7857( 19) - 10/19/98 09:04 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7857 Version: 19 Check-in Date: 19-Oct-1998 09:01:47 1.Proposal Title: Investigating the missing link between disks around Pre Main Sequence and Main Sequence stars ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Anne-Marie Lagrange Groupe D'Astrophysique de Grenoble CoI: David Mouillet Laboratoire d'Astrophysique de Grenoble N CoI: Jean-Luc Beuzit Canada Franc Hawaii Telescope N CoI: Francois Menard Laboratoire d'Astrophysique de Grenoble N CoI: Thierry Forveille Laboratoire d'Astrophysique de Grenoble N CoI: Jean-Charles Augereau Laboratoire d'Astrophysique de Grenoble N ------------------------------------------------------------------------------------ 5. Abstract Various observations and models of the circumstellar (CS) disk around the Main Sequence (MS) IR excess star beta Pictors tend to show that the optically thin dust disk is the result of destruction processes among planetesimals. Such large bodies are also needed to explain the infall of ionized gaz toward the star. Dust disks around young Pre-Main Sequence (PMS) stars have also been evidenced. Theory predicts that these gas-rich disks are blown away before the star reaches the MS. In order to be consistent with the observation of disks around a large number of MS stars and with planetary formation theory, planetesimals should be already efficiently formed before the disk is dissipated. The evolved PMS stars give the missing link between young protoplaneta ry systems and evolved ones. One should expect those stars to be surrounded by a large population of planetesimals, able to substain an optically thin dust disk, such as the beta Pictoris one. To directly investigate the evolution of such CS environments, from PMS to MS stages, we propose to use the unique capabilities of HST- NICMOS in its coronographic mode to look for faint, optically thin disks around a few carefully selected PMS stars close to the MS. This will enable to test the nature and history o f this CS dust, in comparison to younger or older disks proto- types. ------------------------------------------------------------------------------------ 7857( 19) - 10/19/98 09:04 - [ 2] Observations Description ------------------------ The coronographic mode implies the use of camera 2, with on- board precise acquisition of the target. The scientific target will be observed in a single filter. Filter F160W is selected for optimum detectability: the PSF will be well sampled and the back ground (zodiacal light and thermal background) is minimized. The integration time of the exposure in this filter is determined so as to stay below the saturation limit of the detector on the brightest pixels (near the central hole edge area). The subtract ion of residual PSF wings is foreseen and require specific exposures close in time to each corresponding science target observation because of the slight possible variation of the PSF with time. This operation is required to take full benefit of the cor onograph high contrast capabilities. We observe in the same optical configuration a comparison star within a few degrees of the science target, of similar or or slightly higher brightness, and proved to be circumstellar matter free. The comparison star i s observed in the same orbit. One orbit is needed for each science target. The full on target exposure time will be about 13 minutes. As a comparison, the beta Pictoris disk could be detected in coronography with adaptive optics on a 3.6-m telescope an d with a similar NICMOS detector within a total on target integration time of only 3 minutes. In the current case, the circumstellar emission will be probably less angularly extended since the stars are more distant, but the HST unperturbed PSF should abl e to investigate the surrounding area within 1" . Also, the contrast with the star should be more favourable, according to the larger IR excess. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------