Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Search for associations containing young stars (SACY). III. Ages and Li abundances
Context: Our study is a follow-up of the SACY project, an extended highspectral resolution survey of more than two thousand opticalcounterparts to X-ray sources in the southern hemisphere targeted tosearch for young nearby association. Nine associations have either beennewly identified, or have had their member list revised. Groupsbelonging to the Sco-Cen-Oph complex are not considered in the presentstudy. Aims: These nine associations, with ages of between about 6Myr and 70 Myr, form an excellent sample to study the Li depletion inthe pre-main sequence (PMS) evolution. In the present paper, weinvestigate the use of Li abundances as an independent clock toconstrain the PMS evolution. Methods: Using our measurements ofthe equivalent widths of the Li resonance line and assuming fixedmetallicities and microturbulence, we calculated the LTE Li abundancesfor 376 members of various young associations. In addition, weconsidered the effects of their projected stellar rotation.Results: We present the Li depletion as a function of age in the firsthundred million years for the first time for the most extended sample ofLi abundances in young stellar associations. Conclusions: A clearLi depletion can be measured in the temperature range from 5000 K to3500 K for the age span covered by the nine associations studied in thispaper. The age sequence based on the Li-clock agrees well with theisochronal ages, the ?Cha association being the only possibleexception. The lithium depletion patterns for the associations presentedhere resemble those of the young open clusters with similar ages,strengthening the notion that the members proposed for these loose youngassociations have indeed a common physical origin. The observed scatterin the Li abundances hampers the use of Li in determining reliable agesfor individual stars. For velocities above 20 km s-1,rotation seems to play an important role in inhibiting the Li depletion.Based on observations collected at the ESO - La Silla and at theLNA-OPD.Tables [see full textsee full text]-[see full textsee full text] areonly available in electronic form at http://www.aanda.org
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Chemical abundances in six nearby star-forming regions. Implications for galactic evolution and planet searches around very young stars
In this paper we present a study of chemical abundances in sixstar-forming regions. Stellar parameters and metallicities are derivedusing high-resolution, high S/N spectra of weak-line T-Tauri stars ineach region. The results show that nearby star-forming regions have avery small abundance dispersion (only 0.033 dex in [Fe/H]). The averagemetallicity found is slightly below that of the Sun, although compatiblewith solar once the errors are taken into account. The derivedabundances for Si and Ni show that the observed stars have theabundances typical of Galactic thin disk stars of the same metallicity.The impact of these observations is briefly discussed in the context ofthe Galactic chemical evolution, local inter-stellar medium abundances,and in the origin of metal-rich stars in the solar neighbourhood(namely, stars more likely to harbour planets). The implication forfuture planet-search programmes around very young, nearby stars is alsodiscussed.Based on observations collected at Paranal Observatory, eso (Chile) withthe uves and flames/uves spectrographs at the VLT-Kueyen telescope (runIDs 075.C-0272(A) and 076.C-0524(A), respectively), with the ferosspectrograph at the eso/mpi 2.2-m telescope (program ID 070.C-0507(A)),as well as with the sophie spectrograph at the ohp observatory, France.
| On the kinematic evolution of young local associations and the Scorpius-Centaurus complex
Context: Over the last decade, several groups of young (mainly low-mass)stars have been discovered in the solar neighbourhood (closer than ~100pc), thanks to cross-correlation between X-ray, optical spectroscopy andkinematic data. These young local associations - including an importantfraction whose members are Hipparcos stars - offer insights into thestar formation process in low-density environments, shed light on thesubstellar domain, and could have played an important role in the recenthistory of the local interstellar medium. Aims: To study the kinematicevolution of young local associations and their relation to other youngstellar groups and structures in the local interstellar medium, thuscasting new light on recent star formation processes in the solarneighbourhood. Methods: We compiled the data published in theliterature for young local associations. Using a realistic Galacticpotential we integrated the orbits for these associations and theSco-Cen complex back in time. Results: Combining these data with thespatial structure of the Local Bubble and the spiral structure of theGalaxy, we propose a recent history of star formation in the solarneighbourhood. We suggest that both the Sco-Cen complex and young localassociations originated as a result of the impact of the inner spiralarm shock wave against a giant molecular cloud. The core of the giantmolecular cloud formed the Sco-Cen complex, and some small cloudlets ina halo around the giant molecular cloud formed young local associationsseveral million years later. We also propose a supernova in young localassociations a few million years ago as the most likely candidate tohave reheated the Local Bubble to its present temperature.
| Pre-main sequence spectroscopic binaries suitable for VLTI observations
Context: A severe problem for research in star-formation is that themasses of young stars are almost always estimated from evolutionarytracks alone. Since the tracks published by different groups differ, itis often only possible to give a rough estimate of the masses of youngstars. It is thus crucial to test and calibrate the tracks. Up to now,only a few tests of the tracks could be carried out. However, it is nowpossible with the VLTI to set constrains on the tracks by determiningthe masses of many young binary stars precisely. Aims: In order to usethe VLTI efficiently, a first step is to find suitable targets, which isthe purpose of this work. Given the distance of nearby star-formingregions, suitable VLTI targets are binaries with orbital periods betweenat least 50 days and a few years. Although a number of surveys fordetecting spectroscopic binaries have been carried out, most of thebinaries found so far have periods that are too short. Methods: We thussurveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, and ρOphiuci star-forming regions in order to search for spectroscopicbinaries with periods longer than 50 days, which are suitable for theVLTI observations. Results: As a result of the 8 year campaign, wediscovered 8 binaries with orbital periods longer than 50 days. Amongstthe newly discovered long-period binaries is CS Cha, which is one of thefew classical T Tauri stars with a circumbinary disk. The survey islimited to objects with masses higher than 0.1 to 0.2 M_ȯ forperiods between 1 and 8 years. Conclusions: We find that the frequencyof binaries with orbital periods ≤3000 days is of 20±5%. Thefrequency of long and short period pre-main sequence spectroscopicbinaries is about the same as for stars in the solar neighbourhood. Intotal 14 young binaries are now known that are suitable for massdetermination with the VLTI.based on observations obtained at the European Southern Observatory atLa Silla, Chile in program 62.I-0418(A); 63.I-0096(A); 64.I-0294(A);65.I-0012(A); 67.C-0155(A); 68.C-0292(A); 68.C-0561(A); 69.C-0207(A);70.C-0163(A); 073.C-0355(A); 074.A-9018(A); 075.C-0399(A-F). Tables 2,3, 5, 6, 8, 10, 12-20 are only available in electronic form athttp://www.aanda.org
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Identification of New M Dwarfs in the Solar Neighborhood
We present the results from a spectroscopic study of 1080 nearby activeM dwarfs, selected by correlating the Two Micron All Sky Survey andROSAT catalogs. We have derived the spectral types and estimateddistances for all of our stars. The spectral types range between K5 andM6. Nearly half of our stars lie within 50 pc. We have measured theequivalent width of the Hα emission line. Our targets show anincrease in chromospheric activity from early to midspectral types, witha peak in activity around M5. Using the count rate and hardness ratiosobtained from the ROSAT catalog, we have derived the X-ray luminosities.Our stars display a ``saturation-type'' relation between thechromospheric and coronal activity. The relation is such thatlogLX/Lbol remains ``saturated'' at a value ofapproximately -3 for varying Hα equivalent width. We have found568 matches in the USNO-B catalog and have derived the tangentialvelocities for these stars. There is a slight trend of decreasingchromospheric activity with age, such that the stars with highervtan have lower Hα equivalent widths. The coronalemission, however, remains saturated at a value oflogLX/Lbol~-3 for varying tangential velocities,suggesting that the coronal activity remains saturated with age. We donot find any break in the saturation-type relation at the spectral typeat which stars become fully convective (~M3.5). Most of the stars in oursample show more coronal emission than the dMe stars in the Hyades andPraesepe and have vtan<40 km s-1, suggesting ayoung population.
| Fundamental properties of pre-main sequence stars in young, southern star forming regions: metallicities
Aims. The primary motivation for this project is to search formetal-rich star forming regions, in which, stars of super-solarmetallicity will be created, as hopefully, will be extra-solar planetsorbiting them! The two aims of this project are: 1) to show that oursample stars are young, lithium rich, magnetically active andnon-accreting kinematic members of their respective regions. 2) Tomeasure the metallicity for such members. Methods. The feroséchelle spectrograph together with eso's 2.2 m telescope, wasused to obtain high resolution (R = 32 000) spectra for each of ourweak-lined T-Tauri target stars. The wavelength range of the spectra is≃4000{-}8000 Å. Results. We find (pre-main sequence)model-dependent isochronal ages of the Lupus, Chamaeleon and CrA targetsto be 9.1 ± 2.1 Myr, 4.5 ± 1.6 Myr and 9.0 ± 3.9Myr respectively. The majority of the stars have Li i 6707.8 Åequivalent widths similar to, or above those of, their similar massPleiades counterparts, confirming their youthfulness. Most stars arekinematic members, either single or binary, of their regions. We find amean radial velocity for objects in the Lupus cloud to be RV=+2.6± 1.8 km s-1, for the Chamaeleon i & ii clouds,RV=+12.8 ± 3.6 km s-1 whereas for the CrA cloud, wefind RV=-1.1 ± 0.5 km s-1. All stars are coronally andchromospherically active, exhibiting X-ray and Hα emission levelsmarginally less, approximately equal or superior to that of their olderIC 2602/2391 and/or Pleiades counterparts. All bar three of the targetsshow little or no signature of accretion from a circumstellarenvironment, according to their positions in a J-K/H-K'diagram. For the higher quality spectra, we have performed an iron-linemetallicity analysis for five (5) stars in Chamaeleon, four (4) stars inLupus and three (3) stars in the CrA star forming regions. These resultsshow that all three regions are slightly metal-poor, with marginallysub-solar metallicities, with <[Fe/H]> = -0.11 ± 0.14,-0.10 ± 0.04 and -0.04 ± 0.05 respectively. Conclusions. Asample of stars in several nearby, young star-forming regions has beenestablished, the majority of which is young, lithium rich, magneticallyactive and are non-accreting kinematic members of their respectiveclouds. Within the errors, each region is essentially of solarmetallicity.
| Pre-main sequence star Proper Motion Catalogue
We measured the proper motions of 1250 pre-main sequence (PMS) stars andof 104 PMS candidates spread over all-sky major star-forming regions.This work is the continuation of a previous effort where we obtainedproper motions for 213 PMS stars located in the major southernstar-forming regions. These stars are now included in this present workwith refined astrometry. The major upgrade presented here is theextension of proper motion measurements to other northern and southernstar-forming regions including the well-studied Orion and Taurus-Aurigaregions for objects as faint as V≤16.5. We improve the precision ofthe proper motions which benefited from the inclusion of newobservational material. In the PMS proper motion catalogue presentedhere, we provide for each star the mean position and proper motion aswell as important photometric information when available. We providealso the most common identifier. The rms of proper motions vary from 2to 5 mas/yr depending on the available sources of ancient positions anddepending also on the embedding and binarity of the source. With thiswork, we present the first all-sky catalogue of proper motions of PMSstars.
| Young Stars Near the Sun
Until the late 1990s the rich Hyades and the sparse UMa clusters werethe only coeval, comoving concentrations of stars known within 60 pc ofEarth. Both are hundreds of millions of years old. Then beginning in thelate 1990s the TW Hydrae Association, the Tucana/Horologium Association,the Pictoris Moving Group, and the AB Doradus Moving Group wereidentified within 60 pc of Earth, and the Chamaeleontis cluster wasfound at 97 pc. These young groups (ages 8 50 Myr), along with othernearby, young stars, will enable imaging and spectroscopic studies ofthe origin and early evolution of planetary systems.
| Formation scenarios for the young stellar associations between galactic longitudes l = 280degr - 360degr
We investigate the spatial distribution, the space velocities and agedistribution of the pre-main sequence (PMS) stars belonging toOphiuchus, Lupus and Chamaeleon star-forming regions (SFRs), and of theyoung early-type star members of the Scorpius-Centaurus OB association.These young stellar associations extend over the galactic longituderange from 280degr to 360degr , and are at a distance interval ofaround 100 and 200 pc. This study is based on a compilation ofdistances, proper motions and radial velocities from the literature forthe kinematic properties, and of basic stellar data for the constructionof Hertzsprung-Russel diagrams. Although there was no well-known OBassociation in Chamaeleon, the distances and the proper motions of agroup of 21 B- and A-type stars, taken from the Hipparcos Catalogue,lead us to propose that they form a young association. We show that theyoung early-type stars of the OB associations and the PMS stars of theSFRs follow a similar spatial distribution, i.e., there is no separationbetween the low and the high-mass young stars. We find no difference inthe kinematics nor in the ages of these two populations studied.Considering not only the stars selected by kinematic criteria but thewhole sample of young early-type stars, the scattering of their propermotions is similar to that of the PMS stars and all the young starsexhibit a common direction of motion. The space velocities of theHipparcos PMS stars of each SFR are compatible with the mean values ofthe OB associations. The PMS stars in each SFR span a wide range of ages(from 1 to 20 Myr). The ages of the OB subgroups are 8-10 Myr for UpperScorpius (US), and 16-20 Myr for Upper Centaurus Lupus (UCL) and forLower Centaurus Crux (LCC). Thus, our results do not confirm that UCL isolder than the LCC association. Based on these results and theuncertainties associated with the age determination, we cannot say thatthere is indeed a difference in the age of the two populations. Weanalyze the different scenarios for the triggering of large-scalestar-formation that have been proposed up to now, and argue that mostprobably we are observing a spiral arm that passes close to the Sun. Thealignment of young stars and molecular clouds and the average velocityof the stars in the opposite direction to the Galactic rotation agreewith the expected behavior of star formation in nearby spiral arms.Tables 1 to 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
| Multiplicity of X-Ray-selected T Tauri Stars in Chamaeleon
We report on a multiplicity survey of a sample of X-ray-selected youngstars in the Chamaeleon association. We used speckle interferometry anddirect imaging to find companions in the separation range 0.13"-6".After correction for chance alignment with background stars, we find amultiplicity (number of binaries or multiples divided by number ofsystems) of 14.0%+/-4.3% and a companion-star frequency (number ofcompanions divided by number of systems) of 14.7%+/-5.1%. Compared withsolar-type main-sequence stars, the companion-star frequency is lower bya factor of 0.61+/-0.27. This is remarkably different from the highmultiplicity found in the Taurus-Auriga star-forming region and for TTauri stars in Chamaeleon known before ROSAT. We find only a fewbinaries with projected separations of more than 70 AU, also in contrastto the results for stars known before ROSAT. This indicates that theX-ray-selected stars belong to a different population than the starsknown before ROSAT, a hypothesis further supported by their Hipparcosdistances and proper motions. Based on observations obtained at theEuropean Southern Observatory, La Silla, proposal 56.E-0197.
| The Dispersal of Young Stars and the Greater Sco-Cen Association
We review topics related to the dispersal of young stars from theirbirth-sites, and focus in particular on the entourage of young starsrelated to the ongoing star-formation event in the Sco-Cen OBassociation. We conduct a follow-up kinematic study to that presented inMamajek, Lawson, & Feigelson (2000; ApJ 544, 356) amongst nearby,isolated, young stars. In addition to the eta Cha and TW Hya groups, wefind several more intriguing Sco-Cen outlier candidates: most notablyβ Pic, PZ Tel, HD 199143, and HD 100546. We discuss the connectionbetween Sco-Cen and the southern ``150 pc Conspiracy'' molecular clouds,and in particular, Corona Australis. The kinematic evidence suggeststhat many of the nearby, isolated ~10 Myr-old stars were born nearSco-Cen during the UCL and LCC starbursts 10-15 Myr ago. We hypothesizethat these stars inherited 5-10 km/s velocities moving away fromSco-Cen, either through molecular cloud turbulence, or through formationin molecular clouds associated with the expanding Sco-Cen superbubbles(e.g. Loop I).
| The η Chamaeleontis Cluster: Origin in the Sco-Cen OB Association
A young, nearby compact aggregate of X-ray-emitting pre-main-sequencestars was recently discovered in the vicinity of η Chamaeleontis. Inthis paper, we further investigate this cluster: its membership, itsenvirons, and its origins. ROSAT High Resolution Imager X-ray data forthe cluster's T Tauri stars show high levels of magnetic activity andvariability. The cluster has an anomalous X-ray luminosity functioncompared to other young clusters, deficient in stars with low butdetectable X-ray luminosities. This suggests that many low-mass membershave escaped the surveyed core region. Photographic photometry from theUSNO-A2.0 catalog indicates that additional, X-ray-quiet members existin the cluster core region. The components of the eclipsing binary RSCha, previously modeled in the literature as post-main sequence (MS)with discordant ages, are shown to be consistent with being coevalpre-MS stars. We compute the Galactic motion of the cluster fromHipparcos data and compare it to other young stars and associations inthe fourth Galactic quadrant. The kinematic study shows that the ηCha cluster, as well as members of the TW Hya association and a newgroup near ɛ Cha, probably originated near the giant molecularcloud complex that formed the two oldest subgroups of the Sco-Cen OBassociation roughly 10-15 Myr ago. Their dispersal is consistent withthe velocity dispersions seen in giant molecular clouds. A large H Ifilament and dust lane located near η Cha has been identified aspart of a superbubble formed by Sco-Cen OB winds and supernova remnants.The passage of the superbubble may have terminated star formation in theη Cha cluster and dispersed its natal molecular gas.
| Proper motions of pre-main sequence stars { } in southern star-forming regions
We present proper motion measurements of pre-main sequence (PMS) starsassociated with major star-forming regions of the southern hemisphere(Chamaeleon, Lupus, Upper Scorpius - Ophiuchus, Corona Australis),situated in the galactic longitude range l = 290degr to l = 360degr . Alist of PMS stars as complete as possible was established based on theHerbig and Bell catalogue and many new catalogues like the PDS survey,the catalogue of Herbig Ae/Be stars by Thé et al. (\cite{the}),X-rays surveys, etc. The measurements made use of public material(mainly AC2000 and USNO-A2.0 catalogues) as well as scans of SERC-JSchmidt plates with the MAMA measuring machine (Paris) and Valinhos CCDmeridian circle observations (Brazil). We derived proper motions for 213stars, with an accuracy of 5 to 10 mas/yr depending mainly on thedifference of epochs between the position sources. The maincharacteristics of the sample are discussed. We show that systematicmotions of groups of stars exist, which are not explained by the reflexsolar motion. Based on observations made at Valinhos CCD MeridianCircle. Based on measurements made with MAMA automatic measuringmachine. Table 4 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Proper motions of faint ROSAT WTT stars in the Chamaeleon region
We present proper motions of 59 stars of the ROSAT All-Sky Survey (RASS)located in direction of the Chamaeleon star forming region (SFR) in themagnitude range B=5.1 - 17 mag. Proper motions of the fainter stars werenewly derived utilizing survey Schmidt plates from the GSC II platearchive and from a set of special plates taken with the ESO Schmidttelescope. The vector point diagram (VPD) indicates that the certifiedWTT stars cluster away from the region occupied by the brighterpre-main-sequence stars (PMS) in Cha I. The distance to this newassociation is estimated at ~ 100 pc, sensibly smaller than the 150 pcgenerally assumed for the SFR. This yields an upper limit of 2 km s(-1)for the velocity dispersion of this new kinematic group. The de-reddenedCM diagram of the group members suggest the WTT stars are still PMSobjects, but older (3-30 Myr) and less massive than previousdeterminations. These revised age estimates, the newly derived grouppeculiar velocity, and current distance estimates to the Cha I/II/IIIcomplex would favour in-situ formation against that predicted by highvelocity cloud models. Finally, based on a redetermination of thepeculiar motions of stars and gas, we speculate that the whole SFRoriginated from the local Orion spur as a result of more classicalmechanisms like interactions with the spiral arms. Based on observationscollected at the European Southern Observatory (Chile) and on data fromthe Hipparcos astrometry satellite. Table~1 is available only inelectronic form at the CDS via anonymous ftp at 130.79.128.5.
| Kinematics of T Tauri stars in Chamaeleon
We study the kinematics of T Tauri stars (TTS) located in the cores ofthe Chamaeleon clouds as well as far off these clouds. Our samplecomprises 2 early type stars known to be related to Cha i, 6 classical(CTTS) and 6 weak-line T Tauri stars (WTTS) known before the ROSATmission, and 8 bona-fide pre-main sequence (PMS) stars as well as 23presumably older stars discovered with ROSAT (Alcalaet al. 1995; Covinoet al. 1997). Altogether we present proper motions for 45 stars, takenfrom the Hipparcos, ACT and STARNET catalogues. For 12 stars of oursample parallaxes measured by Hipparcos are available, and we use themto derive constraints on the distance distribution of the other stars inour sample. Our analysis of the proper motions allows us to divide thesample into several subgroups. We analyse the motions of the stars inconnection with different star formation scenarios and find themconsistent with both the high velocity cloud (HVC) impact model (Lepine& Duvert 1994) and the cloudlet model (Feigelson 1996), whereas thedata seem to be inconsistent with any kind of a dynamical ejectionmodel.
| HIPPARCOS results for ROSAT-discovered young stars
Out of ~ 500 Lithium-rich ROSAT counterparts, which were presumed to below-mass pre-main sequence stars, 21 stars have been observed byHIPPARCOS. We study their parallaxes, proper motions, and photometricdata. For 7 out of 10 Taurus and Lupus stars in our sample, propermotions and parallaxes are not inconsistent with membership to theseassociations, while most of the stars in Chamaeleon and Scorpius appearto be young foreground stars. Combined with ground based photometry andspectroscopy, HIPPARCOS parallaxes allow us to place 15 stars on an H-Rdiagram. All these 15 stars lie above the Zero-Age-Main-Sequence andthus are indeed pre-main sequence stars with ages from 1 to 15 Myr. Onlytwo of the stars are located on the Hayashi-tracks, whereas the other 13are post-TTauri stars located on radiative tracks. Although the sampleis admittedly small, containing only 3% of the total sample ofLithium-rich ROSAT counterparts, it does not confirm recent predictionsby other authors: We find no stars in the age range from 20 to 100 Myr.The foreground pre-main sequence stars may have been ejected toward us,or they belong to the Gould Belt system, a plane filled with youngstars.
| A study of the Chamaeleon star-forming region from the ROSAT all-sky survey. III. High resolution spectroscopic study
We present the results of a high-resolution spectroscopic study on some70 stars, discovered recently on the basis of the ROSAT all-sky surveyspread over a wide area in the Chamaeleon star forming region andclassified as new weak-line T Tauri stars. We refine the previousspectral type classification, based on low-resolution spectra, andcharacterize each star in the sample according to the Hα lineprofile. We use the strength of Li I 6708, compared to Pleiades stars ofthe same spectral type, as a youth discriminator in order to recognizebona-fide pre-main sequence stars. According to the adopted ``lithiumcriterion'', more than 50% of the stars in our sample are confirmed tobe truly young, PMS stars (most having age less than 5x 10(6) yr), whilethe remaining part seems mostly composed by active, young, foregroundmain-sequence stars (possibly Pleiades-like), which contaminate theoriginal sample. We confirm the existence of some very young stars farfrom the main Chamaeleon clouds, while we do not find clear evidence forthe presence of post-T Tauri stars in our sample. We find that 5 starsin the sample are spectroscopic binaries and 1 is a spectroscopic triplesystem. We derive radial and rotational velocities for all the stars insample and analyse their distributions for different spectral typeintervals. The radial velocity distribution shows a clear peak at about15 km s(-1) , which coincides with the radial velocity of stars and gasin the Cha I cloud. However, the velocity dispersion of the weak-line TTauri stars appears much broader and, possibly, a second peak is presentaround 16-18 km s(-1) . A clear segregation in radial velocity isobserved between the strong-lithium and the weak-lithium stars, with theformer showing radial velocities which, in most cases, fall in theinterval 12
| Spectrophotometry of flares and short time scale variations in weak line, and classical T Tauri stars in Chamaeleon.
Results are presented from a monitoring program looking for variationsof the equivalent width and flux of Hα, and of the continuum flux,in 18 classical and 18 weak line T Tauri stars, and one Herbig Ae/Bestar, on time scales down to 5 minutes. The stars were observedsimultaiously for 14 hours using the multiobject-spectrograph FLAIR onthe UK Schmidt Telescope. The campaign turned up two events in weak lineT Tauri stars. Both events show the characteristics of flares: Theincrease of the Hα emission is faster than the decline, and theincrease of the emission is much larger in Hα than in thecontinuum. The total energies radiated in Hα are2.0+/-0.7x10^33^erg and >=6+/-2x10^32^ erg, or roughly 200 to 700times larger than the largest flares on the Sun. The spectrum of one ofthese events shows a component of Hα which is blue-shifted byabout 600km.s^-1^. We interpret this component as an indication of massloss, and estimate, if the event is a typical one, that the mass lossrate due to flares would be about 10^-13^Msun_/yr for a weakline T Tauri star. We derive lower limits to the magnetic flux densityof between 10 and 200G. Although a number of events have been observedin the classical T Tauri stars as well, none of these has shown thestrong increase within minutes and decrease in an hour characteristic ofa flare. We interpret this result as being due to either an opticalthickness effect in Hα, or to the absence of optical flares inclassical T Tauri stars, or simply due to failing to catch any flares inclassical T Tauri stars.
| A study of the Chamaeleon star-forming region from the ROSAT All-Sky Survey. II. The pre-main sequence population.
We analyse the nature of the optical counterparts of the ROSAT all-skysurvey (RASS) X-ray sources identified with new weak-line T Tauri (WTTS)stars in the Chamaeleon star forming region (SFR). The new WTTS aredistributed throughout the whole SFR, while the classical T Tauri stars(CTTS) are found only in the cloud cores. Adopting a distance of 150pcwe derive the stellar parameters and place the new WTTS in the HRdiagram. By comparison with theoretical pre-main sequence (PMS)evolutionary tracks, we find masses in the range of0.2-2.5Msun_ and ages from a few 10^5^yr to 5x10^7^yr. Manyof the youngest WTTS are located far away from the main Chamaeleon darkclouds. By comparing the properties of the new WTTS with those of thepreviously known Chamaeleon members, we obtain the following results: i)the new WTTS are, on average, the more massive and luminous PMS stars inChamaeleon, while the Cha II population contains the lower-mass PMSstars; ii) for stellar masses between 2.5 and 0.5Msun_, thecombined mass distribution of the PMS stars is consistent with theinitial mass function (IMF) for field stars, but declines rapidly formasses between 0.5 and 0.1Msun_, where the strongestselection effects are expected; iii) a weak trend for increasing agewith increasing angular distance from the cloud cores is observed but wecannot establish an age segregation since very young WTTS are also foundfar away from the molecular clouds; iv) the age distributions of the newWTTS and the Cha I population are nearly identical, while that of theCha II population is shifted towards younger stars indicating that ChaII is probably in an earlier evolutionary phase as compared with Cha Iand the new WTTS; v) no decrease of the number density of WTTS isobserved with increasing distance to the clouds; vi) the level of X-rayemission of the new WTTS is higher than that of the previously knownChamaeleon members, and the fraction of energy released as X-rayemission, is higher in the new WTTS than in the Cha I TTS. The latter issimilar to the X-ray emission level found in open clusters. Finally, wediscuss possible mechanisms which may give rise to the observed spatialdistribution of the PMS stars in Chamaeleon.
| A study of the Chamaeleon star forming region from the ROSAT all-sky survey. I. X-ray observations and optical identifications.
We present the observations of the ROSAT all-sky survey (RASS) in thedirection of the Chamaeleon cloud complex, as well as the spectroscopicidentifications of the detected X-ray sources. The main purpose of thisidentification program was the search for low mass pre-main sequencestars. Sixteen previously known PMS stars were detected with highconfidence by ROSAT. Eight are classical T Tauri stars and eight areweak-line T Tauri stars, Seventy-seven new weak-line T Tauri stars wereidentified on the basis of the presence of strong Li λ 6707absorption, spectral type later than F0 and chromospheric emission. Wegive coordinates and count rates of the X-ray sources, and presentoptical spectra and finding charts for the sources identified opticallyas new pre-main sequence stars. Optical UBV(RI)_c_ and near-infraredJHKLM photometry for this sample of stars is also provided. In addition,6 new dKe-dMe candidates are found among the RASS sources.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Bukalemun |
| Sað Açýklýk: | 11h59m42.28s |
| Yükselim: | -76°01'26.2" |
| Görünürdeki Parlaklýk: | 11.124 |
| özdevim Sað Açýklýk: | -43.9 |
| özdevim Yükselim: | -6.4 |
| B-T magnitude: | 12.87 |
| V-T magnitude: | 11.269 |
Kataloglar ve belirtme:
|
INHABITED SKY
