The Moth: An Unusual Circumstellar Structure Associated with HD 61005

Abstract

We present the discovery of an unusual spatially resolved circumstellar structure associated with the 90 Myr, nearby, G dwarf star HD 61005. Observations from the FEPS Spitzer Legacy Science survey reveal thermal emission in excess of expected stellar photospheric levels. Follow-up 0.1 -->'' resolution HST NICMOS coronagraphic images reveal scattered starlight ≤7 -->'' (~240 AU) from the occulted star (1.1 μm flux density = -->18 ± 3.3 mJy; and -->0.77% ± 0.16% of the starlight). The extremely high near-IR scattering fraction and IR excess luminosity -->f = LIR/L* ≈ 2 × 10−3 suggests scattering particle sizes of order -->a 1.1 μm/2π ~ 0.2 μm , comparable to the blowout size ( -->a ≈ 0.3 μm) due to radiation pressure from the star. Dust-scattered starlight is traced inward to an instrumental limit of ~10 AU. The structure exhibits a strong asymmetry about its morphological major axis but is mirror-symmetric about its minor axis.