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Local interstellar medium kinematics towards the Southern Coalsack and Chamaeleon-Musca dark clouds
We present the results of a spectroscopic programme aiming toinvestigate the kinematics of the local interstellar medium componentstowards the Southern Coalsack and Chamaeleon-Musca dark clouds. Theanalysis is based upon high-resolution (R~ 60000) spectra of theinterstellar Na I D absorption lines towards 63 B-type stars (d<= 500pc) selected to cover these clouds and the connecting area defined bythe Galactic coordinates: 308°>=l>= 294° and-22°<=b<= 5°. The radial velocities, column densities,velocity dispersions, colour excess and photometric distances to thestars are used to understand the kinematics and distribution of theinterstellar cloud components. The analysis indicates that theinterstellar gas is distributed in two extended sheet-like structurespermeating the whole area, one at d<= 60 pc and another around120-150 pc from the Sun. The nearby feature is approaching the localstandard of rest with an average radial velocity of -7 kms-1, has low average column density logNNaI~ 11.2cm-2 and velocity dispersion b~ 5 km s-1. The moredistant feature has column densities between 12.3 <=logNNaI<= 13.2, average velocity dispersion b~ 2.5 kms-1 and seems associated with the dust sheet observed towardsthe Coalsack, Musca and Chamaeleon direction. Its velocity is centredaround 0 km s-1, but there is a trend for increasing from -3km s-1 near b= 1° to 3 km s-1 near b=-18°.The nearby low column density feature indicates a general outflow fromthe Sco-Cen association, in agreement with several independent lines ofdata in the general searched direction. The dust and gas feature around120-150 pc seem to be part of an extended large-scale feature of similarkinematic properties, supposedly identified with the interaction zone ofthe Local and Loop I Bubbles. Assuming that the interface and thering-like volume of dense neutral matter that would have been formedduring the collision of the two bubbles have similar properties, ourresults suggest that the interaction zone between the bubbles is twistedand folded.
| A Census of the Chamaeleon I Star-forming Region
I present a new census of the members of the Chamaeleon I star-formingregion. Optical spectroscopy has been obtained for 179 objects that havebeen previously identified as possible members of the cluster, that lackeither accurate spectral types or clear evidence of membership, and thatare optically visible (I<~18). I have used these spectroscopic dataand all other available constraints to evaluate the spectralclassifications and membership status of a total sample of 288 candidatemembers of Chamaeleon I that have appeared in published studies of thecluster. The latest census of Chamaeleon I now contains 158 members,eight of which are later than M6 and thus are likely to be brown dwarfs.I find that many of the objects identified as members of Chamaeleon I inrecent surveys are actually field stars. Meanwhile, seven of ninecandidates discovered by Carpenter and coworkers are confirmed asmembers, one of which is the coolest known member of Chamaeleon I at aspectral type of M8 (~0.03 Msolar). I have estimatedextinctions, luminosities, and effective temperatures for the membersand used these data to construct an H-R diagram for the cluster.Chamaeleon I has a median age of ~2 Myr according to evolutionary modelsand hence is similar in age to IC 348 and is slightly older than Taurus(~1 Myr). The measurement of an initial mass function for Chamaeleon Ifrom this census is not possible because of the disparate methods withwhich the known members were originally selected and must await anunbiased, magnitude-limited survey of the cluster.Based on observations performed at Las Campanas Observatory. Thispublication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| The Unusual Infrared Colors of a Faint Object in the Chamaeleon I Star-forming Region
We present deep near-infrared (JSHKS) imagingobservations, carried out with the ESO Very Large Telescope (VLT), of afield in the Chamaeleon I star forming region in an attempt to identifypossible members with masses comparable to, or below, the mass ofJupiter. We focus on an object, Cha I J110814.2-773649, which stands outas an outlier in color-color and color-magnitude diagrams of the field,with H=22.16+0.21-0.17,H-KS=-0.01+0.26-0.24, andJS-H=2.00+∞-0.62. H-bandspectroscopy of this object shows that the unusual colors are not due toemission lines in the region; furthermore, this spectroscopy allowsclear detection of the object's continuum. Assuming membership inChamaeleon I and an age of 2 Myr (the age of most members of Cha I), theblue H-KS color and the absolute magnitude are consistentwith model predictions for a cool, sub-Jupiter mass object with strongdust depletion in the atmosphere. However, the very red JS-Hcolor implied by the marginal detection in the JS band isunexpected in an object with such atmospheric properties. We speculatethat this might be due to differences in the properties of dust and itsdepletion under the photosphere with respect to field objects (T dwarfs)that have a similar temperature, these differences resulting from boththe youth and the low surface gravity of a low-mass member of astar-forming region. We also consider the alternative possibility thatCha I J110814.2-773649 might actually be a high-redshift object, whosered JS-H color could result from absorption of the fluxblueward of Lyα in the J band. We find that this possibility wouldbe marginally compatible with the JSHKS photometryof Cha I J110814.2-773649 if it were an unreddened starburst at8.5
| Classification and properties of UV extinction curves
The catalog of Savage et al. (\cite{ref27}) reporting colour excesses of1415 stars from ANS photometry offers the opportunity to deeplyinvestigate the characteristics of UV extinction curves which differfrom the standard extinction of the diffuse interstellar medium. To thisaim we have selected a sample of 252 curves, which have been comparedwith the relations derived by Cardelli et al. (\cite{ref4}; CCM in thefollowing) for a variety of R_V values in the range 2.4-5 and have beenclassified as normal if they fit at least one of the CCM curves oranomalous otherwise. We find that normal curves with small R_V are justas numerous as those with large R_V. The anomalous objects are arrangedinto two groups according to the strength of the bump at 0.217 mu . Fora given value of c_2 this increases along the sequence: type Aanomalous, normals and type B anomalous, suggesting that this sequenceshould correspond to an increase of the amount of small grains along thesightline. Considerations concerning the environmental characteristicsindicate that the anomalous behaviour is not necessarily tied to theexistence of dense gas clouds along the line of sight.
| On the distance to the Chamaeleon I and II associations
Constraints on the distances to the dark clouds Chamaeleon I and II areinvestigated in detail. A compilation of photometric data, spectraltypes and absolute magnitudes for field stars towards each cloud ispresented, and results are used to examine the distribution of reddeningwith distance along each line of sight. The distances to starsassociated with reflection nebulae in each cloud are examined in detail.On the basis of these results, we deduce the most probable distance ofCha I to be 160+/-15pc, and that of Cha II to be 178+/-18pc. Anexamination of the mean fluxes of T Tauri stars in each cloud providesindependent evidence to suggest that Cha II is significantly moredistant than Cha I. Both clouds appear to be embedded in a macroscopicsheet-like structure extending over much of the Chamaeleon-Musca-Cruxregion. The Chamaeleon III and DC\ts 300.2--16.9 clouds are probablypart of the same structure, with probable distances ~ 140--160pc.
| Optical polarimetry, high--resolution spectroscopy and IR analysis of the Chamaeleon I dark cloud
We present optical polarimetry and high resolution spectroscopy of asample of stars toward the Chamaeleon I dark cloud. We use ourpolarimetry which includes 33 stars to study the wavelength dependenceof the degree and position angle of polarization. From fits to thenormalized wavelength dependence of interstellar polarization, we deriveestimates of lambdaMax ranging from 4500 \AA to 6700 \AA, andPMax ranging from 3 to 8%. The values of lambdaMaxwere found to be well correlated with the IRAS 100 micron intensity,while PMax was found to increase with E_{B-V}. Highresolution spectra of the Ca II, CH, and CH^{+} lines were obtained for10 stars, which show two components of Ca II in absorption at 3.0
| Dust Metamorphosis in the Galaxy
Not Available
| A Study of the Chamaeleon-I Dark Cloud and T-Association - Part Six - Interstellar Polarization Grain Alignment and Magnetic Field
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.268....1W&db_key=AST
| Rotation periods of four T Tauri stars
BVR photometry of nine T Tauri and related stars was obtained on 13nights in January 1988, in an attempt to confirm reported periodicitiesand discover new ones. Each star was observed an average of six timesper night. Rotation periods previously reported for T Tau by Herbst etal.(1986) and HM 4 (LHα 332-20) and HM 30 (LHα 332-21) byBouvier et al.(1986) were confirmed. A periodicity of 1.28 days wasdiscovered for TW Hya; it was also the only star to show significantvariations during the course of a single night. Analysis of a similardataset for TW Hya obtained by Rucinski and Krautter (1983) shows asimilar periodic variation, but with a 1.8 day cycle. SU Aur, RY Tau, BFOri, V380 Ori, and Fu Ori all exhibited variability, but withoutdetectable periodicity. This is in agreement with a previous suggestionthat periodicity is more easily found in the later-typepre-main-sequence stars (i.e., later than about KO).
| Merged log of IUE observations.
Not Available
| A study of the Chamaeleon dark cloud and T-association. I - Extinction, distance and membership
The results of an observing program to investigate the nature and degreeof reddening of a number of stars in the field towards the dark cloudand T-association in Chamaeleon are presented. The observations compriseoptical (UBVRI) and infrared (JHK) photometry, and optical spectroscopy.New and previously published data are combined to assess membershipcriteria for 110 stars, and separate catalogues of association membersand field stars are presented. Optical identifications are listed for 27IRAS point sources in the region. The reddening/distance relation forfield stars indicates a distance to the cloud of 140 + or - 12 pc. Thisresult confirms that the extinction law is anomalous towards some starsin the cloud, notably the embedded A0 ZAMS star HD 97300, with values ofup to 5.5 for the ratio of total-to-selective extinction. A comparisonof 21-cm data with the reddening of background field stars indicatesthat the ratio N(H I)/E(B-V) is significantly less than the interstellarmean, indicating that most of the hydrogen in the cloud is molecular.
| The ratio of total-to-selective extinction in the Chamaeleon T1 and R Coronae Australis dark clouds
The ratio of total-to-selective extinction, R = A(V)/E(B-V), in the ChaT1 and R CrA dark clouds is investigated via observations of backgroundfield stars seen through these clouds. The observations consist of UBVRIand JHKL photometry as well as classification spectrograms and yieldindividual R values by the color difference method. R is found to beessentially normal in the outer parts of the Cha T1 dark cloud, butseveral stars near the opaque core of this cloud show evidence ofanomalous extinction with R greater than 4. In the R CrA dark cloud, Rincreases with depth into the cloud and reaches clearly anomalous valuesof about 5 for lines of sight with A(V) = 3 mag. The results providefurther evidence for anomalous interstellar extinction laws in somedense interstellar clouds.
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