6416( 5) - 04/03/96 13:46 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6416 Version: 5 Check-in Date: 03-Apr-1996 13:44:44 1.Proposal Title: FOC Imaging of the Dusty Envelopes of Mass-Losing Supergiants ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle COOL STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Joel Kastner Massachussetts Institute of Technology CoI: David Weintraub Vanderbilt University N ------------------------------------------------------------------------------------ 6. Abstract Stars more massive than ~10 M_\odot are destined to explode as supernovae (SN). Pre- SN mass loss can prolong core buildup, and the rate and duration of mass loss therefore largely determines a massive star's post-main sequence evolution and its position in the H-R diagram prior to SN detonation. The envelope ejected by a mass- losing supergiant also plays an important role in the formation and evolution of a SN remnant. We propose to investigate these processes with HST. We will use the FOC to image two massive stars that are in different stages of post-main sequence evolution: VY CMa, the prototype for a class of heavily mass-losing OH/IR supergiants, and HD 179821, a post-red supergiant that is likely in transition to the Wolf-Rayet phase. Both are known to possess compact reflection nebulae, but ground-based techniques are unable to separate the inner nebulosities from the PSF of the central stars. We will use the unparalleled resolution of the FOC to probe the structure of these nebulae at subarcsecond scales. These data will yield the mass loss histories of the central stars and will demonstrate the presence or absence of axisymmetric mass loss and circumstellar disks. In so doing, our HST/FOC program will define the role of mass loss in determining the fates of SN progenitors and SN remnants. ------------------------------------------------------------------------------------ 6416( 5) - 04/03/96 13:46 - [ 2] Observations Description ------------------------ To estimate orbit requirements, we conservatively assume 45 min of useful observing time per orbit, and we assume the loss of 7.5 min and 7 min per orbit due to guide star acquisition and FOC instrument overheads, respectively. This restricts us to ~1800 sec actual integration time per orbit. >From Eq.\\\\\\\\\\\\\\\\ 4 of the FOC manual, we estimate that the brightest star in our sample (VY CMa), at V = 8.0, would generate ~2400 counts/sec/pixel through filter F550M at the peak of the image PSF. Hence we will use F6ND to reduce the count rate to ~10 counts/sec/pixel. This is still above the non-linearity threshold of the FOC, but it will only affect the central pixel and in any event we are not concerned with accurate photometry of the central stars. Furthermore, we expect that the flux from the central ``stars'' (in particular, from VY CMa) may in part consist of reflection nebulosity that will be resolved by the FOC, in which case the peak count rate could be considerably lower than 10 counts/sec/pixel. We use equ.\\\\\\\\\\\\\\\\ 22 of Jura (1994, ApJ, 434, 713) to estimate the surface brightnesses S(Phi,Lambda) of the reflection nebulae surrounding our program stars. This equation expresses S(Phi,Lambda) of a nebula in terms of the intensity of the central star, its distance from Earth D, envelope mass loss rate dM/dt and expansion velocity v_e, and circumstellar grain mass density Rho and radius a (for the extended reflection nebulae around IRC +10420 and HD 179821, we find Jura's equation well reproduces our measured 1.25 Mum surface brightnesses if each star has lost mass at an average rate of ~ 10^-3 M_\\\\\\\\\\\\\\\\odot/yr over the last 5*10^3 yr; Kastner & Weintraub 1995b).. We take Rho=3 g cm^-3 and a=0.1 Mum; note that this choice for a implies that Rayleigh scattering (S \\\\\\\\\\\\\\\\propto lambda^-4) characterizes the nebulae at the wavelengths of interest. Our goal is to detect scattered flux out to ~2'' from the central stars through the F550M + F6ND filters. For values appropropriate for HD 179821 (V = 8.3, D = 6 kpc, dM/dt ~ 10^-3 M_\\\\\\\\\\\\\\\\odot/yr, v_e = 33 km/s) we find from Jura's equation that the expected V band scattered intensity is 10.5 mag/arcsec^2 at 2''. From eq.\\\\\\\\\\\\\\\\ 9 of the FOC manual, we translate this to an expected count rate of 0.007 counts/sec/pixel through the F550M + F6ND combination in 14''*14'' zoom mode. This is well above the expected background rate, and we find we can achieve a SNR of >3 in the ~1800 sec minimum useable observing time during 1 HST orbit. Since S \\\\\\\\\\\\\\\\propto phi^-3, we do not expect to obtain useful photometry for the inner <0.5'' of the envelope of HD 179821 if its mass loss rate has been constant over the last 200 yr --- but the ring-like morphology of the mid-IR image (Hawkins et al.\\\\\\\\\\\\\\\\ 1994) suggests there has been a dramatic decline in dM/dt over this same period. Similar SNR estimates apply for IRC +10420 and VY CMa. Although the time-averaged mass loss rate of VY CMa is probably an order of magnitude smaller than those of the other two stars (Jura & Kleinmann 1990, ApJS, 73, 769), it is also four times closer, and in fact knots within its reflection nebulosity have been estimated at V of 10--11 (Herbig 1972). Similar SNR estimates also apply for observations of the program objects through the other filter combination, F346M + F6ND, since the dropoff in illuminating flux from the (late-type) central stars is compensated both by the Rayleigh dependence of scattering and the higher QE of the FOC. Thus we will use a total of 4 orbits, 1 to observe each star through each filter. Note that if observations of a given target through different filters are scheduled for consecutive orbits, then our program consists of 3 (rather than 6) visits and overhead time is reduced somewhat. Real Time Justification ----------------------- None. We are continuing to observe HD 179821 as part of our ongoing infrared imaging program at Kitt Peak. The purpose of this study is twofold: (1) to obtain coronagraphic, polarimetric images at somewhat higher resolution than was achieved by Kastner & Weintraub (1995b); and (2) to determine whether the star is part of a young cluster or association. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6416( 5) - 04/03/96 13:46 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: , ------------------------------------------------------------------------------------ 6416( 5) - 04/03/96 13:46 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 VYCMA STAR,M III-I,LONG RA=07H 22M 58.15S +/- 1.0",DEC=- J2000 V = 75 V = 8.0+/-0.5 PERIOD VARIABLE,WIND, 25D 46' 02.6" +/- 1.0" 2 HD179821 STAR,G III-I,WIND, RA=19H 13M 58.51S +/- 1",DEC=00D J2000 V = 99 V = 8.3+/-0.2 07' 31.9" +/- 1" 6416( 5) - 04/03/96 13:46 - [ 5] Visit: 01 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 VYCMA FOC/96 IMAGE 512X512 F550M,F6N CHECK- 1 1000S D FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ 20 VYCMA FOC/96 IMAGE 512X512 F4ND,F1ND CHECK- 1 1100S ,F550M FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ 30 VYCMA FOC/96 IMAGE 512X512 F346M,F4N CHECK- 2 1200S D FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ 6416( 5) - 04/03/96 13:46 - [ 6] Visit: 02 Visit Requirements: On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 40 HD179821 FOC/96 IMAGE 512X512 F550M,F6N CHECK- 1 1000S D FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ 50 HD179821 FOC/96 IMAGE 512X512 F4ND,F1ND CHECK- 1 1100S ,F550M FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ 60 HD179821 FOC/96 IMAGE 512X512 F346M,F6N CHECK- 2 1200S D FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ 6416( 5) - 04/03/96 13:46 - [ 7] Summary Form for Proposal 6416 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOC/96 ------------------------------------------------------------------------------------------------------------------------------------ Opmodes IMAGE ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CHECK-FILTER=YES ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category COOL STARS ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F550M,F6ND F4ND,F1ND,F550M F346M,F4ND F346M,F6ND ------------------------------------------------------------------------------------------------------------------------------------ Target Names VYCMA HD179821 ------------------------------------------------------------------------------------------------------------------------------------