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|A deep photometric survey of the η Chamaeleontis cluster down to the brown dwarf - planet boundary|
We report the outcome of the deep optical/infrared photometric survey ofthe central region (33 × 33 arcmin2 or 0.9pc2) of the η Chamaeleontis (η Cha) pre-main sequencestar cluster. The completeness limits of the photometry are I = 19.1, J= 18.2 and H = 17.6, faint enough to reveal low-mass members down to thebrown dwarf and planet boundary of ~13 MJup. We found no suchlow-mass members in this region. Our result combined with a previousshallower (I = 17) but larger area survey indicates that low-massobjects (0.013 < M/Msolar < 0.075) were either notcreated in the η Cha cluster or lost due to the early dynamicalhistory of the cluster and ejected to outside the surveyed areas.
|Silicon and Nickel Enrichment in Planet Host Stars: Observations and Implications for the Core Accretion Theory of Planet Formation|
We present evidence that stars with planets exhibit statisticallysignificant silicon and nickel enrichment over the general metal-richpopulation. We also present simulations that predict silicon enhancementof planet hosts within the context of the core accretion hypothesis forgiant planet formation. Because silicon and oxygen are bothα-elements, [Si/Fe] traces [O/Fe], so the silicon enhancement inplanet hosts predicts that these stars are oxygen-rich as well. Wepresent new numerical simulations of planet formation by core accretionthat establish the timescale on which a Jovian planet reaches rapid gasaccretion, trga, as a function of solid surface densityσsolid: (trga/1Myr)=(σsolid/25.0 g cm-2)-1.44.This relation enables us to construct Monte Carlo simulations thatpredict the fraction of star-disk systems that form planets as afunction of [Fe/H], [Si/Fe], disk mass, outer disk radius, and disklifetime. Our simulations reproduce both the known planet-metallicitycorrelation and the planet-silicon correlation reported in this paper.The simulations predict that 15% of solar-type stars form Jupiter-massplanets, in agreement with 12% predicted from extrapolation of theobserved planet frequency-semimajor axis distribution. Although a simpleinterpretation of core accretion predicts that the planet-siliconcorrelation should be much stronger than the planet-nickel correlation,we observe the same degree of silicon and nickel enhancement in planethosts. If this result persists once more planets have been discovered,it might indicate a complexity in the chemistry of planet formationbeyond the simple accumulation of solids in the core accretion theory.
|Spitzer/IRAC Photometry of the η Chameleontis Association|
We present IRAC 3.6, 4.5, 5.8, and 8 μm photometry for the 17 A-, K-,and M-type members of the η Chameleontis association. These datashow infrared excesses toward six of the 15 K and M stars, indicatingthe presence of circumstellar disks around 40% of the stars with massesof 0.1-1 Msolar. The two A stars show no infrared excesses.The excess emission around one of the stars is comparable to the medianexcess for classical T Tauri stars in the Taurus association; theremaining five show comparatively weak excess emission. Taking intoaccount published Hα spectroscopy that shows that five of the sixstars are accreting, we argue that the disks with weak mid-infraredexcesses are disks in which the inner disks have been largely depletedof small grains by grain growth or, in one case, the small grains havesettled to the midplane. This suggests that η Cha has a much higherfraction of disks caught in the act of transitioning into optically thindisks than that measured in younger clusters and associations.
|A Moving Cluster Distance to the Exoplanet 2M1207b in the TW Hydrae Association|
A candidate extrasolar planet companion to the young brown dwarf 2MASSWJ1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin etal. They find that the temperature and luminosity of 2M1207b areconsistent with a young, ~5MJ planet. The 2M1207 system ispurported to be a member of the TW Hya association (TWA) and situated~70 pc away. Using a revised space motion vector for TWA and improvedproper motion for 2M1207, I use the moving cluster method to estimatethe distance to the 2M1207 system and other TWA members. The deriveddistance for 2M1207 (53+/-6 pc) forces the brown dwarf and planet to behalf as luminous as previously thought. The inferred masses for 2M1207Aand 2M1207b decrease to ~21 and ~3-4MJ, respectively, withthe mass of 2M1207b well below the observed tip of the planetary massfunction and the theoretical deuterium-burning limit. After removingprobable Lower Centaurus Crux (LCC) members from the TWA sample, as wellas the probable nonmember TWA 22, the remaining TWA membership is foundto have distances of 49+/-3 (s.e.m.)+/-12 (1 σ) pc and an internalone-dimensional velocity dispersion of 0.8+0.3-0.2km s-1. There is weak evidence that the TWA is expanding, andthe data are consistent with a lower limit on the expansion age of 10Myr (95% confidence).
|Constraints on Inner Disk Evolution Timescales: A Disk Census of the η Chamaeleontis Young Cluster|
We present new L'-band (3.8 μm) observations of stars inthe nearby (~97 pc) young (~6 Myr) compact cluster around ηChamaeleontis, obtained with the European Southern Observatory's VeryLarge Telescope in Paranal, Chile. Our data, combined withJ,H,Ks photometry from the Two Micron All Sky Survey, revealthat only two of the 12 members surveyed harbor L'-bandexcesses consistent with optically thick inner disks; both are alsolikely accretors. Intriguingly, two other stars with possible evidenceof ongoing accretion, albeit at very low rates, do not show significantinfrared excess: this may imply substantial grain growth and/or partialclearing of the inner disk region, as expected in planet formationscenarios. Our findings suggest that η Cha stars are in an epochwhen disks are rapidly evolving, perhaps due to processes related toplanet building, and provide further constraints on inner disklifetimes.
|Decay of Planetary Debris Disks|
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
|An empirical temperature calibration for the Δ a photometric system . I. The B-type stars|
We establish an empirical effective temperature calibration of mainsequence, luminosity class V to III B-type stars for the Δ aphotometric system which was originally developed to detect magneticchemically peculiar objects of the upper main sequence (early B-type toearly F-type) at 5200 Å. However, this system provides the index(g_1-y) which shows an excellent correlation with (B-V) as well as (b-y)and can be used as an indicator of the effective temperature. This issupplemented by a very accurate color-magnitude diagram, y or V versus(g_1-y), which can be used, for example, to determine the reddening,distance and age of an open cluster. This makes the Δ aphotometric system an excellent tool to investigate theHertzsprung-Russell-Diagram (HRD) in more detail. Using thereddening-free parameters and already established calibrations withinthe Strömgren uvbyβ, Geneva 7-color and Johnson UBV systems, apolynomial fit of third degree for the averaged effective temperaturesto the individual (g_1-y)0 values was derived. For thispurpose, data from the literature as well as new observations were takenresulting in 225 suitable bright normal B-type objects. The statisticalmean of the error for this sample is 238 K which is sufficient toinvestigate the HRD of distant galactic open clusters as well asextragalactic aggregates in the future.
|The spectroscopic characteristics of intermediate-aged pre-main-sequence stars: the η Chamaeleontis cluster|
We present a study of calibrated low-resolution spectra of the 18 knownprimaries of the ~9-Myr-old η Chamaeleontis (η Cha)pre-main-sequence (PMS) star cluster. Using synthetic broad-band coloursand narrow-band continuum-sensitive, temperature-sensitive andgravity-sensitive indices derived from the spectra, we compare the ηCha stars to standard dwarfs. We find that the VRI colours of the PMSstars are indistinguishable from those of main-sequence stars, but thata B-band excess attaining ~0.2 mag for late-M cluster stars is present,which might be an indicator of gravity, metallicity and/or activitydifferences between the two samples of stars. The narrow-band spectralindices for the η Cha stars possibly indicate higher metallicity andstrongly indicate lower surface gravity than the dwarf indices,consistent with the elevated location of the cluster in theHertzsprung-Russell evolutionary diagram. Using the derived syntheticcolours and indices, we adopt spectral types for the late-type η Chastars. We then produce a table of absolute optical magnitudes andcolours representing the cluster isochrone for comparison with PMSevolutionary models. From our results we also conclude that the ηCha stars are unreddened, consistent with the group being a sample ofolder PMS stars distant from obscuring molecular clouds, except for theA1 member HD 75505 for which we confirm AV= 0.4 mag likelydue to the presence of circumstellar material.
|Population and dynamical state of the η Chamaeleontis sparse young open cluster|
We report new results in our continuing study of the unique compact (1pc extent), nearby (d= 97 pc), young (t= 9 Myr) stellar clusterdominated by the B9 star η Chamaeleontis. An optical photometricsurvey spanning 1.3 × 1.3 pc adds two M5-M5.5 weak-lined T Tauri(WTT) stars to the cluster inventory which is likely to be significantlycomplete for primaries with masses M > 0.15 Msolar. Thecluster now consists of 17 primaries and ~=9 secondaries lying within100 au of their primaries. The apparent distribution of 9:7:1single:binary:triple systems shows 2-4 times higher multiplicity than inthe field main-sequence stars, and is comparable to that seen in otherpre-main-sequence populations. The initial mass function (IMF) isconsistent with that of rich young clusters and field stars. Byextending the cluster IMF to lower masses, we predict 10-14 additionallow-mass stars with 0.08 < M < 0.15 Msolar and 10-15brown dwarfs with 0.025 < M < 0.08 Msolar remain to bediscovered. The η Cha cluster extends the established stellardensity and richness relationship for young open clusters. The radialdistribution of stars is consistent with an isothermal sphere, but masssegregation is present with >50 per cent of the stellar mass residingin the inner 6 arcmin (0.17 pc). Considering that the η Cha clusteris sparse, diffuse and young, the cluster may be an ideal laboratory fordistinguishing between mass segregation that is primordial in nature, orarising from dynamical interaction processes.
|A Survey for Low-Mass Stars and Brown Dwarfs in the η Chamaeleontis and ɛ Chamaeleontis Young Associations|
I present the results of a search for new low-mass stars and browndwarfs in the η Cha and ɛ Cha young associations. Within radiiof 1.5d and 0.5d surrounding η Cha and ɛ Cha, respectively, Ihave constructed color-magnitude diagrams from Deep Near Infrared Survey(DENIS) and Two Micron All Sky Survey (2MASS) photometry and haveobtained spectra of the candidate low-mass members therein. The fivecandidates in η Cha are classified as four field M dwarfs and onecarbon star. No new members are found in this survey, which is completefor M/Msolar=0.015-0.15, according to the evolutionary modelsof Chabrier and Baraffe. Thus, an extended population of low-massmembers is not present in η Cha out to 4 times the radius of theknown membership. Meanwhile, the three candidate members of ɛ Chaare classified as young stars, and thus likely members of theassociation, based on Li absorption and gravity-sensitive absorptionlines. These new sources have spectral types of M2.25, M3.75, and M5.75,corresponding to masses of 0.45, 0.25, and 0.09 Msolar by themodels of Chabrier and Baraffe. For one of these stars, intense Hαemission, forbidden line emission, and strong K-band excess emissionsuggest the presence of accretion, an outflow, and a disk, respectively.This young star is also much fainter than expected for an associationmember at its spectral type, which could indicate that it is seen inscattered light. No brown dwarfs are detected in ɛ Cha down to thecompleteness limit of 0.015 Msolar. The absence of browndwarfs in these associations is statistically consistent with the massfunctions measured in star-forming regions, which exhibit only ~2 and ~1brown dwarfs for stellar samples at the sizes of the η Cha andɛ Cha associations.Based on observations performed at Las Campanas Observatory. Thispublication makes use of data products from the Two Micron All SkySurvey and the Deep Near-Infrared Survey of the Southern Sky.
|Spectroscopy of Candidate Members of the η Chamaeleontis and MBM 12 Young Associations|
We present an analysis of candidate members of the η Cha and MBM 12Ayoung associations. For an area of 0.7 deg2 toward η Cha,we have performed a search for members of the association by combiningJHKs photometry from the Two Micron All Sky Survey and iphotometry from DENIS with follow-up optical spectroscopy at MagellanObservatory. We report the discovery of three new members with spectraltypes of M5.25-M5.75, corresponding to masses of 0.13-0.08Msolar by theoretical evolutionary models. Two and three ofthese members were found independently by Lyo and coworkers and Song andcoworkers, respectively. Meanwhile, no brown dwarfs were detected inη Cha down to the completeness limit of 0.015 Msolar. ForMBM 12A, we have obtained spectra of three of the remaining candidatemembers that lacked spectroscopy at the end of the survey by Luhman, allof which are found to be field M dwarfs. Ogura and coworkers haverecently presented four ``probable'' members of MBM 12A. However, two ofthese objects were previously classified as field dwarfs by thespectroscopy of Luhman. In this work, we find that the other two objectsare field dwarfs as well.Based on observations performed at Las Campanas Observatory and the MMTObservatory. This publication makes use of data products from 2MASS andDENIS.
|Probing the Low-Mass Stellar End of the η Chamaeleontis Cluster|
We have identified three faint new members of the η Chamaeleontiscluster. Spectral types of the new members are estimated to be ~M5,based on their TiO band strengths and broadband colors. With an age of5-8 Myr for the cluster, masses of these new members are estimated to be~0.08 Msolar. All three display strong Li 6708 Åabsorption and Hα emission features, including one with anHα emission equivalent width of ~60 Å, along with He I 6678and 7605 Å emission features that are characteristics of classicalT Tauri stars.
|Constraints on Inner Disk Evolution Timescales: A Disk Census of the Eta Chamaeleontis Young Cluster|
We present new L-prime band (3.8 micron) observations of stars in thenearby ( 97pc) young ( 6 Myr) compact cluster around Eta Chamaeleontis,obtained with the European Southern Observatory's Very Large Telescopein Paranal, Chile. Our data, combined with JHKs photometry from the2-Micron All Sky Survey, reveal that only two of the 12 members surveyedharbor L-prime band excesses consistent with optically thick innerdisks; both are also likely accretors. Intriguingly, two other starswith possible evidence for on-going accretion, albeit at very low rates,do not show significant infrared excess. This may imply substantialgrain growth and/or partial clearing of the inner disk region, asexpected in planet formation scenarios. Our findings suggest that EtaCha stars are in an epoch when disks are rapidly evolving, perhaps dueto processes related to planet building, and provide further constraintson inner disk lifetimes.
|Brown Dwarfs in Southern Star Forming Regions|
Most of the star forming regions within 200 pc from the Sun are locatedSouth of the celestial equator. Brown dwarfs or strong candidates havebeen discovered in all of them, and have been often studied at multiplewavelengths. This paper summarizes some highlights of the research onyoung brown dwarfs in these regions.
|On the Relationship Between Stellar Rotation and Radius in Young Clusters|
We have compiled data on rotational velocities for more than 1000 K andM stars in 12 young clusters ranging in age from Orion to the Hyades.These data enable a search for systematic changes in stellar rotationalvelocity vs. age. Taken together, these data show that most pre-mainsequence (PMS) stars spanning ages from about 0.1 to about 1 Myr do notappear to spin up in response to contraction down their convectivetracks, that spin up between 1 and 3 million years is modest at best,and that some stars do not appear to spin up at all.These results extend and reinforce our earlier study (Rebull et al.2002), based on observations of several hundred stars in the OrionFlanking Fields, NGC 2264, and the Orion Nebula Cluster (ONC), whichshowed that the majority of PMS stars in these three groups apparentlydo not conserve stellar angular momentum as they contract, but insteadevolve at nearly constant angular velocity. This result applies both tostars with and without near-IR I-K excesses indicative of disks.
|The ɛ Chamaeleontis Young Stellar Group and the Characterization of Sparse Stellar Clusters|
We present the outcomes of a Chandra X-Ray Observatory snapshot studyof five nearby Herbig Ae/Be (HAeBe) stars that are kinematically linkedwith the Oph-Sco-Cen association (OSCA). Optical photometric andspectroscopic follow-up was conducted for the HD 104237 field. Theprincipal result is the discovery of a compact group ofpre-main-sequence (PMS) stars associated with HD 104237 and itscodistant, comoving B9 neighbor ɛ Chamaeleontis AB. We name thegroup after the most massive member. The group has five confirmedstellar systems ranging from spectral type B9 to M5, including aremarkably high degree of multiplicity for HD 104237 itself. The HD104237 system is at least a quintet, with four low-mass PMS companionsin nonhierarchical orbits within a projected separation of 1500 AU ofthe HAeBe primary. Two of the low-mass members of the group are activelyaccreting classical T Tauri stars. The Chandra observations alsoincrease the census of companions for two of the other four HAeBe stars,HD 141569 and HD 150193, and identify several additional new members ofthe OSCA. We discuss this work in light of several theoretical issues:the origin of X-rays from HAeBe stars; the uneventful dynamical historyof the high-multiplicity HD 104237 system; and the origin of the ɛCha group and other OSCA outlying groups in the context of turbulentgiant molecular clouds. Together with the similar η Cha cluster, wepaint a portrait of sparse stellar clusters dominated byintermediate-mass stars 5-10 Myr after their formation.
|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 (220.127.116.11) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/404/913
|ECHA J0843.3-7905: Discovery of an `old' classical T Tauri star in the η Chamaeleontis cluster|
A limited-area survey of the η Chamaeleontis cluster has identifiedtwo new late-type members. The more significant of these is ECHAJ0843.3-7905 (=IRAS F08450-7854), a slowly rotating (P=12d) M2 classicalT Tauri (CTT) star with a spectrum dominated by Balmer emission. At adistance of 97pc and cluster age of ~9Myr, the star is a nearby rareexample of an `old' CTT star and promises to be a rewarding laboratoryfor the study of disc structure and evolution in pre-main sequence (PMS)stars. The other new member is the M4 weak-lined T Tauri (WTT) star ECHAJ0841.5-7853, which is the lowest mass (M~0.2Msolar) primaryknown in the cluster.
|Millimeter-wave Searches for Cold Dust and Molecular Gas around T Tauri Stars in MBM 12|
We report results of a sensitive search for cold dust and molecular gasin the disks around eight T Tauri stars in the high-latitude cloud MBM12. Interferometric observations of 3 mm continuum emission in fivefields containing six of the objects and literature values for theremaining two limit the disk masses to Mdisk<0.04-0.09Msolar (gas plus dust), for a gas-to-dust mass ratio of 100and a distance of 275 pc. By co-adding the 3 mm data of our five fields,we set an upper limit to the average disk mass ofMdisk(N=5)<0.03 Msolar. Simultaneousobservation of the CS J=2-1 and the N2H+ 1-0 linesshow no emission. Single-dish observations of the 13CO 2-1line limit the disk mass to (5-10)×10-4Msolar for a standard CO abundance of2×10-4. Depletion of CO by up to 2 orders of magnitudethrough freezing out or photodissociation can reconcile these limits.These mass limits lie within the range found in the Taurus-Auriga andρ Oph star-forming regions (0.001-0.3 Msolar) andpreclude conclusions about possible decrease in disk mass over the 1-2Myr age range spanned by the latter two regions and MBM 12. Ourobservations can exclude the presence in MBM 12 of T Tauri stars withrelatively bright and massive disks such as T Tau, DG Tau, and GG Tau.
|Close Binaries in the η Chamaeleontis Cluster|
We have used speckle interferometry and adaptive optics observations tosearch for multiple systems among 13 stars in the η Chamaeleontiscluster. We discovered two previously unknown subarcsecond binaries.Placing the components in infrared color-magnitude diagrams shows thatmost members of η Cha are coeval. Repeated observations of thebinary RECX 1 allow us to determine a preliminary orbit and derive asystem mass of about 2 Msolar.Based on observations obtained at the European Southern Observatory, LaSilla, proposals 56.E-0197, 62.I-0399, 65.I-0350, 65.I-0086, 67.C-0354, and 68.C-0539.
|Characterization of low-mass pre-main sequence stars in the Southern Cross|
We report high-resolution spectroscopic observations, as well ashigh-resolution near infrared (IR) imaging of six stars previouslyidentified in a ROSAT pointed observation in the direction of the B-typestar beta Cru, and classified as low-mass pre-main sequence (PMS) stars.Four of the stars are confirmed to be low-mass PMS stars, associatedwith the Lower Centaurus-Crux group, while the other two are unrelatedto the Sco-Cen association. The confirmed PMS stars are most likely intheir post-T Tauri evolutionary phase. Although future deep X-rayobservations with high-resolution imagers might detect more new PMSstars, the possibility that the Crux PMS stars are part of a smallaggregate, with beta Crux itself approximately at the center, is ratherunlikely, given the high velocity dispersion and the low spatial densityof the confirmed PMS stars. Instead, these stars may be part of a movinggroup in a more disperse and numerous population of low-mass PMS stars,distributed in the Lower Centaurus-Crux subgroup. New PMS binaries andmultiple systems were also discovered among the stars in the sample,namely a close visual pair and a hierarchical triple system in which oneof the components is a double-lined spectroscopic binary (SB2). Thedetailed orbital solution is reported for the inner short-period (P_orb= 58.3 days) SB2. A preliminary orbital solution for the hierarchicaltriple system yields a systemic orbital period of about 4.6 years, whichmakes this object a very suitable target for follow-up observations withthe Very-Large Telescope Interferometer (VLTI) in the coming years.Based on observations carried out at the European Southern Observatory,La Silla, Chile under proposals numbers 62.I-0368 and 63.I-0045,65.I-0089 and the Swiss Euler telescope at ESO, La Silla.
|The η Chamaeleontis cluster: photometric study of the ROSAT-detected weak-lined T Tauri stars|
We present the results of a photometric study of X-ray-active weak-linedT Tauri (WTT) stars in the η Chamaeleontis star cluster. Multi-epochV-band photometric monitoring during 1999 and 2000 of the 10X-ray-active WTT stars found that all were variable in one or bothyears, with periods ascribed to rotational modulation of starspots.Comparison between the rotational and X-ray properties of these objectsindicates the saturation level, log(LXLbol)~-3,observed in other studies of X-ray-active pre-main-sequence stars,persists in the η Cha stars from the slow- to the fast-rotatorregimes. Cousins VRI photometry of the WTT stars has enabled us toinvestigate further the photometric properties of these stars. The starsappear sufficiently coeval to distinguish near-equal-mass binarieswithin the sample. A new Hertzsprung-Russell diagram for these objectssuggests ages of 4-9Myr for M-type RECX primaries using the tracks ofD'Antona & Mazzitelli.
|Kinematics of Vega-like stars|
We have studied the kinematics of Vega-like stars in the solarneighbourhood. Space velocities are computed for 80 Vega-like stars. Weanalyse the space velocities of these stars in connection with the youngstellar associations and star forming regions in the solarneighbourhood. Our preliminary analysis suggests that many of theVega-like stars are kinematically connected with the members of theabove mentioned associations and star forming regions. This would placea strong constraint on the ages of Vega-like stars which are generallybelieved to be an older population. Details of the analysis andpreliminary results are presented.
|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).
|Research Note Hipparcos photometry: The least variable stars|
The data known as the Hipparcos Photometry obtained with the Hipparcossatellite have been investigated to find those stars which are leastvariable. Such stars are excellent candidates to serve as standards forphotometric systems. Their spectral types suggest in which parts of theHR diagrams stars are most constant. In some cases these values stronglyindicate that previous ground based studies claiming photometricvariability are incorrect or that the level of stellar activity haschanged. Table 2 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/297
|The proper motions of fundamental stars. I. 1535 stars from the Basic FK5|
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (22.214.171.124) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
|Astrophysics in 1999|
The year 1999 saw the arrival of a star with three planets, a universewith three parameters, and a solar corona that could be heated at leastthree ways. In addition, there were at least three papers on everyquestion that has ever been asked in astrophysics, from ``Will theUniverse expand forever?'' to ``Does mantle convection occur in one ortwo layers?'' The answers generally were, ``Yes,'' ``No,'' and ``None ofthe above,'' to each of the questions. The authors have done their bestto organize the richness around centers defined by objects, methods, andmadnesses.
|The discovery of a low-mass, pre-main-sequence stellar association around γ Velorum|
We report the serendipitous discovery of a population of low-mass,pre-main-sequence (PMS) stars in the direction of the Wolf-Rayet/O-starbinary system γ2Vel and the Vela OB2 association. Weargue that γ2Vel and the low-mass stars are trulyassociated and approximately coeval, and that both are at distancesbetween 360 and 490pc, disagreeing at the 2σ level with the recentHipparcos parallax of γ2Vel, but consistent with olderdistance estimates. Our results clearly have implications for thephysical parameters of the γ2Vel system, but also offeran exciting opportunity to investigate the influence of high-mass starson the mass function and circumstellar disc lifetimes of their lowermass PMS siblings.
|Optical, infrared and millimetre-wave properties of Vega-like systems - IV. Observations of a new sample of candidate Vega-like sources|
Photometric observations at optical and near-infrared wavelengths arepresented for members of a new sample of candidate Vega-like systems, ormain sequence stars with excess infrared emission due to circumstellardust. The observations are combined with IRAS fluxes to define thespectral energy distributions of the sources. Most of the sources showonly photospheric emission at near-IR wavelengths, indicating a lack ofhot (~1000K) dust. Mid-infrared spectra are presented for four sourcesfrom the sample. One of them, HD 150193, shows strong silicate emission,while another, HD 176363, was not detected. The spectra of two starsfrom our previous sample of Vega-like sources both show UIR-bandemission, attributed to hydrocarbon materials. Detailed comparisons ofthe optical and IRAS positions suggest that in some cases the IRASsource is not physically associated with the visible star. Alternativeassociations are suggested for several of these sources. Fractionalexcess luminosities are derived from the observed spectral energydistributions. The values found are comparable to those measuredpreviously for other Vega-like sources.
|On the Variability of Late B III-V Stars|
We investigate the Hipparcos Satellite photometry of luminosity classIII-V B6-B9 stars. Most are relatively non-variable. Candidates forwhich further study is desirable are identified.