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Title:
The System Parameters of DW Ursae Majoris
Authors:
Araujo-Betancor, S.; Knigge, C.; Long, K. S.; Hoard, D. W.; Szkody, P.; Rodgers, B.; Krisciunas, K.; Dhillon, V. S.; Hynes, R. I.; Patterson, J.; Kemp, J.
Affiliation:
AA(Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK), AB(Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK), AC(Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218), AD(Space Infrared Telescope Facility Science Center, Infrared Processing and Analysis Center, California Institute of Technology, MC 314-6, 1200 East California Boulevard, Pasadena, CA 91125), AE(Department of Astronomy, University of Washington, Physics/Astronomy Building, Stevens Way, Box 351580, Seattle, WA 98195), AF(Department of Astronomy, University of Washington, Physics/Astronomy Building, Stevens Way, Box 351580, Seattle, WA 98195; Current address: Gemini Observatory, AURA/Chile, P.O. Box 26732, Tucson, AZ 85726.), AG(Department of Astronomy, University of Washington, Physics/Astronomy Building, Stevens Way, Box 351580, Seattle, WA 98195), AH(Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK), AI(Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK), AJ(Department of Astronomy, Columbia University, 1328 Pupin Hall, 550 West 120th Street, New York, NY 10027), AK(Department of Astronomy, Columbia University, 1328 Pupin Hall, 550 West 120th Street, New York, NY 10027)
Publication:
The Astrophysical Journal, Volume 583, Issue 1, pp. 437-445. (ApJ Homepage)
Publication Date:
01/2003
Origin:
UCP
ApJ Keywords:
Stars: Binaries: Close, Stars: Binaries: Eclipsing, Stars: Novae, Cataclysmic Variables, Stars: Fundamental Parameters, Stars: Individual: Constellation Name: DW Ursae Majoris, Ultraviolet: Stars
DOI:
10.1086/345098
Bibliographic Code:
2003ApJ...583..437A

Abstract

We present new constraints on the system parameters of the SW Sextantis star DW Ursae Majoris, based on ultraviolet (UV) eclipse observations with the Hubble Space Telescope. Our data were obtained during a low state of the system, in which the UV light was dominated by the hot white dwarf (WD) primary. The duration of the WD eclipse allows us to set a firm lower limit on the mass ratio, q=M2/M1>0.24 if q<1.5 (as expected on theoretical grounds) the inclination must satisfy i>71deg. We have also been able to determine the duration of WD ingress and egress from our data. This allows us to constrain the masses and radii of the system components and the distance between them to be 0.67<=M1/Msolar<=1.06, 0.008<=R1/Rsolar<=0.014, M2/Msolar>0.16, R2/Rsolar>0.28, and a/Rsolar>1.05. If the secondary follows Smith & Dhillon's mass-period relation for CV secondaries, our estimates for the system parameters become M1/Msolar=0.77+/-0.07, R1/Rsolar=0.012+/-0.001, M2/Msolar=0.30+/-0.10, R2/Rsolar=0.34+/-0.04, q=0.39+/-0.12, i=82deg+/-4deg, and a/Rsolar=1.14+/-0.06. We have also obtained time-resolved I- and K-band photometry of DW UMa during the same low state. Using Bessell's spectral type versus (I-K) color calibration, we estimate the spectral type of the donor star to be M3.5+/-1.0. This latter result helps us to estimate the distance toward the system via Bailey's method as d=930+/-160 pc. Finally, we have repeated Knigge and coworkers' WD model atmosphere fit to the low-state UV spectrum of DW UMa in order to account for the higher surface gravity indicated by our eclipse analysis. The best-fit model with surface gravity fixed at logg=8 has an effective temperature of Teff=50,000+/-1000 K. The normalization of the fit also yields a second distance estimate, d=590+/-100 pc. If we adopt this distance and assume that the mideclipse K-band flux is entirely due to the donor star, we obtain a second estimate for the spectral type of the secondary in DW UMa, M7+/-2.0. After discussing potential sources of systematic errors in both methods, we conclude that the true value for the distance and spectral type will probably be in between the values obtained by the two methods.

Based on observations with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555, and with the Apache Point Observatory (APO) 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium (ARC).


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