Lifting the Iron Curtain : Toward an Understanding of the Iron Stars XX Oph and AS 325

Abstract

We present new optical, near infrared, and archival ultraviolet observations of XX Ophiuchi and AS 325, two proposed iron stars. These unusual stars have optical spectra dominated by emission lines arising from hydrogen as well as ionized metals such as iron, chromium, and titanium. Both stars have been classified as iron stars and a number of exotic models have been presented for their origin. Using two years of moderately high resolution optical spectroscopy, the first high signal-to-noise K-band spectroscopy of these sources (which reveals stellar photospheric absorption lines), and new near-infrared interferometric observations, we confirm that both systems are composed of two stars, likely binaries, containing a hot Be star with an evolved late-type secondary. The hydrogen emission features arise in the hot wind from the Be star while the corresponding P-Cygni absorption lines are produced from dense material in the expanding, radiation driven, wind around each system. The optical FeII emission lines are pumped by ultraviolet FeII absorption lines through fluorescence. Contrary to some claims in the literature, the spectral features of XX Oph and AS 325 are quite similar, evidence that they are comparable systems. We examine the variability of the spectral morphology and radial velocity motions of both sources. We also study the variability of XX Oph during a major photometric event and find that the spectral nature of the system varies during the event. A comparison of the velocity of the absorption line components in our new spectra with those in the literature show that the structure of the stellar wind from XX Oph has changed since the system was observed in 1951.