The Enigmatic Nature of 2MASS J05073893-6826061: A Hot, H-poor Post-AGB Star?

We call attention to the very unusual spectrum of the LMC star 2MASS J05073893-6826061, which shows an unprecedented combination of odd properties, including intense UV and IR excesses, no Balmer transitions, weak C II emission, and a dense thicket of narrow, difficult to identify absorption lines. The spectrum of this V = 15.6 star has recently been reported by Margon et al. (ApJ, 898, 85, 2020), where it is referred to as object 233-1. The position of 233-1 on the H-R diagram implies that this star may be a very hot, H-poor post-AGB object, caught in a brief stage of evolution.

Membership in the LMC seems certain. The object shows weak C II λλ7231, 7236 emission, with the doublet well resolved, appearing at the LMC velocity, and the Gaia proper motion and parallax data are also compatible with membership. There is no evidence for large amplitude photometric variability or surrounding nebulosity. The most remarkable spectral features are a series of dozens of narrow absorption lines, unresolved at our 1~Å resolution, in the 3300–5000 Å range. Identification of these features is problematic due to the sheer number of observed transitions, making chance coincidences with common atomic species and ionization states likely. Cross-correlation of atomic line lists with the spectrum does reveal strong evidence for the presence of O II, and less convincing although still substantial evidence for He I, He II, Si III, and N II. Although the late-type, low-mass [WC] stars do share the UV and IR excesses and C II λλ7231, 7236 emission seen in 233-1, as well as occasional narrow absorption lines, their spectra, dominated by dozens of very strong C II and He I emission lines, are quite different. The combination of the weak C II emission, lack of H, UV and IR excess, and numerous narrow absorption lines in 233-1 seems very unusual. We plan continued monitoring of the object.

P.M. is grateful for the support of NSF grant AST-1612874 and the Mt. Cuba Astronomical Foundation.