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P.J. Lowrance1, E.E. Becklin1, G. Schneider2, D. Hines2, J.D. Kirkpatrick3, D. Koerner4, F. Low,2 D. McCarthy,2 R. Meier5, M. Rieke,2 B.A. Smith5, R. Terrile,6 R. Thompson,2 B. Zuckerman1
1University of California, Los Angeles, CA
2University of Arizona, Tucson, AZ
3Infrared Processing and Analysis Center (IPAC), Pasadena, CA
4University of Pennsylvania, Philadelphia, PA
5Institute for Astronomy (IFA), University of Hawaii, Honolulu, HI
6Jet Propulsion Laboratory (JPL), Pasadena, CA
NICMOS, coronagraph, young stars, brown dwarfs
The best brown dwarf candidate for several years
was the companion to the white dwarf GD 165 (Becklin& Zuckerman 1998, Zuckerman & Becklin 1992).
For eight years it had the coolest temperature (1800K)
of any dwarf star, but it remained unclear
if it was a very low-mass star or high-mass brown dwarf. Recent ``all-sky'' near-infrared
surveys (2MASS & DENIS) may soon clarify whether 1800K objects are minimum-mass stars or
brown dwarfs. In 1995, the
first undisputed brown dwarf was discovered as a companion to the M1V
star GL 229 (Nakajima et al. 1995). The methane feature in
the infrared spectrum of the companion clearly constrains its
temperature below 1000K (Oppenheimer et al 1995), and models place its
mass between 30 and 50 MJ (Burrows et al 1997).
The last two years have brought many discoveries. Presently, we now have several suspected isolated brown dwarf objects in the Pleiades star cluster and solar neighborhood, and several planets around main-sequence stars. Yet, GD 165B and GL 229B remain the only substellar companions to have been directly imaged. Low-mass objects are intrinsically faint and therefore hard to detect directly close to bright stars. With the resolution and current infrared capability of NICMOS on the Hubble Space Telescope, we can now attempt this daunting task.
However, not all sources in the field of view around the primary will necessarily be companions.
Background objects could be stars or galaxies. With the
excellent resolution on NICMOS, galaxies are easy to distinguish since they
appear extended. As for point sources, the ultimate determinant
of companionship
is common proper motion. The target stars have been chosen for location
away from the galactic plane:
.
Therefore, we expect less
than 0.02 background stars within
at H=22 for each star (D.McCarthy,
private comm). Detections will be followed up with additional direct imaging to confirm
companionship. For a candidate close-in
(
), for which the proper motion is enough (
08/yr),
we will follow up with a later orbit on NICMOS. This will require
giving up another object on our list. NICMOS has a limited lifetime and will be
unusable after Dec 98. If the candidate is
farther from the target star than 4
,
then ground-based
follow-up will be possible to check proper motion. For close
objects with smaller proper motions, or those we are unable to
follow-up, we must wait for Adaptive Optics (AO) at Keck or Lick observations for
confirmation. Keck might have a workable AO system by the first part of 1999 (Larkin, private comm).
Because the coronagraph is in a corner of the detector, this survey will only
fully sample 05 to 4
around the primary star. At the average
distance of 30 pc, this corresponds to 15-120 AU. This will sample the empirical
maximum in the binary distribution of stars as well as the average
distance of the giant planets in our own solar system (Duquennoy & Mayor 1991).
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According to models (Burrows et al 1997), a 0.1 Gyr old, 20 MJ brown
dwarf will have an absolute H magnitude = 13.51 which corresponds to an apparent
magnitude of 16.10 at the typical distance of our targets (d=30pc). In figure 1, early
observations demonstrated that the background is 10-5
less than the star at 5. With a background
12 magnitudes fainter than our
typical 6th magnitude target star, a 16.10 magnitude object should be easily detectable.
We present one of of first stars observed, HD 102982, a G3V star, mH=6.9, 42 parsecs
from the Sun. The NICMOS images revealed a stellar object 9 away which is fainter
at H by 5.3 magnitudes. If it is a companion, then MH=9.08 places it between M5 and M6
in photometric spectral class (Kirkpatrick & McCarthy 1994).
The age of the star was thought to be close to the Pleiades age (0.1 Gyr) from X-ray luminosity (Lampton et al 1997) and
Calcium H & K emission (Henry et al 1996). At this young age, the companion would be a
candidate brown dwarf at the edge of 75 MJ.
However, a recent finding (Soderblom et al 1998) shows the primary to be a spectroscopic binary with a period
less than one month. This suggests that the activity once thought to be a signature of youth is more likely
a product of a tidally-locked close binary system.
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The observations of HD 102982 demonstrate our ability to detect substellar objects. The possible companion
has a S/N1600 (it is visible in the 0.2s acquisition image) with the roll subtraction for a
mag
5. From these
observations, we should expect a
mag
12 for a S/N=5 (a clear detection) at this separation of 1
.
Around this star, that would be a brown dwarf, and around some of our
youngest stars, this brightness corresponds to
10MJ
(age=15Myr).
Clearly, we will be exploring down to the high-mass planet range with
this survey.
To detect brown dwarfs or possible extra-solar giant planets,
we are undertaking
a coronagraphic imaging survey of young stars. We present images of a possible companion,
probably a low-mass M star, to the star HD102982 which demonstrates our ability to detect substellar
objects. Every substellar companion discovered with this survey will
add to the mere handful already known, and its spectrum would be of tremendous
value in determining the physics of objects between 10-60MJ.