7395( 4) - 02/04/98 13:28 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 7395 Version: 4 Check-in Date: 04-Feb-1998 13:27:38 1.Proposal Title: Chromospheric Activity in Old Stars: HD 106516 ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 7 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Ruth C. Peterson ASTROPHYSICAL ADVANCES CoI: Karel Schrijver Lockheed N ------------------------------------------------------------------------------------ 5. Abstract All normal stars cool enough to have convection zones share a common minimum level of chromospheric activity which decreases with advancing spectral type; most show higher emission levels which vary with time. The ubiquity of a minimum ``basal'' level suggests that this component of chromospheric activity is produced by some form of time- and metallicity-independent mechanism, such as acoustic waves arising from convection, while activity above this level most probably originates with magnetic fields generated by a rotationally-driven dynamo. Such a picture implies that even very old cool stars should show a minimum level of such activity, and that ones with rotationally-generated magnetic fields should show a higher level. Our Cycle 5 GHRS snapshot program has confirmed the first prediction: all nine of the old, low-metallicity, Population II field subdwarfs that were observed do indeed show chromospheric Mg II\ emission. In this STIS GO proposal we are requesting a Mg II\ observation for the tenth star of that sample, the only one with measurable rotation. Both the profile and the level of Mg II\ emission should help indicate to what extent rotation affects chromospheric emission in very old solar-type stars. This observation failed in Cycle 5 due to a failed guide star acquisition, and was not repeated in accord with snapshot policy. ------------------------------------------------------------------------------------ 7395( 4) - 02/04/98 13:28 - [ 2] Observations Description ------------------------ The 10 solar-type subdwarfs targeted for Mg II\\ measurements, and their (V, B- V, U-B) photometry, temperatures T(eff), and metallicities Fe/H (from Carney Etal\\ 1994), are: \\medskip HD 19445 (8.06, 0.46, -0.16), 5842K, -2.12; HD 84937 (8.32, 0.40, -0.22), 6220K, -2.18; HD 94028 (8.22, 0.48, -0.18), 5898K, - 1.56; HD 106516 (6.11, 0.44, -0.13), 6118K, -0.87; HD 114762 (7.30, 0.52, - 0.08), 5790K, -0.79; HD 140283 (7.22, 0.49, -0.19), subgt, -2.60; HD 184499 (6.63, 0.58, -0.01), 5726K, -0.66; HD 194598 (8.34, 0.48, -0.20), 5858K, -1.28; HD 201891 (7.37, 0.51, -0.15), 5817K, -1.42; HD 219617 (8.16, 0.49, -0.20), 5857K, -1.40. We propose to obtain exposures of the spectral region near the Mg II lines at 2800 Angstrom\\ using STIS and the E230M grating with the F25MAMA clear aperture, which gives a resolution of 5 km/s/pix. The F25MAMA aperture will be used for consistency with the other targets as well as for improved photometric accuracy. The star is in a sufficiently uncrowded field that it can be observed without a slit. Total overhead time is then 9.0min for guide- star acquisition and 8.0min for the first, and only, MAMA exposure. This leaves a generous amount of time for the stellar exposure. We anticipate a flux at the base of the Mg II\\ absorption features of 5 * 10^-13 erg/cm^-2/s/Angstrom. From the STIS Web simulator, an exposure time of 780s should yield a S/N = 30 at 2800Angstrom\\ in the line core. HD 106516 is also a binary with a period of 853 days, according to the radial velocities discussed by Carney & Latham (1987) and Latham Etal\\ (1992). The long period of the system means that the binarism should have little effect on its chromospheric emission. The low limit to the level of activity implied by the Mt.\\ Wilson observations corroborates this. We prefer to compare this spectrum against those to be obtained by STIS for the other subdwarfs, to minimize the difficulties inherent in intercomparing spectra from two different instruments. For example, scattered light could affect one set of profiles or flux levels more than the other. Unfortunately, the existing dataset does not constrain the relationship between rotation and chromospheric activity, nor can that be derived from ground-based measurements of these stars. First, the projected rotation rate vsin(i)\\ of such stars cannot be reliably measured for stars rotating more slowly than the Sun (Soderblom 1982), which, with vsin(i)\\ = 2 km/s, already has a dynamo. Second, these stars are all sufficiently hot that the \\caii\\ emission is overwhelmed by the stellar photospheric contribution to the \\caii\\ line core; high-resolution spectroscopy has yet to detect it. Consequently, photometric monitoring of the \\caii\\ region isn't sensitive enough to detect cyclical variations. Third, stars with no detectable surface rotation may still contain angular momentum; although the surface of a solar-type star spins down with age (Kraft 1967), the interior may continue to spin, possibly driving a dynamo. Indirect but strong evidence for internal angular momentum in metal-poor solar- type stars is provided by the rotation of blue horizontal branch stars which are their descendants (Peterson, Tarbell, & Carney 1983). While helioseismological measurements of the Sun do not show strong rotation, their sensitivity is still low to rotation in the central regions; it is also conceivable that the Sun could be an inherently slow rotator, especially since its planetary system contains far more angular momentum than the Sun itself. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 7395( 4) - 02/04/98 13:28 - [ 3] Data Distribution Media: DAT Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 7395( 4) - 02/04/98 13:28 - [ 4] TARGET LIST a) Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 4 HD106516 STAR,F3-F9 RA=12H 15M 10.56S +/- 0.1S, DEC= J2000 V = +9. V = 6.11+/-0.02 -10D 18' 45.1" +/- 0.5" B-V = 0.46+/-0.02 TYPE=F8 Comments: PPM225575 coordinates and proper motion Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) J2000 0.0021 -1.018 0.0 7395( 4) - 02/04/98 13:28 - [ 5] Visit: 01 Visit Priority: 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 ------------------------------------------------------------------------------------------------------------------------------------ 1 HD106516 STIS/C ACQ F28X50O MIRROR 1 0.1 S ONBOARD ACQ FOR 2-3 CD II ------------------------------------------------------------------------------------------------------------------------------------ 2 HD106516 STIS/N ACCUM 0.2X0.2 E230M 2707 1 240 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 3 HD106516 STIS/F ACCUM 0.2X0.2 E140M 1425 1 1200 S UV-MAM A ------------------------------------------------------------------------------------------------------------------------------------ 7395( 4) - 02/04/98 13:28 - [ 6] Summary Form for Proposal 7395 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 2-3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR E230M E140M ------------------------------------------------------------------------------------------------------------------------------------ Target Names HD106516 ------------------------------------------------------------------------------------------------------------------------------------