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Spatially Resolved Spectroscopy and Coronagraphic Imaging of the TW Hydrae Circumstellar Disk
We present the first spatially resolved spectrum of scattered light fromthe TW Hydrae protoplanetary disk. This nearly face-on disk is opticallythick, surrounding a classical T Tauri star in the nearby 10 Myr old TWHya association. The spectrum was taken with the Hubble Space Telescope(HST) STIS CCD, providing resolution R~360 over the wavelength range5250-10300 Å. Spatially resolved spectroscopy of circumstellardisks is difficult because of the high contrast ratio between the brightstar and faint disk. Our novel observations provide optical spectra ofscattered light from the disk between 40 and 155 AU from the star. Thescattered light has the same color as the star (gray scattering) at allradii except the innermost region. This likely indicates that thescattering dust grains are larger than about 1 ?m all the way out tolarge radii. From the spectroscopic data, we also obtained radialprofiles of the integrated disk brightness at two position angles, overalmost the same region as previously observed in HST WFPC2 and NICMOScoronagraphic images (35 to 173 AU from the star). The profiles have thesame shape as the earlier ones, but show a small azimuthal asymmetry inthe disk not previously noted. Our STIS broadband coronagraphic imagesof TW Hya confirm the reality of this asymmetry, and show that the disksurface brightness inside 140 AU has a sinusoidal dependence onazimuthal angle. The maximum brightness occurs at a position angle of233.6d+/-5.7d east of north. This might be caused by the combination offorward scattering and an increase in inclination in the inner region ofthe disk, suggesting that the TW Hya disk has a warp like that seen inthe ? Pictoris debris disk.
| The brown dwarf population in the Chamaeleon I cloud
We present the results of a multiband survey for brown dwarfs in theChamaeleon I dark cloud with the Wide Field Imager(WFI) camera at the ESO/MPG 2.2-m telescope on La Silla (Chile). Thesurvey has revealed a substantial population of brown dwarfs in thissouthern star-forming region. Candidates were selected from R, I andHα imaging observations. We also observed in two medium-bandfilters, M 855 and M 915, for spectral type determination. The formerfilter covers a wavelength range containing spectral featurescharacteristic of M-dwarfs, while the latter lies in a relativelyfeatureless wavelength region for these late-type objects. A correlationwas found between spectral type and (M 855-M 915) colour index for mid-to late M-type objects and early L-type dwarfs. With this method, weidentify most of our object candidates as being of spectral type M 5 orlater. Our results show that there is no strong drop in the number ofobjects for the latest spectral types, hence brown dwarfs may be asabundant as low-mass stars in this region. Also, both kind of objectshave a similar spatial distribution. We derive an index α = 0.6± 0.1 of the mass function in this region of dispersed starformation, in good agreement with the values obtained in other starforming regions and young clusters. Some of the brown dwarfs have strongHα emission, suggesting mass accretion. For objects with publishedinfrared photometry, we find that strong Hα emission is related toa mid-infrared excess, indicative of the existence of a circumstellardisk.Based on observations collected at the European Southern Observatory, LaSilla, Chile.
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| Hipparcos: The Stars
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