Page 1 PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID 4634c Report Date: 09-May-96:19:45 Version: ********** Check-in Date: ********** 1.Proposal Title: THE RED EDGE OF THE ZZ CETI INSTABILITY STRIP:CYCLE3 MEDIUM ------------------------------------------------------------------------------------ 2. Scientific Category 3. Proposal For 4. Proposal Type 5. Continuation ID HOT STARS GO Sub Category WHITE DWARFS ------------------------------------------------------------------------------------ 6. Principal Investigator Institution Country Telephone Nicole F. Allard OBSERVATOIRE DE MEUDON FRANCE ------------------------------------------------------------------------------------ 7. Abstract ZZ Ceti stars are variable white dwarfs of spectral type DA. The theoretically predicted instability strip depends on several uncertain assumptions, most importantly the efficiency of convective energy transport and the detailed structure of the outer stellar layers (chemical stratification, thickness of hydrogen layers). Empirical determinations are so far not as accurate as desirable for a comparison, because: (i) the Balmer lines in the optical region reach maximum strengths within the instability strip and therefore vary only little with temperature, and (ii) the use of the ultraviolet spectrum, which is dominated by the line wing of Lyman alpha with quasimolecular satellites at 1400 and 1600 A has been hampered by the lack of an adequate theoretical description of the line shape. This theory has now been developed by one of us (N.Allard) and incorporated into stellar atmosphere programs, allowing us to determine effective temperatures in this range with an accuracy of 200 K. In Cycle 2 we will obtain observations of the apparently hottest ZZ Ceti object (G117-B15A). In this project we want to observe G255-2 - possibly the coolest ZZ Ceti - to get a good estimate for the red edge of the instability region. ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ 9. Est obs time (hours) pri: 1.27 par: 0 10. Num targs pri: 1 par: 0 ------------------------------------------------------------------------------------ 11. Instruments requested: FOS ------------------------------------------------------------------------------------ ------------------------------------------------------------------------------------ Page 2 I. GENERAL FORM Proposal 4634c PI: Nicole F. Allard Proposal Title: THE RED EDGE OF THE ZZ CETI INSTABILITY STRIP:CYCLE3 MEDIUM ------------------------------------------------------------------------------------ 1. Proposers: Proposers Institution Country ESA ------------------------------------------------------------------------------------ Pi Nicole F. Allard OBSERVATOIRE DE MEUDON FRANCE X Detlev G. Koester LOUISIANA STATE UNIVERSITY USA Gerard P. Vauclair OBSERVATOIRE MIDI PYRENEES, FRANCE X TOULOUSE ------------------------------------------------------------------------------------ 2. Scientific Justification. The ZZ Ceti stars form a class of non-radially pulsating DA white dwarfs, confined to a narrow instability strip centered around Teff 12000 K. It is very likely that all DA white dwarfs start to pulsate when their evolution leads them through this region. The theory of these oscillations shows that the exact boundaries of the instability strip contain much information about the structure of the outer layers of white dwarfs as well as about fundamental physical processes (see Wesemael et al. 1991, Nato ASI Series C, Vol. 336,p.159 for a recent discussion). We want to determine the red edge of the instability strip using our recent work on the Lyman alpha satellite features for the spectral analysis. We have chosen the object G255-2, which in most studies defines the cool edge. With the observation proposed here we will be able to answer the important question, whether the cool edge is close to 10000 K (suggested by G255- 2 and BPM30551) or much hotter (>11000). G255-2 has been studied extensively in the optical region. Fast photometry data have been obtained by one of us (Vauclair, unpublished) and the results will be available. ------------------------------------------------------------------------------------ 3. Description of proposed observations. We want to obtain spectral coverage of the range dominated by the Lyman alpha red wing satellites, i.e. 1300 - 2300 A, for the variable white dwarf G255-2. With FOS and the G160L grating we will achieve a S/N of >20 around 1500 A, where a secondary flux maximum between the 1400 and 1600 A satellites is extremely temperature sensitive. For lambda > 1650 A the flux rises steeply in the star, and the expected S/N ranges from 40 to 100. The range from 1300 to 2300 A can be covered in one exposure with the G160L grating. The exposure time necessary has been estimated from a model spectrum calculated for different effective temperatures between 10000 and 11000 K and log g = 8.0. The fluxes have been scaled using the observed V magnitude. A 90 min exposure will give a S/N of >20 for the regions most sensitive to temperature and surface gravity. ------------------------------------------------------------------------------------ 4. Justification of need for HST observations. Numerous attempts to determine the boundaries of the instability strip have been performed with optical or IUE data, among others also by one of the current CO- PI's (Weidemann and Koester 1984, Astron.Astrophys. 132, 195). A recent summary of these studies is given in Wesemael et al. 1991 (White Dwarfs, Nato ASI Series C, Vol.336, p.159). The red edge of the instability strip is unfortunately defined by 4 different stars in 4 studies, the coolest ones being BPM30551 and G255-2 around 10000 K (with large uncertainties), while the next coolest stars do only appear above 11000 K, thus precluding any firm conclusions from a comparison with observations. With V = 16.04 G255-2 is too faint to give a useful spectrum with the IUE satellite. For an accurate temperature determination and possibly also a determination of the surface gravity (and thus mass) from the relative strength of the two Lyman alpha satellites (from neutral and charged perturbers) we will need a S/N of about 30 in the UV. Page 3 ------------------------------------------------------------------------------------ 7. Data reduction and analysis plans. The data obtained will be reduced using STSDAS installed on the SPARCstation of the CO-PI (Koester) at Baton Rouge by Allard and Koester. The analysis will consist in the fitting of the final observation result with synthetic spectra calculated by Koester and collaborators. Methods for calculating white dwarf model atmospheres and synthetic spectra have been developed by us over the past 15 years and used and documented in numerous publications. A complete grid of model atmospheres covering the range of ZZ Ceti stars is available and can be expanded at any time if necessary. These calculations include the detailed line profile calculations for Lyman alpha and the quasimolecular satellites at 1400 and 1600 A obtained recently by N.Allard, which are essential for the desired accuracy of the temperature determination of this project. ------------------------------------------------------------------------------------ 9. Description of previous HST work. Data for our project ID3447 were obtained in November 1992. We are currently trying to understand a factor of two difference in the absolute fluxes compared to a (very noisy) IUE spectrum, and at the same time proceeding with the spectral analysis. ------------------------------------------------------------------------------------ 10. Resources to be supplied by investigator's institution(s). The Observatoire de Meudon provides computing facilities and basic support (secretarial, technical programming etc.) for N. Allard. The computing facilities of the Department of Physics (Workstations) and Louisiana State University (Mainframes) will also be available for this project essentially free of charge. ------------------------------------------------------------------------------------ 11. Address Information Name: NICOLE F. ALLARD Category: PI Institution: Observatoire de Meudon Address: OBSERVATOIRE DE MEUDON DEPARTEMENT ATOMES ET MOLECULES EN ASTROPHYSIQUE F-92195 MEUDON PRINCIPAL CEDEX City: MEUDON State: Zip Code: Country: FRANCE Telephone: 33-1-45077449 Telex (or e-mail): ------------------------------------------------------------------------------------ TARGET LIST a) Fixed Targets ID = 4634c [ 4] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial |Acqui|FLX| Flux data No | Name | Description | Position |Eqnx | Vel. |Prblm|REF| | | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------ 1 WD1159+803 A,177 PLATE-ID=01G5, 2000 1 V = 16.04 +/- 0.05 G255-2 RA = 12H01M45.699S +/- 0.3", 2 F-CONT(1700) = 4.00 +/- 0.5 E-15 DEC = +80D05'00.47" +/- 0.3" 3 F-CONT(1660) = 1.41 +/- 0.2 E-14 Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) J 1984.20 -0.0879 +/- 0.0000 -0.0400 +/- 0.0000 0.0000 +/- 0.0000 ------------------------------------------------------------------------------------------------------------------------------------ EXPOSURE LOGSHEET ID = 4634c [ 5] ------------------------------------------------------------------------------------------------------------------------------------ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |10 | 11 | 12 |13 |14| 15 ------------------------------------------------------------------------------------------------------------------------------------ Line | Seq | Target |Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | S/N |Flx|Pr| Special Number | Name | Name |Config| Mode |or FOV |Element |Waveln.| Parameters |Exp| |Rel. Time|Ref| | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 WD1159+803 FOS/BL ACQ/ 4.3 MIRROR NTHSTAR=1 1 13.0S 1 1 CYCLE 3 / 1.00- BINARY 3.00 ONBOARD ACQ FOR 2.00 ------------------------------------------------------------------------------------------------------------------------------------ 2 WD1159+803 FOS/BL ACQ/ 0.25X2. MIRROR TYPE=UP 1 13.0S 1 1 ONBOARD ACQ FOR PEAK 0 3.00 SPATIAL SCAN ------------------------------------------------------------------------------------------------------------------------------------ 3 WD1159+803 FOS/BL ACCUM 0.25X2. G160L 1600 1 76.0M 2 1 0 ------------------------------------------------------------------------------------------------------------------------------------ Scan Paramters Form Proposal ID: 4634c [ 6] ------------------------------------------------------------------------------------------------------------------------------------ Data ID: 1 Exposure Logsheet lines: 2.00 FGS Scan: DWELL Dwell Only: dwell points/line: 7 Seconds per dwell: 1.00 Scan width (arc-secs): 0.0000 Scan length (arc-secs): 0.3000 Angle between sides (degrees) 90.00 Number of lines: 1 Scan rate (arc-sec/sec): 0.0000 PA of first scan line (degrees) 90.000 Scan frame (CEL or S/C): S/C Length Offset (arc-sec): 0.1500 Width Offset (arc-sec): 0.0 ------------------------------------------------------------------------------------------------------------------------------------ Summary Form for Proposal 4634c [ 7] Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Configurations FOS/BL ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/BINARY ACQ/PEAK ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters NTHSTAR=1 TYPE=UP ------------------------------------------------------------------------------------------------------------------------------------ Proposal for GO ------------------------------------------------------------------------------------------------------------------------------------ S/C Hours 1.27 ------------------------------------------------------------------------------------------------------------------------------------ Scientific Category HOT STARS ------------------------------------------------------------------------------------------------------------------------------------ Scientific Sub-category WHITE DWARFS ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements CYCLE 3 / 1.00-3.00; ONBOARD ACQ FOR 2.00; ONBOARD ACQ FOR 3.00; SPATIAL SCAN; ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G160L ------------------------------------------------------------------------------------------------------------------------------------ Target Names WD1159+803 G255-2 ------------------------------------------------------------------------------------------------------------------------------------