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1

Circumstellar habitable zones for deep terrestrial biospheres  

NASA Astrophysics Data System (ADS)

The habitable zone (HZ) is conventionally the thin shell of space around a star within which liquid water is thermally stable on the surface of an Earth-like planet (Kasting et al., 1993). However, life on Earth is not restricted to the surface and includes a "deep biosphere" reaching several km in depth. Similarly, subsurface liquid water maintained by internal planetary heat could potentially support life well outside conventional HZs. We introduce a new term,subsurface-habitability zone (SSHZ) to denote the range of distances from a star within which rocky planets are habitable at any depth below their surfaces up to a stipulated maximum, and show how SSHZs can be estimated from a model relating temperature, depth and orbital distance. We present results for Earth-like, Mars-like and selected extrasolar terrestrial planets, and conclude that SSHZs are several times wider and include many more planets than conventional surface-based habitable zones.

McMahon, Sean; O’Malley-James, Jack; Parnell, John

2013-09-01

2

The Galactic Habitable Zone  

ScienceCinema

We propose the concept of a "Galactic Habitable Zone" (GHZ). Similar to the circumstellar habitable zone (CHZ), the GHZ is that region in a spiral galaxy where life can exist. The width of the GHZ is controlled by two factors. The inner (closest to the center of the galaxy) limit is set by threats to complex life: nearby transient sources of ionizing radiation and comet impacts. Such threats tend to increase close to the galactic center. The outer limit is imposed by galactic chemical evolution, specifically the abundance of heavier elements. Observation of stars in the Milky Way galaxy suggests that the outer reaches of a spiral galaxy may be too poor in heavy elements to allow terrestrial complex life to exist.

3

DETECTABILITY OF EARTH-LIKE PLANETS IN CIRCUMSTELLAR HABITABLE ZONES OF BINARY STAR SYSTEMS WITH SUN-LIKE COMPONENTS  

SciTech Connect

Given the considerable percentage of stars that are members of binaries or stellar multiples in the solar neighborhood, it is expected that many of these binaries host planets, possibly even habitable ones. The discovery of a terrestrial planet in the {alpha} Centauri system supports this notion. Due to the potentially strong gravitational interaction that an Earth-like planet may experience in such systems, classical approaches to determining habitable zones (HZ), especially in close S-type binary systems, can be rather inaccurate. Recent progress in this field, however, allows us to identify regions around the star permitting permanent habitability. While the discovery of {alpha} Cen Bb has shown that terrestrial planets can be detected in solar-type binary stars using current observational facilities, it remains to be shown whether this is also the case for Earth analogs in HZs. We provide analytical expressions for the maximum and rms values of radial velocity and astrometric signals, as well as transit probabilities of terrestrial planets in such systems, showing that the dynamical interaction of the second star with the planet may indeed facilitate the planets' detection. As an example, we discuss the detectability of additional Earth-like planets in the averaged, extended, and permanent HZs around both stars of the {alpha} Centauri system.

Eggl, Siegfried; Pilat-Lohinger, Elke [University of Vienna, Institute for Astrophysics, Tuerkenschanzstr. 17, A-1180 Vienna (Austria)] [University of Vienna, Institute for Astrophysics, Tuerkenschanzstr. 17, A-1180 Vienna (Austria); Haghighipour, Nader, E-mail: siegfried.eggl@univie.ac.at [Institute for Astronomy and NASA Astrobiology Institute, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)] [Institute for Astronomy and NASA Astrobiology Institute, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States)

2013-02-20

4

The habitable zone and extreme planetary orbits.  

PubMed

The habitable zone for a given star describes the range of circumstellar distances from the star within which a planet could have liquid water on its surface, which depends upon the stellar properties. Here we describe the development of the habitable zone concept, its application to our own solar system, and its subsequent application to exoplanetary systems. We further apply this to planets in extreme eccentric orbits and show how they may still retain life-bearing properties depending upon the percentage of the total orbit which is spent within the habitable zone. Key Words: Extrasolar planets-Habitable zone-Astrobiology. PMID:23035897

Kane, Stephen R; Gelino, Dawn M

2012-10-01

5

Habitable Zone Evolution  

NASA Astrophysics Data System (ADS)

The location of the habitable zone around a star depends upon stellar luminosity and upon the properties of a potentially habitable planet such as its mass and near-surface volatile inventory. Stellar luminosity generally increases as a star ages whilst planetary properties change through time as a consequence of biological and geological evolution. Hence, the location of the habitable zone changes through time as a result of both stellar evolution and planetary evolution. Using the Earth's Phanerozoic temperature history as a constraint, it is shown that changes in our own habitable zone over the last 540 My have been dominated by planetary evolution rather than solar evolution. Furthermore, sparse data from earlier times suggests that planetary evolution may have dominated habitable zone development throughout our biosphere's history. Hence, the existence of a continuously habitable zone depends upon accidents of complex bio-geochemical evolution more than it does upon relatively simple stellar-evolution. Evolution of the inner margin of the habitable zone through time using three different estimates for climate sensitivity. The dashed line shows a typical predicted evolution assuming this was driven simply by a steady increase in solar luminosity. Solar evolution does not account for the observations. Evolution of the outer margin of the habitable zone through time using three different estimates for climate sensitivity. The dashed line shows a typical predicted evolution assuming this was driven simply by a steady increase in solar luminosity. Solar evolution does not account for the observations.

Waltham, D.; Lota, J.

2012-12-01

6

Trojans in habitable zones.  

PubMed

With the aid of numerical experiments we examined the dynamical stability of fictitious terrestrial planets in 1:1 mean motion resonance with Jovian-like planets of extrasolar planetary systems. In our stability study of the so-called "Trojan" planets in the habitable zone, we used the restricted three-body problem with different mass ratios of the primary bodies. The application of the three-body problem showed that even massive Trojan planets can be stable in the 1:1 mean motion resonance. From the 117 extrasolar planetary systems only 11 systems were found with one giant planet in the habitable zone. Out of this sample set we chose four planetary systems--HD17051, HD27442, HD28185, and HD108874--for further investigation. To study the orbital behavior of the stable zone in the different systems, we used direct numerical computations (Lie Integration Method) that allowed us to determine the escape times and the maximum eccentricity of the fictitious "Trojan planets." PMID:16225431

Schwarz, Richard; Pilat-Lohinger, Elke; Dvorak, Rudolf; Erdi, Balint; Sándor, Zsolt

2005-10-01

7

Comparable Habitable Zones of Stars  

NASA Video Gallery

The habitable zone is the distance from a star where one can have liquid water on the surface of a planet. If a planet is too close to its parent star, it will be too hot and water would have evapo...

8

Determining Habitable Zones in Binary Star Systems  

NASA Astrophysics Data System (ADS)

Planets in and around binary star systems are subject to potentially large variations in insolation, especially when the planetary orbits become eccentric (Forgan, 2012; Kane & Hinkel, 2013). Eggl et al. (2012) were able to show that even planets on initially circular orbits experience large insolation variations due to the gravitational interaction between the two stars and the planet. It is, thus, not an easy task to identify Habitable Zones in such systems. We present an analytic framework suited to determine Habitable Zones in and around binary star systems using effective insolation values provided by Kopparapu et al. (2013). Accounting for the complex dynamical interaction between the planet and the binary star, we could define our Habitable Zones in such a way that they still remain independent of time up to stellar evolution timescales. Thus, our method provides a simple means of guiding observations to regions of interest, where terrestrial planets can remain habitable in double star systems.

Eggl, S.; Georgakarakos, N.; Pilat-Lohinger, E.; Funk, B.

2013-09-01

9

Habitable zone lifetimes of exoplanets around main sequence stars.  

PubMed

The potential habitability of newly discovered exoplanets is initially assessed by determining whether their orbits fall within the circumstellar habitable zone of their star. However, the habitable zone (HZ) is not static in time or space, and its boundaries migrate outward at a rate proportional to the increase in luminosity of a star undergoing stellar evolution, possibly including or excluding planets over the course of the star's main sequence lifetime. We describe the time that a planet spends within the HZ as its "habitable zone lifetime." The HZ lifetime of a planet has strong astrobiological implications and is especially important when considering the evolution of complex life, which is likely to require a longer residence time within the HZ. Here, we present results from a simple model built to investigate the evolution of the "classic" HZ over time, while also providing estimates for the evolution of stellar luminosity over time in order to develop a "hybrid" HZ model. These models return estimates for the HZ lifetimes of Earth and 7 confirmed HZ exoplanets and 27 unconfirmed Kepler candidates. The HZ lifetime for Earth ranges between 6.29 and 7.79×10? years (Gyr). The 7 exoplanets fall in a range between ?1 and 54.72 Gyr, while the 27 Kepler candidate planets' HZ lifetimes range between 0.43 and 18.8 Gyr. Our results show that exoplanet HD 85512b is no longer within the HZ, assuming it has an Earth analog atmosphere. The HZ lifetime should be considered in future models of planetary habitability as setting an upper limit on the lifetime of any potential exoplanetary biosphere, and also for identifying planets of high astrobiological potential for continued observational or modeling campaigns. PMID:24047111

Rushby, Andrew J; Claire, Mark W; Osborn, Hugh; Watson, Andrew J

2013-09-01

10

Habitable zone limits for dry planets.  

PubMed

Most discussion of habitable planets has focused on Earth-like planets with globally abundant liquid water. For an "aqua planet" like Earth, the surface freezes if far from its sun, and the water vapor greenhouse effect runs away if too close. Here we show that "land planets" (desert worlds with limited surface water) have wider habitable zones than aqua planets. For planets at the inner edge of the habitable zone, a land planet has two advantages over an aqua planet: (i) the tropics can emit longwave radiation at rates above the traditional runaway limit because the air is unsaturated and (ii) the dry air creates a dry stratosphere that limits hydrogen escape. At the outer limits of the habitable zone, the land planet better resists global freezing because there is less water for clouds, snow, and ice. Here we describe a series of numerical experiments using a simple three-dimensional global climate model for Earth-sized planets. Other things (CO(2), rotation rate, surface pressure) unchanged, we found that liquid water remains stable at the poles of a low-obliquity land planet until net insolation exceeds 415 W/m(2) (170% that of modern Earth), compared to 330 W/m(2) (135%) for the aqua planet. At the outer limits, we found that a low-obliquity land planet freezes at 77%, while the aqua planet freezes at 90%. High-obliquity land and aqua planets freeze at 58% and 72%, respectively, with the poles offering the last refuge. We show that it is possible that, as the Sun brightens, an aqua planet like Earth can lose most of its hydrogen and become a land planet without first passing through a sterilizing runaway greenhouse. It is possible that Venus was a habitable land planet as recently as 1 billion years ago. PMID:21707386

Abe, Yutaka; Abe-Ouchi, Ayako; Sleep, Norman H; Zahnle, Kevin J

2011-06-01

11

Potential photosynthetic systems in extraterrestrial habitable zones  

NASA Astrophysics Data System (ADS)

A number of photosynthetic systems have evolved on Earth to harvest various portions of the available spectrum from its G2 star. Currently, the number of confirmed extrasolar planets approaches 300, although many are in orbits well outside their habitable zone. This largely results from an observational bias that tends to more easily spot these "hot Jupiters," but increasingly more Earth-like extrasolar planets are detected. The spectral classes of the stars supporting these planets are generally well-identified, permitting some basic assumptions on the inner and outer habitable zone radii. We can also make some assumptions on the spectrum of photon energy available for potential photosynthesis on these planets, allowing for local atmospheric effects. The absorption spectra of terrestrial photosynthetic systems, both naturally evolved, and artificially created, are matched to the anticipated spectra on extrasolar planets. Further consideration is given to the cooler M class stars, whose large number and long life enhance the likelihood of photosynthesis evolving.

Konesky, Gregory

2008-08-01

12

UV habitable zones around M stars  

Microsoft Academic Search

During the last decade there was a change in paradigm, which led to consider that terrestrial-type planets within liquid-water habitable zones (LW-HZ) around M stars can also be suitable places for the emergence and evolution of life. Since many dMe stars emit large amount of UV radiation during flares, in this work we analyze the UV constrains for living systems

Andrea P. Buccino; Guillermo A. Lemarchand; Pablo J. D. Mauas

2007-01-01

13

Cellular Automation of Galactic Habitable Zone  

NASA Astrophysics Data System (ADS)

We present a preliminary results of our Galactic Habitable Zone (GHZ) 2D probabilistic cellular automata models. The relevant time-scales (emergence of life, it's diversification and evolution influenced with the global risk function) are modeled as the probability matrix elements and are chosen in accordance with the Copernican principle to be well-represented by the data inferred from the Earth's fossil record. With Fermi's paradox as a main boundary condition the resulting histories of astrobiological landscape are discussed.

Vukotic, B.; Cirkovic, M. M.

2010-09-01

14

Cellular Automation of Galactic Habitable Zone  

Microsoft Academic Search

We present a preliminary results of our Galactic Habitable Zone (GHZ) 2D probabilistic cellular automata models. The relevant time-scales (emergence of life, it's diversification and evolution influenced with the global risk function) are modeled as the probability matrix elements and are chosen in accordance with the Copernican principle to be well-represented by the data inferred from the Earth's fossil record.

Branislav Vukotic; Milan M. Cirkovic

2010-01-01

15

Which habitable zones have the most real estate?  

Microsoft Academic Search

Over the last few years, our concept of a habitable world has become much broader. Besides terrestrial planets in the conventional habitable zone roughly 0.7-1.5 AU from a Sun-like star, we realize that a moon of a Jovian planet in the habitable zone may be habitable, as well as a moon like Europa that is tidally heated. Habitable planets may

R. Belikov; M. Kuchner

2009-01-01

16

Radiative habitable zones in martian polar environments.  

PubMed

The biologically damaging solar ultraviolet (UV) radiation (quantified by the DNA-weighted dose) reaches the martian surface in extremely high levels. Searching for potentially habitable UV-protected environments on Mars, we considered the polar ice caps that consist of a seasonally varying CO2 ice cover and a permanent H2O ice layer. It was found that, though the CO2 ice is insufficient by itself to screen the UV radiation, at approximately 1 m depth within the perennial H2O ice the DNA-weighted dose is reduced to terrestrial levels. This depth depends strongly on the optical properties of the H2O ice layers (for instance snow-like layers). The Earth-like DNA-weighted dose and Photosynthetically Active Radiation (PAR) requirements were used to define the upper and lower limits of the northern and southern polar Radiative Habitable Zone (RHZ) for which a temporal and spatial mapping was performed. Based on these studies we conclude that photosynthetic life might be possible within the ice layers of the polar regions. The thickness varies along each martian polar spring and summer between approximately 1.5 and 2.4 m for H2O ice-like layers, and a few centimeters for snow-like covers. These martian Earth-like radiative habitable environments may be primary targets for future martian astrobiological missions. Special attention should be paid to planetary protection, since the polar RHZ may also be subject to terrestrial contamination by probes. PMID:16044598

Córdoba-Jabonero, Carmen; Zorzano, María-Paz; Selsis, Franck; Patel, Manish R; Cockell, Charles S

2005-06-01

17

Detection of Exomoons Inside the Habitable Zone  

NASA Astrophysics Data System (ADS)

Since the discovery of the first exoplanets, those most adequate for life to begin and evolve have been sought. Due to observational bias, however, most of the discovered planets so far are gas giants, precluding their habitability. However, if these hot Jupiters are located in the habitable zones of their host stars, and if rocky moons orbit them, then these moons may be habitable. In this work, we present a model for planetary transit simulation considering the presence of moons around a planet. The moon orbit is considered to be circular and coplanar with the planetary orbit. The other physical and orbital parameters of the star, planet, and moon, can be adjusted in each simulation. It is possible to simulate as many successive transits as desired. Since the presence of spots on the surface of the star may produce a signal similar to that of the presence of a moon, our model also allows for the inclusion of starspots. The goal is to determine the criteria for detectability of moons using photometry with the CoRoT and Kepler telescopes taking into account the stellar activity.

Tusnski, Luis Ricardo M.; Valio, Adriana

2014-04-01

18

Habitable zones around main sequence stars.  

PubMed

A one-dimensional climate model is used to estimate the width of the habitable zone (HZ) around our Sun and around other main sequence stars. Our basic premise is that we are dealing with Earth-like planets with CO2/H2O/N2 atmospheres and that habitability requires the presence of liquid water on the planet's surface. The inner edge of the HZ is determined in our model by loss of water via photolysis and hydrogen escape. The outer edge of the HZ is determined by the formation of CO2 clouds, which cool a planet's surface by increasing its albedo and by lowering the convective lapse rate. Conservative estimates for these distances in our own Solar System are 0.95 and 1.37 AU, respectively; the actual width of the present HZ could be much greater. Between these two limits, climate stability is ensured by a feedback mechanism in which atmospheric CO2 concentrations vary inversely with planetary surface temperature. The width of the HZ is slightly greater for planets that are larger than Earth and for planets which have higher N2 partial pressures. The HZ evolves outward in time because the Sun increases in luminosity as it ages. A conservative estimate for the width of the 4.6-Gyr continuously habitable zone (CHZ) is 0.95 to 1.15 AU. Stars later than F0 have main sequence lifetimes exceeding 2 Gyr and, so, are also potential candidates for harboring habitable planets. The HZ around an F star is larger and occurs farther out than for our Sun; the HZ around K and M stars is smaller and occurs farther in. Nevertheless, the widths of all of these HZs are approximately the same if distance is expressed on a logarithmic scale. A log distance scale is probably the appropriate scale for this problem because the planets in our own Solar System are spaced logarithmically and because the distance at which another star would be expected to form planets should be related to the star's mass. The width of the CHZ around other stars depends on the time that a planet is required to remain habitable and on whether a planet that is initially frozen can be thawed by modest increases in stellar luminosity. For a specified period of habitability, CHZs around K and M stars are wider (in log distance) than for our Sun because these stars evolve more slowly. Planets orbiting late K stars and M stars may not be habitable, however, b ecause they can become trapped in synchronous rotation as a consequence of tidal damping. F stars have narrower (log distance) CHZ's than our Sun because they evolve more rapidly. Our results suggest that mid-to-early K stars should be considered along with G stars as optimal candidates in the search for extraterrestrial life. PMID:11536936

Kasting, J F; Whitmire, D P; Reynolds, R T

1993-01-01

19

Tides, planetary companions, and habitability: habitability in the habitable zone of low-mass stars  

NASA Astrophysics Data System (ADS)

Earth-scale planets in the classical habitable zone (HZ) are more likely to be habitable if they possess active geophysics. Without a constant internal energy source, planets cool as they age, eventually terminating tectonic activity. Planets orbiting low-mass stars can be very old, due to the longevity of such stars, so they may be rendered sterile to life in this way. However, the presence of an outer companion could generate enough tidal heat in the HZ planet to prevent such cooling. The range of mass and orbital parameters for the companion that give adequate long-term heating of the inner HZ planet, while avoiding very early total desiccation, is probably substantial. We locate the ideal location for the outer of a pair of planets, under the assumption that the inner planet has the same incident flux as Earth, orbiting example stars: a generic late M dwarf (Teff = 2670 K) and the M9V/L0 dwarf DEN1048. Thus discoveries of Earth-scale planets in the HZ zone of old small stars should be followed by searches for outer companion planets that might be essential for current habitability.

Van Laerhoven, C.; Barnes, R.; Greenberg, R.

2014-07-01

20

The Habitable Zone of Inhabited Planets  

NASA Astrophysics Data System (ADS)

In this paper we discuss and illustrate the hypothesis that life substantially alters the state of a planetary environment and therefore, modifies the limits of the HZ as estimated for an uninhabited planet. This hypothesis lead to the introduction of the Habitable Zone for Inhabited Planets (hereafter InHZ), defined here as the region where the complex interaction between life and its abiotic environment is able to produce plausible equilibrium states with the necessary physical conditions for the existence and persistence of life itself. We support our hypothesis of an InHZ with three theoretical arguments, multiple evidences coming from observations of the Earth system, several conceptual experiments and illustrative numerical simulations. Conceptually the diference between the InHZ and the Abiotic HZ (AHZ) depends on unique and robust properties of life as an emergent physical phenomenon and not necesarily on the particular life forms bearing in the planet. Our aim here is to provide conceptual basis for the development of InHZ models incorporating consistently life-environment interactions. Although previous authors have explored the effects of life on habitability there is a gap in research developing the reasons why life should be systematically included at determining the HZ limits. We do not provide here definitive limits to the InHZ but we show through simple numerical models (as a parable of an inhabited planet) how the limits of the AHZ could be modified by including plausible interactions between biota and its environment. These examples aim also at posing the question that if limits of the HZ could be modified by the presence of life in those simple dynamical systems how will those limits change if life is included in established models of the AHZ.

Zuluaga, J. I.; Salazar, J. F.; Cuartas-Restrepo, P.; Poveda, G.

2014-06-01

21

Tectonics and the photosynthetic habitable zone (Invited)  

NASA Astrophysics Data System (ADS)

The traditional habitable zone lies between an inner stellar radius where the surface of the planet becomes too hot for liquid water carbon-based life and on outer radius, where the surface freezes. It is effectively the zone where photosynthesis is feasible. The concept extends to putative life on objects with liquid methane at the surface, like Titan. As a practical matter, photosynthesis leaves detectable biosignatures in the geological record; black shale on the Earth indicates that sulfide and probably FeO based photosynthesis existed by 3.8 Ga. The hard crustal rocks and the mantle sequester numerous photosynthetic biosignatures. Photosynthesis can produce detectable free oxygen with ozone in the atmosphere of extrasolar planets. In contrast, there is no outer limit for subsurface life in large silicate objects. Pre-photosynthetic niches are dependable but meager and not very detectable at great antiquity or great distance, with global productivity less than 1e-3 of the photosynthetic ones. Photosynthetic organisms have bountiful energy that modifies their surface environment and even tectonics. For example, metamorphic rocks formed at the expense of thick black shale are highly radioactive and hence self-fluxing. Active tectonics with volcanism and metamorphism prevents volatiles from being sequestered in the subsurface as on Mars. A heat-pipe object, like a larger Io, differs from the Earth in that the volatiles return to the deep interior distributed within massive volcanic deposits rather than concentrated in the shallow oceanic crust. One the Earth, the return of water to the surface by arc volcanoes controls its mantle abundance at the transition between behaving as a trace element and behaving as a major element that affects melting. The ocean accumulates the water that the mantle and crust do not take. The Earth has the “right” amount of water that erosion/deposition and tectonics both tend to maintain near sea level surfaces. The mantle contains carbon (dioxide) that platform carbonates and the deep continental lithosphere do not take. Weathering and formation of carbonates in the oceanic crust dynamically buffers atmospheric CO2 at habitable levels. N2 is an indirect greenhouse gas in that the total pressure increases the effect of CO2. Photosynthetic life affects the nitrogen cycle as NH4+ replaces K+ in subducted black shale. N2 hence correlates with Ar-40 in volcanic gases. The net effect is that atmospheric pressure and hence the greenhouse effect decrease with time. Continents are in part the result of biological weathering. Their presence allows life to directly affect continental albedo as with “Daisy World” and to indirectly affect albedo as high global temperatures lead to reflective deserts.

Sleep, N. H.

2009-12-01

22

Europa, tidally heated oceans, and habitable zones around giant planets  

NASA Technical Reports Server (NTRS)

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. Europa could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such a case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of eons and in which life could perhaps evolve. A zone around a giant planet is defined in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In this solar system, this radiatively-heated habitable zone contains the earth.

Reynolds, Ray T.; Mckay, Christopher P.; Kasting, James F.

1987-01-01

23

What Can The Habitable Zone Gallery Do for You?  

NASA Astrophysics Data System (ADS)

The Habitable Zone Gallery (www.hzgallery.org) has been online since August 2011 as a service to the exoplanet community to provide Habitable Zone (HZ) information for each of the exoplanetary systems with known planetary orbital parameters. The service includes a sortable table, a plot with the period and eccentricity of each of the planets with respect to their time spent in the HZ, a gallery of known systems which plots the orbits and the location of the HZ with respect to those orbits, and orbital movies. Recently, we have added new features including: implementation of both conservative and optimistic HZs, more user-friendly table and movies, movies for circumbinary planets, and a count of planets whose orbits lie entirely within the system’s HZ. Here we discuss various educational and scientific applications of the site such as target selection, exploring planets with eccentric or circumbinary orbits, and investigating habitability.

Gelino, Dawn M.; Kane, Stephen R.

2014-06-01

24

STABILIZING CLOUD FEEDBACK DRAMATICALLY EXPANDS THE HABITABLE ZONE OF TIDALLY LOCKED PLANETS  

SciTech Connect

The habitable zone (HZ) is the circumstellar region where a planet can sustain surface liquid water. Searching for terrestrial planets in the HZ of nearby stars is the stated goal of ongoing and planned extrasolar planet surveys. Previous estimates of the inner edge of the HZ were based on one-dimensional radiative-convective models. The most serious limitation of these models is the inability to predict cloud behavior. Here we use global climate models with sophisticated cloud schemes to show that due to a stabilizing cloud feedback, tidally locked planets can be habitable at twice the stellar flux found by previous studies. This dramatically expands the HZ and roughly doubles the frequency of habitable planets orbiting red dwarf stars. At high stellar flux, strong convection produces thick water clouds near the substellar location that greatly increase the planetary albedo and reduce surface temperatures. Higher insolation produces stronger substellar convection and therefore higher albedo, making this phenomenon a stabilizing climate feedback. Substellar clouds also effectively block outgoing radiation from the surface, reducing or even completely reversing the thermal emission contrast between dayside and nightside. The presence of substellar water clouds and the resulting clement surface conditions will therefore be detectable with the James Webb Space Telescope.

Yang Jun; Abbot, Dorian S. [Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637 (United States); Cowan, Nicolas B., E-mail: abbot@uchicago.edu [Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, 2131 Tech Drive, Evanston, IL 60208 (United States)

2013-07-10

25

Habitable Zones Around Stars and the Search for Extraterrestrial Life  

Microsoft Academic Search

The habitable zone, or HZ, is defined as the region around a star in which liquid water can exist on the surface of a planet. Liquid water can exist in subsurface environments on planets or moons outside of the HZ (e.g., Jupiter's moon, Europa), but this possibility is of little significance, as such environments cannot be observed or otherwise investigated.

J. F. Kasting

2001-01-01

26

Dynamical effects on the habitable zone for Earth-like exomoons  

NASA Astrophysics Data System (ADS)

With the detection of extrasolar moons (exomoons) on the horizon, it is important to consider their potential for habitability. If we consider the circumstellar habitable zone (HZ, often described in terms of planet semi-major axis and orbital eccentricity), we can ask, `How does the HZ for an Earth-like exomoon differ from the HZ for an Earth-like exoplanet?' For the first time, we use 1D latitudinal energy balance modelling to address this question. The model places an Earth-like exomoon in an orbit around a Jupiter mass planet, which in turn orbits a Sun-like star. The exomoon's surface is decomposed into latitudinal strips, and the temperature of each strip is evolved under the action of stellar insolation, atmospheric cooling, heat diffusion, eclipses of the star by the planet and tidal heating. We use this model to carry out two separate investigations. In the first investigation, four test cases are run to investigate in detail the dependence of the exomoon climate on the orbital direction, orbital inclination and the frequency of the stellar eclipse by the host planet. We find that lunar orbits which are retrograde to the planetary orbit exhibit greater climate variations than prograde orbits, with global mean temperatures around 0.1 K higher due to the geometry of eclipses. If eclipses become frequent relative to the atmospheric thermal inertia time-scale, climate oscillations become extremely small. In the second investigation, we carry out an extensive parameter study, running the model many times to study the habitability of the exomoon in the four-dimensional space composed of the planet semi-major axis and eccentricity, and the moon semi-major axis and eccentricity. We find that for zero moon eccentricity, frequent eclipses allow the moon to remain habitable in regions of high planet eccentricity, but tidal heating severely constrains habitability in the limit of high moon eccentricity, making the HZ a sensitive function of the moon semi-major axis.

Forgan, Duncan; Kipping, David

2013-07-01

27

CANDIDATE PLANETS IN THE HABITABLE ZONES OF KEPLER STARS  

SciTech Connect

A key goal of the Kepler mission is the discovery of Earth-size transiting planets in ''habitable zones'' where stellar irradiance maintains a temperate climate on an Earth-like planet. Robust estimates of planet radius and irradiance require accurate stellar parameters, but most Kepler systems are faint, making spectroscopy difficult and prioritization of targets desirable. The parameters of 2035 host stars were estimated by Bayesian analysis and the probabilities p{sub HZ} that 2738 candidate or confirmed planets orbit in the habitable zone were calculated. Dartmouth Stellar Evolution Program models were compared to photometry from the Kepler Input Catalog, priors for stellar mass, age, metallicity and distance, and planet transit duration. The analysis yielded probability density functions for calculating confidence intervals of planet radius and stellar irradiance, as well as p{sub HZ}. Sixty-two planets have p{sub HZ} > 0.5 and a most probable stellar irradiance within habitable zone limits. Fourteen of these have radii less than twice the Earth; the objects most resembling Earth in terms of radius and irradiance are KOIs 2626.01 and 3010.01, which orbit late K/M-type dwarf stars. The fraction of Kepler dwarf stars with Earth-size planets in the habitable zone ({eta}{sub Circled-Plus }) is 0.46, with a 95% confidence interval of 0.31-0.64. Parallaxes from the Gaia mission will reduce uncertainties by more than a factor of five and permit definitive assignments of transiting planets to the habitable zones of Kepler stars.

Gaidos, Eric, E-mail: gaidos@hawaii.edu [Department of Geology and Geophysics, University of Hawai'i at Manoa, Honolulu, HI 96822 (United States)

2013-06-20

28

Europa, tidally heated oceans, and habitable zones around giant planets.  

PubMed

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. In our own solar system, Europa, one of the Galilean satellites of Jupiter, could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such case, sufficient solar energy could reach liquid water that organisms similar to those found under Antarctic ice could grow. In other solar systems, larger satellites with more significant heat flow could represent environments that are stable over an order of Aeons and in which life could perhaps evolve. We define a zone around a giant planet in which such satellites could exist as a tidally-heated habitable zone. This zone can be compared to the habitable zone which results from heating due to the radiation of a central star. In our solar system, this radiatively-heated habitable zone contains the Earth. PMID:11538217

Reynolds, R T; McKay, C P; Kasting, J F

1987-01-01

29

Tidal Heating and the Boundaries of the Habitable Zone  

NASA Astrophysics Data System (ADS)

With the discovery of planetary systems beyond our own, and the interest in their implications for extraterrestrial life, the notion of a habitable zone emerged, based on a requirement of surface liquid water, under an atmosphere warmed by stellar radiation [1]. Widespread use of that definition has persisted even as it became increasing clear that one of the most likely places in our solar system for extraterrestrial life to first be found, Europa, lies outside of that conventional habitable zone [2]. Europa derives its heat from internal tidal dissipation, rather than insolation. And, although there is no atmosphere to maintain liquid water at the surface, a vast global ocean lies below a relatively thin layer of surface ice. We find that transport of radiolytic oxidants and other substances through the permeable ice layer may be adequate to support life, perhaps even an ecology including complex organisms [3]. Clearly the definition of “habitable zone” needs to be expanded to include planetary bodies where tidal heating is significant. Such heating, which requires an adequate orbital eccentricity, may be common and important in extrasolar systems. Eccentricities themselves tend to be damped by tides, which would turn off the heating, but we have demonstrated several ways that tidal heating can be important: (1) As with Europa, interactions with other orbiters can maintain eccentricities [4]; (2) Many planets previously assumed to have damped to circular orbits may still retain some eccentricity [5]; (3) Even among planets now on circular orbits, many underwent tidal heating recently enough that it remains a factor in current geophysics [6]. Tidal heating may make otherwise uninhabitable planets habitable by, for example, creating Europa-like planets, or driving plate tectonics in small, rocky planets [7]. The notion of a habitable zone needs to be modified to account for this broader array of habitable planetary types. References: [1] Kasting, J., et al., Icarus 101, 108, 1993. [2] Greenberg, R., Europa the Ocean Moon, Springer, 2005. [3] Greenberg, R., Astrobiology, submitted 2009. [4] Barnes, R., et al., Astrophys. J. 695, 1006, 2009. [5] Jackson, B., et al., Astrophys. J. 678, 1396, 2008. [6] Jackson, B., et al., Astrophys. J. 681, 1631, 2008. [7] Jackson, B., et al., Mon. Not. Roy. Astron. Soc. 391, 237, 2008.

Greenberg, R.; Barnes, R.; Jackson, B.

2009-12-01

30

Planetary Obliquity Evolution in the 47 Uma Habitable Zone  

NASA Astrophysics Data System (ADS)

In two studies by Laskar and Robuterl (1993) and Laskar et al. (1993) the obliquity evolution of the major planets of the Solar System was investigated. It was found that the terrestrial planets have a high probability of encountering chaotic regimes and thereby initiating large variations in their obliquities. Such variations are expected to have had profound implications for the long-term atmosphere and surface conditions of the planets. The recent discovery of an extra solar planetary system (47 Uma), with a habitable zone that could potentially harbor small terrestrial planets (e.g. Jones and Sleep, 2002 and Thébault et al. 2002), have raised questions about the conditions for habitability in such a system (e.g. Franck et al. 2001 and Cuntz et al. 2003). In that context the effects of the obliquity evolution might be of importance and has been numerically simulated for hypothetical planets located in the habitable zone of 47 Uma. The results found will be presented and discussed in detail. Cuntz et al. 2003. Icarus in press. - Franck et al. 2001. Naturwissenschaften 88, 416. - Jones and Sleep 2002. A&A 393, 1015. - Laskar and Robuterl 1993. Nature 361, 608. - Laskar et al. 1993. Nature 361, 615. - Thébault et al. 2002. A&A 384, 594.

Erikson, A.; Skoglöv, E.

2003-04-01

31

Habitable zones of host stars during the post-MS phase  

NASA Astrophysics Data System (ADS)

A star will become brighter and brighter with stellar evolution, and the distance of its habitable zone will become larger and larger. Some planets outside the habitable zone of a host star during the main sequence phase may enter the habitable zone of the host star during other evolutionary phases. A terrestrial planet within the habitable zone of its host star is generally thought to be suitable for the existence of life. Furthermore, a rocky moon around a giant planet may be also suitable for life to survive, provided that the planet-moon system is within the habitable zone of its host star. Using Eggleton’s code and the boundary flux of the habitable zone, we calculate the habitable zone of our Solar system after the main sequence phase. It is found that Mars’ orbit and Jupiter’s orbit will enter the habitable zone of the Solar system during the subgiant branch phase and the red giant branch phase, respectively. And the orbit of Saturn will enter the habitable zone of Solar during the He-burning phase for about 137 million years. Life is unlikely at any time on Saturn, as it is a giant gaseous planet. However, Titan, the rocky moon of Saturn, may be suitable for biological evolution and become another Earth during that time. For low-mass stars, there are similar habitable zones during the He-burning phase as our Solar, because there are similar core masses and luminosities for these stars during that phase.

Guo, Jianpo; Zhang, Fenghui; Han, Zhanwen

2010-06-01

32

Laminar Accretion in the Habitable Zone of Protoplanetary Disks  

NASA Astrophysics Data System (ADS)

Protoplanetary disks (PPDs) are widely believed to be turbulent as a result of the magnetorotational instability (MRI). We perform magnetohydrodynamical simulations of PPDs that for the first time, take into account both Ohmic resistivity and ambipolar diffusion in a self-consistent manner. We show that in the inner region of PPDs that corresponds the habitable zone, the MRI is completely suppressed due to the interplay between magnetic field and ambipolar diffusion. The gas in this region is laminar throughout the entire vertical extent of the disk. Instead of MRI-driven accretion, a strong magnetocentrifugal wind is launched that efficiently carries away disk angular momentum. A physical wind geometry requires the presence of a strong current layer that is offset from the disk midplane where horizontal magnetic fields flip. We show that the entire accretion flow proceeds through this strong current layer. The non-turbulent nature of the gas flow strongly favors the habitable zone as the site for planetesimal formation, and has important implications for their subsequent growth into terrestrial planets.

Bai, Xue-Ning; Stone, James M.

2014-04-01

33

Toward the Minimum Inner Edge Distance of the Habitable Zone  

NASA Astrophysics Data System (ADS)

We explore the minimum distance from a host star where an exoplanet could potentially be habitable in order not to discard close-in rocky exoplanets for follow-up observations. We find that the inner edge of the Habitable Zone for hot desert worlds can be as close as 0.38 AU around a solar-like star, if the greenhouse effect is reduced (~1% relative humidity) and the surface albedo is increased. We consider a wide range of atmospheric and planetary parameters such as the mixing ratios of greenhouse gases (water vapor and CO2), surface albedo, pressure, and gravity. Intermediate surface pressure (~1-10 bars) is necessary to limit water loss and to simultaneously sustain an active water cycle. We additionally find that the water loss timescale is influenced by the atmospheric CO2 level, because it indirectly influences the stratospheric water mixing ratio. If the CO2 mixing ratio of dry planets at the inner edge is smaller than 10-4, the water loss timescale is ~1 billion years, which is considered here too short for life to evolve. We also show that the expected transmission spectra of hot desert worlds are similar to an Earth-like planet. Therefore, an instrument designed to identify biosignature gases in an Earth-like atmosphere can also identify similarly abundant gases in the atmospheres of dry planets. Our inner edge limit is closer to the host star than previous estimates. As a consequence, the occurrence rate of potentially habitable planets is larger than previously thought.

Zsom, Andras; Seager, Sara; de Wit, Julien; Stamenkovi?, Vlada

2013-12-01

34

Reaching into the Habitable Zones of Kepler Stars  

NASA Astrophysics Data System (ADS)

The field of exoplanets has rapidly expanded from the exclusivity of exoplanet detection to include exoplanet characterization. A key step towards this characterization is the determination of which planets occupy the Habitable Zone (HZ) of their host stars. As the Kepler data continues to be processed, the orbital period sensitivity is increasing and there are now numerous exoplanets known to occupy the HZ of their host stars. In this talk I will describe the properties of the HZ, the dependence on the spectral type properties, and the current state of exoplanet detections in the HZ. I will present several case studies of HZ Kepler planets, including circumbinary planets for which the HZ is a time-dependent function. Finally, I will relate HZ results to the calculation of eta_Earth and eta_Venus.

Kane, Stephen R.; Gelino, D. M.; Hinkel, N. R.

2014-01-01

35

Habitable Zone Super-Earths with Non-Stabilised Spectrographs  

NASA Astrophysics Data System (ADS)

Detecting the small velocity amplitudes (<= 10 m/s) produced by habitable zone rocky planets around M Dwarfs requires radial velocity precisions of a few m s-1. However, an iodine absorption cell, commonly used as a high precision wavelength reference on non-stabilised spectrographs, is not efficient for very red and faint objects like M Dwarfs. Instead, arc lamps have to be used. With the exception of the ultra-stabilised HARPS spectrograph, achieving ~m s-1 calibration with arc lamps has not been possible because typical spectrographs experience drifts of several hundred m s-1 due to local atmospheric changes in pressure and temperature. We outline and present results from an innovative differential wavelength calibration method that enables ~m s-1 precision from non-stabilised, high-resolution spectrographs. This technique allows the detection of rocky planets with radial velocity amplitudes of a few m s-1.

Wright, Duncan J.; Tinney, Christopher G.; Wittenmyer, Robert A.

2014-04-01

36

A Statistical Analysis of Exoplanets in Their Habitable Zones  

NASA Astrophysics Data System (ADS)

The Kepler mission has detected a wealth of planets through planetary transits since its launch in 2009. An important step in the continued study of exoplanets is to characterize planets based on their orbital properties and compositions. As the Kepler mission has progressed the data sensitivity to planetary transits at longer orbital periods has increased. This allows for an enhanced probability of detecting planets which lie in the Habitable Zones (HZs) of their host stars. We present the results of statistical analyses of Kepler planetary candidates to study the percentage of orbital time spent in the HZ as a function of planetary parameters, including planetary mass, radius, and orbital eccentricity. We compare these results to the confirmed exoplanet population.

Adams, Arthur; Kane, S. R.

2014-01-01

37

Assessing circumbinary habitable zones using latitudinal energy balance modelling  

NASA Astrophysics Data System (ADS)

Previous attempts to describe circumbinary habitable zones (HZs) have been concerned with the spatial extent of the zone, calculated analytically according to the combined radiation field of both stars. By contrast to these `spatial HZs', we present a numerical analysis of the `orbital HZ', an HZ defined as a function of planet orbital elements. This orbital HZ is better equipped to handle (for example) eccentric planet orbits, and is more directly connected to the data returned by exoplanet observations. Producing an orbital HZ requires a large number of climate simulations to be run to investigate the parameter space - we achieve this using latitudinal energy balance models, which handle the insolation of the planet by both stars (including mutual eclipses), as well as the planetary atmosphere's ability to absorb, transfer and lose heat. We present orbital HZs for several known circumbinary planetary systems: Kepler-16, Kepler-34, Kepler-35, Kepler-47 and PH-1. Generally, the orbital HZs at zero eccentricity are consistent with spatial HZs derived by other authors, although we detect some signatures of variability that coincide with resonances between the binary and planet orbital periods. We confirm that Earth-like planets around Kepler-47 with Kepler-47c's orbital parameters could possess liquid water, despite current uncertainties regarding its eccentricity. Kepler-16b is found to be outside the HZ, as well as the other circumbinary planets investigated.

Forgan, Duncan

2014-01-01

38

Terrestrial, Habitable-zone Exoplanet Frequency from Kepler  

NASA Astrophysics Data System (ADS)

Data from Kepler's first 136 days of operation are analyzed to determine the distribution of exoplanets with respect to radius, period, and host-star spectral type. The analysis is extrapolated to estimate the percentage of terrestrial, habitable-zone (HZ) exoplanets. The Kepler census is assumed to be complete for bright stars (magnitude <14.0) having transiting planets >0.5 Earth radius and periods <42 days. It is also assumed that the size distribution of planets is independent of orbital period and that there are no hidden biases in the data. Six significant statistical results are found: there is a paucity of small planet detections around faint target stars, probably an instrumental effect; the frequency of mid-size planet detections is independent of whether the host star is bright or faint; there are significantly fewer planets detected with periods <3 days, compared to longer periods, almost certainly an astrophysical effect; the frequency of all planets in the population with periods <42 days is 29%, broken down as terrestrials 9%, ice giants 18%, and gas giants 3%; the population has a planet frequency with respect to period which follows a power-law relation dN/dP ~ P ? - 1, with ? ~= 0.71 ± 0.08; and an extrapolation to longer periods gives the frequency of terrestrial planets in the HZs of FGK stars as ?? ~= (34 ± 14)%. Thus about one-third of FGK stars are predicted to have at least one terrestrial, HZ planet.

Traub, Wesley A.

2012-01-01

39

Kepler Mission: Detecting Earth-sized Planets in Habitable Zones  

NASA Technical Reports Server (NTRS)

The Kepler Mission, which is presently in Phase A, is being proposed for launch in 5 years for a 4-year mission to determine the frequency of Earth-sized or larger planets in habitable zones in our galaxy. Kepler will be placed in an Earth-trailing orbit to provide stable physical environments for the sensitive scientific instruments. The satellite is equipped with a photometric system with the precision of 10E-5, which should be sufficient for detecting the transits of Earth-sized or larger planets in front of dwarf stars similar to the Sun. Approximately 100,000 or more sun-like stars brighter than the 14th apparently magnitude will be monitored continuously for 4 years in a preselected region of the sky, which is about 100 square degrees in size. In addition, Kepler will have a participating scientist program that will enable research in intrinsic variable stars, interacting binaries including cataclysmic stars and X-ray binaries, and a large number of solar analogs in our galaxy. Several ten thousand additional stars may be investigated in the guest observer program open to the whole world.

Kondo, Yoji; Fisher, Richard R. (Technical Monitor)

2001-01-01

40

Probability distribution of terrestrial planets in habitable zones around host stars  

NASA Astrophysics Data System (ADS)

With more and more exoplanets being detected, it is paid closer attention to whether there are lives outside solar system. We try to obtain habitable zones and the probability distribution of terrestrial planets in habitable zones around host stars. Using Eggleton’s code, we calculate the evolution of stars with masses less than 4.00 M ?. We also use the fitting formulae of stellar luminosity and radius, the boundary flux of habitable zones, the distribution of semimajor axis and mass of planets and the initial mass function of stars. We obtain the luminosity and radius of stars with masses from 0.08 to 4.00 M ?, and calculate the habitable zones of host stars, affected by stellar effective temperature. We achieve the probability distribution of terrestrial planets in habitable zones around host stars. We also calculate that the number of terrestrial planets in habitable zones of host stars is 45.5 billion, and the number of terrestrial planets in habitable zones around K type stars is the most, in the Milky Way.

Guo, Jianpo; Zhang, Fenghui; Chen, Xuefei; Han, Zhanwen

2009-10-01

41

Habitable Zones around Main-sequence Stars: New Estimates  

NASA Astrophysics Data System (ADS)

Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et al. However, this model used band models that were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in the Kasting et al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO2 atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67 AU. Here an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars. New H2O and CO2 absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water-loss (inner HZ) and maximum greenhouse (outer HZ) limits for our solar system are at 0.99 and 1.70 AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 and 7200 K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water-loss limits for stars with T eff <~ 5000 K, which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending on the host star's spectral type. We suggest that conservative estimates of the HZ (water-loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on ??, so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

Kopparapu, Ravi Kumar; Ramirez, Ramses; Kasting, James F.; Eymet, Vincent; Robinson, Tyler D.; Mahadevan, Suvrath; Terrien, Ryan C.; Domagal-Goldman, Shawn; Meadows, Victoria; Deshpande, Rohit

2013-03-01

42

HABITABLE ZONES AROUND MAIN-SEQUENCE STARS: NEW ESTIMATES  

SciTech Connect

Identifying terrestrial planets in the habitable zones (HZs) of other stars is one of the primary goals of ongoing radial velocity (RV) and transit exoplanet surveys and proposed future space missions. Most current estimates of the boundaries of the HZ are based on one-dimensional (1D), cloud-free, climate model calculations by Kasting et al. However, this model used band models that were based on older HITRAN and HITEMP line-by-line databases. The inner edge of the HZ in the Kasting et al. model was determined by loss of water, and the outer edge was determined by the maximum greenhouse provided by a CO{sub 2} atmosphere. A conservative estimate for the width of the HZ from this model in our solar system is 0.95-1.67 AU. Here an updated 1D radiative-convective, cloud-free climate model is used to obtain new estimates for HZ widths around F, G, K, and M stars. New H{sub 2}O and CO{sub 2} absorption coefficients, derived from the HITRAN 2008 and HITEMP 2010 line-by-line databases, are important improvements to the climate model. According to the new model, the water-loss (inner HZ) and maximum greenhouse (outer HZ) limits for our solar system are at 0.99 and 1.70 AU, respectively, suggesting that the present Earth lies near the inner edge. Additional calculations are performed for stars with effective temperatures between 2600 and 7200 K, and the results are presented in parametric form, making them easy to apply to actual stars. The new model indicates that, near the inner edge of the HZ, there is no clear distinction between runaway greenhouse and water-loss limits for stars with T{sub eff} {approx}< 5000 K, which has implications for ongoing planet searches around K and M stars. To assess the potential habitability of extrasolar terrestrial planets, we propose using stellar flux incident on a planet rather than equilibrium temperature. This removes the dependence on planetary (Bond) albedo, which varies depending on the host star's spectral type. We suggest that conservative estimates of the HZ (water-loss and maximum greenhouse limits) should be used for current RV surveys and Kepler mission to obtain a lower limit on {eta}{sub Circled-Plus }, so that future flagship missions like TPF-C and Darwin are not undersized. Our model does not include the radiative effects of clouds; thus, the actual HZ boundaries may extend further in both directions than the estimates just given.

Kopparapu, Ravi Kumar; Ramirez, Ramses; Kasting, James F. [Department of Geosciences, Penn State University, 443 Deike Building, University Park, PA 16802 (United States); Eymet, Vincent [Laboratoire d'Astrophysique de Bordeaux, Universite de Bordeaux 1, UMR 5804, F-33270 Floirac (France); Robinson, Tyler D.; Domagal-Goldman, Shawn; Meadows, Victoria [NASA Astrobiology Institute's Virtual Planetary Laboratory (United States); Mahadevan, Suvrath; Terrien, Ryan C.; Deshpande, Rohit [Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, University Park, PA 16802 (United States)

2013-03-10

43

Earth-Sized Planets in the Habitable Zones of Cool Stars  

NASA Astrophysics Data System (ADS)

The primary goal of the Kepler mission is to determine the frequency of Earth-sized planets in the habitable zone of their parent star. Great strides have been made towards achieving this goal, including the discoveries of Earth-sized planets interior to the habitable zone and several super-Earth-sized planets in the habitable zone. A planet that is both Earth-sized and has an orbit within the habitable zone of a main-sequence star, however, has remained elusive. We present updates several promising multi-planet systems that have Earth-sized, and possibly sub-Earth-sized, candidates in the habitable zone of cool low-mass stars in the Kepler field of view. We will present our methods of combining ground-based observations with transit modeling in our quest to confirm these planets and discuss their potential habitability. More than 70% of the stars in our galaxy are M stars, thus confirming these planets will have profound implications on the number of potentially habitable worlds beyond our Solar System.

Quintana, Elisa V.; Barclay, Thomas

2014-06-01

44

Habitable Evaporated Cores: Converting Mini-Neptunes into Super-Earths in the Habitable Zone of M Dwarfs  

NASA Astrophysics Data System (ADS)

We show that photoevaporation and Roche lobe overflow of small gaseous exoplanets (“mini-Neptunes”) in the habitable zone (HZ) of young late M dwarfs can remove several Earth masses of hydrogen/helium from these planets and transform them into potentially habitable worlds, which we call “habitable evaporated cores.” We couple a simple model for the evolution of inflated planets that are partly overflowing their Roche lobes with evaporation due to strong extreme ultraviolet (XUV) irradiation. We also couple the orbital effects of anisotropic mass loss with tidal evolution and show that this coupling can lead to unexpected behavior, such as a net increase of the eccentricity and an outward-then-inward evolution of the semi-major axis, significantly enhancing the mass loss rate. Habitable evaporated cores are most likely to form from planets with up to about 40% gas by mass orbiting M4 stars and later. As terrestrial planet formation by accumulation of local material is likely to form sub-Earth mass planets, evaporation of mini-Neptunes could be the dominant formation mechanism for volatile-rich super-Earths around M dwarfs. Such habitable evaporated cores are likely to be detected in upcoming surveys.

Luger, Rodrigo; Barnes, R.; Lopez, E.; Fortney, J. J.; Jackson, B. K.; Meadows, V.

2014-01-01

45

The Habitability of Our Earth and Other Earths: Astrophysical, Geochemical, Geophysical, and Biological Limits on Planet Habitability  

NASA Astrophysics Data System (ADS)

For life-forms like us, the most important feature of Earth is its habitability. Understanding habitability and using that knowledge to locate the nearest habitable planet may be crucial for our survival as a species. During the past decade, expectations that the universe could be filled with habitable planets have been bolstered by the increasingly large overlap between terrestrial environments known to harbor life and the variety of environments on newly detected rocky exoplanets. The inhabited and uninhabited regions on Earth tell us that temperature and the presence of water are the main constraints that can be used in a habitability classification scheme for rocky planets. Our compilation and review of recent exoplanet detections suggests that the fraction of stars with planets is ˜100%, and that the fraction with rocky planets may be comparably large. We review extensions to the circumstellar habitable zone (HZ), including an abiogenesis habitable zone and the galactic habitable zone.

Lineweaver, Charles H.; Chopra, Aditya

2012-05-01

46

Re-Evaluation of the Inner Edge of Habitable Zone  

NASA Astrophysics Data System (ADS)

Existence of liquid water on the planetary surface is thought to be an important condition for the origin and evolution of life. Planets with oceans (or lakes) are classified in two types: Earth-like `aqua planets' and less water `land planets'. The latter shows stronger resistance than the former to the runaway greenhouse caused by the increase of stellar luminosity. We examined the possibility of evolution from an aqua planet to a land planet by water loss. We showed that an aqua planet with less than about 0.1 present Earth's ocean mass can evolve to a land planet without having experience of the runaway greenhouse, and maintains liquid water on its surface for about 2Gyrs longer than planets with larger amount of water. Our results mean that the initial amount of water is important for their evolution paths and habitability.

Kodama, Takanori; Genda, Hidenori; Abe, Yutaka; Zahnle, Kevin

2014-04-01

47

TERRESTRIAL PLANET FORMATION AROUND THE CIRCUMBINARY HABITABLE ZONE: INWARD MIGRATION IN THE PLANETESIMAL SWARM  

SciTech Connect

According to the core accretion theory, circumbinary embryos can form only beyond a critical semimajor axis (CSMA). However, due to the relatively high density of solid materials in the inner disk, a significant amount of small planetesimals must exist in the inner zone when embryos form outside this CSMA. Thus, embryo migration induced by the planetesimal swarm is possible after gas disk depletion. Through numerical simulations, we found that (1) the scattering-driven inward migration of embryos is robust and planets can form in the habitable zone if we adopt a mass distribution of an MMSN-like disk; (2) the total mass of the planetesimals in the inner region and continuous embryo-embryo scattering are two key factors that cause significant embryo migrations; and (3) the scattering-driven migration of embryos is a natural water-delivery mechanism. We propose that planet detections should focus on the close binary with its habitable zone near CSMA.

Gong Yanxiang; Zhou Jilin; Xie Jiwei, E-mail: yxgong@nju.edu.cn, E-mail: zhoujl@nju.edu.cn [Department of Astronomy and Key Laboratory of Modern Astronomy and Astrophysics in Ministry of Education, Nanjing University, Nanjing 210093 (China)

2013-01-20

48

Habitable Climates  

Microsoft Academic Search

According to the standard liquid-water definition, the Earth is only\\u000apartially habitable. We reconsider planetary habitability in the framework of\\u000aenergy-balance models, the simplest seasonal models in physical climatology, to\\u000aassess the spatial and temporal habitability of Earth-like planets. We quantify\\u000athe degree of climatic habitability of our models with several metrics of\\u000afractional habitability. Previous evaluations of habitable zones

David S. Spiegel; Kristen Menou; Caleb A. Scharf

2007-01-01

49

Exomoon Habitability Constrained by Illumination and Tidal Heating  

PubMed Central

Abstract The detection of moons orbiting extrasolar planets (“exomoons”) has now become feasible. Once they are discovered in the circumstellar habitable zone, questions about their habitability will emerge. Exomoons are likely to be tidally locked to their planet and hence experience days much shorter than their orbital period around the star and have seasons, all of which works in favor of habitability. These satellites can receive more illumination per area than their host planets, as the planet reflects stellar light and emits thermal photons. On the contrary, eclipses can significantly alter local climates on exomoons by reducing stellar illumination. In addition to radiative heating, tidal heating can be very large on exomoons, possibly even large enough for sterilization. We identify combinations of physical and orbital parameters for which radiative and tidal heating are strong enough to trigger a runaway greenhouse. By analogy with the circumstellar habitable zone, these constraints define a circumplanetary “habitable edge.” We apply our model to hypothetical moons around the recently discovered exoplanet Kepler-22b and the giant planet candidate KOI211.01 and describe, for the first time, the orbits of habitable exomoons. If either planet hosted a satellite at a distance greater than 10 planetary radii, then this could indicate the presence of a habitable moon. Key Words: Astrobiology—Extrasolar planets—Habitability—Habitable zone—Tides. Astrobiology 13, 18–46.

2013-01-01

50

ASTROPHYSICAL PARAMETERS AND HABITABLE ZONE OF THE EXOPLANET HOSTING STAR GJ 581  

SciTech Connect

GJ 581 is an M dwarf host of a multiplanet system. We use long-baseline interferometric measurements from the CHARA Array, coupled with trigonometric parallax information, to directly determine its physical radius to be 0.299 {+-} 0.010 R{sub sun}. Literature photometry data are used to perform spectral energy distribution fitting in order to determine GJ 581's effective surface temperature T{sub EFF} = 3498 {+-} 56 K and its luminosity L = 0.01205 {+-} 0.00024 L{sub sun}. From these measurements, we recompute the location and extent of the system's habitable zone and conclude that two of the planets orbiting GJ 581, planets d and g, spend all or part of their orbit within or just on the edge of the habitable zone.

Von Braun, Kaspar; Kane, Stephen R.; Ciardi, David R. [NASA Exoplanet Science Institute, California Institute of Technology, MC 100-22, Pasadena, CA 91125 (United States); Boyajian, Tabetha S.; McAlister, Harold A.; Henry, Todd J.; Jao, Wei-Chun; Riedel, Adric R. [Center for High Angular Resolution Astronomy and Department of Physics and Astronomy, Georgia State University, P.O. Box 4106, Atlanta, GA 30302-4106 (United States); Van Belle, Gerard T. [European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching (Germany); Lopez-Morales, Mercedes [Institut de Ciencies de L'Espai (CSIC-IEEC), Campus UAB, Facultat Ciencies, Torre C5 parell 2, 08193 Bellaterra, Barcelona (Spain); Subasavage, John P. [Cerro Tololo Inter-American Observatory, Casilla 603, La Serena (Chile); Schaefer, Gail; Ten Brummelaar, Theo A.; Sturmann, Laszlo; Sturmann, Judit; Mazingue, Jude; Turner, Nils H.; Farrington, Chris; Goldfinger, P. J. [CHARA Array, Mount Wilson Observatory, Mount Wilson, CA 91023 (United States); Ridgway, Stephen, E-mail: kaspar@caltech.edu [National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726-6732 (United States)

2011-03-10

51

Catalytical properties of liver monoamine oxidases of the Commander squid Berryteuthis magister from various habitation zones  

Microsoft Academic Search

There has been performed kinetic analysis of enzymatic reactions of deamination of tyramine, tryptamine, serotonin, benzylamine,\\u000a ?-phenylethylamine, and histamine under action of liver monoamine oxidase (MAO) of the Commander squid Berryteuthis magister from various habitation zones in the Bering and Japan Seas. A substrate inhibition by high concentrations of all studied\\u000a substrates has been revealed, which seems to indicate mutual

O. V. Yagodina

2009-01-01

52

THE HABITABLE ZONE OF EARTH-LIKE PLANETS WITH DIFFERENT LEVELS OF ATMOSPHERIC PRESSURE  

SciTech Connect

As a contribution to the study of the habitability of extrasolar planets, we implemented a one-dimensional energy balance model (EBM), the simplest seasonal model of planetary climate, with new prescriptions for most physical quantities. Here we apply our EBM to investigate the surface habitability of planets with an Earth-like atmospheric composition but different levels of surface pressure. The habitability, defined as the mean fraction of the planet's surface on which liquid water could exist, is estimated from the pressure-dependent liquid water temperature range, taking into account seasonal and latitudinal variations of surface temperature. By running several thousands of EBM simulations we generated a map of the habitable zone (HZ) in the plane of the orbital semi-major axis, a, and surface pressure, p, for planets in circular orbits around a Sun-like star. As pressure increases, the HZ becomes broader, with an increase of 0.25 AU in its radial extent from p = 1/3 to 3 bar. At low pressure, the habitability is low and varies with a; at high pressure, the habitability is high and relatively constant inside the HZ. We interpret these results in terms of the pressure dependence of the greenhouse effect, the efficiency of horizontal heat transport, and the extent of the liquid water temperature range. Within the limits discussed in the paper, the results can be extended to planets in eccentric orbits around non-solar-type stars. The main characteristics of the pressure-dependent HZ are modestly affected by variations of planetary properties, particularly at high pressure.

Vladilo, Giovanni; Murante, Giuseppe; Silva, Laura [INAF-Trieste Astronomical Observatory, Trieste (Italy)] [INAF-Trieste Astronomical Observatory, Trieste (Italy); Provenzale, Antonello [Institute of Atmospheric Sciences and Climate-CNR, Torino (Italy)] [Institute of Atmospheric Sciences and Climate-CNR, Torino (Italy); Ferri, Gaia; Ragazzini, Gregorio, E-mail: vladilo@oats.inaf.it [Department of Physics, University of Trieste, Trieste (Italy)] [Department of Physics, University of Trieste, Trieste (Italy)

2013-04-10

53

A METHOD FOR COUPLING DYNAMICAL AND COLLISIONAL EVOLUTION OF DUST IN CIRCUMSTELLAR DISKS: THE EFFECT OF A DEAD ZONE  

SciTech Connect

Dust is a major component of protoplanetary and debris disks as it is the main observable signature of planetary formation. However, since dust dynamics are size-dependent (because of gas drag or radiation pressure) any attempt to understand the full dynamical evolution of circumstellar dusty disks that neglect the coupling of collisional evolution with dynamical evolution is thwarted because of the feedback between these two processes. Here, a new hybrid Lagrangian/Eulerian code is presented that overcomes some of these difficulties. The particles representing 'dust clouds' are tracked individually in a Lagrangian way. This system is then mapped on an Eulerian spatial grid, inside the cells of which the local collisional evolutions are computed. Finally, the system is remapped back in a collection of discrete Lagrangian particles, keeping their number constant. An application example of dust growth in a turbulent protoplanetary disk at 1 AU is presented. First, the growth of dust is considered in the absence of a dead zone and the vertical distribution of dust is self-consistently computed. It is found that the mass is rapidly dominated by particles about a fraction of a millimeter in size. Then the same case with an embedded dead zone is investigated and it is found that coagulation is much more efficient and produces, in a short timescale, 1-10 cm dust pebbles that dominate the mass. These pebbles may then be accumulated into embryo-sized objects inside large-scale turbulent structures as shown recently.

Charnoz, Sebastien; Taillifet, Esther, E-mail: charnoz@cea.fr [Laboratoire AIM, Universite Paris Diderot/CEA/CNRS, 91191 Gif-sur-Yvette Cedex (France)

2012-07-10

54

MOA-2011-BLG-293Lb: First Microlensing Planet Possibly in the Habitable Zone  

NASA Astrophysics Data System (ADS)

We used Keck adaptive optics observations to identify the first planet discovered by microlensing to lie in or near the habitable zone, i.e., at projected separation r = 1.1 ± 0.1 AU from its ML = 0.86 ± 0.06 M ? host, being the highest microlensing mass definitely identified. The planet has a mass mp = 4.8 ± 0.3 M Jup, and could in principle have habitable moons. This is also the first planet to be identified as being in the Galactic bulge with good confidence: DL = 7.72 ± 0.44 kpc. The planet/host masses and distance were previously not known, but only estimated using Bayesian priors based on a Galactic model. These estimates had suggested that the planet might be a super-Jupiter orbiting an M dwarf, a very rare class of planets. We obtained high-resolution JHK images using Keck adaptive optics to detect the lens and so test this hypothesis. We clearly detect light from a G dwarf at the position of the event, and exclude all interpretations other than that this is the lens with high confidence (95%), using a new astrometric technique. The calibrated magnitude of the planet host star is HL = 19.16 ± 0.13. We infer the following probabilities for the three possible orbital configurations of the gas giant planet: 53% to be in the habitable zone, 35% to be near the habitable zone, and 12% to be beyond the snow line, depending on the atmospherical conditions and the uncertainties on the semimajor axis.

Batista, V.; Beaulieu, J.-P.; Gould, A.; Bennett, D. P.; Yee, J. C.; Fukui, A.; Gaudi, B. S.; Sumi, T.; Udalski, A.

2014-01-01

55

Validating the First Habitable-Zone Planet Candidates Identified by the NASA Kepler Mission  

NASA Astrophysics Data System (ADS)

At the beginning of Cycle 8, the NASA Kepler Mission will have completed two years of science observations, the minimum baseline sufficient to identify candidate transiting exoplanets orbiting within the habitable-zones of Sun-like stars. The principal task that lies ahead is to reject from this sample the false positives (blends of eclipsing binaries that precisely mimic the signal of a transiting exoplanet), and to confirm the planetary nature of the remaining candidates. For planets more massive than Neptune, the direct confirmation of their planetary status can be accomplished by radial-velocity measurements. However, such planets possess primordial envelopes of hydrogen and helium that make them unsuitable to life as we know it. The most exciting candidates -- and the ones that Kepler is specifically tasked with finding -- are super-Earth and Earth-sized planets orbiting within their stellar habitable zones. Kepler has just begun to identify such planet candidates, and it will identify many more as its baseline increases throughout the coming year. While the Kepler team has developed powerful tools to weed out the impostors, Spitzer possesses the unique ability to provide the final validation of these candidates as planets, namely by measuring the depth of the transit at infrared wavelengths. By combining the infrared and optical measurements of the transit depth with models of hypothetical stellar blends, we can definitively test the stellar-blend hypothesis. We propose to observe the transits of 20 candidate habitable-zone super-Earths to be identified by the Kepler Mission. The results from this Exploration Science Program will be twofold: First, we will definitively validate the first potentially habitable planets ever identified. Second, we will determine the rate of occurrence of impostors. This rate of false positives can then be applied to the much larger sample of candidates identified by Kepler, to deduce the true rate of planetary companions.

Charbonneau, David; Desert, Jean-Michel; Fressin, Francois; Ballard, Sarah; Borucki, William; Latham, David; Gilliland, Ronald; Seager, Sara; Knutson, Heather; Fortney, Jonathan; Brown, Timothy; Ford, Eric; Deming, Drake; Torres, Guillermo

2011-05-01

56

The galactic habitable zone of the Milky Way and M31 from chemical evolution models with gas radial flows  

NASA Astrophysics Data System (ADS)

The galactic habitable zone is defined as the region with sufficient abundance of heavy elements to form planetary systems in which Earth-like planets could be born and might be capable of sustaining life, after surviving to close supernova explosion events. Galactic chemical evolution models can be useful for studying the galactic habitable zones in different systems. We apply detailed chemical evolution models including radial gas flows to study the galactic habitable zones in our Galaxy and M31. We compare the results to the relative galactic habitable zones found with `classical' (independent ring) models, where no gas inflows were included. For both the Milky Way and Andromeda, the main effect of the gas radial inflows is to enhance the number of stars hosting a habitable planet with respect to the `classical' model results, in the region of maximum probability for this occurrence, relative to the classical model results. These results are obtained by taking into account the supernova destruction processes. In particular, we find that in the Milky Way the maximum number of stars hosting habitable planets is at 8 kpc from the Galactic Centre, and the model with radial flows predicts a number which is 38 per cent larger than what was predicted by the classical model. For Andromeda we find that the maximum number of stars with habitable planets is at 16 kpc from the centre and that in the case of radial flows this number is larger by 10 per cent relative to the stars predicted by the classical model.

Spitoni, E.; Matteucci, F.; Sozzetti, A.

2014-05-01

57

Thermal Evolution and Lifetime of Intrinsic Magnetic Fields of Super-Earths in Habitable Zones  

NASA Astrophysics Data System (ADS)

We have numerically studied the thermal evolution of different-mass terrestrial planets in habitable zones, focusing on the duration of dynamo activity to generate their intrinsic magnetic fields, which may be one of the key factors in habitability of the planets. In particular, we are concerned with super-Earths, observations of which are rapidly developing. We calculated the evolution of temperature distributions in the planetary interior using Vinet equations of state, the Arrhenius-type formula for mantle viscosity, and the astrophysical mixing-length theory for convective heat transfer modified for mantle convection. After calibrating the model with terrestrial planets in the solar system, we apply it for 0.1-10 M ? rocky planets with a surface temperature of 300 K (in habitable zones) and Earth-like compositions. With the criterion of heat flux at the core-mantle boundary (CMB), the lifetime of the magnetic fields is evaluated from the calculated thermal evolution. We found that the lifetime slowly increases with planetary mass (Mp ), independent of the initial temperature gap at the CMB (?T CMB), but beyond the critical value M c,p (~O(1) M ?) it abruptly declines from the mantle viscosity enhancement due to the pressure effect. We derived M c,p as a function of ?T CMB and a rheological parameter (activation volume, V*). Thus, the magnetic field lifetime of super-Earths with Mp >M p,c sensitively depends on ?T CMB, which reflects planetary accretion, and V*, which has uncertainty at very high pressure. More advanced high-pressure experiments and first-principle simulation, as well as planetary accretion simulation, are needed to discuss the habitability of super-Earths.

Tachinami, C.; Senshu, H.; Ida, S.

2011-01-01

58

The Instellation Habitable Zone: Liquid Water Stability on a Single Axis  

NASA Astrophysics Data System (ADS)

The habitable zone is the region around a star for which liquid water might be stable at the surface of a planet. This is roughly defined as orbital distances greater than those for which runaway greenhouses are triggered or water loss via H escape becomes rapid, yet less than those for which snowball Earth conditions are unavoidable. Both of these limits are inherently tied to surface temperature of the planet. Exoplanet observers have often defined habitable zones based on estimates of the equilibrium temperature for a planet, using that as a proxy for surface. However, the calculation of equilibrium temperature requires knowledge of the planet's albedo, which is usually not known. Furthermore, translating a planet's equilibrium temperature into a surface temperature requires estimations of greenhouse and anti-greenhouse effects that are also unknown. Venus presents both of these problems: it has a much-higher albedo than the value commonly assumed for Earth-like exoplanets, and yet its surface temperature is hundreds of degrees higher than its equilibrium temperature. Without knowledge of the albedo of a planet or the magnitude of the greenhouse effect, equilibrium temperature is an unknown quantity that provides unreliable estimates of the surface temperature of a planet. For these reasons, atmospheric modelers have incorporated the effects of albedo and of greenhouse effects into definitions of the habitable zone. Historically, these definitions have been based on the luminosity of the host star and the semi-major axis of the planet's orbit. This has served the community well, as planets are treated and analyzed on a case-by-base basis. However, the presence of two criteria for habitability (semi-major axis and stellar luminosity) presents an impediment to plotting planets in 2-dimensional diagrams that also include geophysical parameters such as planetary radius, mass, or density. While such plots were not previously warranted for ~Earth-sized planets because very few were known, the large number of ~Earth-sized planets currently being discovered by exoplanet surveys such as NASA's Kepler mission increase the need for a single metric that represents the possibility for liquid water to be stable at the surface of a planet. In this presentation, we propose the use of installation - the amount of energy reaching the top of a planet's atmosphere - as a metric for habitability that can be calculated strictly from measured properties and that also allows for display of "surface water stability" on the same chart as other geophysical parameters. The habitable zone presented here is primarily derived from information on planets in our solar system, including knowledge of the history of those planets. We compare this new definition of the habitable zone to traditional ones, and apply it to the February 2011 release of data from the Kepler data set and the database of confirmed extrasolar planets.

Domagal-Goldman, S. D.; Virtual Planetary Laboratory

2011-12-01

59

Habitable Climates  

NASA Astrophysics Data System (ADS)

According to the standard liquid-water definition, the Earth is only partially habitable. We reconsider planetary habitability in the framework of energy balance models, the simplest seasonal models in physical climatology, to assess the spatial and temporal habitability of Earth-like planets. We quantify the degree of climatic habitability of our models with several metrics of fractional habitability. Previous evaluations of habitable zones may have omitted important climatic conditions by focusing on close solar system analogies. For example, we find that model pseudo-Earths with different rotation rates or different land-ocean fractions have fractional habitabilities that differ significantly from that of the Earth itself. Furthermore, the stability of a planet's climate against albedo-feedback snowball events strongly impacts its habitability. Therefore, issues of climate dynamics may be central in assessing the habitability of discovered terrestrial exoplanets, especially if astronomical forcing conditions are different from the moderate solar system cases.

Spiegel, David S.; Menou, Kristen; Scharf, Caleb A.

2008-07-01

60

The ultraviolet radiation environment in the habitable zones around low-mass exoplanet host stars  

NASA Astrophysics Data System (ADS)

The EUV (200-911 Å), FUV (912-1750 Å), and NUV (1750-3200 Å) spectral energy distribution of exoplanet host stars has a profound influence on the atmospheres of Earth-like planets in the habitable zone. The stellar EUV radiation drives atmospheric heating, while the FUV (in particular, Ly?) and NUV radiation fields regulate the atmospheric chemistry: the dissociation of H2O and CO2, the production of O2 and O3, and may determine the ultimate habitability of these worlds. Despite the importance of this information for atmospheric modeling of exoplanetary systems, the EUV/FUV/NUV radiation fields of cool (K and M dwarf) exoplanet host stars are almost completely unconstrained by observation or theory. We present observational results from a Hubble Space Telescope survey of M dwarf exoplanet host stars, highlighting the importance of realistic UV radiation fields for the formation of potential biomarker molecules, O2 and O3. We conclude by describing preliminary results on the characterization of the UV time variability of these sources.

France, Kevin; Linsky, Jeffrey L.; Parke Loyd, R. O.

2014-04-01

61

The galactic habitable zone and the age distribution of complex life in the Milky Way.  

PubMed

We modeled the evolution of the Milky Way Galaxy to trace the distribution in space and time of four prerequisites for complex life: the presence of a host star, enough heavy elements to form terrestrial planets, sufficient time for biological evolution, and an environment free of life-extinguishing supernovae. We identified the Galactic habitable zone (GHZ) as an annular region between 7 and 9 kiloparsecs from the Galactic center that widens with time and is composed of stars that formed between 8 and 4 billion years ago. This GHZ yields an age distribution for the complex life that may inhabit our Galaxy. We found that 75% of the stars in the GHZ are older than the Sun. PMID:14704421

Lineweaver, Charles H; Fenner, Yeshe; Gibson, Brad K

2004-01-01

62

An Earth-sized planet in the habitable zone of a cool star.  

PubMed

The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets, all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star's radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form. PMID:24744370

Quintana, Elisa V; Barclay, Thomas; Raymond, Sean N; Rowe, Jason F; Bolmont, Emeline; Caldwell, Douglas A; Howell, Steve B; Kane, Stephen R; Huber, Daniel; Crepp, Justin R; Lissauer, Jack J; Ciardi, David R; Coughlin, Jeffrey L; Everett, Mark E; Henze, Christopher E; Horch, Elliott; Isaacson, Howard; Ford, Eric B; Adams, Fred C; Still, Martin; Hunter, Roger C; Quarles, Billy; Selsis, Franck

2014-04-18

63

An Earth-Sized Planet in the Habitable Zone of a Cool Star  

NASA Astrophysics Data System (ADS)

The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets, all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star’s radiation place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its surface, then some of this water is likely to be in liquid form.

Quintana, Elisa V.; Barclay, Thomas; Raymond, Sean N.; Rowe, Jason F.; Bolmont, Emeline; Caldwell, Douglas A.; Howell, Steve B.; Kane, Stephen R.; Huber, Daniel; Crepp, Justin R.; Lissauer, Jack J.; Ciardi, David R.; Coughlin, Jeffrey L.; Everett, Mark E.; Henze, Christopher E.; Horch, Elliott; Isaacson, Howard; Ford, Eric B.; Adams, Fred C.; Still, Martin; Hunter, Roger C.; Quarles, Billy; Selsis, Franck

2014-04-01

64

Anomalously diverse Early Triassic ichnofossil assemblages in northwest Pangea: A case for a shallow-marine habitable zone  

NASA Astrophysics Data System (ADS)

Early Triassic trace fossil assemblages from the northwest marginof Pangea record a diverse suite of postextinction infauna.These ichnofossil assemblages occurred within well-oxygenated,shallow-marine refuges in a Panthalassa Ocean otherwise characterizedby widespread anoxia. We propose an environmentally controlledmodel for their distribution, in which wave aeration, enhancedby frequent storms, gave rise to an optimal zone for benthiccolonization. Within this habitable zone extinction pressureswere ameliorated and postextinction recovery duration was minimized.

Beatty, Tyler W.; Zonneveld, J.-P.; Henderson, Charles M.

2008-10-01

65

WISE Detections of Dust in the Habitable Zones of Planet-Bearing Stars  

NASA Technical Reports Server (NTRS)

We use data from the Wide-field Infrared Survey Explorer (WISE) all-sky release to explore the incidence of warm dust in the habitable zones around exoplanet-host stars. Dust emission at 12 and/or 22 microns (T(sub dust) approx.300 and/or approx.150 K) traces events in the terrestrial planet zones; its existence implies replenishment by evaporation of comets or collisions of asteroids, possibly stirred by larger planets. Of the 591 planetary systems (728 extrasolar planets) in the Exoplanet Encyclopedia as of 2012 January 31, 350 are robustly detected by WISE at > or = 5(sigma) level. We perform detailed photosphere subtraction using tools developed for Spitzer data and visually inspect all the WISE images to confirm bona fide point sources. We find nine planet-bearing stars show dust excess emission at 12 and/or 22 microns at > or = 3(sigma) level around young, main-sequence, or evolved giant stars. Overall, our results yield an excess incidence of approx.2.6% for stars of all evolutionary stages, but approx.1% for planetary debris disks around main-sequence stars. Besides recovering previously known warm systems, we identify one new excess candidate around the young star UScoCTIO 108.

Morales, Farisa Y.; Padgett, Deborah L.; Bryden, Geoffrey; Werner, M. W.; Furlan, E.

2012-01-01

66

Biomarker Response to Cosmic Rays on Planets orbiting in the Habitable Zone of M-Stars  

NASA Astrophysics Data System (ADS)

Our model predicts that biomarkers from planets orbiting in the Habitable Zone (HZ) of M-Dwarf stars with Earthlike atmospheres (hereafter "M-star worlds") may survive strong fluxes from Galactic Cosmic Rays (GCRs). These calculations included the effects of (albeit weakened) magnetospheric shielding. Our result refers to the effect of cosmic rays producing nitrogen oxides, NOx(=NO+NO2) in earthlike atmospheres which perturb the atmospheric chemistry hence remove ozone. Ozone on the M-star world is resilient to GCR-induced NOx because high methane levels favour the ozoneproducing "smog" mechanism, which is catalysed by NOx as discussed by Grenfell et al. (2007) (Astrobiology Special Issue on M-stars). We have also performed an initial estimate of stellar cosmic ray (SCR) fluxes but these are currently upper estimate fluxes, based on our own Sun, assuming a completely unmagnetised planet scaled to 0.2 AU i.e. in the HZ. SCR results imply that up to 98

Grenfell, J. L.; Griessmeier, J.-M.; Hedelt, P.; Patzer, B.; Rauer, H.; Stracke, B.; von Paris, P.

2007-08-01

67

Distribution of Extrasolar Planets relative to the Habitable Zones of Nearby Stars  

NASA Astrophysics Data System (ADS)

Today ˜102 planets are known to orbit 96 sun-like stars, and 86% of the planets are within 50 parsecs of Earth. The stars themselves range in spectral type from F7 to M5, and from luminosity class V (main sequence, like the Sun) to luminosity class III (sub-giants). The companions to most of these stars have been detected through careful long-term monitoring of stellar radial velocity variations, which reveals the product msini (m is mass and i is the inclination of the planet's orbit relative to our line of sight), not the planetary mass m. Thus far, the companions are all more massive than 0.12 MJ (or 2.2 x Neptune) and are located between 0.037 AU and 4.47 AU from their parent stars. Over 40% (42) of the new objects spend at least a portion of their orbits within the liquid-water habitable zone (HZ) of their stars, and many (25) on orbits with eccentricities e > 0.4. Systems with planets whose orbits cross the HZ boundaries might harbor life on terrestrial-like moons if the seasonal temperature extremes resulting from the planets? large orbital eccentricities are not too severe [cf., Williams and Pollard 2002. Inter. J. Astrobio. 1(1), 61-69]. Here we show the locations and range of orbital distances for the known extrasolar planets relative to the HZs around their parent stars.

Williams, D. M.

2003-05-01

68

DIRECT IMAGING IN THE HABITABLE ZONE AND THE PROBLEM OF ORBITAL MOTION  

SciTech Connect

High contrast imaging searches for exoplanets have been conducted on 2.4-10 m telescopes, typically at H band (1.6 {mu}m) and used exposure times of {approx}1 hr to search for planets with semi-major axes of {approx}> 10 AU. We are beginning to plan for surveys using extreme-AO systems on the next generation of 30 m class telescopes, where we hope to begin probing the habitable zones (HZs) of nearby stars. Here we highlight a heretofore ignorable problem in direct imaging: planets orbit their stars. Under the parameters of current surveys, orbital motion is negligible over the duration of a typical observation. However, this motion is not negligible when using large diameter telescopes to observe at relatively close stellar distances (1-10 pc), over the long exposure times (10-20 hr) necessary for direct detection of older planets in the HZ. We show that this motion will limit our achievable signal-to-noise ratio and degrade observational completeness. Even on current 8 m class telescopes, orbital motion will need to be accounted for in an attempt to detect HZ planets around the nearest Sun-like stars {alpha} Cen A and B, a binary system now known to harbor at least one planet. Here we derive some basic tools for analyzing this problem, and ultimately show that the prospects are good for de-orbiting a series of shorter exposures to correct for orbital motion.

Males, Jared R.; Skemer, Andrew J.; Close, Laird M., E-mail: jrmales@as.arizona.edu [Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States)

2013-07-01

69

Habitable Zones around Main-sequence Stars: Dependence on Planetary Mass  

NASA Astrophysics Data System (ADS)

The ongoing discoveries of extra-solar planets are unveiling a wide range of terrestrial mass (size) planets around their host stars. In this Letter, we present estimates of habitable zones (HZs) around stars with stellar effective temperatures in the range 2600 K-7200 K, for planetary masses between 0.1 M ? and 5 M ?. Assuming H2O-(inner HZ) and CO2-(outer HZ) dominated atmospheres, and scaling the background N2 atmospheric pressure with the radius of the planet, our results indicate that larger planets have wider HZs than do smaller ones. Specifically, with the assumption that smaller planets will have less dense atmospheres, the inner edge of the HZ (runaway greenhouse limit) moves outward (~10% lower than Earth flux) for low mass planets due to larger greenhouse effect arising from the increased H2O column depth. For larger planets, the H2O column depth is smaller, and higher temperatures are needed before water vapor completely dominates the outgoing longwave radiation. Hence the inner edge moves inward (~7% higher than Earth's flux). The outer HZ changes little due to the competing effects of the greenhouse effect and an increase in albedo. New, three-dimensional climate model results from other groups are also summarized, and we argue that further, independent studies are needed to verify their predictions. Combined with our previous work, the results presented here provide refined estimates of HZs around main-sequence stars and provide a step toward a more comprehensive analysis of HZs.

Kopparapu, Ravi Kumar; Ramirez, Ramses M.; SchottelKotte, James; Kasting, James F.; Domagal-Goldman, Shawn; Eymet, Vincent

2014-06-01

70

Delayed gratification habitable zones: when deep outer solar system regions become balmy during post-main sequence stellar evolution.  

PubMed

Like all low- and moderate-mass stars, the Sun will burn as a red giant during its later evolution, generating of solar luminosities for some tens of millions of years. During this post-main sequence phase, the habitable (i.e., liquid water) thermal zone of our Solar System will lie in the region where Triton, Pluto-Charon, and Kuiper Belt objects orbit. Compared with the 1 AU habitable zone where Earth resides, this "delayed gratification habitable zone" (DGHZ) will enjoy a far less biologically hazardous environment - with lower harmful radiation levels from the Sun, and a far less destructive collisional environment. Objects like Triton, Pluto-Charon, and Kuiper Belt objects, which are known to be rich in both water and organics, will then become possible sites for biochemical and perhaps even biological evolution. The Kuiper Belt, with >10(5) objects > or =50 km in radius and more than three times the combined surface area of the four terrestrial planets, provides numerous sites for possible evolution once the Sun's DGHZ reaches it. The Sun's DGHZ might be thought to only be of academic interest owing to its great separation from us in time. However, approximately 10(9) Milky Way stars burn as luminous red giants today. Thus, if icy-organic objects are common in the 20-50 AU zones of these stars, as they are in our Solar System (and as inferred in numerous main sequence stellar disk systems), then DGHZs may form a niche type of habitable zone that is likely to be numerically common in the Galaxy. PMID:14577880

Stern, S Alan

2003-01-01

71

Exoplanet detection. Stellar activity masquerading as planets in the habitable zone of the M dwarf Gliese 581.  

PubMed

The M dwarf star Gliese 581 is believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. The detection of another habitable-zone planet--GJ 581g--is disputed, as its significance depends on the eccentricity assumed for d. Analyzing stellar activity using the H? line, we measure a stellar rotation period of 130 ± 2 days and a correlation for H? modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5 standard deviations) while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g. PMID:24993348

Robertson, Paul; Mahadevan, Suvrath; Endl, Michael; Roy, Arpita

2014-07-25

72

WATER-PLANETS IN THE HABITABLE ZONE: ATMOSPHERIC CHEMISTRY, OBSERVABLE FEATURES, AND THE CASE OF KEPLER-62e AND -62f  

SciTech Connect

Planets composed of large quantities of water that reside in the habitable zone are expected to have distinct geophysics and geochemistry of their surfaces and atmospheres. We explore these properties motivated by two key questions: whether such planets could provide habitable conditions and whether they exhibit discernable spectral features that distinguish a water-planet from a rocky Earth-like planet. We show that the recently discovered planets Kepler-62e and -62f are the first viable candidates for habitable zone water-planets. We use these planets as test cases for discussing those differences in detail. We generate atmospheric spectral models and find that potentially habitable water-planets show a distinctive spectral fingerprint in transit depending on their position in the habitable zone.

Kaltenegger, L. [Max Planck Institute of Astronomy, Koenigstuhl 17, D-69115 Heidelberg (Germany)] [Max Planck Institute of Astronomy, Koenigstuhl 17, D-69115 Heidelberg (Germany); Sasselov, D.; Rugheimer, S., E-mail: kaltenegger@mpia.de [Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States)

2013-10-01

73

A SUPER-EARTH-SIZED PLANET ORBITING IN OR NEAR THE HABITABLE ZONE AROUND A SUN-LIKE STAR  

SciTech Connect

We present the discovery of a super-Earth-sized planet in or near the habitable zone of a Sun-like star. The host is Kepler-69, a 13.7 mag G4V-type star. We detect two periodic sets of transit signals in the 3-year flux time series of Kepler-69, obtained with the Kepler spacecraft. Using the very high precision Kepler photometry, and follow-up observations, our confidence that these signals represent planetary transits is >99.3%. The inner planet, Kepler-69b, has a radius of 2.24{sup +0.44}{sub -0.29} R{sub Circled-Plus} and orbits the host star every 13.7 days. The outer planet, Kepler-69c, is a super-Earth-sized object with a radius of 1.7{sup +0.34}{sub -0.23} R{sub Circled-Plus} and an orbital period of 242.5 days. Assuming an Earth-like Bond albedo, Kepler-69c has an equilibrium temperature of 299 {+-} 19 K, which places the planet close to the habitable zone around the host star. This is the smallest planet found by Kepler to be orbiting in or near the habitable zone of a Sun-like star and represents an important step on the path to finding the first true Earth analog.

Barclay, Thomas; Burke, Christopher J.; Howell, Steve B.; Rowe, Jason F.; Huber, Daniel; Jenkins, Jon M.; Quintana, Elisa V.; Still, Martin; Twicken, Joseph D.; Bryson, Stephen T.; Borucki, William J.; Caldwell, Douglas A.; Clarke, Bruce D.; Christiansen, Jessie L; Coughlin, Jeffrey L. [NASA Ames Research Center, M/S 244-30, Moffett Field, CA 94035 (United States); Isaacson, Howard; Kolbl, Rea; Marcy, Geoffrey W. [Department of Astronomy, University of California at Berkeley, Berkeley, CA 94720 (United States); Ciardi, David [NASA Exoplanet Science Institute, California Institute of Technology, 770 South Wilson Avenue, Pasadena, CA 91125 (United States); Fischer, Debra A. [Department of Astronomy, Yale University, New Haven, CT 06520 (United States); and others

2013-05-10

74

Cosmic ray impact on extrasolar earth-like planets in close-in habitable zones.  

PubMed

Because of their different origins, cosmic rays can be subdivided into galactic cosmic rays and solar/stellar cosmic rays. The flux of cosmic rays to planetary surfaces is mainly determined by two planetary parameters: the atmospheric density and the strength of the internal magnetic moment. If a planet exhibits an extended magnetosphere, its surface will be protected from high-energy cosmic ray particles. We show that close-in extrasolar planets in the habitable zone of M stars are synchronously rotating with their host star because of the tidal interaction. For gravitationally locked planets the rotation period is equal to the orbital period, which is much longer than the rotation period expected for planets not subject to tidal locking. This results in a relatively small magnetic moment. We found that an Earth-like extrasolar planet, tidally locked in an orbit of 0.2 AU around an M star of 0.5 solar masses, has a rotation rate of 2% of that of the Earth. This results in a magnetic moment of less than 15% of the Earth's current magnetic moment. Therefore, close-in extrasolar planets seem not to be protected by extended Earth-like magnetospheres, and cosmic rays can reach almost the whole surface area of the upper atmosphere. Primary cosmic ray particles that interact with the atmosphere generate secondary energetic particles, a so-called cosmic ray shower. Some of the secondary particles can reach the surface of terrestrial planets when the surface pressure of the atmosphere is on the order of 1 bar or less. We propose that, depending on atmospheric pressure, biological systems on the surface of Earth-like extrasolar planets at close-in orbital distances can be strongly influenced by secondary cosmic rays. PMID:16225432

Griessmeier, J-M; Stadelmann, A; Motschmann, U; Belisheva, N K; Lammer, H; Biernat, H K

2005-10-01

75

Bistability of the climate around the habitable zone: a thermodynamic investigation  

NASA Astrophysics Data System (ADS)

The goal of this paper is to explore the potential multistability of the cli- mate of a planet around the habitable zone. We apply our methodology to the Earth system, but our investigation has more general relevance. A thor- ough investigation of the thermodynamics of the climate system is performed for very diverse conditions of energy input and infrared atmosphere opacity. Using PlaSim, an Earth-like general circulation model, the solar constant S is modulated between 1160 and 1510 Wm-2 and the CO2 concentration, [CO2], from 90 to 2880 ppm. It is observed that in such a parameter range the climate is bistable, i.e. there are two coexisting attractors, one char- acterised by warm, moist climates (W) and one by completely frozen sea surface (Snowball Earth, SB). Linear relationships are found for the two transition lines (W?SB and SB ?W) in (S,[CO2]) between S and the log- arithm of [CO2]. The dynamical and thermodynamical properties - energy fluxes, Lorenz energy cycle, Carnot efficiency, material entropy production - of the W and SB states are very different: W states are dominated by the hydrological cycle and latent heat is prominent in the material entropy production; the SB states are eminently dry climates where heat transport is realized through sensible heat fluxes and entropy mostly generated by dis- sipation of kinetic energy. We also show that the Carnot efficiency regularly increases towards each transition between W and SB, with a large decrease in each transition. Finally, we propose well-defined empirical functions al- lowing for expressing the global non-equilibrium thermodynamical properties of the system in terms of either the mean surface temperature or the mean planetary emission temperature. This paves the way for the possibility of proposing efficient parametrisations of complex non-equilibrium properties and of practically deducing fundamental properties of a planetary system from a relatively simple observable.

Pascale, S.; Boschi, R.; Lucarini, V.

2012-12-01

76

Observations of Interstellar Formamide: Availability of a Prebiotic Precursor in the Galactic Habitable Zone  

PubMed Central

Abstract We conducted a study on interstellar formamide, NH2CHO, toward star-forming regions of dense molecular clouds, using the telescopes of the Arizona Radio Observatory (ARO). The Kitt Peak 12?m antenna and the Submillimeter Telescope (SMT) were used to measure multiple rotational transitions of this molecule between 100 and 250?GHz. Four new sources of formamide were found [W51M, M17 SW, G34.3, and DR21(OH)], and complementary data were obtained toward Orion-KL, W3(OH), and NGC 7538. From these observations, column densities for formamide were determined to be in the range of 1.1×1012 to 9.1×1013 cm?2, with rotational temperatures of 70–177?K. The molecule is thus present in warm gas, with abundances relative to H2 of 1×10?11 to 1×10?10. It appears to be a common constituent of star-forming regions that foster planetary systems within the galactic habitable zone, with abundances comparable to that found in comet Hale-Bopp. Formamide's presence in comets and molecular clouds suggests that the compound could have been brought to Earth by exogenous delivery, perhaps with an infall flux as high as ?0.1?mol/km2/yr or 0.18?mmol/m2 in a single impact. Formamide has recently been proposed as a single-carbon, prebiotic source of nucleobases and nucleic acids. This study suggests that a sufficient amount of NH2CHO could have been available for such chemistry. Key Words: Formamide—Astrobiology—Radioastronomy—ISM—Comets—Meteorites. Astrobiology 13, 439–453.

Adande, Gilles R.; Woolf, Neville J.

2013-01-01

77

Detection of a Proto-planetary Clump in the Habitable Zone of GM Cephei  

NASA Astrophysics Data System (ADS)

GM Cephei is an active T Tauri star in the young open cluster Trumpler 37, showing abrupt UX Orionis type of photometric variability. Its light curves exhibit frequent, sporadic brightening events, each of <0.5 mag and lasting for days, which must have been originated from unsteady circumstellar accretion. In addition, the star undergoes a brightness drop up to ~1 mag lasting for about a month, during which the star became bluer when fainter. Moreover, the brightness drops seem to have a recurrence timescale of about 300 days. It is proposed that the brightness drop arises from obscuration of the central star by an orbiting dust concentration, exemplifying disk inhomogeneity in transition between grain coagulation and planetesimal formation in a young circumstellar disk. GM Cep was found to show a few percent polarization in the optical wavelengths, and an enhanced level of polarization during the occultation phase.

Chen, W. P.; Hu, S. C.-L.

2014-04-01

78

Influence of Giant Planets near the 5:2 Resonance in the Habitable Zone of Sun-Like Stars  

NASA Astrophysics Data System (ADS)

From the nearly 190 planets that have been discovered so far we can distinguish the following types of HZs from the dynamicals point of view: 1) The solar-system type (SST) - where the HZ is between the host-star and the detected giant planet. 2) The hot-Jupiter type (HJT) for a close-in giant planet, the HZ is outside its orbit. 3) The giant planet type (GPT) - when the detected planet moves in the HZ. In this case we can only expect "habitable moons" or "habitable trojan-like planets". 4) Furthermore in multi-planet systems there will be combinations of these 3 types. But even if a planet will be detected in the HZ, it has to be verified if it can be considered as habitable planet. An important contribution from the dynamical point of view is to examine the long-term dynamical behavior of the orbital motion in this region, which obviously depends on the motion of the giant planets in the systems. Motivated by the fact, that in some of the discovered multi-planet systems the giant planets move in mean motion resonance, we studied the influence of giant planets, that are close to the 5:2 mean motion resonance on the habitable zone of a sun-like star. Starting with the actual Jupiter-Saturn-Uranus configuration of our Solar-System one can see how the perturbation changes when we increase Saturn's mass (from 1 to 9 Jupiter-masses).

Pilat-Lohinger, E.; Suli, A.; Freistetter, F.; Dvorak, R.; Schwarz, R.; Funk, B.

79

Bistability of the climate around the habitable zone: A thermodynamic investigation  

NASA Astrophysics Data System (ADS)

The goal of this paper is to explore the potential multistability of the climate for a planet around the habitable zone. We apply our methodology to the Earth system, but our investigation has more general relevance. A thorough investigation of the thermodynamics of the climate system is performed for very diverse conditions of energy input and infrared atmosphere opacity. Using PlaSim, an Earth-like general circulation model, the solar constant S? is modulated between 1160 and 1510 W m-2 and the CO2 concentration, [CO2], between 90 and 2880 ppm. It is observed that in such a parameter range the climate is bistable, i.e. there are two coexisting attractors, one characterised by warm, moist climates (W) and one by completely frozen sea surface (Snowball Earth, SB). The tipping points of both the transitions (W ? SB and SB ?W) are located along straight lines in the (S?, log[CO2]) space. The dynamical and thermodynamical properties - energy fluxes, Lorenz energy cycle, Carnot efficiency, material entropy production - of the W and SB states are very different: W states are dominated by the hydrological cycle and latent heat is prominent in the material entropy production; the SB states are eminently dry climates where heat transport is realised through sensible heat fluxes and entropy mostly generated by dissipation of kinetic energy. We also show that the Carnot efficiency regularly increases towards each transition between W and SB, with a large discontinuous decrease at the point of each transition. Finally, we propose well-defined empirical functions allowing for expressing the global non-equilibrium thermodynamical properties of the system in terms of either the mean surface temperature or the mean planetary emission temperature. While the specific results presented in this paper depend on some characteristics of the Earth system (e.g. rotation rate, position of the continents), this paves the way for the possibility of proposing efficient parameterisations of complex non-equilibrium properties and of practically deducing fundamental properties of a planetary system from a relatively simple observable. As a preliminary result, we obtain that when reducing the rotation rate of the planet by a factor of two, the multistability properties, the quantitative estimators of the thermodynamics of the system, and the approximate parameterisations in terms of the surface of emission temperature are only weakly affected.

Boschi, Robert; Lucarini, Valerio; Pascale, Salvatore

2013-11-01

80

EFFECT OF METALLICITY ON THE EVOLUTION OF THE HABITABLE ZONE FROM THE PRE-MAIN SEQUENCE TO THE ASYMPTOTIC GIANT BRANCH AND THE SEARCH FOR LIFE  

SciTech Connect

During the course of stellar evolution, the location and width of the habitable zone changes as the luminosity and radius of the star evolves. The duration of habitability for a planet located at a given distance from a star is greatly affected by the characteristics of the host star. A quantification of these effects can be used observationally in the search for life around nearby stars. The longer the duration of habitability, the more likely it is that life has evolved. The preparation of observational techniques aimed at detecting life would benefit from the scientific requirements deduced from the evolution of the habitable zone. We present a study of the evolution of the habitable zone around stars of 1.0, 1.5, and 2.0 M{sub Sun} for metallicities ranging from Z = 0.0001 to Z = 0.070. We also consider the evolution of the habitable zone from the pre-main sequence until the asymptotic giant branch is reached. We find that metallicity strongly affects the duration of the habitable zone for a planet as well as the distance from the host star where the duration is maximized. For a 1.0 M{sub Sun} star with near solar metallicity, Z = 0.017, the duration of the habitable zone is >10 Gyr at distances 1.2-2.0 AU from the star, whereas the duration is >20 Gyr for high-metallicity stars (Z = 0.070) at distances of 0.7-1.8 AU, and {approx}4 Gyr at distances of 1.8-3.3 AU for low-metallicity stars (Z = 0.0001). Corresponding results have been obtained for stars of 1.5 and 2.0 solar masses.

Danchi, William C. [Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center, Code 667, Greenbelt, MD 20771 (United States); Lopez, Bruno, E-mail: william.c.danchi@nasa.gov, E-mail: bruno.lopez@oca.eu [Observatoire de la Cote d'Azur, Laboratoire Lagrange UMR 7293, BP 4229, F-06034 Nice Cedex 4 (France)

2013-05-20

81

Direct Detection of Nearby Habitable Zone Planets Using Slicer Based Integral Field Spectrographs and EPICS on the E-ELT  

NASA Astrophysics Data System (ADS)

Early design studies for the future Exo-Planet Imaging Camera and Specrotgraph (EPICS) on the European Extremely Large Telescope (E-ELT) show the ability to probe the region of super-Earths in the habitable zone of stars within 5pc (including Gilese 581d). However, these planets will be lost to us if the correct choice of integral field spectrograph (IFS) technology is not selected for such an instrument the ability to fit and remove the speckle noise that remains is crucial to reaching these contrasts. We conclusively demonstrate, though the use of an experimental setup producing an artificial speckle, that slicer based IFSs and post-processing using spectral deconvolution can achieve speckle rejection factors exceeding 103. Contrary to popular belief, we do not find any evidence that this choice of IFS technology limits the achievable contrast. Coupled with extreme adaptive optics and high performance coronographs, a slicer based integral field spectrograph could achieve contrasts exceeding 109, enabling these super-Earths to be detected in the habitable zone of nearby stars, making it an attractive option for the next generation of instruments being designed for the direct detection of extra solar planets.

Salter, Graeme S.; Thatte, Niranjan A.; Tecza, Matthias; Clarke, Fraser; Kasper, Markus E.

2014-04-01

82

The Loss of Nitrogen-rich Atmospheres from Earth-like Exoplanets within M-star Habitable Zones  

NASA Astrophysics Data System (ADS)

After the first discovery of massive Earth-like exoplanets around M-type dwarf stars, the search for exoplanets which resemble more an Earth analogue continues. The discoveries of super-Earth planets pose questions on habitability and the possible origin of life on such planets. Future exoplanet space projects designed to characterize the atmospheres of terrestrial exoplanets will also search for atmospheric species which are considered as bio-markers (e.g. O3, H2O, CH4, etc.). By using the Earth with its atmosphere as a proxy and in agreement with the classical habitable zone concept, one should expect that Earth-like exoplanets suitable for life as we know it should have a nitrogen atmosphere and a very low CO2 content. Whether a water bearing terrestrial planet within its habitable zone can evolve into a habitable world similar than the Earth, depends on the capability of its water-inventory and atmosphere to survive the period of high radiation of the young and/or active host star. Depending on their size and mass, lower mass stars remain at high X-ray and EUV (XUV) activity levels for hundreds of Ma's to Ga's. XUV flux values which are 10 or 20 times higher than that of the present Sun can heat the thermosphere and expand the exobase of N2-rich Earth-like exoplanets to altitudes well above their expected magnetopause distances. This results in magnetically non-protected upper atmospheres and high non-thermal escape rates. We studied this plasma induced N+ ion pick up escape and applied a numerical test-particle stellar wind plasma - exosphere interaction model. Our results indicate that Earth-analogue exoplanets with atmosphere compositions similar to that of present Earth will lose their nitrogen inventories if they are exposed over a sufficient period of time to XUV fluxes ? 10 times that of the present Sun. Because most M-type stars are active in XUV radiation we suggest that these planets will undergo a different atmospheric evolution than the Earth so that life as we know it may not evolve on their surfaces.

Lammer, H.; Lichtenegger, H. I. M.; Khodachenko, M. L.; Kulikov, Y. N.; Griessmeier, J.

2011-12-01

83

Strong Dependence of the Inner Edge of the Habitable Zone on Planetary Rotation Rate  

NASA Astrophysics Data System (ADS)

Planetary rotation rate is a key parameter in determining atmospheric circulation and hence the spatial pattern of clouds. Since clouds can exert a dominant control on planetary radiation balance, rotation rate could be critical for determining the mean planetary climate. Here we investigate this idea using a three-dimensional general circulation model with a sophisticated cloud scheme. We find that slowly rotating planets (like Venus) can maintain an Earth-like climate at nearly twice the stellar flux as rapidly rotating planets (like Earth). This suggests that many exoplanets previously believed to be too hot may actually be habitable, depending on their rotation rate. The explanation for this behavior is that slowly rotating planets have a weak Coriolis force and long daytime illumination, which promotes strong convergence and convection in the substellar region. This produces a large area of optically thick clouds, which greatly increases the planetary albedo. In contrast, on rapidly rotating planets a much narrower belt of clouds form in the deep tropics, leading to a relatively low albedo. A particularly striking example of the importance of rotation rate suggested by our simulations is that a planet with modern Earth's atmosphere, in Venus' orbit, and with modern Venus' (slow) rotation rate would be habitable. This would imply that if Venus went through a runaway greenhouse, it had a higher rotation rate at that time.

Yang, Jun; Boué, Gwenaël; Fabrycky, Daniel C.; Abbot, Dorian S.

2014-05-01

84

Climatic Habitability  

NASA Astrophysics Data System (ADS)

It is likely that, in the next several years, the Corot and Kepler satellites will find many terrestrial planets around other stars. In order to judge what fraction of these planets are likely to be hospitable to life, it is important to reassess the notion of the habitable zone. Classical considerations of habitability, in the context of extrasolar planets, have often regarded it as a binary property (either a planet is or is not habitable). But according to the standard liquid water definition, the Earth itself is only partially habitable. I will describe a way to use energy balance climate models to assess the spatial and temporal habitability of terrestrial planets that are not too different from the Earth. Initial investigations of model planets' temperature distributions indicate that climate varies with observable features of planets (e.g., how far they are from their star) and unobservable features (e.g., how fast they are spinning, how much surface water they have, what their obliquity is). The habitability of model pseudo-Earths with different rotation rates or land-ocean fractions, for instance, generally differs significantly from that of the Earth itself.

Spiegel, David S.; Menou, K.; Scharf, C. A.

2008-05-01

85

The Effect of Planets Beyond the Ice Line on the Accretion of Volatiles by Habitable-zone Rocky Planets  

NASA Astrophysics Data System (ADS)

Models of planet formation have shown that giant planets have a large impact on the number, masses, and orbits of terrestrial planets that form. In addition, they play an important role in delivering volatiles from material that formed exterior to the snow line (the region in the disk beyond which water ice can condense) to the inner region of the disk where terrestrial planets can maintain liquid water on their surfaces. We present simulations of the late stages of terrestrial planet formation from a disk of protoplanets around a solar-type star and we include a massive planet (from 1 M ? to 1 M J) in Jupiter's orbit at ~5.2 AU in all but one set of simulations. Two initial disk models are examined with the same mass distribution and total initial water content, but with different distributions of water content. We compare the accretion rates and final water mass fraction of the planets that form. Remarkably, all of the planets that formed in our simulations without giant planets were water-rich, showing that giant planet companions are not required to deliver volatiles to terrestrial planets in the habitable zone. In contrast, an outer planet at least several times the mass of Earth may be needed to clear distant regions of debris truncating the epoch of frequent large impacts. Observations of exoplanets from radial velocity surveys suggest that outer Jupiter-like planets may be scarce, therefore, the results presented here suggest that there may be more habitable planets residing in our galaxy than previously thought.

Quintana, Elisa V.; Lissauer, Jack J.

2014-05-01

86

The EXoplanetary Circumstellar Disk Environments and Disk Explorer  

NASA Astrophysics Data System (ADS)

We present an overview of the EXoplanetary Circumstellar Environments and Disk Explorer (EXCEDE), selected by NASA for technology development and maturation. EXCEDE will study the formation, evolution and architectures of exoplanetary systems, and characterize circumstellar environments into stellar habitable zones. EXCEDE provides contrast-limited scattered-light detection sensitivities 1000x greater than HST or JWST coronagraphs at a much smaller effective inner working angle (IWA), thus enabling the exploration and characterization of exoplanetary CS disks in currently inaccessible domains. EXCEDE will utilize a laboratory demonstrated high-performance Phase Induced Amplitude Apodized Coronagraph (PIAA-C) integrated with a 70 cm diameter unobscured aperture visible light telescope. The EXCEDE PIAA-C will deliver star-to-disk augmented image contrasts of < 10E-8 and a 1.2 lambda/D IWA of 0.14” with a wavefront control system utilizing a 64x64 element MEMS DM and fast steering mirror. EXCEDE will provide 144 mas spatial resolution at 0.4 microns with dust detection sensitivity to levels of a few tens of zodis with two-band imaging polarimetry. EXCEDE is a science-driven technology pathfinder that will advance our understanding of the formation and evolution of exoplanetary systems, placing our solar system in broader astrophysical context, and will demonstrate the high contrast technologies required for larger-scale follow-on and multi-wavelength investigations on the road to finding and characterizing exo-Earths in the years ahead.

Schneider, Glenn; Guyon, O.; Science Mission, EXCEDE; Technology Team

2012-01-01

87

A Campaign for the Detection of Earth-Mass Planets in the Habitable Zone of Alpha Centauri  

NASA Astrophysics Data System (ADS)

We review the possible formation and orbital stability of Earth-mass or super Earth-mass planets around either of the stars Alpha Centauri A or B and describe a program at Mt John University Observatory using the Doppler method that aims to detect such planets. From New Zealand, we are able to observe the Alpha Centauri system year-round. This is critical in order to acquire data of sufficient quantity and phase coverage to detect the orbit of a terrestrial-mass planet in the habitable zone. Our observations are being made at high resolution (R = 70,000) and high signal-to-noise with the Hercules vacuum echelle spectrograph attached to the 1-m McLellan telescope by a 25-m long optical fibre and using an iodine cell. We discuss the velocity precision and instrumental stability required for success and outline the progress of the observations so far. At present we are collecting about 10,000 observations of each star, A and B, per year with a typical precision of 2.5 m/s per observation.

Wittenmyer, Robert A.; Endl, Michael; Bergmann, Christoph; Hearnshaw, John; Barnes, Stuart I.; Wright, Duncan

2014-04-01

88

The Habitable Zone Planet Finder: A Proposed High Resolution Nir Spectrograph For The Het To Discover Low Mass Exoplanets Around M Stars  

Microsoft Academic Search

The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 9m Hobby Eberly telescope that will be capable of discovering low mass planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution R 50,000 and cover the wavelength range 0.9-1.65mm, the Y, J and H near infrared (NIR) bands where most of the flux is emitted

Suvrath Mahadevan; L. Ramsey; A. Wolszczan; J. Wright; M. Endl; S. Redman

2010-01-01

89

How do changes in the Diurnal Cycle affect Bi-stability and Climate Sensitivity in the Habitable Zone?  

NASA Astrophysics Data System (ADS)

In this study we deal with the effect of varying the length of the diurnal cycle on its bi-stability properties. By using a general circulation model, PlaSim, we consider several values for the diurnal cycle, from tidally locked, to that of 1 Earth day. For each value of the diurnal cycle, we slowly modulate the solar constant between 1510 and 1000 Wm-2 and perform a hysteresis experiment. It is found that the width of the bi-stable region, i.e. the range of climate states - determined here by changes in S* - which support two climatic attractors, reduces when the diurnal cycle is increased in length and disappears - signifying the merging of both attractors - for climates with a diurnal cycle greater than 180 days. Crucial to the loss of bi-stability is the longitudinally asymmetric distribution of solar radiation, incident on the planet's surface, leading to the development of equatorial sea-ice. For diurnal cycles where bi-stability is found, the longitudinally asymmetric heating is sufficiently compensated for by the strength of the zonal winds and the rate of solar distribution, which redistribute heat and maintain the meridional temperature gradient across all longitudes. Conversely, for mono-stable regimes, the energy transport associated with zonal winds becomes insufficient to compensate for the increase in the length of the diurnal cycle, resulting in large zonal temperature gradients along the equatorial band. Furthermore, the results found here confirm and reenforce the robustness of those found in Boschi et al (2013), showing that, for climates which support bistability, it may be possible to parameterise variables such as the material entropy production and the meridional heat transport in terms of the surface and emission temperatures, within reasonably well defined upper and lower bounds, even when considering a wide range of planetary rotation speeds and changes to the infrared opacity. This paves the way for the possibility of practically deducing fundamental properties of planets in the habitable zone from relatively simple observables.

Boschi, R.; Valerio, L.

2013-09-01

90

Looking Through the Ice: Searching for Past and Present Habitable Zones in the Martian North Polar Region Using MOLA DEMs  

NASA Astrophysics Data System (ADS)

Hydrothermal systems have been acknowledged as important gateways to accessing a potential subsurface biology (extant or extinct) on Mars. Groundwater circulation, sustained for up to one billion years by large plutonic bodies (as modeled by previous authors), might well be capable of tapping into a deep subsurface biosphere and subsequently carrying members of microbial communities to the surface. Hence, future robotic missions with near surface drilling capabilities may be able to unearth cryopreserved biosignatures, or perhaps extant organisms, in the midst of the hydrothermal system itself. Digital Elevation Models (DEMs) constructed from Mars Orbiter Laser Altimeter (MOLA) data have proved to be a valuable tool in the search for potential habitable zones for extant and extinct life, and the detection of possible hydrothermal systems on Mars. When formatted for use in a Geographical Information Systems (GIS) software package such as ESRI's ArcView, MOLA data can be used to compose DEMs. Those DEMs can, in turn, be used to create contour maps, to allow profiling through features of interest, and to generate hillshaded views, which provide an image-like perspective of a selected area. Furthermore, DEMs eliminate many problems associated with photographic images such as over-/underexposure, poor focus, and albedo values too high or low for optimal observations. During this study, DEMs were used in the analysis of several regions north of 70° N latitude, in the Martian north polar cap and polar cap margin. The regions were selected during a Viking image survey that concentrated on the location of surface expressions of potential magma-ice interactions, and hence past or present hydrothermal activity. Specific features sought included individual volcanoes and volcanic fields, as well as pseudocrater fields, subglacial volcanic constructs (such as tuyas and tindar ridges), fluvial channels and outwash plains (indicative of j”kulhlaup flooding events), possible subglacial lakes, and impact melt sheets capable of generating and sustaining localized hydrothermal systems. Two candidate sites, both located in the Olympia Planitia region in the remnant margin of the Martian north polar cap, will be discussed. MOLA revealed potential fluvial and volcanic activity beneath the ice of one site, and a possible mix of small volcanoes and rootless pseudocraters in the other region. Hence, those two auspicious sites, significant for astrobiologically driven research, represent a diversity of possible hydrothermal regions. It must be cautioned at all times that interpretations made of the selected sites are done so through polar ice. Nevertheless, the wealth of information imparted by MOLA of the underlying terrain is remarkable. Although MOLA is incapable of revealing present aqueous processes beneath the ice surface, it is an invaluable aid in the location of promising subglacial sites that suggest a history of hydrothermal activity.

Payne, M. C.; Farmer, J. D.

2002-12-01

91

Planet Hunters. V. A Confirmed Jupiter-size Planet in the Habitable Zone and 42 Planet Candidates from the Kepler Archive Data  

NASA Astrophysics Data System (ADS)

We report the latest Planet Hunter results, including PH2 b, a Jupiter-size (R PL = 10.12 ± 0.56 R ?) planet orbiting in the habitable zone of a solar-type star. PH2 b was elevated from candidate status when a series of false-positive tests yielded a 99.9% confidence level that transit events detected around the star KIC 12735740 had a planetary origin. Planet Hunter volunteers have also discovered 42 new planet candidates in the Kepler public archive data, of which 33 have at least 3 transits recorded. Most of these transit candidates have orbital periods longer than 100 days and 20 are potentially located in the habitable zones of their host stars. Nine candidates were detected with only two transit events and the prospective periods are longer than 400 days. The photometric models suggest that these objects have radii that range between those of Neptune and Jupiter. These detections nearly double the number of gas-giant planet candidates orbiting at habitable-zone distances. We conducted spectroscopic observations for nine of the brighter targets to improve the stellar parameters and we obtained adaptive optics imaging for four of the stars to search for blended background or foreground stars that could confuse our photometric modeling. We present an iterative analysis method to derive the stellar and planet properties and uncertainties by combining the available spectroscopic parameters, stellar evolution models, and transiting light curve parameters, weighted by the measurement errors. Planet Hunters is a citizen science project that crowd sources the assessment of NASA Kepler light curves. The discovery of these 43 planet candidates demonstrates the success of citizen scientists at identifying planet candidates, even in longer period orbits with only two or three transit events. .

Wang, Ji; Fischer, Debra A.; Barclay, Thomas; Boyajian, Tabetha S.; Crepp, Justin R.; Schwamb, Megan E.; Lintott, Chris; Jek, Kian J.; Smith, Arfon M.; Parrish, Michael; Schawinski, Kevin; Schmitt, Joseph R.; Giguere, Matthew J.; Brewer, John M.; Lynn, Stuart; Simpson, Robert; Hoekstra, Abe J.; Jacobs, Thomas Lee; LaCourse, Daryll; Schwengeler, Hans Martin; Chopin, Mike; Herszkowicz, Rafal

2013-10-01

92

Optimizing Spectral Resolution and Observation Time for Measurements of Habitability  

Microsoft Academic Search

The Terrestrial Planet Finder (TPF) is a NASA mission concept that will attempt to characterize and search for habitability and life on extrasolar planets. While detection of a planet in the habitable zone increases the probability that the planet is habitable, planetary characterization will be required to confirm habitability and thereby test predictions of the position of the habitable zone.

N. Khalfa; V. S. Meadows; S. D. Domagal-Goldman

2009-01-01

93

Deep Habits  

Microsoft Academic Search

This paper generalizes the standard habit-formation model to an environment in which agents form habits over individual varieties of goods as opposed to over a composite consumption good. We refer to this preference specification as deep habit formation. Under deep habits, the demand function faced by individual producers depends on past sales. This feature is typically assumed ad hoc in

MORTEN RAVN; STEPHANIE SCHMITT-GROHÉ; Mart ´ in Uribe

2006-01-01

94

The Habitable Zone Planet Finder: A Proposed High Resolution Nir Spectrograph For The Het To Discover Low Mass Exoplanets Around M Stars  

NASA Astrophysics Data System (ADS)

The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 9m Hobby Eberly telescope that will be capable of discovering low mass planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution R 50,000 and cover the wavelength range 0.9-1.65mm, the Y, J and H near infrared (NIR) bands where most of the flux is emitted by late type M stars, and where most of the radial velocity information is concentrated. Enclosed in a vacuum tank with active temperature control, fiber scrambling and mechanical agitation, HZPF is designed to achieve a radial velocity precision < 3m/s, with a desire to achieve 1m/s for the brightest targets. This instrument will enable a study of the properties of low mass planets around M dwarfs; discover planets in the habitable zones around these stars, and serve as an essential radial velocity confirmation tool for astrometric and transit detections around late M dwarfs. Radial velocity observation the NIR will also enable a search for close in planets around young active stars, complementing the search space enabled by upcoming high-contrast imaging instruments. Tests with our laboratory prototype have already demonstrated the ability to recover radial velocities in the NIR at 7-10 m/s precision from integrated sunlight. We will discuss lessons learned about calibration and NIR array performance from our tests and how they impact the design of the HZPF.

Mahadevan, Suvrath; Ramsey, L.; Wolszczan, A.; Wright, J.; Endl, M.; Redman, S.

2010-01-01

95

The habitable zone of Earth-mass planets around 47 UMa: results for land and water worlds  

Microsoft Academic Search

In a previous paper, we showed that Earth-type habitable planets around 47 UMa are in principle possible if a distinct set of conditions is warranted. These conditions include that the Earth-type planets have successfully formed and are orbitally stable and, in addition, that the 47 UMa star-planet system is relatively young ([less, similar]6 Gyr). We now extend this study by

S. Franck; M. Cuntz; W. von Bloh; C. Bounama

2003-01-01

96

New circumstellar cyanoacetylene sources  

NASA Astrophysics Data System (ADS)

A search for the J = 10-9 transition of cyanoacetylene (HC3N) in circumstellar molecular envelopes has resulted in new detections of CIT 6 (definite) and GL 3068 (probable). The 17 sources surveyed included a mixture of carbon-rich, oxygen-rich, and S-type stars, and the previously known source IRC + 10216. Four circumstellar shells have now been identified as sources of HC3N emission. IRC + 10216, CIT 6, and GL 3068 are cool, carbon-rich giants; the remaining object, GL 2688, which is not included in this study, consists of a much hotter supergiant surrounded by a carbon-rich nebula. It was not possible to detect HC3N from any oxygen-rich shell, including W Hydrae, once reported as an HC3N source. The prospects for extending the list of molecules and host circumstellar sources and the potential of circumstellar HC3N as a tool for the study of circumstellar shells and the chemistry of long-chain carbon molecules are discussed.

Jewell, P. R.; Snyder, L. E.

1982-04-01

97

Atmospheric dynamics on tidally locked Earth-like planets in the habitable zone of an M dwarf star  

NASA Astrophysics Data System (ADS)

We investigated the large scale atmospheric circulation of Gl581g, a potentially habitable planet around an M dwarf star, using an idealized dry global circulation model (GCM) with simplified thermal forcing as a first step towards a systematic extended parameter study. The results are compared with the work of Joshi et al. (1997) who investigated a tidally-locked habitable Earth analogue with less than half the rotation period of Gl581g. The extent, form and strength of the atmospheric circulation in each model generally agree with each other, even though the models differ in key parameters such as planetary radius, surface gravity, forcing scheme and rotation period. The substellar point is associated with an uprising direct circulation-branch of a Hadley-like cell with return flow over the poles. It is compelling to assume that the substellar point of a tidally locked terrestrial exoplanet behaves dynamically like the Earth's tropic associated with clouds and precipitation, making it an ideal target for habitability.

Carone, Ludmila; Keppens, Rony; Decin, Leen

2014-01-01

98

Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. I. CME impact on expected magnetospheres of Earth-like exoplanets in close-in habitable zones.  

PubMed

Low mass M- and K-type stars are much more numerous in the solar neighborhood than solar-like G-type stars. Therefore, some of them may appear as interesting candidates for the target star lists of terrestrial exoplanet (i.e., planets with mass, radius, and internal parameters identical to Earth) search programs like Darwin (ESA) or the Terrestrial Planet Finder Coronagraph/Inferometer (NASA). The higher level of stellar activity of low mass M stars, as compared to solar-like G stars, as well as the closer orbital distances of their habitable zones (HZs), means that terrestrial-type exoplanets within HZs of these stars are more influenced by stellar activity than one would expect for a planet in an HZ of a solar-like star. Here we examine the influences of stellar coronal mass ejection (CME) activity on planetary environments and the role CMEs may play in the definition of habitability criterion for the terrestrial type exoplanets near M stars. We pay attention to the fact that exoplanets within HZs that are in close proximity to low mass M stars may become tidally locked, which, in turn, can result in relatively weak intrinsic planetary magnetic moments. Taking into account existing observational data and models that involve the Sun and related hypothetical parameters of extrasolar CMEs (density, velocity, size, and occurrence rate), we show that Earth-like exoplanets within close-in HZs should experience a continuous CME exposure over long periods of time. This fact, together with small magnetic moments of tidally locked exoplanets, may result in little or no magnetospheric protection of planetary atmospheres from a dense flow of CME plasma. Magnetospheric standoff distances of weakly magnetized Earth-like exoplanets at orbital distances

Khodachenko, Maxim L; Ribas, Ignasi; Lammer, Helmut; Griessmeier, Jean-Mathias; Leitner, Martin; Selsis, Franck; Eiroa, Carlos; Hanslmeier, Arnold; Biernat, Helfried K; Farrugia, Charles J; Rucker, Helmut O

2007-02-01

99

Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone.  

PubMed

M stars comprise 80% of main sequence stars, so their planetary systems provide the best chance for finding habitable planets, that is, those with surface liquid water. We have modeled the broadband albedo or reflectivity of water ice and snow for simulated planetary surfaces orbiting two observed red dwarf stars (or M stars), using spectrally resolved data of Earth's cryosphere. The gradual reduction of the albedos of snow and ice at wavelengths greater than 1 ?m, combined with M stars emitting a significant fraction of their radiation at these same longer wavelengths, means that the albedos of ice and snow on planets orbiting M stars are much lower than their values on Earth. Our results imply that the ice/snow albedo climate feedback is significantly weaker for planets orbiting M stars than for planets orbiting G-type stars such as the Sun. In addition, planets with significant ice and snow cover will have significantly higher surface temperatures for a given stellar flux if the spectral variation of cryospheric albedo is considered, which in turn implies that the outer edge of the habitable zone around M stars may be 10-30% farther away from the parent star than previously thought. PMID:22181553

Joshi, Manoj M; Haberle, Robert M

2012-01-01

100

The habitable zone planet finder: a proposed high-resolution NIR spectrograph for the Hobby Eberly Telescope to discover low-mass exoplanets around M dwarfs  

NASA Astrophysics Data System (ADS)

The Habitable Zone Planet Finder (HZPF) is a proposed instrument for the 10m class Hobby Eberly telescope that will be capable of discovering low mass planets around M dwarfs. HZPF will be fiber-fed, provide a spectral resolution R~ 50,000 and cover the wavelength range 0.9-1.65?m, the Y, J and H NIR bands where most of the flux is emitted by midlate type M stars, and where most of the radial velocity information is concentrated. Enclosed in a chilled vacuum vessel with active temperature control, fiber scrambling and mechanical agitation, HZPF is designed to achieve a radial velocity precision < 3m/s, with a desire to obtain <1m/s for the brightest targets. This instrument will enable a study of the properties of low mass planets around M dwarfs; discover planets in the habitable zones around these stars, as well serve as an essential radial velocity confirmation tool for astrometric and transit detections around late M dwarfs. Radial velocity observation in the near-infrared (NIR) will also enable a search for close in planets around young active stars, complementing the search space enabled by upcoming high-contrast imaging instruments like GPI, SPHERE and PALM3K. Tests with a prototype Pathfinder instrument have already demonstrated the ability to recover radial velocities at 7-10 m/s precision from integrated sunlight and ~15-20 m/s precision on stellar observations at the HET. These tests have also demonstrated the ability to work in the NIR Y and J bands with an un-cooled instrument. We will also discuss lessons learned about calibration and performance from our tests and how they impact the overall design of the HZPF.

Mahadevan, Suvrath; Ramsey, Larry; Wright, Jason; Endl, Michael; Redman, Stephen; Bender, Chad; Roy, Arpita; Zonak, Stephanie; Troupe, Nathaniel; Engel, Leland; Sigurdsson, Steinn; Wolszczan, Alex; Zhao, Bo

2010-07-01

101

EXOPLANET CHARACTERIZATION BY PROXY: A TRANSITING 2.15 R{sub Circled-Plus} PLANET NEAR THE HABITABLE ZONE OF THE LATE K DWARF KEPLER-61  

SciTech Connect

We present the validation and characterization of Kepler-61b: a 2.15 R{sub Circled-Plus} planet orbiting near the inner edge of the habitable zone of a low-mass star. Our characterization of the host star Kepler-61 is based upon a comparison with a set of spectroscopically similar stars with directly measured radii and temperatures. We apply a stellar prior drawn from the weighted mean of these properties, in tandem with the Kepler photometry, to infer a planetary radius for Kepler-61b of 2.15 {+-} 0.13 R{sub Circled-Plus} and an equilibrium temperature of 273 {+-} 13 K (given its period of 59.87756 {+-} 0.00020 days and assuming a planetary albedo of 0.3). The technique of leveraging the physical properties of nearby ''proxy'' stars allows for an independent check on stellar characterization via the traditional measurements with stellar spectra and evolutionary models. In this case, such a check had implications for the putative habitability of Kepler-61b: the planet is 10% warmer and larger than inferred from K-band spectral characterization. From the Kepler photometry, we estimate a stellar rotation period of 36 days, which implies a stellar age of >1 Gyr. We summarize the evidence for the planetary nature of the Kepler-61 transit signal, which we conclude is 30,000 times more likely to be due to a planet than a blend scenario. Finally, we discuss possible compositions for Kepler-61b with a comparison to theoretical models as well as to known exoplanets with similar radii and dynamically measured masses.

Ballard, Sarah; Charbonneau, David; Fressin, Francois; Torres, Guillermo; Irwin, Jonathan; Newton, Elisabeth [University of Washington, Seattle, WA 98195 (United States); Desert, Jean-Michel; Crepp, Justin R.; Shporer, Avi [California Institute of Technology, Pasadena, CA 91125 (United States); Mann, Andrew W. [Institute for Astronomy, University of Hawai'i, Honolulu, HI 96822 (United States); Ciardi, David R. [NASA Exoplanet Science Institute/Caltech, Pasadena, CA 91125 (United States); Henze, Christopher E.; Bryson, Stephen T.; Howell, Steven B. [NASA Ames Research Center, Moffett Field, CA 94035 (United States); Horch, Elliott P. [Southern Connecticut State University, New Haven, CT 06515 (United States); Everett, Mark E., E-mail: sarahba@uw.edu [National Optical Astronomy Observatory, Tucson, AZ 85719 (United States)

2013-08-20

102

Dynamical Habitability of Known Extrasolar Planetary Systems  

Microsoft Academic Search

Habitability is usually defined as the requirement for a terrestrial planet's atmosphere to sustain liquid water. This definition can be complemented by the dynamical requirement that other planets in the system do not gravitationally perturb terrestrial planets outside of their habitable zone, the orbital region allowing the existence of liquid water. We quantify the dynamical habitability of 85 known extrasolar

Kristen Menou; Serge Tabachnik

2003-01-01

103

Exoplanets, extremophiles and habitability  

NASA Astrophysics Data System (ADS)

Estimates of the average surface temperature and CO2 partial atmospheric pressure of already discovered exoplanets supposed to be in their Habitable Zone of their stars were surveyed from the Exoplanet Encyclopedia database. Moreover, since planetary surface temperature strongly depends on its albedo and geodynamic conditions, we have been feeding exoplanetary data into a comprehensive model of Earth's atmosphere to get better estimations. We also investigated the possible presence of "exomoons" belonging to giant planets capable of harbour dynamic stability and to retain atmospheric layers and keep geodynamic activity for long time spans. Collected information on biological data of micro-organisms classified as "extremophiles" indicate that such kind of microbial species could dwell in many of them. We thus propose an extension of the more astronomically defined "Habitable Zone" concept into the more astrobiologically "Extremophile Zone", taking into account other refined parameters allowing survival of more robust life forms.

Janot Pacheco, E.; Bernardes, L.

2012-09-01

104

Loss of nebula-captured hydrogen envelopes from 'sub'- to 'super-Earths' in the habitable zone of Sun-like stars  

NASA Astrophysics Data System (ADS)

We investigate the escape of captured hydrogen envelopes from protoplanets having masses in a range between `sub-Earth'-like bodies of 0.1 Earth-mass and `super-Earths' with a mass of 5 Earth-masses in the habitable zone at 1 AU of a Sun like G star, assuming that their rocky cores had formed before the nebula gas dissipated. For studying the escape of these accumulated hydrogen-dominated protoatmospheres, we apply a hydrodynamic upper atmosphere model and calculate the loss rates due to the heating by the high soft-X-ray and extreme ultraviolet (XUV) flux of the young Sun/star. The results of our study indicate that under most nebula conditions `sub-Earth' and Earth-mass planets can lose their captured hydrogen envelopes by thermal escape during the first 100 Myr after the disk dissipated. Depending on nebula properties and protoplanetary luminosities, it is possible that even protoplanets with Earth-mass cores may keep their hydrogen envelopes during their whole lifetime. In contrast to lower mass protoplanets, more massive `super-Earths' that can accumulate a huge amount of nebula gas, lose only tiny fractions of their primordial hydrogen envelopes. Our results agree with the fact that Venus, Earth, and Mars are not surrounded by dense hydrogen envelopes, as well as with the recent discoveries of low density `super-Earths' that most likely could not get rid of their dense protoatmospheres.

Lammer, Helmut; Erkaev, Nikolai V.; Odert, Petra; Kulikov, Yuri N.; Kislyakova, Kristina G.; Stoekl, Alexander; Dorfi, Ernst A.; Guedel, Manuel; Leitzinger, Martin

2014-05-01

105

THE LICK-CARNEGIE EXOPLANET SURVEY: A SATURN-MASS PLANET IN THE HABITABLE ZONE OF THE NEARBY M4V STAR HIP 57050  

SciTech Connect

Precision radial velocities (RV) from Keck/HIRES reveal a Saturn-mass planet orbiting the nearby M4V star HIP 57050. The planet has a minimum mass of Msin i {approx} 0.3 M{sub J}, an orbital period of 41.4 days, and an orbital eccentricity of 0.31. V-band photometry reveals a clear stellar rotation signature of the host star with a period of 98 days, well separated from the period of the RV variations and reinforcing a Keplerian origin for the observed velocity variations. The orbital period of this planet corresponds to an orbit in the habitable zone of HIP 57050, with an expected planetary temperature of {approx}230 K. The star has a metallicity of [Fe/H] = 0.32 {+-} 0.06 dex, of order twice solar and among the highest metallicity stars in the immediate solar neighborhood. This newly discovered planet provides further support that the well-known planet-metallicity correlation for F, G, and K stars also extends down into the M-dwarf regime. The a priori geometric probability for transits of this planet is only about 1%. However, the expected eclipse depth is {approx}7%, considerably larger than that yet observed for any transiting planet. Though long on the odds, such a transit is worth pursuing as it would allow for high quality studies of the atmosphere via transmission spectroscopy with Hubble Space Telescope. At the expected planetary effective temperature, the atmosphere may contain water clouds.

Haghighipour, Nader [Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Manoa, Honolulu, HI 96822 (United States); Vogt, Steven S.; Rivera, Eugenio J.; Laughlin, Greg; Meschiari, Stefano [Department of Astronomy and Astrophysics, UCO/Lick Observatory, University of California at Santa Cruz, Santa Cruz, CA 95064 (United States); Paul Butler, R. [Department of Terrestrial Magnetism, Carnegie Institute of Washington, Washington, DC 20015 (United States); Henry, Gregory W. [Center of Excellence in Information Systems, Tennessee State University, Nashville, TN 37209 (United States)

2010-05-20

106

Habitability in Binary Systems: The Role of UV Reduction and Magnetic Protection  

NASA Astrophysics Data System (ADS)

The number of planets found in binary systems is growing rapidly and the discovery of many more planets in binary systems appears inevitable. We use the newly refined and more restrictive, single star habitable zone (HZ) models of Kopparapu et al. (2013) and include planetary magnetic protection calculations in order to investigate binary star habitability. Here we present results on circumstellar or S-type planets, which are planets orbiting a single star member of a binary. P-type planets, on the other hand, orbit the center of mass of the binary. Stable planetary orbits exist in HZs for both types of binaries as long as the semi-major axis of the planet is either greater than (P-type) or less than (S-type) a few times the semi-major axis of the binary. We define two types of S-type binaries for this investigation. The SA-type is a circumstellar planet orbiting the binary’s primary star. In this case, the limits of habitability are dominated by the primary being only slightly affected by the presence of the lower mass companion. Thus, the SA-type planets have habitability characteristics, including magnetic protection, similar to single stars of the same type. The SB-type is a circumstellar planet orbiting the secondary star in a wide binary. An SB-type planet needs to orbit slightly outside the secondary’s single star HZ and remain within the primary’s single star HZ at all times. We explore the parameter space for which this is possible. We have found that planets lying in the combined HZ of SB binaries can be magnetically protected against the effects of stellar winds from both primary and secondary stars in a limited number of cases. We conclude that habitable conditions exist for a subset of SA-type, and a smaller subset of SB-type binaries. However, circumbinary planets (P-types) provide the most intriguing possibilities for the existence of complex life due to the effect of synchronization of binaries with periods in the 20-30 day range which allows for planets with significant magnetic protection.

Clark, Joni; Mason, P. A.; Zuluaga, J. I.; Cuartas, P. A.; Bustamonte, S.

2013-06-01

107

A PLANETARY SYSTEM AROUND THE NEARBY M DWARF GJ 667C WITH AT LEAST ONE SUPER-EARTH IN ITS HABITABLE ZONE  

SciTech Connect

We re-analyze 4 years of HARPS spectra of the nearby M1.5 dwarf GJ 667C available through the European Southern Observatory public archive. The new radial velocity (RV) measurements were obtained using a new data analysis technique that derives the Doppler measurement and other instrumental effects using a least-squares approach. Combining these new 143 measurements with 41 additional RVs from the Magellan/Planet Finder Spectrograph and Keck/High Resolution Echelle Spectrometer spectrometers reveals three additional signals beyond the previously reported 7.2 day candidate, with periods of 28 days, 75 days, and a secular trend consistent with the presence of a gas giant (period {approx}10 years). The 28 day signal implies a planet candidate with a minimum mass of 4.5 M{sub Circled-Plus} orbiting well within the canonical definition of the star's liquid water habitable zone (HZ), that is, the region around the star at which an Earth-like planet could sustain liquid water on its surface. Still, the ultimate water supporting capability of this candidate depends on properties that are unknown such as its albedo, atmospheric composition, and interior dynamics. The 75 day signal is less certain, being significantly affected by aliasing interactions among a potential 91 day signal, and the likely rotation period of the star at 105 days detected in two activity indices. GJ 667C is the common proper motion companion to the GJ 667AB binary, which is metal-poor compared to the Sun. The presence of a super-Earth in the HZ of a metal-poor M dwarf in a triple star system supports the evidence that such worlds should be ubiquitous in the Galaxy.

Anglada-Escude, Guillem; Butler, R. Paul [Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Rd. NW, Washington, DC 20015 (United States); Arriagada, Pamela; Minniti, Dante [Department of Astronomy, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22 (Chile); Vogt, Steven S.; Rivera, Eugenio J. [UCO/Lick Observatory, University of California, Santa Cruz, CA 95064 (United States); Crane, Jeffrey D.; Shectman, Stephen A.; Thompson, Ian B. [Carnegie Observatories, 813 Santa Barbara St., Pasadena, CA 91101-1292 (United States); Haghighipour, Nader [Institute for Astronomy and NASA Astrobiology Institute, University of Hawaii-Monoa, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Carter, Brad D. [Faculty of Sciences, University of Southern Queensland, Toowoomba 4350 (Australia); Tinney, C. G.; Wittenmyer, Robert A.; Bailey, Jeremy A. [Department of Astrophysics, School of Physics, University of New South Wales, Sydney 2052 (Australia); O'Toole, Simon J. [Australian Astronomical Observatory, P.O. Box 296, Epping 1710 (Australia); Jones, Hugh R. A. [Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield, Herts, AL10 9AB (United Kingdom); Jenkins, James S., E-mail: anglada@dtm.ciw.edu [Departamento de Astronomia, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago (Chile)

2012-05-20

108

Exoplanet Habitability: Effects of Planetesimal Carbon Chemistry  

NASA Astrophysics Data System (ADS)

We explore the effects of reported differences in C/O values for exoplanet host stars on the composition of planetesimals formed beyond the snow line in these systems. Since the value of C/O in a planet forming nebula has a strong effect on amount of oxygen available for water ice in an oxidizing nebula, exoplanet systems for host stars with C/O greater than the solar value may have planetesimals with very little or no water ice. We have estimated the composition of volatile and refractory material in extrasolar planetesimals using a set of stars with a wide range of measured C/O abundances (Johnson et al. ApJ. 757(2), 192, 2012). The volatile ice content of planetesimals in these systems varies significantly with C/O, controlled primarily by the availability of O for H2O ice condensation. Systems with C/O less than the solar value (C/O = 0.55) should have very water ice rich planetesimals, while water ice mass fraction decreases rapidly with increasing C/O until only ices of CO and CO2 are left in significant proportions. If a significant fraction of C is in the form of refractory CHON particles, C and O are removed from the gas phase and the condensates for super-solar C/O values will be water-poor mixtures of silicates and metal, carbon, and carbon-bearing volatile ices, depending on temperature. For very carbon-rich systems, oxidizing conditions cannot be sustained beyond about C/O=1, due to the oxygen sequestered in solid silicates, oxides and CHON, for refractory C fractions within the Pollack et al. range of 0.4 - 0.7 (ApJ. 421, 615, 1994). These results have implications for assessing the habitability of exoplanets since they constrain the amount of water available beyond the snow line for dynamical delivery to inner planets, depending on the host star's C/O in the circumstellar nebula. Thus one the key chemical ingredients for habitability may be in short supply in carbon-rich, oxygen-poor systems even if planets exist in the 'habitable zone'. TVJ acknowledges government support at JPL/Caltech, under a contract with NASA. NM acknowledges support from Yale University. JIL was supported by the JWST Project through NASA. O.M. acknowledges support from CNES.

Johnson, Torrence; Mousis, Olivier; Lunine, Jonathan; Madhusudhan, Nikku

2014-05-01

109

Origin and loss of nebula-captured hydrogen envelopes from `sub'- to `super-Earths' in the habitable zone of Sun-like stars  

NASA Astrophysics Data System (ADS)

We investigate the origin and loss of captured hydrogen envelopes from protoplanets having masses in a range between `sub-Earth'-like bodies of 0.1 M? and `super-Earths' with 5 M? in the habitable zone at 1 au of a Sun-like G star, assuming that their rocky cores had formed before the nebula gas dissipated. We model the gravitational attraction and accumulation of nebula gas around a planet's core as a function of protoplanetary luminosity during accretion and calculate the resulting surface temperature by solving the hydrostatic structure equations for the protoplanetary nebula. Depending on nebular properties, such as the dust grain depletion factor, planetesimal accretion rates, and resulting luminosities, for planetary bodies of 0.1-1 M? we obtain hydrogen envelopes with masses between ˜2.5 × 1019 and 1.5 × 1026 g. For `super-Earths' with masses between 2 and 5 M? more massive hydrogen envelopes within the mass range of ˜7.5 × 1023-1.5 × 1028 g can be captured from the nebula. For studying the escape of these accumulated hydrogen-dominated protoatmospheres, we apply a hydrodynamic upper atmosphere model and calculate the loss rates due to the heating by the high soft-X-ray and extreme ultraviolet (XUV) flux of the young Sun/star. The results of our study indicate that under most nebula conditions `sub-Earth' and Earth-mass planets can lose their captured hydrogen envelopes by thermal escape during the first ˜100 Myr after the disc dissipated. However, if a nebula has a low dust depletion factor or low accretion rates resulting in low protoplanetary luminosities, it is possible that even protoplanets with Earth-mass cores may keep their hydrogen envelopes during their whole lifetime. In contrast to lower mass protoplanets, more massive `super-Earths', which can accumulate a huge amount of nebula gas, lose only tiny fractions of their primordial hydrogen envelopes. Our results agree with the fact that Venus, Earth, and Mars are not surrounded by dense hydrogen envelopes, as well as with the recent discoveries of low density `super-Earths' that most likely could not get rid of their dense protoatmospheres.

Lammer, H.; Stökl, A.; Erkaev, N. V.; Dorfi, E. A.; Odert, P.; Güdel, M.; Kulikov, Yu. N.; Kislyakova, K. G.; Leitzinger, M.

2014-04-01

110

Habitable exoplanets statistics in the Milky Way  

NASA Astrophysics Data System (ADS)

We present an exoplanet statistical analysis into the Milky Way. We use the Becanson galactic synthetic model to simulate the Milky Way and the galactic and stellar habitable zones to calculate habitable planets. To assess habitability on the Galactic scale, we model supernova rates and planet formation. Our study, models the SNII and SNIa sterilizations by selecting them from within this preexisting stellar population. Furthermore, we consider habitability on tidally locked and non-tidally locked planets separately, and study habitability as a function of height above and below the Galactic mid-plane. The number of total habitable planets makes Milky Way practically empty of habitable planets. Our results, from these simulations, agree very well with Kepler's discoveries. Finally, we apply our results to the PLATO future space mission.

Anagnos, Th.

2013-09-01

111

Constraints on the Habitability of Extrasolar Moons  

NASA Astrophysics Data System (ADS)

Detections of massive extrasolar moons are shown feasible with the Kepler space telescope. Kepler's findings of about 50 exoplanets in the stellar habitable zone naturally make us wonder about the habitability of their hypothetical moons. Illumination from the planet, eclipses, tidal heating, and tidal locking distinguish remote characterization of exomoons from that of exoplanets. We show how evaluation of an exomoon's habitability is possible based on the parameters accessible by current and near-future technology.

Heller, René; Barnes, Rory

2014-04-01

112

Lifestyle Habits  

PubMed Central

Objectives: This study aimed to investigate the lifestyle habits—physical activity (PA), eating habits (EH), and sleep duration (SD)—of Omani adolescents, and to examine gender differences in such variables. Methods: 802 Omani adolescents (442 females and 360 males), aged 15–18 years were randomly recruited. Anthropometric indices, PA level, and EH and SD were evaluated by the Arab Teenage Lifestyle questionnaire. A semi-quantitative food frequency questionnaire for dietary assessment was also administered. Results: The results showed that although the study subjects had a sedentary lifestyle (lack of PA, average of 6.7 hours sleep, and consumption of high calorie foods), they maintained a normal body mass (less than 25 Kg/m2). Males were more than twice as active as females. With respect to EH, there were few gender differences, except in dairy and meat consumption where 62.5% and 55.5% of males consumed more than 3 servings, respectively, compared to 18.78 % and 35.2% of females, respectively. In addition, waist/height ratio, height, reasons for being active, energy drinks, potato consumption, eating sweets, vigorous PA and breakfast EHs were statistically significant independent predictors for BMI, P <0.05 for both males and females. Conclusion: This study revealed a high prevalence of sedentary behaviors and a low level of physical activity, especially among females. Unhealthy dietary habits were also widely found among both genders. There is an urgent need for more research as well as a national policy promoting active living and healthy eating and discouraging sedentary behaviour among Omani adolescents.

Kilani, Hashem; Al-Hazzaa, Hazzaa; Waly, Mostafa I.; Musaiger, Abdulrahman

2013-01-01

113

Martian Habitability  

NASA Astrophysics Data System (ADS)

Due to the reported Mars surface environmental conditions (Klein, 1978) (oxidative stress, high UV radiation levels, etc.) the possibility for life development in the surface of the red planet is very small. The identification of water-ice on the subsurface on Mars by the Thermal Emission Spectrometer onboard of the Mars Odyssey (Kieffer and Titus, 2001) and from the High Energy Neutron Detector (Litvak, et al., 2006) has important astrobiological connotations, because in addition to be a potential source for water, these locations are shielding habitats against the harsh conditions existing on the planet, like UV radiation (Gomez, et al., 2007; Gomez, et al., 2012). Martian habitability potential could change in particular located micro-niches. Salt deliquescence and hard environmental parameters modification could be relevant for life under protected niches. An example could be endolithic niches inside salt deposits used by phototrophs for taking advantage of sheltering particular light wavelengths. Similar acidic salts deposits are located in Río Tinto extreme environment with shelter life forms which are difficult to localize by eye. Techniques for its localization and study during space missions are needed to develop. Extreme environments are good scenarios where to test and train those techniques and where hypothetical Astrobiological space missions could be simulated for increasing possibilities of micro niches identification. Here we will report some experiments of bacteria exposition to Martian surface conditions in Mars Simulation chamber. Bacteria were shelter and exposed included in simulated salty endolithic micro niches. High percentage of bacteria resistance and adaptation to harsh extreme those conditions was reported (Gómez, F. et al., 2010). These results were used to develop and implement a Habitability Index to study Martian habitability during the next MSL mission to Mars landed on August 2012 on the surface of the red planet.

Gómez, F.

2012-09-01

114

Atmospheric escape, redox evolution, and planetary habitability  

NASA Astrophysics Data System (ADS)

Through the greenhouse effect, the presence and composition of an atmosphere is critical for defining a (conventional) circumstellar habitable zone in terms of planetary surface temperatures suitable for liquid water. Lack of knowledge of planetary atmospheres is likely to frustrate attempts to say with any certainty whether detected terrestrial-sized exoplanets may or may not be habitable. Perhaps an underappreciated role in such considerations is the evolutionary effect of atmospheric escape for determining atmospheric composition or whether an atmosphere exists in the first place. Whether atmospheres exist at all on planets is demonstrably connected to the effect of integrated atmospheric escape. When we observe our own Solar System and transiting exoplanets, the existence of an atmosphere is clearly delineated by a relative vulnerability to thermal escape and impact erosion. The prevalence of thermal escape as a key evolutionary determinant for the presence of planetary atmosphere is shown by a relationship between the relative solar (or stellar) heating and the escape velocity. Those bodies with too much stellar heating and too smaller escape velocity end up devoid of atmospheres. Impact erosion is evident in the relationship between impact velocity and escape velocity. Escape due to impacts is particularly important for understanding the large differences in the atmospheres of giant planet moons, such as Ganymede versus Titan. It is also significant for Mars-sized planets. The oxidation state of atmospheres is important for some theories of the origin of life (where an early reducing atmosphere is helpful for organic synthesis) and the evolution of advanced life (where free molecular oxygen is the best source of high energy metabolism). Surfaces on some relatively small planets and moons are observed to have evolved to an oxidized state, which theory and observation can explain through atmospheric escape. There are several examples in the Solar System where a net escape of hydrogen relative to heavier oxygen is the generally accepted explanation for the present oxidation state: Venus and Mars amongst the planets, and Ganymede, Europa, and Rhea amongst bodies with extremely tenuous atmospheres. We also argue that hydrogen escape was the key factor for oxidizing the Earth and facilitating the increase of photosynthetically-produced oxygen in the Proterozoic atmosphere. Our view about the primacy of hydrogen escape with regard to the Earth's atmospheric oxygenation is perhaps less widely accepted. However, it was inevitable that hydrogen escaped from Earth's early anoxic atmosphere at a significant rate. The result was a very big integrated oxidation consistent with what is observed in the Earth's crust in addition to some export to the mantle. In conclusion, a better understanding of atmospheric escape processes appears critical for understanding the suitability of planets for harboring life from simple to advanced forms.

Catling, D. C.; Zahnle, K. J.

2011-12-01

115

Promoting habit formation  

Microsoft Academic Search

Habits are automatic behavioural responses to environmental cues, thought to develop through repetition of behaviour in consistent contexts. When habit is strong, deliberate intentions have been shown to have a reduced influence on behaviour. The habit concept may provide a mechanism for establishing new behaviours, and so healthy habit formation is a desired outcome for many interventions. Habits also however

Phillippa Lally; Benjamin Gardner

2011-01-01

116

Circumstellar masers in the Magellanic Clouds  

NASA Astrophysics Data System (ADS)

Results are presented of a search for 22 GHz H_2O 616->523, 43 GHz SiOv=1(J=1->0), 86 GHz SiOv=1(J=2->1) and 129 GHz SiOv=1(J=3->2) maser emission from bright IRAS point sources in the Small and Large Magellanic Clouds - mostly circumstellar envelopes around obscured red supergiants and Asymptotic Giant Branch stars (OH/IR stars). The aim of this effort was to test whether the kinematics of the mass loss from these stars depends on metallicity. H_2O maser emission was detected in the red supergiants IRAS 04553-6825 and IRAS 05280-6910, and tentatively in the luminous IR object IRAS 05216-6753 and the AGB star IRAS 05329-6708, all in the LMC. SiOv=1(J=2->1) maser emission was detected in IRAS 04553-6825. The double-peaked H_2O maser line profiles of IRAS 04553-6825 and IRAS 05280-6910, in combination with the OH (and SiO) maser line profiles, yield the acceleration of the outflows from these stars. The outflow velocity increases between the H_2O masing zone near the dust-formation region and the more distant OH masing zone from v ~ 18 to 26 km s-1 for IRAS 04553-6825 and from v ~ 6 to 17 km s-1 for IRAS 05280-6910. The total sample of LMC targets is analysed in comparison with circumstellar masers in the Galactic Centre. The photon fluxes of circumstellar masers in the LMC are found to be very similar to those in the Galactic Centre. The expansion velocities in the LMC appear to be ~ 20% lower than for similarly bright OH masers in the Galactic Centre, but the data are still consistent with no difference in expansion velocity. OH/IR stars in the LMC appear to have slower accelerating envelopes than OH/IR stars in the Galactic Centre. The masers in the LMC have blue-asymmetric emission profiles. This may be due to the amplification of stellar and/or free-free radiation, rather than the amplification of dust emission, and may be more pronounced in low metallicity envelopes. The SiO maser strength increases with the photometric amplitude at 2.2 mu m but is independent of the photometric amplitude at 10 mu m. This suggests a strong connection between shocks in the dust-free SiO masing zone and the dust formation process. The LMC masers obey the same trend as the Galactic Centre masers. Appendices describe H_2O maser emission from the moderately mass-losing AGB star R Dor in the Milky Way, optical echelle spectroscopy of IRAS 04553-6825, and the properties of circumstellar masers in the Galactic Centre.

van Loon, J. Th.; Zijlstra, A. A.; Bujarrabal, V.; Nyman, L.-Å.

2001-03-01

117

Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the 12 Myr HD172555 System  

NASA Astrophysics Data System (ADS)

We have used the InfraRed Spectrograph on the Spitzer Space Telescope to study the fine dust orbiting around the 29 pc distant ? Pic analogue star HD172555. The dust mineralogy is is very peculiar, composed primarily of highly refractory, non-equilibrium materials, with 3/4 of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal emission spectra (non-equilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a-3.95±0.10. This steep a size distribution argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 ± 0.6 AU, within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 x 1019 - 2 x 1020 kg, equivalent to a 150 - 200 km radius asteroid. Significant emission features centered at 4 and 8 µm due to fluorescing SiO gas are also found. Roughly 1022 kg of SiO gas, formed by vaporizing silicate rock, is also present in-system, and a separate population of very large, cool grains, massing 1021 - 1022 kg and equivalent to the largest sized asteroid currently found in the Solar System's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong primary x-ray activity, or an extended disk of ? meteroids argues that the source of the observed circumstellar materials is a giant hypervelocity (> 10 km sec-1) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.

Lisse, Carey M.; Chen, C. H.; Wyatt, M. C.; Morlok, A.; Song, I.; Bryden, G.; Sheehan, P.

2010-01-01

118

Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the 12 Myr HD172555 System  

NASA Astrophysics Data System (ADS)

We have used the IRS Spectrograph onboard the Spitzer Space Telescope to study the warm dust orbiting around the 29 pc distant ? Pic analogue star HD172555. The dust mineralogy is very peculiar, composed primarily of highly refractory, non-equilibrium materials, with 3/4 of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal emission spectra (non-equilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a-3.95±0.10. This steep a size distribution argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 ± 0.6 AU, within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 x 1019 - 2 x 1020 kg, equivalent to a 150 - 200 km radius asteroid. Significant emission features centered at 4 and 8 µm due to fluorescing SiO gas are also found. Roughly 1022 kg of SiO gas, formed by vaporizing silicate rock, is also present in the system, and a separate population of very large, cool grains, massing 1021 - 1022 kg and equivalent to the largest sized asteroid currently found in the Solar System's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong primary x-ray activity, or an extended disk of ? meteroids argues that the source of the observed circumstellar materials is a giant hypervelocity (> 10 km sec-1) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.

Lisse, Carey M.; Chen, C. H.; Wyatt, M. C.; Morlok, A.; Song, I.; Bryden, G.; Sheehan, P.

2009-09-01

119

Imaging Polarimetry of Circumstellar Disks  

NASA Astrophysics Data System (ADS)

Imaging polarimetry of circumstellar disks around young stars is providing new insights into the formation and evolution of planetary systems. Imaging polarimetry can be used to assess the grain growth processes that work within protoplanetary disks to build up planetesimals and planets from primordial material, as well as the collisional processes that counter this growth by grinding larger bodies down to dust. The equilibrium between these processes causes dust grain populations within disks to evolve over time as systems shift from protoplanetary (containing primordial material) to transitional (epoch of planet formation) to debris disk (post-planet formation, collision erosion) phases. When light is scattered from circumstellar material, a signature of the scattering bodies is encoded into the resulting degree and pattern of polarization. Inverting this process enables us infer the nature of the scattering bodies and assess grain growth within the disk. A few dozen young planetary systems have been imaged in polarized light so far, enabling us to see resolved structures in disks on scales of a few AUs to a few hundred AUs. In this contribution, we briefly review the state of the art for imaging polarimetry of circumstellar disks, summarize some of the current questions that are being addressed with this technique, and present some illustrative examples from our research.

Hines, D. C.; Perrin, M. D.

2013-12-01

120

Effects of Exoplanet Planetesimal Carbon Chemistry on Habitability  

NASA Astrophysics Data System (ADS)

We explore the effects of reported differences in C/O values for exoplanet host stars on the composition of planetesimals formed beyond the snow line in these systems. Since the value of C/O in a planet forming nebula has a strong effect on amount of oxygen available for water ice in an oxidizing nebula, exoplanet systems for host stars with C/O greater than the solar value may have planetesimals with very little or no water ice. We have estimated the composition of volatile and refractory material in extrasolar planetesimals using a set of stars with a wide range of measured C/O abundances (Johnson et al. ApJ. 757(2), 192, 2012). The volatile ice content of planetesimals in these systems varies significantly with C/O, controlled primarily by the availability of O for H2O ice condensation. Systems with C/O less than the solar value (C/O = 0.55) should have very water ice rich planetesimals, while water ice mass fraction decreases rapidly with increasing C/O until only ices of CO and CO2 are left in significant proportions. If a significant fraction of C is in the form of refractory CHON particles, C and O are removed from the gas phase and the condensates for super-solar C/O values will be water-poor mixtures of silicates and metal, carbon, and carbon-bearing volatile ices, depending on temperature. For very carbon-rich systems, oxidizing conditions cannot be sustained beyond about C/O=1, due to the oxygen sequestered in solid silicates, oxides and CHON, for refractory C fractions within the Pollack et al. range of 0.4 - 0.7 (ApJ. 421, 615, 1994). These results have implications for assessing the habitability of exoplanets since they constrain the amount of water available beyond the snow line for dynamical delivery to inner planets, depending on the host star's C/O in the circumstellar nebula. Thus one the key chemical ingredients for habitability may be in short supply in carbon-rich, oxygen-poor systems even if planets exist in the ';habitable zone'. TVJ acknowledges government support at JPL/Caltech, under a contract with NASA. NM acknowledges support from Yale University. JIL was supported by the JWST Project through NASA. O.M. acknowledges support from CNES.Planetesimal compositions for several stellar systems with a fraction, Csolid, of C in solid CHON particles

Johnson, T. V.; Mousis, O.; Lunine, J. I.; Madhusudhan, N.

2013-12-01

121

Effects of Exoplanet Planetesimal Carbon Chemistry on Habitability  

NASA Astrophysics Data System (ADS)

We explore the effects of reported differences in C/O values for exoplanet host stars on the composition of planetesimals formed beyond the snow line in these systems. Since the value of C/O in a planet forming nebula has a strong effect on amount of oxygen available for water ice in an oxidizing nebula, exoplanet systems for host stars with C/O greater than the solar value may have planetesimals with very little or no water ice. We have estimated the composition of volatile and refractory material in extrasolar planetesimals using a set of stars with a wide range of measured C/O abundances (Johnson et al. ApJ. 757(2), 192, 2012). The volatile ice content of planetesimals in these systems varies significantly with C/O, controlled primarily by the availability of O for H2 O ice condensation. Systems with C/O less than the solar value (C/O = 0.55) should have very water ice rich planetesimals, while water ice mass fraction decreases rapidly with increasing C/O until only ices of CO and CO2 are left in significant proportions. If a significant fraction of C is in the form of refractory CHON particles, C and O are removed from the gas phase and the condensates for super-solar C/O values will be water-poor mixtures of silicates and metal, carbon, and carbon-bearing volatile ices, depending on temperature. For very carbon-rich systems, oxidizing conditions cannot be sustained beyond about C/O=1, due to the oxygen sequestered in solid silicates, oxides and CHON, for refractory C fractions within the Pollack et al. range of 0.4 - 0.7 (ApJ. 421, 615, 1994). These results have implications for assessing the habitability of exoplanets since they constrain the amount of water available beyond the snow line for dynamical delivery to inner planets, depending on the host star’s C/O in the circumstellar nebula. Thus one the key chemical ingredients for habitability may be in short supply in carbon-rich, oxygen-poor systems even if planets exist in the ‘habitable zone’. TVJ acknowledges government support at JPL/Caltech, under a contract with NASA. NM acknowledges support from Yale University. JIL was supported by the JWST Project through NASA. O.M. acknowledges support from CNES.

Johnson, Torrence V.; Mousis, O.; Lunine, J. I.; Madhusudhan, N.

2013-10-01

122

Habitability: Camelot 4.  

National Technical Information Service (NTIS)

During 1988 to 1989 the NASA/USRA Advanced Design Program sponsored research and design efforts aimed at developing habitability criteria and at defining a habitability concept as a useful tool in understanding and evaluating dwellings for prolonged stays...

W. Alequin A. Barragan M. Carro F. Garcia I. Gonzalez

1990-01-01

123

ADULT READING HABITS  

Microsoft Academic Search

Four aspects of adult reading habits are reviewed and identified: 1. the nature and extent of adult reading habits as a field of inquiry from 1900?1975;2. key findings on adult reading habits are related to amount and motivation in purposes for reading;3. correlation between adult reading habits, education, employment, and socioeconomic status;4. problem and direction theory and research should take

Martin Kling

1982-01-01

124

Changing Circumstances, Disrupting Habits  

Microsoft Academic Search

The present research investigated the mechanisms guiding habitual behavior, specifically, the stimulus cues that trigger habit performance. When usual contexts for performance change, habits cannot be cued by recurring stimuli, and performance should be disrupted. Thus, the exercising, newspaper reading, and TV watching habits of students transferring to a new university were found to survive the transfer only when aspects

Wendy Wood; Leona Tam; Melissa Guerrero Witt

2005-01-01

125

Polytype distribution in circumstellar silicon carbide.  

SciTech Connect

The inferred crystallographic class of circumstellar silicon carbide based on astronomical infrared spectra is controversial. We have directly determined the polytype distribution of circumstellar SiC from transmission electron microscopy of presolar silicon carbide from the Murchison carbonaceous meteorite. Only two polytypes (of a possible several hundred) were observed: cubic 3C and hexagonal 2H silicon carbide and their intergrowths. We conclude that this structural simplicity is a direct consequence of the low pressures in circumstellar outflows and the corresponding low silicon carbide condensation temperatures.

Daulton, T. L.; Bernatowicz, T. J.; Lewis, R. S.; Messenger, S.; Stadermann, F. J.; Amari, S.; Materials Science Division; Naval Research Lab.; Washington Univ.; Univ. of Chicago

2002-06-07

126

Energy Balance Models of planetary climate as a tool for investigating the habitability of terrestrial planets and its evolution  

NASA Astrophysics Data System (ADS)

The study of the habitability and potential for life formation of terrestrial planets requires a considerable work of modelization owing to the limited amount of experimental constraints typical of this type of research. As an example, the paucity of experimental Archean data severely limits the study of the habitability of the primitive Earth at the epoch of the origin of life. In the case of exoplanets the amount of experimental information available is quite limited and the need for modelization strong. Here we focus on the modelization of the surface planetary temperature, a key thermodynamical quantity used to define the habitability. Energy Balance Models (EBM) of planetary climate provide a simple way to calculate the temperature-latitude profile of terrestrial planets with a small amount of computing resources. Thanks to this fact EBMs offer an excellent tool to exploring a wide range of parameter space and therefore testing the effects of variations of physical/chemical quantities unconstrained by experimental data. In particular, one can easily probe possible scenarios of habitability at different stages of planetary evolution. We have recently implemented one-dimensional EBMs featuring the possibility of probing variations of astronomical and geophysical parameters, such as stellar luminosity, orbital semi-major axis and eccentricity, obliquity of the planetary axis, planet rotational velocity, land/ocean surface fractions and thermal capacities, and latitudinal heat diffusion. After testing our models against results obtained in previous work (Williams & Kasting 1997, Icarus, 129, 254; Spiegel et al. 2008, ApJ, 681, 1609), we introduced a novel parametrization of the diffusion coefficient as a function of the stellar zenith distance. Our models have been validated using the mean temperature-latitude profiles of the present Earth and its seasonal variations; the global albedo has been used as an additional constraint. In this work we present specific examples of application of our EBMs to studies of habitability of terrestrial planets. In the first part we focus on the primitive Earth, taking into account the effects of the higher speed of Earth rotation and reduced solar luminosity at the epoch of life formation. In the second part we provide examples of habitability studies of planetary systems discovered in surveys of exoplanets. These examples allow us to critically discuss the concept of circumstellar habitable zone.

Ferri, G.; Murante, G.; Provenzale, A.; Silva, L.; Vladilo, G.

2012-04-01

127

Tides, Planetary Companions, and Habitability  

NASA Astrophysics Data System (ADS)

Earth-scale planets in the classical habitable zone (HZ) are more likely to be habitable if they possess active geophysics to drive processes that regulate their atmosphere. Without a constant internal energy source, planets cool as they age, eventually terminating tectonic activity. Planets orbiting low-mass stars can be very old, due to the longevity of such stars, so they may be rendered sterile to life in this way. However, the presence of an outer companion could generate enough tidal heat in the HZ planet to prevent such cooling. The range of mass and orbital parameters for the companion that give adequate long-term heating of the inner HZ planet, while avoiding very early total desiccation, is substantial. We locate the ideal location for the outer of a pair of planets, under the assumption that the inner planet has the same incident flux as Earth, orbiting example stars: a generic late M dwarf and the M9V/L0 dwarf DEN1048. We also analyze the extent to which systems with ideal parameters for heating will evolve over time. Thus discoveries of Earth-scale planets in the HZ zone of old small stars should be followed by searches for outer companion planets that might be essential for current habitability.

Van Laerhoven, Christa L.; Barnes, Rory; Greenberg, Richard

2014-05-01

128

The HARPS search for southern extra-solar planets. XXXIV. A planetary system around the nearby M dwarf GJ 163, with a super-Earth possibly in the habitable zone  

NASA Astrophysics Data System (ADS)

The meter-per-second precision achieved by today's velocimeters enables us to search for 1-10 M? planets in the habitable zone of cool stars. This paper reports on the detection of three planets orbiting GJ 163 (HIP 19394), a M3 dwarf monitored by our ESO/HARPS search for planets. We made use of the HARPS spectrograph to collect 150 radial velocities of GJ 163 over a period of eight years. We searched the radial-velocity time series for coherent signals and found five distinct periodic variabilities. We investigated the stellar activity and called into question the planetary interpretation for two signals. Before more data can be acquired we concluded that at least three planets are orbiting GJ 163. They have orbital periods of Pb = 8.632 ± 0.002, Pc = 25.63 ± 0.03, and Pd = 604 ± 8 days and minimum masses msini = 10.6 ± 0.6, 6.8 ± 0.9, and 29 ± 3 M?, respectively. We hold our interpretations for the two additional signals with periods P(e) = 19.4 and P(f) = 108 days. The inner pair presents an orbital period ratio of 2.97, but a dynamical analysis of the system shows that it lays outside the 3:1 mean motion resonance. The planet GJ 163c, in particular, is a super-Earth with an equilibrium temperature of Teq = (302 ± 10)(1 - A)1/4 K and may lie in the so-called habitable zone for albedo values (A = 0.34 - 0.89) moderately higher than that of Earth (A? = 0.2-0.3). Based on observations made with the HARPS instrument on the ESO 3.6 m telescope under the program IDs 072.C-0488, 082.C-0718, and 183.C-0437 at Cerro La Silla (Chile).Table 6 is available in electronic form at http://www.aanda.orgRadial-velocity time series (Table 6) are also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/556/A110

Bonfils, X.; Lo Curto, G.; Correia, A. C. M.; Laskar, J.; Udry, S.; Delfosse, X.; Forveille, T.; Astudillo-Defru, N.; Benz, W.; Bouchy, F.; Gillon, M.; Hébrard, G.; Lovis, C.; Mayor, M.; Moutou, C.; Naef, D.; Neves, V.; Pepe, F.; Perrier, C.; Queloz, D.; Santos, N. C.; Ségransan, D.

2013-08-01

129

Planetary Habitability of the Solar System  

NASA Astrophysics Data System (ADS)

Habitability is a qualitative concept generally defined as the suitability of an environment to support life. Although there are many works related to planetary habitability, there is no practical quantitative definition of habitability. The search for habitable environments in the Solar System and beyond requires a method to quantify and compare their significance. Therefore, this study presents a quantitative approach to assess the habitability of Earth and other planetary bodies. A Quantitative Habitability Model (QH Model) was develop and used to model the terrestrial habitability as a standard for comparison. The QH Model provides a simple ecophysiology-based framework that can be used to predict the potential distribution, abundance and productivity of life in planetary bodies from local to global scales. The simplest QH Model calculates habitability from the environment temperature and relative humidity in gas phases (i.e. atmospheres), and from temperature and salinity in liquid phases (i.e. oceans). The model was used to explain the latitudinal gradients of primary producers on Earth and was validated with ground and satellites observations of net primary productivity (NPP). The potential global habitability for prokaryotes of the upper-troposphere of Venus, the subsurface of Mars, Europa, Titan, and Enceladus was compared. Results show that Enceladus has the zone with the highest mean habitability in the Solar System although to deep for direct exploration. Results also show that the current global terrestrial environment of land areas is not optimized for primary producers, but it was during some paleoclimates. The QH Model has applications in ecosystem modeling, global climate studies including paleoclimates and global warming, planetary protection, and astrobiology. It can also be used to quantify the potential for life of any terrestrial-size extrasolar planet as compared to Earth. This study was partially supported by UPR Arecibo and NASA Astrobiology Institute.

Mendez, Abel

2009-09-01

130

Reservoir for Comet Material: Circumstellar Grains  

NASA Astrophysics Data System (ADS)

Primitive meteorites and interplanetary dust particles contain small quantities of dust grains with highly anomalous isotopic compositions. These grains formed in the winds of evolved stars and in the ejecta of stellar explosions, i.e., they represent a sample of circumstellar grains that can be analyzed with high precision in the laboratory. Such studies have provided a wealth of information on stellar evolution and nucleosynthesis, Galactic chemical evolution, grain growth in stellar environments, interstellar chemistry, and the inventory of stars that contributed dust to the Solar System. Among the identified circumstellar grains in primitive solar system matter are diamond, graphite, silicon carbide, silicon nitride, oxides, and silicates. Circumstellar grains have also been found in cometary matter. To date the available information on circumstellar grains in comets is limited, but extended studies of matter returned by the Stardust mission may help to overcome the existing gaps.

Hoppe, Peter

131

Reservoir for Comet Material: Circumstellar Grains  

NASA Astrophysics Data System (ADS)

Primitive meteorites and interplanetary dust particles contain small quantities of dust grains with highly anomalous isotopic compositions. These grains formed in the winds of evolved stars and in the ejecta of stellar explosions, i.e., they represent a sample of circumstellar grains that can be analyzed with high precision in the laboratory. Such studies have provided a wealth of information on stellar evolution and nucleosynthesis, Galactic chemical evolution, grain growth in stellar environments, interstellar chemistry, and the inventory of stars that contributed dust to the Solar System. Among the identified circumstellar grains in primitive solar system matter are diamond, graphite, silicon carbide, silicon nitride, oxides, and silicates. Circumstellar grains have also been found in cometary matter. To date the available information on circumstellar grains in comets is limited, but extended studies of matter returned by the Stardust mission may help to overcome the existing gaps.

Hoppe, Peter

2008-07-01

132

Circumstellar Material Around Evolved Massive Stars  

NASA Astrophysics Data System (ADS)

I review multiwavelength observations of material seen around different types of evolved massive stars (i.e. red supergiants, yellow hypergiants, luminous blue variables, B[e] supergiants, and Wolf-Rayet stars), concentrating on diagnostics of mass, composition, and kinetic energy in both local and distant examples. Circumstellar material has significant implications for the evolutionary state of the star, the role of episodic mass loss in stellar evolution, and the roles of binarity and rotation in shaping the ejecta. This mass loss determines the type of supernova that results via the stripping of the star's outer layers, but the circumstellar gas can also profoundly influence the immediate pre-supernova environment. Dense circumstellar material can actually change the type of supernova that is seen when it is illuminated by the supernova or heated by the blast wave. As such, unresolved circumstellar material illuminated by distant supernovae can provide a way to study mass loss in massive stars in distant environments.

Smith, Nathan

2011-01-01

133

Circumstellar interaction of the type Ia supernova 2002ic  

NASA Astrophysics Data System (ADS)

We propose a model to account for the bolometric light curve, the quasi-continuum and the CaII emission features of the peculiar type Ia supernova (SN) 2002ic, which exploded in a dense circumstellar envelope. The model suggests that the SN Ia had the maximum possible kinetic energy and that the ejecta expand in an approximately spherically symmetric (possibly clumpy) circumstellar environment. The CaII and the quasi-continuum are emitted by shocked SN ejecta that underwent deformation and fragmentation in the intershock layer. Modelling of the CaII triplet implies that the contribution of the OI 8446-Åline is about 25 per cent of the 8500-Åfeature on day 234, which permits us to recover the flux in the CaII 8579-Åtriplet from the flux of 8500-Åblend reported by Deng et al. We use the CaII doublet and triplet fluxes on day 234 to derive the electron temperature (~ 4400 K) in the CaII line-emitting zone and the ratio of the total area of dense fragments to the area of the shell, S/S0~ 102. We argue that CaII bands and the quasi-continuum originate from different zones of the shocked ejecta that reflect the abundance stratification of the supernova.

Chugai, N. N.; Chevalier, R. A.; Lundqvist, P.

2004-12-01

134

Geophysical and atmospheric evolution of habitable planets.  

PubMed

The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planet's host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planet's initial water inventory. Depending on the host star's radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere. PMID:20307182

Lammer, Helmut; Selsis, Frank; Chassefière, Eric; Breuer, Doris; Griessmeier, Jean-Mathias; Kulikov, Yuri N; Erkaev, Nikolai V; Khodachenko, Maxim L; Biernat, Helfried K; Leblanc, Francois; Kallio, Esa; Lundin, Richard; Westall, Frances; Bauer, Siegfried J; Beichman, Charles; Danchi, William; Eiroa, Carlos; Fridlund, Malcolm; Gröller, Hannes; Hanslmeier, Arnold; Hausleitner, Walter; Henning, Thomas; Herbst, Tom; Kaltenegger, Lisa; Léger, Alain; Leitzinger, Martin; Lichtenegger, Herbert I M; Liseau, René; Lunine, Jonathan; Motschmann, Uwe; Odert, Petra; Paresce, Francesco; Parnell, John; Penny, Alan; Quirrenbach, Andreas; Rauer, Heike; Röttgering, Huub; Schneider, Jean; Spohn, Tilman; Stadelmann, Anja; Stangl, Günter; Stam, Daphne; Tinetti, Giovanna; White, Glenn J

2010-01-01

135

Turnout as a Habit  

Microsoft Academic Search

It is conventional to speak of voting as “habitual.” But what does this mean? In psychology, habits are cognitive associations\\u000a between repeated responses and stable features of the performance context. Thus, “turnout habit” is best measured by an index\\u000a of repeated behavior and a consistent performance setting. Once habit associations form, the response can be cued even in\\u000a the absence

John H. Aldrich; Jacob M. Montgomery; Wendy Wood

136

Your Child's Habits  

MedlinePLUS

... who start hair pulling as older kids or teens, the habit is harder to break and may be a ... But if you're concerned about your child's habits, talk with your doctor. Reviewed by: Steven Dowshen, MD Date reviewed: August 2011 Back 1 ? 2 ? 3 ? 4 ? 5 For Teens For Kids For Parents MORE ON THIS TOPIC ...

137

The HARPS search for southern extra-solar planets . XXXII. New multi-planet systems in the HARPS volume limited sample: a super-Earth and a Neptune in the habitable zone  

NASA Astrophysics Data System (ADS)

The vast diversity of planetary systems detected to date is defying our capability of understanding their formation and evolution. Well-defined volume-limited surveys are the best tool at our disposal to tackle the problem, via the acquisition of robust statistics of the orbital elements. We are using the HARPS spectrograph to conduct our survey of ?850 nearby solar-type stars, and in the course of the past nine years we have monitored the radial velocity of HD 103774, HD 109271, and BD-061339. In this work we present the detection of five planets orbiting these stars, with msin (i) between 0.6 and 7 Neptune masses, four of which are in two multiple systems, comprising one super-Earth and one planet within the habitable zone of a late-type dwarf. Although for strategic reasons we chose efficiency over precision in this survey, we have the capability to detect planets down to the Neptune and super-Earth mass range as well as multiple systems, provided that enough data points are made available. Based on observations made with the HARPS instrument on the ESO 3.6 m telescope at La Silla (Chile), under the GTO program ID 072.C-0488 and the regular programs: 085.C-0019, 087.C-0831 and 089.C-0732. RV data are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/551/A59

Lo Curto, G.; Mayor, M.; Benz, W.; Bouchy, F.; Hébrard, G.; Lovis, C.; Moutou, C.; Naef, D.; Pepe, F.; Queloz, D.; Santos, N. C.; Segransan, D.; Udry, S.

2013-03-01

138

Physics and Chemistry of Circumstellar Dust Shells  

NASA Astrophysics Data System (ADS)

Part I. Setting the Stage: 1. Introduction; 2. Evolutionary status of dust-enshrouded objects; Part II. Theoretical Description of Circumstellar Dust Shells: 3. Theory of circumstellar dust shells; 4. The energy equation for matter; 5. Radiative transfer; 6. Interaction between gas and dust particles; 7. Extinction by dust grains and gas; 8. Approaches to the temperature equations; 9. Chemistry in thermodynamic equilibrium; 10. Gas-phase chemical composition; 11. Gas-solid chemical equilibria; 12. Growth of dust grains; 13. Formation of seed nuclei; 14. Moment equations; Part III. Applications: 15. Modeling of circumstellar dust shells; 16. Miras and long-period variables; 17. Mass loss formulae; 18. R Coronae Borealis stars; Part IV. Appendices; Bibliography; Index.

Gail, Hans-Peter; Sedlmayr, Erwin

2013-12-01

139

Physics and Chemistry of Circumstellar Dust Shells  

NASA Astrophysics Data System (ADS)

Part I. Setting the Stage: 1. Introduction; 2. Evolutionary status of dust-enshrouded objects; Part II. Theoretical Description of Circumstellar Dust Shells: 3. Theory of circumstellar dust shells; 4. The energy equation for matter; 5. Radiative transfer; 6. Interaction between gas and dust particles; 7. Extinction by dust grains and gas; 8. Approaches to the temperature equations; 9. Chemistry in thermodynamic equilibrium; 10. Gas-phase chemical composition; 11. Gas-solid chemical equilibria; 12. Growth of dust grains; 13. Formation of seed nuclei; 14. Moment equations; Part III. Applications: 15. Modeling of circumstellar dust shells; 16. Miras and long-period variables; 17. Mass loss formulae; 18. R Coronae Borealis stars; Part IV. Appendices; Bibliography; Index.

Gail, Hans-Peter; Sedlmayr, Erwin

2014-02-01

140

TIDAL LIMITS TO PLANETARY HABITABILITY  

SciTech Connect

The habitable zones (HZs) of main-sequence stars have traditionally been defined as the range of orbits that intercept the appropriate amount of stellar flux to permit surface water on a planet. Terrestrial exoplanets discovered to orbit M stars in these zones, which are close-in due to decreased stellar luminosity, may also undergo significant tidal heating. Tidal heating may span a wide range for terrestrial exoplanets and may significantly affect conditions near the surface. For example, if heating rates on an exoplanet are near or greater than that on Io (where tides drive volcanism that resurfaces the planet at least every 1 Myr) and produce similar surface conditions, then the development of life seems unlikely. On the other hand, if the tidal heating rate is less than the minimum to initiate plate tectonics, then CO{sub 2} may not be recycled through subduction, leading to a runaway greenhouse that sterilizes the planet. These two cases represent potential boundaries to habitability and are presented along with the range of the traditional HZ for main-sequence, low-mass stars. We propose a revised HZ that incorporates both stellar insolation and tidal heating. We apply these criteria to GJ 581 d and find that it is in the traditional HZ, but its tidal heating alone may be insufficient for plate tectonics.

Barnes, Rory [Department of Astronomy, University of Washington, Seattle, WA 98195-1580 (United States); Jackson, Brian; Greenberg, Richard [Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721 (United States); Raymond, Sean N. [Virtual Planetary Laboratory (United States)

2009-07-20

141

Dynamical habitability of planetary systems.  

PubMed

The problem of the stability of planetary systems, a question that concerns only multiplanetary systems that host at least two planets, is discussed. The problem of mean motion resonances is addressed prior to discussion of the dynamical structure of the more than 350 known planets. The difference with regard to our own Solar System with eight planets on low eccentricity is evident in that 60% of the known extrasolar planets have orbits with eccentricity e > 0.2. We theoretically highlight the studies concerning possible terrestrial planets in systems with a Jupiter-like planet. We emphasize that an orbit of a particular nature only will keep a planet within the habitable zone around a host star with respect to the semimajor axis and its eccentricity. In addition, some results are given for individual systems (e.g., Gl777A) with regard to the stability of orbits within habitable zones. We also review what is known about the orbits of planets in double-star systems around only one component (e.g., gamma Cephei) and around both stars (e.g., eclipsing binaries). PMID:20307181

Dvorak, Rudolf; Pilat-Lohinger, Elke; Bois, Eric; Schwarz, Richard; Funk, Barbara; Beichman, Charles; Danchi, William; Eiroa, Carlos; Fridlund, Malcolm; Henning, Thomas; Herbst, Tom; Kaltenegger, Lisa; Lammer, Helmut; Léger, Alain; Liseau, René; Lunine, Jonathan; Paresce, Francesco; Penny, Alan; Quirrenbach, Andreas; Röttgering, Huub; Selsis, Frank; Schneider, Jean; Stam, Daphne; Tinetti, Giovanna; White, Glenn J

2010-01-01

142

Abundant Circumstellar Silica Dust and SiO Gas Created by a Giant Hypervelocity Collision in the ~12 Myr HD172555 System  

NASA Astrophysics Data System (ADS)

The fine dust detected by infrared (IR) emission around the nearby ? Pic analog star HD172555 is very peculiar. The dust mineralogy is composed primarily of highly refractory, nonequilibrium materials, with approximately three quarters of the Si atoms in silica (SiO2) species. Tektite and obsidian lab thermal emission spectra (nonequilibrium glassy silicas found in impact and magmatic systems) are required to fit the data. The best-fit model size distribution for the observed fine dust is dn/da = a -3.95±0.10. While IR photometry of the system has stayed stable since the 1983 IRAS mission, this steep a size distribution, with abundant micron-sized particles, argues for a fresh source of material within the last 0.1 Myr. The location of the dust with respect to the star is at 5.8 ± 0.6 AU (equivalent to 1.9 ± 0.2 AU from the Sun), within the terrestrial planet formation region but at the outer edge of any possible terrestrial habitability zone. The mass of fine dust is 4 × 1019-2 × 1020 kg, equivalent to a 150-200 km radius asteroid. Significant emission features centered at 4 and 8 ?m due to fluorescing SiO gas are also found. Roughly 1022 kg of SiO gas, formed by vaporizing silicate rock, is also present in the system, and a separate population of very large, cool grains, massing 1021-1022 kg and equivalent to the largest sized asteroid currently found in the solar system's main asteroid belt, dominates the solid circumstellar material by mass. The makeup of the observed dust and gas, and the noted lack of a dense circumstellar gas disk, strong stellar X-ray activity, and an extended disk of ? meteoroids argues that the source of the observed circumstellar materials is a giant hypervelocity (>10 km s-1) impact between large rocky planetesimals, similar to the ones which formed the Moon and which stripped the surface crustal material off of Mercury's surface.

Lisse, C. M.; Chen, C. H.; Wyatt, M. C.; Morlok, A.; Song, I.; Bryden, G.; Sheehan, P.

2009-08-01

143

Stellar Activity Masking and Mimicking Habitable Exoplanets  

NASA Astrophysics Data System (ADS)

Future generations of precise radial velocity (RV) surveys aim to attain a sensitivity sufficient to detect Earth mass planets orbiting in their host star's habitable zones. The RV semi-amplitude of such a planet can be significantly smaller than RV variations caused by stellar "jitter". Some RV variations, in particular those caused by starspots rotating in and out of view and those caused by magnetic activity cycles can be periodic in nature and can mimic planetary RV signals. We calculate and compare the relative timescales and amplitudes of RV variations due to activity and habitable planetary companions as a function of stellar mass, and discuss the ramifications for RV surveys.

Vanderburg, Andrew; Plavchan, Peter; Johnson, John A.

2014-06-01

144

Technology Demonstration Milestone #1 for the EXoplanetary Circumstellar Environments and Disk Explorer (EXCEDE) I. Laboratory/Experimental Results  

NASA Astrophysics Data System (ADS)

Coronagraph technology is advancing and promises to enable space telescopes capable of directly detecting and spatially resolving low surface brightness circumstellar debris disks as well as imaging giant planets as close as in the habitable zones of their host stars. One proposed mission capable of doing this is called EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer), which in 2011 was selected by NASA's Explorer program for technology development (Category III). EXCEDE is a 0.7m space telescope concept designed to achieve raw contrasts of 1e6 at an inner working angle of 1.2 l/D and 1e7 at 2 l/D and beyond. In addition to doing fundamental science on debris disks, EXCEDE will also serve as a technological and scientific precursor for an exo-Earth imaging mission. EXCEDE uses a Starlight Suppression System (SSS) based on the PIAA coronagraph, enabling aggressive performance. In this presentation, we report on our continuing progress of developing the SSS for EXCEDE, and in particular the achievement of the first major milestone in our technology development program (1e6 median raw contrast between a 1.2 l/D inner working angle and 2 l/D, simultaneously with 1e7 median raw contrast between 2 l/D and 4 l/D, in monochromatic light and in a controlled and repeatable fashion - see companion paper by Schneider et al. for science drivers). In addition, we will describe the upgrades to our system, such as (a) the Low Order Wavefront Sensor (LOWFS) which enabled achieving deep contrasts at aggressive inner working angles; (b) efficient model-based wavefront control algorithms; (c) a reconfiguration of our DM to be upstream of the coronagraph and the addition of the “inverse PIAA” system that enables better outer working angles. Finally, we report on preliminary demonstrations in a vacuum chamber. Even though this technology development is primarily targeted towards EXCEDE, it is also germane to any exoplanet direct imaging space-based telescopes because of the many challenges common to different coronagraph architectures and mission requirements. This work was supported in part by the NASA Explorer program and Ames Research Center, University of Arizona, and Lockheed Martin SSC.

Belikov, Ruslan; Bendek, E.; Davis, P.; Duncan, A.; Greene, T. P.; Guyon, O.; Hix, T.; Irwin, W.; Kendrick, R.; Lozi, J.; Lynch, D.; Mihara, R.; Pluzhnik, E.; Schneider, G.; Smith, E.; Thomas, S.; Witteborn, F. C.

2014-01-01

145

EVIDENCE FOR STRONG CIRCUMSTELLAR INTERACTION1  

Microsoft Academic Search

Optical and ultraviolet observations of the Type IIn supernova 1995N at epochs between 321 and 1799 days after the explosion show three distinct ve- locity components. The narrow lines come from circumstellar gas and show both low and high ionization. This component has a low filling factor, and is photoionized by X-rays from the shock. The intermediate component, which is

Claes Fransson; Roger A. Chevalier; Alexei V. Filippenko; Bruno Leibundgut; Aaron J. Barth; Robert A. Fesen; Robert P. Kirshner; Douglas C. Leonard; Weidong Li; Peter Lundqvist; Jesper Sollerman; Schuyler D. Van Dyk

146

Isothermal Circumstellar Dust Shell Model for Teaching  

ERIC Educational Resources Information Center

We introduce a model of radiative transfer in circumstellar dust shells. By assuming that the shell is both isothermal and its thickness is small compared to its radius, the model is simple enough for students to grasp and yet still provides a quantitative description of the relevant physical features. The isothermal model can be used in a…

Robinson, G.; Towers, I. N.; Jovanoski, Z.

2009-01-01

147

Shapes of the Circumstellar 'Silicate' Features.  

National Technical Information Service (NTIS)

Around oxygen-rich stars we find that the spectra of most long-period variables (LPV) show an excess infrared emission which is attributed to circumstellar silicate dust grains. These grains produce emission features at about 10 and 18 micrometers due to ...

I. R. Little-Marenin S. D. Price

1986-01-01

148

Radiative transfer in spherical circumstellar dust envelopes  

Microsoft Academic Search

S>The equation of radiative transfer for the seattering of polarized ; light in a spherical circumstellar dust shell is solved with the adjoint Monte ; Carlo methed. Calculations are presented for four colors of light scattering ; from small (1.0- mu radius) particles having a complex index of refraction that ; is assumed independent of wavelength. Scattering by large (5.0-

M. T. II

1973-01-01

149

TEM Studies of a Circumstellar Rock.  

National Technical Information Service (NTIS)

Reported here is the discovery of crystals of titanium carbide in a grain of silicon carbide which formed as a circumstellar dust particle in the atmosphere of a carbon-rich star. Just as in the case of terrestrial rocks, whose assemblage of minerals give...

T. J. Bernatowicz S. Amari R. S. Lewis

1992-01-01

150

Habitability: CAMELOT 4  

NASA Technical Reports Server (NTRS)

During 1988 to 1989 the NASA/USRA Advanced Design Program sponsored research and design efforts aimed at developing habitability criteria and at defining a habitability concept as a useful tool in understanding and evaluating dwellings for prolonged stays in extraterrestrial space. The Circulating Auto sufficient Mars-Earth Luxurious Orbital Transport (CAMELOT) was studied as a case in which the students would try to enhance the quality of life of the inhabitants by applying architectural design methodology. The study proposed 14 habitability criteria considered necessary to fulfill the defined habitability concept, which is that state of equilibrium that results from the interaction between components of the Individual Architecture Mission Complex, which allows a person to sustain physiological homeostatis, adequate performance, and acceptable social relationships. Architecture, design development, refinements and revisions to improve the quality of life, new insights on artificial gravity, form and constitution problems, and the final design concept are covered.

Alequin, W.; Barragan, A.; Carro, M.; Garcia, F.; Gonzalez, I.; Mercado, J. A.; Negron, N.; Lopez, D.; Rivera, L. A.; Rivera, M.

1990-01-01

151

Habitability study shuttle orbiter  

NASA Technical Reports Server (NTRS)

Studies of the habitability of the space shuttle orbiter are briefly summarized. Selected illustrations and descriptions are presented for: crew compartment, hygiene facilities, food system and galley, and storage systems.

1972-01-01

152

NASA: Habitable Worlds  

NSDL National Science Digital Library

NASA's Habitable Worlds website encourages visitors to "search the solar system for signs of life," by selecting a "World to Explore." This creatively designed website smartly displays our solar system's colorful planets or worlds amidst the dark background of space. In order to gain in-depth information specific to each planet, users simply click on the world of their choosing. Each planet page provides beautiful images and information about habitability, moons, and more.

153

Habitable Climates: The Influence of Eccentricity  

NASA Astrophysics Data System (ADS)

In the outer regions of the habitable zone, the risk of transitioning into a globally frozen "snowball" state poses a threat to the habitability of planets with the capacity to host water-based life. Here, we use a one-dimensional energy balance climate model (EBM) to examine how obliquity, spin rate, orbital eccentricity, and the fraction of the surface covered by ocean might influence the onset of such a snowball state. For an exoplanet, these parameters may be strikingly different from the values observed for Earth. Since, for a constant semimajor axis, the annual mean stellar irradiation scales with (1 - e 2)-1/2, one might expect the greatest habitable semimajor axis (for fixed atmospheric composition) to scale as (1 - e 2)-1/4. We find that this standard simple ansatz provides a reasonable lower bound on the outer boundary of the habitable zone, but the influence of both obliquity and ocean fraction can be profound in the context of planets on eccentric orbits. For planets with eccentricity 0.5, for instance, our EBM suggests that the greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending on obliquity, with higher obliquity worlds generally more stable against snowball transitions. One might also expect that the long winter at an eccentric planet's apoastron would render it more susceptible to global freezing. Our models suggest that this is not a significant risk for Earth-like planets around Sun-like stars, as considered here, since such planets are buffered by the thermal inertia provided by oceans covering at least 10% of their surface. Since planets on eccentric orbits spend much of their year particularly far from the star, such worlds might turnout to be especially good targets for direct observations with missions such as TPF-Darwin. Nevertheless, the extreme temperature variations achieved on highly eccentric exo-Earths raise questions about the adaptability of life to marginally or transiently habitable conditions.

Dressing, Courtney D.; Spiegel, David S.; Scharf, Caleb A.; Menou, Kristen; Raymond, Sean N.

2010-10-01

154

High on Habits  

PubMed Central

The neural circuits involved in learning and executing goal-directed actions, which are governed by action-outcome contingencies and sensitive to changes in the expected value of the outcome, have been shown to be different from those mediating habits, which are less dependent on action-outcome relations and changes in outcome value. Extended training, different reinforcement schedules, and substances of abuse have been shown to induce a shift from goal-directed performance to habitual performance. This shift can be beneficial in everyday life, but can also lead to loss of voluntary control and compulsive behavior, namely during drug seeking in addiction. Although the brain circuits underlying habit formation are becoming clearer, the molecular mechanisms underlying habit formation are still not understood. Here, we review a recent study where Hilario et al. (2007) established behavioral procedures to investigate habit formation in mice in order to investigate the molecular mechanisms underlying habit formation. Using those procedures, and a combination of genetic and pharmacological tools, the authors showed that endocannabinoid signaling is critical for habit formation.

Hilario, Monica R. F.; Costa, Rui M.

2008-01-01

155

Technology Demonstration Milestone #1 for the EXoplanetary Circumstellar Environments and Disk Explorer (EXCEDE) II. Science Drivers and Implications  

NASA Astrophysics Data System (ADS)

The EXoplanetary Circumstellar (CS) Environments and Disk Explorer (EXCEDE) is an EX class Explorer mission proposed to study the formation, evolution, architectures, and diversity of exoplanetary systems by characterizing suspected planet-hosting CS environments into and beyond host-star habitable zones using a small (0.7 m diameter) off-axis telescope. EXCEDE was selected by NASA (as a Class III Explorer program) for technology demonstration and maturation to advance key elements of its proposed starlight suppression system (SSS) combining the use of a Phase Induced Amplitude Apodized coronagraph, MEMS Deformable Mirror, closed-loop Low-Order Wavefront Sensing and Control, and mid-spatial frequency wavefront error correction and control using the science camera for electric field conjugation and speckle suppression for image contrast enhancement. To meet the science goals of the EXCEDE mission, the SSS must simultaneously, repeatably, and stably, deliver disk-to-starlight raw image contrast per resel of 1E-6 from 1.2 to 2 lambda/D, and 1E-7 from 2 to ~ 20 lambda/D in optical light, which has now been laboratory demonstrated for monochromatic light in an in-air environment (see paper I. by Belikov et al.) This level of performance when extended to 10% - 20% broadband light (technology demonstration milestone #2 to be pursued over the next year) will enable the EXCEDE mission. Here we discuss the applicability of these performance metrics to studying the current "here be dragons" regions of light-scattering CS debris disks, including those now well-observed as revealed at larger stellocentric angular distances with the Hubble Space Telescope Imaging Spectrograph's coronagraph with multiple-roll PSF-template subtracted coronagraphy as imaged in HST GO program 12228 in the context of the EXCEDE science mission goals. This investigation is funded in part by NASA grant NNX12AH39G, and STScI grant GO-12228.

Schneider, Glenn; Belikov, R.; Guyon, O.; Lozi, J.; Eduardo, B.; Davis, P.; Greene, T. P.; Lynch, D.; Eugene, P.; Sandrine, T.; Witteborn, F.; Duncan, A.; Kendrick, R.; Hix, T.; Mihara, R.; Smith, E.; Irwin, W.; Debes, J. H.; Carson, J.; Hines, D. C.; Grady, C. A.; Perrin, M. D.; Silverstone, M. D.; Wisniewski, J. P.; Hinz, P.; Moro-Martin, A.; Henning, T.; Tamura, M.; Jang-Condell, H.; Weinberger, A. J.; Woodgate, B. E.; Goto, M.; Serabyn, G.; Rodigas, T.; Kuchner, M. J.; Stark, C. C.; EXCEDE Project Technology Development Team; HST GO 12228 Team

2014-01-01

156

X-ray and Hubble/COS UV Measures of Kapteyn's Star: A Crucial Proxy of X-UV Irradiances for Old Red Dwarf Stars that May Host Habitable Zone Planets  

NASA Astrophysics Data System (ADS)

Red dwarfs (dM) stars make up over 80% of the local stellar population and a significant fraction of them are old (age > 4 Gyr). Because of the high frequency of red dwarfs and their longevity, there is a greater possibility of more advanced life in red dwarf planet systems. MEarths, UVES, SDSS-III, and the upcoming TESS mission are some surveys that are targeting these objects. As part of Villanova’s Living with a Red Dwarf program, we have obtained HST/COS spectra and Chandra X-ray observations of Kapteyn's star (M1V, V = 8.853, d = 12.76 +/- 0.05 ly, P_rot = 195 days). This star is crucial to the study of old red dwarfs as it is the nearest halo star with a radial velocity of +245.2 km/s and an estimated age of 10-12 Gyr. In our program, Kapteyn's star is the oldest red dwarf and as such serves as an anchor for our age, rotation, and activity relations. The spectra obtained from HST/COS provide one of the cleanest measurements of Lyman-alpha emission for red dwarfs. This is due to Doppler shift from the high radial velocity, separating the Lyman-alpha line from emission produced by the ISM and geocoronal sources. These observations further provide calibration at the old age/low rotation/low activity extremes for our relations. They also provide insights into the magnetic properties as investigating coronal x-ray and UV emission in very old, slowly rotating dM stars. Kapteyn’s star also serves as a proxy for metal-poor old disk/Pop II M dwarfs by providing information about X-UV emissions. This information is crucial for determining X-UV irradiances of possible habitable zone planets hosted by old red dwarfs. We gratefully acknowledge the support from NSF/RUI Grant AST-1009903, NASA/Chandra Grants GO1-12124X and GO2-13020X, and HST-GO-13020.

Durbin, Allyn J.; Guinan, E. F.; Engle, S. G.

2014-01-01

157

WHERE TO FIND HABITABLE ''EARTHS'' IN CIRCUMBINARY SYSTEMS  

SciTech Connect

Six P-type planets have been found thus far around five binary systems, i.e., Kepler-16b, 34b, 35b, 38b, and 47b and c, which are all Neptune- or Jupiter-like planets. The stability of planets and the habitable zones are influenced by the gravitational and radiative perturbations of binary companions. In this Letter, we check the stability of an additional habitable Earth-mass planet in each system. Based on our simulations in 10 Myr, a habitable ''Earth'' is hardly stable in Kepler-16, while a stable ''Earth'' in Kepler-47 close to the boundaries of the habitable zone is possible. In contrast, Kepler-34, 35, and 38 seem to have high probabilities of being able to tolerante a stable ''Earth'' in their habitable zones. The affects of transit time variations are quite small due to the small mass of an undetected ''Earth,'' except that of Kepler-16b. With a time precision of 10{sup -3} day ({approx}88 s), an ''Earth'' in the corotational resonance with Kepler-16b can be detected in three years, while habitable ''Earths'' in the Kepler-34 and 38 systems can be detected in 10 yr. Habitable ''Earths'' in Kepler-35 and 47 are not likely to be detected in 10 yr under this precision.

Liu Huigen; Zhang Hui; Zhou Jilin, E-mail: huigen@nju.edu.cn [School of Astronomy and Space Science and Key Laboratory of Modern Astronomy and Astrophysics in Ministry of Education, Nanjing University, Nanjing 210093 (China)

2013-04-20

158

Habitable Climates: The Influence of Obliquity  

NASA Astrophysics Data System (ADS)

Extrasolar terrestrial planets with the potential to host life might have large obliquities or be subject to strong obliquity variations. We revisit the habitability of oblique planets with an energy balance climate model (EBM) allowing for dynamical transitions to ice-covered snowball states as a result of ice-albedo feedback. Despite the great simplicity of our EBM, it captures reasonably well the seasonal cycle of global energetic fluxes at Earth's surface. It also performs satisfactorily against a full-physics climate model of a highly oblique Earth-like planet, in an unusual regime of circulation dominated by heat transport from the poles to the equator. Climates on oblique terrestrial planets can violate global radiative balance through much of their seasonal cycle, which limits the usefulness of simple radiative equilibrium arguments. High obliquity planets have severe climates, with large amplitude seasonal variations, but they are not necessarily more prone to global snowball transitions than low obliquity planets. We find that terrestrial planets with massive CO2 atmospheres, typically expected in the outer regions of habitable zones, can also be subject to such dynamical snowball transitions. Some of the snowball climates investigated for CO2-rich atmospheres experience partial atmospheric collapse. Since long-term CO2 atmospheric build-up acts as a climatic thermostat for habitable planets, partial CO2 collapse could limit the habitability of such planets. A terrestrial planet's habitability may thus sensitively depend on its short-term climatic stability.

Spiegel, David S.; Menou, Kristen; Scharf, Caleb A.

2009-01-01

159

Effects of Extreme Obliquity Variations on the Habitability of Exoplanets  

PubMed Central

Abstract We explore the impact of obliquity variations on planetary habitability in hypothetical systems with high mutual inclination. We show that large-amplitude, high-frequency obliquity oscillations on Earth-like exoplanets can suppress the ice-albedo feedback, increasing the outer edge of the habitable zone. We restricted our exploration to hypothetical systems consisting of a solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We verified that these systems are stable for 108 years with N-body simulations and calculated the obliquity variations induced by the orbital evolution of the Earth-mass planet and a torque from the host star. We ran a simplified energy balance model on the terrestrial planet to assess surface temperature and ice coverage on the planet's surface, and we calculated differences in the outer edge of the habitable zone for planets with rapid obliquity variations. For each hypothetical system, we calculated the outer edge of habitability for two conditions: (1) the full evolution of the planetary spin and orbit and (2) the eccentricity and obliquity fixed at their average values. We recovered previous results that higher values of fixed obliquity and eccentricity expand the habitable zone, but we also found that obliquity oscillations further expand habitable orbits in all cases. Terrestrial planets near the outer edge of the habitable zone may be more likely to support life in systems that induce rapid obliquity oscillations as opposed to fixed-spin planets. Such planets may be the easiest to directly characterize with space-borne telescopes. Key Words: Exoplanets—Habitable zone—Energy balance models. Astrobiology 14, 277–291.

Barnes, R.; Domagal-Goldman, S.; Breiner, J.; Quinn, T.R.; Meadows, V.S.

2014-01-01

160

The Role of Planetary System Architecture in Planetary Habitability  

NASA Astrophysics Data System (ADS)

Traditionally, the habitable zone is defined as the region around a star in which liquid water can be stable on a planetary surface. At first these calculations considered planets on circular orbits [1]. More recent investigations into nonzero orbital eccentricities found that the limits of the habitable zone should reflect the orbit-averaged flux a planet receives [2]. However, those studies assumed the potentially habitable planet is isolated. If additional planets are in the system, gravitational interactions between planets can cause eccentricity oscillations on timescales of 103 - 106 years. Furthermore, the known multi-planet systems (generally consisting of giant planets) appear to undergo large amplitude eccentricity oscillations [3]. If rocky exoplanets also experience such large variations, then the orbit-averaged flux may change significantly, impacting habitability. We show that plausible architectures of rocky planet systems can indeed lead to orbits with large eccentricity cycles. Moreover, some planets could cross either the inner or outer habitable zone boundaries due to these oscillations. We therefore suggest that the shape of actual habitable zones depends critically on the configuration (orbits and masses) of the entire planetary system. [1] Kasting, J.F. et al. 1993, Icarus, 101, 108. [2] Williams, D.M. & Pollard, D. 2002, I. J. AsBio, 2, 1. [3] Barnes, R., & Greenberg, R. 2006, Astrophys. J., 652, L53.

Barnes, R.; Jackson, B.; Raymond, S.; Greenberg, R.

2009-12-01

161

A circumstellar disk around Beta pictoris.  

PubMed

A circumstellar disk has been observed optically around the fourth-magnitude star beta Pictoris. First detected in the infrared by the Infrared Astronomy Satellite last year, the disk is seen to extend to more than 400 astronomical units from the star, or more than twice the distance measured in the infrared by the Infrared Astronomy Satellite. The beta Pictoris disk is presented to Earth almost edgeon and is composed of solid particles in nearly coplanar orbits. The observed change in surface brightness with distance from the star implies that the mass density of the disk falls off with approximately the third power of the radius. Because the circumstellar material is in the form of a highly flattened disk rather than a spherical shell, it is presumed to be associated with planet formation. It seems likely that the system is relatively young and that planet formation either is occurring now around beta Pictoris or has recently been completed. PMID:17788996

Smith, B A; Terrile, R J

1984-12-21

162

Proper Motions of Circumstellar Water Masers  

NASA Astrophysics Data System (ADS)

I present results from my dissertation work on circumstellar water masers, including the first measured proper motions for these sources. Three epochs of observation of the stars S Per, IK Tau, NML Cyg, RX Boo, VX Sgr, VY CMa, and U Her with the VLBA show that proper motions of circumstellar water masers can be measured with relative ease. Motions of several milliarcseconds over a year are common in these sources. Preliminary model fits indicate most sources can be represented by ellipsoidal shell-like distributions. Further analysis will yield estimates on the distances to these objects, limits on the physical conditions in the shells, and perhaps structural information of the maser spots themselves. I would like to acknowledge the support of a NRAO pre-doctoral fellowship which enabled the completion of my PhD.

Marvel, K. B.

1995-12-01

163

Dusty Circumstellar Environments: MPI-AMRVAC Simulations  

NASA Astrophysics Data System (ADS)

We highlight results obtained on multi-dimensional modeling of circumstellar environments, where stellar outflows interact mutually or with the surrounding interstellar medium. We use the MPI-AMRVAC code, able to handle Newtonian to relativistic gas and plasma dynamic scenarios. For circumstellar modeling, the code has been extended by coupling the hydro to (possibly multiple) pressureless dust species. The dynamics of pressureless dust poses ultimate challenges to grid-adaptive numerical simulations, as it allows for delta waves in its purest form. We subsequently illustrate pure gas dynamic scenarios, where optically thin radiative losses combined with supersonic outflows cause complex instability dominated circumstellar bubbles. We comment on single star, as well as on binary system scenarios. In the latter, we focus on the wind collision front where distinctly different instabilities manifest themselves in different regions. Finally, we show coupled gas-dust dynamical treatments, as case scenarios for dust-loaded stellar outflows, specifically for moving massive stars. This includes a detailed treatment for dust grains in the stellar wind, accounting for drag forces between dust and gas.

Keppens, R.; van Marle, A. J.; Meliani, Z.

2012-07-01

164

The Cool Hypergiants: Imaging their Circumstellar Environments  

NASA Astrophysics Data System (ADS)

The cool hypergiants are evolved massive stars that lie just below the empirical upper luminosity boundary in the HR Diagram, with spectral types ranging from late A to M. Recent observations of two of these stars (IRC +10420 and VY CMa) have yielded surprising results about their circumstellar environments including evidence for bi-polarity, equatorial disks, and multiple high mass loss events. To better understand their mass loss histories, we obtained high resolution WF/PC2 PC images of 7 of the most luminous known cool stars to sample a variety of stages during their brief evolution as cool hypergiants. the presence or lack of fossil shells, equatorial ejecta, jets and other structures in their circumstellar environments is a record of their current and prior mass loss episodes. NML Cyg, Rho Cas, Mu Cep, VX Sgr, S Per, HR 8752, and HR 5171A were selected on the basis of their infrared emission, strong molecular emission, or peculiar spectroscopic variations to give us a snapshot of different steps in their evolution across the top of the HR Diagram. We have also imaged these same stars in the near and mid-infrared. We present evidence for extensive circumstellar material and discuss the implications for their evolutionary state.

Schuster, M. T.; Humphreys, R. M.

2001-12-01

165

Possible Habitability of Ocean Worlds  

NASA Astrophysics Data System (ADS)

In the last decade, the number of detected exoplanets has increased to over thousand confirmed planets and more as yet unconfirmed planet candidates. The scientific community mainly concentrates on terrestrial planets (up to 10 Earth masses) in the habitable zone, which describes the distance from the host star where liquid water can exist at the surface (Kasting et al., 1993). Another target group of interest are ocean worlds, where a terrestrial-like body (i.e. with an iron core and a silicate mantle) is covered by a thick water-ice layer - similar to the icy moons of our solar system but with several Earth masses (e.g. Grasset et al., 2009). When an exoplanet is detected and confirmed as a planet, typically the radius and the mass of it are known, leading to the mean density of the planet that gives hints to possible interior structures. A planet with a large relative iron core and a thick ocean on top of the silicate mantle for example would have the same average planet density as a planet with a more Earth-like appearance (where the main contributor to the mass is the silicate mantle). In this study we investigate how the radius and mass of a planet depend on the amount of water, silicates and iron present (after Wagner et al., 2011) the occurence of high-pressure-ice in the water-ice layer (note: we only consider surface temperatures at which liquid water exists at the surface) if the ocean layer influences the initiation of plate tectonics We assume that ocean worlds with a liquid ocean layer (and without the occurence of high-pressure ice anywhere in the water layer) and plate tectonics (especially the occurence of subduction zones, hydrothermal vents and continental formation) may be called habitable (Class III/IV habitats after Lammer et al., 2009). References: Kasting, J.F., Whitmire, D.P., and Reynolds, R.T. (1993). Habitable Zones around Main Sequence Stars. Icarus 101, 108-128. Grasset, O., Schneider, J., and Sotin, C. (2009). A study of the accuracy of mass-radius relationships for silicate-rich and ice-rich planets up to 100 Earth masses. The Astrophysical Journal 693, 722-733. Wagner, F.W., Sohl, F., Hussmann, H., Grott, M., and Rauer, H. (2011). Interior structure models of solid exoplanets using material laws in the infinite pressure limit. Icarus 214, 366-376. Lammer, H., Bredehöft, J.H., Coustenis, A., Khodachenko, M.L., Kaltenegger, L., Grasset, O., Prieur, D., Raulin, F., Ehrenfreund, P., Yamauchi, M., Wahlund, J.-E., Grießmeier, J.-M., Stangl, G., Cockell, C.S., Kulikov, Yu.N., Grenfell, J.L., and Rauer, H. (2009). What makes a planet habitable? Astron Astrophys Rev 17, 181-249.

Noack, Lena; Höning, Dennis; Bredehöft, Jan H.; Lammer, Helmut

2014-05-01

166

Radio spectroscopy of circumstellar molecular masers  

NASA Astrophysics Data System (ADS)

Results of observations of circumstellar maser sources in long-period variable stars are reported. We have monitored a sample of Mira-type and semiregular variables in the H_2O (wavelength 1.35 cm) and OH (wavelength 18 cm) radio lines. A comparison with data of optical photometry and spectroscopy have been done. The observations in the H_2O line at 1.35 cm were carried out on the RT-22 radio telescope of the Pushchino Radio Astronomy Observatory in 1980--2011. The sample included sixty late-type variable stars with circumstellar envelopes. For most stars twenty to thirty variability cycles (lasting about a year). There is a correlation of the H_2O line flux density with the light variations: in general, the maser intensity follows the optical variability with a certain phase lag. Figure 1 illustrates variations in the H_2O maser emission versus the optical light curve for the Mira-type star U Her. A model, in which the maser variability is caused by the joint influence of a shock wave propagating in the circumstellar envelope and of the variable radio continuum of the stellar photosphere, is considered. There are indications to a quasiperiodic character of the variations of the integrated H_2O line flux and mean radial velocity of the maser emission at a timescale of 14--15 years. Polarimetry of the OH maser emission has allowed us to draw conclusions about circumstellar magnetic field. Results of observations of circumstellar OH masers in the lines at a wavelength of 18 cm are reported. The observations were carried out on the radio telescope of the Nançay Radio Astronomy Observatory (France). In 2007--2011 seventy late-type stars (including Mira-type and semiregular variables) were observed. For 53 of them the emission in at least one of three OH lines (1612, 1665, or 1667 MHz) was detected. Circular and linear polarization of the maser emission was measured; this yielded all four Stokes parameters. The results obtained for the stars T Lep, R LMi, and R Crt are discussed. The emission of T Lep in the 1665 and 1667 MHz OH lines was observed for the first time. In the OH line profiles of all three stars features probably due to Zeeman splitting were detected. Estimates of the magnetic intensity in the maser sources (0.46--2.32 mG) were obtained.

Rudnitskij, G. M.; Colom, P.; Lekht, E. E.; Pashchenko, M. I.; Samodurov, V. A.; Subaev, I. A. Alexander M. Tolmachev,

2011-05-01

167

Choosing Heart Healthy Habits  

MedlinePLUS Videos and Cool Tools

... player. Choosing Heart Healthy Habits HealthDay July 8, 2014 Related MedlinePlus Pages Coronary Artery Disease Heart Diseases--Prevention Transcript Young adults who improve their lifestyle can enter middle age with a healthier heart new research suggests. In fact those who eat better, slim ...

168

Europa: Processes and Habitability  

Microsoft Academic Search

Europa may be a habitable world. Evidence points strongly to a subsurface ocean beneath an ice shell about 20 km thick. Europa's surface geology as viewed by Galileo imaging suggests a thin brittle lithosphere above a warm ice layer that is at least in part convecting, in turn situated above a liquid water ocean. This configuration is consistent with thermal

R. T. Pappalardo

2006-01-01

169

Habitability Of Europa's Crust  

Microsoft Academic Search

Physical characterization of Europa's crust shows it to be rich in potentially habitable niches, with several timescales for change that would allow stability for organisms to prosper and still require and drive evolution and adaptation. Studies of tectonics on Europa indicate that tidal stress causes much of the surface cracking, that cracks pen- etrate through to liquid water (so the

R. Greenberg; B. R. Tufts; P. Geissler; G. Hoppa

2002-01-01

170

MAGNETIC SHIELDING OF EXOMOONS BEYOND THE CIRCUMPLANETARY HABITABLE EDGE  

SciTech Connect

With most planets and planetary candidates detected in the stellar habitable zone (HZ) being super-Earths and gas giants rather than Earth-like planets, we naturally wonder if their moons could be habitable. The first detection of such an exomoon has now become feasible, and due to observational biases it will be at least twice as massive as Mars. However, formation models predict that moons can hardly be as massive as Earth. Hence, a giant planet's magnetosphere could be the only possibility for such a moon to be shielded from cosmic and stellar high-energy radiation. Yet, the planetary radiation belt could also have detrimental effects on exomoon habitability. Here we synthesize models for the evolution of the magnetic environment of giant planets with thresholds from the runaway greenhouse (RG) effect to assess the habitability of exomoons. For modest eccentricities, we find that satellites around Neptune-sized planets in the center of the HZ around K dwarf stars will either be in an RG state and not be habitable, or they will be in wide orbits where they will not be affected by the planetary magnetosphere. Saturn-like planets have stronger fields, and Jupiter-like planets could coat close-in habitable moons soon after formation. Moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.

Heller, René [McMaster University, Department of Physics and Astronomy, Hamilton, ON L8S 4M1 (Canada)] [McMaster University, Department of Physics and Astronomy, Hamilton, ON L8S 4M1 (Canada); Zuluaga, Jorge I., E-mail: rheller@physics.mcmaster.ca, E-mail: jzuluaga@fisica.udea.edu.co [FACom - Instituto de Física - FCEN, Universidad de Antioquia, Calle 70 No. 52-21, Medellín (Colombia)

2013-10-20

171

Magnetic Shielding of Exomoons beyond the Circumplanetary Habitable Edge  

NASA Astrophysics Data System (ADS)

With most planets and planetary candidates detected in the stellar habitable zone (HZ) being super-Earths and gas giants rather than Earth-like planets, we naturally wonder if their moons could be habitable. The first detection of such an exomoon has now become feasible, and due to observational biases it will be at least twice as massive as Mars. However, formation models predict that moons can hardly be as massive as Earth. Hence, a giant planet's magnetosphere could be the only possibility for such a moon to be shielded from cosmic and stellar high-energy radiation. Yet, the planetary radiation belt could also have detrimental effects on exomoon habitability. Here we synthesize models for the evolution of the magnetic environment of giant planets with thresholds from the runaway greenhouse (RG) effect to assess the habitability of exomoons. For modest eccentricities, we find that satellites around Neptune-sized planets in the center of the HZ around K dwarf stars will either be in an RG state and not be habitable, or they will be in wide orbits where they will not be affected by the planetary magnetosphere. Saturn-like planets have stronger fields, and Jupiter-like planets could coat close-in habitable moons soon after formation. Moons at distances between about 5 and 20 planetary radii from a giant planet can be habitable from an illumination and tidal heating point of view, but still the planetary magnetosphere would critically influence their habitability.

Heller, René; Zuluaga, Jorge I.

2013-10-01

172

Spectral Fingerprints of Habitability  

NASA Astrophysics Data System (ADS)

The emerging field of extrasolar planet search has shown an extraordinary ability to combine research by astrophysics, chemistry, biology and geophysics into a new and exciting interdisciplinary approach to understand our place in the universe. Are there other worlds like ours? How can we characterize those planets and assess if they are habitable? After a decade rich in giant exoplanet detections, observation techniques have now reached the ability to find planets of less than 10 M_Earth (so called Super-Earths) that may potentially be habitable. The detection and characterization of Earth-like planet is approaching rapidly with dedicated space observatories already in operation (Corot) or in development phase (Kepler, James Webb Space Telescope, Extremely Large Telescope (ELT), Darwin/TPF). Space missions like CoRoT (CNES, Rouan et al. 1998) and Kepler (NASA, Borucki et al. 1997) will give us statistics on the number, size, period and orbital distance of planets, extending to terrestrial planets on the lower mass range end as a first step, while missions like Darwin/TPF are designed to characterize their atmospheres. In this chapter we discuss how we can read a planet's spectral fingerprint and characterize if it is potentially habitable. We discuss the first steps to detect a habitable planet and set biomarker detection in context in Section 1. In Section 2 we focus on biomarkers, their signatures at different wavelengths, abiotic sources and cryptic photosynthesis - using Earth as our primary example - the only habitable planet we know of so far. Section 3 concentrates on planets around different stars, and Section 4 summarizes the chapter.

Kaltenegger, L.; Selsis, F.

2010-01-01

173

Application of the Titius-Bode Rule to the 55 Cancri System: Tentative Prediction of a Possibly Habitable Planet  

NASA Astrophysics Data System (ADS)

Following the notion that the Titius-Bode rule (TBR) may also be applicable to some extrasolar planetary systems, although this number could be relatively small, it is applied to 55 Cancri, which is a G-type main-sequence star currently known to host five planets. Following a concise computational process, we tentatively identified four new hypothetical planetary positions, given as 0.081, 0.41, 1.51, and 2.95 AU from the star. The likelihood that these positions are occupied by real existing planets is significantly enhanced for the positions of 1.51 and 2.95 AU in view of previous simulations on planet formation and planetary orbital stability. For example, Raymond, Barnes, and Gorelick (2008, ApJ, 689, 478) argued that additional planets would be possible between 55 Cnc f and 55 Cnc d, which would include planets situated at 1.51 and 2.95 AU. If two additional planets are assumed to exist between 55 Cnc f and 55 Cnc d, the deduced domains of stability would be given as 1.3-1.6 and 2.2-3.3 AU. The possible planet near 1.5 AU appears to be located at the outskirts of the stellar habitable zone, which is, however, notably affected by the stellar parameters as well as the adopted model of circumstellar habitability. We also computed the distance of the next possible outer planet in the 55 Cnc system, which, if existing, is predicted to be located between 10.9 and 12.2 AU, which is consistent with orbital stability constraints. The inherent statistical significance of the TBR was evaluated following a method by Lynch (2003, MNRAS, 341, 1174). Yet it is up to future planetary search missions to verify or falsify the applicability of the TBR to the 55 Cnc system, and to obtain information on additional planets, if existing.

Cuntz, Manfred

2012-08-01

174

Adult Reading Habits and Patterns.  

ERIC Educational Resources Information Center

Examines the reading habits and patterns of White and Asian American adults. Hypothesizes that when grouped by demographic variables, participants' responses about their reading habits and patterns would not differ. Concludes that gender, race, and education were predictors for participants' reading habits; education and race were predictors for…

Scales, Alice M.; Rhee, Ock

2001-01-01

175

Effects of extreme obliquity variations on the habitability of exoplanets.  

PubMed

We explore the impact of obliquity variations on planetary habitability in hypothetical systems with high mutual inclination. We show that large-amplitude, high-frequency obliquity oscillations on Earth-like exoplanets can suppress the ice-albedo feedback, increasing the outer edge of the habitable zone. We restricted our exploration to hypothetical systems consisting of a solar-mass star, an Earth-mass planet at 1 AU, and 1 or 2 larger planets. We verified that these systems are stable for 10(8) years with N-body simulations and calculated the obliquity variations induced by the orbital evolution of the Earth-mass planet and a torque from the host star. We ran a simplified energy balance model on the terrestrial planet to assess surface temperature and ice coverage on the planet's surface, and we calculated differences in the outer edge of the habitable zone for planets with rapid obliquity variations. For each hypothetical system, we calculated the outer edge of habitability for two conditions: (1) the full evolution of the planetary spin and orbit and (2) the eccentricity and obliquity fixed at their average values. We recovered previous results that higher values of fixed obliquity and eccentricity expand the habitable zone, but we also found that obliquity oscillations further expand habitable orbits in all cases. Terrestrial planets near the outer edge of the habitable zone may be more likely to support life in systems that induce rapid obliquity oscillations as opposed to fixed-spin planets. Such planets may be the easiest to directly characterize with space-borne telescopes. PMID:24611714

Armstrong, J C; Barnes, R; Domagal-Goldman, S; Breiner, J; Quinn, T R; Meadows, V S

2014-04-01

176

HABITABILITY OF EXOMOONS AT THE HILL OR TIDAL LOCKING RADIUS  

SciTech Connect

Moons orbiting extrasolar planets are the next class of object to be observed and characterized for possible habitability. Like the host-planets to their host-star, exomoons have a limiting radius at which they may be gravitationally bound, or the Hill radius. In addition, they also have a distance at which they will become tidally locked and therefore in synchronous rotation with the planet. We have examined the flux phase profile of a simulated, hypothetical moon orbiting at a distant radius around the confirmed exoplanets {mu} Ara b, HD 28185 b, BD +14 4559 b, and HD 73534 b. The irradiated flux on a moon at its furthest, stable distance from the planet achieves its largest flux gradient, which places a limit on the flux ranges expected for subsequent (observed) moons closer in orbit to the planet. We have also analyzed the effect of planetary eccentricity on the flux on the moon, examining planets that traverse the habitable zone either fully or partially during their orbit. Looking solely at the stellar contributions, we find that moons around planets that are totally within the habitable zone experience thermal equilibrium temperatures above the runaway greenhouse limit, requiring a small heat redistribution efficiency. In contrast, exomoons orbiting planets that only spend a fraction of their time within the habitable zone require a heat redistribution efficiency near 100% in order to achieve temperatures suitable for habitability. This means that a planet does not need to spend its entire orbit within the habitable zone in order for the exomoon to be habitable. Because the applied systems comprise giant planets around bright stars, we believe that the transit detection method is most likely to yield an exomoon discovery.

Hinkel, Natalie R.; Kane, Stephen R., E-mail: natalie.hinkel@gmail.com [NASA Exoplanet Science Institute, Caltech, MS 100-22, 770 South Wilson Avenue, Pasadena, CA 91125 (United States)

2013-09-01

177

Circumstellar Material in Young Stellar Objects  

NASA Astrophysics Data System (ADS)

In my dissertation I present the results of optical, infrared and submm studies of the circumstellar environment of young stellar objects, mostly of intermediate mass. Both the circumstellar dust and gas are studied, using results from the Hipparcos mission, the Infrared Space Observatory (ISO) and several ground-based telescopes. The basic stellar properties of a sample of Herbig Ae/Be stars (HAeBes) were derived using the astrometric data from the Hipparcos mission. The basic conclusion from this study is that most HAeBes in our sample must be pre-main sequence stars. An evolutionary scenario for the dissipation of dust around Herbig Ae/Be stars is outlined, based on the new stellar parameters derived from the Hipparcos astrometry. Using the photometry obtained by Hipparcos, I also study the circumstellar dust through its effect on the light of the central star as it moves in and out of our line of sight. I show that such an effect can only be seen towards Herbig Ae/Be systems with a central star of spectral type A0 or later, which is explained as being due to the evolutionary effect that Herbig Be stars are not optically visible while still contracting towards the zero-age main sequence. I also looked in more detail at the thermal emission from dust in the disks of two Herbig Ae systems, AB Aur and HD 163296, using ISO spectroscopy and new VLA data. In both these systems, significant grain growth has already occurred. However, the degree in which this has happened, as well as the degree in which the dust has crystallized, differs greatly between these two systems of identical mass and age. Clearly other parameters than just stellar mass and age influence the dissipation speed and degree of processing of the dust in a circumstellar disk. Another way to study circumstellar dust is through submillimeter photometry. I have followed this approach to study the dust in the star forming region associated with the Herbig stars R and T CrA, using 450 and 850 micron maps obtained with SCUBA at the JCMT. Extended emission is present throughout the region at both 450 and 850 microns. The SCUBA maps do not show an enhanced intensity at the positions of the Herbig Ae/Be stars R CrA and T CrA. Six point-like submm sources were detected in the R CrA region, of which four have not been detected before. Two of these could not be identified with a near-infrared source, making them valid new candidates for Class 0 sources. The remainder of my dissertation is devoted to ISO spectroscopy of star forming regions. I first look in some detail at a number of individual objects (S106, Cep A East, BD+40o 4124 region, T Tau). The differences found in the emission line and the solid-state spectra of these objects cannot be explained in terms of differences in orientation and mass, but must reflect an evolution of the circumstellar material. The line of sight towards the embedded YSO LkH? 225 in the BD+40° 4124 region was found to have a CO2 gas/solid ratio that is more than a factor 100 higher than that found in any other line of sight so far and may contain the most evolved molecular core known to date. After these studies of individual systems, I focus on a larger sample, consisting of 10 embedded YSOs and 11 Herbig Ae/Be stars, to study the gas in the circumstellar environment in a more systematic way. I conclude that the infrared atomic fine structure lines and the infrared molecular emission lines are in general dominated by emission from photodissociation regions (PDRs) and/or shocks. The distinction between PDRs and shocks can be made with relative ease using infrared spectroscopy using the presence of [S i] emission (indicative of the presence of a shock), strong [C ii] emission (PDR) and PAH emission (PDR). Based on these results I suggest an evolutionary scenario in which the circumstellar material around a young star changes from being heated mechanically by shocks into heated by radiation from the central star through a PDR as the star clears its surroundings. Electronic version available at: http://zon.wins.uva.nl/~mario/ or http://w

van den Ancker, Mario

1999-09-01

178

A 'dry' condensation origin for circumstellar carbonates.  

PubMed

The signature of carbonate minerals has long been suspected in the mid-infrared spectra of various astrophysical environments such as protostars. Abiogenic carbonates are considered as indicators of aqueous mineral alteration in the presence of CO2-rich liquid water. The recent claimed detection of calcite associated with amorphous silicates in two planetary nebulae and protostars devoid of planetary bodies questions the relevance of this indicator; but in the absence of an alternative mode of formation under circumstellar conditions, this detection remains controversial. The main dust component observed in circumstellar envelopes is amorphous silicates, which are thought to have formed by non-equilibrium condensation. Here we report experiments demonstrating that carbonates can be formed with amorphous silicates during the non-equilibrium condensation of a silicate gas in a H2O-CO2-rich vapour. We propose that the observed astrophysical carbonates have condensed in H2O(g)-CO2(g)-rich, high-temperature and high-density regions such as evolved stellar winds, or those induced by grain sputtering upon shocks in protostellar outflows. PMID:16237436

Toppani, Alice; Robert, François; Libourel, Guy; de Donato, Philippe; Barres, Odile; d'Hendecourt, Louis; Ghanbaja, Jaafar

2005-10-20

179

HABITABLE CLIMATES: THE INFLUENCE OF ECCENTRICITY  

SciTech Connect

In the outer regions of the habitable zone, the risk of transitioning into a globally frozen 'snowball' state poses a threat to the habitability of planets with the capacity to host water-based life. Here, we use a one-dimensional energy balance climate model (EBM) to examine how obliquity, spin rate, orbital eccentricity, and the fraction of the surface covered by ocean might influence the onset of such a snowball state. For an exoplanet, these parameters may be strikingly different from the values observed for Earth. Since, for a constant semimajor axis, the annual mean stellar irradiation scales with (1 - e {sup 2}){sup -1/2}, one might expect the greatest habitable semimajor axis (for fixed atmospheric composition) to scale as (1 - e {sup 2}){sup -1/4}. We find that this standard simple ansatz provides a reasonable lower bound on the outer boundary of the habitable zone, but the influence of both obliquity and ocean fraction can be profound in the context of planets on eccentric orbits. For planets with eccentricity 0.5, for instance, our EBM suggests that the greatest habitable semimajor axis can vary by more than 0.8 AU (78%) depending on obliquity, with higher obliquity worlds generally more stable against snowball transitions. One might also expect that the long winter at an eccentric planet's apoastron would render it more susceptible to global freezing. Our models suggest that this is not a significant risk for Earth-like planets around Sun-like stars, as considered here, since such planets are buffered by the thermal inertia provided by oceans covering at least 10% of their surface. Since planets on eccentric orbits spend much of their year particularly far from the star, such worlds might turnout to be especially good targets for direct observations with missions such as TPF-Darwin. Nevertheless, the extreme temperature variations achieved on highly eccentric exo-Earths raise questions about the adaptability of life to marginally or transiently habitable conditions.

Dressing, Courtney D.; Spiegel, David S. [Department of Astrophysical Sciences, Princeton University, Peyton Hall, Princeton, NJ 08544 (United States); Scharf, Caleb A. [Columbia Astrobiology Center, Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027 (United States); Menou, Kristen [Kavli Institute for Theoretical Physics, UCSB, Santa Barbara, CA 93106-4030 (United States); Raymond, Sean N., E-mail: courtney@astro.princeton.ed, E-mail: dsp@astro.princeton.ed, E-mail: kristen@astro.columbia.ed, E-mail: caleb@astro.columbia.ed, E-mail: raymond@obs.u-bordeaux1.f [Universite Bordeaux, Observatoire Aquitain des Sciences de l'Univers, 2 rue de l'Observatoire, BP 89, F-33271 Floirac Cedex (France)

2010-10-01

180

Molecular content of the circumstellar disk in AB Aurigae. First detection of SO in a circumstellar disk  

Microsoft Academic Search

Aims: Very few molecular species have been detected in circumstellar disks surrounding young stellar objects. We are carrying out an observational study of the chemistry of circumstellar disks surrounding T Tauri and Herbig Ae stars. First results of this study are presented in this note. Methods: We used the EMIR receivers recently installed at the IRAM 30 m telescope to

A. Fuente; J. Cernicharo; M. Agúndez; O. Berné; J. R. Goicoechea; T. Alonso-Albi; N. Marcelino

2010-01-01

181

Beyond the principle of plentitude: a review of terrestrial planet habitability.  

PubMed

We review recent work that directly or indirectly addresses the habitability of terrestrial (rocky) planets like the Earth. Habitability has been traditionally defined in terms of an orbital semimajor axis within a range known as the habitable zone, but it is also well known that the habitability of Earth is due to many other astrophysical, geological, and geochemical factors. We focus this review on (1) recent refinements to habitable zone calculations; (2) the formation and orbital stability of terrestrial planets; (3) the tempo and mode of geologic activity (e.g., plate tectonics) on terrestrial planets; (4) the delivery of water to terrestrial planets in the habitable zone; and (5) the acquisition and loss of terrestrial planet carbon and nitrogen, elements that constitute important atmospheric gases responsible for habitable conditions on Earth's surface as well as being the building blocks of the biosphere itself. Finally, we consider recent work on evidence for the earliest habitable environments and the appearance of life itself on our planet. Such evidence provides us with an important, if nominal, calibration point for our search for other habitable worlds. PMID:15815163

Gaidos, E; Deschenes, B; Dundon, L; Fagan, K; Menviel-Hessler, L; Moskovitz, N; Workman, M

2005-04-01

182

Computer codes for evaluation of control room habitability (HABIT)  

SciTech Connect

This report describes the Computer Codes for Evaluation of Control Room Habitability (HABIT). HABIT is a package of computer codes designed to be used for the evaluation of control room habitability in the event of an accidental release of toxic chemicals or radioactive materials. Given information about the design of a nuclear power plant, a scenario for the release of toxic chemicals or radionuclides, and information about the air flows and protection systems of the control room, HABIT can be used to estimate the chemical exposure or radiological dose to control room personnel. HABIT is an integrated package of several programs that previously needed to be run separately and required considerable user intervention. This report discusses the theoretical basis and physical assumptions made by each of the modules in HABIT and gives detailed information about the data entry windows. Sample runs are given for each of the modules. A brief section of programming notes is included. A set of computer disks will accompany this report if the report is ordered from the Energy Science and Technology Software Center. The disks contain the files needed to run HABIT on a personal computer running DOS. Source codes for the various HABIT routines are on the disks. Also included are input and output files for three demonstration runs.

Stage, S.A. [Pacific Northwest Lab., Richland, WA (United States)

1996-06-01

183

The complex circumstellar environment of HD 142527  

NASA Astrophysics Data System (ADS)

Context. The recent findings of gas giant planets around young A-type stars suggest that disks surrounding Herbig Ae/Be stars will develop planetary systems. An interesting case is HD 142527, for which previous observations revealed a complex circumstellar environment and an unusually high ratio of infrared to stellar luminosity. Its properties differ considerably from other Herbig Ae/Be stars. This suggests that the disk surrounding HD 142527 is in an uncommon evolutionary stage. Aims: We aim for a better understanding of the geometry and evolutionary status of the circumstellar material around the Herbig Ae/Be star HD 142527. Methods: We map the composition and spatial distribution of the dust around HD 142527. We analyze SEST and ATCA millimeter data, VISIR N and Q-band imaging and spectroscopy. We gather additional relevant data from the literature. We use the radiative transfer code MCMax to construct a model of the geometry and density structure of the circumstellar matter, which fits all of the observables satisfactorily. Results: We find that the disk of HD 142527 has three geometrically distinct components separated by a disk gap running from 30 to 130 AU. There is a geometrically flat inner disk running from 0.3 AU up to 30 AU; an optically thin halo-like component of dust in the inner disk regions; and a massive self-shadowed outer disk running from 130 AU up to 200 AU. We derived a total dust mass in small grains of 1.0 × 10-3 M? and a vertical height of the inner wall of the outer disk of h = 60 AU. Owing to the gray extinction of the "halo" we obtained new stellar parameters, including a stellar luminosity of 20 ± 2 L? and age of 106.7 ± 0.4 yr. Conclusions: We find that the disk surrounding HD 142527 is highly evolved despite the relatively young age of the star. The peculiar disk geometry can explain the extreme IR reprocessing efficiency of the disk. Furthermore, the geometry, the large disk mass, and the highly processed dust composition are indicative of on-going planet formation. Based on observations collected at the European Southern Observatory, Chile. Under program IDs: 075.C-0687A, 076.C-0266A and 079.C-0286A.

Verhoeff, A. P.; Min, M.; Pantin, E.; Waters, L. B. F. M.; Tielens, A. G. G. M.; Honda, M.; Fujiwara, H.; Bouwman, J.; van Boekel, R.; Dougherty, S. M.; de Koter, A.; Dominik, C.; Mulders, G. D.

2011-04-01

184

Astrophysical radiation environments of habitable worlds  

NASA Astrophysics Data System (ADS)

Numerous astrophysical sources of radiation affect the environment of planets orbiting within the liquid-water habitable zone of main-sequence stars. This dissertation reaches a number of conclusions about the ionizing radiation environment of the habitable zone with respect to X-rays and gamma-rays from stellar flares and background Galactic cosmic rays. Gamma-rays and X-rays incident on terrestrial-like exoplanet atmospheres can be efficiently reprocessed into diffuse UV emission that, depending on the presence of atmospheric UV absorbers, can reach the surface. Extreme solar X-ray flares over the last 4.6 Gyr could have delivered large enough radiation doses to the Martian surface to sterilize any unprotected organisms, depending on the largest energy releases possible. These flares also pose a significant hazard to manned space missions, since a large flare can occur with little or no warning during an extravehicular activity. A flare as large as the largest observed could deliver radiation doses exceeding safety limits to an astronaut protected by only a spacesuit. With respect to particle radiation, the nature of Galactic cosmic-ray modulation by astrospheres means that habitable-zone cosmic-ray fluxes change by much larger magnitudes when passing through low- densities regions of the interstellar medium. In contrast to the popular idea that passages through dense molecular clouds are required to significantly enhance Galactic cosmic-ray fluxes and affect planets' electrical circuits, background mutation rates, and climates, we find that densities of only 0.1-10 cm -3 , the densities of most interstellar clouds, are sufficient to bring fluxes close to the full, interstellar level. Finally, passages through dense molecular clouds are necessary to shrink astrospheres to within the habitable zone, but such events produce even higher interstellar hydrogen and dust accretion rates than have been estimated because of the combination of enhanced charge-exchange rates between stellar-wind ions and interstellar neutrals and the growing importance of the central star's gravity on particle trajectories as the astrosphere shrinks.

Smith, David Samuel

185

Circumstellar molecular radio line intensity ratios  

NASA Astrophysics Data System (ADS)

We have observed a sample of 61 AGB--stars (39 M--stars and 22 C--stars) in circumstellar CO, CS, HCN, SiO, SiS, and SO radio line emission. The main results presented are based on the use of line intensity ratios, a well defined observational quantity that can be used to infer important conclusions as well as to provide constraints on models. Taken together the data are fully consistent with the facts that for this sample the circumstellar envelopes have the same basic chemistry (i.e., C/O<1 or >1) as the central stars, and that the mass loss rates have not changed drastically over periods between 10(2) --10(3) years. The HCN({\\jtra10})/SiO({\\jtra21}) intensity ratio discriminates unambiguously between {``}normal{''} circumstellar envelopes with C/O<1 (O--CSEs) and >1 (C--CSEs), while the CS({\\jtra21}), HCN({\\jtra10}), SiO({\\jtra21}), and SiS({\\jtra54}) intensity ratios with respect to CO({\\jtra10}) are not perfect for this purpose, and neither is the SiS({\\jtra54})/SiO({\\jtra21}) intensity ratio. The data further shows that SO and the C-bearing molecule HCN are ubiquitously present in O--CSEs, and that their line intensities in O--CSEs are qualitatively consistent with the fact that the molecules are formed in a photo--induced circumstellar chemistry in a quantity that depends on the mass loss rate. Hence, both species can in principle be used to estimate the mass loss rate, and the tight relation between the SO(J_K= 3_2->2_1) and CO({\\jtra10}) intensities in O--CSEs shows that SO line emission may even be a good mass loss rate estimator. On the contrary, the SiO({\\jtra21}) luminosity appears to be essentially independent of the mass loss rate in O--CSEs, possibly due to a larger influence from molecular adhesion onto grains. These results explain why the HCN({\\jtra10})/SiO({\\jtra21}) intensity ratio increases with the mass loss rate in O--CSEs, and there is no need to invoke e.g. a spread in C/O--ratios for the M--stars to explain the large range of this ratio. Maser emission is very likely present in the HCN({\\jtra10}) line in C--CSEs, and it seems to be sensitively dependent on the mass loss rate, i.e., it appears only for dot M la 5x 10(-7) M_sunpyr. Based on time monitoring of this emission towards the C--stars W Ori and X TrA, we suggest that the strongest maser features are due to radial amplification in the {\\ftra21} transition. The predominance of redshifted maser emission could be caused by an additional amplification in the {\\ftra11} transition. We find no evidence for a similar maser in O--CSEs.

Olofsson, H.; Lindqvist, M.; Nyman, L.-A.; Winnberg, A.

1998-01-01

186

Spectroscopic Survey of Circumstellar Disks in Orion  

NASA Astrophysics Data System (ADS)

As a second stage of a project focused on characterizing candidate stars bearing a circumstellar disk in Orion, we present a spectroscopic follow-up of a set of about 170 bright stars. The present set of stars was selected by their optical (UBVRI) and infrared behavior in different color-color and color-magnitude diagrams. Observations were carried out at the Observatorio Astronomico Nacional located at the Sierra San Pedro Martir in B.C., Mexico and at the Observatorio Guillermo Haro in Cananea, Sonora, Mexico. Low-resolution spectra were obtained for all candidates in the sample. Using the SPTCLASS code, we have obtained spectral types and equivalent widths of the Li I 6707 and Halpha lines for each one of the stars. This project is a cornerstone of a large scale survey aimed to obtain stellar parameters in a homogeneous way using spectroscopic data. This work was partially supported by UNAM-PAPIIT grant IN-109311.

Contreras, Maria; Hernandez, Jesus; Olguin, Lorenzo; Briceno, Cesar

2013-07-01

187

Delivery of Volatiles to Habitable Planets in Extrasolar Planetary Systems  

NASA Technical Reports Server (NTRS)

The Earth can support life because: (1) its orbit lies in the Sun's habitable zone', and (2) it contains enough volatile material (e.g. water and organics) for life to flourish. However, it seems likely that the Earth was drier when it formed because it accreted in a part of the Sun's protoplanetary nebula that was too hot for volatiles to condense. If this is correct, water and organics must have been delivered to the habitable zone, after dissipation of the solar nebula, from a 'wet zone' in the asteroid belt or the outer solar system, where the nebula was cool enough for volatiles to condense. Material from the wet zone would have been delivered to the Earth by Jupiter and Saturn. Gravitational perturbations from these giant planets made much of the wet zone unstable, scattering volatile-rich planetesimals and protoplanets across the Solar System. Some of these objects ultimately collided with the inner Planets which themselves lie in a stable part of the Solar System. Giant planets are now being discovered orbiting other sunlike stars. To date, these planets have orbits and masses very different from Jupiter and Saturn, such that few if any of these systems is likely to have terrestrial planets in the star's habitable zone. However, new discoveries are anticipated due to improved detector sensitivity and the increase in the timespan of observations. Here we present numerical experiments examining the range of giant-planet characteristics that: (1) allow stable terrestrial Planets to exist in a star's habitable zone, and (2) make a large part of the star's wet zone weakly unstable, thus delivering volatiles to the terrestrial planets over an extended period of time after the dissipation of the solar nebula.

Chambers, John E.; Kress, Monika E.; Bell, K. Robbins; Cash, Michele; DeVincenzi, Donald L. (Technical Monitor)

2000-01-01

188

A Model of Habitability Within the Milky Way Galaxy  

Microsoft Academic Search

We present a model of the Galactic Habitable Zone (GHZ), described in terms of the spatial and temporal dimensions of the Galaxy that may favour the development of complex life. The Milky Way galaxy is modelled using a computational approach by populating stars and their planetary systems on an individual basis using Monte-Carlo methods. We begin with well-established properties of

M. G. Gowanlock; D. R. Patton; S. M. McConnell

2011-01-01

189

The Extraordinary Circumstellar Environment of U Equ  

NASA Astrophysics Data System (ADS)

We have discovered a star with an optical spectrum unlike any ever observed. Deep, narrow absorption lines of non-photospheric TiO, AlO, and VO riddle the optical spectrum, and indicate a rotation temperature of 700 K. This cold gas is well below the grain condensation temperature, and it takes an unusual circumstellar environment to retain Ti and Al in the gaseous phase. AlO and VO are sometimes seen in the photospheres of only the coldest { 3000 K} M stars, but the underlying spectrum of U Equ seems to be that of a warm mid-G star { 4800 to 5000 K}. The IRAS 25/12 micron colors are consistent with optically thin dust, yet the LRS spectrum shows moderate silicate absorption which is normally associated with optically thick material. Based on the IRAS data, we suspect that the star has an edge-on disk, with extended material giving rise to optically thin emission. We recently obtained an infrared spectrum {1 to 2.4 micron} that includes prominent H2O absorption bands and shows the CO 2-0 overtone band without the bandhead structure normally present in spectra of cool stars, indicating that the material is cold. The absence of CH4 in the spectrum further indicates a low, non-photospheric density. Although there are other warm stars with edge-on disks, none has the cold molecular absorption that U Equ displays. Ground based J and K band {7"} images suggest a bipolar symmetry. With NICMOS we will be able to clearly identify the nature of the circumstellar material around this unusual star.

Barnbaum, Cecilia

1997-12-01

190

Circumstellar dust: From protostars to planetary systems  

NASA Astrophysics Data System (ADS)

A combination of theoretical work and observational discoveries over the past three decades has led to significant advances in our understanding of the star and planet formation process. However, many important questions remain to be addressed, especially regarding the earliest phases of protostellar collapse and the transformation of circumstellar disks into planetary systems. In this thesis, I have undertaken a theoretical study of ``Class 0'' protostars and an observational investigation of the evolution of protoplanetary disks, diversity of planetary debris systems, and the kinship between dusty remnants and planets, using a new generation of infrared and sub- millimeter instruments. I present radiative transfer calculations of infalling envelopes surrounding Class 0 sources, compare them to the observed spectral energy distributions and radial intensity profiles, and derive mass infall rates. The rapid infall, probably inevitable given their dense environments, and the relatively flat inferred density distribution, perhaps due to contributions from external cloud material, lead us to suggest that many Class 0 sources could be the protostars of dense regions. It has been suggested that circumstellar disks evolve from massive, optically thick, actively accreting structures to low-mass, optically thin, passive remnants in about 10 Myr. That transition may mark the assembly of grains into planetesimals, or clearing of the disk by planets. I present mid infrared observation of the TW Hydrate Association, a recently identified nearby group of 10-Myr-old stars. The results suggest rapid evolution of inner disks as does our discovery of a spatially- resolved disk with a central cavity around the young A star HR 4796A. I also present the results of mid-infrared imaging of 11 other Vega-like stars, derive global properties of the dust disks, place constraints on their sizes, and discuss several interesting cases in detail. Finally, I report the detection of dust emission from a possible Kuiper Belt around 55 Cancri, a star with known planetary companion(s).

Jayawardhana, Ray

2000-11-01

191

Encouraging the Lifetime Reading Habit.  

ERIC Educational Resources Information Center

Maintains that teachers must encourage the lifetime reading habit at school in independent reading. Discusses several ways to do so, including cluttering up the classroom with reading materials, providing time for reading, and encouraging the reading habit throughout the year. (SR)

Sanacore, Joseph

1992-01-01

192

How Common are Habitable Planets?  

NASA Technical Reports Server (NTRS)

The Earth is teeming with life, which, occupies a diverse array of environments; other bodies in our Solar System offer fewer, if any, niches which are habitable by life as we know it. Nonetheless, astronomical studies suggest that a large number of habitable planets-are likely to be present within our Galaxy.

Lissauer, Jack J.; DeVincenzi, Donald (Technical Monitor)

2000-01-01

193

Magnetic constraints on the habitability of exoearths and exomoons  

NASA Astrophysics Data System (ADS)

Surface habitability of planetary environments is essentially constrained by two basic and related conditions: 1) the existence of a thick enough atmosphere and 2) proper levels of insolation or other sources of energy able to guarantee the right temperatures required for the existence of surface liquid water. It is customary to assume that the first condition (an atmosphere) is always fulfilled and to focus on the physical factors limiting the second one (insolation or energy sources). Now it is widely accepted that magnetic fields play a central role into determining if a planet is able to preserve a dense enough atmosphere or the right content of volatiles required for habitability. Hence the fulfillment of the first condition could strongly depend on the existence of a relatively strong intrinsic or extrinsic magnetic field. In the Solar System Venus and Mars provide examples of planets that, though located inside the Radiative Habitable Zone (RHZ), lack a protective magnetic field and have lost their inventory of water or most of their early atmospheric content by a combination of thermal and non-thermal atmospheric losses. We present here a review of the role that magnetic fields would have at constraining the habitability of planetary environments, both in the case of Earth-like planets and super-Earths (exoearths) and for the case of exomoons around giant planets in the RHZ of their host stars. In the first case we found that magnetic properties constraining habitability strongly dependent on planetary mass and composition. We present preliminary results of the case of already discovered potentially habitable exoearths and Kepler candidates. In the case of potentially habitable exomoons we found that magnetic protection together with conditions of tidal heating and illumination, constraints the possible range of exomoons planetocentric orbits. Also in this case we present results concerning the magnetic constraints to habitability of hypotetical exomoons of already discovered giant exoplanets in the HZ and Kepler planetary candidates.

Zuluaga, J. I.

2013-05-01

194

TEM studies of a circumstellar rock  

NASA Technical Reports Server (NTRS)

Reported here is the discovery of crystals of titanium carbide in a grain of silicon carbide which formed as a circumstellar dust particle in the atmosphere of a carbon-rich star. Just as in the case of terrestrial rocks, whose assemblage of minerals gives us clues to the composition and conditions of the environment in which they formed, the titanium carbide crystals and their textural relationship to the silicon carbide give us important clues to the nature of the stellar atmosphere in which they formed. From microscopic studies of the relationships between the atomic planes of the silicon carbide and the titanium carbide, we can show that the titanium carbide cannot have existed as already-formed crystals in a gas around which silicon carbide subsequently condensed. An alternative possibility is that both minerals grew quickly and simultaneously from condensing gas in the rapidly cooling and expanding stellar atmosphere. Other microscopic features of the silicon carbide, such as abundant atomic layer disorder and crystal twinning, similarly suggest rapid grain growth. However, another possibility is that the titanium carbide grew inside of the silicon carbide by diffusion of titanium atoms. Our calculations suggest that this scenario is less likely, given the relatively short times (a year or less) for which stellar condensates can be expected to be exposed to temperatures high enough to make diffusion sufficiently rapid.

Bernatowicz, Thomas J.; Amari, Sachiko; Lewis, Roy S.

1992-01-01

195

Polarization observations of circumstellar OH masers  

NASA Astrophysics Data System (ADS)

Results of observations of circumstellar OH masers in lines with wavelengths near 18 cm are reported. The observations were carried out on the radio telescope of the Nan cay Radio Astronomy Observatory (France). In 2007-2009, 70 late-type stars were observed (including Mira and semi-regular variables). For 53 of these, emission was detected in at least one of three OH lines (1612, 1665, or 1667 MHz). Circular and linear polarization of the maser emission was measured, yielding all four Stokes parameters. Polarized emission features were detected in the OH line spectra of 41 stars. A summary of all the observations is given. The results obtained for T Lep, R LMi, and R Crt are discussed. Emission in the 1665 and 1667 MHz OH lines was detected in T Lep for the first time. Features probably due to Zeeman splitting were detected in the OH line profiles of all three stars. Estimates of the magnetic-field strengths in the maser sources were obtained (0.46-2.32 mG). Variability of the polarization characteristics of the maser emission of the stars on time intervals of several months was found.

Rudnitski, G. M.; Pashchenko, M. I.; Colom, P.

2010-05-01

196

Diagnosing the Circumstellar Environment of HD163296  

NASA Astrophysics Data System (ADS)

We present ultraviolet and optical spectropolarimetric observations of the suspected pre-main-sequence Herbig Ae/Be star HD163296. The observations were obtained in March 1995 with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) aboard the Astro-2 mission, and in summer 1995 at the University of Wisconsin's Pine Bluff Observatory (PBO). HD163296 is thought to be observed at an intermediate inclination angle. Interestingly, the polarimetric data show a 90 degree position angle flip between the optical and the ultraviolet, similar to that observed in HD45677 by WUPPE on Astro-1. Since the WUPPE and PBO observations were made 10 weeks apart, there could be a question of whether the PA flip existed at the time of the WUPPE observations; however, optical broad-band (BVRI) polarimetric observations made at Mt. Lemmon Observatory one week after the WUPPE observations confirm that the optical was at the same PA as measured in the PBO data. For HD45677, which is seen equator-on, the PA flip was interpreted as evidence for a bipolar geometry of the circumstellar environment (Schulte-Ladbeck et al. 1992). We will discuss this possibility as well as other potential interpretations for HD163296, especially in light of the evidence for infall detected in our contemporaneous IUE observations (Grady et al, this session). WUPPE is supported under NASA contract NAS5-26777 with the University of Wisconsin.

Bjorkman, K. S.; Meade, M. R.; Babler, B. L.; Anderson, C. M.; Code, A. D.; Fox, G. K.; Johnson, J. J.; Sanders, W. T.; Weitenbeck, A. J.; Zellner, N. E. B.; Nordsieck, K. H.; Pozdell, S.; Hanson, J.; Ager, W.; Putman, M.; Lupie, O. L.; Edgar, R. J.; Johansen, K. A.; Kobulnicky, H. A.

1995-12-01

197

Non-axisymmetric Be star circumstellar disks.  

NASA Astrophysics Data System (ADS)

A new high resolution and high signal-to-noise ratio spectroscopic study of H?, FeII ?5317 and HeI ?5876 emission lines of northern and equatorial Be stars is presented. The line profiles are analyzed in order to test predictions of the recently proposed model of global disk oscillations. The FeII and HeI line profiles are used to derive the kinematics and the radial extension of the circumstellar disk-like plasma. Based on line profile derivatives, we present a new method, the separation of the inflection points (SIP), to resolve the convolved peaks of H? winebottle-type profiles. The newly derived peak positions are used to determine H? emission disk radii and mean optical line depths along the line of sight. We find that these improved H? disk radii are smaller and closer to FeII disk radii than those H? disk radii derived from pure peak separations. We find evidence that class 2 profiles originate from quasi-Keplerian disks with a non-axisymmetric density distribution.

Hummel, W.; Vrancken, M.

1995-10-01

198

Circumstellar features in hot DA white dwarfs  

NASA Astrophysics Data System (ADS)

We present a phenomenological study of highly ionized, non-photospheric absorption features in high spectral resolution vacuum ultraviolet spectra of 23 hot DA white dwarfs. Prior to this study, four of the survey objects (Feige 24, REJ 0457-281, G191-B2B and REJ 1614-085) were known to possess these features. We find four new objects with multiple components in one or more of the principal resonance lines: REJ 1738+665, Ton 021, REJ 0558-373 and WD 2218+706. A fifth object, REJ 2156-546, also shows some evidence of multiple components, though further observations are required to confirm the detection. We discuss possible origins for these features including ionization of the local interstellar environment, the presence of material inside the gravitational well of the white dwarf, mass loss in a stellar wind and the existence of material in an ancient planetary nebula around the star. We propose ionization of the local interstellar medium as the origin of these features in G191-B2B and REJ 1738+665, and demonstrate the need for higher-resolution spectroscopy of the sample, to detect multiple interstellar medium velocity components and to identify circumstellar features that may lie close to the photospheric velocity.

Bannister, N. P.; Barstow, M. A.; Holberg, J. B.; Bruhweiler, F. C.

2003-05-01

199

Interferometric Imaging of Circumstellar Disks with OVRO  

NASA Astrophysics Data System (ADS)

We are conducting an intensitve multi-species imaging study of two T Tauri and two Herbig Ae stars with the Owens Valley Millimeter Array. Interferometric images of several species in each of the important chemical families (C-, N-, O-, and S-bearing), including a number of isotopic variants, have been acquired. Even at moderate resolution (5") there appear to be interesting morphological differences between species expected to follow different (photo)chemical paths. The HCN velocity structure is similar to that seen in CO(2-1) showing that HCN participates in the same disk rotation. The integrated intensity map of HCN, however, shows a double peaked morphology suggestive of depletion of HCN in the inner disk. The H13CO+ emission provides an important lower bound to the gas fractional ionization of a few 10-10. The first detection of DCN in this kind of object has made possible a determination of the critical D/H ratio (DCN/HCN) in the circumstellar gas. At present, the data provides an approximate value of the D/H ratio of 0.01 since even H13CN(1-0) is optically thick, judging by its three hyperfine components. Still, this very high D/H ratio is comparable to those in comets, and suggests an evolutionary history in which cometary materials remain at very low temperatures throughout their assemblage and for the bulk of their lives.

Qi, Charlie; Blake, Geoffrey A.; Sargent, Anneila I.

1999-10-01

200

CIRCUMSTELLAR SHELL FORMATION IN SYMBIOTIC RECURRENT NOVAE  

SciTech Connect

We present models of spherically symmetric recurrent nova shells interacting with circumstellar material (CSM) in a symbiotic system composed of a red giant (RG) expelling a wind and a white dwarf accreting from this material. Recurrent nova eruptions periodically eject material at high velocities ({approx}> 10{sup 3} km s{sup -1}) into the RG wind profile, creating a decelerating shock wave as CSM is swept up. High CSM densities cause the shocked wind and ejecta to have very short cooling times of days to weeks. Thus, the late-time evolution of the shell is determined by momentum conservation instead of energy conservation. We compute and show evolutionary tracks of shell deceleration, as well as post-shock structure. After sweeping up all the RG wind, the shell coasts at a velocity {approx}100 km s{sup -1}, depending on system parameters. These velocities are similar to those measured in blueshifted CSM from the symbiotic nova RS Oph, as well as a few Type Ia supernovae that show evidence of CSM, such as 2006X, 2007le, and PTF 11kx. Supernovae occurring in such systems may not show CSM interaction until the inner nova shell gets hit by the supernova ejecta, days to months after the explosion.

Moore, Kevin; Bildsten, Lars [Department of Physics, Broida Hall, University of California, Santa Barbara, CA 93106 (United States)

2012-12-20

201

Assessing Habitability: Lessons from the Phoenix Mission  

NASA Technical Reports Server (NTRS)

The Phoenix mission's key objective was to search for a habitable zone. The Phoenix lander carried a robotic arm with digging scoop to collect soil and icy material for analysis with an instrument payload that included volatile mineral and organic analysis(3) and soil ionic chemistry analysis (4). Results from Phoenix along with theoretical modeling and other previous mission results were used to evaluate the habitability of the landing site by considering four factors that characterize the environments ability to support life as we know it: the presence of liquid water, the presence of an energy source to support metabolism, the presence of nutrients containing the fundamental building blocks of life, and the absence of environmental conditions that are toxic to or preclude life. Phoenix observational evidence for the presence of liquid water (past or present) includes clean segregated ice, chemical etching of soil grains, calcite minerals in the soil and variable concentrations of soluble salts5. The maximum surface temperature measured was 260K so unfrozen water can form only in adsorbed films or saline brines but warmer climates occur cyclically on geologically short time scales due to variations in orbital parameters. During high obliquity periods, temperatures allowing metabolism extend nearly a meter into the subsurface. Phoenix discovered 1%w/w perchlorate salt in the soil, a chemical energy source utilized by a wide range of microbes. Nutrient sources including C, H, N, O, P and S compounds are supplied by known atmospheric sources or global dust. Environmental conditions are within growth tolerance for terrestrial microbes. Summer daytime temperatures are sufficient for metabolic activity, the pH is 7.8 and is well buffered and the projected water activity of a wet soil will allow growth. In summary, martian permafrost in the north polar region is a viable location for modern life. Stoker et al. presented a formalism for comparing the habitability of various regions visited to date on Mars that involved computing a habitability probability, defined as the product of probabilities for the presence of liquid water (P(sub lw)), energy (P(sub e)), nutrients (P(sub ch)), and a benign environment (P(sub b)). Using this formalism, they argued that the Phoenix site was the most habitable of any site visited to date by landed missions and warranted a follow up mission to search for modern evidence of life. This paper will review that conclusion in view of more recent information from the Mars Exploration Rovers and Mars Science Lander missions.

Stoker, Carol R.

2013-01-01

202

Space weather hazards and planetary habitability  

NASA Astrophysics Data System (ADS)

Space weather related responses of planetary atmospheres include: thermospheric and ionospheric density variations, changes in atmospheric composition including ozone depletion, changes in heating/cooling, temperature and wind perturbations, photochemistry, collisional excitation, deactivation and cooling due to IR-, optical and UV-emissions, magnetospheric compression, enhancement of secondary particles, etc. That these atmospheric effects related to extreme solar events have an influence on satellites (drag, anomalous behaviour / failure), navigation and tracking difficulties, spacecraft/mission design, radiation hazard issues for future manned missions (e.g., Moon, Mars) etc., is only one aspect. Another important aspect is directly related to the evolution of planetary atmospheres and their water inventories. For understanding the principles that generate long-time habitable environments like on Earth or terrestrial type exoplanets such as the recent discovered Gliese 581 inside the habitable zones of lower mass stars, one has to understand the evolutionary influence of the solar/ stellar radiation and particle environment to the atmosphere and surface. The need for a coordinated study of the behaviour of the upper atmosphere, ionosphere, magnetospheric environment and thermal and non-thermal atmospheric loss processes of Venus, Earth, and Mars during "extreme solar events" will be discussed. Such events can serve as a proxy for the influence of the active young Sun with implications for the evolution of planetary atmospheres (solar system and exoplanets), water inventories and habitability. Studies of the evolution of the spectral irradiances (X-ray, EUV, UV) of solar-type stars of different ages are used as a proxy for reconstructing the history of the Sun/stars radiation output. Complementary information concerning solar mass loss (solar/stellar winds) can be derived from studies of the energy distributions of flare related CMEs during low and high solar activity by extrapolating CME related solar mass loss to the early stage of the solar system using solar proxies. The presentation will also discuss why the understanding of space weather related effects are important within the investigation on how different planetary systems and their individual planets - and particularly Earth-like ones - evolve, how they may evolve in their radiation and particle interaction with their host stars under different circumstances, and how often they give rise to conditions that could in principle be benevolent enough for the origin of life, even if such a planet orbits within a climatologically defined habitable zone.

Lammer, H.

2007-08-01

203

On the Habitability of Terrestrial Planets in Binary Star Systems  

NASA Astrophysics Data System (ADS)

The growing number of detected planets in binary star systems requires methods for a quick assessment of possible habitability of planets in such environments. We offer an analytic method to determine habitability of a terrestrial planet in binary star systems. In this context we give an answer to the most important question: Do the radiative and gravitational perturbations of a secondary influence the extent of the habitable zone (HZ)? After we have defined the borders of the HZ, we will show the dynamical behaviour of a terrestrial planet in the HZ when adding a Jupiter to the system. In such a system the HZ shows signs of mean motion resonances and secular resonances, depending on the architecture of the planetary system.

Pilat-Lohinger, Elke; Funk, Barbara; Eggl, Siegfried

2014-04-01

204

Habitability: from stars to cells  

Microsoft Academic Search

To determine where to search for life in our solar system or in other extrasolar systems, the concept of habitability has\\u000a been developed, based on the only sample we have of a biological planet—the Earth. Habitability can be defined as the set\\u000a of the necessary conditions for an active life to exist, even if it does not exist. In astronomy,

Emmanuelle J. Javaux; Véronique Dehant

2010-01-01

205

Collisional Evolution of a Circumstellar Debris Disk  

NASA Astrophysics Data System (ADS)

A simple and fast model for a circumstellar debris disk has been developed. These disks are of considerable interest because they may be sites of ongoing planet formation. Collisions among unseen planetesimals are thought to be the source of these small dust grains, which upon their creation are launched into very wide orbits due to stellar radiation pressure (Strubbe and Chiang 2006). Collisions among dust grains also depletes the disk of its dust. However inspecting all pairs of intersecting dust orbits allows one to assess the collision probabilities, which then leads to a rate equation that accounts for dust production by unseen planetesimals minus losses due to collisions. This results in a system of coupled nonlinear differential equations, with one equation for the dust in each size bin, that is easily solved numerically for the dust abundance over time. The model's principal parameters are the dust production rate, and nature of the source planetesimals, namely, whether they reside in a narrow ring or a broad disk. The model is also sensitive to whether the planetesimal disk experiences `inside-out' erosion, with dust production being more rapid in the inner part of the planetesimal disk, versus `outside-in' erosion. The simulated disks' surface brightness are then compared to HST observations of beta Pictoris (acquired by Golimowski et al 2006), as well as the solar analog HD 107146 (by Ardila et al 2004, 2005). Preliminary results indicate that, for both systems, the source planetesimals reside in a broad disk, and that the planetesimal erosion there is `outside-in', with dust production being more vigorous in the outer parts of the planetesimal disk. Lastly, the inferred dust production rates are prodigious, possibly resulting in the loss of tens of earth-masses during the life of the planetesimal disk, due to collisional grinding and blowout by radiation pressure.

Hahn, Joseph M.

2010-01-01

206

High resolution studies of circumstellar masers  

NASA Astrophysics Data System (ADS)

This thesis describes in detail the observational procedures that we have followed and developed to analyze several research projects organized at Onsala since 1990 to study the regions of SiO maser emission at 43 GHz in the circumstellar envelopes of late-type stars with very high angular resolution. The compact nature of these SiO maser regions has been demonstrated in this work. In fact, we find that they are an order of magnitude smaller than previously reported. As a consequence, they can be studied with the available VLBI instruments. We have used the EVN and VLBA telescopes to produce images of the distribution of the SiO maser features in several sources, in particular R Cassiopeiae and VX Sagittarii. Through this work, we discuss in detail the procedures involved in all the project steps, from the scheduling of the observations to the calibration, imaging, and modelling of the source structure. We compare our results with those of other instruments at different frequencies for the same objects. This thesis also describes the analysis of observations of water masers at 22 GHz in several late-type stars using the VLA. The limited resolution of these observations made us develop a computer program, "FG3", that would search for the maser sources (blended spatially and in frequency) and extract them via 3D least squares fitting to Gaussian functions. This is, to our knowledge, a new approach in this kind of investigations. This program provides information of not only the source structure distribution and kinematics, but also measures parameters that are directly related with the physical conditions in the masing regions. In particular, we conclude by the study of the fitted maser linewidths that water masers in late-type stars are unsaturated, and experiment exponential growth. We are not able to reach a clear conclusion for the SiO masers in the supergiant star VX Sgr, but there are indications that they might be saturated.

Colomer, F.

1996-06-01

207

CO line emission from circumstellar envelopes  

NASA Astrophysics Data System (ADS)

Aims.We present the results of a multi-transition CO observational program conducted on a sample of AGB and post-AGB stars envelopes. We have collected maps and single pointing observations of these envelopes in 5 rotational transitions ranging from J = 1-0 to J = 6-5, including in particular new observations of the CO line at 691 GHz at the CSO. The use of such a set of mm and submm CO line on stellar envelopes is rare and limited to the work of some authors on IRC+10216. Methods: .Using a model for the CO emission of an AGB circumstellar envelope, in combination with a standard LVG approach, we have conducted a systematic modelling analysis using the whole set of CO data collected for a sample of 12 sources. We simultaneously fit all five transitions, taking into account the spatial information provided by the maps. Results: .We find mass-loss rates in the range 1 × 10-7 to 4 × 10-4 M_?/yr, and envelope temperatures ranging from 20 K to 1000 K at a radius of 1016 cm. There seem to be a general anti-correlation between mass loss rates and temperature, the high mass loss rate AGBs having low temperatures, and vice versa. We show that most AGB data can be fitted using a constant mass loss rate, at least within the calibration uncertainties associated with the data collected at different frequencies. For some cases though (e.g. CIT 6, R Hya, ? Cyg), a change in the mass loss rate history needs to be invoked to reconcile data at low- and high-J, a scenario already mentioned by several authors to explain observations of WX Psc.

Teyssier, D.; Hernandez, R.; Bujarrabal, V.; Yoshida, H.; Phillips, T. G.

2006-04-01

208

First Circumstellar Disk around a Massive Star  

NASA Astrophysics Data System (ADS)

Observations with an infrared-sensitive instrument at the ESO 3.6-m telescope at La Silla have for the first time shown the presence of a disk around a hot and massive star, known as G339.88-1.26 . Until now, disks have only been found around less massive stars. Planets are formed in such disks. The new discovery may thus have important implications for our understanding of the formation of planetary systems around stars. TIMMI observations Observations at mid-infrared wavelengths were carried out in July 1997 by Bringfried Stecklum (Landessternwarte Thüringen, Tautenburg, Germany) and Hans-Ulrich Käufl (ESO), using the TIMMI instrument at the ESO 3.6-m telescope. Additional measurements were carried out in March 1998. TIMMI ( T hermal I nfrared M ulti M ode I nstrument) is a general-purpose camera spectrometer operating at a wavelength of 10 µm. To reach sufficient sensitivity, the camera must be cooled to approx. -260 o C, i.e. a few degrees above the absolute minimum, by use of liquid Helium. Astronomical objects whose temperatures are between -120 o C and 300 o C radiate most of their energy at this wavelength. In addition, dust and haze that are absolutely impenetrable for light visible to the human eye, are often found to be nearly transparent at this wavelength. This is why fire-fighters now use similar equipment to look through smoke. G339.88-1.26: A very special object ESO PR Photo 22a/98 ESO PR Photo 22a/98 [JPEG, 800k] This image is a true-color composite of near-infrared observations of the sky region around the radio source G339.88-1.26 with the ESO/MPI 2.2-m telescope at La Silla. In this image, the visible colors red, green and blue have been used to represent the infrared filters J, H and K (at 1.25, 1.63 and 2.2 µm wavelength, respectively). No object is visible at the position of the radio source, even at these near-infrared wavelengths. A dark band of absorbing dust is clearly visible, exactly at the position of the object (indicated by an arrow). Earlier observations with radio telescopes of the object G339.88-1.26 , deeply embedded in an interstellar nebula, had been interpreted in terms of the possible existence of a circumstellar disk around a high-mass star. It was concluded that the star responsible for heating the surrounding gas must be very hot and also that it must be intrinsically very bright. The star, most likely of spectral type O9, would have a luminosity 10,000 times higher than that of the Sun and a mass of about 20 times that of the Sun. From the measured velocity, the likely distance of this object is about 10,000 light-years. The object is associated with several "spots" of very strong radio emission from methanol molecules (methanol masers). Interestingly, they form a chain in the sky and the measured velocities of the individual spots are indicative for orbital motion in a rotating disk around the central star. The circumstellar disk ESO PR Photo 22/98 ESO PR Photo 22b/98 [JPEG, 640k] The TIMMI 10 µm image of the inclined dust disk around a hot O9 star at the G339.88-1.26 radio source. The diameter of the disk is of the order of 5 arcsec, i.e. at the most probable distance to the object (10,000 lightyears) it is 20,000 times larger than the diameter of the Earth's orbit around the Sun. The new TIMMI observations of G339.88-1.26 showed an elliptical object with strong infrared radiation. The peak of this radiation (as seen in the sky) coincides with the peak of the radio emission. Furthermore, the apparent orientation of the disk is well aligned with that of the methanol maser "spots". There is little doubt that this object is indeed the infrared image of a circumstellar disk, viewed at an angle. As far as known, this is the first direct image of a disk around a very massive star. At a wavelength of 10 µm, however, the central star that is responsible for heating the dust disc, cannot be seen in spite of its rather high luminosity. This is because it radiates mostly in the ultra-violet part of the spectrum. Moreover, the dust disk in which the hot star is

1998-06-01

209

Interventions to Break and Create Consumer Habits  

Microsoft Academic Search

Interventions to change everyday behaviors often attempt to change people's beliefs and intentions. As the authors explain, these interventions are unlikely to be an effective means to change behaviors that people have repeated into habits. Successful habit change interventions involve disrupting the environmental factors that automatically cue habit performance. The authors propose two potential habit change interventions. \\

Bas Verplanken; Wendy Wood

2006-01-01

210

The logic of habit in International Relations  

Microsoft Academic Search

IR theory is dominated by the logics of consequentialism and appropriateness. But Max Weber offered four logics of choice, not just two. Beyond the instrumental rationality of Zweckrationalität and the normative rationality of Wertrationalität are affect and habit. Drawing on Weber, James, Dewey, and Bourdieu, and habit’s microfoundations in neurocognitive psychology, I explore the logic of habit and its consequences

Ted Hopf

2010-01-01

211

Planetary Habitability of the Solar System  

Microsoft Academic Search

Habitability is a qualitative concept generally defined as the suitability of an environment to support life. Although there are many works related to planetary habitability, there is no practical quantitative definition of habitability. The search for habitable environments in the Solar System and beyond requires a method to quantify and compare their significance. Therefore, this study presents a quantitative approach

Abel Mendez

2009-01-01

212

Rotational Synchronization May Enhance Habitability for Circumbinary Planets: Kepler Binary Case Studies  

NASA Astrophysics Data System (ADS)

We report a mechanism capable of reducing (or increasing) stellar activity in binary stars, thereby potentially enhancing (or destroying) circumbinary habitability. In single stars, stellar aggression toward planetary atmospheres causes mass-loss, which is especially detrimental for late-type stars, because habitable zones are very close and activity is long lasting. In binaries, tidal rotational breaking reduces magnetic activity, thus reducing harmful levels of X-ray and ultraviolet (XUV) radiation and stellar mass-loss that are able to erode planetary atmospheres. We study this mechanism for all confirmed circumbinary (p-type) planets. We find that main sequence twins provide minimal flux variation and in some cases improved environments if the stars rotationally synchronize within the first Gyr. Solar-like twins, like Kepler 34 and Kepler 35, provide low habitable zone XUV fluxes and stellar wind pressures. These wide, moist, habitable zones may potentially support multiple habitable planets. Solar-type stars with lower mass companions, like Kepler 47, allow for protected planets over a wide range of secondary masses and binary periods. Kepler 38 and related binaries are marginal cases. Kepler 64 and analogs have dramatically reduced stellar aggression due to synchronization of the primary, but are limited by the short lifetime. Kepler 16 appears to be inhospitable to planets due to extreme XUV flux. These results have important implications for estimates of the number of stellar systems containing habitable planets in the Galaxy and allow for the selection of binaries suitable for follow-up searches for habitable planets.

Mason, Paul A.; Zuluaga, Jorge I.; Clark, Joni M.; Cuartas-Restrepo, Pablo A.

2013-09-01

213

Habitability of the Phoenix landing site  

NASA Astrophysics Data System (ADS)

The Phoenix mission's key objective was to search for a habitable zone. Mission results are used to evaluate habitability where Phoenix landed. A habitability probability (HI) is defined as the product of probabilities for the presence of liquid water (Plw), energy (Pe), nutrients (Pch), and a benign environment (Pb). Observational evidence for the presence of liquid water (past or present) includes clean ice at a polygon boundary, chemical etching of soil grains, and carbonate minerals. The presence of surface and near subsurface ice, along with thermodynamic conditions that support melting, suggest that liquid water is theoretically possible. Presently, unfrozen water can form only in adsorbed films or saline brines but more clement conditions recur periodically due to variations in orbital parameters. Energy to drive metabolism is available from sunlight, when semitransparent soil grains provide shielding from UV radiation and chemical energy from the redox couple of perchlorate and reduced iron. Nutrient sources including C, H, N, O, P, and S compounds are supplied by known atmospheric sources or global dust. Environmental conditions are within growth tolerance for terrestrial microbes. Surface soil temperatures currently reach 260 K and are periodically much higher, the pH is 7.8 and is well buffered, and the water activity is high enough to allow growth when sufficient water is available. Computation of HI for the sites visited by landers yields Phoenix, 0.47; Meridiani, 0.23; Gusev, 0.22; Pathfinder, 0.05; Viking 1, 0.01; Viking 2, 0.07. HI for the Phoenix site is the largest of any site explored, but dissimilar measurements limit the comparisons' confidence.

Stoker, Carol R.; Zent, Aaron; Catling, David C.; Douglas, Susanne; Marshall, John R.; Archer, Douglas; Clark, Benton; Kounaves, Samuel P.; Lemmon, Mark T.; Quinn, Richard; Renno, Nilton; Smith, Peter H.; Young, Suzanne M. M.

2010-06-01

214

Constraints on planetary habitability from interior modelling  

NASA Astrophysics Data System (ADS)

The most interesting planetary bodies outside the Solar System regarding the search for life are potentially rocky extrasolar planets. Some of them may feature surface conditions that allow for liquid water, which is the elementary prerequisite for life as we know it. The amount of greenhouse gases, like e.g. carbon dioxide (CO2), plays an important role for the determination of the surface temperature, hence the habitability of an extrasolar planet. The amount of greenhouse gases is strongly influenced by their outgassing from the interior. In this study, we investigate under which conditions the planetary interior structure and dynamics allow for the build-up of planetary atmospheres which may lead to habitable surface conditions. We investigate the evolution of a secondary atmosphere for Earth-sized planets with different interior structures (i.e. iron-silicate mixing ratios) by applying a two-dimensional model of interior dynamics [1], which allows for the calculation of the production of partial melt [2]. From this, we estimate the amount of CO2 outgassing for Earth-sized planets with different core and mantle radii after adapting the total CO2 outgassing in 4.5 Gyr for a Venus reference simulation to the present-day atmosphere of Venus. We furthermore investigate the possible influence of plate tectonics on outgassing and the likelihood of plate tectonics depending on the interior structure of the planet. We find that the size of the iron core has a large impact on the production of partial melt, hence on the possible outgassing of CO2, which is due to the pressure-dependence of the melting temperature of silicate rocks: for planets with a large core the planetary mass is larger than for a planet with a small iron core, leading to larger melting temperatures in the upper mantle. Therefore only little outgassing from the interior can be expected. However, for the determination of the outer edge of the habitable zone it is typically assumed that enough greenhouse gas CO2 is available in the atmosphere to lead to liquid water at the surface - independent of the interior of the planet [3]. Our results on the other hand suggest that the outer boundary of the habitable zone may be constrained by the production of partial melt in the interior for planets with a large iron core and a thin silicate mantle. However, if plate tectonics initiates, several tens of bars of CO2 can be outgassed in a short time also for planets with a large iron core. In this case the outer boundary of the habitable zone would not be limited by outgassing as is the case for stagnant-lid planets. It is, however, questionable if planets with a very thin mantle are able to initiate plate tectonics. References [1] Hüttig, C. and Stemmer, K. (2008), PEPI, 171(1-4):137-146. [2] Plesa, A.-C. and Spohn, T. (2012), Transactions of the HLRS 2011, Springer, 551-565. [3] Kasting, J., Whitmire, D.P. and Reynolds, R.T. (1993), Icarus, 101:108-128.

Noack, Lena; Godolt, Mareike; von Paris, Philip; Plesa, Ana-Catalina; Stracke, Barbara; Breuer, Doris; Rauer, Heike

2013-04-01

215

The Rich Circumstellar Chemistry of SMP LMC 11  

NASA Astrophysics Data System (ADS)

Carbon-rich evolved stars from the asymptotic giant branch to the planetary nebula phase are characterized by a rich and complex carbon chemistry in their circumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC 11, whose Spitzer Infrared Spectrograph spectrum shows remarkable and diverse molecular absorption bands. To study how the molecular composition in this object compares to our current understanding of circumstellar carbon chemistry, we modeled this molecular absorption. We find high abundances for a number of molecules, perhaps most notably benzene. We also confirm the presence of propyne (CH3C2H) in this spectrum. Of all the cyanopolyynes, only HC3N is evident; we can detect at best a marginal presence of HCN. From comparisons to various chemical models, we can conclude that SMP LMC 11 must have an unusual circumstellar environment (a torus rather than an outflow).

Malek, S. E.; Cami, J.; Bernard-Salas, J.

2012-01-01

216

THE RICH CIRCUMSTELLAR CHEMISTRY OF SMP LMC 11  

SciTech Connect

Carbon-rich evolved stars from the asymptotic giant branch to the planetary nebula phase are characterized by a rich and complex carbon chemistry in their circumstellar envelopes. A peculiar object is the preplanetary nebula SMP LMC 11, whose Spitzer Infrared Spectrograph spectrum shows remarkable and diverse molecular absorption bands. To study how the molecular composition in this object compares to our current understanding of circumstellar carbon chemistry, we modeled this molecular absorption. We find high abundances for a number of molecules, perhaps most notably benzene. We also confirm the presence of propyne (CH{sub 3}C{sub 2}H) in this spectrum. Of all the cyanopolyynes, only HC{sub 3}N is evident; we can detect at best a marginal presence of HCN. From comparisons to various chemical models, we can conclude that SMP LMC 11 must have an unusual circumstellar environment (a torus rather than an outflow).

Malek, S. E.; Cami, J. [Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7 (Canada); Bernard-Salas, J., E-mail: smalek2@uwo.ca, E-mail: jcami@uwo.ca [Institut d'Astrophysique Spatiale, CNRS/Universite Paris-Sud 11, 91405 Orsay (France)

2012-01-01

217

Habitable moons around extrasolar giant planets.  

PubMed

Possible planetary objects have now been discovered orbiting nine different main-sequence stars. These companion objects (some of which might actually be brown dwarfs) all have a mass at least half that of Jupiter, and are therefore unlikely to be hospitable to Earth-like life: jovian planets and brown dwarfs support neither a solid nor a liquid surface near which organisms might dwell. Here we argue that rocky moons orbiting these companions could be habitable if the planet-moon system orbits the parent star within the so-called 'habitable zone', where life-supporting liquid water could be present. The companions to the stars 16 Cygni B and 47 Ursae Majoris might satisfy this criterion. Such a moon would, however, need to be large enough (>0.12 Earth masses) to retain a substantial and long-lived atmosphere, and would also need to possess a strong magnetic field in order to prevent its atmosphere from being sputtered away by the constant bombardment of energetic ions from the planet's magnetosphere. PMID:9000072

Williams, D M; Kasting, J F; Wade, R A

1997-01-16

218

Young stellar objects and their circumstellar dust - An overview  

NASA Technical Reports Server (NTRS)

The basic observational classes of young stellar objects are briefly reviewed and some of the theoretical and observational problems in premain sequence stellar evolution are discussed. The evidence which indicates that the infrared excesses in recently formed stars are primarily due to thermal emission from circumstellar dust is summarized. Various indirect lines of evidence on the spatial distribution of the circumstellar dust are examined, and it is concluded that the concentration of the dust in a large disk structure is the most viable model at this time.

Rydgren, A. E.; Cohen, M.

1985-01-01

219

Kepler: A Search for Habitable Planets  

NSDL National Science Digital Library

NASA's Kepler Mission was launched in 2009 with the focused goal of surveying regions of the Milky Way Galaxy to discover Earth-size planets. The spacecraft's detection instrument is a photometer that continually monitors 145,000 stars to locate exoplanets in the "habitable zone" of a star, where liquid water and possibly life might exist. The website tracks information about mission results, with more than 2,000 candidates identified after the first year's operation. Of those, two Earth-size candidates have been confirmed as of January, 2012. Educators will also find classroom activities, interactive resources, simple animations showing how the detection system works, and galleries of photos, videos and 3D images.

2012-01-27

220

Shaping the pre-supernova circumstellar environment  

NASA Astrophysics Data System (ADS)

Recent discoveries of very luminous supernovae associated with luminous blue variable-type objects {LBV's} raise the interesting possibility that an LBV phase may be the prelude to core collapse. Alternatively, the LBV events may be occuring in binary systems where the LBV phase is reached by one of the stars shortly before its more evolved companion becomes a supernova. The Small Magellanic Cloud binary system HD 5980 is believed to consist precisely of such two objects: a massive hydrogen-poor Wolf-Rayet star in orbit around an even more massive variable star that recently underwent an LBV-type eruption. The wind velocity and the mass-loss rate of the LBV-component have changed remarkably over the past ˜40 years, providing a glimpse of the detailed information of how a binary LBV-type star may shape the circumstellar environment into which the eventual supernova ejecta will collide. One process that is shaping the CSM around HD 5980 is the interaction between the slow wind ejected during eruption and the fast wind that was subsequently ``turned on". In order to model the evolution of this interaction region, an accurate determination of the mass-loss rate and the wind velocity of HD 5980 is required. Because the optical emission lines are contaminated by other sources, only the P Cygni profiles observable in the UV spectral region provide unambiguous values for the current wind speed and mass-loss rate. In this proposal we are requesting 2 HST orbits to observe HD 5980 with STIS in order to obtain one FUV MAMA spectrum from which we will determine the current wind velocity and mass-loss rate of the LBV-type star. These data will also allow a more detailed analysis of the atmospheric structure of the LBV-type object in its quiescent state and, combined with ground-based observations, an analysis of the emission arising in the wind-wind collision region may be performed. Although HD5980 may be unique in our Galactic vicinity, it may be typical of massive star systems formed in low-metallicity regions in distant galaxies and thus, an understanding of HD5980 can provide further insight into the energetic phenomena present in the more distant, low-metallicity star-forming regions.;

Koenigsberger, Gloria

2008-07-01

221

Laboratory Studies Of Circumstellar Carbonaceous Grain Formation  

NASA Astrophysics Data System (ADS)

The study of the formation processes of dust is essential to understand the budget of extraterrestrial organic molecules. Although dust with all its components plays an important role in the evolution of interstellar (IS) chemistry and in the formation of organic molecules, little is known on the formation processes of carbonaceous dust. We report the progress that was recently achieved in this domain using NASA Ames’ COSmIC facility (Contreras & Salama 2013, ApJS, 208, 6). PAHs are important chemical building blocks of IS dust. They are detected in IDPs and in meteoritic samples. Additionally, observational, laboratory, and theoretical studies have shown that PAHs are an important, ubiquitous component of the ISM. The formation of PAHs from smaller molecules has not been extensively studied. Therefore, we have performed laboratory experiments to study the dynamic processes of carbon grain formation, starting from the smallest hydrocarbon molecules into the formation of larger PAH and further into nanograins. Studies of IS dust analogs formed from a variety of PAH and hydrocarbon precursors as well as species that include the atoms O, N, and S, have recently been performed in our laboratory using the COSmIC facility to provide conditions that simulate IS and circumstellar environments. The species formed in the COSmiC chamber through a pulsed discharge nozzle plasma source are detected and characterized with a cavity ringdown spectrometer coupled to a time-of-flight mass spectrometer, thus providing both spectroscopic and ion mass information in-situ. Analysis of solid soot particles was also conducted using scanning electron microscopy at the UCSC/NASA Ames’ MACS facility. The SEM analysis of the deposition of soot from methane and acetylene precursors seeded in argon plasmas provide examples on the types of nanoparticles and micrograins that are produced in these gas mixtures under our experimental conditions. From these measurements, we derive information on the size and the structure of IS dust grain particles, the growth and the destruction processes of IS dust and the resulting budget of extraterrestrial organic molecules. Acknowledgements: This work is supported by NASA SMD (APRA; Carbon in the Galaxy).

Contreras, Cesar; Sciamma-O'Brien, Ella; Salama, Farid

2014-06-01

222

Crystalline Silicates in Circumstellar Dust Shells  

NASA Astrophysics Data System (ADS)

The discovery of crystalline silicates outside our own Solar System by the infrared space observatory (ISO) in both young (Waelkens et al. 1996, A&A 315, L245), and evolved stars (Waters et al. 1996, A&A 315, L361) brought new inside in the circumstellar dust formation and evolution. We will present here an extensive overview of the solid state bands found in a sample of 17 stars all with oxygen-rich dust around them. For all stars good ISO-SWS (short wavelength spectrometer 2--45 ? m) spectra were available and for 12 stars also reliable ISO-LWS (long wavelength spectrometer 43--195 ? m) spectra were taken. We could identify about 50 different spectral features, most of them clustered into one of the 7 complexes (which we defined). Most bands could be identified with crystalline silicates and crystalline water ice, however still roughly 20% remains unidentified. An important result was that the presence of strong crystalline silicates bands always correlates with the presence of a disk like structure (N.B. The presence of a disk does not necessary imply a high fraction of crystalline silicates)(Molster et al. 1999, Nature 401, 563). We found that not only the strength but also the shape of the crystalline silicate features is different for sources with and without the presence of a disk. Another surprising result of this research is that the crystalline silicates contain no measurable amount of Fe. The main minerals found, are forsterite (Mg2 SiO4) and enstatite (MgSiO3). We have calculated mean crystalline silicate spectra for both the disk and the non-disk sources. By simple model fitting we derived estimates for the (relative) mass and temperature of the amorphous silicates, forsterite and enstatite. Based on these results we drew the conclusion that the crystalline and amorphous silicate grains are two separate grain populations. This work was part of a PhD-thesis and funded by NWO.

Molster, F. J.; Waters, L. B. F. M.; Tielens, A. G. G. M.

2000-12-01

223

The Water Vapor Abundance in Circumstellar Envelopes  

NASA Astrophysics Data System (ADS)

The maser emission of the para-H2O 313-->220 line at 183 GHz in O-rich evolved stars has been modeled to account for the empirical characteristics of this line reported by González-Alfonso et al. Likewise, efforts have been made to derive water vapor abundance in these sources. The Sobolev or large velocity gradient (LVG) method has been employed to study the intensity of this line as a function of source properties and physical conditions (i.e., mass-loss rate, p-H2O abundance, velocity field, kinetic temperature profile, stellar luminosity, and the set of collisional rates adopted in the calculations). It has been found that the intensity of the 313-->220 line is sensitive to the mass-loss rate, the p-H2O abundance, and the terminal velocity of the envelope, but it is rather insensitive to the rest of the parameters in stars with high mass-loss rates (?>10-6 Msolar yr-1). The models reproduce the main spectral characteristics of the emission at 183 GHz in the latter sources. A global fit to the data proves that the observational luminosities can be explained by assuming an H2O abundance relative to H2 [x(H2O)] of 1-2×10-4. Detailed fitting to the line profile in five selected objects yields a similar value for x(H2O). The validity of the LVG approach has been verified by modeling the maser emission at 183 GHz through a nonlocal radiative transfer code. The model calculations with both methods lead to similar results. The pumping of the first bending mode of water vapor through absorption of photons emitted by the dust and the star has been also simulated. This effect is found to be important in the pumping of the H2O rotational levels. Hence, in order to recover LVG results, the water abundance must be increased by a factor of ~2 for stars with high mass-loss rates. Consequently, x(H2O) has been estimated to be 3×10-4 within a factor ~=2. With this value for x(H2O), the expected near- and far-infrared spectra of the circumstellar envelopes of O-rich stars for several mass-loss rates have also been computed. Hence, it is possible to predict that, in some stars, the ro-vibrational lines of the 6 ?m water vapor band with wavelengths longer that 6.3 ?m--the P-branch--can be observed in emission, rather than in absorption.

González-Alfonso, Eduardo; Cernicharo, José

1999-11-01

224

A postulate to assess ‘habitability  

NASA Astrophysics Data System (ADS)

One principal challenge in biology is defining a postulate by which the habitability of other planets can be assessed. Current assessments suffer from two potential weaknesses. With respect to other planets, either assumptions are made about the physical and chemical conditions of environments that err on the side of biological optimism without empirical constraint by spacecraft observations or novel physiologies of microorganisms are invented to fit extraterrestrial environmental conditions with no demonstrated microbiological counterparts on Earth. Attempts to assess the habitability of the early Earth suffer from similar problems. We discuss the following postulate: ‘the proposition that a planet is or was habitable requires that the physiological requirements of microorganisms on Earth known at the time of assessment match the empirically determined combined physical and chemical conditions in the extraterrestrial or early Earth environment being assessed’ as a means of evaluating ‘habitability’. We use as tests for our postulate the early Earth and the cloud deck of Venus (a habitat that has been a source of optimistic debate for forty years). We conclude that, although the early Earth was habitable, Venus is a dead world.

Cockell, Charles S.; Westall, Frances

2004-04-01

225

Towards a Classification System for Assessing Extrasolar Planet Habitability  

NASA Astrophysics Data System (ADS)

There is currently no quantitatively-based framework on which to focus the search for Earth-like habitable planets, or to assess the habitability of extrasolar planets already discovered. We suggest that previous assessments have been limited by an incomplete understanding of what a habitable planet could be, and that the search for habitable worlds has been overly limited. We propose to create the first quantitative guide to the habitability and observable characteristics of a broad range of extrasolar planets, using a hierarchy of numerical climate models for worlds spanning a parameter space of orbit and spin configurations, gravity, stellar parents, hydrologic cycles, and atmospheric composition. Habitability is defined, at least initially, via terrestrial analogs. We will explore these through a series of 3-D general circulation model (GCM) paleoclimate simulations, based upon available geologic data. The time slices selected represent Earth's own passage through distinct phases of habitability, from the late Archaean (2.8 Ga) through the Early Paleozoic (440 Ma); each phase reflects steps in the co-evolution of climate and life. Various impacts upon the hydrologic cycle of Earth-like worlds, such as changes in rotation rate, land/sea distribution and ocean heat transports will also be explored using generic GCM simulations at a higher resolution (2 x 2.5 degree grid). Our classification effort will then extend to non-Earth-like terrestrial bodies and to gas giants orbiting various types of stars, through the use of simplified GCMs and 1-D energy balance models (EBMs) better suited to exploration of certain conditions as superrotation, tidal lock, and non-Earth-specific radiative schemes. This array of models will not only allow us to explore a broader range of investigations, but will also permit some degree of calibration and physical understanding between the different approaches. In addition to unveiling the physical diversity of potential habitable zones and exoplanet climates, this effort will form a critical basis for the selection of candidate planets for intensive observation by future planet-finding instruments. At the same time, the project will help us to understand both the Earth and our Solar System within the much broader context of exoplanetary habitability and the potential for extrasolar life.

Sohl, L. E.; Chandler, M. A.; Scharf, C. A.; del Genio, A.; Allison, M.; Menou, K.

2005-12-01

226

Exotic Earths: forming habitable worlds with giant planet migration.  

PubMed

Close-in giant planets (e.g., "hot Jupiters") are thought to form far from their host stars and migrate inward, through the terrestrial planet zone, via torques with a massive gaseous disk. Here we simulate terrestrial planet growth during and after giant planet migration. Several-Earth-mass planets also form interior to the migrating jovian planet, analogous to recently discovered "hot Earths." Very-water-rich, Earth-mass planets form from surviving material outside the giant planet's orbit, often in the habitable zone and with low orbital eccentricities. More than a third of the known systems of giant planets may harbor Earth-like planets. PMID:16960000

Raymond, Sean N; Mandell, Avi M; Sigurdsson, Steinn

2006-09-01

227

Pioneering Concepts of Planetary Habitability  

NASA Astrophysics Data System (ADS)

Famous astronomers such as Richard A. Proctor (1837-1888), Jules Janssen (1824-1907), and Camille Flammarion (1842-1925) studied the concept of planetary habitability a century before this concept was updated in the context of the recent discoveries of exoplanets and the development of planetary exploration in the solar system. They independently studied the conditions required for other planets to be inhabited, and these considerations led them to specify the term "habitability." Naturally, the planet Mars was at the heart of the discussion. Our neighboring planet, regarded as a sister planet of Earth, looked like a remarkable abode for life. During the second part of the nineteenth century, the possibility of Martian intelligent life was intensively debated, and hopes were still ardent to identify a kind of vegetation specific to the red planet. In such a context, the question of Mars' habitability seemed to be very valuable, especially when studying hypothetical Martian vegetation. At the dawn of the Space Age, German-born physician and pioneer of space medicine Hubertus Strughold (1898-1987) proposed in the book The Green and Red Planet: A Physiological Study of the Possibility of Life on Mars (1954) to examine the planets of the solar system through a "planetary ecology." This innovative notion, which led to a fresh view of the concept of habitability, was supposed to designate a new field involving biology: "the science of planets as an environment for life" (Strughold 1954). This notion was very close to the concept of habitability earlier designated by our nineteenth-century pioneers. Strughold also coined the term "ecosphere" to name the region surrounding a star where conditions allowed life-bearing planets to exist. We highlight in this chapter the historical aspects of the emergence of the (modern) concept of habitability. We will consider the different formulations proposed by the pioneers, and we will see in what way it can be similar to our contemporary notion of planetary habitability. This study also shows the convergence of the methodological aspects used to examine the concept of habitability, mainly based on analogy.

Raulin Cerceau, Florence

228

Chemical evolution of circumstellar matter around young stellar objects  

NASA Technical Reports Server (NTRS)

Recent observational studies of the chemical composition of circumstellar matter around both high- and low-mass young stellar objects are reviewed. The molecular abundances are found to be a strong function of evolutionary state, but not of system mass or luminosity. The data are discussed with reference to recent theoretical models.

van Dishoeck, E. F.; Blake, G. A.

1995-01-01

229

Detection of Circumstellar Material in a Normal Type Ia Supernova  

Microsoft Academic Search

Type Ia supernovae are important cosmological distance indicators. Each of these bright supernovae supposedly results from the thermonuclear explosion of a white dwarf star that, after accreting material from a companion star, exceeds some mass limit, but the true nature of the progenitor star system remains controversial. Here we report the spectroscopic detection of circumstellar material in a normal type

F. Patat; P. Chandra; R. Chevalier; S. Justham; Ph. Podsiadlowski; C. Wolf; A. Gal-Yam; L. Pasquini; I. A. Crawford; P. A. Mazzali; A. W. A. Pauldrach; K. Nomoto; S. Benetti; E. Cappellaro; N. Elias-Rosa; W. Hillebrandt; D. C. Leonard; A. Pastorello; A. Renzini; F. Sabbadin; J. D. Simon; M. Turatto

2007-01-01

230

A gallery of cool hypergiants --- imaging their circumstellar environments  

NASA Astrophysics Data System (ADS)

High resolution imaging with HST-WFPC2 of seven of the most luminous evolved cool stars shows that the three OH/IR stars NML Cyg, VX Sgr and S Per have circumstellar nebulae while no ejecta was detected for the M-type supergiant ? Cep and the yellow hypergiants ? Cas, HR 8752 and HR 5171a.

Schuster, Michael T.; Humphreys, Roberta M.; Smith, Nathan; Davidson, Kris; Gehrz, Robert D.

231

Circumstellar Ejecta Surrounding Evolved, Intermediate Mass Stars  

NASA Astrophysics Data System (ADS)

In an attempt to catalogue and understand the outflows from evolved, intermediate-mass stars, three projects concerning stellar nebulae are presented. The first of these is a new method for determining distances to planetary nebulae (PNe) known as the expansion parallax algorithm. The ionized shells of 18 PNe were observed at the Very Large Array in the lambda6 cm continuum during two epochs separated by several (~ 5) years. The data from both epochs were subtracted in the Fourier plane and analyzed to determine the angular expansion rate of the nebulae, which is often very small( ~1 mas/yr). By combining this result with the Doppler expansion rate, the distance to a PN can be computed. Though other methods failed to provide distances to PNe that are smaller than the uncertainties, the results of the expansion parallax method yield errors of ~10-30% for several targets. The ionized gas discussed above was once neutral, when the protoplanetary nebula (PPN) phase reigned. Since their lifetimes are short and their central stars are too obscured to be visible optically, PPNe are rare astronomical objects. The circumstellar shroud surrounding PPNe is neutral in character, and thick enough to support molecules. Defying observations at optical and shorter wavelengths due to the high dust density in the nebula, and lacking any radio emission except from a tiny expanding HII region, PPNe are best observed with a millimeter interferometer tuned to any of the copious molecular emission lines emanating from the nebula. For the second project, the protypical PPN CRL 618, has been imaged with the Owens Valley Radio Interferometer in the 98 GHz (3.1 mm) CS(2-1) transition and in the 115 GHz (2.6 mm) CO(1-0) transition. The CS transition is sensitive to high density material, and the images reveal the marginally resolved core of CRL 618. The more diffuse halo of CRL 618 is probed by the CO gas and exhibits previously undetected structure. The final project described herein involves Fast, Low Ionization Emission Regions (FLIERs), which are localized regions of low ionization atomic gas. FLIERs are found in pairs located along the major axis of several PNe. Because they bear anomalous kinematic and chemical signatures, and because there is no known method of collimation and ejection from a PN nucleus, FLIERs are enigmatic objects. Observations are presented of the FLIERs and similar low ionization microstructures along the major axis of 5 PNe obtained with the Palomar 5-m telescope equipped with the Double Spectrograph. Emission lines in the medium resolution spectra were analyzed with a 5 level model atom to reveal nebular diagnostics and chemical abundances in the emitting gas. Results are presented for several conspicuous positions along the nebular major axis for all targets.

Hajian, Arsen Rosdom

232

7 Habits of Developmental Coaches  

ERIC Educational Resources Information Center

In this article, the authors describe how coaches can apply principles of athlete growth and development to the learning and performance of motor skills. They present 7 habits that lead to well-rounded athletes who experience increased enjoyment, self-motivation, skill improvement, and ultimately more success on the playing field. (Contains 1…

Darden, Gibson; Shimon, Jane

2004-01-01

233

The Hunt for Habitable Exoplanets  

Microsoft Academic Search

Fifteen years after the discovery of planets around other stars, the discovery of rocky, 1 Earth-mass, 1 Earth-radius planets around other stars is imminent. Of particular interest are planets that receive similar amounts of stellar radiation as the Earth, as they could support liquid water, probably the most critical requirement for habitability. Current technology favors the discovery of these planets

Rory Barnes

2010-01-01

234

The Integrative Character of Habits  

Microsoft Academic Search

Having eschewed in various journals the subjectivistic, the introspective, the dualistic, the physiological, and the ultra behavioristic tenets, and still treating of a psychobiological organism, Kantor presents a deductive and rationalistic discussion dealing with habits. Dr. Kantor's method of purifying psychology, by deductive discussion and speculation, has been the greatest deterrent to the fact of an accomplished psychological science. Dr.

J. R. Kantor

1922-01-01

235

[Dietary habits and cardiovascular diseases].  

PubMed

Cardiovascular diseases are a major public health problem worldwide. They are the main cause of death in industrialized countries, while the mortality associated with cardiovascular disease is increasing in less developed countries. The modifiable risk factors for cardiovascular disease are cigarette smoking, hypertension, hyperlipidemia, diabetes mellitus and obesity. Obesity has been recorded in 10%-25% of the population, indicating that poor or inappropriate diet is one of the most common causes of cardiovascular disease. Unhealthy dietary habits including place and way of taking meals, number of daily meals and excessive salt intake from processed foods also contribute to body mass gain. In the present study, dietary habits were assessed in cardiovascular patients versus control group by use of Dietary Habits Questionnaire. Study results showed a statistically significantly higher (P < 0.05) prevalence of inappropriate eating habits in cardiovascular patients (lower number of daily meals, more often skipping breakfast and having dinner) than in control group. In conclusion, many lifestyle and individual behavior modifications are needed in most patients with or at a high risk of cardiovascular disease. PMID:20649073

Nola, Iskra Alexandra; Doko Jelini?, Jagoda; Bergovec, Mijo; Ruzi?, Alen; Persi?, Viktor

2010-05-01

236

Diurnal Habitability of Frozen Worlds  

Microsoft Academic Search

In this work we discuss effects allowing local habitability of some extraterrestrial planets of low average surface temperatures. We analyze the problem of diurnal and seasonal changes of temperature and biological productivity at different locations on a hypothetical Earth-like planet. We have found, that under some circumstances the temperature may locally rise well above the average value, allowing periods of

W. von Bloh; K. J. Kossacki; S. Franck; C. Bounama

2010-01-01

237

Habits—A Repeat Performance  

Microsoft Academic Search

Habits are response dispositions that are activated automatically by the context cues that co-occurred with responses during past performance. Experience-sampling diary studies indicate that much of everyday action is characterized by habitual repetition. We consider various mechanisms that could underlie the habitual control of action, and we conclude that direct cuing and motivated contexts best account for the characteristic features

David T. Neal; Wendy Wood; Jeffrey M. Quinn

2006-01-01

238

Optimizing Spectral Resolution and Observation Time for Measurements of Habitability  

NASA Astrophysics Data System (ADS)

The Terrestrial Planet Finder (TPF) is a NASA mission concept that will attempt to characterize and search for habitability and life on extrasolar planets. While detection of a planet in the habitable zone increases the probability that the planet is habitable, planetary characterization will be required to confirm habitability and thereby test predictions of the position of the habitable zone. The TPF-I mission will accomplish this with an interferometer, allowing the detection of Earth-mass planets around stars up to 15 pc away and production of mid-infrared spectra from those planets. The focus on the mid-infrared region of the spectrum (7-20 mm) is beneficial because this is where energy from Earth-like planets is strongest relative to the flux from their parent stars. To discover if such planets are habitable we need to know not only what to look for - biosignatures and indicators of habitability - but also how to look. In other words, we must determine the trade-off in telescope properties that will provide the best science return. Extensive models have been made of Earth-like planets to describe many planetary properties, including atmospheric chemistry and surface temperature. Those properties may be derived for extrasolar planets using these models if spectra are obtained for the target planet. When modeling a planet, we can calculate a very high-resolution spectrum that can show the detailed absorption features of gases such as CO2, H2O, and O3. However, the telescope resolution will necessarily be limited by low photon fluxes from the distant targets. Alternatively, the telescope could spend more time taking in photons from each target planet. A balance may have to be struck between the numbers of targets observed and the quality of the data obtained for each target. We will present a number of simulations of TPF instrument measurements of terrestrial spectra that parametrize spectral resolution and observation time. The relative errors of these various simulated spectra will be shown, and the results will be analyzed for a “sweet spot” between the exposure time for a planet and the spectral precision and signal to noise required to detect certain biosignatures.

Khalfa, N.; Meadows, V. S.; Domagal-Goldman, S. D.

2009-12-01

239

Rotation rate evolution in habitable super-Earths  

NASA Astrophysics Data System (ADS)

Abstract (2,250 Maximum Characters): The search for habitable planets in our neighborwood is one of the most exiting challenges for planetary sciences at present. The recent discovery of Earth-like planets inside the habitable zone of their host stars is an incentive to try to understand how the physical conditions for habitability evolve in these kind of objects. Super Earths, already discovered, GJ581d and GJ667Cc, are possibly habitable because they are located in a close-in orbit (0.22 AU and 0.125 AU respectively), arround low mass stars. The gravitational interaction between the planets and its host stars produce tides that modify their orbits and their rotation periods. We consider the evolution of the tidal torque and the rotation rate as dependent of the rheological properties of the planet's mantle. Several experiments are performed to study the rotation evolution, depending on the the parameters of the rheological model as well as initial conditions. The main interest goal of our investigation is to understand the spin-orbit evolution produced by the gravitational tides between the host star and a close planet and the impact of the rotation rate variation in the thermal and magnetic evolution of the planet.

Melita, Mario; Cuartas Restrepo, P.; Zuluaga, J.; Miloni, O.

2013-05-01

240

Exomoon habitability constrained by energy flux and orbital stability  

NASA Astrophysics Data System (ADS)

Context. Detecting massive satellites that orbit extrasolar planets has now become feasible, which led naturally to questions about the habitability of exomoons. In a previous study we presented constraints on the habitability of moons from stellar and planetary illumination as well as from tidal heating. Aims: Here I refine our model by including the effect of eclipses on the orbit-averaged illumination. I then apply an analytic approximation for the Hill stability of a satellite to identify the range of stellar and planetary masses in which moons can be habitable. Moons in low-mass stellar systems must orbit their planet very closely to remain bounded, which puts them at risk of strong tidal heating. Methods: I first describe the effect of eclipses on the stellar illumination of satellites. Then I calculate the orbit-averaged energy flux, which includes illumination from the planet and tidal heating to parametrize exomoon habitability as a function of stellar mass, planetary mass, and planet-moon orbital eccentricity. The habitability limit is defined by a scaling relation at which a moon loses its water by the runaway greenhouse process. As a working hypothesis, orbital stability is assumed if the moon's orbital period is less than 1/9 of the planet's orbital period. Results: Due to eclipses, a satellite in a close orbit can experience a reduction in orbit-averaged stellar flux by up to about 6%. The smaller the semi-major axis and the lower the inclination of the moon's orbit, the stronger the reduction. I find a lower mass limit of ? 0.2 M? for exomoon host stars that allows a moon to receive an orbit-averaged stellar flux comparable to the Earth's, with which it can also avoid the runaway greenhouse effect. Precise estimates depend on the satellite's orbital eccentricity. Deleterious effects on exomoon habitability may occur up to ? 0.5 M? if the satellite's eccentricity is ? 0.05. Conclusions: Although the traditional habitable zone lies close to low-mass stars, which allows for many transits of planet-moon binaries within a given observation cycle, resources should not be spent to trace habitable satellites around them. Gravitational perturbations by the close star, another planet, or another satellite induce eccentricities that likely make any moon uninhabitable. Estimates for individual systems require dynamical simulations that include perturbations among all bodies and tidal heating in the satellite.

Heller, R.

2012-09-01

241

Experiences of habit formation: A qualitative study  

Microsoft Academic Search

Habit formation is an important goal for behaviour change interventions because habitual behaviours are elicited automatically and are therefore likely to be maintained. This study documented experiences of habit development in 10 participants enrolled on a weight loss intervention explicitly based on habit-formation principles. Thematic analysis revealed three themes: Strategies used to support initial engagement in a novel behaviour; development

Phillippa Lally; Jane Wardle; Benjamin Gardner

2011-01-01

242

Some effects of morphine on habit function  

Microsoft Academic Search

In 2 experiments the morphine habit is compared with a food habit under various conditions of deprivation and satiation. In contrast to the food habit, both injection of morphine and the deprivation of morphine would appear to produce drive. Continuing effects of morphine may lead to learning without drive reduction. And, an injection of morphine counteracts the effect of food

H. D. Beach

1957-01-01

243

Health Habits of School-Age Children  

Microsoft Academic Search

While health habits such as eating breakfast, maintaining desirable weight, sleeping regularly, and wearing seat belts are related to the longevity of adults, very little is known about the health habits of disadvantaged school-age children. Using data from the 1987 National Medical Expenditure Survey (NMES), this paper examines selected health habits of children between the ages of five and 17

1991-01-01

244

Health Habits of School-Age Children  

Microsoft Academic Search

:While health habits such as eating breakfast, maintaining desirable weight, sleeping regularly, and wearing seat belts are related to the longevity of adults, very little is known about the health habits of disadvantaged school-age children. Using data from the 1987 National Medical Expenditure Survey (NMES), this paper examines selected health habits of children between the ages of five and 17

Llewellyn J. Cornelius

1991-01-01

245

HABIT FORMATION, DYNASTIC ALTRUISM, AND POPULATION DYNAMICS  

Microsoft Academic Search

We study the general equilibrium properties of two growth models with overlapping generations, habit formation and endogenous fertility. In the neoclassical model, habits modify the economy's growth rate and generate transitional dynamics in fertility; station- ary income per capita is associated with either increasing or decreasing population and output, depending on the strength of habits. In the AK speci…cation, growing

Andreas Schäfer; Simone Valentey

2010-01-01

246

Habitability potential of icy moons: a comparative study  

NASA Astrophysics Data System (ADS)

Looking for habitable conditions in the outer solar system our research focuses on the natural satellites rather than the planets themselves. Indeed, the habitable zone as traditionally defined may be larger than originally con-ceived. The strong gravitational pull caused by the giant planets may produce enough energy to sufficiently heat the interiors of orbiting icy moons. The outer solar system satellites then provide a conceptual basis within which new theories for understanding habitability can be constructed. Measurements from the ground but also by the Voyager, Galileo and the Cassini spacecrafts revealed the potential of these satellites in this context, and our understanding of habitability in the solar system and beyond can be greatly enhanced by investigating several of these bodies together [1]. Their environments seem to satisfy many of the "classical" criteria for habitability (liquid water, energy sources to sustain metabolism and chemical compounds that can be used as nutrients over a period of time long enough to allow the development of life). Indeed, several of the moons show promising conditions for habitability and the de-velopment and/or maintenance of life. Europa, Callisto and Ganymede may be hiding, under their icy crust, putative undersurface liquid water oceans [3] which, in the case of Europa [2], may be in direct contact with a silicate mantle floor and kept warm by tidally generated heat [4]. Titan and Enceladus, Saturn's satellites, were found by the Cassini-Huygens mission to possess active organic chemistries with seasonal variations, unique geological features and possibly internal liquid water oceans. Titan's rigid crust and the probable existence of a subsurface ocean create an analogy with terrestrial-type plate tectonics, at least surficial [5], while Enceladus' plumes find an analogue in gey-sers. As revealed by Cassini the liquid hydrocarbon lakes [6] distributed mainly at polar latitudes on Titan are ideal isolated environments to look for biomarkers. Currently, for Titan and Enceladus, geophysical models try to explain the possible existence of an oceanic layer that decouples the mantle from the icy crust. If the silicate mantles of Eu-ropa and Ganymede and the liquid sources of Titan and Enceladus are geologically active as on Earth, giving rise to the equivalent of hydrothermal systems, the simultaneous presence of water, geodynamic interactions, chemical en-ergy sources and a diversity of key chemical elements may fulfill the basic conditions for habitability. Titan has been suggested to be a possible cryovolcanic world due to the presence of local complex volcanic-like geomorphol-ogy and the indications of surface albedo changes with time [7,8]. Such dynamic activity that would most probably include tidal heating, possible internal convection, and ice tectonics, is believed to be a pre-requisite of a habitable planetary body as it allows the recycling of minerals and potential nutrients and provides localized energy sources. In a recent study by Sohl et al. [2013], we have shown that tidal forces are a constant and significant source of inter-nal deformation on Titan and the interior liquid water ocean can be relatively warm for reasonable amounts of am-monia concentrations, thus completing the set of parameters needed for a truly habitable planetary body. Such habi-tability indications from bodies at distances of 10 AU, are essential discoveries brought to us by space exploration and which have recently revolutionized our perception of habitability in the solar system. In the solar system's neighborhood, such potential habitats can only be investigated with appropriate designed space missions, like JUICE-Laplace (JUpiter ICy moon Explorer) for Ganymede and Europa [9]. JUICE is an ESA mission to Jupiter and its icy moons, recently selected to launch in 2022. References: [1] Coustenis, A., Encrenaz, Th., in "Life Beyond Earth : the search for habitable worlds in the Universe", Cambridge Univ. Press, 2013. [2] Patterson, G.W., et al.: AGU P41F-09, 2011. [3]

Solomonidou, Anezina; Coustenis, Athena; Encrenaz, Thérèse; Sohl, Frank; Hussmann, Hauke; Bampasidis, Georgios; Wagner, Frank; Raulin, François; Schulze-Makuch, Dirk; Lopes, Rosaly

2014-05-01

247

Trace Element Condensation in Circumstellar Envelopes of Carbon Stars  

NASA Astrophysics Data System (ADS)

It is now well established that meteorites contain reduced presolar grains, such as graphite and silicon carbide (SiC), which are probably formed by condensation of dust in the circumstellar envelopes of carbon-rich AGB stars. Here we model condensation in envelopes of carbon stars, with an emphasis on trace elements. Since absolute elemental abundances in stellar atmospheres are generally not known, we assume solar abundances (Anders and Grevesse 1989), except for carbon. A C/O ratio of 2, consistent with the mean and median values of 2.1 and 1.8 respectively, for 61 carbon stars (Gow 1977) was used. The C/O ratio was increased by adding carbon because astrophysicists believe that carbon produced in helium-burning zones may be mixed to the surfaces of C stars (e.g. Lucy 1976). We used physical parameters for the circumstellar shell of the high mass-loss rate, prototypical carbon star IRC +10216 (e.g. Keady et al. 1988, Dominik et al. 1990) and theoretical considerations by Salpeter (1974a,b) to construct a P-T-model of the envelope (see Fig. 1). Thermodynamic equilibrium condensation calculations for a reduced gas include ~600 gaseous and solid compounds of the elements H, C, N, O, S, P, F, Cl, Fe, Mg, Al, Ti, Si, Ca, Zr, Hf, V, Nb, Ta, Cr, Mo, W, and REE. Refractory oxides, sulfides, nitrides, and carbides were considered as condensates. The calculations were done from T = 800 to 2500 K, and P= 10^-5 to 10^-13 bars. The effects of nucleation on condensation temperatures were calculated using the nucleation model discussed by Salpeter (1974a,b) and Cameron and Fegley (1982). The temperature drop required for condensation depends on (P,T, density) in the expanding envelope and also on the abundance, density, and surface energy (Es) of the nucleating compound. The range of E(sub)s values for NaCl-type carbides are about 800-1700 erg/cm^2 (Livey & Murray 1956); however, these data are generally poorly known. Another important variable is the sticking coefficient (s), taken = 10^-3 here. Results of the equilibrium condensation calculations as a function of pressure at C/O = 2 are shown in Table 1 and Fig. 1. The initial major element condensates are graphite, TiC, SiC, Fe3C, AlN, and CaS (Table 1). The data for C(sub)GR TiC, and SiC are also shown in Fig. 1, together with the P-T profile for the carbon star IRC +10216. Also included are the condensation temperatures if nucleation constraints are applied (dotted lines). Neglecting nucleation effects, C(sub)Gr, TiC, and SiC would be present within 2-3 stellar radii from the photosphere (r/R = 1). With nucleation constraints, TiC and SiC form at lower T at a distance of about 5 stellar radii. The T-drop required for graphite condensation is only about 100 K lower than the equilibrium condensation temperatures at higher P. Therefore, graphite grains would be stable at r/R >1.5. We note that at r = 3-5 R there is observational evidence for SiC, graphite and amorphous carbon in the envelope of the C star IRC +10216 (e.g. Keady et al. 1988, Ridgway and Keady 1988). Of the nitrides, AlN is the only which forms initially. Because of its structural similarity to SiC and TiC one could expect formation of AlN solid solutions with NaCl-type carbides. Most trace elements initially form carbides. The most refractory carbides are TaC, WC, NbC, ZrC, and HfC, condensing about 100-250 K higher than TiC. E(sub)s data are available for TaC and ZrC. Nucleation constraints show that only ZrC would form prior to TiC. Other trace element carbides (Mo(sub)2C, MoC, VC(sub)0.88, YC(sub)2, Cr(sub)3C(sub)2) condense as pure compounds below the equilibrium condensation temperatures of C(sub)Gr, TiC, and SiC. However, they may condense in solid solution in TiC or SiC or in both if allowed by their crystal structures. In any case, nitrides are not initial condensates for these trace elements. However, because the carbides and nitrides can form solid solutions, one could expect carbide-nitride solutions. References: Anders E. and Grevesse N. (1989) Geochim. Cosmochim. Acta 53, 197-214. Cameron A.G.W

Lodders, K.; Fegley, B., Jr.

1992-07-01

248

Circumstellar shells resolved in IRAS survey data. II - Analysis  

NASA Technical Reports Server (NTRS)

IRAS survey data for 512 red giant stars and young planetary nebulae were processed using a computer program which fitted the data to an idealized model of a circumstellar shell. Seventy-six of these stars were found to have circumstellar shells resolved in the 60-micron survey data. Forty pct of the 76 stars are carbon stars. Thirteen are Mira variables. The evolution of these shells, involving the interaction of the expelled material with the ISM, is modeled, and the results suggest that the period during which Mira variables lose mass lasts for approximately 10 exp 5 yr. Carbon stars are found to shed mass for about 2 x 10 exp 5 yr. The expansion velocity of the outer shell for the largest shells will normally be lower by a factor of 3-5 than the expansion velocity obtained from CO observations.

Young, K.; Phillips, T. G.; Knapp, G. R.

1993-01-01

249

CIRCUMSTELLAR ABSORPTION IN DOUBLE DETONATION TYPE Ia SUPERNOVAE  

SciTech Connect

Upon formation, degenerate He core white dwarfs are surrounded by a radiative H-rich layer primarily supported by ideal gas pressure. In this Letter, we examine the effect of this H-rich layer on mass transfer in He+C/O double white dwarf binaries that will eventually merge and possibly yield a Type Ia supernova (SN Ia) in the double detonation scenario. Because its thermal profile and equation of state differ from the underlying He core, the H-rich layer is transferred stably onto the C/O white dwarf prior to the He core's tidal disruption. We find that this material is ejected from the binary system and sweeps up the surrounding interstellar medium hundreds to thousands of years before the SN Ia. The close match between the resulting circumstellar medium profiles and values inferred from recent observations of circumstellar absorption in SNe Ia gives further credence to the resurgent double detonation scenario.

Shen, Ken J. [Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 (United States); Guillochon, James [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States); Foley, Ryan J., E-mail: kenshen@astro.berkeley.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

2013-06-20

250

Investigating the Faraday Rotation Morphologies of Circumstellar and Interstellar Bubbles  

NASA Astrophysics Data System (ADS)

We compute model polarization position angle maps for the effect of Faraday rotation for circumstellar and interstellar bubbles. In the circumstellar case, we consider spherically symmetric bubbles, as examples of wind-blown bubbles, with either azimuthal or split monopole magnetic fields. In the case of an azimuthal field, the polarization position angle maps are antisymmetric about the field axis, because Faraday rotation is sensitive to whether the line-of-sight magnetic field points toward or away from the observer. By contrast a split monopole field produces a map of only one sign for the polarization position angle. We also consider Faraday rotation as a probe of an interstellar magnetic field. In this case a supernova explosion is considered to provide an ionization front that moves through the interstellar medium. We explore how a turbulent magnetic field affects the polarization position angle maps. Our results indicate that it is mainly the largest turbulent scales that are relevant for Faraday rotation.

Pingel, N. M.; Ignace, R.

2012-12-01

251

Preliminary Results from a VLBA Circumstellar Water Maser Monitoring Project  

NASA Astrophysics Data System (ADS)

We report on the first epoch of observations of an ongoing circumstellar water maser monitoring project. The stars included in our program are S Perseus, U Herculis, VY Canis Majoris, VX Sagittarius, RX Bootis, IK Taurus and NML Cygnus. We present the spatial and frequency distributions of the masers. Continued observations are underway. We also present initial models of the spatial distribution of the masing regions and the physical environments in the circumstellar shell. We hope to derive the distance to these stars using the expanding cluster technique. Concurrent observations of the OH and SiO maser distributions are also presented. This work is part of Marvel's dissertation research which will be completed in June 1996.

Marvel, K.; Diamond, P.

1994-12-01

252

Proper Motions of Water Masers in Circumstellar Shells  

NASA Astrophysics Data System (ADS)

We present proper motion measurements of circumstellar water masers obtained with the VLBA. The objects observed include S Persei, VX Sagittarii, U Herculis, VY Canis Majoris, NML Cygni, IK Tauri and RX Bootis. Results of the observations and modeling indicate that the water masers exist in a kinematically complex region of the circumstellar envelope, which is not well fit by the standard model of a uniformly expanding spherical wind. Attempts at fitting an ellipsoidal geometric distribution with a variety of kinematic models are presented. Estimates for the distances of the stars are also discussed. A change in position of the maser spots as a function of velocity has been measured. This effect may be used to place limits on accelerations in the masing gas.

Marvel, K. B.; Diamond, P. J.; Kemball, A. J.

253

Circumstellar CO Emission of Evolved Stars and Mass Loss  

NASA Astrophysics Data System (ADS)

We have calculated the CO J = 1 ? 0 and 2 ? 1 emission of circumstellar molecular envelopes, allowing the physical parameters, namely, the mass-loss rate, the CO abundance, the distance to the envelope, and the parameter specifying the amount of gas-dust collisional heating, to take on values over wide ranges. From these results we establish two relations between the intensity and shape of the J = 1 ? 0 or 2 ? 1 line and the above-mentioned variables. We have also examined the effects of CO dissociation by the interstellar UV field, the presence of an appreciable velocity gradient in the gas flow, and the 4.6 microns continuum flux level. We apply the two relations to the circumstellar envelopes IRC+10216, ? Cyg, and U Cam.

Kwan, John; Webster, Zoe

1993-12-01

254

Cluster headache and lifestyle habits  

Microsoft Academic Search

Cluster headache (CH) has traditionally been associated with certain anthropometric features, personality traits, and lifestyle\\u000a features. This article focuses on lifestyle features in patients with CH. Especially excessive smoking and alcohol consumption\\u000a have been ascribed to patients with CH. Despite country-specific habits and a time trend, smoking is much more prevalent among\\u000a CH patients compared with the general population. Although

Markus Schürks; Hans-Christoph Diener

2008-01-01

255

Diurnal Habitability of Frozen Worlds  

Microsoft Academic Search

In this work we discuss effects allowing local habitability of some extraterrestrial planets of low average surface temperatures.\\u000a We analyze the problem of diurnal and seasonal changes of temperature and biological productivity at different locations on\\u000a a hypothetical Earth-like planet. We have found, that under some circumstances the temperature may locally rise well above\\u000a the average value, allowing periods of

W. von Bloh; K. J. Kossacki; S. Franck; C. Bounama

2010-01-01

256

What makes a planet habitable?  

Microsoft Academic Search

This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the\\u000a host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss\\u000a the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological\\u000a time scales and for planets

H. Lammer; J. H. Bredehöft; A. Coustenis; M. L. Khodachenko; L. Kaltenegger; O. Grasset; D. Prieur; F. Raulin; P. Ehrenfreund; M. Yamauchi; J.-E. Wahlund; J.-M. Grießmeier; G. Stangl; C. S. Cockell; Yu. N. Kulikov; J. L. Grenfell; H. Rauer

2009-01-01

257

The compulsive habit of cars.  

PubMed

The car dependence of people living in contemporary cities is a major concern for policy makers, who often find it difficult to persuade people into more sustainable transport modes. By contrast, recent insights from neuroscience have shown that a broad spectrum of behaviors can become habitual and, thus, resistant to change. Here, we outline the potential of collaboration between neuroscience and human geography aiming at a better understanding of habits that determine everyday commuting routines. PMID:24767180

Yalachkov, Yavor; Naumer, Marcus J; Plyushteva, Anna

2014-05-01

258

Widen the belt of habitability!  

PubMed

Among the key-parameters to characterize habitability are presence or availability of liquid water, an appropriate temperature range, and the time scale of reference. These criteria for habitability are discussed and described from the point of view of water- and ice-physics, and it is shown that liquid water may exist in the sub-surfaces of planetary bodies like Mars, and possibly of inner asteroids and internally heated ice-moons. Water can remain fluid there also at temperatures far below the "canonical" 0 °C. This behaviour is made possible as a consequence of the freezing point depression due to salty solutes in water or "brines", as they can be expected to exist in nature more frequently than pure liquid water. On the other hand, low temperatures cause a slowing down of chemical processes, as can be described by Arrhenius's relation. The resulting smaller reaction rates probably will have the consequence to complicate the detection of low-temperature life processes, if they exist. Furthermore, the adaptation potential of life is to be mentioned in this context as a yet partially unknown process. Resulting recommendations are given to improve the use of criteria to characterize habitable conditions. PMID:22638839

Möhlmann, D

2012-06-01

259

NICMOS Coronagraphic Observations of Circumstellar Environments - Preliminary Results  

NASA Astrophysics Data System (ADS)

The NICMOS camera 2 coronagraph provides a unique instrumental resource for exploring circumstellar environments. The NICMOS Environments of Nearby Stars (EONS) programs recently commenced following the completion of the commissioning of the coronagraph in February, 1998. Under those programs we are conducting systematic surveys of nearby, and young stars searching for companion brown dwarfs and giant planets, and protoplanetary disks around main-sequence stars. We present the status of these and related programs along with a summary of preliminary results.

Schneider, G.; Thompson, R.; Becklin, E.; Smith, B.; Terrile, R.

1998-05-01

260

NASA plans relevant to the study of circumstellar matter  

NASA Astrophysics Data System (ADS)

The Astrophysics program of the National Aeronautics and Space Administration of the United States emphasizes use of vehicles to obtain above-the-atmosphere observational advantages, including expanded electromagnetic frequency access, enhanced sensitivity resulting from reduced or eliminated atmospheric absorption of light and image smearing. Space technology provides a superior means for astrophysical inquiry, particularly in the case of circumstellar material. Much of the flight program is undergoing intensive review following the Space Shuttle disaster of January 1986.

Stencel, Robert E.

261

Structure and Evolution of Pre-main-sequence Circumstellar Disks  

Microsoft Academic Search

We present new subarcsecond (~0farcs7) Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of the 1.3 mm continuum emission from circumstellar disks around 11 low- and intermediate-mass pre-main-sequence stars. High-resolution observations for three additional sources were obtained from the literature. In all cases the disk emission is spatially resolved. We adopt a self-consistent accretion disk model based on the

Andrea Isella; John M. Carpenter; Anneila I. Sargent

2009-01-01

262

Properties of Circumstellar Envelopes of Be Stars from IR lines  

NASA Astrophysics Data System (ADS)

We have analyzed the formation of Hydrogen IR emission lines (Brackett, Pfund and Humphreys series) in Be stars on the basis of expanding extended atmosphere models with different temperature laws. We compared our calculated lines with IR spectroscopic observations in the L-band performed in the ESO Observatory Cerro Paranal, Chile. This allowed us to reveal physical characteristics of the circumstellar envelope and test possible departures of radiative equilibrium.

Granada, A.; Sabogal, B.; Cidale, L. S.; Mennickent, R.

2006-08-01

263

Condensation of Grains in Circumstellar Shells of AGB Stars  

Microsoft Academic Search

I review the gas and condensation chemistry of major (e.g., C, N, O, Si, Mg, Fe, Al, Ca, Ti, Cr, Mn, Na, etc.) and trace (notably s-process) elements in circumstellar shells (CSs) of red giant stars as a function of temperature, total pressure (Ptot)), C\\/O ratio, and metallicity (e.g., [1-4]). Several expected minerals containing the major abundant elements have been

K. Lodders

2006-01-01

264

Observations of Circumstellar Thermochemical Equilibrium: The Case of Phosphorus  

NASA Technical Reports Server (NTRS)

We will present observations of phosphorus-bearing species in circumstellar envelopes, including carbon- and oxygen-rich shells 1. New models of thermochemical equilibrium chemistry have been developed to interpret, and constrained by these data. These calculations will also be presented and compared to the numerous P-bearing species already observed in evolved stars. Predictions for other viable species will be made for observations with Herschel and ALMA.

Milam, Stefanie N.; Charnley, Steven B.

2011-01-01

265

Habitability of extrasolar planets and tidal spin evolution.  

PubMed

Stellar radiation has conservatively been used as the key constraint to planetary habitability. We review here the effects of tides, exerted by the host star on the planet, on the evolution of the planetary spin. Tides initially drive the rotation period and the orientation of the rotation axis into an equilibrium state but do not necessarily lead to synchronous rotation. As tides also circularize the orbit, eventually the rotation period does equal the orbital period and one hemisphere will be permanently irradiated by the star. Furthermore, the rotational axis will become perpendicular to the orbit, i.e. the planetary surface will not experience seasonal variations of the insolation. We illustrate here how tides alter the spins of planets in the traditional habitable zone. As an example, we show that, neglecting perturbations due to other companions, the Super-Earth Gl581d performs two rotations per orbit and that any primordial obliquity has been eroded. PMID:22139513

Heller, René; Barnes, Rory; Leconte, Jérémy

2011-12-01

266

The chemical history of molecules in circumstellar disks. II. Gas-phase species  

NASA Astrophysics Data System (ADS)

Context. The chemical composition of a molecular cloud changes dramatically as it collapses to form a low-mass protostar and circumstellar disk. Two-dimensional (2D) chemodynamical models are required to properly study this process. Aims: The goal of this work is to follow, for the first time, the chemical evolution in two dimensions all the way from a pre-stellar core into a circumstellar disk. Of special interest is the question whether the chemical composition of the disk is a result of chemical processing during the collapse phase, or whether it is determined by in situ processing after the disk has formed. Methods: Our model combines a semi-analytical method to get 2D axisymmetric density and velocity structures with detailed radiative transfer calculations to get temperature profiles and UV fluxes. Material is followed in from the core to the disk and a full gas-phase chemistry network - including freeze-out onto and evaporation from cold dust grains - is evolved along these trajectories. The abundances thus obtained are compared to the results from a static disk model and to observations of comets. Results: The chemistry during the collapse phase is dominated by a few key processes, such as the evaporation of CO or the photodissociation of H2O. Depending on the physical conditions encountered along specific trajectories, some of these processes are absent. At the end of the collapse phase, the disk can be divided into zones with different chemical histories. The disk is not in chemical equilibrium at the end of the collapse, so care must be taken when choosing the initial abundances for stand-alone disk chemistry models. Our model results imply that comets must be formed from material with different chemical histories: some of it is strongly processed, some of it remains pristine. Variations between individual comets are possible if they formed at different positions or different times in the solar nebula.

Visser, R.; Doty, S. D.; van Dishoeck, E. F.

2011-10-01

267

Exoplanet Searches in the Habitable Zone with Gravitational Microlensing  

NASA Astrophysics Data System (ADS)

There are different methods for finding exoplanets such as radial spectral shifts, astrometrical measurements, transits, timing, etc. Gravitational microlensing (including pixel-lensing) is among the most promising techniques with the potential of detecting Earth-like planets at distances about a few astronomical units from their host stars. Here we emphasize the importance of polarization measurements which can help to resolve degeneracies in theoretical models. In particular, the polarization angle could give additional information about the relative position of the lens with respect to the source.

Zakharov, Alexander F.; Ingrosso, Gabriele; De Paolis, Francesco; Nucita, Achille A.; Strafella, Francesco; Novati, Sebastiano Calchi; Jetzer, Philippe

2014-04-01

268

Europa, tidally heated oceans, and habitable zones around giant planets  

Microsoft Academic Search

Tidal dissipation in the satellites of a giant planet may provide sufficient heating to maintain an environment favorable to life on the satellite surface or just below a thin ice layer. Europa could have a liquid ocean which may occasionally receive sunlight through cracks in the overlying ice shell. In such a case, sufficient solar energy could reach liquid water

R. T. Reynolds; C. P. McKay; J. F. Kasting

1987-01-01

269

Additional constraints on circumstellar disks in the Trapezium Cluster  

NASA Technical Reports Server (NTRS)

We discuss the new constraints on the population of compact ionized sources in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequence stars. We present new Hubble Space Telescope (HST) Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around them. One source shows a small-scale (greater than or approximately = 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al. We show that the circumstellar disk model is the most likely one for the majority of sources, although is remains plausible that some of the larger objects could be equilibrium globules. We combine the most complete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associated with a circumstellar disk. Our estimate of 25%-75% is comparable to that found for pre-main sequence (PMS) stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastically reduced the frequency of disks seen around PMS stars.

Stauffer, John R.; Prosser, Charles F.; Hartmann, Lee; Mccaughrean, Mark J.

1994-01-01

270

Additional constraints on circumstellar disks in the Trapezium Cluster  

NASA Technical Reports Server (NTRS)

We discuss new constraints on the population of compact ionized sources in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequence stars. We present new HST Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around them. One source shows a small-scale (greater than 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al. We show that the circumstellar disk model is the most likely one for the majority of sources, although it remains plausible that some of the larger objects could be equilibrium globules. We combine the most complete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associated with a circumstellar disk. Our estimate of 25-75 percent is comparable to that found for PMS stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastically reduced the frequency of disks seen around pre-main sequence stars.

Stauffer, John R.; Prosser, Charles F.; Hartmann, Lee; Mccaughrean, Mark J.

1994-01-01

271

On the Crystallization of Small Silica Particles in Circumstellar Environments  

NASA Astrophysics Data System (ADS)

Infrared Space Observatory (ISO) observations have revealed the presence of crystalline silicate dust in circumstellar environments of some evolved stars (e.g. Waters et al. 1996, A&A 315, L361). Molster et al. (2001, A&A 366, 923), for instance, reported the discovery of a carbon-rich AGB star surrounded by a highly crystalline silicate dust shell as indicated by the high resolution ISO-SWS spectrum. However, it is still a matter of debate, whether the presence of crystalline dust is restricted only to certain phases of the stellar evolution (e.g. Kemper et al. 2000, A&A 369, 132). In order to understand the process, which leads to the formation of crystalline structures in circumstellar environments, the microphysical rearrangement of small silica particles is investigated by means of molecular dynamic calculations. The results indicate a dynamic coexistence of `amorphous'-like and symmetrical, `crystalline' structures. Such rearrangement processes depend on the energy transfer to the grain governed by the physical conditions of the astrophysical environment. Therefore, the effect of the degree of crystallinity on optical properties is additionally exemplified by Mie calculations. Some implications regarding the condensation and the mineralogy of silicate dust particles in the circumstellar environments of AGB and post-AGB objects are discussed.

John, M.; Müller, E.; Patzer, B.; Lüttke, M.; Sedlmayr, E.

272

AGB/PPN/PN circumstellar rings vs. spiral  

NASA Astrophysics Data System (ADS)

With the advent of high-resolution high-sensitivity observations, spiral patterns have been revealed around several asymptotic giant branch (AGB) stars. Such patterns can provide possible evidence for the existence of central binary stars embedded in outflowing circumstellar envelopes. It is, however, not generally recognized that the binary-induced pattern, vertically extended from the orbital plane, exhibits a ring-like pattern with an inclined viewing angle. I will first review the binary-induced spiral-shell patterns on the AGB circumstellar envelopes with the effect of inclination angle with respect to the orbital plane, of which large inclination cases reveal incomplete ring-like patterns. I will describe a method of extracting such spiral-shell from the gas kinematics of an incomplete ring-like pattern to place constraints on the characteristics of the (unknown) central binary stars. This first success may open the possibility of connecting the ring-like patterns commonly found in the AGB circumstellar envelopes and in the outer parts of (pre-)planetary nebulae and pointing to the conceivable presence of central binary systems, which may give a clue for the onset of asymmetrical planetary nebulae.

Kim, H.; Taam, R. E.; Liu, S.-Y.; Hsieh, I.-T.

2014-04-01

273

The Circumstellar Environments of NML Cygni and the Cool Hypergiants  

NASA Astrophysics Data System (ADS)

We present high-resolution HST WFPC2 images of compact nebulosity surrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. The powerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetric nebula that is coincident with the distribution of its H2O vapor masers. We show that NML Cyg's circumstellar envelope is likely shaped by photodissociation from the powerful, nearby association Cyg OB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Per have marginally resolved envelopes. S Per's circumstellar nebula appears elongated in a northeast/southwest orientation similar to that for its OH and H2O masers, while VX Sgr is embedded in a spheroidal envelope. We find no evidence for circumstellar nebulosity around the intermediate-type hypergiants ? Cas, HR 8752, and HR 5171a, nor around the normal M-type supergiant ? Cep. We conclude that there is no evidence for high mass loss events prior to 500-1000 years ago for these four stars.

Schuster, Michael T.; Humphreys, Roberta M.; Marengo, Massimo

2006-01-01

274

Habitability: From solar system planets to Earth-like exoplanets  

NASA Astrophysics Data System (ADS)

For understanding the principles that generated Earth's long-time habitable environment compared with other terrestrial Solar System planets like Venus and Mars and terrestrial exoplanets inside the habitable zones of late-type stars, one has to understand the evolutionary influence of the solar/stellar radiation and particle environment to the atmosphere and surface. Because the spectral type of the star plays a major role in the photochemistry and evolution of planetary atmospheres and their water inventories must be understood within the context of the evolving stellar energy and particle fluxes. An only stable and dense enough atmosphere, which allows water to be liquid over geological time periods and protects the planetary surface from hostile radiation, will allow the evolution of surface life and Earth-like biospheres. Such long-time habitable environments are ideal cases of course, but life may have also originated in other habitats in the young Solar System. Such habitats could have been the environments of early Venus and Mars, subsurface (during formation maybe surface) oceans of icy satellites like in Europa, Titan, Encleadus, in hydrocarbon lakes of Titan, etc. Therefore, studies related to habitability and comparative planetology in the Solar System are essential for precursor studies dedicated to the investigation of habitability of terrestrial exoplanets.With comparative planetology one means the investigation on how different planetary systems and their individual planets - and particularly Earth-like ones - are formed, how they evolve in their radiation and particle interaction with their host stars under different circumstances, how often they give rise to conditions that could in principle be benevolent enough for the origin of life to occur, and even whether life as we know it could have arisen on any world in Earth's neighborhood. The presentation will point out the synergy of these studies and latest theoretical models currently applied for Solar System planets to terrestrial exoplanet atmospheres, which will be characterized by terrestrial planet finding missions like Darwin.

Lammer, H.

2007-08-01

275

A model of habitability within the Milky Way galaxy.  

PubMed

We present a model of the galactic habitable zone (GHZ), described in terms of the spatial and temporal dimensions of the Galaxy that may favor the development of complex life. The Milky Way galaxy was modeled using a computational approach by populating stars and their planetary systems on an individual basis by employing Monte Carlo methods. We began with well-established properties of the disk of the Milky Way, such as the stellar number density distribution, the initial mass function, the star formation history, and the metallicity gradient as a function of radial position and time. We varied some of these properties and created four models to test the sensitivity of our assumptions. To assess habitability on the galactic scale, we modeled supernova rates, planet formation, and the time required for complex life to evolve. Our study has improved on other literature on the GHZ by populating stars on an individual basis and modeling Type II supernova (SNII) and Type Ia supernova (SNIa) sterilizations by selecting their progenitors from within this preexisting stellar population. Furthermore, we considered habitability on tidally locked and non-tidally locked planets separately and studied habitability as a function of height above and below the galactic midplane. In the model that most accurately reproduces the properties of the Galaxy, the results indicate that an individual SNIa is ?5.6× more lethal than an individual SNII on average. In addition, we predict that ?1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy. Of those stars with a habitable planet, ?75% of planets are predicted to be in a tidally locked configuration with their host star. The majority of these planets that may support complex life are found toward the inner Galaxy, distributed within, and significantly above and below, the galactic midplane. PMID:22059554

Gowanlock, M G; Patton, D R; McConnell, S M

2011-11-01

276

Insensitivity of weathering behavior to planetary land fraction and effect on habitability  

NASA Astrophysics Data System (ADS)

It is likely that an increasing number of terrestrial planets of unknown water content will soon be discovered in the habitable zone of their stars. Planetary surface land fraction may, however, influence the functioning of the silicate weathering feedback, which buffers planetary surface climate against changes in stellar luminosity over a star's lifetime. It is therefore worthwhile to consider the effect of land fraction on the planetary carbon cycle and weathering behavior in a general sense. Here a low-order model of weathering and climate is developed that includes both continental silicate weathering and seafloor weathering. This model can be used to gain an intuitive sense of the behavior of terrestrial planets with different land fractions in the habitable zone of main-sequence stars as their star's insolation changes with time. It is found that, as long as seafloor weathering is independent of surface temperature, there can be no weathering feedback on a waterworld. This means that the tenure of a waterworld in the habitable zone (before it undergoes a moist greenhouse) is likely to be much shorter than that of a planet with some land fraction. The silicate weathering feedback, however, is effective even at very low land fractions. A planet with a land fraction of 0.01 should remain in the habitable zone nearly as long as a planet with a land fraction of 0.3. Finally, by comparing the timescale for water loss to space to the weathering timescale, it is found that it is possible for a waterworld to draw down atmospheric CO2 quickly enough as a moist greenhouse is in progress to prevent complete loss of all water. This would imply that waterworlds in the habitable zone of main sequence stars can go through a moist greenhouse stage and end up as planets like Earth with only partial ocean coverage and a habitable climate.

Abbot, D. S.; Archer, D.; Pierrehumbert, R. T.; Ciesla, F. J.; Bean, J. L.

2012-04-01

277

Maintenance of permeable habitable subsurface environments by earthquakes and tidal stresses  

NASA Astrophysics Data System (ADS)

Life inhabits the subsurface of the Earth down to depths where temperature precludes it. Similar conditions are likely to exist within the traditional habitable zone for objects between 0.1 Earth mass (Mars) and 10 Earth masses (superearth). Long-term cooling and internal radioactivity maintain surface heat flow on the Earth. These heat sources are comparable and likely to be comparable in general within old rocky planets. Surface heat flow scales with mass divided by surface area and hence with surface gravity. The average absolute habitable subsurface thickness scales inversely with heat flow and gravity. Surface gravity varies by only 0.4 g for Mars to 3.15 g for a superearth. This range is less than the regional variation of heat flow on the Earth. Still ocean-boiling asteroid impacts (if they occur) are more likely to sterilize the thin habitable subsurface of large objects than thick habitable subsurface of small ones. Tectonics self-organizes to maintain subsurface permeability and habitability within both stable and active regions on the Earth. Small earthquakes within stable regions allow sudden mixing of water masses. Large earthquakes at plate boundaries allow surface water to descend to great habitable depths. Seismic shaking near major faults cracks shallow rock forming permeable regolith. Strong tidal strains form a similar porous regolith on small bodies such as Enceladus with weak stellar heating. This regolith may be water-saturated within rocky bodies and thus habitable.

Sleep, Norman H.

2012-10-01

278

Habitability  

NASA Video Gallery

Students analyze physical processes that occur on Earth and Mars and compare differences on how particular similar physical features occur. Students will use planetary comparisons in understanding ...

279

NASA contributions to the global habitability program  

NASA Technical Reports Server (NTRS)

As a result of developments occurring over the last two decades, the data acquisition, storage, analysis, and transmission facilities are now available for a concerted long-term interdisciplinary and international study of the global environmental system. Such a study is the essence of the 'Global Habitability' concept introduced in 1982. The aims of Global Habitability research are considered, taking into account an understanding of the vital global processes of the earth's energy balance, the global hydrological cycle, and the biogeochemical cycling of carbon, nitrogen, phosphorus, and sulfur. Details of NASA planning for Global Habitability are discussed along with international data exchange arrangements. Attention is given to the possible contributions of satellite data and associated techniques to Global Habitability, examples of specific research conducted by NASA in support of the Global Habitability and the international sharing of data and results for Global Habitability.

Mcconnell, D. G.

1984-01-01

280

Human factor design of habitable space facilities  

NASA Technical Reports Server (NTRS)

Current fundamental and applied habitability research conducted as part of the U.S. space program is reviewed with emphasis on methods, findings, and applications of the results to the planning and design of the International Space Station. The discussion covers the following six concurrent directions of habitability research: operational simulation, functional interior decor research, space crew privacy requirements, interior layout and configuration analysis, human spatial habitability model, and analogous environments research.

Clearwater, Yvonne A.

1987-01-01

281

Habit Breaking Appliance for Multiple Corrections  

PubMed Central

Tongue thrusting and thumb sucking are the most commonly seen oral habits which act as the major etiological factors in the development of dental malocclusion. This case report describes a fixed habit correcting appliance, Hybrid Habit Correcting Appliance (HHCA), designed to eliminate these habits. This hybrid appliance is effective in less compliant patients and if desired can be used along with the fixed orthodontic appliance. Its components can act as mechanical restrainers and muscle retraining devices. It is also effective in cases with mild posterior crossbites.

Abraham, Reji; Kamath, Geetha; Sodhi, Jasmeet Singh; Sodhi, Sonia; Rita, Chandki; Sai Kalyan, S.

2013-01-01

282

Habitability on planetary surfaces: interdisciplinary preparation phase for future Mars missions  

Microsoft Academic Search

Life on Earth is one of the outcomes of the formation and evolution of our solar system and has adapted to every explored environment on planet Earth. Recent discoveries have shown that life can exist in extreme environments, such as hydrothermal vents, in deserts and in ice lakes in Antarctica. These findings challenge the definition of the `planetary habitable zone'.

Z. Peeters; R. Quinn; Z. Martins; M. A. Sephton; L. Becker; M. C. M. van Loosdrecht; J. Brucato; F. Grunthaner; P. Ehrenfreund

2009-01-01

283

Search for Habitable Planets Around Low-Mass Stars Using the InfraRed Doppler Instrument  

NASA Astrophysics Data System (ADS)

We present strategies and plans for a new Doppler exoplanet survey of late-M dwarf stars to search for Earth-mass planets in the habitable zone around low-mass stars using a new near-infrared instrument for the Subaru telescope (IRD).

Omiya, M.; Sato, B.; Harakawa, H.; Kuzuhara, M.; Hirano, T.; Narita, N.; IRD Team

2013-11-01

284

Effects of M dwarf magnetic fields on potentially habitable planets  

NASA Astrophysics Data System (ADS)

We investigate the effect of the magnetic fields of M dwarf (dM) stars on potentially habitable Earth-like planets. These fields can reduce the size of planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. We used a sample of 15 active dM stars, for which surface magnetic-field maps were reconstructed, to determine the magnetic pressure at the planet orbit and hence the largest size of its magnetosphere, which would only be decreased by considering the stellar wind. Our method provides a fast means to assess which planets are most affected by the stellar magnetic field, which can be used as a first study to be followed by more sophisticated models. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (~1 G) orbiting at the inner (outer) edge of the habitable zone of these stars would present magnetospheres that extend at most up to 6 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G to up to a few thousand G. By assuming a magnetospheric size that is more appropriate for the young-Earth (3.4 Gyr ago), the required planetary magnetic fields are one order of magnitude weaker. However, in this case, the polar-cap area of the planet, which is unprotected from transport of particles to/from interplanetary space, is twice as large. At present, we do not know how small the smallest area of the planetary surface is that could be exposed and would still not affect the potential for formation and development of life in a planet. As the star becomes older and, therefore, its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets probably expands. Using an empirically derived rotation-activity/magnetism relation, we provide an analytical expression for estimating the shortest stellar rotation period for which an Earth-analogue in the habitable zone could sustain an Earth-sized magnetosphere. We find that the required rotation rate of the early- and mid-dM stars (with periods ?37-202 days) is slower than the solar one, and even slower for the late-dM stars (?63-263 days). Planets orbiting in the habitable zone of dM stars that rotate faster than this have smaller magnetospheric sizes than that of the Earth magnetosphere. Because many late-dM stars are fast rotators, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early- and mid-dM stars.

Vidotto, A. A.; Jardine, M.; Morin, J.; Donati, J.-F.; Lang, P.; Russell, A. J. B.

2013-09-01

285

The Evolution of Dust in the Terrestrial Planet Region of Circumstellar Disks Around Young Stars  

NASA Astrophysics Data System (ADS)

Circumstellar disks with masses comparable to the primeval solar nebula have been discovered around numerous pre-main sequence stars; it is believed the disks are a natural byproduct of star formation. If most stars originally have massive circumstellar disks, it is very likely planetary systems are common. Orbiting planets are not directly observable owing to their relatively cool temperatures and meager surface area. However, in the early stages of planetary formation, the surface area of debris in the disk may exceed the surface area of the star by many orders of magnitude. Material in the terrestrial zone emits primarily at near-infrared wavelengths; sufficient disk debris may produce detectable excess emission at these wavelengths. As clearing mechanisms, including possible planetary formation, remove the small particles in the disk, the strong infrared emission diminishes. By observing the excess infrared emission as a function of stellar age and spectral type, timescales for inner disk processes which create or remove small particles can be established. This dissertation presents sensitive, simultaneous, near-infrared broadband continuum observations of old pre-main sequence and young main-sequence cluster stars. The stellar ages range from 1-600 Myr, spanning the predicted epoch of planetary formation for solar-type stars. A wide range of spectral types were observed. We detect no excess emission after an age of about 3 × 106yr. Using a model to predict the infrared emission from an optically thin dust disk, we find our measurements are sensitive to 1020 - 1021 g of micron-radius dust grains ( = 2 g cm-3) distributed within the terrestrial zone. Adapting this result to a more realistic particle size distribution, we believe we can detect debris in extra-solar systems until the terrestrial planets are 90-95% complete. Older models of the formation of the Earth which assume orderly growth predict the Earth is 90% complete after about 80 Myr. Newer models allow runaway growth, which shortens the timescale to ~105yr. If the observed clearing in the inner disk reflects the formation of terrestrial planets, our results strongly support models of planetary formation which incorporate runaway growth. Implications are discussed. This thesis is available on the World Wide Web at: http://decoy.phast.umass.edu/

Dutkevitch, Diane

1995-05-01

286

A hydrodynamic study of the circumstellar envelope of ? Scorpii  

NASA Astrophysics Data System (ADS)

Context. Both the absolute mass-loss rates and the mechanisms that drive the mass loss of late-type supergiants are still not well known. Binaries such as ? Sco provide the most detailed empirical information about the winds of these stars. Aims: Our goal was to improve the binary technique for the determination of the mass-loss rate of ? Sco A by including a realistic density distribution and velocity field from hydrodynamic and plasma simulations. Methods: We performed 3D hydrodynamic simulations of the circumstellar envelope of ? Sco in combination with plasma simulations accounting for the heating, ionization, and excitation of the wind by the radiation of ? Sco B. These simulations served as the basis for an examination of circumstellar absorption lines in the spectrum of ? Sco B as well as of emission lines from the Antares nebula. Results: The present model of the extended envelope of ? Sco reproduces some of the structures that were observed in the circumstellar absorption lines in the spectrum of ? Sco B. Our theoretical density and velocity distributions of the outflow deviate considerably from a spherically expanding model, which was used in previous studies. This results in a higher mass-loss rate of (2 ± 0.5) × 10-6 M? yr-1. The hot H ii region around the secondary star induces an additional acceleration of the wind at large distances from the primary, which is seen in absorption lines of Ti ii and Cr ii at -30 km s-1. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under program ID 076.D-0690(A), and on observations made with the NASA/ESA Hubble Space Telescope (program # 5952), obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

Braun, K.; Baade, R.; Reimers, D.; Hagen, H.-J.

2012-10-01

287

3-D Radiation Transfer in Line-Blanketed Circumstellar Media  

NASA Astrophysics Data System (ADS)

We present a fully 3-D Monte Carlo radiation transfer (MCRT) technique for calculating metal line blanketing by rotating, expanding (or contracting) circumstellar envelopes around early-type stars. Current MCRT algorithms follow individual monochromatic photon packets, moving through the envelope. The scattering opacity determines the scattering probability within a given volume element, while the absorptive opacity determines the destruction probability. A difficulty with MCRT is that when large absorptive opacities (i.e., spectral lines) are present, most photons are destroyed before they exit the envelope. Consequently, a very large number of stellar photons must be generated to obtain enough emergent photons to measure the flux. Furthermore, to incorporate the large number of blanketing spectral lines, one requires an extremely fine frequency sampling. Here, we present a technique (for electron scattering atmospheres) that may circumvent these problems. Since electron scattering is wavelength-independent, we extend the monochromatic photon packet to include all frequencies simultaneously (i.e., a spectrum) when calculating the photon scattering locations and subsequent directions. Instead of destroying photons upon absorption, we attenuate the spectrum (after it emerges) by the absorptive optical depth accumulated along the entire path. Finally, owing to Doppler shifts within the expanding atmosphere, we only require the cumulative frequency integral of the line opacity when calculating the attenuation. This allows us to include the large number of lines required for line blanketing. Furthermore, the frequency sampling of our spectra is determined by the expansion velocity rather than the line thermal widths. In this paper, we present a feasibility study of this method applied to the axisymmetric circumstellar disk around the Be star zeta Tau. By matching the amount of excess Fe line absorption within the flux spectrum, as well as the amount of UV depolarization observed by the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), we hope to determine the temperature, column density, and ionization structure of the circumstellar disk.

Bjorkman, J. E.; Wood, Kenneth

1995-12-01

288

IR emission from circumstellar envelopes of C-rich stars  

NASA Technical Reports Server (NTRS)

The reliability of a theoretical model that solves the radiative transfer equation in dust clouds surrounding a central star is checked. In particular, it is found that both classical scattering by dust and the back-heating effects are negligible in the radiative transfer when envelopes similar to IRC+10216 are taken into consideration. In addition, new fits of IRC+10216 spectra are presented which were obtained, when the source is in different luminosity phases, under the assumption that amorphous carbon grains are in the circumstellar envelope. The same model is currently used to simulate the emission from carbon-rich sources showing the silicon carbide feature at 11.3 microns.

Blanco, A.; Borghesi, A.; Bussoletti, E.; Colangeli, Luigi; Fonti, Sergio; Orofino, Vincenso

1989-01-01

289

Formation of Small Iron Clusters in Circumstellar Envelopes  

NASA Astrophysics Data System (ADS)

We have investigated the role of small iron clusters as possible seeds for the formation of dust particles in circumstellar envelopes. Using cluster data available in the literature we constructed a simple cluster model in order to calculate the partition function and the Gibbs energy of formation for FeN clusters (N ? 19). Based on these data we have calculated the equilibrium densities of such clusters under different physical conditions. The results are discussed with respect to the problem of the primary dust condensate in M-type stars.

John, M.; Sedlmayr, E.

1997-07-01

290

Protonated acetylene - An important circumstellar and interstellar ion  

NASA Technical Reports Server (NTRS)

In a circumstellar envelope, a substantial amount of acetylene is transported in a wind to the outer envelope, where it can be photoionized by interstellar radiation and then converted into C2H3(+) by a low-temperature reaction with H2. New chemical modeling calculations indicate that sufficient C2H3(+) may be produced in the outer envelope of IRC + 10216 to be observable. Similar considerations suggest that C2H3(+) should also be detectable in interstellar clouds, provided its rotational spectrum has been measured accurately in the laboratory.

Glassgold, A. E.; Omont, A.; Guelin, M.

1992-01-01

291

The Automatic Component of Habit in Health Behavior: Habit as Cue-Contingent Automaticity  

Microsoft Academic Search

Objective: Habit might be usefully characterized as a form of automaticity that involves the association of a cue and a response. Three studies examined habitual automaticity in regard to different aspects of the cue-response relationship characteristic of unhealthy and healthy habits. Design, Main Outcome Measures, and Results: In each study, habitual automaticity was assessed by the Self-Report Habit Index (SRHI).

Sheina Orbell; Bas Verplanken

2010-01-01

292

A New Look at Habits and the Habit–Goal Interface  

Microsoft Academic Search

The present model outlines the mechanisms underlying habitual control of responding and the ways in which habits interface with goals. Habits emerge from the gradual learning of associations between responses and the features of performance contexts that have historically covaried with them (e.g., physical settings, preceding actions). Once a habit is formed, perception of contexts triggers the associated response without

Wendy Wood; David T. Neal

2007-01-01

293

The Effect of Circumstellar Material on the Light Curves of Eclipsing Binary Systems  

Microsoft Academic Search

This study inspects the influence of various effects and free parameters of the accretion disc and circumstellar material on the emerging light curve of eclipsing binary systems that have a circumstellar disc, by using the shellspec code. The results indicate that some of the parameters, namely the temperature and inclination of the disc, spot, jet, stream and shell, significantly affect

S. M. R. Ghoreyshi; J. Ghanbari; F. Salehi

2011-01-01

294

The light curve and changes in the circumstellar envelope around IRC + 10216  

Microsoft Academic Search

This paper presents results of NIR photometric observations of IRC + 10216 star during the time interval 1965-1990, which were combined with measurements of the circumstellar shell surrounding IRC + 10216 by IR speckle interferometry. Using these data together with published observations, a very precise light curve was constructed, demonstrating the occurrence of long-term changes in the circumstellar shell surrounding

H. M. Dyck; J. A. Benson; R. R. Howell; R. R. Joyce; C. H. Leinert

1991-01-01

295

The set of habitable planets and astrobiological regulation mechanisms  

NASA Astrophysics Data System (ADS)

The number of habitable planets in the Milky Way and its temporal variation are major unknowns in the nascent fields of astrobiology and Search for ExtraTerrestrial Intelligence studies. All numerical models developed thus far have suffered from large uncertainties in the input data, in addition to our lack of understanding of the processes of astrobiological dynamics. Here, we argue that at least the input data can now be specified with more confidence, and use a simple Monte Carlo model of the Galactic Habitable Zone (GHZ) as a flexible platform for their elucidation. Previous papers have described some of the major results of this class of models; in this paper we present its mechanics and input parameters, notably the number of the habitable planets in the GHZ and their temporal distribution, based on the results of Lineweaver et al. (Lineweaver, C.H., Fenner, Y. & Gibson, B.K. (2004). Science 303, 59-62.) Regulation mechanisms (such as gamma-ray bursts or supernovae) and their temporal evolution, assumed to be main agents responsible for large-scale correlation effects, are modelled as type ? (which can sterilize part of or the entire GHZ) and type ? (which are of local importance) events with decreasing mean temporal frequency over the cosmological timescale. The considered global risk function implies as an upper limit that about one out of a hundred habitable sites will achieve high astrobiological complexity. The preliminary results of numerical modelling presented here and elsewhere imply that the lack of a sudden change from an essentially dead Galaxy to a Galaxy filled with complex life - the astrobiological phase transition - in our past (a version of Fermi's paradox) may be understood as a consequence of global astrobiological disequilibrium, strongly indicating such a transitional epoch in our future.

Vukoti?, Branislav

2010-04-01

296

Student Work Habits: An Educational Imperative.  

ERIC Educational Resources Information Center

Major problems in the American workforce are absenteeism, tardiness, disorganization, off-task behavior, and limited teamwork. Attacking such problems by promoting effective work habits in schools should be an educational priority, with teachers, counselors, and school psychologists all playing a role in the process. Student work habits that can…

Williams, Robert L.; Oh, Eun Jung

297

Family Perceptions of Television Viewing Habits.  

ERIC Educational Resources Information Center

Examined families' television viewing habits during prime time and family members' perceptions of their viewing habits. Individually interviewed 24 mother-father-child triads to determine viewing patterns over a four-evening period. Both boys and girls were more concordant with fathers than with mothers. Age differences were not significant for…

Hopkins, M. Nancy; Mullis, Ann K.

1985-01-01

298

Genetic Influences on Adolescent Eating Habits  

ERIC Educational Resources Information Center

Behavioral genetic research shows that variation in eating habits and food consumption is due to genetic and environmental factors. The current study extends this line of research by examining the genetic contribution to adolescent eating habits. Analysis of sibling pairs drawn from the National Longitudinal Study of Adolescent Health (Add Health)…

Beaver, Kevin M.; Flores, Tori; Boutwell, Brian B.; Gibson, Chris L.

2012-01-01

299

Habit Lag: When “Automatization” is Dysfunctional  

Microsoft Academic Search

La Fave's habit lag construct, which specifies conditions under which previously automatized motor responses become disruptive of subsequent performance, was tested. Performance under stress was also examined as a possible factor in the occurrence of habit lag. Following a visual discrimination task, 48 women performed motor responses simultaneously: (a) repeating an invariant lever movement and (b) pushing one of two

Roger C. Mannell; James H. Duthie

1975-01-01

300

Hierarchy of stroking habits at the typewriter  

Microsoft Academic Search

Studied the typing stroking habits of 82 high school students, 103 college undergraduates, and 5 faculty members and secretaries at skill levels ranging between 10 and 114 words\\/min (wpm) in order to identify the habits that account for skill and locate the skill levels at which significant changes in stroking occur. Ss typed (using 3-min timings) a series of materials

Leonard J. West; Yitzchak Sabban

1982-01-01

301

Unsuccessful Study Habits in Foreign Language Courses.  

ERIC Educational Resources Information Center

This study determined which study habits would distinguish successful from unsuccessful foreign language learners. Participants were 219 college students from a variety of disciplinary backgrounds enrolled in either Spanish, French, German, or Japanese classes. The students completed the Study Habits Inventory and the Background Demographic Form.…

Bailey, Phillip D.; Onwuegbuzie, Anthony J.

302

The Leisure Reading Habits of Urban Adolescents  

ERIC Educational Resources Information Center

Research indicates that there is a strong relationship between leisure reading and school achievement, but the leisure reading habits of urban adolescents have rarely been studied. From their investigation of the leisure reading habits of 584 urban minority middle school students, the authors identify these key findings: (1) More than two-thirds…

Hughes-Hassell, Sandra; Rodge, Pradnya

2007-01-01

303

Planetary Conditions for Habitability on Enceladus  

Microsoft Academic Search

The prolific activity and presence of a plume on Saturn's tiny moon Enceladus offers us a unique opportunity to sample the interior composition of an icy satellite, and to look for interesting chemistry and possible signs of life. Based on studies of the potential habitability of Jupiter's moon Europa, icy satellite oceans can be habitable if they are chemically mixed

Christopher D. Parkinson; Amy C. Barr; Mao-Chang Liang; Y. L. Yung

2007-01-01

304

Habitability of Enceladus: Planetary Conditions for Life  

Microsoft Academic Search

The prolific activity and presence of a plume on Saturn's tiny moon Enceladus offers us a unique opportunity to sample the interior composition of an icy satellite, and to look for interesting chemistry and possible signs of life. Based on studies of the potential habitability of Jupiter's moon Europa, icy satellite oceans can be habitable if they are chemically mixed

Christopher D. Parkinson; Mao-Chang Liang; Yuk L. Yung; Joseph L. Kirschivnk

2008-01-01

305

The Habitability of the Moon  

NASA Astrophysics Data System (ADS)

Following the vague guesswork of some writers in antiquity, early telescopic astronomy was strongly preoccupied with the ``World in the Moone.'' About the same time, as Kepler's charming ``Dream'' appeared posthumously, Wilkins set out to prove that there was no contradiction ``with reason or faith'' if we assumed the habitability of the Moon. For about two hundred years, this hypothesis remained quite popular (Cyrano, Fontenelle, Huygens) particularly among the wider public. That in spite of the reverberations, for instance, of the Whewell-Brewster controversy over the habitability of planets, now largely forgotten. On this background, the success of the famous ``Moon hoax of 1835'' seems more understandable. It was only in the middle of the 19th century that this idea began to slowly fade as the lack of lunar atmosphere became more and more obvious. The scientific evidence was mainly in connection with the lunar occultations (Bessel, John Herschel, and others), and also with the well-observed total solar eclipse of 1842. Yet, even later, rather fanciful assumptions about the lunar atmosphere collecting on the invisible far side of the Moon kept a modicum of believability alive for some years. Ultimately, however, the ``Selenites'' wandered over into the domain of science fiction -- the best representative being perhaps Wells' utopia in the ``First Men on the Moon'' exploring the inside of the Moon. The scientific studies concentrated more on the rather frustrating topic of lunar surface variations such as the disappearance of the crater Linnae. Nevertheless, as late as the 1960's, a possibly overly cautious NASA was ready to quarantine the returning Apollo astronauts, paying homage, perhaps, to the panspermia hypothesis.

Herczeg, Tibor

306

PRIMORDIAL CIRCUMSTELLAR DISKS IN BINARY SYSTEMS: EVIDENCE FOR REDUCED LIFETIMES  

SciTech Connect

We combine the results from several multiplicity surveys of pre-main-sequence stars located in four nearby star-forming regions with Spitzer data from three different Legacy Projects. This allows us to construct a sample of 349 targets, including 125 binaries, which we use to to investigate the effect of companions on the evolution of circumstellar disks. We find that the distribution of projected separations of systems with Spitzer excesses is significantly different (P{approx} 2.4e-5, according to the K-S test for binaries with separations less than 400 AU) from that of systems lacking evidence for a disk. As expected, systems with projected separations less than 40 AU are half as likely to retain at least one disk than are systems with projected separations in the 40-400 AU range. These results represent the first statistically significant evidence for a correlation between binary separation and the presence of an inner disk (r{approx} 1 AU). Several factors (e.g., the incompleteness of the census of close binaries, the use of unresolved disk indicators, and projection effects) have previously masked this correlation in smaller samples. We discuss the implications of our findings for circumstellar disk lifetimes and the formation of planets in multiple systems.

Cieza, Lucas A. [Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822 (United States); Padgett, Deborah L.; McCabe, Caer E.; Brooke, Timothy Y.; Carey, Sean J.; Noriga-Crespo, Alberto; Rebull, Luisa M. [Spitzer Science Center, MC 220-6, California Institute of Technology, Pasadena, CA 91125 (United States); Allen, Lori E.; Peterson, Dawn E. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138 (United States); Chapman, Nicholas L. [Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, MS 301-429, Pasadena, CA 91109 (United States); Fukagawa, Misato [Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043 (Japan); Huard, Tracy L. [Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)], E-mail: lcieza@ifa.hawaii.edu

2009-05-01

307

Probing Pre-Supernova Mass Loss With Circumstellar Dust Shells  

NASA Astrophysics Data System (ADS)

Late-time (>100 day) mid-infrared (mid-IR) observations of supernovae (SNe) offer a valuable probe of the progenitor mass-loss. Already, this technique has been exemplified with the Type IIn subclass, which often have large, dusty, pre-existing shells formed in pre-SN eruptions. While other SN subclasses are generally thought of having relatively low density circumstellar environments, a growing number of objects in other subclasses now show evidence for significant pre-SN mass loss and similar mid-IR characteristics. Long after the SN radioactive tail disappears, warm dust can stay bright at mid-IR wavelengths due to alternative heating mechanisms, such as shocks. The success of Spitzer archival studies has already been highlighted by the work of several members of this team. Here we propose a SNAPSHOT survey of a well-studied and high-profile SN sample, extending over a range of subclasses, and including both recent and historical events with evidence of a dense CSM and/or dust. This program will (a) discover new SNe with warm dust and (b) monitor the evolution of warm dust in previously detected SNe. Expanding upon our previous mid-IR work on SNe IIn, these observations will probe the similarities in and differences between the subclasses' circumstellar environments, pre-SN mass-loss, and ultimately, the progenitors themselves.

Fox, Ori; Filippenko, Alex; Skrutskie, Mike; van Dyk, Schuyler; Kelly, Pat

2013-10-01

308

Far-Infrared Water Line Emissions from Circumstellar Outflows  

NASA Technical Reports Server (NTRS)

We have modeled the far-infrared water line emission expected from circumstellar outflows from oxygen-rich late-type stars, as a function of the mass-loss rate and the terminal outflow velocity. For each mass-loss rate and terminal outflow velocity considered, we computed self-consistently the gas density, temperature, outflow velocity, and water abundance as a function of distance from the star. We then used an escape probability method to solve for the equilibrium level populations of 80 rotational states of water and thereby obtained predictions for the luminosity of a large number of far-infrared rotational transitions of water. In common with previous models, our model predicts that water will be copiously produced in the warm circumstellar gas and that water rotational emission will dominate the radiative cooling. However, our use of a realistic radiative cooling function for water leads to a lower gas temperature than that predicted in previous models. Our predictions for the far-infrared water line luminosities are consequently significantly smaller than those obtained in previous studies. Observations to be carried out by the Infrared Space Observatory will provide a crucial test of the models presented here.

Chen, Wesley; Neufeld, David A.

1995-01-01

309

The Circumstellar Envelope of ? Tauri through Optical Interferometry  

NASA Astrophysics Data System (ADS)

We present optical interferometric observations of the Be star ? Tauri obtained using the Navy Prototype Optical Interferometer (NPOI). The multichannel capability of the NPOI allows a high-quality internal calibration of the squared visibilities corresponding to the H? emission from the circumstellar environment. The observations suggest a strong departure from circular symmetry and thus are described by an elliptical Gaussian model. We use a nonlinear least-squares fit to the data to obtain the likeliest parameters, and the corresponding uncertainties are determined using a Monte Carlo simulation. We obtain 3.14+/-0.21 mas for the angular size of the major axis, -62.3d+/-4.4d for the position angle, and 0.310+/-0.072 for the axial ratio. By comparing our results with those already in the literature, we conclude that the model parameters describing the general characteristics of the circumstellar envelope of ? Tau appear to be stable on timescales of years. We also compare our results with the known parameters describing the binary nature of ? Tau, and we conclude that the envelope surrounds only the primary component and is well within its Roche lobe.

Tycner, Christopher; Hajian, Arsen R.; Armstrong, J. T.; Benson, J. A.; Gilbreath, G. C.; Hutter, D. J.; Lester, John B.; Mozurkewich, D.; Pauls, T. A.

2004-02-01

310

On the gas temperature in circumstellar disks around A stars  

NASA Astrophysics Data System (ADS)

In circumstellar disks or shells it is often assumed that gas and dust temperatures are equal where the latter is determined by radiative equilibrium. This paper deals with the question whether this assumption is applicable for tenuous circumstellar disks around young A stars. In this paper the thin hydrostatic equilibrium models described by Kamp & Bertoldi (\\cite{Kamp}) are combined with a detailed heating/cooling balance for the gas. The most important heating and cooling processes are heating through infrared pumping, heating due to the drift velocity of dust grains, and fine structure and molecular line cooling. Throughout the whole disk gas and dust are not efficiently coupled by collisions and hence their temperatures are quite different. Most of the gas in the disk models considered here stays well below 300 K. In the temperature range below 300 K the gas chemistry is not much affected by Tgas and therefore the simplifying approximation Tgas = Tdust can be used for calculating the chemical structure of the disk. Nevertheless the gas temperature is important for the quantitative interpretation of observations, like fine structure and molecular lines.

Kamp, I.; van Zadelhoff, G.-J.

2001-07-01

311

Orbital Dynamics and Habitability I: Triggering a Runaway Greenhouse via Tidal Heating  

NASA Astrophysics Data System (ADS)

The inner edge of the habitable zone is often defined by the tightest orbit which does not initiate a moist or runaway greenhouse. Previously it was believed that only stellar radiation could trigger these phenomena for a long enough duration to desiccate a planet and preclude habitability. We show that for some planets orbiting low-mass stars (<0.3 solar masses), tidal heating can reach levels that induce a runaway greenhouse. We call these planets "Tidal Venuses." As tides circularize the orbit and drive the obliquity to 0 or PI, the heating level drops, but tidal heating can persist long enough to remove all of a planet's water. Therefore, a planet may be discovered in the habitable zone with very low eccentricity (and hence without enough tidal heat to drive a runaway greenhouse), and yet be uninhabitable due to a previous epoch of extreme tidal heating. The range of possible tidal and radiative heating predicts a diversity of planets in and around the habitable zone of low-mass stars. In multi-planet systems, interactions with other companions may maintain non-zero eccentricities and obliquities, increasing the threat of catastrophic tidal heating. As terrestrial planets are discovered around low luminosity primaries, careful consideration of current and past tidal heating will be essential for estimating their likelihoods to be inhabited.

Barnes, Rory; Mullins, K.; Goldblatt, C.; Meadows, V. S.; Kasting, J. F.; Heller, R.

2012-05-01

312

Mars habitability: epochs, processes, remnants  

NASA Astrophysics Data System (ADS)

Following the OMEGA/Mars Express pioneering discovery of hydrated phyllosilicates in a variety of sites spread over the ancient terrains, an impressive set of results, combining refined observations by OMEGA/Mars Express and CRISM/MRO, relevant contextual images by HRSC/Mars Express and HiRiSe/MRO, modeling and simulations, has been acquired. They confirm the initial hypothesis that these minerals record an ancient era during which water might have remained liquid over extended durations, which led to naming this era: phyllosian. It is remarkable that 30 years after the Mariner and Viking images of Mars as a desolated and inhospitable world, a new vision is emerging, for Mars to have possibly hosted environmental conditions favoring the presence of a key ingredient of potential astrobiological relevance. If ever Mars was habitable, it happened during the phyllosian. A variety of coupled information, from compositional, geomorphological and magnetic data, enables to assess the processes that took place, and their chronology. The observations that we will discuss include the specific composition identified, in relation to the stratification observed; the scarcity of phyllosilicates within the cratered terrains where mafics dominate; the large depth of deposits in a variety of sites, in particular within Mawrth Vallis; the distribution of remnant magnetization, with almost none within the large impacts and along the volcanic features. As a consequence, it will be shown that this era is tightly limited in time. It seems to have ended prior to the heavy bombardment to have ceased: the Noachian expanded long after the phyllosian ended. This habitable era could have been enabled by the maintenance of an active dynamo: its drop triggered its end, given the properties of the early Sun, by the lost of suitable atmospheric conditions for surface water to remain stable as a liquid. Up to now, neither with the Moon or any other body, we did not have means to study the complex history of inner planets along their first hundreds of millions years, at a time when the early bombardment took place, while maintaining habitable conditions which, at least on the Earth, led to the emergence of life. Did the impact rate steadily decreased, or did it drop a number of times, up to late events some 3.8/3.9 billion years ago, offering quiet epochs with biologically active standing water reservoirs in place? Mars is unique in offering means to study this era, as its history preserved sites recording this era with potentially biorelics still in place. The occurrence of phyllosilicate-rich sites offers the exciting perspective to address scientifically the question of the emergence of life elsewhere than on the Earth, and to decipher the processes that drove diverging evolutionary pathways for Mars and the Earth.

Bibring, J.; Omega, T.

2008-12-01

313

A Legacy Archive PSF Library And Circumstellar Environments LAPLACE Investigation  

NASA Astrophysics Data System (ADS)

NICMOS coronagraphy, with well-matched template Point Spread Function PSF subtraction, probes the closest environments of occulted targets with the highest imaging sensitivity in intrinsically high contrast fields at the smallest radial distances afforded, uniquely, by HST. NICMOS PSF-subtracted coronagraphy has been invoked in a wide variety of HST programs with science themes as divergent as detecting and characterizing disks of circumstellar material in neo-natal stellar environments, to studying faint nebulosity associated with luminous active galaxies, to searching for planetary-mass companions in extrasolar planetary systems recently born and in the "stellar graveyard." The investment in HST time in the execution of these and other programs, which has resulted in more than 8450 NICMOS coronagraphic images to date, has met with mixed returns. Stunning but infrequent successes, importantly advancing their fields highlight much more frequent, unfortunately common, failures arising from highly compromised technically-achievable performance due to the lack of suitable template PSFs required to produce high-fidelity, photometrically robust, high contrast coronagraphic images. We propose to remedy this situation by undertaking a rigorous, homogeneous, and complete recalibration and analysis of the full archival set of raw NICMOS coronagraphic images previously obtained and residing in the MAST to create a Legacy library of template PSFs enabling the recovery of the large body of science otherwise lost. This PSF library, along with generically applicable analysis software that we will deliver to STScI, will: 1 critically augment the needs of future observational programs reliant on high-fidelity PSF subtractions, 2 increase their yields and photometric efficacy, 3 reduce the observing time HST orbit allocations otherwise required for near-contemporaneous reference PSF observations, and 4 greatly enrich the yet-unrealized potential of the many NICMOS coronagraphic observations already acquired from the broad spectrum of science programs previously executed. We will then use the enabling power of the PSF library to re-reduce and re-analyze all archival NICMOS coronagraphic observations of circumstellar disk and VLM stellar, brown dwarf, and EJP companion candidate stars ~ 400 targets to probe for previously undetected circumstellar disks. Through image analysis and modeling we will ascertain the physical properties of newly-discovered disks and their constituent grains. With a very large and homogeneously contrast-limited sample of optimally PSF-subtracted images, we will also set spatially resolved dust-scattered light flux density limits from non-detections to constrain the properties of the many IR-excess and other sources in this sample.;

Schneider, Glenn

2007-07-01

314

Deep optical imaging of asymptotic giant branch circumstellar envelopes  

NASA Astrophysics Data System (ADS)

We report results of a program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered Galactic light. The goal is to characterize the shapes of the envelopes to probe the mass-loss geometry and the presence of hidden binary companions. The observations consist of deep optical imaging of 22 AGB stars with high mass loss rates: 16 with the ESO 3.6 m NTT telescope, and the remainder with other telescopes. The circumstellar envelopes are detected in 15 objects, with mass loss rates ?2 × 10-6 M? yr-1. The surface brightness of the envelopes shows a strong decrease with Galactic radius, which indicates a steep radial gradient in the interstellar radiation field. The envelopes range from circular to elliptical in shape, and we characterize them by the ellipticity (E = major/minor axis) of iso-intensity contours. We find that ~50% of the envelopes are close to circular with E ? 1.1, and others are more elliptical with ~20% with E ? 1.2. We interpret the shapes in terms of populations of single stars and binaries whose envelopes are flattened by a companion. The distribution of E is qualitatively consistent with expectations based on population synthesis models of binary AGB stars. We also find that ~50% of the sample exhibit small-scale, elongated features in the central regions. We interpret these as the escape of light from the central star through polar holes, which are also likely produced by companions. Our observations of envelope flattening and polar holes point to a hidden population of binary companions within the circumstellar envelopes of AGB stars. These companions are expected to play an important role in the transition to post-AGB stars and the formation of planetary nebulae. Based on observations made at the European Southern Observatory, Chile (programs 078.D-0102, 082.D-0338 and 0.84.D-0302) and on de-archived data obtained with the ESO Very Large Telescope and with the NASA/ESA Hubble Space Telescope.Appendix A and Figs. 5, 6 are available in electronic form at http://www.aanda.org

Mauron, N.; Huggins, P. J.; Cheung, C.-L.

2013-03-01

315

The Impact of SN1987A with its Circumstellar Ring  

NASA Technical Reports Server (NTRS)

Following is final report on the study "The Impact of SN1987A with its Circumstellar Ring", which is now complete. In 1994, it was predicted that the blast wave from SN1987A would strike the circumstellar ring in AD1999+/-3, and that the ring would brighten by several hundreds optically as the shock entered the ring. It was also predicted that the emission lines from the shocked ring would have linewidths of a few hundred km/s. Today, we see a "hot spot" on the ring that first appeared in Hubble Space Telescope (HST) images in 1995 and has doubled in brightness between August 1997 and February 1998. Moreover, spectra from STIS show that the emission lines from the hot spot have widths and blue shifts of order 200 km/s, just as we predicted in 1994. Our guess of 1999+/-3 for the impact time was lucky, because we assumed that the hot gas in the bubble between the supernova and the ring had a low density (approximately 10 cm(exp -3)). But it was pointed out that the ROSAT observations of soft X-rays from SN 1987A implied that the intervening gas had a higher density, (approximately 100 cm(exp -3)), which would delay the impact until ca. 2007. Others developed hydrodynamic models to fit the ROSAT X-ray emission spectrum and came to the same conclusion. But these models were oversimplified in that they assumed that the ring was perfectly round. Now we see clearly that the hot spot is a peninsula that protrudes inward from the ring -- the first spot on the ring to be struck by the blast wave. No doubt there are other protrusions on the ring, which we may expect to light up in the next few years until they finally merge to set the entire ring ablaze, probably within 5 - 10 years. It was also predicted that the X-ray emission must be accompanied by optical and ultraviolet emission from atoms in the supernova debris and in the circumstellar gas that cross the reverse shock and the blast wave, respectively, and that the Lyman-alpha and NV emission lines should be bright enough to see with the STIS.

McCray, Richard

1999-01-01

316

Searching for the Circumstellar Ejecta Around Cool Hypergiants  

NASA Astrophysics Data System (ADS)

We present HST and Spitzer images of several of the most luminous cool stars in the Galaxy. These highly unstable, very massive stars lie on or near the empirical upper luminosity boundary in the H-R diagram, and are characterized by high mass loss phenomena, sometimes violent, which may be responsible for the upper boundary. These observations are designed to search for circumstellar structures close to the star as well as more distant nebulosity. We discuss the presence, or lack of, ejecta around these hypergiants, and the evolutionary implications. Our high-resolution WFPC2 images show compact nebulosity around the cool M-type hypergiants NML Cyg, VX Sgr and S Per. The powerful OH/IR source NML Cyg exhibits a small, peculiar bean-shaped asymmetric nebula that closely matches the distribution of the surrounding H2O vapor masers. NML Cyg's concave outer envelope is likely shaped by photo-dissociation from the powerful, nearby association Cyg OB2 inside the Cygnus X superbubble. VX Sgr and S Per, also OH/IR sources, have marginally resolved envelopes. S Per's circumstellar nebula appears elongated in a NE/SW orientation similar to that for its surrounding OH and H2O masers, while VX Sgr is obscured by a spheroidal envelope. We find no evidence in our WFPC2 images for circumstellar nebulosity around the intermediate-type hypergiants ? Cas, HR 8752, HR 5171a nor the normal M-type supergiant ? Cep. We conclude that very likely, there has been no high mass loss event prior to 500-1000 yrs ago for these four stars. Our IRAC images (? Cas, HR 8752, and R 150) also show no evidence for extended structure. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work is in part provided by NASA through contracts 1256406 and 1215746 issued by JPL/Caltech to the University of Minnesota.

Schuster, M. T.; Humphreys, R. M.; Marengo, M.; Gehrz, R. D.; Woodward, C. E.; Polomski, E.

2005-05-01

317

The Mineralogy of Circumstellar Silicates Preserved in Cometary Dust  

NASA Technical Reports Server (NTRS)

Interplanetary dust particles (IDPs) contain a record of the building blocks of the solar system including presolar grains, molecular cloud material, and materials formed in the early solar nebula. Cometary IDPs have remained relatively unaltered since their accretion because of the lack of parent body thermal and aqueous alteration. We are using coordinated transmission electron microscope (TEM) and ion microprobe studies to establish the origins of the various components within cometary IDPs. Of particular interest is the nature and abundance of presolar silicates in these particles because astronomical observations suggest that crystalline and amorphous silicates are the dominant grain types produced in young main sequence stars and evolved O-rich stars. Five circumstellar grains have been identified including three amorphous silicate grains and two polycrystalline aggregates. All of these grains are between 0.2 and 0.5 micrometers in size. The isotopic compositions of all five presolar silicate grains fall within the range of presolar oxides and silicates, having large (17)O-enrichments and normal (18)O/(16)O ratios (Group 1 grains from AGB and RG stars). The amorphous silicates are chemically heterogeneous and contain nanophase FeNi metal and FeS grains in a Mg-silicate matrix. Two of the amorphous silicate grains are aggregates with subgrains showing variable Mg/Si ratios in chemical maps. The polycrystalline grains show annealed textures (equilibrium grains boundaries, uniform Mg/Fe ratios), and consist of 50-100 nm enstatite and pyrrhotite grains with lesser forsterite. One of the polycrystalline aggregates contains a subgrain of diopside. The polycrystalline aggregates form by subsolidus annealing of amorphous precursors. The bulk compositions of the five grains span a wide range in Mg/Si ratios from 0.4 to 1.2 (avg. 0.86). The average Fe/Si (0.40) and S/Si (0.21) ratios show a much narrower range of values and are approximately 50% of their solar abundances. The latter observation may indicate a decoupling of the silicate and sulfide components in grains that condense in stellar outflows. The amorphous silicate grains described here were not extensively affected by irradiation, sputtering, or thermal processing and may represent relatively pristine circumstellar grains. They are strong candidates for the "dirty silicates" in astronomical observations of circumstellar dust shells. The polycrystalline grains were originally amorphous silicate grains that were likely annealed in the early solar nebula but the processing was not sufficient to erase their anomalous oxygen isotopic compositions.

Keller, L. P.; Messenger, S.

2010-01-01

318

Habitability from a microbial point of view  

NASA Astrophysics Data System (ADS)

We examine here the definition of habitability from the point of view of primitive, anaerobic microorganisms noting that the conditions of habitability are different for the appearance of life, for established life, and for life in dormant mode [1]. Habitability in this sense is clearly distinguished from the 'prebiotic world' that precedes the appearance of life. The differences in the conditions of habitability necessary for life to appear, for life to flourish and for dormant life entrain differences in spatial and temporal scales of habitability. For the origin of life, the ingredients carbon molecules, water, nutrients and energy need to be present on time scales applicable for the origin of life (105 to a few 106 y ?), necessitating the spatial scales of a minimum of ~100 km. Established life can take advantage of short-lived habitats (hours, days) to much longer lived ones on spatial scales of 100s ?m to cm-m, whereas dormant life can survive (but not metabolise) in extreme environments for very long periods (perhaps up to millions of years) at microbial spatial scales (100s ?m - mms). Thus, it is not necessary for the whole of a planet of satellite to be habitable. But the degree of continued habitability will have a strong influence on the possibility of organisms to evolve. For a planet such as Mars, for instance, microbial habitability was (perhaps still is) at different times and in different places. Habitable conditions conducive to the appearance of life, established life and possibly even dormant life could co-exist at different locations. Reference: [1] F. Westall, D. Loizeau, F. Foucher, N. Bost, M. Bertrand, J. Vago, & G. Kminek, Astrobiology 13:9, 887-897 (2013).

Westall, Frances; Loizeau, Damien; Foucher, Frédéric; Bost, Nicolas; Bertrand, Marylène; Vago, Jorge; Kminek, Gerhard

2014-05-01

319

Zoning Study.  

National Technical Information Service (NTIS)

The study analyzes existing zoned properties in the Shreveport Standard Metropolitan Statistical Area to determine if current development justifies the amount of such zoning. It reviews administrative procedures and policies used in carrying out zoning ch...

1968-01-01

320

[Smoking habits in chronic schizophrenics].  

PubMed

The smoking habits of 48 chronic schizophrenics who were inpatients of private psychiatric were examined. Subjects were divided into three groups; non-smokers, moderate smokers and heavy smokers, and tested by Fagerstrom's Tolerance Questionnaire (FTQ) as a tobacco dependence evaluation, and Bender Gestalt Test (BGT) and Benton Visual Retention Test (BVRT) as a neuropsychological battery. In addition, we used the Scales for the Assessment of Negative Symptoms (SANS) to evaluate the degree of schizophrenic negative symptoms, confirming the correlation between SANS score and the amount of nicotine intake or FTQ item. From the results of FTQ scores and their correlation with SANS scores, the amount of nicotine intake was significantly related to the severity of schizophrenic negative symptoms including affective blunting, poverty of thinking and attention impairment. BGT and BVRT showed disturbances in visual-motor gestalt function, and attention and memory, in chronic schizophrenics who smoked, especially in heavy smokers. These results suggest that smoking behavior in chronic schizophrenics might increase the individual vigilance against negative symptoms, from the perspective of self-medication. PMID:8588752

Fukui, K; Kobayashi, T; Hayakawa, S; Koga, E; Okazaki, S; Kawashima, Y; Kawakami, F; Fukui, Y; Tani, N; Kato, A

1995-12-01

321

Journal Reading Habits of Internists  

PubMed Central

We assessed the reading habits of internists with and without epidemiological training because such information may help guide medical journals as they make changes in how articles are edited and formatted. In a 1998 national self-administered mailed survey of 143 internists with fellowship training in epidemiology and study design and a random sample of 121 internists from the American Medical Association physician master file, we asked about the number of hours spent reading medical journals per week and the percentage of articles for which only the abstract is read. Respondents also were asked which of nine medical journals they subscribe to and read regularly. Of the 399 eligible participants, 264 returned surveys (response rate 66%). Respondents reported spending 4.4 hours per week reading medical journal articles and reported reading only the abstract for 63% of the articles; these findings were similar for internists with and without epidemiology training. Respondents admitted to a reliance on journal editors to provide rigorous and useful information, given the limited time available for critical reading. We conclude that internists, regardless of training in epidemiology, rely heavily on abstracts and prescreening of articles by editors.

Saint, Sanjay; Christakis, Dimitri A; Saha, Somnath; Elmore, Joann G; Welsh, Deborah E; Baker, Paul; Koepsell, Thomas D

2000-01-01

322

MgS grain component in circumstellar shells  

NASA Technical Reports Server (NTRS)

In this letter, far-infrared spectrophotometry (30-55 microns) and photometry (53-200 microns) are presented which define, for the first time, the long wavelength limit of the previously unidentified 30 micron emission feature found in certain extreme carbon star spectra. The spectral similarities are sufficiently similar to those of solid MgS that MgS is proposed to be the band carrier. This is interpreted as the first direct evidence that chemical surface reactions occur on dust grains in circumstellar environments. The presence of MgS indicates that either the oxygen abundance is relatively low and/or the sulfur abundance is high in extreme carbon stars.

Goebel, J. H.; Moseley, S. H.

1985-01-01

323

Omicron Aquarii: Numerical Analysis of the Circumstellar Disk  

NASA Astrophysics Data System (ADS)

We present the results of a study of the circumstellar disk surrounding the star Omicron Aquarii, where numerical disk models are compared to the spectroscopic and the interferometric data collected using the Solar Stellar Spectrograph at the Lowell Observatory’s John S. Hall telescope and the Navy Precision Optical Interferometer, respectively. The numerical analysis is based on a quantitative comparison between the Fourier Transform of the synthetic images computed with BEDISK code of Sigut and Jones (2007, ApJ, 668, 481) and the squared visibility data obtained at the NPOI. We also demonstrate how the model dependent synthetic spectrum can be directly constrained by the observed spectrum of the H-alpha emission line. The complementary spectroscopic and interferometric constraints allow for more accurate determination of the density and temperature structure of the disk, including an inclination angle of the disk with respect to the line-of-sight.

Jansen, Brian; Tycner, C.; Sigut, T. A.; Lembryk, L.; Zavala, R. T.

2014-01-01

324

Resolved images of self-gravitating circumstellar discs with ALMA  

NASA Astrophysics Data System (ADS)

In this paper, we present simulated observations of massive self-gravitating circumstellar discs using the Atacama Large Millimetre/sub-millimetre Array (ALMA). Using a smoothed particle hydrodynamics model of a 0.2-Msolar disc orbiting a 1-Msolar protostar, with a cooling model appropriate for discs at temperatures below ~160K and representative dust opacities, we have constructed maps of the expected emission at sub-mm wavelengths. We have then used the Common Astronomy Software Applications ALMA simulator to generate simulated images and visibilities with various array configurations and observation frequencies, taking into account the expected thermal noise and atmospheric opacities. We find that at 345GHz (870?m) spiral structures at a resolution of a few au should be readily detectable in approximately face-on discs out to distances of the Taurus-Auriga star-forming complex.

Cossins, Peter; Lodato, Giuseppe; Testi, Leonardo

2010-09-01

325

Exocomets in the circumstellar gas disk of HD 172555  

NASA Astrophysics Data System (ADS)

The source HD 172555 is a young A7V star surrounded by a debris disk with a gaseous component. Here, we present the detection of variable absorption features detected simultaneously in the Ca II K and H doublet lines (at ?3933 Å and ?3968 Å). We identified the presence of these absorption signatures at four different epochs in the 129 HARPS high-resolution spectra gathered between 2004 and 2011. These transient absorption features are most likely due to falling evaporating bodies (FEBs, or exocomets) that produce absorbing gas observed transiting in front of the central star. We also detect a stable Ca II absorption component at the star's radial velocity. With no corresponding detection in the Na I line, the resulting very low upper limit for the Na I/Ca II ratio suggests that this absorption is due to circumstellar gas.

Kiefer, F.; Lecavelier des Etangs, A.; Augereau, J.-C.; Vidal-Madjar, A.; Lagrange, A.-M.; Beust, H.

2014-01-01

326

A WISE Survey of Circumstellar Disks in Taurus  

NASA Astrophysics Data System (ADS)

Complete samples of circumstellar disks in star-forming regions and accurate classifications of those disks represent a foundation for studies of star and planet formation. Using data from the Wide-field Infrared Survey Explorer All-Sky Release Catalog and other catalogs of ancillary data, we have 1) analyzed all known members of the Taurus star-forming region for infrared excess, indicative of disk presence, and estimated the evolutionary stages of detected disks and 2) searched for new members of Taurus by identifying sources with red mid-infrared colors. Through the latter, we have confirmed 25 new members with optical and infrared spectra. The census of disk-bearing stars in Taurus should now be largely complete.

Esplin, Taran; Luhman, K.; Mamajek, E. E.

2014-01-01

327

PAH formation in carbon-rich circumstellar envelopes  

NASA Technical Reports Server (NTRS)

While there is growing observational evidence that some fraction of interstellar carbon is in polycyclic aromatic hydrocarbons (PAH's), the mechanisms by which these molecules might be formed have not been extensively studied. A detailed investigation of PAH production in the outflowing molecular envelopes of carbon-rich red giant star is presented. The gasphase kinetics of a chemical reaction mechanism developed to study soot production in hydrocarbon flames is modified to apply in circumstellar environments. It was found that astrophysically significant quantities of PAH's can be formed in carbon star envelopes provided the gas is sufficiently dense and resides for a long time in the temperature range of 900 to 1100 k. The precise yield of PAH's is very sensitive to astronomical parameters of the envelope (e.g., mass loss rate, outflow velocity, and acetylene abundance) and certain poorly determined chemical reaction rates.

Feigelson, Eric D.; Frenklach, Michael

1989-01-01

328

Tidally Induced Brown Dwarf and Planet Formation in Circumstellar Disks  

NASA Astrophysics Data System (ADS)

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar disks and therefore the formation of planets and brown dwarfs (BDs). Recent findings suggest that tidal perturbations of typical circumstellar disks due to close encounters may inhibit rather than trigger disk fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disk models in these calculations were restricted to disk radii of 40 AU and disk masses below 0.1 M sun. Here, we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive (~0.5 M sun), extended (>=100 AU) disks that are observed around young stars. Tidal perturbation alone can do this; no disk-disk collision is required. We also show that very low mass binary systems can form through the interaction of objects in the disk. In our computations, otherwise non-fragmenting massive disks, once perturbed, fragment into several objects between about 0.01 and 0.1 M sun, i.e., over the whole BD mass range. Typically, these orbit on highly eccentric orbits or are even ejected. While probably not suitable for the formation of Jupiter- or Neptune-type planets, our scenario provides a possible formation mechanism for BDs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar initial mass function. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.

Thies, Ingo; Kroupa, Pavel; Goodwin, Simon P.; Stamatellos, Dimitrios; Whitworth, Anthony P.

2010-07-01

329

Circumstellar grain extinction properties of recently discovered post AGB stars  

NASA Technical Reports Server (NTRS)

The circumstellar grains of two hot evolved post asymptotic giant branch (post AGB) stars, HD 89353 and HD 213985 were examined. From ultraviolet spectra, energy balance of the flux, and Kurucz models, the extinction around 2175 A was derived. With visual spectra, an attempt was made to detect 6614 A diffuse band absorption arising from the circumstellar grains so that we could examine the relationship of these features to the infrared features. For both stars, we did not detect any diffuse band absorption at 6614 A, implying the carrier of this diffuse band is not the carrier of the unidentified infrared features not of the 2175 A bump. The linear ultraviolet extinction of the carbon-rich star HD 89353 was determined to continue across the 2175 A region with no sign of the bump; for HD 213985 it was found to be the reverse: a strong, wide bump in the mid-ultraviolet. The 213985 bump was found to be positioned at 2340 A, longward of its usual position in the interstellar medium. Since HD 213985 was determined to have excess carbon, the bump probably arises from a carbonaceous grain. Thus, in view of the ultraviolet and infrared properties of the two post AGB stars, ubiquitous interstellar infrared emission features do not seem to be associated with the 2175 A bump. Instead, the infrared features seem related to the linear ultraviolet extinction component: hydrocarbon grains of radius less than 300 A are present with the linear HD 89353 extinction; amorphous anhydrous carbonaceous grains of radius less than 50 A might cause the shifted ultraviolet extinction bump of HD 213985.

Buss, Richard H., Jr.; Lamers, Henny J. G. L. M.; Snow, Theodore P., Jr.

1989-01-01

330

Temporal Evolution of the Circumstellar Shell IRC+10216  

NASA Astrophysics Data System (ADS)

A set of high-resolution 2 microns infrared spectral observations of the prototype obscured carbon star IRC+10216 have been modeled using a co-moving frame-of-reference formalism and monotonic spherically symmetric gas flow. The data set spans more than a decade and shows that the line profile shapes of the CO (0-2) overtone band transitions have changed during the period of observation. Preliminary results indicate that these lines are formed within 15 stellar radii (R_*) of the central star and suggest an evolution of this inner section of the circumstellar shell over a time span of a few years. This supports recent ideas which argue against constant outflow models for mass loss in evolved stars. The presented gas models required to fit the data show distinct phenomena: 1) An abrupt acceleration of the gas flow (Delta v ~ 10 km s(-1) ) is located in approximately the same region where the dust condensation occurs in the model (2.5 to 5 R_*). This observation is consistent with theories which predict that dust grains, accelerated by radiation pressure from the central star, drag the gas molecules outward. 2) A region of almost constant velocity located just past the dust condensation point (5 R_*) slowly accelerates from about 10.5 km s(-1) to the terminal expansion velocity of the shell (14.2 km s(-1) at 12 R_*). The evolution of the line profile shapes during the 12 years spanned by the data require this velocity plateau to expand radially by about 4 R_*. In contrast with the 1.75 year period of pulsation of the central star, these variations in the line profiles suggest a rate of evolution for the inner envelope of the circumstellar shell on the order of decades.

Sada, P. V.; Keady, J. J.; Hinkle, K. H.

1992-12-01

331

Beta Pic-like Circumstellar Gas Disk Around 2 And  

NASA Technical Reports Server (NTRS)

This grant was awarded to support the data analysis and publication of results from our project entitled P Pic-like Circumstellar Gas Disk Around 2 And . We proposed to obtain FUSE observations of 2 And and study the characteristics and origin of its circumstellar gas. We observed 2 Andromedae with FUSE on 3-4 July 2001 in 11 exposures with a total exposure time of 21,289 seconds through the LWRS aperture. Our data were calibrated with Version 1.8.7 of the CALFUSE pipeline processing software. We corrected the wavelength scale for the heliocentric velocity error in this version of the CALFUSE software. The relative accuracy of the calibrated wavelength scale is +/- 9 km/s . We produced a co-added spectrum in the LiF 1B and LiF 2A channels (covering the 1100 to 1180 A region) by cross-correlating the 11 individual exposures and doing an exposure-time weighted average flux. The final co-added spectra have a signal-to-noise ratio in the stellar continuum near 1150 A of about 20. To obtain an absolute wavelength calibration, we cross-correlated our observed spectra with a model spectrum to obtain the best fit for the photospheric C I lines. Because the photospheric lines are very broad, this yields an absolute accuracy for the wavelength scale of approx.+/- 15 km/s. We then rebinned 5 original pixels to yield the optimal sampling of .033 A for each new pixel, because the calibrated spectra oversample the spectral resolution for FUSE+LWRS (R = 20,000 +/- 2,000).

Cheng, Patricia

2003-01-01

332

TIDALLY INDUCED BROWN DWARF AND PLANET FORMATION IN CIRCUMSTELLAR DISKS  

SciTech Connect

Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar disks and therefore the formation of planets and brown dwarfs (BDs). Recent findings suggest that tidal perturbations of typical circumstellar disks due to close encounters may inhibit rather than trigger disk fragmentation and so would seem to rule out planet formation by external tidal stimuli. However, the disk models in these calculations were restricted to disk radii of 40 AU and disk masses below 0.1 M{sub sun}. Here, we show that even modest encounters can trigger fragmentation around 100 AU in the sorts of massive ({approx}0.5 M{sub sun}), extended ({>=}100 AU) disks that are observed around young stars. Tidal perturbation alone can do this; no disk-disk collision is required. We also show that very low mass binary systems can form through the interaction of objects in the disk. In our computations, otherwise non-fragmenting massive disks, once perturbed, fragment into several objects between about 0.01 and 0.1 M{sub sun}, i.e., over the whole BD mass range. Typically, these orbit on highly eccentric orbits or are even ejected. While probably not suitable for the formation of Jupiter- or Neptune-type planets, our scenario provides a possible formation mechanism for BDs and very massive planets which, interestingly, leads to a mass distribution consistent with the canonical substellar initial mass function. As a minor outcome, a possible explanation for the origin of misaligned extrasolar planetary systems is discussed.

Thies, Ingo; Kroupa, Pavel [Argelander-Institut fuer Astronomie (Sternwarte), Universitaet Bonn, Auf dem Huegel 71, D-53121 Bonn (Germany); Goodwin, Simon P. [Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH (United Kingdom); Stamatellos, Dimitrios; Whitworth, Anthony P. [School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA (United Kingdom)

2010-07-01

333

Molecular abundances in carbon-rich circumstellar envelopes  

NASA Astrophysics Data System (ADS)

A millimetre molecular line survey of seven high mass-loss rate carbon stars in both the northern and southern skies is presented. A total of 196 emission lines (47 transitions) from 24 molecular species were detected. The observed CO emission is used to determine mass-loss rates and the physical structure of the circumstellar envelope, such as the density and temperature structure, using a detailed radiative transfer analysis. This enables abundances for the remaining molecular species to be determined. The derived abundances generally vary between the sources by no more than a factor of five indicating that circumstellar envelopes around carbon stars with high mass-loss rates have similar chemical compositions. However, there are some notable exceptions. The most striking difference between the abundances are reflecting the spread in the 12C/13C-ratio of about an order of magnitude between the sample stars, which mainly shows the results of nucleosynthesis. The abundance of SiO also shows a variation of more than an order of magnitude between the sources and is on average more than an order of magnitude more abundant than predicted from photospheric chemistry in thermal equilibrium. The over-abundance of SiO is consistent with dynamical modelling of the stellar atmosphere and the inner parts of the wind where a pulsation-driven shock has passed. This scenario is possibly further substantiated by the relatively low amount of CS present in the envelopes. The chemistry occurring in the outer envelope is consistent with current photochemical models. Figures A.1-A.13 are only available in electronic form at http://www.edpsciences.org

Woods, P. M.; Schöier, F. L.; Nyman, L.-Å.; Olofsson, H.

2003-05-01

334

Finding habitable earths around white dwarfs with a robotic telescope transit survey  

NASA Astrophysics Data System (ADS)

I discuss the possibility that white dwarfs might harbor habitable planets at 0.01 AU that arrive there after the red giant phase. These planets would be detectable with ground-based telescopes via deep transits of the star, and a transit survey of the nearest white dwarfs would favor detection of Earth-temperature planets, assuming they could form as close as twice the Roche limit. I show that a robotic survey is required for habitable planet transit detection around white dwarfs due to the large amount of sky coverage, geographical coverage with multiple telescopes, and significant observing time involved. Robotic telescopes such as LCOGT, ATLAS/WIST, and LSST could place interesting limits on the frequency of planets in the 3 Gyr continuously habitable zone of white dwarfs, possibly constraining frequencies as small as 0.1%.

Agol, Eric

2011-03-01

335

Quantitative Modelling of Human Spatial Habitability.  

National Technical Information Service (NTIS)

A model for the quantitative assessment of human spatial habitability is presented in the space station context. The visual aspect assesses how interior spaces appear to the inhabitants. This aspect concerns criteria such as sensed spaciousness and the af...

J. A. Wise

1985-01-01

336

Habitability of Enceladus: Planetary Conditions for Life  

Microsoft Academic Search

The prolific activity and presence of a plume on Saturn's tiny moon Enceladus offers us a unique opportunity to sample the\\u000a interior composition of an icy satellite, and to look for interesting chemistry and possible signs of life. Based on studies\\u000a of the potential habitability of Jupiter's moon Europa, icy satellite oceans can be habitable if they are chemically mixed

Christopher D. Parkinson; Mao-Chang Liang; Yuk L. Yung; Joseph L. Kirschivnk

2008-01-01

337

Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary  

PubMed Central

Abstract White and brown dwarfs are astrophysical objects that are bright enough to support an insolation habitable zone (IHZ). Unlike hydrogen-burning stars, they cool and become less luminous with time; hence their IHZ moves in with time. The inner edge of the IHZ is defined as the orbital radius at which a planet may enter a moist or runaway greenhouse, phenomena that can remove a planet's surface water forever. Thus, as the IHZ moves in, planets that enter it may no longer have any water and are still uninhabitable. Additionally, the close proximity of the IHZ to the primary leads to concern that tidal heating may also be strong enough to trigger a runaway greenhouse, even for orbital eccentricities as small as 10?6. Water loss occurs due to photolyzation by UV photons in the planetary stratosphere, followed by hydrogen escape. Young white dwarfs emit a large amount of these photons, as their surface temperatures are over 104 K. The situation is less clear for brown dwarfs, as observational data do not constrain their early activity and UV emission very well. Nonetheless, both types of planets are at risk of never achieving habitable conditions, but planets orbiting white dwarfs may be less likely to sustain life than those orbiting brown dwarfs. We consider the future habitability of the planet candidates KOI 55.01 and 55.02 in these terms and find they are unlikely to become habitable. Key Words: Extrasolar terrestrial planets—Habitability—Habitable zone—Tides—Exoplanets. Astrobiology 13, 279–291.

Heller, Rene

2013-01-01

338

Investigating habits: strategies, technologies and models  

PubMed Central

Understanding habits at a biological level requires a combination of behavioral observations and measures of ongoing neural activity. Theoretical frameworks as well as definitions of habitual behaviors emerging from classic behavioral research have been enriched by new approaches taking account of the identification of brain regions and circuits related to habitual behavior. Together, this combination of experimental and theoretical work has provided key insights into how brain circuits underlying action-learning and action-selection are organized, and how a balance between behavioral flexibility and fixity is achieved. New methods to monitor and manipulate neural activity in real time are allowing us to have a first look “under the hood” of a habit as it is formed and expressed. Here we discuss ideas emerging from such approaches. We pay special attention to the unexpected findings that have arisen from our own experiments suggesting that habitual behaviors likely require the simultaneous activity of multiple distinct components, or operators, seen as responsible for the contrasting dynamics of neural activity in both cortico-limbic and sensorimotor circuits recorded concurrently during different stages of habit learning. The neural dynamics identified thus far do not fully meet expectations derived from traditional models of the structure of habits, and the behavioral measures of habits that we have made also are not fully aligned with these models. We explore these new clues as opportunities to refine an understanding of habits.

Smith, Kyle S.; Graybiel, Ann M.

2014-01-01

339

Habitable worlds with no signs of life.  

PubMed

'Most habitable worlds in the cosmos will have no remotely detectable signs of life' is proposed as a biological hypothesis to be tested in the study of exoplanets. Habitable planets could be discovered elsewhere in the Universe, yet there are many hypothetical scenarios whereby the search for life on them could yield negative results. Scenarios for habitable worlds with no remotely detectable signatures of life include: planets that are habitable, but have no biosphere (Uninhabited Habitable Worlds); planets with life, but lacking any detectable surface signatures of that life (laboratory examples are provided); and planets with life, where the concentrations of atmospheric gases produced or removed by biota are impossible to disentangle from abiotic processes because of the lack of detailed knowledge of planetary conditions (the 'problem of exoplanet thermodynamic uncertainty'). A rejection of the hypothesis would require that the origin of life usually occurs on habitable planets, that spectrally detectable pigments and/or metabolisms that produce unequivocal biosignature gases (e.g. oxygenic photosynthesis) usually evolve and that the organisms that harbour them usually achieve a sufficient biomass to produce biosignatures detectable to alien astronomers. PMID:24664917

Cockell, Charles S

2014-04-28

340

Investigating habits: strategies, technologies and models.  

PubMed

Understanding habits at a biological level requires a combination of behavioral observations and measures of ongoing neural activity. Theoretical frameworks as well as definitions of habitual behaviors emerging from classic behavioral research have been enriched by new approaches taking account of the identification of brain regions and circuits related to habitual behavior. Together, this combination of experimental and theoretical work has provided key insights into how brain circuits underlying action-learning and action-selection are organized, and how a balance between behavioral flexibility and fixity is achieved. New methods to monitor and manipulate neural activity in real time are allowing us to have a first look "under the hood" of a habit as it is formed and expressed. Here we discuss ideas emerging from such approaches. We pay special attention to the unexpected findings that have arisen from our own experiments suggesting that habitual behaviors likely require the simultaneous activity of multiple distinct components, or operators, seen as responsible for the contrasting dynamics of neural activity in both cortico-limbic and sensorimotor circuits recorded concurrently during different stages of habit learning. The neural dynamics identified thus far do not fully meet expectations derived from traditional models of the structure of habits, and the behavioral measures of habits that we have made also are not fully aligned with these models. We explore these new clues as opportunities to refine an understanding of habits. PMID:24574988

Smith, Kyle S; Graybiel, Ann M

2014-01-01

341

Habit Interference in the White Rat and in Human Subjects  

Microsoft Academic Search

The literature of habit interference in animals is reviewed and the results are summarized, as a background for Hunter's investigation which sought to solve two problems, namely: (1) To plot a curve for habit interference; and (2) to secure data concerning the correlation existing between the ability to form a habit and the ability to break that habit by establishing

W. S. Hunter

1922-01-01

342

The effect of metrazol shock upon habit systems  

Microsoft Academic Search

2. Twenty-one schizophrenics undergoing metrazol therapy were compared with a control group. The techniques involved setting up a simple motor habit and then training in another habit which was similar to, but incompatible with, the first habit, thus necessitating the suppression of the first habit. The experimental group was then subjected to a metrazol shock, and both groups tested for

E. H. Rodnick

1942-01-01

343

Modelling motivation and habit in stable travel mode contexts  

Microsoft Academic Search

The habitual action perspective suggests that where transport users have strong travel choice habits, motivation will have no effect on behaviour. This has commonly been demonstrated in situations where travel habits and intentions conflict, but in stable decision settings intentions and habits will correspond. This paper reports two studies which demonstrate the moderating effect of habit on the intention–behaviour relationship

Benjamin Gardner

2009-01-01

344

Exploring Asymmetries in Circumstellar Environments: Winds, Disks, and Things that Go Clump in the Light  

NASA Technical Reports Server (NTRS)

The aim of the NASA LTSA grant is to develop Monte Carlo radiation transfer techniques for use in the analysis of data from stellar systems that exhibit evidence for extended, non-spherical circumstellar environments.

Wood, Kenneth

1999-01-01

345

TW HYA ASSOCIATION MEMBERSHIP AND NEW WISE-DETECTED CIRCUMSTELLAR DISKS  

SciTech Connect

We assess the current membership of the nearby, young TW Hydrae association and examine newly proposed members with the Wide-field Infrared Survey Explorer (WISE) to search for infrared excess indicative of circumstellar disks. Newly proposed members TWA 30A, TWA 30B, TWA 31, and TWA 32 all show excess emission at 12 and 22 {mu}m providing clear evidence for substantial dusty circumstellar disks around these low-mass, {approx}8 Myr old stars that were previously shown to likely be accreting circumstellar material. TWA 30B shows large amounts of self-extinction, likely due to an edge-on disk geometry. We also confirm previously reported circumstellar disks with WISE and determine a 22 {mu}m excess fraction of 42{sup +10}{sub -{sub 9}}% based on our results.

Schneider, Adam; Song, Inseok [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 (United States); Melis, Carl, E-mail: aschneid@physast.uga.edu, E-mail: song@physast.uga.edu, E-mail: cmelis@ucsd.edu [Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093 (United States)

2012-07-20

346

Imaging Circumstellar Disks around Young Stars by Multi-Aperture Interferometry.  

National Technical Information Service (NTIS)

Synthetic images of circumstellar accretion disks, computed from a model where the complete disk vertical structure is solved are presented. The emission processes in the disk are: thermal emission arising from accretion process, thermal emission due to r...

F. Malbet J. Monin J. Bouvier

1992-01-01

347

Habitable Extrasolar Planetary Systems; The Case of ? Cancri  

NASA Astrophysics Data System (ADS)

Having the largest number of extrasolar planets, a Neptune-mass object on a 2.8 days orbit, and an outer planet that orbits at 5.3 AU which is comparable to Jupiter's distance from the Sun, ? Cancri has become an interesting system for study of habitability. In this paper, we present the results of a study of the orbital evolution and dynamical stability of Earth-like planets in this system. Numerical integrations of the system, using the orbital parameters reported by McArthur et al. (2004), indicate that the system may not be stable. In search of stable planetary orbits, an extensive search of the parameter-space of the system was carried out, and a stable region was identified. Within this region, dynamical stability of an Earth-like planet in the habitable zone of the system was studied and two regions of stability were recognized. This research took place at the University of Hawaii's Institute for Astronomy during a Research Experience for Undergraduates (REU) internship funded by the National Science Foundation (NSF).

Cotto-Figueroa, D.; Haghighipour, N.

2005-12-01

348

High Resolution Spectroscopy of Vega-like Stars: Abundances and Circumstellar Gas  

NASA Technical Reports Server (NTRS)

Vega-like stars are main-sequence stars exhibiting excess infrared emission. In an effort to improve the information available on this class of star, 13 stars have been analyzed which have been classed as Vega-like, or have an infra-red excess attributable to dust in their circumstellar environment. In a separate paper stellar properties such as effective temperature and log g have been derived and in this poster we highlight the results of the photospheric abundance analysis also carried out during this work. King recently drew attention to the possible link between Vega-like stars and the photospheric metal-depleted class of A-stars, the Lambda Bootis stars. Since Vega-like stars are thought to have disks of dust, it might be expected that accretion of depleted gas onto the surface of these stars may cause this same phenomenon. In the 6 stars studied for depletions, none showed the extreme underabundance patterns observed in Lambda Bootis stars. However, depletions of silicon and magnesium were found in two of the sample, suggesting that these elements are in silicate dust grains in the circumstellar environment of these stars. Absorption lines attributed to circumstellar gas have been positively identified in three stars in our sample. Individual cases show evidence either of high-velocity outflowing gas, variability in the circumstellar lines observed, or evidence of circumstellar gas in excited lines of Fe II. No previous identification of circumstellar material has been made for two of the stars in question.

Dunkin, S. K.; Barlow, M. J.; Ryan, Sean G.

1996-01-01

349

How do habits guide behavior? Perceived and actual triggers of habits in daily life  

Microsoft Academic Search

What are the psychological mechanisms that trigger habits in daily life? Two studies reveal that strong habits are influenced by context cues associated with past performance (e.g., locations) but are relatively unaffected by current goals. Specifically, performance contexts – but not goals that were activated outside of awareness – triggered strongly habitual behaviors in memory (Experiment 1) and triggered overt

David T. Neal; Wendy Wood; Jennifer S. Labrecque; Phillippa Lally

350

Habitable Trinity: a new concept of a habitable environment beyond Earth  

NASA Astrophysics Data System (ADS)

Habitable Trinity, which is a newly proposed concept of a habitable environment for searching for life beyond Earth, is the coexistence of an atmosphere (consisting largely of C and N), an ocean (H and O), and a landmass (supplier of nutrients). It is the minimum requirement for the beginning of life as we know it.

Dohm, J.; Maruyama, S.

2013-09-01

351

Anthropic selection and the habitability of planets orbiting M and K dwarfs  

NASA Astrophysics Data System (ADS)

The Earth may have untypical characteristics which were necessary preconditions for the emergence of life and, ultimately, intelligent observers. This paper presents a rigorous procedure for quantifying such "anthropic selection" effects by comparing Earth's properties to those of exoplanets. The hypothesis that there is anthropic selection for stellar mass (i.e. planets orbiting stars with masses within a particular range are more favourable for the emergence of observers) is then tested. The results rule out the expected strong selection for low mass stars which would result, all else being equal, if the typical timescale for the emergence of intelligent observers is very long. This indicates that the habitable zone of small stars may be less hospitable for intelligent life than the habitable zone of solar-mass stars. Additional planetary properties can also be analyzed, using the approach introduced here, once relatively complete and unbiased statistics are made available by current and planned exoplanet characterization projects.

Waltham, Dave

2011-10-01

352

Combined Molecular and Isotopic Analysis of Circumstellar and Interplanetary Dust  

NASA Astrophysics Data System (ADS)

This thesis is comprised of two distinct lines of study. The first part reports the measurements of polycyclic aromatic hydrocarbons (PAHs) in 58 individual circumstellar graphite grains from primitive meteorites. PAH measurements were performed by two-step laser ionization mass spectrometry by colleagues at Stanford University. Each grain was later measured for C and N isotopic ratios by secondary ion mass spectrometry at Washington University. These grains are shown to have a circumstellar origin on the basis of their extremely anomalous C isotopic ratios. Independent C isotopic measurements of individual PAH molecules shows that most of the PAHs in these grains are isotopically normal. However, in several cases correlated isotopic anomalies are observed in individual PAH molecules and their parent graphite grains, demonstrating that the molecules are indigenous to the grains. The second part of this thesis reports the results of a H, C, and N isotopic survey of 'cluster' interplanetary dust particles (IDPs) collected by high flying aircraft. Cluster IDPs are identified as those IDPs that fragment into numerous pieces upon impact onto the collection surfaces. Cluster IDPs are shown to exhibit large and common excesses in deuterium (D) and 15N relative to terrestrial values. In some cases the D/H ratios of these cluster IDPs exceed previously observed values in meteorites and interplanetary dust particles, reaching a maximum value of D/H ~.008. The D/H and 14N/15N ratios of cluster IDPs are often heterogeneous on a several micron scale, indicating that the materials contained in these particles have remained largely unchanged since their aggregation. The large D/H ratios, and perhaps the large 15N/14N ratios, have resulted from chemical reactions occurring at very low (10 to 100 K) temperatures, probably occurring in the cold molecular cloud predating the formation of the Solar System. Cluster IDPs exhibit larger and much more common H and N isotopic anomalies than previously measured individual IDPs, suggesting that these two types of IDPs have different sources. Constraints on the orbital histories of these particles show that a plausible source of many of D-rich cluster IDPs is a single Earth-crossing comet, Schwassmann-Wachmann 3.

Messenger, Scott Roger

353

Signatures of Exo-solar Planets in Circumstellar Dust Disks  

NASA Astrophysics Data System (ADS)

Using a new numerical approach to the dynamics of minor bodies and dust particles, which enables us to increase, without using a supercomputer, the number of particle positions employed in each model up to 1010 -1011, a factor of 106-10^7 higher than existing numerical simulations. We apply this powerful approach to the high-resolution modeling of the structure and emission of circumstellar dust disks, incorporating all relevant physical processes. Here we examine the resonant structure of a dusty disk induced by the presence of one planet of mass in the range of (5? 10-5 - 5? 10-3) M* . It is shown that the planet, via mean-motion resonances and gravitational scattering, produces a number of characteristic features which include a central cavity void of dust and an asymmetric resonant dust belt with one, two, or more clumps. The results of our study reveal a remarkable similarity with various types of highly asymmetric circumstellar disks observed with the JCMT around Epsilon Eridani and Vega. Using our modeling, we find that Vega may have a massive planet at a distance of 50-60 AU, and ? Eri may have a less massive planet at a similar distance of 55-65 AU. This conclusion is testable: the above asymmetric feature is expected to revolve around the star with an angular velocity of 1.2-1.6 deg/year (Vega) and 0.6-0.8 deg/year (? Eri) -- a prediction that can be tested within several years. In sum, our modeling offers the ability to determine the major orbital parameters and masses of planets in dusty disks. As these disks are common, many planetary systems may be found. Moreover, since the dust disks are often so bright that they prevent direct detection of planets with imaging at visible, IR, or sub-mm wavelengths, our technique may actually be helpful in finding more planets than other methods do. It is well worth the efforts to develop the modeling and interpretation for observations already available or obtainable in the near future. This work has been supported by NASA Grant NAG5-7065 to George Mason University. N.G. acknowledges the NRC-NAS associateship.

Ozernoy, L.; Gorkavyi, N.; Mather, J.; Taidakova, T.

1999-12-01

354

Unveiling the circumstellar environment toward a massive young stellar object  

NASA Astrophysics Data System (ADS)

Aims: As a continuation of a previous work, in which we found strong evidence of massive molecular outflows toward a massive star-forming site, we present a new study of this region based on very high angular resolution observations with the aim of discovering the outflow-driven mechanism. Methods: Using near-IR data acquired with Gemini-NIRI at the broad H- and Ks-bands, we studied a region of 22'' × 22'' around the UCHii region G045.47+0.05, a massive-star forming site at a distance of about 8 kpc. To image the source with the highest spatial resolution possible we employed the adaptative optics system ALTAIR, achieving an angular resolution of about 0.15 arcsec. Results: We discovered a cone shaped nebula that has an opening angle of about 90° and extends eastward of the IR source 2MASS J19142564+1109283, which is very likely a massive young stellar object (MYSO). This morphology suggests a cavity that was cleared in the circumstellar material, and its emission may arise from scattered continuum light, warm dust, and probably also from emission lines from shock-excited gas. The nebula, which presents arc-like features, is connected with the IR source through a jet-like structure, which is aligned with the blueshifted CO outflow found in a previous study. The near-IR structure lies ~3'' north of the radio continuum emission, revealing that it is not spatially coincident with the UCHii region. The observed morphology and structure of the near-IR nebula strongly suggest the presence of a precessing jet. We resolved the circumstellar environment (in scale of a thousand AU) of a distant MYSO, indeed one of the farthest currently known. Reduced NIR data (FITS) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/559/L2

Paron, S.; Fariña, C.; Ortega, M. E.

2013-11-01

355

Formation of Habitable Planets in Inclined Planetary Systems  

NASA Astrophysics Data System (ADS)

We extensively investigate the terrestrial planetary formation for the inclined planetary systems (considering the OGLE-2006-BLG-109L system as example) in the late stage. In the simulations, we show that the occurrence of terrestrial planets appears to be common in the final assembly stage. Moreover, we find that a lot of runs finally occupy at least one planet in the habitable zone (HZ). On the other hand, the numerical results also indicate that the inner region of the planetesimal disk, ranging from ~ 0.1 to 0.3 AU, plays an important role in building up terrestrial planets. The outcomes suggest that it may exist moderate possibility for the inclined systems to harbor terrestrial planets in the HZ.

Ji, Jianghui; Jin, Sheng

2014-04-01

356

The MEarth project: searching for transiting habitable super-Earths around nearby M dwarfs  

Microsoft Academic Search

Due to their small radii, M dwarfs are very promising targets to search for transiting super-Earths, with a planet of 2 Earth radii orbiting an M5 dwarf in the habitable zone giving rise to a 0.5% photometric signal, with a period of two weeks. This can be detected from the ground using modest-aperture telescopes by targeting samples of nearby M

Jonathan Irwin; David Charbonneau; Philip Nutzman; Emilio Falco

2009-01-01

357

An Optical Study of the Circumstellar Environment Around the Crab Nebula  

NASA Technical Reports Server (NTRS)

Long-slit spectra of two peripheral regions around the Crab Nebula show no H(alpha) emission down to a flux level of 1.5 x 10(exp -7)erg/sq cm s sr (0.63 Rayleigh), corresponding to an emission measure limit of 4.2 cm(sup - 6) pc (3(sigma)) assuming A(sub V)= 1.6(sup m) and T(sub e)=7000 K. This is below the flux levels reported by Murdin & Clark (Nature, 294, 543 (198 1)) for an H(alpha) halo around the Crab. Narrow H(beta) emission as described by Murdin (MNRAS, 269, 89 (1994)) is detected but appears to be Galactic emission unassociated with the remnant. A review of prior searches indicates no convincing observational evidence to support either a high- or low-velocity envelope around the remnant. Spectral scans confirm a well-organized, N-S expansion asymmetry of the filaments with a approx. 500 km/s central velocity constriction as described by MacAlpine et al. (ApJ, 342, 364 (1989)) and Lawrence et (it. (AJ, 109, 2635 (1995)] but questioned by Hester et al. (ApJ, 448, 240 (1995)). The velocity pinching appears to coincide with an cast-west chain of bright [O III] and helium-rich filaments. This expansion asymmetry might be the result of ejecta interaction with a disk of circumstellar matter, but such a model may be inconsistent with H and He filament abundances in the velocity constriction zone. A re-analysis of the remnant's total mass suggests that the filaments contain 4.6 +/- 1.8 M(solar) in ionized and neutral cas, about twice that of earlier estimates. For a 10M(solar) progenitor, this suggests that approx.equals 4M(solar) remains to be detected in an extended halo or wind.

Fesen, Robert A.; Shull, J. Michael; Hurford, Alan P.

1997-01-01

358

A circumstellar disk associated with a massive protostellar object.  

PubMed

The formation process for stars with masses several times that of the Sun is still unclear. The two main theories are mergers of several low-mass young stellar objects, which requires a high stellar density, or mass accretion from circumstellar disks in the same way as low-mass stars are formed, accompanied by outflows during the process of gravitational infall. Although a number of disks have been discovered around low- and intermediate-mass young stellar objects, the presence of disks around massive young stellar objects is still uncertain and the mass of the disk system detected around one such object, M17, is disputed. Here we report near-infrared imaging polarimetry that reveals an outflow/disk system around the Becklin-Neugebauer protostellar object, which has a mass of at least seven solar masses (M(o)). This strongly supports the theory that stars with masses of at least 7M(o) form in the same way as lower mass stars. PMID:16136137

Jiang, Zhibo; Tamura, Motohide; Fukagawa, Misato; Hough, Jim; Lucas, Phil; Suto, Hiroshi; Ishii, Miki; Yang, Ji

2005-09-01

359

Evidence for Remnant Circumstellar Disks Around POST T Tauri Stars  

NASA Astrophysics Data System (ADS)

We report infrared imaging photometry at 6.5 and 15 microns of seven Lindroos binaries selected by Pallavicini et al. as ``almost certain physical pairs''. The photometry was obtained in order to asses whether these adolescent (10---100 Myr) low-mass stars still have infrared excess emission which could be caused by a circumstellar disk. Given the current status of the calibration, we could not perform precise photometry or reliable modeling. We could, however, for the subset of the sample that was well observed, compare the ratio of Fs/Fl for the primaries versus the secondaries. This ratio is significantly higher for the primaries than for the companions, and for the primaries it is close to the expected value for a Rayleigh-Jeans spectrum. While some of the difference is due to the cooler energy distribution of the late-type photospheres, we conclude that a significant part of the difference must come from excess emission at 15 microns, and thus we have found clear evidence for remnant disks. Based on observations with ISO, and ESA project funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.

Moneti, A.; Zinnecker, H.; Kunkel, M.; Preibish, T.

360

Rapid disappearance of a warm, dusty circumstellar disk  

NASA Astrophysics Data System (ADS)

Stars form with gaseous and dusty circumstellar envelopes, which rapidly settle into disks that eventually give rise to planetary systems. Understanding the process by which these disks evolve is paramount in developing an accurate theory of planet formation that can account for the variety of planetary systems discovered so far. The formation of Earth-like planets through collisional accumulation of rocky objects within a disk has mainly been explored in theoretical and computational work in which post-collision ejecta evolution typically is ignored, although recent work has considered the fate of such material. Here we report observations of a young, Sun-like star (TYC824126521) where infrared flux from post-collisional ejecta has decreased drastically, by a factor of about 30, over a period of less than two years. The star seems to have gone from hosting substantial quantities of dusty ejecta, in a region analogous to where the rocky planets orbit in the Solar System, to retaining at most a meagre amount of cooler dust. Such a phase of rapid ejecta evolution has not been previously predicted or observed, and no currently available physical model satisfactorily explains the observations.

Melis, Carl; Zuckerman, B.; Rhee, Joseph H.; Song, Inseok; Murphy, Simon J.; Bessell, Michael S.

2012-07-01

361

On circumstellar disks: Spitzer identifies two possible evolutionary paths  

NASA Astrophysics Data System (ADS)

Multi-wavelength surveys have vastly improved our understanding of many astrophysical objects, in particular, circumstellar disks. We present our results for the disk population of the young cluster NGC 2264. Our study was based on data obtained with the Infrared Array Camera (IRAC) and the Multiband Imaging Photometer on board the Spitzer Space Telescope combined with previously published optical data. We divide the disk population into 3 classes based on their spectral energy distribution shapes: optically thick disks, homologously depleted anemic disks, and radially depleted transition disks. We find that there are two distinct evolutionary paths for disks: a homologous one, where the disk emission decreases uniformly in NIR and mid-infrared wavelengths (anemic disks) and throughout which most sources pass, and a radially differential one where the emission from the inner region of the disk decreases more rapidly than from the outer region (transition disks). Whether a disk evolves in a homologously or radially depleted fashion is still unknown and may depend on the nature of planet formation in the disk.

Teixeira, Paula S.; Lada, Charles J.; Marengo, Massimo; Lada, Elizabeth

362

The formation and structure of circumstellar and interstellar dust  

NASA Technical Reports Server (NTRS)

The intriguing abundance of long linear carbon chain molecules in some dark clouds and in circumstellar shells is still not well understood. Recent laboratory studies which have probed this problem indicate that when carbon vapor nucleates to form particles, linear chains and hollow cage molecules (fullerenes) also form at more-or-less the same time. The results have consequences for the formation, structures and spectroscopic properties of the molecular and dust components ejected from cool carbon-rich stars. A most interesting result of the experimental observations relates to the probability that a third character in addition to the chains and grains, the C(sub 60) molecule probably in the form of the ion C(sub 60)(sup +) in the less shielded regions, is present and perhaps responsible for some of the ubiquitously observed interstellar spectroscopic features such as the Diffuse Interstellar Features, the 2170A UV Absorption or perhaps some of the Unidentified Infrared Bands. Further study of small carbon particles which form in the gas phase has resulted in the discovery that they have quasi-icosahedral spiral shell structures. The role that such species may play in the interstellar medium as well as that played by C(sub 60) (or C sub 60 sup +) should soon be accessible to verification by a combination of laboratory experiment and astronomical spectroscopy.

Kroto, H. W.

1990-01-01

363

ON THE EXCITATION AND FORMATION OF CIRCUMSTELLAR FULLERENES  

SciTech Connect

We compare and analyze the Spitzer mid-infrared spectrum of three fullerene-rich planetary nebulae in the Milky Way and the Magellanic Clouds: Tc1, SMP SMC 16, and SMP LMC 56. The three planetary nebulae share many spectroscopic similarities. The strongest circumstellar emission bands correspond to the infrared active vibrational modes of the fullerene species C{sub 60} and little or no emission is present from polycyclic aromatic hydrocarbons. The strengths of the fullerene bands in the three planetary nebulae are very similar, while the ratios of the [Ne III]15.5 {mu}m/[Ne II]12.8 {mu}m fine structure lines, an indicator of the strength of the radiation field, are markedly different. This raises questions about their excitation mechanism and we compare the fullerene emission to fluorescent and thermal models. In addition, the spectra show other interesting and common features, most notably in the 6-9 {mu}m region, where a broad plateau with substructure dominates the emission. These features have previously been associated with mixtures of aromatic/aliphatic hydrocarbon solids. We hypothesize on the origin of this band, which is likely related to the fullerene formation mechanism, and compare it with modeled hydrogenated amorphous carbon that present emission in this region.

Bernard-Salas, J.; Jones, A. P. [Institut d'Astrophysique Spatiale, CNRS/Universite Paris-Sud 11, F-91405 Orsay (France); Cami, J.; Peeters, E.; Micelotta, E. R. [Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7 (Canada); Groenewegen, M. A. T., E-mail: jbernard@ias.u-psud.fr [Royal Observatory of Belgium, Ringlaan 3, B-1180 Brussels (Belgium)

2012-09-20

364

Molecular catastrophes and circumstellar SiO masers  

NASA Technical Reports Server (NTRS)

Understanding the complex SiO maser regions of highly evolved stars can be improved through multiwavelength studies of 'premaser' stars, such as M0-M4 giants and semiregular variables, which can be placed on normal H-R diagrams unlike most of the OH-IR stars. I argue that SiO masers are a key part of the transformation of hot stellar plasma into cold circumstellar silicate dust, in the outflows from evolved, oxygen rich stars. Evidence for this statement rests on the following: (1) red giant mass loss originates in a stochastic, amsotropic manner; (2) SiO maser maps of Miras and red supergiants show numerous maser spots within a few stellar radii; (3) molecules and dust naturally form in a cooling outflow; (4) the IRAS Low Resolution Spectrometer provided evidence for diverse and variable 10 micron silicate features in Miras, and these shapes correlate well with the proposed maser chronology, suggesting a formation and annealing sequence. The theory for the occurrence of SiO masers involving thermal instability, related 'new' physics, recent calculations and a prediction are discussed.

Stencel, Robert E.

1993-01-01

365

Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes  

NASA Technical Reports Server (NTRS)

Production of polycyclic aromatic hydrocarbons in carbon-rich circumstellar envelopes was investigated using a kinetic approach. A detailed chemical reaction mechanism of gas-phase PAH formation and growth, containing approximately 100 reactions of 40 species, was numerically solved under the physical conditions expected in cool stellar winds. The chemistry is based on studies of soot production in hydrocarbon pyrolysis and combustion. Several first-ring and second-ring cyclization processes were considered. A linear lumping algorithm was used to describe PAH growth beyond the second aromatic ring. PAH production using this mechanism was examined with respect to a grid of idealized constant velocity stellar winds as well as several published astrophysical models. The basic result is that the onset of PAH production in the interstellar envelopes is predicted to occur within the temperature interval of 1100 to 900 K. The absolute amounts of the PAHs formed, however, are very sensitive to a number of parameters, both chemical and astrophysical, whose values are not accurately known. Astrophysically meaningful quantities of PAHs require particularly dense and slow stellar winds and high initial acetylene abundance. It is suggested that most of the PAHs may be produced in a relatively small fraction of carbon-rich red giants.

Frenklach, Michael; Feigelson, Eric D.

1989-01-01

366

Light echo detection of circumstellar disks around flaring stars  

NASA Technical Reports Server (NTRS)

Light echoes can be used to detect and characterize disks around flaring stars. Such disks are thought to be a hallmark of planet formation but are very difficult to detect by ordinary means. Dwarf emission-line M stars experience flares with luminosities comparable to their quiescent photospheres on time scales of minutes, less than the light travel time across a disk many astronomical units in extent; they are thus ideal candidates for such a search. Bromley (1992) calculated that the detection of Jupiter-sized companions using light echoes requires photometric accuracies better than 1 part in 10(exp 6). However, a disk consisting of grains or small bodies will scatter a much larger fraction of the light than a planet of similar mass. I estimate the light echo amplitutdes from plausible geometries of circumstellar material and present simulation light curves. The light echo amplitudes are typically 1% of the flare and I conclude that such events will be detected best in cases where the flare is eclipsed by the star. An examination of the time scales associated with internal processes in a protoplanetary disks around dM stars indicates that any primordial disks may become undetectable in 10(exp 4) years and will have completely disappeared by 10(exp 8) years, the estimated age of dMe stars in the solar neighborhood. However, searches for light echoes might constrain the amount of material continuing to fall into these young stellar systems in the form of comet-like objects.

Gaidos, Eric J.

1994-01-01

367

Rotation and Circumstellar Disks in the Upper Scorpius OB Association  

NASA Astrophysics Data System (ADS)

We present projected rotational velocities for 20 early-type (B8-A9) and 53 late-type (F3-M8) members of the 5 Myr old Upper Scorpius OB Association determined using high-resolution optical spectra obtained with HIRES on Keck I and MIKE on the Magellan Clay telescope. The Upper Scorpius sample is composed of primordial and debris disk systems as well as non-excess sources. We combine the derived rotational velocities with Spitzer Space Telescope IRAC 4.5 and 8.0 ?m fluxes and MIPS 24.0 ?m photometry in order to examine the influence of circumstellar disks upon stellar and substellar rotation. Preliminary results using non-parametric statistical tests suggest that the late-type disk-bearing members of Upper Scorpius rotate more slowly than their non-disk counterparts at a high confidence level. Among the early-type Upper Scorpius members, however, the rotational velocities of the debris-disk and non-disk populations are indistinguishable. We also compare the rotational velocities of the Upper Scorpius sample with those of similar sources in Taurus-Auriga, where stars may not have had sufficient time to undergo disk-braking.

Dahm, Scott E.; Slesnick, C. L.

2011-01-01

368

Computer Simulation of Dust Growth in Massive Circumstellar Disks  

NASA Astrophysics Data System (ADS)

In the frame of solving problem of abiogenous synthesis matter in solar system, one of the most important problems is forming of bodies with radii longer 100 km. According to the astrocatalysis hypothesis carbohydrates are synthesized on the surface of the dust particles in the circumstellar protoplanetary disk, which provide coagulation of these particles, and therefore bodies with radii 1-10 m appear. There is problem of principle mechanism of forming of 100-km-sized bodies, as meter-sized bodies are destroyed due to collisions when they are moving with orbital velocities. There is a collisionless mechanism for solving this problem. The main point of it lies in possibility of appearance of gravitational instability in two-phase gas-dust environment. The probability depends on average particles mass. Thus it is necessary to study the influence of particles growth on increasing of the gravitational instability. One of the most suitable instruments for studying evolution of the protoplanetary disk is mathematical modeling with numerical supercomputer experiments. Numerical model was developed with primary particle motion in the equatorial plane of the disk. The model describes gas dynamics and particle motion including coagulation process. Since the numerical algorithm is based on splitting method. It was made investigation of the influence of the coagulation rate on dynamics of gas-dust system for the initial conditions in which gravitational instability appeared. It was found that coagulation rate can increase the instability rate. For some parameters it can significantly influence on the structure of the disk.

Markelova, Tamara; Snytnikov, Valeriy

2013-07-01

369

RESOLVING THE CIRCUMSTELLAR DISK OF HL TAURI AT MILLIMETER WAVELENGTHS  

SciTech Connect

We present results of high-resolution imaging toward HL Tau by the Combined Array for Research in Millimeter-wave Astronomy. We have obtained {lambda} = 1.3 mm and 2.7 mm dust continua with an angular resolution down to 0.''13. Through simultaneous model fitting to the two wavelength data sets in Bayesian inference using a flared viscous accretion disk model, we estimate the physical properties of HL Tau, such as density distribution, dust opacity spectral index, disk mass, disk size, inclination angle, position angle, and disk thickness. HL Tau has a circumstellar disk mass of 0.13 M{sub sun}, a characteristic radius of 79 AU, an inclination of 40{sup 0}, and a position angle of 136{sup 0}. Although a thin disk model is preferred by our two wavelength data sets, a thick disk model is needed to explain the high mid- and far-infrared emission of the HL Tau spectral energy distribution. This could imply large dust grains settled down on the midplane with fine dust grains mixed with gas. The HL Tau disk is likely gravitationally unstable and can be fragmented between 50 and 100 AU of radius. However, we did not detect dust thermal continuum supporting the protoplanet candidate claimed by a previous study using observations of the Very Large Array at {lambda} = 1.3 cm.

Kwon, Woojin; Looney, Leslie W. [Department of Astronomy, University of Illinois, Urbana, IL 61801 (United States); Mundy, Lee G., E-mail: wkwon@illinois.edu [Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)

2011-11-01

370

On the Chemistry of Circumstellar Disk Around MWC349A  

NASA Astrophysics Data System (ADS)

The evolutionary status of MWC 349A - the unique source of hydrogen maser and laser radiation arising in a massive circumstellar disk - is still a matter of debate. One way to shed light on this issue would be to measure the isotopic composition of the disk. We performed computer simulations of the chemical composition of the disk using the package CLOUDY (Ferland et al. 1998). The best agreement between the calculated hydrogen line strength ratios and those measured by Hamann and Simon (1986) was achieved for a model with a central (black body) star of T ? 20,000 K and a disk with the inner radius 1014 cm, hydrogen density at the inner radius 108 cm-3 and the r-2 drop of density with radius. We present the column densities predicted by CLOUDY for observed (CO) and several not yet observed molecules containing the major isotopes of C, N, O and discuss the prospects of detecting and measuring the radio lines of these molecules and their isotopologues with existing and forthcoming facilities. This project was supported by NSF/REU grant AST-0851892, the Presidential Award for Excellence in Science, Mathematics and Engineering Mentoring, and the Nantucket Maria Mitchell Association.

Lagergren, Kristen; Bans, A.; Strelnitski, V.

2012-01-01

371

Thermal structure of circumstellar discs around A stars  

NASA Astrophysics Data System (ADS)

IRAS and ISO observations revealed that a large number of A stars is surrounded by dust discs. At least for a subgroup, the lambda Bootis stars, there is growing evidence that these are pre-main-sequence stars surrounded by gas and dust discs (Holweger & Rentzsch-Holm 1995; Holweger, Hempel & Kamp 1999). In order to understand the physical and chemical processes in such circumstellar discs, we developed thin hydrostatic equilibrium models which contain a sophisticated chemical network and account for the detailed photospheric flux of the central star (Kamp & Bertoldi 1999). In a first approximation we assumed that gas and dust have the same temperature due to strong collisional coupling, and that the temperature is determined by the radiative equilibrium of the dust. Now we present disc models where the chemical equilibrium is solved subject to the energy balance of gas and dust. We take into account a large number of heating and cooling processes in detail, like for example heating by photodissociation or formation of H_2 or cooling by C and O fine structure lines. We find that frictional heating by dust grains may be the dominant heating source depending on the drift velocity of dust grains and hence the efficiency of radiation pressure. In the absence of a large grain drift velocity gas-dust collisions determine the gas temperature for densities larger than 10^6 cm^{-3}.

Kamp, I.; Holweger, H.

372

CO & O Emission Lines In Tenuous Circumstellar Disks  

NASA Astrophysics Data System (ADS)

CO emission lines have been used to observe Circumstellar Disks around A-type stars. Combined CO gas and IR dust observations have detected a gas-to-dust ratio much lower than the interstellar gas-to-dust ratio. The reason is that CO is a poor tracer of gas; most of the gas is in the form of H2. The main motivation of the project was twofold: (1) to show at which disk masses CO ceases to be a good tracer of the disk mass and (2) how reliable and sensitive other tracers such as C, C+ and O will be using future instruments such as SOFIA, Herschel, APEX and ALMA. Emission line calculations are based on the disk models of Kamp & Bertoldi (2000) and Kamp & van Zadelhoff (2001). The 2D Monte Carlo radiative transfer code from Hogerheijde & van der Tak (2000) was used to calculate the line emission from the optically thin disk models. Four different disk masses were used - spanning four orders of magnitude - to create the line profiles for CO and O. The integrated line fluxes for typical distance and inclination are presented here to enable an approximation of the disk mass from the observations. Contrary to previous studies - that assume constant conversion factors - these new results take into account the varying tracer abundance and excitation temperature within the disk. We acknowledge the Summer Student Program at STScI for the support of this project.

Enriquez, J. E.; Kamp, I.

2005-12-01

373

Environmental Signatures for Habitability: What to Measure and How to Rank the Habitability Potential of Mars  

NASA Technical Reports Server (NTRS)

The environmental signatures for habitability are not necessarily biosignatures, even though on Earth, they are definitive proof of habitability. It is the constant overprint of the chemical signatures of life that makes it difficult to recognize the chemical and physical properties of a potentially habitable environment as distinct from an inhabited one. Mars Science Laboratory (MSL) will soon embark on a mission to Mars to assess its past or present habitability, so it is useful to examine how we measure habitability on Earth and prepare for how that approach may differ for Mars. This exercise includes: (a) articulation of fundamental assumptions about habitability, (b) an inventory of factors that affect habitability, (c) development of metrics, measurement approach and implementation, and (d) a new classification scheme for planetary habitability that goes beyond the binary "yes" or "no." There may be dozens of factors that affect habitability and they can be weighted as a function of specific environment. However a robotic, in situ investigation even on Earth has constraints that prevent the measurement of every environmental factor, so metrics must be reduced to the most relevant subset, given available time, cost, technical feasibility and scientific importance. Many of the factors could be measured with a combination of orbital data and the MSL payload. We propose that, at a minimum, a designation of high habitability potential requires the following conditions be met: (a) thermally stable with respect to extremes and frequency of fluctuation, (b) has more than one energy source, (c) sufficient chemical diversity to make compounds with covalent and hydrogen bonding, (d) can moderate ionizing radiation enough to allow a stable or evolving pool of organic molecules, (e) must have water or other high quality polar solvent, (f) must be able to renew chemical resources (e.g., plate tectonics, volcanism or something else we haven't envisioned). A measurement approach we have taken to measure habitability on Earth is : 1. Study remote sensing data, maps, etc. 2. Decide how big an area to measure. 3. Determine the spatial sampling rate. 4. Determine the temporal sampling rate. 5. Determine the order of measurements 6. Decide where to begin measurements 7. Select locations at field site and proceed While science drives each of the steps, there are additional constraints, e.g., technical, time, cost, safety (risk). This approach is also executable on Mars. Measurement of past habitability is more challenging both for Earth and Mars where access to the past means subsurface access and confrontation with unknowns about preservation of the martian past. Some environments preserve evidence of past habitability better than others, and this is where selection of the landing site to maximize the preservation potential of habitability indicators will be key. Mars presents an opportunity to discover transitional states between habitable or not, and we offer a ranking scale for planetary habitability with Mars as the second test subject: CLASS ONE Uninhabitable and likely has never been so CLASS TWO Has a high potential but no confirmed observation of life (as defined above) CLASS THREE Inhabited (we find life) 3-A Globally inhabited 3-B Primitive life; early in its evolution, but not yet globally established 3-C Exists only in refugia -- planet heading toward class four CLASS FOUR Post-habitable (there once was life, but now it's gone) MSL provides an opportunity to carefully investigate the habitability of at least one site on Mars and it will reveal much about the possible states of planetary habitability

Conrad, Pamela G.; Eigenbrode, Jennifer L.; Mahaffy, Paul M.; Steele, Andrew

2011-01-01

374

Deciphering spectral fingerprints of habitable exoplanets.  

PubMed

We discuss how to read a planet's spectrum to assess its habitability and search for the signatures of a biosphere. After a decade rich in giant exoplanet detections, observation techniques have advanced to a level where we now have the capability to find planets of less than 10 Earth masses (M(Earth)) (so-called "super Earths"), which may be habitable. How can we characterize those planets and assess whether they are habitable? This new field of exoplanet search has shown an extraordinary capacity to combine research in astrophysics, chemistry, biology, and geophysics into a new and exciting interdisciplinary approach to understanding our place in the Universe. The results of a first-generation mission will most likely generate an amazing scope of diverse planets that will set planet formation, evolution, and our planet into an overall context. PMID:20307185

Kaltenegger, Lisa; Selsis, Frank; Fridlund, Malcolm; Lammer, Helmut; Beichman, Charles; Danchi, William; Eiroa, Carlos; Henning, Thomas; Herbst, Tom; Léger, Alain; Liseau, René; Lunine, Jonathan; Paresce, Francesco; Penny, Alan; Quirrenbach, Andreas; Röttgering, Huub; Schneider, Jean; Stam, Daphne; Tinetti, Giovanna; White, Glenn J

2010-01-01

375

Information systems - Issues in global habitability  

NASA Technical Reports Server (NTRS)

The present investigation is concerned with fundamental issues, related to information considerations, which arise in an interdisciplinary approach to questions of global habitability. Information system problems and issues are illustrated with the aid of an example involving biochemical cycling and biochemical productivity. The estimation of net primary production (NPP) as an important consideration in the overall global habitability issue is discussed. The NPP model requires three types of data, related to meteorological information, a land surface inventory, and the vegetation structure. Approaches for obtaining and processing these data are discussed. Attention is given to user requirements, information system requirements, workstations, network communications, hardware/software access, and data management.

Norman, S. D.; Brass, J. A.; Jones, H.; Morse, D. R.

1984-01-01

376

Looking for a habitable planet  

NASA Astrophysics Data System (ADS)

Only very favorable combination of many physical parameters may provide the necessary con-ditions for unicellular organisms to evolve into multicellular animals. The main factors of the planet, that is critical for the evolution and existence of life, form a peculiar labyrinth with many impasses. Most important are mass and temperature conditions on the planet. The planet that meets RNA/ DNA life requirements must have: •a mass about 5E27 g; •some zones with a favorable thermal conditions (273-340K); •an atmosphere that is able to absorb an external hard radiation but transparent for photons with 1-3 eV energy; •a sufficient den-sity of a stellar radiation; •presence of other sources of energy, e.g. of oxidation species in the atmosphere; •a moderate gravitation; •open water with big islands or continents; •a moderate rotation period; •a moderate eccentricity of the orbit; •a moderate inclination of equator plane to the orbit plane; •an intensive meteoritic impacts or other cosmic catastrophes that stimulate evolution of the most perfect beings; •one or more massive satellites; •an intensive volcanism and/or plate tectonics.

Ksanfomality, Leonid

377

Returning from the War Zone: A Guide for Families of Military Members  

MedlinePLUS

... more strongly to common family issues . Anger and aggression are common war zone stress reactions . Even minor ... Problems doing regular tasks at work or school ? Aggressive driving habits Understanding the Military Mindset Most service ...

378

Long-term variation in fruiting and the food habits of wild Japanese macaques on Kinkazan Island, northern Japan.  

PubMed

We studied the relationship between the food habits of Japanese macaques (Macaca fuscata) and the availability of nuts (Fagus crenata, Zelkova serrata, Carpinus spp., and Torreya nucifera) on Kinkazan Island, northern Japan, from 1997 to 2005 to examine the long-term variations in both food habits and availability. The food habits of the monkeys showed clear seasonal changes: the staple foods were woody leaves and flowers in spring (May and June), woody leaves and seeds or fruits other than nuts in summer (July and August), nuts and seeds or fruits other than nuts in fall (September-November), and herbaceous plants in winter (December-April). The availability of nuts, combinations of masting species, and energy production varied among years. Food habits varied among years, but the magnitude of variability of food habits differed among seasons, with large variability during summer and winter, and small variability in spring. Food availability was poor in summer and winter, but in several years the monkeys were able to consume nuts during those seasons. We emphasize the importance of conducting long-term studies on both food availability and the food habits of animals in the temperate zone. PMID:17044008

Tsuji, Yamato; Fujita, Shiho; Sugiura, Hideki; Saito, Chiemi; Takatsuki, Seiki

2006-11-01

379

Food Habits: A Selected Annotated Bibliography  

ERIC Educational Resources Information Center

This is a selective annotated bibliography of material on food habits and factors affecting them, published during the period 1928-1972. References are mainly in English, although a few in European languages are included, and represent information primarily from scholarly and professional journals. Entries are organized by subject and author. (LK)

Wilson, Christine S.

1973-01-01

380

Improving Recreational Reading Habits of Elementary Students.  

ERIC Educational Resources Information Center

A study investigated the effectiveness of a program for improving the recreational reading habits of elementary students through the use of cross-age tutoring in critical reading strategies. The targeted population consisted of a kindergarten and a fourth-grade class in the growing upper-middle-class community of Geneva, Illinois, located…

Krug, Marline; Fordonski, Patricia

381

The incidence of nervous habits in children  

Microsoft Academic Search

A method of measuring nervous habits has been described which yields a quantitative score of determinable reliability for each individual. The unit of measurement may be defined in general as one or more specified overt reactions per stated unit of time. Successive observations were made to secure a differentiation in terms of amount for each child. The data obtained indicate

W. C. Olson

1930-01-01

382

Publication Habits of Psychologists in Canadian Universities  

Microsoft Academic Search

A survey of publication habits of psychologists was made. Histograms of publication frequency were prepared for biology, chemistry, history, mathematics, political science, psychology, physics and sociology. Psychologists were most like sociologists both in the mean number of publications per individual and in the form the distribution assumes. Other clusters also appear. Assistant, associate and full professors holding staff appointments in

Thomas M. Nelson; Wayne Poley

1971-01-01

383

Habitual Creativity: Revising Habit, Reconceptualizing Creativity  

Microsoft Academic Search

Current psychological scholarship is based on a dichotomy between habit, associated with automatic reflex behavior, and creativity, which involves deliberation, purpose and heuristic procedures. However, this account is problematic and contradicts everyday experience where mastery, for instance, is one of the highest levels of creative performance achieved within a habitual practice. This article argues that such a separation misrepresents both

Vlad Petre Gl?veanu

2012-01-01

384

A normative study of family media habits  

Microsoft Academic Search

The present study documents family media habits in six areas: electronic and print Media Use, parental Monitoring of children's media, parental Consistency regarding rules for children's media use, parents' reports of observable Media Effects on their children, parents' Knowledge about media and media effects, and how much children participate in Alternative Activities to electronic media. A random national sample of

Douglas A. Gentile; David A. Walsh

2002-01-01

385