Spitzer Space Telescope observations of Var Her 04: Possible detection of dust formation in a superoutbursting tremendous outburst amplitude dwarf nova

摘要

We present four MIPS (24 mu m) and two IRAC (3.6, 4.5, 5.8, and 8.0 mu m) Spitzer Space Telescope observations of the newly discovered tremendous outburst amplitude dwarf nova (TOAD) Var Her 04 during decline from superoutburst. The four MIPS observations span 271 days, and the two IRAC observations span 211 days. Along the line of sight to Var Her 04, there is a foreground M star within 100 of the variable; as a result, all of the Spitzer photometry presented in this paper is a blend of the foreground M star and Var Her 04. We estimate the quiescent level of the TOAD to be Delta V = 4-5 mag below that of the M star. Based on the spectral energy distribution and the Two Micron All Sky Survey colors, we find the M star to be an M3.5 V dwarf at a distance of 80 - 130 pc. Based on its outburst amplitude and quiescent apparent magnitude, we estimate the distance to Var Her 04 to be 200 - 400 pc, suggesting that the line-of-sight foreground star is physically unrelated to the cataclysmic variable. All of the Spitzer photometry is consistent with the photospheric emission of the line-of-sight M3.5 V star, except for one 24 mu m observation obtained after the variable rebrightened. This 24 mu m flux density is 75 mu Jy (4 sigma) above the preceding and following MIPS observations. We tentatively suggest that the mid-infrared brightening of 75 mu Jy may be associated with a dust formation event in the superoutburst ejecta. Assuming a dust temperature of 100 - 400 K, we have estimated the amount of dust required. We find that 10(-13) to 10(-11) M-circle dot of dust is needed, consistent with the amounts of mass ejection in TOADs expected during superoutburst and possibly making TOADs important contributors to the recycling of the interstellar medium.