9047( 2) - 02/15/01 08:39 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 9047 Version: 2 Check-in Date: 15-Feb-2001 08:38:40 1.Proposal Title: The densely spotted photospheres of active cool stars ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 10 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Andrew Cameron University of St. Andrews CoI: Dr. Ron Hilditch University of St Andrews N CoI: Dr. Gaitee Hussain Harvard-Smithsonian Center for N Astrophysics CoI: Dr. Moira Jardine University of St Andrews N CoI: Dr. Jean-Francois Donati Observatoire Midi-Pyrenees N CoI: Dr. Meir Semel Observatoire de Paris-Meudon N ------------------------------------------------------------------------------------ 5. Abstract Rapidly rotating main-sequence stars are so heavily mottled by starspots that rotational modulation of their light may amount to 0.1 magnitude or more. Over the last decade or so, the spatial distributions of the largest starspot groups have been mapped on many active stars using Doppler imaging methods applied to ground- based echelle spectroscopy. In rapidly rotating young main-sequence stars with ages comparable to the Pleiades, Doppler images seldom show more than 10\ methods such as TiO band-strength analysis and broad-band optical and near-IR photometry, however, indicate that 20\ active stars' surfaces are occupied by spots. The discrepancy can be resolved by requiring the brighter regions of an active stellar surface to be packed with a high density of dark spots, too small to be resolved in Doppler images. Here we propose to measure the packing fraction and size distribution of small starspots on the inner face of the G2/3V primary of the eclipsing binary SV Cam (= HD 44982). We will observe three primary eclipses at the exquisite photometric precision (1.3* 10^-4 mag RMS with time resolution 50s) attainable by using STIS as a bright-star photometer. Starspots with radii as small as 15000 km will be detected as marginally-resolved, 5Sigma fluctuations in the eclipse profile as the dark K5 secondary scans across the disk of the primary. ------------------------------------------------------------------------------------ 9047( 2) - 02/15/01 08:39 - [ 2] Observations Description ------------------------ sectionChoice of target: Why SV Cam? We searched the Strassmeier et al (1993) catalogue of chromospherically- active binary stars (CABS2) for active binary systems satisfying three criteria. (1) The period of the binary orbit has to be less than a day, to ensure that the activity levels would be comparable to the single ultra-rapid rotators found in young clusters. (2) The equator and mid- latitudes of the primary have to be eclipsed. (3) It should be possible to obtain complete coverage of the primary eclipse with HST on two consecutive binary orbits. Of the 9 known systems with periods less than a day, SV Cam is the second-brightest. The brightest is ER Vul, but its eclipses are grazing and shallow, failing the second requirement. The other short- period RS CVns are fainter and have orbits that are less well-matched to HST. None are in the CVZ. SV Cam has other significant advantages. It is well established to have variations in light-curve shape and level amounting to 0.1 mag or more, confirming the presence of spot activity. Its high declination (82^degrees) renders it visible to HST for 59 minutes per orbit. The contrast between its components (G2-3V/K4V) is such that the cool secondary acts as a dark occulting disc during primary eclipse. This enhances greatly the contrast of fluctuations due to eclipsed spots. sectionChoice of wavelength range We considered using G750L as this would have given us access to the 7055Angstrom\ TiO band, but rejected this option because of possible problems with temperature-dependent fringing. The STIS exposure-time calculator for G430L CCD spectra gives a peak count of 3.51* 10^4 electrons at 5070.8Angstrom, in a single 35-s exposure on point source with a G2V Kurucz spectrum and V=9.1, viewed in a 52* 2-arcsec slit (ETC 24979). This exposure level is well short of full-well saturation but would saturate the gain amplifier in the CCDGAIN=1 setting, so we will use CCDGAIN=4. The total counts over the entire detector are 5.7* 10^7 electrons per 35-s exposure. Splitting each exposure into two wavelength ranges longward and shortward of 4300Angstrom\ yields S:N = 5900 and 3200 per 35s exposure respectively. beginfigurehtb epsscale0.6 plotoneexptime.eps captionThe solid line shows the smallest spot that can be resolved on the primary of SV Cam in a given exposure time. The dashed curves shows the CCD exposure times needed to obtain a 4Sigma (short-dashed) or 5Sigma (long-dashed) detection of the flux deficit produced by a spot of given radius, using the G430L grating. The optimal tradeoff of surface resolution against detection threshold occurs for an exposure time of 35s. This is sufficient to detect a marginally-resolved spot of radius 15000 km at 5Sigma. labelfig:exptime endfigure sectionOptimal exposure time The choice of 35s as the optimum exposure time is set by a trade- off between maximising signal-to-noise per exposure and the minimizing time interval between observations. The former determines the smallest spot that can be detected, while the latter fixes the size of the smallest feature that can be resolved by the eclipse-mapping process. This trade-off is illustrated in Fig. reffig:exptime, and shows the optimum exposure time to be 35 s, yielding 5Sigma detections of marginally-resolved spots with radii of 15000 km. The spatial resolution is determined by the 55-s cadence of the observations, after allowance for 20s overhead per exposure. To maximise observing efficiency we will read out only a subarray consisting of 32 rows, keeping the overhead between exposures down to 20s. The STIS buffer can accommodate ~ 256 such exposures, so at one exposure per 55s no additional overhead for buffer dumps will be incurred. Of the 59 minutes available per orbit, 6 will be spent on guide-star acquisition, 6 on ACQ and 3 on GO wavecal (since auto-wavecal will be disabled) on the first orbit of each visit. This leaves 45 min on the first orbit of each visit for a sequence of 49 on-target exposures of 35s each. Subsequent orbits will require guide-star reacquisition and GO wavecal (8 minutes total) leaving 51 minutes for a sequence of 58 on- target exposures. The accurate ephemeris for primary eclipses is HJD= 2451179.3435+0.593071E, derived from recent eclipse timings by Pribulla et al (1999) and Agerer &\ Huebscher (1998). First and fourth contact occur at binary phase +/- 0.0894 measured relative to mid-primary eclipse, while second and third contact are at +/- 0.0170. We need to measure part of the light-curve out of eclipse, to verify that no unexpected sources of short-term variability are present outside eclipse, that could be confused with genuine spot fluctuations during the eclipse. Our goal is to observe the entire range of binary phases from 0.8 to 0.2 with enough redundancy to verify the reality of the faintest features detected during primary eclipse. We can do this in a total of 9 orbits, split into 3 blocks of 3 orbits apiece (see ``Special Requirements'' below). sectionGround-based supporting observations We will obtain optical narrow-band photometry around the entire orbit to a precision of +/- 0.003 mag using the James Gregory Telescope in St Andrews and/or the Liverpool Robotic Telescope on La Palma. This will plug the gaps in the HST coverage of the light-curve outside primary eclipse, with sufficient precision to model out any global asymmetry in the light-curve due to uneclipsed spots. We will also apply to use the Utrecht Echelle Spectrograph on the 4.2-m William Herschel Telescope on La Palma to secure contemporaneous (Zeeman-) Doppler images. The high northern declination of the target permits considerable flexibility in the scheduling of the ground- based observations, so the ground-based program will not impact the scheduling of the HST observations. Real Time Justification ----------------------- This program involves time-critical observations, as we are observing primary eclipses of a binary system. The beat period between the HST orbit (P=95.43 min in April 2000) and the binary orbit gives us complete phase coverage and ~50 percent phase redundancy in three blocks of three contiguous HST orbits, centered on primary eclipse at HJD= 2451179.3435+0.593071E. To ensure full phase coverage, the three visits should cover a sequence of three primary eclipses at intervals of (preferably) three or (acceptably) four binary orbits. The entire program should be completed within the space of 5 days or 8 binary orbits. Longer intervals between visits are undesirable, as the solar analogy suggests that the spot distribution will evolve significantly on a timescale of a few days. The ground- based program described above will not affect the scheduling of the HST observations. Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 9047( 2) - 02/15/01 08:39 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 HD44982 STAR, RS CVN STAR RA=06H 41M 18.89S +/- 0.5", J2000 V = 9.1+/-0.40 DEC=82D 16' 3.8" +/- 0.5" B-V = 0.67+/-0.07 Coordinate Source: HIPPARCOS/TYCHO_CATALOGUE Comments: Short-period RS CVn star. V magnitude given outside eclipse. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 1991.25 0.021 -0.153 0.012 9047( 2) - 02/15/01 08:39 - [ 4] Visit: 01 Visit Priority: Visit Requirements: PERIOD .593071 D AND ZERO-PHASE JD2451179.343370 ORIENT 40D TO 110D ORIENT 130D TO 200D ORIENT 220D TO 290D ORIENT 310D TO 20D 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 HD44982 STIS/C ACQ F28X50O MIRROR 1 5 S PHASE 0.851 TO 0.891 CD II Comments: CCD peak counts expected are 15, 200 e- ------------------------------------------------------------------------------------------------------------------------------------ 20 WAVE STIS/C ACCUM 52X0.2 G430L 4300 1 DEF CD Comments: Explicit WAVECAL, since auto-waves disabled ------------------------------------------------------------------------------------------------------------------------------------ 30 HD44982 STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=NO,GAIN=4,SIZEAXIS2=32, 49 35 S END ORBIT CD WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ 40 HD44982 STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=NO,GAIN=4,SIZEAXIS2=32 116 35 S CD ------------------------------------------------------------------------------------------------------------------------------------ 50 WAVE STIS/C ACCUM 52X0.2 G430L 4300 1 DEF CD Comments: Explicit WAVECAL, since auto-waves disabled ------------------------------------------------------------------------------------------------------------------------------------ 9047( 2) - 02/15/01 08:39 - [ 5] Visit: 02 Visit Priority: Visit Requirements: PERIOD .593071 D AND ZERO-PHASE JD2451179.343370 AFTER 01 BY 41 H TO 57 H SAME ORIENT AS 01 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 HD44982 STIS/C ACQ F28X50O MIRROR 1 5 S PHASE 0.805 TO 0.843 CD II Comments: CCD peak counts expected are 15, 200 e- ------------------------------------------------------------------------------------------------------------------------------------ 20 WAVE STIS/C ACCUM 52X0.2 G430L 4300 1 DEF CD Comments: Explicit WAVECAL, since auto-waves disabled ------------------------------------------------------------------------------------------------------------------------------------ 30 HD44982 STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=NO,GAIN=4,SIZEAXIS2=32, 49 35 S END ORBIT CD WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ 40 HD44982 STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=NO,GAIN=4,SIZEAXIS2=32 116 35 S CD ------------------------------------------------------------------------------------------------------------------------------------ 50 WAVE STIS/C ACCUM 52X0.2 G430L 4300 1 DEF CD Comments: Explicit WAVECAL, since auto-waves disabled ------------------------------------------------------------------------------------------------------------------------------------ 9047( 2) - 02/15/01 08:39 - [ 6] Visit: 03 Visit Priority: Visit Requirements: PERIOD .593071 D AND ZERO-PHASE JD2451179.343370 AFTER 02 BY 41 H TO 57 H SAME ORIENT AS 01 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 HD44982 STIS/C ACQ F28X50O MIRROR 1 5 S PHASE 0.835 TO 0.908 CD II Comments: CCD peak counts expected are 15, 200 e- ------------------------------------------------------------------------------------------------------------------------------------ 20 WAVE STIS/C ACCUM 52X0.2 G430L 4300 1 DEF CD Comments: Explicit WAVECAL, since auto-waves disabled ------------------------------------------------------------------------------------------------------------------------------------ 30 HD44982 STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=NO,GAIN=4,SIZEAXIS2=32, 49 35 S END ORBIT CD WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ 40 HD44982 STIS/C ACCUM 52X2 G430L 4300 CR-SPLIT=NO,GAIN=4,SIZEAXIS2=32 116 35 S CD ------------------------------------------------------------------------------------------------------------------------------------ 50 WAVE STIS/C ACCUM 52X0.2 G430L 4300 1 DEF CD Comments: Explicit WAVECAL, since auto-waves disabled ------------------------------------------------------------------------------------------------------------------------------------ 9047( 2) - 02/15/01 08:39 - [ 7] Summary Form for Proposal 9047 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50OII 52X0.2 52X2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters CR-SPLIT=NO GAIN=4 SIZEAXIS2=32 WAVECAL=NO ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements PERIOD .593071 D AND ZERO-PHASE JD2451179.343370 ORIENT 40D TO 110D ORIENT 130D TO 200D ORIENT 220D TO 290D ORIENT 310D TO 20D PHASE 0.851 TO 0.891 END ORBIT PERIOD .593071 D AND ZERO-PHASE JD2451179.343370 AFTER 01 BY 41 H TO 57 H SAME ORIENT AS 01 PHASE 0.805 TO 0.843 PERIOD .593071 D AND ZERO-PHASE JD2451179.343370 AFTER 02 BY 41 H TO 57 H SAME ORIENT AS 01 PHASE 0.835 TO 0.908 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G430L ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD44982 WAVE ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 4300 ------------------------------------------------------------------------------------------------------------------------------------