Shedding light on these, and related questions, is a major element of the NASA's Origins initiative which contains at it's core the establishment of scientific activities to investigate the birth and early evolution of stars and planets (Dressler, 1996). The NICMOS IDT embraced this theme in the establishment and formulation of our EONS search program. From the ill-constrained low-mass end of the mass-luminosity function (0.2 solar masses) through the transitional regime of brown dwarfs (0.08 solar masses), and down into the realm of giant planets NICMOS provides a unique resource for exploration. The EONS search programs were designed to identify low-mass and sub-stellar companions and over two decades in the mass-spectrum as well as circumstellar disks which may be potential harbingers of planetary formation.
The 7226 candidate list contains 43 very young main-sequence stars with mean distances of 30 pc. The median age for the candidates with well established ages is 90 Myrs and contains at least 6 candidates with ages as young as <10 Myrs (see Fig. 1), including several members of the TW Hyd association. Because of the extreme youth of these objects any low-mass brown dwarf and planetary companions will still be in a higher luminosity phase and thus easily detectable. For example, in its first 10 Myrs a 10
MJupiter brown dwarf would have a luminosity exceeding 10-4 solar luminosities (Kulkarni, 1997). Typical separations observable with NICMOS are near the empirical maximum in the binary distribution of stars (20 to 40 AU), which also corresponds to the mean distance of the giant planets in our own solar system.
The 7227 candidate list contains 31 M-dwarf stars which are (a) nearby (d 6 pc) with spectral types later than M3.5, (b) young (age 100 Myr) with (d 25 pc), and (c) spectrally the latest known (i.e.,``ultra-cool" stars later than M8.5), with some overlaps (see Fig. 2). Because of the proximity of many of the stars in this sample to the sun, companions as close as 1.2 AU from their primaries may be detected at the inner radius of our search space.
The 18 candidate stars in the search for circumstellar dust disks (7233) are primarily main sequence stars with IRAS IR excesses,
> 10-3, and other indicators of the possible presence of disks. In addition to H-band imaging for the brighter stars in this program we are obtaining multi-spectral images at 1.71
HCO2 + C2 continuum), 2.04
(methane band), and in the line spectra of Paschen-
Our EONS surveys are obtaining single-color coronagraphic images aimed at discovering sub-stellar companions utilizing the F160W filter (1.4 - 1.8)
where the NICMOS coronagraphic performance is optimized. This wavelength band also corresponds to the strong emission in GL 229B (Burrows, et. al, 1997), GD 165B (Zuckerman & Becklin, 1992) and several of the DENIS survey candidate brown dwarfs (Delfosse, et. al, 1997), as well as to strong reflections in Jupiter and Titan. Putative discoveries of brown dwarf and giant planet companions arising from the above single-epoch searches must be subsequently confirmed and the objects characterized in follow-up second-epoch observations.Such follow-up observations are required to sort out field-objects from true companions by common proper motions and/or color indices and to obtain multi-color imaging to establish luminosity ratios of such companions. All of the stars in the M-dwarf survey, and most of those in the young stars program, have sufficiently high proper motions to allow follow-up confirmatory observations to be carried out in the shortened NICMOS lifetime. It is anticipated that for some regimes of the search-space ground-based follow-up using facilities such as Keck, Lick, or VLT may be possible.
Due to a now recognized, and understood, deficiency in the target acquisitionprocess the anticipated performance levels of differential coronagraphic observations (as demonstrated in SMOV) have been somewhat compromised. As a result the limiting sensitivities, and ultimate detection limits for observations made in the first three months of the EONS programs have fallen short of their performance goals due to locally increased backgrounds of two stellar magnitudes or more in a spatially dependent fashion. Corrective action for subsequent observations has been taken in the form of an operational workaround, and a soon to be implemented flight software upgrade. A strategy for recovery of these early coronagraphic data has been devised by the NICMOS IDT.
The first 34 coronagraphic target acquisitions in the EONS program exhibited large (up to 1 pixel) pointing errors due to the above mentioned flight S/W problem. Acquisitions are done in pairs, approximately 20 minutes apart, in the same target visibility period. The spacecraft is rolled 30o between acquisitions to obtain coronagraphic images at two orientations. The origin of the graph in Fig. 4 is the desired placement of the target in the coronagraphic hole (the ``low scatter point" of the coronagraph). To achieve the detection limits and sensitivities required for the full search space of the EONS programs afforded by the coronagraph the maximum deviation from the low scatter point for an acquisition pair should be no more than 0.25 pixels, with a dispersion for the two points in the pair of no more than 0.1 pixels.
To ``recover" the observations targets with similar mis-pointings may bepaired and subtracted after appropriate flux-scaling. In the absence of large spacecraft ``breathing" excursions, the magnitude of the subtraction residuals is greatly reduced. This is demonstrated in Fig. 5. The subtraction of two identically exposed and reduced, but differentially mis-pointed, coronagraphic images of the same target (805-06 in Fig. 4) is shown in the top left panel. The field was rotated 30 degrees about the target star between observations. The differential pointing error resulted in a mis-registartion of the target in the coronagraphic hole by 0.16 pixel (with an absolute pointing error of 1.3 pixels).The panel on the top right shows the subtraction of the ``positive roll" image of the same target (805) using a much better position-matched flux-scaled PSF (S62).The registration of the two PSF cores was three times better then the roll subtraction shown on the left. (The large dark spot below the target is a moderately bright field star in the reference PSF image). The visually obvious improvement in the image subtraction is quantified in the histogram of subtraction residuals. Using a more closely position matched PSF the FWHM of the residual distribution function is reduced by a factor of 4. Unfortunately, only a few such weii matched mis-pointing pairs exist for the already obtained data. The NICMOS IDT is now designing a post S/W fix observing program to obtain reference/calibration coronagraphic PSFs to recover, to a large degree, the observations already in hand.
This work is supported by the National Aeronautics and Space Administration under NASA grant NAG-5-3042.