Computer Science – Performance
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7952m&link_type=abstract
HST Proposal ID #7952
Computer Science
Performance
Hst Proposal Id #7952
Scientific paper
Very recently the first two free-floating brown dwarfs have been discovered. They have been published in the last few weeks {Delfosse et al. 1997 AA, 327, L25; Ruiz et al. 1997 ApJ, 491, L107}. High resolution Keck spectroscopy has confirmed strong lithium lines in both of them {Martin et al. 1997, AA, 327, L29; Basri et al. 1997, in "Cool Stars X"}. The temperature and lithium of these brown dwarfs imply that they are less massive than 60 Jupiters and younger than 1 Gyr. If they have companions, those would be still less massive and thus would be low-mass brown dwarfs or giant planets. NICMOS observations can detect companions with masses as low as 5 Jupiters around these brown dwarfs at separations comparable to our solar system's outer planets. This proposal was not submitted to the most recent TAC because the two discovery papers had not yet been accepted for publication. The need for NICMOS implies that the proposed observations cannot wait until the next TAC for evaluation. Brown Dwarfs {BDs} have become astronomical objects only in the last 2 years. In 1995, the first two uncontrovertible BDs were announced, one in the Pleiades cluster {Teide1, Rebolo et al. 1995, Nature, 377, 129} and the other in the vicinity of a nearby star {Gliese 229B, Nakajima et al. 1995, Nature, 378, 463}. Both of them have been subsequently confirmed with follow- up spectroscopic observations {Rebolo et al. 1996, ApJ, 469, L53; Matthews et al. 1996, AJ, 112, 1678}. More BDs are continuing to be revealed in the Pleiades cluster {Zapatero- Osorio et al. 1997, 491, L81} indicating that they are quite numerous. Wide field deep searches are also starting to provide the first discoveries of nearby free-floating BDs. The first two such objects have just been published. It is fortunate that we are still in time to use HST NICMOS to look for faint companions to them. The presence of lithium in both BDs confirms that they have substellar masses of less than 60 Jupiters because otherwise this fragile element would had been burned and depleted {Magazzu et al. 1993, ApJ, 404, L17}. It also implies that they are quite young, because BDs cool to lower temperatures and fade to lower luminosities with time. If these BDs had been older they would not have been detected yet. The present surveys are flux-limited and hence they favour the detection of young BDs which are still up in their cooling tracks. The youth of these BDs implies that it is easier to detect even lower mass companions orbiting them. HST NICMOS is needed because of its combination of high spatial resolution and sensitivity to very cool objects. It has already been shown that HST/NICMOS has much superior expected performance for detecting companions to BDs than HST/WFPC or any ground-based instrument currently available {see HST proposal 7899}. The two nearby BDs that we propose to observe have distances of only about 10-15 parsecs. Thus, the likelihood of detecting companions is much increased with respect to the more distant BDs of the Pleiades. Both BDs have high galactic latitude and high proper motions. Thus, we do not expect a high density of background objects, and we will check the proper motion of any faint close companion in a few months.
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