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1

New Candidate Eruptive Young Stars in Lynds 1340  

NASA Astrophysics Data System (ADS)

We report on the discovery of three candidate eruptive young stars, found during our comprehensive multi-wavelength study of the young stellar population of the dark cloud L1340. These stars are as follows. (1) IRAS 02224+7227 (2MASS 02270555+7241167, HH 487S) exhibited FUor-like spectrum in our low-resolution optical spectra. The available photometric data restrict its luminosity to 23 L ? < L bol < 59 L ?. (2) 2MASS 02263797+7304575, identified as a classical T Tauri star during our H? survey, exhibited an EXor-type brightening in 2005 November at the time of the Sloan Digital Sky Survey observations of the region. (3) 2MASS 02325605+7246055, a low-mass embedded young star, associated with a fan-shaped infrared nebula, underwent an outburst between the DSS 1 and DSS 2 surveys, leading to the appearance of a faint optical nebula. Our [S II] and H? images, as well as the Spitzer Infrared Array Camera 4.5 ?m images, revealed Herbig-Haro objects associated with this star. Our results suggest that amplitudes and timescales of outbursts do not necessarily correlate with the evolutionary stage of the stars.

Kun, M.; Apai, D.; O'Linger-Luscusk, J.; Moór, A.; Stecklum, B.; Szegedi-Elek, E.; Wolf-Chase, G.

2014-11-01

2

Radial velocity variations in the young eruptive star EX Lupi  

NASA Astrophysics Data System (ADS)

Context. EX Lup-type objects (EXors) are low-mass pre-main sequence objects characterized by optical and near-infrared outbursts attributed to highly enhanced accretion from the circumstellar disk onto the star. Aims: The trigger mechanism of EXor outbursts is still debated. One type of theory requires a close (sub)stellar companion that perturbs the inner part of the disk and triggers the onset of the enhanced accretion. Here, we study the radial velocity (RV) variations of EX Lup, the prototype of the EXor class, and test whether they can be related to a close companion. Methods: We conducted a five-year RV survey, collecting 54 observations with HARPS and FEROS. We analyzed the activity of EX Lup by checking the bisector, the equivalent width of the Ca 8662 Å line, the asymmetry of the Ca II K line, the activity indicator SFEROS, the asymmetry of the cross-correlation function, the line depth ratio of the VI/FeI lines, and the TiO, CaH 2, CaH 3, CaOH, and H? indices. We complemented the RV measurements with a 14-day optical/infrared photometric monitoring to look for signatures of activity or varying accretion. Results: We found that the RV of EX Lup is periodic (P = 7.417 d), with stable period, semi-amplitude (2.2 km s-1), and phase over at least four years of observations. This period is not present in any of the above-mentioned activity indicators. However, the RVs of narrow metallic emission lines suggest the same period, but with an anti-correlating phase. The observed absorption line RVs can be fitted with a Keplerian solution around a 0.6 M? central star with msini = (14.7 ± 0.7) MJup and eccentricity of e = 0.24. Alternatively, we attempted to model the observations with a cold or hot stellar spot as well. We found that in our simple model, the spot parameters needed to reproduce the RV semi-amplitude are in contradiction with the photometric variability, making the spot scenario unlikely. Conclusions: We qualitatively discuss two possibilities to explain the RV data: a geometry with two accretion columns rotating with the star, and a single accretion flow synchronized with the orbital motion of the hypothetical companion; the second scenario is more consistent with the observed properties of EX Lup. In this scenario, the companion's mass would fall into the brown dwarf desert, which, together with the unusually small separation of 0.06 au would make EX Lup a unique binary system. The companion also has interesting implications on the physical mechanisms responsible for triggering the outburst. This work is based in part on observations made with ESO Telescopes at the La Silla Paranal Observatory under program IDs 079.A-9017, 081.A-9005, 081.A-9023, 081.C-0779, 082.C-0390, 082.C-0427, 083.A-9011, 083.A-9017, 084.A-9011, 085.A-9027, 086.A-9006, 086.A-9012, 087.A-9013, 087.A-9029, and 089.A-9007.Tables 2 and 3 are available in electronic form at http://www.aanda.org

Kóspál, Á.; Mohler-Fischer, M.; Sicilia-Aguilar, A.; Ábrahám, P.; Curé, M.; Henning, Th.; Kiss, Cs.; Launhardt, R.; Moór, A.; Müller, A.

2014-01-01

3

The Progenitor of the FUor-Type Young Eruptive Star 2MASS J06593158–0405277  

NASA Astrophysics Data System (ADS)

Only a dozen confirmed FU Orionis-type young outbursting stars (FUors) are known today; this explains the interest in the recent FUor eruption of 2MASS J06593158–0405277. Its outburst and expected decline will be subject to numerous studies in the future. Almost equally important for the understanding of the eruption mechanism, however, is the physical characterization of the FUor’s precursor. Here we analyze unpublished archival data and summarize—and partly revise—all relevant photometry from optical to submillimeter wavelengths. Our analysis implies that the FUor is possibly associated with eight T Tauri star candidates and a strong Class 0 source. Adopting a distance of 450 pc for the FUor, we derive a quiescent bolometric luminosity and temperature of {{L}bol} = 4.8 L ? and {{T}bol} = 1190 K, typical for young Class II sources. The central star has a temperature of {{T}eff} = 4000 K, a mass of 0.75 M ? , and an age of about 6 × 105 yr. The SED implies a circumstellar mass of 0.01–0.06 M ? , and the system is surrounded by a faint infrared nebulosity. Our results provide an almost complete picture of a FUor progenitor, supporting the interpretation of future post-outburst studies. Based on observations made with the Herschel Space Observatory. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Kóspál, Á.; Ábrahám, P.; Moór, A.; Haas, M.; Chini, R.; Hackstein, M.

2015-03-01

4

The progenitor of the FUor-type young eruptive star 2MASS J06593158-0405277  

E-print Network

Only a dozen confirmed FU Orionis-type young outbursting stars (FUors) are known today; this explains the interest in the recent FUor eruption of 2MASS J06593158-0405277. Its outburst and expected decline will be subject to numerous studies in the future. Almost equally important for the understanding of the eruption mechanism, however, is the physical characterization of the FUor's precursor. Here we analyze unpublished archival data and summarize - and partly revise - all relevant photometry from optical to submillimeter wavelengths. Our analysis implies that the FUor is possibly associated with eight T Tauri star candidates and a strong Class 0 source. Adopting a distance of 450 pc for the FUor, we derive a quiescent bolometric luminosity and temperature of L_bol = 4.8 L_Sun and T_bol = 1190 K, typical for young Class II sources. The central star has a temperature of T_eff = 4000 K, a mass of 0.75 M_Sun, and an age of about 6 x 10^5 yr. The SED implies a circumstellar mass of 0.01 - 0.06 M_Sun, and the sys...

Kóspál, Á; Moór, A; Haas, M; Chini, R; Hackstein, M

2015-01-01

5

Exploring the circumstellar environment of the young eruptive star V2492 Cygni  

NASA Astrophysics Data System (ADS)

Context. V2492 Cyg is a young eruptive star that went into outburst in 2010. The near-infrared color changes observed since the outburst peak suggest that the source belongs to a newly defined sub-class of young eruptive stars, where time-dependent accretion and variable line-of-sight extinction play a combined role in the flux changes. Aims: In order to learn about the origin of the light variations and to explore the circumstellar and interstellar environment of V2492 Cyg, we monitored the source at ten different wavelengths, between 0.55 ?m and 2.2 ?m from the ground and between 3.6 ?m and 160 ?m from space. Methods: We analyze the light curves and study the color-color diagrams via comparison with the standard reddening path. We examine the structure of the molecular cloud hosting V2492 Cyg by computing temperature and optical depth maps from the far-infrared data. Results: We find that the shapes of the light curves at different wavelengths are strictly self-similar and that the observed variability is related to a single physical process, most likely variable extinction. We suggest that the central source is episodically occulted by a dense dust cloud in the inner disk and, based on the invariability of the far-infrared fluxes, we propose that it is a long-lived rather than a transient structure. In some respects, V2492 Cyg can be regarded as a young, embedded analog of UX Orionis-type stars. Conclusions: The example of V2492 Cyg demonstrates that the light variations of young eruptive stars are not exclusively related to changing accretion. The variability provided information on an azimuthally asymmetric structural element in the inner disk. Such an asymmetric density distribution in the terrestrial zone may also have consequences for the initial conditions of planet formation. This work is based on observations made with the Herschel Space Observatory and with the Spitzer Space Telescope. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Spitzer is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.Tables 1 and 2 are available in electronic form at http://www.aanda.org

Kóspál, Á.; Ábrahám, P.; Acosta-Pulido, J. A.; Arévalo Morales, M. J.; Balog, Z.; Carnerero, M. I.; Szegedi-Elek, E.; Farkas, A.; Henning, Th.; Kelemen, J.; Kovács, T.; Kun, M.; Marton, G.; Mészáros, Sz.; Moór, A.; Pál, A.; Sárneczky, K.; Szakáts, R.; Szalai, N.; Szing, A.; Tóth, I.; Turner, N. J.; Vida, K.

2013-03-01

6

The outburst and nature of two young eruptive stars in the North America/Pelican Nebula Complex  

E-print Network

Context. In August 2010, the sudden optical brightening of two young stellar objects, located in the North America/Pelican Nebula Complex, was announced. Early photometric and spectroscopic observations of these objects indicated that they may belong to the FUor or EXor class of young eruptive stars. The eruptions of FUors and EXors are often explained by enhanced accretion of material from the circumstellar disk to the protostar. Aims. In order to determine the true nature of these two objects, we started an optical and near-infrared monitoring program, and complemented our data with archival observations and data from the literature. Methods. We plot and analyze pre-outburst and outburst spectral energy distributions (SEDs), multi-filter light curves, and color-color diagrams. Results. While the quiescent SED of HBC 722 is consistent with that of a slightly reddened normal T Tauri-type star, the quiescent SED of VSX J205126.1+440523 is highly extincted, either due to an envelope, or an edge-on disk geometry...

Kóspál, Á; Acosta-Pulido, J A; Morales, M J Arévalo; Carnerero, M I; Elek, E; Kun, M; Pál, A; Szakáts, R

2010-01-01

7

Dwarf Star Erupts in Giant Flare  

NASA Technical Reports Server (NTRS)

This movie taken by NASA'S Galaxy Evolution Explorer shows one of the largest flares, or star eruptions, ever recorded at ultraviolet wavelengths. The star, called GJ 3685A, just happened to be in the Galaxy Evolution Explorer's field of view while the telescope was busy observing galaxies. As the movie demonstrates, the seemingly serene star suddenly exploded once, then even more intensely a second time, pouring out in total about one million times more energy than a typical flare from our Sun. The second blast of light constituted an increase in brightness by a factor of at least 10,000.

Flares are huge explosions of energy stemming from a single location on a star's surface. They are caused by the brief destruction of a star's magnetic fields. Many types of stars experience them, though old, small, rapidly rotating 'red dwarfs' like GJ 3685A tend to flare more frequently and dramatically. These stars, called flare stars, can experience powerful eruptions as often as every few hours. Younger stars, in general, also erupt more often. One of the reasons astronomers study flare stars is to gain a better picture and history of flare events taking place on the Sun.

A preliminary analysis of the GJ 3685A flare shows that the mechanisms underlying stellar eruptions may be more complex than previously believed. Evidence for the two most popular flare theories was found.

Though this movie has been sped up (the actual flare lasted about 20 minutes), time-resolved data exist for each one-hundredth of a second. These observations were taken at 2 p.m. Pacific time, April 24, 2004. In the still image, the time sequence starts in the upper left panel, continues in the upper right, then moves to the lower left and ends in the lower right.

The circular and linear features that appear below and to the right of GJ 3685A during the flare event are detector artifacts caused by the extreme brightness of the flare.

2005-01-01

8

Identifying Young, Nearby Stars  

NASA Technical Reports Server (NTRS)

Young stars have certain characteristics, e.g., high atmospheric abundance of lithium and chromospheric activity, fast rotation, distinctive space motion and strong X-ray flux compared to that of older main sequence stars. We have selected a list of candidate young (<100Myr) and nearby (<60pc) stars based on their space motion and/or strong X-ray flux. To determine space motion of a star, one needs to know its coordinates (RA, DEC), proper motion, distance, and radial velocity. The Hipparcos and Tycho catalogues provide all this information except radial velocities. We anticipate eventually searching approx. 1000 nearby stars for signs of extreme youth. Future studies of the young stars so identified will help clarify the formation of planetary systems for times between 10 and 100 million years. Certainly, the final output of this study will be a very useful resource, especially for adaptive optics and space based searches for Jupiter-mass planets and dusty proto-planetary disks. We have begun spectroscopic observations in January, 2001 with the 2.3 m telescope at Siding Spring Observatory (SSO) in New South Wales, Australia. These spectra will be used to determine radial velocities and other youth indicators such as Li 6708A absorption strength and Hydrogen Balmer line intensity. Additional observations of southern hemisphere stars from SSO are scheduled in April and northern hemisphere observations will take place in May and July at the Lick Observatory of the University of California. AT SSO, to date, we have observed about 100 stars with a high resolution spectrometer (echelle) and about 50 stars with a medium spectral resolution spectrometer (the "DBS"). About 20% of these stars turn out to be young stars. Among these, two especially noteworthy stars appear to be the closest T-Tauri stars ever identified. Interestingly, these stars share the same space motions as that of a very famous star with a dusty circumstellar disk--beta Pictoris. This new finding better constrains the age of beta Pictoris to be approx. 10 Myr.

Webb, Rich; Song, Inseok; Zuckerman, Ben; Bessell, Mike

2001-01-01

9

Young Massive Star Clusters  

NASA Astrophysics Data System (ADS)

Young massive clusters (YMCs) are dense aggregates of young stars that form the fundamental building blocks of galaxies. Several examples exist in the Milky Way Galaxy and the Local Group, but they are particularly abundant in starburst and interacting galaxies. The few YMCs that are close enough to resolve are of prime interest for studying the stellar mass function and the ecological interplay between stellar evolution and stellar dynamics. The distant unresolved clusters may be effectively used to study the star-cluster mass function, and they provide excellent constraints on the formation mechanisms of young cluster populations. YMCs are expected to be the nurseries for many unusual objects, including a wide range of exotic stars and binaries. So far only a few such objects have been found in YMCs, although their older cousins, the globular clusters, are unusually rich in stellar exotica. In this review, we focus on star clusters younger than ˜100 Myr, more than a few current crossing times old, and more massive than ˜104M?; the size of the cluster and its environment are considered less relevant as distinguishing parameters. We describe the global properties of the currently known young massive star clusters in the Local Group and beyond, and discuss the state of the art in observations and dynamical modeling of these systems. In order to make this review readable by observers, theorists, and computational astrophysicists, we also review the cross-disciplinary terminology.

Portegies Zwart, Simon F.; McMillan, Stephen L. W.; Gieles, Mark

2010-09-01

10

Companions to Young Stars  

E-print Network

Brown dwarfs occupy the important region in the mass range between stars and planets. Their existence, ambigious until only recently, and their properties give insight into stellar and planetary formation. We present statistical results of an infrared, coronagraphic survey of young, nearby stars that includes probable companions to three young G-type stars, Gl 503.2 (G2V), HD 102982 (G3V), and Gl 577 (G5V). The companion to Gl 577 is a possible binary brown dwarf, according to evolution ary models. A dynamical determination of the components' masses will be achievable in the near future and be an excellent test of the predictive ability of the evolutionary models.

Patrick J. Lowrance

2002-08-03

11

Modeling Giant Eruptions and Inflated Envelopes of LBV Stars  

NASA Astrophysics Data System (ADS)

The Luminous Blue Variable (LBV) stage of massive stars can refer to both S-Doradus variations, characterized by blue-red excursions at nearly constant bolometric luminosity, and eta-Carinae type giant eruptions, characterized by a significant increase in Lbol accompanied by large mass ejection. This talk will discuss how mass-loss-limited envelope inflation might account for S-Dor variations, while eta-Car-type giant eruptions can result from super-Eddington, continuum-driven mass loss that is triggered and moderated by porosity of the stellar envelope.

Owocki, Stan

2013-06-01

12

Eruptions at Lone Star Geyser, Yellowstone National Park, USA, part 1: energetics and eruption dynamics  

USGS Publications Warehouse

Geysers provide a natural laboratory to study multiphase eruptive processes. We present results from a four–day experiment at Lone Star Geyser in Yellowstone National Park, USA. We simultaneously measured water discharge, acoustic emissions, infraredintensity, and visible and infrared video to quantify the energetics and dynamics of eruptions, occurring approximately every three hours. We define four phases in the eruption cycle: 1) a 28?±?3 minute phase with liquid and steam fountaining, with maximum jet velocities of 16–28?m s??1, steam mass fraction of less than ??0.01. Intermittently choked flow and flow oscillations with periods increasing from 20 to 40?s are coincident with a decrease in jet velocity and an increase of steam fraction; 2) a 26?±?8 minute post–eruption relaxation phase with no discharge from the vent, infrared (IR) and acoustic power oscillations gliding between 30 and 40?s; 3) a 59?±?13 minute recharge period during which the geyser is quiescent and progressively refills, and 4) a 69?±?14 minute pre–play period characterized by a series of 5–10?minute–long pulses of steam, small volumes of liquid water discharge and 50–70?s flow oscillations. The erupted waters ascend froma 160???170° C reservoir and the volume discharged during the entire eruptive cycle is 20.8?±?4.1 m3. Assuming isentropic expansion, we calculate a heat output from the geyser of 1.4–1.5?MW, which is

Karlstrom, Leif; Hurwitz, Shaul; Sohn, Robert; Vandemeulebrouck, Jean; Murphy, Fred; Rudolph, Maxwell L.; Johnston, Malcolm J.S.; Manga, Michael; McCleskey, R. Blaine

2013-01-01

13

Eruptions at Lone Star Geyser, Yellowstone National Park, USA: 1. Energetics and eruption dynamics  

NASA Astrophysics Data System (ADS)

Geysers provide a natural laboratory to study multiphase eruptive processes. We present results from a 4 day experiment at Lone Star Geyser in Yellowstone National Park, USA. We simultaneously measured water discharge, acoustic emissions, infrared intensity, and visible and infrared video to quantify the energetics and dynamics of eruptions, occurring approximately every 3 h. We define four phases in the eruption cycle (1) a 28±3 min phase with liquid and steam fountaining, with maximum jet velocities of 16-28 m s-1, steam mass fraction of less than ˜0.01. Intermittently choked flow and flow oscillations with periods increasing from 20 to 40 s are coincident with a decrease in jet velocity and an increase of steam fraction; (2) a 26±8 min posteruption relaxation phase with no discharge from the vent, infrared (IR), and acoustic power oscillations gliding between 30 and 40 s; (3) a 59±13 min recharge period during which the geyser is quiescent and progressively refills, and (4) a 69±14 min preplay period characterized by a series of 5-10 min long pulses of steam, small volumes of liquid water discharge, and 50-70 s flow oscillations. The erupted waters ascend from a 160-170°C reservoir, and the volume discharged during the entire eruptive cycle is 20.8±4.1 m3. Assuming isentropic expansion, we calculate a heat output from the geyser of 1.4-1.5 MW, which is <0.1% of the total heat output from Yellowstone Caldera.

Karlstrom, Leif; Hurwitz, Shaul; Sohn, Robert; Vandemeulebrouck, Jean; Murphy, Fred; Rudolph, Maxwell L.; Johnston, Malcolm J. S.; Manga, Michael; McCleskey, R. Blaine

2013-08-01

14

Coronal Mass Ejections and Angular Momentum Loss in Young Stars  

NASA Astrophysics Data System (ADS)

In our own solar system, the necessity of understanding space weather is readily evident. Fortunately for Earth, our nearest stellar neighbor is relatively quiet, exhibiting activity levels several orders of magnitude lower than young, solar-type stars. In protoplanetary systems, stellar magnetic phenomena observed are analogous to the solar case, but dramatically enhanced on all physical scales: bigger, more energetic, more frequent. While coronal mass ejections (CMEs) could play a significant role in the evolution of protoplanets, they could also affect the evolution of the central star itself. To assess the consequences of prominence eruption/CMEs, we have invoked the solar-stellar connection to estimate, for young, solar-type stars, how frequently stellar CMEs may occur and their attendant mass and angular momentum loss rates. We will demonstrate the necessary conditions under which CMEs could slow stellar rotation.

Aarnio, Alicia N.; Stassun, Keivan G.; Matt, Sean P.

2014-01-01

15

Young and Waltzing Binary Stars  

NASA Astrophysics Data System (ADS)

ADONIS Observes Low-mass Eclipsing System in Orion Summary A series of very detailed images of a binary system of two young stars have been combined into a movie . In merely 3 days, the stars swing around each other. As seen from the earth, they pass in front of each other twice during a full revolution, producing eclipses during which their combined brightness diminishes . A careful analysis of the orbital motions has now made it possible to deduce the masses of the two dancing stars . Both turn out to be about as heavy as our Sun. But while the Sun is about 4500 million years old, these two stars are still in their infancy. They are located some 1500 light-years away in the Orion star-forming region and they probably formed just 10 million years ago . This is the first time such an accurate determination of the stellar masses could be achieved for a young binary system of low-mass stars . The new result provides an important piece of information for our current understanding of how young stars evolve. The observations were obtained by a team of astronomers from Italy and ESO [1] using the ADaptive Optics Near Infrared System (ADONIS) on the 3.6-m telescope at the ESO La Silla Observatory. PR Photo 29a/01 : The RXJ 0529.4+0041 system before primary eclipse PR Photo 29b/01 : The RXJ 0529.4+0041 system at mid-primary eclipse PR Photo 29c/01 : The RXJ 0529.4+0041 system after primary eclipse PR Photo 29d/01 : The RXJ 0529.4+0041 system before secondary eclipse PR Photo 29e/01 : The RXJ 0529.4+0041 system at mid-secondary eclipse PR Photo 29f/01 : The RXJ 0529.4+0041 system after secondary eclipse PR Video Clip 06/01 : Video of the RXJ 0529.4+0041 system Binary stars and stellar masses Since some time, astronomers have noted that most stars seem to form in binary or multiple systems. This is quite fortunate, as the study of binary stars is the only way in which it is possible to measure directly one of the most fundamental quantities of a star, its mass. The mass of a star determines its fate . Massive stars (with masses more than 50 times that of the Sun) lead a glorious, but short life. They are hot and very luminous and exhaust their energy supply in just a few million years. At the other end of the scale, low-mass stars like the Sun are more economical with their resources. Being cooler and dimmer, they are able to shine for billions of years [2]. But although the mass determines the fate of a star, it is not a trivial matter to measure this crucial parameter. In fact, it can only be determined directly if the star happens to be gravitationally bound to another star in a binary stellar system. Observations of the orbital motions of the two stars as they circle each other allows to "weigh" them, and also provide other important information, e.g. about their sizes and temperatures. Orbital motions The understanding of orbital motions has a long history in astronomy. The basic laws of Johannes Kepler (1571-1630) are still used to calculate the masses of orbiting objects, in the solar system as well as in binary stellar systems. However, while the observations of the motion of the nine planets and moons have allowed us to measure quite accurately the masses of objects in our vicinity, the information needed to "weigh" the binary stellar systems is not that easy to obtain. As a result, the mass estimates of the stars in binary systems are often rather uncertain. A main problem is that the individual stars in many binary systems can not be visually separated, even in the best telescopes. The information about the orbit may then come from the motions of the stars, if these are revealed by spectroscopic observations of the combined light (such systems are referred to as "spectroscopic binaries"). If absorption lines from both components are present in the spectrum, the measured wavelength of these double lines will shift periodically back and forth. This is the well-known Doppler effect and it directly reflects the changing velocities of the stars, as they move along their orbits and periodically a

2001-10-01

16

Eruptive Mass Loss in Very Massive Stars and Population III Stars  

E-print Network

I discuss the role played by short-duration eruptive mass loss in the evolution of very massive stars. Giant eruptions of Luminous Blue Variables (LBVs) like the 19th century event of eta Carinae can remove large quantities of mass almost instantaneously, making them significant in stellar evolution. They can potentially remove much more mass from the star than line-driven winds, especially if stellar winds are highly clumped such that previous estimates of O star mass-loss rates need to be revised downward. When seen in other galaxies as ``supernova impostors'', these LBV eruptions typically last for less than a decade, and they can remove of order 10 Msun as indicated by massive nebulae around LBVs. Such extreme mass-loss rates cannot be driven by radiation pressure on spectral lines, because the lines will completely saturate during the events. Instead, these outbursts must either be continuum-driven super-Eddington winds or outright hydrodynamic explosions, both of which are insensitive to metallicity. As such, this eruptive mode of mass loss could also play a pivotal role in the evolution and ultimate fate of massive metal-poor stars in the early universe. If they occur in these Population III stars, such eruptions would also profoundly affect the chemical yield and types of remnants from early supernovae and hypernovae thought to be the origin of long gamma ray bursts.

Nathan Smith

2006-07-24

17

Interstellar Extinction Toward Young Stars  

NASA Astrophysics Data System (ADS)

We present work on a molecular hydrogen (H2) fluorescence model to characterize the ultraviolet (UV) extinction curve along the line of sight towards young stars with circumstellar disks. Stellar UV radiation plays a strong role in heating the disk gas and driving chemical reactions, so it is important to measure the UV extinction curve in order to reconstruct the intrinsic stellar UV flux impacting the disk. To measure the extinction, we compare modeled H2 fluorescence spectra to observed H2 lines. Lyman-alpha radiation from the stars pumps electronic transitions of H2 in the disk, and we model the flux that is re-emitted through the subsequent fluorescent cascade. We then extract the extinction along the line-of-sight over the 1100-1700 Angstrom wavelength region from the difference between the modeled H2 fluorescence and the HST-COS data. The shape of the extinction curve allows us to characterize the dust grain distribution in the intervening material as well as to recover the intrinsic spectral energy distribution of the stars over a wide wavelength range.

McJunkin, Matthew; France, Kevin

2015-01-01

18

Young Star Clusters in Starburst Environments  

E-print Network

Recent high-resolution observations with the Hubble Space Telescope (HST) reveal that young star clusters of extraordinary luminosity and compactness ("super star clusters") are commonly found in starburst systems. Cluster formation appears to be a dominant mode of star formation in starbursts. The principal properties of the young clusters are summarized. A new ultraviolet HST imaging survey of the central regions of nearby galaxies indicates that young clusters form in a wide range of environments. Circumnuclear star-forming rings, in particular, are richly populated with clusters, and several examples from recent imaging studies are discussed. There has been much speculation that super star clusters represent present-day analogs of young globular clusters. I will present evidence suggesting that at least some super star clusters indeed have masses and mass densities comparable to those of evolved globular clusters in the Milky Way.

Luis C. Ho

1996-06-04

19

Accretion Disks around Young Stars  

NASA Astrophysics Data System (ADS)

A method to calculate the structure and brightness distribution of accretion disks surrounding low and intermediate mass young stars is introduced and discussed. The method includes a realistic treatment of the energy transport mechanisms and disk heating by radiation from external sources. The disk is assumed steady, geometrically thin and in vertical hydrostatic equilibrium. The turbulent viscosity coefficient is expressed using the ? prescription and the ? parameter and the mass accretion rate are assumed to be constant through the disk. Energy is transported in the vertical direction by: (a) a turbulent flux, computed self-consistently with the viscosity coefficient used to describe the viscous energy dissipation, (b) radiation, using the first moments of the transfer equation, the Eddington approximation, and the Rosseland and Planck Mean Opacities, and (c) convection, taking into account that the convective elements, not necessarily optically thick, lose energy by radiation and turbulent flux. This treatment of the energy transport mechanisms differs from previous work in this field, allowing one to extend, with confidence, the calculation of the disk structure to optically thin regimes. The heating mechanisms considered, which affect the disk's structure and emission, are stellar radiation and a circumstellar envelope which reprocesses and scatters radiation from the star and from the disk itself. In addition to a detailed numerical calculation, an analytical self-consistent formulation of the irradiation of the disk is given. This analytical formulation allows one to understand and extend the numerical results. To evaluate the potential of the method presented in this thesis, a set of models of viscous non-irradiated and irradiated disks are computed. Their predictions are compared with observations of young stellar sources likely to have disks. Given the disk structure and specifying its orientation with respect to the line of sight, the specific intensity distribution is calculated on the plane of the sky, integrating the radiative transfer equation along rays parallel to the line of sight. To this end, monochromatic opacities are used, which also allow us to construct tables of the Rosseland and Planck Mean Opacities. The disk structure and brightness distribution thus obtained are self-consistent with respect to the abundances and optical properties of the gas and the dust. With the disk intensity distribution, its spectrum is constructed and its colors are calculated in different spectral ranges. These are compared to observations of low mass young stars reported in the literature, for which the disk parameters are then inferred. It is found that the observed properties of a large fraction of classical T Tauri stars can be explained as emission from viscous disks irradiated by the central star or by a thin envelope and that the emission in the long wavelength range from a flat spectrum source like HL Tau is consistent with the predictions of a model in which a viscous disk is irradiated by an optically thick infalling envelope.

D'Alessio, Paola

1996-04-01

20

Are all young stars disc accretors?  

E-print Network

We now understand how low mass stars evolve through cloud collapse and disc accretion, but whether higher mass stars are also disc accretors is as yet unknown. Spectropolarimetry observations can help in answering this basic question, as they probe the first few stellar radii around young stars.

Jorick S. Vink; Janet E. Drew; Tim J. Harries; Rene D. Oudmaijer

2001-10-05

21

THE GALEX NEARBY YOUNG-STAR SURVEY  

SciTech Connect

We describe a method that exploits data from the Galaxy Evolution Explorer (GALEX) ultraviolet and Wide-field Infrared Survey Explorer and Two Micron All Sky Survey infrared source catalogs, combined with proper motions and empirical pre-main sequence isochrones, to identify candidate nearby, young, low-mass stars. Applying our method across the full GALEX-covered sky, we identify 2031 mostly M-type stars that, for an assumed age of 10 (100) Myr, all lie within {approx}150 ({approx}90) pc of Earth. The distribution of M spectral subclasses among these {approx}2000 candidate young stars peaks sharply in the range M3-M4; these subtypes constitute 50% of the sample, consistent with studies of the M star population in the immediate solar neighborhood. We focus on a subset of 58 of these candidate young M stars in the vicinity of the Tucana-Horologium association. Only 20 of these 58 candidates were detected in the ROSAT All-Sky X-ray Survey-reflecting the greater sensitivity of GALEX for the purposes of identifying active nearby, young stars, particularly for stars of type M4 and later. Based on statistical analysis of the kinematics and/or spectroscopic followup of these 58 M stars, we find that 50% (29 stars) indeed have properties consistent with Tuc-Hor membership, while 12 are potential new members of the Columba association, and 2 may be AB Dor moving group members. Hence, {approx}75% of our initial subsample of 58 candidates are likely members of young (age {approx} 10-40 Myr) stellar moving groups within 100 pc, verifying that the stellar color- and kinematics-based selection algorithms described here can be used to efficiently isolate nearby, young, low-mass objects from among the field star population. Future studies will focus on characterizing additional subsamples selected from among this list of candidate nearby, young M stars.

Rodriguez, David R.; Faherty, Jacqueline K. [Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago (Chile); Zuckerman, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States); Kastner, Joel H. [Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623 (United States); Bessell, M. S.; Murphy, Simon J., E-mail: drodrigu@das.uchile.cl [Australian National University, Cotter Road, Weston Creek, ACT 2611 (Australia)

2013-09-10

22

Fluid Mechanics in Disks Around Young Stars  

Microsoft Academic Search

This article reviews hydrodynamic and magnetohydrodynamic processes in disks around young stars, encompassing the epochs of molecular-cloud turbulence, dense core collapse, disk formation, disk evolution, and planetesimal formation.

Karim Shariff

2009-01-01

23

Young Star Clusters: Keys to Understanding Massive Stars  

NASA Astrophysics Data System (ADS)

Young, coeval clusters of stars provide the perfect laboratory in which to test our understanding of how massive stars evolve. Early optical observations limited us to a handful of low-mass clusters within 1kpc. However, thanks to the recent progress in infrared astronomy, the Milky Way's population of young massive star clusters is now beginning to be revealed. Here, I will review the recent progress made in this field, what it has told us about the evolution of massive stars to supernova and beyond, the prospects for this field, and some issues that should be taken into account when interpreting the results.

Davies, B.

2012-12-01

24

Rotational velocities of nearby young stars  

Microsoft Academic Search

Context. Stellar rotation is a crucial parameter driving stellar magnetism, activity and mixing of chemical elements. Measuring rotational velocities of young stars can give additional insight in the initial conditions of the star formation process. Furthermore, the evolution of stellar rotation is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are believed to be responsible for rotational deceleration during

Patrick Weise; Ralf Launhardt; Johny Setiawan; Thomas Henning

2010-01-01

25

The GALEX Nearby Young-Star Survey  

E-print Network

We describe a method that exploits data from the GALEX ultraviolet and WISE and 2MASS infrared source catalogs, combined with proper motions and empirical pre-main sequence isochrones, to identify candidate nearby, young, low-mass stars. Applying our method across the full GALEX- covered sky, we identify 2031 mostly M-type stars that, for an assumed age of 10 (100) Myr, all lie within ~150 (~90) pc of Earth. The distribution of M spectral subclasses among these ~2000 candidate young stars peaks sharply in the range M3-M4; these subtypes constitute 50% of the sample, consistent with studies of the M star population in the immediate solar neighborhood. We focus on a subset of 58 of these candidate young M stars in the vicinity of the Tucana-Horologium Association. Only 20 of these 58 candidates were detected in the ROSAT All-Sky X-ray Survey -- reflecting the greater sensitivity of GALEX for purposes of identifying active nearby, young stars, particularly for stars of type M4 and later. Based on statistical ana...

Rodriguez, David R; Kastner, Joel H; Bessel, M S; Faherty, Jacqueline K; Murphy, Simon J

2013-01-01

26

Echography of young stars reveals their evolution  

NASA Astrophysics Data System (ADS)

We demonstrate that a seismic analysis of stars in their earliest evolutionary phases is a powerful method with which to identify young stars and distinguish their evolutionary states. The early star that is born from the gravitational collapse of a molecular cloud reaches at some point sufficient temperature, mass, and luminosity to be detected. Accretion stops, and the pre-main sequence star that emerges is nearly fully convective and chemically homogeneous. It will continue to contract gravitationally until the density and temperature in the core are high enough to start nuclear burning of hydrogen. We show that there is a relationship for a sample of young stars between detected pulsation properties and their evolutionary status, illustrating the potential of asteroseismology for the early evolutionary phases.

Zwintz, K.; Fossati, L.; Ryabchikova, T.; Guenther, D.; Aerts, C.; Barnes, T. G.; Themeßl, N.; Lorenz, D.; Cameron, C.; Kuschnig, R.; Pollack-Drs, S.; Moravveji, E.; Baglin, A.; Matthews, J. M.; Moffat, A. F. J.; Poretti, E.; Rainer, M.; Rucinski, S. M.; Sasselov, D.; Weiss, W. W.

2014-08-01

27

Numerical Simulations of Giant Eruptions from Massive Stars and their Recoveries  

NASA Astrophysics Data System (ADS)

We use a 3D hydro code to model the response of a massive star to a high mass loss episode. Starting with a modified version of the 1D stellar evolution code MESA, we obtain a model of an evolved massive star, with properties similar to those of Eta Carinae, known for its giant eruption in the 19th century.We simulate a giant eruption using two approaches:1.Removing a layer from the star using energy from inner layers.2.Extracting energy from the core to outer layers that spontaneously causes mass loss.We then follow the evolution of the star using the FLASH code. Our hydrodynamical simulation includes radiation transfer with realistic opacities and convection.We find that the star develops a strong wind, powered by pulsation in the inner parts of the star. The strong eruptive mass loss phase lasts for a few years, followed by centuries of continually weakening mass loss.

Kashi, Amit; Davidson, Kris; Humphreys, Roberta M.

2015-01-01

28

Young star clusters: Progenitors of globular clusters!?  

E-print Network

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. Young clusters are observed to form in a variety of such galaxies, a substantial number resembling the progenitors of globular clusters in mass and size, but with significantly enhanced metallicity. From studies of the metal-poor and metal-rich star cluster populations of galaxies, we can therefore learn about the violent star formation history of these galaxies, and eventually about galaxy formation and evolution. We present a new set of evolutionary synthesis models of our GALEV code, with special emphasis on the gaseous emission of presently forming star clusters, and a new tool to compare extensive model grids with multi-color broad-band observations to determine individual cluster masses, metallicities, ages and extinction values independently. First results for young star clusters in the dwarf starburst galaxy NGC 1569 are presented. The mass distributions determined for the young clusters give valuable input to dynamical star cluster system evolution models, regarding survival and destruction of clusters. We plan to investigate an age sequence of galaxy mergers to see dynamical destruction effects in process.

P. Anders; U. Fritze--v. Alvensleben; R. de Grijs

2003-09-04

29

Young stars and clouds in Camelopardalis  

E-print Network

Star formation in the Local spiral arm in the direction of the Galactic longitudes 132--158 deg is reviewed. Recent star-forming activity in this Milky Way direction is evidenced by the presence here of the Cam OB1 association and dense dust and molecular clouds containing H$\\alpha$ emission stars, young irregular variables and infrared stellar objects. The clouds of the Local arm concentrate in two layers at 150-300 pc and at about 900 pc from the Sun. The Perseus arm objects in this direction are at a distance of about 2 kpc.

V. Straizys; V. Laugalys

2008-11-18

30

Chromospherically young, kinematically old stars  

E-print Network

We have investigated a group of stars known to have low chromospheric ages, but high kinematical ages. Isochrone, chemical and lithium ages are estimated for them. The majority of stars in this group show lithium abundances much smaller than expected for their chromospheric ages, which is interpreted as an indication of their old age. Radial velocity measurements in the literature also show that they are not close binaries. The results suggest that they can be formed from the coalescence of short-period binaries. Coalescence rates, calculated taking into account several observational data and a maximum theoretical time scale for contact, in a short-period pair, predict a number of coalesced stars similar to what we have found in the solar neighbourhood.

H. J. Rocha-Pinto; B. V. Castilho; W. J. Maciel

2001-12-19

31

Probing Planet Formation Among Nearby Young Stars  

E-print Network

The recent identification of several groups of young stars within 100 parsecs of the Sun has generated widespread interest. Given their proximity and possible age differences, these systems are ideally suited for detailed studies of disk evolution and planet formation. Here I discuss recent results and prospects for the near-future.

Ray Jayawardhana

2000-10-02

32

Young Massive Star Clusters in Spiral Galaxies  

Microsoft Academic Search

Abstract. Using ground-based UBVRIH,CCD photometry we have been carrying out a search for young massive star clus- ters (YMCs) in a sample consisting of 21 nearby spiral galaxies. We find a large variety concerning the richness of the cluster sys- tems, with some galaxies containing no YMCs at all and others hosting very large numbers of YMCs. Examples of galaxies

Soeren S. Larsen; T. Richtler

1999-01-01

33

Young Massive Star Clusters. II. (Larsen, 1999)  

Microsoft Academic Search

Table 4 lists photometric data for Young Massive Star Clusters identified in a sample of 21 nearby galaxies. The photometric data have been corrected for Galactic foreground extinction. Each cluster is identified by the abbreviated NGC number of its host galaxy and an object number: nxxx-yyy is object number yyy in the galaxy NGC xxx. Effective cluster radii have been

S. S. Larsen

1999-01-01

34

PV Ceph: Young Star Caught Speeding?  

Microsoft Academic Search

Three independent lines of evidence imply that the young star PV Ceph is moving at roughly 20 km s-1 through the interstellar medium. The first, and strongest, suggestion of motion comes from the geometry of the HH knots in the ``giant\\

A. A. Goodman; H. G. Arce

2003-01-01

35

PV Cephei: Young Star Caught Speeding?  

Microsoft Academic Search

Three independent lines of evidence imply that the young star PV Cep is moving at roughly 20 km s-1 through the interstellar medium. The first and strongest suggestion of motion comes from the geometry of the Herbig-Haro (HH) knots in the ``giant'' HH flow associated with PV Cep. Bisectors of lines drawn between pairs of knots at nearly equal distances

Alyssa A. Goodman; Héctor G. Arce

2004-01-01

36

ERUPTIVE VARIABLE STARS AND OUTFLOWS IN SERPENS NW  

SciTech Connect

We study the outflow activity, photometric variability, and morphology of three very young stellar objects in the Serpens NW star-forming region: OO Serpentis, EC 37 (V370 Ser), and EC 53 (V371 Ser). High spatial resolution Keck/NIRC2 laser guide star adaptive optics images obtained in 2007 and 2009 in broadband K and in a narrowband filter centered on the 1-0 S(1) emission line of H{sub 2} allow us to identify the outflows from all three objects. We also present new, seeing-limited data on the photometric evolution of the OO Ser reflection nebula and re-analyze previously published data. We find that OO Ser declined in brightness from its outburst peak in 1995 to about 2003, but that this decline has recently stopped and actually reversed itself in some areas of the reflection nebula. The morphology and proper motions of the shock fronts MHO 2218 near EC 37 suggest that they all originate in EC 37 and that this is an outflow seen nearly along its axis. We identify an H{sub 2} jet emerging from the cometary nebula EC 53. The star illuminating EC 53 is periodically variable with a period of 543 days and has a close-by, non-variable companion at a projected distance of 92 AU. We argue that the periodic variability is the result of accretion instabilities triggered by another very close, not directly observable, binary companion and that EC 53 can be understood in the model of a multiple system developing into a hierarchical configuration.

Hodapp, Klaus W. [Institute for Astronomy, University of Hawaii, 640 N. Aohoku Place, Hilo, HI 96720 (United States); Chini, Rolf; Watermann, Ramon; Lemke, Roland, E-mail: hodapp@ifa.hawaii.edu [Ruhr Universitaet Bochum, Astronomisches Institut, Universitaetsstrasse 150, D-44801 Bochum (Germany)

2012-01-01

37

Captured older stars as the reason for apparently prolonged star formation in young star clusters  

E-print Network

The existence of older stars within a young star cluster can be interpreted to imply that star formation occurs on time scales longer than a free-fall time of a pre-cluster cloud core. Here the idea is explored that these older stars are not related to the star formation process forming the young star cluster but rather that the orbits of older field stars are focused by the collapsing pre-cluster cloud core. Two effects appear: The focussing of stellar orbits leads to an enhancement of the density of field stars in the vicinity of the centre of the young star cluster. And due to the time-dependent potential of the forming cluster some of these stars can get bound gravitationally to the cluster. These stars exhibit similar kinematical properties as the newly formed stars and can not be distinguished from them on the basis of radial-velocity or proper-motion surveys. Such contaminations may lead to a wrong apparent star-formation history of a young cluster. In the case of the ONC the theoretical number of gravitationally bound older low-mass field stars agrees with the number of observed older low-mass stars.

Jan Pflamm-Altenburg; Pavel Kroupa

2006-11-28

38

Triggered Star Formation in the Environment of Young Massive Stars  

E-print Network

Recent observations with the Spitzer Space Telescope show clear evidence that star formation takes place in the surrounding of young massive O-type stars, which are shaping their environment due to their powerful radiation and stellar winds. In this work we investigate the effect of ionising radiation of massive stars on the ambient interstellar medium (ISM): In particular we want to examine whether the UV-radiation of O-type stars can lead to the observed pillar-like structures and can trigger star formation. We developed a new implementation, based on a parallel Smooth Particle Hydrodynamics code (called IVINE), that allows an efficient treatment of the effect of ionising radiation from massive stars on their turbulent gaseous environment. Here we present first results at very high resolution. We show that ionising radiation can trigger the collapse of an otherwise stable molecular cloud. The arising structures resemble observed structures (e.g. the pillars of creation in the Eagle Nebula (M16) or the Horsehead Nebula B33). Including the effect of gravitation we find small regions that can be identified as formation places of individual stars. We conclude that ionising radiation from massive stars alone can trigger substantial star formation in molecular clouds.

M. Gritschneder; T. Naab; F. Heitsch; A. Burkert

2006-09-26

39

Young Star Clusters: Lighthouses in the Dark  

NASA Astrophysics Data System (ADS)

Star cluster formation is a major mode of star formation in the extreme conditions of interacting galaxies and violent starbursts. Young clusters are observed to form in a variety of such galaxies a substantial number resembling the well-studied globular clusters in mass and size. Due to the high angular resolution instruments available at present and in the near future such young clusters can be observed up to much further distances and in more detail than previously possible. Clusters in the inner parts of bright galaxies and along tidal tails are studied (e.g. ""Tadpole"" galaxy). The determination of precise radii and masses is crucial for survivability estimates. By studying young clusters in merger remnants and peculiar galaxies we can learn about the violent star formation history of these galaxies and about galaxy formation and evolution. We will present a new set of evolutionary synthesis models specially developed to account for the gaseous emission of presently forming star clusters and a new tool to compare large model grids with multi-color observations. First results of newly-born clusters in mergers and starburst galaxies are presented and compared to the well-studied old globulars and interpreted in the framework of galaxy formation / evolution.

Anders, Peter; de Grijs, Richard; Fritze-v. Alvensleben, Uta

40

Eruptions at Lone Star geyser, Yellowstone National Park, USA: 2. Constraints on subsurface dynamics  

NASA Astrophysics Data System (ADS)

We use seismic, tilt, lidar, thermal, and gravity data from 32 consecutive eruption cycles of Lone Star geyser in Yellowstone National Park to identify key subsurface processes throughout the geyser's eruption cycle. Previously, we described measurements and analyses associated with the geyser's erupting jet dynamics. Here we show that seismicity is dominated by hydrothermal tremor (~5-40 Hz) attributed to the nucleation and/or collapse of vapor bubbles. Water discharge during eruption preplay triggers high-amplitude tremor pulses from a back azimuth aligned with the geyser cone, but during the rest of the eruption cycle it is shifted to the east-northeast. Moreover, ~4 min period ground surface displacements recur every 26 ± 8 min and are uncorrelated with the eruption cycle. Based on these observations, we conclude that (1) the dynamical behavior of the geyser is controlled by the thermo-mechanical coupling between the geyser conduit and a laterally offset reservoir periodically filled with a highly compressible two-phase mixture, (2) liquid and steam slugs periodically ascend into the shallow crust near the geyser system inducing detectable deformation, (3) eruptions occur when the pressure decrease associated with overflow from geyser conduit during preplay triggers an unstable feedback between vapor generation (cavitation) and mass discharge, and (4) flow choking at a constriction in the conduit arrests the runaway process and increases the saturated vapor pressure in the reservoir by a factor of ~10 during eruptions.

Vandemeulebrouck, Jean; Sohn, Robert A.; Rudolph, Maxwell L.; Hurwitz, Shaul; Manga, Michael; Johnston, Malcolm J. S.; Soule, S. Adam; McPhee, Darcy; Glen, Jonathan M. G.; Karlstrom, Leif; Murphy, Fred

2014-12-01

41

Periodic flow instabilities during Lone Star Geyser (YNP) eruptions, as deduced from acoustic measurements  

NASA Astrophysics Data System (ADS)

We performed continuous acoustic measurements during four days at Lone Star Geyser, Yellowstone National Park, USA. The microphone was located at 10 meters from the geyser's cone, and the acoustic signal was sampled at 1000 Hz. The 3-hour-long eruptive cycle at Lone Star Geyser contains several water fountaining episodes followed by the main eruption, which generally lasts 25 minutes. During the 30 main eruptions that we studied, the acoustic signal patterns are very similar, and indicate the flow is unstable and clearly follows a pulsating regime. The period of the acoustic pulses drastically increases during the liquid to steam transition in the flow. This abrupt change in the flow regime corresponds to the start of the ground deflation recorded by tiltmeters, and could be due to a transition from hydro-static to vapor-static conditions in the vent.

Vandemeulebrouck, J.; Hurwitz, S.; Johnston, M. J.; Rudolph, M. L.; Karlstrom, L.; Sohn, R. A.; Murphy, F.; McPhee, D. K.; Glen, J. M.; Soule, S. A.; Meertens, C. M.

2011-12-01

42

PV Ceph: Young Star Caught Speeding?  

NASA Astrophysics Data System (ADS)

Three independent lines of evidence imply that the young star PV Ceph is moving at roughly 20 km s-1 through the interstellar medium. The first, and strongest, suggestion of motion comes from the geometry of the HH knots in the ``giant" Herbig-Haro flow associated with PV Ceph. Bisectors of lines drawn between pairs of knots at nearly equal distances from PV Ceph imply an E-W motion of the source, and a plasmon model fit to the knot positions gives a good fit of 22 km s-1 motion for the star. The second bit of damning evidence comes from a redshifted ``trail" of molecular gas, pointing in the same E-W direction implied by the HH knot geometry. The third exhibit we offer in accusing PV Ceph of speeding involves the tilt apparent in the high-velocity molecular jet now emanating from the star. This tilt is best explained if the true, current, jet direction is N-S, as it is in HST WFPC images, and the star is moving--again at roughly 20 km s-1. Tracing the motion of PV Ceph backward in time, to the nearest cluster from which it might have been ejected, we find that it is very likely to have been thrown out of the massive star-forming cluster NGC7023--more than 10 pc away. PV Ceph and NGC7023 are at similar distances, and the backward-trace of PV Ceph's motion is astonishingly well-aligned with a dark, previously unexplained, rift in NGC7023. We propose that PV Ceph was ejected, at a speed large enough to escape NGC7023, at least 100,000 years ago, but that it did not enter the molecular cloud in which it now finds itself until more like 10,000 years ago. Our calculations show that currently-observable molecular outflow associated with PV Ceph is about 10,000 years old, so that the flow has had plenty of time to form while in its current molecular cloud. But, the question of what PV Ceph was doing, and what gas/disk it took along with it in the time it was traveling through the low-density region between NGC7023 and its current home is open to question. Recent numerical simulations have suggested that condensed objects should be ejected at high velocity before they have ``finished" forming in a cluster. Prior to this work, a handful of pre-main-sequence stars have been shown to be moving at speeds > 10 km s-1. To the best of our knowledge, though, the analysis of PV Ceph and NGC7023 described here is the first observational work associating a speeding young star with a distant ancestral cluster. These high-speed ejections from clusters will create a class of rapidly-moving young stars in molecluar clouds. If these ejections are at all common, their existence confounds both calculations of clouds' star-forming efficiency and theories of star formation that do not allow for stars to move rapidly through a reservoir of star-forming material while they form.

Goodman, A. A.; Arce, H. G.

2003-12-01

43

PV Cephei: Young Star Caught Speeding?  

NASA Astrophysics Data System (ADS)

Three independent lines of evidence imply that the young star PV Cep is moving at roughly 20 km s-1 through the interstellar medium. The first and strongest suggestion of motion comes from the geometry of the Herbig-Haro (HH) knots in the ``giant'' HH flow associated with PV Cep. Bisectors of lines drawn between pairs of knots at nearly equal distances from PV Cep imply an east-west motion of the source, and a plasmon model fitted to the knot positions gives a good fit of 22 km s-1 motion for the star. The second bit of damning evidence comes from a redshifted trail of molecular gas pointing in the same east-west direction implied by the HH knot geometry. The third exhibit we offer in accusing PV Cep of speeding involves the apparent tilt in the high-velocity molecular jet now emanating from the star. This tilt is best explained if the true, current jet direction is north-south, as it is in Hubble Space Telescope WFPC2 images, and the star is moving, again at roughly 20 km s-1. Tracing the motion of PV Cep backward in time to the nearest cluster from which it might have been ejected, we find that it is very likely to have been thrown out of the massive star-forming cluster NGC 7023, more than 10 pc away. PV Cep and NGC 7023 are at similar distances, and the backward trace of PV Cep's motion is astonishingly well aligned with a dark, previously unexplained rift in NGC 7023. We propose that PV Cep was ejected, at a speed large enough to escape NGC 7023, at least 100,000 yr ago but that it did not enter the molecular cloud in which it now finds itself until more like 35,000 yr ago. Our calculations show that the currently observable molecular outflow associated with PV Cep is about 10,000 yr old, so the flow has had plenty of time to form while in its current molecular cloud. However, the question of what PV Cep was doing and what gas/disk it took along with it in the time it was traveling through the low-density region between NGC 7023 and its current home is an open one. Recent numerical simulations have suggested that condensed objects should be ejected at high velocity before they have ``finished'' forming in a cluster. Prior to this work, a handful of pre-main-sequence stars have been shown to be moving at speeds greater than 10 km s-1. To the best of our knowledge, though, the analysis of PV Cep and NGC 7023 described here is the first observational work associating a speeding young star with a distant ancestral cluster. These high-speed ejections from clusters will create a class of rapidly moving young stars in molecular clouds. If these ejections are at all common, their existence confounds both calculations of clouds' star-forming efficiency and theories of star formation that do not allow for stars to move rapidly through a reservoir of star-forming material while they form.

Goodman, Alyssa A.; Arce, Héctor G.

2004-06-01

44

Young Neutron Stars and Their Wind Nebulae  

E-print Network

With Teragauss magnetic fields, surface gravity sufficiently strong to significantly modify light paths, central densities higher than that of a standard nucleus, and rotation periods of only hundredths of a second, young neutron stars are sites of some of the most extreme physical conditions known in the Universe. They generate magnetic winds with particles that are accelerated to energies in excess of a TeV. These winds form synchrotron-emitting bubbles as the particle stream is eventually decelerated to match the general expansion caused by the explosion that formed the neutron stars. The structure of these pulsar wind nebulae allow us to infer properties of the winds and the pulsating neutron stars themselves. The surfaces of the the stars radiate energy from the rapidly cooling interiors where the physical structure is basically unknown because of our imprecise knowledge of the strong interaction at ultrahigh densities. Here I present a summary of recent measurements that allow us to infer the birth properties of neutron stars and to probe the nature of their winds, the physics of their atmospheres, and the structure of their interiors.

Patrick Slane

2005-05-24

45

Massive binary stars and the kinematics of Young Massive Clusters   

E-print Network

Located in the Large Magellanic Cloud, R136 is a rare example of a nearby young and dense massive star cluster in which individual stars can be resolved. Often suggested as a globular cluster in formation, its study is ...

Henault-Brunet, Vincent; Brunet, Vincent

2013-11-28

46

Young Stars Emerge from Orion's Head  

NASA Technical Reports Server (NTRS)

This image from NASA's Spitzer Space Telescope shows infant stars 'hatching' in the head of the hunter constellation, Orion. Astronomers suspect that shockwaves from a supernova explosion in Orion's head, nearly three million years ago, may have initiated this newfound birth

The region featured in this Spitzer image is called Barnard 30. It is located approximately 1,300 light-years away and sits on the right side of Orion's 'head,' just north of the massive star Lambda Orionis.

Wisps of green in the cloud are organic molecules called polycyclic aromatic hydrocarbons. These molecules are formed anytime carbon-based materials are burned incompletely. On Earth, they can be found in the sooty exhaust from automobile and airplane engines. They also coat the grills where charcoal-broiled meats are cooked.

Tints of orange-red in the cloud are dust particles warmed by the newly forming stars. The reddish-pink dots at the top of the cloud are very young stars embedded in a cocoon of cosmic gas and dust. Blue spots throughout the image are background Milky Way along this line of sight.

This composite includes data from Spitzer's infrared array camera instrument, and multiband imaging photometer instrument. Light at 4.5 microns is shown as blue, 8.0 microns is green, and 24 microns is red.

2007-01-01

47

Massive young disks around Herbig Ae stars  

NASA Astrophysics Data System (ADS)

Context. Herbig Ae stars (HAe) are the precursors of Vega-type systems, hence crucial objects in planet formation studies. Thus far, only a few disks associated with HAe stars have been studied using millimetre interferometers. Aims: Our aim is to determine the dust evolution and the lifetime of the disks associated with Herbig Ae stars. Methods: We imaged the continuum emission at ~3 mm and ~1.3 mm of the Herbig Ae/Be stars BD+61154, RR Tau, VY Mon, and LkH? 198 using the Plateau de Bure Interferometer (PdBI). These stars are in the upper end of the stellar mass range of the Herbig Ae stars (M? > 3 M?). Our measurements were used to complete the spectral energy distribution (SED). The modelling of the SED, in particular the FIR-mm part, allows us to determine the masses and dust properties of these disks. Results: We detected the disks associated with BD+61154, RR Tau, and VY Mon with disk masses of 0.35 M?, 0.05 M?, and 0.40 M?, respectively. The disk around LkH? 198 was not detected with an upper limit to the disk mass of 0.004 M?. We detected, however, the disks associated with the younger stellar objects LkH? 198-IR and LkH? 198-mm that are located in the vicinity of LkH? 198. The fitting of the mm part of the SED reveal that the grains in the mid-plane of the disks around BD+61154, RR Tau, and VY Mon have sizes of ~1-1000 ?m. Therefore, grains have not grown to centimetre sizes in these disks yet. Conclusions: These massive (M? > 3 M?) and young (~1 Myr) HAe stars are surrounded by massive (? 0.04 M?) disks with grains of micron-millimetre sizes. Although grain growth is proceeding in these disks, their evolutionary stage is prior to the formation of planetesimals. These disks are less evolved than those detected around T Tauri and Herbig Be stars. Based on observations carried out with the IRAM Plateau de Bure Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).The data used for the maps of Fig. 1 (FITS files) 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/531/A50

Boissier, J.; Alonso-Albi, T.; Fuente, A.; Berné, O.; Bachiller, R.; Neri, R.; Ginard, D.

2011-07-01

48

JOURNAL OF GEOPHYSICAL RESEARCH: SOLID EARTH, VOL. 118, 115, doi:10.1002/jgrb.50251, 2013 Eruptions at Lone Star Geyser, Yellowstone National Park, USA  

E-print Network

Eruptions at Lone Star Geyser, Yellowstone National Park, USA: 1. Energetics and eruption dynamics Leif 2013; accepted 12 June 2013. [1] Geysers provide a natural laboratory to study multiphase eruptive processes. We present results from a 4 day experiment at Lone Star Geyser in Yellowstone National Park, USA

Manga, Michael

49

3Celestial Fireworks Near Cluster NGC-3603 This young star  

E-print Network

3Celestial Fireworks Near Cluster NGC-3603 This young star cluster, barely 1 million years old,000 light-years away in the constellation Carina, contains a central cluster of 50, huge, hot stars, hot stars in this cluster emit nearly 1/3 of their light at ultraviolet wavelengths and shorter

50

Young volcanoes in the Chilean Southern Volcanic Zone: A statistical approach to eruption prediction based on time series  

NASA Astrophysics Data System (ADS)

Forecasting volcanic activity has long been an aim of applied volcanology with regard to mitigating consequences of volcanic eruptions. Effective disaster management requires both information on expected physical eruption behaviour such as types and magnitudes of eruptions as typical for the individual volcano, usually reconstructed from deposits of past eruptions, and the likelihood that a new eruption will occur within a given time. Here we apply a statistical procedure to provide a probability estimate for future eruptions based on eruption time series, and discuss the limitations of this approach. The statistical investigation encompasses a series of young volcanoes of the Chilean Southern Volcanic Zone. Most of the volcanoes considered have been active in historical times, in addition to several volcanoes with a longer eruption record from Late-Pleistocene to Holocene. Furthermore, eruption rates of neighbouring volcanoes are compared with the aim to reveal possible regional relations, potentially resulting from local to medium-scale tectonic dynamics. One special focus is directed to the two currently most active volcanoes of South America, Llaima and Villarrica, whose eruption records comprise about 50 historical eruptions over the past centuries. These two front volcanoes are considered together with Lanín Volcano, situated in the back-arc of Villarrica, for which the analysis is based on eight eruptions in the past 10 ka. For Llaima and Villarrica, affirmed tests for independence of the repose times between successive eruptions permit to assume Poisson processes; which is hampered for Lanín because of the more limited availability of documented eruptions. The assumption of stationarity reaches varying degrees of confidence depending on the time interval considered, ameliorating towards the more recent and hence probably more complete eruption record. With these pre-requisites of the time series, several distribution functions are fit and the goodness of their fits is evaluated. The mixture of exponentials distribution (MOED), adopted from Mendoza-Rosas and De la Cruz-Reyna (2008), facilitates statistical evaluation of non-stationary eruptive regimes. Despite providing the least good fit of the data, the MOED proves particularly useful for Lanín Volcano, where stationarity can not be assessed because of possible gaps in the eruption record. In general, the Weibull, exponential and log-logistic distributions imply a higher likelihood of future eruptions within a given time, while the Bayesian and MOED analyses predict lower hazard probabilities. This study does not take into account the complexly interacting geophysical and geochemical processes triggering volcanic eruptions. Our aim is to contribute this statistical prediction to the integrative hazard assessment currently performed in the area by the SFB 574 ("Volatiles and Fluids in Subduction Zones"), complementing regional recording of seismic activity and quiescent gas release, as well as tectonic and geochemical characteristics of the investigated volcanic centres.

Dzierma, Y.; Wehrmann, H.

2010-03-01

51

YOUNG STELLAR GROUPS AND THEIR MOST MASSIVE STARS  

SciTech Connect

We analyze the masses and spatial distributions of 14 young stellar groups in Taurus, Lupus3, ChaI, and IC348. These nearby groups, which typically contain 20-40 members, have membership catalogs complete to {approx}0.02 M{sub sun}, and are sufficiently young that their locations should be similar to where they formed. These groups show five properties seen in clusters having many more stars and much greater surface density of stars: (1) a broad range of masses, (2) a concentration of the most massive star toward the center of the group, (3) an association of the most massive star with a high surface density of lower mass stars, (4) a correlation of the mass of the most massive star with the total mass of the group, and (5) the distribution of a large fraction of the mass in a small fraction of the stars.

Kirk, Helen; Myers, Philip C., E-mail: hkirk@cfa.harvard.edu [Radio and Geoastronomy Division, Harvard Smithsonian Center for Astrophysics, MS-42, Cambridge, MA 02138 (United States)

2011-02-01

52

TeV mu Neutrinos from Young Neutron Stars  

E-print Network

Neutron stars are efficient accelerators for bringing charges up to relativistic energies. We show that if positive ions are accelerated to ~1 PeV near the surface of a young neutron star (t_age star's radiation field will produce beamed mu neutrinos with energies of ~50 TeV that could produce the brightest neutrino sources at these energies yet proposed. These neutrinos would be coincident with the radio beam, so that if the star is detected as a radio pulsar, the neutrino beam will sweep the Earth; the star would be a ``neutrino pulsar''. Looking for muon neutrino emission from young neutron stars will provide a valuable probe of the energetics of the neutron star magnetosphere.

B. Link; Fiorella Burgio

2005-05-19

53

TeV mu neutrinos from young neutron stars.  

PubMed

Neutron stars are efficient accelerators for bringing charges up to relativistic energies. We show that if positive ions are accelerated to approximately 1 PeV near the surface of a young neutron star (t(age) less than or nearly 10(5) yr), protons interacting with the star's radiation field produce beamed mu neutrinos with energies of approximately 50 TeV that could produce the brightest neutrino sources at these energies yet proposed. These neutrinos would be coincident with the radio beam, so that, if the star is detected as a radio pulsar, the neutrino beam will sweep the Earth; the star would be a "neutrino pulsar." Looking for nu(mu) emission from young neutron stars will provide a valuable probe of the energetics of the neutron star magnetosphere. PMID:15904352

Link, Bennett; Burgio, Fiorella

2005-05-13

54

Recent outburst of the young star V1180 Cassiopeiae  

NASA Astrophysics Data System (ADS)

Aims: We report on the ongoing outburst of the young variable V1180 Cas, which is known to display characteristics in common with EXor eruptive variables. We present results that support the scenario of an accretion-driven nature of the brightness variations of the object and provide the first evidence of jet structures around the source. Methods: We monitored the recent flux variations of the target in the RC, J, H, and K bands. New optical and near-IR spectra taken during the current high state of V1180 Cas are presented, in conjunction with H2 narrow-band imaging of the source. Results: Observed near-IR colour variations are analogous to those observed in EXors and consistent with excess emission originating from an accretion event. The spectra show numerous emission lines, which indicates accretion, ejection of matter, and an active disc. Using optical and near-IR emission features we derive a mass accretion rate of ~ 3 × 10-8M? yr-1, which is an order of magnitude lower than previous estimates. In addition, a mass loss rate of ~ 4 × 10-9 and ~ 4 × 10-10M? yr-1 are estimated from atomic forbidden lines and H2, respectively. Our H2 imaging reveals two bright knots of emission around the source and the nearby optically invisible star V1180 Cas B, clearly indicative of mass-loss phenomena. Higher resolution observations of the detected jet will help to clarify whether V1180 Cas is the driving source and to determine the relation between the observed knots. Optical and NIR spectra (FITS files) 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/565/L7

Antoniucci, S.; Arkharov, A. A.; Di Paola, A.; Giannini, T.; Harutyunyan, A.; Kopatskaya, E. N.; Larionov, V. M.; Li Causi, G.; Lorenzetti, D.; Morozova, D.; Nisini, B.; Vitali, F.

2014-05-01

55

Young A stars at high distances from the Galactic plane  

NASA Astrophysics Data System (ADS)

Results are presented of observations of many young A stars at high distances from the Galactic plane (z-distances) with intermediate abundances and W velocity dispersion similar to thick disk and halo stars. Hypotheses are proposed for these stars. The two-color diagram in the RGU system of Buser (1978) is used to classify and determine the metal abundances as well as the distances of these stars to the Galactic plane. RGU data obtained for three fields with different Galactic coordinates were applied. While there are only four young stars in M101 with large z-distances, there are 11 such stars in M5 but not even one in the Praesepe field with a lower limiting magnitude (G = 16.2 mag) in a direction almost opposite the Galactic center (l = 205.9 deg).

Karaali, Salih; Güngör, Serap; Karatas, Yuksel

56

Stellar-Dynamics of Young Star Clusters  

E-print Network

The stellar-dynamical evolution of bound star clusters during the first few Myr is dominated by binary-binary and binary-star interactions, the rapid sinking of the most massive stars to the centre of the clusters and mass loss from evolving stars. The consequences of these processes for the binary and stellar population in clusters, and for the star clusters as a whole, are studied by following the evolution over 150 Myr of a library of compact cluster models containing up to 10^4 stars.

Kroupa, P

2000-01-01

57

Stellar-Dynamics of Young Star Clusters  

E-print Network

The stellar-dynamical evolution of bound star clusters during the first few Myr is dominated by binary-binary and binary-star interactions, the rapid sinking of the most massive stars to the centre of the clusters and mass loss from evolving stars. The consequences of these processes for the binary and stellar population in clusters, and for the star clusters as a whole, are studied by following the evolution over 150 Myr of a library of compact cluster models containing up to 10^4 stars.

Pavel Kroupa

2000-02-10

58

Investigating the process of star formation in young LMC star clusters  

E-print Network

The rich star clusters in the Large Magellanic Cloud (LMC) are ideal for studying the process of star formation. Here we focus on the determination of age spreads amongst the massive stars in two young clusters, NGC1818 and NGC1805. We present colour magnitude diagrams (derived from HST data) for these clusters, and discuss the difficulties in age spread determination.

R. A. Johnson; S. F. Beaulieu; R. A. W. Elson; G. Gilmore; N. Tanvir; B. Santiago

1999-07-29

59

Search for associations containing young stars (SACY). II. Chemical abundances of stars in 11 young associations in the solar neighborhood  

Microsoft Academic Search

The recently discovered coeval, moving groups of young stellar objects in the solar neighborhood represent invaluable laboratories for studying recent star formation and searching for high metallicity stars that can be included in future exo-planet surveys. In this study, we derived through an uniform and homogeneous method stellar atmospheric parameters and abundances for iron, silicium, and nickel in 63 post-T

P. Viana Almeida; N. C. Santos; C. Melo; M. Ammler-von Eiff; C. A. O. Torres; G. R. Quast; J. F. Gameiro; M. Sterzik

2009-01-01

60

XMM-Newton view of eight young open star clusters  

NASA Astrophysics Data System (ADS)

The physical origin of X-ray emission from young stars (age<50 Myr) : massive (> 10M?), intermediate mass (10 - 2 M?) and pre-main-sequence (PMS) low mass (< 2M?) stars, and its evolution with time are poorly understood because X-ray studies of clusters with intermediate age (5 to 30 Myr) are few and far between. We investigate X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-Newton, which provides a link between the X-ray properties of young clusters like the Orion (˜5 Myr) and older clusters like the Pleiades (˜100Myr). Overall 152 X-ray sources have been identified with low mass PMS stars, 36 with intermediate mass stars and 16 with massive stars on the basis of multi-wavelength data. Various properties of stars with different masses, like plasma temperatures, X-ray luminosity distributions and their evolution during 4 to 46 Myr, have been investigated. In addition, we have observed interesting flare-like features in the lightcurves of eight stars.The concomitant analysis of the large data sets and detailed X-ray spectral and temporal properties of the cluster members with different masses in these eight young open clusters will be discussed.

Bhatt, H.; Pandey, J.; Singh, K.; Sagar, R.

2014-07-01

61

Young Stars in the Outer HI Disc of NGC 6822  

E-print Network

We present wide-field optical imaging covering the entire neutral hydrogen disc of the Local Group dwarf galaxy NGC 6822. These observations reveal the presence of numerous blue, young stars at large galactocentric radii well beyond R_25. Blue stars are also found that are associated with NGC 6822's companion HI cloud, indicating that star formation was triggered in the companion in the last 10^8 yr. In general, blue stars are present where the HI surface densities reach values > 5 x 10^20 cm^(-2). However, over one-third of the blue stars detected are found at lower surface densities. The young stars trace the distribution of the neutral hydrogen surprisingly well, but seem to be avoiding the supergiant HI shell in NGC 6822, setting a lower limit for its age of 10^8 yr. The extended distribution of young stars implies that stars can form at large galactocentric radii in dwarf galaxies; the HI is therefore not necessarily much more extended than the stellar population. This finding has important consequences for the chemical enrichment of the interstellar medium throughout (dwarf) galaxies

W. J. G. de Blok; F. Walter

2003-03-28

62

DIRECT DETECTIONS OF YOUNG STARS IN NEARBY ELLIPTICAL GALAXIES  

SciTech Connect

Small amounts of star formation in elliptical galaxies are suggested by several results: surprisingly young ages from optical line indices, cooling X-ray gas, and mid-infrared dust emission. Such star formation has previously been difficult to directly detect, but using ultraviolet Hubble Space Telescope Wide Field Camera 3 imaging, we have identified individual young stars and star clusters in four nearby ellipticals. Ongoing star formation is detected in all galaxies, including three ellipticals that have previously exhibited potential signposts of star-forming conditions (NGC 4636, NGC 4697, and NGC 4374), as well as the typical ''red and dead'' NGC 3379. The current star formation in our closest targets, where we are most complete, is between 2.0 and 9.8 Multiplication-Sign 10{sup -5} M{sub Sun} yr{sup -1}. The star formation history was roughly constant from 0.5 to 1.5 Gyr (at (3-5) Multiplication-Sign 10{sup -4} M{sub Sun} yr{sup -1}), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 10{sup 2} and 10{sup 4} M{sub Sun }. The specific star formation rates of {approx}10{sup -16} yr{sup -1} (at the present day) or {approx}10{sup -14} yr{sup -1} (when averaging over the past Gyr) imply that a fraction 10{sup -8} of the stellar mass is younger than 100 Myr and 10{sup -5} is younger than 1 Gyr, quantifying the level of frosting of recent star formation over the otherwise passive stellar population. There is no obvious correlation between either the presence or spatial distribution of postulated star formation indicators and the star formation we detect.

Ford, H. Alyson; Bregman, Joel N., E-mail: aford@nrao.edu [Department of Astronomy, University of Michigan, Ann Arbor, MI 48109 (United States)

2013-06-20

63

Anne S. Young: Professor and Variable Star Observer Extraordinaire  

NASA Astrophysics Data System (ADS)

One of the original eight members of the AAVSO, but not well known today, was Professor Anne Sewell Young of Mount Holyoke College. Miss Young taught there for thirty-seven years, and trained many women astronomers during the first third of the 20th century. This paper will attempt to present her life as an inspiring teacher, as well as a contributor of more than 6,500 variable star observations to the AAVSO.

Bracher, K.

2012-06-01

64

Search for associations containing young stars (SACY). II. Chemical abundances of stars in 11 young associations in the solar neighborhood  

NASA Astrophysics Data System (ADS)

The recently discovered coeval, moving groups of young stellar objects in the solar neighborhood represent invaluable laboratories for studying recent star formation and searching for high metallicity stars that can be included in future exo-planet surveys. In this study, we derived through an uniform and homogeneous method stellar atmospheric parameters and abundances for iron, silicium, and nickel in 63 post-T Tauri stars from 11 nearby young associations. We further compare the results with two different pre-main sequence (PMS) and main sequence (MS) star populations. The stellar atmospheric parameters and the abundances presented here were derived using the equivalent width of individual lines in the stellar spectra by assuming the excitation/ionization equilibrium of iron. Moreover, we compared the observed Balmer lines with synthetic profiles calculated for model atmospheres with a different line-formation code. We found that the synthetic profiles agree reasonably well with the observed profiles, although the Balmer lines of many stars are substantially filled-in, probably by chromospheric emission. Solar metallicity is found to be a common trend in all the nearby young associations studied. The low abundance dispersion within each association strengthens the idea that the origin of these nearby young associations is related to the nearby star-forming regions (SFR). Abundances of elements other than iron are consistent with previous results for Main Sequence stars in the solar neighborhood. The chemical characterization of the members of the newly found nearby young associations, performed in this study and intended to proceed in subsequent works, is essential to understanding and testing the context of local star formation and the evolutionary history of the galaxy. Based on observations collected with the UVES spectrograph at the VLT/UT2 8.2-m Kueyen Telescope (ESO run ID. 079.C-0556(A)) at the Paranal Observatory, Chile. Tables 1, 2 and 5 are only available in electronic form at http://www.aanda.org

Viana Almeida, P.; Santos, N. C.; Melo, C.; Ammler-von Eiff, M.; Torres, C. A. O.; Quast, G. R.; Gameiro, J. F.; Sterzik, M.

2009-07-01

65

CANDIDATE CORONAGRAPHIC DETECTIONS OF PROTOPLANETARY DISKS AROUND FOUR YOUNG STARS  

SciTech Connect

We present potential detections of H-band scattered light emission around four young stars, selected from a total sample of 45 young stars observed with the Coronagraphic Imager with Adaptive Optics coronagraph of the Subaru telescope. Two Classical T Tauri stars, CI Tau and DI Cep, and two weak-lined T Tauri stars, LkCa 14 and RXJ 0338.3+1020, were detected. In all the four cases, the extended emission is within the area of the residual point-spread function halo, and is revealed only through careful data reduction. We compare the observed extended emission with simulations of the scattered light emission to evaluate the plausibility and nature of the detected emission.

Karr, J. L.; Ohashi, N. [Academia Sinica Institute of Astronomy and Astrophysics, P.O. Box 23-141, Taipei 10617, Taiwan (China); Kudo, T.; Tamura, M. [National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)

2010-03-15

66

Binary Stars in Young Clusters - a Theoretical Perspective  

E-print Network

The preponderance of binary systems in all known stellar populations makes them exciting dynamical agents for research on topics as varied as star formation, star-cluster dynamics and the interiors of young and old stars. Today we know that the Galactic-field binary population is probably a dynamically evolved version of the Taurus-Auriga pre-main sequence population, and that the initial distributions of binary-star orbital elements are probably universal. Furthermore, N-body calculations tentatively suggest that OB stars form in energetic binaries near cluster cores, and that binaries with 'forbidden' orbital elements that are produced in stellar encounters, may turn out to be very useful windows into stellar interiors, potentially allowing tests of pre-main sequence evolution theory as well as of models of main-sequence stars.

Kroupa, P

2000-01-01

67

Young viscous flows in the Lowell crater of Orientale basin, Moon: Impact melts or volcanic eruptions?  

NASA Astrophysics Data System (ADS)

Topographical, morphological and spectral reflectance studies have been carried out for a distinct resurface event inside Copernican aged Lowell crater (13.00°S 103.40°W), Orientale basin, using high resolution TC, MI-VIS, LROC-NAC, and M3 data from Kaguya, Lunar Reconnaissance Orbiter (LRO) and Chandrayaan-1 missions. The resurfacing is predominantly gabbroic/basaltic in composition and is confined to nearly a linear ~17 km long, a 3-6 km wide and a ~100 m deep channel, possibly a graben. It is characterised with distinct surface features such as small uplift with melt pond, several lava-like flows, cracks going up to decimetre size, 20-80 m pits/craters with small central uplifts or depressions and ~100 m craters that emanate liquid. A minimum of three generations of flows have been identified within the unit, the oldest one being less viscous and the subsequent younger ones showing well developed lobes due to the high viscosity. There is a conspicuous absence of unambiguously identified primary impact craters on these flows suggesting their fresh nature. On the basis of these integrated observations we hypothesise that at least the younger portions of this amazingly carved resurfaced unit might be composed of volcanic flows erupted from single or multiple sources subsequent to the emplacement of impact melts from a ~9 km diameter crater on the edge of Lowell crater. Gabbroic/basaltic signatures have also been identified at several other locations inside Lowell crater indicating that it would have impacted on a pre-existing basaltic surface or on a gabbroic pluton. These findings have implications to lunar magmatism and understanding of the genesis of young flows on the lunar surface.

Srivastava, N.; Kumar, D.; Gupta, R. P.

2013-10-01

68

RCW 108: Massive Young Stars Trigger Stellar Birth  

NASA Technical Reports Server (NTRS)

RCW 108 is a region where stars are actively forming within the Milky Way galaxy about 4,000 light years from Earth. This is a complicated region that contains young star clusters, including one that is deeply embedded in a cloud of molecular hydrogen. By using data from different telescopes, astronomers determined that star birth in this region is being triggered by the effect of nearby, massive young stars.

This image is a composite of X-ray data from NASA's Chandra X-ray Observatory (blue) and infrared emission detected by NASA's Spitzer Space Telescope (red and orange). More than 400 X-ray sources were identified in Chandra's observations of RCW 108. About 90 percent of these X-ray sources are thought to be part of the cluster and not stars that lie in the field-of-view either behind or in front of it. Many of the stars in RCW 108 are experiencing the violent flaring seen in other young star-forming regions such as the Orion nebula. Gas and dust blocks much of the X-rays from the juvenile stars located in the center of the image, explaining the relative dearth of Chandra sources in this part of the image.

The Spitzer data show the location of the embedded star cluster, which appears as the bright knot of red and orange just to the left of the center of the image. Some stars from a larger cluster, known as NGC 6193, are also visible on the left side of the image. Astronomers think that the dense clouds within RCW 108 are in the process of being destroyed by intense radiation emanating from hot and massive stars in NGC 6193.

Taken together, the Chandra and Spitzer data indicate that there are more massive star candidates than expected in several areas of this image. This suggests that pockets within RCW 108 underwent localized episodes of star formation. Scientists predict that this type of star formation is triggered by the effects of radiation from bright, massive stars such as those in NGC 6193. This radiation may cause the interior of gas clouds in RCW 108 to be compressed, leading to gravitational collapse and the formation of new stars.

2008-01-01

69

Understanding the Spins of Young Stars  

NASA Astrophysics Data System (ADS)

We review the theoretical efforts to understand why pre-main-sequence stars spin much more slowly than expected. The first idea put forward was that massive stellar winds may remove substantial angular momentum. Since then, it has become clear that the magnetic interaction between the stars and their accretion disks explains many of the observed emission properties. The disk locking scenario, which assumes the magnetic star-disk interaction also solves the stellar spin problem, has received the most attention in the literature to date. However, recent considerations suggest that the torques in the star-disk interaction are insufficient for disk locking to explain the slow rotators. This prompts us to revisit stellar winds, and we conclude that stellar winds, working in conjunction with magnetospheric accretion, are a promising candidate for solving the angular momentum problem. We suggest future directions for both observations and theory, to help shed light on this issue.

Matt, S.; Pudritz, R.

2008-04-01

70

Young Star Cluster Aglow With Mysterious X-Ray Cloud  

NASA Technical Reports Server (NTRS)

At a distance of 6,000 light years from Earth, the star cluster RCW 38 is a relatively close star-forming region. This area is about 5 light years across, and contains thousands of hot, very young stars formed less than a million years ago, 190 of which exposed x-rays to Chandra. Enveloping the star cluster, the diffused cloud of x-rays shows an excess of high energy x-rays, which indicates that the x-rays come from trillion-volt electrons moving in a magnetic field. Such particles are typically produced by exploding stars, or in the strong magnetic fields around neutron stars or black holes, none of which are evident in RCW 38. One possible origin for the particles, could be an undetected supernova that occurred in the cluster, possibly thousands of years ago, producing a shock wave that is interacting with the young stars. Regardless of the origin of these energetic electrons, their presence could change the chemistry of the disks that will eventually form planets around the stars in the cluster.

2002-01-01

71

Winds and Accretion in Young Stars  

E-print Network

Establishing the origin of accretion powered winds from forming stars is critical for understanding angular momentum evolution in the star-disk interaction region. Here, the high velocity component of accretion powered winds is launched and accreting stars are spun down, in defiance of the expected spin-up during magnetospheric accretion. T Tauri stars in the final stage of disk accretion offer a unique opportunity to study the connection between accretion and winds and their relation to stellar spindown. Although spectroscopic indicators of high velocity T Tauri winds have been known for decades, the line of He I 10830 offers a promising new diagnostic to probe the magnetically controlled star-disk interaction and wind-launching region. The high opacity and resonance scattering properties of this line offer a powerful probe of the geometry of both the funnel flow and the inner wind that, together with other atomic and molecular spectral lines covering a wide range of excitation and ionization states, suggests that the magnetic interaction between the star and disk, and the subsequent launching of the inner high velocity wind, is sensitive to the disk accretion rate.

Suzan Edwards

2008-09-21

72

Extreme Coronal Mass Ejections in Young Low-Mass Stars  

NASA Astrophysics Data System (ADS)

Two long-standing questions in the study of young, low-mass stars are: (1) What are the mechanisms that govern the observed order-of-magnitude decrease of stellar angular momentum during pre-main-sequence evolution, and (2) What are the physical drivers of X-ray production in these stars at up to 104 times the solar value? Application of solar flare models to the most powerful X-ray flares observed among T Tauri stars in Orion suggests that the flares are produced by magnetic loop structures with lengths of up to tens of stellar radii. We present new results demonstrating that, for the majority of these stars, the extremely large flaring structures are not anchored to or stabilized by circumstellar disks. Given the energy and size scales involved, mass losses (e.g., via stellar coronal mass ejections -- CMEs -- associated with these flares) at such large effective lever arms could shed substantial angular momentum. To begin estimating the attendant angular momentum losses of such extreme CMEs in young stars, we have assembled from the solar literature a database of ˜10,000 X-ray flares and CMEs, from which we determine for the first time the empirical relationship between solar X-ray flare energy and CME ejected mass. Finally, we demonstrate how our flare flux/CME mass relationship can be used to estimate stellar angular momentum loss via extreme CMEs in young stars.

Aarnio, A. N.; Stassun, K. G.; Matt, S. P.; Hughes, W. J.; McGregor, S. L.

2011-12-01

73

A debris disk around an isolated young neutron star.  

PubMed

Pulsars are rotating, magnetized neutron stars that are born in supernova explosions following the collapse of the cores of massive stars. If some of the explosion ejecta fails to escape, it may fall back onto the neutron star or it may possess sufficient angular momentum to form a disk. Such 'fallback' is both a general prediction of current supernova models and, if the material pushes the neutron star over its stability limit, a possible mode of black hole formation. Fallback disks could dramatically affect the early evolution of pulsars, yet there are few observational constraints on whether significant fallback occurs or even the actual existence of such disks. Here we report the discovery of mid-infrared emission from a cool disk around an isolated young X-ray pulsar. The disk does not power the pulsar's X-ray emission but is passively illuminated by these X-rays. The estimated mass of the disk is of the order of 10 Earth masses, and its lifetime (> or = 10(6) years) significantly exceeds the spin-down age of the pulsar, supporting a supernova fallback origin. The disk resembles protoplanetary disks seen around ordinary young stars, suggesting the possibility of planet formation around young neutron stars. PMID:16598251

Wang, Zhongxiang; Chakrabarty, Deepto; Kaplan, David L

2006-04-01

74

M33's Variable A -- A Hypergiant Star More Than 35 Years in Eruption  

E-print Network

Variable A in M33 is a member of a rare class of highly luminous, evolved stars near the upper luminosity boundary that show sudden and dramatic shifts in apparent temperature due to the formation of optically thick winds in high mass loss episodes. Recent optical and infrared spectroscopy and imaging reveal that its ``eruption'' begun in $\\sim$1950 has ended, {\\it lasting $\\approx$ 45 yrs}. Our current observations show major changes in its wind from a cool, dense envelope to a much warmer state surrounded by low density gas with rare emission lines of Ca II, [Ca II] and K I. Its spectral energy distribution has unexpectedly changed, especially at the long wavelengths, with a significant decrease in its apparent flux, while the star remains optically obscured. We conclude that much of its radiation is now escaping out of our line of sight. We attribute this to the changing structure and distribution of its circumstellar ejecta corresponding to the altered state of its wind as the star recovers from a high mass loss event.

Roberta M. Humphreys; Terry J. Jones; Elisha Polomski; Michael Koppelman; Andrew Helton; Kristen McQuinn; Robert D. Gehrz; C. E. Woodward; R. Mark Wagner; Karl Gordon; Joannah Hinz; S. P. Willner

2006-01-05

75

Identification and Characterization of Young, Nearby, Solar-type Stars  

NASA Astrophysics Data System (ADS)

Post-T Tauri stars (PTTSs) are low-mass, pre-MS stars which have ceased accreting, and are not necessarily near star-forming molecular clouds. Historically, they have been difficult to identify due to their benign spectroscopic signatures. With recent all-sky X-ray surveys and proper motion catalogs, it is now possible to find PTTSs in large numbers. The nearest PTTSs will be important targets for future imaging surveys characterizing dust disks and planetary systems around young solar analogs. The goal of this work is to systematically identify samples of PTTSs, investigate the evolution of circumstellar disks, to infer the fossil star-formation history of molecular clouds, and to estimate kinematic distances to young stars lacking trigonometric parallaxes. We present the results of a spectroscopic survey which identified 110 PTTS members of the nearest OB association (Sco-Cen). We find that 2/3rds of the low-mass star-formation in each OB subgroup occurred in <5 Myr, and that only ~1% of solar-type stars with mean age ~13 Myr shows signs of accretion from a circumstellar disk. In order to assess how long circumstellar material is detectable around PTTSs, we conducted a 10 micron imaging survey of post-T Tauri members of the ~30-Myr-old Tuc-Hor association. The goal was to find evidence of either remnant accretion disks or dusty debris disks with orbital radii of <10 AU. Combined with data from other surveys, we conclude that mid-IR emission from warm dust grains in the terrestrial planet zones around young stars become undetectable compared to the stellar photosphere for nearly all stars by age ~20 Myr. Lastly, we present a technique for calculating distances isolated young field stars that currently lack trigonometric parallax measurements. The technique is a generalization of the classical cluster parallax method, but can handle anisotropic velocity dispersions and non-zero Oort parameters. Distances and isochronal ages are estimated for a subsample of PTTSs included in the Formation and Evolution of Planetary Systems (FEPS) Spitzer Space Telescope (SST) Legacy Science program. The techniques developed in this thesis will be useful for identifying the nearest, young star candidates from current databases.

Mamajek, Eric E.

2004-08-01

76

The rotational evolution of young low mass stars  

E-print Network

Star-disk interaction is thought to drive the angular momentum evolution of young stars. In this review, I present the latest results obtained on the rotational properties of low mass and very low mass pre-main sequence stars. I discuss the evidence for extremely efficient angular momentum removal over the first few Myr of pre-main sequence evolution and describe recent results that support an accretion-driven braking mechanism. Angular momentum evolution models are presented and their implication for accretion disk lifetimes discussed.

Jerome Bouvier

2007-12-18

77

Multiplicity of young stars in and around R Coronae Australis  

NASA Astrophysics Data System (ADS)

Context: In star-forming regions like Taurus-Auriga, it has been found that most young stars are born as multiples, which theories of star formation should definitely take into account. The R CrA star-forming region has a small dark cloud with quite a number of protostars, T Tauri stars, and some Herbig Ae/Be stars, plus a number of weak-line T Tauri stars around the cloud found by ROSAT follow-up observations. Aims: We would like to detect multiples among the young stars in and around the R CrA cloud in order to investigate multiplicity in this region. Methods: We performed interferometric and imaging observations with the speckle camera SHARP I at the ESO 3.5 m NTT and adaptive optics observation with ADONIS at the ESO 3.6 m telescope, all in the near-infrared bands JHK obtained in the years 1995, 2000, and 2001. Results: We found 13 new binaries among the young stars in CrA between 0.13 arcsec (the diffraction limit) and 6 arcsec (set as an upper separation limit to avoid contamination by chance alignments). While most multiples in CrA are binaries, there are also one quadruple (TY CrA), and one triple (HR 7170) which may form a quintuple together with the binary HR 7169. One of the newly detected companions with a large magnitude difference found near the M3-5 type T Tauri star [MR 81] H? 17 could be a brown dwarf or an infrared companion with an edge-on disk. Among seven Herbig Ae/Be stars in CrA, six are multiple. Conclusions: The multiplicity frequency in CrA is as high as in similar star forming regions. By comparing with the period distribution of main-sequence stars and extrapolating to separations not probed in this survey, we conclude that the companion-star frequency is (95±23)%; i.e. the average number of companions per primary is 0.95. Based on observations obtained at the European Southern Observatory, La Silla, proposal numbers 55.E-0968, 65.I-0086, and 67.C-0213.

Köhler, R.; Neuhäuser, R.; Krämer, S.; Leinert, Ch.; Ott, T.; Eckart, A.

2008-09-01

78

Young Stellar Groups, Runaway Stars, and Pulsars  

E-print Network

Milli-arcsecond astrometry provided by Hipparcos and by radio observations makes it possible to retrace the orbits of nearby runaway stars and pulsars with sufficient accuracy to identify their parent stellar cluster or association. For two cases it is even possible to deduce the specific formation scenario. The runaway star zeta Oph and PSR J1932+1059 are the result of a supernova explosion which took place 1 Myr ago in a massive binary in the Upper Scorpius association. The pulsar received a kick velocity of about 350 km/s in this event. The runaway stars mu Col and AE Aur and the isolated eccentric massive binary iota Ori result from a binary-binary encounter, most likely inside the Trapezium cluster, 2.5 Myr ago. Future astrometric missions such as DIVA, FAME and in particular GAIA will allow extension of these studies to a significant fraction of the Galactic disk, and will provide new constraints on the formation and evolution of massive stars.

Tim de Zeeuw; Ronnie Hoogerwerf; Jos de Bruijne

2000-07-28

79

Instabilities in Very Young Neutron Stars: Temperature  

NSDL National Science Digital Library

This simulation shows the first 20 milliseconds in the life of a neutron star which is formed in a Type II supernova. After an initial collapse phase, the neutron star becomes unstable to convection. The resulting convective motions destroy the spherical symmetry of the star and rapidly mix the inner regions. In addition, the neutrino flux from the neutron star will be non-spherical and will be significantly enhanced by the convective motions. This may have major implications for the Type II supernova mechanism. The calculation was performed using the Piecewise-Parabolic Method for hydrodynamics. The computational grid contained 300 zones in radius and 200 zones in angle. The inner 200 zones in radius were uniformly spaced, ranging from the inner boundary at 25 km to 175 km. The outer 100 zones were non-uniformly spaced and stretched to 2000 km. Only the inner 200 zones are plotted. The inner boundary was treated as a hard sphere. At the outer boundary, zero gradients for all the variables were assumed. Periodic boundary conditions were used along the sides of the grid. The following sequence shows the temperature structure for 20 milliseconds after the shock stalls. The minimum temperature is approximately 1.35 MeV. The maximum temperature varies from 6 MeV at the beginning of the calculation to 10 MeV at the later times.

Pamela ONeil

1994-02-12

80

Instabilities in Very Young Neutron Stars: Density  

NSDL National Science Digital Library

This simulation shows the first 20 milliseconds in the life of a neutron star which is formed in a Type II supernova. After an initial collapse phase, the neutron star becomes unstable to convection. The resulting convective motions destroy the spherical symmetry of the star and rapidly mix the inner regions. In addition, the neutrino flux from the neutron star will be non-spherical and will be significantly enhanced by the convective motions. This may have major implications for the Type II supernova mechanism. The calculation was performed using the Piecewise-Parabolic Method for hydrodynamics. The computational grid contained 300 zones in radius and 200 zones in angle. The inner 200 zones in radius were uniformly spaced, ranging from the inner boundary at 25 km to 175 km. The outer 100 zones were non-uniformly spaced and stretched to 2000 km. Only the inner 200 zones are plotted. The inner boundary was treated as a hard sphere. At the outer boundary, zero gradients for all the variables were assumed. Periodic boundary conditions were used along the sides of the grid. The following sequence shows the density evolution for 20 milliseconds after the shock stalls. The density is plotted on a log scale. Values range from 10^9 gm-cm^3 at the outer boundary to 1.4 x 10^12 gm-cm^3 at the inner boundary.

Pamela ONeil

1994-02-12

81

Realistic limitations of detecting planets around young active stars  

NASA Astrophysics Data System (ADS)

Current planet hunting methods using the radial velocity method are limited to observing middle-aged main-sequence stars where the signatures of stellar activity are much less than on young stars that have just arrived on the main-sequence. In this work we apply our knowledge from the surface imaging of these young stars to place realistic limitations on the possibility of detecting orbiting planets. In general we find that the magnitude of the stellar jitter is directly proportional to the stellar vsini. For G and K dwarfs, we find that it is possible, for models with high stellar activity and low stellar vsini, to be able to detect a 1 MJupiter mass planet within 50 epochs of observations and for the M dwarfs it is possible to detect a habitable zone Earth-like planet in 10s of observational epochs.

Jeffers, S. V.; Barnes, J. R.; Jones, H.; Pinfield, D.

2013-04-01

82

Energetic mass outflows from young stars  

NASA Technical Reports Server (NTRS)

Winds from low-mass optically visible T Tauri stars are investigated. It is shown that the presence of energetic T Tauri winds is coupled with the presence of solar-system-sized circumstellar disks which are optically thick in their inner regions. It is likely that these disks are in a state of mass accretion, and the source for the T Tauri winds probably derives from the innermost region of the accretion disk. It is suggested that energetic T Tauri winds are unlikely to be the primary agents in the final clearing of gas and other debris in forming planetary systems, as the cessation of energetic winds apparently coincides with the cessation of inner disk accretion. It is argued that energetic winds from both embedded IR sources and T Tauri stars share a common origin.

Edwards, Suzan; Ray, Tom; Mundt, Reinhard

1993-01-01

83

Probing the Early Evolution of Young High-Mass Stars  

E-print Network

Near-infrared imaging surveys of high-mass star-forming regions reveal an amazingly complex interplay between star formation and the environment (Churchwell et al. 2006; Alvarez et al. 2004). By means of near-IR spectroscopy the embedded massive young stars can be characterized and placed in the context of their birth site. However, so far spectroscopic surveys have been hopelessly incomplete, hampering any systematic study of these very young massive stars. New integral field instrumentation available at ESO has opened the possibility to take a huge step forward by obtaining a full spectral inventory of the youngest massive stellar populations in star-forming regions currently accessible. Simultaneously, the analysis of the extended emission allows the characterization of the environmental conditions. The Formation and Early Evolution of Massive Stars (FEMS) collaboration aims at setting up a large observing campaign to obtain a full census of the stellar content, ionized material, outflows and PDR's over a sample of regions that covers a large parameter space. Complementary radio, mm and infrared observations will be used for the characterization of the deeply embedded population. For the first eight regions we have obtained 40 hours of SINFONI observations. In this contribution, we present the first results on three regions that illustrate the potential of this strategy.

E. Puga; A. Bik; L. B. M. F. Waters; Th. Henning; L. Kaper; M. van den Ancker; A. Lenorzer; E. Churchwell; S. Kurtz; J. A. Rodon; T. Vasyunina; M. B. N. Kouwenhoven; H. Beuther; H. Linz; M. Horrobin; A. Stolte; A. de Koter; W. F. Thi; N. L. Martin-Hernandez; B. Acke; F. Comeron; G. van der Plas; Ch. Waelkens; C. Dominik; M. Feldt

2008-03-27

84

Evolution of the central stars of young planetary nebulae  

NASA Astrophysics Data System (ADS)

Aims: The evolution of central stars of planetary nebulae was so far documented in just a few cases. However, spectra collected a few decades ago may provide a good reference for studying the evolution of central stars using the emission line fluxes of their nebulae. We investigated evolutionary changes of the [O iii] 5007 Å line flux in the spectra of planetary nebulae. Methods: We compared nebular fluxes collected during a decade or longer. We used literature data and newly obtained spectra. A grid of cloudy models was computed using existing evolutionary models, and the models were compared with the observations. Results: An increase of the [O iii] 5007 Å line flux is frequently observed in young planetary nebulae hosting H-rich central stars. The increasing nebular excitation is the response to the increasing temperature and hardening radiation of the central stars. We did not observe any changes in the nebular fluxes in the planetary nebulae hosting late-type Wolf-Rayet (WR) central stars. This may indicate a slower temperature evolution (which may stem from a different evolutionary status) of late-[WR] stars. Conclusions: In young planetary nebulae with H-rich central stars, the evolution can be followed using optical spectra collected during a decade or longer. The observed evolution of H-rich central stars is consistent with the predictions of the evolutionary models provided in the literature. Late-[WR] stars possibly follow a different evolutionary path. Reduced spectra from the Torun and SAAO observatories 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/573/A65

Hajduk, M.; van Hoof, P. A. M.; Zijlstra, A. A.

2015-01-01

85

Star and planet formation in young stellar clusters Die meisten Sterne in unserer Milchstrae wer-  

E-print Network

Star and planet formation in young stellar clusters Die meisten Sterne in unserer MilchstraÃ?e wer such as Orion demonstrated that most of the young stars there are found in dense aggregates or clusters around binary stars and in dense clusters will evolve quite differently to those around single stars

86

7mm Observations toward Young Massive Stars  

NASA Astrophysics Data System (ADS)

We present preliminary result of a VLA 7 mm survey toward 14 regions of massive star formation which are in an evolutionary phase prior to that of ultracompact HII regions. The observations were obtained using bright methanol masers as cross calibration sources. Using additional interferometric data at cm and mm wavelengths we compile the radio SEDs for the sources and discuss possible emission mechanisms responsible for the 7 mm emission, as well as the relation to the 44 GHz methanol masers. This research was partially supported by NSF grant AST-0098524 and grant Nr.CC4966 from the Research Corporation (PH).

Hofner, P.; Araya, E.; Linz, H.; Kurtz, S.; Cesaroni, R.; Molinari, S.

2004-12-01

87

Young Stars Emerge from Orion's Head  

NASA Technical Reports Server (NTRS)

This image from NASA's Spitzer Space Telescope shows infant stars 'hatching' in the head of the hunter constellation, Orion. Astronomers suspect that shockwaves from a supernova explosion in Orion's head, nearly three million years ago, may have initiated this newfound birth

The region featured in this Spitzer image is called Barnard 30. It is located approximately 1,300 light-years away and sits on the right side of Orion's 'head,' just north of the massive star Lambda Orionis.

Wisps of red in the cloud are organic molecules called polycyclic aromatic hydrocarbons. These molecules are formed anytime carbon-based materials are burned incompletely. On Earth, they can be found in the sooty exhaust from automobile and airplane engines. They also coat the grills where charcoal-broiled meats are cooked.

This image shows infrared light captured by Spitzer's infrared array camera. Light with wavelengths of 8 and 5.8 microns (red and orange) comes mainly from dust that has been heated by starlight. Light of 4.5 microns (green) shows hot gas and dust; and light of 3.6 microns (blue) is from starlight.

2007-01-01

88

Variability of Young Massive Stars in the Arches Cluster  

E-print Network

We present preliminary results of the first near-infrared variability study of the Arches cluster, using adaptive optics data from NIRI/Gemini and NACO/VLT. The goal is to discover eclipsing binaries in this young (2.5 +- 0.5 Myr), dense, massive cluster for which we will determine accurate fundamental parameters with subsequent spectroscopy. Given that the Arches cluster contains more than 200 Wolf-Rayet and O-type stars, it provides a rare opportunity to determine parameters for some of the most massive stars in the Galaxy.

Markakis, K; Pietrzynski, G; Macri, L; Stanek, K Z

2010-01-01

89

Variability of young massive stars in the Arches cluster  

NASA Astrophysics Data System (ADS)

We present preliminary results of the first near-infrared variability study of the Arches cluster, using adaptive optics data from NIRI/Gemini and NACO/VLT. The goal is to discover eclipsing binaries in this young (2.5 +/- 0.5 Myr), dense, massive cluster for which we will determine accurate fundamental parameters with subsequent spectroscopy. Given that the Arches cluster contains more than 200 Wolf-Rayet and O-type stars, it provides a rare opportunity to determine parameters for some of the most massive stars in the Galaxy.

Markakis, Kostas; Bonanos, Alceste Z.; Pietrzynski, Grzegorz; Macri, Lucas; Stanek, Kris Z.

2011-07-01

90

Variability of Young Massive Stars in the Arches Cluster  

NASA Astrophysics Data System (ADS)

We present preliminary results of the first near-infrared variability study of the Arches cluster, using adaptive optics data from NIRI/Gemini and NACO/VLT. The goal is to discover eclipsing binaries in this young (2.5 +/- 0.5 Myr), dense, massive cluster for which we will determine accurate fundamental parameters with subsequent spectroscopy. Given that the Arches cluster contains more than 200 Wolf-Rayet and O-type stars, it provides a rare opportunity to determine parameters for some of the most massive stars in the Galaxy.

Markakis, K.; Bonanos, A. Z.; Pietrzynski, G.; Macri, L.; Stanek, K. Z.

2012-04-01

91

Search for OB stars running away from young star clusters. II. The NGC 6357 star-forming region  

NASA Astrophysics Data System (ADS)

Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fraction of their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can be revealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a series of papers devoted to the search for OB stars running away from young ( ? several Myr) Galactic clusters and OB associations, we present the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the Midcourse Space Experiment (MSX) satellite and the Spitzer Space Telescope, and the preliminary data release of the Wide-Field Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by stars expelled from the young (~1-2 Myr) star clusters, Pismis 24 and AH03 J1725-34.4, associated with NGC 6357. Two of the seven bow shocks are driven by the already known OB stars, HD 319881 and [N78] 34. Follow-up spectroscopy of three other bow-shock-producing stars showed that they are massive (O-type) stars as well, while the 2MASS photometry of the remaining two stars suggests that they could be B0 V stars, provided that both are located at the same distance as NGC 6357. Detection of numerous massive stars ejected from the very young clusters is consistent with the theoretical expectation that star clusters can effectively lose massive stars at the very beginning of their dynamical evolution (long before the second mechanism for production of runaway stars, based on a supernova explosion in a massive tight binary system, begins to operate) and lends strong support to the idea that probably all field OB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is the detection of three circular shells typical of luminous blue variable and late WN-type Wolf-Rayet stars.

Gvaramadze, V. V.; Kniazev, A. Y.; Kroupa, P.; Oh, S.

2011-11-01

92

Identifying Young Stars in Massive Star-forming Regions for the MYStIX Project  

NASA Astrophysics Data System (ADS)

The Massive Young star-forming Complex Study in Infrared and X-rays (MYStIX) project requires samples of young stars that are likely members of 20 nearby Galactic massive star-forming regions. Membership is inferred from statistical classification of X-ray sources, from detection of a robust infrared excess that is best explained by circumstellar dust in a disk or infalling envelope and from published spectral types that are unlikely to be found among field stars. We present the MYStIX membership lists here, and describe in detail the statistical classification of X-ray sources via a "Naive Bayes Classifier." These membership lists provide the empirical foundation for later MYStIX science studies.

Broos, Patrick S.; Getman, Konstantin V.; Povich, Matthew S.; Feigelson, Eric D.; Townsley, Leisa K.; Naylor, Tim; Kuhn, Michael A.; King, Robert R.; Busk, Heather A.

2013-12-01

93

Uncovering the Properties of Young Neutron Stars and Their Surroundings  

NASA Technical Reports Server (NTRS)

The subject grant provides funding through the NASA LTSA program. This five-year grant involves the study of young neutron stars, particularly those in supernova remnants. In the fifth year of this program, the following studies have been undertaken in support of this effort and are discussed in this report. 1) 3C 58; 2) Chandra Survey for Compact Objects in Supernova Remnants; 3) G327.1-1.1; 4) Infrared Emission from Pulsar Wind Nebulae; and Cas A.

Oliversen, Ronald (Technical Monitor); Slane, Patrick

2005-01-01

94

HOT WHITE DWARF SHINES IN YOUNG STAR CLUSTER  

NASA Technical Reports Server (NTRS)

A dazzling 'jewel-box' collection of over 20,000 stars can be seen in crystal clarity in this NASA Hubble Space Telescope image, taken with the Wide Field and Planetary Camera 2. The young (40 million year old) cluster, called NGC 1818, is 164,000 light-years away in the Large Magellanic Cloud (LMC), a satellite galaxy of our Milky Way. The LMC, a site of vigorous current star formation, is an ideal nearby laboratory for studying stellar evolution. In the cluster, astronomers have found a young white dwarf star, which has only very recently formed following the burnout of a red giant. Based on this observation astronomers conclude that the red giant progenitor star was 7.6 times the mass of our Sun. Previously, astronomers have estimated that stars anywhere from 6 to 10 solar masses would not just quietly fade away as white dwarfs but abruptly self-destruct in torrential explosions. Hubble can easily resolve the star in the crowded cluster, and detect its intense blue-white glow from a sizzling surface temperature of 50,000 degrees Fahrenheit. IMAGE DATA Date taken: December 1995 Wavelength: natural color reconstruction from three filters (I,B,U) Field of view: 100 light-years, 2.2 arc minutes TARGET DATA Name: NGC 1818 Distance: 164,000 light-years Constellation: Dorado Age: 40 million years Class: Rich star cluster Apparent magnitude: 9.7 Apparent diameter: 7 arc minutes Credit: Rebecca Elson and Richard Sword, Cambridge UK, and NASA (Original WFPC2 image courtesy J. Westphal, Caltech) Image files are available electronically via the World Wide Web at: http://oposite.stsci.edu/pubinfo/1998/16 and via links in http://oposite.stsci.edu/pubinfo/latest.html or http://oposite.stsci.edu/pubinfo/pictures.html. GIF and JPEG images are available via anonymous ftp to oposite.stsci.edu in /pubinfo/GIF/9816.GIF and /pubinfo/JPEG/9816.jpg.

2002-01-01

95

New Insights from Aperiodic Variability of Young Stars  

NASA Astrophysics Data System (ADS)

Nearly all young stars are variable, with the variability traditionally divided into two classes: periodic variables and aperiodic or "irregular" variables. Periodic variables have been studied extensively, typically using periodograms, while aperiodic variables have received much less attention due to a lack of standard statistical tools. However, aperiodic variability can serve as a powerful probe of young star accretion physics and inner circumstellar disk structure. For my dissertation, I analyzed data from a large-scale, long-term survey of the nearby North America Nebula complex, using Palomar Transient Factory photometric time series collected on a nightly or every few night cadence over several years. This survey is the most thorough exploration of variability in a sample of thousands of young stars over time baselines of days to years, revealing a rich array of lightcurve shapes, amplitudes, and timescales. I have constrained the timescale distribution of all young variables, periodic and aperiodic, on timescales from less than a day to ~100 days. I have shown that the distribution of timescales for aperiodic variables peaks at a few days, with relatively few (˜15%) sources dominated by variability on tens of days or longer. My constraints on aperiodic timescale distributions are based on two new tools, magnitude- vs. time-difference (Delta m-Deltat) plots and peak-finding plots, for describing aperiodic lightcurves; this thesis provides simulations of their performance and presents recommendations on how to apply them to aperiodic signals in other time series data sets. In addition, I have measured the error introduced into colors or SEDs from combining photometry of variable sources taken at different epochs. These are the first quantitative results to be presented on the distributions in amplitude and time scale for young aperiodic variables, particularly those varying on timescales of weeks to months.

Findeisen, Krzysztof

96

The Search for Young Planetary Systems And the Evolution of Young Stars  

NASA Technical Reports Server (NTRS)

The Space Interferometer Mission (SIM) will provide a census of planetary systems by con- ducting a broad survey of 2,000 stars that will be sensitive to the presence of planets with masses as small as approx. 15 Earth masses (1 Uranus mass) and a deep survey of approx. 250 of the nearest, stars with a mass limit of approx.3 Earth masses. The broad survey will include stars spanning a wide range of ages, spectral types, metallicity, and other important parameters. Within this larger context, the Young Stars and Planets Key Project will study approx. 200 stars with ages from 1 Myr to 100 Myr to understand the formation and dynamical evolution of gas giant planets. The SIM Young Stars and Planets Project will investigate both the frequency of giant planet formation and the early dynamical history of planetary systems. We will gain insight into how common the basic architecture of our solar system is compared with recently discovered systems with close-in giant planets by examining 200 of the nearest (less than 150 pc) and youngest (1-100 Myr) solar-type stars for planets. The sensitivity of the survey for stars located 140 pc away is shown in the planet mass-separation plane. We expect to find anywhere from 10 (assuming that only the presently known fraction of stars. 5-7%, has planets) to 200 (all young stars have planets) planetary systems. W-e have set our sensitivity threshold to ensure the detection of Jupiter-mass planets in the critical orbital range of 1 to 5 AU. These observations, when combined with the results of planetary searches of mature stars, will allow us to test theories of planetary formation and early solar system evolution. By searching for planets around pre-main sequence stars carefully selected to span an age range from 1 to 100 Myr, we will learn a t what epoch and with what frequency giant planets are found at the water-ice snowline where they are expected to form. This will provide insight into the physical mechanisms by which planets form and migrate from their place of birth, and about their survival rate. With these data in hand, we will provide data, for the first time, on such important questions as: What processes affect the formation and dynamical evolution of planets? When and where do planets form? What is initial mass distribution of planetary systems around young stars? How might planets be destroyed? What is the origin of the eccentricity of planetary orbits? What is the origin of the apparent dearth of companion objects between planets and brown dwarfs seen in mature stars? The observational strategy is a compromise between the desire to extend the planetary mass function as low as possible and the essential need to build up sufficient statistics on planetary occurrence. About half of the sample will be used to address the "where" and "when" of planet formation. We will study classical T Tauri stars (cTTs) which have massive accretion disks and post- accretion, weak-lined T Tauri stars (wTTs). Preliminary estimates suggest the sample will consist of approx. 30% cTTs and approx. 70% wTTs, driven in part by the difficulty of making accurate astrometric measurements toward objects with strong variability or prominent disks.

Beichman, Charles A.; Boden, Andrew; Ghez, Andrea; Hartman, Lee W.; Hillenbrand, Lynn; Lunine, Jonathan I.; Simon, Michael J.; Stauffer, John R.; Velusamy, Thangasamy

2004-01-01

97

Misaligned protoplanetary disks in a young binary star system.  

PubMed

Many extrasolar planets follow orbits that differ from the nearly coplanar and circular orbits found in our Solar System; their orbits may be eccentric or inclined with respect to the host star's equator, and the population of giant planets orbiting close to their host stars suggests appreciable orbital migration. There is at present no consensus on what produces such orbits. Theoretical explanations often invoke interactions with a binary companion star in an orbit that is inclined relative to the planet's orbital plane. Such mechanisms require significant mutual inclinations between the planetary and binary star orbital planes. The protoplanetary disks in a few young binaries are misaligned, but often the measurements of these misalignments are sensitive only to a small portion of the inner disk, and the three-dimensional misalignment of the bulk of the planet-forming disk mass has hitherto not been determined. Here we report that the protoplanetary disks in the young binary system HK Tauri are misaligned by 60 to 68 degrees, such that one or both of the disks are significantly inclined to the binary orbital plane. Our results demonstrate that the necessary conditions exist for misalignment-driven mechanisms to modify planetary orbits, and that these conditions are present at the time of planet formation, apparently because of the binary formation process. PMID:25079553

Jensen, Eric L N; Akeson, Rachel

2014-07-31

98

Do accretion discs regulate the rotation of young stars?  

E-print Network

We present a photometric study of I-band variability in the young cluster IC 348. The main purpose of the study was to identify periodic stars. In all we find 50 periodic stars, of which 32 were previously unknown. For the first time in IC 348, we discover periods in significant numbers of lower-mass stars (M 0.25 Msun) showing a bi-modal period distribution concentrated around periods of 2 and 8 days, and the lower-mass stars showing a uni-modal distribution, heavily biassed towards fast rotators. Closer inspection of the period distribution shows that the higher mass stars show a significant dearth of fast rotators, compared to the Orion Nebula Cluster, whilst the low mass stars are rotating significantly faster than those in Orion. We find no correlation between rotation period and K-L colour or H-alpha equivalent width. We also present a discussion of our own IC 348 data in the context of previously published period distributions for the Orion Nebula Cluster, the Orion Flanking Fields and NGC 2264. We find that the previously claimed correlation between infrared excess and rotation period in the ONC might in fact result from a correlation between infrared excess and mass. We also find a marked difference in period distributions between NGC 2264 and IC 348, which presents a serious challenge to the disc locking paradigm, given the similarity in ages and disc fractions between the two clusters.

S. P. Littlefair; Tim Naylor; Ben Burningham; R. D. Jeffries

2005-01-25

99

Is the Galactic center populated with young star clusters?  

E-print Network

We study the evolution and observability of young and compact star clusters near the Galactic center, such as the Arches cluster and the Quintuplet. The star clusters are modeled with a combination of techniques; using direct N-body, integration to calculate the motions of all stars and detailed stellar and binary evolution to follow the evolution of the stars. The modeled star clusters dissolve within 10 to 60 million years in the tidal field of the Galaxy. The projected stellar density in the modeled clusters drops within 5% to 70% of the lifetime to a level comparable to the projected background density towards the Galactic center. And it will be very hard to distinguished these clusters at later age among the background stars. This effect is more severe for clusters at larger distance from the Galactic center but in projection at the same distance. Based on these arguments we conclude that the Galactic center easily hides 10 to 40 clusters with characteristics similar to the Arches and the Quintuplet cluster.

Simon F. Portegies Zwart

2000-06-29

100

The occurrence and properties of disks around young stars  

NASA Technical Reports Server (NTRS)

The paper discusses the occurrence and properties of disks around young stars, emphasizing in particular how these may relate to planet formation and the evolution of the solar system. The global properties of such disks often resemble those attributed to the primitive solar nebula, suggesting that conditions appropriate for planet formation commonly accompany the birth of low-mass stars. Disk masses, between 0.001 and 1 solar mass, are generally lower than those of the stars, and may represent only a fraction (less than about 10 percent) of the total system mass. From the paucity of near-IR radiation from some disks it is inferred that the inner regions there are gaps where the opacity from small particles becomes vanishingly small. Evidence is presented to the effect that gaps in the inner disks develop preferentially in the oldest objects, suggesting that, with time, matter is lost or accumulates into large particles such as planetesimals, which cannot yet be detected.

Beckwith, Steven V. W.; Sargent, Anneila I.

1993-01-01

101

The influence of grain growth in circumstellar dust envelopes on observed colors and polarization of some eruptive stars  

NASA Technical Reports Server (NTRS)

R CrB stars are classical examples of stars where dust envelope formation takes place. Dust envelope formation was detected around the Kuwano-Honda object (PU Vul) in 1980 to 1981 when the star's brightness fell to 8(sup m). Such envelopes are also formed at nova outbursts. The process of dust envelope formation leads to appreciable variations in optical characteristics, which are seen in specific color and polarization variations in the course of light fading and the appearance of IR radiation. It is shown that the model of a circumstellar dust envelope with aligned particles of changing size can be successfully applied to explain most phenomena observed at the time of light minima for a number of eruptive stars. The polarization may arise in a nonspherical dust envelope or be produced by alignment of nonspherical particles.

Efimov, Yu. S.

1989-01-01

102

MAD Science Demonstration Proposal Young massive star clusters in the Carina Nebula  

E-print Network

MAD Science Demonstration Proposal Young massive star clusters in the Carina Nebula Investigators a population of the most massive stars known in the Galaxy. It contains several open clusters that include, shows no evolved star but several O-type stars notably the O2If* star HD93129Aa. Both clusters have

Liske, Jochen

103

Shock-heating of stellar envelopes: a possible common mechanism at the origin of explosions and eruptions in massive stars  

NASA Astrophysics Data System (ADS)

Observations of transient phenomena in the Universe reveal a spectrum of mass-ejection properties associated with massive stars, covering from Type II/Ib/Ic core-collapse supernovae (SNe) to giant eruptions of luminous blue variables (LBV) and optical transients. In this work, we hypothesize that a large fraction of these phenomena may have an explosive origin, the distinguishing ingredient being the ratio of the prompt energy release Edep to the envelope binding energy Ebinding. Using one-dimensional one-group radiation hydrodynamics and a set of 10 -25 Msolar massive-star models, we explore the dynamical response of a stellar envelope subject to a strong, sudden and deeply rooted energy release. Following energy deposition, a shock systematically forms, crosses the progenitor envelope on a time-scale of a day and breaks out with a signal of a duration of hours to days and a 105 -1011 Lsolar luminosity. We identify three different regimes, corresponding to a transition from dynamic to quasi-static diffusion transport. For Edep > Ebinding, full envelope ejection results with an SN-like bolometric luminosity and kinetic energy, modulations being commensurate to the energy deposited and echoing the diversity of Type II-Plateau SNe. For Edep ~ Ebinding, partial envelope ejection results with a small expansion speed and a more modest but year-long luminosity plateau, reminiscent of LBV eruptions or so-called SN impostors. For Edep < Ebinding, we obtain a `puffed-up' star, secularly relaxing back to thermal equilibrium. In parallel with gravitational collapse and Type II SNe, we argue that thermonuclear combustion, for example of as little as a few 0.01 Msolar of C/O, could power a wide range of explosions/eruptions. Besides massive stars close to the Eddington limit and/or critical rotation, 8 -12 Msolar red supergiants, which are amongst the least bound of all stars, represent attractive candidates for transient phenomena.

Dessart, Luc; Livne, Eli; Waldman, Roni

2010-07-01

104

Stellar Content and Star Formation in Young Clusters Influenced by Massive Stars  

NASA Astrophysics Data System (ADS)

Star Formation (SF) in extreme environment is always challenging and can be significantly different from that in quiet environments. This study presents the comprehensive multi-wavelength (optical, NIR, MIR and radio) observational analysis of three Galactic starforming regions associated with H II regions/young clusters and located at > 2 kpc, which are found to be evolving under the influence of massive stars within their vicinity. The candidate massive stars, young stellar objects, their mass, age, age spread, the form of K-band Luminosity Function (KLF), Initial Mass Function (IMF) and a possible formation history of each region are studied. The major results on Sh2-252, an extended H II region that appears to be undergoing multiple episodes of SF, are highlighted. Our analysis shows that all the regions are undergoing complex SF activity and the new generation of stars in each region seem to be an outcome of the influence by the presence of massive stars within them. SF process in these regions are likely to be multi-fold and the results suggest that multiple modes of triggering mechanism and hierarchial modes of SF are a common phenomena within young clusters.

Jose, J.

2014-09-01

105

Abundance Pattern of Metal-Rich Stars from 14 Old and 24 Young Stars  

NASA Astrophysics Data System (ADS)

The metallicities and abundance ratios, [X/Fe], of 12 elements—C, Na, Mg, Si, S, Ca, Ti, Cr, Ni, Y, Zr, and Ce—are determined for 14 old and 24 young metal-rich stars based on high resolution, high S/N spectra obtained with the HIDES spectrograph attached to the 1.88 m telescope of Okayama Astrophysical Observatory (Okayama, Japan). The results show that there is no any significant difference in the [X/Fe] versus [Fe/H] trend for these elements between the two groups of stars. This is consistent with the kinematics of the two groups of stars being similar to that of the thin disk. In connection with the abundance analysis, we found that iron abundances from Fe II lines are generally higher than those from Fe I lines for cool metal-rich stars. In particular, the [S/Fe] ratio strongly depends on stellar temperature. In view of this, the chemical evolution of the element sulfur can only be traced by selecting solar-type stars within a critically narrow temperature range. Inspecting a large sample of metal-rich stars, we found that young metal-rich stars generally have kinematics similar to that of the local thin disk, while old metal-rich stars show two different kinematic distributions, one with V LSR ~ -10kms-1 and the other with V LSR ~ -50kms-1. Abundances of this new population of old metal-rich stars with a slight lag in the Galactic rotation have already been investigated by the present authors. We compared the abundance patterns for the three groups of metal-rich stars, and did not find any significant difference in abundance ratios for the elements investigated. These results indicate an inhomogeneous metallicity enhancement but similar nucleosynthesis history for the Galactic evolution of the thin disk from the beginning to the present.

Chen, Y. Q.; Zhao, G.; Izumiura, H.; Zhao, J. K.; Liu, Y. J.; Honda, S.; Ohkubo, M.

2008-02-01

106

Discovery of solar system-size halos around young stars  

NASA Technical Reports Server (NTRS)

Near-infrared speckle interferometric observations of five pre-main-sequence stars reveal a core-halo structure around two of these stars: HL Tau and R Mon. The halo light distribution is shown to arise from scattered light from small circumstellar particles. Halo sizes of 320 x 200 AU (alpha x delta FWHM) and 1300 x 1300 AU are deduced for HL Tau and R Mon, respectively, and the halo light is substantially bluer than the stellar light. The minimum mass of small particles in the scattering regions is comparable to the earth's mass in HL Tau and ten times greater in R Mon. Mass loss from the stars is almost certainly insufficient to produce the halo matter. The halos probably consist of relatively slowly moving matter bound gravitationally to the stars. From the size and mass of the circumstellar matter, it appears likely that these halos are in the early stage in the formation of planet-forming disks around the young stars.

Beckwith, S.; Skrutskie, M. F.; Zuckerman, B.; Dyck, H. M.

1984-01-01

107

HUNTING FOR YOUNG DISPERSING STAR CLUSTERS IN IC 2574  

SciTech Connect

Dissolving stellar groups are very difficult to detect using traditional surface photometry techniques. We have developed a method to find and characterize non-compact stellar systems in galaxies where the young stellar population can be spatially resolved. By carrying out photometry on individual stars, we are able to separate the luminous blue stellar population from the star field background. The locations of these stars are used to identify groups by applying the HOP algorithm, which are then characterized using color-magnitude and stellar density radial profiles to estimate age, size, density, and shape. We test the method on Hubble Space Telescope Advanced Camera for Surveys archival images of IC 2574 and find 75 dispersed stellar groups. Of these, 20 highly dispersed groups are good candidates for dissolving systems. We find few compact systems with evidence of dissolution, potentially indicating that star formation in this galaxy occurs mostly in unbound clusters or groups. These systems indicate that the dispersion rate of groups and clusters in IC 2574 is at most 0.45 pc Myr{sup -1}. The location of the groups found with HOP correlate well with H I contour map features. However, they do not coincide with H I holes, suggesting that those holes were not created by star-forming regions.

Pellerin, Anne [Department of Physics and Astronomy, Texas A and M University, College Station, TX 77843-4242 (United States); Meyer, Martin M. [International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, CRAWLEY WA 6009 (Australia); Calzetti, Daniella [Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States); Harris, Jason, E-mail: apellerin@mta.ca, E-mail: martin.meyer@uwa.edu.au, E-mail: calzetti@astro.umass.edu, E-mail: jharris@30doradus.org [Illumina, Inc., 25861 Industrial Blvd, Hayward, CA 94545 (United States)

2012-12-01

108

Quantitative near-infrared spectral analysis of young OB stars  

NASA Astrophysics Data System (ADS)

We have recently obtained moderate resolution (R˜8,000-12,000) high signal-to-noise H- and K-band spectroscopy of a number of optically visible, well studied OB stars (Hanson et al. 2005) to test the reliability of a pure near-infrared quantitative analysis (Repolust et al. 2005). The analysis of 25 of these OB stars via near-infrared spectra alone using the NLTE line-blanketed model atmosphere code FASTWIND (Puls et al. 2005) has proved successful at constraining stellar and wind parameters, consistent with results from previous optical analyses of these stars. This opens the door to quantitative analysis of OB stars at an extraordinarily young age, while they are still heavily enshrouded in their birth cocoons. Because the analysis requires good quality spectra at both H and K band, present 8-m class telescopes limit us to sources which are not extremely embedded (A_V ). As a first example, we present a preliminary analysis of the heavily reddened (A_V = 25), early-O star ionizing the UCHII region, G29.96-0.02. Challenges facing such an analysis include contributions from excess thermal emission from circumstellar material (disks, etc.) which weaken or even eliminate photospheric lines used in the analysis, nebular contamination in several of the principle H and He lines and crowding or general confusion in these very young and typically complex regions. Spectrographs coupled with state of the art adaptive optics will be extremely useful in minimizing these challenges, and may allow even fairly complex regions to be directly studied.

Hanson, M. M.; Puls, J.; Repolust, T.

109

ISOPHOT Observations of the Circumstellar Environment of Young Stars  

NASA Astrophysics Data System (ADS)

The young pre-main sequence T Tau and Herbig Ae/Be stars are associated with circumstellar dust, whose thermal emission can be observed at infrared wavelengths. We report on 3.6-200 micron photometric observations performed with ISOPHOT, the photometer on-board the Infrared Space Observatory. Seven Herbig Ae/Be stars were observed at mid- and far-infrared wavelengths. At ??25?m the emission mainly arises from a compact circumstellar region, and the observed spectral energy distributions follow the power-law relationship F???-n with n typically around 1. The peak of the SEDs (in F?) is at 60-100?m, corresponding to temperatures of around 50 K. At longer wavelengths the observed emission is spatially extended, and at ?>100?m the emission observed by ISOPHOT is never dominated by the Herbig Ae/Be stars. The most likely sources of the far-infrared radiation are arcminute size dust cores located in the vicinity of the stars, and probably related to the star forming process. We also analysed ISOPHOT observations of 16 binary T Tau stars. For the 7 brightest objects the broad-band photometry was supplemented by 2.5-11.7?m spectrophotometry obtained by the PHT-S subinstrument. In most cases strong silicate emission at 10?m was detected with some indications for the presence of crystalline silicate. The comparison of the derived SEDs with those of single T Tau stars will help to understand how the presence of companions could affect the evolution of the circumstellar disks.

Ábrahám, P.; Leinert, Ch.; Lemke, D.; Burkert, A.; Henning, T. H.

110

Observational Manifestations of Young Neutron Stars: Spin-powered pulsars  

NASA Astrophysics Data System (ADS)

The largest number of known young neutron stars are observed as spin-powered pulsars. While the majority of those are detected at radio frequencies, an increasing number can be studied in other parts of the electromagnetic spectrum as well. The Crab pulsar is the prototype of a young pulsar which can be observed from radio to ?-ray frequencies, providing a red thread of discussion during a tour through the pulsar properties observed across the electromagnetic spectrum. The basic observational features of pulsar emission are presented, preparing the ground for more detailed reviews given in these proceedings. Here, particular attention will be paid to those emission features which may provide a link between the radio and high-energy emission processes.

Kramer, M.

111

Star Cluster Ecology: VII The evolution of young dense star clusters containing primordial binaries  

E-print Network

We study the first 100Myr of the evolution of isolated star clusters initially containing 144179 stars, including 13107 (10%) primordial hard binaries. Our calculations include the effects of both stellar and binary evolution. Gravitational interactions among the stars are computed by direct N-body integration using high precision GRAPE-6 hardware. The evolution of the core radii and central concentrations of our simulated clusters are compared with the observed sample of young (about 100Myr) star clusters in the large Magellanic cloud. Even though our simulations start with a rich population of primordial binaries, core collapse during the early phase of the cluster evolution is not prevented. Throughout the simulations, the fraction of binaries remains roughly constant (about 10%). Due to the effects of mass segregation the mass function of intermediate-mass main-sequence stars becomes as flat as $\\alpha=-1.8$ in the central part of the cluster (where the initial Salpeter mass function had $\\alpha=-2.35$). About 6--12% of the neutron stars were retained in our simulations; the fraction of retained black holes is 40--70%. In each simulation about three neutron stars become members of close binaries with a main-sequence companion. Such a binary will eventually become an x-ray binary, when the main-sequence star starts to fill its Roche lobe. Black holes are found more frequently in binaries; in each simulated cluster we find about 11 potential x-ray binaries containing a black hole. Abstract abbreviated....

Simon Portegies Zwart; Steve McMillan; Jun Makino

2006-09-27

112

The variable stars of the young LMC cluster NGC 2164  

NASA Technical Reports Server (NTRS)

The present search of the LMC cluster NGC 2164 for variable stars has uncovered one new member classical Cepheid variable with 3.772-day period; attention is also given to photometry for a previously unknown field overtone Cepheid variable with 3.4626-day period, and the 10.6878-day period HV 12078, which may be a member of the young NGC 2156 cluster. The clear separation of fundamental and overtone pulsators in the period-luminosity-color relation of known LMC cluster Cepheids establishes that the NGC 2164 member is a true overtone.

Welch, Douglas L.; Mateo, Mario; Olszewski, Edward W.; Fischer, Philippe; Takamiya, Marianne

1993-01-01

113

Gamma-ray emission from young neutron stars  

NASA Technical Reports Server (NTRS)

The emission models of Cheng et al. (1986) and Harding (1981) are employed to determine likely candidates for pulsed gamma-ray emission on the basis of recent radio data of pulsars. The recent detection of pulsed gamma rays from PSR 1951+32 lends observational support to the scenario of Cheng et al. which also suggests that PSR 1855+09 might be another excellent gamma-ray pulsar candidate. The possible contribution of young neutron stars to the diffuse high energy glow is also discussed.

Hartmann, Dieter H.; Liang, Edison P.; Cordes, J. M.

1991-01-01

114

A Comprehensive Story Of Young Star Cluster NGC 3603  

NASA Astrophysics Data System (ADS)

We study the young massive (104 solar mass) star cluster NGC 3603 in the Galaxy with HST deep photometry. The membership of stars is determined by relative proper motions based on two epoch WFPC2 data. The radially varying mass function of the members shows significant mass segregation ranging from slopes of Gamma=-0.26±0.32 in the inner 5'' to Gamma=-1.49±0.33 in the outermost annulus (15''--20''). The mass segregation is confirmed using the minimum spanning tree technique, which reveals mass segregation down to 5 solar masses. Both analyses show that the mass segregation in NGC 3603 is dependent on stellar mass, such that more massive stars tend to be more mass-segregated than their lower-mass counterparts. Despite the young age (1 Myr) for the main-sequence (MS) stars, we find a possible age spread of up to 3 Myr for pre-main-sequence (PMS) stars in the core and an even larger age spread up to 30 Myr among the PMS stars in the outskirts of the cluster. Since the outer part of NGC 3603 is dominated by molecular clouds, we computed the pixel-to-pixel distribution of the color excess, E(B - V)g, of the gas associated with this cluster from its Halpha/Paschen? flux ratio using WFC3 images. At the assumed distance of 6.9 kpc, the resulting median color excess within 1 pc from the cluster center is E(B - V)g=1.51 ± 0.04 mag. Outside the cluster (at r > 1 pc), the color excess is seen to increase with cluster-centric distance toward both north and south, reaching a value of about 2.2 mag at r=2 pc from the cluster center. We are currently establishing a conversion from gas to stellar reddening, aiming at correcting individual MS and PMS stars for reddening in the WFC3 field of view and achieve an accurate age determination of the cluster.

Pang, Xiaoying; Grebel, E. K.; Pasquali, A.; Altmann, M.; Allison, R.

2012-01-01

115

10 $?$m interferometry of disks around young stars  

E-print Network

This contribution reviews results from interferometric observations of circumstellar disks around young stars of $\\lesssim$3 M$_{\\odot}$, performed with the MIDI instrument operating in the 10 $\\mu$m spectral region. Two main topics, the disk structure on $\\sim$1-10 AU scales and the dust properties in the same region, are illustrated with several examples of MIDI studies, covering various evolutionary stages. The spatially resolved observations largely confirm SED-only based hypotheses on disk structure, yet also reveal degeneracies that may occur in such SED modeling. The properties of the dust on the disk surface show a strong radial dependence: the dust close to the central star has generally larger grain sizes and in particular a much higher crystallinity than the dust in more remote disk regions.

Roy van Boekel

2008-10-30

116

Global Alfven Wave Heating of the Magnetosphere of Young Stars  

E-print Network

Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to explain plasma heating in the magnetosphere of young stars. The wave and basic plasma parameters are compatible with the requirement that the dissipation length of GAWs be comparable to the distance between the shocked region at the star's surface and the truncation region in the accretion disk. A two-fluid magnetohydrodynamic plasma model is used in the analysis. A current carrying filament along magnetic field lines acts as a waveguide for the GAW. The current in the filament is driven by plasma waves along the magnetic field lines and/or by plasma crossing magnetic field lines in the truncated region of the disk of the accreting plasma. The conversion of a small fraction of the kinetic energy into GAW energy is sufficient to heat the plasma filament to observed temperatures.

A. G. Elfimov; R. M. O. Galvao; V. Jatenco-Pereira; R. Opher

2002-10-30

117

Evolution of Young Stars. Physics and Modeling of Accretion  

NASA Astrophysics Data System (ADS)

The stellar structure and evolution of low- and intermediate-mass stars is investigated in a context where the star is gaining mass from a circumstellar accretion disk. The first part of this work, dedicated to standard evolution, presents new evolutionary models for pre-main sequence stars. These new computations are used to generate synthetic HR diagrams. We show that such a tool is well suited to reproduce the morphology of young clusters and allow to derive some physical parameters prevailing in these clusters such as the binarity rate or the duration of the star forming process. The second part describes the stellar evolutionary code and the accretion model which determines how the accreted matter distributes inside the star. The formalism of the model is based on the Richardson criterion and takes into account the mechanical, thermal and chemical properties of the accreted matter. Numerical results indicate that accreted matter easily crosses the radiative atmosphere and that convection favors a rather uniform distribution of the accreted matter in the stellar interior. The last part is devoted to the influence of the accretion process on stellar structure and evolution. Computations show that deuterium nuclear burning plays a crucial role during the pre-main sequence phase. For a specified range of accretion rates, we observe a expansion of the star along its Hayashi line. For higher accretion rates, characteristic of the star forming phase, we reproduce the birthline which represents an upper envelope of the pre-main sequence stellar distribution. Finally, we compare the structural and evolutionary differences between accreting and non-accreting tracks. We show that the arrival of accreted matter accelerates the star contraction on the Hayashi branch and, from its position in the HR diagram, an accreting star appears younger than in a standard scheme. We also analyze the influence of accretion on surface stellar abundances of light elements such as deuterium and lithium. We determine the areas in the HR diagram where accretion significantly increases the abundances of these elements. Finally, we illustrate the opportunities this accretion model opens to the study of the stellar rotational evolution.

Siess, Lionel

1996-12-01

118

Molecular tracers of photo-evaporating disks around young stars  

NASA Astrophysics Data System (ADS)

Disks around massive young stellar objects, and disks around low-mass stars irradiated by nearby OB associations, are eroded by photo-evaporation. In the latter case, this erosion may be an important factor in planet formation. As Johnstone et al. (\\cite{johnstone98a}) have shown, photo-evaporating material is gravitationally retained within a critical radius from the star, and constitutes an envelope similar to a Photon-Dominated Region (PDR) that normally arises at the edge of a molecular cloud irradiated by a massive star. We explore the chemistry of such a PDR/disk system to examine the contribution that it may make to the molecular species that may be observed. The model is in two phases; firstly, a collapse from low density to a high density appropriate for a disk; and, secondly, a 2D calculation of the irradiation of disk material by the radiation field of the central massive star or nearby OB association. The model follows the chemistry self-consistently through both phases. We compute the column densities of species through the PDR/disk system, averaged over the disk. We validate our model by comparing predicted averaged molecular column densities with those of several species detected in the disk around the 10 solar mass star GL 2591, currently the sole example known of this kind of object. Results are in good agreement for a model in which the outer part of the PDR is hot while the inner part is cool, and in which the local ionization rate is comparable with that caused by cosmic rays in the local interstellar medium. We show that in addition to the four detected species, there should be many others also detectable in this system, including HCN, NH3 and CS. Similar conclusions should apply to other disks around massive stars. Disks around low-mass stars are much more common; our models show that when irradiated by a nearby OB association such disks with their attendant PDRs also generate a rich chemistry. No detections of molecules in such objects have yet been reported. However, the models suggest that averaged molecular column densities should be comparable to those detected in disks around massive stars (see references listed in Table \\ref{restab} for molecules in GL 2591). Potential tracers of irradiated disks around low-mass stars include OH, CH3 and C2H. We note that the detection in a disk of PDR-type chemistry is a clear signature that the disk is undergoing erosion. Its duration is therefore limited, with consequences for planet formation.

Nguyen, T. K.; Viti, S.; Williams, D. A.

2002-06-01

119

Young Star May Be Belching Spheres of Gas, Astronomers Say  

NASA Astrophysics Data System (ADS)

A young star more than 2,000 light-years away in the constellation Cepheus may be belching out spheres of gas, say astronomers who observed it with the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope. Not only is the star ejecting spheres of gas, the researchers say, but it also may be ejecting them repeatedly, phenomena not predicted by current theories of how young stars shed matter. Cepheus A star-forming region with blowups of detail In order to remain stable while accumulating matter, young stars have to throw off some of the infalling material to avoid "spinning up" so fast they would break apart, according to current theories. Infalling matter forms a thin spinning disk around the core of the new star, and material is ejected in twin "jets" perpendicular to the plane of the disk. "Twin jets have been seen emerging from many young stars, so we are quite surprised to see evidence that this object may be ejecting not jets, but spheres of gas," said Paul T.P. Ho, an astronomer at the Harvard-Smithsonian Center for Astrophysics. The research is reported in the May 17 edition of the scientific journal Nature. The astronomers observed a complex star-forming region in Cepheus and found an arc of water molecules that act like giant celestial amplifiers to boost the strength of radio signals at a frequency of 22 GHz. Such radio-wave amplifiers, called masers, show up as bright spots readily observed with radio telescopes. "With the great ability of the VLBA to show fine detail, we could track the motions of these maser spots over a period of weeks, and saw that this arc of water molecules is expanding at nearly 20,000 miles per hour," said Ho. "This was possible because we could detect detail equivalent to seeing Lincoln's nose on a penny in Los Angeles from the distance of New York," Ho added. "These observations pushed the tremendous capabilities of the VLBA and of modern computing power to their limits. This is an extremely complex observational project," said Luis F. Rodriguez, of Mexico's National Autonomous University. The arc of water masers can be fit to a nearly-perfect circle to within one part in a thousand. That, the researchers say, means that the water vapor in the arc most likely is part of a complete sphere. "The arc we see fits a circle so well that it is unlikely that any geometry other than that of a sphere would produce it," Ho said. The sphere would be about 1.5 times the size of the Solar System. Because the arc, and presumably the sphere of which it is part, is so thin and so uniform, the researchers say that it came from a single, short-lived ejection. In addition, other evidence suggests that the sphere from an earlier ejection now is being overtaken by a newer spherical bubble that took only about 33 years after being ejected to reach its observed size. "We now have at least one case, we believe, in which a young star has repeatedly ejected mass spherically in short bursts," Guillem Anglada, of the Institute of Astrophysics of Andalucia (CSIC), in Granada, Spain, said. "In light of our current understanding of star formation, we don't yet understand how this can happen, so we have an exciting new scientific challenge. It is surprising that nature can maintain such perfect symmetry, especially since the environment around the young star must be so varied. This appears to be a triumph of order over chaos," he added. The researchers, in addition to Rodriguez, Ho and Anglada, are: Jose M. Torrelles, Institute for Space Studies of Catalonia (IEEC)-Spanish Research Council (CSIC), Spain; Nimesh A. Patel and Lincoln Greenhill, of the Harvard-Smithsonian Center for Astrophysics; Jose F. Gomez, Laboratory for Space Astrophysics and Theoretical Physics of the National Institute for Aerospace Technology, Madrid, Spain; Salvador Curiel and Jorge Canto, of Mexico's National Autonomous University; and Guido Garay, Department of Astronomy of the University of Chile. The VLBA is part of the Natio

2001-05-01

120

YOUNG, ULTRAVIOLET-BRIGHT STARS DOMINATE DUST HEATING IN STAR-FORMING GALAXIES  

SciTech Connect

In star-forming galaxies, dust plays a significant role in shaping the ultraviolet (UV) through infrared (IR) spectrum. Dust attenuates the radiation from stars, and re-radiates the energy through equilibrium and non-equilibrium emission. Polycyclic aromatic hydrocarbons (PAHs), graphite, and silicates contribute to different features in the spectral energy distribution; however, they are all highly opaque in the same spectral region-the UV. Compared to old stellar populations, young populations release a higher fraction of their total luminosity in the UV, making them a good source of the energetic UV photons that can power dust emission. However, given their relative abundance, the question of whether young or old stellar populations provide most of these photons that power the IR emission is an interesting question. Using three samples of galaxies observed with the Spitzer Space Telescope and our dusty radiative transfer model, we find that young stellar populations (on the order of 100 million years old) dominate the dust heating in star-forming galaxies, and old stellar populations (13 billion years old) generally contribute less than 20% of the far-IR luminosity.

Law, Ka-Hei [Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 (United States); Gordon, Karl D. [STScI, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Misselt, K. A., E-mail: klaw@pha.jhu.edu, E-mail: kgordon@stsci.edu, E-mail: misselt@as.arizona.edu [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)

2011-09-10

121

MYStIX: Subclusters of Young Stars in Massive Star Forming Regions  

NASA Astrophysics Data System (ADS)

The MYStIX (Massive Young Star-Forming Complex Study in Infrared and X-ray; Feigelson et al. 2013) project provides improved censuses of young stars in 20 nearby OB-dominated star-forming regions that were observed by the Chandra X-ray observatory, the Spitzer Space Telescope, and the UKIRT/UKIDSS and 2MASS surveys. The sample of >33,000 members reveals new details about the structure of clusters in these regions. Clusters of young stars are identified using finite mixture models -\\ the sums of isothermal ellipsoids used to model individual (sub)clusters. Maximum likelihood estimation is used to estimate the model parameters and the Akaike Information Criterion is used to detemine the number of subclusters. In the MYStIX star-forming regions, ˜150 subclusters are found (1 to >10 per region). The distribution of cluster core radii is log-normal, peaked at 0.18 pc (similar to the ONC) with a standard deviation of 0.4 dex. The locations of subclusters are often correlated with molecular-cloud clumps or cores. We also recover several well-known embedded subclusters such as the BN-KL region in Orion and the KW Object cluster in M 17. MYStIX star-forming regions typically have one dominant cluster surrounded by smaller subclusters and filamentary groups of young stars. Some clusters are well fit by the ellipsoid model (e.g. Flame Nebula), but others have lumpy structure and are poorly fit (e.g. M 17). A few clusters have a core-halo structure modeled with two overlapping ellipsoids (e.g. RCW 36). Clumpy and core-halo structures could originate in the merger of subclusters. There is a power-law relation between the fitted cluster central density and core radius (index slightly shallower than -3), which may be an effect of cluster expansion. There is also a statistically significant negative relation between median gas/dust absorption of a subcluster and the subcluster's size that can also be explained by cluster expansion if absorption acts as a proxy for age.

Kuhn, Michael A.; Feigelson, Eric D.; Getman, Konstantin V.; Baddeley, Adrian; Townsley, Leisa K.; Broos, Patrick S.; Povich, Matthew S.; Luhman, Kevin L.; Busk, Heather A.; Naylor, Tim; King, Robert R.; Garmire, Gordon P.

2013-07-01

122

Gamma-ray bursts from young neutron stars  

NASA Astrophysics Data System (ADS)

Data from the Mar. 5, 1979 gamma ray burst (M5) from the supernova remnant N49 are used to examine the nature of gamma ray burst sources. The association with N49 was statistically linked at the 0.04% level, thus ruling out a superposition phenomenon. The burst was calculated to be emitted from a source with a 10 to the 44th erg/sec peak luminosity, and displayed an 8.1 sec periodicity, which is taken as the free precession period of a rotating neutron star. The rotation interval is suggested to be from 10-100 msec. A connection between the gamma ray events and pulsar glitches, e.g., an abrupt change in the pulsar period, is explored. A 0.001 erg/sq cm pulse is predicted to be emitted from such an event, much smaller than the M5 levels. The possibility that superglitches occur as a result of transitions between interior equilibriums in neutron stars is discussed. Finally, a connection between the gamma ray bursts and young neutron stars is proposed, with bursts being the result of a postformation glitch, phase transition, or other stellar activity. Observational techniques for validating the model are outlined.

Brecher, K.

1982-01-01

123

FAST STAR, SLOW STAR; OLD STAR, YOUNG STAR: SUBGIANT ROTATION AS A POPULATION AND STELLAR PHYSICS DIAGNOSTIC  

SciTech Connect

Stellar rotation is a strong function of both mass and evolutionary state. Missions such as Kepler and CoRoT provide tens of thousands of rotation periods, drawn from stellar populations that contain objects at a range of masses, ages, and evolutionary states. Given a set of reasonable starting conditions and a prescription for angular momentum loss, we address the expected range of rotation periods for cool field stellar populations (?0.4-2.0 M{sub ?}). We find that cool stars fall into three distinct regimes in rotation. Rapid rotators with surface periods less than 10 days are either young low-mass main sequence (MS) stars, or higher mass subgiants which leave the MS with high rotation rates. Intermediate rotators (10-40 days) can be either cool MS dwarfs, suitable for gyrochronology, or crossing subgiants at a range of masses. Gyrochronology relations must therefore be applied cautiously, since there is an abundant population of subgiant contaminants. The slowest rotators, at periods greater than 40 days, are lower mass subgiants undergoing envelope expansion. We identify additional diagnostic uses of rotation periods. There exists a period-age relation for subgiants distinct from the MS period-age relations. There is also a period-radius relation that can be used as a constraint on the stellar radius, particularly in the interesting case of planet host stars. The high-mass/low-mass break in the rotation distribution on the MS persists onto the subgiant branch, and has potential as a diagnostic of stellar mass. Finally, this set of theoretical predictions can be compared to extensive datasets to motivate improved modeling.

Van Saders, Jennifer L.; Pinsonneault, Marc H., E-mail: vansaders@astronomy.ohio-state.edu [Department of Astronomy, The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210 (United States)

2013-10-20

124

YoungStar in Milwaukee County: An Initial Progress Report as of July 2011  

ERIC Educational Resources Information Center

YoungStar is a program of the Department of Children and Families (DCF) created to improve the quality of child care for Wisconsin children. YoungStar is designed to: (1) Evaluate and rate the quality of care given by child care providers; (2) Help parents choose the best child care for their kids; (3) Support providers with tools and training to…

Edie, Dave

2011-01-01

125

YoungStar in Wisconsin: An Initial Progress Report as of July 2011  

ERIC Educational Resources Information Center

YoungStar is a program of the Department of Children and Families (DCF) created to improve the quality of child care for Wisconsin children. YoungStar is designed to: (1) Evaluate and rate the quality of care given by child care providers; (2) Help parents choose the best child care for their kids; (3) Support providers with tools and training to…

Edie, Dave

2011-01-01

126

Astrophysics of Young Star Binaries in the Taurus Star Forming Region  

NASA Astrophysics Data System (ADS)

This paper describes our studies of the individual components of young star binaries. Most stars are found in multiple systems; thus binaries are important to characterize and understand, both as a common mode for star formation and for their suitability for planet formation. We observed each component in 16 systems, located in the nearby Taurus-Auriga star forming region, using low-resolution (R=760) infrared spectroscopy and photometry. We detected photospheric absorption lines and were able to determine the spectral type, extinction, K-band excess, and luminosity for each component. We estimated the masses and ages by locating each star on the HR Diagram and comparing their positions with theoretical models. The extinction was greater for the secondary components in ~80% of the systems. For two thirds of the systems, the K-band excess of the primary components dominates that of their counterparts. A majority of the systems exhibit Brackett gamma emission at 2.16 microns, which implies that at least one star in these pairs is surrounded by an actively accreting circumstellar disk.

Johns, Paula; Prato, L. A.; Greene, T. P.

2013-01-01

127

Where to Find Young Bright Stars in Geosciences: GGD, NSU  

NASA Astrophysics Data System (ADS)

Geology and Geophysics Department (GGD) of Novosibirsk State University (NSU) can be regarded as infant, because it was founded in 1962. On the other hand, if to judge by what have been done - it is not only full-fledged, but well-known department. The unique location and specific educational and scientific traditions make GGD a famous school not only in Siberia, but in Russia, and all over the world. What are the tips to prepare bright stars in geosciences? 1.NSU is located in Academgorodok (Novosibirsk scientific center), unique place in Siberia, where more than 20 scientific institutions are located. This makes the University different from other schools in Russia. Famous Russian scientists, including members of RAS, together with foreign professors give lectures and seminars for NSU students. 2.The bright star hunting starts far below the NSU level. Each year in April there is a special event in Academgorodok -`Geologic Olympiad', where children of all Russian regions, as well as ex-Soviet republics are gathered together to submit their papers, to discuss most interesting geoscience problems and to win prizes for their knowledge. The youngest stars happen to be only 6-7 years old. The event is sponsored by NSU, UIGGM, and the Ministry of Natural Resources. The brightest geostars are grown from `Geologic Olympiad' participants. 3.There is special physics-mathematical high school in Academgorodok. Each summer this school gathers young stars from farthest Siberian and Far East regions and gives classes and seminars in mathematics, physics, chemistry and geology. As the result the most talented children become the students of this school (for two years). The school in turn supplies GGD with the students. 4.NSU has the study curriculum different from other universities in Russia. That is why the entrance examinations are much more difficult as compared to other schools and are taken in July (a month earlier then at other universities). However the entrance examinations are based on free competition and education at GGD is free. For example, to become a student of oil and gas geochemistry a young star should win a competition between nine young persons. 5.GGD scientific research program starts from course paper (second year of study), the next steps being Bachelor's and Master's dissertations and postgraduate course. The scientific advisors are most famous scientists from Academgorodok. Moreover, the GGD students have a possibility to take unique exclusive electives of most modern fields of science. 6.GGD is equipped by a good computer class and SGG workstation. Most computers were granted by Schlumberger, as a sign that best graduates in geosciences in Russia are from GGD NSU. So the students have free Internet access as well as they can use online web educational resources of GGD. The educational system of GGD does not use a conception `to teach something', but the conception `to teach how to learn'. At GGD a tutor has 5-6 students. For some electives and specialties there is one student - one tutor system. GGD students are able to have field practice in all Siberian and Far East regions, huge territory with unique geology. The NSU educational system is flexible enough, so that the graduates are able to adapt to any interdisciplinary science and can successfully work in other fields. The graduators work not only in oil companies and scientific institutions in Russia, but in such companies as Schlumberger, Halliburton, Shell, Total, De Beers, and others. The brightest GGD stars are even head-hunted. The NSU slogan is `WE WILL NOT MAKE YOU SMARTER, WE WILL TEACH YOU HOW TO THINK!'

Rakhmenkoulova, I. F.; Sharapov, V. N.

2004-12-01

128

Accretion Disks around Young Stars: Lifetimes, Disk Locking and Variability  

E-print Network

We report the findings of a comprehensive study of disk accretion and related phenomena in four of the nearest young stellar associations spanning 6-30 million years in age, an epoch that may coincide with the late stages of planet formation. We have obtained ~650 multi-epoch high-resolution optical spectra of 100 low-mass stars that are likely members of the eta Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), beta Pictoris (~12 Myr) and Tucanae-Horologium (~30 Myr) groups. Our data were collected over 12 nights between 2004 December - 2005 July on the Magellan Clay 6.5m telescope. Based on H$\\alpha$ line profiles, along with a variety of other emission lines, we find clear evidence of on-going accretion in three out of 11 eta Cha stars and two out of 32 TW Hydrae members. None of the 57 beta Pic or Tuc-Hor members shows measurable signs of accretion. Together, these results imply significant evolution of the disk accretion process within the first several Myr of a low-mass star's life. While a few disks can continue to accrete for up to ~10 Myr, our findings suggest that disks accreting for beyond that timescale are rather rare. This result provides an indirect constraint on the timescale for gas dissipation in inner disks and, in turn, on gas giant planet formation. All accretors in our sample are slow rotators, whereas non-accretors cover a large range in rotational velocities. This may hint at rotational braking by disks at ages up to ~8 Myr. Our multi-epoch spectra confirm that emission-line variability is common even in somewhat older T Tauri stars, among which accretors tend to show particularly strong variations. Thus, our results indicate that accretion and wind activity undergo significant and sustained variations throughout the lifetime of accretion disks.

Ray Jayawardhana; Jaime Coffey; Alexander Scholz; Alexis Brandeker; Marten H. van Kerkwijk

2006-05-23

129

Absolute parameters of young stars: GG Lup and ?1 Sco  

NASA Astrophysics Data System (ADS)

New high-resolution spectroscopy and BVR photometry, together with literature data, on the Gould's Belt close binary systems GG Lup and ?1 Sco are presented and analysed. In the case of GG Lup, light and radial velocity curve fittings confirm a near-main-sequence picture of a pair of close stars. Absolute parameters are found, to within a few per cent, thus: M1 = 4.16 ± 0.12, M2 = 2.64 ± 0.12 (M?); R1 = 2.42 ± 0.05, R2 = 1.79 ± 0.04 (R?); T1 ˜ 13 000, T2 ˜ 10 600 (K); photometric distance ˜160 (pc). The high eccentricity and relatively short period (105 yr) of apsidal revolution may be related to an apparent `slow B-type pulsator' oscillation. Disturbances of the outer envelope of at least one of the components then compromise comparisons to standard evolutionary models, at least regarding the age of the system. A rate of apsidal advance is derived, which allows a check on the mean internal structure constant overline{k_2} = 0.0058 ± 0.0004. This is in agreement with values recently derived for young stars of solar composition and mass ˜3 M?. For ?1 Sco, we agree with previous authors that the secondary component is considerably oversized for its mass, implying binary (interactive) stellar evolution, probably of the `Case A' type. The primary appears relatively little affected by this evolution, however. Its parameters show consistency with a star of its derived mass at age about 13 Myr, consistent with the star's membership of the Sco-Cen OB2 Association. The absolute parameters are as follows: M1 = 8.3 ± 1.0, M2 = 4.6 ± 1.0 (M?); R1 = 3.9 ± 0.3, R2 = 4.6 ± 0.4 (R?); T1 ˜ 24 000, T2 ˜ 17 000 (K); photometric distance ˜135 (pc).

Budding, E.; Butland, R.; Blackford, M.

2015-04-01

130

High resolution spectroscopy of old stars and young disks  

NASA Astrophysics Data System (ADS)

In Chapter 2, we present the details of LinBrod, a program that calculates synthetic spectra of cool Roche-lobe-filling stars in close binary systems. The program has two modes of operation. In the primary mode it calculates the spectra by adding wavelength-dependent, velocity-shifted specific intensities from the distorted and gravity-darkened surface of the lobe-filling star. The wavelength-dependent specific intensities are calculated externally using the ATLAS9 stellar atmosphere program and a modified version of the MOOG spectrum synthesis program. In its secondary mode LinBrod calculates phase-dependent line-broadening functions that can be convolved with the spectra of non- rotating single stars to yield approximate synthetic spectra. We use the line- broadening functions to isolate and assess the effects of the physical processes that broaden the absorption lines in the spectra of lobe-filling stars. The synthetic spectra can be used to extract radial velocities, mass ratios, and chemical abundances from observed spectra of lobe-filling stars. Originally written to analyze observations of the secondary stars in X-ray binaries containing black holes, the program can also be used to analyze observations of the secondary stars in Algol systems, cataclysmic variables, and low-mass X-ray binaries containing neutron stars. In Chapter 3, we discuss the dwarf nova SS Cygni, a close binary star consisting of a K star transferring mass to a white dwarf by way of an accretion disk. We have obtained new spectroscopic observations of SS Cyg. Fits of synthetic spectra for Roche-lobe-filling stars to the absorption-line spectrum of the K star yield the amplitude of the K star's radial velocity curve and the mass ratio: K K = 162.5 ± 1.0 km s -1 and q = M K /M wd = 0.685 ± 0.015. The fits also show that the accretion disk and white dwarf contribute a fraction f = 0.535 ± 0.075 of the total flux at 5500 Å. Taking the weighted average of our results with previously published results obtained using similar techniques, we find [left angle bracket]K K [right angle bracket] = 163.7 ± 0.7 km s -1 and [left angle bracket] q [right angle bracket] = 0.683 ± 0.012. The orbital light curve of SS Cyg shows an ellipsoidal variation diluted by light from the disk and white dwarf. From an analysis of the ellipsoidal variations we limit the orbital inclination to the range 45° <= i <= 56°. The derived masses of the K star and white dwarf are M K = 0.55 ± 0.13 [Special characters omitted.] and M wd = 0.81 ± 0.19[Special characters omitted.] , where the uncertainties are dominated by systematic errors in the orbital inclination. The K star in SS Cyg is 10% to 50% larger than an unevolved star with the same mass and thus does not follow the mass-radius relation for Zero-Age Main- Sequence stars; nor does it follow the ZAMS mass/spectral-type relation. Its mass and spectral type are, however, consistent with models in which the core hydrogen has been significantly depleted. In Chapters 4 and 5, we report the results of a search for pure rotational molecular hydrogen emission from the circumstellar environments of young stellar objects with disks using the Texas Echelon Cross Echelle Spectrograph (TEXES) on the NASA Infrared Telescope Facility and the Gemini North Observatory. We searched for mid-infrared H 2 emission in the S(1), S(2), and S (4) transitions. Keck NIRSPEC observations of the H2 S(9) transition were included for some sources as an additional constraint on the gas temperature. We detected H 2 emission from 6 of 28 sources observed: AB Aur, DoAr 21, Elias 29, GSS 30 IRS 1, GV Tau N, and HL Tau. Four of the six targets with detected emission are class I sources that show evidence for surrounding material in an envelope in addition to a circumstellar disk. The detected emission lines are narrow (~10 km s -1 ), centered at the stellar velocity, and spatially unresolved at scales of 0.4 inches, which is consistent with origin from a disk at radii 10-50 AU from the star. In cases where we detect multiple emission lines, we

Bitner, Martin Allan, Jr.

131

Adaptive Optics Search for Faint Companions Around Young Nearby Stars  

NASA Astrophysics Data System (ADS)

Results of an ongoing coronographic NIR imaging survey using ADONIS at the 3.6m telescope are presented. The sample consists of young (ages ? 100Myr), and simultaneously nearby (distances ? 100pc) field stars. Several new, low-mass companions are found. Their physical association has been proven by repeated observations at different epochs. Although the sensitivity allows to probe companion masses well below the hydrogen burning limit, companions with large mass ratios appear to be rare. This finding is in general agreement with several previous and ongoing brown-dwarf companion searches. I suggest a possible explanation for the scarcity of close brown-dwarf companions in the framework of stellar dynamical few-body interactions in early phases of their formation and evolution.

Sterzik, Michael F.

132

SUPERSONIC LINE BROADENING WITHIN YOUNG AND MASSIVE SUPER STAR CLUSTERS  

SciTech Connect

The origin of supersonic infrared and radio recombination nebular lines often detected in young and massive superstar clusters is discussed. We suggest that these arise from a collection of repressurizing shocks (RSs), acting effectively to re-establish pressure balance within the cluster volume and from the cluster wind which leads to an even broader although much weaker component. The supersonic lines here are shown to occur in clusters that undergo a bimodal hydrodynamic solution, that is within clusters that are above the threshold line in the mechanical luminosity or cluster mass versus the size of the cluster plane. A plethora of RSs is due to frequent and recurrent thermal instabilities that take place within the matter reinserted by stellar winds and supernovae. We show that the maximum speed of the RSs and of the cluster wind are both functions of the temperature reached at the stagnation radius. This temperature depends only on the cluster heating efficiency (eta). Based on our two-dimensional simulations we calculate the line profiles that result from several models and confirm our analytical predictions. From a comparison between the predicted and observed values of the half-width zero intensity of the two line components, we conclude that the thermalization efficiency in young super star clusters above the threshold line must be lower than 20%.

Tenorio-Tagle, Guillermo; Silich, Sergiy [Instituto Nacional de AstrofIsica Optica y Electronica, AP 51, 72000 Puebla (Mexico); Wuensch, Richard [Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA (United Kingdom); Munoz-Tunon, Casiana [Instituto de AstrofIsica de Canarias, E 38200 La Laguna, Tenerife (Spain); Palous, Jan, E-mail: gtt@inaoep.m, E-mail: richard@wunsch.c, E-mail: palous@ig.cas.c, E-mail: cmt@ll.iac.e [Astronomical Institute, Academy of Sciences of the Czech Republic, BocnI II 1401, 141 31 Prague (Czech Republic)

2010-01-10

133

Star Formation Ecology: YSO Outflow Feedback in Young Clusters  

NASA Astrophysics Data System (ADS)

Energetic outflows associated with young stellar objects exert a strong effect on their parent molecular clouds. The dynamics of this interaction is yet to be well understood. In particular the role of jets and outflows in powering cloud turbulence, modifying the star formation efficiency (SFE) and/or disrupting the parent clouds remains unclear. Spitzer images of young clusters have provided new views of jet-cloud interactions that can help resolve these critical issues. In this proposal we seek to continue a highly successful (cycle 2) theory program to explore theoretical issues of jet-cloud interactions, turbulence and cloud disruption. Our research relies on 3-D Adaptive Mesh Refinement hydrodynamic and MHD simulations developed in house, in concert with Spitzer databases and other complementary observations. The team we have assembled includes computational and analytic theorists (Frank, Blackman) as well as observers who have worked closely with existing Spitzer Datasets (Bally, Quillen, Pipher, Gutermuth) The work funded through the previous TR program revealed fundamentally new aspects of YSO outflow feedback on parent cloud cores including the importance of the temporal evolution of outflow power. In this proposal we seek to extend the understanding gained in those studies to address specific questions on the nature and efficacy of outflow feedback in real systems.

Frank, Adam; Bally, John; Blackman, Eric; Gutermuth, Robert; Pipher, Judy; Quillen, Alice

2007-05-01

134

Formation of IR emission in HII regions around young stars  

NASA Astrophysics Data System (ADS)

We investigate the formation of IR emission and corresponding intensity distributions at 8, 24, and 100 micron in HII regions around young massive stars. The evolution of an HII region is simulated using an advanced chemo-dynamical model. Three dust components are included in the model: large silicate grains, very small graphite grains (VSG), and polycyclic aromatic hydrocarbon (PAH) particles. The emergent SED and intensity distributions are calculated using our RT model where stochastic heating of VSG and PAHs is taken into account. The efficiency of two processes for stochastic heating of VSG and PAHs is studied: the absorption of star emission and interaction with hot gas. We compare the synthetic maps with the observed maps from Spitzer and Herschel for the RCW 120 HII region. It is shown that the model with constant PAH abundance cannot reproduce the ring-like appearance of the observed intensity distribution at 8 micron. In order to explain the observed IR distributions we inspect two models of dust evolution. The first model assumes that PAHs are destroyed inside an HII region. In the second model the drift of the dust particles caused by radiation pressure is taken into account. We show that the model with PAH destruction is consistent with observed profiles given appropriate choice of the PAH destruction time. On the contrary, the model with the dust drift is not consistent with observations.

Pavlyuchenkov, Yaroslav; Kirsanova, Maria; Akimkin, Vitaly; Wiebe, Dmitry

2013-07-01

135

Time-monitoring observations of Br? emission from young stars  

NASA Astrophysics Data System (ADS)

We present multiple epochs of near-infrared spectroscopy for a sample of 25 young stars, including T Tauri, Herbig Ae/Be, and FU Ori objects. Using the FSPEC instrument on the Bok 90-inch telescope, we obtained K-band spectra of the Br? transition of hydrogen, with a resolution of ?3500. Epochs were taken over a span of >1 yr, sampling time-spacings of roughly 1 d, 1 month, and 1 yr. The majority of our targets show Br? emission, and in some cases these are the first published detections. Time variability is seen in approximately half of the targets showing Br? emission. We compare the observed variability with expectations for rotationally modulated accretion on to the central stars and time-variable continuum emission or extinction from matter in the inner disc. Our observations are not entirely consistent with models of rotationally modulated magnetospheric accretion. Further monitoring, over a larger number of epochs, will facilitate more quantitative constraints on variability time-scales and amplitudes, and a more conclusive comparison with theoretical models.

Eisner, J. A.; Rieke, G. H.; Rieke, M. J.; Flaherty, K. M.; Stone, J. M.; Arnold, T. J.; Cortes, S. R.; Cox, E.; Hawkins, C.; Cole, A.; Zajac, S.; Rudolph, A. L.

2015-02-01

136

Spectral Energy Distributions of Young Stars in IC 348: The Role of Disks in Angular Momentum Evolution of Young, Low-Mass Stars  

E-print Network

Theoretical work suggests that a young star's angular momentum and rotation rate may be strongly influenced by magnetic interactions with its circumstellar disk. A generic prediction of these 'disk-locking' (DL) theories is that a disk-locked star will be forced to co-rotate with the Keplerian angular velocity of the inner edge of the disk. These theories have also been interpreted to suggest a correlation between young stars' rotation periods and the structural properties of their disks, such that slowly rotating stars possess close-in disks that enforce the star's slow rotation, whereas rapidly rotating stars possess anemic or evacuated inner disks that are unable to brake the stars and they spin up as they contract. To test these expectations, we model the SEDs of 33 young stars in IC 348 with known rotation periods and infrared excesses indicating the presence of disks. For each star, we match the observed spectral energy distribution, typically sampling 0.6-8.0 \\mum, to a grid of 200,000 pre-computed sta...

Blanc, Thompson S Le; Stassun, Keivan G

2011-01-01

137

Spin Evolution of Accreting Young Stars. I. Effect of Magnetic Star-Disk Coupling  

NASA Astrophysics Data System (ADS)

We present a model for the rotational evolution of a young, solar mass star interacting with an accretion disk. The model incorporates a description of the angular momentum transfer between the star and the disk due to a magnetic connection, and includes changes in the star's mass and radius and a decreasing accretion rate. The model also includes, for the first time in a spin evolution model, the opening of the stellar magnetic field lines, as expected to arise from twisting via star-disk differential rotation. In order to isolate the effect that this has on the star-disk interaction torques, we neglect the influence of torques that may arise from open field regions connected to the star or disk. For a range of magnetic field strengths, accretion rates, and initial spin rates, we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3 Myr. How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk. For the relatively strong coupling (i.e., high magnetic Reynolds number) expected in real systems, all models predict spin periods of less than ~3 days, in the age range of 1-3 Myr. Furthermore, these systems typically do not reach an equilibrium spin rate within 3 Myr, so that the spin at any given time depends upon the choice of initial spin rate. This corroborates earlier suggestions that, in order to explain the full range of observed rotation periods of approximately 1-10 days, additional processes, such as the angular momentum loss from powerful stellar winds, are necessary.

Matt, Sean P.; Pinzón, Giovanni; de la Reza, Ramiro; Greene, Thomas P.

2010-05-01

138

Pulsating B-type stars in the young open cluster h Persei (NGC 869)  

E-print Network

We announce the discovery of six Beta Cephei stars and many other variable stars in the young open cluster h Persei (NGC 869). The cluster seems to be very rich in variable B-type stars, similarly to its twin, Chi Persei (NGC 884).

A. Majewska-Swierzbinowicz; A. Pigulski; R. Szabo; Z. Csubry

2007-11-10

139

Rotation of Jets from Young Stars: New Clues from the Hubble Space Telescope Imaging Spectrograph  

Microsoft Academic Search

We report findings from the first set of data in a current survey to establish conclusively whether jets from young stars rotate. We observed the bipolar jets from the T Tauri stars TH 28 and RW Aur and the blueshifted jet from T Tauri star LkHalpha 321, using the Hubble Space Telescope Imaging Spectrograph. Forbidden emission lines show distinct and

Deirdre Coffey; Francesca Bacciotti; Jens Woitas; Thomas P. Ray; Jochen Eislöffel

2004-01-01

140

Mid-Infrared Imaging of Massive Young Stars  

NASA Astrophysics Data System (ADS)

Formation of massive stars is a poorly understood process. Detailed observations of high-luminosity young stellar objects (YSOs), often embedded in dusty envelopes, could provide valuable clues to their origin. Imaging in the mid-infrared allows us to investigate the multiplicity of sources, characterize the immediate circumstellar material, and connect the overall structure of the YSO environment on both large and small scales. Here we present subarcsecond-resolution mid-infrared images of nine young stellar objects: AFGL 961, AFGL 2136, AFGL 4176, AFGL 7009S, G343.126, I17441, M8E-IR, S269-IRS2, and W33A. The observations were obtained in two runs using the Thermal Infrared Multi-Mode Instrument (TIMMI2) on the European Southern Observatory's 3.6-meter telescope in La Silla, Chile. Each target, with the exception of AFGL 4176, has observations at both 11.9 microns and Q-band. M8E-IR was also observed at 8.9 and 9.8 microns, while AFGL 961 and S269-IRS2 were imaged at 4.6 microns as well. We present photometry and contour plots to show the infrared morphology of each target, several of which reveal multiple sources and/or diffuse nebulosity. For example, we find that M8E-IR has six separate sources clustered around the central point source. In addition to I17441's central point source, there are three fainter sources as well as a large bow-shaped nebulosity. We detect four point sources toward AFGL 961 and extended diffuse mission associated with S269-IRS2. We discuss the relation between our mid-infrared images of these YSOs and previously available data on their near-infrared morphology, silicate features, and CO outflows.

Billmeier, R. R.; Jayawardhana, R.; Marengo, M.; Mardones, D.; Alves, J.

2003-12-01

141

Linear spectropolarimetry of young and other emission line stars  

E-print Network

The aim of this article is to demonstrate the useful role that can be played by spectropolarimetric observations of young and evolved emission line stars that analyse the linearly polarized component in their spectra. At the time of writing, this demonstration has to be made on the basis of optical data since there is no common-user infrared facility, in operation, that offers the desired combination of spectral resolution and sensitivity. Here we focus on what can be learned from linear spectropolarimetry alone at reasonably high spectral resolution and at $10^3 < $S/N$ < 10^4$. And we remind that the near infrared (1--2 micron) has the potential to out-perform the optical as a domain to work in because of the greatly reduced interstellar obscuration at these wavelengths. This point has been reached at a time when theory, exploiting flexible Monte Carlo methods, is fast becoming a powerful tool. In short we have the complex phenomena, and the rise of the modelling capability to match -- good data are the missing link.

Janet Drew; Jorick Vink; Tim Harries; Ryuichi Kurosawa; Rene Oudmaijer

2004-03-30

142

`Tail-end' Bondi-Hoyle accretion in young star clusters: Implications for disks, planets, and stars  

E-print Network

Young stars orbiting in the gravitational potential well of forming star clusters pass through the cluster's dense molecular gas and can experience Bondi-Hoyle accretion from reservoirs outside their individual protostellar cloud cores. Accretion can occur for several million years after the stars form, but before the cluster disperses. This accretion is predominantly onto the disk and not the star. N-body simulations of stars orbiting in three young model clusters containing 30, 300, and 3000 stars are presented. The simulations include the gravitational potential of the molecular gas which smoothly disperses over time. The clusters have a star formation efficiency of 33% and a radius of 0.22 pc. We find that the disks surrounding solar-mass stars in the N=30 cluster accretes ~0.01 M_sol (~1 minimum-mass solar nebula, MMSN) per Myr. The accretion rate scales as M^2.1 for stars of mass M. The accretion rate is ~5 times lower for N=3000 cluster, due to its higher stellar velocities and higher temperature. The Bondi-Hoyle accretion rates onto the disks are several times lower than accretion rates observed directly onto young stars (e.g., Muzerolle et al 2005): these two accretion rates follow the same M^2 behavior and may be related. The accreted disk mass is large enough that it may have a substantial and unappreciated effect on disk structure and the formation of planetary systems. We discuss a variety of implications of this process, including its effect on metallicity differences between cluster stars, compositional differences between a star and its disk, the formation of terrestrial and gas-giant planets, and isotopic anomalies observed in our Solar System.

Henry B. Throop; John Bally

2008-04-03

143

Search for associations containing young stars (SACY):II. Chemical abundances of stars in 11 young Associations in the Solar neighborhood  

Microsoft Academic Search

The recently discovered coeval, moving groups of young stellar objects in the\\u000asolar neighborhood represent invaluable laboratories to study recent star\\u000aformation and to search for high metallicity stars which can be included in\\u000afuture exo-planet surveys. In this study we derived through an uniform and\\u000ahomogeneous method stellar atmospheric parameters and abundances for iron,\\u000asilicium and nickel in 63

P. Viana Almeida; N. C. Santos; C. Melo; M. Ammler-von Eiff; C. A. O. Torres; G. R. Quast; J. F. Gameiro; M. Sterzik

2009-01-01

144

The luminosity function of young star clusters: implications for the maximum mass and luminosity of clusters  

Microsoft Academic Search

We introduce a method to relate a possible truncation of the star cluster mass function at the high mass end to the shape of the cluster luminosity function (LF). We compare the observed LFs of five galaxies containing young star clusters with synthetic cluster population models with varying initial conditions. The LF of the SMC, the LMC and NGC 5236

Mark Gieles; Soeren Larsen; Nate Bastian; I. T. Stein

2006-01-01

145

Chromospheric activity variations in late-type stars members of young stellar kinematic groups  

E-print Network

Chromospheric activity variations in late-type stars members of young stellar kinematic groups J. L determine radial velocities, chromospheric activity and lithium abundance of these objects. The chromospheric activity level of these stars has been analysed using the information provided by several optical

Complutense de Madrid, Universidad

146

SPECTRAL ENERGY DISTRIBUTIONS OF YOUNG STARS IN IC 348: THE ROLE OF DISKS IN ANGULAR MOMENTUM EVOLUTION OF YOUNG, LOW-MASS STARS  

SciTech Connect

Theoretical work suggests that a young star's angular momentum content and rotation rate may be strongly influenced by magnetic interactions with its circumstellar disk. A generic prediction of these 'disk-locking' theories is that a disk-locked star will be forced to co-rotate with the Keplerian angular velocity of the inner edge of the disk; that is, the disk's inner-truncation radius should equal its co-rotation radius. These theories have also been interpreted to suggest a gross correlation between young stars' rotation periods and the structural properties of their circumstellar disks, such that slowly rotating stars possess close-in disks that enforce the star's slow rotation, whereas rapidly rotating stars possess anemic or evacuated inner disks that are unable to brake the stars and instead the stars spin up as they contract. To test these expectations, we model the spectral energy distributions (SEDs) of 33 young stars in IC 348 with known rotation periods and infrared excesses indicating the presence of circumstellar disks. For each star, we match the observed SED, typically sampling 0.6-8.0 {mu}m, to a grid of 200,000 pre-computed star+disk radiative transfer models, from which we infer the disk's inner-truncation radius. We then compare this truncation radius to the disk's co-rotation radius, calculated from the star's measured rotation period. We do not find obvious differences in the disk truncation radii of slow rotators versus rapid rotators. This holds true both at the level of whether close-in disk material is present at all, and in analyzing the precise location of the inner disk edge relative to the co-rotation radius among the subset of stars with close-in disk material. One interpretation is that disk locking is unimportant for the IC 348 stars in our sample. Alternatively, if disk locking does operate, then it must operate on both the slow and rapid rotators, potentially producing both spin-up and spin-down torques, and the transition from the disk-locked state to the disk-released state must occur more rapidly than the stellar contraction timescale.

Le Blanc, Thompson S.; Stassun, Keivan G. [Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Center, Nashville, TN 37235 (United States); Covey, Kevin R. [Department of Astronomy, Cornell University, 226 Space Sciences Building, Ithaca, NY 14853 (United States)

2011-08-15

147

AGE AND MASS STUDIES FOR YOUNG STAR CLUSTERS IN M31 FROM SEDS-FIT  

SciTech Connect

In this paper, we present photometry for young star clusters in M31, which are selected from Caldwell et al. These star clusters have been observed as part of the Beijing-Arizona-Taiwan-Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2008 March. The BATC images including these star clusters are taken with 15 intermediate-band filters covering 3000-10000 A. Combined with photometry in the GALEX far- and near-ultraviolet, broadband UBV RI, SDSS ugriz, and infrared JHK{sub s} of Two Micron All Sky Survey, we obtain their accurate spectral energy distributions (SEDs) from 1538 to 20000 A. We derive these star clusters' ages and masses by comparing their SEDs with stellar population synthesis models. Our results are in good agreement with previous determinations. The mean value of age and mass of young clusters (<2 Gyr) is about 385 Myr and 2 Multiplication-Sign 10{sup 4} M{sub Sun }, respectively. There are two distinct peaks in the age distribution, a highest peak at age {approx}60 Myr and a secondary peak around 250 Myr, while the mass distribution shows a single peak around 10{sup 4} M{sub Sun }. A few young star clusters have two-body relaxation times greater than their ages, indicating that those clusters have not been well dynamically relaxed and therefore have not established the thermal equilibrium. There are several regions showing aggregations of young star clusters around the 10 kpc ring and the outer ring, indicating that the distribution of the young star clusters is well correlated with M31's star-forming regions. The young massive star clusters (age {<=}100 Myr and mass {>=}10{sup 4} M{sub Sun }) show apparent concentration around the ring splitting region, suggesting a recent passage of a satellite galaxy (M32) through M31 disk.

Wang Song; Ma Jun; Fan Zhou; Wu Zhenyu; Zhang Tianmeng; Zou Hu; Zhou Xu, E-mail: majun@nao.cas.cn [National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012 (China)

2012-12-01

148

Chromospherically active stars. II - HD 82558, a young single BY Draconis variable  

NASA Technical Reports Server (NTRS)

It is presently noted that the HD 82558 chromospherically active star is a young and rapidly rotating K2 V single BY Draconis variable with very strong far-UV emission features and an H-alpha line filled to the continuum level by emission. HD 82558 has constant velocity and is not a member of the Hyades Supercluster. Its light curve behavior, which appears to have been stable for several hundred rotation cycles, is reminiscent of that of the young, rapidly rotating, single K V variable H II 1883 in the Pleiades; this stability may be characteristic of young, single, chromospherically active stars.

Fekel, Francis C.; Bopp, Bernard W.; Africano, John L.; Goodrich, Bret D.; Palmer, Leigh Hunter

1986-01-01

149

Temperaments of young stars: rapid mass accretion rate changes in T Tauri and Herbig Ae stars  

NASA Astrophysics Data System (ADS)

Variability in emission lines is a characteristic feature in young stars and can be used as a tool to study the physics of the accretion process. Here, we present a study of H? variability in 15 T Tauri and Herbig Ae stars (K7 - B2) over a wide range of time windows, from minutes, to hours, to days, and years. We assess the variability using linewidth measurements and the time series of line profiles. All objects show gradual, slow profile changes on time-scales of days. In addition, in three cases there is evidence for rapid variations in H? with typical time-scales of 10 min, which occurs in 10 per cent of the total covered observing time. The mean accretion rate changes, inferred from the line fluxes, are 0.01-0.07 dex for time-scales of <1 h, 0.04-0.4 dex for time-scales of days, and 0.13-0.52 dex for time-scales of years. In Costigan et al., we derived an upper limit finding that the intermediate (days) variability dominated over longer (years) variability. Here, our new results, based on much higher cadence observations, also provide a lower limit to accretion rate variability on similar time-scales (days), thereby constraining the accretion rate variability physics in a much more definitive way. A plausible explanation for the gradual variations over days is an asymmetric accretion flow resulting in a rotational modulation of the accretion-related emission, although other interpretations are possible as well. In conjunction with our previous work, we find that the time-scales and the extent of the variability is similar for objects ranging in mass from ˜0.1 to ˜5 M?. This confirms that a single mode of accretion is at work from T Tauri to Herbig Ae stars - across a wide range of stellar masses.

Costigan, G.; Vink, Jorick S.; Scholz, A.; Ray, T.; Testi, L.

2014-06-01

150

Reconstructing the Initial Relaxation Time of Young Star Clusters in the Large Magellanic Cloud: The Evolution of Star Clusters  

NASA Astrophysics Data System (ADS)

We reconstruct the initial two-body relaxation time at the half mass radius for a sample of young ? 300 Myr star clusters in the Large Magellanic cloud. We achieve this by simulating star clusters with 12288 to 131072 stars using direct N-body integration. The equations of motion of all stars are calculated with high precision direct N-body simulations which include the effects of the evolution of single stars and binaries. We find that the initial relaxation times of the sample of observed clusters in the Large Magellanic Cloud ranges from about 200 Myr to about 2 Gyr. The reconstructed initial half-mass relaxation times for these clusters have a much narrower distribution than the currently observed distribution, which ranges over more than two orders of magnitude.

Portegies Zwart, S. F.; Chen, H.-C.

2008-06-01

151

Life and Death of Young Dense Star Clusters near the Galactic Center  

E-print Network

We discuss the structural change and degree of mass segregation of young dense star clusters within about 100pc of the Galactic center. In our calculations, which are performed with GRAPE-6, the equations of motion of all stars and binaries are calculated accurately but the external potential of the Galaxy is solved (semi)analytically. The simulations are preformed to model the Arches star cluster. We find that star clusters with are less strongly perturbed by the tidal field and dynamical friction are much stronger affected by mass segregation; resulting in a significant pile-up of massive stars in the cluster center. At an age of about 3.5Myr more than 90 per cent of the stars more massive than ~10Msun are concentrated within the half-mass radius of the surviving cluster. Star clusters which are strongly perturbed by the tidal field of the parent Galaxy are much less affected by mass segregation.

Simon Portegies Zwart; Stephen McMillan; Holger Baumgardt

2004-03-05

152

ACCRETION RATES ON PRE-MAIN-SEQUENCE STARS IN THE YOUNG OPEN CLUSTER NGC 6530  

SciTech Connect

It is well accepted that during the star formation process, material from a protoplanetary disk is accreted onto the central object during the first {approx}1-5 Myr. Different authors have published measurements of accretion rates for young low- and intermediate-mass stars in several nearby star-forming regions (SFRs). Due to its somewhat larger distance, the SFR M8 (the Lagoon Nebula) has not been studied to the same extent, despite its abundant population of young stellar objects. We have obtained optical band low-resolution spectra of a sample of pre-main-sequence stars in the open cluster NGC 6530 located in the aforementioned nebulae using the Gemini Multi Object Spectrograph at Gemini-South in multi-object mode. Spectra cover the H{sub {alpha}} emission line used to measure the accretion rate, following the method presented by Natta et al. The observed spectral characteristics are fully consistent with pre-main-sequence stars, showing lithium absorption lines, which are very common in young stellar objects, as well as prominent and broad H{sub {alpha}} emission lines, indicating a T Tauri evolutionary stage. This work presents the first determinations of mass accretion rates of young stellar objects in the open cluster NGC 6530, confirming that they are classical T Tauri stars going through the accretion phase. These observations contribute to a better understanding of the stellar content and evolutionary phase of the very active Lagoon Nebula SFR.

Gallardo, Jose; Del Valle, Luciano; Ruiz, Maria Teresa, E-mail: gallardo@das.uchile.cl, E-mail: ldelvall@das.uchile.cl, E-mail: mtruiz@das.uchile.cl [Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago (Chile)

2012-01-15

153

Open Clusters as Laboratories: The Angular Momentum Evolution of Young Stars  

NASA Technical Reports Server (NTRS)

The core group concentrated on three primary research topics: (1) ROSAT observation of the coronal activity of low mass stars in young open clusters; (2) the determination of stellar ages and the determination of the timescale for dissipation of circumstellar disks around young stars; and (3) the determination of rotation velocities of low mass stars in young open cluster and the inferred angular momentum evolution of low mass stars. With accurate ages for the clusters, we can then derive an independent estimate of the timescale for debris disks to dissipate. As the second half of that project, we are using the Caltech/UC/NASA Keck telescopes to obtain spectra of brown dwarf candidates in a number of nearby, young open clusters, from which we can determine new and accurate cluster ages. The final primary program that we have addressed was the determination of rotational velocities for low mass stars in our target open clusters. Our group has obtained rotational velocities for a large number of stars in several open clusters during this LTSA program, and we have published the results in several papers. One particularly time-consuming aspect of our program was the development of a database of the photometry and rotational velocities for nearby open clusters, which we have made available to the community.

Stauffer, John R.

1998-01-01

154

On the Nature of the Peculiar Hot Star in the Young LMC Cluster NGC1818  

E-print Network

The blue star reported in the field of the young LMC cluster NGC1818 by Elson et al. (1998) has the wrong luminosity and radius to be a "luminous white dwarf" member of the cluster. In addition, unless the effective temperature quoted by the authors is a drastic underestimate, the luminosity is much too low for it to be a cluster member in the post-AGB phase. Other possibilities, including that of binary evolution, are briefly discussed. However, the implication that the massive main sequence turnoff stars in this cluster can produce white dwarfs (instead of neutron stars) from single-star evolution needs to be reconsidered.

James Liebert

1999-01-21

155

Analysis of the changing brightness of stars in nearby young stellar clusters  

NASA Astrophysics Data System (ADS)

We present initial results of a photometric variability study of 19 stars in the TW Hydrae Association (TWA). TWA is a stellar moving group made up of young (10 Myr) dispersed stars moving across the sky. By studying individual brightness variations we hope to learn more about intermediate periods of stellar formation. We have identified 19 stars in the TWA in our data set of irregularly spaced, photometric observations of the southern sky obtained over the past 4 years with KELT-South, the Kilodegree Extremely Little Telescope located in Sutherland, South Africa. We will present light curves of these stars and assess their photometric variability.

Rolen, Emily; Rodriguez, Joseph E.; Weintraub, David A.; Pepper, Joshua; Stassun, Keivan; KELT-South Science Team

2015-01-01

156

Nuclear ashes and outflow in the eruptive star Nova Vul 1670  

NASA Astrophysics Data System (ADS)

CK Vulpeculae was observed in outburst in 1670-1672 (ref. 1), but no counterpart was seen until 1982, when a bipolar nebula was found at its location. Historically, CK Vul has been considered to be a nova (Nova Vul 1670), but its similarity to `red transients', which are more luminous than classical novae and thought to be the results of stellar collisions, has re-opened the question of CK Vul's status. Red transients cool to resemble late M-type stars, surrounded by circumstellar material rich in molecules and dust. No stellar source has been seen in CK Vul, though a radio continuum source was identified at the expansion centre of the nebula. Here we report that CK Vul is surrounded by chemically rich molecular gas in the form of an outflow, as well as dust. The gas has peculiar isotopic ratios, revealing that CK Vul's composition was strongly enhanced by the nuclear ashes of hydrogen burning. The chemical composition cannot be reconciled with a nova or indeed any other known explosion. In addition, the mass of the surrounding gas is too large for a nova, though the conversion from observations of CO to a total mass is uncertain. We conclude that CK Vul is best explained as the remnant of a merger of two stars.

Kami?ski, Tomasz; Menten, Karl M.; Tylenda, Romuald; Hajduk, Marcin; Patel, Nimesh A.; Kraus, Alexander

2015-04-01

157

Nuclear ashes and outflow in the eruptive star Nova Vul 1670.  

PubMed

CK Vulpeculae was observed in outburst in 1670-1672 (ref. 1), but no counterpart was seen until 1982, when a bipolar nebula was found at its location. Historically, CK Vul has been considered to be a nova (Nova Vul 1670), but its similarity to 'red transients', which are more luminous than classical novae and thought to be the results of stellar collisions, has re-opened the question of CK Vul's status. Red transients cool to resemble late M-type stars, surrounded by circumstellar material rich in molecules and dust. No stellar source has been seen in CK Vul, though a radio continuum source was identified at the expansion centre of the nebula. Here we report that CK Vul is surrounded by chemically rich molecular gas in the form of an outflow, as well as dust. The gas has peculiar isotopic ratios, revealing that CK Vul's composition was strongly enhanced by the nuclear ashes of hydrogen burning. The chemical composition cannot be reconciled with a nova or indeed any other known explosion. In addition, the mass of the surrounding gas is too large for a nova, though the conversion from observations of CO to a total mass is uncertain. We conclude that CK Vul is best explained as the remnant of a merger of two stars. PMID:25799986

Kami?ski, Tomasz; Menten, Karl M; Tylenda, Romuald; Hajduk, Marcin; Patel, Nimesh A; Kraus, Alexander

2015-04-16

158

PHAT Youths: Metallicity Gradient of M31 using Young Stars in the PHAT Survey  

NASA Astrophysics Data System (ADS)

Many recent studies have revealed metallicity gradients across galaxies, which are used as important constraints for models of chemical evolution. Existing observations of young stellar metallicities in M31 consist of spectroscopic studies (Zurita & Bresolin 2012; Sanders et al. 2012), which were limited to tens to hundreds of measurements of HII regions.We present photometric metallicity measurements of tens of thousands of young main sequence stars in M31 using multi-band photometry from the Panchromactic Hubble Andromeda Treasury (PHAT). We use UV and optical photometry of young massive main sequence stars to determine the extinction and metallicities of each individual star using isochrone fitting. We then use these measurements to examine metallicity gradients within Andromeda.

Deich, Alex; Seth, Anil

2015-01-01

159

Optical Spectroscopic Classification and Membership of Young M Dwarfs in Star-Forming Regions  

E-print Network

The spectral type is a key parameter in calibrating the temperature which is required to estimate the mass of young stars and brown dwarfs. We describe an approach developed to classify low-mass stars and brown dwarfs in the Trapezium Cluster using red optical spectra, which can be applied to other star-forming regions. The classification uses two methods for greater accuracy: the use of narrowband spectral indices which rely on the variation of the strength of molecular lines with spectral type and a comparison with other previously classified young, low-mass objects in the Chamaeleon I star-forming region. We have investigated and compared many different molecular indices and have identified a small number of indices which work well for classifying M-type objects in nebular regions. The indices are calibrated for young, pre-main sequence objects whose spectra are affected by their lower surface gravities compared with those on the main sequence. Spectral types obtained are essentially independent of both reddening and nebular emission lines. Confirmation of candidate young stars and brown dwarfs as bona fide cluster members may be accomplished with moderate resolution spectra in the optical region by an analysis of the strength of the gravity-sensitive Na doublet. It has been established that this feature is much weaker in these very young objects than in field dwarfs. A sodium spectral index is used to estimate the surface gravity and to demonstrate quantitatively the difference between young (1-2Myr) objects, and dwarf and giant field stars.

F. C. Riddick; P. F. Roche; P. W. Lucas

2007-08-09

160

Cataclysmic binaries in star clusters. 1: A search for erupting dwarf novae in the globular cluster M92  

Microsoft Academic Search

Globular clusters are reputed to contain many capture (and possibly primordial) binaries. Close white dwarf-main sequence binaries should be particularly plentiful. Such systems can give rise to dwarf nova eruptions, which are indisputable indicators of interacting, close binaries. We have therefore surveyed the rich Galactic globular cluster M92 (NGC 6341) for erupting dwarf novae. CCD images with limiting magntide approximately

Michael M. Shara; Louis E. Bergeron; Anthony F. J. Moffat

1994-01-01

161

A NEW METHOD TO IDENTIFY NEARBY, YOUNG, LOW-MASS STARS  

SciTech Connect

We describe a new method to identify young, late-type stars within {approx}150 pc of the Earth that employs visual or near-infrared (NIR) data and the GALEX GR4/5 database. For spectral types later than K5, we demonstrate that the ratio of GALEX near-ultraviolet to visual and NIR emission is larger for stars with ages between 10 and 100 Myr than for older, main-sequence stars. A search in regions of the sky encompassing the TW Hya and Scorpius-Centaurus Associations has returned 54 high-quality candidates for follow up. Spectroscopic observations of 24 of these M1-M5 objects reveal Li 6708 A absorption in at least 17 systems. Because GALEX surveys have covered a significant fraction of the sky, this methodology should prove valuable for future young star studies.

Rodriguez, David R.; Zuckerman, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States); Bessell, M. S. [Australian National University, Cotter Road, Weston Creek, ACT 2611 (Australia); Kastner, Joel H., E-mail: drodrigu@astro.ucla.edu [Center for Imaging Science, Rochester Institute of Technology, 54 Lomb Memorial Drive, Rochester, NY 14623 (United States)

2011-02-01

162

A New Method to Identify Nearby, Young, Low-mass Stars  

E-print Network

We describe a new method to identify young, late-type stars within ~150 pc of the Earth that employs visual or near-infrared data and the GALEX GR4/5 database. For spectral types later than K5, we demonstrate that the ratio of GALEX near-ultraviolet (NUV) to visual and near-IR emission is larger for stars with ages between 10 and 100 Myr than for older, main sequence stars. A search in regions of the sky encompassing the TW Hya and Scorpius-Centaurus Associations has returned 54 high-quality candidates for followup. Spectroscopic observations of 24 of these M1-M5 objects reveal Li 6708 angstrom absorption in at least 17 systems. Because GALEX surveys have covered a significant fraction of the sky, this methodology should prove valuable for future young star studies.

Rodriguez, David R; Zuckerman, B; Kastner, Joel H

2010-01-01

163

A Parameter Study of the Dust and Gas Temperature in a Field of Young Stars  

E-print Network

We model the thermal effect of young stars on their surrounding environment in order to understand clustered star formation. We take radiative heating of dust, dust-gas collisional heating, cosmic-ray heating, and molecular cooling into account. Using Dusty, a spherical continuum radiative transfer code, we model the dust temperature distribution around young stellar objects with various luminosities and surrounding gas and dust density distributions. We have created a grid of dust temperature models, based on our modeling with Dusty, which we can use to calculate the dust temperature in a field of stars with various parameters. We then determine the gas temperature assuming energy balance. Our models can be used to make large-scale simulations of clustered star formation more realistic.

Andrea Urban; Neal J. Evans II; Steven D. Doty

2007-10-21

164

A KINEMATIC AND PHOTOMETRIC STUDY OF THE GALACTIC YOUNG STAR CLUSTER NGC 7380  

SciTech Connect

We present proper motions, radial velocities, and a photometric study of the Galactic open cluster NGC 7380, which is associated with prominent emission nebulosity and dark molecular clouds. On the basis of the sample of highly probable member stars, the star cluster is found to be at a distance of 2.6 {+-} 0.4 kpc, has an age of around 4 Myr, and a physical size of {approx}6 pc across with a tidal structure. The binary O-type star DH Cep is a member of the cluster in its late stage of clearing the surrounding material, and may have triggered the ongoing star formation in neighboring molecular clouds which harbor young stars that are coeval and comoving with, but not gravitationally bound by, the star cluster.

Chen, W. P.; Chen, C. W. [Institute of Astronomy, National Central University, Jhongli 32001, Taiwan (China); Pandey, A. K.; Sharma, Saurabh [Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 129, Uttaranchal (India); Chen Li [Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030 (China); Sperauskas, J. [Astronomical Observatory of Vilnius University (Lithuania); Ogura, K. [Department for Natural Sciences, Kokugakuin University, Higashi, Shibuya-ku, Tokyo 150-8440 (Japan); Chuang, R. J. [Department of Statistics and Information Science, Fu Jen University, Taiwan (China); Boyle, R. P. [Vatican Observatory Research Group, Steward Observatory, Tucson, AZ 85721 (United States)

2011-09-15

165

Characterization of young field stars in the vicinity of the CO Cepheus void  

NASA Astrophysics Data System (ADS)

After mixing in the Galactic plane, young stars are barely discernible from old ones. Nonetheless, the characterization of stars in the RasTyc} sample has ledto the discovery of several sources whose lithium content is higher than that of the Pleiades cluster members. In the locus of the CO Cepheus void, we identified four comoving T Tauri stars within a few degrees. They likely form a new young association. Looking for the presence of further members, we performed selections through multivariate analysis. We focused on the late-type stars in the field, which are identified as the optical and infrared counterparts of ROSAT All-Sky Survey/XMM-Newton X-ray sources and are in a 30^circ-wide region encompassing this new moving group. Based on our spectroscopic observations of this dataset, we identified two distinct populations of lithium-rich stars that are spatially and kinematically separated. While the sources having the same lithium content as the members of the Pleiades cluster are mostly projected in front of the Galactic plane, the youngest stars are mainly located towards the sky area surrounding the CO Cepheus void. The latter stars have an age of about 5-15 Myr, which is rather similar to that derived for the four comoving T Tauri stars previously found in this region. The discovery and characterization of all the young stars in the field are of great importance to have new insights into the process of stellar formation outside the typical star-forming regions. The Gaia mission will certainly shed light on this issue and on the origin of this group that could be related to the Cepheus-Cassiopeia complex.

Klutsch, A.; Montes, D.; Guillout, P.; Frasca, A.; Pineau, F.-X.; Grosso, N.; Marilli, E.; López-Santiago, J.

2012-12-01

166

Planetary science: Preventing stars from eating their young  

NASA Astrophysics Data System (ADS)

Researchers have found a mechanism that prevents newly forming giant-planet cores from spiralling in towards their parent stars. The result may explain why planets such as Saturn and Jupiter are where they are today. See Letter p.63

Duncan, Martin J.

2015-04-01

167

IDENTIFYING THE YOUNG LOW-MASS STARS WITHIN 25 pc. II. DISTANCES, KINEMATICS, AND GROUP MEMBERSHIP  

SciTech Connect

We have conducted a kinematic study of 165 young M dwarfs with ages of {approx}<300 Myr. Our sample is composed of stars and brown dwarfs with spectral types ranging from K7 to L0, detected by ROSAT and with photometric distances of {approx}<25 pc assuming that the stars are single and on the main sequence. In order to find stars kinematically linked to known young moving groups (YMGs), we measured radial velocities for the complete sample with Keck and CFHT optical spectroscopy and trigonometric parallaxes for 75 of the M dwarfs with the CAPSCam instrument on the du Pont 2.5 m Telescope. Due to their youthful overluminosity and unresolved binarity, the original photometric distances for our sample underestimated the distances by 70% on average, excluding two extremely young ({approx}<3 Myr) objects found to have distances beyond a few hundred parsecs. We searched for kinematic matches to 14 reported YMGs and identified 10 new members of the AB Dor YMG and 2 of the Ursa Majoris group. Additional possible candidates include six Castor, four Ursa Majoris, two AB Dor members, and one member each of the Her-Lyr and {beta} Pic groups. Our sample also contains 27 young low-mass stars and 4 brown dwarfs with ages {approx}<150 Myr that are not associated with any known YMG. We identified an additional 15 stars that are kinematic matches to one of the YMGs, but the ages from spectroscopic diagnostics and/or the positions on the sky do not match. These warn against grouping stars together based only on kinematics and that a confluence of evidence is required to claim that a group of stars originated from the same star-forming event.

Shkolnik, Evgenya L. [Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001 (United States); Anglada-Escude, Guillem [Institut fuer Astrophysik, Universitaet Goettingen, Friedrich-Hund-Platz 1, D-37077 Goettingen (Germany); Liu, Michael C.; Bowler, Brendan P. [Institute for Astronomy, University of Hawaii at Manoa 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Weinberger, Alycia J.; Boss, Alan P. [Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015 (United States); Reid, I. Neill [Space Telescope Science Institute, Baltimore, MD 21218 (United States); Tamura, Motohide, E-mail: shkolnik@lowell.edu [National Astronomical Observatory of Japan, Tokyo (Japan)

2012-10-10

168

Understanding The Physics Of X-ray Emission In Young Stars  

NASA Astrophysics Data System (ADS)

We present the analysis of Chandra X-ray observations of a sample of young stars, particularly T Tauri stars, in order to better understand the characteristics of the X-ray emitting plasmas of young stars and the physical mechanisms that produce that emission. Our sample includes Chandra High Energy Transmission Grating Spectroscopy (HETGS) relatively long integration, high spectral resolution data for nine young stars, including classical, weak-lined, and post-T Tauri stars as well as a young main sequence star. For five of these nine stars we observed significant flaring during the Chandra observing epochs. We extracted spectra for each observation, and for the observations with significant flaring, we extracted separate spectra for the flaring and quiescent epochs. From these spectra, we measured the line fluxes of the strongest emission lines, most notably the Hydrogen-like Lyman line and the He-like triplet (forbidden, intercombination, and resonance) lines for Si, Mg, and Ne as well as strong Fe lines. From these line flux measurements, we were able to infer densities and temperatures from line flux ratios sensitive to these properties. In general, the quiescent state observations have higher densities (as inferred by the density-sensitive R ratio) than the flaring state observations. By measuring elemental abundance ratios, we find that all the observations of the stars in our sample have a high Ne/Fe ratio, but only one, TW Hya in quiescence, shows an enhanced Ne/O ratio. This work was supported by a grant from NASA's Chandra X-ray Observatory.

Shukla, Sonali J.; Weintraub, D.

2010-01-01

169

Disk Braking in young Stars: Probing Rotation in Chamaeleon i and Taurus-Auriga  

Microsoft Academic Search

We present a comprehensive study of rotation, disk, and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star-forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infrared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster

Duy Cuong Nguyen; Ray Jayawardhana; Marten H. van Kerkwijk; Alexis Brandeker; Alexander Scholz; Ivana Damjanov

2009-01-01

170

Nuclear ashes and outflow in the eruptive star Nova Vul 1670  

E-print Network

CK Vulpeculae was observed in outburst in 1670-16721, but no counterpart was seen until 1982, when a bipolar nebula was found at its location. Historically, CK Vul has been considered to be a nova (Nova Vul 1670), but a similarity to 'red transients', which are more luminous than classical nova and thought to be the result of stellar collisions, has re-opened the question of CK Vul's status. Red transients cool to resemble late M-type stars, surrounded by circumstellar material rich in molecules and dust. No stellar source has been seen in CK Vul, though a radio continuum source was identified at the expansion centre of the nebula. Here we report CK Vul is surrounded by chemically rich molecular gas with peculiar isotopic ratios, as well as dust. The chemical composition cannot be reconciled with a nova or indeed any other known explosion. In addition, the mass of the surrounding gas is too high for a nova, though the conversion from observations of CO to a total mass is uncertain. We conclude that CK Vul is ...

Kaminski, Tomasz; Tylenda, Romuald; Hajduk, Marcin; Patel, Nimesh A; Kraus, Alexander

2015-01-01

171

STAR FORMATION AND YOUNG STELLAR CONTENT IN THE W3 GIANT MOLECULAR CLOUD  

SciTech Connect

In this work, we have carried out an in-depth analysis of the young stellar content in the W3 giant molecular cloud (GMC). The young stellar object (YSO) population was identified and classified in the Infrared Array Camera/Multiband Imaging Photometer color-magnitude space according to the 'Class' scheme and compared to other classifications based on intrinsic properties. Class 0/I and II candidates were also compared to low-/intermediate-mass pre-main-sequence (PMS) stars selected through their colors and magnitudes in the Two Micron All Sky Survey. We find that a reliable color/magnitude selection of low-mass PMS stars in the infrared requires prior knowledge of the protostar population, while intermediate-mass objects can be more reliably identified. By means of the minimum spanning tree algorithm and our YSO spatial distribution and age maps, we investigated the YSO groups and the star formation history in W3. We find signatures of clustered and distributed star formation in both triggered and quiescent environments. The central/western parts of the GMC are dominated by large-scale turbulence likely powered by isolated bursts of star formation that triggered secondary star formation events. Star formation in the eastern high-density layer (HDL) also shows signs of quiescent and triggered stellar activity, as well as extended periods of star formation. While our findings support triggering as a key factor for inducing and enhancing some of the major star-forming activity in the HDL (e.g., W3 Main/W3(OH)), we argue that some degree of quiescent or spontaneous star formation is required to explain the observed YSO population. Our results also support previous studies claiming a spontaneous origin for the isolated massive star(s) powering KR 140.

Rivera-Ingraham, Alana [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada); Martin, Peter G. [Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8 (Canada); Polychroni, Danae [INAF-IFSI, via Fosso del Cavaliere 100, 00133 Roma (Italy); Moore, Toby J. T. [Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD (United Kingdom)

2011-12-10

172

Analysis of MOST light curves of five young stars in Taurus-Auriga and Lupus~3 Star Forming Regions  

E-print Network

Continuous photometric observations of five young stars obtained by the MOST satellite in 2009 and 2010 in the Taurus and Lupus star formation regions are presented. Using light curve modelling under the assumption of internal invariability of spots, we obtained small values of the solar-type differential-rotation parameter (k=0.0005-0.009) for three spotted weak-line T Tau stars, V410 Tau, V987 Tau and Lupus 3-14; for another spotted WTTS, Lupus 3-48, the data are consistent with a rigidly rotating surface (k=0). Three flares of similar rise (4 min 30 sec) and decay (1 h 45 min) times were detected in the light curve of Lupus 3-14. The brightness of the classical T Tau star RY Tau continuously decreased over 3 weeks of its observations with a variable modulation not showing any obvious periodic signal.

Siwak, Michal; Matthews, Jaymie M; Kuschnig, Rainer; Guenther, David B; Moffat, Anthony F J; Sasselov, Dimitar; Weiss, Werner W

2011-01-01

173

Gravitational Slingshot of Young Massive Stars in Orion  

NASA Astrophysics Data System (ADS)

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest among the ONC's ~1000 members are: ?1 Ori C, the most massive binary in the cluster with stars of masses 38 and 9 M ? the Becklin-Neugebauer (BN) object, a 30 km s-1 runaway star of ~8 M ? and the Kleinmann-Low (KL) nebula protostar, a highly obscured, ~15 M ? object still accreting gas while also driving a powerful, apparently "explosive" outflow. The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here, we report the results of a systematic survey using ~107 numerical experiments of gravitational interactions of the ?1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for ?1C. Five other observed properties of ?1C are also consistent with it having ejected BN and altogether we estimate that there is only a <~ 10-5 probability that ?1C has these properties by chance. We conclude that BN was dynamically ejected from the ?1C system about 4500 years ago. BN then plowed through the KL massive star-forming core within the last 1000 years causing its recently enhanced accretion and outflow activity.

Chatterjee, Sourav; Tan, Jonathan C.

2012-08-01

174

M-dwarf Rapid Rotators and the Detection of Relatively Young Multiple M-Star Systems  

NASA Astrophysics Data System (ADS)

We have searched the Kepler light curves of ~3900 M-star targets for evidence of periodicities that indicate, by means of the effects of starspots, rapid stellar rotation. Several analysis techniques, including Fourier transforms, inspection of folded light curves, "sonograms," and phase tracking of individual modulation cycles, were applied in order to distinguish the periodicities due to rapid rotation from those due to stellar pulsations, eclipsing binaries, or transiting planets. We find 178 Kepler M-star targets with rotation periods, P rot, of <2 days, and 110 with P rot < 1 day. Some 30 of the 178 systems exhibit two or more independent short periods within the same Kepler photometric aperture, while several have 3 or more short periods. Adaptive optics imaging and modeling of the Kepler pixel response function for a subset of our sample support the conclusion that the targets with multiple periods are highly likely to be relatively young physical binary, triple, and even quadruple M star systems. We explore in detail the one object with four incommensurate periods all less than 1.2 days, and show that two of the periods arise from one of a close pair of stars, while the other two arise from the second star, which itself is probably a visual binary. If most of these M-star systems with multiple periods turn out to be bound M stars, this could prove a valuable way discovering young hierarchical M-star systems; the same approach may also be applicable to G and K stars. The ~5% occurrence rate of rapid rotation among the ~3900 M star targets is consistent with spin evolution models that include an initial contraction phase followed by magnetic braking, wherein a typical M star can spend several hundred Myr before spinning down to periods longer than 2 days.

Rappaport, S.; Swift, J.; Levine, A.; Joss, M.; Sanchis-Ojeda, R.; Barclay, T.; Still, M.; Handler, G.; Oláh, K.; Muirhead, P. S.; Huber, D.; Vida, K.

2014-06-01

175

Theory of bipolar flows and jets from young stars  

Microsoft Academic Search

Current thinking on the origin, evolution and stability of stellar jets and bipolar flows is reviewed. Particular emphasis is given to the driving mechanism of bipolar molecular and ionised gas outflows from young stellar objects. General constraints on both hydrodynamic and magnetohydrodynamic flows are presented. The interrelationship between the protostellar outflows and others such as those associated with powerful, highly

Colin A. Norman

1987-01-01

176

STAR FORMATION HISTORY OF A YOUNG SUPER-STAR CLUSTER IN NGC 4038/39: DIRECT DETECTION OF LOW-MASS PRE-MAIN SEQUENCE STARS  

SciTech Connect

We present an analysis of the near-infrared spectrum of a young massive star cluster in the overlap region of the interacting galaxies NGC 4038/39 using population synthesis models. Our goal is to model the cluster population as well as provide rough constraints on its initial mass function (IMF). The cluster shows signs of youth, such as thermal radio emission and strong hydrogen emission lines in the near-infrared. Late-type absorption lines are also present which are indicative of late-type stars in the cluster. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence (PMS) stars or red supergiants alone. Thus, we interpret the spectrum as a superposition of two star clusters of different ages, which is feasible since the 1'' spectrum encompasses a physical region of {approx}90 pc and radii of super-star clusters (SSCs) are generally measured to be a few parsecs. One cluster is young (<= 3 Myr) and is responsible for part of the late-type absorption features, which are due to PMS stars in the cluster, and the hydrogen emission lines. The second cluster is older (6 Myr-18 Myr) and is needed to reproduce the overall depth of the late-type absorption features in the spectrum. Both are required to accurately reproduce the near-infrared spectrum of the object. Thus, we have directly detected PMS objects in an unresolved SSC for the first time using a combination of population synthesis models and PMS tracks. This analysis serves as a testbed of our technique to constrain the low-mass IMF in young SSCs as well as an exploration of the star formation history of young UC H II regions.

Greissl, Julia; Meyer, Michael R. [Steward Observatory, University of Arizona, Tucson, AZ 85721 (United States); Christopher, Micol H.; Scoville, Nick Z., E-mail: jgreissl@as.arizona.ed [California Institute of Technology, Pasadena, CA 91125 (United States)

2010-02-20

177

An Investigation of Three Methods for Determining Young Star Spectral Types  

NASA Astrophysics Data System (ADS)

We present an investigation of several spectral typing techniques applied to 6 young, low-mass binary systems in the Taurus star-forming region (2 Myr). Spectra of resolution ~2000 were taken in the K band at Keck II using NIRC2 in grism spectroscopy mode where adaptive optics allowed us to resolve subarcsecond separations. We tested three different methods to determine spectral type to compare and contrast the strengths and weaknesses of each method. First, we used fits to standard star spectra to determine spectral types, extinctions, and K-band excesses. This method resulted in anomalously high extinctions not supported in the literature. It was also often difficult to distinguish between best fits. Second, we used the equivalent width ratios of IRTF SpeX standards to determine linear relationships onto which we plotted the equivalent width ratios of our sample stars. This method was complicated by low signal to noise in weak lines and the presence of significant circumstellar material around some of our sample of young stars, which may have inconsistently veiled and skewed our results. Third, we used K-band spectral indices and solar metallicity models to infer effective temperatures for our sample. This promising approach, applicable for the M-type stars in our sample, yields effective temperatures of several hundred degrees Kelvin lower than the other methods. Our main goal in this work is to highlight the uncertainties inherent in the typical procedures used for determining young star spectral types and encourage a concerted effort to define a more accurate and precise approach to the measurement of pre-main sequence effective temperature. Temperature is a fundamental stellar property without which our calibration of young star evolution, and by inference planet formation, is highly uncertain, even in the face of precisely measured stellar masses.

Bruhns, Sara; Prato, Lisa A.

2015-01-01

178

Gravitational Slingshot of Young Massive Stars in Orion  

NASA Astrophysics Data System (ADS)

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest amongst the ONC's 1000 members are: theta1C, the most massive binary in the cluster with stars of masses 38 and 9 Msun; the Becklin-Neugebauer (BN) object, a 30 km/s runaway star of 8 Msun; and the Kleinmann-Low (KL) nebula protostar, a highly-obscured, 15 Msun object still accreting gas while also driving a powerful, apparently "explosive" outflow. The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here we report the results of a systematic survey using 10^7 numerical experiments of gravitational interactions of the theta1C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for theta1C. Several other observed properties of theta1C are also consistent with it having ejected BN and altogether we estimate there is only a 10^-5 probability that theta1C has these properties by chance. Our results suggest that after being launched from theta1C 4,500 years ago, BN has plowed through the KL massive-star-forming core within the last 1,000 years causing its recently-enhanced accretion and outflow activity.

Tan, Jonathan; Chatterjee, S.

2012-05-01

179

GRAVITATIONAL SLINGSHOT OF YOUNG MASSIVE STARS IN ORION  

SciTech Connect

The Orion Nebula Cluster (ONC) is the nearest region of massive star formation and thus a crucial testing ground for theoretical models. Of particular interest among the ONC's {approx}1000 members are: {theta}{sup 1} Ori C, the most massive binary in the cluster with stars of masses 38 and 9 M{sub Sun }; the Becklin-Neugebauer (BN) object, a 30 km s{sup -1} runaway star of {approx}8 M{sub Sun }; and the Kleinmann-Low (KL) nebula protostar, a highly obscured, {approx}15 M{sub Sun} object still accreting gas while also driving a powerful, apparently 'explosive' outflow. The unusual behavior of BN and KL is much debated: How did BN acquire its high velocity? How is this related to massive star formation in the KL nebula? Here, we report the results of a systematic survey using {approx}10{sup 7} numerical experiments of gravitational interactions of the {theta}{sup 1}C and BN stars. We show that dynamical ejection of BN from this triple system at its observed velocity leaves behind a binary with total energy and eccentricity matching those observed for {theta}{sup 1}C. Five other observed properties of {theta}{sup 1}C are also consistent with it having ejected BN and altogether we estimate that there is only a {approx}< 10{sup -5} probability that {theta}{sup 1}C has these properties by chance. We conclude that BN was dynamically ejected from the {theta}{sup 1}C system about 4500 years ago. BN then plowed through the KL massive star-forming core within the last 1000 years causing its recently enhanced accretion and outflow activity.

Chatterjee, Sourav; Tan, Jonathan C., E-mail: s.chatterjee@astro.ufl.edu, E-mail: jt@astro.ufl.edu [Department of Astronomy, University of Florida, Gainesville, FL 32611 (United States)

2012-08-01

180

A Dynamical Origin for Early Mass Segregation in Young Star Clusters  

Microsoft Academic Search

Some young star clusters show a degree of mass segregation that is inconsistent with the effects of standard two-body relaxation from an initially unsegregated system without substructure, in virial equilibrium, and it is unclear whether current cluster formation models can account for this degree of initial segregation in clusters of significant mass. We show that mergers of small clumps that

Steve McMillan; Enrico Vesperini; Simon Portegies Zwart

2008-01-01

181

Chandra and NTT Observations of Massive Young Stars in the Heavily Reddened Galactic Cluster Westerlund 1  

Microsoft Academic Search

The southern galactic starburst cluster Westerlund 1 (Wd1) contains a rich population of massive young stars that is spectacularly revealed in infrared images. Recent studies give a mean extinction in the range Av = 9.5 - 13.6 mag and age estimates of ˜3 - 5 Myr (Brandner et al. 2005, Clark et al. 2005). The cluster contains numerous supergiants, hypergiants,

S. L. Skinner; A. Damineli; F. Palla; S. A. Zhekov; A. E. Simmons; M. Teodoro

2005-01-01

182

Spin Evolution of Accreting Young Stars. II. Effect of Accretion-powered Stellar Winds  

NASA Astrophysics Data System (ADS)

We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh & Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

Matt, Sean P.; Pinzón, Giovanni; Greene, Thomas P.; Pudritz, Ralph E.

2012-01-01

183

Workshop on Physics of Accretion Disks Around Compact and Young Stars  

NASA Technical Reports Server (NTRS)

The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities.

Liang, E (editor); Stepinski, T. F. (editor)

1995-01-01

184

X-RAY VARIABILITY OF {sigma} ORIONIS YOUNG STARS AS OBSERVED WITH ROSAT  

SciTech Connect

We used the Aladin Virtual Observatory tool and High Resolution Imager ROSAT archival data to search for X-ray variability in scale of days in 23 young stars in the {sigma} Orionis cluster and a background galaxy. Five stars displayed unambiguous flares and had probabilities p {sub var}>> 99% of being actual variables. Two of the detected flares were violent and long lasting, with maximum duration of six days and amplitude of eight times above the quiescent level. We classified another four stars as possible X-ray variables, including the binary system formed by the B2Vp star {sigma} Ori E and its close late-type companion. This makes a minimum frequency of high-amplitude X-ray variability in excess of a day of 39% among {sigma} Orionis stars. The incidence of this kind of X-ray variability seems to be lower among classical T Tauri stars with mid-infrared flux excesses than among fast-rotating, disk-less young stars.

Caballero, J. A.; Lopez-Santiago, J.; De Castro, E.; Cornide, M. [Departamento de AstrofIsica y Ciencias de la Atmosfera, Facultad de FIsica, Universidad Complutense de Madrid, E-28040 Madrid (Spain)], E-mail: caballero@astrax.fis.ucm.es

2009-06-15

185

COOL YOUNG STARS IN THE NORTHERN HEMISPHERE: {beta} PICTORIS AND AB DORADUS MOVING GROUP CANDIDATES  

SciTech Connect

As part of our continuing effort to identify new, low-mass members of nearby, young moving groups (NYMGs), we present a list of young, low-mass candidates in the northern hemisphere. We used our proven proper-motion selection procedure and ROSAT X-ray and GALEX-UV activity indicators to identify 204 young stars as candidate members of the {beta} Pictoris and AB Doradus NYMGs. Definitive membership assignment of a given candidate will require a measurement of its radial velocity and distance. We present a simple system of indices to characterize the young candidates and help prioritize follow-up observations. New group members identified in this candidate list will be high priority targets for (1) exoplanet direct imaging searches, (2) the study of post-T-Tauri astrophysics, (3) understanding recent local star formation, and (4) the study of local galactic kinematics. Information available now allows us to identify eight likely new members in the list. Two of these, a late-K and an early-M dwarf, we find to be likely members of the {beta} Pic group. The other six stars are likely members of the AB Dor moving group. These include an M dwarf triple system, and three very cool objects that may be young brown dwarfs, making them the lowest-mass, isolated objects proposed in the AB Dor moving group to date.

Schlieder, Joshua E.; Simon, Michal [Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794 (United States); Lepine, Sebastien, E-mail: michal.simon@stonybrook.edu, E-mail: schlieder@mpia-hd.mpg.de, E-mail: lepine@amnh.org [Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States)

2012-04-15

186

Instabilities in Very Young Neutron Stars: Electron Fraction  

NSDL National Science Digital Library

This simulation shows the first 20 milliseconds in the life of a neutron star which is formed in a Type II supernova. After an initial collapse phase, the neutron star becomes unstable to convection. The resulting convective motions destroy the spherical symmetry of the star and rapidly mix the inner regions. In addition, the neutrino flux from the neutron star will be non-spherical and will be significantly enhanced by the convective motions. This may have major implications for the Type II supernova mechanism. The calculation was performed using the Piecewise-Parabolic Method for hydrodynamics. The computational grid contained 300 zones in radius and 200 zones in angle. The inner 200 zones in radius were uniformly spaced, ranging from the inner boundary at 25 km to 175 km. The outer 100 zones were non-uniformly spaced and stretched to 2000 km. Only the inner 200 zones are plotted. The inner boundary was treated as a hard sphere. At the outer boundary, zero gradients for all the variables were assumed. Periodic boundary conditions were used along the sides of the grid. The following sequence shows the mixing of composition which results from the convective motions. The variable plotted is the electron fraction Ye, which ranges from 0.2 to 0.5.

Pamela ONeil

1994-02-12

187

Structure of young star clusters in the Large Magellanic Cloud  

Microsoft Academic Search

The surface brightness profiles presented for 10 rich star clusters in the LMC extend over 8-10 mag in surface brightness, and to radii of 4 arcmin. The crossing times are shorter than the ages of the clusters, and the two-body relaxation times, except in a few of the cores, are noted to be significantly longer than the ages; the clusters

Rebecca A. W. Elson; S. Michael Fall; Kenneth C. Freeman

1987-01-01

188

Acneiform facial eruptions  

PubMed Central

OBJECTIVE To summarize clinical recognition and current management strategies for four types of acneiform facial eruptions common in young women: acne vulgaris, rosacea, folliculitis, and perioral dermatitis. QUALITY OF EVIDENCE Many randomized controlled trials (level I evidence) have studied treatments for acne vulgaris over the years. Treatment recommendations for rosacea, folliculitis, and perioral dermatitis are based predominantly on comparison and open-label studies (level II evidence) as well as expert opinion and consensus statements (level III evidence). MAIN MESSAGE Young women with acneiform facial eruptions often present in primary care. Differentiating between morphologically similar conditions is often difficult. Accurate diagnosis is important because treatment approaches are different for each disease. CONCLUSION Careful visual assessment with an appreciation for subtle morphologic differences and associated clinical factors will help with diagnosis of these common acneiform facial eruptions and lead to appropriate management. PMID:15856972

Cheung, Melody J.; Taher, Muba; Lauzon, Gilles J.

2005-01-01

189

Young and intermediate-age massive star clusters.  

PubMed

An overview of our current understanding of the formation and evolution of star clusters is given, with the main emphasis on high-mass clusters. Clusters form deeply embedded within dense clouds of molecular gas. Left-over gas is cleared within a few million years and, depending on the efficiency of star formation, the clusters may disperse almost immediately or remain gravitationally bound. Current evidence suggests that a small percentage of star formation occurs in clusters that remain bound, although it is not yet clear whether this fraction is truly universal. Internal two-body relaxation and external shocks will lead to further, gradual dissolution on time scales of up to a few hundred million years for low-mass open clusters in the Milky Way, while the most massive clusters (>10(5) M(o)) have lifetimes comparable to or exceeding the age of the Universe. The low-mass end of the initial cluster mass function is well approximated by a power-law distribution, dN/dM proportional to M(-2), but there is mounting evidence that quiescent spiral discs form relatively few clusters with masses M > 2 x 10(5) M(o). In starburst galaxies and old globular cluster systems, this limit appears to be higher, at least several x10(6) M(o). The difference is likely related to the higher gas densities and pressures in starburst galaxies, which allow denser, more massive giant molecular clouds to form. Low-mass clusters may thus trace star formation quite universally, while the more long-lived, massive clusters appear to form preferentially in the context of violent star formation. PMID:20083510

Larsen, Søren S

2010-02-28

190

Distances with <4% precision from type Ia supernovae in young star-forming environments  

NASA Astrophysics Data System (ADS)

The luminosities of type Ia supernovae (SNe), the thermonuclear explosions of white-dwarf stars, vary systematically with their intrinsic color and the rate at which they fade. From images taken with the Galaxy Evolution Explorer (GALEX), we identified SNe Ia that erupted in environments that have high ultraviolet surface brightness and star-formation surface density. When we apply a steep model extinction law, we calibrate these SNe using their broadband optical light curves to within ~0.065 to 0.075 magnitude, corresponding to <4% in distance. The tight scatter, probably arising from a small dispersion among progenitor ages, suggests that variation in only one progenitor property primarily accounts for the relationship between their light-curve widths, colors, and luminosities.

Kelly, Patrick L.; Filippenko, Alexei V.; Burke, David L.; Hicken, Malcolm; Ganeshalingam, Mohan; Zheng, WeiKang

2015-03-01

191

Distances with <4% precision from type Ia supernovae in young star-forming environments.  

PubMed

The luminosities of type Ia supernovae (SNe), the thermonuclear explosions of white-dwarf stars, vary systematically with their intrinsic color and the rate at which they fade. From images taken with the Galaxy Evolution Explorer (GALEX), we identified SNe Ia that erupted in environments that have high ultraviolet surface brightness and star-formation surface density. When we apply a steep model extinction law, we calibrate these SNe using their broadband optical light curves to within ~0.065 to 0.075 magnitude, corresponding to <4% in distance. The tight scatter, probably arising from a small dispersion among progenitor ages, suggests that variation in only one progenitor property primarily accounts for the relationship between their light-curve widths, colors, and luminosities. PMID:25814580

Kelly, Patrick L; Filippenko, Alexei V; Burke, David L; Hicken, Malcolm; Ganeshalingam, Mohan; Zheng, WeiKang

2015-03-27

192

A new sub-stellar companion around the young star HD 284149  

E-print Network

Even though only a handful of sub-stellar companions have been found via direct imaging, each of these discoveries has had a tremendous impact on our understanding of the star formation process and the physics of cool atmospheres. Young stars are prime targets for direct imaging searches for planets and brown dwarfs, due to the favorable brightness contrast expected at such ages and also because it is often possible to derive relatively good age estimates for these primaries. Here we present the direct imaging discovery of HD 284149 b, a 18-50 M_Jup companion at a projected separation of 400 AU from a young (25 Myr) F8 star, with which it shares common proper motion

Bonavita, Mariangela; Desidera, Silvano; Jayawardhana, Ray; Janson, Markus; Lafreniere, David

2014-01-01

193

Young stars and brown dwarfs surrounding Alnilam (eps Ori) and Mintaka (del Ori)  

E-print Network

Aims: We look for new regions for the search of substellar objects. Methods: Two circular areas, 45 arcmin-radius each, centred on the young massive star systems Alnilam and Mintaka in the Orion Belt, have been explored. The regions are very young (less than 10 Ma), have low extinction, and are neighbours to sigma Orionis (~3 Ma), a young open cluster very rich in brown dwarfs and planetary-mass objects. We have used Virtual Observatory tools, the astro-photometric Tycho-2, DENIS and 2MASS catalogues, 10 control fields at similar galactic latitudes, and X-ray, mid-infrared and spectroscopic data from the literature. Results: We have compiled exhaustive lists of known young stars and new candidate members in the Ori OB1b association, and of fore- and background sources. A total of 136 stars display features of extreme youth, like early spectral types, lithium in absorption, or mid-infrared flux excess. Other two young brown dwarf and 289 star candidates have been identified from an optical/near-infrared colour-magnitude diagram. We list additional 74 known objects that might belong to the association. This catalogue can serve as an input for characterisation of the stellar and high-mass substellar populations in the Orion Belt. Finally, we have investigated the surface densities and radial distributions of young objects surrounding Alnilam and Mintaka, and compared them with those in the sigma Orionis cluster. We report a new open cluster centred on Mintaka. Conclusions: Both regions can be analogs to the sigma Orionis cluster, but more massive, more extended, slightly older, and less radially concentrated.

J. A. Caballero; E. Solano

2008-04-14

194

SIM PlanetQuest Key Project Precursor Observations to Detect Gas Giant Planets Around Young Stars  

NASA Technical Reports Server (NTRS)

We present a review of precursor observing programs for the SIM PlanetQuest Key project devoted to detecting Jupiter mass planets around young stars. In order to ensure that the stars in the sample are free of various sources of astrometric noise that might impede the detection of planets, we have initiated programs to collect photometry, high contrast images, interferometric data and radial velocities for stars in both the Northern and Southern hemispheres. We have completed a high contrast imaging survey of target stars in Taurus and the Pleiades and found no definitive common proper motion companions within one arcsecond (140 AU) of the SIM targets. Our radial velocity surveys have shown that many of the target stars in Sco-Cen are fast rotators and a few stars in Taurus and the Pleiades may have sub-stellar companions. Interferometric data of a few stars in Taurus show no signs of stellar or sub-stellar companions with separations of <5 mas. The photometric survey suggests that approximately half of the stars initially selected for this program are variable to a degree (1(sigma) >0.1 mag) that would degrade the astrometric accuracy achievable for that star. While the precursor programs are still a work in progress, we provide a comprehensive list of all targets ranked according to their viability as a result of the observations taken to date. By far, the observable that removes the most targets from the SIM-YSO program is photometric variability.

Tanner, Angelle; Beichman, Charles; Akeson, Rachel; Ghez, Andrea; Grankin, Konstantin N.; Herbst, William; Hillenbrand, Lynne; Huerta, Marcos; Konopacky, Quinn; Metchev, Stanimir; Mohanty, Subhanjoy; Prato, L.; Simon, Michal

2008-01-01

195

Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214  

NASA Astrophysics Data System (ADS)

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of MV=-14.45, with a half-light radius of 161 pc, and if it is a single star cluster, it would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~4-5 Myr, its derived mass is less than 1.3×106Msolar. Thus, the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intracluster media in the distant and younger universe.

Tran, H. D.; Sirianni, M.; Ford, H. C.; Illingworth, G. D.; Clampin, M.; Hartig, G.; Becker, R. H.; White, R. L.; Bartko, F.; Benítez, N.; Blakeslee, J. P.; Bouwens, R.; Broadhurst, T. J.; Brown, R.; Burrows, C.; Cheng, E.; Cross, N.; Feldman, P. D.; Franx, M.; Golimowski, D. A.; Gronwall, C.; Infante, L.; Kimble, R. A.; Krist, J.; Lesser, M.; Magee, D.; Martel, A. R.; McCann, Wm. J.; Meurer, G. R.; Miley, G.; Postman, M.; Rosati, P.; Sparks, W. B.; Tsvetanov, Z.

2003-03-01

196

ACS and Keck Observations of Young Star Clusters in the Interacting Galaxy UGC 10214  

NASA Astrophysics Data System (ADS)

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~ 3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of MV = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < ~ 1.3 x 106 M? . Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations in the tail lie at large projected distances from the nucleus and occur at the edges of low surface brightness gaps in the tail, providing strong evidence that the star formation begins in the tail long after it was ejected. ACS was developed under NASA contract NAS 5-32864, and this research is supported by NASA grant NAG5-7697. We are grateful for an equipment grant from the Sun Microsystems, Inc.

Tran, H. D.; Sirianni, M.; Ford, H. C.; Illingworth, G. D.; Clampin, M.; Hartig, G.; Becker, R. H.; White, R. L.; Bartko, F.; Benitez, N.; Blakeslee, J. P.; Bouwens, R.; Broadhurst, T.; Brown, R.; Burrows, C.; Cheng, E.; Cross, N.; Feldman, P. D.; Franx, M.; Golimowski, D. A.; Gronwall, C.; Infante, L.; Kimble, R.; Krist, J.; Lesser, M.; Magee, D.; Martel, A. R.; McCann, Wm. J.; Meurer, G. R.; Miley, G.; Postman, M.; Rosati, P.; Sparks, W. B.; Tsvetanov, Z.

2002-12-01

197

THE NEARBY, YOUNG, ISOLATED, DUSTY STAR HD 166191  

SciTech Connect

We report an in-depth study of the F8-type star HD 166191, identified in an ongoing survey for stars exhibiting infrared emission above their expected photospheres in the Wide-field Infrared Survey Explorer all-sky catalog. The fractional IR luminosity measured from 3.5 to 70 ?m is exceptionally high (L{sub IR}/L{sub bol} ? 10%). Near-diffraction-limited imaging observations with the T-ReCS Si filter set on the Gemini South telescope and adaptive optics imaging with the NIRC2 Lp filter on the Keck II telescope confirmed that the excess emission coincides with the star. Si-band images show a strong solid-state emission feature at ?10 ?m. Theoretical evolutionary isochrones and optical spectroscopic observations indicate a stellar age in the range 10-100 Myr. The large dust mass seen in HD 166191's terrestrial planet zone is indicative of a recent collision between planetary embryos or massive ongoing collisional grinding associated with planet building.

Schneider, Adam; Song, Inseok; Hufford, Tara [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 (United States); Melis, Carl [Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093 (United States); Zuckerman, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States); Bessell, Mike [Research School of Astronomy and Astrophysics, The Australian National University, Weston Creek, ACT 2611 (Australia); Hinkley, Sasha, E-mail: aschneid@physast.uga.edu, E-mail: song@physast.uga.edu, E-mail: tara@physast.uga.edu, E-mail: cmelis@ucsd.edu, E-mail: ben@astro.ucla.edu, E-mail: bessell@mso.anu.edu.au, E-mail: shinkley@astro.caltech.edu [Department of Astronomy, California Institute of Technology, Pasadena, CA 91125 (United States)

2013-11-01

198

Determining the Locations of Brown Dwarfs in Young Star Clusters  

NASA Technical Reports Server (NTRS)

Brown dwarfs are stellar objects with masses less than 0.08 times that of the Sun that are unable to sustain nuclear fusion. Because of the lack of fusion, they are relatively cold, allowing the formation of methane and water molecules in their atmospheres. Brown dwarfs can be detected by examining stars' absorption spectra in the near-infrared to see whether methane and water are present. The objective of this research is to determine the locations of brown dwarfs in Rho Ophiuchus, a star cluster that is only 1 million years old. The cluster was observed in four filters in the near-infrared range using the Wide-Field Infra-Red Camera (WIRC) on the 100" DuPont Telescope and Persson's Auxiliary Nasymith Infrared Camera (PANIC) on the 6.5-m Magellan Telescope. By comparing the magnitude of a star in each of the four filters, an absorption spectrum can be formed. This project uses standard astronomical techniques to reduce raw frames into final images and perform photometry on them to obtain publishable data. Once this is done, it will be possible to determine the locations and magnitudes of brown dwarfs within the cluster.

Porter, Lauren A.

2005-01-01

199

Young Stars in an Old Bulge: A Natural Outcome of Internal Evolution in the Milky Way  

NASA Astrophysics Data System (ADS)

The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have reported that they span a range of ages, emphasizing that the bulge may not be a monolithic structure. In this Letter we demonstrate that the presence of young stars that are located predominantly nearer to the plane is expected for a bulge that has formed from the disk via dynamical instabilities. Using an N-body+ smoothed particle hydrodynamics simulation of a disk galaxy forming out of gas cooling inside a dark matter halo and forming stars, we find a qualitative agreement between our model and the observations of younger metal-rich stars in the bulge. We are also able to partially resolve the apparent contradiction in the literature between results that argue for a purely old bulge population and those that show a population comprised of a range in ages; the key is where to look.

Ness, M.; Debattista, Victor P.; Bensby, T.; Feltzing, S.; Roškar, R.; Cole, D. R.; Johnson, J. A.; Freeman, K.

2014-06-01

200

Magnetic fields on young, moderately rotating Sun-like stars - I. HD 35296 and HD 29615  

NASA Astrophysics Data System (ADS)

Observations of the magnetic fields of young solar-type stars provide a way to investigate the signatures of their magnetic activity and dynamos. Spectropolarimetry enables the study of these stellar magnetic fields and was thus employed at the Télescope Bernard Lyot and the Anglo-Australian Telescope to investigate two moderately rotating young Sun-like stars, namely HD 35296 (V119 Tau, HIP 25278) and HD 29615 (HIP 21632). The results indicate that both stars display rotational variation in chromospheric indices consistent with their spot activity, with variations indicating a probable long-term cyclic period for HD 35296. Additionally, both stars have complex, and evolving, large-scale surface magnetic fields with a significant toroidal component. High levels of surface differential rotation were measured for both stars. For the F8V star HD 35296 a rotational shear of ?? = 0.22^{+0.04}_{-0.02} rad d- 1 was derived from the observed magnetic profiles. For the G3V star HD 29615, the magnetic features indicate a rotational shear of ?? = 0.48_{-0.12}^{+0.11} rad d- 1, while the spot features, with a distinctive polar spot, provide a much lower value of ?? of 0.07_{-0.03}^{+0.10} rad d- 1. Such a significant discrepancy in shear values between spot and magnetic features for HD 29615 is an extreme example of the variation observed for other lower mass stars. From the extensive and persistent azimuthal field observed for both targets, it is concluded that a distributed dynamo operates in these moderately rotating Sun-like stars, in marked contrast to the Sun's interface-layer dynamo.

Waite, I. A.; Marsden, S. C.; Carter, B. D.; Petit, P.; Donati, J.-F.; Jeffers, S. V.; Boro Saikia, S.

2015-05-01

201

Are Superluminous Supernovae and Long GRBs the Products of Dynamical Processes in Young Dense Star Clusters?  

NASA Astrophysics Data System (ADS)

Superluminous supernovae (SLSNe) occur almost exclusively in small galaxies (Small/Large Magellanic Cloud (SMC/LMC)-like or smaller), and the few SLSNe observed in larger star-forming galaxies always occur close to the nuclei of their hosts. Another type of peculiar and highly energetic supernovae are the broad-line Type Ic SNe (SN Ic-BL) that are associated with long-duration gamma-ray bursts (LGRBs). Also these have a strong preference for occurring in small (SMC/LMC-like or smaller) star-forming galaxies, and in these galaxies LGRBs always occur in the brightest spots. Studies of nearby star-forming galaxies that are similar to the hosts of LGRBs show that these brightest spots are giant H II regions produced by massive dense young star clusters with many hundreds of O- and Wolf-Rayet-type stars. Such dense young clusters are also found in abundance within a few hundred parsecs from the nucleus of larger galaxies like our own. We argue that the SLSNe and the SNe Ic-BL/LGRBs are exclusive products of two types of dynamical interactions in dense young star clusters. In our model the high angular momentum of the collapsing stellar cores required for the engines of an SN Ic-BL results from the post-main-sequence mergers of dynamically produced cluster binaries with almost equal-mass components. The merger produces a critically rotating single helium star with sufficient angular momentum to produce an LGRB; the observed "metal aversion" of LGRBs is a natural consequence of the model. We argue that, on the other hand, SLSNe could be the products of runaway multiple collisions in dense clusters, and we present (and quantize) plausible scenarios of how the different types of SLSNe can be produced.

van den Heuvel, E. P. J.; Portegies Zwart, S. F.

2013-12-01

202

Young open clusters in the Galactic star forming region NGC 6357  

NASA Astrophysics Data System (ADS)

Context. NGC 6357 is an active star forming region with very young massive open clusters. These clusters contain some of the most massive stars in the Galaxy and strongly interact with nearby giant molecular clouds. Aims: We study the young stellar populations of the region and of the open cluster Pismis 24, focusing on their relationship with the nearby giant molecular clouds. We seek evidence of triggered star formation "propagating" from the clusters. Methods: We used new deep JHKs photometry, along with unpublished deep Spitzer/IRAC mid-infrared photometry, complemented with optical HST/WFPC2 high spatial resolution photometry and X-ray Chandra observations, to constrain age, initial mass function, and star formation modes in progress. We carefully examine and discuss all sources of bias (saturation, confusion, different sensitivities, extinction). Results: NGC 6357 hosts three large young stellar clusters, of which Pismis 24 is the most prominent. We found that Pismis 24 is a very young (~1-3 Myr) open cluster with a Salpeter-like initial mass function and a few thousand members. A comparison between optical and infrared photometry indicates that the fraction of members with a near-infrared excess (i.e., with a circumstellar disk) is in the range 0.3-0.6, consistent with its photometrically derived age. We also find that Pismis 24 is likely subdivided into a few different subclusters, one of which contains almost all the massive members. There are indications of current star formation triggered by these massive stars, but clear age trends could not be derived (although the fraction of stars with a near-infrared excess does increase towards the Hii region associated with the cluster). The gas out of which Pismis 24 formed must have been distributed in dense clumps within a cloud of less dense gas ~1 pc in radius. Conclusions: Our findings provide some new insight into how young stellar populations and massive stars emerge, and evolve in the first few Myr after birth, from a giant molecular cloud complex. Based on observations made with ESO telescopes at the La Silla Paranal Observatory under programme ID 63.L-0717.Tables 1 and 2 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/573/A95Appendices are available in electronic form at http://www.aanda.org

Massi, F.; Giannetti, A.; Di Carlo, E.; Brand, J.; Beltrán, M. T.; Marconi, G.

2015-01-01

203

Uncovering the Properties of Young Neutron Stars and their Surrounding Supernova Remnants  

NASA Technical Reports Server (NTRS)

This five-year grant involves the study of young neutron stars, particularly those in supernova remnants.In the fourth year of this program, the following studies have been undertaken in support of this effort: 1.CTA 1: Following up on our ROSAT and ASCA studies of this SNR, we obtained observations with the XMM-Newton observatory to investigate the central compact source and surrounding nebula. 2. 3C 58: Based upon our earlier Chandra observations, we submitted a successful Chandra Large Project proposal for a 350 ks observation of this young neutron star and its wind nebula. 3. G347.3 - - 0.5: Our Chandra observations of portions of this SNR were aimed at studying the nonthermal X-ray emission from the remnant shell. 4. Chandra Survey for Compact Objects in Supernova Remnants: We have formed a collaboration to carry out an extensive search for young neutron stars in nearby supernova remnants. Using X-ray observations from an approved Chandra Large Project, as well as from additional approved XMM observations, we are investigating a volume-limited sample of SNRs for which there is currently no evidence of associated neutron stars.

Oliversen, Ronald J. (Technical Monitor); Slane, Patrick O.

2004-01-01

204

Discovery of close companions to the nearby young stars HD 199143 and HD 358623  

E-print Network

Young stellar systems in the solar neighborhood provide valuable laboratories for detailed studies of star and planet formation. The bright F8V star HD 199143 and the Li-rich late-type emission line star HD 358623 are among the nearest young stars identified to date, and may be members of a young association in Capricornus. We present high-resolution near-infrared images of these two sources, obtained using the adaptive optics system on the 3.6-meter telescope at the European Southern Observatory in La Silla, Chile. Our observations reveal that both are in fact close binary systems. The newly discovered companion at a separation of $\\sim$1'' may account for the unusual characteristics of HD 199143 --rapid rotation, emission lines, ultraviolet variability, and excess infrared emission-- recently discussed by van den Ancker and co-workers. HD 199143 may be a rare example of a close binary with only a circum{\\it secondary} disk. With the detection of a $\\sim$2'' companion, HD 358623 is now possibly one of the closest known T Tauri binaries. Both binary systems are prime targets for follow-up spectroscopic and astrometric observations.

Ray Jayawardhana; Alexis Brandeker

2001-09-17

205

DD 13 - A very young and heavily reddened early O star in the Large Magellanic Cloud  

NASA Technical Reports Server (NTRS)

This paper investigates the Large Magellanic Cloud star DD 13, which is likely the major ionizing source of the nebula N159A. New optical spectroscopy and new estimates of the broadband photometric properties of DD 13 are obtained. A spectral type of O3-O6 V, E(B-V) = 0.64, and M(V) = -6.93 is found. The spectral type cannot be more precisely defined due to contamination of the spectral data by nebular emission, obliterating the important He I classification lines. These results, plus a published estimate of the Lyman continuum photon injection rate into N159A, suggest that DD 13 actually consists of about 2-4 young, early O stars still enshrouded by their natal dust cloud. The star DD 13 may be a younger example of the type of tight cluster represented by the LMC 'star' Sk-66 deg 41, recently revealed to be composed of six or more components.

Conti, Peter S.; Fitzpatrick, Edward L.

1991-01-01

206

Analysis of MOST light curves of five young stars in Taurus-Auriga and Lupus 3 star-forming regions  

NASA Astrophysics Data System (ADS)

Continuous photometric observations of five young stars obtained by the MOST satellite in 2009 and 2010 in the Taurus and Lupus star formation regions are presented. Using light-curve modelling under the assumption of internal invariability of spots, we obtained small values of the solar-type differential-rotation parameter (k = 0.0005-0.009) for three spotted weak-line T Tauri stars, V410 Tau, V987 Tau and Lupus 3-14; for another spotted weak-line T Tauri star (WTTS), Lupus 3-48, the data are consistent with a rigidly rotating surface (k = 0). Three flares of similar rise (4 min and 30 s) and decay (1 h and 45 min) times were detected in the light curve of Lupus 3-14. The brightness of the classical T Tauri star RY Tau continuously decreased over 3 weeks of its observations with a variable modulation not showing any obvious periodic signal. Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Dynacon Inc., the University of Toronto Institute of Aerospace Studies and the University of British Columbia, with the assistance of the University of Vienna.

Siwak, Michal; Rucinski, Slavek M.; Matthews, Jaymie M.; Kuschnig, Rainer; Guenther, David B.; Moffat, Anthony F. J.; Sasselov, Dimitar; Weiss, Werner W.

2011-08-01

207

Statistical fractal analysis of 25 young star clusters  

E-print Network

A large sample of young stellar groups is analysed aiming to investigate their clustering properties and dynamical evolution. A comparison of the Q statistical parameter, measured for the clusters, with the fractal dimension estimated for the projected clouds shows that 52% of the sample has substructures and tends to follow the theoretically expected relation between clusters and clouds, according to calculations for artificial distribution of points. The fractal statistics was also compared to structural parameters revealing that clusters having radial density profile show a trend of parameter s increasing with mean surface stellar density. The core radius of the sample, as a function of age, follows a distribution similar to that observed in stellar groups of Milky Way and other galaxies. They also have dynamical age, indicated by their crossing time that is similar to unbound associations. The statistical analysis allowed us to separate the sample into two groups showing different clustering characteristi...

Gregorio-Hetem, J; Santos-Silva, T; Fernandes, B

2015-01-01

208

Probing the circumstellar environments of very young low-mass stars using water masers  

NASA Technical Reports Server (NTRS)

The VLA is used to search nearby very young low-mass stars for water maser emission. The sample consists of 26 low-luminosity IRAS sources embedded in dense molecular cores, a class of sources suspected to be newly forming low-mass stars on the order of a few hundred thousand years old. Three sources were detected. High spatial resolution maps show the region of maser emission is generally confined to an area smaller than about 0.5 arcsec near the star, and the velocities of individual components span intervals ranging from 20 to 40 km/s. It is inferred from the fact that the maser velocities are too large to be due to gravitational motions in at least two of the sources that the masers are associated with the winds from the young low-mass stars. A comparison of the high spatial resolution maser data to lower-resolution CO data shows no evidence for higher collimation close to the star; the stellar wind cavity appears to have similar collimation at 10 exp 15 cm as at 10 exp 7 to 10 exp 18 cm.

Terebey, S.; Vogel, S. N.; Myers, P. C.

1992-01-01

209

Variability of Young Massive Stars in the Arches Cluster: Accurate Photometry with Adaptive Optics  

E-print Network

We present preliminary results of the first near-infrared variability study of the Arches cluster, using adaptive optics data from NIRI/Gemini and NACO/VLT. The goal is to discover eclipsing binaries in this young (2.5 $\\pm$ 0.5 Myr), dense, massive cluster for which we will determine accurate fundamental parameters with subsequent spectroscopy. Given that the Arches cluster contains more than 200 Wolf-Rayet and O-type stars, it provides a rare opportunity to determine parameters for some of the most massive stars in the Galaxy.

Markakis, K; Pietrzynski, G; Macri, L; Stanek, K Z

2011-01-01

210

Main-sequence Stars Masquerading as Young Stellar Objects in the Central Molecular Zone  

NASA Astrophysics Data System (ADS)

In contrast to most other galaxies, star formation rates in the Milky Way can be estimated directly from young stellar objects (YSOs). In the central molecular zone the star formation rate calculated from the number of YSOs with 24 ?m emission is up to an order of magnitude higher than the value estimated from methods based on diffuse emission (such as free-free emission). Whether this effect is real or whether it indicates problems with either or both star formation rate measures is not currently known. In this paper, we investigate whether estimates based on YSOs could be heavily contaminated by more evolved objects such as main-sequence stars. We present radiative transfer models of YSOs and of main-sequence stars in a constant ambient medium which show that the main-sequence objects can indeed mimic YSOs at 24 ?m. However, we show that in some cases the main-sequence models can be marginally resolved at 24 ?m, whereas the YSO models are always unresolved. Based on the fraction of resolved MIPS 24 ?m sources in the sample of YSOs previously used to compute the star formation rate, we estimate the fraction of misclassified "YSOs" to be at least 63%, which suggests that the star formation rate previously determined from YSOs is likely to be at least a factor of three too high.

Koepferl, Christine M.; Robitaille, Thomas P.; Morales, Esteban F. E.; Johnston, Katharine G.

2015-01-01

211

The Mass Function of Young Star Clusters in the "Antennae" Galaxies.  

PubMed

We determine the mass function of young star clusters in the merging galaxies known as the "Antennae" (NGC 4038/9) from deep images taken with the Wide Field Planetary Camera 2 on the refurbished Hubble Space Telescope. This is accomplished by means of reddening-free parameters and a comparison with stellar population synthesis tracks to estimate the intrinsic luminosity and age, and hence the mass, of each cluster. We find that the mass function of the young star clusters (with ages less, similar160 Myr) is well represented by a power law of the form psi&parl0;M&parr0;~M-2 over the range 104 less, similarM less, similar106 M middle dot in circle. This result may have important implications for our understanding of the origin of globular clusters during the early phases of galactic evolution. PMID:10577944

Zhang; Fall

1999-12-20

212

Statistical fractal analysis of 25 young star clusters  

NASA Astrophysics Data System (ADS)

A large sample of young stellar groups is analysed to investigate their clustering properties and dynamical evolution. A comparison of the Q statistical parameter, measured for the clusters, with the fractal dimension estimated for the projected clouds, shows that 52 per cent of the sample has substructures and tends to follow the theoretically expected relation between clusters and clouds, according to calculations for the artificial distribution of points. The fractal statistics was also compared to structural parameters, revealing that clusters having a radial density profile show a trend of parameter overline{s} increasing with mean surface stellar density. The core radius of the sample, as a function of age, follows a similar distribution to that observed in stellar groups of the Milky Way and other galaxies. They also have dynamical age, indicated by their crossing time, which is similar to unbound associations. The statistical analysis allowed us to separate the sample into two groups showing different clustering characteristics. However, they have the same dynamical evolution, since the whole sample has been revealed as expanding objects, for which the substructures seem to have not been erased. These results are in agreement with simulations that adopt low surface densities and models under supervirial conditions.

Gregorio-Hetem, J.; Hetem, A.; Santos-Silva, T.; Fernandes, B.

2015-04-01

213

Formation of massive black holes through runaway collisions in dense young star clusters  

Microsoft Academic Search

A luminous X-ray source is associated with MGG 11-a cluster of young stars ~200pc from the centre of the starburst galaxy M 82 (refs 1, 2). The properties of this source are best explained by invoking a black hole with a mass of at least 350 solar masses (350Msolar), which is intermediate between stellar-mass and supermassive black holes. A nearby

Simon F. Portegies Zwart; Holger Baumgardt; Piet Hut; Junichiro Makino; Stephen L. W. McMillan

2004-01-01

214

The Mass Function of Young Star Clusters in the ``Antennae'' Galaxies  

Microsoft Academic Search

We determine the mass function of young star clusters in the merging galaxies the ``Antennae'' (NGC 4038\\/39) from deep images taken with the WFPC2 on the refurbished Hubble Space Telescope (HST). This is accomplished by means of reddening-free parameters and a comparison with stellar population synthesis tracks to estimate the intrinsic luminosity and age and hence the mass of each

Qing Zhang; S. Michael Fall

1999-01-01

215

Dynamical masses of young star clusters in NGC 4038\\/4039  

Microsoft Academic Search

In order to estimate the masses of the compact, young star clusters in the merging galaxy pair, NGC 4038\\/4039 (``the Antennae''), we have obtained medium and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of five such clusters. The velocity dispersions were estimated using the stellar absorption features of CO at 2.29 mu m and metal absorption

Sabine Mengel; M. D. Lehnert; Niranjan Thatte; Reinhard Genzel

2002-01-01

216

A dynamical origin for early mass segregation in young star clusters  

Microsoft Academic Search

Some young star clusters show a degree of mass segregation that is\\u000ainconsistent with the effects of standard two-body relaxation from an initially\\u000aunsegregated system without substructure, in virial equilibrium, and it is\\u000aunclear whether current cluster formation models can account for this degree of\\u000ainitial segregation in clusters of significant mass. In this Letter we\\u000ademonstrate that mergers of

Stephen L. W. McMillan; Enrico Vesperini; Simon F. Portegies Zwart

2006-01-01

217

Background X-ray Radiation Fields Produced by Young Embedded Star Clusters  

NASA Astrophysics Data System (ADS)

Most star formation in our galaxy occurs within embedded clusters, and these background environments can affect the star and planet formation processes occurring within them. In turn, young stellar members can shape the background environment and thereby provide a feedback mechanism. This work explores one aspect of stellar feedback by quantifying the background X-ray radiation fields produced by young stellar objects. Specifically, the distributions of X-ray luminosities and X-ray fluxes produced by cluster environments are constructed as a function of cluster membership size N. Composite flux distributions, for given distributions of cluster sizes N, are also constructed. The resulting distributions are wide and the X-ray radiation fields are moderately intense, with the expected flux levels exceeding the cosmic and galactic X-ray backgrounds by factors of ˜10-1000 (for energies 0.2--15 keV). For circumstellar disks that are geometrically thin and optically thick, the X-ray flux from the background cluster dominates that provided by a typical central star in the outer disk where r ? 9-14 AU. In addition, the expectation value of the ionization rate provided by the cluster X-ray background is ?X ˜ 8 × 10-17 s-1, about 4-8 times larger than the canonical value of the ionization rate from cosmic rays. These elevated flux levels in clusters indicate that X-rays can affect ionization, chemistry, and heating in circumstellar disks and in the material between young stellar objects.

Adams, Fred C.; Fatuzzo, Marco; Holden, Lisa

2012-09-01

218

Radiation-driven Warping of Circumbinary Disks around Eccentric Young Star Binaries  

NASA Astrophysics Data System (ADS)

We study a warping instability of a geometrically thin, non-self-gravitating, circumbinary disk around young binary stars on an eccentric orbit. Such a disk is subject to both the tidal torques due to a time-dependent binary potential and the radiative torques due to radiation emitted from each star. The tilt angle between the circumbinary disk plane and the binary orbital plane is assumed to be very small. We find that there is a radius within/beyond which the circumbinary disk is unstable to radiation-driven warping, depending on the disk density and temperature gradient indices. This marginally stable warping radius is very sensitive to viscosity parameters, a fiducial disk radius and the temperature measured there, the stellar luminosity, and the disk surface density at a radius where the disk changes from optically thick to thin for the irradiation from the central stars. On the other hand, it is insensitive to the orbital eccentricity and binary irradiation parameter, which is a function of the binary mass ratio and luminosity of each star. Since the tidal torques can suppress the warping in the inner part of the circumbinary disk, the disk starts to be warped in the outer part. While the circumbinary disks are most likely to be subject to the radiation-driven warping on an AU to kilo-AU scale for binaries with young massive stars more luminous than 104 L ?, the radiation-driven warping does not work for those around young binaries with the luminosity comparable to the solar luminosity.

Hayasaki, Kimitake; Sohn, Bong Won; Okazaki, Atsuo T.; Jung, Taehyun; Zhao, Guangyao; Naito, Tsuguya

2014-12-01

219

The structure of protoplanetary discs around evolving young stars  

NASA Astrophysics Data System (ADS)

The formation of planets with gaseous envelopes takes place in protoplanetary accretion discs on time scales of several million years. Small dust particles stick to each other to form pebbles, pebbles concentrate in the turbulent flow to form planetesimals and planetary embryos and grow to planets, which undergo substantial radial migration. All these processes are influenced by the underlying structure of the protoplanetary disc, specifically the profiles of temperature, gas scale height, and density. The commonly used disc structure of the minimum mass solar nebula (MMSN) is a simple power law in all these quantities. However, protoplanetary disc models with both viscous and stellar heating show several bumps and dips in temperature, scale height, and density caused by transitions in opacity, which are missing in the MMSN model. These play an important role in the formation of planets, since they can act as sweet spots for forming planetesimals via the streaming instability and affect the direction and magnitude of type-I migration. We present 2D simulations of accretion discs that feature radiative cooling and viscous and stellar heating, and they are linked to the observed evolutionary stages of protoplanetary discs and their host stars. These models allow us to identify preferred planetesimal and planet formation regions in the protoplanetary disc as a function of the disc's metallicity, accretion rate, and lifetime. We derive simple fitting formulae that feature all structural characteristics of protoplanetary discs during the evolution of several Myr. These fits are straightforward for applying to modelling any growth stage of planets where detailed knowledge of the underlying disc structure is required. Appendix A is available in electronic form at http://www.aanda.org

Bitsch, Bertram; Johansen, Anders; Lambrechts, Michiel; Morbidelli, Alessandro

2015-03-01

220

Astrometric and spectroscopic surveys to identify new members of young nearby stellar associations and a time sequence study of the young variable star TWA 30  

NASA Astrophysics Data System (ADS)

Young nearby associations of stars are exciting and important observational opportunities to explore the development of young stars and proto-planetary or early planetary systems. The first step in exploring these systems is to locate them. Each group member was born from the same molecular cloud with its own original momentum and most stars born in this cloud will have similar space motions to the cloud which spawned them, at least for the early part of their lives. Since the associations are young, gravitational interactions have not altered the overall space motion of most member stars to the point where the velocity dispersion of the group is too high to kinematically identify members. It is the low velocity dispersions that are exploited to determine potential group membership across a 435 square degree area of the sky. An astrometric study was conducted to determine the position of thousands of stars to high precision such that they may be compared with earlier position measurements of the Two Micron All Sky Survey (2MASS) to determine their proper motions over a six to eight year period. A method of projecting the mean proper motion of a nearby association to any point on the sky has been developed and by comparison to the actual proper motion of a star at that point, individual stars can be ranked as probable or not probable members of separate young nearby associations. The projected proper motion of a group allows the calculation of a probable distance to the star if it were to be a group member. Thus a star's potential 3D spatial position can be compared to existing group members and its correlation with the spatial extent of the group can be determined. Some probable group members determined by their proper motions are studied spectroscopically to determine if their intrinsic brightness (due to projected distance) places them in an acceptable location on a color-magnitude diagram within an acceptable spectral type. If a star meets all of these criteria it is a strong candidate for further study as a potential young nearby association member. A time series spectroscopic study of the young nearby variable star TWA 30 is presented showing a probable circumstellar disk very close to the star. The spectra variations show indications of a disk that is not always along our line of sight and appears to obscure the stellar surface unequally.

Shaw, John David

221

An infrared coronagraphic search for substellar companions to young nearby stars  

NASA Astrophysics Data System (ADS)

This dissertation presents a comprehensive infrared coronagraphic search for substellar companions to stars. The research consists of: (1) a 102 star infrared survey using the Keck telescope, capable of detecting extrasolar brown dwarfs and planets typically more massive than 10 MJupiter , orbiting between about 75 and 300 AU, (2) a 178 star infrared survey at Steward and Lick Observatories, with optical followup from Keck Observatory, capable of detecting companions with masses greater than 30 MJupiter, orbiting between about 120 to 1200 AU and (3) a 24 star survey of two very young stellar associations TW Hydrae and MBM 12, capable of detecting companions down to 2 MJupiter, between about 25 and 1000 AU. These searches are the first to place limits on the frequency of massive planets orbiting beyond 75 AU, and the most sensitive to encompass such a large number of stars. This research resulted in the discovery of one brown dwarf companion, zero planets and 23 double stars. The frequency of brown dwarf companions to G, K & M stars orbiting between 75 and 300 AU is measured to be 1 +/- 1%, the most precise measurement of this quantity to date. The frequency of massive (>30MJupiter) brown dwarf companions is found to be 0.6% +/- 0.6%. The frequency of giant planet companions with masses between 5 and 10 MJupiter orbiting between 75 and 300 AU is measured here for the first time to be no more than 3%. Taken together with results of other searches encompassing a wide range of orbital separations, this research implies that objects with masses between 12 and 75 MJupiter form very rarely as companions to stars. Theories of star formation which could explain these data are only now beginning to emerge.

McCarthy, Chris

2001-08-01

222

Eruptive dermal clear cell desmo-plastic mesenchymal tumors with perivascular myoid differentiation in a young boy. A clinical, histopathologic, immunohistochemical and electron microscopy study of 17 lesions.  

PubMed

Clear cell tumors of the skin are observed in a wide variety of benign and malignant conditions with different histogenesis, sharing the presence of cells with abundant clear cytoplasm. Herein, we report the clinicopathologic features of a healthy young patient affected by asymptomatic, eruptive and disseminated, benign clear cell dermal tumors since early infancy. Neither family history nor genetic testing and counseling provided further useful information. The lesions were mostly confined to the face and lower left extremity with pink teleangiectatic papules and small nodules. Over a 4-year period, a total of 16 different cutaneous lesions were biopsied and histopathologic and immunohistochemical studies carried out; an additional lesion was also removed for electron microscopy examination. Histopathology evidenced multiple perivascular growths of spindle to oval and round cells intermingled with clear/granular cells throughout the dermis, with prominent desmoplasia and numerous capillary-like vessels with focal hemangiopericytoma-like features. Immunohistochemical neoplastic cells were uniformly positive for h-caldesmon and focally smooth muscle ?-actin and CD13 indicating myoid differentiation whereas the consistent diffuse cytoplasmic staining for lysosome antigen, such as CD68PG-M1 and NKI/C3 along with the ultrastructural findings supported the view of a lysosome-mediated apoptotic process. The differential diagnosis with other clear cell cutaneous neoplasms is discussed. PMID:24117956

Tomasini, Carlo; Metze, Dieter; Osella-Abate, Simona; Novelli, Mauro; Kutzner, Heinz

2014-02-01

223

VizieR Online Data Catalog: IC 2391 and Argus young stars (de Silva+, 2013)  

NASA Astrophysics Data System (ADS)

We explore the possible connection between the open cluster IC 2391 and the unbound Argus association identified by the search for associations containing young stars survey. In addition to common kinematics and ages between these two systems, here we explore their chemical abundance patterns to confirm if the two substructures shared a common origin. We carry out a homogeneous high-resolution elemental abundance study of eight confirmed members of IC 2391 as well as six members of the Argus association using UVES spectra. We derive spectroscopic stellar parameters and abundances for Fe, Na, Mg, Al, Si, Ca, Ti, Cr, Ni and Ba. All stars in the open cluster and Argus association were found to share similar abundances with the scatter well within the uncertainties, where [Fe/H]=-0.04+/-0.03 for cluster stars and [Fe/H]=-0.06+/-0.05 for Argus stars. Effects of overionization/excitation were seen for stars cooler than roughly 5200K as previously noted in the literature. Also, enhanced Ba abundances of around 0.6dex were observed in both systems. The common ages, kinematics and chemical abundances strongly support the fact that the Argus association stars originated from the open cluster IC 2391. Simple modelling of this system finds this dissolution to be consistent with two-body interactions. (4 data files).

de Silva, G. M.; D'Orazi, V.; Melo, C.; Torres, C. A. O.; Gieles, M.; Quast, G. R.; Sterzik, M.

2014-01-01

224

Spectroscopic characterization of X-ray emitting young stars associated with the Sh 2-296 nebula  

NASA Astrophysics Data System (ADS)

We studied a sample of stars associated with the Sh 2-296 nebula, part of the reflection nebulae complex in the region of Canis Major R1 (CMa R1). Our sample corresponds to optical counterparts of X-ray sources detected from observations with the XMM-Newton satellite, which revealed dozens of possible low-mass young stars not yet known in this region. A sample of 58 young star candidates were selected based on optical spectral features, mainly H? and lithium lines, observed with multiobjects spectroscopy performed by the Gemini South telescope. Among the candidates, we find 41 confirmed T Tauri and 15 very likely young stars. Based on the H? emission, the T Tauri stars were distinguished between classical (17 per cent) and weak-lined (83 per cent), but no significant difference was found in the age and mass distribution of these two classes. The characterization of the sample was complemented by near- and mid-infrared data, providing an estimate of ages and masses from the comparison with pre-main-sequence evolutionary models. While half of the young stars have an age of 1-2 Myr or less, only a small fraction (˜25 per cent) shows evidence of IR excess revealing the presence of circumstellar discs. This low fraction is quite rare compared to most young star-forming regions, suggesting that some external factor has accelerated the disc dissipation.

Fernandes, B.; Gregorio-Hetem, J.; Montmerle, T.; Rojas, G.

2015-03-01

225

Precise High-cadence Time Series Observations of Five Variable Young Stars in Auriga with MOST  

NASA Astrophysics Data System (ADS)

To explore young star variability on a large range of timescales, we have used the MOST satellite to obtain 24 days of continuous, sub-minute cadence, high-precision optical photometry on a field of classical and weak-lined T Tauri stars (TTSs) in the Taurus-Auriga star formation complex. Observations of AB Aurigae, SU Aurigae, V396 Aurigae, V397 Aurigae, and HD 31305 reveal brightness fluctuations at the 1%-10% level on timescales of hours to weeks. We have further assessed the variability properties with Fourier, wavelet, and autocorrelation techniques, identifying one significant period per star. We present spot models in an attempt to fit the periodicities, but find that we cannot fully account for the observed variability. Rather, all stars exhibit a mixture of periodic and aperiodic behavior, with the latter dominating stochastically on timescales less than several days. After removal of the main periodicity, periodograms for each light curve display power-law trends consistent with those seen for other young accreting stars. Several of our targets exhibited unusual variability patterns not anticipated by prior studies, and we propose that this behavior originates with the circumstellar disks. The MOST observations underscore the need for investigation of TTS light variations on a wide range of timescales in order to elucidate the physical processes responsible; we provide guidelines for future time series observations. Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Systems Canada Inc. (MSCI), formerly part of Dynacon, Inc., the University of Toronto Institute for Aerospace Studies, and the University of British Columbia with the assistance of the University of Vienna.

Cody, Ann Marie; Tayar, Jamie; Hillenbrand, Lynne A.; Matthews, Jaymie M.; Kallinger, Thomas

2013-03-01

226

Star Formation and Young Population of the H II Complex Sh2-294  

NASA Astrophysics Data System (ADS)

The Sh2-294 H II region ionized by a single B0V star features several infrared excess sources, a photodissociation region, and also a group of reddened stars at its border. The star formation scenario in this region seems to be quite complex. In this paper, we present follow-up results of Sh2-294 H II region at 3.6, 4.5, 5.8, and 8.0 ?m observed with the Spitzer Space Telescope Infrared Array Camera (IRAC), coupled with H2 (2.12 ?m) observation, to characterize the young population of the region and to understand its star formation history. We identified 36 young stellar object (YSO, Class I, Class II, and Class I/II) candidates using IRAC color-color diagrams. It is found that Class I sources are preferentially located at the outskirts of the H II region and associated with enhanced H2 emission; none of them are located near the central cluster. Combining the optical to mid-infrared (MIR) photometry of the YSO candidates and using the spectral energy distribution fitting models, we constrained stellar parameters and the evolutionary status of 33 YSO candidates. Most of them are interpreted by the model as low-mass (<4 M ?) YSOs; however, we also detected a massive YSO (~9 M ?) of Class I nature, embedded in a cloud of visual extinction of ~24 mag. Present analysis suggests that the Class I sources are indeed a younger population of the region relative to Class II sources (age ~ 4.5 × 106 yr). We suggest that the majority of the Class I sources, including the massive YSOs, are second-generation stars of the region whose formation is possibly induced by the expansion of the H II region powered by a ~4 × 106 yr B0 main-sequence star.

Samal, M. R.; Pandey, A. K.; Ojha, D. K.; Chauhan, N.; Jose, J.; Pandey, B.

2012-08-01

227

Evolution of X-ray emission from young massive star clusters  

E-print Network

The evolution of X-ray emission from young massive star clusters is modeled, taking into account the emission from the stars as well as from the cluster wind. It is shown that the level and character of the soft (0.2-10 keV) X-ray emission change drastically with cluster age and are tightly linked with stellar evolution. Using the modern X-ray observations of massive stars we show that the correlation between bolometric and X-ray luminosity known for single O stars also holds for O+O and O+Wolf-Rayet (WR) binaries. The diffuse emission originates from the cluster wind heated by the kinetic energy of stellar winds and supernova explosions. To model the evolution of the cluster wind, the mass and energy yields from a population synthesis are used as input to a hydrodynamic model. It is shown that in a very young clusters the emission from the cluster wind is low. When the cluster evolves, WR stars are formed. Their strong stellar winds power an increasing X-ray emission of the cluster wind. Subsequent supernova explosions pump the level of diffuse emission even higher. Clusters at this evolutionary stage may have no X-ray bright stellar point sources, but a relatively high level of diffuse emission. A supernova remnant may become a dominant X-ray source, but only for a short time interval of a few thousand years. We retrieve and analyse Chandra and XMM-Newton observations of six massive star clusters located in the Large Magellanic Cloud. Our model reproduces the observed diffuse and point-source emission from these LMC clusters, as well as from the Galactic clusters Arches, Quintuplet and NGC 3603.

L. M. Oskinova

2005-05-25

228

Chandra Observations of a Young Embedded Magnetic B Star in the p Ophiuchus Cloud  

NASA Technical Reports Server (NTRS)

This paper reports the analysis of two Chandra X-ray observations of the young magnetic B star rho Ophiuchus S1. X-ray emission from the star was detected in both observations. The average flux is almost the same in both, but during each observation the flux shows significant time variations by a factor of two on timescales of 20-40 ksec. Each spectrum can be fit by either an absorbed power law model with a photon index of approx. -3 or a thin-thermal plasma model with a temperature of approx. 2 keV and an extremely low metal abundance (approx. less than 0.1 solar). The spectrum of the first observation has an apparent line feature at about 6.8 keV, which likely corresponds to highly ionized iron K alpha. In contrast, the spectrum of the second observation shows an anomalous edge absorption component at E approx. 1 keV. The continuum emission and log (L(sub X)/L(sub bol)) approx. -6 are similar to those of young intermediate-mass stars (Herbig Ae/Be stars) although the presence of the magnetic field inferred from the detection of non-thermal radio emission has drawn an analogy between rho Ophiuchus S1 and magnetic chemically peculiar (MCP) stars. If the X-ray emission is thermal, the highest plasma temperature observed is too high to be explained by the conventional theories of magnetic stars, and favors some kind of magnetic dynamo activity, while if the emission is nonthermal, it might be related to mass infall. The 6.8 keV line and 4 keV edge features are marginal but they give important information near the stellar body if they are real. Their physical interpretation is discussed.

Hamaguchi, Kenji; Imanishi, Kensuke

2002-01-01

229

Bayesian Analysis to Identify New Star Candidates in Nearby Young Stellar Kinematic Groups  

NASA Astrophysics Data System (ADS)

We present a new method based on a Bayesian analysis to identify new members of nearby young kinematic groups. The analysis minimally takes into account the position, proper motion, magnitude, and color of a star, but other observables can be readily added (e.g., radial velocity, distance). We use this method to find new young low-mass stars in the ? Pictoris and AB Doradus moving groups and in the TW Hydrae, Tucana-Horologium, Columba, Carina, and Argus associations. Starting from a sample of 758 mid-K to mid-M (K5V-M5V) stars showing youth indicators such as H? and X-ray emission, our analysis yields 214 new highly probable low-mass members of the kinematic groups analyzed. One is in TW Hydrae, 37 in ? Pictoris, 17 in Tucana-Horologium, 20 in Columba, 6 in Carina, 50 in Argus, 32 in AB Doradus, and the remaining 51 candidates are likely young but have an ambiguous membership to more than one association. The false alarm rate for new candidates is estimated to be 5% for ? Pictoris and TW Hydrae, 10% for Tucana-Horologium, Columba, Carina, and Argus, and 14% for AB Doradus. Our analysis confirms the membership of 58 stars proposed in the literature. Firm membership confirmation of our new candidates will require measurement of their radial velocity (predicted by our analysis), parallax, and lithium 6708 Å equivalent width. We have initiated these follow-up observations for a number of candidates, and we have identified two stars (2MASSJ01112542+1526214, 2MASSJ05241914-1601153) as very strong candidate members of the ? Pictoris moving group and one strong candidate member (2MASSJ05332558-5117131) of the Tucana-Horologium association; these three stars have radial velocity measurements confirming their membership and lithium detections consistent with young age. Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientique of France, and the University of Hawaii.

Malo, Lison; Doyon, René; Lafrenière, David; Artigau, Étienne; Gagné, Jonathan; Baron, Frédérique; Riedel, Adric

2013-01-01

230

Star formation rates from young-star counts and the structure of the ISM across the NGC 346/N66 complex in the SMC  

NASA Astrophysics Data System (ADS)

The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a comparison of the surface density of young stars (??) and dust surface density (?dust) across NGC 346 (N66) in 115 independent pixels of 6 × 6 pc2. We find a correlation between ?? and ?dust with a considerable scatter. A power-law fit to the data yields a steep relation with an exponent of 2.6 ± 0.2. We convert ?dust to gas surface density (?gas) and ?? to star formation rate (SFR) surface densities (?SFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4 ± 1×10-3 M? yr-1) is consistent with SFR estimated from the H? emission integrated over the H? nebula. On small scales the ?SFR derived using H? systematically underestimates the count-based ?SFR, by up to a factor of 10. This is due to ionizing photons escaping the area, where the stars are counted. We find that individual 36 pc2 pixels fall systematically above integrated disc galaxies in the Schmidt-Kennicutt diagram by on average a factor of ˜7. The NGC 346 average SFR over a larger area (90 pc radius) lies closer to the relation but remains high by a factor of ˜3. The fraction of the total mass (gas plus young stars) locked in young stars is systematically high (˜10 per cent) within the central 15 pc and systematically lower outside (2 per cent), which we interpret as variations in star formation efficiency. The inner 15 pc is dominated by young stars belonging to a centrally condensed cluster, while the outer parts are dominated by a dispersed population. Therefore, the observed trend could reflect a change of star formation efficiency between clustered and non-clustered star formation.

Hony, S.; Gouliermis, D. A.; Galliano, F.; Galametz, M.; Cormier, D.; Chen, C.-H. R.; Dib, S.; Hughes, A.; Klessen, R. S.; Roman-Duval, J.; Smith, L.; Bernard, J.-P.; Bot, C.; Carlson, L.; Gordon, K.; Indebetouw, R.; Lebouteiller, V.; Lee, M.-Y.; Madden, S. C.; Meixner, M.; Oliveira, J.; Rubio, M.; Sauvage, M.; Wu, R.

2015-04-01

231

Uncovering The Properties of Young Neutron Stars and Their Surrounding Supernova A Remnants  

NASA Technical Reports Server (NTRS)

In the third year of this program, the following studies have been undertaken in support of this effort: G292.0+1.8: In our previous work on this SNR, we discovered a young neutron star and its associated pulsar wind nebula. Radio observations by Camilo et al. (2002) have identified a young 136 ms pulsar in the direction of G292.0+1.8. We have used Chandra HRC observations of the central source to identify X-ray pulsations at the same period, thus establishing the neutron star as the radio pulsar counterpart. We have also set limits on the cooling of this young neutron star based on the unpulsed component of the X-ray emission. We find that the limit falls slightly below standard cooling models in which the modified Urca process is responsible for the bulk of the interior neutrino emission. A paper summarizing these results is currently being circulated amongst co-authors for review prior to publication. 3c 58: Our Chandra observations of this Crab-like SNR revealed the presence of a young, rapidly rotating pulsar as well as a central compact nebula which we interpret as a toroidal structure associated with the pulsar wind termination shock. Our modeling of this structure has allowed us to establish a temperature upper limit for the neutron star which falls well below predictions from standard cooling models, and implies the presence of exotic particles (such as pion condensates) or other processes that increase the neutrino production rate in the interior. A paper summarizing this work has been published in the Astrophysical Journal (Slane, Helfand, & Murray 2002, ApJ, 571, L45), and the results were the subject of a NASA Space Science Update (4/10/2002) which led to extensive media coverage. Based upon our initial observations, we submitted a successful Chandra Large Project proposal for a 350 ks observation of this young neutron star and its wind nebula. Kes 79: Our Chandra observations of this SNR reveal a compact central source which appears to be the neutron star formed in the explosion that produced the remnant. There is no evidence for a surrounding pulsar wind nebula. The source properties are similar to the central source in Cas A even though the Kes 79 remnant is considerably older. The results have been published in the Astrophysical Journal (Seward, Slane, Smith, and Sun 2003, ApJ, 584,414). Chandra Survey for Compact Objects in Supernova Remnants: We have formed a collaboration to carry out an extensive search for young neutron stars in nearby supernova remnants. Using X-ray observations from an approved Chandra Large Project, as well as from additional approved XMM observations, we are investigating a volume-limited sample of SNRs for which there is currently no evidence of associated neutron stars. We have obtained extensive optical and 1R data to complement the project, and analysis of these data is currently underway.

Slane, Patrick O.; Oliversen, Ronald J. (Technical Monitor)

2003-01-01

232

KEPLER-63b: A GIANT PLANET IN A POLAR ORBIT AROUND A YOUNG SUN-LIKE STAR  

E-print Network

We present the discovery and characterization of a giant planet orbiting the young Sun-like star Kepler-63 (KOI-63, m [subscript Kp] = 11.6, T [subscript eff] = 5576 K, M [star] = 0.98 M [subscript ?]). The planet transits ...

Marcy, Geoffrey W.

233

A Search for Hot Massive Extrasolar Planets around Nearby Young Stars with the Adaptive Optics System NACO  

Microsoft Academic Search

We report on a survey devoted to the search of exoplanets around young and nearby stars carried out with NACO at the VLT. The detection limit for 28 among the best available targets versus the angular separation from the star is presented. The nondetection of any planetary mass companion in our survey is used to derive, for the first time,

E. Masciadri; R. Mundt; Th. Henning; C. Alvarez; D. Barrado y Navascués

2005-01-01

234

An X-ray census of young stars in the Chamaeleon I North cloud  

E-print Network

Sensitive X-ray imaging surveys provide a new and effective tool to establish the census of pre-main sequence (PMS) stars in nearby young stellar clusters. We report here a deep Chandra X-ray Observatory observation of PMS stars in the Chamaeleon I North cloud, achieving a limiting luminosity of log L_t ~ 27 erg/s (0.5-8 keV band) in a 0.8x0.8 pc region. Of the 107 X-ray sources, 37 are associated with Galactic stars of which 27 are previously recognized cloud members. These include three PMS brown dwarfs; the protostellar brown dwarf ISO 192 has a particularly high level of magnetic activity. Followup optical photometry and spectroscopy establishes that 9-10 of the Chandra sources are probably magnetically active background stars. Several previously proposed cloud members are also inferred to be interlopers due to the absence of X-ray emission at the level expected from the L_t - K correlation. No new X-ray discovered stars were confidently found despite the high sensitivity. From these findings, we argue that the sample of 27 PMS cloud members in the Chandra field is uncontaminated and complete down to K = 12 or M ~ 0.1 Mo. The initial mass function (IMF) derived from our sample is deficient in 0.1-0.3 Mo stars compared to the IMF of the rich Orion Nebula Cluster and other Galactic populations. We can not discriminate whether this is due to different star formation processes, mass segregation, or dynamical ejection of lower mass stars.

Eric D. Feigelson; Warrick A. Lawson

2004-06-23

235

The INfrared Survey of Young Nebulous Clusters (IN-SYNC): Surveying the Dynamics and Star Formation Histories of Young Clusters with APOGEE  

NASA Astrophysics Data System (ADS)

Young clusters are the most prolific sites of star formation in the Milky Way, but demographic studies indicate that relatively few of the Milky Way's stellar clusters persist as bound structures for 100 Myrs or longer. Uniform & precise measurements of the stellar populations and internal dynamics of these regions are difficult to obtain, however, particularly for extremely young clusters whose optical visibility is greatly hampered by their parental molecular cloud. The INfrared Survey of Young Nebulous Clusters (IN-SYNC), an SDSS-III ancillary science program, leverages the stability and multiplex capability of the APOGEE spectrograph to obtain high resolution spectra at near-infrared wavelengths, where photospheric emission is better able to penetrate the dusty shrouds that surround sites of active star formation. We summarize our recent measurements of the kinematics and stellar populations of IC 348 and NGC 1333, two young clusters in the Perseus Molecular Cloud, and of the members of the Orion Nebula Cluster (ONC) and L1641 filament in the Orion molecular complex. These measurements highlight the dynamically 'warm' environment within these young clusters, and suggest a range of stellar radii within these quasi-single-age populations. We close with a preview of plans for continuing this work as part of the APOGEE-2 science portfolio: self-consistent measurements of the kinematics and star formation histories for clusters spanning a range of initial conditions and ages will provide a opportunity to disentangle the mechanisms that drive the formation and dissolution of sites of active star formation.

Covey, Kevin R.; Cottaar, Michiel; Foster, Jonathan B.; Da Rio, Nicola; Tan, Jonathan; Meyer, Michael; Nidever, David L.; Flaherty, Kevin M.; Arce, Hector G.; Rebull, Luisa M.; Chojnowski, S. Drew; Frinchaboy, Peter M.; Hearty, Fred R.; Majewski, Steven R.; Skrutskie, Michael F.; Stassun, Keivan; Wilson, John C.; Zasowski, Gail

2015-01-01

236

Strong biases in estimating the time dependence of mass accretion rates in young stars  

NASA Astrophysics Data System (ADS)

The temporal decay of mass accretion in young stars is a fundamental tracer of the early evolution of circumstellar discs. Through population syntheses, we study how correlated uncertainties between the estimated parameters of young stars (luminosity, temperature, mass and age) and mass accretion rates dot{M}_acc, as well as observational selection effects, can bias the temporal decay of mass accretion rates (dot{M}_acc? t^{-? }) inferred from a comparison of measured dot{M}_acc with isochronal ages in young stellar clusters. We find that the presence of realistic uncertainties reduces the measured value of ? by up to a factor of 3, leading to the inference of shallower decays than the true value. This suggests a much faster temporal decay of dot{M}_acc than generally assumed. When considering the minimum uncertainties in ages affecting the Orion Nebula Cluster, the observed value ? ˜ 1.4, typical of Galactic star-forming regions, can only be reproduced if the real decay exponent is ? ? 4. This effect becomes more severe if one assumes that observational uncertainties are larger, as required by some fast star formation scenarios. Our analysis shows that while selection effects due to sample incompleteness do bias ?, they cannot alter this main result and strengthen it in many cases. A remaining uncertainty in our work is that it applies to the most commonly used and simple relationship between dot{M}_acc, the accretion luminosity and the stellar parameters. We briefly explore how a more complex interplay between these quantities might change the results.

Da Rio, N.; Jeffries, R. D.; Manara, C. F.; Robberto, M.

2014-04-01

237

Be phenomenon in open clusters: Results from a survey of emission-line stars in young open clusters  

E-print Network

Emission-line stars in young open clusters are identified to study their properties, as a function of age, spectral type and their evolutionary state. 207 open star clusters were observed using slitless spectroscopy method and 157 emission stars were identified in 42 clusters. We have found 54 new emission-line stars in 24 open clusters, out of which 19 clusters are found to house emission stars for the first time. About 20% clusters harbour emission stars. The fraction of clusters housing emission stars is maximum in both the 0--10 and 20--30 Myr age bin ($\\sim$ 40% each) and in the other age bins, this fraction ranges between 10% -- 25%, upto 80 Myr. We have used optical colour magnitude diagram (CMD) along with Near-IR Colour-Colour diagram (NIR CCDm) to classify the emission stars into Classical Be (CBe) stars and Herbig Be (HBe) stars. Most of the emission stars in our survey belong to CBe class ($\\sim$ 92%) while a few are HBe stars ($\\sim$ 6%) and HAe stars ($\\sim$1%). The CBe stars are located all along the MS in the optical CMDs of clusters of all ages, which indicates that the Be phenomenon is unlikely due to core contraction near the turn-off. Most of the clusters which contain emission stars are found in Cygnus, Perseus & Monoceros region of the Galaxy, which are locations of active star formation. The distribution of CBe stars as a function of spectral type shows peaks at B1-B2 and B6-B7. Our results indicate there could be two mechanisms responsible for the CBe phenomenon. Some are born CBe stars (fast rotators), as indicated by their presence in clusters younger than 10 Myr. Some stars evolve to CBe stars, as indicates by the enhancement in the fraction of clusters with CBe stars in the 20-30 Myr age bin.

Blesson Mathew; Annapurni Subramaniam; Bhuwan Chandra Bhatt

2008-04-09

238

Commissioning COSMOS: Detection of Lithium in Young Stars in Lupus 3 through Multi-Object Spectroscopy  

NASA Astrophysics Data System (ADS)

COSMOS, a multi-object spectrograph and imager, is a new instrument on the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory. In order to demonstrate the instrument's operations during commissioning, we used COSMOS, its red grism and three custom slit masks to conduct a spectroscopic survey of the star-forming core of the Lupus 3 dark cloud in an effort to detect the presence of Lithium in the T Tauri stars that have been previously identified in that region. We detected the Li I 6708 Angstrom resonance transition in several (but not all) stars that were observed, consistent with prior studies that have observed Lithium in other young stars at the center of the Lupus 3 dark cloud and in other star-forming regions. These results also demonstrate the ability of COSMOS to significantly reduce the time required to complete spectroscopic surveys, relative to single-object instruments.Lackey was supported by the NOAO/KPNO Research Experiences for Undergraduates (REU) Program which is funded by the National Science Foundation Research Experiences for Undergraduates Program (AST-1262829).

Lackey, Kyle; Briceno, Cesar; Elias, Jonathan H.

2015-01-01

239

IDENTIFYING NEARBY, YOUNG, LATE-TYPE STARS BY MEANS OF THEIR CIRCUMSTELLAR DISKS  

SciTech Connect

It has recently been shown that a significant fraction of late-type members of nearby, very young associations (age {approx}<10 Myr) display excess emission at mid-IR wavelengths indicative of dusty circumstellar disks. We demonstrate that the detection of mid-IR excess emission can be utilized to identify new nearby, young, late-type stars including two definite new members ('TWA 33' and 'TWA 34') of the TW Hydrae Association (TWA). Both new TWA members display mid-IR excess emission in the Wide-field Infrared Survey Explorer catalog and they show proper motion and youthful spectroscopic characteristics-namely, H{alpha} emission, strong lithium absorption, and low surface gravity features consistent with known TWA members. We also detect mid-IR excess-the first unambiguous evidence of a dusty circumstellar disk-around a previously identified UV-bright, young, accreting star (2M1337) that is a likely member of the Lower-Centaurus Crux region of the Scorpius-Centaurus Complex.

Schneider, Adam; Song, Inseok [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602 (United States); Melis, Carl [Center for Astrophysics and Space Sciences, University of California, San Diego, CA 92093 (United States); Zuckerman, B. [Department of Physics and Astronomy, University of California, Los Angeles, CA, 90095 (United States); Bessell, Mike, E-mail: aschneid@physast.uga.edu, E-mail: song@physast.uga.edu, E-mail: cmelis@ucsd.edu, E-mail: ben@astro.ucla.edu, E-mail: bessell@mso.anu.edu.au [Research School of Astronomy and Astrophysics, The Australian National University, Weston Creek, ACT 2611 (Australia)

2012-10-01

240

Spectral evolution and dust mixing in the recently erupted disk of EXLup  

NASA Astrophysics Data System (ADS)

This proposal is a follow-up of our previous Spitzer DDT PID 477, where we observed the young eruptive star EX Lup during its outburst, and detected on-going silicate crystal formation for the first time in a young eruptive star. We argue that spectral evolution at mid-infrared wavelengths takes place also in the fading phase of the outburst, and we request two new IRS observations to complete our spectral monitoring program and document the spectral changes. The data will help to characterize the newly formed crystal population, and provide important insight into the mixing processes in the disk. Spitzer/IRS is the only instrument we could use for our program. We request 0.5 hours observing time. Since the propose date of the first observation is early October 2008, this is a time-critical program.

Abraham, Peter; Bouwman, Jeroen; Dullemond, Cornelis; Henning, Thomas; Juhasz, Attila; Kospal, Agnes; Moor, Attila; Mosoni, Laszlo; Sicilia Aguilar, Aurora; Sipos, Nikoletta

2008-09-01

241

A double-lined spectroscopic orbit for the young star HD 34700  

E-print Network

We report high-resolution spectroscopic observations of the young star HD 34700, which confirm it to be a double-lined spectroscopic binary. We derive an accurate orbital solution with a period of 23.4877 +/- 0.0013 days and an eccentricity of e = 0.2501 +/- 0.0068. The stars are found to be of similar mass (M2/M1 = 0.987 +/- 0.014) and luminosity. We derive also the effective temperatures (5900 K and 5800 K) and projected rotational velocities (28 km/s and 22 km/s) of the components. These values of v sin i are much higher than expected for main-sequence stars of similar spectral type (G0), and are not due to tidal synchronization. We discuss also the indicators of youth available for the object. Although there is considerable evidence that the system is young --strong infrared excess, X-ray emission, Li I 6708 absorption (0.17 Angstroms equivalent width), H alpha emission (0.6 Angstroms), rapid rotation-- the precise age cannot yet be established because the distance is unknown.

Guillermo Torres

2003-11-07

242

Direct Measurement of Interstellar Extinction toward Young Stars Using Atomic Hydrogen Ly? Absorption  

NASA Astrophysics Data System (ADS)

Interstellar reddening corrections are necessary to reconstruct the intrinsic spectral energy distributions (SEDs) of accreting protostellar systems. The stellar SED determines the heating and chemical processes that can occur in circumstellar disks. Measurement of neutral hydrogen absorption against broad Ly? emission profiles in young stars can be used to obtain the total H I column density (N(H I)) along the line of sight. We measure N(H I) with new and archival ultraviolet observations from the Hubble Space Telescope (HST) of 31 classical T Tauri and Herbig Ae/Be stars. The H I column densities range from log10(N(H I)) ?19.6-21.1, with corresponding visual extinctions of AV =0.02-0.72 mag, assuming an RV of 3.1. We find that the majority of the H I absorption along the line of sight likely comes from interstellar rather than circumstellar material. Extinctions derived from new HST blue-optical spectral analyses, previous IR and optical measurements, and new X-ray column densities on average overestimate the interstellar extinction toward young stars compared to the N(H I) values by ~0.6 mag. We discuss possible explanations for this discrepancy in the context of a protoplanetary disk geometry.

McJunkin, Matthew; France, Kevin; Schneider, P. C.; Herczeg, Gregory J.; Brown, Alexander; Hillenbrand, Lynne; Schindhelm, Eric; Edwards, Suzan

2014-01-01

243

Millimeter continuum measurements of circumstellar dust around very young low-mass stars  

NASA Technical Reports Server (NTRS)

We investigate the question of disk formation during the protostar phase. We build on the results of Keene and Masson (1990) whose analysis of L1551 showed the millimeter continuum emission comes from both an unresolved circumstellar component, i.e., a disk and an extended cloud core. We model the dust continuum emission from the cloud core and show how it is important at 1.3 mm but negligible at 2.7 mm. Combining new 2.7 mm Owens Valley Interferometer data of IRAS-Dense cores with data from the literature we conclude that massive disks are also seen toward a number of other sources. However, 1.3 mm data from the IRAM 30 m telescope for a larger sample shows that massive disks are relatively rare, occurring around perhaps 5% of young embedded stars. This implies that either massive disks occur briefly during the embedded phase or that relatively few young stars form massive disks. At 1.3 mm the median flux of IRAS-Dense cores is nearly the same as T Tauri stars in the sample of Beckwith et al. (1990). We conclude that the typical disk mass during the embedded phase is nearly the same or less than the typical disk mass during the T Tauri phase.

Terebey, S.; Chandler, C. J.; Andre, P.

1994-01-01

244

X-Ray and Infrared Observations of Embedded Young Stars in NGC 2264  

NASA Technical Reports Server (NTRS)

Images of the NGC 2264 star-forming region, which we have acquired with the XMM-Newton spacecraft, reveal strong X-ray emission from three deeply embedded (Av > 10 mag) young stellar objects in the vicinity of Allen's infrared source (AFGL 989 = IRS 1) and Castelaz & Grasdalen s infrared source (RNO-EW = IRS 2). Thermal plasma models for the brightest source in X-rays, located 11 southwest of Allen's star, yield a quasi-steady luminosity of Lx = 10 ergs s-1 and an extraordinarily high X-ray temperature of 100 MK. The high temperature is consistent with the presence of emission lines of Fe xxv and Fe xxvi at photon energies of 6.7 and 6.9 keV, respectively. An even higher temperature of nearly 140 MK was observed during the rise phase of a powerful impulsive X-ray flare of another young star in the IRS 2 region. Moderate-resolution near-infrared (1-4 um) spectra of the embedded objects, obtained at the NASA Infrared Telescope Facility, exhibit deep water ice absorption bands, as well as a variety of emission and absorption features of H I, CO, and both neutral and ionized metals.

Simon, Theordore; Dahm, S. E.

2005-01-01

245

Warm Gas in the Inner Disks around Young Intermediate Mass Stars  

E-print Network

The characterization of gas in the inner disks around young stars is of particular interest because of its connection to planet formation. In order to study the gas in inner disks, we have obtained high-resolution K-band and M-band spectroscopy of 14 intermediate mass young stars. In sources that have optically thick inner disks, i.e. E(K-L)>1, our detection rate of the ro-vibrational CO transitions is 100% and the gas is thermally excited. Of the five sources that do not have optically thick inner disks, we only detect the ro-vibrational CO transitions from HD 141569. In this case, we show that the gas is excited by UV fluorescence and that the inner disk is devoid of gas and dust. We discuss the plausibility of the various scenarios for forming this inner hole. Our modeling of the UV fluoresced gas suggests an additional method by which to search for and/or place stringent limits on gas in dust depleted regions in disks around Herbig Ae/Be stars.

Sean Brittain; Theodore Simon; Joan Najita; Terrence Rettig

2006-12-08

246

Magnetocentrifugally driven flows from young stars and disks. 1: A generalized model  

NASA Technical Reports Server (NTRS)

We propose a generalized model for stellar spin-down, disk accretion, and truncation, and the origin of winds, jets, and bipolar outflows from young stellar objects. We consider the steady state dynamics of accretion of matter from a viscous and imperfectly conducting disk onto a young star with a strong magnetic field. For an aligned stellar magnetosphere, shielding currents in the surface layers of the disk prevent stellar field lines from penetrating the disk everywhere except for a range of radii about pi = R(sub x), where the Keplerian angular speed of rotation Omega(sub x) equals the angular speed of the star Omega(sub *). For the low disk accretion rates and high magnetic fields associated with typical T Tauri stars, R(sub x) exceeds the radius of the star R(sub *) by a factor of a few, and the inner disk is effectively truncated at a radius R(sub t) somewhat smaller than R(sub x). Where the closed field lines between R(sub t) and R(sub x) bow sufficiently inward, the accreting gas attaches itself to the field and is funneled dynamically down the effective potential (gravitational plus centrifugal) onto the star. Contrary to common belief, the accompanying magnetic torques associated with this accreting gas may transfer angular momentum mostly to the disk rather than to the star. Thus, the star can spin slowly as long as R(sub x) remains significantly greater than R(sub *). Exterior to R(sub x) field lines threading the disk bow outward, which makes the gas off the mid-plane rotate at super-Keplerian velocities. This combination drives a magnetocentrifugal wind with a mass-loss rate M(sub w) equal to a definite fraction f of the disk accretion rate M(sub D). For high disk accretion rates, R(sub x) is forced down to the stellar surface, the star is spun to breakup, and the wind is generated in a manner identical to that proposed by Shu, Lizano, Ruden, & Najita in a previous communication to this journal. In two companion papers (II and III), we develop a detailed but idealized theory of the magnetocentrifugal acceleration process.

Shu, Frank; Najita, Joan; Ostriker, Eve; Wilkin, Frank; Ruden, Steven; Lizano, Susana

1994-01-01

247

Episodic formation of cometary material in the outburst of a solar-like young star  

E-print Network

Our Solar System originated in interstellar gas and dust; the latter is in the form of amorphous silicate particles and carbonaceous dust. The composition of cometary material shows that a significant fraction of the amorphous silicates was transformed into crystalline form during the early evolution of the protosolar nebula. How and when this transformation happened has been controversial, with the main options being heating by the young Sun or shock heating. Here we report mid-infrared features in the outburst spectrum of the young solar-like star EX Lupi that were not present in quiescence. We attribute them to crystalline forsterite; the crystals were produced via thermal annealing in the surface layer of the inner disk by heat from the outburst, a process that has hitherto not been considered. The observed lack of cold crystals excludes shock heating at larger radii.

Ábrahám, P; Dullemond, C P; Kóspál, Á; Van Boekel, R; Bouwman, J; Henning, T; Moór, A; Mosoni, L; Sicilia-Aguilar, A; Sipos, N; 10.1038/nature08004

2009-01-01

248

NEAR-INFRARED VARIABILITY IN YOUNG STARS IN CYGNUS OB7  

SciTech Connect

We present the first results from a 124 night J, H, K near-infrared monitoring campaign of the dark cloud L 1003 in Cygnus OB7, an active star-forming region. Using three seasons of UKIRT observations spanning 1.5 years, we obtained high-quality photometry on 9200 stars down to J = 17 mag, with photometric uncertainty better than 0.04 mag. On the basis of near-infrared excesses from disks, we identify 30 pre-main-sequence stars, including 24 which are newly discovered. We analyze those stars and find that the NIR excesses are significantly variable. All 9200 stars were monitored for photometric variability; among the field star population, {approx}160 exhibited near-infrared variability (1.7% of the sample). Of the 30 young stellar objects (YSOs), 28 of them (93%) are variable at a significant level. Of the 30 YSOs, twenty-five have near-infrared excess consistent with simple disk-plus-star classical T Tauri models. Nine of these (36%) drift in color space over the course of these observations and/or since Two Micron All Sky Survey observations such that they cross the boundary defining the NIR excess criteria; effectively, they have a transient near-infrared excess. Thus, time-series JHK observations can be used to obtain a more complete sample of disk-bearing stars than single-epoch JHK observations. About half of the YSOs have color-space variations parallel to either the classical T Tauri star locus or a hybrid track which includes the dust reddening trajectory. This indicates that the NIR variability in YSOs that possess accretion disks arises from a combination of variable extinction and changes in the inner accretion disk: either in accretion rate, central hole size, and/or the inclination of the inner disk. While some variability may be due to stellar rotation, the level of variability on the individual stars can exceed a magnitude. This is a strong empirical suggestion that protoplanetary disks are quite dynamic and exhibit more complex activity on short timescales than is attributable to rotation alone or captured in static disk models.

Rice, Thomas S. [Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138 (United States); Wolk, Scott J. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Aspin, Colin [Institute for Astronomy, University of Hawaii at Manoa, 640 N Aohoku Pl, Hilo, HI 96720 (United States)

2012-08-10

249

Bolometric temperature and young stars in the Taurus and Ophiuchus complexes  

NASA Technical Reports Server (NTRS)

We calculated bolometric temperature (T(sub bol)) and luminosity (L(sub bol)) for 128 young stellar objects (YSOs) in Taurus, 74 in the Ophiuchus 'core', and 33 in the Ophiuchus 'off-core' region. We have constructed the bolometric luminosity-temperature (BLT) diagram, the log-log plot of L(sub bol) versus T(sub bol), for the three samples. T(sub bol) is defined as the temperature of a blackbody having the same frequency as the observed continuum spectrum. It measures the redness (or coldness) of an astronomical source. The BLT diagram is analogous to the H-R diagram and allows for a direct and quantitative comparison of YSOs at a wide variety of evolutionary states, ranging from the most deeply embedded stars to T Tauri stars nearly on the main sequence. We found (1) T(sub bol) increases monotonically from embedded sources (approximately 60-500 K) to classical T Tauri stars (approximately 1000-3000 K) to weak-line T Tauri stars (approximately 2000-5000 K); (2) T(sub bol) correlates reasonably well with the age inferred from the evolutionary models of pre-main-sequence stars and protostars for embedded 'protostars' and weak-line T Tauri stars. There is no significant correlation for the classical T Tauri stars. These results can be understood in terms of dissipation of circumstellar dust envelope and disk during the early stages of stellar evolution. Sources in the three regions have different distributions in the BLT diagram. The Ophiuchus core has the highest fraction of cold sources among the three regions. These cold sources are also more luminous than the YSOs in the other regions. The Ophiuchus off-core sample is dominated by the more evolved pre-main-sequence stars. The Taurus sources have distributions intermediate in L(sub bol), T(sub bol), and age between the Ophiuchus core and off-core distributions. These may suggest differences in the star formation history, and possibly in the stellar masses and mass accretion rates in these star-forming regions.

Chen, H.; Myers, P. C.; Ladd, E. F.; Wood, D. O. S.

1995-01-01

250

Eruptive Vent  

USGS Multimedia Gallery

Near-vertical view into the eruptive vent inset within the southeast edge of Halema'uma'u Crater. The crusting, slowly circulating lava surface was about 115 m (375 ft) below the floor of Halema'uma'u....

251

A COMPREHENSIVE GALEX ULTRAVIOLET CATALOG OF STAR CLUSTERS IN M31 AND A STUDY OF THE YOUNG CLUSTERS  

SciTech Connect

We present a comprehensive catalog of 700 confirmed star clusters in the field of M31 compiled from three major existing catalogs. We detect 418 and 257 star clusters in Galaxy Evolution Explorer near-ultraviolet and far-ultraviolet (FUV) imaging, respectively. Our final catalog includes photometry of star clusters in up to 16 passbands ranging from FUV to NIR as well as ancillary information such as reddening, metallicity, and radial velocities. In particular, this is the most extensive and updated catalog of UV-integrated photometry for M31 star clusters. Ages and masses of star clusters are derived by fitting the multi-band photometry with model spectral energy distribution (SED); UV photometry enables more accurate age estimation of young clusters. Our catalog includes 182 young clusters with ages less than 1 Gyr. Our estimated ages and masses of young clusters are in good agreement with previously determined values in the literature. The mean age and mass of young clusters are about 300 Myr and 10{sup 4} M{sub Sun }, respectively. We found that the compiled [Fe/H] values of young clusters included in our catalog are systematically lower (by more than 1 dex) than those from recent high-quality spectroscopic data and our SED-fitting result. We confirm that most of the young clusters' kinematics shows systematic rotation around the minor axis and association with the thin disk of M31. The young cluster distribution exhibits a distinct peak in the M31 disk around 10-12 kpc from the center and follows a spatial distributions similar to other tracers of disk structure such as OB stars, UV star-forming regions, and dust. Some young clusters also show concentration around the ring splitting regions found in the southern part of the M31 disk and most of them have systematically younger (<100 Myr) ages. Considering the kinematical properties and spatial distribution of young clusters, they might be associated with the well-known 10 kpc star formation ring structure in the M31 disk. Consequently, we suggest that various properties of young clusters in M31 might be in line with the scenarios that a satellite galaxy had passed through the disk of M31 less than few hundred million years ago.

Kang, Yongbeom; Rey, Soo-Chang; Lee, Kyungsook; Kim, YoungKwang [Department of Astronomy and Space Science, Chungnam National University, Daejeon 305-764 (Korea, Republic of); Bianchi, Luciana [Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States); Sohn, Sangmo Tony, E-mail: ybkang@cnu.ac.kr, E-mail: screy@cnu.ac.kr [Space Telescope Science Institute, Baltimore, MD 21218 (United States)

2012-04-01

252

High Angular Resolution Mid-Infrared Imaging of Young Stars in Orion BN/KL  

NASA Technical Reports Server (NTRS)

The authors present Keck LWS images of the Orion BN/KL star forming region obtained in the first multi-wavelength study to have 0.3--0.5 resolution from 4.7 (micro)m to 22 (micro)m. The young stellar objects designed infrared source n and radio source I are believed to dominate the BN/KL region. They have detected extended emission from a probable accretion disk around source n but infer a stellar luminosity on the order of only 2000 L(sub (center-dot)).

Greenhill, L. J.; Gezari, D. Y.; Danchi, W. C.; Najita, J.; Monnier, J. D.

2004-01-01

253

Ring shaped 6.7 GHz methanol maser emission around a young high-mass star  

E-print Network

We report on EVN imaging of the 6.7 GHz methanol maser emission from the candidate high-mass protostar G23.657-0.127. The masers originate in a nearly circular ring of 127 mas radius and 12 mas width. The ring structure points at a central exciting object which characteristics are typical for a young massive star; its bolometric luminosity is estimated to be methanol masers originate in a spherical bubble or in a rotating disc seen nearly face-on.

A. Bartkiewicz; M. Szymczak; H. J. van Langevelde

2005-09-21

254

VizieR Online Data Catalog: Young Massive Star Clusters. II. (Larsen, 1999)  

NASA Astrophysics Data System (ADS)

Table 4 lists photometric data for Young Massive Star Clusters identified in a sample of 21 nearby galaxies. The photometric data have been corrected for Galactic foreground extinction. Each cluster is identified by the abbreviated NGC number of its host galaxy and an object number: nxxx-yyy is object number yyy in the galaxy NGC xxx. Effective cluster radii have been obtained by modeling the cluster images as MOFFAT15 functions convolved with the point-spread function measured on the CCD images. (1 data file).

Larsen, S. S.

1999-07-01

255

A circumstellar molecular gas structure associated with the massive young star Cepheus A-HW 2  

NASA Technical Reports Server (NTRS)

We report the detection via VLA-D observations of ammonia of a circumstellar high-density molecular gas structure toward the massive young star related to the object Cepheus A-HW 2, a firm candidate for the powering source of the high-velocity molecular outflow in the region. We suggest that the circumstellar molecular gas structure could be related to the circumstellar disk previously suggested from infrared, H2O, and OH maser observations. We consider as a plausible scenario that the double radio continuum source of HW 2 could represent the ionized inner part of the circumstellar disk, in the same way as proposed to explain the double radio source in L1551. The observed motions in the circumstellar molecular gas can be produced by bound motions (e.g., infall or rotation) around a central mass of about 10-20 solar masses (B0.5 V star or earlier).

Torrelles, Jose M.; Rodriguez, Luis F.; Canto, Jorge; Ho, Paul T. P.

1993-01-01

256

Economic Development Activities at the Young - Rainey Science, Technology, & Research (STAR) Center  

SciTech Connect

Project mission was to mitigate the adverse economic effects of closing the U.S. Department of Energy's Pinellas Plant in Largo, Florida. This project was to facilitate the physical renovation of the plant and to help maintain and create jobs for the employees that worked at the plant when DOE terminated its operations. It also included finding and attracting high technology, industrial manufacturing and related firms to utilize the space and high tech equipment to remain at the plant. Stakeholders included the affected plant employees, local government and related public organizations, and businesses and universities in the Tampa Bay Florida area. The $17.6 million funded for this project helped produce 2,780 jobs at the Young - Rainey STAR Center at an average cost of $6,328. Rental income from STAR Center tenants and third party cash input amounted to approximately $66 million over the project period of 13.3 years.

Paul S. Sacco; Carl Smeigh; John Caponiti, Jr.

2008-06-30

257

Disks, young stars, and radio waves: the quest for forming planetary systems  

E-print Network

Kant and Laplace suggested the Solar System formed from a rotating gaseous disk in the 18th century, but convincing evidence that young stars are indeed surrounded by such disks was not presented for another 200 years. As we move into the 21st century the emphasis is now on disk formation, the role of disks in star formation, and on how planets form in those disks. Radio wavelengths play a key role in these studies, currently providing some of the highest spatial resolution images of disks, along with evidence of the growth of dust grains into planetesimals. The future capabilities of EVLA and ALMA provide extremely exciting prospects for resolving disk structure and kinematics, studying disk chemistry, directly detecting proto-planets, and imaging disks in formation.

Claire J. Chandler; Debra S. Shepherd

2008-01-24

258

H_alpha emission fluxes and lithium abundances of low mass stars in the young open cluster IC 4665  

E-print Network

As part of a long term effort to understand pre-main sequence Li burning, we have obtained high resolution spectroscopic observations of 14 late type stars (G0--M1) in the young open cluster IC~4665. Most of the stars have \\ha filled-in and \\li absorption, as expected for their young age. From the equivalent widths of \\ha emission excess (obtained using the spectral subtraction technique) and the \\lii feature, we have derived \\ha emission fluxes and photospheric Li abundances. The mean Li abundance of IC~4665 solar-type stars is log N(Li)=3.1; the same as in other young clusters ($\\alpha$~Per, Pleiades) and T Tauri stars. Our results support the conclusions from previous works that PMS Li depletion is very small for masses $\\sim$ 1 \\msun . Among the IC 4665 late-G and early K-type stars, there is a spread in Li abundances of about one order of magnitude. The Li-poor IC~4665 members have low \\ha excess and vsin{\\it i}$\\le$10. Hence, the Li-activity-rotation connection which has been clearly established in the Pleiades also seems to hold in IC 4665. One M-type IC~4665 star that we have observed does not show Li, implying a very efficient Li depletion as observed in $\\alpha$~Per stars of the same spectral type. The level of chromospheric activity and Li depletion among the low mass stars of IC 4665 is similar to that in the Pleiades. In fact, we note that the Li abundance distributions in several young clusters ($\\alpha$~Per, Pleiades, IC~2391, IC~4665) and in post T Tauri stars are strikingly similar. This result suggests that \\ha emission and Li abundance not well correlated with age for low mass stars between 20 and 100 Myr old. We argue that a finer age indicator, the ``LL-clock", would be the luminosity at

E. L. Martin; D. Montes

1996-05-08

259

Angular Momentum Evolution of Young Low-Mass Stars and Brown Dwarfs: Observations and Theory  

NASA Astrophysics Data System (ADS)

This chapter aims at providing the most complete review of both the emerging concepts and the latest observational results regarding the angular momentum evolution of young low-mass stars and brown dwarfs. In the time since Protostars and Planets V (Reipurth et al., 2007) (PPV), there have been major developments in the availability of rotation-period measurements at multiple ages and in different star-forming environments that are essential for testing theory. In parallel, substantial theoretical developments have been carried out in the last few years, including the physics of the star-disk interaction, numerical simulations of stellar winds, and the investigation of angular momentum transport processes in stellar interiors. This chapter reviews both the recent observational and theoretical advances that prompted the development of renewed angular momentum evolution models for cool stars and brown dwarfs. While the main observational trends of the rotational history of low-mass objects seem to be accounted for by these new models, a number of critical open issues remain that are outlined in this review.

Bouvier, J.; Matt, S. P.; Mohanty, S.; Scholz, A.; Stassun, K. G.; Zanni, C.

260

Substantial reservoirs of molecular hydrogen in the debris disks around young stars.  

PubMed

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the 'debris' disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value. PMID:11343110

Thi, W F; Blake, G A; van Dishoeck, E F; van Zadelhoff, G J; Horn, J M; Becklin, E E; Mannings, V; Sargent, A I; van Den Ancker, M E; Natta, A

2001-01-01

261

Substantial reservoirs of molecular hydrogen in the debris disks around young stars  

NASA Technical Reports Server (NTRS)

Circumstellar accretion disks transfer matter from molecular clouds to young stars and to the sites of planet formation. The disks observed around pre-main-sequence stars have properties consistent with those expected for the pre-solar nebula from which our own Solar System formed 4.5 Gyr ago. But the 'debris' disks that encircle more than 15% of nearby main-sequence stars appear to have very small amounts of gas, based on observations of the tracer molecule carbon monoxide: these observations have yielded gas/dust ratios much less than 0.1, whereas the interstellar value is about 100 (ref. 9). Here we report observations of the lowest rotational transitions of molecular hydrogen (H2) that reveal large quantities of gas in the debris disks around the stars beta Pictoris, 49 Ceti and HD135344. The gas masses calculated from the data are several hundreds to a thousand times greater than those estimated from the CO observations, and yield gas/dust ratios of the same order as the interstellar value.

Thi, W. F.; Blake, G. A.; van Dishoeck, E. F.; van Zadelhoff, G. J.; Horn, J. M.; Becklin, E. E.; Mannings, V.; Sargent, A. I.; van Den Ancker, M. E.; Natta, A.

2001-01-01

262

NEW BROWN DWARF COMPANIONS TO YOUNG STARS IN SCORPIUS-CENTAURUS  

SciTech Connect

We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of {approx}40-100 M{sub jup} complements previous work in the same region, reporting detections of similarly wide companions with lower masses, in the range of {approx}10-30 M{sub jup}. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass companions to stars.

Janson, Markus [Department of Astrophysical Sciences, Princeton University, Princeton, NJ (United States); Jayawardhana, Ray; Bonavita, Mariangela [Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON (Canada); Girard, Julien H. [European Southern Observatory, Santiago (Chile); Lafreniere, David [Department of Physics, University of Montreal, Montreal, QC (Canada); Gizis, John [Department of Physics and Astronomy, University of Delaware, Newark, DE (United States); Brandeker, Alexis, E-mail: janson@astro.princeton.edu [Department of Astronomy, Stockholm University, Stockholm (Sweden)

2012-10-10

263

Young Stars in the Camelopardalis Dust and Molecular Clouds. I. The Cam OB1 Association  

E-print Network

The distribution of dust and molecular clouds in the direction of Galactic longitudes 132--158 deg and latitudes pm 12 deg is investigated. The maps of dust distribution in the area were plotted from the following surveys: the star counts in the DSS I database by Dobashi et al. (2005), the survey of the average infrared color excesses by Froebrich et al. (2007) and the thermal dust emission survey at 100 micrometers by Schlegel et al. (1998). The distribution of molecular clouds was taken from the whole sky CO survey by Dame et al. (2001). All these surveys show very similar cloud patterns in the area. Using the radial velocities of CO, the distances to separate clouds are estimated. A revised list of the Cam OB1 association members contains 43 stars and the open cluster NGC 1502. 18 young irregular variable and H alpha emission stars are identified in the area. All this proves that the star forming process in the Camelopardalis clouds is still in progress.

V. Straizys; V. Laugalys

2008-03-31

264

VizieR Online Data Catalog: Probable young stars in the MYStIX project (Broos+, 2013)  

NASA Astrophysics Data System (ADS)

The Massive Young star-forming complex Study in Infrared and X-rays (MYStIX) project, described by Feigelson et al. (2013ApJS..209...26F), seeks to identify and study samples of young stars in 20 nearby (0.4star-forming regions (MSFRs). These samples are derived using X-ray data from the Chandra X-ray Observatory, near-infrared (NIR) photometry from the United Kingdom InfraRed Telescope (UKIRT) and from the Two Micron All Sky Survey (2MASS), mid-infrared (MIR) photometry from the Spitzer Space Telescope, and published spectroscopically identified massive stars. Our purpose here is to describe our efforts to minimize contaminants in the MYStIX catalogs of young stars, which we refer to as "MYStIX Probable Complex Members" (MPCMs), and to present the MPCM catalog for each MYStIX MSFR. An MPCM catalog is the union of three sets of probable members identified by three established methods for identifying young stars (Feigelson et al. 2013ApJS..209...26F, Fig. 3). (3 data files).

Broos, P. S.; Getman, K. V.; Povich, M. S.; Feigelson, E. D.; Townsley, L. K.; Naylor, T.; Kuhn, M. A.; King, R. R.; Busk, H. A.

2014-01-01

265

The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets around Young B and A Stars  

NASA Astrophysics Data System (ADS)

We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around high-mass stars (~1.5-2.5 M?) conducted to date and includes the planet hosts ? Pic and Fomalhaut. Despite detecting two new brown dwarfs, our observations did not detect new planets around our target stars, and we present upper limits on the fraction of high-mass stars that can host giant planets that are consistent with our null result.

Nielsen, Eric L.; Liu, Michael C.; Wahhaj, Zahed; Biller, Beth A.; Hayward, Thomas L.; Hayward

2014-01-01

266

MASS DISTRIBUTIONS OF STARS AND CORES IN YOUNG GROUPS AND CLUSTERS  

SciTech Connect

We investigate the relation of the stellar initial mass function and the dense core mass function (CMF), using stellar masses and positions in 14 well-studied young groups. Initial column density maps are computed by replacing each star with a model initial core having the same star formation efficiency (SFE). For each group the SFE, core model, and observational resolution are varied to produce a realistic range of initial maps. A clump-finding algorithm parses each initial map into derived cores, derived core masses, and a derived CMF. The main result is that projected blending of initial cores causes derived cores to be too few and too massive. The number of derived cores is fewer than the number of initial cores by a mean factor of 1.4 in sparse groups and 5 in crowded groups. The mass at the peak of the derived CMF exceeds the mass at the peak of the initial CMF by a mean factor of 1.0 in sparse groups and 12.1 in crowded groups. These results imply that in crowded young groups and clusters, the mass distribution of observed cores may not reliably predict the mass distribution of protostars that will form in those cores.

Michel, Manon [Formation Interuniversitaire de Physique, Departement de Physique de l'Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris Cedex 05 (France); Kirk, Helen; Myers, Philip C., E-mail: manon.michel@ens.fr, E-mail: hkirk@cfa.harvard.edu, E-mail: pmyers@cfa.harvard.edu [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138 (United States)

2011-07-01

267

Discovery of a Wide Planetary-mass Companion to the Young M3 Star GU Psc  

NASA Astrophysics Data System (ADS)

We present the discovery of a comoving planetary-mass companion ~42'' (~2000 AU) from a young M3 star, GU Psc, a likely member of the young AB Doradus Moving Group (ABDMG). The companion was first identified via its distinctively red i - z color (>3.5) through a survey made with Gemini-S/GMOS. Follow-up Canada-France-Hawaii Telescope/WIRCam near-infrared (NIR) imaging, Gemini-N/GNIRS NIR spectroscopy and Wide-field Infrared Survey Explorer photometry indicate a spectral type of T3.5 ± 1 and reveal signs of low gravity which we attribute to youth. Keck/Adaptive Optics NIR observations did not resolve the companion as a binary. A comparison with atmosphere models indicates T eff = 1000-1100 K and log g = 4.5-5.0. Based on evolution models, this temperature corresponds to a mass of 9-13 M Jup for the age of ABDMG (70-130 Myr). The relatively well-constrained age of this companion and its very large angular separation to its host star will allow its thorough characterization and will make it a valuable comparison for planetary-mass companions that will be uncovered by forthcoming planet-finder instruments such as Gemini Planet Imager and SPHERE.

Naud, Marie-Eve; Artigau, Étienne; Malo, Lison; Albert, Loïc; Doyon, René; Lafrenière, David; Gagné, Jonathan; Saumon, Didier; Morley, Caroline V.; Allard, France; Homeier, Derek; Beichman, Charles A.; Gelino, Christopher R.; Boucher, Anne

2014-05-01

268

IS THE YOUNG STAR CLUSTER NGC 376 DISSOLVING IN THE FIELD OF THE SMALL MAGELLANIC CLOUD?  

SciTech Connect

We use deep images acquired with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope in the filters F555W and F814W to characterize the properties of NGC 376, a young star cluster located in the wing of the Small Magellanic Cloud. Using isochrone fitting we derive for NGC 376 an age of 28 {+-} 7 Myr, in good agreement with previous studies. The high spatial resolution ACS data allow us to determine the center of gravity of the cluster and to construct extended surface brightness and radial density profiles. Neither of these profiles can be fitted with a theoretical model, suggesting that the cluster is not in virial equilibrium. Considering the young age of the cluster, we speculate that the distortion of the radial profiles may be the result of the rapid gas dispersal that follows the initial phase of star formation (SF). The cluster shows clear evidence of dynamical mass segregation. From the properties of the radial profiles and the present-day mass function we conclude that NGC 376 appears to have already lost nearly 90% of its initial stellar mass, probably as a consequence of the sudden gas dispersal that follows the early phase of SF.

Sabbi, E.; Nota, A.; Smith, L. J. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Tosi, M.; Cignoni, M. [Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127, Bologna (Italy); Gallagher, J., E-mail: sabbi@stsci.edu [Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706 (United States)

2011-09-20

269

Young stars in old galaxies - surprising discovery with the world's leading telescopes  

NASA Astrophysics Data System (ADS)

Do elliptical galaxies only contain old stars? One of the challenges of modern astronomy is to understand how galaxies - large systems of stars, gas and dust - form and evolve. When did most of the stars in the Universe form? Did this happen at a very early stage, within a few billion years of the Big Bang? Have a significant number of the stars we now observe formed much more recently? Spectacular collisions between galaxies take place all the time, triggering the formation of thousands or even millions of stars. However, when looking at the Universe as a whole, most of its stars are found in elliptical galaxies whose overall appearance has so far led us to believe that they, and their stars and as well, are old. These elliptical galaxies do shine with the diffuse, reddish glow normally associated with stars that are many thousand million years old. However, what is the underlying mix of stars that produces this elderly appearance? Could a significant number of much younger stars be 'hiding' among the older ones? Detailed observations with the world's premier telescopes have now cast new light on this central question about the behaviour of some of the major building blocks of the Universe. Cosmic paleonthology To break the stellar 'cocktail' in elliptical galaxies down into its different constituents, a team of European and American astronomers observed massive stellar clusters in and around nearby galaxies. These "globular" clusters, so called because of their shape, exist in large numbers around all observed galaxies and form a kind of 'skeleton' within their host galaxies. These 'bones' receive an imprint for every episode of star formation they undergo. By reading the ages of the globular clusters in a galaxy, it is possible to identify the past epoch(s) of active star formation in a galaxy. Reading the imprints and deducing the distribution of ages of the globular clusters, astronomers can reveal when many of the stars in elliptical galaxies formed. This is similar to the way a palaeontologist uses the skeletons of dinosaurs to deduce information about the era in which they lived. A surprising discovery The team combined images of a number of galaxies from Hubble's Wide Field and Planetary Camera 2 with infrared images obtained from the multi-mode ISAAC instrument on the 8.2m VLT Antu telescope at the ESO Paranal Observatory (Chile). To their great surprise, they discovered that many of the globular clusters in one of these galaxies, NGC 4365, a member of the large Virgo cluster of galaxies, were only a few thousand million years old, much younger than most of the other stars in this galaxy (roughly 12 thousand million years old). The astronomers were able to identify three major groups of stellar clusters. There is an old population of clusters of metal-poor stars, some clusters of old but metal-rich stars and now, seen for the first time, a population of clusters with young and metal-rich stars. These results have been fully confirmed by spectroscopic observations made with another of the world's giant telescopes, the 10-metre Keck on Hawaii. "It is a great pleasure to see two projects wholly or partly funded by Europe - VLT and Hubble - work in concert to produce such an important scientific result", says Piero Benvenuti, ESA Hubble Project Scientist. "The synergy between the most advanced ground and space telescopes continues to prove its effectiveness, paving the way to impressive new discoveries that would not otherwise be possible." The discovery of young globular clusters within old galaxies is surprising since the stars in the giant elliptical galaxies were until now believed to have formed during a single period early in the history of the Universe. It is now clear that some of the galaxies may be hiding their true nature and have indeed experienced much more recent periods of major star formation. Notes for editors This press release is issued in coordination between ESA and ESO. The Hubble Space Telescope project is an international cooperation between ESA and NASA. The te

2002-06-01

270

Hubble space telescope observations of young star clusters in NGC-4038/4039, 'the antennae' galaxies  

NASA Technical Reports Server (NTRS)

New, high-resolution images of the disks of NGC 4038/4039 obtained with the Wide Field Camera of the Hubble Space Telescope (HST) are presented. NGC 4038/4039, nicknamed The Antennae, is a prototypical example of a pair of colliding galaxies believed to be at an early stage of a merger. Down to the limiting magnitude of V approximately 23 mag, the HST images reveal a population of over 700 blue pointlike objects within the disks. The mean absolute magnitude of these objects is M(sub V) = -11 mag, with the brightest objects reaching M(sub V) approximately -15. Their mean apparent color indices ar U - V = -0.7 mag and V - 1 = 0.8 mag on the Johnson UVI passband system, while their mean indices corrected for internal reddening are (u - v)(sub 0) = -1.0 mag and (V - I(sub 0) = 0.5. Their mean effective radius, determined from slightly resolved images, is 18 pc (for H(sub 0) = 50 km/s /Mpc). Based on their luminosities and resolution, most of these objects cannot be individual stars, but are likely young compact star clusters. The brighter ones are similar to the objects found in NGC 1275 and NGC 7252, which appear to be young globular clusters formed during recent galazy mergers. Based on their U - V and V - I colors, the brightest, bluest clusters of NGC 4038/4039 appear to be less than 10 Myr old. Most of these bright clusters are relatively tightly clustered themselves, with typically a dozen individual clusters belonging to a complex identified as a giant H II region from ground-based observations. The cluster luminosity function (LF) is approximately a power law, phi(L)dL proportional to L(exp -1.78+/-0.05)dL, with no hint of a turnover at fainter magnitudes. This power-law shape agrees with the LF of Magellanic Cloud clusters and Galactic open clusters, but differs from the LF of old globular cluster systems that is typically Gaussian with a Full Width at Half Maximum (FWHM) of approximately 3 mag. Besides the blue clusters, we also find about a dozen extremely red objects with V - I greater than 3.0. The highest number density of these red objects is found in the SE quadrant, where star formation appears to be most recent. We propose that these objects may be very young star clusters still embedded in their placental dust cocoons.

Whitmore, Bradley C.; Schweizer, Francois

1995-01-01

271

A Wide-field near- and Mid-infrared Census of Young Stars in NGC 6334  

NASA Astrophysics Data System (ADS)

This paper presents a study of the rate and efficiency of star formation in the NGC 6334 star-forming region. We obtained observations at J, H, and Ks taken with the NOAO Extremely Wide-Field Infrared Imager and combined them with observations taken with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope at wavelengths = 3.6, 4.5, 5.8, and 8.0 ?m. We also analyzed previous observations taken at 24 ?m using the Spitzer MIPS camera as part of the MIPSGAL survey. We have produced a point source catalog with >700, 000 entries. We have identified 2283 young stellar object (YSO) candidates, 375 Class I YSOs, and 1908 Class II YSOs using a combination of existing IRAC-based color classification schemes that we have extended and validated to the near-IR for use with warm Spitzer data. We have identified multiple new sites of ongoing star formation activity along filamentary structures extending tens of parsecs beyond the central molecular ridge of NGC 6334. By mapping the extinction, we derived an estimate for the gas mass, 2.2 × 105 M ?. The heavy concentration of protostars along the dense filamentary structures indicates that NGC 6334 may be undergoing a "mini-starburst" event with ?SFR > 8.2 M ? Myr-1 pc-2 and SFE > 0.10. We have used these estimates to place NGC 6334 in the Kennicutt-Schmidt diagram to help bridge the gap between observations of local low-mass star-forming regions and star formation in other galaxies.

Willis, S.; Marengo, M.; Allen, L.; Fazio, G. G.; Smith, H. A.; Carey, S.

2013-12-01

272

The masses of young stars: CN as a probe of dynamical masses  

NASA Astrophysics Data System (ADS)

Aims: We attempt to determine the masses of single or multiple young T Tauri and HAeBe stars from the rotation of their Keplerian disks. Methods: We used the IRAM PdBI interferometer to perform arcsecond resolution images of the CN N = 2-1 transition with good spectral resolution. Integrated spectra from the 30 m radiotelescope show that CN is relatively unaffected by contamination from the molecular clouds. Our sample includes 12 sources, among which isolated stars like DM Tau and MWC 480 are used to demonstrate the method and its accuracy. We derive the dynamical mass by fitting a disk model to the emission, a process giving M/D, the mass-to-distance ratio. We also compare the CN results with higher resolution CO data, that are however affected by contamination. Results: All disks are found in nearly perfect Keplerian rotation. We determine accurate masses for 11 stars, in the mass range 0.5 to 1.9 M?. The remaining one, DG Tau B, is a deeply embedded object for which CN emission partially arises from the outflow. With previous determinations, this leads to 14 (single) stars with dynamical masses. Comparison with evolutionary tracks, in a distance independent modified HR diagram, show good overall agreement (with one exception, CW Tau), and indicate that measurement of effective temperatures are the limiting factor. The lack of low mass stars in the sample does not allow to distinguish between alternate tracks. Based on observations carried out with the IRAM Plateau de Bure interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).Appendices are available in electronic form at http://www.aanda.org

Guilloteau, S.; Simon, M.; Piétu, V.; Di Folco, E.; Dutrey, A.; Prato, L.; Chapillon, E.

2014-07-01

273

NGC 2782: A Merger Remnant with Young Stars in its Gaseous Tidal Tail  

NASA Technical Reports Server (NTRS)

We have searched for young star-forming regions around the merger remnant NGC 2782. By using GALEX FUV and NUV imaging and HI data we found seven UV sources, located at distances greater than 26 kpc from the center of NGG 2782, and coinciding with its western HI tidal tail. These regions were resolved in several smaller systems when Gemini/GMOS r-band images were used. We compared the observed colors to stellar population synthesis models and we found that these objects have ages of l to ll11yr and masses ranging from 10(exp 3.9) to l0(exp 4.6) Solar Mass. By using Gemini/GMOS spectroscopic data we confirm memberships and derive high metallicities for three of the young regions in the tail (12+log(O/H)=8.74+/-0.20, 8.81+/-0.20 and 8.78+/-0.20). These metallicities are similar to the value presented by the nuclear region of NGG 2782 and also similar to the value presented for an object located close to the main body of NGG 2782. The high metallicities measured for the star-forming regions in the gaseous tidal tail of NGG 2782 could be explained if they were formed out of highly enriched gas which was once expelled from the center of the merging galaxies when the system collided. An additional possibility is that the tail has been a nursery of a few generations of young stellar systems which ultimately polluted this medium with metals, further enriching the already pre-enriched gas ejected to the tail when the galaxies collided.

Torres-Flores, S.; de Oliveira, C. Mendes; de Mello, D. F.; Scarano, S. Jr.; Urrutia-Viscarra, R.

2012-01-01

274

Keck Adaptive Optics Imaging of Nearby Young Stars: Detection of Close Multiple Systems  

E-print Network

Using adaptive optics on the Keck II 10-meter telescope on Mauna Kea, we have surveyed 24 of the nearest young stars known in search of close companions. Our sample includes members of the MBM 12 and TW Hydrae young associations and the classical T Tauri binary UY Aurigae in the Taurus star-forming region. We present relative photometry and accurate astrometry for 10 close multiple systems. The multiplicity frequency in the TW Hydrae and MBM 12 groups are high in comparison to other young regions, though the significance of this result is low because of the small number statistics. We resolve S 18 into a triple system including a tight 63 mas (projected separation of 17 AU at a distance of 275 pc) binary for the first time, with a hierarchical configuration reminiscent of VW Chamaeleontis and T Tauri. Another tight binary in our sample -- TWA 5Aab (54 mas or 3 AU at 55 pc) -- offers the prospect of dynamical mass measurement using astrometric observations within a few years, and thus could be important for testing pre-main sequence evolutionary models. Our observations confirm with 9-sigma confidence that the brown dwarf TWA 5B is bound to TWA 5A. We find that the flux ratio of UY Aur has changed dramatically, by more than a magnitude in the H-band, possibly as a result of variable extinction. With a smaller flux ratio, the system may once again become detectable as an optical binary, as it was at the time of its discovery in 1944. Taken together, our results demonstrate that adaptive optics on large telescopes is a powerful tool for detecting tight companions, and thus exploring the frequency and configurations of close multiple systems.

Alexis Brandeker; Ray Jayawardhana; Joan Najita

2003-06-25

275

Direct Imaging of Jupiter and Saturn-mass planets in wide orbit around nearby young stars  

NASA Astrophysics Data System (ADS)

The recent discovery of planetary-mass objects on very wide orbits (hundreds of AU and more) around young stars (e.g. Naud et al. 2014) demonstrates that planets can be found even with arcsecond-level resolution imaging. These massive ( 10MJup) companions are likely formed in-situ via hierarchical collapse and it is not yet known whether this mechanism can form lighter objects. However, dynamical modelling of young planetary systems (Veras et al. 2009) and the relatively large fraction of massive planets in eccentric orbits found by radial velocity surveys suggest that a few percent of planetary systems should host planets, comparable in mass to Jupiter and Saturn, on orbits wide enough to be imaged as isolated objects. We propose to obtain deep IRAC observations combined with J-band imaging gathered by our team to search for such planets around all known nearby young stars (< 70 pc, < 120Myr; 172 stellar systems). This survey will be sensitive to planets down to the mass of Jupiter for all systems and down to the mass of Saturn for 80 of them. Planets lighter than 2MJup are much too faint in the near-infrared to be identified from the ground; Spitzer is the only facility where such a survey can be undertaken. This survey is a unique opportunity to bring direct imaging in a new era with the detection of analogs to our own Solar System Giants, is complementary to the work done on the ground with high-contrast imagers such as GPI and Sphere, and is critical to identify new planets that will be optimally characterized with JWST.

Artigau, Etienne; Lafreniere, David; Baron, Frederique; Malo, Lison; Doyon, Rene; Beichman, Charles; Delorme, Philippe; Rameau, Julien; Janson, Markus; Gagne, Jonathan; Naud, Marie-Eve; Albert, Loic

2014-12-01

276

Orion in a New Light - VISTA exposes high-speed antics of young stars  

NASA Astrophysics Data System (ADS)

The Orion Nebula reveals many of its hidden secrets in a dramatic image taken by ESO's new VISTA survey telescope. The telescope's huge field of view can show the full splendour of the whole nebula and VISTA's infrared vision also allows it to peer deeply into dusty regions that are normally hidden and expose the curious behaviour of the very active young stars buried there. VISTA - the Visible and Infrared Survey Telescope for Astronomy - is the latest addition to ESO's Paranal Observatory (eso0949). It is the largest survey telescope in the world and is dedicated to mapping the sky at infrared wavelengths. The large (4.1-metre) mirror, wide field of view and very sensitive detectors make VISTA a unique instrument. This dramatic new image of the Orion Nebula illustrates VISTA's remarkable powers. The Orion Nebula [1] is a vast stellar nursery lying about 1350 light-years from Earth. Although the nebula is spectacular when seen through an ordinary telescope, what can be seen using visible light is only a small part of a cloud of gas in which stars are forming. Most of the action is deeply embedded in dust clouds and to see what is really happening astronomers need to use telescopes with detectors sensitive to the longer wavelength radiation that can penetrate the dust. VISTA has imaged the Orion Nebula at wavelengths about twice as long as can be detected by the human eye. As in the many visible light pictures of this object, the new wide field VISTA image shows the familiar bat-like form of the nebula in the centre of the picture as well as the fascinating surrounding area. At the very heart of this region lie the four bright stars forming the Trapezium, a group of very hot young stars pumping out fierce ultraviolet radiation that is clearing the surrounding region and making the gas glow. However, observing in the infrared allows VISTA to reveal many other young stars in this central region that cannot be seen in visible light. Looking to the region above the centre of the picture, curious red features appear that are completely invisible except in the infrared. Many of these are very young stars that are still growing and are seen through the dusty clouds from which they form. These youthful stars eject streams of gas with typical speeds of 700 000 km/hour and many of the red features highlight the places where these gas streams collide with the surrounding gas, causing emission from excited molecules and atoms in the gas. There are also a few faint, red features below the Orion Nebula in the image, showing that stars form there too, but with much less vigour. These strange features are of great interest to astronomers studying the birth and youth of stars. This new image shows the power of the VISTA telescope to image wide areas of sky quickly and deeply in the near-infrared part of the spectrum. The telescope is just starting to survey the sky and astronomers are anticipating a rich harvest of science from this unique ESO facility. Notes [1] The Orion Nebula lies in the sword of the famous celestial hunter and is a favourite target both for casual sky watchers and astrophysicists alike. It is faintly visible to the unaided eye and appeared to early telescopic observers as a small cluster of blue-white stars surrounded by a mysterious grey-green mist. The object was first described in the early seventeenth century although the identity of the discoverer is uncertain. The French comet-hunter Messier made an accurate sketch of its main features in the mid-eighteenth century and gave it the number 42 in his famous catalogue. He also allocated the number 43 to the smaller detached region just above the main part of the nebula. Later William Herschel speculated that the nebula might be "the chaotic material of future suns" and astronomers have since discovered that the mist is indeed gas glowing under the fierce ultraviolet light from young hot stars that have recently formed there. More information ESO, the European Southern Observatory, is the foremost intergovernmental astronomy organisation in Europe and the

2010-02-01

277

Multiwavelength Study of the Starburst Galaxy NGC 7714. II: The Balance between Young, Intermediate Age and Old Stars  

E-print Network

We combine existing multiwavelength data (incl. an HST/GHRS UV spectrum, an optical spectrum, far-IR, Xray and radio fluxes) with new HST/WFPC2 images, near-IR photometry and K band spectroscopy. We use these data to constrain the young, the intermediate age and the old stellar populations in the central 330 pc of the starburst galaxy NGC 7714. [...] We find that the young burst responsible for the UV light is only a small part of an extended episode of enhanced star formation (SF) [...]. The mass of young and intermediate age stars thus formed equals at least 10% of the mass locked in pre-existing stars of the underlying galaxy nucleus [...]. The spectrophotometric SF timescale is long compared to the ~110 Myr elapsed since closest contact with NGC 7715. The trigger of the starburst remains elusive. NGC 7714 owes its brightness in the UV to a few low extinction lines of sight towards young stars. [...] The different extinction estimates obtained from different indicators result naturally from the coexistence of populations with various ages and obscurations. The near-IR continuum image looks smoothest, as a consequence of lower sensitivity to extinction and of a larger contribution of old stars. We compare the nuclear properties of NGC 7714 with results from studies in larger apertures. We emphasize that the global properties of starburst galaxies are the result of the averaging over many lines of sight with diverse properties in terms of obscuration and stellar ages.

Ariane Lancon; Jeff D. Goldader; Claus Leitherer; Rosa M. Gonzalez Delgado

2001-01-18

278

New Brown Dwarf Companions to Young Stars in Scorpius-Centaurus  

E-print Network

We present the discoveries of three faint companions to young stars in the Scorpius-Centaurus region, imaged with the NICI instrument on Gemini South. We have confirmed all three companions through common proper motion tests. Follow-up spectroscopy has confirmed two of them, HIP 65423 B and HIP 65517 B, to be brown dwarfs, while the third, HIP 72099 B, is more likely a very low-mass star just above the hydrogen burning limit. The detection of wide companions in the mass range of ~40--100 Mjup complements previous work in the same region, reporting detections of similarly wide companions with lower masses, in the range of ~10--30 Mjup. Such low masses near the deuterium burning limit have raised the question of whether those objects formed like planets or stars. The existence of intermediate objects as reported here could represent a bridge between lower-mass companions and stellar companions, but in any case demonstrate that mass alone may not provide a clear-cut distinction for the formation of low-mass comp...

Janson, Markus; Girard, Julien H; Lafreniere, David; Bonavita, Mariangela; Gizis, John; Brandeker, Alexis

2012-01-01

279

Near-IR Variability of Young Stars in Orion OB1  

NASA Astrophysics Data System (ADS)

We present preliminary results of a multi-epoch near-IR study of a sample of 137 T Tauri stars (TTS) in the Orion OB1 association, belonging to the ~10 Myr old 25 Ori cluster and the 4 Myr old OB1b subassociation. We used the NEWFIRM near-IR wide field imager on the Kitt Peak 4m telescope to obtain data in the JHKs bands for up to 10 epochs spanning 22 days during Oct.-Nov. 2012. We find that on average, that TTS in the 25 Ori cluster vary with amplitudes of ~0.1mag in J and H, and ~0.06 mag in Ks. Among the interesting cases is a Classical T Tauri star in this region with a large variation in all three bands, with J=0.63mag, H=0.39mag and Ks=0.35mag, which we speculate could originate in the innermost region of the disk surrounding this young star.

Contreras, Alexander; Briceno, Cesar

2015-01-01

280

Young Stars in the Camelopardalis Dust and Molecular Clouds. II. Infrared Objects  

E-print Network

Using infrared photometric data extracted from the 2MASS, IRAS and MSX databases, 142 suspected young stellar objects (YSOs) are selected from about 2 million stars in the Camelopardalis segment of the Milky Way limited by Galactic longitude 132-158 deg, latitude pm 12 deg. According to radial velocities of the associated CO clouds, the objects are attributed to three molecular and dust cloud layers at 150--300 pc, 900 pc and 2.2 kpc distances from the Sun. These objects concentrate into dust and molecular clouds and exhibit extremely large reddenings (A_V up to 25 mag) which can be caused by the dust in foreground clouds and circumstellar envelopes or disks. In the J-H vs. H-K diagram these objects lie above the intrinsic line of T Tauri variables, roughly along the black-body line. Among the identified objects, some already known YSOs are present, including the well investigated massive object GL 490. The spectral energy distributions between 700 nm and 100 mum suggest that the objects may be YSOs of classes I, II and III. However, we do not exclude the possibility that a small fraction of the objects, especially those without IRAS and MSX photometry, may be unrecognized heavily reddened OB-stars, late-type AGB stars or even galaxies.

V. Straizys; V. Laugalys

2008-03-17

281

Spitzer IRS Spectra of Young Stars Near the Hydrogen Burning Mass Limit  

NASA Astrophysics Data System (ADS)

We present Spitzer Infrared Spectrograph* (IRS) measurements for two young stars in the Taurus star-forming region, with masses near the hydrogen-burning mass limit . While one of the objects displays no mid-infrared excess, the other one shows a large excess at wavelengths longward of 10 microns. The latter spectrum resembles in detail the spectra of flared, optically-thick accretion disks around more massive classical T Tauri stars. A bright and complex 10 micron silicate emission feature in this spectrum indicates the presence in the disk photosphere of small dust grains that contain significant amounts of crystalline silicates. This work is based 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 was provided by NASA through Contract Number 1257184 issued by JPL/Caltech, and Cornell subcontracts 31419-5714 to the University of Rochester. * The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.

Furlan, E.; Calvet, N.; D'Alessio, P.; Hartmann, L.; Forrest, W. J.; Uchida, K. I.; Watson, D. M.; Luhman, K. L.; Green, J. D.; Sargent, B.; Najita, J.; Sloan, G. C.; Keller, L. D.; Herter, T. L.

2004-12-01

282

VizieR Online Data Catalog: Stellar physical parameters for young stars (Monguio+, 2014)  

NASA Astrophysics Data System (ADS)

A uvbyH? Stromgren photometric survey covering 16 square degrees in the anticenter direction was carried out using the Wide Field Camera at the Isaac Newton Telescope. Physical parameters like stellar distances and extinctions for the young stars of our survey are presented here. We developed a new method for deriving physical parameters from Stromgren photometry and also implemented and tested it. This is a model-based method that uses the most recent available stellar atmospheric models and evolutionary tracks to interpolate in a 3D grid of the unreddened indexes [m1], [c1] and H?. Distances derived from both this method and the classical pre-Hipparcos calibrations were tested against Hipparcos parallaxes and found to be accurate. Furthermore, a shift in the atmospheric grids in the range Teff=[7000,9000]K was detected and a correction is proposed. The two methods were used to compute distances and reddening for around 12000 OBA-type stars in our Stromgren anticenter survey. Data from the IPHAS and 2MASS catalogs were used to complement the detection of emission line stars and to break the degeneracy between early and late photometric regions. We note that photometric distances can differ by more than 20%, those derived from the empirical calibrations being smaller than those derived with the new method, which agree better with the Hipparcos data. (1 data file).

Monguio, M.; Figueras, F.; Grosbol, P.

2014-08-01

283

Ultra-High-Energy Cosmic Rays from Young Neutron Star Winds.  

PubMed

The long-held notion that the highest energy cosmic rays are of distant extragalactic origin is challenged by observations that events above approximately 1020 eV do not exhibit the expected high-energy cutoff from photopion production off the cosmic microwave background. We suggest that these unexpected ultra-high-energy events are due to iron nuclei accelerated from young strongly magnetized neutron stars through relativistic MHD winds. We find that neutron stars whose initial spin periods are shorter than approximately 10 ms and whose surface magnetic fields are in the 1012-1014 G range can accelerate iron cosmic rays to greater than approximately 1020 eV. These ions can pass through the remnant of the supernova explosion that produced the neutron star without suffering significant spallation reactions or energy loss. For plausible models of the Galactic magnetic field, the trajectories of the iron ions curve sufficiently to be consistent with the observed, largely isotropic arrival directions of the highest energy events. PMID:10770705

Blasi; Epstein; Olinto

2000-04-20

284

An XMM-Newton view of the young open cluster NGC 6231 - II. The OB star population  

Microsoft Academic Search

In this second paper of the series, we pursue the analysis of the 180-ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30 per cent can be associated

H. Sana; G. Rauw; Y. Nazé; E. Gosset; J.-M. Vreux

2006-01-01

285

Erupting Fizz  

NSDL National Science Digital Library

This is a highly visual demonstration that illustrates both the effects of density and chemical reactions. Like a density column, volcano eruption, and Alka-seltzer reaction all in one, learners will observe layering between immiscible liquids, a volcanic eruption from salt crystals and a lava-lamp bubble effect caused by Alka-seltzer. This instruction guide includes a helpful video illustrating each step of the demonstration as well as an explanation of how the substances in the glass are interacting with each other. Note: this activity calls for beer, but any other light-colored fizzy drink will work as well.

Institute of Physics

2014-06-24

286

Young [$\\alpha$/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?  

E-print Network

We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [alpha/Fe] abundance ratios (as determined from APOGEE spectra) with respect to Solar values. Their existence is not explained by standard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be robustly obtained, although in smaller relative numbers, which could explain why these stars have not received prior attention. The young [alpha/Fe]-rich stars are much more numerous in the CoRoT-APOGEE (CoRoGEE) inner-field sample than in any other high-resolution sample available at present, as only CoRoGEE can explore the inner-disc regions and provide ages for its field stars. The kinematic properties of the young [$\\alpha$/Fe]-rich stars are not clearly thick-disc like, despite their rather large distances from the Galactic mid-plane. Our tentative interpretat...

Chiappini, C; Rodrigues, T S; Miglio, A; Montalban, J; Mosser, B; Girardi, L; Valentini, M; Noels, A; Morel, T; Minchev, I; Steinmetz, M; Santiago, B X; Schultheis, M; Martig, M; da Costa, L N; Maia, M A G; Prieto, C Allende; Peralta, R de Assis; Hekker, S; Themeßl, N; Kallinger, T; Garcia, R A; Mathur, S; Baudin, F; Beers, T C; Cunha, K; Harding, P; Holtzman, J; Majewski, S; Meszaros, Sz; Nidever, D; Pan, K; Schiavon, R P; Shetrone, M D; Schneider, D P; Stassun, K

2015-01-01

287

Interacting supernovae and supernova impostors. SN 2007sv: the major eruption of a massive star in UGC 5979  

NASA Astrophysics Data System (ADS)

We report the results of the photometric and spectroscopic monitoring campaign of the transient SN 2007sv. The observables are similar to those of Type IIn supernovae, a well-known class of objects whose ejecta interact with pre-existing circumstellar material (CSM). The spectra show a blue continuum at early phases and prominent Balmer lines in emission; however, the absolute magnitude at the discovery of SN 2007sv (MR = -14.25 ± 0.38) indicate it to be most likely a supernova impostor. This classification is also supported by the lack of evidence in the spectra of very high velocity material as expected in supernova ejecta. In addition, we find no unequivocal evidence of broad lines of ?- and/or Fe-peak elements. The comparison with the absolute light curves of other interacting objects (including Type IIn supernovae) highlights the overall similarity with the prototypical impostor SN 1997bs. This supports our claim that SN 2007sv was not a genuine supernova, and was instead a supernova impostor, most likely similar to the major eruption of a luminous blue variable.

Tartaglia, L.; Pastorello, A.; Taubenberger, S.; Cappellaro, E.; Maund, J. R.; Benetti, S.; Boles, T.; Bufano, F.; Duszanowicz, G.; Elias-Rosa, N.; Harutyunyan, A.; Hermansson, L.; Höflich, P.; Maguire, K.; Navasardyan, H.; Smartt, S. J.; Taddia, F.; Turatto, M.

2015-02-01

288

GEMINI SPECTROSCOPIC SURVEY OF YOUNG STAR CLUSTERS IN MERGING/INTERACTING GALAXIES. III. THE ANTENNAE  

SciTech Connect

We present optical spectroscopy of 16 star clusters in the merging galaxies NGC 4038/39 ('the Antennae') and supplement this data set with Hubble Space Telescope imaging. The age and metallicity of each cluster is derived through a comparison between the observed Balmer and metal line strengths with simple stellar population models. We then estimate extinctions and masses using the photometry. We find that all but three clusters have ages between {approx}3 and 200 Myr, consistent with the expected increase in the star formation rate (SFR) due to the merger. Most of the clusters have velocities in agreement with nearby molecular and H I gas that has been previously shown to be rotating within the progenitor galaxies, hence star/cluster formation is still taking place within the galactic disks. However, three clusters have radial velocities that are inconsistent with being part of the rotating gas disks, which is surprising given their young (200-500 Myr) ages. Interestingly, we find a stellar association with the same colors (V - I) near one of these three clusters, suggesting that the cluster and association were formed concurrently and have remained spatially correlated. We find evidence for spatially distributed cluster formation throughout the duration of the merger. The impact of various assumptions about the SFR/cluster formation rate on the interpretation of the cluster age distribution is explored, and we do not find evidence for long-term 'infant mortality' as has been previously suggested. Models of galaxy mergers that include a prescription for star formation can provide an overall good fit to the observed cluster age distribution.

Bastian, Nate [Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom); Trancho, Gelys; Konstantopoulos, Iraklis S.; Miller, Bryan W. [Gemini Observatory, Casilla 603, La Serena (Chile)], E-mail: bastian@ast.cam.ac.uk, E-mail: gtrancho@gemini.edu

2009-08-10

289

Popocatepetl Erupts  

NASA Technical Reports Server (NTRS)

The Popocatepetl Volcano, almost 30 miles south of Mexico City, erupted yesterday (December 18, 2000) in what authorities are calling its most spectacular eruption since 800 A.D. This morning, Popocatepetl (pronounced poh-poh-kah-TEH-peh-til) continued spewing red-hot rocks as well as a column of smoke and ash about 2.5 miles high into the atmosphere. This true-color image of the volcano was acquired today by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) flying aboard the OrbView-2 satellite. In this image, Popocatepetl's plume (greyish pixels) can be seen blowing southward, away from Mexico City. There is a large cloud bank (bright white pixels) just to the east of the volcanic plume. Although Popocatepetl has been active since 1994-when it awoke from a 70-year slumber-this most recent eruption is most concerning to the greater Mexico City region's 20 million residents. The volcano demonstrated what it can do in 800 A.D. when it belched forth enough lava to fill many of the valleys in the surrounding region. Earlier, scientists warned the citizens of Mexico that there is a dome of lava at the base of the volcano that is causing pressure to build inside. They are concerned that, if it continues to build unabated, this pressure could cause even larger eruptions in the future. Image provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE

2002-01-01

290

Direct imaging search for planets around low-mass stars and spectroscopic characterization of young exoplanets  

NASA Astrophysics Data System (ADS)

Low--mass stars between 0.1--0.6 M? are the most abundant members our galaxy and may be the most common sites of planet formation, but little is known about the outer architecture of their planetary systems. We have carried out a high-contrast adaptive imaging search for gas giant planets between 1--13 MJup around 122 newly identified young M dwarfs in the solar neighborhood ( ? 35 pc). Half of our targets are younger than 145 Myr, and 90% are younger than 580 Myr. After removing 39 resolved stellar binaries, our homogeneous sample of 83 single young M dwarfs makes it the largest imaging search for planets around low--mass stars to date. Our H- and K- band coronagraphic observations with Subaru/HiCIAO and Keck/NIRC2 achieve typical contrasts of 9--13 mag and 12--14 mag at 100, respectively, which corresponds to limiting masses of ˜1--10 M Jup at 10--30 AU for most of our sample. We discovered four brown dwarfs with masses between 25--60 MJup at projected separations of 4--190 AU. Over 100 candidate planets were discovered, nearly all of which were found to be background stars from follow-up second epoch imaging. Our null detection of planets nevertheless provides strong statistical constraints on the occurrence rate of giant planets around M dwarfs. Assuming circular orbits and a logarithmically-flat power law distribution in planet mass and semi--major axis of the form d 2N=(dloga dlogm) infinity m0 a0, we measure an upper limit (at the 95% confidence level) of 8.8% and 12.6% for 1--13 MJup companions between 10--100 AU for hot start and cold start evolutionary models, respectively. For massive gas giant planets in the 5--13 M Jup range like those orbiting HR 8799, GJ 504, and beta Pictoris, we find that fewer than 5.3% (7.8%) of M dwarfs harbor these planets between 10--100 AU for a hot start (cold start) formation scenario. Our best constraints are for brown dwarf companions; the frequency of 13--75 MJup companions between (de--projected) physical separations of 10--100 AU is 2.1+2.1-1.2 %. Altogether, our results show that gas giant planets, especially massive ones, are rare in the outskirts of M dwarf planetary systems. If disk instability is a viable way to form planets, our constraints for the most common type of star imply that overall it is an inefficient mechanism.

Bowler, Brendan Peter

291

CSI 2264: Characterizing Accretion-burst Dominated Light Curves for Young Stars in NGC 2264  

NASA Astrophysics Data System (ADS)

Based on more than four weeks of continuous high-cadence photometric monitoring of several hundred members of the young cluster NGC 2264 with two space telescopes, NASA's Spitzer and the CNES CoRoT (Convection, Rotation, and planetary Transits), we provide high-quality, multi-wavelength light curves for young stellar objects whose optical variability is dominated by short-duration flux bursts, which we infer are due to enhanced mass accretion rates. These light curves show many brief—several hours to one day—brightenings at optical and near-infrared wavelengths with amplitudes generally in the range of 5%-50% of the quiescent value. Typically, a dozen or more of these bursts occur in a 30 day period. We demonstrate that stars exhibiting this type of variability have large ultraviolet (UV) excesses and dominate the portion of the u - g versus g - r color-color diagram with the largest UV excesses. These stars also have large H? equivalent widths, and either centrally peaked, lumpy H? emission profiles or profiles with blueshifted absorption dips associated with disk or stellar winds. Light curves of this type have been predicted for stars whose accretion is dominated by Rayleigh-Taylor instabilities at the boundary between their magnetosphere and inner circumstellar disk, or where magneto-rotational instabilities modulate the accretion rate from the inner disk. Among the stars with the largest UV excesses or largest H? equivalent widths, light curves with this type of variability greatly outnumber light curves with relatively smooth sinusoidal variations associated with long-lived hot spots. We provide quantitative statistics for the average duration and strength of the accretion bursts and for the fraction of the accretion luminosity associated with these bursts. Based on data from the Spitzer and CoRoT missions, as well as the Canada-France-Hawaii Telescope (CFHT) MegaCam CCD, and the European Southern Observatory Very Large Telescope, Paranal Chile, under program 088.C-0239. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain. MegaCam is a joint project of CFHT and CEA/DAPNIA, which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

Stauffer, John; Cody, Ann Marie; Baglin, Annie; Alencar, Silvia; Rebull, Luisa; Hillenbrand, Lynne A.; Venuti, Laura; Turner, Neal J.; Carpenter, John; Plavchan, Peter; Findeisen, Krzysztof; Carey, Sean; Terebey, Susan; Morales-Calderón, María; Bouvier, Jerome; Micela, Giusi; Flaccomio, Ettore; Song, Inseok; Gutermuth, Rob; Hartmann, Lee; Calvet, Nuria; Whitney, Barbara; Barrado, David; Vrba, Frederick J.; Covey, Kevin; Herbst, William; Furesz, Gabor; Aigrain, Suzanne; Favata, Fabio

2014-04-01

292

New Debris Disks Around Young, Low-Mass Stars Discovered with the Spitzer Space Telescope  

NASA Astrophysics Data System (ADS)

We present 24 ?m and 70 ?m Multiband Imaging Photometer for Spitzer (MIPS) observations of 70 A through M-type dwarfs with estimated ages from 8 Myr to 1.1 Gyr, as part of a Spitzer guaranteed time program, including a re-analysis of some previously published source photometry. Our sample is selected from stars with common youth indicators such as lithium abundance, X-ray activity, chromospheric activity, and rapid rotation. We compare our MIPS observations to empirically derived Ks -[24] colors as a function of the stellar effective temperature to identify 24 ?m and 70 ?m excesses. We place constraints or upper limits on dust temperatures and fractional infrared luminosities with a simple blackbody dust model. We confirm the previously published 70 ?m excesses for HD 92945, HD 112429, and AU Mic, and provide updated flux density measurements for these sources. We present the discovery of 70 ?m excesses for five stars: HD 7590, HD 10008, HD 59967, HD 73350, and HD 135599. HD 135599 is also a known Spitzer IRS (InfraRed Spectrograph) excess source, and we confirm the excess at 24 ?m. We also present the detection of 24 ?m excesses for 10 stars: HD 10008, GJ 3400A, HD 73350, HD 112429, HD 123998, HD 175742, AT Mic, BO Mic, HD 358623 and Gl 907.1. We find that large 70 ?m excesses are less common around stars with effective temperatures of less than 5000 K (3.7+7.6 -1.1%) than around stars with effective temperatures between 5000 K and 6000 K (21.4+9.5 -5.7%), despite the cooler stars having a younger median age in our sample (12 Myr vs. 340 Myr). We find that the previously reported excess for TWA 13A at 70 ?m is due to a nearby background galaxy, and the previously reported excess for HD 177724 is due to saturation of the near-infrared photometry used to predict the mid-infrared stellar flux contribution. In the Appendix, we present an updated analysis of dust grain removal timescales due to grain-grain collisions and radiation pressure, Poynting-Robertson (P-R) drag, stellar wind drag, and planet-dust dynamical interaction. We find that drag forces can be important for disk dynamics relative to grain-grain collisions for L IR/L * < 10-4, and that stellar wind drag is more important than P-R drag for K and M dwarfs, and possibly for young (<1 Gyr) G dwarfs as well.

Plavchan, Peter; Werner, M. W.; Chen, C. H.; Stapelfeldt, K. R.; Su, K. Y. L.; Stauffer, J. R.; Song, I.

2009-06-01

293

Evolutionary studies of the young star clusters: NGC 1960, NGC 2453 and NGC 2384  

NASA Astrophysics Data System (ADS)

We report the analysis of the young star clusters NGC 1960, NGC 2453 and NGC 2384 observed in the J (1.12 ?m), H (1.65 ?m) and K' (2.2 ?m) bands. Estimates of reddening, distance and age as E( B- V)=0.25, d=1380 pc and t=31.6 to 125 Myr for NGC 1960, E( B- V)=0.47, d=3311 pc and t=40 to 200 Myr for NGC 2453 and E( B- V)=0.25, d=3162 pc and t=55 to 125 Myr for NGC 2384 have been obtained. Also, we have extended the color magnitude diagrams of these clusters to the fainter end and thus extended the luminosity functions to fainter magnitudes. The evolution of the main sequence and luminosity functions of these clusters have been compared with themselves as well as Lyngå 2 and NGC 1582.

Hasan, Priya; Kilambi, G. C.; Hasan, S. N.

2008-02-01

294

Predicted Colors And Spectral Energy Distributions Of Young Stars In The Herschel Pacs And Spire Filters.  

NASA Astrophysics Data System (ADS)

The Herschel Space Observatory, the most sensitive far-IR/sub-mm observatory to date, is expected to revolutionize our view of the earliest phases of star formation. Upcoming surveys with Herschel's PACS and SPIRE instruments will yield far-IR photometry of large samples of young stellar objects associated with molecular clouds. We present theoretical predictions for the colors and luminosities of protostellar envelopes and protoplanetary disks in the passbands of the PACS and SPIRE instruments. These predictions are based on a grid of model calculations using the code of Whitney et al. (2003, ApJ, 591, 1049). From these predictions, we will assess what properties of protostars can be constrained from the Herschel photometry by itself and combined with mid-IR photometry from Spitzer surveys. We will also discuss uncertainties in the models and how these may affect the predicted fluxes.

Ali, Babar; Tobin, J. J.; Fischer, W.; Poteet, C.; Megeath, T.; Allen, L.; Hartmann, L.; Calvert, N.

2010-01-01

295

Deep Near-Infrared Surveys and Young Brown Dwarf Populations in Star-Forming Regions  

NASA Astrophysics Data System (ADS)

We are currently conducting three kinds of IR surveys of star forming regions (SFRs) in order to seek for very low-mass young stellar populations. First is a deep JHKs-bands (simultaneous) survey with the SIRIUS camera on the IRSF 1.4m or the UH 2.2m telescopes. Second is a very deep JHKs survey with the CISCO IR camera on the Subaru 8.2m telescope. Third is a high resolution companion search around nearby YSOs with the CIAO adaptive optics coronagraph IR camera on the Subaru. In this contribution, we describe our SIRIUS camera and present preliminary results of the ongoing surveys with this new instrument.

Tamura, M.; Naoi, T.; Oasa, Y.; Nakajima, Y.; Nagashima, C.; Nagayama, T.; Baba, D.; Nagata, T.; Sato, S.; Kato, D.; Kurita, M.; Sugitani, K.; Itoh, Y.; Nakaya, H.; Pickles, A.

2003-06-01

296

Multiwavelength Study of the Starburst Galaxy NGC 7714. II The Balance between Young, Intermediate Age and Old Stars  

E-print Network

We combine existing multiwavelength data (incl. an HST/GHRS UV spectrum, an optical spectrum, far-IR, Xray and radio fluxes) with new HST/WFPC2 images, near-IR photometry and K band spectroscopy. We use these data to constrain the young, the intermediate age and the old stellar populations in the central 330 pc of the starburst galaxy NGC 7714. [...] We find that the young burst responsible for the UV light is only a small part of an extended episode of enhanced star formation (SF) [...]. The mass of young and intermediate age stars thus formed equals at least 10% of the mass locked in pre-existing stars of the underlying galaxy nucleus [...]. The spectrophotometric SF timescale is long compared to the ~110 Myr elapsed since closest contact with NGC 7715. The trigger of the starburst remains elusive. NGC 7714 owes its brightness in the UV to a few low extinction lines of sight towards young stars. [...] The different extinction estimates obtained from different indicators result naturally from the coexistence...

Lançon, A; Leitherer, C; González-Delgado, R M; Lancon, Ariane; Goldader, Jeff D.; Leitherer, Claus; Delgado, Rosa M. Gonzalez

2001-01-01

297

A Pulsation Search among Young Brown Dwarfs and Very-low-mass Stars  

NASA Astrophysics Data System (ADS)

In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very-low-mass stars (VLMSs; < 0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of one to four hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters ? Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to observable amplitudes in the early pre-main sequence. In spite of the nondetection, we did uncover a rich set of variability behavior—both periodic and aperiodic—on day to week timescales. We present new compilations of variable sources from our sample, as well as three new candidate cluster members in Chamaeleon I.

Cody, Ann Marie; Hillenbrand, Lynne A.

2014-12-01

298

A Pulsation Search Among Young Brown Dwarfs and Very Low Mass Stars  

E-print Network

In 2005, Palla & Baraffe proposed that brown dwarfs (BDs) and very low mass stars (VLMSs; <0.1 solar masses) may be unstable to radial oscillations during the pre-main-sequence deuterium burning phase. With associated periods of 1-4 hours, this potentially new class of pulsation offers unprecedented opportunities to probe the interiors and evolution of low-mass objects in the 1-15 million year age range. Following up on reports of short-period variability in young clusters, we designed a high-cadence photometric monitoring campaign to search for deuterium-burning pulsation among a sample of 348 BDs and VLMSs in the four young clusters $\\sigma$ Orionis, Chamaeleon I, IC 348, and Upper Scorpius. In the resulting light curves we achieved sensitivity to periodic signals of amplitude several millimagnitudes, on timescales from 15 minutes to two weeks. Despite the exquisite data quality, we failed to detect any periodicities below seven hours. We conclude that D-burning pulsations are not able to grow to obs...

Cody, Ann Marie

2014-01-01

299

THE GEMINI NICI PLANET-FINDING CAMPAIGN: THE FREQUENCY OF GIANT PLANETS AROUND YOUNG B AND A STARS  

SciTech Connect

We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around high-mass stars (?1.5-2.5 M{sub ?}) conducted to date and includes the planet hosts ? Pic and Fomalhaut. We obtained follow-up astrometry of all candidate companions within 400 AU projected separation for stars in uncrowded fields and identified new low-mass companions to HD 1160 and HIP 79797. We have found that the previously known young brown dwarf companion to HIP 79797 is itself a tight (3 AU) binary, composed of brown dwarfs with masses 58{sup +21}{sub -20} M{sub Jup} and 55{sup +20}{sub -19} M{sub Jup}, making this system one of the rare substellar binaries in orbit around a star. Considering the contrast limits of our NICI data and the fact that we did not detect any planets, we use high-fidelity Monte Carlo simulations to show that fewer than 20% of 2 M{sub ?} stars can have giant planets greater than 4 M{sub Jup} between 59 and 460 AU at 95% confidence, and fewer than 10% of these stars can have a planet more massive than 10 M{sub Jup} between 38 and 650 AU. Overall, we find that large-separation giant planets are not common around B and A stars: fewer than 10% of B and A stars can have an analog to the HR 8799 b (7 M{sub Jup}, 68 AU) planet at 95% confidence. We also describe a new Bayesian technique for determining the ages of field B and A stars from photometry and theoretical isochrones. Our method produces more plausible ages for high-mass stars than previous age-dating techniques, which tend to underestimate stellar ages and their uncertainties.

Nielsen, Eric L.; Liu, Michael C.; Chun, Mark; Ftaclas, Christ [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Wahhaj, Zahed [European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago (Chile); Biller, Beth A. [Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany); Hayward, Thomas L.; Hartung, Markus [Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena (Chile); Close, Laird M.; Males, Jared R.; Skemer, Andrew J. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Alencar, Silvia H. P. [Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, Av. Antonio Carlos 6627, 30270-901 Belo Horizonte, MG (Brazil); Artymowicz, Pawel [University of Toronto at Scarborough, 1265 Military Trail, Toronto, Ontario M1C 1A4 (Canada); Boss, Alan [Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015 (United States); Clarke, Fraser [Department of Astronomy, University of Oxford, DWB, Keble Road, Oxford OX1 3RH (United Kingdom); De Gouveia Dal Pino, Elisabete; Gregorio-Hetem, Jane [Departamento de Astronomia, Universidade de Sao Paulo, IAG/USP, Rua do Matao 1226, 05508-900 Sao Paulo, SP (Brazil); Ida, Shigeru [Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550 (Japan); Kuchner, Marc [NASA Goddard Space Flight Center, Exoplanets and Stellar Astrophysics Laboratory, Greenbelt, MD 20771 (United States); Lin, Douglas N. C. [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA (United States); and others

2013-10-10

300

GEMINI SPECTROSCOPIC SURVEY OF YOUNG STAR CLUSTERS IN MERGING/INTERACTING GALAXIES. IV. STEPHAN's QUINTET  

SciTech Connect

We present a spectroscopic survey of 21 young massive clusters and complexes and one tidal dwarf galaxy (TDG) candidate in Stephan's Quintet, an interacting compact group of galaxies. All of the selected targets lie outside the main galaxies of the system and are associated with tidal debris. We find clusters with ages between a few and 125 Myr and confirm the ages estimated through Hubble Space Telescope photometry by Fedotov et al., as well as their modeled interaction history of the Quintet. Many of the clusters are found to be relatively long-lived, given their spectrosopically derived ages, while their high masses suggest that they will likely evolve to eventually become intergalactic clusters. One cluster, T118, is particularly interesting, given its age ({approx}125 Myr), high mass ({approx}2 Multiplication-Sign 10{sup 6} M{sub Sun }), and position in the extreme outer end of the young tidal tail. This cluster appears to be quite extended (R{sub eff} {approx} 12-15 pc) compared to clusters observed in galaxy disks (R{sub eff} {approx} 3-4 pc), which confirms an effect we previously found in the tidal tails of NGC 3256, where clusters are similarly extended. We find that star and cluster formation can proceed at a continuous pace for at least {approx}150 Myr within the tidal debris of interacting galaxies. The spectrum of the TDG candidate is dominated by a young population ({approx}7 Myr), and, assuming a single age for the entire region, has a mass of at least 10{sup 6} M{sub Sun }.

Trancho, Gelys [Gemini Observatory, Casilla 603, La Serena (Chile); Konstantopoulos, Iraklis S.; Mullan, Brendan; Charlton, Jane C. [Department of Astronomy and Astrophysics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802 (United States); Bastian, Nate [Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching (Germany); Fedotov, Konstantin; Gallagher, Sarah, E-mail: gtrancho@gmto.org [Physics and Astronomy Department, University of Western Ontario, London, ON N6A 3K7 (Canada)

2012-04-01

301

Suitability of North Star Ambulatory Assessment in young boys with Duchenne muscular dystrophy.  

PubMed

The aim of this study was to establish the suitability of the North Star Ambulatory Assessment for use in young boys with Duchenne muscular dystrophy. We studied 147 typically developing and 144 boys affected by Duchenne muscular dystrophy between the ages of 3 and 5 years. More than 85% of the typically developing boys by the age of 4 years had full scores on all the items with total scores ?33/34. Before the age of 4 years more than 15% of the typically developing boys did not achieve full scores on all the items. Some items, such as standing on one leg, showed significant improvement with age. In contrast, other activities were rarely achieved even in the older boys. Even if there was a progressive increase in scores with age, both total and individual item scores in Duchenne were still far from those obtained in the typically developing children of the same age. Our findings suggest that the North Star Ambulatory Assessment can be reliably used at least from the age of 4 years. Longitudinal natural history data studies are needed to assess possible changes over time and the possible effect of early steroids. PMID:25454732

De Sanctis, Roberto; Pane, Marika; Sivo, Serena; Ricotti, Valeria; Baranello, Giovanni; Frosini, Silvia; Mazzone, Elena; Bianco, Flaviana; Fanelli, Lavinia; Main, Marion; Corlatti, Alice; D'Amico, Adele; Colia, Giulia; Scalise, Roberta; Palermo, Concetta; Alfonsi, Chiara; Tritto, Giovanna; Romeo, Domenico M; Graziano, Alessandra; Battini, Roberta; Morandi, Lucia; Bertini, Enrico; Muntoni, Francesco; Mercuri, Eugenio

2015-01-01

302

Periodic Disk Eclipsing Stars: A New Class of Variable Young Stellar Objects  

NASA Astrophysics Data System (ADS)

We present the discovery of 92.6 day periodic near-infrared flu variability for the Class I T Tauri star YLW 16A. Our data are from the 2MASS Calibration Point Source Working Database and constitute 1582 observations in J, H and Ks of a field in Rho Ophiuchus used to calibrate the 2MASS All-Sky Survey. We identify a light curve that brightens from a quiescent faint state by 0.4 mag for only 20% of the period. The long period cannot be explained by stellar rotation. We propose that YLW 16A is a triple YSO system, with an inner binary orbital period of 92.6 days. We postulate that we are observing a component of the binary being eclipsed by a circumbinary disk with respect to our line of site. YLW 16A joins WL 4 and KH-15D as a third member of a new class of disk-eclipsing young stars. Both YLW 16A and WL 4 have been identified to have tertiary companions with projected separations of 20--50 AU. We propose that the tertiary companion can warp the inner circumbinary disk to produce the disk eclipses. We present NIRSPEC observations of WL 4 that suggest three distinct radial velocity components, and present a model SED for YLW 16A. These systems will be useful in investigating terrestrial-zone YSO disk properties and dynamics at 1 Myr.

Plavchan, Peter; Laohakunakorn, N.; Seifahrt, A.; Staplefeldt, K.; Gee, A. H.

2010-01-01

303

VLT/NACO Deep imaging survey of young, nearby austral stars  

E-print Network

Since November 2002, we have conducted the largest deep imaging survey of the young, nearby associations of the southern hemisphere. Our goal is detection and characterization of substellar companions at intermediate (10--500 AU) physical separations. We have observed a sample of 88 stars, mostly G to M dwarfs, that we essentially identify as younger than 100 Myr and closer to Earth than 100 pc. The VLT/NACO adaptive optics instrument of the ESO Paranal Observatory was used to explore the faint circumstellar environment between typically 0.1 and 10''. We report the discovery of 17 new close (0.1-5.0'') multiple systems. HIP108195AB and C (F1III-M6), HIP84642AB (a~14 AU, K0-M5) and TWA22AB (a~1.8 AU; M6-M6) confirmed comoving systems. TWA22AB is likely to be a astrometric calibrator that can be used to test evolutionary predictions. Among our complete sample, a total of 65 targets observed with deep coronagraphic imaging. About 240 faint candidates were detected around 36 stars. Follow-up observations VLT or H...

Chauvin, G; Bonavita, M; Zuckerman, B; Dumas, C; Bessell, M S; Beuzit, J -L; Bonnefoy, M; Desidera, S; Farihi, J; Lowrance, P; Mouillet, D; Song, I

2009-01-01

304

Camping with Stars - A FullDome show for a young audience  

NASA Astrophysics Data System (ADS)

Digital systems are playing an increasing role in the world of planetariums. The possibilities offered by these systems when compare with the more traditional ones based on star machines and still images projectors are enormous. The number of planetariums available with this system is steeply increasing as well as the number of shows produced for them. Following this trend, the Espinho Planetarium, together with Appia Films, an animation company, is producing a show for a young audience between 10 and 14 years old entitled Camping with the Stars. It is a show where science and entertainment are mixed together in order to produce an appealing content. It tells a story of a class that is camping in the middle of the nature with their teachers. At night, when almost everyone is asleep, a teacher and a small group of students stay awake, admiring the night sky, an opportunity that they seldom have in the city they live. The students, with their curious questions, and the teacher with its wise answers, make a trip through some of the most important concepts of the Universe. In this talk we would like to present the show, the scientific goals as well as the approach and techniques used in its production.

Pedrosa, A.; Barrosa, M.; Calcada, L.; Russo, P.

305

YOUNG STELLAR OBJECTS IN LYNDS 1641: DISKS, ACCRETION, AND STAR FORMATION HISTORY  

SciTech Connect

We investigate the young stellar objects (YSOs) in the Lynds 1641 (L1641) cloud using multi-wavelength data including Spitzer, WISE, the Two Micron All Sky Survey, and XMM covering {approx}1390 YSOs across a range of evolutionary stages. In addition, we targeted a sub-sample of YSOs for optical spectroscopy with the MMT/Hectospec and the MMT/Hectochelle. We use these data, along with archival photometric data, to derive spectral types, extinction values, masses, ages, and accretion rates. We obtain a disk fraction of {approx}50% in L1641. The disk frequency is almost constant as a function of stellar mass with a slight peak at log (M{sub *}/M{sub Sun }) Almost-Equal-To -0.25. The analysis of multi-epoch spectroscopic data indicates that the accretion variability of YSOs cannot explain the two orders of magnitude of scatter for YSOs with similar masses. Forty-six new transition disk (TD) objects are confirmed in this work, and we find that the fraction of accreting TDs is lower than for optically thick disks (40%-45% versus 77%-79%, respectively). We confirm our previous result that the accreting TDs have a median accretion rate similar to normal optically thick disks. We confirm that two star formation modes (isolated versus clustered) exist in L1641. We find that the diskless YSOs are statistically older than the YSOs with optically thick disks and the TD objects have a median age that is intermediate between those of the other two populations. We tentatively study the star formation history in L1641 based on the age distribution and find that star formation started to be active 2-3 Myr ago.

Fang Min [Purple Mountain Observatory and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing (China); Kim, Jinyoung Serena; Flaherty, Kevin [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065 (United States); Van Boekel, Roy; Henning, Thomas [Max-Planck Institute for Astronomy, Koenigstuhl 17, D-69117 Heidelberg (Germany); Sicilia-Aguilar, Aurora, E-mail: mfang@pmo.ac.cn [Departamento de Fisica Teorica, Facultad de Ciencias, Universidad Autonoma de Madrid, 28049 Cantoblanco, Madrid (Spain)

2013-07-01

306

On the time dependence of differential rotation in young late-type stars  

NASA Astrophysics Data System (ADS)

A model for the angular momentum transfer within the convection zone of a rapidly rotating star is introduced and applied to the analysis of recent observations of temporal fluctuations of the differential rotation on the young late-type stars AB Doradus (AB Dor) and LQ Hydrae (LQ Hya). Under the hypothesis that the mean magnetic field produced by the stellar dynamo rules the angular momentum exchanges and that the angular velocity depends only on the distance s from the rotation axis and the time, the minimum azimuthal Maxwell stress |BsB?|, averaged over the convection zone, is found to range from ~0.04 to ~0.14T2. If the poloidal mean magnetic field Bs is of the order of 0.01T, as indicated by the Zeeman-Doppler imaging maps of those stars, then the azimuthal mean field B? can reach an intensity of several teslas, which significantly exceeds equipartition with the turbulent kinetic energy. Such strong fields can account also for the orbital period modulation observed in cataclysmic variables and RS Canum Venaticorum systems with a main-sequence secondary component. Moreover, the model allows us to compute the kinetic energy dissipation rate during the maintenance of the differential rotation. Only in the case of the largest surface shear observed on LQ Hya may the dissipated power exceed the stellar luminosity, but the lack of a sufficient statistic on the occurrence of such episodes of large shear does not allow us to estimate their impact on the energy budget of the convection zone.

Lanza, Antonino F.

2006-12-01

307

Stellar physical parameters from Strömgren photometry. Application to the young stars in the Galactic anticenter survey  

NASA Astrophysics Data System (ADS)

Aims: The aim is to derive accurate stellar distances and extinctions for young stars of our survey in the Galactic anticenter direction using the Strömgren photometric system. This will allow a detailed mapping of the stellar density and absorption toward the Perseus arm. Methods: We developed a new method for deriving physical parameters from Strömgren photometry and also implemented and tested it. This is a model-based method that uses the most recent available stellar atmospheric models and evolutionary tracks to interpolate in a 3D grid of the unreddened indexes [m1], [c1] and H?. Distances derived from both this method and the classical pre-Hipparcos calibrations were tested against Hipparcos parallaxes and found to be accurate. Results: Systematic trends in stellar photometric distances derived from empirical calibrations were detected and quantified. Furthermore, a shift in the atmospheric grids in the range Teff = [7000,9000] K was detected and a correction is proposed. The two methods were used to compute distances and reddening for ~12 000 OBA-type stars in our Strömgren anticenter survey. Data from the IPHAS and 2MASS catalogs were used to complement the detection of emission line stars and to break the degeneracy between early and late photometric regions. We note that photometric distances can differ by more than 20%, those derived from the empirical calibrations being smaller than those derived with the new method, which agree better with the Hipparcos data. Appendices are available in electronic form at http://www.aanda.orgThe catalog of the physical parameters is only available in electronic form 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/568/A119

Monguió, M.; Figueras, F.; Grosbøl, P.

2014-08-01

308

Dynamical star-disk interaction in the young stellar system V354 Monocerotis  

NASA Astrophysics Data System (ADS)

Aims: The main goal of this work is to characterize the mass accretion and ejection processes of the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. Methods: In March 2008, photometric and spectroscopic observations of V354 Mon were obtained simultaneously with the CoRoT satellite, the 60 cm telescope at the Observatório Pico dos Dias (LNA, Brazil) equipped with a CCD camera and Johnson/Cousins BV(RI)c filters, and the SOPHIE échelle spectrograph at the Observatoire de Haute-Provence (CNRS, France). Results: The light curve of V354 Mon shows periodical minima (P = 5.26 ± 0.50 days) that vary in depth and width at each rotational cycle. The BV(RI)c observations indicate that the system becomes slightly bluer as the flux increases. The spectra of this T Tauri star exhibit variable emission lines, with blueshifted and redshifted absorption components associated with a disk wind and with the accretion process, respectively, confirming the magnetospheric accretion scenario. From the analysis of the photometric and spectroscopic data, it is possible to identify correlations between the emission line variability and the light-curve modulation of the young system, such as the occurrence of pronounced redshifted absorption in the H? line at the epoch of minimum flux. This is evidence that during photometric minima we see the accretion funnel projected onto the stellar photosphere in our line of sight, implying that the hot spot coincides with the light-curve minima. We applied models of cold and hot spots and a model of occultation by circumstellar material to investigate the source of the observed photometric variations. Conclusions: We conclude that nonuniformly distributed material in the inner part of the circumstellar disk is the main cause of the photometric modulation, which does not exclude the presence of hot and cold spots at the stellar surface. It is believed that the distortion in the inner part of the disk is created by the dynamical interaction between the stellar magnetosphere, inclined with respect to the rotation axis, and the circumstellar disk, as also observed in the classical T Tauri star AA Tau and predicted by magnetohydrodynamical numerical simulations. Based on the observations obtained with the CoRoT satellite, at the Observatório Pico dos Dias, Brazil, and at the Observatoire de Haute Provence, France. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.

Fonseca, N. N. J.; Alencar, S. H. P.; Bouvier, J.; Favata, F.; Flaccomio, E.

2014-07-01

309

Young [?/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?  

NASA Astrophysics Data System (ADS)

We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [?/Fe] abundance ratios (as determined from APOGEE spectra) with respect to solar values. Their existence is not explained bystandard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be reliably obtained, although in smaller relative numbers. This might explain why these stars have not previously received attention. The young [?/Fe]-rich stars are much more numerous in the CoRoT-APOGEE (CoRoGEE) inner-field sample than in any other high-resolution sample available at present because only CoRoGEE can explore the inner-disc regions and provide ages for its field stars. The kinematic properties of the young [?/Fe]-rich stars are not clearly thick-disc like, despite their rather large distances from the Galactic mid-plane. Our tentative interpretation of these and previous intriguing observations in the Milky Way is that these stars were formed close to the end of the Galactic bar, near corotation - a region where gas can be kept inert for longer times than in other regions that are more frequently shocked by the passage of spiral arms. Moreover, this is where the mass return from older inner-disc stellar generations is expected to be highest (according to an inside-out disc-formation scenario), which additionally dilutes the in-situ gas. Other possibilities to explain these observations (e.g., a recent gas-accretion event) are also discussed. Appendix A is available in electronic form at http://www.aanda.org

Chiappini, C.; Anders, F.; Rodrigues, T. S.; Miglio, A.; Montalbán, J.; Mosser, B.; Girardi, L.; Valentini, M.; Noels, A.; Morel, T.; Minchev, I.; Steinmetz, M.; Santiago, B. X.; Schultheis, M.; Martig, M.; da Costa, L. N.; Maia, M. A. G.; Allende Prieto, C.; de Assis Peralta, R.; Hekker, S.; Themeßl, N.; Kallinger, T.; García, R. A.; Mathur, S.; Baudin, F.; Beers, T. C.; Cunha, K.; Harding, P.; Holtzman, J.; Majewski, S.; Mészáros, Sz.; Nidever, D.; Pan, K.; Schiavon, R. P.; Shetrone, M. D.; Schneider, D. P.; Stassun, K.

2015-04-01

310

Exploring the origins of the young stars in the central parsec of the Galaxy with stellar dynamics  

NASA Astrophysics Data System (ADS)

One of the most perplexing problems associated with the supermassive black hole at the center of our Galaxy is the origin of the young stars in its close vicinity. In this thesis, the question of the young stars' origins is addressed using high-resolution infrared images obtained at the W. M. Keck telescopes to study both the distribution and kinematics of the young stellar population. First, using proper motion measurements and stellar number density counts based on 9 years of diffraction-limited K(2.2 microm)-band speckle imaging at the W. M. Keck 10-meter telescopes, we have identified a new comoving group of stars, which we call the IRS 16SW comoving group, located 1'' .9 (0.08 pc, in projection) from the central black hole. Four of the five members of this comoving group have been spectroscopically identified as massive young stars, specifically He I emission-line stars and OBN stars. This is the second young comoving group within the central parsec of the Milky Way to be recognized and is the closest, by a factor of 2, in projection to the central black hole. Second, we present new proper motions from the 10 m Keck telescopes for a puzzling population of massive, young stars located within a parsec of the supermassive black hole at the Galactic Center. Our proper motion measurements have uncertainties of only 0.07 mas/yr (3 km/s), which is ? 7 times better than previous proper motion measurements for these stars, and enables us to measure accelerations as low as 0.2 mas/yr 2 (7 km/s/yr). These measurements, along with stellar line-of-sight velocities from the literature, constrain the true orbit of each individual star and allow us to directly test the hypothesis that the massive stars reside in two stellar disks as has been previously proposed. Analysis of the stellar orbits reveals only one disk of young stars. No second disk was detected using a method that is capable of detecting disks with half-opening angles of 19° and containing at least 7 stars. The detected disk contains 50% of the young stars and is inclined by ˜115° from the plane of the sky and oriented at a position angle of ˜100° East of North. Additionally, the on-disk and off-disk populations have similar K-band luminosity functions and radial distributions that decrease at larger radii as r-2 . The disk has a finite thickness as expressed by an out-of-the-disk velocity dispersion of 28 +/- 6 km/s, and several candidate disk members have eccentricities greater than 0.2. The comoving group, IRS 16SW, makes up the South-Eastern component of the disk as stars on the other side of the disk are less apparent due to higher extinction. Our findings suggest that the young stars may have formed in situ but in a more complex geometry than a simple thin circular disk. Finally, future astrometric studies of the young stars in the Galactic Center will take advantage of the factor of 5 improvement in astrometric precision that is possible with new laser guide star adaptive optics imaging techniques. We show that careful data calibration and analysis, including correcting for differential atmospheric refraction, results in a relative astrometric accuracy of ˜0.2 mas over multi-year time scales. To achieve this level of accuracy, we find that many individual images of the Galactic Center should be combined to produce a long integration time (˜25--50 minutes) in order to average down the short time scale, high-order astrometric fluctations that are apparent in individual 30 second exposures. These improvements in astrometric precision and accuracy not only offers the opportunity to measure the accelerations for more young stars at larger radii than has previously been possible, but is also applicable to a much broader range of scientific experiments beyond the Galactic Center.

Lu, Jessica Ryan

311

Evolution of dynamo-generated magnetic fields in accretion disks around compact and young stars  

NASA Technical Reports Server (NTRS)

Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized objects because the accreted matter, whether it comes from the companion star (binaries) or from a collapsing molecular cloud core (single young stars), carries an embedded magnetic field. In addition, most accretion disks are hot and turbulent, thus meeting the condition for the MHD turbulent dynamo to maintain and amplify any seed field magnetic field. In fact, for a disk's magnetic field to persist long enough in comparison with the disk viscous time it must be contemporaneously regenerated because the characteristic diffusion time of a magnetic field is typically much shorter than a disk's viscous time. This is true for most thin accretion disks. Consequently, studying magentic fields in thin disks is usually synonymous with studying magnetic dynamos, a fact that is not commonly recognized in the literature. Progress in studying the structure of many accretion disks was achieved mainly because most disks can be regarded as two-dimensional flows in which vertical and radial structures are largely decoupled. By analogy, in a thin disk, one may expect that vertical and radial structures of the magnetic field are decoupled because the magnetic field diffuses more rapidly to the vertical boundary of the disk than along the radius. Thus, an asymptotic method, called an adiabatic approximation, can be applied to accretion disk dynamo. We can represent the solution to the dynamo equation in the form B = Q(r)b(r,z), where Q(r) describes the field distribution along the radius, while the field distribution across the disk is included in the vector function b, which parametrically depends on r and is normalized by the condition max (b(z)) = 1. The field distribution across the disk is established rapidly, while the radial distribution Q(r) evolves on a considerably longer timescale. It is this evolution that is the subject of this paper.

Stepinski, Tomasz F.

1994-01-01

312

OBSERVED LUMINOSITY SPREAD IN YOUNG CLUSTERS AND FU Ori STARS: A UNIFIED PICTURE  

SciTech Connect

The idea that non-steady accretion during the embedded phase of protostar evolution can produce the observed luminosity spread in the Herzsprung-Russell diagram (HRD) of young clusters has recently been called into question. Observations of FU Ori, for instance, suggest an expansion of the star during strong accretion events, whereas the luminosity spread implies a contraction of the accreting objects, decreasing their radiating surface. In this paper, we present a global scenario based on calculations coupling episodic accretion histories derived from numerical simulations of collapsing cloud prestellar cores of various masses and subsequent protostar evolution. Our calculations show that, assuming an initial protostar mass M{sub i} {approx} 1 M{sub Jup}, typical of the second Larson's core, both the luminosity spread in the HRD and the inferred properties of FU Ori events (mass, radius, accretion rate) can be explained by this scenario, providing two conditions. First, there must be some variation within the fraction of accretion energy absorbed by the protostar during the accretion process. Second, the range of this variation should increase with increasing accretion burst intensity and thus with the initial core mass and final star mass. The numerical hydrodynamics simulations of collapsing cloud prestellar cores indeed show that the intensity of the accretion bursts correlates with the mass and initial angular momentum of the prestellar core. Massive prestellar cores with high initial angular momentum are found to produce intense bursts characteristic of FU Ori-like events. Our results thus suggest a link between the burst intensities and the fraction of accretion energy absorbed by the protostar, with some threshold in the accretion rate, of the order of 10{sup -5} M{sub Sun} yr{sup -1}, delimitating the transition from 'cold' to 'hot' accretion. Such a transition might reflect a change in the accretion geometry with increasing accretion rate, i.e., a transition from magnetospheric or thin-disk to thick-disk accretion, or in the magnetospheric interaction between the star and the disk. Conversely, the luminosity spread can also be explained by a variation of the initial protostar mass within the {approx}1-5 M{sub Jup} range, although it is unclear for now whether such a spread among the second Larson's core can be produced during the prestellar core second collapse. This unified picture confirms the idea that early accretion during protostar and proto-brown dwarf formation/evolution can explain the observed luminosity spread in young clusters without invoking any significant age spread, and that the concept of a well-defined birthline does not apply for low-mass objects. Finally, we examine the impact of accretion on the determination of the initial mass function in young clusters.

Baraffe, I.; Chabrier, G. [Astrophysics Group, University of Exeter, EX4 4QL Exeter (United Kingdom); Vorobyov, E., E-mail: i.baraffe@ex.ac.uk, E-mail: eduard.vorobiev@univie.ac.at, E-mail: gilles.chabrier@ens-lyon.fr [Institute of Astrophysics, University of Vienna, Vienna A-1180 (Austria)

2012-09-10

313

A Search for Very Low-mass Stars and Brown Dwarfs in the Young sigma Orionis Cluster  

Microsoft Academic Search

We present a CCD-based photometric survey covering 870 sq. arcmin in a young\\u000astellar cluster around the young multiple star sigma Orionis. Our survey\\u000alimiting R, I, and Z magnitudes are 23.2, 21.8, and 21.0, respectively. From\\u000aour colour-magnitude diagrams, we have selected 49 faint objects, which\\u000asmoothly extrapolate the photometric sequence defined by more massive known\\u000amembers. Adopting the

V. J. S. Bejar; M. R. Zapatero Osorio; R. Rebolo

1999-01-01

314

Effects of Turbulence on Cosmic Ray Propagation in Protostars and Young Star/Disk Systems  

NASA Astrophysics Data System (ADS)

The magnetic fields associated with young stellar objects are expected to have an hour-glass geometry, i.e., the magnetic field lines are pinched as they thread the equatorial plane surrounding the forming star but merge smoothly onto a background field at large distances. With this field configuration, incoming cosmic rays experience both a funneling effect that acts to enhance the flux impinging on the circumstellar disk and a magnetic mirroring effect that acts to reduce that flux. To leading order, these effects nearly cancel out for simple underlying magnetic field structures. However, the environments surrounding young stellar objects are expected to be highly turbulent. This paper shows how the presence of magnetic field fluctuations affects the process of magnetic mirroring, and thereby changes the flux of cosmic rays striking circumstellar disks. Turbulence has two principle effects: (1) the (single) location of the magnetic mirror point found in the absence of turbulence is replaced with a wide distribution of values. (2) The median of the mirror point distribution moves outward for sufficiently large fluctuation amplitudes (roughly when ?B/B 0 > 0.2 at the location of the turbulence-free mirror point); the distribution becomes significantly non-Gaussian in this regime as well. These results may have significant consequences for the ionization fraction of the disk, which in turn dictates the efficiency with which disk material can accrete onto the central object. A similar reduction in cosmic ray flux can occur during the earlier protostellar stages; the decrease in ionization can help alleviate the magnetic braking problem that inhibits disk formation.

Fatuzzo, Marco; Adams, Fred C.

2014-05-01

315

Starbursts near supermassive black holes: young stars in the Galactic Center, and gravitational waves in LISA band  

E-print Network

We propose a scenario in which massive stars form in a self-gravitating gaseous disc around a supermassive black hole. We find that once the surface density of the disc exceeds a critical value, the disc fragments into dense clumps. The clumps accrete material from the remaining disc and merge into larger clumps; the upper mass of a merged clump is a few tens to a few hundreds of solar mass. This picture fits well with the observed young stellar discs near the SgrA* black hole in the Galactic Center. In particular, we show how the masses and spatial distribution of the young stars, and the total mass in the Galactic Center discs can be explained. However, explaining the origin of the several young stars closest to the black hole (the S-stars) is more problematic: their orbits are compact, eccentric, and have random orientation. We propose that the S-stars were born in a previous starburst(s), and then migrated through their parent disc via type I or runaway migration. Their orbits were then randomized by the Rauch-Tremaine resonant relaxation. We then explore the consequences of the star-formation scenario for AGN discs, which are continuously resupplied with gas. We argue that some compact remnants generated by the starburst will get embedded in the disc. The disc-born stellar-mass black holes will interact gravitationally with the massive accretion disc and be dragged towards the central black hole. Merger of a disc-born black hole with the central black hole will produce a burst of gravitational waves. If the central black hole is accreting at a rate comparable to the Eddington limit, the gas drag from the accretion disc will not alter significantly the dynamics of the final year of merger, and the gravitational waves should be observable by LISA.

Yuri Levin

2006-03-21

316

THE FIRST X-SHOOTER OBSERVATIONS OF JETS FROM YOUNG STARS  

SciTech Connect

We present the first pilot study of jets from young stars conducted with X-shooter, on the ESO/Very Large Telescope. As it offers simultaneous, high-quality spectra in the range 300-2500 nm, X-shooter is uniquely important for spectral diagnostics in jet studies. We chose to probe the accretion/ejection mechanisms at low stellar masses examining two targets with well-resolved continuous jets lying on the plane of the sky: ESO-HA 574 in Chameleon I and Par-Lup3-4 in Lupus III. The mass of the latter is close to the sub-stellar boundary (M{sub *} = 0.13 M{sub sun}). A large number of emission lines probing regions of different excitation are identified, position-velocity diagrams are presented, and mass outflow/accretion rates are estimated. Comparison between the two objects is striking. ESO-HA 574 is a weakly accreting star for which we estimate a mass accretion rate of log ( M-dot{sub acc}) = -10.8{+-}0.5 (in M{sub sun} yr{sup -1}), yet it drives a powerful jet with M-dot{sub out} {approx} 1.5-2.7 x 10{sup -9} M{sub sun} yr{sup -1}. These values can be reconciled with a magneto-centrifugal jet acceleration mechanism assuming that the presence of the edge-on disk severely depresses the luminosity of the accretion tracers. In comparison, Par-Lup3-4, with stronger mass accretion ( log ( M-dot{sub acc}) = -9.1{+-}0.4 M{sub sun} yr{sup -1}), drives a low-excitation jet with about M-dot{sub out} {approx} 3.2 x 10{sup -10} M{sub sun} yr{sup -1} in both lobes. Despite the low stellar mass, M-dot{sub out}/ M-dot{sub acc} for Par-Lup3-4 is at the upper limit of the range usually measured for young objects, but still compatible with a steady magneto-centrifugal wind scenario if all uncertainties are considered.

Bacciotti, F.; Randich, S. [INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze (Italy); Whelan, E. T. [Laboratoire d'Astrophysique de l'Observatoire de Grenoble, UMR 5521 du CNRS, 38041 Grenoble Cedex (France); Alcala, J. M. [INAF-Osservatorio di Capodimonte, Via Moiariello 16, 80131 Napoli (Italy); Nisini, B. [INAF-Osservatorio di Roma, Via di Frascati 33, 00040, Monteporzio Catone (Italy); Podio, L. [Kapteyn Astronomical Institute, Landleven 12, 9747 AD Groningen (Netherlands); Stelzer, B. [INAF-Osservatorio di Palermo, Piazza del Parlamento 1, 90134 Palermo (Italy); Cupani, G. [INAF-Osservatorio di Trieste, Via Tiepolo 11, 34143 Trieste (Italy)

2011-08-20

317

A direct imaging search for close stellar and sub-stellar companions to young nearby stars  

NASA Astrophysics Data System (ADS)

A total of 28 young nearby stars (ages {? 60} Myr) have been observed in the K_s-band with the adaptive optics imager Naos-Conica of the Very Large Telescope at the Paranal Observatory in Chile. Among the targets are ten visual binaries and one triple system at distances between 10 and 130 pc, all previously known. During a first observing epoch a total of 20 faint stellar or sub-stellar companion-candidates were detected around seven of the targets. These fields, as well as most of the stellar binaries, were re-observed with the same instrument during a second epoch, about one year later. We present the astrometric observations of all binaries. Their analysis revealed that all stellar binaries are co-moving. In two cases (HD 119022 AB and FG Aqr B/C) indications for significant orbital motions were found. However, all sub-stellar companion candidates turned out to be non-moving background objects except PZ Tel which is part of this project but whose results were published elsewhere. Detection limits were determined for all targets, and limiting masses were derived adopting three different age values; they turn out to be less than 10 Jupiter masses in most cases, well below the brown dwarf mass range. The fraction of stellar multiplicity and of the sub-stellar companion occurrence in the star forming regions in Chamaeleon are compared to the statistics of our search, and possible reasons for the observed differences are discussed. Based on observations made with ESO telescopes at Paranal Observatory under programme IDs 083.C-0150(B), 084.C-0364(A), 084.C-0364(B), 084.C-0364(C), 086.C-0600(A) and 086.C-0600(B).

Vogt, N.; Mugrauer, M.; Neuhäuser, R.; Schmidt, T. O. B.; Contreras-Quijada, A.; Schmidt, J. G.

2015-01-01

318

Dynamical Masses of Young Star Clusters in NGC 4038/4039  

E-print Network

In order to estimate the masses of the compact, young star clusters in the merging galaxy pair, NGC 4038/4039 (``the Antennae''), we have obtained medium and high resolution spectroscopy using ISAAC on VLT-UT1 and UVES on VLT-UT2 of five such clusters. The velocity dispersions were estimated using the stellar absorption features of CO at 2.29 microns and metal absorption lines at around 8500 \\AA, including lines of the Calcium Triplet. The size scales and light profiles were measured from HST images. From these data and assuming Virial equilibrium, we estimated the masses of five clusters. The resulting masses range from 6.5 x 10^5 to 4.7 x 10^6 M_sun. These masses are large, factor of a few to more than 10 larger than the typical mass of a globular cluster in the Milky Way. The mass-to-light ratios for these clusters in the V- and K-bands in comparison with stellar synthesis models suggest that to first order the IMF slopes are approximately consistent with Salpeter for a mass range of 0.1 to 100 M_sun. However, the clusters show a significant range of possible IMF slopes or lower mass cut-offs and that these variations may correlate with the interstellar environment of the cluster. Comparison with the results of Fokker-Planck simulations of compact clusters with properties similar to the clusters studied here, suggest that they are likely to be long-lived and may lose a substantial fraction of their total mass. This mass loss would make the star clusters obtain masses which are comparable to the typical mass of a globular cluster.

Sabine Mengel; Matthew D. Lehnert; Niranjan Thatte; Reinhard Genzel

2001-11-29

319

A UKIDSS-based search for low-mass stars and small stellar clumps in off-cloud parts of young star-forming regions *  

NASA Astrophysics Data System (ADS)

The form and universality of the mass function of young and nearby star-forming regions is still under debate. Its relation to the stellar density, its mass peak and the dependency on most recent models shows significant differencies for the various regions and remains unclear up to date. We aim to get a more complete census of two of such regions. We investigate yet unexplored areas of Orion and Taurus-Auriga, observed by the UKIDSS survey. In the latter, we search for low-mass stars via photometric and proper motion criteria and signs for variability. In Orion, we search for small stellar clumps via nearest-neighbor methods. Highlights in Taurus would be the finding of the missing low-mass stars and the detection of a young cluster T dwarf. In Orion, we discovered small stellar associations of its OB1b and OB1c populations. Combined with what is known in literature, we will provide by this investigations a general picture of the results of the star-forming processes in large areas of Taurus and Orion and probe the most recent models. Based on data of the UKIRT (operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the U.K.) Infrared Deep Sky Survey (UKIDSS).Supported by the Marie Curie Research Training Network `CONSTELLATION' under grant no. MRTN-CT-2006-035890.

Perger, M.; Lodieu, N.; Martín, E. L.; Barrado Y Navascués, D.

2011-07-01

320

Young Stars in Orion May Solve Mystery of Our Solar System  

NASA Astrophysics Data System (ADS)

Scientists may have to give the Sun a little more credit. Exotic isotopes present in the early Solar System--which scientists have long-assumed were sprinkled there by a powerful, nearby star explosion--may have instead been forged locally by our Sun during the colossal solar-flare tantrums of its baby years. The isotopes--special forms of atomic nuclei, such as aluminum-26, calcium-41, and beryllium-10--can form in the X-ray solar flares of young stars in the Orion Nebula, which behave just like our Sun would have at such an early age. The finding, based on observations by the Chandra X-ray Observatory, has broad implications for the formation of our own Solar System. Eric Feigelson, professor of astronomy and astrophysics at Penn State, led a team of scientists on this Chandra observation and presents these results in Washington, D.C., today at a conference entitled "Two Years of Science with Chandra". "The Chandra study of Orion gives us the first chance to study the flaring properties of stars resembling the Sun when our solar system was forming," said Feigelson. "We found a much higher rate of flares than expected, sufficient to explain the production of many unusual isotopes locked away in ancient meteorites. If the young stars in Orion can do it, then our Sun should have been able to do it too." Scientists who study how our Solar System formed from a collapsed cloud of dust and gas have been hard pressed to explain the presence of these extremely unusual chemical isotopes. The isotopes are short-lived and had to have been formed no earlier than the creation of the Solar System, some five billion years ago. Yet these elements cannot be produced by a star as massive as our Sun under normal circumstances. (Other elements, such as silver and gold, were created long before the creation of the solar system.) The perplexing presence of these isotopic anomalies, found in ancient meteoroids orbiting the Earth, led to the theory that a supernova explosion occurred very close to the Solar System's progenitor gas cloud, simultaneously triggering its collapse and seeding it with short-lived isotopes. Solar flares could produce such isotopes, but the flares would have to be hundreds of thousands of times more powerful and hundreds of times more frequent than those our Sun generates. Enter the stars in the Orion Nebula. This star-forming region has several dozen new stars nearly identical to our Sun, only much younger. Feigelson's team used Chandra to study the flaring in these analogs of the early Sun and found that nearly all exhibit extremely high levels of X-ray flaring--powerful and frequent enough to forge many of the kinds of isotopes found in the ancient meteorites from the early solar system. "This is a very exciting result for space X-ray astronomy," said Donald Clayton, Centennial Professor of Physics and Astronomy at Clemson University. "The Chandra Penn State team has shown that stellar-flare acceleration produces radioactive nuclei whether we want them or not. Now the science debate can concentrate on whether such irradiation made some or even all of the extinct radioactivities that were present when our solar system was formed, or whether some contamination of our birth molecular cloud by external material is also needed." "This is an excellent example of how apparently distant scientific fields, like X-ray astronomy and the origins of solar systems, can in fact be closely linked," said Feigelson. The Orion observation was made with Chandra's Advanced CCD Imaging Spectrometer, which was conceived and developed for NASA by Penn State and Massachusetts Institute of Technology under the leadership of Gordon Garmire, the Evan Pugh Professor of Astronomy and Astrophysics at Penn State. The Penn State observation team includes Pat Broos, James Gaffney, Gordon Garmire, Leisa Townsley and Yohko Tsuboi. Collaborators also include Lynne Hillenbrand of CalTech and Steven Pravdo of the NASA Jet Propulsion Laboratory. Background: Isotopes are atoms whose nuclei have d

2001-09-01

321

THE IMPORTANCE OF NEBULAR CONTINUUM AND LINE EMISSION IN OBSERVATIONS OF YOUNG MASSIVE STAR CLUSTERS  

SciTech Connect

In this spectroscopic study of infant massive star clusters, we find that continuum emission from ionized gas rivals the stellar luminosity at optical wavelengths. In addition, we find that nebular line emission is significant in many commonly used broadband Hubble Space Telescope (HST) filters including the F814W I-band, the F555W V-band, and the F435W B-band. Two young massive clusters (YMCs) in the nearby starburst galaxy NGC 4449 were targeted for follow-up spectroscopic observations after Reines et al. discovered an F814W I-band excess in their photometric study of radio-detected clusters in the galaxy. The spectra were obtained with the Dual Imaging Spectrograph (DIS) on the 3.5 m Apache Point Observatory (APO) telescope and have a spectral range of approx3800-9800 A. We supplement these data with HST and Sloan Digital Sky Survey photometry of the clusters. By comparing our data to the Starburst99 and GALEV evolutionary synthesis models, we find that nebular continuum emission competes with the stellar light in our observations and that the relative contribution from the nebular continuum is largest in the U- and I-bands, where the Balmer (3646 A) and Paschen jumps (8207 A) are located. The spectra also exhibit strong line emission including the [S III] lambdalambda9069, 9532 lines in the HST F814W I-band. We find that the combination of nebular continuum and line emission can account for the F814W I-band excess previously found by Reines et al. In an effort to provide a benchmark for estimating the impact of ionized gas emission on photometric observations of young massive stellar populations, we compute the relative contributions of the stellar continuum, nebular continuum, and emission lines to the total observed flux of a 3 Myr old cluster through various HST filter/instrument combinations, including filters in the Wide Field Camera 3. We urge caution when comparing observations of YMCs to evolutionary synthesis models since nebular continuum and line emission can have a large impact on magnitudes and colors of young (approx<5 Myr) clusters, significantly affecting inferred properties such as ages, masses and extinctions.

Reines, Amy E.; Nidever, David L.; Whelan, David G. [Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325 (United States); Johnson, Kelsey E., E-mail: areines@virginia.ed [Adjunct at National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA. (United States)

2010-01-01

322

X-RAY FLARES IN ORION YOUNG STARS. II. FLARES, MAGNETOSPHERES, AND PROTOPLANETARY DISKS  

E-print Network

-developed analytical theory assumes that a dipolar field extends to several stellar radii where the Keplerian orbits: individual (Orion Nebula Cluster) -- planetary systems: protoplanetary disks -- stars: flare -- stars

Micela, Giusi

323

Astronomical Observations and Laboratory Analogs of Shock Waves in Supersonic Collimated Jets From Young Stars  

NASA Astrophysics Data System (ADS)

Jets from young stars offer a rich environment for studying shock waves in collimated jets. These remarkable supersonic flows consist of multiple bow shocks which form in response to velocity perturbations. In at least two cases, jets deflect from obstacles located along the path of the flow and produce spectacular shocked wakes and shear. The hot gas behind the shocks in stellar jets radiates optically thin forbidden lines, from which one can measure turbulent line widths, densities, ionization fractions, and temperature, and even watch how the jets and shocks evolve in real time. On the laboratory side, experiments at Omega, Magpie, and other laser facilities have begun to produce analogs of stellar jets, which, when appropriately scaled, have dimensionless fluid parameters similar to those present in the astrophysical flows. The laboratory experiments provide insights into several aspects of jets, including morphologies and time evolution of deflection shocks, dynamics of shocked material, and interactions between colliding winds. New experiments will probe the roles of clumps in defining the morphologies of jets, and will explore how magnetic fields influence the shock waves. This talk summarizes recent observational developments that pertain to stellar jets, and outlines areas where laboratory experiments are most likely to improve our understanding of this fundamental astrophysical process.

Hartigan, Patrick M.

2008-05-01

324

Young massive star clusters in the era of the Hubble Space Telescope  

E-print Network

The Hubble Space Telescope (HST) has been instrumental in the discovery of large numbers of extragalactic young massive star clusters (YMCs), often assumed to be proto-globular clusters (GCs). As a consequence, the field of YMC formation and evolution is thriving, generating major breakthroughs as well as controversies on annual (or shorter) time-scales. Here, I review the long-term survival chances of YMCs, hallmarks of intense starburst episodes often associated with violent galaxy interactions. In the absence of significant external perturbations, the key factor determining a cluster's long-term survival chances is the shape of its stellar initial mass function (IMF). It is, however, not straightforward to assess the IMF shape in unresolved extragalactic YMCs. I also discuss the latest progress in worldwide efforts to better understand the evolution of entire cluster populations, predominantly based on HST observations, and conclude that there is an increasing body of evidence that GC formation appears to be continuing until today; their long-term evolution crucially depends on their environmental conditions, however.

Richard de Grijs

2007-06-13

325

Evolutionary models for low-mass stars and brown dwarfs: Uncertainties and limits at very young ages  

Microsoft Academic Search

We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses m <= 1.4 Msun based on the Baraffe et al. (\\\\cite{Bar98}) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists,

I. Baraffe; G. Chabrier; F. Allard; P. H. Hauschildt

2002-01-01

326

NGC 2782: a merger remnant with young stars in its gaseous tidal tail  

NASA Astrophysics Data System (ADS)

We have searched for young star-forming regions around the merger remnant NGC 2782. By using Galaxy Evolution Explorer far-ultraviolet and near-ultraviolet imaging and H I data we found seven ultraviolet sources, located at distances greater than 26 kpc from the centre of NGC 2782, and coinciding with its western H I tidal tail. These regions were resolved in several smaller systems when Gemini/Gemini multi-object spectrograph (GMOS) r-band images were used. We compared the observed colours to stellar population synthesis models and found that these objects have ages of ˜1 to 11 Myr and masses ranging from 103.9 to 104.6 M?. By using Gemini/GMOS spectroscopic data we confirm memberships and derive high metallicities for three of the young regions in the tail (12+log(O/H) = 8.74 ± 0.20, 8.81 ± 0.20 and 8.78 ± 0.20). These metallicities are similar to the value presented by the nuclear region of NGC 2782 and also similar to the value presented for an object located close to the main body of NGC 2782. The high metallicities measured for the star-forming regions in the gaseous tidal tail of NGC 2782 could be explained if they were formed out of highly enriched gas which was once expelled from the centre of the merging galaxies when the system collided. An additional possibility is that the tail has been a nursery of a few generations of young stellar systems which ultimately polluted this medium with metals, further enriching the already pre-enriched gas ejected to the tail when the galaxies collided. a r-band magnitude inside a fixed aperture of 4 arcsec radius and corrected by MW extinction. Region 2 appears to have diffuse emission in the r-band image. b Fixed FUV-band magnitudes corrected by apertures (as described in Section 3.2) and MW extinction. c FUV-r colours. d Magnitudes were measured in a fixed aperture of 4 arcsec radius and corrected by MW extinction. FUV and NUV magnitudes were corrected by aperture, as described in Section 3.2. e Left values: E(B-V) estimated as in Section 3.3. Right values: E(B-V) estimated by using the H I data of NGC 2782. Total extinction is the sum of the internal and Galactic extinctions (0.13 mag in this band). f Ages estimated from FUV-NUV and FUV-r. In parenthesis we show the ages estimated from the H? equivaland SB99 models. g Distance from the centre of the galaxy, assuming a position angle and inclination of 0°.

Torres-Flores, S.; de Oliveira, C. Mendes; de Mello, D. F.; Scarano, S.; Urrutia-Viscarra, F.

2012-04-01

327

A survey of young, nearby, and dusty stars to understand the formation of wide-orbit giant planets  

E-print Network

Direct imaging has confirmed the existence of substellar companions on wide orbits. To understand the formation and evolution mechanisms of these companions, the full population properties must be characterized. We aim at detecting giant planet and/or brown dwarf companions around young, nearby, and dusty stars. Our goal is also to provide statistics on the population of giant planets at wide-orbits and discuss planet formation models. We report a deep survey of 59 stars, members of young stellar associations. The observations were conducted with VLT/NaCo at L'-band (3.8 micron). We used angular differential imaging to reach optimal detection performance. A statistical analysis of about 60 % of the young and southern A-F stars closer than 65 pc allows us to derive the fraction of giant planets on wide orbits. We use gravitational instability models and planet population synthesis models following the core-accretion scenario to discuss the occurrence of these companions. We resolve and characterize new visual ...

Rameau, J; Lagrange, A -M; Klahr, H; Bonnefoy, M; Mordasini, C; Bonavita, M; Desidera, S; Dumas, C; Girard, J H

2013-01-01

328

Dynamic Young Stars and their Disks: A Temporal View of NGC 2264 with Spitzer and CoRoT  

NASA Astrophysics Data System (ADS)

Variability is a signature feature of young stars. Among the well known light curve phenomena are periodic variations attributed to surface spots and irregular changes associated with accretion or circumstellar disk material. While decades of photometric monitoring have provided a framework for classifying young star variability, we still know surprisingly little about its underlying mechanisms and connections to the surrounding disks. In the past few years, dedicated photometric monitoring campaigns from the ground and space have revolutionized our view of young stars in the time domain. We present a selection of optical and infrared time series from several recent campaigns, highlighting the Coordinated Synoptic Investigation of NGC 2264 ("CSI 2264")- a joint30-day effort with the Spitzer, CoRoT, and MOST telescopes. The extraordinary photometric precision, high cadence, and long time baseline of these observations is now enabling correlation of variability properties at very different wavelengths, corresponding to locations from the stellar surface to the inner 0.1 AU of the disk. We present some results of the CSI 2264 program, including new classes of optical/infrared behavior. Further efforts to tie observed variability features to physical models will provide insights into the inner disk environment at a time when planet formation may be underway. Based on data from the Spitzer and CoRoT missions. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA-s RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.

Cody, Ann Marie; Stauffer, John; Bouvier, Jèrôme

2014-01-01

329

Erupting Volcanoes!  

NSDL National Science Digital Library

This lesson presents volcanoes through the making of volcano models. While students are constructing their physical representations of volcanoes, they will be filled with questions about volcanoes as well as how to build their models. This process will provide students with a tangible reference for learning about volcanoes and give them a chance to problem-solve as they build their models. Students will be able to observe how the eruption changes the original form of their volcano model. In this way, students see first hand how this type of phenomenon creates physical change. While students at this level may struggle to understand larger and more abstract geographical concepts, they will work directly with material that will help them build a foundation for understanding concepts of phenomena that sculpt the Earth.

330

KEPLER-63b: A GIANT PLANET IN A POLAR ORBIT AROUND A YOUNG SUN-LIKE STAR  

SciTech Connect

We present the discovery and characterization of a giant planet orbiting the young Sun-like star Kepler-63 (KOI-63, m{sub Kp} = 11.6, T{sub eff} = 5576 K, M{sub *} = 0.98 M{sub ?}). The planet transits every 9.43 days, with apparent depth variations and brightening anomalies caused by large starspots. The planet's radius is 6.1 ± 0.2 R{sub ?}, based on the transit light curve and the estimated stellar parameters. The planet's mass could not be measured with the existing radial-velocity data, due to the high level of stellar activity, but if we assume a circular orbit, then we can place a rough upper bound of 120 M{sub ?} (3?). The host star has a high obliquity (? = 104°), based on the Rossiter-McLaughlin effect and an analysis of starspot-crossing events. This result is valuable because almost all previous obliquity measurements are for stars with more massive planets and shorter-period orbits. In addition, the polar orbit of the planet combined with an analysis of spot-crossing events reveals a large and persistent polar starspot. Such spots have previously been inferred using Doppler tomography, and predicted in simulations of magnetic activity of young Sun-like stars.

Sanchis-Ojeda, Roberto; Winn, Joshua N.; Albrecht, Simon [Department of Physics, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States); Marcy, Geoffrey W.; Isaacson, Howard [Astronomy Department, University of California, Berkeley, CA 94720 (United States); Howard, Andrew W. [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Johnson, John Asher [Department of Astronomy, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Torres, Guillermo; Carter, Joshua A.; Dawson, Rebekah I.; Geary, John C. [Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138 (United States); Campante, Tiago L.; Chaplin, William J.; Davies, Guy R. [School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT (United Kingdom); Lund, Mikkel N. [Stellar Astrophysics Centre (SAC), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C (Denmark); Buchhave, Lars A. [Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, DK-2100 Copenhagen (Denmark); Everett, Mark E. [National Optical Astronomy Observatory, 950 N. Cherry Ave, Tucson, AZ 85719 (United States); Fischer, Debra A. [Astronomy Department, Yale University, New Haven, CT (United States); Gilliland, Ronald L. [Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802 (United States); Horch, Elliott P. [Southern Connecticut State University, New Haven, CT 06515 (United States); and others

2013-09-20

331

CLOSE COMPANIONS TO YOUNG STARS. I. A LARGE SPECTROSCOPIC SURVEY IN CHAMAELEON I AND TAURUS-AURIGA  

SciTech Connect

We present the results of a multiplicity survey of 212 T Tauri stars in the Chamaeleon I and Taurus-Auriga star-forming regions, based on high-resolution spectra from the Magellan Clay 6.5 m telescope. From these data, we achieved a typical radial velocity (RV) precision of {approx}80 m s{sup -1} with slower rotators yielding better precision, in general. For 174 of these stars, we obtained multi-epoch data with sufficient time baselines to identify binaries based on RV variations. We identified eight close binaries and four close triples, of which three and two, respectively, are new discoveries. The spectroscopic multiplicity fractions we find for Chamaeleon I (7%) and Taurus-Auriga (6%) are similar to each other, and to the results of field star surveys in the same mass and period regime. However, unlike the results from imaging surveys, the frequency of systems with close companions in our sample is not seen to depend on primary mass. Additionally, we do not find a strong correlation between accretion and close multiplicity. This implies that close companions are not likely the main source of the accretion shut down observed in weak-lined T Tauri stars. Our results also suggest that sufficient RV precision can be achieved for at least a subset of slowly rotating young stars to search for hot Jupiter planets.

Nguyen, Duy Cuong; Brandeker, Alexis; Van Kerkwijk, Marten H.; Jayawardhana, Ray, E-mail: nguyen@astro.utoronto.ca, E-mail: mhvk@astro.utoronto.ca, E-mail: rayjay@astro.utoronto.ca, E-mail: dcnguyen@pas.rochester.edu, E-mail: duy.nguyen@astro.su.se, E-mail: alexis@astro.su.se [Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4 (Canada)

2012-02-01

332

A "diamond-ring" star: the unusual morphologic structure of a young (multiple?) object  

E-print Network

We have observed IRAS06468-0325 obtaining optical and infrared images through IJHKs and L' filters, K-band low-resolution spectroscopy, together with millimetre line observations of CO and CS. IRAS06468-0325 has a very unusual and enigmatic morphology with two components: a bright, close to point-like source (the diamond) and a sharp-edge ring-like structure (the ring). The source is not detected in the optical, at wavelengths shorter than the I-band. The diamond is seen in all the imaging bands observed. The ring-like structure in IRAS06468-0325 is clearly seen in the I, J, H, and Ks. It is not detected in the L'-band image. Infrared colours of the diamond are compatible with excess circumstellar emission and a young stellar nature. A strongly non-gaussian and moderately bright CO(1-0) and {13}CO(2-1) lines are seen towards IRAS06468-0325, at v_{LSR} of 30.5 km s{-1} (corresponding to a kinematic distance of 3 kpc). Very weak C{18}O(2-1) and CS(2-1) lines were detected. K-band spectra of the diamond and of the ring are similar both in the slope of the continuum and in the presence of lines supporting the idea that the ring is reflected light from the diamond. With the current data, a few different scenarios are possible to explain the morphology of this object. However, the available data seem to favour that the morphology of IRAS06468-0325 correspond to a young stellar multiple system in a transient stage where a binary co-exists with a circumbinary disc, similar to the case of GG Tau. In this case, the sharpness of the well-defined ring may be due to tidal truncation from dynamic interactions between components in a binary or multiple stellar system. IRAS06468-0325 may be an important rare case that illustrates a short-lived stage of the process of binary or multiple star formation.

Joao L. Yun; Jose M. Torrelles; Nuno C. Santos

2007-04-24

333

Dynamics and multiplicity of brown dwarfs and young, low mass stars  

NASA Astrophysics Data System (ADS)

This thesis addresses the formation and evolution of very low mass (VLM) stellar objects using their multiplicity and dynamics. First, we surveyed thirteen VLM [Special characters omitted.] objects in the Taurus star-forming region using near-infrared speckle imaging techniques on the W.M. Keck 10 m telescope. Of these thirteen, five were found to be binary. These new systems have properties that differ significantly from older field VLM binaries in that the young systems have wider separations and lower mass ratios, supporting the idea that VLM binaries undergo significant dynamical evolution ~5-10 Myr after their formation. Second, we present a pilot study for our dynamics work in which we completed a five year monitoring campaign of the close binary TWA 5Aab in the TW Hydrae association, using speckle and adaptive optics on the Keck telescopes. Our observations allowed us to determine this system's astrometric orbit. We calculate a total mass of 0.71 ± 0.14 [Special characters omitted.] (D/44 pc) 3 for this system. Finally, we present the results of a 3 year monitoring program of a sample of 26 VLM field binaries using both astrometric and spectroscopic data obtained in conjunction with the laser guide star adaptive optics system on the Keck telescope. By combining astrometry and spectroscopy, degeneracies in orbital parameters can be resolved and individual component masses can be obtained. We present relative orbits for 15 of these systems, which allow us to derive the total system mass. In addition, we find the absolute orbits for 6 systems in our sample, which allows us to derive individual masses. We compare our dynamical mass measurements to the predictions of theoretical evolutionary models and find that there are systematic discrepancies, where both models considered either underpredict or overpredict the correct mass. The discrepancies are a function of spectral type, with late M through mid L systems tending to have their masses underpredicted, while one T type system has its mass overpredicted. These discrepancies imply that either the temperatures predicted by evolutionary and atmosphere models are inconsistent for an object of a given mass, or the mass-radius relationship or cooling timescales predicted by the evolutionary models are incorrect.

Konopacky, Quinn Morgan

334

Refining the asteroseismic model for the young ? Scuti star HD 144277 using HARPS spectroscopy  

NASA Astrophysics Data System (ADS)

Context. HD 144277 was previously discovered by Microvariability and Oscillations of Stars (MOST) space photometry to be a young and hot ? Scuti star showing regular groups of pulsation frequencies. The first asteroseismic models required lower than solar metallicity to fit the observed frequency range based on a purely photometric analysis. Aims: The aim of the present paper is to determine, by means of high-resolution spectroscopy, fundamental stellar parameters required for the asteroseismic model of HD 144277, and subsequently, to refine it. Methods: High-resolution, high signal-to-noise spectroscopic data obtained with the HARPS spectrograph were used to determine the fundamental parameters and chemical abundances of HD 144277. These values were put into context alongside the results from asteroseismic models. Results: The effective temperature, Teff, of HD 144277 was determined as 8640 +300-100 K, log g is 4.14 ± 0.15 and the projected rotational velocity, ?sini, is 62.0 ± 2.0 km s-1. As the ?sini value is significantly larger than previously assumed, we refined the first asteroseimic model accordingly. The overall metallicity Z was determined to be 0.011 where the light elements He, C, O, Na, and S show solar chemical composition, but the heavier elements are significantly underabundant. In addition, the radius of HD 144277 was determined to be 1.55 ± 0.65 R? from spectral energy distribution fitting, based on photometric data taken from the literature. Conclusions: From the spectroscopic observations, we could confirm our previous assumption from asteroseismic models that HD 144277 has less than solar metallicity. The fundamental parameters derived from asteroseismology, Teff, log g, L/L? and R/R? agree within one sigma to the values found from spectroscopic analysis. As the ?sini value is significantly higher than assumed in the first analysis, near-degeneracies and rotational mode coupling were taken into account in the new models. These suggest that HD 144277 has an equatorial rotational velocity of about 80 km s-1 and is seen equator-on. The observed frequencies are identified as prograde modes. This work is based on ground-based observations made with the 3.6 m telescope at La Silla Observatory under the ESO Large Programme LP185.D-0056.

Zwintz, K.; Ryabchikova, T.; Lenz, P.; Pamyatnykh, A. A.; Fossati, L.; Sitnova, T.; Breger, M.; Poretti, E.; Rainer, M.; Hareter, M.; Mantegazza, L.

2014-07-01

335

Magnetocentrifugally driven flows from young stars and disks. 2: Formulation of the dynamical problem  

NASA Technical Reports Server (NTRS)

We formulate the dynamical problem of a cool wind centrifugally driven from the magnetic interface of a young star and an adjoining Keplerian disk. We examine the situation for mildly accreting T Tauri stars that rotate slowly as well as rapidly accreting protostars that rotate near break-up. In both cases a wind can be driven from a small X-region just outside the stellar magnetopause, where the field lines assume an open geometry and are rooted to material that rotates at an angular speed equal both to the local Keplerian value and to the stellar angular speed. Assuming axial symmetry for the ideal magnetohydrodynamic flow, which requires us to postpone asking how the (lightly ionized) gas is loaded onto field lines, we can formally integrate all the governing equations analytically except for a partial equation that describes how streamlines spread in the meridional plane. Apart from the difficulty of dealing with PDEs of mixed type, finding the functional forms of the conserved quantities along streamlines - the ratio beta of magnetic field to mass flux, the specific energy H of the fluid in the rotating frame, and the total specific angular momentum J carried in the matter and the field - constitutes a standard difficulty in this kind of (Grad-Shafranov) formalism. Fortunately, because the ratio of the thermal speed of the mass-loss regions to the Keplerian speed of rotation of the interface constitutes a small parameter epsilon, we can attack the overall problem by the method of matched asymptotic expansions. This procedure leads to a natural and systematic technique for obtaining the relevant functional dependences of beta, H, and J. Moreover, we are able to solve analytically for the properties of the flow emergent from the small transsonic region driven by gas pressure without having to specify the detailed form of any of the conserved functions, beta, H, and J. This analytical solution provides inner boundary conditions for the numerical computation in a companion paper by Najita & Shu of the larger region where the main acceleration to terminal speeds occurs.

Shu, Frank H.; Najita, Joan; Ruden, Steven P.; Lizano, Susana

1994-01-01

336

NEAR-INFRARED VARIABILITY AMONG YOUNG STELLAR OBJECTS IN THE STAR FORMATION REGION CYGNUS OB7  

SciTech Connect

We present an analysis of near-infrared time-series photometry in J, H, and K bands for about 100 epochs of a 1 Degree-Sign Multiplication-Sign 1 Degree-Sign region of the Lynds 1003/1004 dark cloud in the Cygnus OB7 region. Augmented by data from the Wide-field Infrared Survey Explorer, we identify 96 candidate disk bearing young stellar objects (YSOs) in the region. Of these, 30 are clearly Class I or earlier. Using the Wide-Field Imaging Camera on the United Kingdom Infrared Telescope, we were able to obtain photometry over three observing seasons, with photometric uncertainty better than 0.05 mag down to J Almost-Equal-To 17. We study detailed light curves and color trajectories of {approx}50 of the YSOs in the monitored field. We investigate the variability and periodicity of the YSOs and find the data are consistent with all YSOs being variable in these wavelengths on timescales of a few years. We divide the variability into four observational classes: (1) stars with periodic variability stable over long timescales, (2) variables which exhibit short-lived cyclic behavior, (3) long-duration variables, and (4) stochastic variables. Some YSO variability defies simple classification. We can explain much of the observed variability as being due to dynamic and rotational changes in the disk, including an asymmetric or changing blocking fraction, changes to the inner disk hole size, as well as changes to the accretion rate. Overall, we find that the Class I:Class II ratio of the cluster is consistent with an age of <1 Myr, with at least one individual, wildly varying source {approx}100, 000 yr old. We have also discovered a Class II eclipsing binary system with a period of 17.87 days.

Wolk, Scott J.; Rice, Thomas S. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States); Aspin, Colin [Institute for Astronomy, University of Hawaii at Manoa, 640 North Aohoku Place, Hilo, HI 96720 (United States)

2013-08-20

337

Young star clusters in interacting galaxies - NGC 1487 and NGC 4038/4039  

E-print Network

We estimate the dynamical masses of several young (~10 Myr) massive star clusters in two interacting galaxies, NGC 4038/4039 ("The Antennae") and NGC 1487, under the assumption of virial equilibrium. These are compared with photometric mass estimates from K-band photometry and assuming a standard Kroupa IMF. The clusters were selected to have near-infrared colors dominated by red supergiants, and hence to be old enough to have survived the earliest phases of cluster evolution when the interstellar medium is rapidly swept out from the cluster, supported by the fact that there is no obvious Halpha emission associated with the clusters. All but one of the Antennae clusters have dynamical and photometric mass estimates which are within a factor ~2 of one another, implying both that standard IMFs provide a good approximation to the IMF of these clusters, and that there is no significant extra-virial motion, as would be expected if they were rapidly dispersing. These results suggest that almost all of the Antennae clusters in our sample have survived the gas removal phase as bound or marginally bound objects. Two of the three NGC 1487 clusters studied here have M_dyn estimates which are significantly larger than the photometric mass estimates. At least one of these two clusters, and one in the Antennae, may be actively in the process of dissolving. The process of dissolution contributes a component of non-virial motion to the integrated velocity measurements, resulting in an estimated M_dyn which is too high relative to the amount of measured stellar light. The dissolution candidates in both galaxies are amongst the clusters with the lowest pressures/densities measured in our sample.

Sabine Mengel; Matthew D. Lehnert; Niranjan A. Thatte; William D. Vacca; Brad Whitmore; Rupali Chandar

2008-05-16

338

GAS SURFACE DENSITY, STAR FORMATION RATE SURFACE DENSITY, AND THE MAXIMUM MASS OF YOUNG STAR CLUSTERS IN A DISK GALAXY. I. THE FLOCCULENT GALAXY M 33  

SciTech Connect

We analyze the relationship between maximum cluster mass M{sub max} and surface densities of total gas ({Sigma}{sub gas}), molecular gas ({Sigma}{sub H{sub 2}}), and star formation rate ({Sigma}{sub SFR}) in the flocculent galaxy M 33, using published gas data and a catalog of more than 600 young star clusters in its disk. By comparing the radial distributions of gas and most massive cluster masses, we find that M{sub max}{proportional_to}{Sigma}{sup 4.7{+-}0.4}{sub gas}, M{sub max}{proportional_to}{Sigma}{sup 1.3{+-}0.1}{sub H{sub 2}}, and M{sub max}{proportional_to}{Sigma}{sup 1.0{+-}0.1}{sub SFR}. We rule out that these correlations result from the size of the sample; hence, the change of the maximum cluster mass must be due to physical causes.

Gonzalez-Lopezlira, Rosa A.; Pflamm-Altenburg, Jan; Kroupa, Pavel, E-mail: r.gonzalez@crya.unam.mx [Argelander Institut fuer Astronomie, Universitaet Bonn, Auf dem Huegel 71, D-53121 Bonn (Germany)

2012-12-20

339

Jupiter Eruptions  

NASA Technical Reports Server (NTRS)

[figure removed for brevity, see original site] Click on the image for high resolution image of Nature Cover

Detailed analysis of two continent-sized storms that erupted in Jupiter's atmosphere in March 2007 shows that Jupiter's internal heat plays a significant role in generating atmospheric disturbances. Understanding these outbreaks could be the key to unlock the mysteries buried in the deep Jovian atmosphere, say astronomers.

This visible-light image is from NASA's Hubble Space Telescope taken on May 11, 2007. It shows the turbulent pattern generated by the two plumes on the upper left part of Jupiter.

Understanding these phenomena is important for Earth's meteorology where storms are present everywhere and jet streams dominate the atmospheric circulation. Jupiter is a natural laboratory where atmospheric scientists study the nature and interplay of the intense jets and severe atmospheric phenomena.

According to the analysis, the bright plumes were storm systems triggered in Jupiter's deep water clouds that moved upward in the atmosphere vi gorously and injected a fresh mixture of ammonia ice and water about 20 miles (30 kilometers) above the visible clouds. The storms moved in the peak of a jet stream in Jupiter's atmosphere at 375 miles per hour (600 kilometers per hour). Models of the disturbance indicate that the jet stream extends deep in the buried atmosphere of Jupiter, more than 60 miles (approximately100 kilometers) below the cloud tops where most sunlight is absorbed.

2008-01-01

340

Dynamics of stellar black holes in young star clusters with different metallicities - I. Implications for X-ray binaries  

NASA Astrophysics Data System (ADS)

We present N-body simulations of intermediate-mass (3000-4000 M?) young star clusters (SCs) with three different metallicities (Z = 0.01, 0.1 and 1 Z?), including metal-dependent stellar evolution recipes and binary evolution. Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. In particular, massive metal-poor stars (Z ? 0.3 Z?) are enabled to form massive stellar black holes (MSBHs, with mass ?25 M?) through direct collapse. We find that three-body encounters, and especially dynamical exchanges, dominate the evolution of the MSBHs formed in our simulations. In SCs with Z = 0.01 and 0.1 Z?, about 75 per cent of simulated MSBHs form from single stars and become members of binaries through dynamical exchanges in the first 100 Myr of the SC life. This is a factor of ?3 more efficient than in the case of low-mass (<25 M?) stellar black holes. A small but non-negligible fraction of MSBHs power wind-accreting (10-20 per cent) and Roche lobe overflow (RLO, 5-10 per cent) binary systems. The vast majority of MSBH binaries that undergo wind accretion and/or RLO were born from dynamical exchange. This result indicates that MSBHs can power X-ray binaries in low-metallicity young SCs, and is very promising to explain the association of many ultraluminous X-ray sources with low-metallicity and actively star-forming environments.

Mapelli, M.; Zampieri, L.; Ripamonti, E.; Bressan, A.

2013-03-01

341

Star-formation rates from young-star counts and the structure of the ISM across the NGC346/N66 complex in the SMC  

E-print Network

The rate at which interstellar gas is converted into stars, and its dependence on environment, is one of the pillars on which our understanding of the visible Universe is build. We present a comparison of the surface density of young stars (Sigma_*) and dust surface density (Sigma_d) across NGC346 (N66) in 115 independent pixels of 6x6 pc^2. We find a correlation between Sigma_* and Sigma_d with a considerable scatter. A power law fit to the data yields a steep relation with an exponent of 2.6+-0.2. We convert Sigma_d to gas surface density (Sigma_g) and Sigma_* to star formation rate (SFR) surface densities (Sigma_SFR), using simple assumptions for the gas-to-dust mass ratio and the duration of star formation. The derived total SFR (4+-1 10^-3 M_sun/yr) is consistent with SFR estimated from the Ha emission integrated over the Ha nebula. On small scales the Sigma_SFR derived using Ha systematically underestimates the count-based Sigma_SFR, by up to a factor of 10. This is due to ionizing photons escaping the ...

Hony, S; Galliano, F; Galametz, M; Cormier, D; Chen, C -H R; Dib, S; Hughes, A; Klessen, R S; Roman-Duval, J; Smith, L; Bernard, J -P; Bot, C; Carlson, L; Gordon, K; Indebetouw, R; Lebouteiller, V; Lee, M -Y; Madden, S C; Meixner, M; Oliveira, J; Rubio, M; Sauvage, M; Wu, R

2015-01-01

342

X-ray Flares Observed from Six Young Stars Located in the Region of Star Clusters NGC 869 and IC 2602  

NASA Astrophysics Data System (ADS)

We present, for the first time, an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car). These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10-40 min) and a slow decay (20-90 min). The X-ray luminosities during the flares in the energy band 0.3-7.5 keV are in the range of 1029.9 to 1031.7 erg s-1. The strongest flare was observed with the ratio ˜13 for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be ˜100 MK. The semi-loop lengths for the flaring loops are estimated to be of the order of 1010 cm. The physical parameters of the flaring structure, the peak density, pressure and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.

Bhatt, Himali; Pandey, J. C.; Singh, K. P.; Sagar, Ram; Kumar, Brijesh

2014-03-01

343

An Extraordinary Cluster of Massive Young Stars in the Milky Way's Nucleus  

NASA Technical Reports Server (NTRS)

The mass distribution of newborn stars is key to the evolution of galaxies, as it determines whether a galaxy's interstellar medium is funneled predominantly into dim, long-lived, low-mass stars, as is the case in normal galactic disks, or into bright, short-lived, massive stars, as is perhaps the case in starburst nuclei.

Serabyn, E.; Shupe, D.; Figer, D. F.

1998-01-01

344

ON THE DYNAMICAL FORMATION OF VERY YOUNG, X-RAY EMITTING BLACK HOLE BINARIES IN DENSE STAR CLUSTERS  

SciTech Connect

We recently discovered a population of very young ({tau} {approx}< 6-8 Myr), X-ray emitting black hole binaries (BHBs) in the nearby starburst galaxy NGC 4449. These BHBs are located within or near to very young star clusters, indicating that they form within the clusters, but that some fraction are dynamically ejected. Here we present results from a suite of N-body simulations of N = 16,384 ({approx}6000 M{sub Sun }) star clusters, similar to the masses of BHB hosts in NGC 4449, through the first 10 Myr of their lives. Our goal is to determine whether dynamical interactions are responsible for the observed population of BHBs in NGC 4449. Our simulations span a wide range of initial size and density profiles, both with and without primordial mass segregation, testing both realistic initial conditions and extreme ones. We find that clusters without primordial mass segregation only dynamically produce BHBs within 10 Myr when they are extremely compact and centrally concentrated. Preliminary results that include primordial binaries support this conclusion. The introduction of strong primordial mass segregation, however, greatly increases the rapidity with which the binaries form, although these are still not tight enough that they will emit X-rays. We conclude that X-ray emitting BHBs are unlikely to form dynamically in clusters of this mass under realistic conditions. Instead, they probably originate from binaries that contain two massive stars with small orbital separations, which are present from the cluster's birth.

Garofali, Kristen [Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824 (United States); Converse, Joseph M.; Chandar, Rupali; Rangelov, Blagoy, E-mail: garofal4@msu.edu [Department of Physics and Astronomy, University of Toledo, 2801 W Bancroft Street, Toledo, OH 43606 (United States)

2012-08-10

345

THE GEMINI NICI PLANET-FINDING CAMPAIGN: DISCOVERY OF A MULTIPLE SYSTEM ORBITING THE YOUNG A STAR HD 1160  

SciTech Connect

We report the discovery of two low-mass companions to the young A0V star HD 1160 at projected separations of 81 {+-} 5 AU (HD 1160 B) and 533 {+-} 25 AU (HD 1160 C) by the Gemini NICI Planet-Finding Campaign. Very Large Telescope images of the system taken over a decade for the purpose of using HD 1160 A as a photometric calibrator confirm that both companions are physically associated. By comparing the system to members of young moving groups and open clusters with well-established ages, we estimate an age of 50{sup +50}{sub -40} Myr for HD 1160 ABC. While the UVW motion of the system does not match any known moving group, the small magnitude of the space velocity is consistent with youth. Near-IR spectroscopy shows HD 1160 C to be an M3.5 {+-} 0.5 star with an estimated mass of 0.22{sup +0.03}{sub -0.04} M{sub Sun }, while NIR photometry of HD 1160 B suggests a brown dwarf with a mass of 33{sup +12}{sub -9} M{sub Jup}. The very small mass ratio (0.014) between the A and B components of the system is rare for A star binaries, and would represent a planetary-mass companion were HD 1160 A to be slightly less massive than the Sun.

Nielsen, Eric L.; Liu, Michael C.; Wahhaj, Zahed; Bowler, Brendan; Kraus, Adam; Chun, Mark; Ftaclas, Christ [Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822 (United States); Biller, Beth A. [Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, 69117 Heidelberg (Germany); Hayward, Thomas L. [Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena (Chile); Boss, Alan [Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W., Washington, DC 20015 (United States); Shkolnik, Evgenya L. [Lowell Observatory, 1400 West Mars Road, Flagstaff, AZ 86001 (United States); Tecza, Matthias; Clarke, Fraser [Department of Astronomy, University of Oxford, DWB, Keble Road, Oxford OX1 3RH (United Kingdom); Close, Laird M.; Hartung, Markus; Males, Jared R.; Skemer, Andrew J. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Reid, I. Neill [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Alencar, Silvia H. P. [Departamento de Fisica, ICEx, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, 30270-901, Belo Horizonte, MG (Brazil); Burrows, Adam [Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States); and others

2012-05-01

346

Kinematics of the inner thousand AU region around the young massive star AFGL 2591-VLA3: a massive disk candidate?  

NASA Astrophysics Data System (ADS)

Context. Recent detections of disks around young high-mass stars support the idea of massive star formation through accretion rather than coalescence, but the detailed kinematics in the equatorial region of the disk candidates is not well known, which limits our understanding of the accretion process. Aims: This paper explores the kinematics of the gas around a young massive star with millimeter-wave interferometry to improve our understanding of the formation of massive stars though accretion. Methods: We use Plateau de Bure interferometric images to probe the environment of the nearby (~1 kpc) and luminous (~20 000 L?) high-mass (10-16 M?) young star AFGL 2591-VLA3 in continuum and in lines of HDO, H_218O and SO2 in the 115 and 230 GHz bands. Radiative transfer calculations are employed to investigate the kinematics of the source. Results: At ~0.5? (500 AU) resolution, the line images clearly resolve the velocity field of the central compact source (diameter of ~800 AU) and show linear velocity gradients in the northeast-southwest direction. Judging from the disk-outflow geometry, the observed velocity gradient results from rotation and radial expansion in the equatorial region of VLA3. Radiative transfer calculations suggest that the velocity field is consistent with sub-Keplerian rotation plus Hubble-law like expansion. The line profiles of the observed molecules suggest a layered structure, with HDO emission arising from the disk mid-plane, H_218O from the warm mid-layer, and SO2 from the upper disk. Conclusions: We propose AFGL 2591-VLA3 as a new massive disk candidate, with peculiar kinematics. The rotation of this disk is sub-Keplerian, probably due to magnetic braking, while the stellar wind may be responsible for the expansion of the disk. The expansion motion may also be an indirect evidence of disk accretion in the very inner region because of the conservation of angular momentum. The sub-Keplerian rotation discovered in our work suggests that AFGL 2591-VLA3 may be a special case linking transition of velocity field of massive disks from pure Keplerian rotation to solid-body rotation though definitely more new detections of circumstellar disks around high-mass YSOs are required to examine this hypothesis. Our results support the idea that early B-type stars could be formed with a circumstellar disk from the point of view of the disk-outflow geometry, though the accretion processes in the disk need to be further investigated.

Wang, K.-S.; van der Tak, F. F. S.; Hogerheijde, M. R.

2012-07-01

347

Kinds of Volcanic Eruptions  

NSDL National Science Digital Library

At this site, different kinds of eruptions are compared with Hawaiian eruptions. Students will learn that volcanoes can erupt from vents on their summits or flanks, eruptions interact with water in submarine environments as new islands form beneath glaciers and in crater lakes, and volcanic eruptions can be classified by the character of the eruption, including the following types: basaltic flood, Hawaiian, Strombolian, Vulcanian, Peleean, Plinian, rhyolitic flood, ultravulcanian, gas, and fumarolic. As a result of this lesson, students will be able to recognize summit and flank eruptions, know the types of water-related eruptions, and describe different types of volcanic eruptions. The site contains seven activities that range in grade level from kindergarten to grade twelve, and a virtual fieldtrip to Crater Rim Drive.

348

Population Synthesis of Young Isolated Neutron Stars: The Effect of Fallback Disk Accretion and Magnetic Field Evolution  

NASA Astrophysics Data System (ADS)

The spin evolution of isolated neutron stars (NSs) is dominated by their magnetic fields. The measured braking indices of young NSs show that the spin-down mechanism due to magnetic dipole radiation with constant magnetic fields is inadequate. Assuming that the NS magnetic field is buried by supernova fallback matter and re-emerges after accretion stops, we carry out a Monte Carlo simulation of the evolution of young NSs, and show that most of the pulsars have braking indices ranging from -1 to 3. The results are compatible with the observational data of NSs associated with supernova remnants. They also suggest that the initial spin periods of NSs might occupy a relatively wide range.

Fu, Lei; Li, Xiang-Dong

2013-10-01

349

Age-Related Observations of Low Mass Pre-Main and Young Main Sequence Stars (Invited Review)  

E-print Network

This overview summarizes the age dating methods available for young sub-solar mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell (HR) diagram, spectroscopic surface gravity indicators, and lithium depletion; asteroseismology is also showing recent promise. Near and beyond the zero-age main sequence, rotation period or vsini and activity (coronal and chromospheric) diagnostics along with lithium depletion serve as age proxies. Other authors in this volume present more detail in each of the aforementioned areas. Herein, I focus on pre-main sequence HR diagrams and address the questions: Do empirical young cluster isochrones match theoretical isochrones? Do isochrones predict stellar ages consistent with those derived via other independent techniques? Do the observed apparent luminosity spreads at constant effective temperature correspond to true age spreads? While definitive answers to these questions are not provided, some methods of progression are outlined.

Lynne A. Hillenbrand

2008-12-06

350

POPULATION SYNTHESIS OF YOUNG ISOLATED NEUTRON STARS: THE EFFECT OF FALLBACK DISK ACCRETION AND MAGNETIC FIELD EVOLUTION  

SciTech Connect

The spin evolution of isolated neutron stars (NSs) is dominated by their magnetic fields. The measured braking indices of young NSs show that the spin-down mechanism due to magnetic dipole radiation with constant magnetic fields is inadequate. Assuming that the NS magnetic field is buried by supernova fallback matter and re-emerges after accretion stops, we carry out a Monte Carlo simulation of the evolution of young NSs, and show that most of the pulsars have braking indices ranging from –1 to 3. The results are compatible with the observational data of NSs associated with supernova remnants. They also suggest that the initial spin periods of NSs might occupy a relatively wide range.

Fu, Lei; Li, Xiang-Dong, E-mail: lixd@nju.edu.cn [Department of Astronomy, Nanjing University, Nanjing 210093 (China)

2013-10-01

351

The VLTI/MIDI survey of massive young stellar objects . Sounding the inner regions around intermediate- and high-mass young stars using mid-infrared interferometry  

NASA Astrophysics Data System (ADS)

Context. Because of inherent difficulties involved in observations and numerical simulations of the formation of massive stars, an understanding of the early evolutionary phases of these objects remains elusive. In particular, observationally probing circumstellar material at distances ?100 AU from the central star is exceedingly difficult, as such objects are rare (and thus, on average, far away) and typically deeply embedded. Long-baseline mid-infrared interferometry provides one way of obtaining the necessary spatial resolution at appropriate wavelengths for studying this class of objects; however, interpreting such observations is often difficult due to sparse spatial-frequency coverage. Aims: We aim to characterize the distribution and composition of circumstellar material around young massive stars and to investigate exactly which physical structures in these objects are probed by long-baseline mid-infrared interferometric observations. Methods: We used the two-telescope interferometric instrument MIDI of the Very Large Telescope Interferometer of the European Southern Observatory to observe a sample of 24 intermediate- and high-mass young stellar objects in the N band (8-13 ?m). We had successful fringe detections for 20 objects and present spectrally-resolved correlated fluxes and visibility levels for projected baselines of up to 128 m. We fit the visibilities with geometric models to derive the sizes of the emitting regions, as well as the orientation and elongation of the circumstellar material. Fourteen objects in the sample show the 10 ?m silicate feature in absorption in the total and correlated flux spectra. For 13 of these objects, we were able to fit the correlated flux spectra with a simple absorption model, allowing us to constrain the composition and absorptive properties of the circumstellar material. Results: Nearly all of the massive young stellar objects observed show significant deviations from spherical symmetry at mid-infrared wavelengths. In general, the mid-infrared emission can trace both disks and outflows, and in many cases it may be difficult to disentangle these components on the basis of interferometric data alone, because of the sparse spatial frequency coverage normally provided by current long-baseline interferometers. For the majority of the objects in this sample, the absorption occurs on spatial scales larger than those probed by MIDI. Finally, the physical extent of the mid-infrared emission around these sources is correlated with the total luminosity, albeit with significant scatter. Conclusions: Circumstellar material is ubiquitous at distances ?100 AU around young massive stars. Long-baseline mid-infrared interferometry provides the resolving power necessary for observing this material directly. However, in particular for deeply-embedded sources, caution must be used when attempting to attribute mid-infrared emission to specific physical structures, such as a circumstellar disk or an outflow. Appendix A is available in electronic form at http://www.aanda.orgBased on observations with the Very Large Telescope Interferometer of the European Southern Observatory.The reduced interferometric data presented here are 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/558/A24

Boley, Paul A.; Linz, Hendrik; van Boekel, Roy; Henning, Thomas; Feldt, Markus; Kaper, Lex; Leinert, Christoph; Müller, André; Pascucci, Ilaria; Robberto, Massimo; Stecklum, Bringfried; Waters, L. B. F. M.; Zinnecker, Hans

2013-10-01

352

An Icy Kuiper-Belt Around the Young Solar-Type Star HD 181327  

NASA Technical Reports Server (NTRS)

HD 181327 is a young Main Sequence F5/F6 V star belonging to the Beta Pictoris moving group (age approx 12 Myr). It harbors an optically thin belt of circumstellar material at approx90 AU, presumed to result from collisions in a populat.ion of unseen planetesimals. Aims. We aim to study the dust properties in the belt in great details, and to constrain the gas-to-dust ratio. Methods. We obtained far-IR photometric observations of HD 181327 with the PACS instrument onboard the Herschel Space Observatory, complemented by new 3.2 nun observations carried with the ATCA array. The geometry of the belt is constrained with newly reduced HST /NICMOS scattered light images that break the degeneracy between the disk geometry and the dust properties. We then use the radiative transfer code GRaTer to compute a large grid of dust models, and we apply a Bayesian inference method to identify the grain models that best reproduce the SED. We attempt to detect the oxygen and ionized carbon fine-structure lines with Herschel/PACS spectroscopy, providing observables to our photochemical code ProDiMo. Results. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected with Herschel/PACS completing nicely the SED in the far-infrared. The disk is marginally resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the [OI] and [CII] line fluxes. We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an import.ant layer of ice for a total dust mass of approx 0.05 stellar Mass. We discuss evidences that the grains consists of fluffy aggregates. The upper limits on the gas atomic lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than approx 17 Stellar Mass Conclusions. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Edgeworth-Kuiper Belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.

Lebreton, J.; Augereau, J.-C.; Thi, W.-F.; Roberge, A.; Donaldson, J.; Schneider, G.; Maddison, S. T.; Menard, F.; Riviere-Marichalar, P.; Mathews, G. S.; Kamp, I.; Pinte, C.; Dent, W. R. F.; Barrado, D.; Duchene, G.; Gonzalez, J.-F.; Grady, C. A.; Meeus, G.; Pantin, E.; Williams, J. P.; Woitke, P.

2011-01-01

353

An Icy Kuiper Belt Around the Young Solar-type Star HD 181327  

NASA Technical Reports Server (NTRS)

Context. HD 181327 is a young main sequence F5/F6 V star belonging to the Beta Pictoris moving group (age approx.. 12 Myr). It harbors an optically thin belt of circumstellar material at radius approx.. 90 AU, presumed to result from collisions in a population of unseen planetesimals. Aims. We aim to study the dust properties in the belt in details, and to constrain the gas-to-dust ratio. Methods. We obtained far-infrared photometric observations of HD 181327 with the PACS instrument onboard the Herschel Space Observatory, complemented by new 3.2 mm observations carried with the ATCA array. The geometry of the belt is constrained with newly reduced HST/NICMOS scattered light images that allow the degeneracy between the disk geometry and the dust properties to be broken. We then use the radiative transfer code GRaTeR to compute a large grid of models, and we identify the grain models that best reproduce the spectral energy distribution (SED) through a Bayesian analysis. We attempt to detect the oxygen and ionized carbon fine-structure lines with Herschel/PACS spectroscopy, providing observables to our photochemical code ProDiMo. Results. The HST observations confirm that the dust is confined in a narrow belt. The continuum is detected with Herschel/PACS completing nicely the SED in the far-infrared. The disk is marginally resolved with both PACS and ATCA. A medium integration of the gas spectral lines only provides upper limits on the [OI] and [CII] line fluxes.We show that the HD 181327 dust disk consists of micron-sized grains of porous amorphous silicates and carbonaceous material surrounded by an important layer of ice, for a total dust mass of approx.. 0.05 Solar Mass (in grains up to 1 mm). We discuss evidences that the grains consists of fluffy aggregates. The upper limits on the gas atomic lines do not provide unambiguous constraints: only if the PAH abundance is high, the gas mass must be lower than approx. 17 Solar Mass. Conclusions. Despite the weak constraints on the gas disk, the age of HD 181327 and the properties of the dust disk suggest that it has passed the stage of gaseous planets formation. The dust reveals a population of icy planetesimals, similar to the primitive Edgeworth-Kuiper belt, that may be a source for the future delivery of water and volatiles onto forming terrestrial planets.

Lebreton, J.; Augereau, J.-C.; Thi, W.-F.; Roberge, A.; Donaldson, J; Schneider, G.; Maddison, S. T.; Menard, F.; Riviere-Marichalar, P.; Matthews, G. S.; Kamp, I.; Pinte, C.; Dent, W. R. F.; Barrado, D.; Duchene, G.; Gonzalez, J.-F.; Grady C. A.; Meeus,G.; Pantin, E.; Williams, J. P.; Woitke, P.

2012-01-01

354

On the origin of kinematic distribution of the sub-parsec young stars in the Galactic center  

E-print Network

Within a half-parsec from the Galactic center (GC), there is a population of coeval young stars which appear to reside in a coherent disk. Surrounding this dynamically-cool stellar system, there is a population of stars with a similar age and much larger eccentricities and inclinations relative to the disk. We propose a hypothesis for the origin of this dynamical dichotomy. Without specifying any specific mechanism, we consider the possibility that both stellar populations were formed within a disk some 6 Myr ago. But this orderly structure was dynamically perturbed outside-in by an intruding object with a mass ~10^4 Msun, which may be an intermediate-mass black hole (IMBH) or a dark stellar cluster hosting an IMBH. We suggest that the perturber migrated inward to ~0.15-0.3pc from the GC under the action of dynamical friction. Along the way, it captured many stars in the outer disk region into its mean-motion resonance, forced them to migrate with it, closely encountered with them, and induced the growth of their eccentricity and inclination. But stars in the inner regions of the disk retain their initial coplanar structure. We predict that some of the inclined and eccentric stars surrounding the disk may have similar Galactocentric semimajor axis. Future precision determination of their kinematic distribution of these stars will not only provide a test for this hypothesis but also evidences for the presence of an IMBH or a dark cluster at the immediate proximity of the massive black hole at the GC. (abridged)

Qingjuan Yu; Youjun Lu; D. N. C. Lin

2007-06-17

355

Dynamics of stellar black holes in young star clusters with different metallicities - II. Black hole-black hole binaries  

E-print Network

In this paper, we study the formation and dynamical evolution of black hole-black hole (BH-BH) binaries in young star clusters (YSCs), by means of N-body simulations. The simulations include metallicity-dependent recipes for stellar evolution and stellar winds, and have been run for three different metallicities (Z = 0.01, 0.1 and 1 Zsun). Following recent theoretical models of wind mass-loss and core-collapse supernovae, we assume that the mass of the stellar remnants depends on the metallicity of the progenitor stars. We find that BH-BH binaries form efficiently because of dynamical exchanges: in our simulations, we find about 10 times more BH-BH binaries than double neutron star binaries. The simulated BH-BH binaries form earlier in metal-poor YSCs, which host more massive black holes (BHs) than in metal-rich YSCs. The simulated BH-BH binaries have very large chirp masses (up to 80 Msun), because the BH mass is assumed to depend on metallicity, and because BHs can grow in mass due to the merger with stars. The simulated BH-BH binaries span a wide range of orbital periods (10^-3-10^7 yr), and only a small fraction of them (0.3 per cent) is expected to merge within a Hubble time. We discuss the estimated merger rate from our simulations and the implications for Advanced VIRGO and LIGO.

Brunetto Marco Ziosi; Michela Mapelli; Marica Branchesi; Giuseppe Tormen

2014-05-20

356

Limits of detection in debris disks around young stars with NaCo/Sparse Aperture Masking observations  

NASA Astrophysics Data System (ADS)

To understand the formation and evolution of solar systems and planets formations in the stars neighbourhood, we need to obtain information of their state at different time of their evolution. Here, we focus on debris disks around young stars aged of ten to few tens of Myr, we analyze NaCo/Sparse Aperture Masking (SAM) observations in the L' band (3.8 ?m) of eight objects (beta Pictoris, AU Mic, 49 Ceti, eta Tel, Fomalhaut, G Lupi, HD182327 and HR8799). The aim is to get limits of detection about the mass of the debris orbiting around their stars. The SAM technique consists in transforming a single telescope into a Fizeau interferometer using a non redundant mask inserted in a pupil plane of the instrument. The analysis of the observations was completed with the sparse aperture mode pipeline. Interference fringes are fitted to obtain complex visibilities of the object, then the closure phases are calibrated and evaluated. Finally, a map of the detection limits is obtained as it is related to the closure phases previously estimated. In order to obtain an estimation of the mass corresponding to the luminosity measured with the reduction pipeline we are using theoretical isochrones interpolated into synthetic color tables. The results are maps of detection limits in unit of Jupiter Mass in a range of up to 450 mas around the stars.

Gauchet, L.; Lacour, S.

2014-09-01

357

A search for He-weak stars in very young clusters.  

NASA Technical Reports Server (NTRS)

Spectral classification has been carried out for B-type stars in Ori OBl Ib, NGC 2362, IC 5146 and NGC 2264. Six stars with weak helium lines for their UBV colors were found in Ori OBl Ib, one in NGC 2362 which might be helium variable, and none in IC 5146. The star Walker No 109 (B6V) in NGC 2264 is a helium variable which has developed an outstanding absorption feature between 4144 and 4156 A.

Bernacca, P. L.; Ciatti, F.

1972-01-01

358

YOUNG STARS NEAR EARTH: THE OCTANS-NEAR ASSOCIATION AND CASTOR MOVING GROUP  

SciTech Connect

All cataloged stellar moving groups and associations with ages ?100 Myr and within 100 pc of Earth have Galactic space motions (UVW) situated in a 'good box' with dimensions ?20 km s{sup –1} on a side. Torres et al. defined the Octans Association as a group of 15 stars with age '20 Myr?' and located ?140 pc from Earth, but with average V space velocity –3.6 km s{sup –1} that is well outside of the good box. We present a list of 14 Hipparcos star systems within 100 pc of Earth that we call {sup O}ctans-Near{sup ;} these systems have UVW similar to those of the much more distant Octans Association. The Octans-Near stars have apparent ages between about 30 and 100 Myr and their relationship to the Octans Association stars is unclear. Six additional star systems have UVW similar to those of Octans-Near stars and likely ages ?200 Myr. These six systems include the late-type binary star EQ Peg—6.2 pc from Earth with likely age ?100 Myr and thus likely to be the nearest known pre-main sequence star system. The UVW of stars in a previously proposed ?200 Myr old Castor moving group are not too dissimilar from the UVW of Octans-Near stars. However, stars in the Castor group—if it exists at all—are mostly substantially older than 200 Myr and thus generally can readily be distinguished from the much younger Octans-Near stars.

Zuckerman, B.; Vican, Laura [Department of Physics and Astronomy, University of California, Los Angeles, CA 90095 (United States); Song, Inseok [Department of Physics and Astronomy, University of Georgia, Athens, GA 30602-2451 (United States); Schneider, Adam, E-mail: ben@astro.ucla.edu, E-mail: lvican@ucla.edu, E-mail: song@uga.edu, E-mail: Adam.Schneider@Utoledo.edu [Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606 (United States)

2013-11-20

359

Do All Stars Form in Clusters?: Masses and Ages of Young Supergiants in Andromeda  

NASA Astrophysics Data System (ADS)

Currently it is not understood whether seemingly isolated stars formed in situ or were ejected from star clusters as runaway stars. Previous studies determined the origins of isolated stars by measuring their velocities, but past research was limited to OB stars in the Milky Way and Magellanic Clouds due to the difficulty of computing velocities of distant objects. This study proposed an innovative velocity test to statistically determine whether six seemingly isolated BA-type supergiants in Andromeda are runaways. We calculated the minimum relative transverse velocity needed for each supergiant to travel to its current location from the nearest open cluster. By comparing the minimum velocity with Andromeda’s known velocity dispersion, a statistical measure of the stars’ actual velocities, we determined whether the star had the necessary velocity to be a runaway. Minimum velocity was computed from the age of the star, which was calculated from its effective temperature and surface gravity. To compute effective temperature and surface gravity, we applied three new techniques based on Balmer absorption features. The results suggest that all six supergiants had the necessary velocities to be runaways. Although the proposed velocity test is a statistical assessment, it offers a valuable new tool for future investigation of isolated stars beyond the Milky Way and its satellites. This research was supported by the Science Internship Program (SIP) at UCSC, the National Science Foundation, NASA, and Palomar Observatory.

Choudhury, Zareen; Debs, C.; Kirby, E. N.; Guhathakurta, P.

2013-01-01

360

Filament Eruption Onset  

NASA Technical Reports Server (NTRS)

We have been investigating filament eruptions in recent years. Use filament eruptions as markers of the coronal field evolution. Data from SoHO, Yohkoh, TRACE, Hinode, and other sources. We and others have observed: (1)Filaments often show slow rise, followed by fast rise, (2) Brightenings, preflares, microflares during slow rise (3) Magnetic evolution in hours prior to eruption onset. We investigated What do Hinode and SDO show for filament eruptions?

Sterling, Alphonse C.; Moore, Ronald L.

2011-01-01

361

On the absence of young white dwarf companions to five technetium stars  

NASA Technical Reports Server (NTRS)

A search for hot companions to five stars of type MS and S has been carried out using the IUE satellite. No hot companions were detected for the MS stars HR 85, 4647, 6702, and 8062, and the S star HR 8714. Limits on the luminosities of possible white dwarf companions provide lower limits of 2-5x10 to the 8th yr to the ages of any degenerate companions. All five stars exhibit strong Tc I lines, and the presence of technetium, with a half-life of 2.1x10 to the 5th yr, signifies recent nucleosynthesis. The limits on the ages of possible white dwarf companions that are equal to or greater than 1000 half-lives of Tc exclude the possibility that the s-process elemental enhancement seen in these MS and S stars resulted from mass transfer from a more highly evolved companion (as is probably the mechanism by which barium stars are created). These MS and S stars represent a sample of true thermally pulsing asymptotic giant-branch stars.

Smith, Verne V.; Lambert, David L.

1987-01-01

362

Search for x ray emitting young stars outside of massive molecular clouds  

NASA Technical Reports Server (NTRS)

This project is intended to determine whether X-ray surveys of the sky can uncover previously unrecognized populations of pre-main sequence stars outside of large well-known star forming regions. X-ray observations of large regions such as the Taurus-Auriga complex, Orion molecular cloud, Ophiuchi and Chamaeleon clouds had revealed that low mass pre-main sequence emit X-rays 10(exp 2)-10(exp 4) above main sequence levels, and that X-ray surveys select a large population of 'weak' T Tauri stars that are not easily found in traditional optical and infrared surveys. The present project sought to find 'weak' T Tauri stars around smaller and more distant molecular clouds. X-ray surveys potentially could elucidate the star forming capabilities of small clouds, which are not well understood.

Feigelson, Eric D.

1991-01-01

363

Quark-hadron phase transitions in Young and old neutron stars  

NASA Astrophysics Data System (ADS)

The mixed phase of quarks and hadrons which might exist in the dense matter encountered in the varying conditions of temperature and trapped neutrino fraction in proto-neutron stars is studied. We show that hadronic equations of state that maximize the quark content of matter at a given density generally minimize the extent of the mixed phase region in a neutron star of a given mass, and that only in extreme cases could a pure quark star result. Neutrino trapping inhibits the appearance of a mixed phase which leads to possible proto-neutron star metastability. We also demonstrate that the temperature along adiabats in the quark-hadron mixed phase is much smaller than what is found for the kaon condensate-hadron mixed phase. This could lead to core temperatures which are significantly lower in stars containing quarks than in those not containing quarks.

Steiner, A. W.; Prakash, M.; Lattimer, J. M.

2000-08-01

364

An Assessment of HR Diagram Constraints on Ages and Age Spreads in Star-Forming Regions and Young Clusters  

E-print Network

Pre-main sequence evolutionary theory is not well-calibrated to observations. With care, the observed quantities can be converted into effective temperature and luminosity (i.e. the Hertzsprung-Russell diagram) which the theoretical calculations also predict as a function of stellar mass and age. For a sample of nearby young stellar clusters and associations ranging in age from 100 Myr, we have tested the loci of luminosity as a function of effective temperature against various sets of predicted pre-main sequence isochrones. As we found in Hillenbrand & White (2004) which tested stellar masses, here for the stellar ages there are two conclusions: some evolutionary calculations fare better than others in reproducing the empirical sequences, and systematic differences between all pre-main sequence evolutionary calculations and the data are apparent. We also simulate hypothetical clusters of varying star formation history and compare the resulting HR diagram predictions to observed clusters. Our efforts are directed towards quantitative assessment of **apparent** luminosity spreads in star forming regions and young clusters, which are often erroneously interpreted as **true** luminosity spreads indicative of **true** age spreads.

Lynne A. Hillenbrand; Amber Bauermeister; Russel J. White

2007-03-26

365

Volcanic Eruptions Alan Robock  

E-print Network

) in more than 50 years, and inspired many modern scientific studies. While the Mt. St. Helens eruptionVolcanic Eruptions Alan Robock Volume 1, The Earth system: physical and chemical dimensions, Ltd, Chichester, 2002 #12;Volcanic Eruptions Alan Robock Rutgers University, New Brunswick, NJ, USA

Robock, Alan

366

The star fish twins: Two young planetary nebulae with extreme multipolar morphology  

NASA Technical Reports Server (NTRS)

We present alpha images of two objects, He 2-47 and M1-37, obtained during a Hubble Space Telescope imaging survey of young planetary nebulae (PNs) selected on the basis of their low-excitation characteristics.

Sahai, R.

2000-01-01

367

Generation and evolution of stable stellar magnetic fields in young A-type stars  

NASA Astrophysics Data System (ADS)

While the presence of magnetic fields on low-mass stars is attributed to a dynamo process that is primarily driven by convective motion, the existence of magnetic fields on intermediate-mass stars very probably has other explanations. This paper focuses on the generation of stable magnetic configurations at the early stages of stellar evolution, and presumes that the fields we detect are nearly constant in time. The convective processing of an initial magnetic field during the pre-main-sequence phase is studied in a very simple model star. Azimuthal magnetic fields are found to be typical remnants in the upcoming radiative envelope after convection has receded.

Arlt, R.

2014-11-01

368

Regular frequency patterns in the young ? Scuti star HD 261711 observed by the CoRoT and MOST satellites  

NASA Astrophysics Data System (ADS)

Context. The internal structure of pre-main-sequence (PMS) stars is poorly constrained at present. This could change significantly through high-quality asteroseismological observations of a sample of such stars. Aims: We concentrate on an asteroseismological study of HD 261711, a rather hot ? Scuti-type pulsating member of the young open cluster NGC 2264 located at the blue border of the instability region. HD 261711 was discovered to be a PMS ? Scuti star using the time series photometry obtained by the MOST satellite in 2006. Methods: High-precision, time-series photometry of HD 261711 was obtained by the MOST and CoRoT satellites in four separate new observing runs that are put into context with the star's fundamental atmospheric parameters obtained from spectroscopy. Frequency Analysis was performed using Period04. The spectral analysis was performed using equivalent widths and spectral synthesis. Results: With the new MOST data set from 2011/12 and the two CoRoT light curves from 2008 and 2011/12, the ? Scuti variability was confirmed and regular groups of frequencies were discovered. The two pulsation frequencies identified in the data from the first MOST observing run in 2006 are confirmed and 23 new ? Scuti-type frequencies were discovered using the CoRoT data. Weighted average frequencies for each group were determined and are related to l = 0 and l = 1 p-modes. Evidence for amplitude modulation of the frequencies in two groups is seen. The effective temperature (Teff) was derived to be 8600 ± 200 K, log g is 4.1 ± 0.2, and the projected rotational velocity (?sini) is 53 ± 1 km s-1. Using our Teff value and the radius of 1.8 ± 0.5 R? derived from spectral energy distribution (SED) fitting, we get a luminosity log L/L? of 1.20 ± 0.14 which agrees well to the seismologically determined values of 1.65 R? and, hence, a log L/L? of 1.13. The radial velocity of 14 ± 2 km s-1 we derived for HD 261711, confirms the star's membership to NGC 2264. Conclusions: Our asteroseismic models suggest that HD 261711 is a ? Scuti-type star close to the zero-age main sequence (ZAMS) with a mass of 1.8 to 1.9 M?. With an age of about 10 million years derived from asteroseismology, the star is either a young ZAMS star or a late PMS star just before the onset of hydrogen-core burning. The observed splittings about the l = 0 and 1 parent modes may be an artifact of the Fourier derived spectrum of frequencies with varying amplitudes. The CoRoT space mission was developed and is operated by the French space agency CNES, with participation of ESA's RSSD and Science Programmes, Austria, Belgium, Brazil, Germany, and Spain.Based on data from the MOST satellite, a Canadian Space Agency mission, jointly operated by Microsatellite Systems Canada Inc. (MSCI), formerly part of Dynacon, Inc., the University of Toronto Institute for Aerospace Studies and the University of British Columbia with the assistance of the University of Vienna.Reduced spectra 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/552/A68

Zwintz, K.; Fossati, L.; Guenther, D. B.; Ryabchikova, T.; Baglin, A.; Themessl, N.; Barnes, T. G.; Matthews, J. M.; Auvergne, M.; Bohlender, D.; Chaintreuil, S.; Kuschnig, R.; Moffat, A. F. J.; Rowe, J. F.; Rucinski, S. M.; Sasselov, D.; Weiss, W. W.

2013-04-01

369

Star clusters  

Microsoft Academic Search

Star clusters are observed in almost every galaxy. In this thesis we address several fundamental problems concerning the formation, evolution and disruption of star clusters. From observations of (young) star clusters in the interacting galaxy M51, we found that clusters are formed in complexes of stars and star clusters. These complexes share similar properties with giant molecular clouds, from which

M. Gieles

2006-01-01

370

From Stars to Super-Planets: The Low-Mass IMF in the Young Cluster IC348  

NASA Technical Reports Server (NTRS)

We investigate the low-mass population of the young cluster IC348 down to the deuterium-burning limit, a fiducial boundary between brown dwarf and planetary mass objects, using a new and innovative method for the spectral classification of late-type objects. Using photometric indices, constructed from HST/NICMOS narrow-band imaging, that measure the strength of the 1.9 micron water band, we determine the spectral type and reddening for every M-type star in the field, thereby separating cluster members from the interloper population. Due to the efficiency of our spectral classification technique, our study is complete from approximately 0.7 solar mass to 0.015 solar mass. The mass function derived for the cluster in this interval, dN/d log M alpha M(sup 0.5), is similar to that obtained for the Pleiades, but appears significantly more abundant in brown dwarfs than the mass function for companions to nearby sun-like stars. This provides compelling observational evidence for different formation and evolutionary histories for substellar objects formed in isolation vs. as companions. Because our determination of the IMF is complete to very low masses, we can place interesting constraints on the role of physical processes such as fragmentation in the star and planet formation process and the fraction of dark matter in the Galactic halo that resides in substellar objects.

Najita, Joan R.; Tiede, Glenn P.; Carr, John S.

2000-01-01