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Spitzer Far-Infrared Detections of Cold Circumstellar Disks
Observations at 70 μm with the Spitzer Space Telescope have detectedseveral stellar systems within 65 pc of the Sun. Of 18 presumably youngsystems detected in this study, as many as 15 have 70 μm emission inexcess of that expected from their stellar photospheres. Five of thesystems with excesses are members of the Tucanae association. The 70μm excesses range from a factor of ~2 to nearly 30 times the expectedphotospheric emission from these stars. In contrast to the 70 μmproperties of these systems, there is evidence for an emission excess at24 μm for only HD 3003, confirming previous results for this star.The lack of a strong 24 μm excess in most of these systems suggeststhat the circumstellar dust producing the IR excesses is relatively cool(Tdust<~150 K) and that there is little IR-emittingmaterial within the inner few AU of the primary stars. Many of thesesystems lie close enough to Earth that the distribution of the dustproducing the IR excesses might be imaged in scattered light at opticaland near-IR wavelengths.
| 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.
| 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
| 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.
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Observation and Astrometry data
| Constellation: | Chamaleon |
| Right ascension: | 09h36m17.82s |
| Declination: | -78°20'41.6" |
| Apparent magnitude: | 8.601 |
| Distance: | 62.972 parsecs |
| Proper motion RA: | -73.6 |
| Proper motion Dec: | 50.5 |
| B-T magnitude: | 9.305 |
| V-T magnitude: | 8.66 |
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