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1

An Infrared and Optical View of Young Eruptive Stars  

NASA Astrophysics Data System (ADS)

Young eruptive stars (FU Orionis- and EX Lupi-type objects) form a small, but remarkable subgroup of pre-main sequence stars. They are characterized by eruptions which may increase the brightness of the system by as much as a factor of 100. Outbursts probably correspond to increased accretion from the circumstellar disk to the star. In my dissertation I present the detailed analysis of three young eruptive stars using ground-based and space-borne observations at optical and infrared wavelengths. The outburst of OO Serpentis between 1995 and 2006. OO Ser is a deeply embedded young star that produced an eruption in 1995. Using archival (ISO, Spitzer) and new observations, I constructed the light curves of the star at ten different infrared wavelengths. My results show that the eruption caused brightening in the whole infrared regime, which is followed by a slow, wavelength-independent fading. The fading is still ongoing and the star will probably not return to quiescence before 2011. The timescale of the outburst is shorter than that of typical FU Ori-type objects, but longer than that of EX Lupi-type stars. Based on the spectral energy distribution, OO Ser seems to be an early Class I object with an age of <10^5 yr. In accordance with outburst models, the star is probably surrounded by an accretion disk and a dense envelope. Due to the shorter timescales, outburst models developed for FU Ori-type stars can only work for OO Ser if the viscosity of the circumstellar disk is set to an order of magnitude larger value than usual for FU Ori objects. The 2004-2006 outburst and environment of V1647 Orionis. V1647 Ori is a pre-main sequence star that suddenly brightened up in less than 4 months, reaching its peak brightness in 2004 February. I participated in a monitoring program, in which we obtained V R_C I_C J H K_s-band observations. After reaching its peak brightness, the star was fading very slowly. Then, in 2005 October it suddenly faded back to its quiescence level. The timescale of the outburst and the moderate increase in bolometric luminosity suggest that V1647 Ori differs both from FU Ori- and EX Lupi-type objects. By calculating the time delay between the brightness variations of the star and a nebular position, I deduced the geometry of the system. The near-infrared color maps of the nebula suggest that the star is surrounded by a disk and an extended nebula. High-resolution polarimetry of Parsamian 21. Parsamian 21 is an FU Orionis-type object consisting of a central star and an extended reflection nebula. We obtained high spatial resolution adaptive optics assisted near-infrared direct and polarimetric observations with the NACO instrument at ESO's Very Large Telescope. I complemented these measurements with archival Hubble, Spitzer and ISO data. The results revealed that (1) the star is probably indeed an FU Orionis-type object; (2) the star is not associated with any known rich cluster of young stars; (3) the star is surrounded by an edge-on disk, and a circumstellar envelope with a polar cavity; (4) the disk seems to be flat and extends from at least 48 to 360 AU from the star; (5) the spectral energy distribution can be successfully modeled with a circumstellar disk and an envelope; (6) within the framework of an evolutionary sequence of FU Ori-type stars, Parsamian 21 seems to be an intermediate-aged object.

Kóspál, Á.

2009-01-01

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 2008 Extreme Outburst of the Young Eruptive Variable Star EX Lupi  

NASA Astrophysics Data System (ADS)

In early 2008, the young low-mass star EX Lupi, the prototype of the EXor class of eruptive variables, optically brightened by over 5 mag for a period of seven months. The previous time a change of such amplitude had been observed in EX Lup was over 50 years ago. In this Letter, we present new optical and near-IR high-resolution spectroscopy of EX Lup during the 2008 outburst. We investigate the physical characteristics of the outburst both soon after it began and some four months later, and consider the observed energetics and kinematics. Emission line strengths, widths, and profiles significantly changed between the two observations. Also, modeling of the 2.2935 ?m CO overtone band head emission suggests that an inner gap in the circumstellar gas disk around the star may be present and that it is from the inner edge of the gas disk that the CO overtone emission probably arises. We derive a mass accretion luminosity and rate during the extreme outburst of ~2 ± 0.5 L sun and ~(2 ± 0.5) × 10-7 M sun yr-1, respectively, which suggests that this outburst was indeed one of the strongest witnessed in EX Lup, yet not as strong as those observed in FU Orionis stars.

Aspin, Colin; Reipurth, Bo; Herczeg, Gregory J.; Capak, Peter

2010-08-01

4

Defining Pre-Outburst and Post-Outburst Characteristics of Eruptive Young Stars  

NASA Astrophysics Data System (ADS)

EXORs are extreme T Tauri stars that experience major UV-optical eruptions (3-5 mag) that last about a year. These recurring outbursts driven by high accretion rates assist in dispersal of the circumstellar disk and the large X-ray-UV flux injected into the local environment modifies the physical and chemical history, influencing planet formation and time scales. Little X-ray data exists on these stars during either quiescent or outburst phases, so basic details such as how much X-ray flux is produced during eruption is unknown. We will characterize the X-ray properties of 6 EXORs in their quiescent phase. If an eruption of any known EXOR occurs during AO6, a high-quality spectrum will be obtained. Contrasting quiescent and outburst properties provides crucial insight into the physics.

Stringfellow, Guy

2006-10-01

5

Dynamics during outburst. VLTI observations of the young eruptive star V1647 Orionis during its 2003-2006 outburst  

NASA Astrophysics Data System (ADS)

Context. It is hypothesized that low-mass young stellar objects undergo eruptive phases during their early evolution. These eruptions are thought to be caused by highly increased mass accretion from the disk onto the star, and therefore play an important role in the early evolution of Sun-like stars, of their circumstellar disks (structure, dust composition), and in the formation of their planetary systems. The outburst of V1647 Ori between 2003 and 2006 offered a rare opportunity to investigate such an accretion event. Aims: By means of our interferometry observing campaign during this outburst, supplemented by other observations, we investigate the temporal evolution of the inner circumstellar structure of V1647 Ori, the region where Earth-like planets could be born. We also study the role of the changing extinction in the brightening of the object and separate it from the accretional brightening. Methods: We observed V1647 Ori with MIDI on the VLTI at two epochs in this outburst. First, during the slowly fading plateau phase (2005 March) and second, just before the rapid fading of the object (2005 September), which ended the outburst. We used the radiative transfer code MC3D to fit the interferometry data and the spectral energy distributions from five different epochs at different stages of the outburst. The comparison of these models allowed us to trace structural changes in the system on AU-scales. We also considered qualitative alternatives for the interpretation of our data. Results: We found that the disk and the envelope are similar to those of non-eruptive young stars and that the accretion rate varied during the outburst. We also found evidence for the increase of the inner radii of the circumstellar disk and envelope at the beginning of the outburst. Furthermore, the change of the interferometric visibilities indicates structural changes in the circumstellar material. We test a few scenarios to interpret these data. We also speculate that the changes are caused by the fading of the central source, which is not immediately followed by the fading of the outer regions. Conclusions: We found that most of our results fit in the canonical picture of young eruptive stars. Our study provided dynamical information from the regions of the innermost few AU of the system: changes of the inner radii of the disk and envelope. However, if the delay in the fading of the disk is responsible for the changes seen in the MIDI data, the effect should be confirmed by dynamical modeling. Based on observations made with ESO telescopes at the Paranal Observatory under program IDs 274.C-5026 and 076.C-0736. In addition, this work is based in part on archival data obtained with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.

Mosoni, L.; Sipos, N.; Ábrahám, P.; Moór, A.; Kóspál, Á.; Henning, Th.; Juhász, A.; Kun, M.; Leinert, Ch.; Quanz, S. P.; Ratzka, Th.; Schegerer, A. A.; van Boekel, R.; Wolf, S.

2013-04-01

6

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

NASA Astrophysics Data System (ADS)

Context. The sudden optical brightening of two young stellar objects, HBC 722 and VSX J205126.1+440523, located in the North America/Pelican Nebula Complex, was announced in August 2010. 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: The quiescent SED of HBC 722 is consistent with that of a slightly reddened normal T Tauri-type star. The source brightened monotonically in about two months, and the SED obtained during maximum brightness indicates the appearance of a hot, single-temperature blackbody. The current fading rate implies that the star will return to quiescence in about a year, which questions its classification as a bone fide FUor. The quiescent SED of VSX J205126.1+440523 looks like that of a highly embedded Class I source. The outburst of this source happened more gradually, but reached an unprecedentedly high amplitude. Its light curves showed a deep minimum two and a half months after the peak, when the object was close to its pre-outburst optical brightness. Further monitoring indicates that it is still far from being quiescent. Conclusions: The shape of the light curves as well as the bolometric luminosities and accretion rates suggest that these objects do not fit into the classic FUor group. Although HBC 722 exhibits all spectral characteristics of a bona fide FUor, its luminosity and accretion rate is too low and its timescale is too fast compared with classical FUors. VSX J205126.1+440523 seems to be an example where quick extinction changes modulate the light curve. Figure 1 and Tables 1-3 are only available in electronic form at http://www.aanda.org

Kóspál, Á.; Ábrahám, P.; Acosta-Pulido, J. A.; Arévalo Morales, M. J.; Carnerero, M. I.; Elek, E.; Kelemen, J.; Kun, M.; Pál, A.; Szakáts, R.; Vida, K.

2011-03-01

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

V1647 Orionis: Accretion in an Eruptive Variable Star  

NASA Astrophysics Data System (ADS)

Eruptions in young stars are rare and can provide us with new insight into the mechanics of star formation. High-resolution near-IR observations were taken of the 2004-2006 outburst of V1647 Orionis, a young variable which illuminates what is now termed McNeil's nebula. From these observations, we are able to follow the decrease in the mass accretion rate onto the star as the outburst subsides using Br-gamma and Pa-beta emission line fluxes. Accretion rates were found to range from 5 x 10-6 solar masses per year during the outburst to 3 x 10-7 solar masses per year one year following the outburst, showing that there is a definite relationship between accretion of circumstellar material onto the disk and the eruptive event. We present these results and consider their implications for pre-main sequence stellar evolution. This work was conducted by a Research Experience for Undergraduates (REU) position at the University of Hawaii's Institute for Astronomy and funded by the NSF.

Beerman, Lori; Aspin, C.

2009-01-01

9

Eruptive star V1180 Cas now in outburst  

NASA Astrophysics Data System (ADS)

In the framework of our optical/near-IR EXor monitoring program dubbed EXORCISM (EXOR optiCal Infrared Systematic Monitoring - Antoniucci et al. PPVI), we have been observing since two months the variable star V1180 Cas, associated with the dark cloud Lynds 1340. This source has been originally recognized as a young eruptive object by Kun et al. (2011, ApJ 733, L8), who observed a powerful outburst (5-6 mag in the Ic band) in the period 2005-2008.

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

2013-09-01

10

Starspots on Young pms Stars  

NASA Astrophysics Data System (ADS)

Long-term, multiband photometric observations of 8 young PMS stars are used to construct models for their starspots. It is shown that the average density of starspots is up to 40 % of the total surface of a star (V824 Ara), while the difference in temperatures between a quiet photosphere and a spot ranges from 870 K (AB Dor) to 1700-1800 K (PZ Tel, V1321 Ori, V395 Cep). The spots lie at low (2-8°, V343 Nor) and medium (25-61°) latitudes, while the largest latitude of starspots is 16-80°. A cyclical activity that shows up as changes in the total area and average latitude of the starspots is observed in the stars PZ Tel, TY Col, V824 Ara, and AB Dor. A latitudinal drift of the starspots and differential rotation of the star are observed which are analogous to those of the sun.

Alekseev, I. Yu.

2014-06-01

11

Molecular Disks around Young Stars  

NASA Astrophysics Data System (ADS)

Understanding how planets and life appeared is one of the older dreams of Mankind. Today, more and more circumstellar disks are found around Young Stellar Objects (YSOs) called TTauri stars. These stars are indeed young suns at the stage where our Sun was still surrounded by a flattened structure of rotating gas and dust: the so-called proto-solar Nebula which provided the material to build the Solar System. Therefore, understanding the physics, the chemistry and the evolution of these disks, is the important clue to find how planetary systems form around Solar-type stars. In protoplanetary disks, except very close to the star, the gas and the dust remain at low temperatures and radiate at long wavelengths, from the Far-Infrared to millimeter waves. Unfortunately, these systems are relatively far away. With a sensitivity 30-40 times larger than that of the best mm array (IRAM interferometer), ALMA will provide images with details as small as a few astronomical units, allowing to image disks at the scale at which planetary formation is believed to occur. In this talk, I will review the kinematics and the physical properties of the gas surrounding YSOs, from the early stages of planet formation to more evolved ones such as the beta Pic-like disks, showing how our knowledges are definitely limited by the possibilities of current mm arrays. In conclusion, I will show that ALMA will allow the first quantitative studies of gas evolution towards planet formation.

Dutrey, A.

1999-10-01

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

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

15

Dustiest Star Could Harbor a Young Earth  

NSF Publications Database

... Biology Chemistry & Materials Computing Earth & Environment Education Engineering Mathematics ... 05-122Dustiest Star Could Harbor a Young Earth The thick dust shrouding BD +20 307 is thought to ...

16

Star Formation and Young Clusters in Cygnus  

NASA Astrophysics Data System (ADS)

The Great Cygnus Rift harbors numerous very active regions of current or recent star formation. In this part of the sky we look down a spiral arm, s= o regions from only a few hundred pc to several kpc are superposed. The North America and Pelican nebulae, parts of a single giant HII region, are the best known of the Cygnus regions of star formation and are located at a distance of only about 600 pc. Adjacent, but at a distance of about 1.7 kpc, is the Cygnus X region, a ˜10° complex of actively star forming molecular clouds and young clusters. The most massive of these clusters is the 3-4 Myr old Cyg OB2 association, containing several thousand OB stars and akin to the young globular clusters in the LMC. The rich populations of young low and high mass stars and protostars associated with the massive cloud complexes in Cygnus are largely unexplored and deserve systematic study.

Reipurth, B.; Schneider, N.

2008-12-01

17

The inner disks of EXor-type eruptive stars  

NASA Astrophysics Data System (ADS)

EX Lupi-type young stars (EXors) show sporadic brightenings of several magnitudes, caused by the episodic increase in the accretion rate of the circumstellar matter onto the young star. As the inner disk plays a crucial role during the onset of the outburst, we examined the quiescent properties of the circumstellar environment of EXors, focusing on the inner regions. We found that in case of three EXors (VY Tau, V1143 Ori and EX Lup) the spectral energy distributions show no or weak excess above the stellar photosphere at NIR-MIR wavelengths, indicative of inner disk clearing. A detailed radiative transfer modeling of the sources revealed that the inner regions of these disks had to go through significant evolution, either the inner radius of the dusty disk is beyond the sublimation radius and/or the inner disks are flattened.

Sipos, Nikoletta; Kóspál, Ágnes

2014-01-01

18

Young stars in the Galactic center  

NASA Astrophysics Data System (ADS)

The central parsec of our Galaxy hosts not only a supermassive black hole, but also a large population of young stars (age <6 Myr) whose presence is puzzling given how inhospitable the region is for star formation. The strong tidal forces require gas densities many orders of magnitude higher than is found in typical molecular clouds. Kinematic observations of this young nuclear cluster show complex structures, including a well-defined inner disk, but also a substantial off-disk population. Spectroscopic and photometric measurements indicate the initial mass function (IMF) differs significantly from the canonical IMF found in the solar neighborhood. These observations have led to a number of proposed star formation scenarios, such as an infalling massive star cluster, a single infalling molecular cloud, or cloud-cloud collisions. I will review recent works on the young stars in the central parsec and discuss connections with young nuclear star clusters in other galaxies, such as M31, and with star formation in the larger central molecular zone.

Lu, Jessica R.; Ghez, Andrea M.; Morris, Mark R.; Clarkson, Will; Stolte, Andrea; Do, Tuan; Yelda, Sylvana; Anderson, Jay

2014-05-01

19

Young Star Populations in the Kepler Field  

NASA Astrophysics Data System (ADS)

The Kepler satellite is providing spectacular optical photometric light-curves of unprecedented precision and duration that routinely allow detailed studies of stellar magnetic activity on late-type stars that were difficult previously. Kepler provides multi-year duration light-curves that allow investigation of how activity phenomena -- such as the growth, migration, and decay of star-spots, differential rotation, activity cycles, and flaring -- operate on a wide variety of single and binary stars. The 105 square degree Kepler Field contains tens of thousands of late-type stars showing rotational modulation due to star-spots with periods ranging from one day to a ``solar-like'' month. Short rotation periods and high levels of magnetic activity are strongly correlated. However, there are only two basic reasons why stars with rotation periods of a few days possess such high angular momentum --- either they are close binaries or they are young stars. During Kepler GO Cycles 1 through 4 we have been studying the Long-cadence (30 minute sampling) photometry of hundreds of active late-type stars and as an absolutely essential complement we have been obtaining high resolution optical spectra to understand the physical properties of these stars. We present results from a spectroscopic survey using the MMT Hectochelle multi-object echelle of 4 square degrees of the Kepler Field. We have discovered a significant population of young stars with Li I absorption indicating ages of ~100 Myr or less at a spatial density of at least 20 stars per square degree. Our detected young star sample comprises at least 80 stars and represents a dramatic advance compared to the previously known sample over the full Kepler Field of three stars in this age range. Roughly one sixth of the stars observed are young and a similar number short-period binaries based on 2-4 radial velocities. We show how the rotational properties of the stars and their physical properties are related. This work is based on data obtained with the NASA Kepler satellite and the MMT Hectochelle spectrograph using NOAO community access time. Support by NASA Kepler grants to the University of Colorado and by NSF grant to the College of Charleston.

Brown, Alexander; Neff, J. E.; Wells, M.; Saar, S.; Furesz, G.; Walkowicz, L. M.; Ayres, T. R.; Basri, G. S.; Berdyugina, S.; Harper, G.; Hawley, S. L.; Korhonen, H.; Kowalski, A.; Micela, G.; Piskunov, N. E.; Ramsey, L. W.

2013-01-01

20

Unraveling the Origins of Nearby Young Stars  

NASA Astrophysics Data System (ADS)

A systematic search for close conjunctions and clusterings in the past of nearby stars younger than the Pleiades is undertaken, which may reveal the time, location, and mechanism of formation of these often isolated, disconnected from clusters and star-forming regions, objects. The sample under investigation includes 101 T Tauri, post-TT, and main-sequence stars and stellar systems with signs of youth, culled from the literature. Their Galactic orbits are traced back in time and near approaches are evaluated in time, distance, and relative velocity. Numerous clustering events are detected, providing clues to the origin of very young, isolated stars. Each star's orbit is also matched with those of nearby young open clusters, OB and TT associations and star-forming molecular clouds, including the Ophiuchus, Lupus, Corona Australis, and Chamaeleon regions. Ejection of young stars from open clusters is ruled out for nearly all investigated objects, but the nearest OB associations in Scorpius-Centaurus, and especially, the dense clouds in Ophiuchus and Corona Australis have likely played a major role in the generation of the local streams (TWA, Beta Pic, and Tucana-Horologium) that happen to be close to the Sun today. The core of the Tucana-Horologium association probably originated from the vicinity of the Upper Scorpius association 28 Myr ago. A few proposed members of the AB Dor moving group were in conjunction with the coeval Cepheus OB6 association 38 Myr ago.

Makarov, Valeri V.

2007-03-01

21

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

22

Eruptive Variable Stars and Outflows in Serpens NW  

NASA Astrophysics Data System (ADS)

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 H2 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 H2 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.; Chini, Rolf; Watermann, Ramon; Lemke, Roland

2012-01-01

23

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

24

Probing Young Star Physics with Aperiodic Variability  

NASA Astrophysics Data System (ADS)

Ongoing time domain surveys such as PTF, CRTS, and Pan-STARRS1, as well as upcoming surveys such as LSST, promise to revolutionize optical astronomy by providing a comprehensive picture of the variability properties of everything from local flare stars to distant quasars. Time domain surveys have already proven a boon for studies of young stars, whose variability is frequently aperiodic and may have time scales of days to decades, depending on the physics underlying the variability. I present an overview of the PTF-NAN (North America Nebula) survey, which allows us, for the first time, to simultaneously resolve day-scale variability and to monitor changes in photometric behavior in young stars over several years, without large data gaps and without any assumptions about periodicity. I describe preliminary results of the survey, including a search for episodic stellar behavior, a study of the most robust methods for identifying the characteristic time scale(s) of an aperiodic signal, and a characterization of the full range of amplitudes and time scales represented in optical variability of young stars.

Findeisen, Krzysztof

2014-01-01

25

Limited climate impact of the Young Toba Tuff eruption  

NASA Astrophysics Data System (ADS)

Super eruptions have been linked to global climate change, biotic turnover, and, for the Younger Toba Tuff (YTT) eruption 74,000 years ago, near-extinction of modern humans. Very large volcanic eruptions produce extremely strong radiative forcing, which can affect the Earth system for longer times than the pure atmospheric residence time of the volcanic aerosol. This leads to large negative temperature anomalies at the surface and significant warming of the aerosol containing layers altering substantial atmospheric and ocean circulation and composition. Here we present and discuss Earth system model simulations of the YTT eruption taking into account also the temporal evolution of the volcanic aerosol size distribution, one of the largest uncertainties in prior calculations. We demonstrate that there is a large negative feedback that has heretofore not been considered and which greatly reduces the climate impact of the aerosol cloud. The temperature response of the YTT is shorter and weaker as previously estimated. The smaller response, plus its geographic patchiness, suggests that most biota have escaped threshold extinction pressures from the eruption.

Timmreck, C.; Zanchettin, D.; Graf, H.; Lorenz, S.; Niemeier, U.; Matei, D.; Jungclaus, J. H.; Crowley, T. J.

2010-12-01

26

Early Earth closely surrounded by young stars  

Microsoft Academic Search

Knowledge of the physical and chemical conditions on the primeval Earth is important for the study of the origin of the biosphere.\\u000a This paper discusses the latest modification of the theory of the origin of the Earth and other planets. Possible consequences\\u000a of the formation of the Sun in the area of the star formation closely surrounded by neighboring young

A. V. Vityazev; G. V. Pechernikova

2010-01-01

27

Dispersal of Disks Around Young Stars  

NASA Technical Reports Server (NTRS)

We review the evidence pertaining to the lifetimes of planet-forming disks and discuss possible disk dispersal mechanisms: 1) viscous accretion of material onto the central source, 2) close stellar encounters, 3) stellar winds, and 4) photoevaporation by ultraviolet radiation. We focus on 3) and 4) and describe the quasi-steady state appearance and the overall evolution of disks under the influence of winds and radiation from the central star and of radiation from external OB stars. Viscous accretion likely dominates disk dispersal in the inner disk (r < or approx. equals 10 AU), while photoevaporation is the principal process of disk dispersal outside of r > or approx. equals 10 AU for low mass stars. Disk dispersal timescales are compared and discussed in relation to theoretical estimates for planet formation timescales. Photoevaporation may explain the large differences in the hydrogen content of the giant planets in the solar system. The commonly held belief that our early sun's stellar wind dispersed the solar nebula is called into question. Finally, we model the small bright objects ("proplyds") observed in the Orion Nebula as disks around young, low mass stars which are externally illuminated by the UV photons from the nearby massive star Theta(sup 1)C.

Hollenbach, David

2001-01-01

28

Dispersal of Disks Around Young Stars  

NASA Technical Reports Server (NTRS)

We review the evidence pertaining to the lifetimes of planet-forming disks and discuss possible disk dispersal mechanisms: 1) viscous accretion of material onto the central source; 2) close stellar encounters; 3) stellar winds; and 4) photoevaporation by ultraviolet radiation. We focus on 3) and 4) and describe the quasi-steady state appearance and the overall evolution of disks under the influence of winds and radiation from the central star and of radiation from external OB stars. Viscous accretion likely dominates disk dispersal in the inner disk (r approx. or less than 10 AU), while photoevaporation is the principal process of disk dispersal outside of r approx. or greater than 10 AU for low mass stars. Disk dispersal timescales are compared and discussed in relation to theoretical estimates for planet formation timescales. Photoevaporation may explain the large differences in the hydrogen content of the giant planets in the solar system. The commonly held belief that our early sun's stellar wind dispersed he solar nebula is called into question. Finally, we model the small bright objects ('proplyds') observed in the Orion Nebula as disks around young, low mass stars which are externally illuminated by the UV (ultraviolet) photons from the nearby massive star Theta(1)C.

Hollenbach, David; DeVincenzi, D. (Technical Monitor)

2002-01-01

29

HUBBLE SEES DISKS AROUND YOUNG STARS  

NASA Technical Reports Server (NTRS)

[Top left]: This Wide Field and Planetary Camera 2 (WFPC2) image shows Herbig-Haro 30 (HH 30), the prototype of a young star surrounded by a thin, dark disk and emitting powerful gaseous jets. The disk extends 40 billion miles from left to right in the image, dividing the nebula in two. The central star is hidden from direct view, but its light reflects off the upper and lower surfaces of the disk to produce the pair of reddish nebulae. The gas jets are shown in green. Credit: Chris Burrows (STScI), the WFPC2 Science Team and NASA [Top right]: DG Tauri B appears very similar to HH 30, with jets and a central dark lane with reflected starlight at its edges. In this WFPC2 image, the dust lane is much thicker than seen in HH 30, indicating that dusty material is still in the process of falling onto the hidden star and disk. The bright jet extends a distance of 90 billion miles away from the system. Credit: Chris Burrows (STScI), the WFPC2 Science Team and NASA [Lower left]: Haro 6-5B is a nearly edge-on disk surrounded by a complex mixture of wispy clouds of dust and gas. In this WFPC2 image, the central star is partially hidden by the disk, but can be pinpointed by the stubby jet (shown in green), which it emits. The dark disk extends 32 billion miles across at a 90-degree angle to the jet. Credit: John Krist (STScI), the WFPC2 Science Team and NASA [Lower right]: HK Tauri is the first example of a young binary star system with an edge-on disk around one member of the pair. The thin, dark disk is illuminated by the light of its hidden central star. The absence of jets indicates that the star is not actively accreting material from this disk. The disk diameter is 20 billion miles. The brighter primary star appears at top of the image. Credit: Karl Stapelfeldt (JPL) and colleagues, and NASA

2002-01-01

30

Many faces of young neutron stars  

NASA Astrophysics Data System (ADS)

The hardware aspect of this thesis consists in the design, fabrication and assembly of twin analog Flexible Filter Banks at Caltech. These are user-friendly, workhorse, radio-pulsar search and timing instruments. Novel features include the flexibility in configuring channel center-frequencies and widths, the rapid sampling down to 25 ?s and a total instrument bandwidth ranging from a narrow 0.2 MHz to a mammoth 100 MHz. Frequency synthesis is used to downconvert, detect and sample the telescope receiver bandpass as 32 separate time-series in each polarization. The collected data are later subjected to standard pulsar search and timing algorithms in software.The vital scientific issue addressed here is the nature of young neutron stars. In the standard picture, young neutron stars are rapidly spinning radio-luminous pulsars, which may also display pulsed emission at high X-ray and ?-ray energies. However there is no evidence that all neutron stars are born according to this standard picture. We present radio or X-ray investigations of steady nebular emission produced by three clearly non-standard and ill-understood objects. In all likelihood, these are young neutron stars, a notion upheld by their association with young Galactic supernova remnants.Based on its display of high energy transients, the soft ?-ray repeater SGR 1806-20 is posited to be a seismically active "magnetar", i.e., a neutron star with a super-strong magnetic field (10[superscript 15] G) nearly three orders of magnitude greater than pulsar dipolar fields. Our VLA observations of fleeting small-scale structure around SGR 1806-20 provide intriguing, although preliminary, support for the magnetar model. In time, similar observations could unravel the riddle of soft ?-ray repeaters and possibly establish the reality of magnetars.X-ray observations of the remnant of the historical supernova of 386 A.D., SNR G 11.2-0.3 are presented. The nature of an embedded underlumnious plerion discovered in these observations argues for a central neutron star very different from the prototypical Crab pulsar. The urgency to undertake a large scale study of young and hollow Galactic shells in broadband X-rays with fine spatial resolution is elucidated.X-ray spectroscopy of the object 1E 1207.4-5209 at the core of the large remnant PKS 1209-51/52 has revealed a non-thermal source with a very steep spectrum. After considering various scenarios for lE 1207.4-5209, we conclude that its spectral signature, its lack of optical emission and its position at the center of a supernova remnant make it a source similar to the mysterious anomalous X-ray pulsars.A large and sensitive search for radio pulsar companions of massive stars was undertaken. Primary motivation stems from the recent discovery of binary radio pulsar B 1259-63 as the first member of such a population and a "missing link" in the current models of evolution. Prevalent expectations, based on binary evolution scenarios, suggested that many more such systems should exist and would be uncovered in sensitive targeted searches. Together with other smaller searches, this survey uncovered no pulsars orbiting early-type stars. We conclude that such binary systems must be rare.

Vasisht, Gautam

31

NEW YOUNG STAR CANDIDATES IN CG4 AND Sa101  

SciTech Connect

The CG4 and Sa101 regions together cover a region of {approx}0.5 deg{sup 2} in the vicinity of a 'cometary globule' that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6-70 {mu}m) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two Micron All Sky Survey. We find infrared excesses in all six of the previously identified young stars in our maps and identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the 'fingers' of the dust being disturbed by the ionization front from the heart of the Gum Nebula.

Rebull, L. M.; Laine, S.; Laher, R.; Legassie, M. [Spitzer Science Center/Caltech, M/S 220-6, 1200 E. California Blvd., Pasadena, CA 91125 (United States); Johnson, C. H. [Breck School Science Department, 123 Ottawa Ave., N., Golden Valley, MN 55422 (United States); Hoette, V. [The University of Chicago Department of Astronomy and Astrophysics, Yerkes Observatory Education and Outreach, 373 W. Geneva St., Williams Bay, WI 53191 (United States); Kim, J. S.; Foster, M. [Steward Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States); Mallory, C. R. [Department of Astronomy, Pierce College, 6201 Winnetka Ave., Woodland Hills, CA 91371 (United States); McCarron, K. [Science Division, Oak Park and River Forest High School, 201 N. Scoville Ave., Oak Park, IL 60302 (United States); Sherry, W. H., E-mail: luisa.rebull@jpl.nasa.gov [NOAO/NSO, 950 N. Cherry Ave., Tucson, AZ 85719 (United States)

2011-07-15

32

Fallback Debris Disks Around Young Neutron Stars  

NASA Astrophysics Data System (ADS)

Our Spitzer/IRAC discovery of a dusty debris disk around the young neutron star 4U 0142+61 is likely the first detection of a supernova fallback disk around a neutron star. Here we propose to test three aspects of this hypothesis with targeted Spitzer observations: (1) IRAC imaging of the magnetar 1E 2259+586, to see if debris disks are ubiquitous; (2) deep MIPS 24 micron imaging of 4U 0142+61 to probe the long-wavelength tail of the disk and measure its radial extent; and (3) IRAC subarray observations of 4U 0142+61 designed to detect for 8.7 sec pulsations or set an upper limit on their amplitude. This last measurement will allow us to definitively discriminate whether the mid-IR emission arises from an X-ray heated debris disk or the pulsar magnetosphere.

Chakrabarty, Deepto; Kaplan, David; Wachter, Stefanie; Wang, Zhongxiang

2007-05-01

33

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

34

Embedded young stars in northern NGC 3372  

NASA Astrophysics Data System (ADS)

Deep narrow-band (Br? at 2.17?m and H_2 at 2.12?m) and broad-band (JHKs) near-infrared images are presented of the young regions Tr14-N4 and Car I. The observations were made with PANIC, attached to the Baade 6.5 m Magellan Telescope at Las Campanas Observatory. These were supplemented by archive Spitzer/IRAC images. Evidence is given of an embedded young compact cluster (r=23'') of medium-to-low mass stars in Tr14-N4 but that includes at least one high mass protostar. Its properties are derived from the 1.2 to 11?m photometry. The presence of outflows is evinced by knotty H_2 emission near the edge of a cavity. We also presented updates of near-IR light-curves of Car I-136 and 125 embedded in the dark cloud associated with Car I.

Tapia, M.; Roth, M.; Persi, P.

2011-10-01

35

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

36

A-Train Observations of Young Volcanic Eruption Clouds  

NASA Astrophysics Data System (ADS)

NASA's A-Train satellite constellation (including Aqua, CloudSat, CALIPSO, and Aura) has been flying in formation since 2006, providing unprecedented synergistic observations of numerous volcanic eruption clouds in various stages of development. Measurements made by A-Train sensors include total column SO2 by the Ozone Monitoring Instrument (OMI) on Aura, upper tropospheric and stratospheric (UTLS) SO2 column by the Atmospheric Infrared Sounder (AIRS) on Aqua and Microwave Limb Sounder (MLS) on Aura, ash mass loading from AIRS and the Moderate resolution Imaging Spectroradiometer (MODIS) on Aqua, UTLS HCl columns and ice water content (IWC) from MLS, aerosol vertical profiles from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument aboard CALIPSO, and hydrometeor profiles from the Cloud Profiling Radar (CPR) on CloudSat. The active vertical profiling capability of CALIPSO, CloudSat and MLS sychronized with synoptic passive sensing of trace gases and aerosols by OMI, AIRS and MODIS provides a unique perspective on the structure and composition of volcanic clouds. A-Train observations during the first hours of atmospheric residence are particularly valuable, as the fallout, segregation and stratification of material in this period determines the concentration and altitude of constituents that remain to be advected downwind. This represents the eruption 'source term' essential for dispersion modeling, and hence for aviation hazard mitigation. In this presentation we show examples of A-Train data collected during recent eruptions including Chaitén (May 2008), Kasatochi (August 2008), Redoubt (March 2009), Eyjafjallajökull (April 2010) and Cordón Caulle (June 2011). We interpret the observations using the canonical three-stage view of volcanic cloud development [e.g., Rose et al., 2000] from initial rapid ash fallout to far-field dispersion of fine ash, gas and aerosol, and results from numerical modeling of volcanic plumes [e.g., Textor et al., 2003] and discuss the degree to which the observations validate existing theory and models. We also describe plans for advanced SO2 and ash retrieval algorithms that will exploit the synergy between UV and IR sensors in the A-Train for improved quantification of ash and SO2 loading by volcanic eruptions.

Carn, S. A.; Prata, F.; Yang, K.; Rose, W. I.

2011-12-01

37

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

38

The Sizes of the Nearest Young Stars  

NASA Astrophysics Data System (ADS)

We present moderate resolution (R ~ 3575) optical spectra of 19 known or suspected members of the AB Doradus and ? Pictoris Moving Groups, obtained with the DeVeny Spectrograph on the 72 inch Perkins telescope at Lowell Observatory. For four of five recently proposed members, signatures of youth such as Li I 6708 Å absorption and H? emission further strengthen the case for youth and membership. The lack of detected lithium in the proposed ? Pic member TYC 2211-1309-1 implies that it is older than all other K-type members and weakens the case for membership. Effective temperatures are determined via line ratio analyses for the 11 F, G, and early-K stars observed, and via spectral comparisons for the eight late-K and M stars observed. We assemble updated candidate membership lists for these moving groups that account for known binarity. Currently, the AB Dor Moving Group contains 127 proposed members and the ? Pic Moving Group holds 77 proposed members. We then use temperature, luminosity, and distance estimates to predict angular diameters for these stars; the motivation is to identify stars that can be spatially resolved with long-baseline optical/infrared interferometers in order to improve age estimates for these groups and to constrain evolutionary models at young ages. Considering the portion of the sky accessible to northern hemisphere facilities (decl. > - 30), six stars have diameters large enough to be spatially resolved (? > 0.4 mas) with the CHARA Array, which currently has the world's longest baseline of 331 m; this subsample includes the low-mass M2.5 member of AB Dor, GJ 393, which is likely to still be pre-main sequence. For southern hemisphere facilities (decl. < + 30), 18 stars have diameters larger than this limiting size, including the low-mass debris disk star AU Mic (0.72 mas). However, the longest baselines of southern hemisphere interferometers (160 m) are only able to resolve the largest of these, the B6 star ? Gru (1.17 mas) proposed long-baseline stations may alleviate the current limitations.

McCarthy, Kyle; White, Russel J.

2012-06-01

39

New optical rebrightening of the young eruptive object V2492 Cyg (IRAS 20496+4354)  

NASA Astrophysics Data System (ADS)

We report results from VRI optical observations of the recently discovered PMS eruptive star V2492 Cyg in the region of Pelican Nebula (Itagaki and Yamaoka, CBET #2426, Munari et al. CBET #2428 Covey et al. 2011, AJ, 141, 40). Starting from the mid-2009 the object shows a strong outburst by ~6 mag, which reaches its maximum in the summer of 2010. Such large amplitude eruptions are very rare phenomenon and they can be grouped into two main types, named FUors and EXors.

Semkov, E.; Peneva, S.

2012-06-01

40

Massive Young Cluster and Super Star Cluster  

NASA Astrophysics Data System (ADS)

As astronomical observational technologies have been developing, more and more massive young clusters (MYC) and super star clusters (SSC) in different galaxies including normal ones, starburst ones and mergers have been found, some properties of which are different from either open or globular clusters in the Galaxy, mainly their ages being significantly younger than those of globular ones in the Milky Way galaxy on one side and they being obviously more massive than open ones on the other side. Up to now, at least one SSC has been found in our own galaxy. The main observational characteristics of MYCs or SSCs are as follows: 1) They are young and massive star clusters. Usually their ages are several million years, the oldest ones of which are not over a few hundred million years, their masses are 3 × 104 - 107 Msolar, and the range of their effective radii is a few parsecs to 20 pc, even a bit bigger. 2) The distribution of MYCs in host galaxies is quite different from one galaxy to another. In some galaxies most of MYCs are close to the galactic centers, and in other galaxies they have been observed at places far from the centers. 3) A bright compact nucleus can be easily observed in some MYCs and there is no obvious nucleus for other MYCs. 4) The numbers of MYCs in different galaxies are of significant difference, from zero to a few dozens, even more than 100. 5) The distribution in ages is quite smooth for spiral galaxies, being no more the case of star burst galaxies, in which MYCs are apparently formed in a relative short period. The formation efficiency of MYCs is in close relationship with star formation rate (SFR) for both, normal galaxies and interacting ones, and the higher the SFR, the higher the formation efficiency of such clusters. So far as the evolutionary significance of MYCs is concerned, MYCs, at least some of them, are possible precursor objects of globular star clusters for some reasons, which can be used to explain the origin of globular clusters. Since observational data for MYCs or SSCs in galaxies are limited by now, some important problems for such clusters remain to be further studied and solved, such as what is the intrinsic relationship between the SFR and the formation efficiency of MYCs, is there some relationship between the formation and evolution of MYCs and their host galaxies, and whether can MYCs exist for long enough time to be finally evolved to form globular clusters?

Zhao, Jun-Liang

2007-03-01

41

Gravitational waves from hot young rapidly rotating neutron stars  

Microsoft Academic Search

Gravitational radiation drives an instability in the r-modes of young rapidly rotating neutron stars. This instability is expected to carry away most of the angular momentum of the star by gravitational radiation emission, leaving a star rotating at about 100 Hz. In this paper we model in a simple way the development of the instability and evolution of the neutron

Benjamin J. Owen; Lee Lindblom; Curt Cutler; Bernard F. Schutz; Alberto Vecchio; Nils Andersson

1998-01-01

42

Young stellar clusters and star formation throughout the Galaxy  

Microsoft Academic Search

Most stars are born in rich young stellar clusters (YSCs) embedded in giant molecular clouds. The most massive stars live out their short lives there, profoundly influencing their natal environments by ionizing HII regions, inflating wind-blown bubbles, and soon exploding as supernovae. Thousands of lower-mass pre-main sequence stars accompany the massive stars, and the expanding HII regions paradoxically trigger new

Eric Feigelson; Fred Adams; Lori Allen; Edwin Bergin; John Bally; Zoltan Balog; Tyler Bourke; Crystal Brogan; You-Hua Chu; Edward Churchwell; Marc Gagne; Konstantin Getman; Todd Hunter; Larry Morgan; Philip Massey; Mordecai-Mark Mac Low; Eric Mamajek; S. Thomas Megeath; C. Robert O'Dell; Jill Rathborne; Luisa Rebull; Steven Stahler; Leisa Townsley; Junfeng Wang; Jonathan Williams

2009-01-01

43

Nearby Post-T Tauri Stars: Young Solar System Analogs  

Microsoft Academic Search

Post-T Tauri stars 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 compared to T Tauri stars. The advent of the ROSAT All-Sky Survey and recent astrometric surveys has, however, accelerated the pace of discovery of these young suns. The

E. E. Mamajek

2003-01-01

44

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

45

Young Stars far from the Galactic Plane: Runaways from Clusters  

NASA Astrophysics Data System (ADS)

Quite recently, a significant number of OB stars far from the galactic plane have been found, situated at z-distances ranging from several hundreds of pc to several kpc. The short lifetimes of these stars pose problems for their interpretation in terms of the standard picture of star formation. Different mechanisms have been put forward to explain the existence of these stars, either within the conventional view, or postulating star formation in the galactic halo itself. These mechanisms range from arguing that they are misidentified evolved or abnormal stars, to postulating powerful ejection mechanisms for young disk stars; in situ formation also admits several variants. We have collected from the literature a list of young stars far from the plane, for which the evidence of youth seems convincing. We discuss two possible formation mechanisms for these stars: ejection from the plane as the result of dynamical evolution of small clusters (Poveda et al. 1967) and in situ formation, via induced shocks created by spiral density waves (Martos et al. 1999). We compute galactic orbits for these stars, and identify the stars that could be explained by one or the other mechanism. We find that about 90 percent of the stars can be accounted for by the cluster ejection mechanism, that is, they can be regarded as runaway stars in the galactic halo.

Allen, C.; Kinman, T.

2004-08-01

46

Evolution of the binary population in young dense star clusters  

NASA Astrophysics Data System (ADS)

Context. Field stars are not always single stars, but can often be found in bound double systems. Since binary frequencies in the birth places of stars, young embedded clusters, are sometimes even higher than on average the question arises of how binary stars form in young dense star clusters and how their properties evolve to those observed in the field population. Aims: We assess, the influence of stellar dynamical interactions on the primordial binary population in young dense cluster environments. Methods: We perform numerical N-body simulations of the Orion nebula cluster like star cluster models including primordial binary populations using the simulation code nbody6++. Results: We find two remarkable results that have yet not been reported: The first is that the evolution of the binary frequency in young dense star clusters is independent predictably of its initial value. The time evolution of the normalized number of binary systems has a fundamental shape. The second main result is that the mass of the primary star is of vital importance to the evolution of the binary. The more massive a primary star, the lower the probability that the binary is destroyed by gravitational interactions. This results in a higher binary frequency for stars more massive than 2 M? compared to the binary frequency of lower mass stars. The observed increase in the binary frequency with primary mass is therefore most likely not due to differences in the formation process but can be entirely explained as a dynamical effect. Conclusions: Our results allow us to draw conclusions about the past and the future number of binary systems in young dense star clusters and demonstrate that the present field stellar population has been influenced significantly by its natal environments.

Kaczmarek, T.; Olczak, C.; Pfalzner, S.

2011-04-01

47

Gravitational Radiation Instability in Hot Young Neutron Stars  

Microsoft Academic Search

We show that gravitational radiation drives an instability in hot young\\u000arapidly rotating neutron stars. This instability occurs primarily in the l=2\\u000ar-mode and will carry away most of the angular momentum of a rapidly rotating\\u000astar by gravitational radiation. On the timescale needed to cool a young\\u000aneutron star to about T=10^9 K (about one year) this instability can

Lee Lindblom; Benjamin J. Owen; Sharon M. Morsink

1998-01-01

48

A young massive planet in a star-disk system  

Microsoft Academic Search

There is a general consensus that planets form within disks of dust and gas around newly born stars. Details of their formation process, however, are still a matter of ongoing debate. The timescale of planet formation remains unclear, so the detection of planets around young stars with protoplanetary disks is potentially of great interest. Hitherto, no such planet has been

J. Setiawan; Th. Henning; R. Launhardt; A. Müller; P. Weise; M. Kürster

2008-01-01

49

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

50

THE FORMATION OF YOUNG DENSE STAR CLUSTERS THROUGH MERGERS  

SciTech Connect

Young star clusters such as NGC 3603 and Westerlund 1 and 2 in the Milky Way and R136 in the Large Magellanic Cloud are dynamically more evolved than expected based on their current relaxation times. In particular, the combination of a high degree of mass segregation, a relatively low central density, and the large number of massive runaway stars in their vicinity are hard to explain with the monolithic formation of these clusters. Young star clusters can achieve such a mature dynamical state if they formed through the mergers of a number of less massive clusters. The shorter relaxation times of less massive clusters cause them to dynamically evolve further by the time they merge, and the merger product preserves the memory of the dynamical evolution of its constituent clusters. With a series of N-body simulations, we study the dynamical evolution of single massive clusters and those that are assembled through merging smaller clusters together. We find that the formation of massive star clusters through the mergers of smaller clusters can reproduce the currently observed spatial distribution of massive stars, the density, and the characteristics (number and mass distribution) of the stars ejected as runaways from young dense clusters. We therefore conclude that these clusters and possibly other young massive star clusters formed through the mergers of smaller clusters.

Fujii, M. S.; Portegies Zwart, S. F. [Leiden Observatory, Leiden University, NL-2300RA Leiden (Netherlands); Saitoh, T. R. [Interactive Research Center of Science, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro, Tokyo 152-8551 (Japan)

2012-07-01

51

Stars on the Edge -- Variable A in M33, 35 Years (or More) in Eruption  

NASA Astrophysics Data System (ADS)

Variable A is one of the highly luminous but unstable stars that define the upper luminosity limit in the HRD for evolved stars. At its maximum light in 1950 Var A was one of the visually brightest stars in M33 with an F-type spectrum and colors. It then rapidly declined by more than 3 magnitudes and became red. Optical and near-infrared photometry from 1977 - 1986 showed that Var A had gotten even fainter, was still red, and its spectrum from 1985 showed strong TiO bands. It had a strong infrared excess and was as bright at 10 microns as at its visual maximum in 1950 with a luminosity of 5 x 105 Lsun and a high mass loss rate ( 2 x 10-4 Msun yr-1). Its late-type spectrum was produced in a cool pseudo-photosphere or optically thick wind. Thus, Var A appeared to be a more extreme, more luminous version of rho Cas, well known for its "shell episodes" during which it fades and its normal F supergiant spectrum shows TiO bands. While in rho Cas these events last 1 - 2 years at most, in Var A, the outburst or eruption phase lasted at least 35 years and probably 40 - 45 years! A recent spectrum shows that the star's wind has returned to a warmer apparent temperature; however, it has remained faint. Its present faintness in the optical is presumably due to obscuration by circumstellar dust, although it is not red. The fading of its near-IR flux very likely corresponds to the changes in its wind possibly accompanied by the dissipation of warm dust near the star. Var A thus presents us with the opportunity to observe the formation and destruction of dust accompanying a major eruptive episode.

Humphreys, R. M.; Koppleman, M.; Helton, A.; Jones, T. J.; Gehrz, R. D.; Wagner, R. M.

2005-05-01

52

Young low mass stars in the vicinity of Sigma Scorpii  

NASA Technical Reports Server (NTRS)

The region near Sigma Scorpii, a member of the Sco-Cen OB association, is examined for signs of recent star formation. Thirteen candidate young stellar objects are identified over an 80 x 80 arcmin region centered on Sigma Sco using the Point source Catalog and a recent survey for H-alpha emission-line stars. Near-infrared photometry, improved IRAS fluxes, and optical spectra are used to determine the nature of these objects. Four definite young stars are revealed, as well as one additional such object that fell just outside of the present target region. These stars, all of spectral type K or M, are argued to have formed in the vicinity of the B1 giant star Sigma Scorpii and to represent a subsample of the low-mass members of the association.

Meyer, Michael R.; Wilking, Bruce A.; Zinnecker, Hans

1993-01-01

53

Asteroseismology. Echography of young stars reveals their evolution.  

PubMed

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. PMID:24993346

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

54

Development of dental charts according to tooth development and eruption for Turkish children and young adults  

PubMed Central

Purpose In this study, we aimed to develop dental charts for Turkish children and young adults of both genders within the age group of 4.5-22.5 years according to tooth mineralization and eruption in a format similar to that proposed by AlQahtani et al. Materials and Methods In total, 753 digital panoramic radiographs from 350 males and 403 females were assessed. The permanent teeth were evaluated according to the classification system described by Demirjian et al. The eruption stage was assessed with Bengston's system, which was modified by AlQahtani et al at four points. Results Teeth generally developed earlier in females than in males. This was particularly notable in the age group of 5-14 years. However, this difference was usually visible in only one stage, not in all teeth. It has been determined that the mixed dentition period ended with the shedding of the second deciduous molars in both genders. Conclusion The dental charts presented here included information that could be beneficial to dental clinicians in making appropriate diagnosis and planning orthodontic and surgical procedures. These charts also provided datasets for preliminary dental age estimation in Turkish children and young adults.

Afsin, Huseyin; Ozaslan, Abdi; Karaday?, Sukriye

2014-01-01

55

Circumstellar dust disks around young stars  

SciTech Connect

Kuiper Airborne Observatory observations of several resolved far-infrared structures that are likely to represent disks formed during the process of the collapse of a cloud to form a star are discussed. An infrared radiation nebula, star HH57, and SVS13 were studied. Forty-seven and 95-micron maps of far infrared radiation are given.

Cohen, M.; Schwartz, R.; Harvey, P.M.

1984-12-01

56

Direct Detections of Young Stars in Nearby Elliptical Galaxies  

NASA Astrophysics Data System (ADS)

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 × 10-5 M ? yr-1. The star formation history was roughly constant from 0.5 to 1.5 Gyr (at (3-5) × 10-4 M ? yr-1), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 102 and 104 M ?. The specific star formation rates of ~10-16 yr-1 (at the present day) or ~10-14 yr-1 (when averaging over the past Gyr) imply that a fraction 10-8 of the stellar mass is younger than 100 Myr and 10-5 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program 11583.

Ford, H. Alyson; Bregman, Joel N.

2013-06-01

57

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

58

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

59

Chemistry in the envelopes around massive young stars  

NASA Astrophysics Data System (ADS)

Recent chemical studies of high-mass star-forming regions at submillimeter and infrared wavelengths reveal large variations in the abundances depending on evolutionary state. Such variations can be explained by freezing out of molecules onto grains in the cold collapse phase, followed by evaporation and high-temperature chemical reactions when the young star heats the envelope. Thus, the chemical composition can be a powerful diagnostic tool. A detailed study of a set of infrared-bright massive young stars reveals systematic increases in the gas/solid ratios and abundances of evaporated molecules with temperature. This "global heating" plausibly results from the gradual dispersion of the envelopes. We argue that these objects form the earliest phase of massive star formation, before the "hot core" and ultracompact H II region phase.

van Dishoeck, Ewine F.; van der Tak, Floris F. S.

60

Two Populations of Young Massive Star Clusters in Arp 220  

Microsoft Academic Search

We present new optical observations of young massive star clusters in Arp\\u000a220, the nearest ultraluminous infrared galaxy, taken in UBVI with the Hubble\\u000aSpace Telescope ACS\\/HRC camera. We find a total of 206 probable clusters whose\\u000aspatial distribution is centrally concentrated toward the nucleus of Arp 220.\\u000aWe use model star cluster tracks to determine ages, luminosities, and masses

Christine D. Wilson; William E. Harris; Rebecca Longden; N. Z. Scoville

2005-01-01

61

The Rotational Evolution of Young, Solar-Type Stars  

NASA Astrophysics Data System (ADS)

Although stars have been known to rotate since the time of Galileo, some fundamental issues concerning stellar rotation have not been understood. Of these, the existence of both rapid and slow rotators in the same star clusters is perhaps the most problematic. I present theoretical models of rotating stars which explain the origin of both the ultra-fast and the slow rotators observed in young star clusters. I also present new observations of two young open clusters, which are subsequently interpreted using the models. The principal conclusions of this work are: (1) The ultra-fast rotators are distributed uniformly across the color axis. (2) The existence of the ultra-fast rotators implies that angular momentum loss from young stars is inconsistent with a Skumanich-type slowdown. It may be interpreted either as evidence of magnetic saturation at high rotation rates or of a different magnetic field configuration during young stellar stages. (3) The slow rotators are not distributed uniformly across the color axis. (4) The existence of slow rotators may be understood via disk-regulation of stellar rotation during pre-main sequence evolution. The non-uniform distribution of these slow rotators suggests that disk lifetimes are a function of stellar mass. (5) The amplitude of variability has been delineated as a function of both color and period leading to an understanding of the selection effects in the rotation period database and of the accuracy necessary to do similar work in other, perhaps older, clusters.

Barnes, Sydney Agnello

62

Lithium-depleted stars in the young ? Orionis cluster  

NASA Astrophysics Data System (ADS)

Context: Knowledge of the age distribution of stars in young clusters and associations is crucial to constrain models of star formation. HR diagrams of different young clusters and associations suggest the presence of age spreads, but the influence of errors on the derived ages is still largely debated. Determination of lithium abundances in low-mass stars represents an alternative and robust way to infer stellar ages. Aims: We measured lithium in a sample of low mass members of the young (4-5 Myr) ? Ori cluster with the main goal of investigating its star formation history. Methods: Using the FLAMES multi-object spectrograph on VLT/UT2, we obtained spectra of 98 candidate cluster members. The spectra were used to determine radial velocities, to infer the presence of H? emission, and to measure the strength of the Li i 670.8 nm absorption line. Results: Using radial velocities, H? and Li, together with information on X-ray emission, we identified 59 high probability cluster members. Three of them show severe Li depletion. The nuclear ages inferred for these highly depleted stars exceed 10-15 Myr; for two of them these values are in good agreement with the isochronal age, while for the third star the nuclear age exceeds the isochronal one. Based on data collected at the ESO Very Large Telescope, Paranal Observatory, Chile [program 074.D-0136(A)].

Sacco, G. G.; Randich, S.; Franciosini, E.; Pallavicini, R.; Palla, F.

2007-02-01

63

Pulsating pre-Main sequence stars in young open clusters  

NASA Astrophysics Data System (ADS)

New pulsating pre-main sequence (PMS) stars have been discovered in the young open clusters IC 4996 and NGC 6383 using CCD time series photometry in Johnson B and V filters. As the cluster ages are both smaller than 10 million years, all members later than spectral type A0 are still contracting towards the ZAMS, hence providing ideal candidates for searches of pulsation. A dozen stars in NGC 6383 and 35 stars in IC 4996 lie within the boundaries of the classical instability region in the Hertzsprung-Russell (HR) diagram, but pulsation was detected for only two of them in each cluster.

Zwintz, Konstanze; Marconi, Marcella; Kallinger, Thomas; Weiss, Werner W.

2004-12-01

64

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.

Oneil, Pamela; Fryxell, Bruce; Burrows, Adam

1994-02-12

65

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.

Oneil, Pamela; Fryxell, Bruce; Burrows, Adam

1994-02-12

66

Young, Massive Stars at Low Metallicity  

NASA Technical Reports Server (NTRS)

We report on an extensive analysis of O-type stars in the Small Magellanic Cloud that have been observed by HST and FUSE. The analysis, which involves detailed NLTE atmospheric models recently constructed by Lanz & Hubeny, has the following goals: (1) calibration of spectral properties in terms of fundamental parameters; (2) resolution of the discrepancy between spectro-scopic masses and evolutionary masses; and (3) evidence for mixing of nuclear-processed elements. We derive a significantly lower temperature scale for O-type stars than previously assumed; the lower temperatures help to resolve the mass discrepancy. We describe the results of our analysis and discuss their implications for stellar evolution at low metallicity.

Oegerle, William R. (Technical Monitor); Heap, S. R.; Lanz, T.

2002-01-01

67

Evolution of massive stars in very young clusters and associations  

NASA Technical Reports Server (NTRS)

Statistics concerning the stellar content of young galactic clusters and associations which show well defined main sequence turnups have been analyzed in order to derive information about stellar evolution in high-mass galaxies. The analytical approach is semiempirical and uses natural spectroscopic groups of stars on the H-R diagram together with the stars' apparent magnitudes. The new approach does not depend on absolute luminosities and requires only the most basic elements of stellar evolution theory. The following conclusions are offered on the basis of the statistical analysis: (1) O-tupe main-sequence stars evolve to a spectral type of B1 during core hydrogen burning; (2) most O-type blue stragglers are newly formed massive stars burning core hydrogen; (3) supergiants lying redward of the main-sequence turnup are burning core helium; and most Wolf-Rayet stars are burning core helium and originally had masses greater than 30-40 solar mass. The statistics of the natural spectroscopic stars in young galactic clusters and associations are given in a table.

Stothers, R. B.

1985-01-01

68

The Beta Pictoris Phenomenon in Young Stars With Accreting Gas  

NASA Technical Reports Server (NTRS)

Program Hae2BPIC resulted in usable ISO spectra of three young, Herbig Ae stars: HR 5999 (A7e, t=0.6 Myr), SV Cep (al-2e, t=1-3 Myr), and MW Vul (Al-2e, t=1-3 Myr). While too small a sample to pursue our original goal of surveying the silicate emission in these young, protoplanetary disk systems, comparison of these data with ground-based IR spectra, and published ISO observations of other HAe stars (especially the posters at PPIV) reveals the following: The known binary stars in the sample show signatures of partially crystal line silicate features by t=0.6 Myr, at an epoch when ostensibly single Herbig Ae stars have substantially stronger silicate emission dominated by amorphous grains. The known binary stars also show deficits in the optically thick continuum flux relative to coeval single stars. Comparison of ISO spectra indicates that the flux deficit seen in WD 163296 over 10-100 microns relative to AB Aur reflects a real deficit of material interior to 300.

Grady, Carol A.

1999-01-01

69

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

70

Observations of hydrocarbon emission in disks around young stars  

NASA Astrophysics Data System (ADS)

I give an overview of the recent scientific results based on observations of PAH emission from circumstellar disks around young stars. The stellar radiation field plays a key role in the excitation and destruction of the PAH molecules in the disk. The detection rate of PAH emission in disks is optimal for stars of spectral type A. Around stars of similar temperature, the disk structure determines the PAH emission strength: disks with a flared geometry produce stronger PAH emission than flattened disks. The spectral properties of the emission features, indicative of the chemistry of the emitting hydrocarbons, is closely linked to the central star radiation field. The main PAH features shift to redder wavelengths with decreasing stellar effective temperature. This trend has been interpreted as an indication for a higher aliphatic/aromatic ratio of the hydrocarbon mixture around cool stars with respect to hot stars. An alternative explanation may be a more significant contribution to the infrared emission of very small grains around cooler stars.

Acke, B.

2011-03-01

71

Young stars and outflows : case studies of three different regions  

NASA Astrophysics Data System (ADS)

Mass is clearly the most important parameter of a star as it determines the phases it will go through, its physical parameters, the elements produced and its life span. Two important processes that occur simultaneously in young stars are directly related to the final mass a star ends up with; the accretion of material onto the star from a circumstellar disk and the bipolar outflow of material from the star. These processes are related since the infalling material drives the outflow. The life span of circumstellar disks is also important for the possibility of life, as this limits the time available for planet formation. This may depend on the stellar mass and its environment. Three detailed observational case-studies are presented, a globule (B335) that is forming only one or two stars, a medium-sized star formation region (L1551) and a large cluster of young stars (YSOs) in a part of our nearest giant molecular cloud (L1641N). The first part of this thesis concerns finding YSOs by looking for mid-IR excess, indicative of circumstellar disks, using the ISO and Spitzer satellites in combination with ground-based observations. Other signs of youth are also used; H-alpha in emission (accretion), Li 6707 in absorption (Li is destroyed with age) and cloud membership based on extinction. The detection of 15 new YSO candidates in L1551 (Paper I) and 89 YSOs in L1641N (Paper II) is presented. In L1641N, distributions are presented of stellar age and mass. An empirical extinction law is found and the star formation history is presented. There seems to be an accelerated star formation with time. This can however also be explained, at least partly, by migration of older stars from the cluster - a migration that may very well be mass dependent. The second part concerns bipolar outflows. The discovery of six Herbig-Haro objects and at least 15 H2 objects in B335 is presented (Paper III) as well as proper motions for previously known objects. A planar shock model is used to calculate shock velocities. In L1641N (Paper IV) we discover a new outflow source and connect this to a number of H2 objects that are found to be part of this flow from proper motion measurements.

Gålfalk, Magnus

72

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

73

Special Session 7 Young Stars, Brown Dwarfs, and Protoplanetary Disks  

NASA Astrophysics Data System (ADS)

In recent years our knowledge of star, brown dwarf and planet formation has progressed immensely due to new data in the IR domain (Spitzer telescope), new X-ray campaigns such as the Chandra Orion Ultradeep Project (COUP) and the X-ray Emission Survey of Taurus (XEST), with XMM-Newton, as well as adaptive optics results and synoptic studies of young stellar and substellar objects.

Gregorio-Hetem, Jane; Alencar, Silvia

2010-11-01

74

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

75

Twinkle, Twinkle: Characterizing Variable Stars in Young Open Clusters  

NASA Astrophysics Data System (ADS)

The usefulness of identifying and characterizing variable stars has proven timeless since the first variable discovery in the sixteenth century. From distance calculations to binary star discoveries to development of stellar evolution theories, variable star observations have made, and continue to make, significant contributions to astronomy. Here we present the results of an ongoing investigation of four young open star clusters: ASCC 105, ASCC 109, Collinder 359, and IC 4665. Observations of the four clusters were collected with the Maria Mitchell Observatory’s 17-inch reflecting telescope. The data collected were used primarily to characterize known variable stars and to identify new variables. Images of each of the four fields were also used to characterize non-variable members in order to refine estimates of the cluster’s membership, distance, reddening, age, and chemical abundance. The main goals of the project were to determine these characteristics of the cluster fields to higher precision than previously measured, and to provide a foundational list of variable stars for future spectroscopic data collection.

Nava, Chantanelle; Stetson, P. B.; Walker, G. E.; West, M.

2014-01-01

76

Pulsating Pre-Main Sequence Stars In Young Open Clusters  

NASA Astrophysics Data System (ADS)

For the search for pre-main sequence (PMS) stars in the young open cluster NGC 6383, 14 nights were granted last year at the CTIO 0.9m telescope. In a preliminary analysis of the data we found ~30% variable stars among the cluster members. We also detected (delta) Scuti like pulsation in a PMS star with an amplitude of ~ 14.8 mmag which could be the 11^th PMS star of which pulsation has been detected. The goal of the project is to determine observationally the boundaries of the PMS instability strip. Only 10 (11 or more by March 2002) pulsating PMS stars are known. This small sample is insufficient for a profound theoretical investigation of the structure of PMS stars using asteroseismic techniques. Additional telescope time is necessary to improve on this situation. We therefore apply for observing time for photometric time series of the open clusters IC 4996 and NGC 6530. Both clusters contain members with spectral types of interest (A-F) which lie between the birthline and the zero-age main sequence (ZAMS). Hence, they are perfect targets for our project.

Zwintz, Konstanze; Weiss, Werner W.; Kallinger, Thomas

2002-08-01

77

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

2014-07-30

78

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

79

The Evolution of X-Ray Emission in Young Stars  

NASA Astrophysics Data System (ADS)

The evolution of magnetic activity in late-type stars is part of the intertwined rotation-age-activity relation, which provides an empirical foundation to the theory of magnetic dynamos. We study the age-activity relation in the pre-main-sequence (PMS) regime, for the first time using mass-stratified subsamples. The effort is based on the Chandra Orion Ultradeep Project (COUP), which provides very sensitive and homogenous X-ray data on a uniquely large sample of 481 optically well-characterized low-extinction low-mass members of the Orion Nebula Cluster, for which individual stellar masses and ages could be determined. More than 98% of the stars in this sample are detected as X-ray sources. Within the PMS phase for stellar ages in the range ~0.1-10 Myr, we establish a mild decay in activity with stellar age ? roughly as LX~?-1/3. On longer timescales, when the Orion stars are compared to main-sequence stars, the X-ray luminosity decay law for stars in the 0.5 Msolarstars. The magnetic activity history for M stars with masses 0.1 Msolarstars, we find that the activity-age decay is strong across the entire history of solar-type stars but is not attributable to rotational deceleration during the early epochs. A combination of tachocline and distributed convective dynamos may be operative in young solar-type stars. The results for the lowest mass stars are most easily understood by the dominance of convective dynamos during both the PMS and main-sequence phases.

Preibisch, Thomas; Feigelson, Eric D.

2005-10-01

80

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

81

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

82

Absolute parameters of young stars - II. V831 Centauri  

NASA Astrophysics Data System (ADS)

Literature photometry and new high-resolution spectroscopy of V831 Cen are presented and analysed. Light and radial velocity curve fittings confirm the central pair of this young multiple system to be close to contact. Absolute parameters are found as follows: M1 = 4.08 +/- 0.07Msolar, M2 = 3.35 +/- 0.06Msolar, R1 = 2.38 +/- 0.03Rsolar, R2 = 2.25 +/- 0.03Rsolar, T1 = 13000 +/- 300K, T2 = 11800 +/- 300K distance of 110 +/- 10pc and age of ~20 +/- 5Myr. Detailed examination of the spectrograms indicates the third component (V831 Cen B) to be an Ap star. The orbit of the third star about the close binary is analysed using historic astrometric measurements. This allows an estimate of the third star's mass to be about 2.5Msolar, but this is sensitive to the adopted distance and inclination values. It is, however, confirmed by the measured radial velocity of the third star. To some extent, such analysis can also be applied to the fourth star (V831 Cen C). The derived properties can be checked against the system's membership of the Scorpius-Centaurus OB2 association.

Budding, E.; Erdem, A.; Inlek, G.; Demircan, O.

2010-04-01

83

Multiplicity among Young Brown Dwarfs and Very Low Mass Stars  

NASA Astrophysics Data System (ADS)

We report on a near-infrared adaptive optics imaging survey of 31 young brown dwarfs and very low mass (VLM) stars, 28 of which are in the Chamaeleon I star-forming region, using the ESO Very Large Telescope. We resolve the suspected 0.16'' (~26 AU) binary Cha H? 2 and present two new binaries, Hn 13 and CHXR 15, with separations of 0.13'' (~20 AU) and 0.30'' (~50 AU), respectively; the latter is one of the widest VLM systems known. We find a binary frequency of 11+9-6%, thus confirming the trend for a lower binary frequency with decreasing mass. By combining our work with previous surveys, we arrive at the largest sample of young VLM objects (72) with high angular resolution imaging to date. Its multiplicity fraction is in statistical agreement with that for VLM objects in the field. Furthermore, we note that many field stellar binaries with lower binding energies and/or wider cross sections have survived dynamical evolution and that statistical models suggest tidal disruption by passing stars is unlikely to affect the binary properties of our systems. Thus, we argue that there is no significant evolution of multiplicity with age among brown dwarfs and VLM stars in OB and T associations between a few megayears to several gigayears. Instead, the observations so far suggest that VLM objects are either less likely to be born in fragile multiple systems than solar-mass stars or such systems are disrupted very early. We dedicate this paper to the memory of our coauthor, Eduardo Delgado-Donate, who died in a hiking accident in Tenerife earlier this year.

Ahmic, Mirza; Jayawardhana, Ray; Brandeker, Alexis; Scholz, Alexander; van Kerkwijk, Marten H.; Delgado-Donate, Eduardo; Froebrich, Dirk

2007-12-01

84

HUBBLE PICTURES SHOW HOT GAS BUBBLE EJECTED BY YOUNG STAR  

NASA Technical Reports Server (NTRS)

These images taken with the Hubble Space Telescope's Wide Field and Planetary Camera 2 reveal the evolution of bubbles of glowing gas being blown out from the young binary star system XZ Tauri. Gas from an unseen disk around one or both of the stars is channeled through magnetic fields surrounding the binary system and then is forced out into space at nearly 300,000 miles per hour (540,000 kilometers per hour). This outflow, which is only about 30 years old, extends nearly 60 billion miles (96 billion kilometers). Hubble first discovered this unique bubble in 1995, and additional observations were made between 1998 and 2000. These images show that there was a dramatic change in its appearance between 1995 and 1998. In 1995, the bubble's edge was the same brightness as its interior. However, when Hubble took another look at XZ Tauri in 1998, the edge was suddenly brighter. This brightening is probably caused by the hot gas cooling off, which allows electrons in the gas to recombine with atoms, a process that gives off light. This is the first time that astronomers have seen such a cooling zone 'turn on.' These images provide an unprecedented opportunity to study the development of a very recent outflow from young (about 1 million years old) stars. Credits: NASA, John Krist (Space Telescope Science Institute), Karl Stapelfeldt (Jet Propulsion Laboratory), Jeff Hester (Arizona State University), Chris Burrows (European Space Agency/Space Telescope Science Institute)

2002-01-01

85

Probing the nature of optically obscured companions to young stars  

NASA Astrophysics Data System (ADS)

We propose spatially-resolved R 50 000 PHOENIX observations of Glass I, a member of the class of Infrared Companions (IRCs) to T Tauri stars. The nature of these objects remains elusive, as they apparently defy the paradigm of coevality in young multiple systems. The main goals are (1) to search for the potentially highly-veiled photospheric absorption lines with unprecedented spectral resolution in order to derive the intrinsic stellar nature (spectral type, extinction, continuum excess) of one of these intriguing young objects, (2) to derive the accretion properties (accretion rate, kinematics of brackett gamma line), and (3) characterize the inner circumstellar environment through the search for molecular absorption/emission like HCN and/or non-stellar ^12CO features, as recently achieved for the prototype IRC, namely T Tau Sa (Duchene et al. 2005). These observations will allow us to better constrain the nature and geometry of the obscuring material towards IRCs and to probe the organic chemistry, gas temperatures, and gas kinematics in the planet-forming zones close to a young star.

Duchene, Gaspard; Correia, Serge; Ratzka, Thorsten

2009-08-01

86

Dust around young stars. Photopolarimetric activity of the classical Herbig Ae/Be star RR Tauri.  

NASA Astrophysics Data System (ADS)

The classical Herbig Ae/Be (HAeBe) star RR Tau is known as a highly variable young star with an amplitude of variability of about 4mag in the V pass-band. In this paper we present the results of coordinated Crimea-Dodaira multi-band photo-polarimetric observations of this star which cover fully the observed interval of its brightness changes. Within the observed interval of its light variations the linear polarization of RR Tau anti-correlates with its brightness changes. The dependence of the linear polarization on the stellar magnitude agrees well with the model according to which the main source of the intrinsic polarization of RR Tau is scattered radiation by the circumstellar (CS) disk-like dust envelope (probably the protoplanetary disk) seen edge-on or under a small inclination to the line-of-sight, and that the brightness variations are caused by variable obscuration of the star by revolving circumstellar dust clouds. A comparison with previous photo-polarimetric observations of this star shows that the Stokes parameters of its polarized radiation are quite stable on a time scale of about 7 years. By using this fact we have separated the interstellar and intrinsic components of the observed polarization. The numerical modeling of the intrinsic linear polarization together with the colour-magnitude diagrams of RR Tau show that the circumstellar disk-like envelope around this star is strongly flattened and that the characteristic size of the grains is intermediate between that of interstellar dust and dust in the old protoplanetary disk of ? Pictoris. We assume on the basis of this analysis that RR Tau is surrounded by a young protoplanetary disk and that it can be considered as a young progenitor of ? Pictoris.

Rostopchina, A. N.; Grinin, V. P.; Okazaki, A.; The, P. S.; Kikuchi, S.; Shakhovskoy, D. N.; Minikhulov, N. Kh.

1997-11-01

87

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

88

Fast Star, Slow Star; Old Star, Young Star: Subgiant Rotation as a Population and Stellar Physics Diagnostic  

NASA Astrophysics Data System (ADS)

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 ?). 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.

2013-10-01

89

Observing Young Variable Stars Using WFCAM at UKIRT  

NASA Astrophysics Data System (ADS)

We present the results from the analysis of 120 epoch time-series photometry of a 1 square degree region of the Lynds 1003 dark cloud in the Cygnus OB7 association. Using the Wide-Field imaging camera (WFCAM) on UKIRT we were able to obtain almost-nightly J,H,K' photometry over three observing seasons of over 100,000 stars with photometric uncertainty better than 0.02 mag in the range J=10-16 mags and better than 0.5 mag down to J=19.5. From the data we establish criteria for determining variability based on the least-varying sources. We report the discovery of both periodic and stochastic variability for a number of young T Tauri stars. We compare statistical properties of known cluster members with the general field population, and discuss physical models for some of the more interesting sources. This work was funded by the NSF REU program.

Rice, Thomas; Aspin, C.; Wolk, S. J.

2011-01-01

90

Near-Infrared Variability in Young Stars with Disks  

NASA Astrophysics Data System (ADS)

We present the results from the analysis of 120 epoch time-series photometry of a 1 square degree region of the Lynds 1003 dark cloud in the Cygnus OB7 association. Using the Wide-Field imaging camera (WFCAM) on UKIRT we were able to obtain almost-nightly J,H,K' photometry over three observing seasons of over 100,000 stars with photometric uncertainty better than 0.02 mag in the range J=10-16 mags and better than 0.1 mag down to J=18. We have identified over 300 candidate disk-bearing YSOs from color criteria and have investigated variability and periodicity for these stars relative to the field population. We have uncovered a population of young stars whose K-band infrared excess varies significantly over our 2-year observations. We report the discovery of periodic and stochastic variables among T Tauri stars, and present rotation periods for a number of YSOs. Part of this work was funded by the NSF REU program.

Rice, Thomas; Wolk, S. J.; Aspin, C.

2011-05-01

91

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

92

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

93

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

94

Multiplicity study of young pre-main sequence stars in the Lupus star-forming Region  

NASA Astrophysics Data System (ADS)

We have conducted a high contrast imaging search for (sub)stellar companions among 63 young pre-main sequence stars in the Lupus star forming region, using the adaptive optics imager NACO at UT4 of the ESO Paranal observatory. We detected faint co-moving companions around our targets at angular separations between about 0.1 up to several arc seconds (binaries and triple systems). Some of these companions are in the sub stellar mass regime, according to their apparent near infrared photometry at the distance of the Lupus star forming region (about 140pc). We give a progress report to our long-term project, still in execution with the follow-up spectroscopy of detected substellar companion-candidates, and present some first results.

Vogt, Nikolaus; Mugrauer, Markus; Schmidt, Tobias O. B.; Neuhaeuser, Ralph; Ginski, Christian

2013-07-01

95

JET FORMATION FROM MASSIVE YOUNG STARS: MAGNETOHYDRODYNAMICS VERSUS RADIATION PRESSURE  

SciTech Connect

Observations indicate that outflows from massive young stars are more collimated during their early evolution compared to later stages. Our paper investigates various physical processes that impact the outflow dynamics, i.e., its acceleration and collimation. We perform axisymmetric magnetohydrodynamic (MHD) simulations particularly considering the radiation pressure exerted by the star and the disk. We have modified the PLUTO code to include radiative forces in the line-driving approximation. We launch the outflow from the innermost disk region (r < 50 AU) by magnetocentrifugal acceleration. In order to disentangle MHD effects from radiative forces, we start the simulation in pure MHD and later switch on the radiation force. We perform a parameter study considering different stellar masses (thus luminosity), magnetic flux, and line-force strength. For our reference simulation-assuming a 30 M{sub Sun} star-we find substantial de-collimation of 35% due to radiation forces. The opening angle increases from 20 Degree-Sign to 32 Degree-Sign for stellar masses from 20 M{sub Sun} to 60 M{sub Sun }. A small change in the line-force parameter {alpha} from 0.60 to 0.55 changes the opening angle by {approx}8 Degree-Sign . We find that it is mainly the stellar radiation that affects the jet dynamics. Unless the disk extends very close to the star, its force is too small to have much impact. Essentially, our parameter runs with different stellar masses can be understood as a proxy for the time evolution of the star-outflow system. Thus, we have shown that when the stellar mass (thus luminosity) increases with age, the outflows become less collimated.

Vaidya, Bhargav; Porth, Oliver [Member of the International Max Planck Research School for Astronomy and Cosmic Physics at the University of Heidelberg (IMPRS-HD) and the Heidelberg Graduate School of Fundamental Physics (HGSFP). (Germany); Fendt, Christian; Beuther, Henrik, E-mail: vaidya@mpia.de, E-mail: fendt@mpia.de [Max Planck Institute for Astronomy, Koenigstuhl 17, D-69117 Heidelberg (Germany)

2011-11-20

96

Multiple jets from the young star IRAS 21334 + 5039  

NASA Technical Reports Server (NTRS)

The source IRAS 21334 + 5039, a young stellar object (YSO) with broad CO outflow velocity profiles, was imaged in the 1-2 micron region with broad-band J, H, and K filters, and with a Fabry-Perot set to the molecular hydorgen v = 1-0 S(1) line, the hydrogen Br-gamma recombination line, and the neighboring continua. At 2 microns the source has an elongated continuum emission structure centered on the star. The structure has very blue tips, with a weak VLA 6 cm continuum source coincident with one of them. Strong molecular hydrogen emission appears as bow-shaped arcs oriented along an axis perpendicular to the continuum emission, and as a weaker structure aligned with it. Both the continuum and the molecular hydrogen emission are most likely produced by jets, possibly emanating in multiple directions from the central source at large angles to each other. While jets are a common feature of young stars, this source provides solid evidence for multiple jet structures.

Smith, Howard A.; Fischer, Jacqueline

1992-01-01

97

Molecular clouds associated with young open star clusters  

SciTech Connect

Regions around 69 open star clusters were searched for spectral line emission from the J = 1 ..-->.. 0 rotational transition of /sup 12/CO. Of these, 47 regions were completely or nearly completely mapped with a sampling interval of 7.5 arcminutes. The region surveyed around each cluster was at least 10 cluster diameters in size, typically approx.3 square degrees. Survey sensitivity is sufficient to detect lines as weak as 1 K over a range in velocity +/- 83 km s/sup -1/. Clusters in this sample have well-determined distances ranging from approx.1 to 5 kpc and ages approximately less than or equal to 100 million years. The physical characteristics and the kinematics of the ensemble of molecular clouds associated with the clusters are examined. This enables empirical determination of the systematic interaction between young star clusters and the interstellar matter from which they form. Molecular clouds associated with clusters whose ages are less than 5 million years are expanding away from those clusters at approximately 10 km s/sup -1/, thus explaining the relative paucity of emission detected in the neighborhoods of older clusters. Molecular clouds associated with clusters older than approx.5 Myr are smaller, less massive, and colder than clouds associated with extremely young clusters. Evidence is found that the clouds, to some extent, dissolve in the stellar radiation field.

Leisawitz, D.T.

1985-01-01

98

Young Photodissociation Complexes in NGC 6822: Stars and PDRs  

NASA Astrophysics Data System (ADS)

I examine Photodissociation Region (PDR) properties in relation to stellar populations in three regions of NGC 6822. This Local Group dwarf galaxy has a metallicity less than half Solar and lies 490kpc away. It is close enough that stellar populations are resolved as are nebular structures of evolving young HII regions; we can see that these regions are being driven by O/B stars. We model the radiation field directly from the stellar content and find that it matches the radiation structure determined from far-infrared (FIR) line ratios from Herschel/PACS spectral maps (in [CII], [OI] 63micron, and [OIII] 88micron) and derived total FIR maps from dust spectral energy distribution fitting. This allows us to constrain the radiation and density structure of the PDR. Finally, with mid-IR [SIII] line ratios, we map the ionized gas density. At this distance, Spitzer images are insufficient to confirm continuing star formation in these regions via the identification of Young Stellar Objects (YSOs), but the evolutionary picture and ISM density distribution indicate that we are likely to find YSOs with the advent of JWST.

Carlson, Lynn; Dwarf Galaxy Survey Team

2014-01-01

99

Photometric monitoring of the young star Par 1724 in Orion  

NASA Astrophysics Data System (ADS)

We report new photometric observations of the ˜ 200 000 year old naked weak-line run-away T Tauri star Par 1724, located north of the Trapezium cluster in Orion. We observed in the broad band filters B, V, R, and I using the 90 cm Dutch telescope on La Silla, the 80 cm Wendelstein telescope, and a 25 cm telescope of the University Observatory Jena in Großschwabhausen near Jena. The photometric data in V and R are consistent with a ˜ 5.7 day rotation period due to spots, as observed before between 1960ies and 2000. Also, for the first time, we present evidence for a long-term 9 or 17.5 year cycle in photometric data (V band) of such a young star, a cycle similar to that to of the Sun and other active stars. Based on observations obtained with telescopes of the University Observatory Jena, which is operated by the Astrophysical Institute of the Friedrich-Schiller-University; a telescope of the University Observatory Munich on Mount Wendelstein, the 0.9m ESO-Dutch telescope on La Silla, Chile, and with the All Sky Automated Survey (ASAS) project (www.astrouw.edu.pl/asas).

Neuhäuser, R.; Koeltzsch, A.; Raetz, St.; Schmidt, T. O. B.; Mugrauer, M.; Young, N.; Bertoldi, F.; Roell, T.; Eisenbeiss, T.; Hohle, M. M.; Va?ko, M.; Ginski, C.; Rammo, W.; Moualla, M.; Broeg, C.

2009-05-01

100

Transformed photometry of young stars in Cha requested  

NASA Astrophysics Data System (ADS)

Dr. Peter Abraham (Konkoly Observatory, Budapest, Hungary) requested the assistance of AAVSO observers in monitoring eight young stars in Chamaeleon in support of photometry he and his colleagues will be obtaining with the VLT/ISAAC (infrared) and Herschel Space Observatory (far-infrared) during January-February 2013. The targets are CR Cha, CT Cha, HP Cha (Glass I), VW Cha, VZ Cha, WW CHa, WX Cha, XX Cha. Calibrated, transformed VRI photometry is requested to precisely monitor changes in the optical brightness and colors of these objects. Calibration and transformation of the photometry is crucial - if all of the data are not on the same system and particularly if the colors are not transformed, it will be extremely difficult to correlate the data usefully. Previous observations indicate that the stars are highly variable. Brightness changes can be expected from a few tenths of a magnitude to up to 1-2 magnitudes on a timescale of a few days to a few weeks. Observers are asked to try to obtain one to two sets of VRI images per night. Finder charts with sequences may be created using the AAVSO Variable Star Plotter (http://www.aavso.org/vsp). Observations should be submitted to the AAVSO International Database. See full Alert Notice for more details.

Waagen, Elizabeth O.

2013-01-01

101

Properties of stellar clusters around high-mass young stars  

NASA Astrophysics Data System (ADS)

Context: Twenty-six high-luminosity IRAS sources believed to be collection of stars in the early phases of high-mass star formation have been observed in the near-IR (J, H, K_s) to characterize the clustering properties of their young stellar population and compare them with those of more evolved objects (e.g., Herbig Ae/Be stars) of comparable mass. All the observed sources possess strong continuum and/or line emission in the millimeter, being therefore associated with gas and dust envelopes. Nine sources have far-IR colors characteristic of UCHII regions, while the other 17 are probably experiencing an evolutionary phase that precedes the hot-cores, as suggested by a variety of evidence collected in the past decade. Aims: We attempt to gain insight into the initial conditions of star formation in these clusters (initial mass function [IMF], star formation history [SFH]), and to determine mean cluster ages. Methods: For each cluster, we complete aperture photometry. We derive stellar density profiles, color-color and color-magnitude diagrams, and color (HKCF) and luminosity (KLF) functions. These two functions are compared with simulated KLFs and HKCFs from a model that generates populations of synthetic clusters starting from assumptions about the IMF, SFH, and Pre-MS evolution, and using the average properties of the observed clusters as boundary conditions (bolometric luminosity, dust distribution, infrared excess, extinction). Results: Twenty-two sources show evidence of clustering with a stellar richness indicator that varies from a few up to several tens of objects, and a median cluster radius of 0.7 pc. A considerable number of cluster members present an infrared excess characteristic of young pre-main-sequence objects. For a subset of 9 detected clusters, we could perform a statistically significant comparison of the observed KLFs with those resulting from synthetic cluster models; for these clusters, we find that the median stellar age ranges between 2.5×105 and 5×106 years, with evidence of an age spread of the same entity within each cluster. We also find evidence that older clusters tend to be smaller in size, in agreement with our clusters being on average larger than those around relatively older Herbig Ae/Be stars. Our models allow us to explore the relationship between the mass of the most massive star in the cluster and both the cluster richness and the total stellar mass. Although these relationships are predicted by several classes of cluster formation models, their detailed analysis suggests that the properties of our modeled clusters may not be consistent with them resulting from random sampling of the IMF. Conclusions: Our results are consistent with star formation having occurred continuously over a period of time longer than the typical crossing time. Appendices are only available in electronic form at http://www.aanda.org Based on observations obtained at the Palomar Observatory and at the ESO La Silla Observatory (Chile), programme 65.I-0310(A).

Faustini, F.; Molinari, S.; Testi, L.; Brand, J.

2009-09-01

102

Analysis of Star-Disk Interaction in Young Stellar Systems  

NASA Astrophysics Data System (ADS)

We present the study of star-disk interaction in the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. As part of an international campaign of observation of NGC 2264 organized from December 2011 to February 2012, high resolution photometric and spectroscopic data of this object were obtained simultaneously with the Chandra, CoRoT and Spitzer satellites, and ground-based telescopes, as CFHT and VLT/FLAMES at ESO. The optical and infrared light curves of V354 Mon show periodic brightness minima that vary in depth and width every rotational cycle. We found evidence that the H/alpha emission line profile changes according to the period of photometric variations, indicating that the same phenomenon causes both modulations. Such correlation was also identified in a previous observational campaign on the same object, where we concluded that material non-uniformly distributed in the inner part of the disk is the main cause of the photometric modulation. This assumption is supported by the fact that the system is seen at high inclination. It is believed that this distortion of the inner part of the disk results from 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 magnetohydrodynamic numerical simulations. A model of occultation by circumstellar material was applied to the photometric data in order to determine the parameters of the obscuring material during both observational campaigns, thus providing an investigation of its stability on a timescale of a few years. We also studied V422 Mon, a classical T Tauri star with photometric variations similar to those of V354 Mon at optical wavelengths, but with a distinct behavior in the infrared. The mechanism that produces such difference is investigated, testing the predictions of magnetospheric accretion models.

Fonseca, Nathalia; Alencar, Silvia; Bouvier, Jérôme

2013-07-01

103

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

104

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

105

Jets from young stars and z-pinch machines  

NASA Astrophysics Data System (ADS)

Outflows and jets are intimately related to the formation of stars, and play a central role in redistributing mass, energy and angular momentum within the core, disk and parent cloud. The interplay between magnetic field and rotation is widely thought to be responsible for launching and collimating these outflows. Shear induced by differential rotation along initially poloidal field lines results in an azimuthal component of the magnetic field being generated; the magnetic pressure gradient then accelerates the plasma, and inflates bipolar magnetic cavities within the circumstellar matter. However, the resulting winding of the magnetic field can be potentially disrupted by magneto-hydrodynamic instabilities. To better understand the role of magnetic fields in shaping these ouflows, a series of experiments on pulsed-power z-pinch machines have been developed. In this talk I will present results related to the formation of jets in young stellar objects and in the laboratory, and draw a parallel between the two systems.

Ciardi, A.

2012-02-01

106

An Infrared Examination of Young Stars in Upper Centaurus Lupus  

NASA Astrophysics Data System (ADS)

Optical studies of the Upper Centaurus Lupus (UCL) region of the Scorpius-Centaurus (Sco-Cen) complex have found many young stellar objects. The nearby G/K/M Sco-Cen members have been estimated to be much younger 10 Myr) than similar star associations (Song, et al 2012). We have assembled infrared data for the objects thought to be members of UCL by mining various archives including the 2-Micron All-Sky Survey (2MASS), the Spitzer Heritage Archive (SHA), specifically the Spitzer Enhanced Imaging Products Source List, and the Wide-field Infrared Survey Explorer (WISE) all-sky source catalog. We created spectral energy distributions (SEDs) and color-magnitude diagrams (CMDs) with multiple wavelengths to identify infrared excesses and determine what fraction of these stars have circumstellar disks. Students from three high schools collaborated on this project, which is a follow-up project made possible through the NASA/IPAC Teacher Archive Research Project (NITARP; http://nitarp.ipac.caltech.edu).

Johnson, Chelen H.; Linahan, M.; Barge, J.; Rebull, L. M.; Aranda, D.; Canlas, N. G.; Donahoe, K. E.; Ernst, M. K.; Ford, S.; Fox, M. E.; Gutierrez, E.; Haecker, L. W.; Hibbs, C. A.; Maddaus, M. R.; Martin, T. A.; Ng, E.; Niedbalec, A. P.; O'Bryan, S. E.; Searls, E. F.; Zeidner, A. B.; Zegeye, D.

2014-01-01

107

Analysis of star-disk interaction in young stellar systems  

NASA Astrophysics Data System (ADS)

We present preliminary results of the study of star-disk interaction in the classical T Tauri star V354 Mon, a member of the young stellar cluster NGC 2264. As part of an international campaign of observation of NGC 2264 organized from December 2011 to February 2012, high resolution photometric and spectroscopic data of this object were obtained simultaneously with the Chandra, CoRoT and Spitzer satellites, and ground-based telescopes, as CFHT and VLT at ESO. The optical and infrared light curves of V354 Mon show periodic brightness minima that vary in depth and width every rotational cycle. We found evidence that the H? emission line profile changes according to the period of photometric variations, indicating that the same phenomenon causes both modulations. Such a correlation between emission line variability and light curve modulation was also identified in a previous observational campaign on the same object, where we concluded that material non-uniformly distributed in the inner part of the disk is the main cause of the photometric modulation. This assumption is supported by the fact that the system is seen at high inclination. It is believed that this distortion of the inner part of the disk results from 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 magnetohydrodynamic numerical simulations. A model of occultation by circumstellar material was applied to the photometric data in order to determine the parameters of the obscuring material during both observational campaigns, thus providing an investigation of its stability on a timescale of a few years.

Fonseca, N. N. J.; Alencar, S. H. P.; Bouvier, J.

2014-01-01

108

Fossil magnetic field of accretion disks of young stars  

NASA Astrophysics Data System (ADS)

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

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

2014-03-01

109

Fossil magnetic field of accretion disks of young stars  

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

110

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

111

StarChild: A Learning Center for Young Astronomers  

NSDL National Science Digital Library

The StarChild website, a service of the High Energy Astrophysics Science Archive Research Center (HEASARC)/NASA, offers a wealth of information on space science for young astronomers and teachers who are searching for space science activities for young learners. Each of these major topics are covered in an age appropriate manner, either for Level 1 or Level 2 learners: Solar System, Universe, Space Stuff, and a Glossary. Links are provided between Level 1 and Level 2 versions of pages so that differences in difficulty can be easily compared. The basics of the Solar System and Universe are covered, as well as human endeavors in Space Stuff, such as astronauts, space wardrobes, space travel, space probes, the Hubble Space Telescope, and Who's Who in Space. Difficult words are linked from the text to a glossary. Questions are posed after many of the textual explanations and interactive activities can be accessed within each topic. The activities can be used via the Web or with pencil and paper. Links are provided to other educational space science sites.

Dejoie, Joyce; Truelove, Elizabeth; Whitlock, Laura

112

Young and very young stars in NGC 3372, the Carina Nebula  

NASA Astrophysics Data System (ADS)

Results are presented of a large-scale imaging photometric study of the stellar population in the northern part of NGC 3372 with a wavelength coverage from 0.33 to 2.5 ? m. All observations were made at Las Campanas Observatory. The sizes of the three stellar clusters, Tr 14, Tr 15 and Tr 16, were determined by means of star counts. Two-colour and colour-magnitude diagrams are presented and analyzed for each individual cluster. The three clusters were found to be at a similar distance from the Sun, < d > = 2.7 kpc, but with very large scatter in both A[V] and d. suggesting drastic variations in intracluster dust density. Dust particle size distribution variations are evident resulting in wide variations in extinction law. We determined ages between 3 and 60 million years for Tr 15 and between less than 1 and 6 million years for Tr 14 and Tr 16. The Tr 14 cluster is partially embedded in a dense molecular cloud that extends towards the southwest reaching its highest density some three arcmin from the cluster nucleus. The rich UV field created by the Tr14 stars ionizes most of the visible HII region in its vicinity and most of the radio HII region Car I. Deep JHK images of the Car I region reveal the presence of a young, embedded stellar population that includes several O9-B0 stars and an ultracompact HII region.

Tapia, M.; Roth, M.; Vázquez, R. A.; Persi, P.

2004-12-01

113

Variable stars in the field of the young open cluster Roslund 2  

NASA Astrophysics Data System (ADS)

The study of variable stars in open clusters via asteroseismology is a powerful tool for the study of stellar evolution and stars in general. That is because stars in clusters can be assumed to originate from the same interstellar cloud, so they share similar properties such as age and overall metallicity. We performed a search for variable stars in the field of the young open star cluster Roslund 2, with photoelectric and CCD photometry acquired at two different telescopes. Within the resulting light curves we have found 12 variable stars. Our measurements confirm three previously known variables.

Sowicka, P.; Handler, G.; Taubner, R.; Brunner, M.; Passegger, V.-M.; Bauer, F.; Paunzen, E.

2014-02-01

114

The Young Population of the Lambda Orionis Star-Forming Region  

NASA Astrophysics Data System (ADS)

To investigate the effect that massive stars have on low-mass star formation, we have conducted a detailed study of the young stellar population of the ? Orionis star-forming complex. To do this, we first surveyed the complex to enumerate the young stellar population, including both high- and low-mass stars. Measuring the ages and masses of these stars by photometric comparison with stellar evolution models, we have reconstructed the star-formation history and initial mass function of the region. The stellar ages demonstrate that the current episode of star formation started gradually about 8--10 Myr ago, showing no signs of triggered or sequential star formation. The birth rate increased continuously until 1 Myr ago when a supernova exploded, cleared away the gas from which stars were forming in the center of the star-forming region. This terminated low-mass star formation within 15 pc of the massive stars while further away stars continue to form today. Globally, the mass function resembles that of the field but it shows substantial local variation across the star-forming region, favoring massive stars in the center and low-mass stars elsewhere. However, we do not see strictly bimodal star formation, as the low-mass stellar density is highest in the same location as the OB stars. We have discovered a marked lack of accretion disks (diagnosed by stellar H? emission) around the low-mass stars in the vicinity of the massive stars. We suspect that close encounters with OB stars or the supernova shock diminished the disks of those stars. Since the stars with or without accretion disks span all ages, we conclude that the absence of disks is the product of environment, not just evolution. Thus we find that the star-forming environment near OB stars is detrimental to further formation. The massive stars may disperse the gas from which stars form and they may destroy the disks by which low-mass stars accrete that gas. Nonetheless, low-mass stars do form in great numbers both near to and far from their massive neighbors. http://www.astro.wisc.edu/~dolan/thesis/

Dolan, Christopher Jon

2000-08-01

115

Structure, stratigraphy, and eruption dynamics of a young tuff ring: Hanauma Bay, O'ahu, Hawai'i  

NASA Astrophysics Data System (ADS)

The Hanauma Bay-Koko Head complex is one of several young volcanic landforms along the Koko fissure, in southeastern O'ahu. The Hanauma Bay region of the complex comprises two nested tuff rings, inner and outer Hanauma Bay, and multiple smaller vents. The internal structure of the inner tuff ring, well exposed due to subsequent breaching by the ocean and wave erosion, indicates that it formed during a minimum of five distinct phases of deposition that produced five mappable units. Significant inward collapses generated major unconformities that separate the units exposed in the inner wall. The planes of failure are cut by narrow steep-walled, locally overhung channels and gullies, suggesting that the collapse events were each followed by short time breaks during which the deposits were eroded by rainfall runoff. Within each pyroclastic unit, there are many local slump scars and unconformities, suggesting that minor instability of the inner wall was a near-constant feature. From bedding sags and surge bed forms, it is apparent that the vent shifted at least twice during tuff ring growth. Ballistic blocks in the youngest unit indicate that the eruption overlapped in time with a separate eruption to the north, most likely to be that of the Kahauloa tuff ring 880 m away.

Rottas, K. M.; Houghton, B. F.

2012-09-01

116

Star formation in the region of young open cluster - NGC 225  

NASA Astrophysics Data System (ADS)

NGC 225 is believed to be a 120 Myr old open cluster located at tilda 650 pc. Eight stars with H alphaemission are found to be located around the cluster, of which two are probable Herbig Be stars, indicating a very young age for the cluster. To explore whether the Herbig Be stars, which are pre- main sequence (PMS) stars are part of this cluster, we re-estimated the cluster parameters using optical (UBV)pg and 2MASS JHK photometry. We combined the above data to detect the presence of any possible PMS stars in the cluster region. Among the identified 28 proper motion members, 15 stars were found to have near-infrared (NIR) excess indicating that they are PMS stars. Also, most of the upper MS stars were found to show NIR excess suggesting that the brighter proper motion member stars have not yet reached the MS. PMS isochrones were used to estimate the age of stars with NIR excess and is found to be between 0.5-10 Myr. Thus, the cluster NGC 225 is a very young cluster, younger than 10 Myr and its age is not 120 Myr as previously believed. We propose that a recent star formation has resulted in the formation of NGC 225, two Herbig Be stars, stars with H emission, dust lanes and nebulosity in the vicinity of the cluster.

Subramaniam, Annapurni; Mathew, Blesson; Kartha, Sreeja S.

2006-12-01

117

A CCD Search for Variable Stars in Young Open Cluster IC 4996 II  

NASA Astrophysics Data System (ADS)

New photometric observations of the young open cluster IC 4996 are presented. Two new variable stars have been discovered. More complete light curves and unambiguous periods for two variable discovered earlier are also obtained.

Pietrzynski, G.

1996-10-01

118

Near-Ir Spectroscopy of Young Stars in the Braid Nebula Star Formation Region in Cygnus OB7  

NASA Astrophysics Data System (ADS)

We present 1.4-2.5 ?m integral field spectroscopy of 16 stars in the Braid Nebula star formation region in Cygnus OB7. These data form one aspect of a large-scale multiwavelength survey aimed at determining an unbiased estimate of the number, mass distribution, and evolutionary state of the young stars within this 1 deg2 area of the previously poorly studied Lynds 1003 molecular cloud. Our new spectroscopic data, when combined with Two Micron All Sky Survey near-IR photometry, provide evidence of membership of many of these objects in the regions' pre-main-sequence population. We discuss both the characteristics of the young stars found in the region and the level of starforming activity present.

Aspin, Colin; Beck, Tracy L.; Pyo, Tae-Soo; Davis, Chris J.; Schieven, Gerald M.; Khanzadyan, Tigran; Magakian, Tigran Yu.; Movsessian, Tigran A.; Nikogossian, Elena G.; Mitchison, Sharon; Smith, Michael D.

2009-01-01

119

Extreme Star Formation in the Massive Young Cluster Westerlund 1  

NASA Astrophysics Data System (ADS)

We propose to extend Spitzer's study of massive star formation to the massive cluster Westerlund 1, which at ~4 kpc is the closest and most massive 'Super Star Cluster' known in the Galaxy. Star formation may have proceeded differently in this region, having created a higher overall density of coeval massive stars. The proposed observations will allow us to compare star formation in this region to that seen near the Sun, in the massive Cygnus-X complex, and in the outer Galaxy (coming from the studies of W5, the Cycle-5 SMOG project, and GLIMPSE360), and therefore to complete a more representative view of star formation in the Galaxy.

Hora, Joseph; Kraemer, Kathleen; Megeath, Tom; Gutermuth, Rob; Smith, Howard; Martinez Galarza, Juan Rafael; Guzman Fernandez, Andres; Carey, Sean; Koenig, Xavier; Schneider, Nicola; Motte, Frederique; Bontemps, Sylvain; Adams, Joseph; Simon, Robert; Nguyen-Luong, Quang; Schilke, Peter; Keto, Eric; Fazio, Giovanni; Allen, Lori

2012-12-01

120

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

NASA Astrophysics Data System (ADS)

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 ?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.; Covey, Kevin R.; Stassun, Keivan G.

2011-08-01

121

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

122

A Brgamma Probe of Disk Accretion in T Tauri Stars and Embedded Young Stellar Objects  

Microsoft Academic Search

We report on observations of Pabeta and Brgamma for a sample of classical T Tauri stars in Taurus and find a tight correlation between the emission-line luminosities and the accretion luminosity as measured from the hot continuum excess. We use the Brgamma luminosity correlation to calculate accretion luminosities in highly reddened young stars with existing line measurements. The distribution of

James Muzerolle; Lee Hartmann; Nuria Calvet

1998-01-01

123

The Spatial Distribution of Young Stars in the Small Magellanic Cloud  

NASA Astrophysics Data System (ADS)

We present a statistical analysis of the spatial distribution of young stars in the Small Magellanic Cloud using data from the Magellanic Clouds Photometric Survey. In particular we investigate the utility of the integrated two-point autocorrelation function for resolved stars with ages determined by the color magnitude diagram. This work is partially supported by the Skidmore College Student Opportunity Fund.

Chen, Y.; Crone, M. M.

2005-12-01

124

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

125

FAKE STAR FORMATION BURSTS: BLUE HORIZONTAL BRANCH STARS MASQUERADE AS YOUNG MASSIVE STARS IN OPTICAL INTEGRATED LIGHT SPECTROSCOPY  

SciTech Connect

Model color-magnitude diagrams of low-metallicity globular clusters (GCs) usually show a deficit of hot evolved stars with respect to observations. We investigate quantitatively the impact of such modeling inaccuracies on the significance of star formation history reconstructions obtained from optical integrated spectra. To do so, we analyze the sample of spectra of galactic globular clusters of Schiavon et al. with STECKMAP (Ocvirk et al.), and the stellar population models of Vazdekis et al. and Bruzual and Charlot, and focus on the reconstructed stellar age distributions. First, we show that background/foreground contamination correlates with E(B - V), which allows us to define a clean subsample of uncontaminated GCs, on the basis of an E(B - V) filtering. We then identify a 'confusion zone' where fake young bursts of star formation pop up in the star formation history although the observed population is genuinely old. These artifacts appear for 70%-100% of cases depending on the population model used, and contribute up to 12% of the light in the optical. Their correlation with the horizontal branch (HB) ratio indicates that the confusion is driven by HB morphology: red HB clusters are well fitted by old stellar population models while those with a blue HB require an additional hot component. The confusion zone extends over [Fe/H] = [ - 2, - 1.2], although we lack the data to probe extreme high and low metallicity regimes. As a consequence, any young starburst superimposed on an old stellar population in this metallicity range could be regarded as a modeling artifact, if it weighs less than 12% of the optical light, and if no emission lines typical of an H II region are present. This work also provides a practical method for constraining HB morphology from high signal to noise integrated light spectroscopy in the optical. This will allow post-asymptotic giant branch evolution studies in a range of environments and at distances where resolving stellar populations is impossible with current and planned telescopes.

Ocvirk, P. [Astrophysikalisches Institut Potsdam, an der Sternwarte 16, D-14482 Potsdam (Germany)

2010-01-20

126

The Spatial Distribution of Resolved Young Stars in Blue Compact Dwarf Galaxies  

NASA Astrophysics Data System (ADS)

We present the first results from a survey of the distribution of resolved young stars in Blue Compact Dwarf Galaxies. In order to identify the dominant physical processes driving star formation in these puzzling galaxies, we use a multi-scale cluster-finding algorithm to quantify the characteristic scales and properties of star-forming regions, from sizes smaller than 10 pc up to the size of each entire galaxy. This project was partially funded by the Lubin Chair at Skidmore College.

Murphy, K.; Crone, M. M.

2002-12-01

127

X-ray Emission from YSOs, Protostellar Jets, and Accretion Eruptive Variables  

NASA Astrophysics Data System (ADS)

Imaging in X-rays has become an extremely useful tool to identify YSOs residing in star forming regions. X-ray emission is also being measured in eruptive young stars, the FUOr-EXOr type stars, and in protostellar jets. Recent deep near-IR imaging of the North American and Pelican nebulae in JHKs and narrowband emission lines of H2 and [FeII] have revealed one of the most active, richest star forming regions in the Galaxy. Within a single EPIC FOV lies dozens of resolved outflows, jets, clusters of YSOs, and even eruptive FUOR-EXOr stars currently undergoing outbursts. I propose to obtain XMM-Newton imaging of three regions rich in all three types of objects to render x-ray detections to assist with confirming the YSOs, and to measure the x-ray flux of the eruptive stars and shocked outflows.

Stringfellow, Guy

2010-10-01

128

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

129

Variable stars in the young open cluster IC 4996  

NASA Astrophysics Data System (ADS)

The brightest stars of the cluster IC 4996 have been monitored for variability. These observations have led to the discovery of a new ? Cephei multiperiodic variable star, lying outside the so-called "instability strip". Another member star shows variations in a longer time scale, which could be due to binarity.

Delgado, A. J.; Alfaro, E. J.; Garrido, R.

1985-07-01

130

Magnetic Feilds of Young Stars in NGC 752  

Microsoft Academic Search

The magnetic fields of stars change in direction and strength not only over short time scales, as in the Sun's twenty-two year cycle, but also as the star ages. As characteristics of a star like rotational velocity and depth of convection zone change, the magnetic field must alter as well. To shed light on this ageing process, we studied the

C. Christensen; T. Simon

2004-01-01

131

Discovery at Young Star Hints Magnetism Common to All Cosmic Jets  

NASA Astrophysics Data System (ADS)

Astronomers have found the first evidence of a magnetic field in a jet of material ejected from a young star, a discovery that points toward future breakthroughs in understanding the nature of all types of cosmic jets and of the role of magnetic fields in star formation. Throughout the Universe, jets of subatomic particles are ejected by three phenomena: the supermassive black holes at the cores of galaxies, smaller black holes or neutron stars consuming material from companion stars, and young stars still in the process of gathering mass from their surroundings. Previously, magnetic fields were detected in the jets of the first two, but until now, magnetic fields had not been confirmed in the jets from young stars. "Our discovery gives a strong hint that all three types of jets originate through a common process," said Carlos Carrasco-Gonzalez, of the Astrophysical Institute of Andalucia Spanish National Research Council (IAA-CSIC) and the National Autonomous University of Mexico (UNAM). The astronomers used the National Science Foundation's Very Large Array (VLA) radio telescope to study a young star some 5,500 light-years from Earth, called IRAS 18162-2048. This star, possibly as massive as 10 Suns, is ejecting a jet 17 light-years long. Observing this object for 12 hours with the VLA, the scientists found that radio waves from the jet have a characteristic indicating they arose when fast-moving electrons interacted with magnetic fields. This characteristic, called polarization, gives a preferential alignment to the electric and magnetic fields of the radio waves. "We see for the first time that a jet from a young star shares this common characteristic with the other types of cosmic jets," said Luis Rodriguez, of UNAM. The discovery, the astronomers say, may allow them to gain an improved understanding of the physics of the jets as well as of the role magnetic fields play in forming new stars. The jets from young stars, unlike the other types, emit radiation that provides information on the temperatures, speeds, and densities within the jets. This information, combined with the data on magnetic fields, can improve scientists' understanding of how such jets work. "In the future, combining several types of observations could give us an overall picture of how magnetic fields affect the young star and all its surroundings. This would be a big advance in understanding the process of star formation," Rodriguez said. Carrasco-Gonzalez and Rodriguez worked with Guillem Anglada and Mayra Osorio of the Astrophysical Institute of Andalucia, Josep Marti of the University of Jaen in Spain, and Jose Torrelles of the University of Barcelona. The scientists reported their findings in the November 26 edition of Science.

2010-11-01

132

What's happening around Herbig Ae stars? Investigating circumstellar activity in young intermediate mass stars with optical and near infrared spectroscopy  

NASA Astrophysics Data System (ADS)

We have investigated the optical and near-infrared spectral behavior of the intermediate mass, pre-main sequence Herbig Ae stars, with particular emphasis on variability and evolution. The evolutionary state of Herbig Ae (HAe) stars is intermediate between very young, pre-main sequence objects and the young main sequence Vega, or ? Pic-like, stars. The majority of HAe stars are estimated to span a small range in ages, between 2 and 6 million years. Near-infrared spectra reveal an evolutionary sequence from strong Brackett emission in the youngest sources to photospheric absorption in main sequence Vega-like stars. HAe stars exhibit nearly featureless spectra in the near-infrared, a combination of photospheric absorption veiled by strong excess continuum, and weak Brackett line emission. In the evolutionary sequence, the hot gas creating the Brackett emission disappears before the hot dust responsible for the thermal emission. The data suggest that the strength of the Brackett emission decreases with age between 1 and 10 Myrs, while the thermal emission drops for stars >10 Myrs. Our high resolution spectra of Br? in 2 HAe stars indicate that the Brackett emission is associated with accreting gas. Therefore, near-infrared Brackett emission offers an important diagnostic of inner disk evolution in late pre-main sequence intermediate mass stars. A large fraction (~25-50%) of HAe stars, called UXORs after the prototype UX Ori, exhibit large optical variability (?V > 1.5 mag), commonly attributed to obscuration of the central star by circumstellar dust. Optical spectra of the UXOR star RR Tau over factor of 10 in brightness (V = 11 to V ~ 13.5 mag), reveal a system in which the photosphere of the star (Balmer wings and weak metal lines) and the wind flux ([OI], [FeII], and H? lines) are unaffected by the brightness minima while permitted metal lines (FeII, CaII, OI, NaI) change from absorption to emission when the star fades. We suggest a model that attributes the metal absorption to a higher density region obscured with the star, while the metal emission lines come from a more extended unobscured region. More importantly, this result exemplifies the value of spectral monitoring in constraining the location of obscuring material and its effects on the circumstellar environment.

Rodgers, Bernadette Marie

2001-11-01

133

Young, high-velocity a stars. II - Misidentified, ejected, or unique?  

NASA Astrophysics Data System (ADS)

Rodgers, Harding, and Sadler identified a group of A stars at the south Galactic (SGP) pole with properties unlike those of any stellar population. They found the stars to be at distances from the disk of 1 kpc to more than 4 kpc, with a velocity dispersion perpendicular to the plane of 66 km s-1, yet they appear to be young stars. In this study, the ages, abundances, and kinematics of a large number of early-type stars at the SGP have been derived in order to examine the properties and augment the sample of high-velocity stars. In striking contrast to a comparative group of normal A stars near the disk the high-velocity A stars were all formed within the last 6.5×108yr. Their calcium abundances range from -0.5 dex to 0.0 dex, and their W velocity dispersion is 62 km s-1. It is shown that the stars are not misidentified horizontal-branch stars. It is suggested that at around 6.5×108yr ago, a major source of relatively low abundance hydrogen was accreted by the Galactic disk, forming young high-velocity stars that do not partake of the age-abundance-kinematics relationships shown by other stellar groupings.

Lance, Catherine M.

1988-11-01

134

A CCD Search for Variable Stars in a Young Open Cluster IC 4996  

NASA Astrophysics Data System (ADS)

The results of CCD search for variables stars in the young open cluster IC 4996 based on 18 nights are presented. Three variable stars have been discovered. Variable v1 is probably an RR Lyr type variable with the period of 0.236 d and semi-amplitude of 0.12 mag in filter I. v2 is an eclipsing systems. The most probable period of this star is 1.499458 d, but period two times shorter cannot be excluded. The period and type of variability of third star cannot be established from the present data.

Pietrzynski, G.

1996-07-01

135

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

136

The Spatial Structure of Young Stellar Clusters in High-Mass Star-Forming Regions  

NASA Astrophysics Data System (ADS)

The spatial distributions of young stars in star-forming regions can be linked to the theory of clustered star formation using spatial statistical methods. The MYStIX project (Massive Young stellar clusters Study in Infrared and X-rays) provides rich samples of young stars from the nearest high-mass star-forming regions. Maps of stellar surface density reveal diverse structure and subclustering. Young stellar clusters and subclusters are fit with isothermal spheres and ellipsoids using the Bayesian Information Criterion to estimate the number of subclusters. We investigate the relation between (sub)cluster size and density and examine evidence for dynamical relaxation in some of the (sub)clusters. Clustering properties are also measured using Cartwright and Whitworth's Q statistic and the inhomogeneous two-point correlation function, which can be used to distinguish between centrally concentrated and fractal distributions. Mass segregation is detected in several cases, in both centrally concentrated and fractally structured star clusters, but a few clusters are not mass segregated. We discuss the astrophysical implications of the comparison of cluster structures in the different regions.

Kuhn, Michael

2013-01-01

137

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

138

Magnetic Feilds of Young Stars in NGC 752  

NASA Astrophysics Data System (ADS)

The magnetic fields of stars change in direction and strength not only over short time scales, as in the Sun's twenty-two year cycle, but also as the star ages. As characteristics of a star like rotational velocity and depth of convection zone change, the magnetic field must alter as well. To shed light on this ageing process, we studied the magnetic flux of stars 1.78 gigayears old, an age between that of the Hyades and the Sun, both of which have been extensively studied. This study was done using data from a deep-field Chandra observation to find the coronal luminosity, a proxy for the magnetic flux, of a total of 130 stars, many of which are known to be members of NGC 752. Photon count rate was converted to luminosity using the Raymond-Smith model. Convective stars were found to have a range of coronal luminosities averaging only slightly higher then that of the Sun. Stars with B-V < 0.45, however showed a trend towards brighter luminosities. We determined the average coronal luminosity of G-type stars to be 2.98 *102{^8} ergs/sec with a range of plus or minus 2.21 *102{^8} ergs/sec, which corresponds to an exponential decline in coronal luminosities from those of the Hyades. This research was funded by the National Science Foundation's Research Experience for Undergraduates program.

Christensen, C.; Simon, T.

2004-12-01

139

The scale height of low mass stars in the galactic disc - Evidence for a young population  

NASA Astrophysics Data System (ADS)

Samples of M stars have been obtained in the direction of the SGP and at galactic latitude b = -47° from R and I band UK Schmidt plates. The absolute magnitudes MR were obtained from (R-I) colour, and the space density evaluated as a function of height above the galactic plane, for a range of luminosities MR = 9 - 14. This was then used to obtain the relation between scale height and absolute magnitude. Combining the data with previously published results for more massive stars shows a steady decline in the scale height from a maximum around 300 pc for early M stars down to 80 pc for the least luminous stars in the sample. This result is interpreted as implying a small age of 5×108yr for these low luminosity stars. If these stars are indeed young, it is argued that they are probably "brown dwarfs" which have failed to ignite hydrogen and are cooling rapidly to very low luminosities.

Hawkins, M. R. S.

1988-10-01

140

Warm Dust in Circumstellar Disks around Young Binary and Single Stars  

NASA Astrophysics Data System (ADS)

We present a study of young T Tauri stars in the Lupus, Corona Australis, and Ophiuchus star forming regions that is intended to indentify and characterize circumstellar disks. Twenty-one pre-main-sequence binary systems were observed, as well as thirty-three young X-ray sources. Photometry was taken in the K and L bands and K-L excesses were used to determine whether targets had circumstellar disks. For the X-ray sample, spectral lines were used to estimate vsin(i) and to look for a correlation between the presence of a disk and slow stellar rotation. The goals of this project were to determine the frequency of circumstellar disks around individual stars in binary systems, to investigate variations of NIR colors in these disks, and to search for disks around young X-ray sources in Ophiuchus.

Spreng, Rachel; Prato, L.

2009-01-01

141

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

142

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

143

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

144

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

145

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

NASA Astrophysics Data System (ADS)

We describe a new method to identify young, late-type stars within ~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 Å 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.; Bessell, M. S.; Zuckerman, B.; Kastner, Joel H.

2011-02-01

146

On the origin of young stars at the Galactic center  

NASA Astrophysics Data System (ADS)

The center of our Galaxy is home to a massive black hole, Sgr A*, and a nuclear star cluster containing stellar populations of various ages. While the late type stars may be too old to have retained memory of their initial orbital configuration, and hence formation mechanism, the kinematics of the early type stars should reflect their original distribution. In this contribution we present a new statistic which uses directly-observable kinematic stellar data to infer orbital parameters for stellar populations, and is capable of distinguishing between different origin scenarios. We use it on a population of B-stars in the Galactic center that extends out to large radii (˜0.5 pc) from the massive black hole. We find that the high K-magnitude population (?15 M?) form an eccentric distribution, suggestive of a Hills binary-disruption origin.

Madigan, Ann-Marie; Pfuhl, Oliver; Levin, Yuri; Gillessen, Stefan; Genzel, Reinhard; Perets, Hagai B.

2014-05-01

147

APOD: Companion of a Young, Sun-like Star Confirmed  

NSDL National Science Digital Library

This APOD page features the first confirmed image of an extrasolar planet orbiting a sun-like star. Text and numerous links provide additional information; one link goes to the press release of the confirmation.

2012-01-21

148

Microwave H2O emission from young stars  

NASA Technical Reports Server (NTRS)

H2O emission has been detected from the Ae or Be stars HD 250550, LkH-alpha 234, and LkH-alpha 198. Comparison of the H2O velocity with that of associated interstellar CO indicates that the H2O originates in an infall region, possibly the contracting parent cloud. H2O emission has also been detected in OH 205.1-14.1, and OH source near the T Tauri star LkH-alpha 308.

Schwartz, P. R.; Buhl, D.

1975-01-01

149

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

150

Low-Mass Star Formation and the Initial Mass Function in Young Clusters  

Microsoft Academic Search

I have used optical and near-infrared spectroscopy and imaging to measure spectral types and luminosities for young (\\/tau<10 Myr), embedded (AV=0[-]50), low-mass (0.1-1 Msolar) stars in three nearby (d<300 pc) clusters: L1495E, IC 348, and rho Ophiuchi. In conjunction with theoretical evolutionary tracks, I have derived the star formation history and initial mass function for each stellar population. A large

Kevin Lee Luhman

1998-01-01

151

Spectacular Spitzer images of the Trifid Nebula: Protostars in a young, massive-star-forming region  

Microsoft Academic Search

Spitzer IRAC and MIPS images of the Trifid Nebula (M20) reveal its spectacular appearance in infrared light, demonstrating its special evolutionary stage: recently-formed massive protostars and numerous young stars, including a single O star that illuminates the surrounding molecular cloud from which it formed and unveiling large-scale, filamentary dark clouds. The hot dust grains show contrasting infrared colors in shells,

Jeonghee Rho; W. T. Reach; B. Lefloch; G. Fazio

2005-01-01

152

Serendipitous Chandra X-ray Spectroscopy of GALEX Nearby Young-Star Survey (GALNYSS) Candidates  

NASA Astrophysics Data System (ADS)

More than 2000 candidate young (age 10-100 Myr) low-mass stars within ~100 pc of Earth have been identified by the Galex Nearby Young-Star Survey (GALNYSS), via the combination of ultraviolet (Galex) and near-IR (WISE and 2MASS) photometry and kinematic data. Among these candidates, we find more than a dozen objects for which serendipitous archival Chandra X-ray observations are available. The spectral types for these objects, if stellar, range from early- to mid-M. Hence, this serendipitously observed subsample affords the opportunity to study the X-ray emission characteristics of young stars at the low-mass end of the stellar mass spectrum. We present preliminary results of spectral analysis, including estimates of plasma temperature, intervening absorption, and intrinsic X-ray luminosities, for these Chandra X-ray counterparts to GALNYSS candidates. These results will be used both to confirm young, late-type star status and to investigate the evolution of magnetic (coronal) activity in stars whose masses potentially range from a few tenths of a solar mass down to near the H-burning limit. This work is supported by NASA Astrophysics Data Analysis Program award NNX12AH37G to RIT and UCLA and Chilean FONDECYT grant 3130520 to Universidad de Chile.

Kastner, Joel H.; Baum, N.; Principe, D.; Rodriguez, D.

2014-01-01

153

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

154

Constraints on the low-mass IMF in young super-star clusters in starburst galaxies  

NASA Astrophysics Data System (ADS)

As evidence for variations in the initial mass function (IMF) in nearby star forming regions remains elusive we are forced to expand our search to more extreme regions of star formation. Starburst galaxies, which contain massive young clusters have in the past been reported to have IMFs different than that characterizing the field star IMF. In this thesis we use high signal-to-noise near-infrared spectra to place constraints on the shape of the IMF in extreme regions of extragalactic star formation and also try to understand the star formation history in these regions. Through high signal-to-noise near-infrared spectra it is possible to directly detect low-mass PMS stars in unresolved young super-star clusters, using absorption features that trace cool stars. Combining Starburst99 and available PMS tracks it is then possible to constrain the IMF in young super-star clusters using a combination of absorption lines each tracing different ranges of stellar masses and comparing observed spectra to models. Our technique can provide a direct test of the universality of the IMF compared to the Milky Way. We have obtained high signal-to-noise H- and K-band spectra of two young super-star clusters in the starburst galaxies NGC 4039/39 and NGC 253 in order to constrain the low-mass IMF and star formation history in the clusters. The cluster in NGC 4038/39 shows signs of youth such as thermal radio emission and strong hydrogen emission lines as well as late-type absorption lines indicative of cool stars. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence stars or red supergiants alone. We interpret the spectrum as a superposition of two star clusters of different ages over the physical region of 90 pc our spectrum represents. 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. While the superposition of clusters does not allow us to place stringent constraints on the IMF there is no evidence of a low-mass cutoff in the cluster and the IMF is consistent with a Chabrier and Kroupa IMF typical of the field. The cluster in NGC 253 shows the same signs of youth as the cluster in NGC 4038/39 and sits in front of a background population of older stars. The background population has an age of ? 12 Myr and thus contains red supergiants. After carefully subtracting this background we model the spectrum of the young cluster. We find that its IMF is consistent with a Chabrier and Kroupa IMF with a best-fit power-law slope of 1.0 in linear units. Slopes of 0.0 - 1.5 are also formally consistent with the cluster spectrum. We conclude that there is no strong evidence for an unusual IMF or a lack of low-mass stars (? 1 M? ) in either of these galaxies.

Greissl, Julia Jennifer

2010-12-01

155

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

156

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

157

A Wide-Field Census of Young Stars in NGC 6334  

NASA Astrophysics Data System (ADS)

NGC 6334 is a giant molecular cloud with high far-infrared luminosity and a complex history of star formation located approximately 1.6 kpc away in the plane of the Milky Way Galaxy. We have obtained and analyzed near- and mid-infrared observations using deep J,H,K observation and Spitzer and Herschel datasets of this region which uncovered over 2,000 young stars nestled within the intricate structure of the cloud complex. We have used this YSO census to estimate the overall rate and efficiency of star formation in this region. By comparing these with other Galactic star forming regions and other galaxies, we have identified NGC 6334 as a potential "mini-starburst" with SFE > 0.15 and ?SFR > 40 M? Myr-1 pc2. We have also compared the YSO population to the previously identified cold dust clumps and filaments to search for the youngest clusters and massive stars within this region.

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

2013-06-01

158

HUBBLE IMAGES REVEAL A YOUNG STAR'S DYNAMIC DISK AND JETS  

NASA Technical Reports Server (NTRS)

These images of HH 30 show changes over only a five-year period in the disk and jets of this newborn star, which is about half a million years old. The pictures were taken between 1995 and 2000 with the Wide Field and Planetary Camera 2 aboard NASA's Hubble Space Telescope. Astronomers are interested in the disk because it is probably similar to the one from which the Sun and the planets in our solar system formed. Hubble reveals an edge-on disk (located at the bottom of the images), which appears as a flattened cloud of dust split into two halves by a dark lane. The disk blocks light from the central star. All that is visible is the reflection of the star's light by dust above and below the plane of the disk. The disk's diameter is 450 astronomical units (one astronomical unit equals the Earth-Sun distance). Shadows billions of miles in size can be seen moving across the disk. In 1995 and 2000, the left and right sides of the disk were about the same brightness, but in 1998 the right side was brighter. These patterns may be caused by bright spots on the star or variations in the disk near the star. The dust cloud near the top of these frames is illuminated by the star and reflects changes in its brightness. The star's magnetic field plays a major role in forming the jets (located above and below the disk), which look like streams of water from a fire hose. The powerful magnetic field creates the jets by channeling gas from the disk along the magnetic poles above and below the star. The gaps between the compact knots of gas seen in the jet above the disk indicate that this is a sporadic process. By tracking the motion of these knots over time, astronomers have measured the jet's speed at between 200,000 to 600,000 miles per hour (160,000 and 960,000 kilometers per hour). Oddly, the jet below the disk is moving twice as fast as the one above it. Credits: NASA, Alan Watson (Universidad Nacional Autonoma de Mexico), Karl Stapelfeldt (Jet Propulsion Laboratory), John Krist and Chris Burrows (European Space Agency/Space Telescope Science Institute)

2002-01-01

159

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

160

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.

Oneil, Pamela; Fryxell, Bruce; Burrows, Adam

1994-02-12

161

BVI photometry of young stars in 10 clusters (Mayne+, 2007)  

Microsoft Academic Search

We present photometry in V, V-I and in some cases B-V for a range of star forming regions. NGC2547, NGC7160, NGC2264, Cepheus OB3b, Sigma Orionis, IC348 and h and chi Per. A full description of the data can be found at: http:\\/\\/www.astro.ex.ac.uk\\/people\\/timn\\/Catalogues (8 data files).

N. J. Mayne; T. Naylor; S. P. Littlefair; E. S. Saunders; R. D. Jeffries

2007-01-01

162

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

163

Near-IR integral field spectroscopy of ionizing stars and young stellar objects on the borders of H II regions  

Microsoft Academic Search

Aims: We study three Galactic H ii regions - RCW 79, RCW 82, and RCW 120 - where triggered star formation is taking place. Two stellar populations are observed: the ionizing stars of each H ii region and young stellar objects on their borders. Our goal is to show that they represent two distinct populations, as expected from successive star-forming

F. Martins; M. Pomarès; L. Deharveng; A. Zavagno; J. C. Bouret

2010-01-01

164

Evolved GK stars near the Sun. 2: The young disk population  

Microsoft Academic Search

From a sample of nearly 2000 GK giants a group of young disk stars with well determined space motions has been selected. The zero point of the luminosity calibrations, both from the ultraviolet flux (modified Stroemgren system) and that in the region of 4200 to 4900 A (DDO system), show a discontinuity of about a half magnitude at the border

O. J. Eggen

1994-01-01

165

The young star cluster system in the Antennae: evidence for a turnover in the luminosity function  

Microsoft Academic Search

The luminosity functions (LFs) of star cluster (SC) systems (i.e. the number of clusters per luminosity interval) are vital diagnostics to probe the conditions of SC formation. Early studies have revealed a clear dichotomy between old globular clusters and young clusters, with the former characterized by Gaussian-shaped LFs, and the latter following a power law. Recently, this view was challenged

P. Anders; N. Bissantz; L. Boysen; R. de Grijs; U. Fritze-von Alvensleben

2007-01-01

166

Evidence of Planetesimal Infall onto the Very Young Herbig Be Star LkHalpha 234  

Microsoft Academic Search

We report here the first evidence of planetesimal infall onto the very young Herbig Be star LkHalpha 234. These results are based on observations acquired over 31 days using spectroscopy of the sodium D lines, the He I 5876 Å line, and hydrogen Halpha lines. We find redshifted absorption components (RACs) with velocities up to 200 km s-1 and very

Abhijit Chakraborty; Jian Ge; Suvrath Mahadevan

2004-01-01

167

Spectacular Spitzer Images of the Trifid Nebula: Protostars in a Young, Massive-Star-forming Region  

Microsoft Academic Search

Spitzer IRAC and MIPS images of the Trifid Nebula (M20) reveal its spectacular appearance in infrared light, highlighting the nebula's special evolutionary stage. The images feature recently formed massive protostars and numerous young stellar objects, and a single O star that illuminates the surrounding molecular cloud from which it formed, and unveil large-scale, filamentary dark clouds. Multiple protostars are detected

Jeonghee Rho; William T. Reach; Bertrand Lefloch; Giovanni G. Fazio

2006-01-01

168

A Photometric Study of the Young Stellar Population throughout the ? Orionis Star-Forming Region  

NASA Astrophysics Data System (ADS)

We present VRI photometry of 320,917 stars with 11<~R<~18 throughout the ? Ori star-forming region. Using the more spatially limited spectroscopic surveys of Dolan & Mathieu to define the color-magnitude domain of young low-mass members of the association and removing statistically the field stars in this domain, we use our photometry to identify a representative pre-main-sequence (PMS) population throughout the interior of the molecular ring. The spatial distribution of this population shows a concentration of PMS stars around ? Ori and in front of the B35 dark cloud. However, few PMS stars are found outside these pockets of high stellar density, suggesting that star formation was concentrated in an elongated cloud extending from B35 through ? Ori to the B30 cloud. We find a lower limit for the global stellar mass of about 500 Msolar. We find that the global ratio of low- to high-mass stars is similar to that predicted by the field initial mass function, but this ratio varies strongly as a function of position in the star-forming region. Locally, the star formation process does not produce a universal initial mass function. Using our derived stellar ages across the region, we construct a history of the star-forming complex. This history incorporates a recent supernova to explain the distribution of stars and gas today. We infer that most of the present molecular ring was formed by ejecta from the center driven by the supernova blast about 1 Myr ago. However, we suggest that the B30 and B35 clouds were primordial, and massive enough to be mostly little disturbed by the shock. The stars that we see today trace the former extent of the cloud complex. Given the kinematics of the stellar population, we predict that the association will disperse into the field within a few tens of megayears.

Dolan, Christopher J.; Mathieu, Robert D.

2002-01-01

169

Get A Bite On The "Delicious" Young Star Cluster NGC 3603  

NASA Astrophysics Data System (ADS)

Young star cluster NGC3603 is one of the most massive star clusters in the Milky Way. It hosts 10 times more OB stars than the Orion Nebular Cluster, among which two are the most massive binaries in the Galaxy (Schnurr et al. 2008). To investigate this star formation arena, we utilize the HST/WFPC2 data. Those data are 10 years apart, which permits us to determine star membership. The cluster displays a significant degree of mass segregation (Pang et al. 2010). To quantify the lower limit in stellar mass at which we see segregation, we apply the minimum spanning tree analysis to cluster stars. The result shows that the stars above 5 solar mass exhibit pronounced mass segregation. What's the origin of this mass segregation? Simulations by Moeckel & Bate (2010) show that primordial segregation is transient and exists within the first 1 Myr. The cluster pre-main sequence (PMS) stars display an age spread up to 3 Myr, while the main sequence stars are consistent with an age of 1 Myr (Pang et al. 2010). And Beccari et al. (2010) derive an age spread as large as 10 Myr among PMS stars. Therefore, at present, the age of NGC3603 is still highly uncertain. A way to improve the age determination of the cluster is to quantify the differential reddening across the cluster. The differential reddening is about 0.8 mag from the core of NGC3603 to the outer region (Sung & Bessel 2004). This affects the PMS stars, which are spatially distributed more widely than the MS stars. We are currently deriving an extinction map of the cluster through Halpha and Paschenbeta images from WFC3 (work in progress), in order to correct the color magnitude diagram, and thus to better constrain the age of the cluster and the masses of its members.

Pang, Xiaoying; Grebel, E.; Altmann, M.; Pasquali, A.

2011-01-01

170

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

171

Young stars and brown dwarfs in Ori OB1b (Caballero+, 2008)  

NASA Astrophysics Data System (ADS)

We present here exhaustive lists of known young stars and new candidate members around Alnilam and Mintaka in the Ori OB1b association as well as of fore- and background sources. A total of 133 stars display features of extreme youth, including 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 compilation of tables can serve as an input for characterisation of the stellar and high-mass substellar populations in the Orion Belt. (20 data files).

Caballero, J. A.; Solano, E.

2008-06-01

172

Evolution of Young Stars in the Large Massive Cluster Cep OB3b  

NASA Astrophysics Data System (ADS)

We propose deep ASIC-I imaging of the CepOB3b cluster, a large (> 1000 member), 5 Myr old cluster, 700 pc from the Sun. Spitzer observations have mapped the full extent of CepOB3b, showing two distinct sub-clusters. We will target each sub-clusters to identify the young stars without disks and determine the X-ray properties of the stars. These observations will probe the properties of a young cluster and its constituent stars at a poorly studied evolutionary age, thereby complementing the COUP survey of the 2 Myr old Orion Nebula Cluster. We will study the dynamical evolution of clusters emerging from their parental clouds, the evolution of stellar coronae, the affect of circumstellar disks on the X-ray emission, and the role of X-rays in creating inner holes in disks.

Allen, Thomas

2008-09-01

173

BAYESIAN ANALYSIS TO IDENTIFY NEW STAR CANDIDATES IN NEARBY YOUNG STELLAR KINEMATIC GROUPS  

SciTech Connect

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 {beta} 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{alpha} 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 {beta} 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 {beta} 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 A 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 {beta} 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.

Malo, Lison; Doyon, Rene; Lafreniere, David; Artigau, Etienne; Gagne, Jonathan; Baron, Frederique [Departement de physique and Observatoire du Mont-Megantic, Universite de Montreal, Montreal, QC H3C 3J7 (Canada)] [Departement de physique and Observatoire du Mont-Megantic, Universite de Montreal, Montreal, QC H3C 3J7 (Canada); Riedel, Adric, E-mail: malo@astro.umontreal.ca, E-mail: doyon@astro.umontreal.ca, E-mail: david@astro.umontreal.ca, E-mail: artigau@astro.umontreal.ca, E-mail: gagne@astro.umontreal.ca, E-mail: baron@astro.umontreal.ca, E-mail: riedel@phy-astr.gsu.edu [Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States)] [Department of Astrophysics, American Museum of Natural History, Central Park West at 79th Street, New York, NY 10024 (United States)

2013-01-10

174

Clustering Properties of Young Stellar Objects in the Massive Star Forming Region W49  

NASA Astrophysics Data System (ADS)

Massive stars play a vital role in the star formation process, yet their own formation and their effects on subsequent generations of star formation is not well understood. To improve our understanding, we have begun a detailed study of massive and active star forming complexes in giant molecular clouds outside the Galactic Center. One of the main goals of this study is to identify and classify the Young Stellar Objects (YSOs) in each region by using Spitzer Space Telescope IRAC & MIPS data. Following this,YSO clusters will be identified based on spatial distributions of the detected sources. Studying clusters with different evolutionary stages will help us to understand the formation and evolution processes from beginning to end. This study will also provide significant information on how massive stars interact with their environment and how they affect the low-mass star formation in the cloud. Within this context, we present the initial results of our investigation on the star-forming complex W49 that is one of the youngest, most luminous and most massive star formation region in the Galaxy. We used a combination of Spitzer Space Telescope IRAC & MIPS data, Two Micron All Sky Survey (2MASS) and UKIRT Deep Infrared Sky Survey (UKIDSS) data to identify and classify the Young Stellar Objects (YSOs) and generated a final catalog with a photometry of sources containing more than 2 million sources within an area of size ?lx?b = 2°.6 x 3°.4, centered at (l,b) = (42°.7,0°.04) over a wavelength range from 1.2 to 24 ?m. With a preliminary source classification we identified thousands of YSO candidates. In addition, to understand the evolution of star formation in W49 we analyzed the distributions of YSOs to identify the clusters based on spatial distributions of the detected sources.

Saral, Gozde; Hora, Joseph L.; Koenig, Xavier; Saygac, Talat

2014-06-01

175

On the Constancy of the Characteristic Mass of Young Stars  

Microsoft Academic Search

The characteristic mass M_c in the stellar initial mass function (IMF) is\\u000aabout constant for most star-forming regions. Numerical simulations\\u000aconsistently show a proportionality between M_c and the thermal Jeans mass M_J\\u000aat the time of cloud fragmentation, but no models have explained how it can be\\u000athe same in diverse conditions. Here we show that M_J depends weakly on

Bruce G. Elmegreen; Ralf S. Klessen; Christine D. Wilson

2008-01-01

176

IRAS 06562{minus}0337, The Ironclad Nebula: A New Young Star Cluster  

SciTech Connect

IRAS 06562{minus}0337 has been the recent subject of a classic debate: is it a proto{endash}planetary nebula or a young stellar object? We present the first 2 {mu}m image of IRAS 06562{minus}0337, which reveals an extended diffuse nebula containing approximately 70 stars inside a 30{double_prime} radius around a bright, possibly resolved, central object. The derived stellar luminosity function is consistent with that expected from a single coeval population, and the brightness of the nebulosity is consistent with the predicted flux of unresolved low-mass stars. The stars and nebulosity are spatially coincident with strong CO line emission. We therefore identify IRAS 06562{minus}0337 as a new young star cluster embedded in its placental molecular cloud. The central object is likely a Herbig Be star, {ital M} {approx} 20 {ital M}{sub {circle_dot}}, which may be seen in reflection. We present medium-resolution high signal-to-noise ratio 1997 epoch optical spectra of the central object. Comparison with previously published spectra shows new evidence for time-variable permitted and forbidden line emission, including Si ii, Fe ii, [Fe ii], and [O i]. We suggest that the origin is a dynamic stellar wind in the extended stratified atmosphere of the massive central star in IRAS 06562{minus}0337. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}

Alves, D.R. [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States)] [Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States); Hoard, D.W.; Rodgers, B. [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States)] [Department of Astronomy, University of Washington, Box 351580, Seattle, WA 98195 (United States)

1998-07-01

177

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

178

Modeling Tracers of Young Stellar Population Age in Star-forming Galaxies  

NASA Astrophysics Data System (ADS)

The young stellar population of a star-forming galaxy is the primary engine driving its radiative properties. As a result, the age of a galaxy's youngest generation of stars is critical for a detailed understanding of its star formation history, stellar content, and evolutionary state. Here we present predicted equivalent widths for the H?, H?, and Br? recombination lines as a function of stellar population age. The equivalent widths are produced by the latest generations of stellar evolutionary tracks and the Starburst99 stellar population synthesis code, and are the first to fully account for the combined effects of both nebular emission and continuum absorption produced by the synthetic stellar population. Our grid of model stellar populations spans six metallicities (0.001 < Z < 0.04), two treatments of star formation history (a 106 M ? instantaneous burst and a continuous star formation rate of 1 M ? yr-1), and two different treatments of initial rotation rate (v rot = 0.0v crit and 0.4v crit). We also investigate the effects of varying the initial mass function. Given constraints on galaxy metallicity, our predicted equivalent widths can be applied to observations of star-forming galaxies to approximate the age of their young stellar populations.

Levesque, Emily M.; Leitherer, Claus

2013-12-01

179

Eruptive style of the young high-Mg basaltic-andesite Pelagatos scoria cone, southeast of México City  

Microsoft Academic Search

The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico\\u000a City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long\\u000a lava flow, and continued with a phase of alternating and\\/or simultaneous explosive and effusive activity that built a 50-m-high\\u000a scoria cone

Marie-Noëlle Guilbaud; Claus Siebe; Javier Agustín-Flores

2009-01-01

180

Discovering young stars in the Gum 31 region with infrared observations  

NASA Astrophysics Data System (ADS)

Context. The Gum 31 bubble, which contains the stellar cluster NGC 3324, is a poorly studied young region close to the Carina Nebula. Aims: We are aiming to characterise the young stellar and protostellar population in and around Gum 31 and to investigate the star-formation process in this region. Methods: We identified candidate young stellar objects from Spitzer, WISE, and Herschel data. Combining these, we analysed the spectral energy distributions of the candidate young stellar objects. With density and temperature maps obtained from Herschel data and comparisons to a collect-and-collapse scenario for the region we are able to further constrain the characteristics of the region as a whole. Results: We find 661 candidate young stellar objects from WISE data; 91 protostar candidates are detected through Herschel observations in a 1.0° × 1.1° area. Most of these objects are found in small clusters or are well aligned with the H II bubble. We also identify the sources of Herbig-Haro jets. The infrared morphology of the region suggests that it is part of the larger Carina Nebula complex. Conclusions: The location of the candidate young stellar objects on the rim of the H II bubble is suggestive of their being triggered according to a collect-and-collapse scenario, which agrees well with the observed parameters of the region. Some candidate young stellar objects are found in the heads of pillars, which indicates radiative triggering of star formation. All in all, we find evidence that in the region different mechanisms of triggered star formation are at work. Correcting the number of candidate young stellar objects for contamination, we find ~600 young stellar objects in Gum 31 above our completeness limit of about 1 M?. Extrapolating the initial mass function down to 0.1 M?, we estimate a total population of ~5000 young stars for the region. This work is based in part on data collected by Herschel, an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA, and on data observed by VISTA (ESO run number 088.C-0117(A)), an ESO survey telescope developed by a consortium of 18 universities in the United Kingdom, led by Queen Mary, University of London.Tables 1-3 are available in electronic form at http://www.aanda.org

Ohlendorf, H.; Preibisch, T.; Gaczkowski, B.; Ratzka, T.; Ngoumou, J.; Roccatagliata, V.; Grellmann, R.

2013-04-01

181

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

182

Near-infrared images of IC 348 and the luminosity functions of young embedded star clusters  

NASA Astrophysics Data System (ADS)

We present the results of a sensitive near-infrared (JHK) imaging survey of the young cluster IC 348 and a nearby control field. From comparison of the cluster and control field observations we estimate that 380 sources, the majority of the stars observed in the cluster field, are members of the cluster. The spatial density of these stars is found to be significantly larger than that typical of the classical open clusters but comparable to that which characterizes young embedded clusters such as NGC 2024 and the Trapezium. Overall, we find the surface density distribution of stars in IC 348 to be centrally concentrated and to decrease inversely with the distance from the inner (r approximately 0.1 pc) to the outer (r approximately 1.0 pc) regions of the cluster. In detail the stellar surface density distribution of this cluster exhibits significant structure. Roughly half the stars are contained within a central subcluster with a radius of 0.5 pc. Outside this half-mass radius we identify eight small subclusters which contain 10-20 stars and have radii 0.1-0.2 pc in extent. We construct the K luminosity function (KLF) for IC 348 and find it to increase with magnitude in a nonlinear, power-law fashion in the range 8 less than or equal mK less than or equal 11 mag. We construct evolutionary models for the near-infrared luminosity functions of young (taucl less than or equal 107 yr) star clusters containing pre-main-sequence stars. We find that the KLFs of very young synthetic clusters evolve in a systematic and predictable manner as the clusters age. In general we find that the luminosity functions of young clusters broaden with age. For coeval models (i.e., tausf much less than taucl) the slopes of the power-law portion of the KLFs exhibit significant variation with time, while models with continuous star fromation (i.e., tausf approximately taucl) maintain more or less constant slopes as they age. From comparison of four models with our observations of IC 348 we conclude that star formation in IC 348 has been a continuous process over the last 5-7 X 106 yr and that the overall rate of star formation and the rate of star formation as a function of mass has been constant over the cluster lifetime. From a comparative analysis of published observations of the Trapezium cluster with our models and observations of IC 348, we find that the underlying mass function of both clusters is similar to the IMF for field stars down to the hydrogen burning limit with little evidence for a significant population of single, lower mass objects (brown dwarfs). In addition we also find that despite the similarities in their mass functions, stellar densities, and sizes, IC 348 and the Trapezium have been characterized by significantly different rates of star formation over their lifetimes. The rate of star formation in the younger trapezium cluster has been a factor of 20 greater than that in IC 348. Finally, analysis of the JHK colors of the stars in IC 348 reveals that approximately 20% of the cluster sources are infrared excess sources.

Lada, Elizabeth A.; Lada, Charles J.

1995-04-01

183

YOUNG STELLAR POPULATIONS AND STAR CLUSTERS IN NGC 1705  

SciTech Connect

We present Hubble Space Telescope (HST) photometry of the late-type dwarf galaxy NGC 1705 observed with the Wide-Field Planetary Camera 2 (WFPC2) in the F380W and F439W bands and with the Advanced Camera for Surveys/High-Resolution Channel (HRC) in the F330W, F555W, and F814W broad-band filters. We cross-correlate these data with previous ones acquired with the WFPC2 in the F555W, F814W bands, and derive multiband color-magnitude diagrams (CMDs) of the cross-identified individual stars and candidate star clusters. For the central regions of the galaxy, where HST-NICMOS F110W and F160W photometry is also available, we present U, B, V, I, J, H CMDs of the 256 objects with magnitudes measured in all bands. While our previous study based on F555W, F814W, F110W, and F160W data allowed us to trace the star formation history of NGC 1705 back to a Hubble time, the new data provide a better insight on its recent evolution. With the method of the synthetic CMDs, we confirm the presence of two strong bursts of star formation (SF). The older of the two bursts (B1) occurred between {approx}10 and 15 Myr ago, coeval to the age of the central super star cluster (SSC). The younger burst (B2) started {approx}3 Myr ago, and it is still active. The stellar mass produced by B2 amounts to {approx}10{sup 6} M {sub sun}, and it is a factor of {approx}3 lower for B1. The interburst phase was likely characterized by a much lower level of SF rather than by its complete cessation. The two bursts show distinct spatial distributions: while B1 is centrally concentrated, B2 is more diffused, and presents ring and arclike structures that remind of an expanding shell. This suggests a feedback mechanism, in which the expanding superbubble observed in NGC 1705, likely generated by the 10-15 Myr burst, triggered the current strong SF activity. The excellent spatial resolution of the HRC allowed us to reliably identify 12 star clusters (plus the SSC) in the central {approx}26'' x 29'' region of NGC 1705, 10 of which have photometry in all the UBVIJH bands. The comparison of the cluster photometry with the GALEV populations synthesis models provides ages from {approx}10 Myr to {approx}1 Gyr, and masses between {approx}10{sup 4} and 10{sup 5} M {sub sun}. The conspicuous cluster population in the central regions, with one SSC, one populous cluster, and several regular ones, confirm the strong star-forming activity of NGC 1705.

Annibali, F.; Greggio, L. [INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, I-35122 Padova (Italy); Tosi, M.; Montegriffo, P. [INAF-Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna (Italy); Monelli, M. [Instituto de Astrofisica de Canarias, Calle Via Lactea, E38200 La Laguna (Spain); Sirianni, M.; Aloisi, A. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States)

2009-07-15

184

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

185

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

186

Spitzer and Variable Young Stars: Shining a Spotlight on Circumstellar Disks  

NASA Astrophysics Data System (ADS)

Since its launch in 2003, the Spitzer Space Telescope has helped to uncover hundreds of disk bearing young stars in clusters by detecting their infrared excesses. Study of the spectral energy distributions of these objects has shed light on disk evolution, dispersal, and the relationship to planet formation. With the start of the Warm Spitzer Mission, mid-infrared time series observations have opened up a new window into the dynamic nature of these systems. Not only are young stellar objects (YSOs) highly variable, but so are their disks! I will review recent findings on mid-infrared variability in young stars, highlighting the Young Stellar Object Variability project and the joint Spitzer/CoRoT Coordinated Synoptic Investigation of NGC 2264. These efforts have resulted in a comprehensive census and categorization of YSO flux behavior at 0.5 through 4.5 microns, on timescales from hours to months. We now have evidence for multiple simultaneous variability mechanisms, supporting the picture of a highly dynamic star-disk system.

Cody, Ann Marie; CSI 2264 Team

2014-01-01

187

A newly discovered young massive star cluster at the far end of the Galactic Bar  

NASA Astrophysics Data System (ADS)

We present a near-infrared study of the candidate star cluster Mercer 81, located at the centre of the G338.4+0.1 H II region and close to the TeV gamma-ray source HESS 1640-465. Using Hubble Space Telescope/NICMOS imaging and VLT/ISAAC spectroscopy, we have detected a compact and highly reddened cluster of stars, although the bright stars in the centre of the field are in fact foreground objects. The cluster contains nine stars with strong P? emission, one of which we identify as a Wolf-Rayet (WR) star, as well as an A-type supergiant. The line-of-sight extinction is very large, AV˜ 45, illustrating the challenges of locating young star clusters in the Galactic plane. From a quantitative analysis of the WR star, we argue for a cluster age of 3.7? Myr, and, assuming that all emission-line stars are WR stars, a cluster mass of ?104 M?. A kinematic analysis of the cluster's surrounding H II region shows that the cluster is located in the Galactic disc at a distance of 11 ± 2 kpc. This places the cluster close to where the far end of the Bar intersects the Norma spiral arm. This cluster, as well as the nearby cluster [DBS2003]179, represents the first detections of active star cluster formation at this side of the Bar, in contrast to the near side which is well known to have recently undergone a ˜106 M? starburst episode.

Davies, Ben; de La Fuente, Diego; Najarro, Francisco; Hinton, Jim A.; Trombley, Christine; Figer, Donald F.; Puga, Elena

2012-01-01

188

YOUNG STELLAR OBJECTS AND TRIGGERED STAR FORMATION IN THE VULPECULA OB ASSOCIATION  

SciTech Connect

The Vulpecula OB association, Vul OB1, is a region of active star formation located in the Galactic plane at 2.3 kpc from the Sun. Previous studies suggest that sequential star formation is propagating along this 100 pc long molecular complex. In this paper, we use Spitzer MIPSGAL and GLIMPSE data to reconstruct the star formation history of Vul OB1, and search for signatures of past triggering events. We make a census of young stellar objects (YSOs) in Vul OB1 based on IR color and magnitude criteria, and we rely on the properties and nature of these YSOs to trace recent episodes of massive star formation. We find 856 YSO candidates, and show that the evolutionary stage of the YSO population in Vul OB1 is rather homogeneous-ruling out the scenario of propagating star formation. We estimate the current star formation efficiency to be {approx}8%. We also report the discovery of a dozen pillar-like structures, which are confirmed to be sites of small scale triggered star formation.

Billot, N.; Latter, W. [NASA Herschel Science Center, IPAC, MS 100-22, California Institute of Technology, Pasadena, CA 91125 (United States); Noriega-Crespo, A.; Carey, S.; Guieu, S.; Shenoy, S.; Paladini, R., E-mail: nbillot@ipac.caltech.ed [Spitzer Science Center, IPAC, MS 220-6, California Institute of Technology, Pasadena, CA 91125 (United States)

2010-04-01

189

A Near-infrared Study of the Obscured, Young, Massive Star Cluster, Mercer 23  

NASA Astrophysics Data System (ADS)

We determine the physical parameters and investigate the high-mass stellar content of the infrared star cluster Mercer 23, situated near the Galactic plane (l=53.772, b=+0.164). Our analysis is based on new Baade/PANIC JHK imaging of Mercer 23 and ISAAC/VLT moderate resolution (R 4000) spectroscopy of the brightest cluster members in the H- and K-bands. The cluster age is determined from isochrone main-sequence (MS) and pre-MS fitting. We derive stellar parameters for eight of the stellar members, using a full non-LTE modeling of the obtained spectra. Mercer 23 is a very young cluster, with age of t=2-4 Myr. The cluster suffers reddening of E(J-Ks)=1.35, Av = 7.2 mag. The derived distance is d=6.5 +/- 0.2 kpc. Our spectral modeling allows us to conclude that the three most luminous member are evolved, highly massive stars: a WR star, and two mid-O supergiant stars, based on their derived luminosity. Mercer 23, is not a super-massive cluster, such as those recently recognized to exist in the Milky Way. However, its mass estimate of 4-6 x 103 Msun puts it in the unique class of young, very massive Galactic clusters hosting WR stars.

Hanson, Margaret M.; Kurtev, R.; Borissova, J.; Georgiev, L.; Ivanov, V. D.; Hillier, D. J.; Minniti, D.

2010-05-01

190

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

191

Sulphur chemistry in the envelopes of massive young stars  

NASA Astrophysics Data System (ADS)

The sulphur chemistry in nine regions in the earliest stages of high-mass star formation is studied through single-dish submillimeter spectroscopy. The line profiles indicate that 10-50% of the SO and SO2 emission arises in high-velocity gas, either infalling or outflowing. For the low-velocity gas, excitation temperatures are 25 K for H2S, 50 K for SO, H2cs, NS and HCS+, and 100 K for OCS and SO2, indicating that most observed emission traces the outer parts (T<100 K) of the molecular envelopes, except high-excitation OCS and SO2 lines. Abundances in the outer envelopes, calculated with a Monte Carlo program, using the physical structures of the sources derived from previous submillimeter continuum and CS line data, are ~ 10-8 for OCS, ~ 10-9 for H2S, H2cs, SO and SO2, and ~ 10-10 for HCS+ and NS. In the inner envelopes (T>100 K) of six sources, the SO2 abundance is enhanced by a factor of ~ 100-1000. This region of hot, abundant SO2 has been seen before in infrared absorption, and must be small, la 0\\farcs2 (180 AU radius). The derived abundance profiles are consistent with models of envelope chemistry which invoke ice evaporation at T ~ 100 K. Shock chemistry is unlikely to contribute. A major sulphur carrier in the ices is probably OCS, not H2S as most models assume. The source-to-source abundance variations of most molecules by factors of ~ 10 do not correlate with previous systematic tracers of envelope heating. Without observations of H2S and SO lines probing warm (>~ 100 K) gas, sulphur-bearing molecules cannot be used as evolutionary tracers during star formation.

van der Tak, F. F. S.; Boonman, A. M. S.; Braakman, R.; van Dishoeck, E. F.

2003-12-01

192

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

National Technical Information Service (NTIS)

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 su...

C. A. Beichman A. Boden A. Ghez L. W. Hartman L. Hillenbrand J. I. Lunine M. J. Simon J. R. Stauffer T. Velusamy

2004-01-01

193

A search for young stars in the halo of M31  

NASA Astrophysics Data System (ADS)

Previous studies have identified young hot stars that appear to have been formed in the halo of our Galaxy. Here we present preliminary results of a spectroscopic search for similar objects in the halo of M31. Radial velocities have been measured for 23 apparently blue objects in a B(v) magnitude range of about 17-21. For one of these targets, AND 0029+413, the radial velocity suggests that the object is gravitationally bound to M31 and hence that it may be in its halo. However, recent CCD photometry for this object indicates that it is redder than implied by the original photographic observations. A number of possible scenarios are presented to explain this object, including one consistent with it being a young star in the halo of M31.

McCausland, R. J. H.; Conlon, E. S.; Dufton, P. L.; Fitzsimmons, A.; Irwin, M. J.; Keenan, F. P.

1993-07-01

194

Sulphur Chemistry in the Envelopes of Massive Young Stars  

NASA Astrophysics Data System (ADS)

The sulphur chemistry in eight regions of massive star formation is studied through single-dish submillimeter spectroscopy. The SO line profiles show that up to 50% of the SO emission arises in bipolar outflows, much more than found previously for CS. Rotation temperatures of SO and SO2 range from 26 to 75 K for both envelope and outflow, while two sources show additional hot (>sim 200 K) SO2 in their envelopes. Column densities are ˜1014 cm-2 for H2S, OCS, SO and SO2, ˜1013 cm-2 for H2CS, and ˜1012 cm-2 for HCS+ and NS. The hot, abundant SO2 seen previously in infrared absorption is not picked up, so its angular size must be small. Molecular abundances are calculated with a Monte Carlo program, using the physical structures of the sources derived from previous submillimeter continuum and CS line data. Abundances are ˜10-8 for H2S, OCS, SO and SO2, ˜10-9 for H2CS, and ˜10-10 for HCS+ and NS. For SO and SO2, the number of detected lines is large enough to derive radial abundance profiles, and therefore to study their chemistry as a function of temperature. While the SO abundance appears constant over the range T = 20-80 K, the SO2 abundance may increase at T >sim 100 K. The derived abundances are compared to models of shock chemistry, hot core chemistry and high-temperature envelope chemistry.

van der Tak, Floris F. S.; Boonman, Annemieke M. S.; Braakman, Rogier; van Dishoeck, Ewine F.

195

Elemental abundances of low-mass stars in the young clusters 25 Orionis and ? Orionis  

NASA Astrophysics Data System (ADS)

Aims: We aim to derive the chemical pattern of the young clusters 25 Orionis and ? Orionis through homogeneous and accurate measurements of elemental abundances. Methods: We present flames/uves observations of a sample of 14 K-type targets in the 25 Ori and ? Ori clusters. We measured their radial velocities to confirm cluster membership. We derived stellar parameters and abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code MOOG. Results: All 25 Ori stars are confirmed cluster members without evidence of binarity; in ? Ori we identify one non member and one possible single-lined binary star. We find an average metallicity [Fe/H] = -0.05 ± 0.05 for 25 Ori, where the error is the 1? standard deviation from the average. The ? Ori members have a mean Fe abundance value of 0.01 ± 0.01. The other elements show close-to-solar ratios and no star-to-star dispersion. Conclusions: Our results, along with previous metallicity determinations in the Orion complex, show a small but detectable dispersion in the [Fe/H] distribution of the complex. This appears to be compatible with large-scale star formation episodes and initial nonuniformity in the precloud medium. We show that, as expected, the abundance distribution of star-forming regions is consistent with the chemical pattern of the Galactic thin disk. Based on flames observations collected at the Paranal Observatory (Chile). Program 082.D-0796(A).

Biazzo, K.; Randich, S.; Palla, F.; Briceño, C.

2011-06-01

196

Search for associations containing young stars: chemical tagging IC 2391 and the Argus association  

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 homogenous 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 5200 K as previously noted in the literature. Also, enhanced Ba abundances of around 0.6 dex 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.

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

2013-05-01

197

The star-forming history of the young cluster NGC 2264  

NASA Technical Reports Server (NTRS)

UBVRI H-alpha photographic photometry was obtained for a sample of low-mass stars in the young open cluster NGC 2264 in order to investigate the star-forming history of this region. A theoretical H-R diagram was constructed for the sample of probable cluster members. Isochrones and evolutionary tracks were adopted from Cohen and Kuhi (1979). Evidence for a significant age spread in the cluster was found amounting to over ten million yr. In addition, the derived star formation rate as a function of stellar mass suggests that the principal star-forming mass range in NGC 2264 has proceeded sequentially in time from the lowest to the highest masses. The low-mass cluster stars were the first cluster members to form in significant numbers, although their present birth rate is much lower now than it was about ten million yr ago. The star-formation rate has risen to a peak at successively higher masses and then declined.

Adams, M. T.; Strom, K. M.; Strom, S. E.

1983-01-01

198

SPATIALLY EXTENDED BRACKETT GAMMA EMISSION IN THE ENVIRONMENTS OF YOUNG STARS  

SciTech Connect

The majority of atomic hydrogen Br{gamma} emission detected in the spectra of young stellar objects is believed to arise from the recombination regions associated with the magnetospheric accretion of circumstellar disk material onto the forming star. In this paper, we present the results of a K-band integral field unit spectroscopic study of Br{gamma} emission in eight young protostars: CW Tau, DG Tau, Haro 6-10, HL Tau, HV Tau C, RW Aur, T Tau, and XZ Tau. We spatially resolve Br{gamma} emission structures in half of these young stars and find that most of the extended emission is consistent with the location and velocities of the known Herbig-Haro flows associated with these systems. At some velocities through the Br{gamma} line profile, the spatially extended emission comprises 20% or more of the integrated flux in that spectral channel. However, the total spatially extended Br{gamma} is typically less than {approx}10% of the flux integrated over the full emission profile. For DG Tau and Haro 6-10 S, we estimate the mass outflow rate using simple assumptions about the hydrogen emission region and compare this to the derived mass accretion rate. We detect extended Br{gamma} in the vicinity of the more obscured targets in our sample and conclude that spatially extended Br{gamma} emission may exist toward other stars, but unattenuated photospheric flux probably limits its detectability.

Beck, Tracy L. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 (United States); Bary, Jeffery S. [Colgate University, Department of Physics and Astronomy, 13 Oak Drive, Hamilton, NY 13346 (United States); McGregor, Peter J., E-mail: tbeck@stsci.ed, E-mail: jbary@colgate.ed, E-mail: peter@mso.anu.edu.a [Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston, ACT 2611 (Australia)

2010-10-20

199

Anatomy of a Flaring Proto-Planetary Disk Around a Young Intermediate-Mass Star  

NASA Astrophysics Data System (ADS)

Although planets are being discovered around stars more massive than the Sun, information about the proto-planetary disks where such planets have built up is sparse. We have imaged mid-infrared emission from polycyclic aromatic hydrocarbons at the surface of the disk surrounding the young intermediate-mass star HD 97048 and characterized the disk. The disk is in an early stage of evolution, as indicated by its large content of dust and its hydrostatic flared geometry, indicative of the presence of a large amount of gas that is well mixed with dust and gravitationally stable. The disk is a precursor of debris disks found around more-evolved A stars such as ?-Pictoris and provides the rare opportunity to witness the conditions prevailing before (or during) planet formation.

Lagage, Pierre-Olivier; Doucet, Coralie; Pantin, Eric; Habart, Emilie; Duchêne, Gaspard; Ménard, François; Pinte, Christophe; Charnoz, Sébastien; Pel, Jan-Willem

2006-10-01

200

The Massive Young Star-forming Complex Study in Infrared and X-Ray: X-Ray Sources in 10 Star-forming Regions  

NASA Astrophysics Data System (ADS)

The Massive Young star-forming complex Study in Infrared and X-ray (MYStIX) uses data from the Chandra X-Ray Observatory to identify and characterize the young stellar populations in 20 Galactic (d < 4 kpc) massive star-forming regions. Here, the X-ray analysis for Chandra ACIS-I observations of 10 of the MYStIX fields is described, and a catalog of >10,000 X-ray sources is presented. In comparison to other published Chandra source lists for the same regions, the number of MYStIX-detected faint X-ray sources in a region is often doubled. While the higher catalog sensitivity increases the chance of false detections, it also increases the number of matches to infrared stars. X-ray emitting contaminants include foreground stars, background stars, and extragalactic sources. The X-ray properties of sources in these classes are discussed.

Kuhn, Michael A.; Getman, Konstantin V.; Broos, Patrick S.; Townsley, Leisa K.; Feigelson, Eric D.

2013-12-01

201

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

202

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

203

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

204

Young Super Star Clusters in the Starburst of M82: The Catalog  

NASA Astrophysics Data System (ADS)

Recent results from the Hubble Space Telescope (HST) have resolved starbursts as collections of compact young stellar clusters. Here we present a photometric catalog of the young stellar clusters in the nuclear starburst of M82, observed with the HST WFPC2 in H? (F656N) and in four optical broadband filters. We identify 197 young super stellar clusters. The compactness and high density of the sources led us to develop specific techniques to measure their sizes. Strong extinction lanes divide the starburst into five different zones, and we provide a catalog of young super star clusters for each of these. In the catalog we include relative coordinates, radii, fluxes, luminosities, masses, equivalent widths, extinctions, and other parameters. Extinction values have been derived from the broadband images. The radii range between 3 and 9 pc, with a mean value of 5.7+/-1.4 pc, and a stellar mass between 104 and 106 Msolar. The inferred masses and mean separation, comparable to the size of the super star clusters, together with their high volume density, provide strong evidence for the key ingredients postulated by Tenorio-Tagle and coworkers as required for the development of a supergalactic wind. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

Melo, V. P.; Muñoz-Tuñón, C.; Maíz-Apellániz, J.; Tenorio-Tagle, G.

2005-01-01

205

Galex-selected Nearby Young Stars: X-ray Counterparts and Potential New eps Cha Members  

NASA Astrophysics Data System (ADS)

We are searching the GALEX (UV) and 2MASS+WISE (infrared) sky survey data for nearby young, low-mass stars. We select candidates on the basis of proper motions (PMs), infrared colors and magnitudes, and UV excesses that are all indicative of young M dwarfs within ~100 pc of Earth. Here, we describe the preliminary results of searches of available ROSAT, XMM, and Chandra archival data for X-ray detections that might establish high levels of coronal activity and, hence, help confirm the youth of UV/IR/PM-selected candidates. We also present an analysis of candidate young stars in the vicinity of the ~7-Myr-old epsilon Chamaeleonis Association, in an effort to identify possible new members (and companions to known members) of this young stellar group. Support for this work was provided in part by the National Science Foundation under a Research Experience for Undergraduates program grant (PHY-1062874) to RIT and by NASA Astrophysics Data Analysis Program award NNX12AH37G to RIT and UCLA.

Diaz, Mariangelly; Rodriguez, D.; Darling, S.; Principe, D.; Kastner, J. H.; Montez, R.; Zuckerman, B. M.

2013-01-01

206

A Confirmed Directly Imaged Planet Orbiting a Nearby Young, Dusty Star  

NASA Astrophysics Data System (ADS)

We present new VLT/NaCo infrared (Lp/3.8 micron) high-contrast imaging observations of a nearby, young (13--21 Myr old) star known to be surrounded by a luminous Kuiper belt-like debris disk. Using multiple reduction pipelines, we unambiguously detect a faint companion located interior to the disk at a projected separation of ~55 AU in four separate data sets between 2012 and 2013. The companion’s astrometry is decisively inconsistent with that of a background object. Combining our Lp photometry with sensitive upper limits at shorter wavelengths shows that the companion has red colors characteristic of young jovian planets with an inferred mass of 3--7 Mj, making it potentially the lowest mass planet imaged thus far. This planet will be a benchmark for further physical and orbital characterization of young gas giants.

Currie, Thayne M.; Rameau, J.; Chauvin, G.; Lagrange, A.; Boccaletti, A.; Meshkat, T.; Quanz, S.; Girard, J.; Bonnefoy, M.; Kenworthy, M. A.

2014-01-01

207

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

208

Direct Exoplanet\\/Disk Search Around Young & Nearby Early-Type Stars; The International Deep Planet Survey (IDPS)  

Microsoft Academic Search

We propose to continue a deep AO survey for exoplanets around young and close A- and F-type stars using a refined version of the very successful Angular Differential Imaging technique, which distinguishes true companions\\/disks from artifacts through sidereal rotation. Stars as massive as A- and early F-type stars have been neglected in AO searches, including the Gemini Deep Planet Survey,

Christian Marois; Bruce Macintosh; Jennifer Patience; Rene Doyon; Benjamin Zuckerman; Inseok Song; David Lafreniere; Travis Barman

2008-01-01

209

X-ray emission from young stars in Taurus-Auriga-Perseus: Luminosity functions and the rotation - activity - age - relation  

Microsoft Academic Search

We report on a systematic search for X-ray emission from pre-main sequence and young main sequence stars in the Taurus-Auriga-Perseus region. Our stellar sample consists of all T Tauri stars from the Taurus-Auriga region, and all late-type stars from the Pleiades and Hyades clusters which have been observed by the ROSAT PSPC in pointed observations. We present the X-ray parameters

B. Stelzer; R. Neuhäuser

2001-01-01

210

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

211

Eruptive and Intrusive History of Mount Mazama and the Crater Lake Region Explains Growth of a Young Silicic Magma Body  

NASA Astrophysics Data System (ADS)

Crater Lake caldera (subsidence area >25 km2) collapsed into a shallow long-lived magmatic system during the ~7.7 ka climactic eruption of Mount Mazama that ejected ~50 km3 of magma dominated by homogeneous crystal-poor rhyodacite pumice. Geologic mapping, Ar and U-Th geochronology, and geochemistry provide volume-time-composition information on the growth and evolution of the Mazama system. Granodiorite and related plutonic rocks in the climactic ejecta give a window into the intrusive component of that system. We interpret the eruptive and intrusive history in terms of the competition between (1) crystallization driven by degassing and hydrothermal cooling and (2) thermal input from a regional magma flux focused at Mazama. Throughout its ~400 kyr history, the andesite-dacite volcano known as Mount Mazama, built astride an earlier silicic dome field, was accompanied by nearby basaltic to andesitic effusive volcanism representing a range of differentiation of melts of depleted mantle containing varied amounts of water and subduction-related fluid-mobile elements. Similar parent magmas appear to have differentiated to form the products of Mount Mazama. The Mazama edifice was constructed in many comparatively brief episodes, some of the more voluminous being approximately coeval with volcanic pulses in the surrounding region. Magmas as evolved as dacite erupted many times, commonly associated with or following voluminous andesite effusion. It was not until ~30 ka, after 400 kyr of preconditioning, that a shallow rhyodacite-dominated magma reservoir began to grow. Between ~30 ka and the climactic eruption, rhyodacitic magmas vented mainly north of the edifice, unaccompanied by other compositions. From ~35 ka into postglacial time unusually primitive lavas (magnesian basaltic andesite and tholeiitic basalt) erupted west of Mazama suggest increased thermal input. Reconstruction of the Mazama climactic magma chamber from preclimactic rhyodacites and their enclaves and from andesitic scoriae and gabbroic cumulates ejected near the end of the climactic eruption suggests incremental generation of an average of 2.5 km3/kyr of rhyodacite dominantly by crystallization differentiation of basaltic to andesitic magma that was repeatedly intruded between cumulate mush and overlying silicic derivative magma. The mineralogy and elemental and radiogenic isotopic compositions of the granodiorite blocks are similar to those of volcanic rocks of Mount Mazama. Zircon SHRIMP U-Th geochronology gives a range of crystallization ages for granodiorites that correspond to times of dacite effusion. Because erupted silicic magmas were zircon undersaturated, zircon apparently grew late when magmas were mostly crystallized. Variation in crystallization age of blocks from different localities around the caldera suggests a composite pluton underlies Mount Mazama at shallow depth. The granodiorite blocks commonly are partially melted up to 50%, showing that the climactic magma chamber was melting its walls. Retention of zircon antecrysts in pre-35-ka magmas of the granodiorites implies recycling of zircon into high-crystallinity magmas whereas climactic rhyodacite appears to have consumed virtually all zircon from any assimilated wallrock. We conclude that (1) the composite pluton formed from relatively small volumes of derivative magma that crystallized before the upper crust had been heated sufficiently to sustain voluminous convecting crystal-poor melt and (2) that a large volume of eruptible silicic magma accumulated because of heating associated with input of relatively primitive magma to the roots of the system.

Bacon, C. R.; Lanphere, M. A.; Lowenstern, J. B.

2005-05-01

212

An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT  

Microsoft Academic Search

We present the results of a survey of 45 young (<~250 Myr), close (<~50 pc) stars with the Simultaneous Differential Imager (SDI) implemented at the VLT and the MMT for the direct detection of extrasolar planets. As part of the survey, we observed 54 objects, consisting of 45 close, young stars; two more distant (<150 pc), extremely young (<=10 Myr)

Beth A. Biller; Laird M. Close; Elena Masciadri; Eric Nielsen; Rainer Lenzen; Wolfgang Brandner; Donald McCarthy; Markus Hartung; Stephan Kellner; Eric Mamajek; Thomas Henning; Douglas Miller; Matthew Kenworthy; Craig Kulesa

2007-01-01

213

Neutron Stars From Young Nearby Associations: The Origin of RX J1605.3+3249  

NASA Astrophysics Data System (ADS)

Many neutron stars (NSs) and runaway stars apparently come from the same regions on the sky. This suggests that they share the same birth places, namely associations and clusters of young massive stars. To identify NS birth places, we attempt to find NS-runaway pairs that could be former companions that were disrupted in a supernova (SN). The remains of recent (stars HIP 68228 and HIP 89394 - as well as the appearance of a feature in the ?-ray emission from 26Al decay at the predicted SN place. Both, the progenitor masses estimated by comparison with theoretical 26Al yields as well as derived from the life-time of the progenitor star, are found to be ~11Msolar.

Tetzlaff, N.; Schmidt, J. G.; Hohle, M. M.; Neuhäuser, R.

2012-03-01

214

Protoplanetary disc evolution affected by star-disc interactions in young stellar clusters  

NASA Astrophysics Data System (ADS)

Most stars form in a clustered environment. Therefore, it is important to assess how this environment influences the evolution of protoplanetary discs around young stars. In turn, this affects their ability to produce planets and ultimately life. We present here for the first time 3D smoothed particle hydrodynamics/N-body simulations that include both the hydrodynamical evolution of the discs around their natal stars, as well as the dynamics of the stars themselves. The discs are viscously evolving, accreting mass on to the central star and spreading. We find penetrating encounters to be very destructive for the discs as in previous studies, although the frequency of such encounters is low. We also find, however, that encounter influence the disc radii more strongly than other disc properties such as the disc mass. The disc sizes are set by the competition between viscous spreading and the disruptive effect of encounters. As discs spread, encounters become more and more important. In the regime of rapid spreading, encounters simply truncate the discs, stripping the outer portions. In the opposite regime, we find that the effect of many distant encounters is able to limit the disc size. Finally, we predict from our simulations that disc sizes are limited by encounters at stellar densities exceeding ˜2-3 × 103 pc-2.

Rosotti, Giovanni P.; Dale, James E.; de Juan Ovelar, Maria; Hubber, David A.; Kruijssen, J. M. Diederik; Ercolano, Barbara; Walch, Stefanie

2014-07-01

215

Pair-instability supernovae via collision runaway in young dense star clusters  

NASA Astrophysics Data System (ADS)

Stars with helium cores between ˜64 and 133 M? are theoretically predicted to die as pair-instability supernovae. This requires very massive progenitors, which are theoretically prohibited for Pop II/I stars within the Galactic stellar mass limit due to mass-loss via line-driven winds. However, the runaway collision of stars in a dense, young star cluster could create a merged star with sufficient mass to end its life as a pair-instability supernova, even with enhanced mass-loss at non-zero metallicity. We show that the predicted rate from this mechanism is consistent with the inferred volumetric rate of roughly ˜2 × 10-9 Mpc-3 yr-1 of the two observed pair-instability supernovae, SN 2007bi and PTF 10nmn, neither of which has metal-free host galaxies. Contrary to prior literature, only pair-instability supernovae at low redshifts z < 2 will be observable with the Large Synoptic Survey Telescope. We estimate that the telescope will observe ˜102 such events per year that originate from the collisional runaway mergers in clusters.

Pan, Tony; Loeb, Abraham; Kasen, Daniel

2012-07-01

216

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

PubMed

A luminous X-ray source is associated with MGG 11--a cluster of young stars approximately 200 pc 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 (350 M(o)), which is intermediate between stellar-mass and supermassive black holes. A nearby but somewhat more massive cluster (MGG 9) shows no evidence of such an intermediate-mass black hole, raising the issue of just what physical characteristics of the clusters can account for this difference. Here we report numerical simulations of the evolution and motion of stars within the clusters, where stars are allowed to merge with each other. We find that for MGG 11 dynamical friction leads to the massive stars sinking rapidly to the centre of the cluster, where they participate in a runaway collision. This produces a star of 800-3,000 M(o) which ultimately collapses to a black hole of intermediate mass. No such runaway occurs in the cluster MGG 9, because the larger cluster radius leads to a mass segregation timescale a factor of five longer than for MGG 11. PMID:15085124

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

2004-04-15

217

Angular momentum regulation in low-mass young stars surrounded by accretion disks  

NASA Technical Reports Server (NTRS)

From study of a sample of 34 T Tauri stars with photometrically derived rotation periods and spectral types later than KS, we find that the observed periods appear to be related to the presence or absence of an accretion disk. Those stars which we infer to be surrounded by accretion disks have rotation periods P(rot) over 4 days with a most probable P(rot) of about 8.5 days, while those stars which lack accretion disk signatures cover a wide range of P(rot) from 1.5 to 16 days, including a significant number of objects with P(rot) less than 4 days. This suggests the possibility that the 'initial' angular momentum of a star is not established until it dissipates its circumstellar accretion disk. During the disk accretion phase, the stellar angular velocity appears to be regulated at a low value, countering the tendency of the star to spin up both from contraction toward the main sequence and from the accretion of inner disk material of high specific angular momentum. When the accretion disk is dissipated, this regulation mechanism will cease to function. At this point, the star is no longer maintained at a low angular velocity, but is 'free' to conserve its angular momentum, and thus to increase its angular velocity in response to contraction and changes in moment of inertia. This hypothesis, combined with a spread in disk dispersal time scales, provides a context for explaining the observed distribution of stellar rotational velocities for stars on the ZAMS in young clusters.

Edwards, Suzan; Strom, Stephen E.; Hartigan, Patrick; Strom, Karen M.; Hillenbrand, Lynne A.; Herbst, William; Attridge, Joanne; Merrill, K. M.; Probst, Ron; Gatley, Ian

1993-01-01

218

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

219

PP 13S, a young, low-mass FU Orionis-type pre-main sequence star  

NASA Astrophysics Data System (ADS)

We present near-IR imaging and spectroscopy, together with sub-mm/mm photometry and {(12) CO} molecular line maps of the cometary nebula PP 13S. Previous models have been unable to resolve whether PP 13S is a young, pre-main sequence star or an old, evolved object. Our new observations prove conclusively that PP 13S is a young stellar object with a luminosity of ~ 30Lsun and an accretion disk with an inclination of ~ 40°. It has a {(12) CO} J=2-1 outflow, strong and broadened CO overtone band absorption, and vibrationally excited H_2 emission. We conclude that PP 13S has all the characteristics of an FU Orionis-type star, and although no outburst has been observed, we suggest that PP 13S should be added to the list of FUor pre-main sequence stars. Our study also includes PP13N, which we resolve into a young double star system.

Sandell, Goeran; Aspin, Colin

1998-05-01

220

Near-infrared Variability in Young Stars in Cygnus OB7  

NASA Astrophysics Data System (ADS)

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, ~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.; Wolk, Scott J.; Aspin, Colin

2012-08-01

221

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

222

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

223

Photometric properties of stars clusters with young or mixed age stellar populations  

NASA Astrophysics Data System (ADS)

The main goal of this work is to present and discuss the synthetic photometrical properties of stellar clusters resulting from the PopStar code. Colors in Johnson and SDSS systems, H? and H? luminosities and equivalent widths, and ionizing region size, have been computed for a wide range of metallicities Z = 0.0001, 0.0004, 0.004,0.008,0.02 and 0.05, and ages, from 0.1 Myr to 20 Gyr in Mollá, Garc{í}a-Vargas, & Bressan (2009, MNRAS, 398, 451). Emission lines are shown in Mart{í}n-Manj{ó}n et al. (2010, MNRAS, 403, 2012). Now we calculate colors with the emission lines contribution to the broad band color, so colors include stellar and nebular components, plus the emission lines following the evolution of the cluster and the region geometry in a consistent way. We compare the Single Stellar Populations contaminated and uncontaminated colors (in both Johnson and SDSS systems) and show the importance of emission lines contribution when photometry is used as a tool to characterize stellar populations. With these models we may determine the physical properties of young ionizing clusters when only photometrical observations are available and these correspond to the isolated star forming regions, subtracted the contribution of the underlying population In most cases, however, the ionizing population is usually embedded in a large and complex system, and the observed photometrical properties are the result of the combination of both the young star-forming burst and the host-underlying older population. The second objective of our work is therefore to provide a grid of models for nearby galaxies able to interpret mixed regions where the separation of young and old population is not possible or reliable enough. We obtain a set of PopStar Spectral Energy Distributions (available at PopStar site and also in VO) and derived colors for mixed populations where an underlying host population is combined in different mass ratios with a recent, metal-rich ionizing burst. These colors, together with other photometrical parameters, like H? radius of the ionized region, and Balmer lines equivalent width and luminosity allow to infer the physical properties of star-forming regions without any spectroscopic information. For details and a complete set of tables and figures see Mollá, García-Vargas, & Martín-Manjón (2012, MNRAS, submitted).

Mollá, M.; García-Vargas, M. L.; Martín-Manjón, M. L.

2013-05-01

224

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

225

Young Star Cluster Found Aglow With Mysterious X-Ray Cloud  

NASA Astrophysics Data System (ADS)

A mysterious cloud of high-energy electrons enveloping a young cluster of stars has been discovered by astronomers using NASA's Chandra X-ray Observatory. These extremely high-energy particles could cause dramatic changes in the chemistry of the disks that will eventually form planets around stars in the cluster. Known as RCW 38, the star cluster covers a region about 5 light years across. It contains thousands of stars formed less than a million years ago and appears to be forming new stars even today. The crowded environment of a star cluster is thought to be conducive to the production of hot gas, but not high-energy particles. Such particles are typically produced by exploding stars, or in the strong magnetic fields around neutron stars or black holes, none of which is evident in RCW 38. "The RCW 38 observation doesn't agree with the conventional picture," said Scott Wolk of the Harvard-Smithsonian Center for Astrophysics in Cambridge, MA, lead author of an Astrophysical Journal Letters paper describing the Chandra observation. "The data show that somehow extremely high-energy electrons are being produced there, although it is not clear how." RCW 38 RCW 38 X-ray, Radio, Infrared Composite Electrons accelerated to energies of trillions of volts are required to account for the observed X-ray spectrum of the gas cloud surrounding the ensemble of stars, which shows an excess of high-energy X-rays. As these electrons move in the magnetic field that threads the cluster, they produce X-rays. One possible origin for the high-energy electrons is a previously undetected supernova that occurred in the cluster. Although direct evidence for the supernova could have faded away thousands of years ago, a shock wave or a rapidly rotating neutron star produced by the outburst could be acting in concert with stellar winds to produce the high-energy electrons. "Regardless of the origin of the energetic electrons," said Wolk, "their presence would change the chemistry of proto-stellar disks in ways that could still be manifest billions of years later." For example, in our own solar system, we find evidence of certain short-lived radioactive nuclides (Aluminum 26 being the most well known). This implies the existence of a high-energy process late in the evolution of our solar system. If our solar system was immersed for a time in a sea of energetic particles, this could explain the rare nuclides present in meteorites found on the Earth today. RCW 38, at a distance of 6,000 light years from Earth, is one of the nearest star-forming regions with very young, hot stars. Other authors of the paper, which appeared in the 2002 December 1 issue of the Astrophysical Journal Letters, are Tyler Bourke, Randall Smith and Bradley Spitzbart of the Harvard-Smithsonian Center for Astrophysics, and Joao Alves of the European Southern Observatory in Garching, Germany. NASA's Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for the Office of Space Science in Washington. TRW, Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.

2002-12-01

226

Rotational modulation of X-ray emission in Orion Nebula young stars  

NASA Astrophysics Data System (ADS)

I will present results of a search for rotational modulation of X-ray light-curves in a sample of young Orion Nebula Cluster stars. The study, aiming at understanding the spatial distribution of X-ray emitting plasma on pre-main sequence stars, is part of the Chandra Orion Ultradeep Project (COUP) and was made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17x17 arcmin field of view. I will focus on a subsample of 233 X-ray-bright stars with known rotational periods. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period while in seven cases it is about half that value, possibly due to the presence of X-ray emitting structures at opposite stellar longitudes. These results constitute the largest sample of low mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (LX/Lbol ˜ 10-3) showing that saturation is not due to the filling of the stellar surface with X-ray emitting regions.

Flaccomio, Ettore; Micela, G.; Sciortino, S.; Feigelson, E. D.; Herbst, W.; Favata, F.; Harnden, F. R., Jr.; Vrtilek, S. D.

2005-07-01

227

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

228

Bright X-Ray Flares in Orion Young Stars from COUP: Evidence for Star-Disk Magnetic Fields?  

NASA Astrophysics Data System (ADS)

We have analyzed a number of intense X-ray flares observed in the Chandra Orion Ultradeep Project (COUP), a 13 day observation of the Orion Nebula Cluster (ONC), concentrating on the events with the highest statistics (in terms of photon flux and event duration). Analysis of the flare decay allows to determine the physical parameters of the flaring structure, particularly its size and (using the peak temperature and emission measure of the event) the peak density, pressure, and minimum confining magnetic field. A total of 32 events, representing the most powerful ~=1% of COUP flares, have sufficient statistics and are sufficiently well resolved to grant a detailed analysis. A broad range of decay times are present in the sample of flares, with ?lc (the 1/e decay time) ranging from 10 to 400 ks. Peak flare temperatures are often very high, with half of the flares in the sample showing temperatures in excess of 100 MK. Significant sustained heating is present in the majority of the flares. The magnetic structures that are found, from the analysis of the flare's decay, to confine the plasma are in a number of cases very long, with semilengths up to ~=1012 cm, implying the presence of magnetic fields of hundreds of G (necessary to confine the hot flaring plasma) extending to comparable distance from the stellar photosphere. These very large sizes for the flaring structures (length L>>R*) are not found in more evolved stars, where, almost invariably, the same type of analysis results in structures with L<=R*. As the majority of young stars in the ONC are surrounded by disks, we speculate that the large magnetic structures that confine the flaring plasma are actually the same type of structures that channel the plasma in the magnetospheric accretion paradigm, connecting the star's photosphere with the accretion disk.

Favata, F.; Flaccomio, E.; Reale, F.; Micela, G.; Sciortino, S.; Shang, H.; Stassun, K. G.; Feigelson, E. D.

2005-10-01

229

Hokupa'a-Gemini Discovery of Two Ultracool Companions to the Young Star HD 130948  

NASA Astrophysics Data System (ADS)

We report the discovery of two faint ultracool companions to the nearby (d~17.9 pc) young G2 V star HD 130948 (HR 5534, HIP 72567) using the Hokupa'a adaptive optics (AO) instrument mounted on the Gemini North 8 m telescope. Both objects have the same common proper motion as the primary star as seen over a 7 month baseline and have near-IR photometric colors that are consistent with an early L classification. Near-IR spectra taken with the NIRSPEC AO instrument on the Keck II telescope reveal K I lines, FeH, and H2O band heads. Based on these spectra, we determine that both objects have a spectral type of dL2 with an uncertainty of two spectral subclasses. The position of the new companions on the H-R diagram in comparison with theoretical models is consistent with the young age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity, lithium abundance, and fast rotation. HD 130948B and C likely constitute a pair of young contracting brown dwarfs with an orbital period of about 10 yr and will yield dynamical masses for L dwarfs in the near future. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (US), the Particle Physics and Astronomy Research Council (UK), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina).

Potter, D.; Martín, E. L.; Cushing, M. C.; Baudoz, P.; Brandner, W.; Guyon, O.; Neuhäuser, R.

2002-03-01

230

Chemical analysis of giant stars in the young open cluster NGC 3114  

NASA Astrophysics Data System (ADS)

Context. Open clusters are very useful targets for examining possible trends in galactocentric distance and age, especially when young and old open clusters are compared. Aims: We carried out a detailed spectroscopic analysis to derive the chemical composition of seven red giants in the young open cluster NGC 3114. Abundances of C, N, O, Li, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, and Nd were obtained, as well as the carbon isotopic ratio. Methods: The atmospheric parameters of the studied stars and their chemical abundances were determined using high-resolution optical spectroscopy. We employed the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code MOOG. The abundances of the light elements were derived using the spectral synthesis technique. Results: We found that NGC 3114 has a mean metallicity of [Fe/H] = -0.01 ± 0.03. The isochrone fit yielded a turn-off mass of 4.2 M?. The [N/C] ratio is in good agreement with the models predicted by first dredge-up. We found that two stars, HD 87479 and HD 304864, have high rotational velocities of 15.0 km s-1 and 11.0 km s-1; HD 87526 is a halo star and is not a member of NGC 3114. Conclusions: The carbon and nitrogen abundance in NGC 3114 agree with the field and cluster giants. The oxygen abundance in NGC 3114 is lower compared to the field giants. The [O/Fe] ratio is similar to the giants in young clusters. We detected sodium enrichment in the analyzed cluster giants. As far as the other elements are concerned, their [X/Fe] ratios follow the same trend seen in giants with the same metallicity. Based on observations made with the 2.2 m telescope at the European Southern Observatory (La Silla, Chile).Tables 2 and 5 are available in electronic form at http://www.aanda.org

Santrich, O. J. Katime; Pereira, C. B.; Drake, N. A.

2013-06-01

231

A general catalogue of molecular hydrogen emission-line objects (MHOs) in outflows from young stars  

NASA Astrophysics Data System (ADS)

We present a catalogue of Molecular Hydrogen emission-line Objects (MHOs) in outflows from young stars, most of which are deeply embedded. All objects are identified in the near-infrared lines of molecular hydrogen, all reside in the Milky Way, and all are associated with jets or molecular outflows. Objects in both low and high-mass star forming regions are included. This catalogue complements the existing database of Herbig-Haro objects; indeed, for completeness, HH objects that are detected in H2 emission are included in the MHO catalogue. http://www.jach.hawaii.edu/UKIRT/MHCat/ Tables A.1 to A.15 are also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/511/A24

Davis, C. J.; Gell, R.; Khanzadyan, T.; Smith, M. D.; Jenness, T.

2010-02-01

232

Pulsation of Pre-Main Sequence Stars in Young Open Clusters  

NASA Astrophysics Data System (ADS)

The aim of this proposal is to determine observationally the parameter space of the pre-main sequence instability strip. For that purpose we intend to obtain photometric timeseries with high time resolution and low noise level of the stars in young open clusters (IC 4996, NGC 6910 and NGC 6383) and to identify pre-main sequence pulsators. Several cluster members have the spectral types of interest (A-F) and lie between the birthline and the zero-age main sequence. Up to now the number of pre-main sequence pulsators is absolutely inadequate to determine reliably the hot and cool border of the according instability region. Its definition is indispensable for a better understanding of the internal structure and evolution of such stars.

Zwintz, Konstanze; Weiss, Werner W.

2001-08-01

233

Rotational Velocities of Post T Tauri Stars in Young Stellar Associations  

NASA Astrophysics Data System (ADS)

This contribution summarizes the preliminary results concerning vsini measurements for a sample of G-K solar type stars in the solar neighborhood based on observations made with the European Southern Observatory telescopes obtained from ESO/ST-ECF Science Archive Facility. We conducted measurements of rotational projected velocities for a sample of 33 young low mass post-T Tauri stars with ages between 10 and 30 Myr belonging to the nearby associations: Beta Pictoris Moving Group (BPMG), Upper Centaurus Lupus (UCL), Lower Centaurus Crux (LCC), and Tucana Horologium (THA). When compared with data of members of the older clusters, the Pleiades and the Hyades, our vsini measurements appear to be consistent with an increase of the rotation between the age of BPMG and THA in agreement with the spin up expected after the disk disappearance.

Rodríguez, J.; Pinzón, G.

2013-04-01

234

X-ray emission in Orion Nebula young stars (Flaccomio+, 2005)  

NASA Astrophysics Data System (ADS)

We investigate the spatial distribution of X-ray-emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP, Cat. ), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17'x17' field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. (1 data file).

Flaccomio, E.; Micela, G.; Sciortino, S.; Feigelson, E. D.; Herbst, W.; Favata, F.; Harnden, F. R. Jr; Vrtilek, S. D.

2006-07-01

235

Rotational modulation of X-ray emission in Orion Nebula young stars.  

NASA Astrophysics Data System (ADS)

We present the detection of rotational modulation of X-ray emission from the Orion Nebula Cluster (ONC) young stars. This result is obtained with the Chandra Orion Ultradeep Project (COUP) thanks to the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days. We apply the Lomb Periodogram method to the X-ray light curves; comparing the X-ray modulation periods with published rotation periods, we find that the two are statistically related. X-ray rotational modulation is often observed in stars with saturated activity levels, indicating a substantial inhomogeneity in the spatial distribution of active regions. Saturation is thus not due to the filling of the stellar surface with active regions.

Flaccomio, E.; Micela, G.; Sciortino, S.; Feigelson, E. D.; Herbst, W.; Favata, F.; Harnden, F. R., Jr.; Vrtilek, S. D.

236

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

237

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

238

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

SciTech Connect

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) = {minus}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. 22 refs.

Conti, P.S.; Fitzpatrick, E.L. (Joint Institute for Laboratory Astrophysics, Boulder, CO (USA) Princeton University Observatory, NJ (USA))

1991-05-01

239

Direct VLBI detection of the magnetosphere surrounding the young star S1 in Rho Ophiuchi  

NASA Technical Reports Server (NTRS)

VLBI 6-mm data are presently used to investigate the circularly polarized radio core previously identified around the young B3 star S1 in Rho Ophiuchi. The measured angular diameter and brightness temperature are found to be consistent with gyrosynchrotrom radiation emission from mildly relativistic electrons. A simple model based on a pole-on dipolar magnetic field of about 2 kG at the stellar surface suggests itself as consistent with the main observed features of the S1 magnetosphere; an important feature of the model is its taking the influence of the X-ray-emitting plasma into account. S1 may represent a new type of young stellar object, characterized by very extended magnetic fields.

Andre, Philippe; Phillips, Robert B.; Lestrade, Jean-Francois; Klein, Karl-Ludwig

1991-01-01

240

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

PubMed

The Solar System originated in a cloud of interstellar gas and dust. The dust is in the form of amorphous silicate particles and carbonaceous dust. The composition of cometary material, however, shows that a significant fraction of the amorphous silicate dust was transformed into crystalline form during the early evolution of the protosolar nebula. How and when this transformation happened has been a question of debate, with the main options being heating by the young Sun and shock heating. Here we report mid-infrared features in the outburst spectrum of the young Sun-like star EX Lupi that were not present in quiescence. We attribute them to crystalline forsterite. We conclude that the crystals were produced through 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. PMID:19444209

Abrahám, P; Juhász, A; Dullemond, C P; Kóspál, A; van Boekel, R; Bouwman, J; Henning, Th; Moór, A; Mosoni, L; Sicilia-Aguilar, A; Sipos, N

2009-05-14

241

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

242

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.

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

2005-01-01

243

TRIGGERED STAR FORMATION AND YOUNG STELLAR POPULATION IN BRIGHT-RIMMED CLOUD SFO 38  

SciTech Connect

We have investigated the young stellar population in and around SFO 38, one of the massive globules located in the northern part of the Galactic H II region IC 1396, using the Spitzer IRAC and MIPS observations (3.6-24 {mu}m), and followed up with ground-based optical photometric and spectroscopic observations. Based on the IRAC and MIPS colors and H{alpha} emission, we identify {approx}45 young stellar objects (Classes 0/I/II) and 13 probable pre-main-sequence candidates. We derive the spectral types (mostly K- and M-type stars), effective temperatures, and individual extinction of the relatively bright and optically visible Class II objects. Most of the Class II objects show variable H{alpha} emission as well as optical and near-infrared photometric variability, which confirm their 'youth'. Based on optical photometry and theoretical isochrones, we estimate the spread in stellar ages to be between 1 and 8 Myr with a median age of 3 Myr and a mass distribution of 0.3-2.2 M{sub sun} with a median value around 0.5 M{sub sun}. Using the width of the H{alpha} emission line measured at 10% peak intensity, we derive the mass accretion rates of individual objects to be between 10{sup -10} and 10{sup -8} M{sub sun} yr{sup -1}. From the continuum-subtracted H{alpha} line image, we find that the H{alpha} emission of the globule is not spatially symmetric with respect to the O-type ionizing star HD 206267, and the interstellar extinction toward the globule is also anomalous. We clearly detect an enhanced concentration of YSOs closer to the southern rim of SFO 38 and identify an evolutionary sequence of YSOs from the rim to the dense core of the cloud, with most of the Class II objects located at the bright rim. The YSOs appear to be aligned along two different directions toward the O6.5V type star HD 206267 and the B0V type star HD 206773. This is consistent with the Radiation Driven Implosion (RDI) model for triggered star formation. Further, the apparent speed of sequential star formation is consistent with the speed of propagation of shocks in dense globules as derived from numerical simulations of RDI.

Choudhury, Rumpa; Bhatt, H. C. [Indian Institute of Astrophysics, II Block, Koramangala, Bangalore 560 034 (India); Mookerjea, Bhaswati, E-mail: rumpa@iiap.res.i, E-mail: hcbhatt@iiap.res.i, E-mail: bhaswati@tifr.res.i [Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005 (India)

2010-07-10

244

STAR FORMATION AND YOUNG POPULATION OF THE H II COMPLEX Sh2-294  

SciTech Connect

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 {mu}m observed with the Spitzer Space Telescope Infrared Array Camera (IRAC), coupled with H{sub 2} (2.12 {mu}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 H{sub 2} 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{sub Sun }) YSOs; however, we also detected a massive YSO ({approx}9 M{sub Sun }) of Class I nature, embedded in a cloud of visual extinction of {approx}24 mag. Present analysis suggests that the Class I sources are indeed a younger population of the region relative to Class II sources (age {approx} 4.5 Multiplication-Sign 10{sup 6} 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 {approx}4 Multiplication-Sign 10{sup 6} yr B0 main-sequence star.

Samal, M. R.; Pandey, A. K.; Chauhan, N.; Jose, J. [Aryabhatta Research Institute of Observational Sciences, Nainital 263129 (India); Ojha, D. K. [Tata Institute of Fundamental Research, Mumbai 400005 (India); Pandey, B., E-mail: manash.samal@oamp.fr [Physics Department, D.S.B. Campus, Kumaun University, Nainital 263129 (India)

2012-08-10

245

Young stars in nearby early-type galaxies: SED fitting based on ultraviolet (UV) and optical imaging  

NASA Astrophysics Data System (ADS)

Recent studies from the Galaxy Evolution Explore (GALEX) ultraviolet (UV) data have demonstrated that the recent star formation is more common in early-type galaxies (ETGs) than we used to believe. The UV is one order of magnitude more sensitive than the optical to the presence of young stellar populations. The near-ultraviolet (NUV) lights of ETGs, especially, are used to reveal their residual star formation history. Here we used the GALEX UV data of 34 nearby early-type galaxies from the SAURON sample, all of which have optical data from MDM Observatory. At least 15% of the galaxies in this sample show blue UV-optical colours suggesting recent star formation (Jeong et al. 2009). These NUV blue galaxies are generally low velocity dispersion systems and change the slopes of scaling relations (colour-magnitude relations and fundamental planes) and increase the scatters. To quantify the amount of recent star formation in our sample, we assume two bursts of star formation, allowing us to constrain the age and mass fraction of the young component pixel by pixel (Jeong et al. 2007). The pixel-by-pixel SED fitting based on UV and optical imaging reveals that the mass fraction of young (< 1 Gyr old) stars in ETGs varies between 1 and 3% in the nearby universe (Jeong et al. in prep.). We will compare our results with the prediction from the hierarchical merger paradigm to understand the mechanism of low-level recent star formation observed in early-type galaxies.

Jeong, Hyunjin; Yi, Sukyoung K.; Bureau, Martin; Davies, Roger L.

2012-08-01

246

Young stars in ? Chamaleontis and their disks: disk evolution in sparse associations  

NASA Astrophysics Data System (ADS)

Context. The nearby young stellar association ? Cha has an estimated age of 3-5 Myr, making it an ideal laboratory to study the disk dissipation process and provide empirical constraints on the timescale of planet formation. Aims: We wish to complement existing optical and near-infrared data of the ? Cha association, which provide the stellar properties of its members, with mid-infrared data that probe the presence, geometry, and mineralogical composition of protoplanetary disks around individual stars. Methods: We combine the available literature data with our Spitzer/IRS spectroscopy and VLT/VISIR imaging data. We use proper motions to refine the membership of ? Cha. Masses and ages of individual stars are estimated by fitting model atmospheres to the optical and near-infrared photometry, followed by placement in the Hertzsprung-Russell diagram. The Spitzer/IRS spectra are analyzed using the two-layer temperature distribution spectral decomposition method. Results: Two stars previously identified as members, CXOU J120152.8 and 2MASS J12074597, have proper motions that are very different from those of the other stars. But other observations suggest that the two stars are still young and thus might still be related to ? Cha. HD 104237C is the lowest mass member of ? Cha with an estimated mass of ~13-15 Jupiter masses. The very low mass stars USNO-B120144.7 and 2MASS J12005517 show globally depleted spectral energy distributions, pointing at strong dust settling. 2MASS J12014343 may have a disk with a very specific inclination, where the central star is effectively screened by the cold outer parts of a flared disk, but the 10 ?m radiation of the warm inner disk can still reach us. We find that the disks in sparse stellar associations are dissipated more slowly than those in denser (cluster) environments. We detect C2H2 rovibrational band around 13.7 ?m on the IRS spectrum of USNO-B120144.7. We find strong signatures of grain growth and crystallization in all ? Cha members with 10 ?m features detected in their IRS spectra. We combine the dust properties derived in the ? Cha sample with those found using identical or similar methods in the MBM 12, Coronet, ? Cha associations, and in the cores-to-disks legacy program. We find that disks around low-mass young stars show a negative radial gradient in the mass-averaged grain size and mass fraction of crystalline silicates. A positive correlation exists between the mass-averaged grain sizes of amorphous silicates and the accretion rates if the latter is above ~10-9 M? yr-1, possibly indicating that those disks are sufficiently turbulent to prevent grains of several microns in size to sink into the disk interior. Based on observations performed at ESO's La Silla-Paranal observatory under programme 076.C-0470.

Fang, M.; van Boekel, R.; Bouwman, J.; Henning, Th.; Lawson, W. A.; Sicilia-Aguilar, A.

2013-01-01

247

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

248

Rotational Modulation of X-Ray Emission in Orion Nebula Young Stars  

NASA Astrophysics Data System (ADS)

We investigate the spatial distribution of X-ray-emitting plasma in a sample of young Orion Nebula Cluster stars by modulation of their X-ray light curves due to stellar rotation. The study, part of the Chandra Orion Ultradeep Project (COUP), is made possible by the exceptional length of the observation: 10 days of ACIS integration during a time span of 13 days, yielding a total of 1616 detected sources in the 17'×17' field of view. We here focus on a subsample of 233 X-ray-bright stars with known rotational periods. We search for X-ray modulation using the Lomb Normalized Periodogram method. X-ray modulation related to the rotation period is detected in at least 23 stars with periods between 2 and 12 days and relative amplitudes ranging from 20% to 70%. In 16 cases, the X-ray modulation period is similar to the stellar rotation period, while in 7 cases it is about half that value, possibly due to the presence of X-ray-emitting structures at opposite stellar longitudes. These results constitute the largest sample of low-mass stars in which X-ray rotational modulation has been observed. The detection of rotational modulation indicates that the X-ray-emitting regions are distributed inhomogeneneously in longitude and do not extend to distances significantly larger than the stellar radius. Modulation is observed in stars with saturated activity levels (LX/Lbol~10-3) showing that saturation is not due to the filling of the stellar surface with X-ray-emitting regions. Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138. Universe Division, Science Mission Directorate, NASA Headquarters.

Flaccomio, E.; Micela, G.; Sciortino, S.; Feigelson, E. D.; Herbst, W.; Favata, F.; Harnden, F. R., Jr.; Vrtilek, S. D.

2005-10-01

249

Eruption Variability  

NSDL National Science Digital Library

This resource provides information about variances in size and explosiveness of volcanoes that have erupted during the past 10,000 years. Included is a table showing that small eruptions are more frequent than larger ones. A feature of this site is the introduction of a scheme for estimating the magnitude of historic eruptions, called the Volcanic Explosivity Index (VEI). Historical eruptions can be assigned a VEI number on a scale of 0 to 8, using one or more of the criteria which include volume of ejecta, height of the eruptive column, qualitative descriptions, style of past activity, and height of spreading of the eruptive plume head. A chart shows an example for each VEI from 0 through 8 along with the plume height, volume, and eruption type classification. Each volcano name provides a link to detailed information about it.

Camp, Victor

250

Is the Young Star Cluster NGC 376 Dissolving in the Field of the Small Magellanic Cloud?  

NASA Astrophysics Data System (ADS)

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. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. These observations are associated with program 10248.

Sabbi, E.; Nota, A.; Tosi, M.; Smith, L. J.; Gallagher, J.; Cignoni, M.

2011-09-01

251

YOUNG STELLAR OBJECTS IN THE LARGE MAGELLANIC CLOUD STAR-FORMING REGION N206  

SciTech Connect

We present analysis of the energetic star-forming region Henize 206 (N206) located near the southern edge of the Large Magellanic Cloud (LMC) based on photometric data from the Spitzer Surveying the Agents of Galaxy Evolution (SAGE-LMC; IRAC 3.6, 4.5, 5.8, 8.0 {mu}m and MIPS 24 {mu}m), Infrared Survey Facility near-infrared survey (J, H, K{sub s}), and the Magellanic Clouds Photometric Survey (MCPS UBVI) covering a wavelength range of 0.36-24 {mu}m. Young stellar object (YSO) candidates are identified based upon their location in infrared color-magnitude space and classified by the shapes of their spectral energy distributions in comparison with a pre-computed grid of YSO models. We identify 116 YSO candidates: 102 are well characterized by the YSO models, predominately Stage I, and 14 may be multiple sources or young sources with transition disks. Careful examination of the individual sources and their surrounding environment allows us to identify a factor of {approx}14.5 more YSO candidates than have already been identified. The total mass of these well-fit YSO candidates is {approx}520 M{sub sun}. We calculate a current star formation rate of 0.27 x 10{sup -1} M{sub sun} yr{sup -1} kpc{sup -2}. The distribution of YSO candidates appears to follow shells of neutral material in the interstellar medium.

Romita, Krista Alexandra; Meixner, M.; Sewilo, M.; Shiao, B. [Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD (United States); Carlson, Lynn Redding [Johns Hopkins University, 3400 N. Charles St., Baltimore, MD (United States); Whitney, B. [Space Science Institute, Boulder, CO (United States); Babler, B.; Meade, M. [Department of Astronomy, University of Wisconsin, Madison, WI (United States); Indebetouw, R. [Department of Astronomy, University of Virginia, Charlottesville, VA (United States); Hora, J. L., E-mail: k.a.romita@gmail.co, E-mail: carlson@stsci.ed, E-mail: bwhitney@spacescience.or, E-mail: brian@sal.wisc.ed, E-mail: remy@virginia.ed, E-mail: jhora@cfa.harvard.ed [Harvard Smithsonian Center for Astrophysics, Cambridge, MA (United States)

2010-09-20

252

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

253

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

254

High contrast spectral imaging of sub-stellar companions around nearby young stars  

NASA Astrophysics Data System (ADS)

This thesis is a compilation of my work on OSIRIS, resolved spectroscopy of binary systems with low mass components, and high contrast imaging for the purpose of direct detection of extrasolar planets. I begin this publication with an overview of OSIRIS and a description of my contributions towards the development, commissioning, and support of this facility-class instrument. In short, OSIRIS is an innovative near-IR integral field spectrograph (IFS) that operates behind the Keck II adaptive optics (AO) system. Its IFS nature has proven very successful at accomplishing a wide variety of science objectives. As an OSIRIS team member, I have participated in multiple science programs throughout the commissioning process. Two closely separated binary systems are studied in detail-the new M/L binary system 2MASSJ17072343-0558249AB and the previously proposed planetary-mass candidate to the T-Tauri star GQ Lup. Resolved spectroscopy was obtained for each of the binary systems. The 2MASSJ17072343-0558249AB system probes the M/ L transition at a fixed metallicity and age. The extremely young GQ Lup system harbors a low-mass companion. As presented by the discoverers, the proposed planetary-mass companion challenged the current theoretical evolutionary models for low-mass objects and defied an explanation by the planet formation scenarios. OSIRIS imaging and spectroscopy suggest GQ Lup B is not of planetary-mass, but it is probably a brown dwarf that formed through cloud fragmentation. Finally, this thesis presents a Keck II adaptive optics (AO) search to explore the formation and evolution of non-stellar (brown dwarf and planetary) companions around nearby young stars. OSIRIS is used to suppress the bright halo and speckle pattern of the host star by employing various IFS techniques for high-contrast imaging. The sample is comprised of nearby (< 100 pc), young (<= 100 Myr) stars, which enables OSIRIS to detect companions at small separations (> 2 AU). The algorithms used for speckle suppression are relevant for the next generation of planet detection instruments, such as the Gemini Planet Imager (GPI) and NASA's Terrestrial Planet Finder Coronagraph (TPF-C).

McElwain, Michael William

255

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

256

OUTFLOWS AND YOUNG STARS IN ORION'S LARGE COMETARY CLOUDS L1622 AND L1634  

SciTech Connect

New observations of protostellar outflows associated with young stars in two of Orion's outlying cometary clouds, L1622 and L1634, are presented. The H{alpha} surface brightness of the bright rims are used to argue that both clouds are located at a distance of about 400 pc in the interior of the Orion superbubble where they are illuminated by Orion's massive stars. Spitzer IRAC and MIPS images reveal 28 candidate young stellar objects (YSOs). Combined with the 14 spectroscopically confirmed T Tauri stars, there are at least 34 YSOs in L1622. Narrow-band images have led to the identification of about a dozen shock complexes in L1622. At least six belong to a highly collimated externally irradiated, bipolar jet, HH 963, that is powered by a low-luminosity Class II YSO located outside the projected edge of the L1622 dark cloud. However, the sources of most shocks remain unclear. The Spitzer/IRAC images reveal a compact, highly obscured, S-symmetric outflow brightest in the 4.5 {mu}m images. A faint [S II] counter part, HH 962, is associated with the western end of this flow which appears to be powered by an obscured source in the L1622 cloud interior. The currently identified sample of YSOs implies a star formation efficiency of about 4% for L1622. The L1634 cloud contains nine YSOs and three outflows, including the well known HH 240/241 system. A new flow, HH 979, is powered by the embedded YSO IRS7 in L1634 and crosses the eastern lobe of the HH 240 outflow. Spitzer/IRAC images show 4.5 {mu}m emission indicating molecular shocks from the Herbig-Haro objects closest to IRS 7. A YSO embedded in a condensation located 5' north of HH 240/241 is identified in the Spitzer images. This source drives an irradiated outflow, HH 980, whose lobes emerge into the ionized environment of the Orion-Eridanus superbubble interior. The star formation efficiency of L1634 is estimated to be about 3%.

Bally, John [Department of Astrophysical and Planetary Sciences, University of Colorado, UCB 389 CASA, Boulder CO 80309-0389 (United States); Walawender, Josh; Reipurth, Bo [Institute for Astronomy, University of Hawaii, HI (United States); Megeath, S. Thomas [Department of Physics and Astronomy, University of Toledo, OH (United States)], E-mail: John.Bally@colorado.edu, E-mail: joshw@ifa.hawaii.edu, E-mail: reipurth@ifa.hawaii.edu, E-mail: megeath@physics.utoledo.edu

2009-04-15

257

Young stellar population and ongoing star formation in the H II complex Sh2-252  

NASA Astrophysics Data System (ADS)

In this paper, an extensive survey of the star-forming complex Sh2-252 has been undertaken with an aim to explore its hidden young stellar population as well as to understand the structure and star formation history for the first time. This complex is composed of five prominent embedded clusters associated with the subregions A, C, E, NGC 2175s and Teu 136. We used Two Micron All Sky Survey-near-infrared and Spitzer-Infrared Array Camera, Multiband Imaging Photometer for Spitzer photometry to identify and classify the young stellar objects (YSOs) by their infrared (IR) excess emission. Using the IR colour-colour criteria, we identified 577 YSOs, of which, 163 are Class I, 400 are Class II and 14 are transition disc YSOs, suggesting a moderately rich number of YSOs in this complex. Spatial distribution of the candidate YSOs shows that they are mostly clustered around the subregions in the western half of the complex, suggesting enhanced star formation activity towards its west. Using the spectral energy distribution and optical colour-magnitude diagram-based age analyses, we derived probable evolutionary status of the subregions of Sh2-252. Our analysis shows that the region A is the youngest (˜0.5 Myr), the regions B, C and E are of similar evolutionary stage (˜1-2 Myr) and the clusters NGC 2175s and Teu 136 are slightly evolved (˜2-3 Myr). Morphology of the region in the 1.1 mm map shows a semicircular shaped molecular shell composed of several clumps and YSOs bordering the western ionization front of Sh2-252. Our analyses suggest that next generation star formation is currently under way along this border and that possibly fragmentation of the matter collected during the expansion of the H II region as one of the major processes is responsible for such stars. We observed the densest concentration of YSOs (mostly Class I, ˜0.5 Myr) at the western outskirts of the complex, within a molecular clump associated with water and methanol masers and we suggest that it is indeed a site of cluster formation at a very early evolutionary stage, sandwiched between the two relatively evolved CH II regions A and B.

Jose, Jessy; Pandey, A. K.; Samal, M. R.; Ojha, D. K.; Ogura, K.; Kim, J. S.; Kobayashi, N.; Goyal, A.; Chauhan, N.; Eswaraiah, C.

2013-07-01

258

The Galactic Center Cloud G2—a Young Low-mass Star with a Stellar Wind  

NASA Astrophysics Data System (ADS)

We explore the possibility that the G2 gas cloud falling in toward SgrA* is the mass-loss envelope of a young T Tauri star. As the star plunges to smaller radius at 1000-6000 km s-1, a strong bow shock forms where the stellar wind is impacted by the hot X-ray emitting gas in the vicinity of SgrA*. For a stellar mass-loss rate of 4 × 10-8 M ? yr-1 and wind velocity 100 km s-1, the bow shock will have an emission measure (EM = n 2 vol) at a distance ~1016 cm, similar to that inferred from the IR emission lines. The ionization of the dense bow shock gas is potentially provided by collisional ionization at the shock front and cooling radiation (X-ray and UV) from the post shock gas. The former would predict a constant line flux as a function of distance from SgrA*, while the latter will have increasing emission at lesser distances. In this model, the star and its mass-loss wind should survive pericenter passage since the wind is likely launched at 0.2 AU and this is much less than the Roche radius at pericenter (~3 AU for a stellar mass of 2 M ?). In this model, the emission cloud will probably survive pericenter passage, discriminating this scenario from others.

Scoville, N.; Burkert, A.

2013-05-01

259

Exo-comet Detection in Debris Disks Around Young A-type Stars  

NASA Astrophysics Data System (ADS)

We present details of the successful search for comet-like bodies (i.e. exo-comets) in orbit around several nearby stars. These objects have been found in young stellar systems that are in the transitional stage of evolution between possession of a gaseous protoplanetary disk to that of a dust-rich debris disk. During this period it is thought that large planetesimals of ~ 1000 km diameter may cause dynamical perturbations in the population of smaller bodies (such as asteroids and comets), such that they are sent on highly eccentric orbits towards their parent star resulting in the liberation of large amounts of evaporating gas and dust. By observing the varying spectral absorption signature of the CaII K-line at 3933Å due to this liberated gas, we have been able to track the trajectory of these exo-comets over a time-frame of several nights as they approach (and sometimes pass around) the central star. The youngest debris disks (1 - 50 Myr) are thought to represent the last stage in the formation of planetary systems and they may resemble our solar system’s own debris disk at the time of the Late Heavy Bombardment when the terrestrial worlds were subject to frequent collisions with asteroids and comets. Collisions with water-rich comets from the outer regions of our solar system may have delivered water to thee Earth’s oceans.

Welsh, Barry; Montgomery, S. L.

2013-01-01

260

CLOSE STELLAR ENCOUNTERS IN YOUNG, SUBSTRUCTURED, DISSOLVING STAR CLUSTERS: STATISTICS AND EFFECTS ON PLANETARY SYSTEMS  

SciTech Connect

Both simulations and observations indicate that stars form in filamentary, hierarchically clustered associations, most of which disperse into their galactic field once feedback destroys their parent clouds. However, during their early evolution in these substructured environments, stars can undergo close encounters with one another that might have significant impacts on their protoplanetary disks or young planetary systems. We perform N-body simulations of the early evolution of dissolving, substructured clusters with a wide range of properties, with the aim of quantifying the expected number and orbital element distributions of encounters as a function of cluster properties. We show that the presence of substructure both boosts the encounter rate and modifies the distribution of encounter velocities compared to what would be expected for a dynamically relaxed cluster. However, the boost only lasts for a dynamical time, and as a result the overall number of encounters expected remains low enough that gravitational stripping is unlikely to be a significant effect for the vast majority of star-forming environments in the Galaxy. We briefly discuss the implications of this result for models of the origin of the solar system, and of free-floating planets. We also provide tabulated encounter rates and orbital element distributions suitable for inclusion in population synthesis models of planet formation in a clustered environment.

Craig, Jonathan; Krumholz, Mark R., E-mail: krumholz@ucolick.org [Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)

2013-06-01

261

A New Method for Measuring Metallicities of Young Super Star Clusters  

NASA Astrophysics Data System (ADS)

We demonstrate how the metallicities of young super star clusters (SSC) can be measured using novel spectroscopic techniques in the J-band. The near-infrared flux of SSCs older than ~6 Myr is dominated by tens to hundreds of red supergiant stars. Our technique is designed to harness the integrated light of that population and produces accurate metallicities for new observations in galaxies above (M83) and below (NGC 6946) solar metallicity. In M83 we find [Z] = +0.28 ± 0.14 dex using a moderate resolution (R ~ 3500) J-band spectrum and in NGC 6496 we report [Z] = -0.32 ± 0.20 dex from a low resolution spectrum of R ~ 1800. Recently commissioned low resolution multiplexed spectrographs on the Very Large Telescope (KMOS) and Keck (MOSFIRE) will allow accurate measurements of SSC metallicities across the disks of star-forming galaxies up to distances of 70 Mpc with single night observation campaigns using the method presented in this paper.

Gazak, J. Zachary; Davies, Ben; Bastian, Nate; Kudritzki, Rolf; Bergemann, Maria; Plez, Bertrand; Evans, Chris; Patrick, Lee; Bresolin, Fabio; Schinnerer, Eva

2014-06-01

262

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

263

3-D Structure of Outflows from Young Stars: GMOS IFU Observations of HH 32  

NASA Astrophysics Data System (ADS)

To better understand the 3-D physics and kinematics of collimated outflows from young stars, we have observed two knots in the HH 32 outflow with the GMOS Integral Field Unit (IFU) at the The Frederick C. Gillett Gemini Telescope. These observations were obtained for System Verification of the dithered IFU observations of a purely emission line object. HH 32 is a collimated Herbig-Haro outflow from the active young binary AS 353. In one spectral setting we were easily able to detect the H alpha, [SII], [NII] and [OI] emission lines in the spectra of knots A and B. In this poster we present the 6275-6785 Angstrom spectra of HH 32 averaged over the A and B knots, images of the knots in each emisson line, and several images of the H alpha emission with velocities ranging from -15 km/s to 415km/s. These observations reveal a complicated 3-D velocity structure that could not be discerned with longslit spectroscopy alone, and thus demonstrate the success of observing outflows from active young stars with optical integral field technology. We will be collaborating closely with with theorists in this field to intepret this data within the confines of current HH flow excitation and kinematic models. This study is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil), and CONICET (Argentina)

Beck, T. L.; Aspin, C.

2002-12-01

264

A Survey of Classical and Weak-line T Tauri Stars in the Young Cluster NGC 2264  

Microsoft Academic Search

We present initial results from an H-alpha and X-ray emission survey of the young cluster NGC 2264 in the Mon OB1 association. Approximately 450 emission H-alpha stars were identified in a 25'X40' field roughly centered between the O7V multiple star S Mon and the Cone Nebula. The H-alpha emission survey was carried out using the wide-field grism spectrograph on the

S. E. Dahm; T. Simon; E. David

2003-01-01

265

The Massive Young Star-Forming Complex Study in Infrared and X-Ray: Mid-infrared Observations and Catalogs  

NASA Astrophysics Data System (ADS)

Spitzer IRAC observations and stellar photometric catalogs are presented for the Massive Young Star-Forming Complex Study in the Infrared and X-ray (MYStIX). MYStIX is a multiwavelength census of young stellar members of 20 nearby (d < 4 kpc), Galactic, star-forming regions that contain at least one O star. All regions have data available from the Spitzer Space Telescope consisting of GLIMPSE or other published catalogs for 11 regions and results of our own photometric analysis of archival data for the remaining 9 regions. This paper seeks to construct deep and reliable catalogs of sources from the Spitzer images. Mid-infrared study of these regions faces challenges of crowding and high nebulosity. Our new catalogs typically contain fainter sources than existing Spitzer studies, which improves the match rate to Chandra X-ray sources that are likely to be young stars, but increases the possibility of spurious point-source detections, especially peaks in the nebulosity. IRAC color-color diagrams help distinguish spurious detections of nebular polycyclic aromatic hydrocarbon emission from the infrared excess associated with dusty disks around young stars. The distributions of sources on the mid-infrared color-magnitude and color-color diagrams reflect differences between MYStIX regions, including astrophysical effects such as stellar ages and disk evolution.

Kuhn, Michael A.; Povich, Matthew S.; Luhman, Kevin L.; Getman, Konstantin V.; Busk, Heather A.; Feigelson, Eric D.

2013-12-01

266

Puzzling wind properties of young massive stars in SMC-N81  

NASA Astrophysics Data System (ADS)

We present a quantitative study of massive stars in the High Excitation Blob N81, a compact star forming region in the SMC. The stellar content was resolved by HST, and STIS was used to obtain medium resolution spectra. The qualitative analysis of the stellar properties presented in Heydari-Malayeri et al. (\\cite{papI}) is extended using non-LTE spherically extended atmosphere models including line-blanketing computed with the code CMFGEN (Hillier & Miller \\cite{hm98}), and the wind properties are investigated. The main results are the following: The SMC-N81 components are young (˜0-4 Myrs) O stars with effective temperatures compatible with medium to late subtypes and with luminosities lower than those of average Galactic O dwarfs, rendering them possible ZAMS candidates. The winds are extremely weak: with values of the order of 10-8/10-9 M? yr-1, the mass loss rates are lower than observed so far for Galactic dwarfs. Only the recent study of SMC stars by Bouret et al. (\\cite{jc03}) show the same trend. The modified wind momenta (\\dot{M} v?\\sqrt{R}) are also 1 to 2 orders of magnitude lower than observed for Galactic stars. Both the mass loss rates and the modified wind momenta are lower than the predictions of the most recent hydrodynamical models. The accuracy of the UV based mass loss rate determination, relying in particular on the predicted ionisation fractions, are carefully examined. We find that \\dot{M} could be underestimated by a factor of up to 10. Even in this unlikely case, the above conclusions remain valid qualitatively. The reasons for such weak winds are investigated with special emphasis on the modified wind momenta: There may be a break-down of the wind momentum-luminosity relation (WLR) for dwarf stars at low luminosity (log L/L?? 5.5). However, reasons for such a breakdown remain unknown. The slope of the WLR may be steeper at low metallicity. This is predicted by the radiation driven wind theory, but the current hydrodynamical simulations do not show any change of the slope at SMC metallicity. Moreover, there are indications that some Galactic objects have wind momenta similar to those of the SMC stars. Decoupling may take place in the atmosphere of the SMC-N81 stars, leading to multicomponent winds. However, various tests indicate that this is not likely to be the case. The origin of the weakness of the wind observed in the SMC-N81 stars remains unknown. We suggest that this weakness may be linked with the youth of these stars and represents possibly the onset of stellar winds in recently formed massive stars. Appendix A is only available in electronic form at http://www.edpsciences.org

Martins, F.; Schaerer, D.; Hillier, D. J.; Heydari-Malayeri, M.

2004-06-01

267

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

268

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

NASA Astrophysics Data System (ADS)

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

Dutkevitch, Diane

1995-05-01

269

Observed Luminosity Spread in Young Clusters and FU Ori Stars: A Unified Picture  

NASA Astrophysics Data System (ADS)

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 Mi ~ 1 M 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 10-5 M ? yr-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 ~1-5 M 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.; Vorobyov, E.; Chabrier, G.

2012-09-01

270

Elemental abundances of low-mass stars in nearby young associations: AB Doradus, Carina Near and Ursa Major  

NASA Astrophysics Data System (ADS)

We present stellar parameters and abundances of 11 elements (Li, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni and Zn) of 13 F6-K2 main-sequence stars in the young groups AB Doradus, Carina Near and Ursa Major. The exoplanet-host star ? Horologii is also analysed. The three young associations have lithium abundance consistent with their age. All other elements show solar abundances. The three groups are characterized by a small scatter in all abundances, with mean [Fe/H] values of 0.10 (? = 0.03), 0.08 (? = 0.05) and 0.01 (? = 0.03) dex for AB Doradus, Carina Near and Ursa Major, respectively. The distribution of elemental abundances appears congruent with the chemical pattern of the Galactic thin disc in the solar vicinity, as found for other young groups. This means that the metallicity distribution of nearby young stars, targets of direct-imaging planet-search surveys, is different from that of old, field solar-type stars, i.e. the typical targets of radial velocity surveys. The young planet-host star ? Horologii shows a lithium abundance lower than that found for the young association members. It is found to have a slightly super-solar iron abundance ([Fe/H] = 0.16 ± 0.09), while all [X/Fe] ratios are similar to the solar values. Its elemental abundances are close to those of the Hyades cluster derived from the literature, which seems to reinforce the idea of a possible common origin with the primordial cluster. Based on observations performed with European Southern Observatory (ESO) telescopes [programme IDs: 70.D-0081(A), 082.A-9007(A), 083.A-9011(B), 084.A-9011(B)].

Biazzo, K.; D'Orazi, V.; Desidera, S.; Covino, E.; Alcalá, J. M.; Zusi, M.

2012-12-01

271

Overview of the Massive Young Star-Forming Complex Study in Infrared and X-Ray (MYStIX) Project  

NASA Astrophysics Data System (ADS)

The Massive Young Star-Forming Complex Study in Infrared and X-ray (MYStIX) seeks to characterize 20 OB-dominated young clusters and their environs at distances d <= 4 kpc using imaging detectors on the Chandra X-ray Observatory, Spitzer Space Telescope, and the United Kingdom InfraRed Telescope. The observational goals are to construct catalogs of star-forming complex stellar members with well-defined criteria and maps of nebular gas (particularly of hot X-ray-emitting plasma) and dust. A catalog of MYStIX Probable Complex Members with several hundred OB stars and 31,784 low-mass pre-main sequence stars is assembled. This sample and related data products will be used to seek new empirical constraints on theoretical models of cluster formation and dynamics, mass segregation, OB star formation, star formation triggering on the periphery of H II regions, and the survivability of protoplanetary disks in H II regions. This paper gives an introduction and overview of the project, covering the data analysis methodology and application to two star-forming regions: NGC 2264 and the Trifid Nebula.

Feigelson, Eric D.; Townsley, Leisa K.; Broos, Patrick S.; Busk, Heather A.; Getman, Konstantin V.; King, Robert R.; Kuhn, Michael A.; Naylor, Tim; Povich, Matthew S.; Baddeley, Adrian; Bate, Matthew R.; Indebetouw, Remy; Luhman, Kevin L.; McCaughrean, Mark J.; Pittard, Julian M.; Pudritz, Ralph E.; Sills, Alison; Song, Yong; Wadsley, James

2013-12-01

272

Photospheric and chromospheric activity on the young solar-type star HD 171488 (V889 Herculis)  

NASA Astrophysics Data System (ADS)

Aims: We present the results of contemporaneous spectroscopic and photometric monitoring of the young solar-type star HD 171488 (Prot ? 1.337 days) aimed at studying surface inhomogeneities at both photospheric and chromospheric levels. Methods: Echelle FOCES spectra (R ? 40 000) and Johnson BV photometry have been performed in August 2006, with a good coverage of rotational phases. Spectral type, rotational velocity, metalicity, and gravity were determined with a code developed by us (Rotfit) and a library of high-resolution spectra of slowly-rotating reference stars. The metalicity was measured from the analysis of iron lines with the Moog code. The spectral subtraction technique was applied to the most relevant chromospheric diagnostics included in the FOCES spectral range, namely Ca II IRT, H?, He I D3, H?, and Ca II H&K lines. Results: A simple model with two large high-latitude spots is sufficient to reproduce the B and V light curves as well as the radial velocity modulation if a temperature difference between photosphere and spots of about 1500 K is used. A Doppler-imaging analysis of photospheric lines basically confirms a similar spot distribution. With the help of an analogous geometric two-spot model, we are able to reproduce the observed modulations in the residual chromospheric emissions adopting different values of ratios between the flux of plages and the quiet chromosphere (about 5 for H? and 3 for Ca II diagnostics). Facular regions of solar type appear to be the main responsible features for the modulations of chromospheric diagnostics. Both the spot/plage model and the cross-correlation between the light curve and the chromospheric line fluxes display a significant lead effect of plages with respect to spots (from 20° to 40° in longitude), as already observed in some active solar-type stars and RS CVn systems. Conclusions: The contemporaneous monitoring of photospheric and chromospheric diagnostics in the young and rapidly rotating solar-type star HD 171488 allowed us to detect active regions which have nearly the same location at both atmospheric layers, with plages slightly leading spots in longitudes. These active regions are similar to the solar ones in some respects, because the spot temperature is close to that of sunspot umbrae and the plage flux-contrast is consistent with the average solar values. The main differences with respect to the Sun are larger sizes and higher latitudes. Based on observations collected at Calar Alto Astronomical Observatory (Spain) and Catania Astrophysical Observatory (Italy).Table 5 and Fig. 7 are only available in electronic form at http://www.aanda.org

Frasca, A.; Biazzo, K.; K?vári, Zs.; Marilli, E.; Çak?rl?, Ö.

2010-07-01

273

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. 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^{+21}_{-20} M Jup and 55^{+20}_{-19} M 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 ? stars can have giant planets greater than 4 M 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 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 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.; Wahhaj, Zahed; Biller, Beth A.; Hayward, Thomas L.; Close, Laird M.; Males, Jared R.; Skemer, Andrew J.; Chun, Mark; Ftaclas, Christ; Alencar, Silvia H. P.; Artymowicz, Pawel; Boss, Alan; Clarke, Fraser; de Gouveia Dal Pino, Elisabete; Gregorio-Hetem, Jane; Hartung, Markus; Ida, Shigeru; Kuchner, Marc; Lin, Douglas N. C.; Reid, I. Neill; Shkolnik, Evgenya L.; Tecza, Matthias; Thatte, Niranjan; Toomey, Douglas W.

2013-10-01

274

Topics in theoretical astrophysics: Precession of warped disks, oscillations of presupernova stars, and thermal evolution and nucleosynthesis of young neutron stars  

NASA Astrophysics Data System (ADS)

This thesis consists of three parts. In the first part, we study the magnetically driven precession of warped disks. An accretion disk around a rotating magnetized star is subjected to the magnetic torques that induce warping and precession of the disk. We study the global hydrodynamical warping/ precession modes of the disk under the combined influences of the magnetic torques, relativistic frame dragging, and the classical precession due to oblateness of the neutron star. We apply our analysis to two types of accreting systems: low-mass X-ray binaries (LMXBs) and accreting X-ray pulsars. We argue that some features of low-frequency quasi-periodic oscillations (QPOs) in LMXBs and milli-Hertz QPOs in accreting X-ray pulsars can be explained by the magnetically driven precession of warped disks. The second part is related to the hydrodynamically-driven mechanism for asymmetric supernova explosions/neutron star kicks. We explore the possibility that the gravity modes in the core of a presupernova star may be amplified in the silicon burning shell to produce the global asymmetric perturbations that lead to an asymmetric supernova explosion. By performing a linear analysis of the oscillations in the cores of presupernova stars, we estimate the growth/ damping rates of the modes. We find that most of the modes are damping modes with a few exceptions. We also find that, even for a growing mode, the timescale of mode growth is much longer than the remaining time before the core collapse. We conclude that the gravity modes in a presupernova core cannot provide the global asymmetric perturbations that lead to an asymmetric supernova explosion. In the last part, we attempt to predict the innate chemical composition of a neutron star atmosphere. There has been great progress in X-ray observations and now thermal radiation from neutron stars is being studied in detail. There has also been significant progress in modeling thermal spectra from neutron stars. However, the unknown chemical composition of the neutron star surface is assumed in these models. Our goal is to predict the chemical composition of a neutron star atmosphere by evolving its thermal structure and the chemical composition from the earliest possible time. We study necessary steps to achieve this goal. We study models for a static atmosphere of a young neutron star, cooling of the bulk of a young neutron star, the nuclear statistical equilibrium abundances, nucleosynthesis, and the possible role of diffusion.

Shirakawa, Akiko

275

On the Equivalent Effective Temperatures of Massive Young Star Clusters: The Case of NGC 595  

NASA Astrophysics Data System (ADS)

The softness parameter is based on the relative intensity of several optical emission lines emitted by the gas ionized by young massive star clusters and can be used to derive the equivalent effective temperature (T_*) in those objects whose stellar population cannot be resolved. This method has several uncertainties due to the disagreement between different synthesis model atmospheres but it is robust to study the relative variations between objects. Following the 2D photoionization models of the giant Hii region NGC 595 (Pérez-Montero et al. 2011) we show that the determination of T_* with the Eta parameter is also robust in different regions of a same object with large variations in the geometry of the gas and in the dust-to-gas ratio.

Pérez-Montero, E.; Vílchez, J. M.; Relaño, M.; Monreal-Ibero, A.

276

High-resolution CO observations of young low-mass stars  

SciTech Connect

Through the use of the Owens Valley Millimeter Interferometer, CO(1-0) emission near very young low-mass stars was observed. The data consist of a snapshot visibility survey of 25 low-luminosity IRAS sources associated with dense molecular cores and full aperture synthesis maps of two cores, L1262 and L1681B. The two sources mapped show spatially compact low-velocity outflows. The momentum flux in the compact outflows is comparable to that powering larger scale low-luminosity outflows. Extrapolating from the results of the snapshot survey, at least 64 percent and perhaps all dense cores with embedded infrared sources have outflows. L1681B contains an unresolved source with velocity extent and emission asymmetries which are consistent with gravitational infall. 24 refs.

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

1989-05-01

277

Search for Wide Planetary-Mass Companions in Young Star-Forming Regions with UKIDSS and Pan-STARRS  

NASA Astrophysics Data System (ADS)

Over the past decade, planetary-mass (<15 MJup) companions have been discovered in very wide orbits (>100 AU) around young stars. It is unclear whether these objects formed like planets or like stars. If these are planets, then modifications to core accretion or disk instability models are needed to allow formation at such wide orbits, or planet scattering must be an important mechanism. On the other hand, if these objects formed like stars, we need to understand the frequency of these extremely low mass ratio binary companions which challenge brown dwarf formation models. Regardless of their origins, these wide companions are easier to observe than close-in planets and can be used as benchmarks to understand the properties of young planets. We have combined optical and NIR photometry from UKIDSS and Pan-STARRS-1 to search the young star-forming region of Upper Scorpius and Taurus for new planetary-mass objects, going ?3 mag deeper than previous work with 2MASS. We identified several candidates with very wide separations (?400-4000 AU) from known members using a combination of color selection and spectral energy distribution (SED) fitting to templates of known low-mass stars and brown dwarfs. Furthermore, we have obtained followup NIR spectra of several Upper Scorpius candidates to spectroscopically identify three new wide very low-mass companions (?15-25 MJup spectral type of M8-L0).

Aller, Kimberly M.; Kraus, A. L.; Liu, M. C.; Bowler, B. P.

2013-01-01

278

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

279

GIANO-TNG spectroscopy of red supergiants in the young star cluster RSGC2  

NASA Astrophysics Data System (ADS)

Aims: The inner disk of the Galaxy has a number of young star clusters dominated by red supergiants that are heavily obscured by dust extinction and observable only at infrared wavelengths. These clusters are important tracers of the recent star formation and chemical enrichment history in the inner Galaxy. Methods: During the technical commissioning and as a first science verification of the GIANO spectrograph at the Telescopio Nazionale Galileo, we secured high-resolution (R ? 50 000) near-infrared spectra of three red supergiants in the young Scutum cluster RSGC2. Results: Taking advantage of the full YJHK spectral coverage of GIANO in a single exposure, we were able to identify several tens of atomic and molecular lines suitable for chemical abundance determinations. By means of spectral synthesis and line equivalent width measurements, we obtained abundances of Fe and other iron-peak elements such as V, Cr, Ni, of alpha (O, Mg, Si, Ca and Ti) and other light elements (C, N, Na, Al, K, Sc), and of some s-process elements (Y, Sr). We found iron abundances between half and one third solar and solar-scaled [X/Fe] abundance patterns of iron-peak, alpha and most of the light elements, consistent with a thin-disk chemistry. We found a depletion of [C/Fe] and enhancement of [N/Fe], consistent with CN burning, and low 12C/13C abundance ratios (between 9 and 11), requiring extra-mixing processes in the stellar interiors during the post-main-sequence evolution. Finally, we found a slight [Sr/Fe] enhancement and a slight [Y/Fe] depletion (by a factor of ?2), with respect to solar. Table 3 is available in electronic form at http://www.aanda.org

Origlia, L.; Oliva, E.; Maiolino, R.; Mucciarelli, A.; Baffa, C.; Biliotti, V.; Bruno, P.; Falcini, G.; Gavriousev, V.; Ghinassi, F.; Giani, E.; Gonzalez, M.; Leone, F.; Lodi, M.; Massi, F.; Montegriffo, P.; Mochi, I.; Pedani, M.; Rossetti, E.; Scuderi, S.; Sozzi, M.; Tozzi, A.

2013-12-01

280

Revealing the full young stellar population in the Carina Nebula, the nearest laboratory of massive star feedback, with VISTA  

NASA Astrophysics Data System (ADS)

The Carina Nebula (NGC 3372) represents one of the most massive star forming regions in our Galaxy. With a distance of 2.3 kpc, it has the most extreme stellar population within a few kpc of the sun (at least 65 O-type stars). It is our best connection between the nearby star forming regions like the Orion Nebula and the even larger and extremer, but more distant regions like 30 Doradus in the Large Magellanic Cloud. Therefore it is a unique target and our richest nearby laboratory for detailed studies of violent massive star formation and its resulting feedback effects of cloud dispersal and triggered star formation. Our recent Herschel far-infrared survey of the Carina Nebula showed that the cloud complex extends over some 2 × 2.5 deg on the sky. Most of the recent investigations of the Carina Nebula had, however, been focused on the central, ? 1 square-degree area of the region, leaving the periphery of the cloud complex poorly studied. In order to solve this problem and to allow a characterization of the young stars throughout the entire extent of the complex, we have used the ESO Visible and Infrared Survey Telescope for Astronomy (VISTA) to map a ˜2 × 3 deg area around the Carina Nebula in the near-infrared J-, H-, Ks bands. Our NIR survey is large enough to cover the full spatial extent of the Carina Nebula complex and is deep enough to detect all young stars down to masses of 0.1 Msun through extinctions of at least Av = 10 mag. We detected in more than ˜ 4 million individual point sources. The data has a typical completeness limit of J ? 18, H ? 18, and Ks ? 17. In combination with a recent Chandra X-ray survey, Spitzer-IRAC, and Herschel observations we have now a sample of data, which reaches from X-ray to the FIR. It will allow us to distinguish between young stars and background contaminating objects and it will allow the identification and characterization of all X-ray selected young stars and the embedded young stellar objects revealed by Herschel.

Zeidler, Peter; Preibisch, Thomas; Ratzka, Thorsten; Gaczkowski, Benjamin; Roccatagliata, Veronica

2013-07-01

281

X-ray Emission From Young Supernovae as a Probe of their Progenitor Stars  

NASA Astrophysics Data System (ADS)

After several decades of study, the progenitor stars of supernovae (SNe) have still proven difficult to identify. The identification of progenitors has generally been the purview of optical astronomy, aided by stellar evolution models. But observations at other wavelengths can provide strong clues about the progenitors.We have aggregated together data available in the literature, or analysed by us, to compute the X-ray lightcurves of almost all young SNe. We use these, coupled with analytical and numerical simulations, to explore the various SN types, investigate SN expansion, explore the characteristics of the medium into which SNe are expanding, and examine the implications for their progenitors. We show that the low X-ray luminosity of IIPs sets a limit on the mass-loss rate, and thereby constrains the maximum initial mass of a red supergiant star which can become a Type IIP progenitor to be lower than 19 solar masses. We discuss how current stellar evolution models relate to the X-ray emission from various types of SNe, and where discrepancies appear to arise between observations and theory.

Dwarkadas, Vikram

2014-08-01

282

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

283

Discovery of Young, Isolated Planetary Mass Objects in the ? Orionis Star Cluster  

NASA Astrophysics Data System (ADS)

We present the discovery by optical and near-infrared imaging of an extremely red, low-luminosity population of isolated objects in the young, nearby stellar cluster around the multiple, massive star ? Orionis. The proximity (352 parsecs), youth (1 million to 5 million years), and low internal extinction make this cluster an ideal site to explore the substellar domain from the hydrogen mass limit down to a few Jupiter masses. Optical and near-infrared low-resolution spectroscopy of three of these objects confirms the very cool spectral energy distribution (atmospheric effective temperatures of 1700 to 2200 kelvin) expected for cluster members with masses in the range 5 to 15 times that of Jupiter. Like the planets of the solar system, these objects are unable to sustain stable nuclear burning in their interiors, but in contrast they are not bound to stars. This new kind of isolated giant planet, which apparently forms on time scales of less than a few million years, offers a challenge to our understanding of the formation processes of planetary mass objects.

Zapatero Osorio, M. R.; Béjar, V. J. S.; Martín, E. L.; Rebolo, R.; Barrado y Navascués, D.; Bailer-Jones, C. A. L.; Mundt, R.

2000-10-01

284

Does the mass distribution in discs influence encounter-induced losses in young star clusters?  

NASA Astrophysics Data System (ADS)

Context. One mechanism for the external destruction of protoplanetary discs in young dense clusters is tidal disruption during the flyby of another cluster member. The degree of mass loss in such an encounter depends, among other parameters, on the distribution of the material within the disc. Previous work showed that this is especially so in encounters that truncate large parts of the outer disc. The expectation is that the number of completely destroyed discs in a cluster also depends on the mass distribution within the discs. Aims: Here we test this hypothesis by determining the influence of encounters on the disc fraction and average disc mass in clusters of various stellar densities for different mass distributions in the discs. Methods: This is done by performing nbody6simulations of a variety of cluster environments, where we track the encounter dynamics and determine the mass loss due to these encounters for different disc-mass distributions. Results: We find that although the disc-mass distribution has a significant impact on the disc losses for specific star-disc encounters, the overall disc frequency generally remains unaffected. The reason is that in single encounters the dependence on the mass distribution is strongest if both stars have very different masses. Such encounters are rather infrequent in sparse clusters. In dense clusters these encounters are more common; however, here the disc frequency is largely determined by encounters between low-mass stars such that the overall disc frequency does not change significantly. Conclusions: For tidal disruption the disc destruction in clusters is fairly independent of the actual distribution of the material in the disc. The all determining factor remains the cluster density.

Steinhausen, Manuel; Pfalzner, Susanne

2014-05-01

285

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

286

How Do Astronomers Know That? Educating Teachers, Students & the Public on HOW You Discover Young Stars  

NASA Astrophysics Data System (ADS)

Every day amazing astronomical facts are taught to thousands of curious people. Students learn them in the classroom. Museum visitors hear them in a Planetarium show or lecture. When it’s time for questions, many intuitively ask, “how do you know that?” NITARP helps close this gap in astronomy education. NITARP stands for NASA/IPAC Teacher Archive Research Program. NITARP brings together an astronomer with a small group of teachers and students to do real astronomical research. After the year long program is completed, the education and experiences gained the teachers are brought back to their classrooms and museums across America. Our NITARP group researched apparent infrared (IR) excesses to identify Young Stellar Objects (YSOs). Our science results are presented in a companion paper, Novatne et al, at this AAS conference. We concentrated our search in the Bright Rimmed Cloud (BRC) 27, located in the constellation Canis Major. Our main focus was to use data from the Wide-field Infrared Survey Explorer (WISE), along with other archived infrared data such as Spitzer and 2MASS. Thus, our NITARP group was called C-WAYS—standing for Cool, WISE and Young Stars. In this poster, we present our educational plan to connect real science by astronomers to educators, students, and ultimately our communities.

Bonadurer, Robert; Piper, M.; French, D.; Barge, J.; Novatne, L. J.; Rebull, L. M.; Ali, B.; Laher, R.; Armstrong, J.

2013-01-01

287

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

288

A DEEP CHANDRA X-RAY SPECTRUM OF THE ACCRETING YOUNG STAR TW HYDRAE  

SciTech Connect

We present X-ray spectral analysis of the accreting young star TW Hydrae from a 489 ks observation using the Chandra High Energy Transmission Grating. The spectrum provides a rich set of diagnostics for electron temperature T{sub e} , electron density N{sub e} , hydrogen column density N{sub H} , relative elemental abundances, and velocities, and reveals its source in three distinct regions of the stellar atmosphere: the stellar corona, the accretion shock, and a very large extended volume of warm postshock plasma. The presence of Mg XII, Si XIII, and Si XIV emission lines in the spectrum requires coronal structures at {approx}10 MK. Lower temperature lines (e.g., from O VIII, Ne IX, and Mg XI) formed at 2.5 MK appear more consistent with emission from an accretion shock. He-like Ne IX line ratio diagnostics indicate that T{sub e} = 2.50 +- 0.25 MK and N{sub e} = 3.0 +- 0.2 x 10{sup 12} cm{sup -3} in the shock. These values agree well with standard magnetic accretion models. However, the Chandra observations significantly diverge from current model predictions for the postshock plasma. This gas is expected to cool radiatively, producing O VII as it flows into an increasingly dense stellar atmosphere. Surprisingly, O VII indicates N{sub e} = 5.7{sup +4.4}{sub -1.2} x 10{sup 11} cm{sup -3}, 5 times lower than N{sub e} in the accretion shock itself and {approx}7 times lower than the model prediction. We estimate that the postshock region producing O VII has roughly 300 times larger volume and 30 times more emitting mass than the shock itself. Apparently, the shocked plasma heats the surrounding stellar atmosphere to soft X-ray emitting temperatures and supplies this material to nearby large magnetic structures-which may be closed magnetic loops or open magnetic field leading to mass outflow. Our model explains the soft X-ray excess found in many accreting systems as well as the failure to observe high N{sub e} signatures in some stars. Such accretion-fed coronae may be ubiquitous in the atmospheres of accreting young stars.

Brickhouse, N. S.; Cranmer, S. R.; Dupree, A. K.; Luna, G. J. M.; Wolk, S. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)

2010-02-20

289

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

290

Young Galaxy Surrounded by Material Needed to Make Stars, VLA Reveals  

NASA Astrophysics Data System (ADS)

Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope have discovered a massive reservoir of cold gas from which a primeval galaxy formed its first stars. Looking more than 12 billion years into the past, the scientists found that the young galaxy experiencing a "burst" of star formation was surrounded by enough cold molecular gas to make 100 billion suns. Optical and Radio Images of APM 08279+5255 at About the Same Scale "This is the first time anyone has seen the massive reservoir of cold gas required for these incredible 'starbursts' to produce a galaxy," said Chris Carilli, an astronomer at the NSF's National Radio Astronomy Observatory (NRAO) in Socorro, NM. "There is much more gas here than we anticipated," Carilli added. The research team was led by Padeli Papadoupoulos of Leiden Observatory in the Netherlands and also included Rob Ivison of University College London and Geraint Lewis of the Anglo-Australian Observatory in Australia. The scientists reported their findings in the January 4 edition of the journal Nature. The astronomers found the gas when studying a quasar called APM 08279+5255, discovered in 1998. Observations with optical and infrared telescopes revealed that the quasar, a young galaxy with a voracious black hole at its center, was forming new stars rapidly in a starburst. At a distance of more than 12 billion light-years, the quasar is seen as it was more than 12 billion years ago, just a billion or so years after the Big Bang. "This thing is at the edge of the dark ages," before the first stars in the universe were born, said Carilli. The year after its discovery, APM 08279+5255 was found to have warm carbon monoxide (CO) gas near its center, heated by the energy released as the galaxy's black hole devours material. The VLA observations revealed cold CO gas much more widely distributed than its warmer counterpart. Based on observations of closer objects, the astronomers presume the CO gas is accompanied by large amounts of molecular hydrogen gas (H2). Cold CO gas never has been detected before in such a distant object. Though APM 08279+5255 is a young galaxy undergoing its first massive burst of star formation, the CO gas indicates that very massive stars formed quickly, lived through their short lifetimes, and exploded as supernovae. Carbon and Oxygen, the component elements of CO, are formed in the cores of stars, so their presence in the cold gas tells the astronomers that massive, short-lived stars had to have exploded already, spreading these elements throughout the galaxy's interstellar gas. "The original discovery of this quasar was quite a surprise, as observations revealed it is among the most luminous objects known in the universe. The discovery of this massive reservoir of cold gas is equally surprising. It provides vital clues to the birth of galaxies, such as our own Milky Way," Lewis said. Discovery of the gas was made possible by the galaxy's great distance. The expansion of the universe "stretches" light and radio waves to longer wavelengths -- the more distant the object, the more stretching is seen. Radio waves emitted by the cold CO gas originally had wavelengths of about 1.3 and 2.6 millimeters, but were "redshifted" to wavelengths of 7 and 13 millimeters -- wavelengths the VLA can receive. "It took eight years to refine this technique, but the effort has been worthwhile. This is the golden age of cosmology. We are learning more and more about our universe, from the smallest planets to the largest galaxy clusters. This new result is a crucial piece in the jigsaw and may help resolve many misconceptions about how galaxies form and evolve" Ivison said. "Because of its sensitivity and its ability to make detailed images, the VLA is the only telescope able to unveil these large reservoirs of cold molecular gas in the distant universe," Carilli said. "In addition, as we expand the technical capabilities of the VLA in the coming years, making it even m

2001-01-01

291

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

292

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

293

Spitzer View of Young Massive Stars in the Large Magellanic Cloud H II Complexes. II. N 159  

NASA Astrophysics Data System (ADS)

The H II complex N 159 in the Large Magellanic Cloud is used to study massive star formation in different environments, as it contains three giant molecular clouds (GMCs) that have similar sizes and masses but exhibit different intensities of star formation. We identify candidate massive young stellar objects (YSOs) using infrared photometry, and model their spectral energy distributions to constrain mass and evolutionary state. Good fits are obtained for less evolved Type I, I/II, and II sources. Our analysis suggests that there are massive embedded YSOs in N 159B, a maser source, and several ultracompact H II regions. Massive O-type YSOs are found in GMCs N 159-E and N 159-W, which are associated with ionized gas, i.e., where massive stars formed a few Myr ago. The third GMC, N 159-S, has neither O-type YSOs nor evidence of previous massive star formation. This correlation between current and antecedent formation of massive stars suggests that energy feedback is relevant. We present evidence that N 159-W is forming YSOs spontaneously, while collapse in N 159-E may be triggered. Finally, we compare star formation rates determined from YSO counts with those from integrated H? and 24 ?m luminosities and expected from gas surface densities. Detailed dissection of extragalactic GMCs like the one presented here is key to revealing the physics underlying commonly used star formation scaling laws.

Chen, C.-H. Rosie; Indebetouw, Remy; Chu, You-Hua; Gruendl, Robert A.; Testor, Gérard; Heitsch, Fabian; Seale, Jonathan P.; Meixner, Margaret; Sewilo, Marta

2010-10-01

294

Core-Halo Age Gradients and Star Formation in the Orion Nebula and NGC 2024 Young Stellar Clusters  

NASA Astrophysics Data System (ADS)

We analyze age distributions of two nearby rich stellar clusters, the NGC 2024 (Flame Nebula) and Orion Nebula cluster (ONC) in the Orion molecular cloud complex. Our analysis is based on samples from the MYStIX survey and a new estimator of pre-main sequence (PMS) stellar ages, AgeJX , derived from X-ray and near-infrared photometric data. To overcome the problem of uncertain individual ages and large spreads of age distributions for entire clusters, we compute median ages and their confidence intervals of stellar samples within annular subregions of the clusters. We find core-halo age gradients in both the NGC 2024 cluster and ONC: PMS stars in cluster cores appear younger and thus were formed later than PMS stars in cluster peripheries. These findings are further supported by the spatial gradients in the disk fraction and K-band excess frequency. Our age analysis is based on AgeJX estimates for PMS stars and is independent of any consideration of OB stars. The result has important implications for the formation of young stellar clusters. One basic implication is that clusters form slowly and the apparent age spreads in young stellar clusters, which are often controversial, are (at least in part) real. The result further implies that simple models where clusters form inside-out are incorrect and more complex models are needed. We provide several star formation scenarios that alone or in combination may lead to the observed core-halo age gradients.

Getman, Konstantin V.; Feigelson, Eric D.; Kuhn, Michael A.

2014-06-01

295

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

296

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

297

Near-Infrared Polarimetric Imaging of Disks around Young Intermediate-mass Stars in SEEDS  

NASA Astrophysics Data System (ADS)

We present our recent results to directly image circumstellar disks around Herbig Fe/Ae/Be stars in scattered light with Subaru. Observations of such young disks are critically important to understand how disks evolve possibly under the mutual interaction with new-born planets. One of the observational approaches is direct imaging in scattered light, and the progress in this field since PPV can be found in the ability to prove inner regions of disks. This improvement largely owes to the technique of polarization differential imaging (PDI) which provides higher contrast by extracting scattered light from the disk while suppressing unpolarized stellar light. Strategic Exploration of Exoplanets and Disks with Subaru (SEEDS) is the project dedicated to exoplanet hunting and study of circumstellar disks by direct imaging. Since its beginning in 2009, thirteen Herbig Fe/Ae/Be stars have been observed primarily in H band (1.6 micron). The PDI method has been employed with adaptive optics, enabling us to look into the inner region as close as 0.2 arcsec (˜30 AU) in radius with the typical angular resolution of 0.06 arcsec (˜8 AU). As a result, the SEEDS imagery has newly uncovered rich structures such as spiral arms, inner holes, and gaps for (pre-)transitional disks while suggested the variably illuminated disks for primordial systems. The highlight is the discovery of two spiral arms each for SAO 206462 and MWC 758. The spiral feature has been uniquely found toward Herbig Fe/Aes so far, which might be due to their warmer disks producing arms loosely wound and more easily detected. The observed morphology can be interpreted by the density-wave model, and those disks are implied to harbor Jupiter-mass companions as the exciting sources of the spiral structures according to these models.

Fukagawa, Misato; Hashimoto, Jun; Grady, C. A.; Momose, Munetake; Wisniewski, J. P.; Okamoto, Yoshiko; Muto, Takayuki; Kusakabe, Nobuhiko; Bonnefoy, Mickael; Kotani, Takayuki; Maruta, Yayoi; Tamura, Motohide; Seeds/Hiciao/Ao188 Collaboration,

2013-07-01

298

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

299

Magnetic activity and differential rotation in the very young star KIC 8429280  

NASA Astrophysics Data System (ADS)

Aims: We present a spectroscopic and photometric analysis of the rapid rotator KIC 8429280, discovered by ourselves as a very young star and observed by the NASA Kepler mission, designed to determine its activity level, spot distribution, and differential rotation. Methods: We use ground-based data, such as high-resolution spectroscopy and multicolor broad-band photometry, to derive stellar parameters (vsini, spectral type, Teff, log g, and [Fe/H]), and we adopt a spectral subtraction technique to highlight the strong chromospheric emission in the cores of hydrogen H? and Ca ii H&K and infrared triplet (IRT) lines. We then fit a robust spot model to the high-precision Kepler photometry spanning 138 days. Model selection and parameter estimation is performed in a Bayesian manner using a Markov chain Monte Carlo method. Results: We find that KIC 8429280 is a cool (K2 V) star with an age of about 50 Myr, based on its lithium content, that has passed its T Tau phase and is spinning up approaching the ZAMS on its radiative track. Its high level of chromospheric activity is clearly indicated by the strong radiative losses in Ca ii H&K and IRT, H?, and H? lines. Furthermore, its Balmer decrement and the flux ratio of Ca ii IRT lines imply that these lines are mainly formed in optically-thick regions similar to solar plages. The analysis of the Kepler data uncovers evidence of at least seven enduring spots. Since the star's inclination is rather high - nearly 70° - the assignment of the spots to either the northern or southern hemisphere is not unambiguous. We find at least three solutions with nearly the same level of residuals. Even in the case of seven spots, the fit is far from being perfect. Owing to the exceptional precision of the Kepler photometry, it is not possible to reach the noise floor without strongly enhancing the degrees of freedom and, consequently, the non-uniqueness of the solution. The distribution of the active regions is such that the spots are located around three latitude belts, i.e. around the star's equator and around ± (50°-60°), with the high-latitude spots rotating slower than the low-latitude ones. The equator-to-pole differential rotation d? ? 0.27 rad d-1 is at variance with some recent mean-field models of differential rotation in rapidly rotating main-sequence stars, which predict a much smaller latitudinal shear. Our results are consistent with the scenario of a higher differential rotation, which changes along the magnetic cycle, as proposed by other models. Based on public Kepler data, on observations made with the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundación Galileo Galilei of INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque del los Muchachos of the Instituto de Astrofisica de Canarias, and on observations collected at the Catania Astrophysical Observatory (Italy).

Frasca, A.; Fröhlich, H.-E.; Bonanno, A.; Catanzaro, G.; Biazzo, K.; Molenda-?akowicz, J.

2011-08-01

300

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

301

THE HIDDEN MAGNETIC FIELD OF THE YOUNG NEUTRON STAR IN KESTEVEN 79  

SciTech Connect

Recent observations of the central compact object in the Kesteven 79 supernova remnant show that this neutron star (NS) has a weak dipole magnetic field (a few Multiplication-Sign 10{sup 10} G) but an anomalously large ({approx}64%) pulse fraction in its surface X-ray emission. We explore the idea that a substantial sub-surface magnetic field exists in the NS crust, which produces diffuse hot spots on the stellar surface due to anisotropic heat conduction, and gives rise to the observed X-ray pulsation. We develop a general-purpose method, termed 'Temperature Template with Full Transport' (TTFT), that computes the synthetic pulse profile of surface X-ray emission from NSs with arbitrary magnetic field and surface temperature distributions, taking into account magnetic atmosphere opacities, beam pattern, vacuum polarization, and gravitational light bending. We show that a crustal toroidal magnetic field of order a few Multiplication-Sign 10{sup 14} G or higher, varying smoothly across the crust, can produce sufficiently distinct surface hot spots to generate the observed pulse fraction in the Kes 79 NS. This result suggests that substantial sub-surface magnetic fields, much stronger than the 'visible' dipole fields, may be buried in the crusts of some young NSs, and such hidden magnetic fields can play an important role in their observational manifestations. The general TTFT tool we have developed can also be used for studying radiation from other magnetic NSs.

Shabaltas, Natalia; Lai Dong [Center for Space Research, Department of Astronomy, Cornell University, Ithaca, NY 14853 (United States)

2012-04-01

302

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

303

Star formation in the outer Galaxy: membership and fundamental parameters of the young open cluster NGC 1893  

NASA Astrophysics Data System (ADS)

Context. Different environmental conditions can play a crucial role in determining final products of the star formation process, and in this context, less favorable activities of star formation are expected in the external regions of our Galaxy. Aims: We studied the properties of the young open cluster NGC 1893 located about 12 Kpc from the galactic center, to investigate how different physical conditions can affect the process of star formation. Methods: By adopting a multiwavelength approach, we compiled a catalog extending from X-rays to NIR data to derive the cluster membership. In addition, optical and NIR photometric properties are used to evaluate the cluster parameters. Results: We find 415 diskless candidate members and 1061 young stellar objects with a circumstellar disk or class II candidate members, 125 of which are also H? emitters. Considering the diskless candidate members, we find that the cluster distance is 3.6 ± 0.2 kpc and the mean interstellar reddening is E(B - V) = 0.6 ± 0.1 with evidence of differential reddening in the whole surveyed region. Conclusions: NGC 1893 contains a conspicuous population of pre-main sequence stars, together with the well-studied main sequence cluster population. We found a disk fraction of about 70% similar to the one found in clusters of similar age in the solar neighbor and then, despite expected unfavorable conditions for star formation, we conclude that very rich young clusters can also form in the outer regions of our Galaxy. Full Tables 5-8 are only available in electronic form 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/527/A77

Prisinzano, L.; Sanz-Forcada, J.; Micela, G.; Caramazza, M.; Guarcello, M. G.; Sciortino, S.; Testi, L.

2011-03-01

304

PMS objects in the star formation region Cep OB3. II. Young stellar objects in the H? nebula Cep B  

NASA Astrophysics Data System (ADS)

Models for the spectral energy distributions of four stellar objects in the bright compact H? nebula Cep B are constructed. With a high probability, three of them are found to be very young stellar objects of evolutionary class 0/I with ages of 104-105 years, comparable to the kinematic age of the ionization front of the nebula itself. The IRAS 22551+6221 source associated with Cep B is initiated by heated dust. An intermediate-mass (B2-B3) star of evolutionary class III lies at the center of the ionization front. The local density of PMS stars in the immediate neighborhood of the Cep B nebula exceeds that for the cluster as a whole. It is highly probable that this zone is a local source of a new star-formation stage.

Nikoghosyan, E. H.

2013-06-01

305

Photometric metal abundances of high-luminosity red stars in young and intermediate-age open clusters  

NASA Astrophysics Data System (ADS)

UBV, DDO, and Washinton photometry has been obtained for G and K stars located in or near 22 young and intermediate-age open clusters. Nearly 65 percent of the observed stars are found to have a high probability of being cluster members, while the remaining 35 percent are likely to be red field stars. Five clusters (NGC 2383, NGC 3033, Ruprecht 20, NGC 5168, and NGC 6249) probably do not contain any red giants. Sixteen clusters are found to be nearly solar in composition; three are slightly metal-poor or metal-rich; one (Ruprecht 20) is moderately metal-poor (Fe/H = -0.3); and another (NGC 5617) is moderately metal-rich (Fe/H = 0.3). None of the clusters with derived Washington abundances appear to be enriched in elements of the CNO group.

Claria, J. J.; Lapasset, E.; Minniti, D.

1989-06-01

306

Old Stars in Young Clusters: Lithium-depleted Low-Mass Stars of the Orion Nebula Cluster  

NASA Astrophysics Data System (ADS)

We measured lithium in a sample of low-mass stars (~0.1-0.3 Msolar) of the Orion Nebula cluster. We find evidence for significant Li depletion in four high-probability members, corresponding to nuclear ages between ~15 and 30 Myr. In two cases, there is excellent agreement between the mass and age based on models of Li burning and those derived from the H-R diagram, reinforcing our early findings. For the two other stars, the nuclear age is significantly larger than the isochronal one. Several Li-depleted stars display accretion activity, veiling, and emission lines. We discuss empirical evidence in favor of the old nuclear age and the implications on the star formation history of the Orion cluster. Based on data collected at ESO-VLT, Paranal Observatory, Chile [ID 074.C-0757(A)].

Palla, F.; Randich, S.; Pavlenko, Ya. V.; Flaccomio, E.; Pallavicini, R.

2007-04-01

307

The massive disk around the young B-star AFGL 490  

NASA Astrophysics Data System (ADS)

The AFGL 490 is a key target in the class of deeply embedded young stellar objects with masses of 8x10 Msol being in a transition stage to the pre-main-sequence Herbig Be stars. We observe this system at sub-arcsecond resolution with the Plateau de Bure Interferometer (PdBI) in the C17O(2-1) transition in order to confirm the presence of a rotating disk. The C17O(2-1) data show clear evidence for a rotating ~ 1 500 AU disk centered at the the 1 mm continuum point source. We model these data iteratively. First, the physical structure of the disk is obtained using a two-dimensional radiative transfer code. Second, with this model in hand, we calculate time-dependent abundances in the disk using a gas-grain chemical network. Finally, the beam-convolved C17O(2-1) interferometric map is synthesized by a 2D line transfer code, and these results are compared with the observations in the step-by-step way (Semenov et al. 2005). We estimate that (1) the disk inclination angle is 30 ± 5?, (2) its positional angle is 150 ± 10?, (3) the disk is in Keplerian rotation, (4) it has a radius of about 1 400 AU, and (5) the disk mass is about 0.2-0.4 Msol (depending on the assumed surface density gradient p ~ -1). This is in contrast to the value of 4 ± 2 Msol estimated from the intensity of the PdBI 1 mm continuum emission within 1000 AU around the star, and ~ 8 Msol for the mass of the gas located within about 4 000 AU around the star, as it has been found using our CS(2-1) PdBI data (Schreyer et al. 2002). It is interesting that the geometry of the detected C17O emission map speaks in favor of a larger inclination angle, > 60?. Thus, the C17O interferometric map traces the densest part of a more extended and likely asymmetric disklike structure. Moreover, we find clear indication for on-going accretion in this object, since the detected PdBI CH3OH J=2(0,2)-1(0,1) A+ emission lines show characteristic inverse P Cygni profiles. Given the large mass of the circumstellar gas, comparable to the mass of the central star, we suggest that accretion in AFGL 490 is globally regulated by gravitational instabilities, inevitably leading to the formation of several spiral arms (as predicted by Fromang et al. 2004), while the detected C17O emission arises in a small inner region that is already fully relaxed to the Keplerian state.

Schreyer, Katharina; Semenov, Dmitry; Henning, Thomas; Pavlyuchenkov, Yaroslav; Dullemond, Cornelius

308

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

309

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

2014-04-01

310

Near-IR [Fe II] emission diagnostics applied to cold disk winds in young stars  

NASA Astrophysics Data System (ADS)

We investigate the emissivity properties of the main near-IR transitions of the Fe+ ion in the conditions prevailing in the inner regions of jets from young stars, based on a simplified 16-level atom model. We present new diagnostic diagrams involving prominent near-IR line ratios that allow us to constrain the electronic density, temperature, and Fe gas phase abundance ratio, independently of the heating process. Comparison with recent near-IR observations of a sample of HH objects indicates gas phase Fe abundances ranging from 15-50% up to 100% of the solar value (depending on the assumed temperature and on the HH object), in agreement with the moderate depletions previously derived from optical line ratios or shock models. Hence, it appears that Fe-bearing dust is efficiently destroyed in stellar jets. We then use our Fe+ emissivity model to predict near-IR [Fe II] emission maps for self-similar, cold MHD disk wind models. We show that near-IR [Fe II] lines are stronger than [S II] lambda 6731 and [O I] lambda 6300 in the cool regions (T <=7000 K) near the wind base, and that observations in [Fe II] with AMBER on the VLTI could severely constrain the MHD solution and the inner launch radius of the jet. We also compare theoretical predictions with recent observations in the near-IR [Fe II] lines of the L1551-IRS5 and DG Tau jets. The cold disk wind model reproduces quite well the two velocity components observed at -100 and -300 km s-1, although the high velocity component appears overestimated by a factor of 1.5 in the DG Tau jet. However, the model predicts too little emission at intermediate velocities and insufficient densities. Similar problems were encountered in previous model comparisons in the optical range with jets from T Tauri stars. Denser disk winds with stronger heating at the jet base, which have been invoked for optical jets, also appear needed in younger, embedded Class I jet sources.

Pesenti, N.; Dougados, C.; Cabrit, S.; O'Brien, D.; Garcia, P.; Ferreira, J.

2003-10-01

311

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

312

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

313

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

NASA Astrophysics Data System (ADS)

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 are characterized by a power-law behavior N d L ? L-? d L, with a mean exponent of < ?> = 2.0 ± 0.2. This can be explained by a cluster population formed with a constant cluster formation rate, in which the maximum cluster mass per logarithmic age bin is determined by the size-of-sample effect and therefore increases with log (age/yr). The LFs of NGC 6946 and M 51 are better described by a double power-law distribution or a Schechter function. When a cluster population has a mass function that is truncated below the limit given by the size-of-sample effect, the total LF shows a bend at the magnitude of the maximum mass, with the age of the oldest cluster in the population, typically a few Gyr due to disruption. For NGC 6946 and M 51 this suggests a maximum mass of M_max = 0.5-1×10^6 M?, although the bend is only a 1-2 ? detection. Faint-ward of the bend the LF has the same slope as the underlying initial cluster mass function and bright-ward of the bend it is steeper. This behavior can be well explained by our population model. We compare our results with the only other galaxy for which a bend in the LF has been observed, the "Antennae" galaxies (NGC 4038/4039). There the bend occurs brighter than in NGC 6946 and M 51, corresponding to a maximum cluster mass of M_max = 1.3-2.5×106 M?. Hence, if the maximum cluster mass has a physical limit, then it can vary between different galaxies. The fact that we only observe this bend in the LF in the "Antennae" galaxies, NGC 6946 and M 51 is because there are enough clusters available to reach the limit. In other galaxies there might be a physical limit as well, but the number of clusters formed or observed is so low, that the LF is not sampled up to the luminosity of the bend. The LF can then be approximated with a single power-law distribution, with an index similar to the initial mass function index.

Gieles, M.; Larsen, S. S.; Bastian, N.; Stein, I. T.

2006-04-01

314

Flame Throwers of the Galaxy - Collimated Jets from Stars Young and Old  

NASA Astrophysics Data System (ADS)

The jets of fiery fluid that shoot out of flame throwers have astronomical counterparts: the jets of hot gas that pour forth from baby stars, dead stars, and galaxies. They are one of the biggest mysteries in astronomy.

Rodriguez, Luis F.

1995-03-01

315

Young Stars near Earth: The Octans-Near Association and Castor Moving Group  

NASA Astrophysics Data System (ADS)

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-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-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 "Octans-Near"; 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; Song, Inseok; Schneider, Adam

2013-11-01

316

Stellar Activity on the Young Suns of Orion: COUP Observations of K5-7 Pre-Main-Sequence Stars  

NASA Astrophysics Data System (ADS)

In 2003 January, the Chandra Orion Ultradeep Project (COUP) detected about 1400 young stars during a 13.2 day observation of the Orion Nebula Cluster (ONC). This paper is a study of the X-ray properties of a well-defined sample of 28 solar-mass ONC stars based on COUP data. Our goals are to characterize the magnetic activity of analogs of the young Sun and thereby to improve understanding of the effects of solar X-rays on the solar nebula during the era of planet formation. Given the length of the COUP observation we are able to clearly distinguish characteristic and flare periods for all stars. We find that active young suns spend 70% of their time in a characteristic state with relatively constant flux and magnetically confined plasma with temperatures kT2~=2.1×kT1. During characteristic periods, the 0.5-8 keV X-ray luminosity is about 0.03% of the bolometric luminosity. One or two powerful flares per week with peak luminosities logLX~30-32 ergs s-1 are typically superposed on this characteristic emission accompanied by heating of the hot plasma component from ~=2.4 to ~=7 keV at the flare peak. The energy distribution of flares superposed on the characteristic emission level follows the relationship dN/dE~E-1.7. The flare rates are consistent with the production of sufficiently energetic protons to spawn a spallogenic origin of some important short-lived radionuclides found in ancient meteorites. The X-rays can ionize gas in the circumstellar disk at a rate of 6×10-9 ionizations per second at 1 AU from the central star, orders of magnitude above cosmic-ray ionization rates. The estimated energetic particle fluences are sufficient to account for many isotopic anomalies observed in meteoritic inclusions.

Wolk, S. J.; Harnden, F. R., Jr.; Flaccomio, E.; Micela, G.; Favata, F.; Shang, H.; Feigelson, E. D.

2005-10-01

317

RESOLVED RADIO EMISSION FROM MODELS OF PHOTOEVAPORATED DISKS AROUND MASSIVE YOUNG STARS  

SciTech Connect

We study the radio continuum and thermal hydrogen radio recombination line (RRL) emission from photoevaporated disk wind models around massive young stars. We applied the models of Lugo and coworkers to the source MWC 349A. The resolved synthetic radio continuum maps reproduce the observed hourglass morphology at low frequency but are more flattened than the observations at high frequency because the density in the model decreases too fast. These photoevaporated wind models naturally produce RRLs with FWHM {Delta}v {approx} 60 km s{sup -1}. Nevertheless, recent H66{alpha} line observations of MWC 349A by Loinard and Rodriguez have an FWHM {Delta}v {approx} 89 km s{sup -1}. We propose that such wide lines could be produced by an extra magnetocentrifugal acceleration of the flow due to a poloidal magnetic field anchored in the disk. Such fields could also prevent the flow divergence and the fast density drop of the photoevaporated disk wind model. To mimic this effect we include in this model a large non-thermal velocity dispersion {sigma}{sub nt} {approx} 70 km s{sup -1}. The width of the RRLs of this modified model increases with quantum number. This is in contrast with the observed H76{alpha} and H92{alpha} lines which are narrower than the H66{alpha} line. We argue that the low-frequency observations could have suffered from insufficient bandwidth and that new measurements of these lines would be very valuable to constrain the models. Finally, the resolved H66{alpha} and H53{alpha} line emission maps show the velocity asymmetry expected from flow rotation.

Avalos, Martin; Lizano, Susana [Centro de Radioastronomia y Astrofisica, UNAM, Apartado Postal 3-72, 58089 Morelia, Michoacan (Mexico)

2012-05-20

318

Subaru/COMICS Study on Silicate Dust Processing around Young Low-Mass Stars  

NASA Astrophysics Data System (ADS)

We have obtained 8-13 ?m spectra of 30 young (1-10 Myr) low-mass pre-main-sequence stars using COMICS on the 8.2 m Subaru Telescope to examine dust evolution in protoplanetary disks. Most spectra show silicate emission features of various strengths and shapes, indicative of dust processing during the different stages of protoplanetary disk evolution. We have analyzed the observed silicate emission features using a simple model previously applied to the more massive and luminous Herbig Ae/Be systems. We determined the feature strength and shape and derived the composition and typical size of the silicate dust grains. We confirm the previously reported dependency of the silicate feature strength and shape on the grain size of the amorphous silicate dust. We examine the relation between the derived dust properties and stellar and circumstellar disk parameters, such as systemic age, luminosity of H? (LH?), disk mass, and opacity power-law index ? at radio wavelengths. A possible relation is found between silicate feature strength (grain size indicator) and the LH?, which may be an indicator of accretion activity. It implies that the turbulence induced by accretion activity may be important for grain size evolution in the disk. No clear correlation between the crystallinity and the stellar/disk parameters is found. We find that on average 5%-20% in mass of the silicate dust grains is in crystalline form, irrespective of systemic age. This latter finding supports the idea that crystalline silicate is formed at an early evolutionary phase, probably at the protostellar phase, and is remaining during the later stages. Based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

Honda, Mitsuhiko; Kataza, Hirokazu; Okamoto, Yoshiko K.; Yamashita, Takuya; Min, Michiel; Miyata, Takashi; Sako, Shigeyuki; Fujiyoshi, Takuya; Sakon, Itsuki; Onaka, Takashi

2006-08-01

319

Near-infrared Variability among Young Stellar Objects in the Star Formation Region Cygnus OB7  

NASA Astrophysics Data System (ADS)

We present an analysis of near-infrared time-series photometry in J, H, and K bands for about 100 epochs of a 1° × 1° 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 ? 17. We study detailed light curves and color trajectories of ~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 ~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.; Aspin, Colin

2013-08-01

320

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

321

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

322

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

323

On the Dynamical Formation of Very Young, X-Ray Emitting Black Hole Binaries in Dense Star Clusters  

NASA Astrophysics Data System (ADS)

We recently discovered a population of very young (? <~ 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 (~6000 M ?) 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; Converse, Joseph M.; Chandar, Rupali; Rangelov, Blagoy

2012-08-01

324

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

325

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

NASA Astrophysics Data System (ADS)

We propose a scenario in which massive stars form in a self-gravitating gaseous disc around a supermassive black hole (SMBH). We analyse the dynamics of a disc forming around an SMBH, in which the angular momentum is transported by turbulence induced by the disc's self-gravity. We find that once the surface density of the disc exceeds a critical value, the disc fragments into dense clumps. We argue that 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 Centre. In particular, we show how the masses and spatial distribution of the young stars, and the total mass in the Galactic Centre 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 Laser Interferometer Space Antenna (LISA). For a reasonable range of parameters such mergers will be detected monthly, and that the gravitational-wave signal from these mergers is distinct from that of other merger scenarios. Also, for some plausible black hole masses and accretion rates, the burst of gravitational waves should be accompanied by a detectable change in the optical luminosity of the central engine.

Levin, Yuri

2007-01-01

326

Variability of young stars: Determination of rotational periods of weak-line T Tauri stars in the Cepheus-Cassiopeia star-forming region  

NASA Astrophysics Data System (ADS)

We report on observation and determination of rotational periods of ten weak-line T Tauri stars in the Cepheus-Cassiopeia star-forming region. Observations were carried out with the Cassegrain-Teleskop-Kamera (CTK) at University Observatory Jena between 2007 June and 2008 May. The periods obtained range between 0.49 d and 5.7 d, typical for weak-line and post T Tauri stars. Based on observations obtained with telescopes of the University Observatory Jena, which is operated by the Astrophysical Institute of the Friedrich-Schiller-University.

Koeltzsch, A.; Mugrauer, M.; Raetz, St.; Schmidt, T. O. B.; Roell, T.; Eisenbeiss, T.; Hohle, M. M.; Va?ko, M.; Ginski, Ch.; Marka, C.; Moualla, M.; Schreyer, K.; Broeg, Ch.; Neuhäuser, R.

2009-05-01

327

Extremely Active Cool Stars in the Young Open Cluster NGC 2362  

NASA Astrophysics Data System (ADS)

NGC 2362 is one of the youngest open stellar clusters in our Galaxy and is centered around the massive O-type star tau CMa. This cluster exhibits several hundred cool stars in the pre-main sequence phase. We used the ROSAT PSPC and HRI to obtain deep X-ray images of NGC 2362. Here we present first results of an in-depth study of the X-ray properties of the cool star cluster members.

Berghofer, T. W.; Schmitt, J. H. M. M.

328

EMBEDDED YOUNG STELLAR OBJECT CANDIDATES IN THE ACTIVE STAR-FORMING COMPLEX W51: MASS FUNCTION AND SPATIAL DISTRIBUTION  

SciTech Connect

We present 737 candidate young stellar objects (YSOs) near the W51 giant molecular cloud over an area of 1.{sup 0}25 x 1.{sup 0}00 selected from Spitzer Space Telescope data. We use spectral energy distribution fits to identify YSOs and distinguish them from main-sequence (MS) or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses >5 M{sub sun} (corresponding to mid- to early-B MS spectral types), 46 of which are located in the central active star-forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M{sub sun} and 18 M{sub sun}. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

Kang, Miju; Lee, Youngung [International Center for Astrophysics, Korea Astronomy and Space Science Institute, Hwaam 61-1, Yuseong, Daejeon 305-348 (Korea, Republic of); Bieging, John H. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Povich, Matthew S., E-mail: mjkang@kasi.re.k [Department of Astronomy, University of Wisconsin at Madison, 475 N. Charter Street, Madison, WI 53706 (United States)

2009-11-20

329

Dynamics of stellar black holes in young star clusters with different metallicities - II. Black hole-black hole binaries  

NASA Astrophysics Data System (ADS)

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 Z?). 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 M?), 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 (10-3-107 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.

Ziosi, Brunetto Marco; Mapelli, Michela; Branchesi, Marica; Tormen, Giuseppe

2014-07-01

330

THE PRESENT-DAY STAR FORMATION RATE OF THE MILKY WAY DETERMINED FROM SPITZER-DETECTED YOUNG STELLAR OBJECTS  

SciTech Connect

We present initial results from a population synthesis model aimed at determining the star formation rate (SFR) of the Milky Way. We find that a total SFR of 0.68-1.45 M {sub sun} yr{sup -1} is able to reproduce the observed number of young stellar objects (YSOs) in the Spitzer/IRAC GLIMPSE survey of the Galactic plane, assuming simple prescriptions for the three-dimensional Galactic distributions of YSOs and interstellar dust, and using model spectral energy distributions to predict the brightness and color of the synthetic YSOs at different wavelengths. This is the first Galaxy-wide measurement derived from pre-main-sequence objects themselves, rather than global observables such as the total radio continuum, H{alpha}, or FIR flux. The value obtained is slightly lower than, but generally consistent with previously determined values. We will extend this method in the future to fit the brightness, color, and angular distribution of YSOs, and simultaneously make use of multiple surveys, to place constraints on the input assumptions, and reduce uncertainties in the SFR estimate. Ultimately, this will be one of the most accurate methods for determining the Galactic SFR, as it makes use of stars of all masses (limited only by sensitivity) rather than solely massive stars or indirect tracers of massive stars.

Robitaille, Thomas P. [Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA, 02138 (United States); Whitney, Barbara A. [Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States)], E-mail: trobitaille@cfa.harvard.edu

2010-02-10

331

Embedded Young Stellar Object Candidates in the Active Star-Forming Complex W51: Mass Function and Spatial Distribution  

NASA Astrophysics Data System (ADS)

We present 737 candidate young stellar objects (YSOs) near the W51 giant molecular cloud over an area of 1fdg25 × 1fdg00 selected from Spitzer Space Telescope data. We use spectral energy distribution fits to identify YSOs and distinguish them from main-sequence (MS) or red giant stars, asymptotic giant branch stars, and background galaxies. Based on extinction of each YSO, we separate a total of 437 YSOs associated with the W51 region from the possible foreground sources. We identify 69 highly embedded Stage 0/I candidate YSOs in our field with masses >5 M sun (corresponding to mid- to early-B MS spectral types), 46 of which are located in the central active star-forming regions of W51A and W51B. From the YSOs associated with W51, we find evidence for mass segregation showing that the most massive YSOs are concentrated on the W51 H II region complex. We find a variation in the spatial distribution of the mass function (MF) of YSOs in the mass range between 5 M sun and 18 M sun. The derived slopes of the MF are -1.26 and -2.36 in the active star-forming region and the outer region, respectively. The variation of the MF for YSOs embedded in the molecular cloud implies that the distribution of stellar masses in clusters depends on the local conditions in the parent molecular cloud.

Kang, Miju; Bieging, John H.; Povich, Matthew S.; Lee, Youngung

2009-11-01

332

Optical manifestations of mass outflows from young stars - At atlas of CCD images of Herbig-Haro objects  

NASA Technical Reports Server (NTRS)

An attempt is made to provide deep CCD images suitable for tracing the shock-excited and scattered-light components characterizing extended Herbig-Haro complexes. Tabular data include astrometric positions for all stars, shock-excited emission knots, prominent scattered-light patches, as well as an identification for all H-alpha emission objects within the CCD images. It is concluded that: (1) optical outflows are not always accompanied by molecular flows, (2) optical outflows show a higher degree of collimation than their molecular counterparts, and (3) the Herbig-Haro phenomenon is not restricted to low-mass young stellar objects.

Strom, K. M.; Strom, S. E.; Wenz, M.; Wolff, S. C.; Morgan, J.

1986-01-01

333

Evidence of Planetesimal infall on to the very young Herbig Be star LkH$_\\\\alpha$234  

Microsoft Academic Search

We report here the first evidence for planetesimal infall onto the very young\\u000aHerbig Be star LkH$_\\\\alpha$234. These results are based on observations\\u000aacquired over 31 days using spectroscopy of the sodium D lines, the He I\\u000a5876\\\\AA, and hydrogen H$_\\\\alpha$ lines. We find Redshifted Absorption\\u000aComponents (RAC) with velocities up to 200 km\\/s and very mild Blueshifted\\u000aAbsorption Components

Abhijit Chakraborty; Jian Ge; Suvrath Mahadevan

2004-01-01

334

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

335

On the origin of the O and B-type stars with high velocities. II. Runaway stars and pulsars ejected from the nearby young stellar groups  

NASA Astrophysics Data System (ADS)

We use milli-arcsecond accuracy astrometry (proper motions and parallaxes) from Hipparcos and from radio observations to retrace the orbits of 56 runaway stars and nine compact objects with distances less than 700 pc, to identify the parent stellar group. It is possible to deduce the specific formation scenario with near certainty for two cases. (i) We find that the runaway star zeta Ophiuchi and the pulsar PSR J1932+1059 originated about 1 Myr ago in a supernova explosion in a binary in the Upper Scorpius subgroup of the Sco OB2 association. The pulsar received a kick velocity of ~ 350 km s-1 in this event, which dissociated the binary, and gave zeta Oph its large space velocity. (ii) Blaauw & Morgan and Gies & Bolton already postulated a common origin for the runaway-pair AE Aur and mu Col, possibly involving the massive highly-eccentric binary iota Ori, based on their equal and opposite velocities. We demonstrate that these three objects indeed occupied a very small volume ~ 2.5 Myr ago, and show that they were ejected from the nascent Trapezium cluster. We identify the parent group for two more pulsars: both likely originate in the ~ 50 Myr old association Per OB3, which contains the open cluster alpha Persei. At least 21 of the 56 runaway stars in our sample can be linked to the nearby associations and young open clusters. These include the classical runaways 53 Arietis (Ori OB1), xi Persei (Per OB2), and lambda Cephei (Cep OB3), and fifteen new identifications, amongst which a pair of stars running away in opposite directions from the region containing the lambda Ori cluster. Other currently nearby runaways and pulsars originated beyond 700 pc, where our knowledge of the parent groups is very incomplete.

Hoogerwerf, R.; de Bruijne, J. H. J.; de Zeeuw, P. T.

2001-01-01

336

A Treasury Study of Star-forming Regions in the Local Group. I. HST Photometry of Young Populations in Six Dwarf Galaxies  

NASA Astrophysics Data System (ADS)

We present a comprehensive study of young stellar populations in six dwarf galaxies in or near the Local Group: Phoenix, Pegasus, Sextans A, Sextans B, WLM, and NGC 6822. Their star-forming regions, selected from GALEX wide-field far-UV imaging, were imaged (at sub-pc resolution) with the WFPC2 camera on board the Hubble Space Telescope (HST) in six bandpasses from far-UV to I to detect and characterize their hot massive star content. This study is part of HST treasury survey program HST-GO-11079; the general data characteristics and reduction procedures are detailed in this paper and results are presented for the first six galaxies. From a total of 180 HST images, we provide catalogs of the multi-band stellar photometry and derive the physical parameters of massive stars by analyzing it with model-atmosphere colors. We use the results to infer ages, number of massive stars, extinction, and spatial characteristics of the young stellar populations. The hot massive star content varies largely across our galaxy sample, from an inconspicuous presence in Phoenix and Pegasus to the highest relative abundance of young massive stars in Sextans A and WLM. Albeit to a largely varying extent, most galaxies show a very young population (a few Myrs, except for Phoenix), and older ones (a few 107 years in Sextans A, Sextans B, NGC 6822, and WLM, ~108yr in Phoenix and Pegasus), suggesting discrete bursts of recent star formation in the mapped regions. The hot massive star content (indicative of the young populations) broadly correlates with the total galaxy stellar mass represented by the integrated optical magnitude, although it varies by a factor of ~3 between Sextans A, WLM, and Sextans B, which have similar MV . Extinction properties are also derived.

Bianchi, Luciana; Efremova, Boryana; Hodge, Paul; Massey, Philip; Olsen, K. A. G.

2012-03-01

337

A TREASURY STUDY OF STAR-FORMING REGIONS IN THE LOCAL GROUP. I. HST PHOTOMETRY OF YOUNG POPULATIONS IN SIX DWARF GALAXIES  

SciTech Connect

We present a comprehensive study of young stellar populations in six dwarf galaxies in or near the Local Group: Phoenix, Pegasus, Sextans A, Sextans B, WLM, and NGC 6822. Their star-forming regions, selected from GALEX wide-field far-UV imaging, were imaged (at sub-pc resolution) with the WFPC2 camera on board the Hubble Space Telescope (HST) in six bandpasses from far-UV to I to detect and characterize their hot massive star content. This study is part of HST treasury survey program HST-GO-11079; the general data characteristics and reduction procedures are detailed in this paper and results are presented for the first six galaxies. From a total of 180 HST images, we provide catalogs of the multi-band stellar photometry and derive the physical parameters of massive stars by analyzing it with model-atmosphere colors. We use the results to infer ages, number of massive stars, extinction, and spatial characteristics of the young stellar populations. The hot massive star content varies largely across our galaxy sample, from an inconspicuous presence in Phoenix and Pegasus to the highest relative abundance of young massive stars in Sextans A and WLM. Albeit to a largely varying extent, most galaxies show a very young population (a few Myrs, except for Phoenix), and older ones (a few 10{sup 7} years in Sextans A, Sextans B, NGC 6822, and WLM, {approx}10{sup 8}yr in Phoenix and Pegasus), suggesting discrete bursts of recent star formation in the mapped regions. The hot massive star content (indicative of the young populations) broadly correlates with the total galaxy stellar mass represented by the integrated optical magnitude, although it varies by a factor of {approx}3 between Sextans A, WLM, and Sextans B, which have similar M{sub V}. Extinction properties are also derived.

Bianchi, Luciana; Efremova, Boryana [Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States); Hodge, Paul [Astronomy Department, University of Washington, Seattle, WA (United States); Massey, Philip [Lowell Observatory, Flagstaff, AZ (United States); Olsen, K. A. G., E-mail: bianchi@pha.jhu.edu [National Optical Astronomy Observatory, Tucson, AZ 85719 (United States)

2012-03-15

338

Disk Dissipation in Single and Binary Young Star Systems in Taurus  

Microsoft Academic Search

We compare the timescales for dissipation of circumstellar disks radiating in the thermal IR in single and binary star systems in the Taurus star-forming region. We accomplish this using data from the multiplicity surveys, available and new 10 microns photometry of the unresolved systems, and the approach of Simon, Ghez, & Leinert to estimate the ages of the singles and

M. Simon; L. Prato

1995-01-01

339

X-ray spectral and timing characteristics of the stars in the young open cluster IC 2391  

NASA Astrophysics Data System (ADS)

We present X-ray spectral and timing analysis of members of the young open cluster IC 2391 observed with the XMM-Newton observatory. We detected 99 X-ray sources by analysing the summed data obtained from MOS1, MOS2 and pn detectors of the EPIC camera; 24 of them are members, or probable members, of the cluster. Stars of all spectral types have been detected, from the early-types to the late-M dwarfs. Despite the capability of the instrument to recognize up to 3 thermal components, the X-ray spectra of the G, K and M members of the cluster are well described with two thermal components (at kT1 ˜ 0.3-0.5 keV and kT2 ˜ 1.0-1.2 keV respectively) while the X-ray spectra of F members require only a softer 1-T model. The Kolmogorov-Smirnov test applied to the X-ray photon time series shows that approximately 46% of the members of IC 2391 are variable with a confidence level >99%. The comparison of our data with those obtained with ROSAT/PSPC, nine years earlier, and ROSAT/HRI, seven years earlier, shows that there is no evidence of significant variability on these time scales, suggesting that long-term variations due to activity cycles similar to that on the Sun are not common, if present at all, among these young stars.

Marino, A.; Micela, G.; Peres, G.; Pillitteri, I.; Sciortino, S.

2005-01-01

340

Stars  

NASA Astrophysics Data System (ADS)

We will begin our study with a more or less superficial inspection of the "forest" of stars that we see in the skies. The first thing we notice is that, as sources of light, they are much weaker than the Sun. Second, their apparent colors vary; from a bluish-white in most of them to a reddish-yellow, which is rarer. There is also a third aspect, though it is not very obvious to the naked eye: most of the stars group themselves in small families of two, three or more members. A good example is the Alpha Centauri, the closest star to us, which, in fact, is a triple system of stars. Another is the group of 7 stars that make up the Pleiades, which will be discussed later on. In fact, almost half of the stars are double systems with only two members, called binary stars. Most of these double stars, though together, are separated by several astronomical units (one astronomical unit, AU, is the distance from Earth to the sun: see Chapter 1), and revolve around each other over periods of several years. And yet the revolutions of some binary stars, separated by much smaller distances, occur in only a few hours! These stars are so close to each other that they can share enveloping material. Often this exchange occurs in a somewhat violent manner. Local explosions may occur, expelling matter away from the system. In other binary systems, where one of the components is a very compact, dense star, companion material flows more calmly, making up a light disk around the compact star.

Capelato, Hugo Vicente

1999-01-01

341

STAR FORMATION IN THE CENTRAL 400 PC OF THE MILKY WAY: EVIDENCE FOR A POPULATION OF MASSIVE YOUNG STELLAR OBJECTS  

SciTech Connect

The central kpc of the Milky Way might be expected to differ significantly from the rest of the Galaxy with regard to gasdynamics and the formation of young stellar objects (YSOs). We probe this possibility with mid-infrared observations obtained with Infrared Array Camera and Multiband Imaging Photometer on Spitzer and with Midcourse Space Experiment. We use color-color diagrams and spectral energy distribution (SED) fits to explore the nature of YSO candidates (including objects with 4.5 {mu}m excesses possibly due to molecular emission). There is an asymmetry in the distribution of the candidate YSOs, which tend to be found at negative Galactic longitudes; this behavior contrasts with that of the molecular gas, approximately 2/3 of which is at positive longitudes. The small-scale height of these objects suggests that they are within the Galactic center region and are dynamically young. They lie between two layers of infrared dark clouds and may have originated from these clouds. We identify new sites for this recent star formation by comparing the mid-IR, radio, submillimeter, and methanol maser data. The methanol masers appear to be associated with young, embedded YSOs characterized by 4.5 {mu}m excesses. We use the SEDs of these sources to estimate their physical characteristics; their masses appear to range from {approx}10 to {approx}20 M{sub sun}. Within the central 400 x 50 pc (|l| < 1.{sup 0}3 and |b| < 10') the star formation rate (SFR) based on the identification of Stage I evolutionary phase of YSO candidates is about 0.14 M{sub sun} yr{sup -1}. Given that the majority of the sources in the population of YSOs are classified as Stage I objects, we suggest that a recent burst of star formation took place within the last 10{sup 5} yr. This suggestion is also consistent with estimates of SFRs within the last {approx}10{sup 7} yr showing a peak around 10{sup 5} yr ago. Lastly, we find that the Schmidt-Kennicutt Law applies well in the central 400 pc of the Galaxy. This implies that star formation does not appear to be dramatically affected by the extreme physical conditions in the Galactic center region.

Yusef-Zadeh, F.; Hewitt, J. W. [Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208 (United States); Arendt, R. G. [Science Systems and Applications, Inc., Baltimore County, GSFC, Code 665, Greenbelt, MD 20771 (United States); Whitney, B. [Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, CO 80301 (United States); Rieke, G.; Hinz, J. L. [Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States); Wardle, M. [Department of Physics and Engineering, Macquarie University, Sydney, NSW 2109 (Australia); Stolovy, S.; Ramirez, S. [Spitzer Science Center, California Institute of Technology, Mail Code 220-6, 1200 East California Boulevard, Pasadena, CA 91125 (United States); Lang, C. C. [Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52245 (United States); Burton, M. G. [School of Physics, University of New South Wales, Sydney, NSW 2052 (Australia)], E-mail: zadeh@northwestern.edu

2009-09-01

342

Star warriors: The young scientists who are inventing the weaponry of space  

SciTech Connect

Broad's account of a week spent at Livermore nuclear lab in California offers a picture of the breakthrough weaponry-their theoretical origins and technology-that gave rise to the President's ''Star Wars'' proposal.

Broad, W.J.

1985-01-01

343

Asteroseismology of massive stars in the young open cluster NGC 884: a status report  

Microsoft Academic Search

To improve our comprehension of the beta Cephei stars, we set up a photometric multi-site campaign on the open cluster NGC 884 (chi Persei). Thirteen telescopes joined the 2005-2007 campaign which resulted in almost 78 000 CCD frames. We present an up-to-date status of the analysis of these data, in which several interesting oscillating stars are pointed out. We end

S. Saesen; F. Carrier; A. Pigulski; C. Aerts; G. Handler; A. Narwid; J. N. Fu; C. Zhang; X. J. Jiang; G. Kopacki; J. Vanautgaerden; M. Steslicki; B. Acke; E. Poretti; K. Uytterhoeven; W. De Meester; M. D. Reed; Z. Kolaczkowski; G. Michalska; E. Schmidt; R. Östensen; C. Gielen; K. Yakut; A. Leitner; B. Kalomeni; S. Prins; V. van Helshoecht; W. Zima; R. Huygen; B. Vandenbussche; P. Lenz; E. Ladjal; E. Puga Antolín; T. Verhoelst; J. De Ridder; P. Niarchos; A. Liakos; D. Lorenz; S. Dehaes; M. Reyniers; G. Davignon; S.-L. Kim; D. H. Kim; Y.-J. Lee; C.-U. Lee; J.-H. Kwon; E. Broeders; H. van Winckel; E. Vanhollebeke; G. Raskin; Y. Blom; J. R. Eggen; P. Beck; J. Puschnig; L. Schmitzberger; G. A. Gelven; B. Steininger; R. Drummond

2009-01-01

344

On Young Neutron Stars as Propellers and Accretors with Conventional Magnetic Fields  

Microsoft Academic Search

The similarity of rotation periods of, the anomalous X-ray pulsars (AXPs),\\u000athe soft gamma ray repeaters (SGRs) and the dim thermal neutron stars (DTNs)\\u000asuggests a common mechanism with an asymptotic spindown phase through the\\u000apropeller and early accretion stages. The DTNs are in the propeller stage.\\u000aTheir luminosities arise from frictional heating in the neutron star. If the\\u000a8.4

M. Ali Alpar

2000-01-01

345

Main-sequence variable stars in young open cluster NGC 1893  

NASA Astrophysics Data System (ADS)

In this paper, we present time series photometry of 104 variable stars in the cluster region NGC 1893. The association of the present variable candidates to the cluster NGC 1893 has been determined by using (U - B)/(B - V) and (J - H)/(H - K) two colour diagrams, and V/(V - I) colour-magnitude diagram. 45 stars are found to be main-sequence variables and these could be B-type variable stars associated with the cluster. We classified these objects as ? Cep, slowly pulsating B stars and new class variables as discussed by Mowlavi et al. These variable candidates show ˜0.005 to ˜0.02 mag brightness variations with periods of <1.0 d. 17 new class variables are located in the H - R diagram between the slowly pulsating B stars and ? Scuti variables. Pulsation could be one of the causes for periodic brightness variations in these stars. The X-ray emission of present main-sequence variables associated with the cluster lies in the saturated region of X-ray luminosity versus period diagram and follows the general trend by Pizzolato et al.

Lata, Sneh; Yadav, Ram Kesh; Pandey, A. K.; Richichi, Andrea; Eswaraiah, C.; Kumar, Brajesh; Kappelmann, Norbert; Sharma, Saurabh

2014-07-01

346

Magnetic activity and differential rotation in the young Sun-like stars KIC 7985370 and KIC 7765135  

NASA Astrophysics Data System (ADS)

Aims: We present a detailed study of the two Sun-like stars KIC 7985370 and KIC 7765135, to determine their activity level, spot distribution, and differential rotation. Both stars were previously discovered by us to be young stars and were observed by the NASA Kepler mission. Methods: The fundamental stellar parameters (vsini, spectral type, Teff, log g, and [Fe/H]) were derived from optical spectroscopy by comparison with both standard-star and synthetic spectra. The spectra of the targets allowed us to study the chromospheric activity based on the emission in the core of hydrogen H? and Ca ii infrared triplet (IRT) lines, which was revealed by the subtraction of inactive templates. The high-precision Kepler photometric data spanning over 229 days were then fitted with a robust spot model. Model selection and parameter estimation were performed in a Bayesian manner, using a Markov chain Monte Carlo method. Results: We find that both stars are Sun-like (of G1.5 V spectral type) and have an age of about 100-200 Myr, based on their lithium content and kinematics. Their youth is confirmed by their high level of chromospheric activity, which is comparable to that displayed by the early G-type stars in the Pleiades cluster. The Balmer decrement and flux ratio of their Ca ii-IRT lines suggest that the formation of the core of these lines occurs mainly in optically thick regions that are analogous to solar plages. The spot model applied to the Kepler photometry requires at least seven persistent spots in the case of KIC 7985370 and nine spots in the case of KIC 7765135 to provide a satisfactory fit to the data. The assumption of the longevity of the star spots, whose area is allowed to evolve with time, is at the heart of our spot-modelling approach. On both stars, the surface differential rotation is Sun-like, with the high-latitude spots rotating slower than the low-latitude ones. We found, for both stars, a rather high value of the equator-to-pole differential rotation (d? ? 0.18 rad d-1), which disagrees with the predictions of some mean-field models of differential rotation for rapidly rotating stars. Our results agree instead with previous works on solar-type stars and other models that predict a higher latitudinal shear, increasing with equatorial angular velocity, that can vary during the magnetic cycle. Based on public Kepler data, on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated by the Fundación Galileo Galilei - INAF at the Observatorio del Roque del los Muchachos, La Palma (Canary Islands), on observations collected at the 2.2-m telescope of the Centro Astronómico Hispano Alemán (CAHA) at Calar Alto (Almería, Spain), operated jointly by the Max-Planck-Institut für Astronomie and the Instituto de Astrofísica de Andalucía (CSIC), and on observations collected at the Catania Astrophysical Observatory (Italy).

Fröhlich, H.-E.; Frasca, A.; Catanzaro, G.; Bonanno, A.; Corsaro, E.; Molenda-?akowicz, J.; Klutsch, A.; Montes, D.

2012-07-01

347

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

348

Star Formation  

NASA Astrophysics Data System (ADS)

Stars are one of the most important constituents of the Universe, and understanding their formation is crucial to many areas of astrophysics. Stars form from dense molecular gas, and they tend not to form in isolation. Stars often form in binary and multiple systems, and these systems tend to form in clusters with 102-105 members. Stars also form with a wide range of masses, from substellar brown dwarfs with masses < 0. 1 M ? to massive stars > 100 M ?, and wherever stars form the distribution of their masses seems always to be the same. This chapter will review our current understanding of star formation from cold gas to young star clusters.

Goodwin, Simon

349