YY Hya and its interstellar environment

Abstract

Context. During a search for previously unknown Galactic emission nebulae, we discovered a faint 36′ diameter Hα emission nebula centered around the periodic variable YY Hya. Although this star has been classified as an RR-Lyr variable, such a classification is inconsistent with a Gaia distance of ≃450 pc. The GALEX image data also show YY Hya as having a strong UV excess, suggesting the existence of a hot and compact binary companion. Aims. We aim to clarify the nature of YY Hya and its nebula. Methods. In addition to our discovery image data, we obtained a 2.°5 × 2.°5 image mosaic of the whole region with CHILESCOPE facilities and time-series spectroscopy at MDM observatory. Also, we used data from various space missions to derive an exact orbital period and a spectral energy distribution. Results. We find that YY Hya is a compact binary system containing a K dwarf star that is strongly irradiated by a hot white dwarf companion. The spectral characteristics of the emission lines that are visible only during the maximum light of the perfectly sinusoidal optical light curve show signatures resembling those of members of the BE UMa variable family. These are post-common-envelope pre-cataclysmic variables. However, the companion star here is more massive than that found in other group members and, thus, the progenitor of the white dwarf must have been a star between 3 and 4 M⊙. The nebula appears to be an ejected common-envelope shell with a mass on the order of one M⊙ and an age of 500 000 yr. This makes it the biggest such shell known thus far. The alignment of neighboring nebulosities some 45′ to the northeast and southwest of YY Hya suggests that the system has had strong bipolar outflows. We also briefly speculate that it might be related to the 1065 BP “guest-star” reported in ancient Chinese records.