Searches for Brown Dwarfs in the Hyades

Details Searches for Brown Dwarfs in the Hyades Searches for Brown Dwarfs in the Hyades

May 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...180.2903m&link_type=abstract

American Astronomical Society, 180th AAS Meeting, #29.03; Bulletin of the American Astronomical Society, Vol. 24, p.773

Astronomy and Astrophysics

Astronomy

2

Scientific paper

We have been conducting a search for very low mass (VLM) objects in the Hyades. Using IRCAM on the UKIRT together with the UCLA coronagraph, and a CCD camera at Lick observatory, we have imaged 90"x90" fields surrounding 49 known Hyades stars. So far results are tantalizing but inconclusive: after rejecting red galaxies, three candidate VLM objects have been detected with I-K colors of 3.2 to 4.2, and K magnitudes from 14.6 to 15.7, which agree with VLM models at the Hyades age and distance (Burrows et al. 1989, ApJ, 345, 939; Burrows 1992, priv. comm.) However, it possible that these are background objects; follow-on observations using UCLA's new KCam IR imager (McLean et al. 1992, this meeting) will measure the proper motion of these candidates relative to their primaries and determine their Hyades membership. We detected only one candidate in the range K=15 to 16.5, which may indicate a level or decreasing IMF below the lowest main-sequence star mass. Substantial interest was generated by the announcement of the discovery of seven candidate brown dwarfs in the Hyades (Bryja et al 1992, ApJ, 388, L23) These were identified first through proper motion studies, an unreliable indicator of cluster membership (Giffen et al. 1988, AJ, 96, 172). The candidate objects all have IJHK colors similar to M stars, and were identified as VLM objects solely on the basis of derived luminosity. We calculate, based on star count models (Bahcall and Soneira 1981, ApJ Suppl, 47, 357), that in the area surveyed by Bryja et al. there should be approximately 6 background M-stars fitting their proper motion criterion, with the same apparent K magnitudes and colors as the candidate brown dwarfs. Finally, models with appropriate helium abundance, Y=0.28, rather than the Y=0.22 models used by Bryja et al. shift the minimum main sequence mass and luminosity downward (Burrows 1992, priv. comm.). Therefore we would classify any objects in the Bryja et al. survey that actually are Hyades members as VLM stars rather than brown dwarfs.

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