EMBARGOED -- FOR RELEASE: 09:20
A.M. PST, January 10, 2005
To Be Presented at: AAS 205th Meeting, San Diego, California
Session 11.14 - Brown Dwarfs and Exoplanets -- Monday, January
NICMOS Imaging of 2MASSWJ 1207334-393254 - A Planetary-Mass
Paper Presented by:
Glenn Schneider (Steward Observatory, University of Arizona)
Inseok Song (Gemini Observatory)
Ben Zuckerman, E. Becklin (University of California, Los Angeles)
Patrick Lowrance (California Institute of Technology)
Bruce Macintosh (Lawrence Livermore National Laboratory)
Michael Bessell (Australian National University)
Christophe Dumas, Gail Chauvin (European Southern Observatory)
Contact: Glenn Schneider,
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OF PAPER to be presented.
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Available via the web after the embargo date: http://nicmosis.as.arizona.edu:8000/AAS_JAN_2004/PR_11_14.html
2M1207 - Closing in on an Extra-Solar Planet: the HST/AO
2MASSWJ 1207334-393254 (aka 2M1207) is a young brown dwarf (estimated
age 8 million years) approximately 70 parsecs (225 light years) from the
Earth. In April 2004 , using an infra-red camera on the Very Large
Telescope (VLT) in Chile augmented with an adaptive optics (AO) system,
astronomers spotted a feeble point of light (1/100th as bright as 2M1207
itself at the wavelengths of their observations) in close angular proximity
to 2M1207, 778 thousandths of an arc second (less than a mile at the distance
of the moon) away. Those observations suggested the possibility of
the object being a giant planet companion to 2M1207, as recently reported
in Astronomy & Astrophysics by Chauvin et al. If confirmed 2M1207"b"
would represent the first image of a planetary mass object gravitationally
bound to a star - other than the Sun. Contemporaneously, the Hubble
Space Telescope (HST) was preparing to execute an imaging survey of 116 comparably
young and nearby stars (then to begin in July 2004 and now under way) to
search for such objects - a survey that also included 2M1207. The HST
survey team was advised by the VLT researchers of the Giant Planet Companion
Candidate (GPCC) they had detected, and the HST observation of this star
was re-planned from a survey/detection mode to one of characterization and,
to the extent possible on a very short timescale, confirmation.
"Second epoch" observations were carried out with HST's Near Infrared
Camera and Multi-Object Spectrometer (NICMOS) in August 2004. Those
observations of 2M1207 and its GPCC provided high resolution images at
the short wavelength end of the near-infrared spectrum which cannot be obtained
from the ground. Ground-based AO, though a potentially powerful technique
to sharpen "fuzzy" infra- red images blurred by the Earth's atmosphere,
degrades in capability with decreasing wavelength, specifically at those
wavelengths where key "color" diagnostics would specifically inform on the
physical nature of 2M1207's GPCC. Thus, HST/NICMOS images were obtained at
three "short" near-IR wavelengths: 0.9 microns where the GPCC was 44 times
fainter than the shortest (1.6 micron) wavelength where it was detected
with VLT, 1.1 microns to provide a second color diagnostic (also undetectable
with VLT), and also longer 1.6 micron (H-band) images to compare directly
with the shortest wavelength VLT image in the same spectral region. These
observations found the object to be extremely "red" and relatively much
brighter at longer wavelengths - both within the NICMOS spectral bands but
also in comparison to the even longer wavelength VLT observations.
The multi-band color indices and wavelength-dependent brightnesses (flux
densities) match theoretical expectations for an 8 million year old object
about five times as massive as Jupiter.
Earlier, the VLT researchers had also obtained a low-resolution spectrum
of the GPCC at wavlengths from 1.5 to 1.8 microns. At these wavelengths,
this spectrum is similar in many respects to what one might expect from
an L-dwarf star with a temperature of approximately 1250 degrees on the
Kelvin scale, included a broad spectral absorption feature which is attributed
to water thus further implicating the possibility of a the object's true
nature as a hot giant planet. Additional broad spectral diagnostics at
shorter wavelengths could not be obtained due to the limitations of ground-based
AO, hence the need for space observations.
Conservatively, the colors, flux densities, and possibility of H2O
cannot unambiguously identify the GPCC as a planet. The ultimate
arbiter is the establishment of common proper motion of 2M1207 and the
GPCC - i.e., if both objects are seen to move through space as a gravitationally
bound pair. 2M1207 has the spectral signature of a very low mass star
which, because of its youth, belies its true nature as a brown dwarf only
25 times the mass of Jupiter. The GPCC, if gravitationally bound
is at a projected (minimum) distance of approximately 54 Astronomical Units
from 2M1207 (about 1/3 again further away than Pluto is from the Sun).
Given the mass of 2M1207 (appx 2.4% that of the Sun), as inferred from its
spectral type (M8.5) and age, the GPCC would have an orbital period of at
least 2500 years. Hence, any relative motion seen between the two on much
shorter time scales would reveal the GPCC to be a background interloper.
The very high precision relative position measures obtained with
the HST/NICMOS imaging observations were used in combination with
the earlier VLT observations to test the possibility of physical companionship
by common proper motion. The August 28, 2004 NICMOS observation placed
the GPCC 773.7 +/- 2.2 thousandths of an arc second from 2M1207. Despite
the only four months between the VLT and NICMOS observations the likelihood
of the object being not being an ill-placed background object (without
invoking the implications of colors and flux densities indicative of a
planetary mass object) was confirmed at the 99.1% level of confidence. But,
even those "odds" are not good enough for conservative astronomers to claim
victory. The precision of this measurement with the images now in hand is
predominantly limited by the systemic astrometric calibration of the VLT/NACO
AO observations and our current knowledge of the proper motion of 2M1207
itself. The uncertainty in follow-up differential position measures would
naturally decline with a sufficiently longer temporal baseline. Such follow-up
HST/NICMOS observations are planned for April 2005 (the next possible scheduling
opportunity). If the 2nd/3rd epoch (NICMOS/NICMOS) differential proper motions
continue to track as suggested by the 1st/2nd epoch (VLT/NICMOS) observations
a differential measure would be secured to reject the possibility of a the
GPCC being a background source at greater than the 7 sigma level (i.e.,
effectively at the 100% confidence level).
The HST/NICMOS and VLT/NACO observations of 2M1207 and its GPCC clearly
demonstrate the complementary nature and symbiosis between space and ground-based
astronomy using state of the art observing techniques and instrumentation.
Each is contributing importantly where uniquely suited to assembling the
pieces of the jigsaw puzzle in the quest for images of extrasolar planets.
Figure 1 Caption - NICMOS Camera 1 images of 2M1207's GPCC at three
increasingly longer near-infrared wavelengths. Left to right 0.9, 1.1,
and 1.6 microns. The light from 2M1207 itself has been greatly reduced
by subtracting a second image of the star taken with HST rolled to a different
orientation in the sky. Imperfections in the "differential roll"
subtraction technique give rise to optical artifacts at the location of
2M1207 which is centered in the red circle of 0.2 arc second radius. In
subtraction, a negative imprint of the GPCC also arises (see Figure 3)
which has been removed from these images in the differential roll image
recombination process. 2M1207, a young brown dwarf of spectral type
M8.5, is itself quite red -- 11 times fainter at 0.9 microns than it is
at 1.6 microns. The GPCC at 0.9 microns is significantly redder in comparison,
44 times fainter at 0.9 microns than it is at 1.6 microns where, at that
shorter wavelength, it is also 720 times fainter than 2M1207.
- Each image is stretched to show the brightness of the GPCC with
respect to the noise in the image background at the -2 sigma (from zero)
- In the 1.1 and 1.6 micron images the first Airy ring, arising
the HST diffraction pattern of an unresolved point source which scales
linearly with wavelength, around the GPCC is seen.
- Each image is 2.5" x 2.5".
- Before subtraction, four images were obtained at each of the two
field orientations in each filter and combined to provide better image
- Total exposure times are as follows: 0.9 microns = 2560 seconds,
1.1 microns = 2048 seconds, 1.6 microns = 448 seconds.
- At 0.9 microns the apparent brightness of 2M1207's GPCC is approximately
one billionth that of the star Vega.
Figure 2 Caption - Three spectral band "color" composite image of the
GPCC. The background light due to 2M1207 in the individually filtered
images has been greatly reduced by the differential roll subtraction
and image recombination technique. Red = 1.6 microns (F160W
filter), Green = 1.1 microns (F110M filter), Blue = 0.90 microns (F090M
filter). The magnitude of the image processing artifacts within the gray
masked region (inside the red circle) and extending to an angular distance
of 0.2 arc seconds from 2M1207 (at the location of the "+" sign) at the shorter
(bluer) wavelengths dominate the roll-subtraction residuals and represents
a "here be dragons" regions which cannot be probed in this manner. This
corresponds to a distance of approximately 14 Astronomical Units at the
distance of 2M1207 from the Earth, which would be mid-way between the orbits
of Saturn and Uranus in our own solar system.
Figure 3 Caption - Demonstration of Process. NICMOS 0.9 micron
(left), 1.1 micron (middle), 1.6 micron (right) images of 2M1207 and
its GPCC. Top two rows are camera 1 images at two field orientations
differing by 9.9 degrees. Most of the structures seen, including point
like "speckles" are optical artifacts due to the point spread function
of the telescope. These artifacts are rotationally invariant. i.e.,
they are fixed in the frame of the NICMOS detectors as the telescope is
rotated, but disappear when one image is subtracted from another (middle
row). Upon subtraction any objects in the field appear as positive/negative
image pairs separated in angle by the amount the telescope was rolled about
the target. Synthetic point sources (differing by wavelengths) generated
using a high fidelity optical model of HST+NICMOS, were separately subtracted
from the difference images, thereby isolating the positive and negative
images of the GPCC (bottom two panels). These images were then added
together after inverting, and rotating the rightmost images by -9.9 degrees
about the position of 2M1207, resulting in the images shown in Figure 1.
This work is supported through grants to the GO 10176 and 10177 teams
from STScI, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
Link to Glenn Schneider's