SIRTF Brown Dwarf Studies

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Session 25 -- SIRTF
Display presentation, Tuesday, 31, 1994, 9:20-6:30

[25.04] SIRTF Brown Dwarf Studies

\def\sun{\ifmmode_{\mathord\odot} \else$_{\mathord\odot}$\fi} \def\mum{\ifmmode{\mu {\rm m}} \else{$\mu$m} \fi} William J. Forrest (U. Rochester)

Cooled space telescopes, such as the planned Space Infrared Telescope Facility (SIRTF), are uniquely suited to detecting and studying objects with temperatures less than about 1000K. This category includes brown dwarfs (superplanets, self-gravitating objects with masses less than the lowest mass stars, about 0.1 M\sun). The recent detection of gravitational lensing events of LMC stars by foreground objects indicates the objects are of low mass ($<$ 0.5 M\sun). It seems most likely that these objects are sub-stellar, and quite possibly are brown dwarfs. Their location is not certain, they could be disk or halo objects. In the latter case, these objects could represent the long-sought halo dark matter. Brown dwarfs in the likely mass range 0.01 to 0.06 M\sun\ will have cooled to 225 to 750 K after 10 billion years. For masses above 0.02 M\sun, a survey at 4.5 \mum\ will be the most effective, while lower mass objects are better detected at 12.5 \mum. A 3 month survey of high galactic latitudes should detect several hundred brown dwarfs if they dominate the dark halo (local density 0.008 M\sun/pc$^3$), or if they provide 10\% of the known local density in the disk (stars, white dwarfs, gas and dust). Candidate brown dwarfs will be selected on the basis of stellar images and very red colors. Halo brown dwarfs will be distinguishable based on their large proper motions (several arcsec/year at a distance of 10 pc) in a later re-survey of the candidate objects. Disk brown dwarfs will be distinguished from dust-shrouded, compact galaxies through 1\% resolution spectroscopy. The optimal brown dwarf survey will employ relatively short integration times (10-100 sec), so spectra of the 100 most likely candidates can be obtained in about 100 hours of additional observations. For the halo objects, a rough measure of mass will be possible through estimates of the temperatures, as noted above. The spectra, in conjunction with theoretical stellar atmosphere calculations, will be crucial in the temperature determinations. SIRTF can locate and take spectra of 0.01-0.06 M\sun\ brown dwarfs in the Hyades and Pleiades in a few days of surveying, even if brown dwarfs are significantly less numerous than the known stellar members.

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