7735( 4) - 11/25/97 13:50 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7735 Version: 4 Check-in Date: 25-Nov-1997 13:49:17 1.Proposal Title: Ultraviolet Tomography of the Chromosphere of the K2 Dwarf in V471 Tauri ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO/CAR 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Frederick M. Walter STATE UNIVERSITY OF NEW YORK CoI: Edward F. Guinan Villanova University N CoI: Jeffery L. Linsky Joint Institute for Laboratory Astrophyics N CoI: Edward M. Sion Villanova University N CoI: Howard E. Bond Space Telescope Science Institute N CoI: Serena Kim State University of New York N ------------------------------------------------------------------------------------ 5. Abstract The solar atmosphere is composed of structures on scales of the solar radius down to smaller than we can resolve. Presumably, such a wealth of detail exists on other stars, but it is difficult to observe directly. It is important to observe the scale and filling factor of this structure to determine the heating rate and energy balance in the magnetic active regions. We propose to use the passage of the DA white dwarf in the V471 Tau system behind the atmosphere of the K star to probe the atmospheric structure of the active K2 dwarf star. The atmosphere on the line of sight to the DA white dwarf will be observed in absorption; variations in the equivalent widths of the transition region (C IV, Si IV) and chromospheric (O I, C II) lines will yield the run of temperature and density with height integrated along this line of sight. We will use the STISD in time-tagged mode to obtain the best possible time resolution. The white dwarf traverses the distance of its own radius (7*10^8 cm) in 30 seconds, and the radius of the K star in ~24 minutes. We will be able to resolve structures on spatial scales down to 2% of the K dwarf radius. Finally, we will study the 555 s rotation of the white dwarf, and use the edge of the K dwarf as a knife-edge during the partial eclipse phases to study the surface brightness variations of the white dwarf. ------------------------------------------------------------------------------------ 7735( 4) - 11/25/97 13:50 - [ 2] Observations Description ------------------------ This is a continuation of cycle 5 program 6088, which was not completed with the GHRS. We will make 2 visits, each of 3 orbits. The observations are time- critical, with the critical phase (ingress to eclipse) occurring in the third exposure of the first visit, and the ssecind exposure of the second visit (egress from eclipse). All observations use STIS/E140M in time-tagged mode. We observed this target with the HRS/G140L in cycle 5, and there are many IUE observations of this target. We folded these data through the STIS response to predict the count rates. The monochromatic count rate (counts/resolution element/s) for the STIS/E140M peaks at about 0.18 c/s near 1280 A. The global count rate is about 3300 c/s. These rates violate no bright limits. We take a conservative BUFFER-TIME value of 450 seconds (suitable for a global count rate 33% higher than expected). ** 11/11/97 Increased throughput gives global count rate about 6500 c/s, and requires buffer time reduction to 240 sec *** We estimated ACQ times using the STIS ECT. The source is a V=9.5 K0 star (The white dwarf adds about 10% to the flux at visible wavelengths). We will acquire the stars through the F28X50OIII filter. A 2 second exposure gives S/N=123, with 15,400 electrons in the central 9 pixels. For ACQ/PEAK, we use 5 second exposures, which give 38,500 electrons in the central 9 pixels. We will use the offset from the visit 1 ACQ to save time during the second acquisition. Visit 1 covers the ingress to eclipse. The phase is selected such that ingress into eclipse occurs 35+/-5 minutes into exposure 14, or about 8+/-5 minutes before the end of the observation. The 3 orbits will yield about 29.5, 43, and 40 minutes of time-tagged data, respectively, after acquisitions and allowing for the automatic wavecals. The first orbit (phase 0.692-0.732) samples the line of sight near quadrature, and any gas that extends out beyond the orbit of the white dwarf. The line of sight observed in orbit 2 (phase 0.797-0.854) passes through the L4 Lagrange point. In orbit 3 (phase 0.922-0.977) we will sample the line of sight just above the limb of the K star, and obtain some data on the K star chromosphere (eclipse occurs at phase 0.969). The eclipse timing will be used to compute detailed phases (O-C variations lead to uncertainties in the ephemeris at scales of a few tens of seconds), as well as to study the limb darkening of the white dwarf. Visit 2 covers the egress from eclipse. The phase is selected such that egress from eclipse occurs 10+/-5 minutes after the start of observation 23. The 3 orbits will yield about 32.5, 43, and 41 minutes of time-tagged data, respectively, after acquisitions and allowing for the automatic wavecals. During the first orbit (phase 0.910-0.953) we observe the K star chromosphere in absorption at the same phases we observed in visit 1, permitting a comparison of the density of chromospheric structures. The second orbit opens with the white dwarf in eclipse for 10+/-5 minutes, and then samples the line of sight just above the limb of the K star. The eclipse timing will be used to compute detailed phases (O-C variations lead to uncertainties in the ephemeris at scales of a few tens of seconds). Orbit 2 phase run from 0.018-0.075; egress is at phase 0.0313. Orbit 3 (phase 0.141-0.198) samples the line of sight through the L5 Lagrange point. We will primarily observe the chromospheric and transition region lines (N V, O I, C II, Si IV, C IV, and He II) in absorption against the disk of the white dwarf, and thereby probe the temperature and density structure of the atmosphere of the K star. Real Time Justification ----------------------- These are phase critical observations. V471 Tau has a period of 0.52118334 days. Total eclipse (centered at Phi=0) lasts 47 minutes. The ephemeris is well known (cf Bois, Lanning, & Mochnacki 1988 AJ 96,157). The O-C variation is also well-characterized (Ibanoglu et al 1994 A&A 281, 811), and increases from about 20 to 60 seconds during cycle 7. We have built this into our observing phases. THE TIME OF ZERO-PHASE IS HELIOCENTRIC; A TIMING CORRECTION OF UP TO +/- 500 SECONDS MUST BE APPLIED, DEPENDING ON THE TIME OF YEAR OF THE OBSERVATION. PHASE CONSTRAINTS ARE HELIOCENTRIC, NOT GEOCENTRIC. HELIOCENTRIC CORRECTION IS +500 SECONDS ON MAY 20 (ECLIPSE LATE) AND -500 SECONDS ON NOVEMBER 23 (ECLIPSE EARLY). We must observe the actual ingress to or egress from totality, in order to establish the absolute location of the DAWD relative to the K dwarf. We seek to maximize the exposure of the uneclipsed DAWD. Because of the rapidity of the eclipse, the scheduling of these observations may be technically challenging. We have built in a +/- 5 minute (+/- 0.0067 cycle) tolerance in the phase to aid with scheduling. We will pursue supportive ground-based photometry and spectroscopy, but these will in no way constrain the scheduling of the HST. We will use the Fairborn-Villanova 0.8m APT to obtain UBVRI light curves around the times of the HST observations. These light curves will be used to establish the longitude of the starspots on the surface, their areas, and the spot distributions. Such knowledge is crucial to the interpretation on the HST/GHRS data. Calibration Justification ------------------------- Additional Comments ------------------- THE HELIOCENTRIC CORRECTION MUST BE APPLIED PRIOR TO SCHEDULING. THE MAGNITUDE OF THE CORRECTION (+/- 8.3 MINUTES) EXCEEDS OUR +/- 5 MINUTE TOLERANCE. ------------------------------------------------------------------------------------ 7735( 4) - 11/25/97 13:50 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: Dept. of Physics and Astronomy*Z=3800*SUNY*Stony Brook NY 11794-3800 Ship Via: Email: fwalter@astro.sunysb.edu ------------------------------------------------------------------------------------ 7735( 4) - 11/25/97 13:50 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 V471-TAU STAR, DA, K V-IV, RA=3H 50M 24.85S +/- .7", J2000 V=+35 V = 9.50+/-0.2 INTERACTING BINARY DEC=17D 14' 47.7" +/- .7", PLATE B-V = 0.92+/-0.1 -ID=00FH F-CONT(1300)=1.5+/-0.1e-12 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1982 0.0062 -0.014 0.0 7735( 4) - 11/25/97 13:50 - [ 5] Visit: 01 Visit Priority: Visit Requirements: PERIOD 0.52118334D AND ZERO-PHASE JD2440610.06413 On Hold Comments: Additional Comments: HELIOCENTRIC CORRECTION MUST BE APPLIED TO ZERO-PHASE, VISIT 1 OF 2 Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 10 V471-TAU STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 2.0 S ONBOARD ACQ FOR 11 CD III ------------------------------------------------------------------------------------------------------------------------------------ 11 V471-TAU STIS/C ACQ/PEA 0.2X0.0 G750M 7283 1 4.0 S ONBOARD ACQ FOR 12-14 CD K 6 ------------------------------------------------------------------------------------------------------------------------------------ 12 V471-TAU STIS/F TIME-TA 0.2X0.0 E140M 1425 BUFFER-TIME=240 1 28.0 M UV-MAM G 6 A ------------------------------------------------------------------------------------------------------------------------------------ 13 V471-TAU STIS/F TIME-TA 0.2X0.0 E140M 1425 BUFFER-TIME=240 1 43.0 M UV-MAM G 6 A ------------------------------------------------------------------------------------------------------------------------------------ 14 V471-TAU STIS/F TIME-TA 0.2X0.0 E140M 1425 BUFFER-TIME=240 1 39.5 M PHASE 0.9114 TO 0.9248 UV-MAM G 6 A ------------------------------------------------------------------------------------------------------------------------------------ 7735( 4) - 11/25/97 13:50 - [ 6] Visit: 02 Visit Priority: Visit Requirements: PERIOD 0.52118334D AND ZERO-PHASE JD2440610.06413; On Hold Comments: Additional Comments: HELIOCENTRIC CORRECTION MUST BE APPLIED TO ZERO-PHASE, VISIT 2 OF 2, EGRESS Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exposure| Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Number | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 20 V471-TAU STIS/C ACQ F28X50O MIRROR ACQTYPE=POINT 1 2.0 S ONBOARD ACQ FOR 21 CD III ------------------------------------------------------------------------------------------------------------------------------------ 21 V471-TAU STIS/C ACQ/PEA 0.2X0.0 G750M 7283 1 4.0 S ONBOARD ACQ FOR 22-24 CD K 6 ------------------------------------------------------------------------------------------------------------------------------------ 22 V471-TAU STIS/F TIME-TA 0.2X0.0 E140M 1425 BUFFER-TIME=240 1 28.0 M UV-MAM G 6 A ------------------------------------------------------------------------------------------------------------------------------------ 23 V471-TAU STIS/F TIME-TA 0.2X0.0 E140M 1425 BUFFER-TIME=240 1 43.0 M PHASE 0.0113 to 0.0247 UV-MAM G 6 A ------------------------------------------------------------------------------------------------------------------------------------ 24 V471-TAU STIS/F TIME-TA 0.2X0.0 E140M 1425 BUFFER-TIME=240 1 41 M UV-MAM G 6 A ------------------------------------------------------------------------------------------------------------------------------------ 7735( 4) - 11/25/97 13:50 - [ 7] Summary Form for Proposal 7735 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT BUFFER-TIME=240 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO/CAR ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PERIOD 0.52118334D AND ZERO-PHASE JD2440610.06413 ONBOARD ACQ FOR 11 ONBOARD ACQ FOR 12-14 PHASE 0.9114 TO 0.9248 PERIOD 0.52118334D AND ZERO-PHASE JD2440610.06413; ONBOARD ACQ FOR 21 ONBOARD ACQ FOR 22-24 PHASE 0.0113 to 0.0247 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G750M E140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names V471-TAU ------------------------------------------------------------------------------------------------------------------------------------