8159( 3) - 11/15/00 13:16 - [ 1] PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS ST ScI Use Only ID: 8159 Version: 3 Check-in Date: 17-Feb-1999 14:59:10 1.Proposal Title: The UV Spectrum of an Elliptical Accretion Disk Devoid of Hydrogen ------------------------------------------------------------------------------------ 2. Proposal For 3. Cycle GO 8 ------------------------------------------------------------------------------------ 4. Investigators Contact? PI: Richard Wade The Pennsylvania State University CoI: Dr. Michael Eracleous The Pennsylvania State University N CoI: Dr. Jerome Orosz The Pennsylvania State University N ------------------------------------------------------------------------------------ 5. Abstract AM CVn (HZ 29) is the prototype of the interacting double- degenerate binaries, or so-called Helium Cataclysmic Variables (He CVs), which could be a channel to form Type Ia supernovae. These are short-orbital period systems in which a low-mass He white dwarf orbits a more massive white dwarf and transfers matter devoid of hydrogen to an accretion disk around that star. AM CVn is the best He CV to measure the mass transfer rate (dM/dt) and the chemical abundance pattern of the transferred material. Lines from different ions of the same element arise at different radii in the disk, and have different velocity widths. This makes a mapping of temperature onto radius possible, and with it a determination of dM/dt. This modeling is best done using ultraviolet spectra, where the necessary lines can be found. Observations by HST/STIS are needed, to provide the combination of high spectral and temporal resolution and signal-to-noise ratio, and wide spectral coverage that neither IUE nor prior HST/GHRS spectra provide. The data can be used in a variety of ways. We will undertake a modeling study of the steady-state spectrum of AM CVn. We will learn whether only H-burning took place in the mass donor or whether 3Alpha He- burning also occurred. Comparing H-rich and H-depleted disks will also tell us about the stability of disks and their ability to drive winds. ------------------------------------------------------------------------------------ 8159( 3) - 11/15/00 13:16 - [ 2] Observations Description ------------------------ The primary scientific goal of the proposed observations is to produce a UV spectrum of AM CVn with a signal-to-noise ratio of 50 to 100, which will serve to test models of emission from hydrogen-deficient accretion disks. Our modeling experience using Hubeny's structure and spectral codes includes, in addition to accretion disks in normal CVs chemically peculiar stars, including the extreme He star PG 0900+400. A specific modeling objective is to use the profiles of the absorption lines to constrain the ionization state of the disk as a function of radius. The latter goal will be pursued by constructing synthetic line profiles based on kinematic models of the flow in an elliptical disk (cf, Patterson, Halpern, & Shambrook 1993; Eracleous et al. 1995) and comparing them with the observations. The above scientific requirements dictate the observing strategy described below. In accordance with the above requirements, we propose to observe AM CVn with STIS for two consecutive s pacecraft orbits. We will use the NUV and FUV MAMA detectors in TIME-TAG mode along with gratings G140L and G230L respectively to cover the wavelength range 1150-3180 Angstrom (NUV MAMA+G230L in the first orbit and FUV MAMA+G140L in the second orbit). We will use the 52"x0.1" slit to get the highest possible spectral resolution. By observing in TIME-TAG mode we will obtain an arrival- time record for each photon so that we can construct time-resolved spectra at any time resolution. More specifically, we will be able to fold the data about the known orbital period of the system to construct phase-resolved spectra. An additional use of the time-resolved spectra is to correct any (small) radial velocity variations resulting from the orbital motion of the white dwarf, before constructing the time-averaged spectrum. The velocity resolution of 200- 300 km/s offered by the STIS+G140L combination is adequate for our purposes. We note that the lower velocity resolution of the G230L gr ating (300-650 km/s) will not have any impact on our analys is because virtually all of the strong absorption lines in the spectrum of AM CVn fall in the range of the G140L grating (see Figure 1). The NUV spectrum is needed for (a) time-resolved studies of superhump modulations, (b) accurate measurement of interstellar reddening from the 2200 Angstrom feature, and (c) study of the He II 2050 Angstrom series limit and continuum jump. None of these goals is attainable with the existing IUE/LWP spectra. After considering the visibility of the star during a spacecraft orbit and all the necessary overheads (guide star and target acquisition and instrument overheads) the time available for science exposures is 30 minutes in the first orbit and 40 minutes in the second orbit. These correspond to 1.6 and 2.4 orbital cycles of the binary, respectively, which means that we will cover the complete orbital cycle of the binary with both gratings. To assess whether the final, time-averaged spectrum will be adequate for our purposes we used the ST IS exposure- time calculator to simulate the spectrum of detected counts and the signal-to-noise ratio as a function of wavelength. We assumed exposure times of 27 minutes for the NUV MAMA+G230L combination and 41 minutes for the FUV MAMA+G140L combination. By assigning the G140L observations to the second orbit we are able to obtain the highest signal-to-noise ratio in the spectral range that includes most of the absorption lines. The signal-to-noise ratio is greater than 50 almost everywhere and greater than 70 over most of the spectrum, as required. The predicted count rates are 5, 100 per second in the NUV MAMA and 3, 400 per second in the FUV MAMA, well below the prescribed upper limit of 21, 000 per sec. Moreover, we note that the source does not vary significantly; the FUV flux measured in 9 separate IUE exposures is within 2 percent of the mean. At the higher of the two count rates we can observe in TIME-TAG mode for 196 minutes, i.e., we will not face a problem wi th on-board data storage. We will, however, have to transfer data from the STIS buffer to the on-board data recorder every 6-10 minutes. Real Time Justification ----------------------- Calibration Justification ------------------------- Additional Comments ------------------- ------------------------------------------------------------------------------------ 8159( 3) - 11/15/00 13:16 - [ 3] Data Distribution Paper Products: YES Media: 8MM Blocking Factor: 10 Ship To: PI_Address Ship Via: Email: ------------------------------------------------------------------------------------ 8159( 3) - 11/15/00 13:16 - [ 4] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 AM-CVN STAR, INTERACTING RA=12H 34M 54.58S +/- 0.4", J2000 V = 14.2+/-0.1 BINARY, NOVA-LIKE DEC=37D 37' 43.4" +/- 0.4", U-B = -1.0+/-0.2 PLATE-ID=01PQ B-V = -0.2+/-0.2 F(1202)=2.3+/-0.1e-13 F(1601)=1.6+/-0.1e-13 F(2004)=1+/-0.1e-13 F(2409)=6+/-0.5e-14 F(2803)=4+/-0.5e-14 F(3208)=4+/-0.5e-14 Coordinate Source: Comments: Flux measurements at specific UV wavelengths come from IUE spectra. 8159( 3) - 11/15/00 13:16 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 10 AM-CVN STIS/C ACQ F28X50L MIRROR ACQTYPE=POINT 1 .3 S ONBOARD ACQ FOR 20 CD P ------------------------------------------------------------------------------------------------------------------------------------ 20 AM-CVN STIS/C ACQ/PEA 52X0.1 MIRROR 1 .3 S ONBOARD ACQ FOR 30, 40 CD K ------------------------------------------------------------------------------------------------------------------------------------ 30 AM-CVN STIS/N TIME-TA 52X0.1 G230L 2376 BUFFER-TIME=360 1 33 M NO SPLIT UV-MAM G A Comments: The purpose of the NO SPLIT requirements is to ensure that we get complete phase coverage of the orbital cycle of this binary. This cannot be accomplished if there are any interruprions. ------------------------------------------------------------------------------------------------------------------------------------ 40 AM-CVN STIS/F TIME-TA 52X0.1 G140L 1425 BUFFER-TIME=360 1 40 M NO SPLIT UV-MAM G A Comments: The purpose of the NO SPLIT requirements is to ensure that we get complete phase coverage of the orbital cycle of this binary. This cannot be accomplished if there are any interruprions (e.g., for wavecals). ------------------------------------------------------------------------------------------------------------------------------------ 8159( 3) - 11/15/00 13:16 - [ 6] Summary Form for Proposal 8159 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures F28X50LP 52X0.1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD STIS/NUV-MAMA STIS/FUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ ACQ/PEAK TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ACQTYPE=POINT BUFFER-TIME=360 ------------------------------------------------------------------------------------------------------------------------------------ Proposal Category GO ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ONBOARD ACQ FOR 20 ONBOARD ACQ FOR 30, 40 NO SPLIT ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements MIRROR G230L G140L ------------------------------------------------------------------------------------------------------------------------------------ Target Names AM-CVN ------------------------------------------------------------------------------------------------------------------------------------ Wavelengths 2376 1425 ------------------------------------------------------------------------------------------------------------------------------------