5881( 11) - 12/28/95 11:19 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 5881 Version: 11 Check-in Date: 28-Dec-1995 11:12:55 1.Proposal Title: The Gaseous Circumstellar Disk of Beta Pictoris ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle Interstellar Medium GO 5 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Lewis Hobbs University of Chicago CoI: Herve Beust Observatoire de Grenoble N CoI: Magali Deleuil Laboratoire d'Astronomie Spatiale de N Marseille CoI: Paul Feldman Johns Hopkins University Y CoI: Roger Ferlet Institut d'Astrophysique de Paris N CoI: Cecile Gry Laboratoire d'Astronomie Spatiale de N Marseille CoI: Anne Marie Lagrange Observatoire de Grenoble N CoI: Jack Lissauer State University of New York at Stony N Brook CoI: Warren Moos Johns Hopkins University N CoI: Melissa McGrath Space Telescope Science Institute N CoI: Alfred Vidal-Madjar Institut d'Astrophysique de Paris N ------------------------------------------------------------------------------------ 6. Abstract Using the high spectral and angular resolution of the HST, we propose a small pilot program. Its purpose is to demonstrate the feasibility of mapping the distribution of gas within the circumstellar dust disk which was discovered around Beta\:Pictoris with the IRAS observatory. The echelle grating of the GHRS will be used to measure the faint, narrow emission lines formed by resonant scattering of the light from the parent star by Si II, S I, Zn II, and Fe II atoms at various positions in the disk. The 0.22 arcsec entrance aperture of the GHRS corresponds to 4.2 AU at the distance of Beta\:Pic. The scattering lines should constitute the easiest means available for detecting and studying the internal structure of the gaseous component of the disk. For example, the detection of annular minima or maxima in the gas density might indicate the orbits of planets. The surface brightness of the inner regions of the circumstellar disk in appropriately chosen, resonantly scattered lines is predicted to be comparable to that of the optical O I airglow lines present in ground-based observations. One-hour echelle exposures are predicted to yield S/N = 20 in suitable lines. The principal barrier to detecting the gas-scattered lines may be set by the light from the much brighter parent star which directly enters the spectrograph aperture, owing to diffraction and scattering in the telescope. ------------------------------------------------------------------------------------ 5881( 11) - 12/28/95 11:19 - [ 2] Observations Description ------------------------ The following array of lines should permit at least one definite detection of the gas at any location within much of the inner CS disk: Si II/S I 1808, Zn II 2062, and Fe II 2374 A. All of these lines are ground-level members of the permitted multiplets of lowest energy, for the respective species. From our existing GHRS observations of the Zn II 2026 Angstrom\\ line in the spectrum of Beta\\:Pictoris itself, we know empirically that the optical depth of a full radius of the disk is near unity, at line center. If the total gaseous abundances of Si and Fe in the disk are in their solar ratios with respect to Zn, the Si II 1808 Angstrom\\ and Fe II 2374 Angstrom\\ optical depths should be larger by factors of about 10 and 60, respectively. Depending on the ionization balance S I/S II, the optical depth of the S I 1807 Angstrom\\ line may be anywhere between those of Zn II 2026 Angstrom\\ and Fe II 2374 Angstrom\\ for a solar S/Zn ratio. Thus, at least one of these four lines is likely to be detectable in resonance scattering from the disk, at a few arcsec (or some tens of AU) from the parent star., The angular radius of the Beta\\:Pic disk is about 25 arcsec. At one of the two positions offset by a few arcsec from Beta Pic, two half-hour echelle exposures are to be obtained through the Small Science Aperture in the Accumulation Mode, at each of three wavelenghths. The observations are made in the ECH-B Mode at the Si II/S I 1808, Zn II 2062, and Fe II 2374 Angstrom\\ lines. For each of these three wavelength settings, two separate observations must be made, with the spectrum moved parallel to the dispersion to two different positions on the photocathode, to allow some correction for non-uniformities of the cathode. At the other offset position, time permits only a pair of 1808 A exposures., As an example, consider the observation of the 2374 Angstrom\\ line of Fe II at a few arcsec from Beta\\:Pic. From Table 8-4 of version 5.0 of the GHRS Instrument Handbook, a sensitivity S = 6 x 10(+11) (counts/diode/s) per (erg/s/cm(+2)/A) applies to the ECH-B mode of the GHRS, where a factor of 0.65 for the blaze function has been taken into account. From the necessarily rather approximate calculation of the line's surface brightness, B = 4 x 10(-3) erg/s/cm(2)/ster, indicated above in the Scientific Justification, the absolute flux within the full line profile which is received through the 0.22 arcsec SSA aperture is roughly F = B x solid angle = 4 x 10(-15) erg/s/cm(+2). In an integration time of 30 min on a line with an intrinsic width (FWHM) of w = 0.02 Angstrom the number of counts within the whole line therefore is (F/w) x S x t = 200 counts/diode, recorded by a single diode. After a slight displacement of the spectrum along the Digicon array, a second identical exposure is then to be obtained. In a total time of 1 hr, about 400 counts are recorded within the line, giving S/N = 20. The total exposure time utilized at 3 such wavelength settings is 3 hr. The low counting rate of about 0.1 count/s/diode exceeds the dark current rate by a factor of about 10., Information about the repaired telescope's point spread function has been used to estimate the surface brightness of the extended image of the parent star, at the two positions in the disk preliminarily chosen for observation. At 1 arcsec from the star, the azimuthally averaged surface brightness of the Beta\\:Pic image should be somewhat fainter than 10(-3) erg/s/cm(+2)/ster/A, the estimated brightness of suitably chosen, resonantly scattered lines. Therefore, at a 4 arcsec offset, the average stellar image brightness should be much fainter than that of a suitable scattered line, whose brightness probably declines outward in the disk relatively slowly, compared to the HST point spread function. We conclude that, within the unavoidable uncertainties in our knowledge of the brightness of the scattered lines, suitable lines should be detectable at both positions., Both target positions must be reached by blind offsets of a few arcseconds from Beta\\:Pic, since the narrow emission lines presumably will be too faint to be recorded in the relatively broad spectral bands of the WF/PC and the FOC. Blind offsetting is a standard part of the Onboard Acquisition Mode of the GHRS. Imaging of this region may be carried out as part of other HST programs, and any resulting data of this kind should be consulted during the detailed planning of our offsets. Real Time Justification ----------------------- none Calibration Justification ------------------------- none Additional Comments ------------------- For ten years, our group has been carrying out extensive absorption-line studies of the gaseous disk in the spectrum of Beta\\:Pictoris, with ground- based spectrographs, IUE, or, recently, HST/GHRS. Such observations cannot map the gaseous disk, however. We plan to continue a vigorous program of complementary absorption-line work. A few references to our previous work are included at the end of the Scientific Justification. ------------------------------------------------------------------------------------ 5881( 11) - 12/28/95 11:19 - [ 3] Data Distribution Media: 6250BPI Blocking Factor: 10 Ship To: PI_address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 5881( 11) - 12/28/95 11:19 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD39060- STAR, A4-A9 V-IV RA = 05H 47M 17.08S +/- 0.01S, J2000 V = +21 V = 3.85+/-0.02 OFFSET DEC = -51D 03' 59.4" +/- 0.1" B-V = 0.17+/-0.02 TYPE=A5V E(B-V) = 0.00 F(2000)=6.6+/-0.9e-11 Comments: Coordinates from Hipparcos INCA Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000.000 0.0005 0.087 0.061 2 BET-PIC-DISK ISM, Reflection Nebula RA-OFF = -0.025S +/- 0.002S, DEC 2000 V = +21 -1 -OFF = -0.44" +/- 0.04", FROM 1 Comments: 0.5 ARCSEC OFFSET TO SOUTHWEST 3 BET-PIC-DISK ISM, Reflection Nebula RA-OFF = -0.077S +/- 0.007S, DEC 2000 V = +21 -2 -OFF = -1.31" +/- 0.12", FROM 1 Comments: 1.5 ARCSEC OFFSET TO SOUTHWEST 5881( 11) - 12/28/95 11:19 - [ 5] Visit: 01 Visit Requirements: ORIENT 73D TO 75D On Hold Comments: Additional Comments: Target PA = 29 + 45 = 74 deg; offset angle (to U3 axis) = 0 deg. The star has an edge-on circumstellar disk with PA = 29 deg. To minimize diffracted light from the central star, the disk must be placed at 45 deg to the 4 vanes of the spider of the secondary mirror, which are aligned with the U2, U3, V2, and V3 axes (Steve Hulbert, 2/20/95). Thus, the offset angles could ALTERNATIVELY be 90, 180, or 270 deg, with corresponding orientation angles of 164, 254, or 344 deg for the U3 axis. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 HD39060- HRS ACQ 2.0 MIRROR-A2 SEARCH- 1 1.8S ONBOARD ACQ FOR 2 OFFSET SIZE=3,BRIGHT=R ETURN,LOCATE=YE S Comments: STEP-TIME=0.2S ------------------------------------------------------------------------------------------------------------------------------------ 2 HD39060- HRS ACQ/PEA 0.25 MIRROR-A2 SEARCH-SIZE=5 1 5S ONBOARD ACQ FOR 29-34 OFFSET KUP Comments: STEP-TIME=0.2S ------------------------------------------------------------------------------------------------------------------------------------ 29 HD39060- HRS ACCUM 0.25 ECH-B 1807.8 FP- 1 230.5S OFFSET SPLIT=DSTWO,STE P-PATT=6 Comments: S I/Si II, ON STAR ------------------------------------------------------------------------------------------------------------------------------------ 30 HD39060- HRS ACCUM 0.25 ECH-B 2379.0 FP- 1 172.9S OFFSET SPLIT=DSTWO,STE P-PATT=6 Comments: Fe II, ON STAR ------------------------------------------------------------------------------------------------------------------------------------ 31 BET-PIC- HRS ACCUM 0.25 ECH-B 2379.0 FP- 2 1008.1S DISK-1 SPLIT=DSTWO,STE P-PATT=6 Comments: Fe II ------------------------------------------------------------------------------------------------------------------------------------ 32 BET-PIC- HRS ACCUM 0.25 ECH-B 1807.8 FP- 2 1036.9S DISK-1 SPLIT=DSTWO,STE P-PATT=6 Comments: S I, Si II ------------------------------------------------------------------------------------------------------------------------------------ 33 BET-PIC- HRS ACCUM 0.25 ECH-B 1807.8 FP- 2 1785.7S DISK-2 SPLIT=DSTWO,STE P-PATT=6 Comments: S I, Si II ------------------------------------------------------------------------------------------------------------------------------------ 34 BET-PIC- HRS ACCUM 0.25 ECH-A 1657.0 FP- 2 1267.3S DISK-1 SPLIT=DSTWO,STE P-PATT=6 Comments: C I, C I*, C I** ------------------------------------------------------------------------------------------------------------------------------------ 5881( 11) - 12/28/95 11:19 - [ 6] Summary Form for Proposal 5881 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAKUP ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE=3 BRIGHT=RETURN LOCATE=YES SEARCH-SIZE=5 STEP-PATT=6 FP-SPLIT=DSTWO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category Interstellar Medium ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ORIENT 73D TO 75D ONBOARD ACQ FOR 2 ONBOARD ACQ FOR 29-34 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-A2 ECH-B ECH-A ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD39060-OFFSET BET-PIC-DISK-1 BET-PIC-DISK-2 ------------------------------------------------------------------------------------------------------------------------------------