The Formation of Brown Dwarfs

We have obtained series of images in the near infrared J and Ks bands for seven L-type dwarfs with a duration of 3 to 6 hours. We present results on: 1) the amplitude of variability associated with atmospheric changes over time scales from minutes to several hours; 2) the search for cool companions in wide orbits; 3) the search for transits of brown dwarfs and planetary companions in very close orbits.

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Very Low-Luminosity Objects in Star-Forming Regions

Highlights of Astronomy ◽

10.1017/s1539299600021626 ◽

1998 ◽

Vol 11 (1) ◽

pp. 423-424

Author(s):

Motohide Tamura ◽

Yoichi Itoh ◽

Yumiko Oasa ◽

Alan Tokunaga ◽

Koji Sugitani

Keyword(s):

Brown Dwarfs ◽

Mass Function ◽

T Tauri Stars ◽

Initial Mass Function ◽

Formation Mechanisms ◽

Star Forming ◽

Star Forming Regions ◽

T Tauri ◽

Low Mass ◽

Stellar Sources

Abstract In order to tackle the problems of low-mass end of the initial mass function (IMF) in star-forming regions and the formation mechanisms of brown dwarfs, we have conducted deep infrared surveys of nearby molecular clouds. We have found a significant population of very low-luminosity sources with IR excesses in the Taurus cloud and the Chamaeleon cloud core regions whose extinction corrected J magnitudes are 3 to 8 mag fainter than those of typical T Tauri stars in the same cloud. Some of them are associated with even fainter companions. Follow-up IR spectroscopy has confirmed for the selected sources that their photospheric temperature is around 2000 to 3000 K. Thus, these very low-luminosity young stellar sources are most likely very low-mass T Tauri stars, and some of them might even be young brown dwarfs.

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A Deep Large area Search for Low Luminosity Stars

Highlights of Astronomy ◽

10.1017/s1539299600021675 ◽

1998 ◽

Vol 11 (1) ◽

pp. 435-435

Author(s):

Hugh R.A. Jones ◽

Mike R.S. Hawkins

Keyword(s):

Spectral Type ◽

Low Temperatures ◽

Current Model ◽

Brown Dwarfs ◽

Evolutionary Models ◽

Spectral Indices ◽

Late Type ◽

Brown Dwarf ◽

Large Area ◽

M Dwarfs

In a recent survey for faint red stars from a digital stack of Schmidt plates a number of candidate objects were identified. Parallax’s for three of these objects have been reported showing them to have luminosities which interpreted within the available evolutionary models indicate them to be good brown dwarf candidates. Here we examine spectra of these objects and others from the plate stack. Using standard spectral indices we find that for a given spectral type their spectra are more consistent with the Pleiades brown dwarfs (PPL 15, Teide 1 and Calar 3) than with standard late-type M dwarfs. Our interpretation is that this is due to their selection by RF IN colours which at values > 3 preferentially selects objects with relatively low gravities. For late-type M dwarfs and brown dwarfs low gravities are expected to be a reliable indication of youth. We also notice that the stack objects generally have strong FeH absorption for their spectral type. Current model atmospheres suggest that FeH strongly increases in strength toward lower metallicities and lower temperatures. We believe that this is not consistent with the available observational evidence from late-type M dwarfs. It is possible that solid Fe is forming inthe low temperature atmospheres relatively depleting FeH strengths toward lower temperatures. We find some evidence that for dwarfs at low temperatures dust formation is less prevalent in lower gravity objects suggesting that dwarfs at low temperatures stronger FeH may be an indication of youth. In addition to the spectral evidence the three stack objects whose parallax’s have been measured show small tangential velocities which is a further indication of youth.

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Spectra of M Dwarfs - Dimmed by Dust

Highlights of Astronomy ◽

10.1017/s1539299600021717 ◽

1998 ◽

Vol 11 (1) ◽

pp. 439-440

Author(s):

T. Tsuji ◽

K. Ohnaka ◽

W. Aoki ◽

H.R.A. Jones

Keyword(s):

Chemical Composition ◽

Brown Dwarfs ◽

Surface Chemical ◽

Basic Information ◽

M Dwarfs ◽

Surface Chemical Composition ◽

Molecular Lines ◽

Low Mass

Spectra of M dwarfs are rich in atomic and molecular lines. These spectra provide such basic information as Teff (or radius), log g (or mass), surface chemical composition, and something more (e.g. activity) if properly interpreted. It is recognized, however, that spectra of M dwarfs are already dimmed by the dust formed in their photospheres (Tsuji et al. 1996a) and this effect, which has been overlooked until recently, should be taken into account in any interpretation and analysis of the spectra of very low mass objects (VLMOs) including late M dwarfs and brown dwarfs.

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