A Multicolor Photometric Study of the Deeply Eclipsing Dwarf Nova EX Draconis

Abstract

ABSTRACTMulticolor (BVRI) light curves have been obtained for the 5.04 hr eclipsing dwarf nova EX Dra. The eclipse profiles have been analyzed with a parameter‐fitting model that assumes four sources of luminosity: a white dwarf primary star, an assumed main‐sequence secondary star, an accretion disk, and a bright spot at the intersection of the mass‐transfer stream and the disk periphery. Model parameters include the temperatures of the white dwarf (T1) and the secondary star (T2), the radius (Rd) and temperature (Td) of the disk periphery, the disk power‐law temperature exponent (α), and the bright‐spot temperature (Ts). The large eclipse phase width (Δϕ = 0.109) constrains the orbital inclination i>83° for plausible mass ratios q (= M2/M1) < 0.81. A matrix of model solutions was computed, covering an extensive range of plausible parameter values. The solution matrix was then explored to determine the optimum values for the fitting parameters and their associated errors. Specifically, the analysis reveals optimum values of T1 = 50,000 ± 20,000 K, Ts = 40,000 ± 10,000 K, Td = 6500 ± 1000 K, Rd/RL1 = 0.50 ± 0.05, α = 0.3 ± 0.2, and B− V = 0.15 ± 0.05 for the EX Dra system. A value of T2 = 3750(± 150) K has been fixed in the analysis on the basis of the observed spectral type of the secondary star (M0–M2). An estimate of the absolute magnitude of the secondary star based on this spectral type [8.8<MV(2)<9.7] coupled with the observed brightness at mideclipse (V = 16.57 ± 0.05) and an estimated extinction AV = 0.45 ± 0.63 yields a distance to EX Dra of 240 ± 90 pc.