Ramarao Tata

Spectroscopic Rotational Velocities of Brown Dwarfs

We have obtained projected rotation velocities (vrot sin i) of a sample of 19 ultracool dwarfs with spectral types in the interval M6.5-T8 using high-resolution, near-infrared spectra obtained with NIRSPEC and the Keck II telescope. Among our targets there are two young brown dwarfs, two likely field stars, and 15 likely brown dwarfs (30-72 MJup) in the solar neighborhood. Our results indicate that the T-type dwarfs are fast rotators in marked contrast to M-type stars. We have derived vrot sin i velocities between ≤15 and 40 km s-1 for them and have found no clear evidence for T dwarfs rotating strongly faster than L dwarfs. However, there is a hint for an increasing lower envelope on moving from mid-M to L spectral types in the vrot sin i-spectral-type diagram that was previously reported in the literature; our vrot sin i results extend it to even cooler types. Assuming that field brown dwarfs have a size of 0.08-0.1 R☉, we can place an upper limit of 12.5 hr on the equatorial rotation period of T-type brown dwarfs. In addition, we have compared our vrot sin i measurements to spectroscopic rotational velocities of very young brown dwarfs of similar mass available in the literature. The comparison, although model dependent, suggests that brown dwarfs lose some angular momentum during their contraction; however, their spin-down time seems to be significantly longer than that of solar-type to early M stars.

Discovery of new nearby L and late-M dwarfs at low Galactic latitude from the DENIS data base

We report on new nearby L and late-M dwarfs (d phot ≤ 30 pc) discovered in our search for nearby ultracool dwarfs (I-J ≥ 3.0, later than M8.0) at low Galactic latitude (|b| < 15°) over 4800 deg 2 in the Deep Near Infrared Survey of the Southern Sky (DENIS) data base. We used late-M (≥ M8.0), L and T dwarfs with accurate trigonometric parallaxes to calibrate the Mj versus I-J colour-luminosity relation. The resulting photometric distances have standard errors of ∼ 15 per cent, which we used to select candidates d phot ≤ 30 pc. We measured proper motions from multi-epoch images found in the public archives ALADIN, DSS, 2MASS and DENIS, with at least three distinct epochs and time baselines of 10-21 yr. We then used a maximum reduced proper motion cut-off to select 28 candidates as ultracool dwarfs (M8.0-L8.0) and to reject one as a distant red star. No T dwarf candidates were found in this search, which required an object to be detected in all three DENIS bands. Our low-resolution optical spectra confirmed that 26 were indeed ultracool dwarfs, with spectral types from M8.0 to L5.5. Two contaminants and one rejected by the maximum reduced proper motion cut-off were all reddened F-K main sequence stars. 20 of these 26 ultracool dwarfs are new nearby ultracool dwarf members, three L dwarfs within 15 pc with one L3.5 at only ∼ 10 pc. We determine a stellar density of Φ J cor = (1.64 ± 0.46) x 10 -3 dwarfs pc -3 mag -1 over 11.1 ≤ M J ≤ 13.1 based on this sample of M8-L3.5 ultracool dwarfs. Our ultracool dwarf density value is in good agreement with the measurement by Cruz et al. of the ultracool dwarf density at high Galactic latitude.

Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Scorpii OB association

Aims. We search for new ultracool dwarfs and report here our observations of 78 ultracool dwarf candidates that have been photometrically selected using the DENIS survey point source catalogue. We analyze low-resolution optical spectroscopic observations to estimate spectral types of all candidates. Methods. We derive spectral types for each object using measurements of the PC3 spectral index as defined in Martin et al. They range from M6 to L3. The Hα emission and NaI subordinate doublet (818.3 nm and 819.9 nm) equivalent widths are measured in the spectra to identify young stellar objects. Spectroscopic indices of TiO, VO, CrH, and FeH molecular features are also reported. Results. A rule-of-thumb criterion for selecting young very low-mass objects using the NaI doublet equivalent width is given. It is used to confirm seven new members of the Upper Sco OB association and two new members of the R Cr-A star-forming region. Four of our field objects are also classified as very young, but are not members of any known nearby young association. The frequency of lower-gravity young objects in our field ultracool sample is 8.5%. Our results provide the first spectroscopic classification for 42 ultracool dwarfs in the solar vicinity with spectrophotometric distances in the range 17 pc to 65 pc (3 of them being new L dwarfs within 20 pc).

Optical linear polarization in ultra cool dwarfs - A tool to probe dust in the ultra cool dwarf atmospheres

Aims. Recent studies have detected linear polarization in L dwarfs in the optical I band. Theoretical models have been developed to explain this polarization. These models predict higher polarization at shorter wavelengths. We discuss the polarization in the R and I band of 4 ultra cool dwarfs. Methods. We report linear polarization measurements of 4 ultra cool dwarfs in the R and I bands using the Intermediate dispersion Spectrograph and Imaging System (ISIS) mounted on the 4.2 m William Herschel Telescope (WHT). Results. As predicted by theoretical models, we find a higher degree of polarization in the R band when compared to polarization in the I band for 3/4 of these ultra cool dwarfs. This suggests that dust scattering asymmetry is caused by oblateness . We also show how these measurements fit the theoretical models. A case for variability of linear polarization is found, which suggests the presence of randomly distributed dust clouds. We also discuss one case for the presence of a cold debris disk.

The Least Massive (Sub)Stellar Component of the Milky Way

This review presents a panorama of the research topics that are currently being developed by our strategic research group at the IAC in the field of very low-mass stars, brown dwarfs, extrasolar giant planets and the solar system. Our main goal is to investigate the cosmogony of the least massive stellar and substellar component of the Milky Way. We are using multiwavelength observations and theoretical modeling to provide constrains to different scenarios of star and planet formation. We present summaries of the following results: (1) the study of a deeply-embedded low-mass protostar in the B59 molecular cloud; (2) the discovery of accreting very low-mass objects in the IPHAS survey; (3) the identification of faint planetary-mass candidates in the cores of young open clusters using multi-conjugate adaptive optics; (4) the discovery of a widely separated companion of a young brown dwarf; (5) the search for reflected light from hot Jupiters; and (6) the spectroscopic study of relics of the formation of our Solar System.