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Disk Evolution in the Three Nearby Star-forming Regions of Taurus, Chamaeleon, and Ophiuchus
We analyze samples of Spitzer Infrared Spectrograph spectra of T Tauristars in the Ophiuchus, Taurus, and Chamaeleon I star-forming regions,whose median ages lie in the <1-2 Myr range. The median mid-infraredspectra of objects in these three regions are similar in shape,suggesting, on average, similar disk structures. When normalized to thesame stellar luminosity, the medians follow each other closely, implyingcomparable mid-infrared excess emission from the circumstellar disks. Weuse the spectral index between 13 and 31 μm and the equivalent widthof the 10 μm silicate emission feature to identify objects whose diskconfiguration departs from that of a continuous, optically thickaccretion disk. Transitional disks, whose steep 13-31 μm spectralslope and near-IR flux deficit reveal inner disk clearing, occur withabout the same frequency of a few percent in all three regions. Objectswith unusually large 10 μm equivalent widths are more common(20%-30%); they could reveal the presence of disk gaps filled withoptically thin dust. Based on their medians and fraction of evolveddisks, T Tauri stars in Taurus and Chamaeleon I are very alike. Diskevolution sets in early, since already the youngest region, theOphiuchus core (L1688), has more settled disks with larger grains. Ourresults indicate that protoplanetary disks show clear signs of dustevolution at an age of a few Myr, even as early as ~1 Myr, but age isnot the only factor determining the degree of evolution during the firstfew million years of a disk's lifetime.
| A Multiplicity Census of Young Stars in Chamaeleon I
We present the results of a multiplicity survey of 126 stars spanning~0.1-3 Msolar in the ~2 Myr old Chamaeleon I star-formingregion, based on adaptive optics imaging with the ESO Very LargeTelescope. Our observations have revealed 30 binaries and six triples,of which 19 and four, respectively, are new discoveries. The overallmultiplicity fraction we find for Cha I (~30%) is similar to thosereported for other dispersed young associations, but significantlyhigher than seen in denser clusters and the field, for comparablesamples. Both the frequency and the maximum separation of Cha I binariesdecline with decreasing mass, while the mass ratios approach unity;conversely, tighter pairs are more likely to be equal mass. We confirmthat brown dwarf companions to stars are rare, even at young ages atwide separations. Based on follow-up spectroscopy of two low-masssubstellar companion candidates, we conclude that both are likelybackground stars. The overall multiplicity fraction in Cha I is in roughagreement with numerical simulations of cloud collapse andfragmentation, but its observed mass dependence is less steep thanpredicted. The paucity of higher order multiples, in particular,provides a stringent constraint on the simulations, and seems toindicate a low level of turbulence in the prestellar cores in Cha I.
| Towards the main sequence: detailed analysis of weak line and post-T Tauri stars
A detailed study was performed for a sample of low-masspre-main-sequence (PMS) stars, previously identified as weak-line TTauri stars, which are compared to members of the Tucanae and HorologiumAssociations. Aiming to verify if there is any pattern of abundanceswhen comparing the young stars at different phases, we selected objectsin the range from 1 to 100 Myr, which covers most of PMS evolution.High-resolution optical spectra were acquired at European SouthernObservatory and Observatório do Pico dos Dias. The stellarfundamental parameters effective temperature and gravity were calculatedby excitation and ionization equilibria of iron absorption lines.Chemical abundances were obtained via equivalent width calculations andspectral synthesis for 44 per cent of the sample, which showsmetallicities within 0.5 dex solar. A classification was developed basedon equivalent width of LiI 6708 Å and Hα lines and spectraltypes of the studied stars. This classification allowed a separation ofthe sample into categories that correspond to different evolutive stagesin the PMS. The position of these stars in the Hertzsprung-Russelldiagram was also inspected in order to estimate their ages and masses.Among the studied objects, it was verified that our sample actuallycontains seven weak-line T Tauri stars, three are Classical T Tauri, 12are Fe/Ge PMS stars and 21 are post-T Tauri or young main-sequencestars. An estimation of circumstellar luminosity was obtained using adisc model to reproduce the observed spectral energy distribution. Mostof the stars show low levels of circumstellar emission, corresponding toless than 30 per cent of the total emission.
| Quiescent H2 Emission From Pre-Main-Sequence Stars in Chamaeleon I
We report the discovery of quiescent emission from molecular hydrogengas located in the circumstellar disks of six pre-main-sequence stars,including two weak-line T Tauri stars (TTSs) and one Herbig AeBe star,in the Chamaeleon I star-forming region. For two of these stars, we alsoplace upper limits on the 2-->1 S(1)/1-->0 S(1) line ratios of~0.4 and 0.5. Of the 11 pre-main-sequence sources now known to besources of quiescent near-infrared hydrogen emission, four possesstransitional disks, which suggests that detectable levels ofH2 emission and the presence of inner disk holes arecorrelated. These H2 detections demonstrate that these innerholes are not completely devoid of gas, in agreement with the presenceof observable accretion signatures for all four of these stars and therecent detections of [Ne II] emission from three of them. The overlap in[Ne II] and H2 detections hints at a possible correlationbetween these two features and suggests a shared excitation mechanism ofhigh-energy photons. Our models, combined with the kinematic informationfrom the H2 lines, locate the bulk of the emitting gas at afew tens of AU from the stars. We also find a correlation betweenH2 detections and those targets which possess the largestHα equivalent widths, suggesting a link between accretion activityand quiescent H2 emission. We conclude that quiescentH2 emission from relatively hot gas within the disks of TTSsis most likely related to ongoing accretion activity, the production ofUV photons and/or X-rays, and the evolutionary status of the dust grainpopulations in the inner disks.
| The Stellar Population of the Chamaeleon I Star-forming Region
I present a new census of the stellar population in the Chamaeleon Istar-forming region. Using optical and near-IR photometry and follow-upspectroscopy, I have discovered 50 new members of Chamaeleon I,expanding the census of known members to 226 objects. Fourteen of thesenew members have spectral types later than M6, which doubles the numberof known members that are likely to be substellar. I have estimatedextinctions, luminosities, and effective temperatures for the knownmembers, used these data to construct an H-R diagram for the cluster,and inferred individual masses and ages with the theoreticalevolutionary models of Baraffe and Chabrier. The distribution ofisochronal ages indicates that star formation began 3-4 and 5-6 Myr agoin the southern and northern subclusters, respectively, and hascontinued to the present time at a declining rate. The IMF in ChamaeleonI reaches a maximum at a mass of 0.1-0.15 Msolar and thusclosely resembles the IMFs in IC 348 and the Orion Nebula Cluster. Inlogarithmic units where the Salpeter slope is 1.35, the IMF is roughlyflat in the substellar regime and shows no indication of reaching aminimum down to a completeness limit of 0.01 Msolar. Thelow-mass stars are more widely distributed than members at other massesin the northern subcluster, but this is not the case in the southernsubcluster. Meanwhile, the brown dwarfs have the same spatialdistribution as the stars out to a radius of 3° (8.5 pc) from thecenter of Chamaeleon I.Based on observations performed with the Magellan Telescopes at LasCampanas Observatory, Cerro Tololo Inter-American Observatory (CTIO),Gemini Observatory, and the NASA/ESA Hubble Space Telescope (HST). CTIOis operated by the Association of Universities for Research in Astronomy(AURA) under a contract with the National Science Foundation (NSF).Gemini Observatory is operated by AURA under a cooperative agreementwith the NSF on behalf of the Gemini partnership: the NSF (US), theParticle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina). The HSTobservations are associated with proposal ID 10138 and were obtained atthe Space Telescope Science Institute, which is operated by AURA underNASA contract NAS 5-26555.
| Lithium abundances of very low mass members of Chamaeleon I
Aims:We present the first study of the lithium abundances of very lowmass objects in Chamaeleon I close to the hydrogen burning mass limitbased on atmospheric models and high-resolution spectroscopicobservations. The studied objects, Cha Hα 2, 3, 4, 5, 6 and 8, arevery young brown dwarf candidates and very low mass stars on the vergeof lithium depletion. Methods: For this analysis, we havecomputed a new “GAIA-cond” class model grid over effectivetemperatures from 2600 K to 3100 K, surface gravities from log(g) = 3.5to 5.5, and lithium abundances from logɛ = 0.0 to 3.7, for twodifferent line profile setups introduced in previous work. Calculatedsynthetic spectra are compared with high-resolution UVES / VLT echellespectra of the objects. Results: We find good descriptions of thelithium resonance doublet lines at 6708 Å and of the surroundingpseudo-continuum and determine a consistent set of lithium abundances(log(ɛ) = 1.55). However, the derived lithium abundances arelower than the meteoritic one (log(ɛ) = 3.31) and that of highermass stars in Cha I (log(ɛ) = 3.1/3.4 forLTE-/non-LTE-calculations). By modeling the TiO-line, we demonstratethat veiling does not make the lithium lines appear weaker. We can alsorule out that the results are spoiled by the presence of spots. Conclusions: A possible explanation for these results would be thatthe objects are either more massive, or much older, than previouslythought, so that the lithium depletion has already started. Although theuncertainties of the masses and ages are large, they are not largeenough as to explain the observed lithium depletion. Therefore, the mostlikely explanation is either a lack of understanding of the details ofthe formation of the lithium line, or a lack of understanding of theinternal structure of the very young low-mass objects.Based on observations obtained at the European Southern Observatory atParanal, Chile in program 65.L-0629(A,B) and 65.I-0011(A).
| The Role of Mass and Environment in Multiple-Star Formation: A 2MASS Survey of Wide Multiplicity in Three Young Associations
We present the results of a search for wide binary systems among 783members of three nearby young associations: Taurus-Auriga, Chamaeleon I,and two subgroups of Upper Scorpius. Near-infrared (JHK) imagery from2MASS was analyzed to search for wide (1"-30" ~150-4500 AU) companionsto known association members, using color-magnitude cuts to rejectlikely background stars. We identify a total of 131 candidate binarycompanions with colors consistent with physical association, of which 39have not been identified previously in the literature. Our resultssuggest that the wide binary frequency is a function of both mass andenvironment, with significantly higher frequencies among high-mass starsthan lower mass stars and in the T associations than in the OBassociation. We discuss the implications for wide binary formation andconclude that the environmental dependence is not a direct result ofstellar density or total association mass, but instead might depend onanother environmental parameter like the gas temperature. The binarypopulations in these associations generally follow the empiricalmass-maximum separation relation observed for field binaries, but wehave found one candidate low-mass system (USco 160611.9-193532Mtot~0.4 Msolar) that has a projected separation(10.8" 1550 AU) much larger than the suggested limit for its mass.Finally, we find that the binary frequency in the USco-B subgroup issignificantly higher than in the USco-A subgroup and is consistent withthe measured values in Taurus and ChamI. This discrepancy, the absenceof high-mass stars in USco-B, and its marginally distinct kinematicssuggest that it might not be directly associated with the OBassociations of Sco-Cen but instead represents an older analog of theyounger ρ Oph or Lupus associations.
| 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
| Investigating grain growth in disks around southern T Tauri stars at millimetre wavelengths
Context: .Low-mass stars form with disks in which the coagulation ofgrains may eventually lead to the formation of planets. It is not knownwhen and where grain growth occurs, as models that explain theobservations are often degenerate. A way to break this degeneracy is toresolve the sources under study. Aims: .Our aim is to findevidence for the existence of grains of millimetre sizes in disks aroundT Tauri stars, implying grain growth. Methods: .The AustraliaTelescope Compact Array (ATCA) was used to observe 15 southern T Tauristars, five in the constellation Lupus and ten in Chamaeleon, at 3.3 mm.The five Lupus sources were also observed with the SubMillimeter Array(SMA) at 1.4 mm. Our new data are complemented with data from theliterature to determine the slopes of the spectral energy distributionsin the millimetre regime. Results: .Ten sources were detected atbetter than 3σ with the ATCA, with σ ≈ 1-2 mJy, and allsources that were observed with the SMA were detected at better than15σ, with σ ≈ 4 mJy. Six of the sources in our sample areresolved to physical radii of ~100 AU. Assuming that the emission fromsuch large disks is predominantly optically thin, the millimetre slopecan be related directly to the opacity index. For the other sources, theopacity indices are lower limits. Four out of six resolved sources haveopacity indices ⪉1, indicating grain growth to millimetre sizes andlarger. The masses of the disks range from <0.01 to 0.08 M_ȯ,which is comparable to the minimum mass solar nebula. A tentativecorrelation is found between the millimetre slope and the strength andshape of the 10-μm silicate feature, indicating that grain growthoccurs on similar (short) timescales in both the inner and outerdisk.
| 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.
| A VLT/NACO survey for triple and quadruple systems among visual pre-main sequence binaries
Aims.This paper describes a systematic search for high-ordermultiplicity among wide visual Pre-Main Sequence (PMS) binaries. Methods: .We conducted an Adaptive Optics survey of a sample of 58 PMSwide binaries from various star-forming regions, which include 52 TTauri systems with mostly K- and M-type primaries, with the NIRinstrument NACO at the VLT. Results: .Of these 52 systems, 7 arefound to be triple (2 new) and 7 quadruple (1 new). The new closecompanions are most likely physically bound based on their probabilityof chance projection and, for some of them, on their position on acolor-color diagram. The corresponding degree of multiplicity among widebinaries (number of triples and quadruples divided by the number ofsystems) is 26.9 ± 7.2% in the projected separation range ~0.07arcsec -12'', with the largest contribution from the Taurus-Aurigacloud. We also found that this degree of multiplicity is twice in Tauruscompared to Ophiuchus and Chamaeleon for which the same number ofsources are present in our sample. Considering a restricted samplecomposed of systems at distance 140-190 pc, the degree of multiplicityis 26.8 ± 8.1%, in the separation range 10/14 AU-1700/2300 AU (30binaries, 5 triples, 6 quadruples). The observed frequency agrees withresults from previous multiplicity surveys within the uncertainties,although a significant overabundance of quadruple systems compared totriple systems is apparent. Tentatively including the spectroscopicpairs in our restricted sample and comparing the multiplicity fractionsto those measured for solar-type main-sequence stars in the solarneighborhood leads to the conclusion that both the ratio of triples tobinaries and the ratio of quadruples to triples seems to be in excessamong young stars. Most of the current numerical simulations of multiplestar formation, and especially smoothed particles hydrodynamicssimulations, over-predict the fraction of high-order multiplicity whencompared to our results. The circumstellar properties around theindividual components of our high-order multiple systems tend to favormixed systems (i.e. systems including components of wTTS and cTTS type),which is in general agreement with previous studies of disks inbinaries, with the exception of Taurus, where we find a preponderance ofsimilar type of components among the multiples studied.
| Deep Imaging Surveys of Star-forming Clouds. IV. The Meek and the Mighty: Outflows from Young Stars in Chamaeleon I
We present a survey of shocks and outflows in the Chamaeleon Istar-forming complex using Hα, [S II], and SDSS i' images obtainedfrom the ground, an i' image obtained with the Hubble Space Telescope,and 4.5 μm images obtained with the Infrared Array Camera (IRAC) onthe Spitzer Space Telescope. We find new Herbig-Haro (HH) objects andextensions to the previously cataloged shocks that trace parts of atleast 20 distinct outflows from young stars. Some HH objects mark thepresence of giant outflows, the largest of which is powered by Cha-MMS1and associated with HH 49/50 near the Ced 110 region. Other large flowsare powered by Cha-MMS2 in the Ced 112 region and the IRN in the Ced 111region. Although some shocks exhibit infrared emission in the IRACbands, most notably HH 49/50 (the ``tornado''), most outflows in the ChaI clouds are not detected in the Spitzer IRAC bands. This result isconsistent with the general lack of extensive 2.12 μm H2emission from Cha I.
| Improved kinematics for brown dwarfs and very low-mass stars in Chamaeleon I and a discussion of brown dwarf formation
We present a precise kinematic study of very young brown dwarfs in theCha I cloud based on radial velocities (RVs) measured with UVES at theVLT. The kinematics of the brown dwarfs in Cha I are compared to thekinematics of T Tauri stars in the same field, based on both UVESmeasurements for very low-mass ones and on RVs from the literature. MoreUVES spectra were taken compared with a former paper (Joergens &Guenther 2001, A&A, 379, L9), and the reduction of the spectra wasimproved, while studying the literature for RVs of T Tauri stars in ChaI led to a cleaned and enlarged sample of T Tauri stars. The result isan improved empirical RV distribution of brown dwarfs, as well as of TTauri stars in Cha I. We found that the RVs of the nine brown dwarfs andvery low-mass stars (M 6-M 8) in ChaI that were studied have a meanvalue of 15.7 km s-1 and a dispersion measured in terms of astandard deviation of 0.9 km s-1, and they cover a totalrange of 2.6 km s-1. The standard deviation is consistentwith the dispersion measured earlier in terms of fwhm of 2.1 kms-1. The studied sample of 25 T Tauri stars (G2-M 5) has amean RV of 14.7 km s-1, a dispersion in terms of standarddeviation of 1.3 km s-1 and in terms of fwhm of 3.0 kms-1, and a total range of 4.5 km s-1. The RVdispersion of the brown dwarfs is consistent within the errors with thatof T Tauri stars, which is in line with the finding of no massdependence in some theoretical models of the ejection-scenario for theformation of brown dwarfs. In contrast to current N-body simulations, wedid not find a high-velocity tail for the brown dwarfs RVs. We foundhints suggesting different kinematics for binaries compared topredominantly single objects in Cha I, as suggested by some models. Theglobal RV dispersion for Cha I members (1.24 km s-1) issignificantly lower than for Taurus members (2.0 km s-1),despite higher stellar density in Cha I showing that a fundamentalincrease in velocity dispersion with stellar density of the star-formingregion is not established observationally. The RVs of brown dwarfsobserved in Cha I are less dispersed than predicted by existing modelsfor the ejection-scenario.
| 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.
| XMM-Newton probes the stellar population in Chamaeleon I South
We report on a 30 ks XMM-Newton observation of the central region of theCha I star forming cloud. The field includes a substantial fraction ofthe known pre-main-sequence population of Cha I South, including allthirteen known very-low mass Hα emitters. We detect two bona-fidebrown dwarfs (spectral types M 7.5 and M 8) and seven Hα emittingobjects near the hydrogen burning mass limit, including six of sevenearlier detections by ROSAT. Three objects classified as Cha I candidatemembers according to their NIR photometry are revealed by XMM-Newton,providing further evidence for them being truly young stars. A total of11 new X-ray sources without known optical/IR counterpart may comprisefurther as yet unrecognized faint cloud members. Spectral analysis ofthe X-ray bright stars shows that previous X-ray studies in Cha I haveunderestimated the X-ray luminosities, as a result of simplifiedassumptions on the spectral shape. In particular, the extinction isvariable over the field, such that the choice of a uniform value for thecolumn density is inappropriate. We establish that the X-ray saturationlevel for the late-type stars in Cha I is located nearLx/Lbol ˜ 10-2.5, with a possibledecline to Lx/Lbol ˜ 10-3 for thelowest mass stars. A group of strongly absorbed stars with unusuallyhard X-ray emission is clustered around HD 97048, a HAeBe star and theonly confirmed intermediate-mass star in the field. While the X-rayproperties of HD 97048 are indistinguishable from those of itslower-mass neighbors, another presumably A-type star (identified as suchbased on NIR photometry) stands out as the softest X-ray emitter in thewhole sample. This suggests that various X-ray emission mechanisms maybe at work in intermediate-mass pre-main-sequence stars. We find thatX-ray luminosity follows a tight correlation with age, effectivetemperature, and mass. No dramatic changes in these correlations areseen at the substellar boundary, suggesting that the same dynamomechanism operates in both low-mass stars and brown dwarfs, at least atyoung ages. The variability of the lowest-mass objects is also similarto that of higher-mass T Tauri stars. X-ray flares are seen in about1/10th of the Cha I members in the field.Tables \ref{tab:x-sources}-\ref{tab:lx} are only available in electronicform at http://www.edpsciences.org
| A search for shock-excited molecular hydrogen knots in Chamaeleon I very low mass YSOs
We have obtained narrow-band images of three selected areas of theChamaeleon I dark cloud which harbor very low mass young stars, centeredon the H2 and Brγ lines and neighboring continuum aswell as on the broad band Ks. One region is located in thenorthern part of the cloud, roughly coinciding with the densest area.The other two regions are in the southern section of the cloud. Our aimis to search for H2 outflows associated with these objects.In the northern region, we found seven new H2 knots, five ofwhich are aligned in the direction of a previously known 12COmolecular bipolar outflow. Further evidence that the class I low massstellar object ISO-ChaI 192 is the driving source of the molecular flowis given by the presence of a 960 AU long elongated structure at 2.2μm emanating from this star and oriented parallel to the bipolarstructure. Another pair of H2 knots, although lyingrelatively nearby, is not aligned with the outflow direction. They arelocated on opposite sides of C1-6, a low mass class II object in thenorthern part of the Chamaeleon I dark cloud. In contrast, we fail todetect any H2 emission object brighter than our sensitivitylimit (˜6 × 10-32 W/m2 Hzarcsec2) in the two southern areas of the cloud that alsoharbor several very low mass stars, including two transitionstellar/sub-stellar objects. This negative result is probably notsurprising in view of the extremely low accretion rates measured forbrown dwarfs (dot{M} ˜ 10-12 -10-9Mȯ yr-1). Deeper H2 observationsare required to better constraint the outflow event in sub-stellarobjects.Based on observations collected at the European Southern Observatory, LaSilla, Chile, ESO proposal 71.C-0144 and at Las Campanas Observatory,Chile.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| The short period multiplicity among T Tauri stars
We present the results of high-resolution spectroscopic observationscarried out over three years aimed at estimating the short-period(P_orb<100 days) binary frequency of a sample of T Tauri stars inOph-Sco, Cha, Lup, CrA star forming regions (SFRs), already observedwith high angular resolution techniques by Ghez et al.(\cite{Ghezetal93}) and by Ghez et al. (\cite{Ghezetal97}) to detectwider components. When combining all four SFRs, the short-period binaryfrequency is indistinguishable from that found by Duquennoy & Mayor(\cite{DuqMay91}) for the solar-type field stars which is alsoconsistent with the previous result obtained by Mathieu(\cite{Mathieu92}, \cite{Mathieu94}). When Oph-Sco is analyzedseparately, it seems that there is an excess of short-period binaries ofa factor 2-2.5. On the contrary, short-period binary systems seem to beabsent in the sample containing stars in Cha/Lup/CrA. Such a trend wasequally found by Mathieu (\cite{Mathieu92}) in Taurus. An excess ofspectroscopic systems among the components of visual multiple systems isalso observed.Based on observations collected with the Swiss Euler Telescope and 1.5-mESO, proposal 63.I-0112.
| An IUE Atlas of Pre-Main-Sequence Stars. III. Co-added Final Archive Spectra from the Long-Wavelength Cameras
We identified 137 T Tauri stars (TTS) and 97 Herbig Ae/Be (HAEBE) starsobserved by IUE in the wavelength interval 1900-3200 Å. Eachlow-resolution (R~6 Å) spectrum was visually inspected for sourcecontamination and data quality, and then all usable spectra werecombined to form a single time averaged spectrum for each star. Forsources with multiple observations, we characterized variability andcompared with previously published amplitudes at shorter wavelengths. Wecombined several co-added spectra of diskless TTS to produce a pair ofintrinsic stellar spectra unaffected by accretion. We then fittedspectra of TTS with the reddened sum of an intrinsic spectrum and aschematic veiling continuum, measuring emission line fluxes from theresiduals. We used extinction and distance estimates from the literatureto convert measured Mg II line fluxes into intrinsic line luminosities,noting that the IUE detection limit introduces a sample bias such thatintrinsic line luminosity is correlated with extinction. This samplebias complicates any physical interpretation of TTS intrinsicluminosities. We measured extinction toward HAEBE stars by fitting ourco-added IUE spectra with reddened spectra of main-sequence stars andalso from V band minus 3000 Å color excess. We measured excessline emission and absorption in spectra of HAEBE stars divided by fittedspectra of main-sequence stars, noting that HAEBE stars with an infraredexcess indicating circumstellar material typically also have anomalousUV line strengths. In the latter situation, Mg II is usually shallowerthan in a main-sequence star of the same spectral class, whereas Fe IIlines are equally likely to be deeper or shallower. Our co-added spectraof TTS, HAEBE stars, and main-sequence templates are availableelectronically.
| 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
| Near-Infrared Spectra of Chamaeleon I Stars
We present low-resolution (R~500) near-infrared spectra of 46 candidateyoung stellar objects in the Chamaeleon I star-forming region recentlydetected in several deep photometric surveys of the cloud. Most of thesestars have K<12. In addition, we present spectra of 63 previouslyknown southern hemisphere young stars mainly belonging to the ChamaeleonI and Lupus dark clouds. We describe near-infrared spectroscopiccharacteristics of these stars and use the water vapor indexes to derivespectral types for the new objects. Photometric data from the literatureare used to estimate the bolometric luminosities of all sources. Weapply the pre-main-sequence evolutionary tracks and isochrones ofD'Antona & Mazzitelli to derive masses and ages. We detect twoobjects with mass below the H-burning limit among the 46 new candidates.One of these objects (PMK99 IR Cha INa1) is the likely driving source ofa bipolar outflow in the northern region of the cloud. Combining ourtargets with previously known members of the cloud we analyze the massand age distributions for 145 stars in the Chamaeleon I dark could. Themass histogram rises from about 2.5 up to 0.4 Msolar and thenfalls off. The median mass is 0.30 Msolar. The currentpopulation with masses greater than 0.4 Msolar is essentiallycomplete. The scarcity of very low mass members is interpreted aspopulation bias toward the least massive and fainter objects. If weassume the true Chamaeleon I initial mass function is flat (inlogarithmic mass bins) in the interval 0.4-0.04 Msolar asrecently found by Comerón et al. in the central 300arcmin2 region, then we estimate that ~100 stars remain to befound in that mass range. The distribution of ages indicates an activestar-formation episode within the last ~5×105 yr and adecreasing rate at older ages (a few times 107 yr).Based on observations collected at the European Southern Observatory,Chile, (ESO proposal N.63.I-0269[A]).
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| UVES spectra of young brown dwarfs in Cha I: Radial and rotational velocities
Based on high-resolution UVES spectra we found that the radial velocity(RV) dispersion of nine of twelve known young bona fide and candidatebrown dwarfs in the Cha I dark cloud is 2.0 km s-1, i.e.significantly smaller than the RV dispersion of T Tauri stars in Cha I(3.6 km s-1) and only slightly larger than the dispersion ofthe surrounding molecular gas (1.2 km s-1) (Mizuno et al.\cite{miz}). This result indicates that the majority of these browndwarfs are not ejected with high velocity out of a dense region asproposed by some formation scenarios for brown dwarfs. The mean RVvalues are consistent with the objects being kinematic members of Cha I.The RV dispersion of the T Tauri stars confined to the Cha I region isbased on a compilation of T Tauri stars with known RVs from theliterature plus three T Tauri stars observed with UVES and unpublishedRVs for nine T Tauri stars. Time-resolved spectroscopy revealed RVvariations for five out of nine of the bona fide and candidate browndwarfs in Cha I, which could be due to orbiting planets or surfacefeatures. Furthermore we derived rotational velocities v sin i and theLithium 6708 Å equivalent width. Based on observations obtained atthe European Southern Observatory at Paranal, Chile in program 65.L-0629and 65.I-0011.
| Mass ratios of the components in T Tauri binary systems and implications for multiple star formation
Using near-infrared speckle interferometry we have obtained resolvedJHK-photometry for the components of 58 young binary systems. From thesemeasurements, combined with other data taken from literature, we derivemasses and particularly mass ratios of the components. We use theJ-magnitude as an indicator for the stellar luminosity and assign theoptical spectral type of the system to the primary. On the assumptionthat the components within a binary are coeval we can then also placethe secondaries into the HRD and derive masses and mass ratios for bothcomponents by comparison with different sets of current theoreticalpre-main sequence evolutionary tracks. The resulting distribution ofmass ratios is comparatively flat for M2/M1>=0.2, but depends on assumed evolutionary tracks. The mass ratio isneither correlated with the primary's mass or the components'separation. These findings are in line with the assumption that for mostmultiple systems in T associations the components' masses areprincipally determined by fragmentation during formation and not by thefollowing accretion processes. Only very few unusually red objects werenewly found among the detected companions.This finding shows that theobserved overabundance of binaries in the Taurus-Auriga associationcompared to nearby main sequence stars should be real and not theoutcome of observational biases related to infrared observing. Based onobservations collected at the German-Spanish Astronomical Center onCalar Alto, Spain, and at the European Southern Observatory, La Silla,Chile. Full Tables A.1 and A.2 are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/376/982 and Figs. B.1 andC.1 are only available in electronic form at http://www.edpsciences.org
| Dusty Debris around Solar-Type Stars: Temporal Disk Evolution
Using ISO-ISOPHOT, we carried out a survey of almost 150 stars to searchfor evidence of emission from dust orbiting young main-sequence stars,both in clusters and isolated systems. Over half of the detections arenew examples of dusty stellar systems and demonstrate that such dust canbe detected around numerous stars older than a few times 106yr. Fluxes at 60 and either 90 or 100 μm for the new excess sourcestogether with improved fluxes for a number of IRAS-identified sourcesare presented. Analysis of the excess luminosity relative to the stellarphotosphere shows a systematic decline of this excess with stellar ageconsistent with a power-law index of -2.
| A 3 μM Survey of the Chamaeleon I Dark Cloud
We describe an L-band photometric survey of ~ 0.5 deg2 of theChamaeleon I dark cloud. The survey has a completeness limit ofL<11.0. Our survey detects 124 sources, including all knownpre-main-sequence stars with L<=11. The fraction of sources withnear-IR excess emission is 58%+/-4% for K=9-11. Cha I sources have bluerH-K and K-L colors than pre-main-sequence stars in Taurus-Auriga. Thesesources also have a strong correlation between EW(Hα) and K-L.Stars with K-L<=0.6 have weak Hα emission; stars withK-L>=0.6 have strong Hα emission. Because many Cha I sourcesare heavily reddened, this division between weak emission T Tauri starsand classical T Tauri stars occurs at a redder K-L than inTaurus-Auriga. Based on observations obtained with the SPIREX/Abu systemin Antarctica. SPIREX/Abu was operated for the 1999 observing seasonunder agreement between the National Optical Astronomy Observatories(NOAO) and the Center for Astrophysical Research in Antarctica (CARA).
| 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
| An IUE Atlas of Pre-Main-Sequence Stars. I. Co-added Final Archive Spectra from the SWP Camera
We have identified 50 T Tauri stars (TTS) and 74 Herbig Ae/Be (HAEBE)stars observed in the IUE short-wavelength bandpass (1150-1980 Å).Each low-resolution (R~6 Å) spectrum was visually inspected forsource contamination and data quality, and then all good spectra werecombined to form a single time-averaged spectrum for each star. Use ofIUE Final Archive spectra processed with NEWSIPS reduces fixed patternnoise in individual spectra, allowing significant signal-to-noise ratiogains in our co-added spectra. For the TTS observed by IUE, we measuredfluxes and uncertainties for 17 spectral features, including twocontinuum windows and four fluoresced H2 complexes. Thirteenof the 32 accreting TTS observed by IUE have detectable H2emission, which until now had been reported only for T Tau. Using anempirical correlation between H2 and C IV line flux, we showthat lack of sensitivity can account for practically all nondetections,suggesting that H2 fluorescence may be intrinsically strongin all accreting TTS systems. Comparison of IUE and GHRS spectra of TTau show extended emission primarily, but not exclusively, in lines ofH2. We also fit reddened main-sequence templates to 72 HAEBEstars, determining extinction and checking spectral types. Several ofthe HAEBE stars could not be fitted well or yielded implausibly lowextinctions, suggesting the presence of a minority emission componenthotter than the stellar photosphere, perhaps caused by white dwarfcompanions or heating in accretion shocks. We identified broadwavelength intervals in the far-UV that contain circumstellar absorptionfeatures ubiquitous in B5-A4 HAEBE stars, declining in prominence forearlier spectral types, perhaps caused by increasing ionization of metalresonance lines. For 61 HAEBE stars, we measured or set upper limits ona depth index that characterizes the strength of circumstellarabsorption and compared this depth index with published IR properties.
| Probing the brown dwarf population of the Chamaeleon I star forming region
We present observations of a sample of 13 very low mass stars and browndwarfs in the central region of the Chamaeleon I star forming cloud. Theobservations include slitless spectroscopy around Hα to identifynew members, low resolution long-slit visible and near-infraredspectroscopy, deep ROSAT PSPC X-ray observations, and ISOCAMmid-infrared observations. Our sample adds seven new objects to thosediscussed by Comerón, Rieke, and Neuhäuser (1999, A&A,343, 477) and extends the range of spectral types up to M8. We studydifferent narrow-band indices as a tool for detecting and classifyingvery late-type young stellar objects. As to K-band spectra, we find thatthe visible features are not appropriate to yield a spectralclassification more accurate than a few subclasses at best beyond M6.None of our sources displays K-band excess emission, but four haveexcess at 6.7 mu m suggesting that, although circumstellar disks arecommon around young very low mass stars, their inner regions are ingeneral not hot enough to radiate significantly in the K band.Mid-infrared emission loosely correlates with Hα emission: sourceswithout mid-IR excesses are always weak Hα emitters, while mid-IRexcess sources have a broad range of Hα equivalent widths. X-rayemission is detected for 7 objects with spectral type M6 or later,including one bona-fide brown dwarf and three objects near the borderseparating stars and brown dwarfs. X-ray to bolometric luminosity ratiosare typical of low mass, fully convective stars. The non-detection ofX-ray emission at comparable levels from more evolved brown dwarfssuggests that X-ray activity may be restricted to early stages of browndwarf evolution. We discuss in detail the temperatures and luminositiesof our objects based on their magnitudes and spectra, and use thederived values to estimate masses and ages according to two differentsets of pre-main sequence evolutionary tracks. Both sets of models arein good agreement concerning the mass derived for our objects, showingthat four of them are bona-fide brown dwarfs, six are transitionobjects, and three are low mass stars. Derived ages differ significantlydepending on the adopted models, especially at the lowest masses. Thisis mainly due to the objects lying on opposite sides of thedeuterium-burning main sequence depending on whether one or another setis used. Using Baraffe et al. (1998, A&A, 337, 403) models for thedating of each object in the area of our survey with mass below 1 M_sun,we find that most have ages near 2 * 106 years, with a smallspread around that value. However, a few objects appear to have agesnear 2 * 107 years, suggesting that most, but not all, starformation in that region of Chamaeleon I may have happened almostsimultaneously in a recent burst. Comparing predictions on members ofthe star forming region based on K-band star counts with the number ofmembers actually identified through Hα emission suggests thatsensitive Hα surveys are very efficient in producing a complete ornearly complete magnitude-limited census of young stellar objects inChamaeleon I. Under the assumption that our sample is complete, wederive a mass function of Chamaeleon I between 0.03 and 1 M_sun whichcan be approximated by a nearly flat powerlaw in logarithmic mass units,in agreement with results for other young aggregates. Based onobservations collected at the European Southern Observatory (La Silla,Chile), programs 63.L-0023 and 63.I-0546; with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, the Netherlands and the United Kingdom) with theparticipation of ISAS and NASA; and with ROSAT, a X-ray satellitesupported by the Max-Planck-Society and the German Government(BMBF/DLR).
| ISOCAM observations of the Chamaeleon I dark cloud
We present the results of an ISOCAM survey of the Chamaeleon I darkcloud conducted in two broad-band filters at 6.7 and 14.3 mu m. In anarea of 0.59 sq.deg. we have detected a total of 282 mid-IR sources with103 sources observed in both filters. Combining the ISOCAM observationswith the I, J, and K_s data obtained with DENIS, we have found 108pre-main-sequence (PMS) stars in the region, of which 34 were previouslyunidentified. Several of these newly discovered young stellar objectsare relatively faint suggesting a population in Cha I of very low massobjects that probably includes brown dwarfs in their early contractionphases. Finally, most of the PMS stars show the spectral index computedbetween 2.2 and 14.3 mu m typical of Class II sources. The luminosityfunction (LF) derived for our detected PMS stars is discussed. ISO is anESA project with instruments funded by ESA Member States (especially thePI countries: France, Germany, the Netherlands and the United Kingdom)and with the participation of ISAS and NASA. Tables 1 and 2 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Tables 3 and 4 are onlyavailable with the on-line publication athttp://link.springer.de/link/service/journals/00230/
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