6446( 4) - 01/25/96 16:34 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 6446 Version: 4 Check-in Date: 25-Jan-1996 15:55:19 1.Proposal Title: The High Chromospheres of the Late A Stars, Altair and Alpha Cephei ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Cycle COOL STARS GO 6 ------------------------------------------------------------------------------------ 5. Investigators Contact? PI: Theodore Simon University of Hawaii CoI: Wayne Landsman Hughes STX Corp./NASA GSFC N CoI: Thierry Lanz USRA/NASA GSFC N ------------------------------------------------------------------------------------ 6. Abstract Along the lower half of the main sequence, Altair and Alpha Cephei are the hottest stars known to have chromospheres. We propose to use the Goddard High Resolution Spectrograph, in medium resolution mode, to record the strengths and profiles of their Si IV chromospheric emission lines at 1394Angstrom\ and 1403Angstrom. These features have not been observed before in the spectrum of any late A star, and may be the highest temperature chromospheric lines (T_e ~ 50000 K) that can be detected in a normal A star spectrum. We will use these spectra, along with GHRS Ly-alpha\ spectra from our Cycle 5 GO program and C II spectra from our Cycle 2 program, to model the chromospheric structure of both stars, using a NLTE model atmosphere computer code. Our models for Altair and Alpha Cephei will be the first to take advantage of high S/N spectra available from HST for a broad range of chromospheric temperatures. These calculations should help to improve our understanding of the chromospheric and coronal heating processes in stars with very shallow convection zones. ------------------------------------------------------------------------------------ 6446( 4) - 01/25/96 16:34 - [ 2] Observations Description ------------------------ Our program includes two targets, Altair and Alpha Cep. For each star we will obtain one medium resolution spectrum centered on the Si IV lines at 1400Angstrom, using the solar -blind D1 detector, the G140M grating, and the large science aperture (LSA). The table below lists the exposure times (in seconds) we are requesting for these observations and the estimated S/N ratios (per diode). We have used the Phase II RPS2 software to check the feasibility of both observations. To estimate an approximate flux level at 1400Angstrom (in units of 10^-11 erg cm^-2 s^-1 ang^-1), we have used existing IUE and GHRS spectra to interpolate between 1335Angstrom\\ and 1550Angstrom. The count rate follows from the appropriate sensitivity given by the GHRS handbook, 5.1 * 10^11 counts s^ -1 diode^-1 per incident erg cm^-2 s^-1 ang^-1 at 1400Angstrom. The high S/N we expect to achieve in these Si IV spectra should allow us to combine them with the existing C II spectra and with Ly-alpha\\ spectra planned for our Cycle 5 GO program in order to compute semi-empirical chromospheric models for both stars, using a moving atmosphere NLTE computer code (Lanz & Hubeny 1993 ASP Conf. Ser. 44, 517). To assist in the theoretical modelling of the UV spectra, we will also obtain one low dispersion G140L spectrum of the 1150--1440Angstrom\\ region for each star (in the same pointing as the Si IV observation). These low dispersion observations will serve as useful tests of the model photospheric spectra we will compute. And since they will have 10* better resolution and much less long-wavelength scattered light than low dispersion IUE spectra, we may detect emission in the high temperature chromospheric lines of C III at 1175Angstrom\\ or N V at 1240Angstrom. These lines are very weak in IUE spectra of solar-type stars, they have not been detected before in the spectra of A or early F stars, and they may not be detectable even with the GHRS. In the very best IUE spectra of active solar- type stars, N V is 25--50\\% as strong as Si IV but the measurement uncertainties are always very large. The integrated fluxes, or flux upper limits, for the N V lines (located at 1239Angstrom\\ and 1243Angstrom) would help to constrain the stellar models at temperatures near 10^5 K, i.e., at heights just above where the Si IV lines are formed. The estimated exposure times for these G140L spectra are very short. To derive them (see table below) we used a Kurucz model atmosphere spectrum to scale from the observed flux level at 1335Angstrom\\ to a photospheric flux at 1240Angstrom. We then adjusted the times to give the desired S/N, 20:1 for Altair and 10:1 for Alpha Cep. Alpha Cep lies inside the Continuous Viewing Zone (CVZ). The table below lists two separate exposure times and S/N ratios for this star, one for the normal viewing time and another for an extended viewing time. The longer exposure time provides a moderate gain in S/N, which is then more comparable to the S/N we expect to achieve for Altair. Because this would be advantageous, if there are no unanticipated scheduling constraints imposed on GHRS observations in Cycle 6, we would prefer a CVZ observation for Alpha Cep. begincenter begintabularlllrrrrrr Star & Grating & Lambda(Angstrom) & Flux & ct s^-1 &Expo.& S/N & CVZ? & Orbits , Altair & G140M & 1400 & 0.45 & 2.30 & 1197 & 53 & & , & G140L & 1240 & 0.015 & 1.80 & 218 & 20 & & 1 , Alpha Cep & G140M & 1400 & 0.09 & 0.47 & 1306 & 24 & & , (1) & G140L & 1240 & 0.003 & 0.36 & 326 & 11 & & 1 ,2mm ...or... & G140M & 1400 & 0.09 & 0.47 & 3590 & 41 & CVZ & , (2) & G140L & 1240 & 0.003 & 0.36 & 326 & 11 & CVZ & 1 , Total & Orbits & & & & & & & 2 , endtabular endcenter Real Time Justification ----------------------- None. None. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 6446( 4) - 01/25/96 16:34 - [ 3] Data Distribution Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: UPS Email: , ------------------------------------------------------------------------------------ 6446( 4) - 01/25/96 16:34 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD187642 STAR, A4-A9 V-IV RA= 19H 50M 47.0S +/- 0.3", 2000 V = -26 V = 0.77+/-0.02 DEC=+08D 52' 06.0" +/- 0.3" B-V = 0.22+/-0.02 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0363 0.386 0.202 2 HD203280 STAR, A4-A9 V-IV RA= 21H 18M 34.8S +/- 0.3", 2000 V = -10 V = 2.44+/-0.02 DEC=+62D 35' 08.1" +/- 0.3" B-V = 0.22+/-0.02 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0218 0.049 0.068 6446( 4) - 01/25/96 16:34 - [ 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 HD187642 HRS ACQ 2.0 MIRROR-N1 SEARCH-SIZE = 1 1.8S ONBOARD ACQ FOR 11-12 3,BRIGHT = RETURN ------------------------------------------------------------------------------------------------------------------------------------ 11 HD187642 HRS ACCUM 2.0 G140L 1300 FP-SPLIT = 1 217.6S STD,DOPPLER = ON,STEP-PATT = 5 ------------------------------------------------------------------------------------------------------------------------------------ 12 HD187642 HRS ACCUM 2.0 G140M 1400 SPYBAL = DEF,FP 1 1196.8S -SPLIT = STD,DOPPLER = ON,STEP-PATT = 5 ------------------------------------------------------------------------------------------------------------------------------------ 6446( 4) - 01/25/96 16:34 - [ 6] Visit: 02 Visit Requirements: CVZ 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 ------------------------------------------------------------------------------------------------------------------------------------ 20 HD203280 HRS ACQ 2.0 MIRROR-N1 SEARCH-SIZE = 1 1.8S ONBOARD ACQ FOR 21-22 3,BRIGHT = RETURN ------------------------------------------------------------------------------------------------------------------------------------ 21 HD203280 HRS ACCUM 2.0 G140L 1300 FP-SPLIT = 1 326.4S STD,DOPPLER = ON,STEP-PATT = 5 ------------------------------------------------------------------------------------------------------------------------------------ 22 HD203280 HRS ACCUM 2.0 G140M 1400 SPYBAL = DEF,FP 3 1196.8S -SPLIT = STD,DOPPLER = ON,STEP-PATT = 5 ------------------------------------------------------------------------------------------------------------------------------------ 6446( 4) - 01/25/96 16:34 - [ 7] Summary Form for Proposal 6446 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations HRS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters SEARCH-SIZE = 3 BRIGHT = RETURN STEP-PATT = 5 DOPPLER = ON FP-SPLIT = STD SPYBAL = DEF ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category COOL STARS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 11-12 CVZ ONBOARD ACQ FOR 21-22 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR-N1 G140L G140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD187642 HD203280 ------------------------------------------------------------------------------------------------------------------------------------