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NICMOS Coronagraphic Surveys-Preliminary Results

Glenn Schneider University of Arizona, Tucson, AZ

 

 

Abstract:

A search for previously unknown low-mass (giant planet and brown dwarf) companions to stars in the solar neighborhood, and circumstellar dust disks around main sequence stars by direct coronagraphic imaging has been undertaken by the NICMOS Instrument Definition Team (IDT). Observations of a carefully selected candidate list of $\sim$100 stars began on February 28, 1998, more than a year after the start of the NICMOS mission. Using a differential imaging strategy, as originally demonstrated in the second Servicing Mission/Observatory Verification (SMOV) program, we were able to achieve statistically significant detections of substellar companions at 1.6$\micron$ with a ${\Delta}H$ of $\sim$10 at separations as close as 0.5" (corresponding to 2.5 AU at 5 pc), with increasingly better performance at increasing radii. With nearly two dozen candidates now observed, and a better understanding of the focal plane stability and target acquisition precision we report on the efficacy of the program for detecting transitional objects into the 3 to 5 Jupiter mass regime.

Introduction

A systematic survey of nearby stars for giant planet and brown dwarf companions, and circumstellar (or protoplanetary) disks, is a cornerstone of the NICMOS IDT's Environments of Nearby Stars (EONS) program. In carrying out this survey we are attempting to address some fundamental questions in the arena of stellar and planetary system formation. We seek to learn if indeed there is there a continuity of objects across the sub-stellar mass spectrum bridging the main sequence to planetary objects. If so then at what distances will such objects be found from their primaries and how might this be biased by the characteristics of their primaries or companion objects? What implications will such discoveries have for our understanding of formation mechanisms?

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 ($<\sim$0.2 solar masses) through the transitional regime of brown dwarfs ($<\sim$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 EONS Coronagraphic Surveys

The design, fabrication, and incorporation of a coronagraph into NICMOS camera 2, as discussed in detail by Schneider, et al. (1998), allows us to push our investigations into previously and other-wise unreachable corners of the observational parameter spaces associated with the close environments of stars. The NICMOS coronagraph is capable of providing detections of non-stellar companions as faint as H = 22, 10 to $\sim$13 magnitudes fainter then their primaries at separations of 0.375" to $\sim$3.0", should they exist. The EONS programs concentrate on these astrophysically and dynamically interesting spatial regions of the search parameter space. For the stars in our sample this covers minimum physical separations of 1.2-50 AU at the inner spatial detection limit. The lower mass limit depends on age, distance, and spectral type, but can be as low as 3 to 5 M_Jupiter for many of our targets. The EONS surveys are segregated into three complementary observing programs (designated 7226, 7227 and 7233 by STScI), delineated primarily by the principle characteristics of the candidate stars,which cover the full accessible range of the observable sub-stellar search space.

The 7226 candidate list contains 43 very young main-sequence stars with mean distances of $\sim$30 pc. The median age for the candidates with well established ages is $<\sim$90 Myrs and contains at least 6 candidates with ages as young as <10 Myrs (see Fig. 1), including several members of the TW Hyd association. Because of the extreme youth of these objects any low-mass brown dwarf and planetary companions will still be in a higher luminosity phase and thus easily detectable. For example, in its first 10 Myrs a 10 M_Jupiter brown dwarf would have a luminosity exceeding 10^-4 solar luminosities (Kulkarni, 1997). Typical separations observable with NICMOS are near the empirical maximum in the binary distribution of stars ($\sim$20 to 40 AU), which also corresponds to the mean distance of the giant planets in our own solar system.

  

Figure 1: Distibution of stellar ages for the young stars candidates (7226)

The 7227 candidate list contains 31 M-dwarf stars which are (a) nearby (d $<\sim$6 pc) with spectral types later than $\sim$M3.5, (b) young (age $<\sim$100 Myr) with (d $<\sim$25 pc), and (c) spectrally the latest known (i.e.,``ultra-cool" stars later than $\sim$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.

  

Figure 2: M-dwarf candidate distances and spectral sub-types (7227).

The 18 candidate stars in the search for circumstellar dust disks (7233) are primarily main sequence stars with IRAS IR excesses, $\tau(dust)$ > 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$\micron$ ( HCO₂ + C₂ continuum), 2.04$\micron$ (methane band), and in the line spectra of Paschen-$\alpha$ (1.87$\micron$) and Bracket-$\gamma$ (2.15$\micron$).

Our EONS surveys are obtaining single-color coronagraphic images aimed at discovering sub-stellar companions utilizing the F160W filter (1.4 - 1.8$\micron$) 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.

Anticipated and Current Coronagraphic Performance

An aggressive 17-orbit commissioning, performance verification, and calibrationprogram executed during SMOV allowed us to ascertain the true performance limits of the coronagraph (Schneider & Lowrance, 1997; Schneider, et. al, 1997) and optimize both its operation and our observing programs. Performance profiles for NICMOS camera 2 direct, coronagraphic, and differential imaging derived from that test program are shown in Figure 1 of Lowrance, et. al. (1998), elsewhere in these conference Proceedings. By subtracting coronagraphic PSFs at different spacecraft roll angles the background diffracted and scattered energy in the unocculted PSF-wings of an occulted primary star can be reduced to approximately one-millionth the peak intensity of the star at a radial distance of 1", or about 10^-5 at 0.5". This is an improvement of more than two orders of magnitude over the already low backgrounds afforded by the diffraction-limited, highly stable, PSF. This represents a very significant gain over the current generation of AO ground-based coronagraphs (Sandler, 1997) and is unique to NICMOS+HST. The basic observational strategy, also described by Lowrence,et al., is illustrated in Fig. 3.

  

Figure 3: A candidate star is placed in the coronagraph and observed at two orientations differing by 30^o, by rolling the spacecraft. The coronagraphic PSF co-rotates with the detector, a companion rotates about the center of the coronagraphic hole. The images are subtracted to remove theprimary PSF, and the image conjugates (positive and negative residuals) are un-rotated and coadded. Bi-cubic interpolative resampling is done only to show the recovery of the first Airy ring for this low-mass companion star.

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 $\sim$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 30^o 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.

  

Figure 4: Pointing errors for the first 17 targets observed in the EONS programs. Paired acquisitions should be < 0.1 pixels apart and centered within 0.25 pixels of the low-scatter point (0,0) of the coronagraph. The geometrical radius of the coronagraphic hole is 4 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 $\sim$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.

  

Figure 5: Significant degradation in performance (increase in PSF subtraction residuals) results from small pointing errors. The improvement in performance by subtracting a better position matched (flux-scaled) PSF is illustrated (right). The residual brightness gradient near the hole is due to the small remaining pointing offsets between the two differenced images.

Summary

Due to the complexities of commissioning the camera 2 coronagraph the EONS search programs received a late start in the NICMOS mission. At this date approximately 1/5 of the sub-stellar companion and circumstellar disk survey observations have been completed. The problem in the target acquisition process resulted in degraded pointing and target centration performance for these observations. None-the-less, the instrumental performance levels of the coronagraph itself, as originally demonstrated in SMOV, continue to meet the needs of the EONS programs. Remedial actions have been taken to mitigate the pointing problem for the remainder of the NICMOS lifetime, and an effective strategy to recover the sensitivity and detection limits from observations taken in the first three months of the program has been developed. With these in place weare confident that NICMOS will allow us to successfully probe the circumstellar environments of nearby stars and address the fundamental questions related to theorigins of stellar and planetary systems raised in our introductory comments.

Acknowledgments

We would like to thank both STScI and Goddard Space Flight Center's Code 512 for their responsiveness in the consideration and implementation of corrective actions for the augmented target acquisition pointing deficiencies.

This work is supported by the National Aeronautics and Space Administration under NASA grant NAG-5-3042.

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