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By two different methods, we show that LHS 4033 is an extremely massive whitedwarf near its likely upper mass limit for destruction by unstable electroncaptures. From the accurate trigonometric parallax reported herein, theeffective temperature (T=10,900 K) and the stellar radius (R=0.00368 R_sun) aredirectly determined from the broad-band spectral energy distribution -- theparallax method. The effective temperature and surface gravity are alsoestimated independently from the simultaneous fitting of the observed Balmerline profiles with those predicted from pure-hydrogen model atmospheres -- thespectroscopic method (T=10,760 K, log g=9.46). The mass of LHS 4033 is theninferred from theoretical mass-radius relations appropriate for white dwarfs.The parallax method yields a mass estimate of 1.310--1.330 M_sun, for interiorcompositions ranging from pure magnesium to pure carbon, respectively, whilethe spectroscopic method yields an estimate of 1.318--1.335 M_sun for the samecore compositions. This star is the most massive white dwarf for which a robustcomparison of the two techniques has been made.Comment: 17 pages, including 4 figures, Accepted for Ap.

Analysis of a Very Massive DA White Dwarf via the Trigonometric Parallax and Spectroscopic Methods