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 10, 2005

NICMOS Imaging of 2MASSWJ 1207334-393254 - A Planetary-Mass Companion Candidate

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, 520-621-5865, gschneider@as.arizona.edu
Click for ABSTRACT OF PAPER to be presented.

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 Symbiosis

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.

Technical notes:
  1. 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) level.
  2. 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.
  3. Each image is 2.5" x 2.5".
  4. Before subtraction, four images were obtained at each of the two field orientations in each filter and combined to provide better image sampling.
  5. Total exposure times are as follows: 0.9 microns = 2560 seconds, 1.1 microns = 2048 seconds, 1.6 microns = 448 seconds.
  6. 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 Home Page