NV Aurigae: An optically thick Mira star

Abstract

Using the 1.26m infrared telescope and the 60 90 cm Schmidt telescope + CCD at the Xinglong Station, Beijing Astronomical Observatory, near-infrared and optical photometric data for the optically thick Mira variable NV Aur at phase 0.206 were obtained. Assuming that the infrared emission is re-radiation of UV and optical photons from the central star by a spheric shell of circumstellar dust, the central star was found to have an effective temperature of T ∗ = 1750 K , and the dust shell, T d = 250 K. We show that, for O-rich AGB stars, their periods and luminosities at 25 μm are tightly correlated. The distance of NV Aur is estimated from this correlation to be 590 pc, and the luminosity and radius of the central star are found to be L ∗ = 9.4 × 10 4L ⊙ , R ∗ = 3.4 × 10 3R ⊙ . The optical depth of the dust shell is found to be 2.7. This confirms it to be an optically thick Mira star and that the central star is a red supergiant. The mass loss rate we derived is 3.1×10 −6 M ⊙/yr, consistent in order of magnitude with the value of 5.7 × 10 −6 M ⊙/yr obtained by Knapp et al. from CO observations.