The System Parameters of DW Ursae Majoris

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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> 71 ◦. We have also been able to determine the duration of WD ingress and egress from our data. This allows us to constrain the – 2 – masses and radii of the system components and the distance between them to be 0.67 ≤ M1/M ⊙ ≤ 1.06, 0.008 ≤ R1/R ⊙ ≤ 0.014, M2/M ⊙> 0.16, R2/R ⊙> 0.28 and a/R ⊙> 1.05. If the secondary follows Smith & Dhillon’s mass-period relation for CV secondaries, our estimates for the system parameters become M1/M ⊙ = 0.77 ±0.07, R1/R ⊙ = 0.012 ±0.001, M2/M ⊙ = 0.30 ±0.10, R2/R ⊙ = 0.34 ±0.04, q = 0.39 ± 0.12, i = 82 ◦ ± 4 ◦ and a/R ⊙ = 1.14 ± 0.06.

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