A time-resolved photometric study of the eclipsing dwarf nova RX J0909+1849

Multicolor (BVRI) light curves have been obtained for the newly discovered, 4.21 hr eclipsing dwarf nova RX J0909+1849. The eclipse profiles have been analyzed with a parameter-fitting model to constrain properties of the sys tem. The model assumes four sources of luminosity : the white dwarf primary star and the secondary star (both assumed to radiate as blackbodies) and a n accretion disk characterized as a blackbody whose temperature follows a r adial power-law distribution: T(r) = T-d(R-d/r)(alpha), where T-d and R-d a re the outer disk temperature and radius, respectively. The model also incl udes an optically thick bright spot at the intersection of the mass transfe r stream and the disk periphery. A matrix of model solutions is computed, w hich covers an extensive range of plausible parameter values. The solution matrix is then explored to determine the optimum values for the fitting par ameters and their associated errors. The mass ratio of RX J0909 + 1849 is unknown, thus the orbital inclination is not tightly constrained by the model. Five mass ratios have been conside red spanning a range of plausible values, 0.3 less than or equal to q (= M- 2/M-1) less than or equal to 0.7. Over this range of q, the inclination var ies between roughly 74 degrees and 80 degrees. Model parameters include the temperatures of the white dwarf (T-1) and the secondary star (T-2), the ra dius (R-d) and temperature (T-d) of the disk periphery, the disk power-law temperature exponent (alpha), and the bright spot temperature (T-s). With t he exception of the q = 0.3 models, which required a relatively hot white d warf (80,000 K), the optimum values of the parameters are nearly constant w ith mass ratio. For a representative mass ratio of q = 0.5, values of T-1 = 26,000 +/- 14,000 K, T-s = 18,000 +/- 7000 K, T-d = 3200 +/- 600 K, R-d/R- L1 = 0.54 +/- 0.08, alpha = 0.53 +/- 0.11, and B-V = 0.45 +/- 0.07 are foun d. A value of T-2 = 3400 K has been adopted for all models based on the spe ctral type of the secondary star (M3-M4). The observed color (B-V = 0.4 +/- 0.2) is consistent with the colors given by the model; thus, it is unlikel y that RX J0909+1849 suffers significant interstellar absorption. The observed spectral type of the secondary star suggests an absolute magni tude in the range 10.6 < MV(2) < 11.0. After correcting the apparent V magn itude at mideclipse by the fraction of light originating from the secondary star (similar to 94%), a distance of similar to 200-250 pc is derived for the RX J0909+1849 system.