7897( 9) - 10/19/98 11:27 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7897 Version: 9 Check-in Date: 19-Oct-1998 11:19:10 1.Proposal Title: Probing planetary formation around main- sequence stars: A snapshot survey ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle SNAP 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Mark Clampin Space Telescope Science Institute CoI: Paul Kalas Max-Planck-Institut fur Astronomie N CoI: Anneila Sargent California Institute of Technology N CoI: Pawel Artymowicz Stockholm Observatory N ------------------------------------------------------------------------------------ 5. Abstract IR excess Main-Sequence (MS) stars host cool circumstellar dust disks, which are most likely replenished by the debris of unseen planetesimals. Although IRAS discovered more than 100 MS stars with IR excesses only one star, Beta Pictoris, appears to have an optically detectable, disk-like nebulosity. We propose to undertake a NICMOS snapshot survey of MS IR excess stars designed to discover new circumstellar disks in the H band, address the statistics of disk detection, and correlate MS disk properties with spectral type, age and fractional far-IR luminosity. With its diffraction-limited coronograph NICMOS will allow us to significantly improve upon previous ground-based surveys, by exploiting the advantages of suppression of diffracted light in the PSF, coupled with the increased surface brightness of the disks in the near-IR, with respect to the visible. Successful detections of circumstellar disks and modelling of the scattered and re- emitted thermal radiation will allow us to determine detailed disk properties. Any positive detection will represent a giant step forward in understanding how planetary systems form and evolve. In combination with models of scattered light from disks, non detections will enable us to constrain the properties of the constituent grains." ------------------------------------------------------------------------------------ 7897( 9) - 10/19/98 11:27 - [ 2] Observations Description ------------------------ We propose to conduct a snapshot survey of a sample of IRAS IR-excess sources to search for circumstellar disks using the coronograph in NICMOS Camera 2 for these observations. Since true coronographic imaging is only obtained when the occulting mask is used (Ftaclas et al. 1994), our baseline plan is to use the coronographic occulting spot. Each observation in the snapshot program will comprise a nominal exposure time of 600 seconds, using the F160W filter and the MULTIACCUM detector readout mode. The MULTIACCUM readout mode is ideally suited to this experiment. We will set SAMP- TIME parameters to deliver appropriately spaced intermediate readout images. Since NICMOS saturates on very bright sources, we will use the narrow F187N filter for target acquisition. The very brightest IR excess stars will be excluded from our survey e.g. Alpha Lyr. However these are being observed in the NICMOS GTO program. Example Observation, A example snapshot observation from our survey, in this case looking at HR 451, V mag=5.6, will comprise the following steps: , begincenter begintabularlrcl Step & Time & Filter & Notes , Guide star acquisition & 540 secs & & Overhead , Coronographic acquisition & 2*0.6 secs & F187N & Two acquisition exposures , & 96 secs & & Overhead , & & & Low sensitivity filter for acq. , Science exposure & 600 secs & F160W & MULTIACCUM readout , & 12 secs & & Instrument setup overhead , endtabular endcenter The overall length of the exposure is ~21 minutes, a typical time for a snapshot program observation. Some observations will be shorter than 600 seconds, because of the IR brightness of their central star, some may be longer. , In order to demonstrate that our exposures times are appropriate, we have calculated our expected signal to noise for a disk 2 magnitudes/arcsec^-2 fainter than Beta Pictoris at 6" from the star. Since the far field wings of the PSF are not yet known for NICMOS, we have adopted a factor of 1times10^-5 arcsec^-2 for the total stellar flux at 6", as measured by Hasan for the HST point spread function (STScI OTA Instrument Science Report 92-06.1), and confirmed by Krist and Burrows (STScI WFPC2 Instrument Science Report 94-01). The advertised suppression of the PSF wings for NICMOS is a factor of 10. However, initial results suggest that this factor is closer to 2.5 at distances greater than 1" from the central star. We have used the NICMOS simulator to estimate the exposure levels. The background counts from the wings of Beta Pictoris (H=3.51), assuming 1times10^-5 arcsec^-2 and an additional suppression factor of 2.5, are 1.3 counts/sec/pixel. The R band surface brightness of the Beta Pictoris disk is 15.2 mag/arcsec^2 (Kalas and Jewitt 1995), and is 14.4 mag/arcsec^2 in the H band using the disk colors reported by Nischan and Backman (1992). A disk 2 magnitudes fainter gives a count rate of 2.9 counts/sec/pix, yielding a signal to noise of >30 pixel^-1 on the disk in 600 secs. Closer to the star where the coronographic suppression is reported to be higher (NICMOS Web page Advisory) the signal to noise is correspondingly higher i.e. >100 at 1.0". Thus the combination of a coronograph and the relatively higher disk brightness in the near-IR will produce the required increase in sensitivity compared to ground-based coronographic observations. , Data Analysis , We aim to obtain a homogeneous data set in this program by using the same observational sequence and F160W filter for all the snapshot observations. In addition to our science targets, we will also include stars with no known IR excess to provide PSF templates. We plan to use these template PSFs to aid in the subtraction of the PSF wings from our targets. The proposing team has considerable experience in coronographic observing (Clampin et al. 1994; Clampin et al. 1995; Kalas and Jewitt 1995; Kalas and Jewitt 1996; Artymowicz et al. 1989) and have computer codes available for the reduction of coronographic observations, using several different methods including PSF subtraction, model PSF subtraction (using TINYTIM) and median azimuthal profile subtraction. , Proprietary Period, This snapshot survey will provide the best dataset ever obtained for detecting circumstellar disks around MS stars, and will be a valuable resource to the planetary formation community. It is also timely since results of ISO are appearing, and it will provide a resource to the ISO community. In regard to the proprietary period, we propose a period of 4 months in order to fully analyze each observation., Choice of instrument , NICMOS is the first HST instrument with a true coronograph. Compared to WFPC2, it offers the significant advantages that HST PSF is apodized, there is an occulting mask and the detector does not bleed. Thus, diffracted light is reduced and exposure times are not limited by pixel saturation. In addition, internal scattering within WFPC2 increases the large angle scattering significantly (Krist and Burrows, 1994 WFPC2 Instrument Science Report 94-01). STIS also offers limited coronographic capabilties, but the coronographic aperture is unfiltered and images over 300 - 1000 nm. The STIS apodizing mask does not apodize diffraction from the secondary mirror assembly, the secondary mirror supports, or the mirror pads. A typical STIS coronographic observation also offers a relatively limited discovery space (clear field of view) due to the diffraction spikes from the telescope and the configuration of the occulting masks. Finally, STIS coronographic observations incur significant penalties due to target acqusition, post-observation CCD cleanup overheads (1 orbit), and multiple observations to effectively cover the 360 ^o field of view. Given these considerations, The NICMOS camera is the undisputed choice for our proposed snapshot survey., 0.5in Real Time Justification ----------------------- Calibration Justification ------------------------- Real_Time_Justification Additional Comments ------------------- ------------------------------------------------------------------------------------