissauer, J. J.; Bodenheimer, P.; Hubickyj, O.

    2012-12-01

    We are performing calculations of the formation of Jupiter via core nucleated accretion and gas capture. The core starts as a seed body of a few hundred kilometers in radius and orbits within a swarm of planetesimals whose initial size distribution ranges from ~10 m to ~100 km. The planetesimals are immersed in a gaseous disk, representative of an early solar nebula. The evolution of the swarm of planetesimals accounts for collisions and gravitational stirring due to mutual interactions among bodies, and for migration and velocity damping due to interactions with the nebula gas. Collisions among planetesimals lead to growth and/or fragmentation, altering the size distribution of the swarm over time. Collisions of planetesimals with the seed body lead to its growth, resulting in the formation of a planetary core. Gas capture by the core leads to the accumulation of a tenuous atmosphere, which later becomes a massive envelope, increasing the size-dependent effective cross-section of the planet for planetesimals' accretion. Planetesimals that travel through the core's envelope release energy, affecting the thermal budget of the envelope, and deliver mass, affecting the opacity of the envelope. The calculation of dust opacity, which is especially important for envelope contraction, is performed self-consistently, accounting for coagulation and sedimentation of dust and small particles that are released in the envelope as passing planetesimals are ablated. We find that, in a disk of planetesimals with a surface density of about 10 g/cm2 at 5.2 AU, a one Earth mass core accumulates in less than 1e5 years, and that it takes over 1.5e6 years to accumulate a core of 3 Earth masses, when the core's geometrical cross-section is used for the accretion of planetesimals. Gas drag in the core's envelope increases the ability of the planet to accrete planetesimals. Smaller planetesimals are affected to a greater extent than are larger planetesimals. We find that the effective

  15. Corotation resonance and overstable oscillations in black hole accretion discs: general relativistic calculations

    NASA Astrophysics Data System (ADS)

    Horák, Jiří; Lai, Dong

    2013-10-01

    We study the dynamics of spiral waves and oscillation modes in relativistic rotating discs around black holes. Generalizing the Newtonian theory, we show that wave absorption can take place at the corotation resonance, where the pattern frequency of the wave matches the background disc rotation rate. We derive the general relativistic expression for the disc vortensity (vorticity divided by surface density), which governs the behaviour of density perturbation near corotation. Depending on the gradient of the generalized disc vortensity, corotational wave absorption can lead to the amplification or damping of the spiral wave. We apply our general theory of relativistic wave dynamics to calculate the non-axisymmetric inertial-acoustic modes (also called p modes) trapped in the innermost region of a black hole accretion disc. Because general relativity changes the profiles of the radial epicyclic frequency and disc vortensity near the inner disc edge close to the black hole, these p modes can become overstable under appropriate conditions. We present the numerical results of the frequencies and growth rates of p modes for various black hole spin and model disc parameters (the surface density profile and sound speed), and discuss their implications for understanding the enigmatic high-frequency quasi-periodic oscillations observed in black hole X-ray binaries.

  16. How to Cope with Sheltering in Place

    MedlinePlus

    ... your own or a relative’s home, school, or work. Sheltering in place may be required because of an emergency such ... things to keep yourself calm while sheltering in place. Relax your body often by doing things that work for you—take deep breaths, stretch, meditate or ...

  17. Vertical accretion and shallow subsidence in a mangrove forest of southwestern Florida, U.S.A

    USGS Publications Warehouse

    Cahoon, D.R.; Lynch, J.C.

    1997-01-01

    Simultaneous measurements of vertical accretion from artificial soil marker horizons and soil elevation change from sedimentation-erosion table (SET) plots were used to evaluate the processes related to soil building in range, basin, and overwash mangrove forests located in a low-energy lagoon which recieves minor inputs of terregenous sediments. Vertical accretion measures reflect the contribution of surficial sedimentation (sediment deposition and surface root growth). Measures of elevation change reflect not only the contributions of vertical accretion but also those of subsurface processes such as compaction, decomposition and shrink-swell. The two measures were used to calculate amounts of shallow subsidence (accretion minus elevation change) in each mangrove forest. The three forest types represent different accretionary envrionments. The basin forest was located behind a natural berm. Hydroperiod here was controlled primarily by rainfall rather than tidal exchange, although the basin flooded during extreme tidal events. Soil accretion here occurred primarily by autochthonous organic matter inputs, and elevation was controlled by accretion and shrink-swell of the substrate apparently related to cycles of flooding-drying and/or root growth-decomposition. This hydrologically-restricted forest did not experience an accretion or elevation deficit relative to sea-level rise. The tidally dominated fringe and overwash island forests accreted through mineral sediment inputs bound in place by plant roots. Filamentous turf algae played an important role in stabilizing loose muds in the fringe forest where erosion was prevalent. Elevation in these high-energy environments was controlled not only by accretion but also by erosion and/or shallow subsidence. The rate of shallow subsidence was consistently 3-4 mm y-1 in the fringe and overwash island forests but was negligible in the basin forest. Hence, the vertical development of mangrove soils was influenced by both

  18. Place and Pedagogy

    ERIC Educational Resources Information Center

    Orr, David

    2013-01-01

    David Orr's classic article links education to living in the outdoors and studying all disciplines through the unifying lens of place. Pedagogy of place counters abstraction, it is the natural world embodying principles of learning that involve direct observation, investigation, experimentation, and manual skills. Place is the laboratory providing…

  19. SUPERNOVA LIGHT CURVES POWERED BY FALLBACK ACCRETION

    SciTech Connect

    Dexter, Jason; Kasen, Daniel

    2013-07-20

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time ({approx}>days) power potentially associated with the accretion of this 'fallback' material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as M-dot {proportional_to}t{sup -5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous ({approx}> 10{sup 44} erg s{sup -1}) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  20. Accretion onto Pre-Main-Sequence Stars

    NASA Astrophysics Data System (ADS)

    Hartmann, Lee; Herczeg, Gregory; Calvet, Nuria

    2016-09-01

    Accretion through circumstellar disks plays an important role in star formation and in establishing the properties of the regions in which planets form and migrate. The mechanisms by which protostellar and protoplanetary disks accrete onto low-mass stars are not clear; angular momentum transport by magnetic fields is thought to be involved, but the low-ionization conditions in major regions of protoplanetary disks lead to a variety of complex nonideal magnetohydrodynamic effects whose implications are not fully understood. Accretion in pre-main-sequence stars of masses ≲1M⊙ (and in at least some 2–3-M⊙ systems) is generally funneled by the stellar magnetic field, which disrupts the disk at scales typically of order a few stellar radii. Matter moving at near free-fall velocities shocks at the stellar surface; the resulting accretion luminosities from the dissipation of kinetic energy indicate that mass addition during the T Tauri phase over the typical disk lifetime ˜3 Myr is modest in terms of stellar evolution, but is comparable to total disk reservoirs as estimated from millimeter-wave dust emission (˜10‑2 M⊙). Pre-main-sequence accretion is not steady, encompassing timescales ranging from approximately hours to a century, with longer-timescale variations tending to be the largest. Accretion during the protostellar phase—while the protostellar envelope is still falling onto the disk—is much less well understood, mostly because the properties of the central obscured protostar are difficult to estimate. Kinematic measurements of protostellar masses with new interfometric facilities should improve estimates of accretion rates during the earliest phases of star formation.

  1. Collapsar Accretion and the Gamma-Ray Burst X-Ray Light Curve

    NASA Astrophysics Data System (ADS)

    Lindner, Christopher C.; Milosavljević, Miloš; Couch, Sean M.; Kumar, Pawan

    2010-04-01

    We present axisymmetric hydrodynamical simulations of the long-term accretion of a rotating gamma-ray burst (GRB) progenitor star, a "collapsar," onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive-mesh-refinement code FLASH in two spatial dimensions and with an explicit shear viscosity. The evolution of the central accretion rate exhibits phases reminiscent of the long GRB γ-ray and X-ray light curve, which lends support to the proposal by Kumar et al. that the luminosity is modulated by the central accretion rate. In the first "prompt" phase, the black hole acquires most of its final mass through supersonic quasiradial accretion occurring at a steady rate of ~0.2 M sun s-1. After a few tens of seconds, an accretion shock sweeps outward through the star. The formation and outward expansion of the accretion shock is accompanied with a sudden and rapid power-law decline in the central accretion rate \\dot{M}∝ t^{-2.8}, which resembles the L X vprop t -3 decline observed in the X-ray light curves. The collapsed, shock-heated stellar envelope settles into a thick, low-mass equatorial disk embedded within a massive, pressure-supported atmosphere. After a few hundred seconds, the inflow of low angular momentum material in the axial funnel reverses into an outflow from the thick disk. Meanwhile, the rapid decline of the accretion rate slows down, which is potentially suggestive of the "plateau" phase in the X-ray light curve. We complement our adiabatic simulations with an analytical model that takes into account the cooling by neutrino emission and estimate that the duration of the prompt phase can be ~20 s. The model suggests that the steep decline in GRB X-ray light curves is triggered by the circularization of the infalling stellar envelope at radii where the virial temperature is below 1010 K, such that neutrino cooling is inefficient and an outward expansion of the accretion shock becomes imminent

  2. A woman's rightful place?

    PubMed

    1993-04-01

    Rural development projects in sub-Saharan Africa tend not to succeed because they do not consider women's role and their significance, even though women constitute 70% of agricultural workers, 80% of food producers, 100% of people who prepare meals, and 60-90% do food marketing. Development specialists ignore women because they are not involved in political activities and in decision making. As long as women and women's contributions are not considered, rural development projects will remain inefficient and development will not take place. Thus, projects must include women as agents and beneficiaries of development in key sectors of the economy. Rural development specialists must also consider the effect male labor emigration has on rural women. For example, drought has forced many men to leave their villages, leaving a work force consisting of 95% women to fight desertification. All too often, women have no or limited land ownership rights, thereby keeping them from improving the land, e.g., planting perennial fruit crops. They also tend to be hired hands rather than food producers. They cannot obtain bank loans because they do not own land, and because they are often illiterate (over 90% female illiteracy in 28 African countries), they can neither understand nor complete bank loan forms. Rural development projects further alienate women by aiming training programs to men or by using male agricultural extension agents. Women react to this alienation by rejecting projects that do not benefit them and follow more profitable activities which sometimes interfere with projects. Thus, rural development programs need to invest in women to ensure viable and efficient sustainable development.

  3. Numerical simulations of two-dimensional and three-dimensional accretion flows

    NASA Astrophysics Data System (ADS)

    Matsuda, Takuya; Ishii, Takanori; Sekino, Nobuhiro; Sawada, Keisuke; Shima, Eiji; Livio, Mario; Anzer, Ulrich

    1992-03-01

    Numerical simulations of 2D and 3D accretion flows past a gravitating compact object from a uniform flow at a large distance upstream are performed by solving the Eulerian equations. The 2D flows exhibit a 'flip-flop instability' if the central accreting body is small. If the central body is enlarged at some instance in the oscillating flow, then the accretion shock shows a rather periodic oscillation similar to the von Karman vortex street. In the case of 3D flows, it is found that the shock cone is much more robust than in 2D, and the flip-flop instability takes a different, probably less violent, form. The causes of the instabilities are discussed.

  4. Accreting neutron stars by QFT

    NASA Astrophysics Data System (ADS)

    Chen, Shao-Guang

    layer with thickness of 1 km then q = 1 (N1S1), the gravity from N1S1 inside and exterior will be completely shielded. Because of net nuν _{0} flux is the medium to produce and transmit gravity, q obstructed by the shielding layer lie on the density of layer matter and the section of single nucleon to electronic neutrino obtained by nuclear physics experiments is about 1.1*10 ({-) 43} cm (2) . The mass inside N1S1 for exterior has not gravity interaction, it equivalent to has not inertia as the mass vanish. The neutron star is as a empty shell thereby may rapidly rotating and has not upper limit of mass and radii by the gravity accretion of N1S1, which will influence the mechanisms of pulsars, quasars and X-rays generated. At N1S1 interior the mass for exterior has not gravity which is just we searching dark matter. The mass each part will each other shielding and gravity decrease to less than the pressure of the degenerate neutron gas. The neutron star cannot collapse into a singular point with infinite density, i.e., the black hole with infinite gravity cannot be formed or the neutron star is jest the black hole in observational meaning. By the gravity accrete of N1S1 the neutron star may enlarge its shell radii but thickness keep. Only a shell gravity may be not less than any a observed value which to be deemed as black hole. The neutron star has powerful gravity certainly accompany with great surface negative charge and it may rapidly to rotate, so that there is a powerful magnetic field surround it. The accreting neutron star is as a slowly expand empty shell with fixed thickness of 1 km, its spin period depend on its radii or total accretion mass.

  5. Chaotic cold accretion on to black holes

    NASA Astrophysics Data System (ADS)

    Gaspari, M.; Ruszkowski, M.; Oh, S. Peng

    2013-07-01

    Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in

  6. Accretion of Moon and Earth and the emergence of life

    NASA Technical Reports Server (NTRS)

    Arrhenius, G.; Lepland, A.

    2000-01-01

    The discrepancy between the impact records on the Earth and Moon in the time period, 4.0-3.5 Ga calls for a re-evaluation of the cause and localization of the late lunar bombardment. As one possible explanation, we propose that the time coverage in the ancient rock record is sufficiently fragmentary, so that the effects of giant, sterilizing impacts throughout the inner solar system, caused by marauding asteroids, could have escaped detection in terrestrial and Martian records. Alternatively, the lunar impact record may reflect collisions of the receding Moon with a series of small, original satellites of the Earth and their debris in the time period about 4.0-3.5 Ga. The effects on Earth of such encounters could have been comparatively small. The location of these tellurian moonlets has been estimated to have been in the region around 40 Earth radii. Calculations presented here, indicate that this is the region that the Moon would traverse at 4.0-3.5 Ga, when the heavy and declining lunar bombardment took place. The ultimate time limit for the emergence of life on Earth is determined by the effects of planetary accretion--existing models offer a variety of scenarios, ranging from low average surface temperature at slow accretion of the mantle, to complete melting of the planet followed by protracted cooling. The choice of accretion model affects the habitability of the planet by dictating the early evolution of the atmosphere and hydrosphere. Further exploration of the sedimentary record on Earth and Mars, and of the chemical composition of impact-generated ejecta on the Moon, may determine the choice between the different interpretations of the late lunar bombardment and cast additional light on the time and conditions for the emergence of life.

  7. Accretion of Moon and Earth and the emergence of life.

    PubMed

    Arrhenius, G; Lepland, A

    2000-08-15

    The discrepancy between the impact records on the Earth and Moon in the time period, 4.0-3.5 Ga calls for a re-evaluation of the cause and localization of the late lunar bombardment. As one possible explanation, we propose that the time coverage in the ancient rock record is sufficiently fragmentary, so that the effects of giant, sterilizing impacts throughout the inner solar system, caused by marauding asteroids, could have escaped detection in terrestrial and Martian records. Alternatively, the lunar impact record may reflect collisions of the receding Moon with a series of small, original satellites of the Earth and their debris in the time period about 4.0-3.5 Ga. The effects on Earth of such encounters could have been comparatively small. The location of these tellurian moonlets has been estimated to have been in the region around 40 Earth radii. Calculations presented here, indicate that this is the region that the Moon would traverse at 4.0-3.5 Ga, when the heavy and declining lunar bombardment took place. The ultimate time limit for the emergence of life on Earth is determined by the effects of planetary accretion--existing models offer a variety of scenarios, ranging from low average surface temperature at slow accretion of the mantle, to complete melting of the planet followed by protracted cooling. The choice of accretion model affects the habitability of the planet by dictating the early evolution of the atmosphere and hydrosphere. Further exploration of the sedimentary record on Earth and Mars, and of the chemical composition of impact-generated ejecta on the Moon, may determine the choice between the different interpretations of the late lunar bombardment and cast additional light on the time and conditions for the emergence of life.

  8. Aerodynamic Simulation of Ice Accretion on Airfoils

    NASA Technical Reports Server (NTRS)

    Broeren, Andy P.; Addy, Harold E., Jr.; Bragg, Michael B.; Busch, Greg T.; Montreuil, Emmanuel

    2011-01-01

    This report describes recent improvements in aerodynamic scaling and simulation of ice accretion on airfoils. Ice accretions were classified into four types on the basis of aerodynamic effects: roughness, horn, streamwise, and spanwise ridge. The NASA Icing Research Tunnel (IRT) was used to generate ice accretions within these four types using both subscale and full-scale models. Large-scale, pressurized windtunnel testing was performed using a 72-in.- (1.83-m-) chord, NACA 23012 airfoil model with high-fidelity, three-dimensional castings of the IRT ice accretions. Performance data were recorded over Reynolds numbers from 4.5 x 10(exp 6) to 15.9 x 10(exp 6) and Mach numbers from 0.10 to 0.28. Lower fidelity ice-accretion simulation methods were developed and tested on an 18-in.- (0.46-m-) chord NACA 23012 airfoil model in a small-scale wind tunnel at a lower Reynolds number. The aerodynamic accuracy of the lower fidelity, subscale ice simulations was validated against the full-scale results for a factor of 4 reduction in model scale and a factor of 8 reduction in Reynolds number. This research has defined the level of geometric fidelity required for artificial ice shapes to yield aerodynamic performance results to within a known level of uncertainty and has culminated in a proposed methodology for subscale iced-airfoil aerodynamic simulation.

  9. Episodic Accretion among the Orion Protostars

    NASA Astrophysics Data System (ADS)

    Fischer, William J.; Safron, Emily; Megeath, S. Thomas

    2016-06-01

    Episodic accretion, where a young stellar object undergoes stochastic spikes in its disk-to-star accretion rate one or more times over its formation period, may be a crucial process in the formation of low-mass stars. These spikes result in a factor of 10 to 100 increase in the source luminosity over the course of several months that may persist for years. Six years after the Spitzer survey of the Orion molecular clouds, the WISE telescope mapped Orion with similar wavelength coverage. Thus, the two surveys can be used to explore the mid-infrared variability of young stars on this timescale, which is suitable for discovering episodic accretion events. Out of 319 Orion protostars that were targets of the Herschel Orion Protostar Survey, we identified two examples of episodic accretion with this method. One of them, HOPS 223, was previously known. The other, HOPS 383, is the first known example of episodic accretion in a Class 0 protostar (age < 0.2 Myr). With these and one other outburst that began early in the Spitzer mission, we estimate that the most likely interval between protostellar outbursts is 740 years, with a 90% confidence interval of 470 to 6200 years. These outbursts are weaker than the optically revealed FU Ori events. We will update the mid-infrared light curves of HOPS 223 and HOPS 383 with recent data from FORCAST aboard SOFIA; HOPS 223 shows signs of fading.

  10. The Final Fates of Accreting Supermassive Stars

    NASA Astrophysics Data System (ADS)

    Umeda, Hideyuki; Hosokawa, Takashi; Omukai, Kazuyuki; Yoshida, Naoki

    2016-10-01

    The formation of supermassive stars (SMSs) via rapid mass accretion and their direct collapse into black holes (BHs) is a promising pathway for sowing seeds of supermassive BHs in the early universe. We calculate the evolution of rapidly accreting SMSs by solving the stellar structure equations including nuclear burning as well as general relativistic (GR) effects up to the onset of the collapse. We find that such SMSs have a less concentrated structure than a fully convective counterpart, which is often postulated for non-accreting ones. This effect stabilizes the stars against GR instability even above the classical upper mass limit ≳105 M ⊙ derived for the fully convective stars. The accreting SMS begins to collapse at the higher mass with the higher accretion rate. The collapse occurs when the nuclear fuel is exhausted only for cases with \\dot{M}≲ 0.1 {M}ȯ {{{yr}}}-1. With \\dot{M}≃ 0.3{--}1 {M}ȯ {{{yr}}}-1, the star becomes GR unstable during the helium-burning stage at M ≃ 2–3.5 × 105 M ⊙. In an extreme case with 10 {M}ȯ {{{yr}}}-1, the star does not collapse until the mass reaches ≃8.0 × 105 M ⊙, where it is still in the hydrogen-burning stage. We expect that BHs with roughly the same mass will be left behind after the collapse in all the cases.

  11. Effects of Ice Accretion on Aircraft Aerodynamics

    NASA Technical Reports Server (NTRS)

    Bragg, Michael B.

    1998-01-01

    The primary objective of this research was to support the development of a new ice accretion model by improving our physical understanding of the ice accretion process through experimental measurements. The focus was on the effect of the initial ice roughness (smooth/rough boundary) on the accretion process. This includes understanding the boundary-layer development over the roughness and especially its effect on the heat transfer which is fundamental to the ice accretion process. The research focused on acquiring the experimental data needed to formulate a new ice accretion physical model. Research was conducted to analyze boundary-layer data taken on a NACA 0012 airfoil with roughness to simulate the smooth/rough boundary. The effect of isolated roughness on boundary-layer transition was studied experimentally to determine if the classical critical roughness Reynolds number criteria could be applied to transition in the airfoil leading-edge area. The effect of simulated smooth/rough boundary roughness on convective heat transfer was studied to complete the study. During the course of this research the effect of free-stream wind tunnel turbulence on the boundary layer was measured. Since this quantity was not well known, research to accurately measure the wind tunnel turbulence in an icing cloud was undertaken. Preliminary results were attained and the final data were acquired, reduced and presented under a subsequent grant.

  12. SIMULATIONS OF THE BOUNDARY LAYER BETWEEN A WHITE DWARF AND ITS ACCRETION DISK

    SciTech Connect

    Balsara, Dinshaw S.; Fisker, Jacob Lund; Godon, Patrick; Sion, Edward M. E-mail: jfisker@nd.edu E-mail: edward.sion@villanova.edu

    2009-09-10

    Using a 2.5D time-dependent numerical code we recently developed, we solve the full compressible Navier-Stokes equations to determine the structure of the boundary layer (BL) between the white dwarf (WD) and the accretion disk in nonmagnetic cataclysmic variable systems. In this preliminary work, our numerical approach does not include radiation. In the energy equation, we either take the dissipation function ({phi}) into account or we assume that the energy dissipated by viscous processes is instantly radiated away ({phi} = 0). For a slowly rotating nonmagnetized accreting WD, the accretion disk extends all the way to the stellar surface. There, the matter impacts and spreads toward the poles as new matter continuously piles up behind it. We carry out numerical simulations for different values of the alpha-viscosity parameter ({alpha}), corresponding to different mass accretion rates. In the high viscosity cases ({alpha} = 0.1), the spreading BL sets off a gravity wave in the surface matter. The accretion flow moves supersonically over the cusp making it susceptible to the rapid development of gravity wave and/or Kelvin-Helmholtz shearing instabilities. This BL is optically thick and extends more than 30 deg. to either side of the disk plane after only 3/4 of a Keplerian rotation period (t{sub K} = 19 s). In the low viscosity cases ({alpha} = 0.001), the spreading BL does not set off gravity waves and it is optically thin.

  13. Dependence of kHz quasi-periodic oscillation frequencies on accretion-related parameters

    NASA Astrophysics Data System (ADS)

    Erkut, M. Hakan; Catmabacak, Onur; Duran, Sivan; Çatmabacak, Önder

    2016-07-01

    To study the possible dependence of kHz QPO frequencies on the parameters such as the mass accretion rate, the surface magnetic field strength, mass, and radius of the neutron star, we consider the up-to-date distribution of neutron star LMXBs in the kHz QPO frequency versus X-ray luminosity plane. We confirm the absence of any correlation between QPO frequencies and luminosity in the ensemble of LMXBs. Searching for the dependence of QPO data on accretion-related parameters, we find a correlation between the lower kHz QPO frequency and the parameter combining mass accretion rate with magnetic field strength. The correlation cannot be adequately described by a simple power law due to observed scattering of individual source data in the ensemble of Z and atoll sources. Based on disk-magnetosphere boundary region, the model function for QPO frequency can delineate the correlation taking into account the scattering of individual sources. In addition to mass accretion rate and magnetic field strength, the model function also depends on the radial width of the boundary region near the magnetopause. Modelling the variation of the width with mass accretion rate, we also provide an explanation for the parallel tracks phenomenon observed in the case of individual sources.

  14. Characteristics of transonic spherical symmetric accretion flow in Schwarzschild-de Sitter and Schwarzschild anti-de Sitter backgrounds, in pseudo-general relativistic paradigm

    NASA Astrophysics Data System (ADS)

    Ghosh, Shubhrangshu; Banik, Prabir

    2015-07-01

    dominant (CD) galaxies directly from ambient intercluster medium (ICM). However, for high mass accretion rate, the influence of cosmological constant on Bondi accretion dynamics, generically, diminishes. As active galactic nuclei (AGN)/ICM feedback can be advertently linked to Bondi type spherical accretion, any proper modeling of AGN feedback or megaparsecs-scale jet dynamics or accretion flow from ICM onto the central regions of host galaxies should take into account the relevant information of repulsive Λ, especially in context to supergiant elliptical galaxies or CD galaxies present in rich galaxy clusters. This could also explore the feasibility to limit the value of Λ, from the kinematics in local galactic-scales.

  15. MAGNETIZED ACCRETION AND DEAD ZONES IN PROTOSTELLAR DISKS

    SciTech Connect

    Dzyurkevich, Natalia; Henning, Thomas; Turner, Neal J.; Kley, Wilhelm

    2013-03-10

    The edges of magnetically dead zones in protostellar disks have been proposed as locations where density bumps may arise, trapping planetesimals and helping form planets. Magneto-rotational turbulence in magnetically active zones provides both accretion of gas on the star and transport of mass to the dead zone. We investigate the location of the magnetically active regions in a protostellar disk around a solar-type star, varying the disk temperature, surface density profile, and dust-to-gas ratio. We also consider stellar masses between 0.4 and 2 M{sub Sun }, with corresponding adjustments in the disk mass and temperature. The dead zone's size and shape are found using the Elsasser number criterion with conductivities including the contributions from ions, electrons, and charged fractal dust aggregates. The charged species' abundances are found using the approach proposed by Okuzumi. The dead zone is in most cases defined by the ambipolar diffusion. In our maps, the dead zone takes a variety of shapes, including a fish tail pointing away from the star and islands located on and off the midplane. The corresponding accretion rates vary with radius, indicating locations where the surface density will increase over time, and others where it will decrease. We show that density bumps do not readily grow near the dead zone's outer edge, independently of the disk parameters and the dust properties. Instead, the accretion rate peaks at the radius where the gas-phase metals freeze out. This could lead to clearing a valley in the surface density, and to a trap for pebbles located just outside the metal freezeout line.

  16. Accretion disks in Algols: Progenitors and evolution

    NASA Astrophysics Data System (ADS)

    Van Rensbergen, W.; De Greve, J. P.

    2016-08-01

    Context. There are only a few Algols with measured accretion disk parameters. These measurements provide additional constraints for tracing the origin of individual systems, narrowing down the initial parameter space. Aims: We investigate the origin and evolution of six Algol systems with accretion disks to find the initial parameters and evolutionary constraints for them. Methods: With a modified binary evolution code, series of close binary evolution are calculated to obtain the best match for observed individual systems. Results: Initial parameters for six Algol systems with accretion disks were determined matching both the present system parameters and the observed disk characteristics. Conclusions: When Roche lobe overflow (RLOF) starts during core hydrogen burning of the donor, the disk lifetime was found to be short. The disk luminosity is comparable to the luminosity of the gainer during a large fraction of the disk lifetime.

  17. Thermal structure of the accreting earth

    SciTech Connect

    Turcotte, D.L.; Pflugrath, J.C.

    1985-02-15

    The energy associated with the accretion of the earth and the segregation of the core is more than sufficient to melt the entire earth. In order to understand the thermal evolution of the early earth it is necessary to study the relevant heat transfer mechanisms. In this paper we postulate the existence of a global magma ocean and carry out calculations of the heat flux through it in order to determine its depth. In the solid mantle heat is transferred by the upward migration of magma. This magma supplies the magma ocean. The increase in the mantle liquidus with depth (pressure) is the dominant effect influencing heat transfer through the magma ocean. We find that a magma ocean with a depth of the order of 20 km would have existed as the earth accreted. We conclude that the core segregated and an atmosphere was formed during accretion.

  18. HST spectrophotometry of accreting white dwarf pulsators

    NASA Astrophysics Data System (ADS)

    Mukadam, Anjum S.; Szkody, Paula; Gaensicke, Boris T.

    2015-01-01

    The discovery of non-radial pulsations in cataclysmic variables has opened a new venue of opportunity to probe the stellar parameters of accreting variable white dwarfs using asteroseismic techniques. A unique model fit to the observed periods of the variable white dwarf can reveal information about the stellar mass, core composition, age, rotation rate, magnetic field strength, and distance. Mode identification is an essential step in determining an unambiguous model fit, that could be achieved by determining optical and ultra-violet pulsation amplitudes. We will be presenting our results on ultra-violet HST observations acquired with contemporaneous ground based optical data for several cataclysmic variables. The HST spectrophotometry also yields the effective temperatures of the accreting white dwarfs, allowing us to improve our present determination of the instability strip for accreting pulsators. We thank NASA for the grant HST-GO12870 that has supported this research.

  19. Strongly magnetized accretion discs require poloidal flux

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Armitage, Philip J.; Simon, Jacob B.; Begelman, Mitchell C.

    2016-08-01

    Motivated by indirect observational evidence for strongly magnetized accretion discs around black holes, and the novel theoretical properties of such solutions, we investigate how a strong magnetization state can develop and persist. To this end, we perform local simulations of accretion discs with an initially purely toroidal magnetic field of equipartition strength. We demonstrate that discs with zero net vertical magnetic flux and realistic boundary conditions cannot sustain a strong toroidal field. However, a magnetic pressure-dominated disc can form from an initial configuration with a sufficient amount of net vertical flux and realistic boundary conditions. Our results suggest that poloidal flux is a necessary prerequisite for the sustainability of strongly magnetized accretion discs.

  20. SPECTRAL ENERGY DISTRIBUTIONS OF ACCRETING PROTOPLANETS

    SciTech Connect

    Eisner, J. A.

    2015-04-10

    Planets are often invoked as the cause of inferred gaps or inner clearings in transition disks. These putative planets would interact with the remnant circumstellar disk, accreting gas and generating substantial luminosity. Here I explore the expected appearance of accreting protoplanets at a range of evolutionary states. I compare synthetic spectral energy distributions with the handful of claimed detections of substellar-mass companions in transition disks. While observed fluxes of candidate companions are generally compatible with accreting protoplanets, challenges remain in reconciling the extended structure inferred in observed objects with the compact emission expected from protoplanets or circumplanetary disks. I argue that a large fraction of transition disks should harbor bright protoplanets, and that more may be detected as larger telescopes open up additional parameter space.

  1. Accretion-powered Compact Binaries

    NASA Astrophysics Data System (ADS)

    Mauche, Christopher W.

    2003-12-01

    Preface; The workshop logo; A short history of the CV workshop F. A. Córdova; Part I. Observations: 1. Low mass x-ray binaries A. P. Cowley, P. C. Schmidtke, D. Crampton, J. B. Hutchings, C. A. Haswell, E. L. Robinson, K. D. Horne, H. M. Johnston, S. R. Kulkarni, S. Kitamoto, X. Han, R. M. Hjellming, R. M. Wagner, S. L. Morris, P. Hertz, A. N. Parmar, L. Stella, P. Giommi, P. J. Callanan, T. Naylor, P. A. Charles, C. D. Bailyn, J. N. Imamura, T. Steiman-Cameron, J. Kristian, J. Middleditch, L. Angelini and J. P. Noris; 2. Nonmagnetic cataclysmic variables R. S. Polidan, C. W. Mauche, R. A. Wade, R. H. Kaitchuck, E. M. Schlegel, P. A. Hantzios, R. C. Smith, J. H. Wood, F. Hessman, A. Fiedler, D. H. P. Jones, J. Casares, P. A. Charles, J. van Paradijs, E. Harlaftis, T. Naylor, G. Sonneborn, B. J. M. Hassall, K. Horne, C. A. la Dous, A. W. Shafter, N. A. Hawkins, D. A. H. Buckley, D. J. Sullivan, F. V. Hessman, V. S. Dhillon, T. R. Marsh, J. Singh, S. Seetha, F. Giovannelli, A. Bianchini, E. M. Sion, D. J. Mullan, H. L. Shipman, G. Machin, P. J. Callanan, S. B. Howell, P. Szkody, E. M. Schlegel and R. F. Webbink; 3. Magnetic cataclysmic variables C. Hellier, K. O. Mason, C. W. Mauche, G. S. Miller, J. C. Raymond, F. K. Lamb, J. Patterson, A. J. Norton, M. G. Watson, A. R. King, I. M. McHardy, H. Lehto, J. P. Osborne, E. L. Robinson, A. W. Shafter, S. Balachandran, S. R. Rosen, J. Krautter, W. Buchholz, D. A. H. Buckley, I. R. Tuoly, D. Crampton, B. Warner, R. M. Prestage, B. N. Ashoka, M. Mouchet, J. M. Bonnet-Bidaud, J. M. Hameury, P. Szkody, P. Garnavich, S. Howell, T. Kii, M. Cropper, K. Mason, J. Bailey, D. T. Wickramasinghe, L. Ferrario, K. Beuermann, A. D. Schwope, H.-C. Thomas, S. Jordan, J. Schachter, A. V. Filippenko, S. M. Kahn, F. B. S. Paerels, K. Mukai, M. L. Edgar, S. Larsson, R. F. Jameson, A. R. King, A. Silber, R. Remillard, H. Bradt, M. Ishida, T. Ohashi and G. D. Schmidt; Part II. Accretion Theory: 4. Nonmagnetic W. Kley, F. Geyer, H. Herold, H

  2. The signature of single-degenerate accretion-induced collapse

    SciTech Connect

    Piro, Anthony L.

    2014-10-10

    The accretion-induced collapse (AIC) of a white dwarf to a neutron star has long been suggested as a natural theoretical outcome in stellar evolution, but there has never been a direct detection of such an event. This is not surprising since the small amount of radioactive nickel synthesized (∼10{sup –3} M {sub ☉}) implies a relatively dim optical transient. Here we argue that a particularly strong signature of an AIC would occur for an oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the ∼10{sup 50} erg explosion from the AIC collides with and shock-heats the surface of the extended companion, creating an X-ray flash lasting ∼1 hr followed by an optical signature that peaks at an absolute magnitude of ∼ – 16 to –18 and lasts for a few days to a week. These events would be especially striking in old stellar environments where hydrogen-rich supernova-like transients would not normally be expected. Although the rate of such events is not currently known, we describe observing strategies that could be utilized with high cadence surveys that should either detect these events or place strong constraints on their rates.

  3. Generalized Similarity for Accretion/Decretion Disks

    NASA Astrophysics Data System (ADS)

    Rafikov, Roman R.

    2016-10-01

    Decretion (or external) disks are gas disks freely expanding to large radii due to their internal stresses. They are expected to naturally arise in tidal disruption events, around Be stars, in mass-losing post-main-sequence binaries, as a result of supernova fallback, etc. Their evolution is theoretically understood in two regimes: when the central object does not exert torque on the disk (a standard assumption for conventional accretion disks) or when no mass inflow (or outflow) occurs at the disk center. However, many astrophysical objects—circumbinary disks, Be stars, neutron stars accreting in a propeller regime, etc.—feature non-zero torque simultaneously with the non-zero accretion (or ejection of mass) at the disk center. We provide a general description for the evolution of such disks (both linear and nonlinear) in the self-similar regime, to which the disk should asymptotically converge with time. We identify a similarity parameter λ, which is uniquely related to the degree, to which the central mass accretion is suppressed by the non-zero central torque. The known decretion disk solutions correspond to the two discrete values of λ, while our new solutions cover a continuum of its physically allowed values, corresponding to either accretion or mass ejection by the central object. A direct relationship between λ and central \\dot{M} and torque is also established. We describe the time evolution of the various disk characteristics for different λ, and show that the observable properties (spectrum and luminosity evolution) of the decretion disks, in general, are different from the standard accretion disks with no central torque.

  4. Maximal possible accretion rates for slim disks

    NASA Astrophysics Data System (ADS)

    Lin, Yiqing; Jiao, Chengliang

    2009-12-01

    It was proved in the previous work that there must be a maximal possible accretion rate dot M_{max} for a slim disk. Here we discuss how the value of dot M_{max} depends on the two fundamental parameters of the disk, namely the mass of the central black hole M and the viscosity parameter α. It is shown that dot M_{max} increases with decreasing α, but is almost independent of M if dot M_{max} is measured by the Eddington accretion rate dot M_{Edd} , which is in turn proportional to M.

  5. Splashback in accreting dark matter halos

    SciTech Connect

    Adhikari, Susmita; Dalal, Neal; Chamberlain, Robert T. E-mail: dalaln@illinois.edu

    2014-11-01

    Recent work has shown that density profiles in the outskirts of dark matter halos can become extremely steep over a narrow range of radius. This behavior is produced by splashback material on its first apocentric passage after accretion. We show that the location of this splashback feature may be understood quite simply, from first principles. We present a simple model, based on spherical collapse, that accurately predicts the location of splashback without any free parameters. The important quantities that determine the splashback radius are accretion rate and redshift.

  6. Mechanisms resulting in accreted ice roughness

    NASA Technical Reports Server (NTRS)

    Bilanin, Alan J.; Chua, Kiat

    1992-01-01

    Icing tests conducted on rotating cylinders in the BF Goodrich's Icing Research Facility indicate that a regular, deterministic, icing roughness pattern is typical. The roughness pattern is similar to kernels of corn on a cob for cylinders of diameter typical of a cob. An analysis is undertaken to determine the mechanisms which result in this roughness to ascertain surface scale and amplitude of roughness. Since roughness and the resulting augmentation of the convected heat transfer coefficient has been determined to most strongly control the accreted ice in ice prediction codes, the ability to predict a priori, location, amplitude and surface scale of roughness would greatly augment the capabilities of current ice accretion models.

  7. The main sequences of NGC 2808: constraints on the early disc accretion scenario

    NASA Astrophysics Data System (ADS)

    Cassisi, Santi; Salaris, Maurizio

    2014-03-01

    A new scenario - early disc accretion - has been proposed very recently to explain the origin of the multiple population phenomenon in Galactic globular clusters. It envisages the possibility that a fraction of low- and very low-mass cluster stars may accrete the ejecta of interacting massive binary (and possibly also fast rotating massive) stars during the fully convective, pre-main sequence stage, to reproduce the CN and ONa anticorrelations observed among stars in individual clusters. This scenario is assumed to be able to explain the presence (and properties) of the multiple populations in the majority of globular clusters in the Milky Way. Here we have considered the well studied cluster NGC 2808, which displays a triple main sequence with well defined and separate He abundances. Knowledge of these abundances allowed us to put strong constraints on the He mass fraction and amount of matter to be accreted by low-mass pre-main sequence stars. We find that the minimum He mass fraction in the accreted gas has to be ~0.44 to produce the observed sequences and that at fixed initial mass of the accreting star, different efficiencies for the accretion are required to produce stars placed onto the multiple main sequences. This may be explained by differences in the orbital properties of the progenitors and/or different spatial distribution of intracluster gas with varying He abundances. Both O-Na and C-N anticorrelations appear naturally along the main sequences, once considering the predicted relationship between He and CNONa abundances in the ejecta of the polluters. As a consequence of the accretion, we predict no discontinuity between the abundance ranges covered by intermediate and blue main sequence stars, but we find a sizeable (several 0.1 dex) discontinuity of the N and Na abundances between objects on the intermediate and red main sequences. There is in principle enough polluting gas with the right He abundances to explain the observed main sequences by early

  8. Diversity of Planets in the Aftermath of Late Stage Accretion (Invited)

    NASA Astrophysics Data System (ADS)

    Asphaug, E. I.; Reufer, A.

    2013-12-01

    It is a mistake to view giant impacts or similar sized collisions between planets or planetary embryos as large-scale versions of impact cratering. Although much of the physics is the same (material mechanics, and shocks in the case of hypervelocity events), fundamentally a similar sized collision is a gravity-regime phenomenon where too much angular momentum is trying to accrete into one place, as the video linked below illustrates. When we see material 'escaping' from a collision, in simulations, it is not so much that it was blasted from the target (this is rarely the case) than that it was not accreted by the target. When we see a disk forming around the proto-Earth, in Moon-forming simulations, it was not ejected from the Earth (in the standard model) but rather it forms from impactor material that was only partially accreted (c.f. Canup 2004). Hence the quandary of oxygen isotopes in Moon formation. With this perspective, a number of novel pathways for meteorite petrogenesis and planetary origins open up: hit and run collisions, which can leave behind iron-rich relics (or rock-rich relics in the case of KBO collisions); partial accretion, which is the standard model for the silicate-rich Moon; and the formation of families of bodies (self-gravitating clumps formed in escaping tidal arms) which offer a new way of forming isotopically closely related bodies from a common parent that may have dramatically different bulk compositions. The concept of imperfect accretion creates a coeval timescale for planetary disruption, one that occurs while the planets are coming together, rather than long afterwards as battered pieces of the Main Belt or the Kuiper Belt -- disruption by accretion, and in many cases, disruption without shock. As time permits we shall present hypotheses invoking imperfect pairwise accretion as an origin scenario for specific planetary bodies such as Mercury and the satellites of Saturn. The linked video is from an origins model by Asphaug and

  9. The value of place

    NASA Astrophysics Data System (ADS)

    Dentzau, Michael W.

    2014-03-01

    This commentary seeks to expand the dialogue on place-based science education presented in Katie Lynn Brkich's article, where the connections fifth grade students make between their formal earth science curriculum and their lived experiences are highlighted. The disconnect between the curriculum the students are offered and their immediate environment is clear, and we are presented with examples of how they strive to make connections between the content and what they are familiar with—namely their surroundings. "Place" is identified as a term with complex meanings and interpretations, even in the scope of place-based science education, and understanding how the term is used in any given scenario is essential to understanding the implications of place-based education. Is place used as a location, locale or a sense of place? To understand "place" is to acknowledge that for the individual, it is highly situational, cultural and personal. It is just such attributes that make place-based education appealing, and potentially powerful, pedagogically on one hand, yet complex for implementation on the other. The argument is posed that place is particularly important in the context of education about the environment, which in its simplest manifestation, connects formal science curriculum to resources that are local and tangible to students. The incorporation of place in such a framework seeks to bridge the gap between formal school science subjects and students' lived experiences, yet acknowledges the tensions that can arise between accommodating place meanings and the desire to acculturate students into the language of the scientific community. The disconnect between guiding policy frameworks and the reality of the Next Generation Science Standards is addressed opening an avenue for further discussion of the importance of socio-cultural frameworks of science learning in an ever increasing era of accountability.

  10. Probing Cosmic Gas Accretion with RESOLVE and ECO

    NASA Astrophysics Data System (ADS)

    Kannappan, Sheila; Eckert, Kathleen D.; Stark, David; Lagos, Claudia; Nasipak, Zachary; Moffett, Amanda J.; Baker, Ashley; Berlind, Andreas A.; Hoversten, Erik A.; Norris, Mark A.; RESOLVE Team

    2016-01-01

    We review results bearing on the existence, controlling factors, and mechanisms of cosmic gas accretion in the RESOLVE and ECO surveys. Volume-limited analysis of RESOLVE's complete census of HI-to-stellar mass ratios and star formation histories for ~1500 galaxies points to the necessity of an "open box" model of galaxy fueling, with the most gas-dominated galaxies doubling their stellar masses on ~Gyr timescales in a regime of rapid accretion. Transitions in gas richness and disk-building activity for isolated or central galaxies with halo masses near ~10^11.5 Msun and ~10^12 Msun plausibly correspond to the endpoints of a theoretically predicted transition in halo gas temperature that slows accretion across this range. The same mass range is associated with the initial grouping of isolated galaxies into common halos, where "isolated" is defined relative to the survey baryonic mass limits of >~10^9 Msun. Above 10^11.5 Msun, patterns in central vs. satellite gas richness as a function of group halo mass suggest that galaxy refueling is valved off from the inside out as the halo grows, with total quenching beyond the virial radius for halo masses >~10^13-13.5 Msun. Within the transition range from ~10^11.5-10^12 Msun, theoretical models predict >3 dex dispersion in ratios of uncooled halo gas to cold gas in galaxies (or more generally gas and stars). In RESOLVE and ECO, the baryonic mass function of galaxies in this transitional halo mass range displays signs of stripping or destruction of satellites, leading us to investigate a possible connection with halo gas heating using central galaxy color and group dynamics to probe group evolutionary state. Finally, we take a first look at how internal variations in metallicity, dynamics, and star formation constrain accretion mechanisms such as cold streams, induced extraplanar gas cooling, isotropic halo gas cooling, and gas-rich merging in different mass and environment regimes. The RESOLVE and ECO surveys have been

  11. Accretion dynamics of EX Lupi in quiescence. The star, the spot, and the accretion column

    NASA Astrophysics Data System (ADS)

    Sicilia-Aguilar, Aurora; Fang, Min; Roccatagliata, Veronica; Collier Cameron, Andrew; Kóspál, Ágnes; Henning, Thomas; Ábrahám, Peter; Sipos, Nikoletta

    2015-08-01

    Context. EX Lupi is a young, accreting M0 star and the prototype of EXor variable stars. Its spectrum is very rich in emission lines, including many metallic lines with narrow and broad components. The presence of a close companion has also been proposed, based on radial velocity signatures. Aims: We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. Methods: We analyse 54 spectra obtained during five years of quiescence time. We study the line profile variability and the radial velocity of the narrow and broad metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated with accretion. Results: We observe periodic velocity variations in the broad and the narrow line components, consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials (e.g. He II), which are likely produced in a confined area very close to the accretion shock. Conclusions: We propose that the narrow line components are produced in the post-shock region, while the broad components originate in the more extended, pre-shock material in the accretion column. All the emission lines suffer velocity modulation due to the rotation of the star. The broad components are responsible for the line-dependent veiling observed in EX Lupi. We demonstrate that a rotationally modulated line-dependent veiling can explain the radial velocity signature of the photospheric absorption lines, making the close-in companion hypothesis unnecessary. The accretion structure is locked to the star and very stable during the five years of observations. Not all stars with similar spectral types and accretion rates show the same metallic emission lines, which could be related to differences in temperature and density in their accretion structure(s). The contamination of photospheric signatures by accretion

  12. Where a Neutron Star's Accretion Disk Ends

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-03-01

    In X-ray binaries that consist of a neutron star and a companion star, gas funnels from the companion into an accretion disk surrounding the neutron star, spiraling around until it is eventually accreted. How do the powerful magnetic fields threading through the neutron star affect this accretion disk? Recent observations provide evidence that they may push the accretion disk away from the neutron stars surface.Truncated DisksTheoretical models have indicated that neutron star accretion disks may not extend all the way in to the surface of a neutron star, but may instead be truncated at a distance. This prediction has been difficult to test observationally, however, due to the challenge of measuring the location of the inner disk edge in neutron-star X-ray binaries.In a new study, however, a team of scientists led by Ashley King (Einstein Fellow at Stanford University) has managed to measure the location of the inner edge of the disk in Aquila X-1, a neutron-star X-ray binary located 17,000 light-years away.Iron line feature detected by Swift (red) and NuSTAR (black). The symmetry of the line is one of the indicators that the disk is located far from the neutron star; if the inner regions of the disk were close to the neutron star, severe relativistic effects would skew the line to be asymmetric. [King et al. 2016]Measurements from ReflectionsKing and collaborators used observations made by NuSTAR and Swift/XRT both X-ray space observatories of Aquila X-1 during the peak of an X-ray outburst. By observing the reflection of Aquila X-1s emission off of the inner regions of the accretion disk, the authors were able to estimate the location of the inner edge of the disk.The authors find that this inner edge sits at ~15 gravitational radii. Since the neutron stars surface is at ~5 gravitational radii, this means that the accretion disk is truncated far from the stars surface. In spite of this truncation, material still manages to cross the gap and accrete onto the

  13. Implications of pebble accretion on the composition of hot and cold Jupiters

    NASA Astrophysics Data System (ADS)

    Bitsch, Bertram; Johansen, Anders; Madhusudhan, Nikku

    2016-10-01

    The formation of the planetary cores of gas giants via the accretion of planetesimals takes very long and is not compatible with the lifetime of protoplanetary discs (Levison et al. 2010). This time-scale problem can be solved through the accretion of pebbles onto a planetary seed. Contrary to planetesimals, pebbles feel the headwind from the gas which robs them of angular momentum allowing an efficient growth from the entire Hill sphere, which reduces the growth time-scale by several orders of magnitude (Lambrechts & Johansen, 2012; 2014). However, pebble accretion self-terminates when the planets start to open a partial gap in the disc, which accelerates the gas outside of the planets orbit to super-Keplerian speeds and thus stops the flow of pebbles onto the planetary core (Lambrechts et al. 2014). Typically this mass is of the order of 10-20 Earth masses, depending on the local disc properties. The planet can then start to accrete a gaseous envelope without a pollution of pebbles. During its growth, the planet migrates through the disc, which evolves in time (Bitsch et al. 2015a,b).Different volatile species like CO2 or H2O have different condensation temperatures and are thus present in either solid or gaseous form at different locations in the disc. A pebble accreting planet can thus only accrete volatiles that are in solid form, while a gas accreting planet will only accrete volatiles which are in gaseous form. Therefore the final chemical composition of the planetary atmosphere of a giant planet is strongly influenced by the formation location of the initial planetary seed and its subsequent migration path through the disc. Additionally, the envelope can be enriched through the erosion of the planetary core.I will discuss the implications of the formation of planets via pebble accretion and their subsequent migration through the disc on the composition of gas giants. In particular I will focus on the carbon to oxygen ratio of hot Jupiters around other stars

  14. Constraints on continental accretion from sedimentation

    NASA Technical Reports Server (NTRS)

    Abbott, Dallas

    1988-01-01

    Heat loss in the ancient Earth was discussed assuming that classical sea floor spreading was the only mechanism. This may be expressed as faster spreading or longer total ridge length. These have important implications as to the size and number of cratonic plates in the distant past, the degree to which they are flooded, the kinds of sediments and volcanics that would be expected, and the amount of recycling of continental material taking place. The higher proportion of marine sedimentary rocks and oceanic volcanics in the Archean, and the relative paucity of evaporites and continental volcanics may in part be due to smaller cratonic blocks. A model was developed of the percentage of continental flooding which utilizes round continents and a constant width of the zone of flooding. This model produces a reasonable good fit to the percentage of flooding on the present day continents.

  15. Place as Library?

    ERIC Educational Resources Information Center

    Davenport, Nancy

    2006-01-01

    Digital technology is redrawing the library's blueprint. Planners are thinking in new ways about how to design libraries as places for learning rather than primarily as storehouses of information. This thinking has given rise to much discussion--and to many publications--about the "library as place." In this article, the author asks why not also…

  16. The Case for Place

    ERIC Educational Resources Information Center

    Thomas, Lisa Carlucci

    2012-01-01

    Bookstores, record stores, libraries, Facebook: these places--both physical and virtual--demonstrate an established and essential purpose as centers of community, expertise, convenience, immediacy, and respect. Yet as digital, mobile, and social shifts continue to transform culture and interactions, these spaces and places transform, too.…

  17. Teaching Bodies in Place

    ERIC Educational Resources Information Center

    Jones, Stephanie; Woglom, James F.

    2013-01-01

    Background/Context: This piece draws on literature in justice-oriented teacher education, feminist pedagogy, and postmodern notions of bodies and place to make sense of data generated from a three-year study of an undergraduate teacher education course. A feminist lens was used to engage a body- and place-focused pedagogy that aimed to engage…

  18. Understanding Place Value

    ERIC Educational Resources Information Center

    Cooper, Linda L.; Tomayko, Ming C.

    2011-01-01

    Developing an understanding of place value and the base-ten number system is considered a fundamental goal of the early primary grades. For years, teachers have anecdotally reported that students struggle with place-value concepts. Among the common errors cited are misreading such numbers as 26 and 62 by seeing them as identical in meaning,…

  19. Schooling Out of Place

    ERIC Educational Resources Information Center

    McConaghy, Cathryn

    2006-01-01

    Education in rural communities is an interesting site for an analysis of the relationship between place and the cultural politics of schooling. In particular the movements of people, ideas and practices to and from, and also within, rural places suggest the need for theorizing on rural education to consider the relevance of new mobility…

  20. Accretion of the Earth and segregation of its core.

    PubMed

    Wood, Bernard J; Walter, Michael J; Wade, Jonathan

    2006-06-15

    The Earth took 30-40 million years to accrete from smaller 'planetesimals'. Many of these planetesimals had metallic iron cores and during growth of the Earth this metal re-equilibrated with the Earth's silicate mantle, extracting siderophile ('iron-loving') elements into the Earth's iron-rich core. The current composition of the mantle indicates that much of the re-equilibration took place in a deep (> 400 km) molten silicate layer, or 'magma ocean', and that conditions became more oxidizing with time as the Earth grew. The high-pressure nature of the core-forming process led to the Earth's core being richer in low-atomic-number elements, notably silicon and possibly oxygen, than the cores of the smaller planetesimal building blocks. PMID:16778882

  1. Accretion Disks and Jets Around Black Holes

    NASA Astrophysics Data System (ADS)

    Narayan, Ramesh

    2008-04-01

    Some of the most luminous objects in the universe involve accretion disks around black holes. In these systems, gas spirals into the black hole and converts a fraction of its gravitational binding energy into thermal energy and radiation. Sometimes, twin relativistic jets are ejected along the angular momentum axis of the disk. Understanding the physics of black hole accretion disks and jets is a major focus of modern astrophysics. Because the object at the center is a black hole, one must work with a relativistic theory. More importantly, one must allow for the effects of magnetic fields. These play an extremely important role, both in the extraction of angular momentum from the accreting gas -- which is what allows the gas to fall into the hole -- and in the launching, acceleration and collimation of the relativistic jets. Thus, at a minimum, one must work with the relativistic single-fluid MHD equations. The talk will briefly summarize our current understanding of black hole accretion, and outline some of the major unsolved problems.

  2. VARIABLE ACCRETION OUTBURSTS IN PROTOSTELLAR EVOLUTION

    SciTech Connect

    Bae, Jaehan; Hartmann, Lee; Zhu, Zhaohuan; Gammie, Charles E-mail: lhartm@umich.edu E-mail: gammie@illinois.edu

    2013-02-20

    We extend the one-dimensional, two-zone models of long-term protostellar disk evolution with infall of Zhu et al. to consider the potential effects of a finite viscosity in regions where the ionization is too low for the magnetorotational instability (MRI) to operate (the {sup d}ead zone{sup )}. We find that the presence of a small but finite dead zone viscosity, as suggested by simulations of stratified disks with MRI-active outer layers, can trigger inside-out bursts of accretion, starting at or near the inner edge of the disk, instead of the previously found outside-in bursts with zero dead zone viscosity, which originate at a few AU in radius. These inside-out bursts of accretion bear a qualitative resemblance to the outburst behavior of one FU Ori object, V1515 Cyg, in contrast to the outside-in burst models, which more closely resemble the accretion events in FU Ori and V1057 Cyg. Our results suggest that the type and frequency of outbursts are potentially a probe of transport efficiency in the dead zone. Simulations must treat the inner disk regions, R {approx}< 0.5 AU, to show the detailed time evolution of accretion outbursts in general and to observe the inside-out bursts in particular.

  3. Jets at lowest mass accretion rates

    NASA Astrophysics Data System (ADS)

    Maitra, Dipankar; Cantrell, Andrew; Markoff, Sera; Falcke, Heino; Miller, Jon; Bailyn, Charles

    2011-02-01

    We present results of recent observations and theoretical modeling of data from black holes accreting at very low luminosities (L/LEdd <~ 10-8). We discuss our newly developed time-dependent model for episodic ejection of relativistic plasma within a jet framework, and a successful application of this model to describe the origin of radio flares seen in Sgr A*, the Galactic center black hole. Both the observed time lags and size-frequency relationships are reproduced well by the model. We also discuss results from new Spitzer data of the stellar black hole X-ray binary system A0620-00. Complemented by long term SMARTS monitoring, these observations indicate that once the contribution from the accretion disk and the donor star are properly included, the residual mid-IR spectral energy distribution of A0620-00 is quite flat and consistent with a non-thermal origin. The results above suggest that a significant fraction of the observed spectral energy distribution originating near black holes accreting at low luminosities could result from a mildly relativistic outflow. The fact that these outflows are seen in both stellar-mass black holes as well as in supermassive black holes at the heart of AGNs strengthens our expectation that accretion and jet physics scales with mass.

  4. Accretion and star formation in RQQs

    NASA Astrophysics Data System (ADS)

    White, Sarah; Jarvis, Matt; Häußler, Boris; Maddox, Natasha; Kalfountzou, Eleni; Hardcastle, Martin

    2016-06-01

    Active Galactic Nuclei (AGN) and star-forming galaxies are well-traced in the radio part of the electromagnetic spectrum, due to emission at these wavelengths being unaffected by dust obscuration. The key processes involved in producing the radio emission are black-hole accretion and star formation, both of which are thought to be crucial in determining how galaxies evolve. Disentangling the two contributions requires multi-wavelength data, and this is the approach we use for our work on radio-quiet quasars (RQQs). In contrast to previous studies, we find that accretion-connected radio emission dominates over that due to star formation, even at very low radio flux-densities. The first sample we describe is selected from the VISTA Deep Extragalactic Observations (VIDEO) survey, whose depth allows the study of very low accretion rates and/or lower-mass black holes. A second sample is obtained from the Spitzer-Herschel Active Galaxy Survey, spanning a factor of ~100 in optical luminosity over a narrow redshift range at z ~ 1. This enables evolutionary effects to be decoupled when comparisons are made with the VIDEO sample. Using radio data from the Karl G. Jansky Very Large Array (JVLA), we find further support that the AGN makes a significant contribution to the radio emission in RQQs. In addition, the levels of accretion and star formation appear to be weakly correlated with each other, and with optical luminosity.

  5. Earth, Moon, Sun, and CV Accretion Disks

    NASA Astrophysics Data System (ADS)

    Montgomery, M. M.

    2009-11-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting cataclysmic variable (CV) dwarf novae (DN) systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths' equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar, and black hole systems. We find that spinning, tilted CV DN systems cannot be described by a precessing ring or by a precessing rigid disk. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our analysis indicates that the best description of a retrogradely precessing spinning, tilted, CV DN accretion disk is a differentially rotating, tilted disk with an attached rotating, tilted ring located near the innermost disk annuli. In agreement with the observations and numerical simulations by others, we find that our numerically simulated CV DN accretion disks retrogradely precess as a unit. Our final, reduced expression for retrograde precession agrees well with our numerical simulation results and with selective observational systems that seem to have main-sequence secondaries. Our results suggest that a major source to retrograde precession is tidal torques like that by the Moon and the Sun on the Earth. In addition, these tidal torques should be common to a variety of systems where one member is spinning and tilted, regardless if

  6. EARTH, MOON, SUN, AND CV ACCRETION DISKS

    SciTech Connect

    Montgomery, M. M.

    2009-11-01

    Net tidal torque by the secondary on a misaligned accretion disk, like the net tidal torque by the Moon and the Sun on the equatorial bulge of the spinning and tilted Earth, is suggested by others to be a source to retrograde precession in non-magnetic, accreting cataclysmic variable (CV) dwarf novae (DN) systems that show negative superhumps in their light curves. We investigate this idea in this work. We generate a generic theoretical expression for retrograde precession in spinning disks that are misaligned with the orbital plane. Our generic theoretical expression matches that which describes the retrograde precession of Earths' equinoxes. By making appropriate assumptions, we reduce our generic theoretical expression to those generated by others, or to those used by others, to describe retrograde precession in protostellar, protoplanetary, X-ray binary, non-magnetic CV DN, quasar, and black hole systems. We find that spinning, tilted CV DN systems cannot be described by a precessing ring or by a precessing rigid disk. We find that differential rotation and effects on the disk by the accretion stream must be addressed. Our analysis indicates that the best description of a retrogradely precessing spinning, tilted, CV DN accretion disk is a differentially rotating, tilted disk with an attached rotating, tilted ring located near the innermost disk annuli. In agreement with the observations and numerical simulations by others, we find that our numerically simulated CV DN accretion disks retrogradely precess as a unit. Our final, reduced expression for retrograde precession agrees well with our numerical simulation results and with selective observational systems that seem to have main-sequence secondaries. Our results suggest that a major source to retrograde precession is tidal torques like that by the Moon and the Sun on the Earth. In addition, these tidal torques should be common to a variety of systems where one member is spinning and tilted, regardless if

  7. GAS ACCRETION FROM A CIRCUMBINARY DISK

    SciTech Connect

    Hanawa, Tomoyuki; Ochi, Yasuhiro; Ando, Koichi

    2010-01-01

    A new computational scheme is developed to study gas accretion from a circumbinary disk. The scheme decomposes the gas velocity into two components one of which denotes the Keplerian rotation and the other of which does the deviation from it. This scheme enables us to solve the centrifugal balance of a gas disk against gravity with better accuracy, since the former inertia force cancels the gravity. It is applied to circumbinary disk rotating around binary of which primary and secondary has mass ratio, 1.4:0.95. The gravity is reduced artificially softened only in small circular regions around the primary and secondary. The radii are 7% of the binary separation and much smaller than those in the previous grid based simulations. Seven models are constructed to study dependence on the gas temperature and the initial inner radius of the disk. The gas accretion shows both fast and slow time variations while the binary is assumed to have a circular orbit. The time variation is due to oscillation of spiral arms in the circumbinary disk. The masses of primary and secondary disks increase while oscillating appreciably. The mass accretion rate tends to be higher for the primary disk although the secondary disk has a higher accretion rate in certain periods. The accretion rates onto the two components are similar within the fluctuations in late times, i.e., after the binary rotates more than 20 times. The primary disk is perturbed intensely by the impact of gas flow so that the outer part is removed. The secondary disk is quiet in most of time on the contrary. Both the primary and secondary disks have traveling spiral waves which transfer angular momentum within them.

  8. Fundamental properties of accreting compact objects

    NASA Astrophysics Data System (ADS)

    Blum, Jennifer L.

    Galactic accreting compact objects, such as stellar-mass black holes and neutron stars, can give us a unique perspective into the behavior of matter in extreme conditions. However, the exact nature of accretion onto these objects is not yet well understood. X-ray studies provide us with a means to observe the innermost regions around these objects and to explore our theories of general relativistic physics. Through X-ray analyses we can constrain the physical parameters necessary to make logical deductions regarding compact object properties, such as disk winds, relativistic jets, the Kerr metric, and the neutron star equation of state. Here we present spectral modeling results from three accreting X-ray binaries. Specifically, we analyze Suzaku spectra from two stellar-mass black hole X-ray binaries, GRS 1915+105 and H1743-322, and one neutron star X-ray binary, 4U 1636-53. For GRS 1915+105 and 4U 1636-53, we use the relativistic iron line, which is part of a reflection spectrum, as a diagnostic for measuring black hole spin and neutron star radius, respectively. We find that while we can exclude a spin of zero at the 2σ level of confidence for GRS 1915+105, data selection and disk reflection modeling nuances can be important when estimating the spin value. For 4U 1636-53, we provide upper limits on the neutron star radius by estimating the radial extent of the inner accretion disk, which are important for constraining models for the neutron star equation of state. Moreover, when testing for the presence of disk winds in H1743-322 (which are key to understanding the nature of accretion disk outflow), we do not detect Fe XXV or Fe XXVI absorption lines in its spectra of H1743-322; implying that disk winds may be state dependent.

  9. Fundamental Properties of Accreting Compact Objects

    NASA Astrophysics Data System (ADS)

    Blum, Jennifer L.

    2011-01-01

    Galactic accreting compact objects, such as stellar-mass black holes and neutron stars can give us a unique perspective into the behavior of matter in extreme conditions. However, the exact nature of accretion onto these objects is not yet well understood. X-ray studies provide us with a means to observe the innermost regions around these objects and to test our theories of general relativistic physics. Through X-ray analyses we can constrain the physical parameters necessary to make logical deductions regarding compact object properties, such as disk winds, relativistic jets, the Kerr metric, and the neutron star equation of state. Here we present spectral modeling results from three accreting X-ray binaries. Specifically, we analyze Suzaku spectra from two stellar-mass black hole X-ray binaries, GRS 1915+105 and H1743-322, and one neutron star X-ray binary, 4U 1636-53. For GRS 1915+105 and 4U 1636-53, we use the relativistic iron line, which is part of a reflection spectrum, as a diagnostic for measuring black hole spin and neutron star radius, respectively. We find that while we can exclude a spin of zero at the 2 sigma level of confidence for GRS 1915+105, data selection and disk reflection modeling nuances can be important when estimating the spin value. For 4U 1636-53, we provide upper limits on the neutron star radius by estimating the radial extent of the inner accretion disk, which are important for constraining models for the neutron star equation of state. Moreover, when testing for the presence of disk winds in H1743-322 (which are key to understanding the nature of accretion disk outflow), we do not detect Fe XXV or Fe XXVI absorption lines in its spectra of H1743-322; implying that disk winds may be state dependent.

  10. Stellar explosions from accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    Moore, Kevin L.

    Unstable thermonuclear burning on accreting white dwarfs (WDs) can lead to a wide variety of outcomes, and induce shock waves in several contexts. In classical and recurrent novae, a WD accreting hydrogen-rich material from a binary companion can experience thermonuclear runaways, ejecting mass into the interstellar/circumbinary environment at ~1000 km/s. This highly supersonic ejecta drives shock waves into the interstellar gas which may be relevant for sweeping out gas from globular clusters or forming circumstellar absorption regions in interacting supernovae. While runaway nuclear burning in novae releases enough energy for these objects to brighten by a factor of ~10 4 over roughly a weeklong outburst, it does not become dynamically unstable. In contrast, certain helium accretion scenarios may allow for dynamical burning modes, in part due to the higher temperature sensitivity of helium burning reactions and larger accreted envelopes. The majority of this thesis involves such dynamical burning modes, specifically detonations - shock waves sustained by nuclear energy release behind the shock front. We investigate when steady-state detonations are realizable in accreted helium layers on WDs, and model their strength and burning products using both semi-analytic and numerical models. We find the minimum helium layer thickness that will sustain a steady laterally propagating detonation and show that it depends on the density and composition of the helium layer, specifically 12 C and 16O. Though gravitationally unbound, the ashes still have unburned helium (~80% in the thinnest cases) and only reach up to heavy elements such as 40Ca, 44Ti, 48Cr, and 52Fe. It is rare for these thin shells to generate large amounts of radioactive isotopes necessary to power light curves, such as 56Ni. This has important implications on whether the unbound helium burning ashes may create faint and fast peculiar supernovae or events with virtually no radioactivity, as well as on off

  11. SIMULATIONS OF ACCRETION POWERED SUPERNOVAE IN THE PROGENITORS OF GAMMA-RAY BURSTS

    SciTech Connect

    Lindner, Christopher C.; Milosavljevic, Milos; Kumar, Pawan; Shen, Rongfeng

    2012-05-10

    Observational evidence suggests a link between long-duration gamma-ray bursts (LGRBs) and Type Ic supernovae. Here, we propose a potential mechanism for Type Ic supernovae in LGRB progenitors powered solely by accretion energy. We present spherically symmetric hydrodynamic simulations of the long-term accretion of a rotating gamma-ray burst progenitor star, a 'collapsar', onto the central compact object, which we take to be a black hole. The simulations were carried out with the adaptive mesh refinement code FLASH in one spatial dimension and with rotation, an explicit shear viscosity, and convection in the mixing length theory approximation. Once the accretion flow becomes rotationally supported outside of the black hole, an accretion shock forms and traverses the stellar envelope. Energy is carried from the central geometrically thick accretion disk to the stellar envelope by convection. Energy losses through neutrino emission and nuclear photodisintegration are calculated but do not seem important following the rapid early drop of the accretion rate following circularization. We find that the shock velocity, energy, and unbound mass are sensitive to convective efficiency, effective viscosity, and initial stellar angular momentum. Our simulations show that given the appropriate combinations of stellar and physical parameters, explosions with energies {approx}5 Multiplication-Sign 10{sup 50} erg, velocities {approx}3000 km s{sup -1}, and unbound material masses {approx}> 6 M{sub Sun} are possible in a rapidly rotating 16 M{sub Sun} main-sequence progenitor star. Further work is needed to constrain the values of these parameters, to identify the likely outcomes in more plausible and massive LRGB progenitors, and to explore nucleosynthetic implications.

  12. Ion viscosity mediated by tangled magnetic fields in Accretion flows onto black holes

    NASA Astrophysics Data System (ADS)

    Subramanian, P.; Becker, P. A.; Kafatos, M.

    1995-12-01

    We examine the viscosity mechanism due to momentum transfer between ions in the presence of tangled magnetic fields in accretion flows onto black holes. We do not attempt to present a self-consistent description of the tangled magnetic field; we merely assume its existence and take its coherence length lambda_ {coh} (which is the maximum length a given magnetic field line can be expected to be straight, on average) to be a free parameter. We arrive at a formulation for a hybrid viscosity in which the tangled magnetic field plays the role of an intermediary in momentum transfer between ions. It does so by affecting the mean free path of the ions; the effective mean free path of the ions in our formulation lies in between the usual ion-ion mean free path lambda_ {ii} and lambda_ {coh}. Our calculations are relevant in the scenario of accretion disks around BLACK HOLES, which are expected to have near equipartition tangled magnetic fields embedded in them. We apply this viscosity mechanism to a steady state model of a two temperature quasi-Keplerian accretion disk. The values of the USUAL alpha parameter arising from this mechanism are FOUND to range from =~ 0.01 to =~ 0.5. We find the viscosity to be influenced both by the degree to which the magnetic fields are tangled (represented by xi = lambda_ {coh}/r) and the relative accretion rate dot {M}/dot {M}Eddington. Our results suggest the viability of quasi-spherical accretion for near and super-Eddington accretion rates.

  13. Black Hole Advective Accretion Disks with Optical Depth Transition

    SciTech Connect

    Artemove, Y.V.; Bisnovatyi-Kogan, G.S.; Igumenshchev, I.V.; Novikov, I.D.

    2006-02-01

    We have constructed numerically global solutions of advective accretion disks around black holes that describe a continuous transition between the effectively optically thick outer and optically thin inner disk regions. We have concentrated on models of accretion flows with large mass accretion rates, and we have employed a bridging formula for radiative losses at high and low effective optical depths.

  14. Development of 3D Ice Accretion Measurement Method

    NASA Technical Reports Server (NTRS)

    Lee, Sam; Broeren, Andy P.; Addy, Harold E., Jr.; Sills, Robert; Pifer, Ellen M.

    2012-01-01

    Icing wind tunnels are designed to simulate in-flight icing environments. The chief product of such facilities is the ice accretion that forms on various test articles. Documentation of the resulting ice accretion key piece of data in icing-wind-tunnel tests. Number of currently used options for documenting ice accretion in icing-wind-tunnel testing.

  15. Sharing Knowledge About Places as Community Building

    NASA Astrophysics Data System (ADS)

    Willis, Katharine S.; O'Hara, Kenton; Giles, Thierry; Marianek, Mike

    Our experience of places is one that goes hand-in-hand with social exchange. It is rare that we visit a place purely to experience it as an isolated encounter with a physical setting. Instead, we visit places in groups, use a myriad of ways to tell others about the experience, and we often seek out and take pleasure from encounters with local people in the setting. Our experience of place is embedded within a social framework for sharing knowledge. In this chapter, we discuss the motivations for sharing place-based knowledge and how this can contribute to community building. We then proceed to review three projects that create platforms for knowledge exchange and discuss their different approaches. This is followed by a discussion on the range of practices for sharing knowledge about places and in particular those that support social frameworks in a community setting. In conclusion, we propose directions for future work in the area.

  16. About Maggie's Place.

    ERIC Educational Resources Information Center

    Emmens, Carol E.

    1982-01-01

    Describes "Maggie's Place," the library computer system of the Pikes Peak Library District, Colorado Springs, Colorado, noting its use as an electronic card catalog and community information file, accessibility by home users and library users, and terminal considerations. (EJS)

  17. Artist Place Settings

    ERIC Educational Resources Information Center

    Pellegrino, Linda

    2009-01-01

    Art history can be a little dry at times, but the author is always trying to incorporate new ways of teaching it. In this article, she describes a project in which students were to create a place setting out of clay that had to be unified through a famous artist's style. This place setting had to consist of at least five pieces (dinner plate, cup…

  18. Spherical Accretion in a Uniformly Expanding Universe

    NASA Astrophysics Data System (ADS)

    Colpi, Monica; Shapiro, Stuart L.; Wasserman, Ira

    1996-10-01

    We consider spherically symmetric accretion of material from an initially homogeneous, uniformly expanding medium onto a Newtonian point mass M. The gas is assumed to evolve adiabatically with a constant adiabatic index F, which we vary over the range Γ ɛ [1, 5/3]. We use a one-dimensional Lagrangian code to follow the spherical infall of material as a function of time. Outflowing shells gravitationally bound to the point mass fall back, giving rise to a inflow rate that, after a rapid rise, declines as a power law in time. If there were no outflow initially, Bondi accretion would result, with a characteristic accretion time-scale ta,0. For gas initially expanding at a uniform rate, with a radial velocity U = R/t0 at radius R, the behavior of the flow at all subsequent times is determined by ta,0/t0. If ta,0/t0 ≫ 1, the gas has no time to respond to pressure forces, so the fluid motion is nearly collisionless. In this case, only loosely bound shells are influenced by pressure gradients and are pushed outward. The late-time evolution of the mass accretion rate Mdot is close to the result for pure dust, and we develop a semianalytic model that accurately accounts for the small effect of pressure gradients in this limit. In the opposite regime, ta,0/t0 ≪ 1, pressure forces significantly affect the motion of the gas. At sufficiently early times, t ≤ ttr, the flow evolved along a sequence of quasi-stationary, Bondi-like states, with a time-dependent Mdot determined by the slowly varying gas density at large distances. However, at later times, t ≥ ttr, the fluid flow enters a dustllke regime; ttr is the time when the instantaneous Bondi accretion radius reaches the marginally bound radius. The transition time ttr depends sensitively on ta,0/t0 for a given Γ and can greatly exceed t0. We show that there exists a critical value Γ = 11/9, below which the transition from fluid to ballistic motion disappears. As one application of our calculations, we consider the

  19. Spatial mapping takes time.

    PubMed

    Whishaw, I Q

    1998-01-01

    The experiment tested the prediction that spatial mapping takes time and asked whether time use is reflected in the overt behavior of a performing animal. The study examines this question by exploiting the expected behavioral differences of control rats and rats with hippocampal formation damage induced with fimbria-fornix (FF) lesions on a spatial navigation task. Previous studies have shown that control rats use a mapping strategy, in which they use the relative positions of environmental cues to reach places in space, whereas FF rats use a cue-based strategy, in which they are guided by a single cue or their own body orientation. Therefore, control and FF rats were overtrained on a complex foraging task in which they left a burrow to retrieve eight food pellets hidden around the perimeter of a circular table. The control rats retrieved the food pellets in order of their distance from the burrow, took direct routes to the food, and made few errors, all of which suggested they used a spatial strategy. The FF rats were less likely to retrieve food as a function of its distance, took a circular path around the perimeter of the table, and made many errors, suggesting they used a cue-based strategy. Despite taking shorter routes than the FF rats, the control rats had proportionally slower response speeds. Their slow response speeds support the hypothesis that spatial mapping takes time and that mapping time is reflected in behavior. The results are discussed in relation to their relevance to spatial mapping theory, hippocampal function, and the evolution of foraging strategies.

  20. Accretional Heating of Asymmetric Supernova Cores

    NASA Astrophysics Data System (ADS)

    Thompson, Christopher

    2000-05-01

    The role of accretion in heating a stalled bounce shock in a core-collapse supernova is investigated. We show that effective accretional heating causes an asymmetric expansion of the shock, sufficient to impart a net impulse of ~300-400 km s-1 to the neutron core. To simplify the analysis, we consider a failed accretion shock. Below such a shock, inward advection is faster than neutrino heating and the usual gain criterion does not suffice to determine a successful explosion. A mechanism that enhances buoyancy and inhibits mixing between hot and cold postshock fluid elements is required to revive the shock. We focus on the response of a magnetic field to the accretion flow. Ram heating and shearing of a low-density, magnetized fluid phase (``M-fluid'') is shown to be faster than neutrino cooling. The long duration of the accretion flow compared with the dynamical time allows for a large amplification of the magnetic energy. We calculate the stability of a spherical shock in the presence of a low-density hydrostatic atmosphere below it and show that below a critical atmospheric density the shock is unstable to a global Rayleigh-Taylor mode. We then calculate the equilibrium structure of this Rayleigh-Taylor plume as it accumulates energy and the critical size beyond which quasi-static expansion is no longer possible and its outer boundary converts to a running shock. Accretion continues while the shock expands, and an energy of ~1051 ergs is a direct consequence of the efficiency of ram heating close to the neutron core. The linear momentum imparted to the core is directly related to the mass profile of the precollapse core and explains the proper motions of (most) radio pulsars. We also estimate the net circulation imparted to the last 0.1-0.2 Msolar of collapsing material, which appears sufficient to torque the core down to a spin period of 1-100 ms. The effect of photodissociation on the shock jump conditions is calculated, and the implications for

  1. Investigation of surface water behavior during glaze ice accretion

    NASA Technical Reports Server (NTRS)

    Hansman, R. John, Jr.; Turnock, Stephen R.

    1988-01-01

    Microvideo observations of glaze ice accretions on 1-in-diameter cylinders in a closed-loop refrigerated wind tunnel were obtained to study factors controlling the behavior of unfrozen surface water during glaze ice accretion. Three zones of surface water behavior were noted, each with a characteristic roughness. The effect of substrate thermal and roughness properties on ice accretions was also studied. The contact angle and hysteresis were found to increase sharply at temperatures just below 0 C, explaining the high resistance to motion of water beads observed on accreting glaze ice surfaces. Based on the results, a simple multizone modification to the current glaze ice accretion model is proposed.

  2. Seeing to the Event Horizon: Probing Accretion Physics with X-ray Reflection

    NASA Astrophysics Data System (ADS)

    Wilkins, Dan

    2015-09-01

    Accretion onto supermassive black holes in active galactic nuclei is known to power some of the most luminous objects we see in the Universe, which through their vast energy outputs must have played an important role in shaping the large scale structure of the Universe we see today. Much remains unknown, however, about the fine details of this process; exactly how energy is liberated from accretion flows onto black holes, how the 'corona' that produces the intense X-ray continuum is formed and what governs this process over time. I will outline how the detection of X-rays reflected from the discs of accreting material around black holes by the present generation of large X-ray observatories, shifted in energy and blurred by relativistic effects in the strong gravitational field close to the black hole, has enabled measurements of the inner regions of the accretion flow in unprecedented detail. In particular, exploiting the shift in energy of atomic emission lines by relativistic effects as a function of location on the disc has enabled the measurement of the illumination pattern of the accretion flow by the X-ray continuum from which the geometry of the emitting region can be inferred and how the detection of time lags between the primary and reflected X-rays owing to the additional path the reflected rays must travel between the corona and the disc places further constraints on the nature of the emitting corona. These techniques allow the evolution of the corona that accompanies transitions from high to low X-ray flux to be studied, giving clues to the physical process that forms and powers the intense X-ray source and uncovering evidence for the potential launching of jets. I will discuss the great steps forward in understanding accretion physics that can be made with the Athena X-ray observatory, combining detailed analysis of observations with predictions and models from general relativistic ray tracing simulations. In particular, I will discuss how high

  3. Inner Accretion Disk Regions of Black Hole X-ray Binaries

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg

    2015-01-01

    The innermost regions of accretion disks in black hole X-ray binaries dominate the observed X-ray emission, which is the main diagnostic that one uses to gain insights into the physics of black holes and accretion. The standard spectrum predicted from a geometrically thin, optically thick disk experiences non-trivial modification due to conspiring physical effects operating within the vertical disk structure such as Comptonization, free-free emission/absorption, bound-free opacities, and energy dissipation by magnetic processes. The complicated interplay of these effects cause the seed accretion disk spectrum to become hardened and it is this hardened emergent spectrum that we observe. To zeroth order, this hardening can be described by a phenomenological parameter called the spectral hardening factor.In practice, the adopted degree of spectral hardening is confined to lie within a rather restrictive range. I will discuss the following consequences of relaxing this criterion, while still requiring the spectral hardening factor to take on physically plausible values. Examining multiple state transitions of the black hole X-ray binary GX 339-4 with archival data from the Rossi X-ray Timing Explorer, I will show that appealing to a spectral hardening factor that varies during state transitions provides a viable alternative to a truncated disk model for the evolution of the inner accretion disk. Having demonstrated that moderate degrees of accretion disk spectral hardening cannot be ruled out by observations, I will explore this possibility from a theoretical standpoint. Extending previous work on radiative transfer modeling coupled to the vertical disk structure, I present the impacts on the emergent accretion disk spectrum caused by disk inclination and by allowing accretion power to be dissipated in the corona. Using magnetohydrodynamic simulations of a localized patch of the accretion disk (i.e., shearing box) performed with the Athena code, I will present the

  4. Evolution of Titan's Major Atmospheric Gases and Cooling Since Accretion

    NASA Astrophysics Data System (ADS)

    Gilliam, A.; Lerman, A.

    2013-12-01

    Titan, the largest moon of Saturn, has been to date explored by only one spacecraft, giving us an incomplete picture of its atmospheric structure and surface conditions. Using available information about Titan's present-day atmosphere composition and internal structure, we present a new model for the chemical and physical composition of primordial Titan and its atmosphere. We propose that Titan had a nearly homogeneous structure shortly after accretion, consisting of an inner solid core of antigorite and brucite, of radius 1898 km, and an outer fluid shell containing H2O, NH3, (NH4)2SO4, and CH4, of thickness 677 km. From a balance of the release of gravitational accretion energy and cooling by an ideal black body emission, with no other internal heat sources, we calculate from a published range of accretion times an accretion temperature of 355 to 300 K. We also consider the gas dynamics in Titan's atmosphere and calculate the escape rates of the two main gases in Titan's primordial atmosphere, NH3 and CH4, as a function of time. At 355 K, the atmosphere would be depleted of NH3 and CH4 in short times, except under certain hypothetical conditions or by their supply by emissions from the interior over certain periods of time. However, a lower accretion temperature of 300 K allows the gases to escape thermally down to their present-day masses. Our results show that the Titan temperature decreases to 150 K, corresponding to the reported lower limit of NH3 to N2 conversion, during the relatively fast initial cooling period between 0.5 and 0.6 Myr. Furthermore, it takes about 5 Myr for the temperature to decrease to 90 K. We present two atmospheric models at 355 and 300 K, and calculate the composition of a primordial atmosphere where the initial masses of NH3 and CH4 satisfy the conditions of gas thermal escape to the present-day levels, including calculations of the partial and total pressures, the volume, thickness, and outer surface area of the atmosphere. The two

  5. Aerodynamic characteristics of airfoils with ice accretions

    NASA Technical Reports Server (NTRS)

    Bragg, M. B.; Gregorek, G. M.

    1982-01-01

    Results of a wind tunnel test to evaluate the performance of an airfoil with simulated rime ice are presented with theoretical comparisons. A NACA 65A413 airfoil was tested in the OSU 6 x 22 inch Transonic Airfoil Wind Tunnel at a Reynolds number near three million and Mach numbers from 0.20 to 0.80. The model was tested in four configurations to determine the aero-dynamic effects of the roughness and shape of a rime ice accretion. The simulated rime ice shape was obtained analytically using a time-stepping dry ice accretion computer code. Lift, drag, moment coefficients, and pressure distributions for the clean and simulated rime ice cases are reported. The measured degradation in airfoil performance is compared to an analytical method which uses existing airfoil analysis computer codes with empirical corrections for the surface roughness. A discussion of the empirical surface roughness correction and uses of other airfoil computer methods is included.

  6. Volatile accretion history of the Earth.

    PubMed

    Wood, B J; Halliday, A N; Rehkämper, M

    2010-10-28

    It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarède paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a small veneer of volatile-rich material at ∼100 Myr (here and elsewhere, ages are relative to the origin of the Solar System). Here we argue that the Earth's mass balance of moderately volatile elements is inconsistent with Albarède's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation.

  7. Implications of nonlinearity for spherically symmetric accretion

    NASA Astrophysics Data System (ADS)

    Sen, Sourav; Ray, Arnab K.

    2014-03-01

    We subject the steady solutions of a spherically symmetric accretion flow to a time-dependent radial perturbation. The equation of the perturbation includes nonlinearity up to any arbitrary order and bears a form that is very similar to the metric equation of an analogue acoustic black hole. Casting the perturbation as a standing wave on subsonic solutions, and maintaining nonlinearity in it up to the second order, we get the time dependence of the perturbation in the form of a Liénard system. A dynamical systems analysis of the Liénard system reveals a saddle point in real time, with the implication that instabilities will develop in the accreting system when the perturbation is extended into the nonlinear regime. The instability of initial subsonic states also adversely affects the temporal evolution of the flow toward a final and stable transonic state.

  8. RELATIVISTIC ACCRETION MEDIATED BY TURBULENT COMPTONIZATION

    SciTech Connect

    Socrates, Aristotle E-mail: socrates@astro.princeton.ed

    2010-08-10

    Black hole and neutron star accretion flows display unusually high levels of hard coronal emission in comparison to all other optically thick, gravitationally bound, turbulent astrophysical systems. Since these flows sit in deep relativistic gravitational potentials, their random bulk motions approach the speed of light, therefore allowing turbulent Comptonization to be an important effect. We show that the inevitable production of hard X-ray photons results from turbulent Comptonization in the limit where the turbulence is trans-sonic and the accretion power approaches the Eddington limit. In this regime, the turbulent Compton y-parameter approaches unity and the turbulent Compton temperature is a significant fraction of the electron rest mass energy, in agreement with the observed phenomena.

  9. Volatile accretion history of the Earth.

    PubMed

    Wood, B J; Halliday, A N; Rehkämper, M

    2010-10-28

    It has long been thought that the Earth had a protracted and complex history of volatile accretion and loss. Albarède paints a different picture, proposing that the Earth first formed as a dry planet which, like the Moon, was devoid of volatile constituents. He suggests that the Earth's complement of volatile elements was only established later, by the addition of a small veneer of volatile-rich material at ∼100 Myr (here and elsewhere, ages are relative to the origin of the Solar System). Here we argue that the Earth's mass balance of moderately volatile elements is inconsistent with Albarède's hypothesis but is well explained by the standard model of accretion from partially volatile-depleted material, accompanied by core formation. PMID:20981045

  10. MECHANISM OF OUTFLOWS IN ACCRETION SYSTEM: ADVECTIVE COOLING CANNOT BALANCE VISCOUS HEATING?

    SciTech Connect

    Gu, Wei-Min

    2015-01-20

    Based on the no-outflow assumption, we investigate steady-state, axisymmetric, optically thin accretion flows in spherical coordinates. By comparing the vertically integrated advective cooling rate with the viscous heating rate, we find that the former is generally less than 30% of the latter, which indicates that the advective cooling itself cannot balance the viscous heating. As a consequence, for radiatively inefficient flows with low accretion rates such as M-dot ≲10{sup −3} M-dot {sub Edd}, where M-dot {sub Edd} is the Eddington accretion rate, the viscous heating rate will be larger than the sum of the advective cooling rate and the radiative cooling one. Thus, no thermal equilibrium can be established under the no-outflow assumption. We therefore argue that in such cases outflows ought to occur and take away more than 70% of the thermal energy generated by viscous dissipation. Similarly, for optically thick flows with extremely large accretion rates such as M-dot ≳10 M-dot {sub Edd}, outflows should also occur owing to the limited advection and the low efficiency of radiative cooling. Our results may help to understand the mechanism of outflows found in observations and numerical simulations.

  11. TIME-DEPENDENT, COMPOSITIONALLY DRIVEN CONVECTION IN THE OCEANS OF ACCRETING NEUTRON STARS

    SciTech Connect

    Medin, Zach; Cumming, Andrew E-mail: cumming@physics.mcgill.ca

    2015-03-20

    We discuss the effect of convection driven by chemical separation at the ocean-crust boundary of accreting neutron stars. We extend the steady-state results of Medin and Cumming to transient accretors, by considering the time-dependent cases of heating during accretion outbursts and cooling during quiescence. During accretion outbursts, inward heat transport has only a small effect on the temperature profile in the outer layers until the ocean is strongly enriched in light elements, a process that takes hundreds of years to complete. During quiescence, however, inward heat transport rapidly cools the outer layers of the ocean while keeping the inner layers hot. We find that this leads to a sharp drop in surface emission at around a week followed by a gradual recovery as cooling becomes dominated by the crust. Such a dip should be observable in the light curves of these neutron star transients, if enough data is taken at a few days to a month after the end of accretion. If such a dip is definitively observed, it will provide strong constraints on the chemical composition of the ocean and outer crust.

  12. Links between the Shock Instability in Core-collapse Supernovae and Asymmetric Accretions of Envelopes

    NASA Astrophysics Data System (ADS)

    Takahashi, Kazuya; Iwakami, Wakana; Yamamoto, Yu; Yamada, Shoichi

    2016-11-01

    The explosion mechanism of core-collapse supernovae (CCSNe) has not been fully understood yet, but multidimensional fluid instabilities such as standing accretion shock instability and convection are now believed to be crucial for shock revival. Another multidimensional effect that has been recently argued is the asymmetric structures in progenitors, which are induced by violent convections in silicon/oxygen layers that occur before the onset of collapse, as revealed by recent numerical simulations of the last stage of massive star evolutions. Furthermore, it has been also demonstrated numerically that accretions of such nonspherical envelopes could facilitate shock revival. These two multidimensional effects may hence hold a key to successful explosions. In this paper, we performed a linear stability analysis of the standing accretion shock in CCSNe, taking into account nonspherical, unsteady accretion flows onto the shock to clarify the possible links between the two effects. We found that such preshock perturbations can excite the fluid instabilities efficiently and hence help the shock revive in CCSNe.

  13. Accretion disk thermal instability in galactic nuclei

    NASA Astrophysics Data System (ADS)

    Mineshige, S.; Shields, G. A.

    1990-03-01

    The nonlinear evolution and spatial propagation of the thermal instability in accretion disks in galactic nuclei are investigated. Integrations of the vertical structure of the disks are described for different alpha prescriptions, and the thermal stability is examined. Global time-dependent calculations of the unstable disks are performed which show that there are two distinct types of behavior according to the assumed prescription for the viscosity parameter: the 'purr' type and the 'roar' type. The roar type is analyzed in some detail.

  14. Transient phenomena from accreting magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Klochkov, Dmitry

    In this contribution, I will review the recent progress in the research of accreting magnetized neutron stars (observed as X-ray pulsars) based on the study of their variability on different time scales. Specifically, I will focus on the properties of the X-ray emitting region. In recent years, the high-quality observational data accumulated with the new generation of X-ray observatories have triggered a renewed interest in these systems. The new studies are primarily focused on the detailed structure of the two physical regions of the objects: (i) the emitting area above the polar caps of the neutron star and (ii) the magnetospheric boundary, where the infalling matter couples to the accretoŕs magnetic field. The modulation of the matter supply from the binary companion as well as the instabilities in the accretion flow lead to the transient character of the majority of X-ray pulsars. The observations show that the "persistent" pulsars also exhibit numerous types of variabilities over a broad range of time scales (off-states, pulse-to-pulse variability, switches of spectral states, alternation of the pulsar's spin-up/spin-down episodes etc.). Of particular importance are the observed variations of the cyclotron absorption features (cyclotron lines), whose centroid energies are directly proportional to the magnetic field strength at the site of the line formation. The detailed studies of these variabilities have lead to the development of new theoretical models describing the physics in the emitting region and at the magnetospheric boundary. It has been proposed that the configuration and geometry of the two areas may change abruptly when the mass accretion rate reaches certain critical values. Such changes cause transitions between different accretion modes. A particular mode is expected to be characterized by certain variability patterns and can thus be inferred from the observations. I will describe these recent observations and the models which are aimed at

  15. Airfoil Ice-Accretion Aerodynamics Simulation

    NASA Technical Reports Server (NTRS)

    Bragg, Michael B.; Broeren, Andy P.; Addy, Harold E.; Potapczuk, Mark G.; Guffond, Didier; Montreuil, E.

    2007-01-01

    NASA Glenn Research Center, ONERA, and the University of Illinois are conducting a major research program whose goal is to improve our understanding of the aerodynamic scaling of ice accretions on airfoils. The program when it is completed will result in validated scaled simulation methods that produce the essential aerodynamic features of the full-scale iced-airfoil. This research will provide some of the first, high-fidelity, full-scale, iced-airfoil aerodynamic data. An initial study classified ice accretions based on their aerodynamics into four types: roughness, streamwise ice, horn ice, and spanwise-ridge ice. Subscale testing using a NACA 23012 airfoil was performed in the NASA IRT and University of Illinois wind tunnel to better understand the aerodynamics of these ice types and to test various levels of ice simulation fidelity. These studies are briefly reviewed here and have been presented in more detail in other papers. Based on these results, full-scale testing at the ONERA F1 tunnel using cast ice shapes obtained from molds taken in the IRT will provide full-scale iced airfoil data from full-scale ice accretions. Using these data as a baseline, the final step is to validate the simulation methods in scale in the Illinois wind tunnel. Computational ice accretion methods including LEWICE and ONICE have been used to guide the experiments and are briefly described and results shown. When full-scale and simulation aerodynamic results are available, these data will be used to further develop computational tools. Thus the purpose of the paper is to present an overview of the program and key results to date.

  16. Dynamics of flux tubes in accretion disks

    NASA Technical Reports Server (NTRS)

    Vishniac, E. T.; Duncan, R. C.

    1994-01-01

    The study of magnetized plasmas in astrophysics is complicated by a number of factors, not the least of which is that in considering magnetic fields in stars or accretion disks, we are considering plasmas with densities well above those we can study in the laboratory. In particular, whereas laboratory plasmas are dominated by the confining magnetic field pressure, stars, and probably accretion disks, have magnetic fields whose beta (ratio of gas pressure to magnetic field pressure) is much greater than 1. Observations of the Sun suggest that under such circumstances the magnetic field breaks apart into discrete flux tubes with a small filling factor. On the other hand, theoretical treatments of MHD turbulence in high-beta plasmas tend to assume that the field is more or less homogeneously distributed throughout the plasma. Here we consider a simple model for the distribution of magnetic flux tubes in a turbulent medium. We discuss the mechanism by which small inhomogeneities evolve into discrete flux tubes and the size and distribution of such flux tubes. We then apply the model to accretion disks. We find that the fibrilation of the magnetic field does not enhance magnetic buoyancy. We also note that the evolution of an initially diffuse field in a turbulent medium, e.g., any uniform field in a shearing flow, will initially show exponential growth as the flux tubes form. This growth saturates when the flux tube formation is complete and cannot be used as the basis for a self-sustaining dynamo effect. Since the typical state of the magnetic field is a collection of intense flux tubes, this effect is of limited interest. However, it may be important early in the evolution of the galactic magnetic field, and it will play a large role in numerical simulations. Finally, we note that the formation of flux tubes is an essential ingredient in any successful dynamo model for stars or accretion disks.

  17. Accretion tectonics and crustal structure in Alaska

    USGS Publications Warehouse

    Coney, P.J.; Jones, D.L.

    1985-01-01

    The entire width of the North American Cordillera in Alaska is made up of "suspect terranes". Pre-Late Cretaceous paleogeography is poorly constrained and the ultimate origins of the many fragments which make up the state are unclear. The Prince William and Chugach terranes accreted since Late Cretaceous time and represent the collapse of much of the northeast Pacific Ocean swept into what today is southern Alaska. Greater Wrangellia, a composite terrane now dispersed into fragments scattered from Idaho to southern Alaska, apparently accreted into Alaska in Late Cretaceous time crushing an enormous deep-marine flysch basin on its inboard side. Most of interior eastern Alaska is the Yukon Tanana terrane, a very large entirely fault-bounded metamorphic-plutonic assemblage covering thousands of square kilometers in Canada as well as Alaska. The original stratigraphy and relationship to North America of the Yukon-Tanana terrane are both obscure. A collapsed Mesozoic flysch basin, similar to the one inboard of Wrangellia, lies along the northern margin. Much of Arctic Alaska was apparently a vast expanse of upper Paleozoic to Early Mesozoic deep marine sediments and mafic volcanic and plutonic rocks now scattered widely as large telescoped sheets and Klippen thrust over the Ruby geanticline and the Brooks Range, and probably underlying the Yukon-Koyukuk basin and the Yukon flats. The Brooks Range itself is a stack of north vergent nappes, the telescoping of which began in Early Cretaceous time. Despite compelling evidence for thousands of kilometers of relative displacement between the accreted terranes, and large amounts of telescoping, translation, and rotation since accretion, the resulting new continental crust added to North America in Alaska carries few obvious signatures that allow application of currently popular simple plate tectonic models. Intraplate telescoping and strike-slip translations, delamination at mid-crustal levels, and large-scale lithospheric

  18. Modeling the Accretion Structure of AU Mon

    NASA Astrophysics Data System (ADS)

    Atwood-Stone, Corwin; Miller, Brendan P.; Richards, Mercedes T.; Budaj, Ján; Peters, Geraldine J.

    2012-12-01

    AU Mon is a long-period (11.113 days) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked Hα emission. We present previously unpublished optical spectra of AU Mon which were obtained over 20 years from 1991-2011 with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including Hα, Hβ, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R ⊙, thickness of 5.2 R ⊙, density of 1.0 × 10-13 g cm-3, and maximum temperature of 14,000 K, along with a gas stream at a temperature of ~8000 K transferring ~2.4 × 10-9 M ⊙ yr-1. We show Hα Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions from the stars and accretion structures. The tomograms provide independent support for the Shellspec modeling, while also illustrating that residual emission at sub-Keplerian velocities persists even after subtracting the disk and stream emission. Spectral variability in the Hα profile beyond that expected from either the orbital or the long-period cycle is present on both multi-week and multi-year timescales, and may reflect quasi-random changes in the mass transfer rate or the disk structure. Finally, a transient UV spectral absorption feature may be modeled as an occasional outflow launched from the vicinity of the disk-stream interaction region.

  19. Accretion/underplating, detachment and exhumation: short/long-term rheology of the subduction plate interface

    NASA Astrophysics Data System (ADS)

    Agard, Philippe; Angiboust, Samuel; Plunder, Alexis; Guillot, Stéphane; Yamato, Philippe; Oncken, Onno; Ruh, Jonas; Burov, Evgueni; Bonnet, Guillaume

    2016-04-01

    The presence of km-scale accreted terranes/units in both ancient and present-day subduction zones attests to changes in strain localization along the plate interface, whereby these terranes/units get detached from the downgoing slab (or, in places, are eroded away from the tip of the upper plate) and either directly exhumed or accreted/underplated below the upper plate before final exhumation. The rock record (P-T-t data) indicates that, for a given subduction zone, exhumation is episodic: no more than a few My compared to the ~100 My lifetime of typical subduction zones. Not much is known, however, regarding this process and important open questions remain: what exactly is episodic (i.e., detachment from the slab and/or exhumation?), for how long and where? How is mechanical coupling impacted by the initial structure of the incoming plates (structural/lithological heterogeneities, thermo-fluid regime, geodynamic boundary conditions, etc...)? We herein present both new and literature structural and P-T-t data ranging from shallow (i.e., 15-20 km) to intermediate depths (~100 km) along the subduction interface, that span a range from long-term to short-lived events of underplating and/or exhumation, and confront them with the recent wealth of geophysical data gathered on subduction zones. Structural and petrological data indicate that the slicing of km-scale units mostly occurs at specific depths where major mechanical changes occur along the plate interface: at 30-40 km (downdip of the seismogenic zone) and 70-80 km (where mechanical coupling between the two plates resumes and where eclogites get critically dense). This suggests that switches in mechanical coupling (i.e., in the rheology of the material) are key in controlling the ability to detach pieces from the slab (and that later exhumation is rather controlled by large-scale, lithospheric-scale boundary conditions). The study of rock remnants detached from the slab and underplated during subduction infancy (i

  20. Gateway to New Atlantis Attraction Takes Shape

    NASA Video Gallery

    The home of space shuttle Atlantis continues taking shape at the Kennedy Space Center Visitor Complex. Crews placed the nose cone atop the second of a replica pair of solid rocket boosters. A life-...

  1. Accretion and primary differentiation of Mars

    NASA Technical Reports Server (NTRS)

    Drake, Michael J.

    1988-01-01

    In collecting samples from Mars to address questions such as whether Mars accreted homogeneously or heterogeneously, how Mars segregated into a metallic core and silicate mantle, and whether Mars outgassed catastrophically coincident with accretion or more serenely on a longer timescale, we must be guided by our experience in addressing these questions for the Earth, Moon, and igneous meteorite parent bodies. A key measurement to be made on any sample returned from Mars is its oxygen isotopic composition. A single measurement will suffice to bind the SNC meteorites to Mars or demonstrate that they cannot be samples of that planet. A positive identification of Mars as the SNC parent planet will permit all that has been learned from the SNC meteorites to be applied to Mars with confidence. A negative result will perhaps be more exciting in forcing us to look for another object that has been geologically active in the recent past. If the oxygen isotopic composition of Earth and Mars are established to be distinct, accretion theory must provide for different compositions for two planets now separated by only 0.5 AU.

  2. Relativistic particle transport in hot accretion disks

    NASA Technical Reports Server (NTRS)

    Becker, Peter A.; Kafatos, Menas; Maisack, Michael

    1994-01-01

    Accretion disks around rapidly rotating black holes provide one of the few plausible models for the production of intense radiation in Acitve Galactic Nuclei (AGNs) above energies of several hundred MeV. The rapid rotation of the hole increases the binding energy per nucleon in the last stable orbit relative to the Schwarzschild case, and naturally leads to ion temperatures in the range 10(exp 12) - 10(exp 13) K for sub-Eddington accretion rates. The protons in the hot inner region of a steady, two-temperature disk form a reservoir of energy that is sufficient to power the observed Energetic Gamma Ray Experiment Telescope (EGRET) outbursts if the black hole mass is 10(exp 10) solar mass. Moreover, the accretion timescale for the inner region is comparable to the observed transient timescale of approximately 1 week. Hence EGRET outbursts may be driven by instabilities in hot, two-temperature disks around supermassive black holes. In this paper we discuss turbulent (stochastic) acceleration in hot disks as a possible source of GeV particles and radiation. We constrain the model by assuming the turbulence is powered by a collective instability that drains energy from the hot protons. We also provide some ideas concerning new, high-energy Penrose processes that produce GeV emission be directly tapping the rotational energy of Kerr black holes.

  3. Cosmic dust synthesis by accretion and coagulation

    NASA Technical Reports Server (NTRS)

    Praburam, G.; Goree, J.

    1995-01-01

    The morphology of grains grown by accretion and coagulation is revaled by a new laboratory method of synthesizing cosmic dust analogs. Submicron carbon particles, grown by accretion of carbon atoms from a gas, have a spherical shape with a cauliflower-like surface and an internal micro-structure of radial columns. This shape is probably common for grains grown by accretion at a temperature well below the melting point. Coagulated grains, consisting of spheres that collided to form irregular strings, were also synthesized. Another shape we produced had a bumpy non- spherical morphology, like an interplanetary particle collected in the terrestrial stratosphere. Besides these isolated grains, large spongy aggregates of nanometer-size particles were also found for various experimental conditions. Grains were synthesized using ions to sputter a solid target, producing an atomic vapor at a low temperature. The ions were provided by a plasma, which also provided electrostatic levitation of the grains during their growth. The temporal development of grain growth was studied by extinguishing the plasma after various intervals.

  4. Self-Consistent Models of Accretion Disks

    NASA Technical Reports Server (NTRS)

    Narayan, Ramesh

    1997-01-01

    The investigations of advection-dominated accretion flows (ADAFs), with emphasis on applications to X-ray binaries containing black holes and neutron stars is presented. This work is now being recognized as the standard paradigm for understanding the various spectral states of black hole X-ray Binaries (BHXBs). Topics discussed include: (1) Problem in BHXBS, namely that several of these binaries have unusually large concentrations of lithium in their companion stars; (2) A novel test to show that black holes have event horizons; (3) Application of the ADAF model to the puzzling X-ray delay in the recent outburst of the BHXB, GRO J1655-40; (4) Description of the various spectral states in BHXBS; (5) Application of the ADAF model to the famous supermassive black hole at the center of our Galaxy, Sgr A(*); (6) Writing down and solving equations describing steady-state, optically thin, advection-dominated accretion onto a Kerr black hole; (7) The effect of "photon bubble" instability on radiation dominated accretion disks; and (8) Dwarf nova disks in quiescence that have rather low magnetic Reynolds number, of order 10(exp 3).

  5. A COMMON SOURCE OF ACCRETION DISK TILT

    SciTech Connect

    Montgomery, M. M.; Martin, E. L.

    2010-10-20

    Many different system types retrogradely precess, and retrograde precession could be from a tidal torque by the secondary on a misaligned accretion disk. However, a source that causes and maintains disk tilt is unknown. In this work, we show that accretion disks can tilt due to a force called lift. Lift results from differing gas stream supersonic speeds over and under an accretion disk. Because lift acts at the disk's center of pressure, a torque is applied around a rotation axis passing through the disk's center of mass. The disk responds to lift by pitching around the disk's line of nodes. If the gas stream flow ebbs, then lift also ebbs and the disk attempts to return to its original orientation. To first approximation, lift does not depend on magnetic fields or radiation sources but does depend on the mass and the surface area of the disk. Also, for disk tilt to be initiated, a minimum mass transfer rate must be exceeded. For example, a 10{sup -11} M{sub sun} disk around a 0.8 M{sub sun} compact central object requires a mass transfer rate greater than {approx} 8 x 10{sup -11} M{sub sun} yr{sup -1}, a value well below the known mass transfer rates in cataclysmic variable dwarf novae systems that retrogradely precess and exhibit negative superhumps in their light curves and a value well below mass transfer rates in protostellar-forming systems.

  6. THE COSMIC BATTERY IN ASTROPHYSICAL ACCRETION DISKS

    SciTech Connect

    Contopoulos, Ioannis; Nathanail, Antonios; Katsanikas, Matthaios

    2015-06-01

    The aberrated radiation pressure at the inner edge of the accretion disk around an astrophysical black hole imparts a relative azimuthal velocity on the electrons with respect to the ions which gives rise to a ring electric current that generates large-scale poloidal magnetic field loops. This is the Cosmic Battery established by Contopoulos and Kazanas in 1998. In the present work we perform realistic numerical simulations of this important astrophysical mechanism in advection-dominated accretion flows, ADAFs. We confirm the original prediction that the inner parts of the loops are continuously advected toward the central black hole and contribute to the growth of the large-scale magnetic field, whereas the outer parts of the loops are continuously diffusing outward through the turbulent accretion flow. This process of inward advection of the axial field and outward diffusion of the return field proceeds all the way to equipartition, thus generating astrophysically significant magnetic fields on astrophysically relevant timescales. We confirm that there exists a critical value of the magnetic Prandtl number between unity and 10 in the outer disk above which the Cosmic Battery mechanism is suppressed.

  7. Terrane accretion: Insights from numerical modelling

    NASA Astrophysics Data System (ADS)

    Vogt, Katharina; Gerya, Taras

    2016-04-01

    The oceanic crust is not homogenous, but contains significantly thicker crust than norm, i.e. extinct arcs, spreading ridges, detached continental fragments, volcanic piles or oceanic swells. These (crustal) fragments may collide with continental crust and form accretionary complexes, contributing to its growth. We analyse this process using a thermo-mechanical computer model (i2vis) of an ocean-continent subduction zone. In this model the oceanic plate can bend spontaneously under the control of visco-plastic rheologies. It moreover incorporates effects such as mineralogical phase changes, fluid release and consumption, partial melting and melt extraction. Based on our 2-D experiments we suggest that the lithospheric buoyancy of the downgoing slab and the rheological strength of crustal material may result in a variety of accretionary processes. In addition to terrane subduction, we are able to identify three distinct modes of terrane accretion: frontal accretion, basal accretion and underplating plateaus. We show that crustal fragments may dock onto continental crust and cease subduction, be scrapped off the downgoing plate, or subduct to greater depth prior to slab break off and subsequent exhumation. Direct consequences of these processes include slab break off, subduction zone transference, structural reworking, formation of high-pressure terranes, partial melting and crustal growth.

  8. Disc accretion in star forming regions

    NASA Astrophysics Data System (ADS)

    Kalari, V. M.

    2015-05-01

    In this thesis, I present new ultraviolet/optical/infrared photometric and spectroscopic observations of pre-main sequence stars that have formed in either metal-poor conditions, or in the vicinity of strong ionising radiation. This includes observations of 235 Classical T Tauri stars in the Lagoon Nebula; 63 Classical T Tauri/Herbig Ae stars in the Carina Nebula open cluster Trumpler 14; 24 intermediate mass T Tauri stars in the low-Z Sh 2-284 SFR; and one Herbig B[e] PMS candidate in the metal-poor 30 Doradus SFR. I measure the accretion rates of these PMS stars using the intensities of the U/Hα band excess measured through either optical spectra or imaging. Where possible, I use archive infrared photometry in the 1.2-8 micron wavelength range to measure the PMS disc evolutionary stage. The influence of the surrounding environment on the accretion rate evolution of pre-main sequence stars in these regions is explored using the spatial, and temporal distributions of accretion rate, mass, age and disc stage of PMS stars.

  9. Accretion shock geometries in the magnetic variables

    NASA Technical Reports Server (NTRS)

    Stockman, H. S.

    1988-01-01

    The first self consistent shock models for the AM Herculis-type systems successfully identified the dominant physical processes and their signatures. These homogenous shock models predict unpolarized, Rayleigh-Jeans optical spectra with sharp cutoffs and rising polarizations as the shocks become optically thin in the ultraviolet. However, the observed energy distributions are generally flat with intermediate polarizations over a broad optical band. These and other observational evidence support a non-homogenous accretion profile which may extend over a considerable fraction of the stellar surface. Both the fundamental assumptions underlying the canonical 1-D shock model and the extension of this model to inhomogenous accretion shocks were identified, for both radial and linear structures. The observational evidence was also examined for tall shocks and little evidence was found for relative shock heights in excess of h/R(1) greater than or equal to 0.1. For several systems, upper limits to the shock height can be obtained from either x ray or optical data. These lie in the region h/R(1) is approximately 0.01 and are in general agreement with the current physical picture for these systems. The quasi-periodic optical variations observed in several magnetic variables may eventually prove to be a major aid in further understanding their accretion shock geometries.

  10. Partial accretion in the propeller stage of accreting millisecond X-ray pulsars

    NASA Astrophysics Data System (ADS)

    Gungor, Can; Gogus, Ersin; Eksi, Kazim Yavuz; Guver, Tolga

    2016-07-01

    Accreting millisecond X-ray pulsars (AMXPs) are very important objects for studying the stages of disk - magnetosphere interaction as these objects may show different stages in an observable duration. A typical X-ray light curve of an outburst of AMXP has a fast rise and an exponential decay phases. Most of the outbursts have a knee where the flux goes from the slow decay stage to the rapid decay stage. This knee may be linked to the transition from accretion to propeller stage. Since, after the knee, the X-ray luminosity of the source is still higher than its quiescent level, the accretion from inner disc must be continuing in the propeller stage with a lower fraction than in the accretion stage. The X-ray does not only come from accretion onto the poles but the inner parts of the disk may also contribute to the total X-ray luminosity. To infer what fraction (f) of the inflowing matter accretes onto the star the light curve in the propeller stage, one should first separate the emission originating from the disk and obtain a light curve of X-ray emission only from the magnetic poles. We provide a new method to infer from the observational data the fraction of accreting matter onto the neutron star pole to the mass transferring from outer layers of the disc to the inner disc (f), as a function of the fastness parameter (ω_{*}), assuming the knee is due to the transition from accretion to the propeller stage. We transform X-ray luminosities to the mass fraction, f, and the time scale of outburst to fastness parameter, ω_*. It allows us to compare different types of outbursts of an AMXP in f - ω_* space which is universal for a unique system. We analysed the Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) observations of the 2000 and the 2011 outbursts and the Swift Gamma-Ray Burst Mission/X-ray Telescope (SWIFT/XRT) data of the 2013 outburst of the most known AMXP, Aql X-1 using a combination of blackbody representing hot spot, disk blackbody

  11. Effect of Gas Accretion Disc Profile on Orbital Parameters of the Accreted Stars

    NASA Astrophysics Data System (ADS)

    Shukirgaliyev, Bekdaulet T.; Panamarev, Taras P.; Naurzbaeva, Aisha Zh.; Kalambay, Mukhagali T.; Makukov, Maxim A.; Vilkoviskij, Emmanuil Y.; Omarov, Chingis T.; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-10-01

    The results of studies of the effect of the gas disk and its profile on the dynamics of active galactic nuclei are presented. The study was conducted with a numerical model of galactic nucleus based on phiGRAPE+GPU comprising three subsystems - a central supermassive black hole, gaseous accretion disc, and compact stellar cluster. The evolution of the compact stellar cluster is modeled with direct integration (N-body simulation), while the black hole and gaseous disc are represented phenomenologically: the black hole is introduced as an external potential (fixed in space but variable in time due to black hole mass growth), and the gaseous disc is introduced as spatial time-independent density distribution. We examined and compared with each other orbital parameters of accreting stars for model of the galactic nucleus with gas disc of constant and variable thickness, as well as without gas. It was found that in the presence of a gaseous disk almost half of the accreted particles interact strongly with the gas and are captured by the disc before accretion, while more than 85% of particles are affected to some extent by the disc prior to accretion. This suggests that interaction of the stellar cluster with the gas disk in the galactic nucleus might lead to the formation of stellar disk in the central part of the nucleus.

  12. Finding Place in Education

    ERIC Educational Resources Information Center

    Peters, Chris

    2011-01-01

    As a society, we are less and less comfortable in our localities. We have embraced the idea of a globalized placelessness, where everything, everywhere, resonates with a sameness. What do we lose, educationally and in society at large, when we reduce our inhabited places to those components that provide material wealth alone? If students and…

  13. Schools as Dangerous Places

    ERIC Educational Resources Information Center

    Potts, Anthony

    2006-01-01

    When students set off for school each day how many of them or their caregivers consider for a moment that they will spend the day at a potentially dangerous place? On the contrary, students and caregivers probably view schools as safe havens, and official research suggests that this is the case for the majority of teachers and pupils. However,…

  14. Tracking Self into Place

    ERIC Educational Resources Information Center

    Piersol, Laura

    2010-01-01

    In an effort to figure out what it means to educate "ecologically," I decided to track down some of the stories that I was living, telling and making as an educator. I ended up lost in the house of environmental education, stuck within the rooms of ecological science and political advocacy. Outside on the lawn sat the story of place based…

  15. Teaching With Historic Places.

    ERIC Educational Resources Information Center

    Greenberg, Ronald M., Ed.

    1993-01-01

    Designed for social studies educators, this theme issue presents 11 articles about historic places that feature a variety of ideas for elementary and secondary lesson plans, curricula, and program development. The articles are: (1) "Where did History Happen?" (Beth M. Boland); (2) "Creating a Partnership" (Carol D. Shull); (3) "Heritage Education:…

  16. The Value of Place

    ERIC Educational Resources Information Center

    Dentzau, Michael W.

    2014-01-01

    This commentary seeks to expand the dialogue on place-based science education presented in Katie Lynn Brkich's article, where the connections fifth grade students make between their formal earth science curriculum and their lived experiences are highlighted. The disconnect between the curriculum the students are offered and their immediate…

  17. Designing Places for Learning.

    ERIC Educational Resources Information Center

    Meek, Anne, Ed.

    This book presents information about the condition of schools around the United States. It also describes the link between architecture and academic success and offers suggestions for improving the design of existing and future school buildings. Eleven articles look at schools as places of deep meaning and show how that view can alter approaches…

  18. Creativity: Does Place Matter?

    ERIC Educational Resources Information Center

    Bradley, Finbarr

    2012-01-01

    This article argues that creativity has the greatest potential to flourish if a learning environment is embedded within a community that emphasises a deep sense of place. Yet in a globalised world, rootedness is often regarded as antithetical to creativity. But far from representing dead artefacts that are anti-modern and non-economic, culture and…

  19. Universities Are Funny Places!

    ERIC Educational Resources Information Center

    Lawless, Ann

    2006-01-01

    Universities are funny places. They have a strong sense of hierarchy and rank. They have an amazing disparity in salary levels and status between staff, are class conscious, and are run by a large bureaucracy that oils and keeps the machinery going. They operate as educational institutions and yet also are entrepreneurial, marketing themselves in…

  20. Microstructure of Ice Accretions Grown on Aluminum Conductors.

    NASA Astrophysics Data System (ADS)

    Laforte, Jean-Louis; Phan, Luan C.; Felin, Béatrice

    1983-07-01

    In order to study the microstructure of glaze and rime deposits formed on the conductors of power lines, ice accretions are grown on a slowly rotating aluminum cylinder placed in the working section of a wind tunnel. The growth conditions cover both dry and wet regimes in the air temperature interval between 2 and 15°C and are close to those commonly prevailing in natural icing storms near the ground: liquid water contents vary from 0.4 to 0.8 g m3 and wind speed from 4 to 20 m s1; thew values are lower than those usually used in hail simulation studies. Four droplet spectra of 12, 22, 38 and 98 m mean volume diameter were used. The air bubble features of glaze and rime deposits grown in the abovementioned conditions show that the degree of transparency and the layering of the fine air bubbles strongly depend on the deposit temperatures and the intensity of accretion. On the other hand, crystal textures reveal that the mean width of ice crystals depends mainly on the ambient temperature and to a lesser degree on the wind velocity, particularly at low wind speeds. Mean droplet size and liquid water content seem to have little effect upon the crystal mean width. In addition, it is found that in the layer of ice closest to the aluminium cylinder, the crystal mean width increases with the radial distance from the conductor surface and this increase is more marked at low air temperatures. This effect cannot be attributed to the thermal conductivity of the substrate, but probably to the nucleation rate of ice crystals near the ice-conductor interface.

  1. Systematic Risk-Taking.

    ERIC Educational Resources Information Center

    Neihart, Maureen

    1999-01-01

    Describes systematic risk-taking, a strategy designed to develop skills and increase self-esteem, confidence, and courage in gifted youth. The steps of systematic risk-taking include understanding the benefits, initial self-assessment for risk-taking categories, identifying personal needs, determining a risk to take, taking the risk, and…

  2. Multi-dimensional structure of accreting young stars

    NASA Astrophysics Data System (ADS)

    Geroux, C.; Baraffe, I.; Viallet, M.; Goffrey, T.; Pratt, J.; Constantino, T.; Folini, D.; Popov, M. V.; Walder, R.

    2016-04-01

    This work is the first attempt to describe the multi-dimensional structure of accreting young stars based on fully compressible time implicit multi-dimensional hydrodynamics simulations. One major motivation is to analyse the validity of accretion treatment used in previous 1D stellar evolution studies. We analyse the effect of accretion on the structure of a realistic stellar model of the young Sun. Our work is inspired by the numerical work of Kley & Lin (1996, ApJ, 461, 933) devoted to the structure of the boundary layer in accretion disks, which provides the outer boundary conditions for our simulations. We analyse the redistribution of accreted material with a range of values of specific entropy relative to the bulk specific entropy of the material in the accreting object's convective envelope. Low specific entropy accreted material characterises the so-called cold accretion process, whereas high specific entropy is relevant to hot accretion. A primary goal is to understand whether and how accreted energy deposited onto a stellar surface is redistributed in the interior. This study focusses on the high accretion rates characteristic of FU Ori systems. We find that the highest entropy cases produce a distinctive behaviour in the mass redistribution, rms velocities, and enthalpy flux in the convective envelope. This change in behaviour is characterised by the formation of a hot layer on the surface of the accreting object, which tends to suppress convection in the envelope. We analyse the long-term effect of such a hot buffer zone on the structure and evolution of the accreting object with 1D stellar evolution calculations. We study the relevance of the assumption of redistribution of accreted energy into the stellar interior used in the literature. We compare results obtained with the latter treatment and those obtained with a more physical accretion boundary condition based on the formation of a hot surface layer suggested by present multi

  3. Settings: In a Variety of Place. . .

    ERIC Educational Resources Information Center

    Cairo, Peter; And Others

    This document consists of the fourth section of a book of readings on issues related to adult career development. The four chapters in this fourth section focus on settings in which adult career development counseling may take place. "Career Planning and Development in Organizations" (Peter Cairo) discusses several concepts and definitions…

  4. The Right Place, The Right Time

    ERIC Educational Resources Information Center

    O'Callaghan, William G.; Irish, Charles M.

    2006-01-01

    Superintendents are in the right place at the right time to take advantage of a golden opportunity to reverse a trend that threatens the nation's schools and communities. They are now uniquely positioned to stop the retreat of good-hearted, well-intentioned citizens from public life. In this article, the authors present perpetual traps that…

  5. BONDI-HOYLE-LYTTLETON ACCRETION ONTO A PROTOPLANETARY DISK

    SciTech Connect

    Moeckel, Nickolas; Throop, Henry B.

    2009-12-10

    Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this 'tail-end' accretion for planet formation was recently highlighted by Throop and Bally. While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper, we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such that outer disk migrates inward, increasing the inner disk surface density and reducing the outer radius. The accretion flow also triggers the development of spiral density features, and changes to the disk inclination. The mean accretion rate onto the star remains roughly the same with and without the presence of a disk. We discuss the potential impact of this process on planet formation, including the possibility of triggered gravitational instability, inclination differences between the disk and the star, and the appearance of spiral structure in a gravitationally stable system.

  6. Fast accretion of the earth with a late moon-forming giant impact.

    PubMed

    Yu, Gang; Jacobsen, Stein B

    2011-10-25

    Constraints on the formation history of the Earth are critical for understanding of planet formation processes. (182)Hf-(182)W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and (182)Hf-(182)W and (87)Rb-(87)Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to > 100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the (182)W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (> 52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in < 10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes. PMID:22006299

  7. UV spectral variability in the Herbig Ae star HR 5999. 11: The accretion interpretation

    NASA Technical Reports Server (NTRS)

    Perez, M. R.; Grady, C. A.; The, P. S.

    1993-01-01

    We report recent IUE high- and low-dispersion observations with the IUE long wavelength camera (LWP) and short wavelength camera (SWP) of the Herbig Ae star HR 5999. We have found a dramatic change in the structure of the Mg II h and k lines (2795.5, 2802.7 A) along with some continuum flux excesses especially at the short end of the SWP camera. LWP high dispersion observations of HR 5999 obtained between 1979 and 1990, at times of comparatively low UV continuum fluxes, exhibit P Cygni type m profiles in the Mg II resonance doublet. In contrast, observations made from September 1990 through March 16-18, 1992, with high W continuum fluxes, present Mg II lines with reverse P Cygni profiles indicative of some active episodic accretion. Accreting gas can also be detected in the additional red wings of the various Fe II and Mn II absorption lines, with velocities up to +300-350 km/s (September 1990). By September 10, 1992 the Mg II profile had returned to the type III P Cygni profile similar to those from earlier spectra. The correlation between the presence of large column densities of accreting gas and the continuum light variations supports suggestions by several authors that HR 5999 is surrounded by an optically thick, viscously heated accretion disk. Detection of accreting gas in the line of sight to HR 5999 permits us to place constraints on our viewing geometry for this system. A discussion is included comparing the spectral and physical similarities between HR 5999 and the more evolved proto-planetary candidate system, beta Pictoris.

  8. Fast accretion of the earth with a late moon-forming giant impact.

    PubMed

    Yu, Gang; Jacobsen, Stein B

    2011-10-25

    Constraints on the formation history of the Earth are critical for understanding of planet formation processes. (182)Hf-(182)W chronometry of terrestrial rocks points to accretion of Earth in approximately 30 Myr after the formation of the solar system, immediately followed by the Moon-forming giant impact (MGI). Nevertheless, some N-body simulations and (182)Hf-(182)W and (87)Rb-(87)Sr chronology of some lunar rocks have been used to argue for a later formation of the Moon at 52 to > 100 Myr. This discrepancy is often explained by metal-silicate disequilibrium during giant impacts. Here we describe a model of the (182)W isotopic evolution of the accreting Earth, including constraints from partitioning of refractory siderophile elements (Ni, Co, W, V, and Nb) during core formation, which can explain the discrepancy. Our modeling shows that the concentrations of the siderophile elements of the mantle are consistent with high-pressure metal-silicate equilibration in a terrestrial magma ocean. Our analysis shows that the timing of the MGI is inversely correlated with the time scale of the main accretion stage of the Earth. Specifically, the earliest time the MGI could have taken place right at approximately 30 Myr, corresponds to the end of main-stage accretion at approximately 30 Myr. A late MGI (> 52 Myr) requires the main stage of the Earth's accretion to be completed rapidly in < 10.7 ± 2.5 Myr. These are the two end member solutions and a continuum of solutions exists in between these extremes.

  9. Space Place Prime

    NASA Technical Reports Server (NTRS)

    Fitzpatrick, Austin J.; Novati, Alexander; Fisher, Diane K.; Leon, Nancy J.; Netting, Ruth

    2013-01-01

    Space Place Prime is public engagement and education software for use on iPad. It targets a multi-generational audience with news, images, videos, and educational articles from the Space Place Web site and other NASA sources. New content is downloaded daily (or whenever the user accesses the app) via the wireless connection. In addition to the Space Place Web site, several NASA RSS feeds are tapped to provide new content. Content is retained for the previous several days, or some number of editions of each feed. All content is controlled on the server side, so features about the latest news, or changes to any content, can be made without updating the app in the Apple Store. It gathers many popular NASA features into one app. The interface is a boundless, slidable- in-any-direction grid of images, unique for each feature, and iconized as image, video, or article. A tap opens the feature. An alternate list mode presents menus of images, videos, and articles separately. Favorites can be tagged for permanent archive. Face - book, Twitter, and e-mail connections make any feature shareable.

  10. Take Steps Toward a Healthier Life | Poster

    Cancer.gov

    The National Institutes of Health (NIH) is promoting wellness by encouraging individuals to take the stairs. In an effort to increase participation in this program, NIH has teamed up with Occupational Health Services (OHS). OHS is placing NIH-sponsored “Take the Stairs” stickers on stair entrances, stair exits, and elevators.

  11. Enhancement of Core Accretion by an Extended Low-Mass Envelope

    NASA Astrophysics Data System (ADS)

    D'Angelo, G.; Lissauer, J. J.; Hubickyj, O.; Weidenschilling, S. J.; Bodenheimer, P.

    2013-12-01

    The early stages of the formation of Jupiter are modeled via core nucleated accretion and gas capture. The core is initially a seed body with a radius of 350 kilometers, i.e., 1e-4 Earth masses (Me), and orbits in a disk of planetesimals whose initial size distribution ranges from ~10 meters to 100 kilometers. The size distribution of solids evolves through growth and fragmentation of planetesimals, whose orbits are affected by viscous and gravitational stirring, velocity damping, and drag-assisted migration. The seed body has an orbital semi-major axis of 5.2 AU and the initial surface density of solids at that distance is 10 grams per square centimeters. The mass growth of the core is initially fast, reaching 1 Me in about 7e4 years, but the core does not grow larger than about 4 Me in ~1 Myr if the accretion of solids is determined by the geometrical cross-section of the core. The formation of a gaseous envelope via gas capture by the core substantially enhances the size-dependent cross-section of the planet for accretion of planetesimals. The calculation of the envelope structure includes a self-consistent treatment for dust opacity, which takes inot account coagulation and sedimentation of dust grains released in the envelope as passing planetesimals are ablated. The envelope-enhanced accretion rate of solids results in a core mass of about 7 Me after about 0.5 Myr, when the envelope mass is approximately 0.3 Me, at which point the accretion rate of gas surpasses that of solids. Support from NASA Outer Planets Research Program is gratefully acknowledged.

  12. Building the giant planet cores by convergent migration of pebble-accreting embryos

    NASA Astrophysics Data System (ADS)

    Chrenko, Ondrej; Broz, Miroslav

    2016-10-01

    An explanation of the accretion buildup of giant planet cores on rather short (~Myr) time scales remains a long-standing challenge for scenarios of planetary system formation. One of the recently proposed processes that can take part during this evolutionary stage is the convergent Type I migration of Earth-sized embryos towards the zero-torque radius, occurring at an opacity transition within the dusty-gaseous protoplanetary disk (e.g. Pierens et al. 2013). Inconveniently, simulations show that such groups of embryos do not merge easily because they often get locked in mutual mean-motion resonances and consequently form an inward-migrating convoy.We revise this possibility of merging embryos while taking into account their ongoing growth by pebble accretion. Our aim is to check whether the rapid changes of masses combined with the migration of embryos through the feeding zone can break the resonant chain and allow for the giant planet core formation.The environment of the protoplanetary disk is modeled with the 2D FARGO code (Masset 2000), which we modified in order to perform non-isothermal hydrodynamic simulations, assuming flux-limited radiative diffusion (Levermore & Pomraning 1981). The embedded massive bodies are evolved simultaneously in 3D using the hybrid Wisdom-Holman/Gauss-Radau integrator from the Rebound package (Rein & Spiegel 2015). A semi-analytic method is used to evolve the masses of embryos by pebble accretion (e.g. Levison et al. 2015).

  13. Sustained Accretion on Gas Giants Surrounded by Low-Turbulence Circumplanetary Disks

    NASA Astrophysics Data System (ADS)

    D'Angelo, Gennaro; Marzari, Francesco

    2015-11-01

    Gas giants more massive than Saturn acquire most of their envelope while surrounded by a circumplanetary disk (CPD), which extends over a fraction of the planet’s Hill radius. Akin to circumstellar disks, CPDs may be subject to MRI-driven turbulence and contain low-turbulence regions, i.e., dead zones. It was suggested that CPDs may inhibit sustained gas accretion, thus limiting planet growth, because gas transport through a CPD may be severely reduced by a dead zone, a consequence at odds with the presence of Jupiter-mass (and larger) planets. We studied how an extended dead zone influences gas accretion on a Jupiter-mass planet, using global 3D hydrodynamics calculations with mesh refinements. The accretion flow from the circumstellar disk to the CPD is resolved locally at the length scale Rj, Jupiter's radius. The gas kinematic viscosity is assumed to be constant and the dead zone around the planet is modeled as a region of much lower viscosity, extending from ~Rj out to ~60Rj and off the mid-plane for a few CPD scale heights. We obtain accretion rates only marginally smaller than those reported by, e.g., D'Angelo et al. (2003), Bate et al. (2003), Bodenheimer et al. (2013), who applied the same constant kinematic viscosity everywhere, including in the CPD. As found by several previous studies (e.g., D’Angelo et al. 2003; Bate et al. 2003; Tanigawa et al. 2012; Ayliffe and Bate 2012; Gressel et al. 2013; Szulágyi et al. 2014), the accretion flow does not proceed through the CPD mid-plane but rather at and above the CPD surface, hence involving MRI-active regions (Turner et al. 2014). We conclude that the presence of a dead zone in a CPD does not inhibit gas accretion on a giant planet. Sustained accretion in the presence of a CPD is consistent not only with the formation of Jupiter but also with observed extrasolar planets more massive than Jupiter. We place these results in the context of the growth and migration of a pair of giant planets locked in the 2

  14. Cold Accretion from the Cosmic Web

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-06-01

    The cosmic web is a vast, foam-like network of filaments and voids stretching throughout the universe. How did the first galaxies form within the cosmic web, at the intersections of filaments? New observations of a protodisk a galaxy in the early stages of formation may provide a clue.Models for Galaxy FormationNarrowband image of the candidate protodisk (marked with a white ellipse) and filaments (outlined in white). [Adapted from Martin et al. 2016]The standard model for galaxy formation, known as the hot accretion model, argues that galaxies form out of collapsing, virialized gas that forms a hot halo and then slowly cools, fueling star and galaxy formation at its center.But what if galaxies are actually formed from cool gas? In this contrasting picture, the cold accretion model, cool (temperature of ~104 K) unshocked gas from cosmic web filaments flows directly onto galactic disks forming at the filamentary intersections. The narrow streams of cold gas deliver fuel for star formation.A signature of the cold accretion model is that the streams of cold gas form a disk as the gas spirals inward, sinking toward the central protogalaxy. Detecting these cold-flow disks could be strong evidence in support of this model and last year, a team of authors reported just such a detection! This year theyre back again with a second object that may provide confirmation of cold accretion from the cosmic web.A Candidate ProtodiskThe team, led by Christopher Martin (California Institute of Technology), made the discovery using the Palomar Cosmic Web Imager, an instrument designed to observe faint emission from the intergalactic medium. Martin and collaborators found a large (R 100 kpc, more than six times the radius of the Milky Way), rotating structure of hydrogen gas, illuminated by the nearby quasi-stellar object QSO HS1549+1919. The system is located at a redshift of z~2.8.The authors testthree potential kinematic models of the candidate protodisk and filaments. In (a) two

  15. Early solar system. Early accretion of water in the inner solar system from a carbonaceous chondrite-like source.

    PubMed

    Sarafian, Adam R; Nielsen, Sune G; Marschall, Horst R; McCubbin, Francis M; Monteleone, Brian D

    2014-10-31

    Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth's isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event. PMID:25359971

  16. Early solar system. Early accretion of water in the inner solar system from a carbonaceous chondrite-like source.

    PubMed

    Sarafian, Adam R; Nielsen, Sune G; Marschall, Horst R; McCubbin, Francis M; Monteleone, Brian D

    2014-10-31

    Determining the origin of water and the timing of its accretion within the inner solar system is important for understanding the dynamics of planet formation. The timing of water accretion to the inner solar system also has implications for how and when life emerged on Earth. We report in situ measurements of the hydrogen isotopic composition of the mineral apatite in eucrite meteorites, whose parent body is the main-belt asteroid 4 Vesta. These measurements sample one of the oldest hydrogen reservoirs in the solar system and show that Vesta contains the same hydrogen isotopic composition as that of carbonaceous chondrites. Taking into account the old ages of eucrite meteorites and their similarity to Earth's isotopic ratios of hydrogen, carbon, and nitrogen, we demonstrate that these volatiles could have been added early to Earth, rather than gained during a late accretion event.

  17. Volatile loss from accreting icy protoplanets

    NASA Technical Reports Server (NTRS)

    Stevenson, D. J.

    1993-01-01

    A large self-gravitating body does not easily lose significant mass because the escape velocity is much larger than the sound speed of atmosphere-forming species under ambient thermal conditions. The most significant exceptions to this are giant impacts or impact jetting by fast-moving projectiles. A very small object (e.g. a comet) also does not easily lose significant volatile mass upon formation because the energy release associated with its accretion is so small. (It can however lose a great deal of mass if it is subsequently moved closer to the Sun.) I argue that there is an intermediate mass range (corresponding to bodies with radii of approximately 300-800 km) for which the ambient steady-state mass loss is a maximum. By ambient, I mean those conditions pertaining to the formation region of the body. By steady state, I mean to exclude infrequent traumas (giant impacts). The existence of a preferred intermediate mass arises through the competition of growing gravitational containment and growing energy release by accretion; it corresponds typically to GM/(Rc(sub s)(exp 2)) approximately equals 2 to 4, where M is the protoplanet mass of radius R, and c(sub s) is the sound speed. Several factors determine the amount of volatile loss is this vulnerable zone during accretion but in general the loss is a substantial fraction of the volatiles, sometimes approaching 100 percent. The principal implication is that bodies larger than a few hundred kilometers in radius will not have a 'primitive' (i.e. cometary) composition. This is relevant for understanding Triton, Pluto, Charon, and perhaps Chiron.

  18. Neutrino oscillation above a black hole accretion disk

    SciTech Connect

    Malkus, A.; Kneller, J. P.; McLaughlin, G. C.; Surman, R.

    2015-05-15

    We examine neutrino oscillations in the context of an accretion disk surrounding a black hole. Because accretion disks produce large quantities of neutrinos, they may be home to interesting neutrino oscillation as well. We model accretion disks associated with stellar collapse for the sake of understanding neutrino oscillations. We find that the neutrino oscillations include phenomena seen in the protoneutron star setting as well as phenomena not seen elsewhere.

  19. Slim accretion discs with different viscosity prescriptions

    NASA Astrophysics Data System (ADS)

    Szuszkiewicz, Ewa

    1990-05-01

    The variability of X-ray sources powered by accretion may be connected to thermal instabilities in the innermost parts of slim disks. The time-scales of variability predicted by the theory with the standard alpha-viscosity prescription agree with those observed in a wide range of sources. The amplitudes (3-4 orders of magnitude in luiminosity) are correctly predicted for X-ray transient sources, but in general are too big for quasars, Seyferts, galactic blackhole candidates and LMXBs. It is shown that a slight modification of the viscosity prescription can offer a much better agreement with observations.

  20. Normal Modes of Black Hole Accretion Disks

    SciTech Connect

    Ortega-Rodriguez, Manuel; Silbergleit, Alexander S.; Wagoner, Robert V.; /Stanford U., Phys. Dept. /KIPAC, Menlo Park

    2006-11-07

    This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigen frequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and examine the role of viscosity, as it appears to render some of the modes unstable to rapid growth.

  1. Zircon dating of oceanic crustal accretion.

    PubMed

    Lissenberg, C Johan; Rioux, Matthew; Shimizu, Nobumichi; Bowring, Samuel A; Mével, Catherine

    2009-02-20

    Most of Earth's present-day crust formed at mid-ocean ridges. High-precision uranium-lead dating of zircons in gabbros from the Vema Fracture Zone on the Mid-Atlantic Ridge reveals that the crust there grew in a highly regular pattern characterized by shallow melt delivery. Combined with results from previous dating studies, this finding suggests that two distinct modes of crustal accretion occur along slow-spreading ridges. Individual samples record a zircon date range of 90,000 to 235,000 years, which is interpreted to reflect the time scale of zircon crystallization in oceanic plutonic rocks.

  2. Accretion Disks in Massive Binary Systems

    NASA Astrophysics Data System (ADS)

    Djurašević, G. R.; Vince, I.; Atanacković, O.

    2010-12-01

    The results of our investigations of some massive close binaries (CB) (RY Sct, V448 Cyg, UU Cas and V455 Cyg), based on the photometric and spectroscopic observations indicate the existance of the accretion disk around the more massive component, located deep inside the Roche lobe. The light curve shapes of some of this systems are similar to the ones of the overcontact systems like W UMa, but the nature of these massive CBs is completely different. Here we present the models of these systems and their basic elements.

  3. The Take Action Project

    ERIC Educational Resources Information Center

    Boudreau, Sue

    2010-01-01

    The Take Action Project (TAP) was created to help middle school students take informed and effective action on science-related issues. The seven steps of TAP ask students to (1) choose a science-related problem of interest to them, (2) research their problem, (3) select an action to take on the problem, (4) plan that action, (5) take action, (6)…

  4. Analysis of surface roughness generation in aircraft ice accretion

    NASA Technical Reports Server (NTRS)

    Hansman, R. J., Jr.; Reehorst, Andrew; Sims, James

    1992-01-01

    Patterns of roughness evolution have been studied analysis of high magnification video observations of accreting ice surfaces provided by the NASA Lewis Research Center. Three distinct patterns of surface roughness generation have been identified within the parametric regions studied. They include: Rime, Multi-Zone Glaze, and Uniform Glaze. Under most icing conditions, a brief period of transient rime ice growth was observed caused by heat conduction into the body. The resulting thin rime layer explains previously observed insensitivity of some ice accretions to substrate insensitivity of some ice accretions to substrate surface chemistry and may provide justification for simplifying assumptions in ice accretion sailing and modeling effects.

  5. Gamma-burst emission from neutron-star accretion

    NASA Technical Reports Server (NTRS)

    Colgate, S. A.; Petschek, A. G.; Sarracino, R.

    1983-01-01

    A model for emission of the hard photons of gamma bursts is presented. The model assumes accretion at nearly the Eddington limited rate onto a neutron star without a magnetic field. Initially soft photons are heated as they are compressed between the accreting matter and the star. A large electric field due to relatively small charge separation is required to drag electrons into the star with the nuclei against the flux of photons leaking out through the accreting matter. The photon number is not increased substantially by Bremsstrahlung or any other process. It is suggested that instability in an accretion disc might provide the infalling matter required.

  6. Limits on luminosity and mass accretion rate of a radiation-pressure-dominated accretion disc

    NASA Astrophysics Data System (ADS)

    Cao, Xinwu; Gu, Wei-Min

    2015-04-01

    There is a maximum for the gravity of a black hole in the vertical direction in the accretion disc. Outflows may probably be driven from the disc if the radiation flux of the disc is greater than a critical value corresponding to the maximal vertical gravity. We find that outflows are driven by the radiation force from the disc if the dimensionless mass accretion rate at the outer radius dot{m}_out≳ 1 (dot{m}=dot{m}/dot{m}_Edd, dot{m} is the mass accretion rate, dot{m}_Edd=L_Edd/0.1c^2, and LEdd is the Eddington luminosity). Assuming the outflow to be strong to carry away sufficient gas from the disc surface, we find that the radiation of the disc is limited by such outflows. The disc luminosity, L_disc/L_Edd∝ ln dot{m}_out, at large-dot{m}_out cases. The Eddington ratio of the disc is ˜3 for dot{m}_out˜ 100, which is significantly lower than that of a conventional slim disc without outflows (but it is comparable with that given in the study by Kawaguchi). This implies that the emission from some ultraluminous X-ray sources with highly super Eddington luminosity should be Doppler beamed, or intermediate-mass black holes are in these sources instead of stellar mass black holes. The spectra of the discs surrounding massive black holes with outflows are saturated in the high-frequency end provided dot{m}_out≳ 2. We suggest that the saturated emission can be observed to estimate the masses of the black holes accreting at high rates, such as the narrow-line Seyfert galaxies, with the model calculations. The rate of the mass accreted by the black hole always dot{m}_in˜eq dot{m}_Edd even if the mass accretion rate at the outer radius dot{m}_out≫ dot{m}_Edd, because most of the gas is removed into the outflows by the radiation force. If this is the case, the luminous quasars at high redshifts z ≳ 6 should have grown up through persistent accretion at a rate close to the Eddington rate.

  7. Advection/diffusion of large scale magnetic field in accretion disks

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Bisnovatyi-Kogan, G. S.; Rothstein, D. M.

    2009-02-01

    Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr(z) which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P=viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work.

  8. Modelling the thermal history of Vesta: time scale of accretion and differentiation.

    NASA Astrophysics Data System (ADS)

    Formisano, M.; Federico, C.; Turrini, D.; Capaccioni, F.

    2012-04-01

    Vesta is the only known intact primordial asteroid in the Solar System showing an internal differentiated structure, as inferred by its spectral connection with HED meteorites (Keil et al. 2002, Scott 2007, Coradini et al. 2011). Recent results (Schiller et al. 2011) indicate a faster cooling of the interior of Vesta than previously thought. If confirmed, this would imply that the thermal history of Vesta diverges from the generally accepted picture (Ghosh & Mc Sween 1998). Using the thermal evolution code (Formisano et al. 2011) we developed, we are simulating several thermal and structural evolution scenarios of Vesta, by varying the delay Δtd in the injection of 26Al in Vesta, which controls the strength of radiogenic sources. We consider a primordial Vesta as a sphere of radius 270 Km with the initial temperature fixed to 200 K. The initial composition is a homogeneous mixture of silicatic (77%) and metallic (23%) material. Our code solves contemporary the heat equation with the source term (i.e. the energy is supplied by the decay of 26Al, 60Fe ) and the equation for the mass transfer in a porous medium, by using a finite difference 1D method. When the melting temperature of Fe-FeS is reached, the percolation of the iron into the silicatic matrix takes place while, when the melting temperature of silicate is reached, the differentiation and subsequent core formation occur. Since our model does not take in account heat removal mechanisms other than the conduction and the irradiation at the surface, our results supply a reliable picture of the thermal history of Vesta up until the onset of the differentiation. When compared to the data provided by HED meteorites, our results suggest short accretion and differentiation times of Vesta respect to the condensation of CAIs. The scenarios characterized by Δtd > 2 Ma show temperatures not reaching the melting temperature of silicate and for this reason they are incompatible with the basaltic magmatism suggest by HEDs

  9. A Model to Assess the Risk of Ice Accretion Due to Ice Crystal Ingestion in a Turbofan Engine and its Effects on Performance

    NASA Technical Reports Server (NTRS)

    Jorgenson, Philip C. E.; Veres, Joseph P.; Wright, William B.; Struk, Peter M.

    2013-01-01

    The occurrence of ice accretion within commercial high bypass aircraft turbine engines has been reported under certain atmospheric conditions. Engine anomalies have taken place at high altitudes that were attributed to ice crystal ingestion, partially melting, and ice accretion on the compression system components. The result was one or more of the following anomalies: degraded engine performance, engine roll back, compressor surge and stall, and flameout of the combustor. The main focus of this research is the development of a computational tool that can estimate whether there is a risk of ice accretion by tracking key parameters through the compression system blade rows at all engine operating points within the flight trajectory. The tool has an engine system thermodynamic cycle code, coupled with a compressor flow analysis code, and an ice particle melt code that has the capability of determining the rate of sublimation, melting, and evaporation through the compressor blade rows. Assumptions are made to predict the complex physics involved in engine icing. Specifically, the code does not directly estimate ice accretion and does not have models for particle breakup or erosion. Two key parameters have been suggested as conditions that must be met at the same location for ice accretion to occur: the local wet-bulb temperature to be near freezing or below and the local melt ratio must be above 10%. These parameters were deduced from analyzing laboratory icing test data and are the criteria used to predict the possibility of ice accretion within an engine including the specific blade row where it could occur. Once the possibility of accretion is determined from these parameters, the degree of blockage due to ice accretion on the local stator vane can be estimated from an empirical model of ice growth rate and time spent at that operating point in the flight trajectory. The computational tool can be used to assess specific turbine engines to their susceptibility to

  10. Dust Coagulation in Protoplanetary Accretion Disks

    NASA Technical Reports Server (NTRS)

    Schmitt, W.; Henning, Th.; Mucha, R.

    1996-01-01

    The time evolution of dust particles in circumstellar disk-like structures around protostars and young stellar objects is discussed. In particular, we consider the coagulation of grains due to collisional aggregation. The coagulation of the particles is calculated by solving numerically the non-linear Smoluchowski equation. The different physical processes leading to relative velocities between the grains are investigated. The relative velocities may be induced by Brownian motion, turbulence and drift motion. Starting from different regimes which can be identified during the grain growth we also discuss the evolution of dust opacities. These opacities are important for both the derivation of the circumstellar dust mass from submillimeter/millimeter continuum observations and the dynamical behavior of the disks. We present results of our numerical studies of the coagulation of dust grains in a turbulent protoplanetary accretion disk described by a time-dependent one-dimensional (radial) alpha-model. For several periods and disk radii, mass distributions of coagulated grains have been calculated. From these mass spectra, we determined the corresponding Rosseland mean dust opacities. The influence of grain opacity changes due to dust coagulation on the dynamical evolution of a protostellar disk is considered. Significant changes in the thermal structure of the protoplanetary nebula are observed. A 'gap' in the accretion disk forms at the very frontier of the coagulation, i.e., behind the sublimation boundary in the region between 1 and 5 AU.

  11. Dynamo Activity in Strongly Magnetized Accretion Disks

    NASA Astrophysics Data System (ADS)

    Salvesen, Greg; Simon, Jacob B.; Armitage, Philip J.; Begelman, Mitchell C.

    2016-01-01

    Strongly magnetized accretion disks around black holes have many attractive features that may explain the enigmatic behavior observed from X-ray binaries. The physics and structure of these disks are governed by a dynamo-like mechanism, which channels the accretion power liberated by the magnetorotational instability into an ordered toroidal magnetic field. To study dynamo activity, we performed three-dimensional, stratified, isothermal, ideal magnetohydrodynamic shearing box simulations. In our simulations, the strength of this self-sustained toroidal magnetic field depends on the net vertical magnetic flux we impose, which allows us to study weak-to-strong magnetization regimes. We find that the entire disk develops into a magnetic pressure-dominated state for a sufficiently strong net vertical magnetic flux. Over the two orders of magnitude in net vertical magnetic flux that we consider, the effective α-viscosity parameter scales as a power-law. We quantify dynamo properties of toroidal magnetic flux production and its buoyant escape as a function of disk magnetization. Finally, we compare our simulations to an analytic model for the vertical structure of strongly magnetized disks applicable to the high/soft state of X-ray binaries.

  12. Accretion disc origin of the Earth's water.

    PubMed

    Vattuone, Luca; Smerieri, Marco; Savio, Letizia; Asaduzzaman, Abu Md; Muralidharan, Krishna; Drake, Michael J; Rocca, Mario

    2013-07-13

    Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10⁻⁸ bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars. PMID:23734050

  13. The lamppost model of accreting black holes

    NASA Astrophysics Data System (ADS)

    Zdziarski, A.

    2016-06-01

    Niedzwiecki, Zdziarski & Szanecki (2016, ApJL, submitted) have studied the lamppost model, in which the X-ray source in accreting black-hole systems is located on the rotation axis close to the horizon. We point out a number of inconsistencies in the widely used lamppost model relxilllp. They appear to invalidate those model fitting results for which the source distances from the horizon are within several gravitational radii. Furthermore, we note that if those results were correct, most of the photons produced in the lamppost would be trapped by the black hole, and the source luminosity as measured at infinity would be much larger than that observed. This appears to be in conflict with the observed smooth state transitions between the hard and soft states of X-ray binaries. The required increase of the accretion rate and the associated efficiency reduction present also a problem for AGNs. Then, those models imply the luminosity measured in the local frame much higher than the dissipated power due to time dilation and redshift, and the electron temperature significantly higher than that observed. We show that these conditions imply that the fitted sources would be out of the pair equilibrium.

  14. Black hole accretion disks with coronae

    NASA Technical Reports Server (NTRS)

    Svensson, Roland; Zdziarski, Andrzej A.

    1994-01-01

    Observations suggest the existence of both hot and cold dark matter in the centers of active galactic nuclei. Recent spectral models require a major fraction of power to be dissipated in the hot matter. We study the case when the hot matter forms a corona around a standard cold alpha-disk. In particular, we investigate the case when a major fraction, f, of the power released when the cold matter accretes is transported to and dissipated in the corona. This has major effects on the cold disk, making it colder, more geometrically thin, denser, and having larger optical depths. One important consequence is the disappearance of the effectively optically thin zone as well as of the radiation pressure dominated zone for values of f sufficiently closed to unity. The disappearance of the radiation pressure dominated zone will result in a cold disk with only a gas pressure dominated zone that is stable against thermal and viscous instabilities. We also show that the pressure ( and the radiation) from the corona will only affect the surface layers of the cold disk. Our results disagree with those of other recent work on accretion disks with coronae. We find those works to be based on unphysical assumptions.

  15. Accretion disc origin of the Earth's water.

    PubMed

    Vattuone, Luca; Smerieri, Marco; Savio, Letizia; Asaduzzaman, Abu Md; Muralidharan, Krishna; Drake, Michael J; Rocca, Mario

    2013-07-13

    Earth's water is conventionally believed to be delivered by comets or wet asteroids after the Earth formed. However, their elemental and isotopic properties are inconsistent with those of the Earth. It was thus proposed that water was introduced by adsorption onto grains in the accretion disc prior to planetary growth, with bonding energies so high as to be stable under high-temperature conditions. Here, we show both by laboratory experiments and numerical simulations that water adsorbs dissociatively on the olivine {100} surface at the temperature (approx. 500-1500 K) and water pressure (approx. 10⁻⁸ bar) expected for the accretion disc, leaving an OH adlayer that is stable at least up to 900 K. This may result in the formation of many Earth oceans, provided that a viable mechanism to produce water from hydroxyl exists. This adsorption process must occur in all disc environments around young stars. The inevitable conclusion is that water should be prevalent on terrestrial planets in the habitable zone around other stars.

  16. The Physics of Wind-Fed Accretion

    SciTech Connect

    Mauche, Christopher W.; Liedahl, Duane A.; Akiyama, Shizuka

    2008-09-30

    We provide a brief review of the physical processes behind the radiative driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and accretion of a fraction of the stellar wind by a compact object, typically a neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we describe a program to develop global models of the radiatively-driven photoionized winds and accretion flows of HMXBs, with particular attention to the prototypical system Vela X-l. The models combine XSTAR photoionization calculations, HULLAC emission models appropriate to X-ray photoionized plasmas, improved models of the radiative driving of photoionized winds, FLASH time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo radiation transport. We present two- and three-dimensional maps of the density, temperature, velocity, ionization parameter, and emissivity distributions of representative X-ray emission lines, as well as synthetic global Monte Carlo X-ray spectra. Such models help to better constrain the properties of the winds of HMXBs, which bear on such fundamental questions as the long-term evolution of these binaries and the chemical enrichment of the interstellar medium.

  17. Magnetic fields in primordial accretion disks

    NASA Astrophysics Data System (ADS)

    Latif, M. A.; Schleicher, D. R. G.

    2016-01-01

    Magnetic fields are considered a vital ingredient of contemporary star formation and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations and are subsequently amplified by the small-scale dynamo, leading to a strong, tangled magnetic field. We explore how the magnetic field provided by the small-scale dynamo is further amplified via the α-Ω dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop. III star with 10M⊙ and an accretion rate of 10-3M⊙ yr-1, and a supermassive star with 105M⊙ and an accretion rate of 10-1M⊙ yr-1. For the 10M⊙ Pop. III star, we find that coherent magnetic fields can be produced on scales of at least 100 AU, which are sufficient to drive a jet with a luminosity of 100L⊙ and a mass outflow rate of 10-3.7M⊙ yr-1. For the supermassive star, the dynamical timescales in its environment are even shorter, implying smaller orbital timescales and an efficient magnetization out to at least 1000 AU. The jet luminosity corresponds to ~106.0L⊙ and a mass outflow rate of 10-2.1M⊙ yr-1. We expect that the feedback from the supermassive star can have a relevant impact on its host galaxy.

  18. ECCENTRICITY EVOLUTION THROUGH ACCRETION OF PROTOPLANETS

    SciTech Connect

    Matsumoto, Yuji; Nagasawa, Makiko; Ida, Shigeru E-mail: nagasawa.m.ad@m.titech.ac.jp

    2015-09-10

    Most super-Earths detected by the radial velocity (RV) method have significantly smaller eccentricities than the eccentricities corresponding to velocity dispersion equal to their surface escape velocity (“escape eccentricities”). If orbital instability followed by giant impacts among protoplanets that have migrated from outer regions is considered, it is usually considered that eccentricities of the merged bodies become comparable to those of orbital crossing bodies, which are excited up to their escape eccentricities by close scattering. However, the eccentricity evolution in the in situ accretion model has not been studied in detail. Here, we investigate the eccentricity evolution through N-body simulations. We have found that the merged planets tend to have much smaller eccentricities than escape eccentricities due to very efficient collision damping. If the protoplanet orbits are initially well separated and their eccentricities are securely increased, an inner protoplanet collides at its apocenter with an outer protoplanet at its pericenter. The eccentricity of the merged body is the smallest for such configurations. Orbital inclinations are also damped by this mechanism and planets tend to share a same orbital plane, which is consistent with Kepler data. Such efficient collision damping is not found when we start calculations from densely packed orbits of the protoplanets. If the protoplanets are initially in the mean-motion resonances, which corresponds to well separated orbits, the in situ accretion model well reproduces the features of eccentricities and inclinations of multiple super-Earths/Earth systems discovered by RV and Kepler surveys.

  19. Use and abuse of crustal accretion calculations

    NASA Astrophysics Data System (ADS)

    Pallister, John S.; Cole, James C.; Stoeser, Douglas B.; Quick, James E.

    1990-01-01

    Recent attempts to calculate the average growth rate of continental crust for the Late Proterozoic shield of Arabia and Nubia are subject to large geological uncertainties, and widely contrasting conclusions result from dissimilar boundary conditions. The four greatest sources of divergence are (1) the extent of 620-920 Ma arc-terrane crust beneath Phanerozoic cover; (2) the extent of pre-920 Ma continental crust within the arc terranes; (3) the amount of postaccretion magmatic addition and erosion; and (4) the aggregate length and average life span of Late Proterozoic magmatic-arc systems that formed the Arabian-Nubian Shield. Calculations restricted to the relatively well known Arabian segment of the Arabian-Nubian Shield result in average crustal growth rates and arc accretion rates comparable to rates for modern arc systems, but we recognize substantial uncertainty in such results. Critical review of available geochemical, isotopic, and geochronological evidence contradicts the often stated notion that intact, pre-920 Ma crust is widespread in the eastern Arabian Shield. Instead, the arc terranes of the region apparently were "contaminated" with sediments derived, in part, from pre-920 Ma crust. Available geologic and radiometric data indicate that the Arabian-Nubian Shield and its "Pan-African" extensions constitute the greatest known volume of arc-accreted crust on Earth that formed in the period 920-620 Ma. Thus, the region may truly represent a disproportionate share of Earth's crustal growth budget for this time period.

  20. Bulk Comptonization by turbulence in accretion discs

    NASA Astrophysics Data System (ADS)

    Kaufman, J.; Blaes, O. M.

    2016-06-01

    Radiation pressure dominated accretion discs around compact objects may have turbulent velocities that greatly exceed the electron thermal velocities within the disc. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. Bulk Comptonization by divergenceless turbulence is due to radiation viscous dissipation only. It can be treated as thermal Comptonization by solving the Kompaneets equation with an equivalent `wave' temperature, which is a weighted sum over the power present at each scale in the turbulent cascade. Bulk Comptonization by turbulence with non-zero divergence is due to both pressure work and radiation viscous dissipation. Pressure work has negligible effect on photon spectra in the limit of optically thin turbulence, and in this limit radiation viscous dissipation alone can be treated as thermal Comptonization with a temperature equivalent to the full turbulent power. In the limit of extremely optically thick turbulence, radiation viscous dissipation is suppressed, and the evolution of local photon spectra can be understood in terms of compression and expansion of the strongly coupled photon and gas fluids. We discuss the consequences of these effects for self-consistently resolving and interpreting turbulent Comptonization in spectral calculations in radiation magnetohydrodynamic simulations of high luminosity accretion flows.

  1. The Physics of Wind-Fed Accretion

    SciTech Connect

    Mauche, C W; Liedahl, D A; Akiyama, S; Plewa, T

    2008-05-27

    We provide a brief review of the physical processes behind the radiative driving of the winds of OB stars and the Bondi-Hoyle-Lyttleton capture and accretion of a fraction of the stellar wind by a compact object, typically a neutron star, in detached high-mass X-ray binaries (HMXBs). In addition, we describe a program to develop global models of the radiatively-driven photoionized winds and accretion flows of HMXBs, with particular attention to the prototypical system Vela X-1. The models combine XSTAR photoionization calculations, HULLAC emission models appropriate to X-ray photoionized plasmas, improved models of the radiative driving of photoionized winds, FLASH time-dependent adaptive-mesh hydrodynamics calculations, and Monte Carlo radiation transport. We present two- and three-dimensional maps of the density, temperature, velocity, ionization parameter, and emissivity distributions of representative X-ray emission lines, as well as synthetic global Monte Carlo X-ray spectra. Such models help to better constrain the properties of the winds of HMXBs, which bear on such fundamental questions as the long-term evolution of these binaries and the chemical enrichment of the interstellar medium.

  2. In-place HEPA filter penetration test

    SciTech Connect

    Bergman, W.; Wilson, kK.; Elliott, J.; Bettencourt, B.; Slawski, J.W.

    1997-01-01

    We have demonstrated the feasibility of conducting penetration tests on high efficiency particulate air (HEPA) filters as installed in nuclear ventilation systems. The in-place penetration test, which is designed to yield equivalent penetration measurements as the standard DOP efficiency test, is based on measuring the aerosol penetration of the filter installation as a function of particle size using a portable laser particle counter. This in-place penetration test is compared to the current in-place leak test using light scattering photometers for single HEPA filter installations and for HEPA filter plenums using the shroud method. Test results show the in-place penetration test is more sensitive than the in-place leak test, has a similar operating procedure, but takes longer to conduct. Additional tests are required to confirm that the in-place penetration test yields identical results as the standard dioctyl phthalate (DOP) penetration test for HEPA filters with controlled leaks in the filter and gasket and duct by-pass leaks. Further development of the procedure is also required to reduce the test time before the in- place penetration test is practical.

  3. In-place HEPA filter penetration test

    SciTech Connect

    Bergman, W.; Wilson, K.; Elliott, J.

    1997-08-01

    We have demonstrated the feasibility of conducting penetration tests on high efficiency particulate air (HEPA) filters as installed in nuclear ventilation systems. The in-place penetration test, which is designed to yield equivalent penetration measurements as the standard DOP efficiency test, is based on measuring the aerosol penetration of the filter installation as a function of particle size using a portable laser particle counter. This in-place penetration test is compared to the current in-place leak test using light scattering photometers for single HEPA filter installations and for HEPA filter plenums using the shroud method. Test results show the in-place penetration test is more sensitive than the in-place leak test, has a similar operating procedure, but takes longer to conduct. Additional tests are required to confirm that the in-place penetration test yields identical results as the standard dioctyl phthalate (DOP) penetration test for HEPA filters with controlled leaks in the filter and gasket and duct by-pass leaks. Further development of the procedure is also required to reduce the test time before the in-place penetration test is practical. 14 refs., 14 figs., 3 tabs.

  4. Breastfeeding in public places.

    PubMed

    McIntyre, E; Turnbull, D; Hiller, J E

    1999-06-01

    This study reports the results of a survey of restaurant and shopping center managers concerning breastfeeding in their facilities. Managers from 66 restaurants and 27 shopping centers were interviewed by telephone. One-third of the restaurant managers and 48% of the shopping center managers stated that a mother could breastfeed anywhere in their facility regardless of what other customers might say. The remaining managers would either discourage breastfeeding anywhere in their facility, suggest a mother move to a more secluded area if she wished to breastfeed, or were unsure how they would react. The variability in support for breastfeeding by managers of restaurants and shopping centers will continue to create uncertainty for mothers wishing to breastfeed in these public places.

  5. Place memory in crickets

    PubMed Central

    Wessnitzer, Jan; Mangan, Michael; Webb, Barbara

    2008-01-01

    Certain insect species are known to relocate nest or food sites using landmarks, but the generality of this capability among insects, and whether insect place memory can be used in novel task settings, is not known. We tested the ability of crickets to use surrounding visual cues to relocate an invisible target in an analogue of the Morris water maze, a standard paradigm for spatial memory tests on rodents. Adult female Gryllus bimaculatus were released into an arena with a floor heated to an aversive temperature, with one hidden cool spot. Over 10 trials, the time taken to find the cool spot decreased significantly. The best performance was obtained when a natural scene was provided on the arena walls. Animals can relocate the position from novel starting points. When the scene is rotated, they preferentially approach the fictive target position corresponding to the rotation. We note that this navigational capability does not necessarily imply the animal has an internal spatial representation. PMID:18230590

  6. Ice Accretions and Icing Effects for Modern Airfoils

    NASA Technical Reports Server (NTRS)

    Addy, Harold E., Jr.

    2000-01-01

    Icing tests were conducted to document ice shapes formed on three different two-dimensional airfoils and to study the effects of the accreted ice on aerodynamic performance. The models tested were representative of airfoil designs in current use for each of the commercial transport, business jet, and general aviation categories of aircraft. The models were subjected to a range of icing conditions in an icing wind tunnel. The conditions were selected primarily from the Federal Aviation Administration's Federal Aviation Regulations 25 Appendix C atmospheric icing conditions. A few large droplet icing conditions were included. To verify the aerodynamic performance measurements, molds were made of selected ice shapes formed in the icing tunnel. Castings of the ice were made from the molds and placed on a model in a dry, low-turbulence wind tunnel where precision aerodynamic performance measurements were made. Documentation of all the ice shapes and the aerodynamic performance measurements made during the icing tunnel tests is included in this report. Results from the dry, low-turbulence wind tunnel tests are also presented.

  7. Binary accretion rates: dependence on temperature and mass ratio

    NASA Astrophysics Data System (ADS)

    Young, M. D.; Clarke, C. J.

    2015-09-01

    We perform a series of 2D smoothed particle hydrodynamics simulations of gas accretion on to binaries via a circumbinary disc, for a range of gas temperatures and binary mass ratios (q). We show that increasing the gas temperature increases the accretion rate on to the primary for all values of the binary mass ratio: for example, for q = 0.1 and a fixed binary separation, an increase of normalized sound speed by a factor of 5 (from our `cold' to `hot' simulations) changes the fraction of the accreted gas that flows on to the primary from 10 to ˜40 per cent. We present a simple parametrization for the average accretion rate of each binary component accurate to within a few per cent and argue that this parametrization (rather than those in the literature based on warmer simulations) is relevant to supermassive black hole accretion and all but the widest stellar binaries. We present trajectories for the growth of q during circumbinary disc accretion and argue that the period distribution of stellar `twin' binaries is strong evidence for the importance of circumbinary accretion. We also show that our parametrization of binary accretion increases the minimum mass ratio needed for spin alignment of supermassive black holes to q ˜ 0.4, with potentially important implications for the magnitude of velocity kicks acquired during black hole mergers.

  8. POSSIBLE SIGNATURES OF MAGNETOSPHERIC ACCRETION ONTO YOUNG GIANT PLANETS

    SciTech Connect

    Lovelace, R. V. E.; Covey, K. R.; Lloyd, J. P. E-mail: kcovey@astro.cornell.edu

    2011-02-15

    Magnetospheric accretion is an important process for a wide range of astrophysical systems and may play a role in the formation of gas giant planets. Extending the formalism describing stellar magnetospheric accretion into the planetary regime, we demonstrate that magnetospheric processes may govern accretion onto young gas giants in the isolation phase of their development. Planets in the isolation phase have cleared out large gaps in their surrounding circumstellar disks and settled into a quasi-static equilibrium with radii only modestly larger than their final sizes (i.e., r {approx} 1.4r{sub final}). Magnetospheric accretion is less likely to play a role in a young gas giant's main accretion phase, when the planet's envelope is predicted to be much larger than the planet's Alfven radius. For a fiducial 1 M{sub J} gas giant planet with a remnant isolation phase accretion rate of M-dot{sub sun}= 10{sup -10} M{sub sun} yr{sup -1} = 10{sup -7} M{sub J} yr{sup -1}, the disk accretion will be truncated at {approx}2.7r{sub J} (with r{sub J} is Jupiter's radius) and drive the planet to rotate with a period of {approx}7 hr. Thermal emission from planetary magnetospheric accretion will be difficult to observe; the most promising observational signatures may be non-thermal, such as gyrosynchrotron radiation that is clearly modulated at a period much shorter than the rotation period of the host star.

  9. m-Accretive extensions of a sectorial operator

    SciTech Connect

    Arlinskii, Yu M; Popov, A B

    2013-08-31

    A description of all the maximal accretive extensions and their resolvents is given for a densely defined closed sectorial operator in terms of abstract boundary conditions. These results are applied to parametrize all the m-accretive extensions of a symmetric operator in a planar model of one-centre point interaction. Bibliography: 40 titles.

  10. Does mass accretion lead to field decay in neutron stars?

    NASA Technical Reports Server (NTRS)

    Shibazaki, N.; Murakami, T.; Shaham, J.; Nomoto, K.

    1989-01-01

    Adopting the hypothesis of accretion-induced magnetic field decay in neutron stars, the consequent evolution of a neutron star's spin and magnetic field are calculated. The results are consistent with observations of binary and millisecond radio pulsars. Thermomagnetic effects could provide a possible physical mechanism for such accretion-induced field decay.

  11. ACCRETION IN PROTOPLANETARY DISKS BY COLLISIONAL FUSION

    SciTech Connect

    Wettlaufer, J. S.

    2010-08-10

    The formation of a solar system such as ours is believed to have followed a multi-stage process around a protostar and its associated accretion disk. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag, and Cuzzi and colleagues have shown that when midplane particle mass densities approach or exceed those of the gas, solid-solid interactions dominate the drag effect. The size dependence of the drag creates a 'bottleneck' at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Independent of whether the origin of the drag is angular momentum exchange with gas or solids in the disk, successful planetary accretion requires rapid planetesimal growth to kilometer scales. A commonly accepted picture is that for collisional velocities V{sub c} above a certain threshold value, V {sub th{approx}} 0.1-10 cm s{sup -1}, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all ranges of interparticle collision speeds the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter-phase diagrams, amorphs, and polymorphs-has been neglected. Here, it is demonstrated for compact bodies that novel aspects of surface phase transitions provide a physical basis for efficient sticking through collisional melting/amorphization/polymorphization and subsequent fusion/annealing to extend the collisional velocity range of primary accretion to {Delta}V{sub c} {approx} 1-100 m s{sup -1} >> V {sub th}, which encompasses both typical turbulent rms speeds and the velocity differences between boulder-sized and small grains {approx}1-50 m s{sup -1}. Therefore, as inspiraling meter-sized bodies collide

  12. Magnetised accretion discs in Kerr spacetimes

    NASA Astrophysics Data System (ADS)

    Ranea-Sandoval, Ignacio F.; García, Federico

    2015-01-01

    Context. Observational data from X-ray binary systems provide strong evidence of astronomical objects that are too massive and compact to be explained as neutron or hybrid stars. When these systems are in the thermal (high/soft) state, they emit mainly in the 0.1-5 keV energy range. This emission can be explained by thin accretion discs that formed around compact objects like black holes. The profile of the fluorescent iron line is useful to obtain insight into the nature of the compact object. General relativity does not ensure that a black hole must form after the complete gravitational collapse of very massive stars, and other theoretical models such as naked singularities cannot be discarded. The cosmic censorship conjecture was proposed by Penrose to avoid these possibilities and is yet to be proven. Aims: We study the effect caused by external magnetic fields on the observed thermal spectra and iron line profiles of thin accretion discs formed around Kerr black holes and naked singularities. We aim to provide a tool that can be used to estimate the presence of magnetic fields in the neighbourhood of a compact object and to probe the cosmic censorship conjecture in these particular astrophysical environments. Methods: We developed a numerical scheme able to calculate thermal spectra of magnetised Page-Thorne accretion discs formed around rotating black holes and naked singularities as seen by an arbitrary distant observer. We incorporated two different magnetic field configurations: uniform and dipolar, using a perturbative scheme in the coupling constant between matter and magnetic field strength. Under the same assumptions, we obtained observed synthetic line profiles of the 6.4 keV fluorescent iron line. Results: We show that an external magnetic field produces potentially observable modifications on the thermal energy spectrum and the fluorescent iron line profile. Thermal energy spectra of naked singularities are harder and brighter than those from black

  13. Accretion in Protoplanetary Disks by Collisional Fusion

    NASA Astrophysics Data System (ADS)

    Wettlaufer, J. S.

    2010-08-01

    The formation of a solar system such as ours is believed to have followed a multi-stage process around a protostar and its associated accretion disk. Whipple first noted that planetesimal growth by particle agglomeration is strongly influenced by gas drag, and Cuzzi and colleagues have shown that when midplane particle mass densities approach or exceed those of the gas, solid-solid interactions dominate the drag effect. The size dependence of the drag creates a "bottleneck" at the meter scale with such bodies rapidly spiraling into the central star, whereas much smaller or larger particles do not. Independent of whether the origin of the drag is angular momentum exchange with gas or solids in the disk, successful planetary accretion requires rapid planetesimal growth to kilometer scales. A commonly accepted picture is that for collisional velocities Vc above a certain threshold value, V th~ 0.1-10 cm s-1, particle agglomeration is not possible; elastic rebound overcomes attractive surface and intermolecular forces. However, if perfect sticking is assumed for all ranges of interparticle collision speeds the bottleneck can be overcome by rapid planetesimal growth. While previous work has dealt with the influences of collisional pressures and the possibility of particle fracture or penetration, the basic role of the phase behavior of matter-phase diagrams, amorphs, and polymorphs—has been neglected. Here, it is demonstrated for compact bodies that novel aspects of surface phase transitions provide a physical basis for efficient sticking through collisional melting/amorphization/polymorphization and subsequent fusion/annealing to extend the collisional velocity range of primary accretion to ΔVc ~ 1-100 m s-1 Gt V th, which encompasses both typical turbulent rms speeds and the velocity differences between boulder-sized and small grains ~1-50 m s-1. Therefore, as inspiraling meter-sized bodies collide with smaller particles in this high velocity collisional fusion

  14. The East African Orogen: Accretion versus Collision

    NASA Astrophysics Data System (ADS)

    Kröner, A.; Muhongo, S.; Sommer, H.; Vogt, M.

    2003-04-01

    The East African Orogen is an extensive Neoproterozoic (Pan-African) orogenic belt extending from Arabia to Mozambique and containing elements of both accretion and collision tectonics. The predominantly upper crustal northern part (Arabian-Nubian Shield, ANS) consists of Neoproterozoic juvenile arc assemblages that accreted onto the African continent along ophiolite-decorated sutures. In contrast, the tectonic evolution of the predominantly middle to lower crustal southern part (Mozambique belt, MB) is still poorly understood, and simple continental collision models as previously applied are not compatible with new isotopic and petrological data. Published Nd isotopic systematics and our new zircon ages demonstrate that large parts of the high-grade MB in Tanzania consists of late Archaean to Palaeoproterozoic granitoid gneisses which may either constitute an extension of the Tanzania craton to the E, reworked during the Pan-African orogeny, or these rocks may constitute a separate terrane, or terranes, tectonically interdigitated with Neoproterozoic gneisses, similar to the situation in Madagascar. Small-scale tectonic interlayering of >1800 Ma and 650-800 Ma gneisses have been documented at several localities, and the amount of pre-Neoproterozoic crust in the MB appears to be ˜70% or more. Relatively small volumes of ˜1000-1100 Ma granitoids have so far only been recorded in southern Tanzania, and their significance in the orogenic evolution and their relation to more extensive rocks of this age in northern Mozambique are not known. High-grade metamorphism in the MB of Tanzania led to granulite and charnockite formation and occurred at 620-640 Ma, slightly earlier than in Mozambique (˜615 Ma) but significantly earlier than in Malawi (˜550-580 Ma) and in Madagascar (˜550-560 Ma). The significance of these age differences is not understood. Petrological data suggest both clockwise and anti-clockwise PT path for the metamorphic assemblages, suggesting that

  15. Accretion Discs Show Their True Colours

    NASA Astrophysics Data System (ADS)

    2008-07-01

    Quasars are the brilliant cores of remote galaxies, at the hearts of which lie supermassive black holes that can generate enough power to outshine the Sun a trillion times. These mighty power sources are fuelled by interstellar gas, thought to be sucked into the hole from a surrounding 'accretion disc'. A paper in this week's issue of the journal Nature, partly based on observations collected with ESO's Very Large Telescope, verifies a long-standing prediction about the intensely luminous radiation emitted by these accretion discs. Uncovering the disc ESO PR Photo 21/08 Uncovering the inner disc "Astronomers were puzzled by the fact that the best models of these discs couldn't quite be reconciled with some of the observations, in particular, with the fact that these discs did not appear as blue as they should be," explains lead-author Makoto Kishimoto. Such a discrepancy could be the signal that there was something very wrong with the models. With his colleagues, he investigated this discrepancy by studying the polarised light from six quasars. This enabled them to demonstrate that the disc spectrum is as blue as predicted. "The crucial observational difficulty here has been that the disc is surrounded by a much larger torus containing hot dust, whose light partly outshines that of the disc," says Kishimoto. "Because the light coming from the disc is scattered in the disc vicinity and thus polarised, by observing only polarised light from the quasars, one can uncover the buried light from the disc." In a similar way that a fisherman would wear polarised sunglasses to help get rid of the glare from the water surface and allow him to see more clearly under the water, the filter on the telescope allowed the astronomers to see beyond surrounding clouds of dust and gas to the blue colour of the disc in infrared light. The observations were done with the FORS and ISAAC instruments on one of the 8.2-m Unit Telescopes of ESO's Very Large Telescope, located in the Atacama

  16. Aerodynamic Simulation of Runback Ice Accretion

    NASA Technical Reports Server (NTRS)

    Broeren, Andy P.; Whalen, Edward A.; Busch, Greg T.; Bragg, Michael B.

    2010-01-01

    This report presents the results of recent investigations into the aerodynamics of simulated runback ice accretion on airfoils. Aerodynamic tests were performed on a full-scale model using a high-fidelity, ice-casting simulation at near-flight Reynolds (Re) number. The ice-casting simulation was attached to the leading edge of a 72-in. (1828.8-mm ) chord NACA 23012 airfoil model. Aerodynamic performance tests were conducted at the ONERA F1 pressurized wind tunnel over a Reynolds number range of 4.7?10(exp 6) to 16.0?10(exp 6) and a Mach (M) number ran ge of 0.10 to 0.28. For Re = 16.0?10(exp 6) and M = 0.20, the simulated runback ice accretion on the airfoil decreased the maximum lift coe fficient from 1.82 to 1.51 and decreased the stalling angle of attack from 18.1deg to 15.0deg. The pitching-moment slope was also increased and the drag coefficient was increased by more than a factor of two. In general, the performance effects were insensitive to Reynolds numb er and Mach number changes over the range tested. Follow-on, subscale aerodynamic tests were conducted on a quarter-scale NACA 23012 model (18-in. (457.2-mm) chord) at Re = 1.8?10(exp 6) and M = 0.18, using low-fidelity, geometrically scaled simulations of the full-scale castin g. It was found that simple, two-dimensional simulations of the upper- and lower-surface runback ridges provided the best representation of the full-scale, high Reynolds number iced-airfoil aerodynamics, whereas higher-fidelity simulations resulted in larger performance degrada tions. The experimental results were used to define a new subclassification of spanwise ridge ice that distinguishes between short and tall ridges. This subclassification is based upon the flow field and resulting aerodynamic characteristics, regardless of the physical size of the ridge and the ice-accretion mechanism.

  17. Power Spectrum Density of Stochastic Oscillating Accretion Disk

    NASA Astrophysics Data System (ADS)

    Long, G. B.; Ou, J. W.; Zheng, Y. G.

    2016-06-01

    In this paper, we employ a stochastic oscillating accretion disk model for the power spectral index and variability of BL Lac object S5 0716+714. In the model, we assume that there is a relativistic oscillation of thin accretion disks and it interacts with an external thermal bath through a friction force and a random force. We simulate the light curve and the power spectrum density (PSD) at (i) over-damped, (ii) critically damped and (iii) under-damped cases, respectively. Our results show that the simulated PSD curves depend on the intrinsic property of the accretion disk, and it could be produced in a wide interval ranging from 0.94 to 2.05 by changing the friction coefficient in a stochastic oscillating accretion disk model. We argue that accretion disk stochastic oscillating could be a possible interpretation for observed PSD variability.

  18. Ice Accretions on a Swept GLC-305 Airfoil

    NASA Technical Reports Server (NTRS)

    Vargas, Mario; Papadakis, Michael; Potapczuk, Mark; Addy, Harold; Sheldon, David; Giriunas, Julius

    2002-01-01

    An experiment was conducted in the Icing Research Tunnel (IRT) at NASA Glenn Research Center to obtain castings of ice accretions formed on a 28 deg. swept GLC-305 airfoil that is representative of a modern business aircraft wing. Because of the complexity of the casting process, the airfoil was designed with three removable leading edges covering the whole span. Ice accretions were obtained at six icing conditions. After the ice was accreted, the leading edges were detached from the airfoil and moved to a cold room. Molds of the ice accretions were obtained, and from them, urethane castings were fabricated. This experiment is the icing test of a two-part experiment to study the aerodynamic effects of ice accretions.

  19. Challenging accretion models with an HETG observation of T Tau

    NASA Astrophysics Data System (ADS)

    Schneider, Christian

    2014-09-01

    Young, accreting stars often emit soft X-rays from the accretion shock on the stellar surface. The favored model predicts high densities in the accretion spot compatible with most observations. However, X-ray data of the nearest accreting star (TW Hya) and of the eponymous T Tau show signatures incompatible with shock models. Proposed model modifications require an enhanced density of higher temperature tracers compared to cooler plasma. We propose to test this with a 130 ks HETG observation of T Tau to (a) provide the first significant measurement of its Ne IX triplet (high temperature) to augment XMM-Newton data of the O VII triplet (low temperature) and (b) to search for spatial offsets of the soft X-rays which could explain the lack of accretion signatures in T Tau by jet emission.

  20. Diapirs of the Mediterranean ridge: The tectonic regime of an incipient accreted terrane

    NASA Technical Reports Server (NTRS)

    Mart, Y.

    1988-01-01

    The occurrence of diapirs in the Mediterranean ridge stems mostly from the massive deposition of salt and gypsum in the Mediterranean basin during the late Miocean. The diapiric emplacement of the evaporitic sequence is not obvious, because the mobilization of the salt beds and the initiation of the diapiric upward flow are constrained by the relatively shallow thickness of the Plio-Pleistocene sedimentary overburden and by the low heat flow that prevails in the eastern Mediterranean. The diapirs consist also of early Cretaceous shales as well as other gravitationally metastable strata which are less mobile than salt. Studies of subduction trenches and their surroundings show that shallow ridges occur seaward of the trenches in many places. The collisional motion between the African and the Eurasian plates would further enhance accretion of sediments in the Mediterranean ridge, which would attain subaerial exposure, and eventually would become a mountain range accreted to southern Europe. The numerous diapirs of salt and shales that occur in the ridge would be common features in the future accreted terrane, indicating an intermediate extensional phase in the tectonic history of the development of crustal growth.

  1. Accretion Disk Coronal Heating Along The Z-pattern In Cygnus X-2

    NASA Astrophysics Data System (ADS)

    Schulz, Norbert S.; Huenemoerder, D. P.; Nowak, M.; Ji, L.; Yao, Y.; Canizares, C. R.

    2009-01-01

    Z-sources are low mass X-ray binaries containing accreting neutron stars which emit luminous X-rays near Eddington luminosities (L_Edd). Their X-ray spectra change in a systematic Z-shaped pattern from the horizontal (HB), to the normal (NB), and to the flaring branch (FB). Cyg X-2 is a prototype Z-source which we observed in X-rays twice along the entire X-ray spectral variation pattern at very high spectral resolution. We find a wealth of resolved and broad emission line features which we attribute to a hot accretion disk corona (ADC). The lines have Doppler velocities between 400 and 2700 km/s, which place the origin of emission within 10^10 cm of the accretion disk. We also find evidence for a flux evolution along the Z-pattern. The line emissivities indicate that the source luminosity changes from about 0.7 L_Edd in the HB to 2.6 L_Edd on the FB. The observed line flux evolution provides direct evidence for ongoing heating of the ADC along the spectral branches.

  2. Structural health monitoring approach for detecting ice accretion on bridge cable using the Haar Wavelet Transform

    NASA Astrophysics Data System (ADS)

    Andre, Julia; Kiremidjian, Anne; Liao, Yizheng; Georgakis, Christos; Rajagopal, Ram

    2016-04-01

    Ice accretion on cables of bridge structures poses serious risk to the structure as well as to vehicular traffic when the ice falls onto the road. Detection of ice formation, quantification of the amount of ice accumulated, and prediction of icefalls will increase the safety and serviceability of the structure. In this paper, an ice accretion detection algorithm is presented based on the Continuous Wavelet Transform (CWT). In the proposed algorithm, the acceleration signals obtained from bridge cables are transformed using wavelet method. The damage sensitive features (DSFs) are defined as a function of the wavelet energy at specific wavelet scales. It is found that as ice accretes on the cables, the mass of cable increases, thus changing the wavelet energies. Hence, the DSFs can be used to track the change of cables mass. To validate the proposed algorithm, we use the data collected from a laboratory experiment conducted at the Technical University of Denmark (DTU). In this experiment, a cable was placed in a wind tunnel as ice volume grew progressively. Several accelerometers were installed at various locations along the testing cable to collect vibration signals.

  3. Modeling Gas Distribution in Protoplanetary Accretion Disks

    NASA Astrophysics Data System (ADS)

    Kronberg, Martin; Lewis, Josiah; Brittain, Sean

    2010-07-01

    Protoplanetary accretion disks are disks of dust and gas which surround and feed material onto a forming star in the earliest stages of its evolution. One of the most useful methods for studying these disks is near infrared spectroscopy of rovibrational CO emission. This paper presents the methods in which synthetically generated spectra are modeled and fit to spectral data gathered from protoplanetary disks. This paper also discussed the methods in which this code can be improved by modifying the code to run a Monte Carlo analysis of best fit across the CONDOR cluster at Clemson University, thereby allowing for the creation of a catalog of protoplanetary disks with detailed information about them as gathered from the model.

  4. Particle rings and astrophysical accretion discs

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Romanova, M. M.

    2016-03-01

    Norman Rostoker had a wide range of interests and significant impact on the plasma physics research at Cornell during the time he was a Cornell professor. His interests ranged from the theory of energetic electron and ion beams and strong particle rings to the related topics of astrophysical accretion discs. We outline some of the topics related to rings and discs including the Rossby wave instability which leads to formation of anticyclonic vortices in astrophysical discs. These vorticies are regions of high pressure and act to trap dust particles which in turn may facilitate planetesimals growth in proto-planetary disks and could be important for planet formation. Analytical methods and global 3D magneto-hydrodynamic simulations have led to rapid advances in our understanding of discs in recent years.

  5. Pair production and escape in accretion disks.

    NASA Astrophysics Data System (ADS)

    Meirelles Filho, C.; Liang, E. P.

    It is shown that, in the absence of confining mechanisms, there will be a non-negligible amount of pairs escaping from the inner region of a Comptonized soft photon two-temperature accretion disk, when pair production is not balanced by annihilation. Assuming conditions such that the photons and particles in the disk can be regarded as close to a Wien plasma (Svensson, 1984), the authors calculate the rate of pair escape from the disk for both a situation close to pair balance and a situation with the rate of escape exceeding annihilation. The pairs are assumed to be created by photon-photon processes. Within this model one can account for the 511 keV γ-ray luminosity due to pair annihilation in the ISM, as recently observed in the Einstein source.

  6. Observational constraints on black hole accretion disks

    NASA Technical Reports Server (NTRS)

    Liang, Edison P.

    1994-01-01

    We review the empirical constraints on accretion disk models of stellar-mass black holes based on recent multiwavelength observational results. In addition to time-averaged emission spectra, the time evolutions of the intensity and spectrum provide critical information about the structure, stability, and dynamics of the disk. Using the basic thermal Keplerian disk paradigm, we consider in particular generalizations of the standard optically thin disk models needed to accommodate the extremely rich variety of dynamical phenomena exhibited by black hole candidates ranging from flares of electron-positron annihilations and quasiperiodic oscillations in the X-ray intensity to X-ray novae activity. These in turn provide probes of the disk structure and global geometry. The goal is to construct a single unified framework to interpret a large variety of black hole phenomena. This paper will concentrate on the interface between basic theory and observational data modeling.

  7. Compact stars and accretion disks: Workshop summary

    NASA Astrophysics Data System (ADS)

    Li, J.

    1998-07-01

    A workshop on `Compact Stars and Accretion Disks' was held on 11-12 August 1997 at the Australian National University. The workshop was opened by Professor Jeremy Mould, the Director of Mount Stromlo Observatory. The workshop was organised to coincide with visits to the ANU Astrophysical Theory Centre by Professor Ron Webbink from the University of Illinois, Professor Rainer Wehrse from the University of Heidelberg and Dr Chris Tout from the University of Cambridge. The workshop attracted over 25 participants nationwide. Participants included members of the Special Research Centre for Theoretical Astrophysics, University of Sydney, led by Professor Don Melrose, Professor Dick Manchester from the ATNF, Professor Ravi Sood from ADFA, Dr John Greenhill from the University of Tasmania and Dr Rosemary Mardling from Monash University. Dr Helen Johnston from AAO and Dr Kurt Liffman from AFDL also attended the workshop. The abstracts of twelve of the workshop papers are presented in this summary.

  8. Dead Zone Accretion Flows in Protostellar Disks

    NASA Technical Reports Server (NTRS)

    Turner, Neal; Sano, T.

    2008-01-01

    Planets form inside protostellar disks in a dead zone where the electrical resistivity of the gas is too high for magnetic forces to drive turbulence. We show that much of the dead zone nevertheless is active and flows toward the star while smooth, large-scale magnetic fields transfer the orbital angular momentum radially outward. Stellar X-ray and radionuclide ionization sustain a weak coupling of the dead zone gas to the magnetic fields, despite the rapid recombination of free charges on dust grains. Net radial magnetic fields are generated in the magnetorotational turbulence in the electrically conducting top and bottom surface layers of the disk, and reach the midplane by ohmic diffusion. A toroidal component to the fields is produced near the midplane by the orbital shear. The process is similar to the magnetization of the solar tachocline. The result is a laminar, magnetically driven accretion flow in the region where the planets form.

  9. Evolution and precession of accretion disk in tidal disruption events

    NASA Astrophysics Data System (ADS)

    Shen, R.-F.; Matzner, C. D.

    2012-12-01

    In a supermassive black hole (BH) tidal disruption event (TDE), the tidally disrupted star feeds the BH via an accretion disk. Most often it is assumed that the accretion rate history, hence the emission light curve, tracks the rate at which new debris mass falls back onto the disk, notably the t-5/3 power law. But this is not the case when the disk evolution due to viscous spreading - the driving force for accretion - is carefully considered. We construct a simple analytical model that comprehensively describes the accretion rate history across 4 different phases of the disk evolution, in the presence of mass fallback and disk wind loss. Accretion rate evolves differently in those phases which are governed by how the disk heat energy is carried away, early on by advection and later by radiation. The accretion rate can decline as steeply as t-5/3 only if copious disk wind loss is present during the early advection-cooled phase. Later, the accretion rate history is t-8/7 or shallower. These have great implications on the TDE flare light curve. A TDE accretion disk is most likely misaligned with the equatorial plane of the spinning BH. Moreover, in the TDE the accretion rate is super- or near-Eddington thus the disk is geometrically thick, for which case the BH's frame dragging effect may cause the disk precess as a solid body, which may manifest itself as quasi-periodic signal in the TDE light curve. Our disk evolution model predicts the disk precession period increases with time, typically as ∝ t. The results are applied to the recently jetted TDE flare Swift transient J1644 + 57 which shows numerous, quasi-periodic dips in its long-term X-ray light curve. As the current TDE sample increases, the identification of the disk precession signature provides a unique way of measuring BH spin and studying BH accretion physics.

  10. Testing Convergence for Global Accretion Disks

    NASA Astrophysics Data System (ADS)

    Hawley, John F.; Richers, Sherwood A.; Guan, Xiaoyue; Krolik, Julian H.

    2013-08-01

    Global disk simulations provide a powerful tool for investigating accretion and the underlying magnetohydrodynamic turbulence driven by magneto-rotational instability (MRI). Using them to accurately predict quantities such as stress, accretion rate, and surface brightness profile requires that purely numerical effects, arising from both resolution and algorithm, be understood and controlled. We use the flux-conservative Athena code to conduct a series of experiments on disks having a variety of magnetic topologies to determine what constitutes adequate resolution. We develop and apply several resolution metrics: langQz rang and langQ phirang, the ratio of the grid zone size to the characteristic MRI wavelength, αmag, the ratio of the Maxwell stress to the magnetic pressure, and \\langle B_R^2\\rangle /\\langle B_\\phi ^2\\rangle, the ratio of radial to toroidal magnetic field energy. For the initial conditions considered here, adequate resolution is characterized by langQz rang >= 15, langQ phirang >= 20, αmag ≈ 0.45, and \\langle B_R^2\\rangle /\\langle B_\\phi ^2\\rangle \\approx 0.2. These values are associated with >=35 zones per scaleheight H, a result consistent with shearing box simulations. Numerical algorithm is also important. Use of the Harten-Lax-van Leer-Einfeldt flux solver or second-order interpolation can significantly degrade the effective resolution compared to the Harten-Lax-van Leer discontinuities flux solver and third-order interpolation. Resolution at this standard can be achieved only with large numbers of grid zones, arranged in a fashion that matches the symmetries of the problem and the scientific goals of the simulation. Without it, however, quantitative measures important to predictions of observables are subject to large systematic errors.

  11. Local outflows from turbulent accretion disks

    NASA Astrophysics Data System (ADS)

    Fromang, S.; Latter, H.; Lesur, G.; Ogilvie, G. I.

    2013-04-01

    Aims: The aim of this paper is to investigate the properties of accretion disks threaded by a weak vertical magnetic field, with a particular focus on the interplay between magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI) and outflows that might be launched from the disk. Methods: For that purpose, we use a set of numerical simulations performed with the MHD code RAMSES in the framework of the shearing box model. We concentrate on the case of a rather weak vertical magnetic field such that the initial ratio β0 of the thermal and magnetic pressures in the disk midplane equals 104. Results: As reported recently, we find that MHD turbulence drives an efficient outflow out of the computational box. We demonstrate a strong sensitivity of that result to the box size: enlargements in the radial and vertical directions lead to a reduction of up to an order of magnitude in the mass-loss rate. Such a dependence prevents any realistic estimates of disk mass-loss rates being derived using shearing-box simulations. We find however that the flow morphology is robust and independent of the numerical details of the simulations. Its properties display some features and approximate invariants that are reminiscent of the Blandford & Payne launching mechanism, but differences exist. For the magnetic field strength considered in this paper, we also find that angular momentum transport is most likely dominated by MHD turbulence, the saturation of which scales with the magnetic Prandtl number, the ratio of viscosity and resistivity, in a way that is in good agreement with expectations based on unstratified simulations. Conclusions: This paper thus demonstrates for the first time that accretion disks can simultaneously exhibit MRI-driven MHD turbulence along with magneto-centrifugally accelerated outflows. However, in contradiction with previously published results, such outflows probably have little impact on the disk dynamics.

  12. Rover Takes a Sunday Drive

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This animation, made with images from the Mars Exploration Rover Spirit hazard-identification camera, shows the rover's perspective of its first post-egress drive on Mars Sunday. Engineers drove Spirit approximately 3 meters (10 feet) toward its first rock target, a football-sized, mountain-shaped rock called Adirondack. The drive took approximately 30 minutes to complete, including time stopped to take images. Spirit first made a series of arcing turns totaling approximately 1 meter (3 feet). It then turned in place and made a series of short, straightforward movements totaling approximately 2 meters (6.5 feet).

  13. Formation of Phobos and Deimos in an extended circum-Martian accretion disk

    NASA Astrophysics Data System (ADS)

    Rosenblatt, Pascal; Charnoz, Sébastien; Dunseath-Terao, Mariko; Dunseath, Kevin; Trinh, Antony

    2015-11-01

    Recent exploration of Phobos by Mars Express has renewed the interest in the origin of the Martian moons, which remains an open issue. New studies have recently focused on accretion of debris in a circum-Martian disk. Craddock (Icarus, 2011) has proposed a scenario of formation of such a disk (about 10-4 to 10-3 % the mass of Mars) from a giant collision occurred early in Mars' history. Rosenblatt and Charnoz (Icarus, 2012) have shown that moonlets can be formed, with Phobos and Deimos masses, in a compact disk close to the planet. Nevertheless, the orbit of these moonlets rapidly recedes back to Mars and the moonlet system vanishes in much less than 1 Gy.Citron at al. (Icarus, 2015) have modeled the formation of a disk of debris resulting from a giant collision but with a larger mass (0.1 % of Mars' mass) than in Craddock (2011). They have shown that most of the disk mass is concentrated close to the planet but a small part can extend beyond the synchronous limit (6 Mars' radii).We study here the accretion of debris in an extended disk, focusing in the edge of this disk where surface density of material is the lowest. However, we take into account the gravitational influence of the more massive bodies formed in the denser part of the disk. Our results highlight the driven mechanisms of accretion in a circum-Martian disk of debris.

  14. Coronal accretion: the power of X-ray emission in AGN

    NASA Astrophysics Data System (ADS)

    Liu, B.-F.; Taam, R. E.; Qiao, E.; Yuan, W.

    2016-02-01

    The optical/UV and X-ray emissions in luminous AGN are commonly believed to be produced in an accretion disk and an embedded hot corona respectively. We explore the possibility that a geometrically thick coronal gas flow, which is supplied by gravitational capture of interstellar medium or stellar wind, condenses partially to a geometrically thin cold disk and accretes via a thin disk and a corona onto the supermassive black hole. We found that for mass supply rates less than about 0.01 (expressed in Eddington units), condensation does not occur and the accretion flow takes the form of a corona/ADAF. For higher mass supply rates, corona gas condenses to the disk. As a consequence, the coronal mass flow rate decreases and the cool mass flow rate increases towards the black hole. Here the thin disk is characterized by the condensation rate of hot gas as it flows towards the black hole. With increase of mass supply rate, condensation becomes more efficient, while the mass flow rate of the coronal gas attains values of order 0.02 in the innermost regions of the disk, which can help to elucidate the production of strong X-ray with respect to the optical and ultraviolet radiation in high luminosity AGN.

  15. Modeling the Accretion and Feedback Processes of Galaxies Similar to the Milky Way

    NASA Astrophysics Data System (ADS)

    Hyatt, Steven; Arielle Phillips, Lara

    2015-01-01

    Composed of sheets, walls, and filaments, the cosmic web connects clusters of galaxies together and is responsible for regulating galaxy evolution by the accreting mass into galaxies via filaments. In an effort to further understand the evolution of galaxies, we first study filaments and devise an interactive analytical model for disk galaxies with accurate inflow and outflow rates of matter. Using accretion rates and feedback rates from different regions in the Galaxy, it is now possible to model and predict galactic behavior for a galaxy with similar mass and morphology as the Milky Way. Other models simulate feedback or accretion processes numerically and in greater detail. Here we consolidate the rates to make one single model for the galaxy as a whole. In this model, we take the rates from other papers and use them to calculate the total mass flowed, energy used, distance travelled, and current location of the gas from the following parameters: change in time, redshift value, morphology of the galaxy, and type of active galactic nuclei (AGN) the galaxy has at its center. Although, we have just begun to make this detailed model, it will serve as the foundation for future work to be done to further understand galaxy evolution.

  16. RADIATION PRESSURE-SUPPORTED ACCRETION DISKS: VERTICAL STRUCTURE, ENERGY ADVECTION, AND CONVECTIVE STABILITY

    SciTech Connect

    Gu Weimin

    2012-07-10

    By taking into account the local energy balance per unit volume between the viscous heating and the advective cooling plus the radiative cooling, we investigate the vertical structure of radiation pressure-supported accretion disks in spherical coordinates. Our solutions show that the photosphere of the disk is close to the polar axis and therefore the disk seems to be extremely thick. However, the density profile implies that most of the accreted matter exists in a moderate range around the equatorial plane. We show that the well-known polytropic relation between the pressure and the density is unsuitable for describing the vertical structure of radiation pressure-supported disks. More importantly, we find that the energy advection is significant even for slightly sub-Eddington accretion disks. We argue that the non-negligible advection may help us understand why the standard thin disk model is likely to be inaccurate above {approx}0.3 Eddington luminosity, which was found by some works on black hole spin measurement. Furthermore, the solutions satisfy the Solberg-Hoiland conditions, which indicate the disk to be convectively stable. In addition, we discuss the possible link between our disk model and ultraluminous X-ray sources.

  17. The Greenhouse: A Place for Year-Round Plant Investigations.

    ERIC Educational Resources Information Center

    Hanif, Muhammad

    1989-01-01

    Activities that may take place in a greenhouse are discussed. Included are learning how to grow plants, plant growth, soil, vegetative reproduction, and plant habitat adaptations. Materials, procedures, and results are presented for the activities. (CW)

  18. Bondi-Hoyle accretion in an isothermal magnetized plasma

    SciTech Connect

    Lee, Aaron T.; McKee, Christopher F.; Klein, Richard I.; Cunningham, Andrew J.

    2014-03-01

    In regions of star formation, protostars and newborn stars will accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma β—the ratio of thermal and magnetic pressures. Observations show that molecular clouds have β ≲ 1, so magnetic fields have the potential to play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbers of up to M≈45; magnetic fields that are either parallel, perpendicular, or oriented 45° to the flow; and β as low as 0.01. Our simulations utilize adaptive mesh refinement in order to obtain high spatial resolution where it is needed; this also allows the boundaries to be far from the accreting object to avoid unphysical effects arising from boundary conditions. Additionally, we show that our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of β and M for the parallel and perpendicular orientations. Using typical molecular cloud values of M∼5 and β ∼ 0.04 from the literature, our fits suggest that a 0.4 M {sub ☉} star accretes ∼4 × 10{sup –9} M {sub ☉} yr{sup –1}, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity can grow to orders of magnitude for stronger fields and lower Mach numbers. We also discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and under what conditions a steady state is possible. The reduction in the accretion rate in a magnetized medium leads to an increase in the time required to form stars in competitive accretion models, making such models less efficient than predicted by

  19. Powerful radiative jets in supercritical accretion discs around non-spinning black holes

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Narayan, Ramesh

    2015-11-01

    We describe a set of simulations of supercritical accretion on to a non-rotating supermassive black hole (BH). The accretion flow takes the form of a geometrically thick disc with twin low-density funnels around the rotation axis. For accretion rates {gtrsim } 10 dot{M}_Edd, there is sufficient gas in the funnel to make this region optically thick. Radiation from the disc first flows into the funnel, after which it accelerates the optically thick funnel gas along the axis. The resulting jet is baryon loaded and has a terminal density-weighted velocity ≈0.3c. Much of the radiative luminosity is converted into kinetic energy by the time the escaping gas becomes optically thin. These jets are not powered by BHrotation or magnetic driving, but purely by radiation. Their characteristic beaming angle is ˜0.2 rad. For an observer viewing down the axis, the isotropic equivalent luminosity of total energy is as much as 1048 erg s- 1 for a 107 M⊙ BH accreting at 103 Eddington. Therefore, energetically, the simulated jets are consistent with observations of the most powerful tidal disruption events, e.g. Swift J1644. The jet velocity is, however, too low to match the Lorentz factor γ > 2 inferred in J1644. There is no such conflict in the case of other tidal disruption events. Since favourably oriented observers see isotropic equivalent luminosities that are highly super-Eddington, the simulated models can explain observations of ultraluminous X-ray sources, at least in terms of luminosity and energetics, without requiring intermediate-mass BHs.

  20. Numerical modeling of the subduction initiation after accretion of oceanic island.

    NASA Astrophysics Data System (ADS)

    Simakin, A.

    2012-04-01

    Accretion of the large terrains leads to the temporal blockage of subduction and accumulation of the oceanic slab material. New subduction front started in the thickened contact zone. This process is modeled numerically in 2D. We apply constant velocity condition at the inlet vertical boundary. Another vertical boundary is treated as a free slide one, Winkler boundary condition is applied to the lower boundary. Sticky air used to represent stress free upper boundary. Scenario of the new front initiation depends on the assumed rheology of oceanic slab and docked island. At the application of the purely viscous rheology of all components of the system: island viscosity 1023 Pas, upper mantle viscosity 3·1019 Pas and slab viscosity in the range 6·1020-6·1022 Pas, we find a variety of dynamic styles. At low slab viscosity in the time scale of several millions years plate is thickened and experienced RTI instability. New subduction front is started after plate break up near the island edge. At the more realistic η=6·1022 Pas oceanic slab is folded before plunging into the mantle. In the intermediate range thickening of the oceanic slab takes place with low angle subduction followed by accelerated submergence of the widen slab tip. Too large time of the transient process and too large scale of oceanic slab accumulation contradict to the observations. Visco-plastic rheology of the crustal rocks brings model closer to the real world. At the early stage of deformation conjugate "viscous faults" form in the oceanic slab in respond to the shortening. Later on sliding along these faults doubles oceanic plate thickness at the contact with docked island. Permanent fault (with dip away from island) was created to accommodate bending of oceanic plate. Thickened plate tip starts to descend with low angle of ca 35o. The most important observation is breakage of island edge that is carried downward with subducted oceanic plate. We compare our results with data on the current

  1. THE LARGE-SCALE MAGNETIC FIELDS OF THIN ACCRETION DISKS

    SciTech Connect

    Cao Xinwu; Spruit, Hendrik C. E-mail: henk@mpa-garching.mpg.de

    2013-03-10

    Large-scale magnetic field threading an accretion disk is a key ingredient in the jet formation model. The most attractive scenario for the origin of such a large-scale field is the advection of the field by the gas in the accretion disk from the interstellar medium or a companion star. However, it is realized that outward diffusion of the accreted field is fast compared with the inward accretion velocity in a geometrically thin accretion disk if the value of the Prandtl number P{sub m} is around unity. In this work, we revisit this problem considering the angular momentum of the disk to be removed predominantly by the magnetically driven outflows. The radial velocity of the disk is significantly increased due to the presence of the outflows. Using a simplified model for the vertical disk structure, we find that even moderately weak fields can cause sufficient angular momentum loss via a magnetic wind to balance outward diffusion. There are two equilibrium points, one at low field strengths corresponding to a plasma-beta at the midplane of order several hundred, and one for strong accreted fields, {beta} {approx} 1. We surmise that the first is relevant for the accretion of weak, possibly external, fields through the outer parts of the disk, while the latter one could explain the tendency, observed in full three-dimensional numerical simulations, of strong flux bundles at the centers of disk to stay confined in spite of strong magnetororational instability turbulence surrounding them.

  2. GRAVITATIONAL ACCRETION OF PARTICLES ONTO MOONLETS EMBEDDED IN SATURN's RINGS

    SciTech Connect

    Yasui, Yuki; Ohtsuki, Keiji; Daisaka, Hiroshi E-mail: ohtsuki@tiger.kobe-u.ac.jp

    2014-12-20

    Using a local N-body simulation, we examine gravitational accretion of ring particles onto moonlet cores in Saturn's rings. We find that gravitational accretion of particles onto moonlet cores is unlikely to occur in the C ring and probably difficult in the inner B ring as well provided that the cores are rigid water ice. Dependence of particle accretion on ring thickness changes when the radial distance from the planet and/or the density of particles is varied: the former determines the size of the core's Hill radius relative to its physical size, while the latter changes the effect of self-gravity of accreted particles. We find that particle accretion onto high-latitude regions of the core surface can occur even if the rings' vertical thickness is much smaller than the core radius, although redistribution of particles onto the high-latitude regions would not be perfectly efficient in outer regions of the rings such as the outer A ring, where the size of the core's Hill sphere in the vertical direction is significantly larger than the core's physical radius. Our results suggest that large boulders recently inferred from observations of transparent holes in the C ring are not formed locally by gravitational accretion, while propeller moonlets in the A ring would be gravitational aggregates formed by particle accretion onto dense cores. Our results also imply that the main bodies of small satellites near the outer edge of Saturn's rings may have been formed in rather thin rings.

  3. Fingering Convection and its Consequences for Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Vauclair, Sylvie; Vauclair, Gérard; Deal, Morgan; Wachlin, F. C.

    2015-06-01

    A number of white dwarf stars show absoption lines of heavy elements in their spectra. Many of them also exhibit infra-red excess in their spectral energy distribution. These observations prove that these white dwarfs are surrounded by an orbiting debris disk resulting from the disruption of rocky planetesimals, remnants of the primordial planetary system. Part of the material from the debris disk is accreted onto the white dwarfs, explaining the presence of heavy elements in their outer layers. Previous attempts to estimate the accretion rates have overlooked the importance of the fingering convection. The fingering convection is an instability triggered by the accumulation in the white dwarf outer layers of material heavier than the underlying H-rich (for the DA) or the He-rich (for the DB) composition. The fingering convection induces a deep mixing of the accreted material. Our preliminary simulations of the fingering convection show that the effect may be important in DA white dwarfs. The accretion rates needed in order to reproduce the observed heavy element abundances exceed by order of magnitudes the accretion rates estimated when this extra-mixing is ignored. By contrast, in the cases of the DB white dwarfs that we have considered in our simulations the fingering convection either does not occur or has very little effects on the derived accretion rates. We have undertaken a systematic exploration of the consequences of the fingering convection in accreting white dwarfs.

  4. Taking centre stage...

    NASA Astrophysics Data System (ADS)

    1998-11-01

    HAMLET (Highly Automated Multimedia Light Enhanced Theatre) was the star performance at the recent finals of the `Young Engineer for Britain' competition, held at the Commonwealth Institute in London. This state-of-the-art computer-controlled theatre lighting system won the title `Young Engineers for Britain 1998' for David Kelnar, Jonathan Scott, Ramsay Waller and John Wyllie (all aged 16) from Merchiston Castle School, Edinburgh. HAMLET replaces conventional manually-operated controls with a special computer program, and should find use in the thousands of small theatres, schools and amateur drama productions that operate with limited resources and without specialist expertise. The four students received a £2500 prize between them, along with £2500 for their school, and in addition they were invited to spend a special day with the Royal Engineers. A project designed to improve car locking systems enabled Ian Robinson of Durham University to take the `Working in industry award' worth £1000. He was also given the opportunity of a day at sea with the Royal Navy. Other prizewinners with their projects included: Jun Baba of Bloxham School, Banbury (a cardboard armchair which converts into a desk and chair); Kobika Sritharan and Gemma Hancock, Bancroft's School, Essex (a rain warning system for a washing line); and Alistair Clarke, Sam James and Ruth Jenkins, Bishop of Llandaff High School, Cardiff (a mechanism to open and close the retractable roof of the Millennium Stadium in Cardiff). The two principal national sponsors of the competition, which is organized by the Engineering Council, are Lloyd's Register and GEC. Industrial companies, professional engineering institutions and educational bodies also provided national and regional prizes and support. During this year's finals, various additional activities took place, allowing the students to surf the Internet and navigate individual engineering websites on a network of computers. They also visited the

  5. Mergers of accreting stellar-mass black holes

    NASA Astrophysics Data System (ADS)

    Tagawa, H.; Umemura, M.; Gouda, N.

    2016-11-01

    We present post-Newtonian N-body simulations on mergers of accreting stellar-mass black holes (BHs), where such general relativistic effects as the pericentre shift and gravitational wave (GW) emission are taken into consideration. The attention is concentrated on the effects of the dynamical friction and the Hoyle-Lyttleton mass accretion by ambient gas. We consider a system composed of 10 BHs with initial mass of 30 M⊙. As a result, we show that mergers of accreting stellar-mass BHs are classified into four types: a gas drag-driven, an interplay-driven, a three-body-driven, or an accretion-driven merger. We find that BH mergers proceed before significant mass accretion, even if the accretion rate is ˜10 Eddington accretion rate, and then all BHs can merge into one heavy BH. Using the simulation results for a wide range of parameters, we derive a critical accretion rate (dot{m}_c), below which the BH growth is promoted faster by mergers. Also, it is found that the effect of the recoil by the GW emission can reduce dot{m}_c especially in gas number density higher than 108 cm-3, and enhance the escape probability of merged BHs. Very recently, a gravitational wave event, GW150914, as a result of the merger of a ˜30 M⊙ BH binary has been detected. Based on the present simulations, the BH merger in GW150914 is likely to be driven by three-body encounters accompanied by a few M⊙ of gas accretion, in high-density environments like dense interstellar clouds or galactic nuclei.

  6. ACCRETION RATE AND THE PHYSICAL NATURE OF UNOBSCURED ACTIVE GALAXIES

    SciTech Connect

    Trump, Jonathan R.; Impey, Christopher D.; Gabor, Jared M.; Diamond-Stanic, Aleksandar M.; Kelly, Brandon C.; Civano, Francesca; Hao, Heng; Lanzuisi, Giorgio; Merloni, Andrea; Salvato, Mara; Urry, C. Megan; Jahnke, Knud; Nagao, Tohru; Taniguchi, Yoshi; Koekemoer, Anton M.; Liu, Charles; Mainieri, Vincenzo; Scoville, Nick Z.

    2011-05-20

    We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rates by using accurate intrinsic accretion luminosities (L{sub int}) from well-sampled multiwavelength spectral energy distributions from the Cosmic Evolution Survey, and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L{sub int}/L{sub Edd} > 10{sup -2}), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L{sub int}/L{sub Edd} < 10{sup -2}) are unobscured and yet lack a broad-line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L{sub int}/L{sub Edd} < 10{sup -2} narrow-line and lineless AGNs to have ratios of radio-to-optical/UV emission that are 10 times higher than L{sub int}/L{sub Edd} > 10{sup -2} broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L{sub int}/L{sub Edd} < 10{sup -2} AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together, these results suggest that specific accretion rate is an important physical 'axis' of AGN unification, as described by a simple model.

  7. Creating Sacred Places for Students in Grades 9-12.

    ERIC Educational Resources Information Center

    Fox, Sandra J.

    This guide attempts to help teachers of American Indian students in grades 9-12 provide a culturally relevant education that takes place in the regular classroom, includes content related to Indian students' lives, makes students proud, expands to other experiences, and enhances learning. Creating sacred places means responding appropriately to…

  8. Creating Sacred Places for Students in Grades 7&8.

    ERIC Educational Resources Information Center

    Fox, Sandra J.

    This guide attempts to help teachers of American Indian students in grades 7-8 provide a culturally relevant education that takes place in the regular classroom, includes content related to Indian students' lives, makes students proud, expands to other experiences, and enhances learning. Creating sacred places means responding appropriately to…

  9. Creating Sacred Places for Children in Grades 4-6.

    ERIC Educational Resources Information Center

    Fox, Sandra J.

    This guide attempts to help teachers of American Indian children in grades 4-6 provide a culturally relevant education that takes place in the regular classroom, includes content related to Indian students' lives, makes students proud, expands to other experiences, and enhances learning. Creating sacred places means responding appropriately to…

  10. LAMBDA BOO ABUNDANCE PATTERNS: ACCRETION FROM ORBITING SOURCES

    SciTech Connect

    Jura, M.

    2015-12-15

    The abundance anomalies in λ Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically inferred bounds for interstellar accretion. Therefore, a λ Boo star’s thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some λ Boo stars accrete from the winds of hot Jupiters.

  11. GRMHD simulations of black hole accretion and jets

    NASA Astrophysics Data System (ADS)

    Tchekhovskoy, Alexander

    2014-03-01

    As black holes accrete surrounding gas, they often produce relativistic, collimated outflows, or jets. Jets are expected to form in the vicinity of a black hole, making them powerful probes of strong-field gravity. However, how the properties of a jet connect to those of the accretion flow and the black hole (e.g. black hole spin) remains an area of active research. I will discuss recent progress in first-principles general relativistic magnetohydrodynamic (GRMHD) models of black hole accretion-jet systems, specifically the emerging picture of how jets form and the factors that determine jet properties. The speaker is supported by NASA through Einstein Postdoctoral Fellowship.

  12. Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana

    SciTech Connect

    Hatton, R.S.; DeLaune, R.D.; Patrick, W.H. Jr.

    1983-05-01

    Vertical accretion and sediment accumulation rates were determined from the distribution of /sup 137/Cs in cores collected from fresh water, intermediate, brackish, and salt marshes in the Barataria Basin, Louisiana. Vertical accretion rates vary from about 1.3 cm.yr/sup -1/ in levee areas to 0.7 in backmarshes. Mineral sediment content of the marsh soil profile decreased with distance from the coast. Except in natural levee areas, marsh accretion rates are less than subsidence measured by water level data, however this alone cannot account for observed land-loss patterns in the basin area.

  13. Recent Observational Progress on Accretion Disks Around Compact Objects

    NASA Astrophysics Data System (ADS)

    Miller, Jon M.

    2016-04-01

    Studies of accretion disks around black holes and neutron stars over the last ten years have made remarkable progress. Our understanding of disk evolution as a function of mass accretion rate is pushing toward a consensus on thin/thick disk transitions; an apparent switching between disk-driven outflow modes has emerged; and monitoring observations have revealed complex spectral energy distributions wherein disk reprocessing must be important. Detailed studies of disk winds, in particular, have the potential to reveal the basic physical processes that mediate disk accretion, and to connect with numerical simulations. This talk will review these developments and look ahead to the potential of Astro-H.

  14. Lambda Boo Abundance Patterns: Accretion from Orbiting Sources

    NASA Astrophysics Data System (ADS)

    Jura, M.

    2015-12-01

    The abundance anomalies in λ Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically inferred bounds for interstellar accretion. Therefore, a λ Boo star’s thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some λ Boo stars accrete from the winds of hot Jupiters.

  15. Does mass accretion lead to field decay in neutron stars

    NASA Technical Reports Server (NTRS)

    Shibazaki, N.; Murakami, T.; Shaham, Jacob; Nomoto, K.

    1989-01-01

    The recent discovery of cyclotron lines from gamma-ray bursts indicates that the strong magnetic fields of isolated neutron stars might not decay. The possible inverse correlation between the strength of the magnetic field and the mass accreted by the neutron star suggests that mass accretion itself may lead to the decay of the magnetic field. The spin and magnetic field evolution of the neutron star was calculated under the hypothesis of the accretion-induced field decay. It is shown that the calculated results are consistent with the observations of binary and millisecond radio pulsars.

  16. Accretion driven outflows across the black hole mass scale

    NASA Astrophysics Data System (ADS)

    King, Ashley L.

    2016-04-01

    Pumping highly relativistic particles and radiation into their environment, accreting black holes co-evolve with their surroundings through their powerful outflows. These outflows are divided into highly collimated, relativistic jets and wide-angle winds, and are primarily associated with a particular accretion states. Understanding just how these outflows couple to the accretion flow will enable us to assess the amount of energy and feedback that is injected into the vicinity of a black hole. During this talk, I will discuss our studies of both stellar-mass and supermassive black hole outlfows, and how the similarities of these flows across the mass scale may point to common driving mechanisms.

  17. Tearing up a misaligned accretion disc with a binary companion

    NASA Astrophysics Data System (ADS)

    Doğan, Suzan; Nixon, Chris; King, Andrew; Price, Daniel J.

    2015-05-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. We calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. We run hydrodynamical simulations to check these results, and confirm that disc breaking is widespread and generally enhances accretion on to the central object. This applies in many cases of astrophysical accretion, e.g. supermassive black hole binaries and X-ray binaries.

  18. Optically thin accretion disks in the Kerr metric

    NASA Technical Reports Server (NTRS)

    Bjornsson, G.

    1995-01-01

    We calculate the structure of an effectively optically thin and geometrically thin accretion disk in the Kerr geometry, including electron-positron pairs. We show that the properties of the disk solutions are strongly dependent on the angular momentum of the central black hole. We find that close to a rapidly rotating hole there can be an appreciable pair density even for modest accretion rates. Pair critical accretion rates recently discovered in Newtonian disk models are also shown to be present in the general relativistic models, and we show that the geometrically thin disk approximations easily break down for rapidly rotating holes.

  19. Accreting binary population synthesis and feedback prescriptions

    NASA Astrophysics Data System (ADS)

    Fragos, Tassos

    2016-04-01

    Studies of extagalactic X-ray binary populations have shown that the characteristics of these populations depend strongly on the characteristics of the host galaxy's parent stellar population (e.g. star-formation history and metallicity). These dependencies not only make X-ray binaries promising for aiding in the measurement of galaxy properties themselves, but they also have important astrophysical and cosmological implications. For example, due to the relatively young stellar ages and primordial metallicities in the early Universe (z > 3), it is predicted that X-ray binaries were more luminous than today. The more energetic X-ray photons, because of their long mean-free paths, can escape the galaxies where they are produced, and interact at long distances with the intergalactic medium. This could result in a smoother spatial distribution of ionized regions, and more importantly in an overall warmer intergalactic medium. The energetic X-ray photons emitted from X-ray binaries dominate the X-ray radiation field over active galactic nuclei at z > 6 - 8, and hence Χ-ray binary feedback can be a non-negligible contributor to the heating and reionization of the inter-galactic medium in the early universe. The spectral energy distribution shape of the XRB emission does not change significantly with redshift, suggesting that the same XRB subpopulation, namely black-hole XRBs in the high-soft state, dominates the cumulative emission at all times. On the contrary, the normalization of the spectral energy distribution does evolve with redshift. To zeroth order, this evolution is driven by the cosmic star-formation rate evolution. However, the metallicity evolution of the universe and the mean stellar population age are two important factors that affect the X-ray emission from high-mass and low-mass XRBs, respectively. In this talk, I will review recent studies on the potential feedback from accreting binary populations in galactic and cosmological scales. Furthermore, I

  20. WIND-ACCRETION DISKS IN WIDE BINARIES, SECOND-GENERATION PROTOPLANETARY DISKS, AND ACCRETION ONTO WHITE DWARFS

    SciTech Connect

    Perets, Hagai B.; Kenyon, Scott J.

    2013-02-20

    Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long-term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind accretion leads to long-lived stable disks over the lifetime of the asymptotic giant branch donor star. The disks have masses of a few times 10{sup -5}-10{sup -3} M {sub Sun }, with surface density and temperature profiles that follow broken power laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly, 50%-80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M {sub Sun }. When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks, suggesting that coordinated observational programs might improve our understanding of uncertain disk physics.

  1. Taking the Long View

    ERIC Educational Resources Information Center

    Bennett, Robert B., Jr.

    2010-01-01

    Legal studies faculty need to take the long view in their academic and professional lives. Taking the long view would seem to be a cliched piece of advice, but too frequently legal studies faculty, like their students, get focused on meeting the next short-term hurdle--getting through the next class, grading the next stack of papers, making it…

  2. Ultrasonic techniques for aircraft ice accretion measurement

    NASA Technical Reports Server (NTRS)

    Hansman, R. John, Jr.; Kirby, Mark S.; Lichtenfelts, Fred

    1990-01-01

    Results of tests to measure ice growth in natural (flight) and artificial (icing wind tunnel) icing conditions are presented. Ice thickness is measured using an ultrasonic pulse-echo technique. Two icing regimes, wet and dry ice growth, are identified and the unique ultrasonic signal characteristics associated with these different types of ice growth are described. Ultrasonic measurements of ice growth on cylinders and airfoils exposed to artificial and natural icing conditions are presented. An accuracy of plus or minus 0.5 mm is achieved for ice thickness measurement using the pulse-echo technique. The performance of two-probe type ice detectors is compared to the surface mounted ultrasonic system. The ultrasonically measured ice accretion rates and ice surface condition (wet or dry) are used to compare the heat transfer characteristics for flight and icing wind tunnel environments. In general the heat transfer coefficient is inferred to be higher in the wind tunnel environment, not likely due to higher freestream turbulence levels. Finally, preliminary results of tests to measure ice growth on airfoil using an array of ultrasonic transducers are described. Ice profiles obtained during flight in natural icing conditions are shown and compared with mechanical and stereo image measurements.

  3. Measurement and Correlation of Ice Accretion Roughness

    NASA Technical Reports Server (NTRS)

    Anderson, David N.; Hentschel, Daniel B.; Ruff, Gary A.

    2003-01-01

    Measurements were taken of the roughness characteristics of ice accreted on NACA 0012 airfoils in the NASA Glenn Icing Research Tunnel (IRT). Tests were conducted with size scaled, using models with chords of 26.7, 53.3, and 80.0 cm, and with liquid-water content scaled, both according to previously-tested scaling methods. The width of the smooth zone which forms on either side of the leading edge of the airfoil and the diameter of the roughness elements are presented in non-dimensional form as functions of the accumulation parameter. The smooth-zone width was found to decrease with increasing accumulation parameter. The roughness-element diameter increased with accumulation parameter until a plateau was reached. This maximum diameter was about 0.06 times twice the model leading-edge radius. Neither smooth-zone width nor element diameter were affected by a change in freezing fraction from 0.2 to 0.4. Both roughness characteristics appeared to scale with model size and with liquid-water content.

  4. Stability of MRI-turbulent Accretion Disks

    NASA Astrophysics Data System (ADS)

    Takahashi, Hiroyuki R.; Masada, Youhei

    2011-02-01

    Based on the characteristics of the magnetorotational instability (MRI) and the MRI-driven turbulence, we construct a steady model for a geometrically thin disk using "non-standard" α-prescription. The efficiency of the angular momentum transport depends on the magnetic Prandtl number, Pm = ν/η, where ν and η are the microscopic viscous and magnetic diffusivities. In our disk model, Shakura-Sunyaev's α-parameter has a power-law dependence on the magnetic Prandtl number, that is α vprop Pm δ, where δ is the constant power-law index. Adopting Spitzer's microscopic diffusivities, the magnetic Prandtl number becomes a decreasing function of the disk radius when δ>0. The transport efficiency of the angular momentum and the viscous heating rate are thus smaller in the outer part of the disk, while these are impacted by the size of index δ. We find that the disk becomes more unstable to the gravitational instability for a larger value of index δ. The most remarkable feature of our disk model is that the thermal and secular instabilities can grow in its middle part even if the radiation pressure is negligibly small in the condition δ>2/3. In the realistic disk systems, it would be difficult to maintain the steady mass accretion state unless the Pm dependence of MRI-driven turbulence is relatively weak.

  5. Universal Accretion Growth Using Sandpile Models

    NASA Astrophysics Data System (ADS)

    Datta, Srabani; McKie, Shane; Spencer, Ralph

    2015-08-01

    The Bak-Tang- Wiesenfeld (BTW) sandpile process is a model of a complex dynamical system with a large collection of particles or grains in a node that sheds load to their neighbours when they reach capacity. The cascades move around thesystem till it reaches stability with a critical point as an attractor. The BTW growth process shows self-organized criticality (SOC) with power-law distribution in cascade sizes having slope -5/3. This self-similarity of structureis synonymous with the fractal structure found in molecular clouds of Kolmogorov dimension 1.67 and by treating cascades as waves, scaling functions are found to be analogous to those observed for velocity structure functions influid turbulence. We apply the BTW sandpile model to study growth on a 2 dimensional rotating lattice in a magnetic field. In this paper, we show that this is a naturally occuring universal process giving rise to scale-freestructures with size limited only by the number of infalling grains. We also compare the BTW process with other sandpile models such as the Manna and Zhang processes. We find that the BTW sandpile model can be applied to a widerange of objects including molecular clouds, accretion disks and perhaps galaxies.

  6. Viscosity Prescription for Gravitationally Unstable Accretion Disks

    NASA Astrophysics Data System (ADS)

    Rafikov, Roman R.

    2015-05-01

    Gravitationally unstable accretion disks emerge in a variety of astrophysical contexts—giant planet formation, FU Orioni outbursts, feeding of active galactic nuclei, and the origin of Pop III stars. When a gravitationally unstable disk is unable to cool rapidly, it settles into a quasi-stationary, fluctuating gravitoturbulent state, in which its Toomre Q remains close to a constant value {{Q}0}∼ 1. Here we develop an analytical formalism describing the evolution of such a disk, which is based on the assumptions of Q={{Q}0} and local thermal equilibrium. Our approach works in the presence of additional sources of angular momentum transport (e.g., MRI), as well as external irradiation. Thermal balance dictates a unique value of the gravitoturbulent stress {{α }gt} driving disk evolution, which is a function of the local surface density and angular frequency. We compare this approach with other commonly used gravitoturbulent viscosity prescriptions, which specify the explicit dependence of stress {{α }gt} on Toomre Q in an ad hoc fashion, and identify the ones that provide consistent results. We nevertheless argue that our Q={{Q}0} approach is more flexible, robust, and straightforward and should be given preference in applications. We illustrate this with a couple of analytical calculations—locations of the snow line and of the outer edge of the dead zone in a gravitoturbulent protoplanetary disk—which clearly show the simplicity and versatility of the Q={{Q}0} approach.

  7. Accreting Binary Populations in the Earlier Universe

    NASA Technical Reports Server (NTRS)

    Hornschemeier, Ann

    2010-01-01

    It is now understood that X-ray binaries dominate the hard X-ray emission from normal star-forming galaxies. Thanks to the deepest (2-4 Ms) Chandra surveys, such galaxies are now being studied in X-rays out to z approximates 4. Interesting X-ray stacking results (based on 30+ galaxies per redshift bin) suggest that the mean rest-frame 2-10 keV luminosity from z=3-4 Lyman break galaxies (LBGs), is comparable to the most powerful starburst galaxies in the local Universe. This result possibly indicates a similar production mechanism for accreting binaries over large cosmological timescales. To understand and constrain better the production of X-ray binaries in high-redshift LBGs, we have utilized XMM-Newton observations of a small sample of z approximates 0.1 GALEX-selected Ultraviolet-Luminous Galaxies (UVLGs); local analogs to high-redshift LBGs. Our observations enable us to study the X-ray emission from LBG-like galaxies on an individual basis, thus allowing us to constrain object-to-object variances in this population. We supplement these results with X-ray stacking constraints using the new 3.2 Ms Chandra Deep Field-South (completed spring 2010) and LBG candidates selected from HST, Swift UVOT, and ground-based data. These measurements provide new X-ray constraints that sample well the entire z=0-4 baseline

  8. Magnetohydrodynamic stability of stochastically driven accretion flows.

    PubMed

    Nath, Sujit Kumar; Mukhopadhyay, Banibrata; Chattopadhyay, Amit K

    2013-07-01

    We investigate the evolution of magnetohydrodynamic (or hydromagnetic as coined by Chandrasekhar) perturbations in the presence of stochastic noise in rotating shear flows. The particular emphasis is the flows whose angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows, however, are Rayleigh stable but must be turbulent in order to explain astrophysical observed data and, hence, reveal a mismatch between the linear theory and observations and experiments. The mismatch seems to have been resolved, at least in certain regimes, in the presence of a weak magnetic field, revealing magnetorotational instability. The present work explores the effects of stochastic noise on such magnetohydrodynamic flows, in order to resolve the above mismatch generically for the hot flows. We essentially concentrate on a small section of such a flow which is nothing but a plane shear flow supplemented by the Coriolis effect, mimicking a small section of an astrophysical accretion disk around a compact object. It is found that such stochastically driven flows exhibit large temporal and spatial autocorrelations and cross-correlations of perturbation and, hence, large energy dissipations of perturbation, which generate instability. Interestingly, autocorrelations and cross-correlations appear independent of background angular velocity profiles, which are Rayleigh stable, indicating their universality. This work initiates our attempt to understand the evolution of three-dimensional hydromagnetic perturbations in rotating shear flows in the presence of stochastic noise.

  9. Magnetic flux stabilizing thin accretion discs

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander

    2016-10-01

    We calculate the minimal amount of large-scale poloidal magnetic field that has to thread the inner, radiation-over-gas pressure dominated region of a thin disc for its thermal stability. Such a net field amplifies the magnetization of the saturated turbulent state and makes it locally stable. For a 10 M⊙ black hole the minimal magnetic flux is 10^{24}(dot{M}/dot{M}_Edd)^{20/21} G cm2. This amount is compared with the amount of uniform magnetic flux that can be provided by the companion star - estimated to be in the range 1022-1024 G cm2. If accretion rate is large enough, the companion is not able to provide the required amount and such a system, if still sub-Eddington, must be thermally unstable. The peculiar variability of GRS 1915+105, an X-ray binary with the exceptionally high BH mass and near-Eddington luminosity, may result from the shortage of large-scale poloidal field of uniform polarity.

  10. Hydraulic jumps in 'viscous' accretion disks

    NASA Astrophysics Data System (ADS)

    Michel, F. C.

    1984-04-01

    It is proposed that the dissipative process necessary for rapid accretion disk evolution is driven by hydraulic jump waves on the surface of the disk. These waves are excited by the asymmetric nature of the central rotator (e.g., neutron star magnetosphere) and spiral out into the disk to form a pattern corotating with the central object. Disk matter in turn is slowed slightly at each encounter with the jump and spirals inward. In this process, the disk is heated by true turbulence produced in the jumps. Additional effects, such as a systematic misalignment of the magnetic moment of the neutron star until it is nearly orthogonal, and systematic distortion of the magnetosphere in such a way as to form an even more asymmetric central 'paddle wheel', may enhance the interaction with inflowing matter. The application to X-ray sources corresponds to the 'slow' solutions of Ghosh and Lamb, and therefore to rms magnetic fields of about 4 x 10 to the 10th gauss. Analogous phenomena have been proposed to act in the formation of galactic spiral structure.

  11. Outflows from Thick, Turbulent Accretion in High Accretion-Rate Protostellar Systems

    NASA Astrophysics Data System (ADS)

    Williams, Peter T.

    2012-01-01

    In previous work we argued that jets might not be produced through magnetocentrifugal acceleration, but rather through the toroidal stresses of magnetorotational instability (MRI)-driven turbulent magnetohydrodynamic (MHD) accretion in a geometrically thick disk or flow. High accretion-rate protostars are among the best candidates for this process because a geometrically thick accretion disk that extends down to the central star is more plausible in this context than in other protostellar systems. These systems are also cleaner objects to study than active galactic nuclei (AGN), microquasars, and the like, which necessarily involve more exotic relativistic physics. Here we present a novel in-depth analysis of the laboratory analog that inspired our work on this topic. This analog consists of the meridional flow around a rotating sphere in a viscoelastic fluid. We examine in detail the fluxes of mass, angular momentum, linear momentum, and energy, and how these depend upon system parameters. We find that the presence of an axial outflow depends critically upon the ratio of the analogous turbulent magnetic stress to turbulent Reynolds stress, which must be roughly equal to or larger than unity to drive an outflow. We also find that the flux of angular momentum can actually be opposed to the flow of matter within the outflow, despite the fact that the outflow is ultimately powered by the radial transport of angular momentum from the central object. In particular we show that, in contrast with magnetocentrifugal acceleration, the angular momentum of the outflow actually decreases even while the material is being axially accelerated. This translates to observational tests in protostellar jets. There are some obvious enormous differences between compressible flow in a gravitational field and incompressible flow in a laboratory. We address this and we and argue why, despite these differences, there is much to learn from this laboratory system.

  12. Pouring 'Cold Water' on Hot Accretion

    NASA Astrophysics Data System (ADS)

    Rubin, A. E.

    1995-09-01

    The extensive recrystallization of type-6 OC has been interpreted as having resulted either from prograde thermal metamorphism of initially cold, unequilibrated material [1,2] or from autometamorphism due to slow cooling of material that accreted while still hot (1000-1200 K). Although the physical implausibility of hot accretion has been addressed [3], no comprehensive evaluation has been made of arguments in its favor. As shown below, these arguments are based on incomplete data, flawed experiments or improbable interpretations. Correlation between petrologic type and Ca in low-Ca pyroxene. Models of prograde metamorphism assume that, with increasing temperature, opx acquires Ca at the expense of diopside. Analyses of pyroxene in 10 H chondrites showed no correlation between Ca in pyroxene cores and increasing petrologic type [4], but more extensive data sets show such correlations [1,5,6]. A review of data for 51 OC [7] shows a progressive increase in the Wo content of low-Ca pyroxene with petrologic type: Wo 0.4-1.2 in type-3 and -4; Wo 1.2-1.6 in type-5; and Wo 1.6-2.2 in type-6. Striated opx. Undeformed striated opx were interpreted as having formed from inverted protopyroxene during slow cooling [8]; striated opx from H4 Quenggouk were found to convert into normal opx within 1 week during annealing at 1100 K [9]. Because prograde metamorphism probably lasted ~60 Ma [10], there should be no striated opx remaining in type-4 or -5 OC. However, samples of 99% twinned clinopyroxene (analogous to that in chondrules in type-3 OC) annealed for >3 weeks at <=1250 K exhibited only very minor inversion to opx [11-13]. These experiments are consistent with prograde metamorphism; it seems likely that Quenggouk pyroxene probably had a substantial proportion of opx lamellae to begin with. Spinodal decomposition textures and cooling rates. Spinodal decomposition textures in pyroxene in type 4-5 OC were observed to have the same periodicities as those in type-3 OC [14]; it

  13. Silicon Isotopes in Achondrites and Planetary Accretion and Differentiation

    NASA Astrophysics Data System (ADS)

    Pringle, E. A.; Savage, P. S.; Badro, J.; Barrat, J.-A.; Moynier, F.

    2015-07-01

    We present new high-precision Si isotope data for an extended suite of achondrites to assess the processes affecting the Si isotope system during the accretion and early geochemical modification of planetesimals.

  14. Gas accretion from halos to disks: observations, curiosities, and problems

    NASA Astrophysics Data System (ADS)

    Elmegreen, Bruce G.

    2016-08-01

    Accretion of gas from the cosmic web to galaxy halos and ultimately their disks is a prediction of modern cosmological models but is rarely observed directly or at the full rate expected from star formation. Here we illustrate possible large-scale cosmic HI accretion onto the nearby dwarf starburst galaxy IC10, observed with the VLA and GBT. We also suggest that cosmic accretion is the origin of sharp metallicity drops in the starburst regions of other dwarf galaxies, as observed with the 10-m GTC. Finally, we question the importance of cosmic accretion in normal dwarf irregulars, for which a recent study of their far-outer regions sees no need for, or evidence of, continuing gas buildup.

  15. Dynamically important magnetic fields near accreting supermassive black holes.

    PubMed

    Zamaninasab, M; Clausen-Brown, E; Savolainen, T; Tchekhovskoy, A

    2014-06-01

    Accreting supermassive black holes at the centres of active galaxies often produce 'jets'--collimated bipolar outflows of relativistic particles. Magnetic fields probably play a critical role in jet formation and in accretion disk physics. A dynamically important magnetic field was recently found near the Galactic Centre black hole. If this is common and if the field continues to near the black hole event horizon, disk structures will be affected, invalidating assumptions made in standard models. Here we report that jet magnetic field and accretion disk luminosity are tightly correlated over seven orders of magnitude for a sample of 76 radio-loud active galaxies. We conclude that the jet-launching regions of these radio-loud galaxies are threaded by dynamically important fields, which will affect the disk properties. These fields obstruct gas infall, compress the accretion disk vertically, slow down the disk rotation by carrying away its angular momentum in an outflow and determine the directionality of jets.

  16. Spinning up black holes with super-critical accretion flows

    NASA Astrophysics Data System (ADS)

    Sądowski, A.; Bursa, M.; Abramowicz, M.; Kluźniak, W.; Lasota, J.-P.; Moderski, R.; Safarzadeh, M.

    2011-08-01

    We study the process of spinning up black holes by accretion from slim disks for a wide range of accretion rates. We show that for super-Eddington accretion rates and low values of the viscosity parameter α ( ≲ 0.01) the limiting value of the dimensionless spin parameter a∗ can reach values higher than a∗ = 0.9978 inferred by Thorne in his seminal study. For Ṁ = 10 ṀEdd and α = 0.01, spin equilibrium is reached at a∗ = 0.9994. We show that the equilibrium spin value depends strongly on the assumed value of α. We also prove that for high accretion rates the impact of captured radiation on spin evolution is negligible.

  17. Prospect of the Theory of Black Hole Accretion Disks

    NASA Astrophysics Data System (ADS)

    Ju, Q.; Wang, W.

    2011-12-01

    The theory of black hole accretion disks is one of the most important basic theories and advanced topics in astrophysics. There are four known models of black hole accretion disks: standard thin disk (SSD), Shapiro-Lightman-Eardley (SLE) disk, optically thick advection dominated accretion flows (ADAF) theory (slim disk) and optically thin ADAF theory. During the past decades, they have made great contributions to the development of astrophysics. Despite their great successes in both theory and application, there are still many open questions. First of all, this paper will briefly introduce these four models. Then it will discuss several limitations of these models and review present possible solutions to address these problems. Our review work will contribute to further research of black hole accretion disks to a certain extent.

  18. New Spectral State of Supercritical Accretion Flow with Comptonizing Outflow

    NASA Astrophysics Data System (ADS)

    Kawashima, Tomohisa; Ohsuga, Ken; Mineshige, Shin; Heinzeller, Dominikus; Takabe, Hideaki; Matsumoto, Ryoji

    2009-08-01

    Supercritical accretion flows inevitably produce radiation-pressure driven outflows, which Compton up-scatter soft photons from the underlying accretion flow, thereby making hard emission. We performed two-dimensional radiation hydrodynamic simulations of supercritical accretion flows and outflows, while incorporating such Compton scattering effects, and demonstrated that there appears a new hard spectral state at higher photon luminosities than that of the slim-disk state. In this state, as the photon luminosity increases, the photon index decreases and the fraction of the hard emission increases. The Compton y-parameter is on the order of unity (and thus the photon index will be ˜2) when the apparent photon luminosity is ˜30LE (with LE being the Eddington luminosity) for nearly face-on sources. This explains the observed spectral hardening of the ULX NGC 1313 X-2 in its brightening phase, and thus supports the model of supercritical accretion onto stellar-mass black holes in this ULX.

  19. X-ray radiation from accreting, magnetized neutron stars

    SciTech Connect

    Pavlov, G.G.

    1984-01-01

    A review is given of recent developments in the theory of emission from a magnetized plasma for accreting neutron star conditions. Some observational data on X-ray pulsars are discussed, and present problems are indicated. 26 references.

  20. A Systems-Level Perspective on Engine Ice Accretion

    NASA Technical Reports Server (NTRS)

    May, Ryan David; Guo, Ten-Huei; Simon, Donald L.

    2012-01-01

    Talk covers: (1) Problem of Engine Power Loss;(2) Modeling Engine Icing Effects; (3) Simulation of Engine Rollback; (4) Icing/Engine Control System Interaction; (5) Detection of Ice Accretion; (6) Potential Mitigation Strategies.

  1. Accretion of a ghost condensate by black holes

    SciTech Connect

    Frolov, Andrei V.

    2004-09-15

    The intent of this paper is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as a tenth of a solar mass per second for 10 MeV scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

  2. The magnetic nature of disk accretion onto black holes.

    PubMed

    Miller, Jon M; Raymond, John; Fabian, Andy; Steeghs, Danny; Homan, Jeroen; Reynolds, Chris; van der Klis, Michiel; Wijnands, Rudy

    2006-06-22

    Although disk accretion onto compact objects-white dwarfs, neutron stars and black holes-is central to much of high-energy astrophysics, the mechanisms that enable this process have remained observationally difficult to determine. Accretion disks must transfer angular momentum in order for matter to travel radially inward onto the compact object. Internal viscosity from magnetic processes and disk winds can both in principle transfer angular momentum, but hitherto we lacked evidence that either occurs. Here we report that an X-ray-absorbing wind discovered in an observation of the stellar-mass black hole binary GRO J1655 - 40 (ref. 6) must be powered by a magnetic process that can also drive accretion through the disk. Detailed spectral analysis and modelling of the wind shows that it can only be powered by pressure generated by magnetic viscosity internal to the disk or magnetocentrifugal forces. This result demonstrates that disk accretion onto black holes is a fundamentally magnetic process.

  3. Accretion Disk Dynamics in X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Schulz, Norbert S.; Ji, Li; Nowak, M.; Canizares, C. R.; Kallman, T.

    2009-09-01

    The last decade of X-ray observations was an era of true discovery in the study of accretion phenomena in X-ray binaries. With the launch of high resolution X-ray spectrometers on board the Chandra X-ray Observatory and XMM Newton we gained novel insights in feedback processes in accretion disks. At the forefront are dynamics in winds and outflows. Recent observations now also not only reveal properties of accretion disk coronal phenomena but point us to highly variable activity in their appearance. Amongst others these include heating along the spectral branches in the Z-source Cyg X-2, short and longterm variations in the photo-ionized emissions in Cir X-1, highly variable and dynamic Ne edges in the ultra-compact binary 4U 0614+091. This presentation summarizes these recent developments and provides an outlook towards more dynamical accretion disk coronal models and perspectives for future missions.

  4. Polarized X-rays from accreting neutron stars

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Dipankar

    2016-07-01

    Accreting neutron stars span a wide range in X-ray luminosity and magnetic field strength. Accretion may be wind-fed or disk-fed, and the dominant X-ray flux may originate in the disk or a magnetically confined accretion column. In all such systems X-ray polarization may arise due to Compton or Magneto-Compton scattering, and on some occasions polarization of non-thermal emission from jet-like ejection may also be detectable. Spectral and temporal behaviour of the polarized X-rays would carry information regarding the radiation process, as well as of the matter dynamics - and can assist the detection of effects such as the Lense-Thirring precession. This talk will review our current knowledge of the expected X-ray polarization from accreting neutron stars and explore the prospects of detection with upcoming polarimetry missions.

  5. Giant Planet Accretion in a Low-Turbulence Circumplanetary Disk

    NASA Astrophysics Data System (ADS)

    D'Angelo, Gennaro; Marzari, Francesco

    2014-06-01

    At least 5% of confirmed planets discovered by the Kepler Mission have a mass greater than Jupiter's. Gas giants more massive than Saturn account for at least 18% of all confirmed planets.The final stages of gas accretion of a giant planet occur in the presence of a circumplanetary disk (CPD). Recently, it was proposed that turbulence (and hence transport) in these disks is driven by MRI, possibly generating low-turbulence regions known as Dead Zones. It was thus suggested that gas accretion through a CPD and on the planet can be severely reduced by a Dead Zone. If CPDs create a bottleneck for the accretion of gas, then the growth of planets more massive than Jupiter may become problematic.We investigate how gas accretion on a Jupiter-mass planet is affected by a Dead Zone by means of global 3D hydrodynamics calculations. We model both the CPD and the protoplanetary disk. The accretion flow is resolved at a length scale smaller than Jupiter's radius, Rj, by using a nested-grid technique. We assume that the kinematic viscosity is constant and equal to nu=1e-5 Omega a^2, where a and Omega are respectively the planet's orbital radius and frequency. A Dead Zone around the planet is represented by a region of low viscosity (nu=1e-8 Omega a^2), extending out to ~60Rj and above and below the CPD mid-plane for a few local scale heights. We obtain an accretion rate of ~5e-5 Omega Sigma a^2, where Sigma is the unperturbed protoplanetary disk density. Calculations by D'Angelo et al. (2003) and Bate et al. (2003), which used nu=1e-5 Omega a^2 everywhere but applied a much coarser resolution and different accretion parameters, found an accretion rate of ~2e-4 Omega Sigma a^2. Accounting for variations of several tens of percent, arising from differences (between these and previous calculations) in numerical parameters and resolution, we argue that a CPD Dead Zone, as modeled here, does not significantly affect the gas accretion rate of a giant planet. This result is compatible

  6. Accreting planets as dust dams in 'transition' disks

    SciTech Connect

    Owen, James E.

    2014-07-01

    We investigate under what circumstances an embedded planet in a protoplanetary disk may sculpt the dust distribution such that it observationally presents as a 'transition' disk. We concern ourselves with 'transition' disks that have large holes (≳ 10 AU) and high accretion rates (∼10{sup –9}-10{sup –8} M {sub ☉} yr{sup –1}), particularly, those disks which photoevaporative models struggle to explain. Adopting the observed accretion rates in 'transition' disks, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small (s ≲ 1 μm) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disk with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a 'transition' disk (NIR dip in the spectral energy distribution (SED), millimeter cavity, and high accretion rate). At suitably high planet masses (≳ 3-4 M{sub J} ), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disk that is optically thin to infrared radiation. The planet-disk system will present as a 'transition' disk with a dip in the SED only when the planet mass and planetary accretion rate are high enough. At other times, it will present as a disk with a primordial SED, but with a cavity in the millimeter, as observed in a handful of protoplanetary disks.

  7. TLUSTY: Stellar Atmospheres, Accretion Disks, and Spectroscopic Diagnostics

    NASA Astrophysics Data System (ADS)

    Hubeny, Ivan; Lanz, Thierry

    2011-09-01

    TLUSTY is a user-oriented package written in FORTRAN77 for modeling stellar atmospheres and accretion disks and wide range of spectroscopic diagnostics. In the program's maximum configuration, the user may start from scratch and calculate a model atmosphere of a chosen degree of complexity, and end with a synthetic spectrum in a wavelength region of interest for an arbitrary stellar rotation and an arbitrary instrumental profile. The user may also model the vertical structure of annuli of an accretion disk.

  8. Continental accretion: From oceanic plateaus to allochthonous terranes

    USGS Publications Warehouse

    Ben-Avraham, Z.; Nur, A.; Jones, D.; Cox, A.

    1981-01-01

    Some of the regions of the anomalously high sea-floor topography in today's oceans may be modern allochthonous terranes moving with their oceanic plates. Fated to collide with and be accreted to adjacent continents, they may create complex volcanism, cut off and trap oceanic crust, and cause orogenic deformation. The accretion of plateaus during subduction of oceanic plates may be responsible for mountain building comparable to that produced by the collision of continents. Copyright ?? 1981 AAAS.

  9. Simulations of accretion disks in pseudo-complex General Relativity

    NASA Astrophysics Data System (ADS)

    Hess, P. O.; Algalán B., M.; Schönenbach, T.; Greiner, W.

    2015-11-01

    After a summary on pseudo-complex General Relativity (pc-GR), circular orbits and stable orbits in general are discussed, including predictions compared to observations. Using a modified version of a model for accretions disks, presented by Page and Thorne in 1974, we apply the raytracing technique in order to simulate the appearance of an accretion disk as it should be observed in a detector. In pc-GR we predict a dark ring near a very massive, rapidly rotating object.

  10. TW Hya: SPECTRAL VARIABILITY, X-RAYS, AND ACCRETION DIAGNOSTICS

    SciTech Connect

    Dupree, A. K.; Brickhouse, N. S.; Cranmer, S. R.; Luna, G. J. M.; Schneider, E. E.; Bessell, M. S.; Bonanos, A.; Crause, L. A.; Lawson, W. A.; Mallik, S. V.; Schuler, S. C.

    2012-05-01

    The nearest accreting T Tauri star, TW Hya was intensively and continuously observed over {approx}17 days with spectroscopic and photometric measurements from four continents simultaneous with a long segmented exposure using the Chandra satellite. Contemporaneous optical photometry from WASP-S indicates a 4.74 day period was present during this time. The absence of a similar periodicity in the H{alpha} flux and the total X-ray flux which are dominated by accretion processes and the stellar corona, respectively, points to a different source of photometric variations. The H{alpha} emission line appears intrinsically broad and symmetric, and both the profile and its variability suggest an origin in the post-shock cooling region. An accretion event, signaled by soft X-rays, is traced spectroscopically for the first time through the optical emission line profiles. After the accretion event, downflowing turbulent material observed in the H{alpha} and H{beta} lines is followed by He I ({lambda}5876) broadening near the photosphere. Optical veiling resulting from the heated photosphere increases with a delay of {approx}2 hr after the X-ray accretion event. The response of the stellar coronal emission to an increase in the veiling follows {approx}2.4 hr later, giving direct evidence that the stellar corona is heated in part by accretion. Subsequently, the stellar wind becomes re-established. We suggest a model that incorporates the dynamics of this sequential series of events: an accretion shock, a cooling downflow in a supersonically turbulent region, followed by photospheric and later, coronal heating. This model naturally explains the presence of broad optical and ultraviolet lines, and affects the mass accretion rates determined from emission line profiles.

  11. Black hole accretion discs and screened scalar hair

    NASA Astrophysics Data System (ADS)

    Davis, Anne-Christine; Gregory, Ruth; Jha, Rahul

    2016-10-01

    We present a novel way to investigate scalar field profiles around black holes with an accretion disc for a range of models where the Compton wavelength of the scalar is large compared to other length scales. By analysing the problem in ``Weyl" coordinates, we are able to calculate the scalar profiles for accretion discs in the static Schwarzschild, as well as rotating Kerr, black holes. We comment on observational effects.

  12. Orion Spacecraft Takes Shape

    NASA Video Gallery

    Technicians move the two halves of the Orion crew exploration vehicle's crew module into place to fuse them together at NASA's Michoud Assembly Facility in New Orleans, La. The Lockheed Martin Orio...

  13. Give/Take

    SciTech Connect

    2007-09-12

    Give and Take are set of companion utilities that allow a secure transfer of files from one user to another without exposing the files to third parties. The named files are copied to a spool area. The reciever can retrieve the files by running the "take" program. Ownership of the files remains with the giver until they are taken. Certain users may be limited to take files only from specific givers. For these users, files may only be taken from givers who are members of the gt-uid-group where uid is the UNIX id of the limited user.

  14. Give/Take

    2007-09-12

    Give and Take are set of companion utilities that allow a secure transfer of files from one user to another without exposing the files to third parties. The named files are copied to a spool area. The reciever can retrieve the files by running the "take" program. Ownership of the files remains with the giver until they are taken. Certain users may be limited to take files only from specific givers. For these users, filesmore » may only be taken from givers who are members of the gt-uid-group where uid is the UNIX id of the limited user.« less

  15. Continuous MHD Jet Launching from Resistive Accretion Disk

    NASA Astrophysics Data System (ADS)

    Casse, Fabien L.; Keppens, Rony

    We present numerical MHD simulations of a magnetized accretion disk launching super-fastmagnetosonic jets. These axisymmetric simulations model a time-dependant resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk and is prescribed as an alpha-type law where the alpha coefficient αm is smaller than unity. We show that the launching of a collimated outflow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the first ever 2.5D simulations of resistive accretion disks launching non-transient ideal MHD jets. This outflow is safely characterized as a jet since the flow becomes super-fastmagnetosonic well-collimated and reaches a quasi-stationary state. We present a complete illustration and explanation of the `accretion-ejection' mechanism that leads to jet formation from a magnetized accretion disk. In particular the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion while it is responsible for an effective magneto-centrifugal acceleration in the jet. As such the magnetic field channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved.

  16. Implementation and Validation of 3-D Ice Accretion Measurement Methodology

    NASA Technical Reports Server (NTRS)

    Lee, Sam; Broeren, Andy P.; Kreeger, Richard E.; Potapczuk, Mark; Utt, Lloyd

    2014-01-01

    A research program has been implemented to develop and validate the use of a commercial 3-D laser scanning system to record ice accretion geometry in the NASA Icing Research Tunnel. A main component of the program was the geometric assessment of the 3- D laser scanning system on a 2-D (straight wing) and a 3-D (swept wing) airfoil geometries. This exercise consisted of comparison of scanned ice accretion to castings of the same ice accretion. The scan data were also used to create rapid prototype artificial ice shapes that were scanned and compared to the original ice accretion. The results from geometric comparisons on the straight wing showed that the ice shape models generated through the scan/rapid prototype process compared reasonably well with the cast shapes. Similar results were obtained with the geometric comparisons on the swept wing. It was difficult to precisely compare the scans of the cast shapes to the original ice accretion scans because the cast shapes appear to have shrunk during the mold/casting process by as much as 0.10-inch. However the comparison of the local ice-shape features were possible and produced better results. The rapid prototype manufacturing process was shown to reproduce the original ice accretion scan normally within 0.01-inch.

  17. Freddi: Fast Rise Exponential Decay accretion Disk model Implementation

    NASA Astrophysics Data System (ADS)

    Lipunova, G. V.; Malanchev, K. L.

    2016-10-01

    Freddi (Fast Rise Exponential Decay: accretion Disk model Implementation) solves 1-D evolution equations of the Shakura-Sunyaev accretion disk. It simulates fast rise exponential decay (FRED) light curves of low mass X-ray binaries (LMXBs). The basic equation of the viscous evolution relates the surface density and viscous stresses and is of diffusion type; evolution of the accretion rate can be found on solving the equation. The distribution of viscous stresses defines the emission from the source. The standard model for the accretion disk is implied; the inner boundary of the disk is at the ISCO or can be explicitely set. The boundary conditions in the disk are the zero stress at the inner boundary and the zero accretion rate at the outer boundary. The conditions are suitable during the outbursts in X-ray binary transients with black holes. In a binary system, the accretion disk is radially confined. In Freddi, the outer radius of the disk can be set explicitely or calculated as the position of the tidal truncation radius.

  18. X-Shooter study of accretion in Chamaeleon I

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Fedele, D.; Herczeg, G. J.; Teixeira, P. S.

    2016-01-01

    We present the analysis of 34 new VLT/X-Shooter spectra of young stellar objects in the Chamaeleon I star-forming region, together with four more spectra of stars in Taurus and two in Chamaeleon II. The broad wavelength coverage and accurate flux calibration of our spectra allow us to estimate stellar and accretion parameters for our targets by fitting the photospheric and accretion continuum emission from the Balmer continuum down to ~700 nm. The dependence of accretion on stellar properties for this sample is consistent with previous results from the literature. The accretion rates for transitional disks are consistent with those of full disks in the same region. The spread of mass accretion rates at any given stellar mass is found to be smaller than in many studies, but is larger than that derived in the Lupus clouds using similar data and techniques. Differences in the stellar mass range and in the environmental conditions between our sample and that of Lupus may account for the discrepancy in scatter between Chamaeleon I and Lupus. Complete samples in Chamaeleon I and Lupus are needed to determine whether the difference in scatter of accretion rates and the lack of evolutionary trends are not influenced by sample selection. This work is based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 084.C-1095 and 094.C-0913.

  19. Bondi-like Accretion in Magnetized Supersonic Isothermal Turbulence

    NASA Astrophysics Data System (ADS)

    Burleigh, Kaylan J.; McKee, Christopher F.; Klein, Richard I.

    2016-01-01

    The Bondi and Bondi-Hoyle-Lytlleton formulas give the order of magnitude steady-accretion rate onto a point mass at rest or moving, respectively, in a uniform density gas in the limit of negligible gas self-gravity. This applies in star-forming clouds where self-gravity is negligible near protostars and new-born stars, but instead of being uniform the gas is supersonically turbulent and threaded by dynamically important (Alven Mach number ˜ 1) large-scale magnetic fields. To determine the Bondi-like accretion rate in these environments, we used the ORION2 code to carry out grid-based 3D adaptive mesh refinement (AMR) magnetohydrodynamic (MHD) simulations of accretion onto sink particles embedded in an environment of fully developed, magnetized supersonic isothermal turbulence. We evolved the models until the median and mean accretion rates, over particles, became steady. We present a simple semi-analytic model that predicts the median and mean accretion rate from the turbulent properties of the background medium, such as the 3D Mach number and RMS plasma-β, and show that it is highly consistent with our simulations. Numerical codes can use our semi-analytic model as an accurate sub-grid model for accretion in magnetized supersonic isothermal turbulence.

  20. A Systems-Level Perspective on Engine Ice Accretion

    NASA Technical Reports Server (NTRS)

    May, Ryan D.; Guo, Ten-Huei; Simon, Donald L.

    2013-01-01

    The accretion of ice in the compression system of commercial gas turbine engines operating in high ice water content conditions is a safety issue being studied by the aviation sector. While most of the research focuses on the underlying physics of ice accretion and the meteorological conditions in which accretion can occur, a systems-level perspective on the topic lends itself to potential near-term operational improvements. This work focuses on developing an accurate and reliable algorithm for detecting the accretion of ice in the low pressure compressor of a generic 40,000 lbf thrust class engine. The algorithm uses only the two shaft speed sensors and works regardless of engine age, operating condition, and power level. In a 10,000-case Monte Carlo simulation, the detection approach was found to have excellent capability at determining ice accretion from sensor noise with detection occurring when ice blocks an average of 6.8% of the low pressure compressor area. Finally, an initial study highlights a potential mitigation strategy that uses the existing engine actuators to raise the temperature in the low pressure compressor in an effort to reduce the rate at which ice accretes.

  1. High mass accretion disks: ATCA's potential for deep impact II

    NASA Astrophysics Data System (ADS)

    Walsh, Andrew; Beuther, Henrik; Longmore, Steven; Fallscheer, Cassandra

    2010-10-01

    The understanding of accretion processes and in particular of massive accretion disks is one of the most important topics in high-mass star formation. Based on our successful ATCA disk studies of high mass star formation, we now propose to investigate higher J inversion transitions of NH3 at high angular resolution (~1'') to complement our NH3 (4,4) and (5,5) data obtained last year. Last year's data showed a number of regions with clear rotational profiles, but no flattened structures that would indicate an edge-on accretion disk. We interpret our results to show rotating surrounding envelopes of any accretion disks. We were not able to see the accretion disks themselves because the (4,4) and (5,5) lines are optically thick. With observations of NH3 (7,7) and (8,8), which occur under even more extreme conditions than (4,4) or (5,5), we hope to peer through the surrounding envelope to see the accretion disks.

  2. High mass accretion disks: ATCA's potential for deep impact II

    NASA Astrophysics Data System (ADS)

    Walsh, Andrew; Beuther, Henrik; Longmore, Steven; Fallscheer, Cassandra

    2009-10-01

    The understanding of accretion processes and in particular of massive accretion disks is one of the most important topics in high-mass star formation. Based on our successful ATCA disk studies of high mass star formation, we now propose to investigate higher J inversion transitions of NH3 at high angular resolution (~1'') to complement our NH3 (4,4) and (5,5) data obtained last year. Last year's data showed a number of regions with clear rotational profiles, but no flattened structures that would indicate an edge-on accretion disk. We interpret our results to show rotating surrounding envelopes of any accretion disks. We were not able to see the accretion disks themselves because the (4,4) and (5,5) lines are optically thick. With observations of NH3 (7,7) and (8,8), which occur under even more extreme conditions than (4,4) or (5,5), we hope to peer through the surrounding envelope to see the accretion disks.

  3. THE GRAVO-MAGNETO LIMIT CYCLE IN ACCRETION DISKS

    SciTech Connect

    Martin, R. G.; Lubow, S. H.

    2011-10-10

    Previous theoretical studies have found that repeating outbursts can occur in certain regions of an accretion disk due to sudden transitions in time from gravitationally produced turbulence to magnetically produced turbulence. We analyze the disk evolution in a state diagram that plots the mass accretion rate versus disk surface density. We determine steady state accretion branches that involve gravitational and magnetic sources of turbulence. Using time-dependent numerical disk simulations, we show that cases having outbursts track along a nonsteady 'dead zone' branch and some steady state accretion branches. The outburst is the result of a rapid inter-branch transition. The gravo-magneto outbursts are then explained on this diagram as a limit cycle that is analogous to the well-known S-curve that has been applied to dwarf nova outbursts. The diagram and limit cycle provide a conceptual framework for understanding the nature of the outbursts that may occur in accretion disks of all scales, from circumplanetary to protoplanetary to active galactic nucleus accretion disks.

  4. The Magnetohydrodynamics of Convection-dominated Accretion Flows

    NASA Astrophysics Data System (ADS)

    Narayan, Ramesh; Quataert, Eliot; Igumenshchev, Igor V.; Abramowicz, Marek A.

    2002-09-01

    Radiatively inefficient accretion flows onto black holes are unstable due to both an outwardly decreasing entropy (``convection'') and an outwardly decreasing rotation rate (the ``magnetorotational instability'' [MRI]). Using a linear MHD stability analysis, we show that long-wavelength modes with λ/H>>β-1/2 are primarily destabilized by the entropy gradient and that such ``convective'' modes transport angular momentum inward (λ is the wavelength of the mode, H is the scale height of the disk, and β is the ratio of the gas pressure to the magnetic pressure). Moreover, the stability criteria for the convective modes are the standard Høiland criteria of hydrodynamics. By contrast, shorter wavelength modes with λ/H~β-1/2 are primarily destabilized by magnetic tension and differential rotation. These ``MRI'' modes transport angular momentum outward. The convection-dominated accretion flow (CDAF) model, which has been proposed for radiatively inefficient accretion onto a black hole, posits that inward angular momentum transport and outward energy transport by long-wavelength convective fluctuations are crucial for determining the structure of the accretion flow. Our analysis suggests that the CDAF model is applicable to an MHD accretion flow provided that the magnetic field saturates at a value sufficiently below equipartition (β>>1), so that long-wavelength convective fluctuations with λ/H>>β-1/2 can fit inside the accretion disk. Numerical MHD simulations are required to determine whether such a subequipartition field is in fact obtained.

  5. A pure hydrodynamic origin of accretion disk turbulence

    NASA Astrophysics Data System (ADS)

    Nath, Sujit Kumar; Mukhopadhyay, Banibrata

    2016-07-01

    Accretion disks consist of flows for which angular velocity decreases but specific angular momentum increases with increasing radial coordinate. Such flows are Rayleigh stable, but must be turbulent in order to explain observed data. Since molecular viscosity is negligible in these systems, scientists have argued for turbulent viscosity for energy dissipation and hence to explain infall of matter. However, so far, the success to explain the origin of turbulence in accretion disks is done with caveats. Here we investigate the evolution of pure hydrodynamic perturbations in stochastically driven accretion disks. We show that the accretion flows, which are inevitably driven by stochastic noise, are hydrodynamically unstable under linear perturbations. We also argue that in accretion disks, stochastic forcing appears generically due to the presence of shear between different annuli of the disk. This work resolves the turbulence problem of accretion disks from pure hydrodynamics and explains the infall of matter for both hot and cold disks. This would help in explaining the origin of timing and spectral features in the disk flows generically.

  6. TURBULENT MIXING ON HELIUM-ACCRETING WHITE DWARFS

    SciTech Connect

    Piro, Anthony L.

    2015-03-10

    An attractive scenario for producing Type Ia supernovae (SNe Ia) is a double detonation, where detonation of an accreted helium layer triggers ignition of a C/O core. Whether or not such a mechanism can explain some or most SNe Ia depends on the properties of the helium burning, which in turn is set by the composition of the surface material. Using a combination of semi-analytic and simple numerical models, I explore when turbulent mixing due to hydrodynamic instabilities during the accretion process can mix C/O core material up into the accreted helium. Mixing is strongest at high accretion rates, large white dwarf (WD) masses, and slow spin rates. The mixing would result in subsequent helium burning that better matches the observed properties of SNe Ia. In some cases, there is considerable mixing that can lead to more than 50% C/O in the accreted layer at the time of ignition. These results will hopefully motivate future theoretical studies of such strongly mixed conditions. Mixing also has implications for other types of WD surface explosions, including the so-called .Ia supernovae, the calcium-rich transients (if they arise from accreting WDs), and metal-enriched classical novae.

  7. TEARING UP THE DISK: HOW BLACK HOLES ACCRETE

    SciTech Connect

    Nixon, Chris; King, Andrew; Price, Daniel; Frank, Juhan

    2012-10-01

    We show that in realistic cases of accretion in active galactic nuclei or stellar-mass X-ray binaries, the Lense-Thirring effect breaks the central regions of tilted accretion disks around spinning black holes into a set of distinct planes with only tenuous flows connecting them. If the original misalignment of the outer disk to the spin axis of the hole is 45 Degree-Sign {approx}< {theta} {approx}< 135 Degree-Sign , as in {approx}70% of randomly oriented accretion events, the continued precession of these disks sets up partially counterrotating gas flows. This drives rapid infall as angular momentum is canceled and gas attempts to circularize at smaller radii. Disk breaking close to the black hole leads to direct dynamical accretion, while breaking further out can drive gas down to scales where it can accrete rapidly. For smaller tilt angles breaking can still occur and may lead to other observable phenomena such as quasi-periodic oscillations. For such effects not to appear, the black hole spin must in practice be negligibly small, or be almost precisely aligned with the disk. Qualitatively similar results hold for any accretion disk subject to a forced differential precession, such as an external disk around a misaligned black hole binary.

  8. Binary interactions with high accretion rates onto main sequence stars

    NASA Astrophysics Data System (ADS)

    Shiber, Sagiv; Schreier, Ron; Soker, Noam

    2016-07-01

    Energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the Great Eruption of Eta Carinae, and other intermediate luminosity optical transients (ILOTs; red novae; red transients). These powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. We propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. By examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10‑2 M ⊙ yr‑1 for solar type stars, and up to ≈ 1 M ⊙ yr‑1 for very massive stars. We speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. It is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars.

  9. Binary interactions with high accretion rates onto main sequence stars

    NASA Astrophysics Data System (ADS)

    Shiber, Sagiv; Schreier, Ron; Soker, Noam

    2016-07-01

    Energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the Great Eruption of Eta Carinae, and other intermediate luminosity optical transients (ILOTs; red novae; red transients). These powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. We propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. By examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10-2 M ⊙ yr-1 for solar type stars, and up to ≈ 1 M ⊙ yr-1 for very massive stars. We speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. It is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars.

  10. Azimuthal Stress and Heat Flux In Radiatively Inefficient Accretion Flows

    NASA Astrophysics Data System (ADS)

    Devlen, Ebru

    2016-07-01

    Radiatively Inefficient Accretion Flows (RIAFs) have low radiative efficiencies and/or low accretion rates. The accreting gas may retain most of its binding energy in the form of heat. This lost energy for hot RIAFs is one of the problems heavily worked on in the literature. RIAF observations on the accretion to super massive black holes (e.g., Sagittarius A* in the center of our Galaxy) have shown that the observational data are not consistent with either advection-dominated accretion flow (ADAF) or Bondi models. For this reason, it is very important to theoretically comprehend the physical properties of RIAFs derived from observations with a new disk/flow model. One of the most probable candidates for definition of mass accretion and the source of excess heat energy in RIAFs is the gyroviscous modified magnetorotational instability (GvMRI). Dispersion relation is derived by using MHD equations containing heat flux term based on viscosity in the energy equation. Numerical solutions of the disk equations are done and the growth rates of the instability are calculated. This additional heat flux plays an important role in dissipation of energy. The rates of the angular momentum and heat flux which are obtained from numerical calculations of the turbulence brought about by the GVMRI are also discussed.

  11. Teachable Moment: Google Earth Takes Us There

    ERIC Educational Resources Information Center

    Williams, Ann; Davinroy, Thomas C.

    2015-01-01

    In the current educational climate, where clearly articulated learning objectives are required, it is clear that the spontaneous teachable moment still has its place. Authors Ann Williams and Thomas Davinroy think that instructors from almost any discipline can employ Google Earth as a tool to take advantage of teachable moments through the…

  12. Taking a Pulse on Your Practice.

    PubMed

    Hoagland-Smith, Leanne

    2015-01-01

    Each medical practice, like a living organism, occasionally requires reading of its vital signs. As with human beings, one of those vital signs is the pulse. For your medical practice, just like your patients, there are numerous places from which to take that reading. This article reviews seven key pulses that provide insight into what is happening within the workplace culture of your practice.

  13. Ion Viscosity Mediated by Tangled Magnetic Fields: An Application to Black Hole Accretion Disks

    NASA Technical Reports Server (NTRS)

    Subramanian, Prasad; Becker, Peter A.; Kafatos, Menas

    1996-01-01

    We examine the viscosity associated with the shear stress exerted by ions in the presence of a tangled magnetic field. As an application, we consider the effect of this mechanism on the structure of black hole accretion disks. We do not attempt to include a self-consistent description of the magnetic field. Instead, we assume the existence of a tangled field with coherence length lambda(sub coh), which is the average distance between the magnetic 'kinks' that scatter the particles. For simplicity, we assume that the field is self-similar, and take lambda(sub coh) to be a fixed fraction zeta of the local disk height H. Ion viscosity in the presence of magnetic fields is generally taken to be the cross-field viscosity, wherein the effective mean free path is the ion Larmor radius lambda(sub L), which is much less than the ion-ion Coulomb mean free path A(sub ii) in hot accretion disks. However, we arrive at a formulation for a 'hybrid' viscosity in which the tangled magnetic field acts as an intermediary in the transfer of momentum between different layers in the shear flow. The hybrid viscosity greatly exceeds the standard cross-field viscosity when (lambda/lambda(sub L)) much greater than (lambda(sub L)/lambda(sub ii)), where lambda = ((lambda(sub ii)(sup -1) + lambda(sub (coh)(sup -1))(sup -1) is the effective mean free path for the ions. This inequality is well satisfied in hot accretion disks, which suggests that the ions may play a much larger role in the momentum transfer process in the presence of magnetic fields than was previously thought. The effect of the hybrid viscosity on the structure of a steady-state, two-temperature, quasi-Keplerian accretion disk is analyzed. The hybrid viscosity is influenced by the degree to which the magnetic field is tangled (represented by zeta = lambda(sub coh)), and also by the relative accretion rate M/M(sub E), where M(sub E) = L(sub E)/c(sup 2) and L(sub E) is the Eddington luminosity. We find that ion viscosity in the

  14. Take Your Medicines Safely

    MedlinePlus Videos and Cool Tools

    ... better, the antibiotic is working in killing the bacteria, but it might not completely give what they call a "bactericidal effect." That means taking the bacteria completely out of the system. It might be ...

  15. A Study of Ice Accretion Physics to Improve the Ice Accretion on Airfoils

    NASA Technical Reports Server (NTRS)

    Bragg, Michael B.

    2001-01-01

    This three-year grant began on November 7, 1996 and was no-cost extended to end on October 30, 2000. The objectives of the grant were: (1) To examine the effect of wind tunnel turbulence on ice accretion; (2) To determine the relationship between ice accretion geometry and airfoil performance; and (3) To determine if the wake-survey method was an appropriate experimental technique for iced-airfoil drag measurement. As specified in the grant the primary deliverables for this research were annual reports in the form of AIAA papers presented at national meetings each year. Masters theses and annual oral reports to be given at NASA Lewis (now Glenn) were also deliverables. Six AIAA papers documented the research findings from this study, Mr. Chad Henze's Masters thesis describes the wind tunnel turbulence work in detail, and a summary of the icing wind tunnel turbulence work was published in the archival AIAA Journal of Aircraft. A brief summary of the findings is given. Please refer to the reports for the details of the studies and findings.

  16. Adjusting to New Places: International Student Adjustment and Place Attachment

    ERIC Educational Resources Information Center

    Terrazas-Carrillo, Elizabeth C.; Hong, Ji Y.; Pace, Terry M.

    2014-01-01

    Using data obtained from in-depth semistructured interviews, we examined international students' attachments to place in the local American Midwestern community where they have attended college for at least 2 years. The results of this study suggest that participants engage in a process of renegotiation of meanings attached to new places in…

  17. Chemistry and Accretion History of Mars

    NASA Astrophysics Data System (ADS)

    Wanke, Heinrich; Dreibus, Gerlind

    1994-11-01

    Using element correlations observed in SNC meteorites and general cosmochemical constraints, Wanke & Dreibus (1988) have estimated the bulk composition of Mars. The mean abundance value for moderately volatile elements Na, P, K, F, and Rb and most of the volatile elements like Cl, Br, and I in the Martian mantle exceed the terrestrial values by about a factor of two. The striking depletion of all elements with chalcophile character (Cu, Co, Ni, etc.) indicates that Mars, contrary to the Earth, accreted homogeneously, which also explains the obvious low abundance of water and carbon. SNC meteorites and especially the shergottites are very dry rocks, they also contain very little carbon, while the concentrations of chlorine and especially sulphur are higher than those in terrestrial rocks. As a consequence we should expect SO2 and HCl to be the most abundant compounds in Martian volcanic gases. This might explain the dominance of sulphur and chlorine in the Viking soils. In turn SO2, being an excellent greenhouse gas, may have been of major importance for the warm and wet period in the ancient Martian history. Episodic release of larger quantities of SO2 stored in liquid or solid SO2 tables in the Martian regolith triggered by volcanic intrusions as suggested here could lead to a large number of warm and wet climate periods of the order of a hundred years, interrupted by much longer cold periods characterized by water ice and liquid of solid SO2. Sulphur (FeS) probably also governs the oxygen fugacity of the Martian surface rocks.

  18. Fractionation and Accretion of Meteorite Parent Bodies

    NASA Technical Reports Server (NTRS)

    Weidenschilling, Stuart J.

    2005-01-01

    Senior Scientist Stuart J. Weidenschilling presents his final administrative report for the research program on which he was the Principal Investigator. The research program resulted in the following publications: 1) Particle-gas dynamics and primary accretion. J. N. Cuzzi and S. J . Weidenschilling. To appear in Meteorites and the Early Solar System 11 (D. Lauretta et a]., Eds.), Univ. Arizona Press. 2005; 2) Timescales of the solar protoplanetary disk. S. Russell, L. Hartmann, J . N. Cuzzi, A. Krot, M. Gounelle and S. J. Weidenschilling. To appear in Meteorites and the Early Solar System II (D. Lauretta et al., Eds.), Univ. Arizona Press, 2005; 3) Nebula evolution of thermally processed solids: Reconciling astrophysical models and chondritic meteorites. J. N. Cuzzi, F. J. Ciesla, M. I. Petaev, A. N. Krot, E. R. D. Scott and S . J. Weidenschilling. To appear in Chondrites and the Protoplanetary Disk (A. Krot et a]., Eds.), ASP Conference Series, 2005; 4) Possible chondrule formation in planetesimal bow shocks: Physical processes in the near vicinity of the planetesimal. L. L. Hood, F. J. Ciesla and S. J. Weidenschilling. To appear in Chondrites and the Protoplanetary Disk (A. Krot et al., Eds.), ASP Conference Series, 2005; 5) From icy grains to comets. In Comets II (M. Festou et al., Eds.), Univ. Arizona Press, pp. 97- 104, 2005; 6) Evaluating planetesimal bow shocks as sites for chondrule formation. F. J . Ciesla, L. L. Hood and S. J. Weidenschilling. Meteoritics & Planetary Science 39, 1809-1 821, 2004; and 7) Radial drift of particles in the solar nebula: Implications for planetesimal formation. Icarus 165, 438-442, 2003.

  19. FORMING AN O STAR VIA DISK ACCRETION?

    SciTech Connect

    Qiu Keping; Zhang Qizhou; Beuther, Henrik; Fallscheer, Cassandra

    2012-09-10

    We present a study of outflow, infall, and rotation in a {approx}10{sup 5} L{sub Sun} star-forming region, IRAS 18360-0537, with Submillimeter Array and IRAM 30 m observations. The 1.3 mm continuum map shows a 0.5 pc dust ridge, of which the central compact part has a mass of {approx}80 M{sub Sun} and harbors two condensations, MM1 and MM2. The CO (2-1) and SiO (5-4) maps reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC) with a gas temperature of 320 {+-} 50 K and a mass of {approx}13 M{sub Sun }. The outflow has a gas mass of 54 M{sub Sun} and a dynamical timescale of 8 Multiplication-Sign 10{sup 3} yr. The kinematics of the HMC are probed by high-excitation CH{sub 3}OH and CH{sub 3}CN lines, which are detected at subarcsecond resolution and unveil a velocity gradient perpendicular to the outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope around the HMC is evidenced by CN lines exhibiting a profound inverse P Cygni profile, and the estimated mass infall rate, 1.5 Multiplication-Sign 10{sup -3} M{sub Sun} yr{sup -1}, is well comparable to that inferred from the mass outflow rate. A more detailed investigation of the kinematics of the dense gas around the HMC is obtained from the {sup 13}CO and C{sup 18}O (2-1) lines; the position-velocity diagrams of the two lines are consistent with the model of a free-falling and Keplerian-like rotating envelope. The observations suggest that the protostar of a current mass {approx}10 M{sub Sun} embedded within MM1 will develop into an O star via disk accretion and envelope infall.

  20. Forming an O Star via Disk Accretion?

    NASA Astrophysics Data System (ADS)

    Qiu, Keping; Zhang, Qizhou; Beuther, Henrik; Fallscheer, Cassandra

    2012-09-01

    We present a study of outflow, infall, and rotation in a ~105 L ⊙ star-forming region, IRAS 18360-0537, with Submillimeter Array and IRAM 30 m observations. The 1.3 mm continuum map shows a 0.5 pc dust ridge, of which the central compact part has a mass of ~80 M ⊙ and harbors two condensations, MM1 and MM2. The CO (2-1) and SiO (5-4) maps reveal a biconical outflow centered at MM1, which is a hot molecular core (HMC) with a gas temperature of 320 ± 50 K and a mass of ~13 M ⊙. The outflow has a gas mass of 54 M ⊙ and a dynamical timescale of 8 × 103 yr. The kinematics of the HMC are probed by high-excitation CH3OH and CH3CN lines, which are detected at subarcsecond resolution and unveil a velocity gradient perpendicular to the outflow axis, suggesting a disk-like rotation of the HMC. An infalling envelope around the HMC is evidenced by CN lines exhibiting a profound inverse P Cygni profile, and the estimated mass infall rate, 1.5 × 10-3 M ⊙ yr-1, is well comparable to that inferred from the mass outflow rate. A more detailed investigation of the kinematics of the dense gas around the HMC is obtained from the 13CO and C18O (2-1) lines; the position-velocity diagrams of the two lines are consistent with the model of a free-falling and Keplerian-like rotating envelope. The observations suggest that the protostar of a current mass ~10 M ⊙ embedded within MM1 will develop into an O star via disk accretion and envelope infall.

  1. Chaotic cold accretion on to black holes in rotating atmospheres

    NASA Astrophysics Data System (ADS)

    Gaspari, M.; Brighenti, F.; Temi, P.

    2015-07-01

    The fueling of black holes is one key problem in the evolution of baryons in the universe. Chaotic cold accretion (CCA) profoundly differs from classic accretion models, as Bondi and thin disc theories. Using 3D high-resolution hydrodynamic simulations, we now probe the impact of rotation on the hot and cold accretion flow in a typical massive galaxy. In the hot mode, with or without turbulence, the pressure-dominated flow forms a geometrically thick rotational barrier, suppressing the black hole accretion rate to ~1/3 of the spherical case value. When radiative cooling is dominant, the gas loses pressure support and quickly circularizes in a cold thin disk; the accretion rate is decoupled from the cooling rate, although it is higher than that of the hot mode. In the more common state of a turbulent and heated atmosphere, CCA drives the dynamics if the gas velocity dispersion exceeds the rotational velocity, i.e., turbulent Taylor number Tat< 1. Extended multiphase filaments condense out of the hot phase via thermal instability (TI) and rain toward the black hole, boosting the accretion rate up to 100 times the Bondi rate (Ṁ• ~ Ṁcool). Initially, turbulence broadens the angular momentum distribution of the hot gas, allowing the cold phase to condense with prograde or retrograde motion. Subsequent chaotic collisions between the cold filaments, clouds, and a clumpy variable torus promote the cancellation of angular momentum, leading to high accretion rates. As turbulence weakens (Tat > 1), the broadening of the distribution and the efficiency of collisions diminish, damping the accretion rate ∝ Tat-1, until the cold disk drives the dynamics. This is exacerbated by the increased difficulty to grow TI in a rotating halo. The simulated sub-Eddington accretion rates cover the range inferred from AGN cavity observations. CCA predicts inner flat X-ray temperature and r-1 density profiles, as recently discovered in M 87 and NGC 3115. The synthetic Hα images

  2. Analyzing the Spectra of Accreting X-Ray Pulsars

    NASA Astrophysics Data System (ADS)

    Wolff, Michael

    This proposal seeks funding for the analysis of accretion-powered X-ray pulsar spectra from NASA/ HEASARC archived X-ray data. Spectral modeling of accreting X-ray pulsars can tell us a great deal about the physical conditions in and near high mass X-ray binary systems. Such systems have accretion flows where plasma is initially channeled from an accretion disk by the strong neutron star magnetic field, eventually falling onto the magnetic polar cap of the neutron star compact object. Many of these accreting X-ray pulsars have X-ray spectra that consist of broad power-law continua with superposed cyclotron resonant scattering features indicating magnetic field strengths above 10^12 G. The energies of these cyclotron line features have recently been shown to vary with X-ray luminosity in a number of sources such as Her X-1 and V 0332+53, a phenomenon not well understood. Another recent development is the relatively new analytic model for the spectral continuum formation in accretion-powered pulsar systems developed by Becker & Wolff. In their formalism the accretion flows are assumed to go through radiation- dominated radiative shocks and settle onto the neutron star surface. The radiation field consists of strongly Comptonized bremsstrahlung emission from the entire plasma, Comptonized cyclotron emission from the de-excitations of Landau-excited electrons in the neutron star magnetic field, and Comptonized black-body emission from a thermal mound near the neutron star surface. We seek to develop the data analysis tools to apply this model framework to the X-ray data from a wide set of sources to make progress characterizing the basic accretion properties (e.g., magnetic field strength, plasma temperatures, polar cap size, accretion rate per unit area, dominance of bulk vs. thermal Comptonization) as well as understanding the variations of the cyclotron line energies with X-ray luminosity. The three major goals of our proposed work are as follows: In the first year

  3. Advection/Diffusion of Large-Scale B Field in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Lovelace, R. V. E.; Rothstein, D. M.; Bisnovatyi-Kogan, G. S.

    2009-08-01

    Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magnetorotational instability, MRI), which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field, which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity, which acts to prevent the buildup of a significant large-scale field. Recent work has pointed out that the disk's surface layers are nonturbulent, and thus highly conducting (or nondiffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components) and the profiles of the large-scale magnetic field, taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr (z), which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P= viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work. For a wide range of parameters β>1 and P≥ 1, we find stationary channel-type flows where the flow is radially outward near the midplane of the disk and radially inward in the top and bottom parts of the disk. Channel flows with inward flow near the midplane and outflow in the top and bottom

  4. PLACE NAMES IN THE CLASSROOM.

    ERIC Educational Resources Information Center

    HARDER, KELSIE B.

    ALTHOUGH "PLACE-NAMING" IS A BASIC HUMAN FUNCTION, THE STUDY OF THE ORIGIN OF PROPER NAMES OF PERSONS AND PLACES (ONOMASTICS) HAS BEEN LARGELY IGNORED BY AMERICAN PHILOLOGISTS AND TEACHERS OF ENGLISH. DESPITE A PAUCITY OF RESEARCH, HOWEVER, ANY ENGLISH TEACHER WHO WANTS TO INTEREST STUDENTS IN ONOMASTIC INVESTIGATION CAN EXPLOIT THE GREAT BODY OF…

  5. Implications of the β Lyrae accretion disk rim Teff

    NASA Astrophysics Data System (ADS)

    Linnell, A. P.

    2000-12-01

    Photometric evidence indicates that the massive gainer in the β Lyrae system is hidden from the observer by a thick accretion disk (Linnell, Hubeny, & Harmanec, 1998, ApJ, 509, 379). It is believed that the gainer approximates a main sequence star of Teff= 30000K. Spectroscopic analysis by Balachrandan et al. (1986, MNRAS, 219, 479) establishes a Teff of 13,300K for the donor. System synthetic spectra, fitted via the BINSYN suite to spectrophotometric scan data and IUE spectra, establish a mean rim Teff of 9000K. Assuming conservative mass transfer, Harmanec & Scholz (1993, A&A, 279, 131) use the rate of period change to derive a mass transfer rate of 20x10-6M⊙ yr-1. Connecting the rim Teff to the accretion disk face Teff with the Hubeny theory (Hubeny & Plavec 1991, AJ, 102, 1156) and using the standard accretion disk relations (Frank, King & Raine), the adopted mass transfer rate predicts a rim Teff of 4500K. The BINSYN-derived 9000K rim Teff would require a mass transfer rate 30 times larger than the adopted value. The observed rate of period change excludes such a large mass transfer rate. The bolometric luminosity of the rim, from the BINSYN model, is 5.6x1036erg sec-1. The bolometric luminosity of the gainer, on the adopted model, is 9.8x1037erg sec-1. Thus, the luminosity of the rim is 6% of the luminosity of the gainer. On the BINSYN model, the accretion disk covers 26% of the sky, as seen by the gainer. Absorption of radiation from the gainer, and its reradiation by the accretion disk, could explain the derived Teff of the rim. The conclusion is that the β Lyrae accretion disk structure must be strongly affected by radiation from the hot gainer (unseen by the observer) at the center of the accretion disk.

  6. ORBITING CIRCUMGALACTIC GAS AS A SIGNATURE OF COSMOLOGICAL ACCRETION

    SciTech Connect

    Stewart, Kyle R.; Kaufmann, Tobias; Bullock, James S.; Barton, Elizabeth J.; Maller, Ariyeh H.; Diemand, Juerg; Wadsley, James

    2011-09-01

    We use cosmological smoothed particle hydrodynamic simulations to study the kinematic signatures of cool gas accretion onto a pair of well-resolved galaxy halos. We find that cold-flow streams and gas-rich mergers produce a circumgalactic component of cool gas that generally orbits with high angular momentum about the galaxy halo before falling in to build the disk. This signature of cosmological accretion should be observable using background-object absorption-line studies as features that are offset from the galaxy's systemic velocity by {approx}100 km s{sup -1}. In most cases, the accreted gas co-rotates with the central disk in the form of a warped, extended cold flow disk, such that the observed velocity offset will be in the same direction as galaxy rotation, appearing in sight lines that avoid the galactic poles. This prediction provides a means to observationally distinguish accreted gas from outflow gas: the accreted gas will show large one-sided velocity offsets in absorption-line studies while radial/bi-conical outflows will not (except possibly in special polar projections). Such a signature of rotation has already been seen in studies of intermediate-redshift galaxy-absorber pairs, and we suggest that these observations may be among the first to provide indirect observational evidence for cold accretion onto galactic halos. This cold-mode halo gas typically has {approx}3-5 times more specific angular momentum than the dark matter. The associated cold-mode disk configurations are likely related to extended H I/extended UV disks that are seen around galaxies in the local universe. The fraction of galaxies with extended cold flow disks and associated offset absorption-line gas should decrease around bright galaxies at low redshift as cold-mode accretion dies out.

  7. Formation of primordial supermassive stars by rapid mass accretion

    SciTech Connect

    Hosokawa, Takashi; Yoshida, Naoki; Yorke, Harold W.; Inayoshi, Kohei; Omukai, Kazuyuki E-mail: hosokwtk@gmail.com

    2013-12-01

    Supermassive stars (SMSs) forming via very rapid mass accretion ( M-dot {sub ∗}≳0.1 M{sub ⊙} yr{sup −1}) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10{sup 4–5} M {sub ☉}. Our stellar evolution calculations show that a star becomes supermassive while passing through the 'supergiant protostar' stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ≅ 100 AU for M {sub *} ≳ 10{sup 4} M {sub ☉}, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 10{sup 4} K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M {sub *} ≳ 10{sup 5} M {sub ☉} can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10{sup 5} M {sub ☉}. An extremely massive black hole should form after the collapse of the inner core.

  8. DISTRIBUTION OF ACCRETING GAS AND ANGULAR MOMENTUM ONTO CIRCUMPLANETARY DISKS

    SciTech Connect

    Tanigawa, Takayuki; Ohtsuki, Keiji; Machida, Masahiro N.

    2012-03-01

    We investigate gas accretion flow onto a circumplanetary disk from a protoplanetary disk in detail by using high-resolution three-dimensional nested-grid hydrodynamic simulations, in order to provide a basis of formation processes of satellites around giant planets. Based on detailed analyses of gas accretion flow, we find that most of gas accretion onto circumplanetary disks occurs nearly vertically toward the disk surface from high altitude, which generates a shock surface at several scale heights of the circumplanetary disk. The gas that has passed through the shock surface moves inward because its specific angular momentum is smaller than that of the local Keplerian rotation, while gas near the midplane in the protoplanetary disk cannot accrete to the circumplanetary disk. Gas near the midplane within the planet's Hill sphere spirals outward and escapes from the Hill sphere through the two Lagrangian points L{sub 1} and L{sub 2}. We also analyze fluxes of accreting mass and angular momentum in detail and find that the distributions of the fluxes onto the disk surface are well described by power-law functions and that a large fraction of gas accretion occurs at the outer region of the disk, i.e., at about 0.1 times the Hill radius. The nature of power-law functions indicates that, other than the outer edge, there is no specific radius where gas accretion is concentrated. These source functions of mass and angular momentum in the circumplanetary disk would provide us with useful constraints on the structure and evolution of the circumplanetary disk, which is important for satellite formation.

  9. Gas content of transitional disks: a VLT/X-Shooter study of accretion and winds

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Testi, L.; Natta, A.; Rosotti, G.; Benisty, M.; Ercolano, B.; Ricci, L.

    2014-08-01

    Context. Transitional disks are thought to be a late evolutionary stage of protoplanetary disks whose inner regions have been depleted of dust. The mechanism responsible for this depletion is still under debate. To constrain the various models it is mandatory to have a good understanding of the properties of the gas content in the inner part of the disk. Aims: Using X-Shooter broad band - UV to near-infrared - medium-resolution spectroscopy, we derive the stellar, accretion, and wind properties of a sample of 22 transitional disks. The analysis of these properties allows us to place strong constraints on the gas content in a region very close to the star (≲0.2 AU) that is not accessible with any other observational technique. Methods: We fitted the spectra with a self-consistent procedure to simultaneously derive spectral type, extinction, and accretion properties of the targets. From the continuum excess at near-infrared wavelength we distinguished whether our targets have dust free inner holes. By analyzing forbidden emission lines, we derived the wind properties of the targets. We then compared our findings with results for classical T Tauri stars. Results: The accretion rates and wind properties of 80% of the transitional disks in our sample, which is strongly biased toward stongly accreting objects, are comparable to those of classical T Tauri stars. Thus, there are (at least) some transitional disks with accretion properties compatible with those of classical T Tauri stars, irrespective of the size of the dust inner hole. Only in two cases are the mass accretion rates much lower, while the wind properties remain similar. We detected no strong trend of the mass accretion rates with the size of the dust-depleted cavity or with the presence of a dusty optically thick disk very close to the star. These results suggest that, close to the central star, there is a gas-rich inner disk with a density similar to that of classical T Tauri star disks. Conclusions: The

  10. Simulation numerique de l'accretion de glace sur une pale d'eolienne

    NASA Astrophysics Data System (ADS)

    Fernando, Villalpando

    The wind energy industry is growing steadily, and an excellent place for the construction of wind farms is northern Quebec. This region has huge wind energy production potential, as the cold temperatures increase air density and with it the available wind energy. However, some issues associated with arctic climates cause production losses on wind farms. Icing conditions occur frequently, as high air humidity and freezing temperatures cause ice to build up on the blades, resulting in wind turbines operating suboptimally. One of the negative consequences of ice accretion is degradation of the blade's aerodynamics, in the form of a decrease in lift and an increase in drag. Also, the ice grows unevenly, which unbalances the blades and induces vibration. This reduces the expected life of some of the turbine components. If the ice accretion continues, the ice can reach a mass that endangers the wind turbine structure, and operation must be suspended in order to prevent mechanical failure. To evaluate the impact of ice on the profits of wind farms, it is important to understand how ice builds up and how much it can affect blade aerodynamics. In response, researchers in the wind energy field have attempted to simulate ice accretion on airfoils in refrigerated wind tunnels. Unfortunately, this is an expensive endeavor, and researchers' budgets are limited. However, ice accretion can be simulated more cost-effectively and with fewer limitations on airfoil size and air speed using numerical methods. Numerical simulation is an approach that can help researchers acquire knowledge in the field of wind energy more quickly. For years, the aviation industry has invested time and money developing computer codes to simulate ice accretion on aircraft wings. Nearly all these codes are restricted to use by aircraft developers, and so they are not accessible to researchers in the wind engineering field. Moreover, these codes have been developed to meet aeronautical industry

  11. Smart skin technology development for measuring ice accretion, stall, and high AOA aircraft performance. Part 1: Capacitive ice detector development

    NASA Technical Reports Server (NTRS)

    Pruzan, Daniel A.; Khatkhate, Ateen A.; Gerardi, Joseph J.; Hickman, Gail A.

    1993-01-01

    A reliable way to detect and measure ice accretion during flight is required to reduce the hazards of icing currently threatening present day aircraft. Many of the sensors used for this purpose are invasive (probe) sensors which must be placed in areas of the airframe where ice does not naturally form. Due to the difference in capture efficiency of the exposed surface, difficulties result in correlating the ice accretion on the probe to what is happening on a number of vastly different airfoil sections. Most flush mounted sensors in use must be integrated into the aircraft surface by cutting or drilling the aircraft surface. An alternate type of ice detector which is based on a NASA patent is currently being investigated at Innovative Dynamics, Inc. (IDI). Results of the investigation into the performance of different capacitive type sensor designs, both rigid as well as elastic, are presented.

  12. Propagation of nuclear burning fronts on accreting neutron stars: X-ray bursts and sub-hertz noise

    NASA Technical Reports Server (NTRS)

    Bildsten, Lars

    1995-01-01

    We identify a new regime of time dependent helium burning for high accretion rate neutron stars and suggest that this burning is the origin of the low-level luminosity variations (on timescales of 10-10(exp 4) s, designated the 'very low-frequency noise'(VLFN) by van der Klis and collaborators) always detected in the brightest accreting X-ray sources. Only two nuclear burning regimes were previously recognized. At accretion rates in excess of the Eddington limit (dot-M approximately greater than (1-3) x 10(exp -8) solar mass/yr), the accreted matter fuses steadily. At very low dot-M, the star's entire surface is rapidly (approximately less than 10 s) burned by a fast propagating convective burning front at regular intervals, giving quasi-periodic Type I X-ray bursts. We show that for the observationally interesting range of 5 x 10(exp -10) solar mass/yr approximately less than dot-M approximately less than 10(exp -8) solar mass/yr, parts of the stellar surface burn slowly. At these accretion rates, a local thermonuclear instability starts a fire which propagates horizontally at v approximately 300 cm/s. The fire propagates around the flammable surface in roughly the same time it takes to accrete enough fuel for the next instability (approximately 10(exp 3)-10(exp 4), so that only a few fires are burning at once, giving rise to large luminosity flares. Nuclear burning is always time dependent for sub-Eddington local accretion rates: a local patch undergoes a recurrent cycle, accumulation fuel for hours until it becomes thermally unstable or is 'ignited' by a nearby burning region. The global pattern of burning and the resulting luminosity are thus very dependent on how fast nuclear fires spread around the star. The nuclear burning luminosity is not uniform over the stellar surface and so may provide a handle on measuring, or constraining, the spin periods of these neutron stars.

  13. A STATE-DEPENDENT INFLUENCE OF TYPE I BURSTS ON THE ACCRETION IN 4U 1608-52?

    SciTech Connect

    Ji, Long; Zhang, Shu; Chen, YuPeng; Zhang, Shuang-Nan; Li, Jian; Torres, Diego F.; Kretschmar, Peter

    2014-08-20

    We investigated the possible feedback of type I bursts on the accretion process during the spectral evolution of the atoll source 4U 1608-52. By fitting the burst spectrum with a blackbody and an adjustable, persistent spectral component, we found that the latter is significantly state-dependent. In the banana state, the persistent flux increases along the burst evolution, while in the island state this trend holds only when the bursts are less luminous and start to reverse at higher burst luminosities. We speculate that, by taking into account both the Poynting-Robertson drag and radiation pressure, these phenomena may arise from the interactions between the radiation field of the type I burst and the inner region of the accretion disk.

  14. Taking a Broader View

    ERIC Educational Resources Information Center

    Flannery, Maura C.

    2005-01-01

    A study on stem cells is presented by understanding the environment in which they are found, the support cells and blood vessels as well as the protein scaffolds and other molecules. Researchers found that stem cells in reproductive tissue are surrounded by a pocket of support cells that hold them in place and align them to divide properly, so…

  15. Evolution Takes a Leap

    ERIC Educational Resources Information Center

    Sloan, Bernie

    2005-01-01

    Thirty years ago most resource sharing took place between libraries, with a few formal organizations (library consortia) providing logistical support. Today there are hundreds, perhaps thousands, of organizations around the world facilitating resource sharing. Automation has greatly accelerated the growth of resource sharing. As access to…

  16. Migration of accreting planets in radiative discs from dynamical torques

    NASA Astrophysics Data System (ADS)

    Pierens, A.; Raymond, S. N.

    2016-11-01

    We present the results of hydrodynamical simulations of the orbital evolution of planets undergoing runaway gas accretion in radiative discs. We consider accreting disc models with constant mass flux through the disc, and where radiative cooling balances the effect of viscous heating and stellar irradiation. We assume that 20-30 M⊕ giant planet cores are formed in the region where viscous heating dominates and migrate outward under the action of a strong entropy-related corotation torque. In the case where gas accretion is neglected and for an α viscous stress parameter α = 2 × 10-3, we find evidence for strong dynamical torques in accreting discs with accretion rates {dot{M}}≳ 7× 10^{-8} M_{⊙} yr{}^{-1}. Their main effect is to increase outward migration rates by a factor of ˜2 typically. In the presence of gas accretion, however, runaway outward migration is observed with the planet passing through the zero-torque radius and the transition between the viscous heating and stellar heating dominated regimes. The ability for an accreting planet to enter a fast migration regime is found to depend strongly on the planet growth rate, but can occur for values of the mass flux through the disc of {dot{M}}≳ 5× 10^{-8} M_{⊙} yr{}^{-1}. We find that an episode of runaway outward migration can cause an accreting planet formed in the 5-10 au region to temporarily orbit at star-planet separations as large as ˜60-70 au. However, increase in the amplitude of the Lindblad torque associated with planet growth plus change in the streamline topology near the planet systematically cause the direction of migration to be reversed. Subsequent evolution corresponds to the planet migrating inward rapidly until it becomes massive enough to open a gap in the disc and migrate in the type II regime. Our results indicate that a planet can reach large orbital distances under the combined effect of dynamical torques and gas accretion, but an alternative mechanism is required to

  17. Quasispherical subsonic accretion in X-ray pulsars

    NASA Astrophysics Data System (ADS)

    Shakura, Nikolai I.; Postnov, Konstantin A.; Kochetkova, A. Yu; Hjalmarsdotter, L.

    2013-04-01

    A theoretical model is considered for quasispherical subsonic accretion onto slowly rotating magnetized neutron stars. In this regime, the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasistatic shell. Angular momentum transfer in the shell occurs via large-scale convective motions resulting, for observed pulsars, in an almost iso-angular-momentum \\omega \\sim 1/R^2 rotation law inside the shell. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instabilities, with allowance for cooling. A settling accretion regime is possible for moderate accretion rates \\dot M \\lesssim \\dot M_* \\simeq 4\\times 10^{16} g s ^{-1}. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and the accretion becomes highly nonstationary. Observations of spin-up/spin-down rates of quasispherically wind accreting equilibrium X-ray pulsars with known orbital periods (e.g., GX 301-2 and Vela X-1) enable us to determine the main dimensionless parameters of the model, as well as to estimate surface magnetic field of the neutron star. For equilibrium pulsars, the independent measurements of the neutron star magnetic field allow for an estimate of the stellar wind velocity of the optical companion without using complicated spectroscopic measurements. For nonequilibrium pulsars, a maximum value is shown to exist for the spin-down rate of the accreting neutron star. From observations of the spin-down rate and the X-ray luminosity in such pulsars (e.g., GX 1+4, SXP 1062, and 4U 2206+54), a lower limit can be put on the neutron star magnetic field, which in all cases turns out to be close to the standard value and which agrees with cyclotron line measurements. Furthermore, both explains the spin-up/spin-down of the pulsar frequency on large time-scales and also accounts for the irregular short

  18. Magnetized Accretion-Ejection Structures: 2.5-dimensional Magnetohydrodynamic Simulations of Continuous Ideal Jet Launching from Resistive Accretion Disks

    NASA Astrophysics Data System (ADS)

    Casse, Fabien; Keppens, Rony

    2002-12-01

    We present numerical magnetohydrodynamic (MHD) simulations of a magnetized accretion disk launching trans-Alfvénic jets. These simulations, performed in a 2.5-dimensional time-dependent polytropic resistive MHD framework, model a resistive accretion disk threaded by an initial vertical magnetic field. The resistivity is only important inside the disk and is prescribed as η=αmVAHexp(- 2Z2/H2), where VA stands for Alfvén speed, H is the disk scale height, and the coefficient αm is smaller than unity. By performing the simulations over several tens of dynamical disk timescales, we show that the launching of a collimated outflow occurs self-consistently and the ejection of matter is continuous and quasi-stationary. These are the first ever simulations of resistive accretion disks launching nontransient ideal MHD jets. Roughly 15% of accreted mass is persistently ejected. This outflow is safely characterized as a jet since the flow becomes superfast magnetosonic, well collimated, and reaches a quasi-stationary state. We present a complete illustration and explanation of the ``accretion-ejection'' mechanism that leads to jet formation from a magnetized accretion disk. In particular, the magnetic torque inside the disk brakes the matter azimuthally and allows for accretion, while it is responsible for an effective magnetocentrifugal acceleration in the jet. As such, the magnetic field channels the disk angular momentum and powers the jet acceleration and collimation. The jet originates from the inner disk region where equipartition between thermal and magnetic forces is achieved. A hollow, superfast magnetosonic shell of dense material is the natural outcome of the inward advection of a primordial field.

  19. LUNAR ACCRETION FROM A ROCHE-INTERIOR FLUID DISK

    SciTech Connect

    Salmon, Julien; Canup, Robin M. E-mail: robin@boulder.swri.edu

    2012-11-20

    We use a hybrid numerical approach to simulate the formation of the Moon from an impact-generated disk, consisting of a fluid model for the disk inside the Roche limit and an N-body code to describe accretion outside the Roche limit. As the inner disk spreads due to a thermally regulated viscosity, material is delivered across the Roche limit and accretes into moonlets that are added to the N-body simulation. Contrary to an accretion timescale of a few months obtained with prior pure N-body codes, here the final stage of the Moon's growth is controlled by the slow spreading of the inner disk, resulting in a total lunar accretion timescale of {approx}10{sup 2} years. It has been proposed that the inner disk may compositionally equilibrate with the Earth through diffusive mixing, which offers a potential explanation for the identical oxygen isotope compositions of the Earth and Moon. However, the mass fraction of the final Moon that is derived from the inner disk is limited by resonant torques between the disk and exterior growing moons. For initial disks containing <2.5 lunar masses (M{sub Last-Quarter-Moon }), we find that a final Moon with mass > 0.8 M{sub Last-Quarter-Moon} contains {<=}60% material derived from the inner disk, with this material preferentially delivered to the Moon at the end of its accretion.

  20. Accretion of solid materials onto circumplanetary disks from protoplanetary disks

    SciTech Connect

    Tanigawa, Takayuki; Maruta, Akito; Machida, Masahiro N.

    2014-04-01

    We investigate the accretion of solid materials onto circumplanetary disks from heliocentric orbits rotating in protoplanetary disks, which is a key process for the formation of regular satellite systems. In the late stage of the gas-capturing phase of giant planet formation, the accreting gas from protoplanetary disks forms circumplanetary disks. Since the accretion flow toward the circumplanetary disks affects the particle motion through gas drag force, we use hydrodynamic simulation data for the gas drag term to calculate the motion of solid materials. We consider a wide range of size for the solid particles (10{sup –2}-10{sup 6} m), and find that the accretion efficiency of the solid particles peaks around 10 m sized particles because energy dissipation of drag with circum-planetary disk gas in this size regime is most effective. The efficiency for particles larger than 10 m becomes lower because gas drag becomes less effective. For particles smaller than 10 m, the efficiency is lower because the particles are strongly coupled with the background gas flow, which prevents particles from accretion. We also find that the distance from the planet where the particles are captured by the circumplanetary disks is in a narrow range and well described as a function of the particle size.

  1. Electromagnetic signatures of thin accretion disks in wormhole geometries

    SciTech Connect

    Harko, Tiberiu; Kovacs, Zoltan; Lobo, Francisco S. N.

    2008-10-15

    In this paper, we study the physical properties and characteristics of matter forming thin accretion disks in static and spherically symmetric wormhole spacetimes. In particular, the time averaged energy flux, the disk temperature, and the emission spectra of the accretion disks are obtained for these exotic geometries and are compared with the Schwarzschild solution. It is shown that more energy is emitted from the disk in a wormhole geometry than in the case of the Schwarzschild potential and the conversion efficiency of the accreted mass into radiation is more than a factor of 2 higher for the wormholes than for static black holes. These effects in the disk radiation are confirmed in the radial profiles of temperature corresponding to theses flux distributions, and in the emission spectrum {omega}L({omega}) of the accretion disks. We conclude that specific signatures appear in the electromagnetic spectrum, thus leading to the possibility of distinguishing wormhole geometries by using astrophysical observations of the emission spectra from accretion disks.

  2. Glancing through the accretion column of EXO 2030+375

    NASA Astrophysics Data System (ADS)

    Ferrigno, C.; Pjanka, P.; Bozzo, E.; Klochkov, D.; Ducci, L.; Zdziarski, A.

    2016-06-01

    The current generation of X-ray instruments is revealing more and more details about the complex magnetic field topology and the geometry of the accretion flows in highly magnetized accretion powered pulsars. We took advantage of the large collecting area and timing capabilities of the EPIC cameras to investigate the accretion geometry onto the magnetized neutron star in the high mass X-ray binary EXO 2030+375 during the rise of one of the source outburst. The X-ray luminosity was 2×10^{36} erg/s and the timing analysis revealed the presence of a narrow dip-like feature in its pulse profile that was never reported before. The width of this feature corresponds to about one hundredth of the neutron star spin period. From the results of the phase-resolved spectral analysis we suggest that this feature can be ascribed to the self-obscuration of the accretion stream passing in front of the observer line of sight. We inferred from Suzaku observation carried out in 2007 that the self-obscuration of the accretion stream might produce a significantly wider feature in the neutron star pulsed profile at higher luminosities (>˜2×10^{37} erg/s). The presence of such feature is so far unique among all known high mass X-ray binaries hosting strongly magnetized neutron stars.

  3. Search for Signatures of Accreted Planetary Material in the Hyades

    NASA Astrophysics Data System (ADS)

    Fulbright, J. P.

    2002-12-01

    We present early results of a study to determine whether accretion of planetary material is a significant contributor to the surface abundances of stars. We have analyzed high-resolution, high-S/N spectra of ~ 80 Hyades members for the abundances of both volatile and refractory elements. Stars that host planets (Gonzalez et al. 2001, Santos et al. 2001) show abundance enhancements with respect to the rest of the nearby disk population. Smith et al. (2001) further showed that a subsample of these host stars are enhanced in refractory elements. Both results may be a sign of the accretion of planetary material, but it may be difficult to disentangle the effects of accretion against the effects of galactic chemical evolution when comparing individual field stars. Cluster stars, however, should form out of material with nearly identical initial abundances. Stars with shallow surface convection zones (F and G stars) could show signs of accretion, while stars with deep surface convective zones (K dwarfs and giants) should be immune to the abundance effects of planetary accretion.

  4. Kronos: A Multiwavelength Observatory for Mapping Accretion-Driven Sources

    NASA Technical Reports Server (NTRS)

    Peterson, Bradley M.; Polidan, Ronald S.; Robinson, Edward L.

    2002-01-01

    Kronos is a multiwavelength observatory proposed as a NASA Medium Explorer. Kronos is designed to make use of the natural variability of accreting sources to create microarcsecond-resolution maps of the environments of supermassive black holes in active galaxies and stella-size black holes in binary systems and to characterize accretion processes in Galactic compact binaries. Kronos will obtain broad energy range spectroscopic data with co-aligned X-ray, ultraviolet, and optical spectrometers. The high-Earth orbit of Kronos enables well-sampled, high time-resolution observations, critical for the innovative and sophisticated methods that are used to understand the accretion flows, mass outflows, jets, and other phenomena found in accreting sources. By utilizing reverberation mapping analysis techniques, Kronos produces advanced high-resolution maps of unprecedented resolution of the extreme environment in the inner cores of active galaxies. Similarly, Doppler tomography and eclipse mapping techniques characterize and map Galactic binary systems, revealing the details of the physics of accretion processes in black hole, neutron star, and white dwarf binary systems. The Kronos instrument complement, sensitivity, and orbital environment make it suitable to aggressively address time variable phenomena in a wide range of astronomical objects from nearby flare stars to distant galaxies.

  5. Dynamical structure of magnetized dissipative accretion flow around black holes

    NASA Astrophysics Data System (ADS)

    Sarkar, Biplob; Das, Santabrata

    2016-09-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several supermassive black hole sources and the observational implications of our present analysis are discussed.

  6. Investigation of surface water behavior during glaze ice accretion

    NASA Technical Reports Server (NTRS)

    Hansman, R. John, Jr.; Turnock, Stephen R.

    1990-01-01

    A series of experimental investigations that focused on isolating the primary factors that control the behavior of unfrozen surface water during glaze ice accretion were conducted. Detailed microvideo observations were made of glaze ice accretions on 2.54 cm diam cylinders in a closed-loop refrigerated wind tunnel. Distinct zones of surface water behavior were observed; a smooth wet zone in the stagnation region with a uniform water film, a rough zone where surface tension effects caused coalescence of surface water into stationary beads, and a zone where surface water ran back as rivulets. The location of the transition from the smooth to the rough zone was found to migrate towards the stagnation point with time. Comparative tests were conducted to study the effect of the substrate thermal and roughness properties on ice accretion. The importance of surface water behavior was evaluated by the addition of a surface tension reducing agent to the icing tunnel water supply, which significantly altered the accreted glaze ice shape. Measurements were made to determine the contact angle behavior of water droplets on ice. A simple multizone modification to current glaze ice accretion models was proposed to include the observed surface roughness behavior.

  7. Ubiquitous equatorial accretion disc winds in black hole soft states

    NASA Astrophysics Data System (ADS)

    Ponti, G.; Fender, R. P.; Begelman, M. C.; Dunn, R. J. H.; Neilsen, J.; Coriat, M.

    2012-05-01

    High-resolution spectra of Galactic black holes (GBHs) reveal the presence of highly ionized absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are a ubiquitous component of the jet-free soft states of all GBHs. We furthermore demonstrate that these winds have an equatorial geometry with opening angles of few tens of degrees, and so are only observed in sources in which the disc is inclined at a large angle to the line of sight. The decrease in Fe XXV/Fe XXVI line ratio with Compton temperature, observed in the soft state, suggests a link between higher wind ionization and harder spectral shapes. Although the physical interaction between the wind, accretion flow and jet is still not fully understood, the mass flux and power of these winds and their presence ubiquitously during the soft X-ray states suggest they are fundamental components of the accretion phenomenon.

  8. Accretion disk emission from a BL Lacertae object

    NASA Technical Reports Server (NTRS)

    Wandel, Amri; Urry, C. Megan

    1991-01-01

    It is suggested here that the UV and X-ray emission of BL Lac objects may originate in an accretion disk. Using detailed calculations of accretion disk spectra, the best-measured ultraviolet and soft X-ray spectra of the BL Lac object PKS 2155-304 are fitted, and the mass and accretion rate required is determined. The ultraviolet through soft X-ray continuum is well fitted by the spectrum of an accretion disk, but near-Eddington accretion rates are required to produce the soft X-ray excess. A hot disk or corona could Comptonize soft photons from the cool disk and produce the observed power-law spectrum in the 1-10 keV range. The dynamic time scale in the disk regions that contribute most of the observed ultraviolet and soft X-ray photons are consistent with the respective time scales for intensity variations observed in these two wave bands; the mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard X-ray variability.

  9. SR-71 Taking Off

    NASA Technical Reports Server (NTRS)

    1990-01-01

    One of three U.S. Air Force SR-71 reconnaissance aircraft originally retired from operational service and loaned to NASA for a high-speed research program retracts its landing gear after taking off from NASA's Ames-Dryden Flight Research Facility (later Dryden Flight Research Center), Edwards, California, on a 1990 research flight. One of the SR-71As was later returned to the Air Force for active duty in 1995. Data from the SR-71 high-speed research program will be used to aid designers of future supersonic/hypersonic aircraft and propulsion systems. Two SR-71 aircraft have been used by NASA as testbeds for high-speed and high-altitude aeronautical research. The aircraft, an SR-71A and an SR-71B pilot trainer aircraft, have been based here at NASA's Dryden Flight Research Center, Edwards, California. They were transferred to NASA after the U.S. Air Force program was cancelled. As research platforms, the aircraft can cruise at Mach 3 for more than one hour. For thermal experiments, this can produce heat soak temperatures of over 600 degrees Fahrenheit (F). This operating environment makes these aircraft excellent platforms to carry out research and experiments in a variety of areas -- aerodynamics, propulsion, structures, thermal protection materials, high-speed and high-temperature instrumentation, atmospheric studies, and sonic boom characterization. The SR-71 was used in a program to study ways of reducing sonic booms or over pressures that are heard on the ground, much like sharp thunderclaps, when an aircraft exceeds the speed of sound. Data from this Sonic Boom Mitigation Study could eventually lead to aircraft designs that would reduce the 'peak' overpressures of sonic booms and minimize the startling affect they produce on the ground. One of the first major experiments to be flown in the NASA SR-71 program was a laser air data collection system. It used laser light instead of air pressure to produce airspeed and attitude reference data, such as angle of

  10. Simulating Price-Taking

    ERIC Educational Resources Information Center

    Engelhardt, Lucas M.

    2015-01-01

    In this article, the author presents a price-takers' market simulation geared toward principles-level students. This simulation demonstrates that price-taking behavior is a natural result of the conditions that create perfect competition. In trials, there is a significant degree of price convergence in just three or four rounds. Students find this…

  11. Take Pride in America.

    ERIC Educational Resources Information Center

    Indiana State Dept. of Education, Indianapolis. Center for School Improvement and Performance.

    During the 1987-88 school year the Indiana Department of Education assisted the United States Department of the Interior and the Indiana Department of Natural Resources with a program which asked students to become involved in activities to maintain and manage public lands. The 1987 Take Pride in America (TPIA) school program encouraged volunteer…

  12. Take a Bow

    ERIC Educational Resources Information Center

    Spitzer, Greg; Ogurek, Douglas J.

    2009-01-01

    Performing-arts centers can provide benefits at the high school and collegiate levels, and administrators can take steps now to get the show started. When a new performing-arts center comes to town, local businesses profit. Events and performances draw visitors to the community. Ideally, a performing-arts center will play many roles: entertainment…

  13. VERTICAL STRUCTURE OF STATIONARY ACCRETION DISKS WITH A LARGE-SCALE MAGNETIC FIELD

    SciTech Connect

    Bisnovatyi-Kogan, G. S.; Lovelace, R. V. E. E-mail: RVL1@cornell.edu

    2012-05-10

    In earlier works we pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the hydrodynamic and/or magnetorotational instabilities are suppressed high in the disk where the magnetic and radiation pressures are larger than the plasma thermal pressure. Here, we calculate the vertical profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity and the fact that the turbulence vanishes at the surface of the disk. Also, here we require that the radial accretion speed be zero at the disk's surface and we assume that the ratio of the turbulent viscosity to the turbulent magnetic diffusivity is of order unity. Thus, at the disk's surface there are three boundary conditions. As a result, for a fixed dimensionless viscosity {alpha}-value, we find that there is a definite relation between the ratio R of the accretion power going into magnetic disk winds to the viscous power dissipation and the midplane plasma-{beta}, which is the ratio of the plasma to magnetic pressure in the disk. For a specific disk model with R of order unity we find that the critical value required for a stationary solution is {beta}{sub c} Almost-Equal-To 2.4r/({alpha}h), where h is the disk's half thickness. For weaker magnetic fields, {beta} > {beta}{sub c}, we argue that the poloidal field will advect outward while for {beta} < {beta}{sub c} it will advect inward. Alternatively, if the disk wind is negligible (R<<1), there are stationary solutions with {beta} >> {beta}{sub c}.

  14. Stationary axisymmetric configuration of the resistive thick accretion tori around a Schwarzschild black hole

    NASA Astrophysics Data System (ADS)

    Shaghaghian, M.

    2016-02-01

    We examine a thick accretion disc in the presence of external gravity and intrinsic dipolar magnetic field due to a non-rotating central object. In this paper, we generalize the Newtonian theory of stationary axisymmetric resistive tori of Tripathy, Prasanna & Das by including the fully general relativistic features. If we are to obtain the steady state configuration, we have to take into account the finite resistivity for the magnetofluid in order to avoid the piling up of the field lines anywhere in the accretion discs. The efficient value of conductivity must be much smaller than the classical conductivity to be astrophysically interesting. The accreting plasma in the presence of an external dipole magnetic field gives rise to a current in the azimuthal direction. The azimuthal current produced due to the motion of the magnetofluid modifies the magnetic field structure inside the disc and generates a poloidal magnetic field for the disc. The solutions we have found show that the radial inflow, pressure and density distributions are strongly modified by the electrical conductivity both in relativistic and Newtonian regimes. However, the range of conductivity coefficient is different for both regimes, as well as that of the angular momentum parameter and the radius of the innermost stable circular orbit. Furthermore, it is shown that the azimuthal velocity of the disc which is not dependent on conductivity is sub-Keplerian in all radial distances for both regimes. Owing to the presence of pressure gradient and magnetic forces. This work may also be important for the general relativistic computational magnetohydrodynamics that suffers from the lack of exact analytic solutions that are needed to test computer codes.

  15. Place recognition using batlike sonar.

    PubMed

    Vanderelst, Dieter; Steckel, Jan; Boen, Andre; Peremans, Herbert; Holderied, Marc W

    2016-01-01

    Echolocating bats have excellent spatial memory and are able to navigate to salient locations using bio-sonar. Navigating and route-following require animals to recognize places. Currently, it is mostly unknown how bats recognize places using echolocation. In this paper, we propose template based place recognition might underlie sonar-based navigation in bats. Under this hypothesis, bats recognize places by remembering their echo signature - rather than their 3D layout. Using a large body of ensonification data collected in three different habitats, we test the viability of this hypothesis assessing two critical properties of the proposed echo signatures: (1) they can be uniquely classified and (2) they vary continuously across space. Based on the results presented, we conclude that the proposed echo signatures satisfy both criteria. We discuss how these two properties of the echo signatures can support navigation and building a cognitive map. PMID:27481189

  16. Place recognition using batlike sonar.

    PubMed

    Vanderelst, Dieter; Steckel, Jan; Boen, Andre; Peremans, Herbert; Holderied, Marc W

    2016-01-01

    Echolocating bats have excellent spatial memory and are able to navigate to salient locations using bio-sonar. Navigating and route-following require animals to recognize places. Currently, it is mostly unknown how bats recognize places using echolocation. In this paper, we propose template based place recognition might underlie sonar-based navigation in bats. Under this hypothesis, bats recognize places by remembering their echo signature - rather than their 3D layout. Using a large body of ensonification data collected in three different habitats, we test the viability of this hypothesis assessing two critical properties of the proposed echo signatures: (1) they can be uniquely classified and (2) they vary continuously across space. Based on the results presented, we conclude that the proposed echo signatures satisfy both criteria. We discuss how these two properties of the echo signatures can support navigation and building a cognitive map.

  17. Rayleigh-Taylor-unstable accretion to and variability of magnetized stars: Global three-dimensional magnetohydrodynamic simulations

    NASA Astrophysics Data System (ADS)

    Kulkanarni, Akshay Kishor

    We present results of three-dimensional (3D) simulations of magnetohydrodynamic (MHD) instabilities at the accretion disk-magnetosphere boundary in accreting magnetized stars. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It manifests itself in the form of tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane, instead of flowing around the magnetosphere as in the canonical accretion picture. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, theta ≲ 30°, between the star's rotation and magnetic axes, and is associated with relatively high accretion rates. We then calculate the photometric variability due to emission from the hot spots that the accreting matter produces on the stellar surface. For neutron stars, we take relativistic effects into account in calculating the observed energy flux. Our goal is to compare the features of the lightcurve during stable and unstable accretion, and to look for possible quasi-periodic oscillations (QPOs), which produce broad peaks in the Fourier power spectra of these objects. The lightcurves during stable accretion show periodicity at the star's frequency and sometimes twice that, due to the presence of two funnel streams that produce antipodal hotspots near the magnetic poles. On the other hand, lightcurves during unstable accretion are more chaotic due to the stochastic behaviour of the tongues, and produce noisier power spectra. However, the power spectra do show some signs of quasi-periodic variability. Most importantly, the rotation frequency of the tongues and the resulting hotspots is close to the inner-disk orbital frequency, except in the most

  18. The Instability in Accretion Flows: GvMRI

    NASA Astrophysics Data System (ADS)

    Yardimci, Melis; Ebru Devlen, Doç.

    2016-07-01

    In this study, we discuss the physical instability defining the expected turbulence in Radiatively Inefficient Accretion Flows (RIAFs) around the supermassive black holes (e.g., Sagittarius A* in the center of our Galaxy). These flows, with a high probability, include weakly collisional hot, optically thin and dilute plasmas. Within these flows, gravitational potential energy brought about by turbulent stresses is trapped as heat energy. Thus, in order accretion to be realized, outward transport of heat as well as angular momentum is required. This outward heat transport may reduce the mass inflow rate on black hole. We solve MHD equations including variation of viscosity coefficients with pressure in the momentum conservation equation. We plot the wave number-frequency diagrams for the wave modes. We show that one of the most probable candidates for definition of mass accretion and the source of excess heat energy in RIAFs is the gyroviscous modified magnetorotational instabilitiy (GvMRI).

  19. Nonlinear dynamics of accretion disks with stochastic viscosity

    SciTech Connect

    Cowperthwaite, Philip S.; Reynolds, Christopher S.

    2014-08-20

    We present a nonlinear numerical model for a geometrically thin accretion disk with the addition of stochastic nonlinear fluctuations in the viscous parameter. These numerical realizations attempt to study the stochastic effects on the disk angular momentum transport. We show that this simple model is capable of reproducing several observed phenomenologies of accretion-driven systems. The most notable of these is the observed linear rms-flux relationship in the disk luminosity. This feature is not formally captured by the linearized disk equations used in previous work. A Fourier analysis of the dissipation and mass accretion rates across disk radii show coherence for frequencies below the local viscous frequency. This is consistent with the coherence behavior observed in astrophysical sources such as Cygnus X-1.

  20. The Influence of Outflow in Supercritical Accretion Flows

    NASA Astrophysics Data System (ADS)

    Zahra Zeraatgari, Fatemeh; Abbassi, Shahram; Mosallanezhad, Amin

    2016-06-01

    We solve the radiation-hydrodynamic equations of supercritical accretion flows in the presence of radiation force and outflow by using self-similar solutions. Similar to the pioneering works, in this paper we consider a power-law function for mass inflow rate as \\dot{M}\\propto {r}s. We found that s = 1 when the radiative cooling term is included in the energy equation. Correspondingly, the effective temperature profile with respect to the radius was obtained as {T}{eff}\\propto {r}-1/2. In addition, we investigated the influence of the outflow on the dynamics of the accretion flow. We also calculated the continuum spectrum emitted from the disk surface as well as the bolometric luminosity of the accretion flow. Furthermore, our results show that the advection parameter, f, depends strongly on mass inflow rate.