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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
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
We are using NICMOS to search for very low-mass stellar and
sub-stellar companions around nearby, young, main-sequence stars. With
the coronagraph on Camera 2 and the F160W filter (1.4-1.8
search in the region 0.4
from the target star.
To greatly reduce the scattered light near the hole, we image at two
roll angles centered on the star and separated by 30 degrees.
The lower mass limit of a detectable
companion depends on the mass, age, and distance of the target star as well as
the angular separation between star and companion.
For several of our targets that are 10-50 Myr old, this lower mass limit
could be as low as 10 Jupiter masses.
We display preliminary results using this technique.
A stellar-like object with
H=5.3 was discovered 0.
9 from HD 102982.
From the distance and age of the primary, this
companion is most likely a low-mass M star. Recent observations demonstrate
that we can obtain a S/N = 5 (clear detection) on a secondary with
H=12 at a separation of 1''.
NICMOS, coronagraph, young stars, brown dwarfs
Searches for sub-stellar companions to main sequence stars and white dwarfs began about a decade ago
utilizing both infrared (photometric) and optical (radial velocity) techniques.
Brown dwarfs are objects thought to form like stars, but with
insufficient mass (
to sustain hydrogen (H)
their interior. In contrast, giant planets like those in our
solar system are thought to form through agglomeration of H and He around
a rocky, icy core within a protoplanetary disk around a star. Planets are usually split
into Jovian (gaseous)
and Terrestrial (rocky), but brown dwarfs are more like Jovian planets than Jovian
planets are like Terrestrial planets. Some have suggested brown dwarfs might be a third class
of planets, since they should not be classified as stars, as they do not have
thermonuclear fusion in their cores.
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
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.
Brown dwarfs and giant planets are more luminous when they are young, as
they are still at a large radius, gravitationally contracting, and hot. Models
of substellar objects (Burrows et al 1997) show that
based on cooling curves derived from atmospheric physics. If these models
are correct, we can expect a brown dwarf/planet of 10MJ will be above
when it is
Numerous young stars within
50 pc of the sun have ages
similar to the Pleiades (
yrs). This is deduced via
inter-comparison of lithium abundance, H
Ca H & K line emission, kinematics, X-ray emission, and rotational
velocity. Age estimates can be calibrated against
samples of T Tauri stars and stars in young, nearby clusters such as
the Pleiades and the Hyades and distances can be determined from parallax
measurements. As part of our GTO program we have assembled a
list of 50 stars
thought to be young and close, with median age and distance
of 0.1 Gyrs and 33pc, respectively.
The main problem with trying to image brown dwarfs or giant planets around
main-sequence stars is the
overwhelming brightness of the primary. A substellar companion could
be as much as 108 times fainter than the star it orbits.
A brown dwarf makes up some of this in the infrared, where it radiates most
of its power, and is brighter with youth, but the primary
can still be 104 times brighter.
The orbit of a high mass planet is thought to be rather close to
the star, as well, so high resolution, achievable from space, is needed.
We will conduct our imaging survey
with the coronagraph on Camera 2 at 1.6
(F160W), which corresponds roughly
to the H-band filter on the ground.
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
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 (
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,
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).
Our observational strategy places the target stars behind the coronagraph on Camera 2.
The coronagraph is actually a hole in one of the mirrors of
the optical path and combined with a Lyot stop, reduces the amount of light in the Airy
rings of the point spread function rather than merely blocking
light. With the target behind the coronagraph, we reduce the amount of
light at the edge of the coronagraph by a factor of 4-6 (figure 1) compared to direct
imaging. We have developed an observing strategy to go even deeper, as
The Hubble Space Telescope has the ability to slew in three directions:
x, y, and .
In order to keep the target star in the same
position behind the coronagraph, as well as simplify the removal of the
stellar profile, we designed two observations of the star at
different spacecraft orientations (
separated by 30 degrees.
Therefore, the observing strategy is to place the young star
behind the coronagraph, observe in the F160W filter (1.4-1.8)
for about 700s, roll the telescope 30
degrees, and observe again for 700s. When we subtract the two images,
the background from the star should subtract out (not quite completely
due to some scattering) and if there are any additional objects in the field,
two images (positive and negative) of each should remain (see figure 2). In the Orbital
Verification testing (SMOV) of the camera, we found that this strategy worked as expected.
Measured background as an azimuthal average
relative to the central pixel of a unocculted image in F160W (H-band) on the NICMOS
camera. The background flux is reduced by placing the star behind the coronagraph
(dashed line), and reduced even more from the roll subtraction of two images (dotted line).
Results of SMOV test 7052 (Schneider & Lowrance, 1997)
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.
HD 102982 images with direct imaging (top panels) and
with coronagraphic imaging (bottom panels). The direct images are from the acquisition with the
F165M filter and are scaled to the F160W image. The far right panels are the subtraction of the
two roll orientations; thus a positive and negative image exist.
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
mag5. From these
observations, we should expect a mag12 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
Clearly, we will be exploring down to the high-mass planet range with
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.
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