LkH$α$ 225 (V1318 Cyg) South in Outburst

Abstract

Magakian et al. called attention to the current bright state of LkHα 225 South, a well-known highly embedded,intermediate-mass young stellar object that over the past two decades has brightened visually from >20m to <13m.We present recent optical photometric monitoring showing colorless, nonsinusoidal, periodic brightnessoscillations occurring every 43 days with amplitude ∼0.7 mag. We also present new flux-calibrated optical andnear-infrared spectroscopy, which we model in terms of a Keplerian accretion disk, and high-dispersion spectrathat demonstrate similarity to some categories of “mixed-temperature” accretion-outburst objects. At bluewavelengths, LkHα 225 South has a pure absorption spectrum and is a good spectral match to the FU Ori starsV1515 Cyg and V1057 Cyg. At red optical and infrared wavelengths, however, the spectrum is more similar toGaia 19ajj, showing emission in TiO, CO, and metals. Sr II absorption indicates a low-surface-gravity atmosphere.There are also signatures of a strong wind/outflow. LkHα 225 South was moderately bright in the early 1950s aswell as in the late 1980s, with evidence for deep fades during intervening epochs. The body of evidence suggeststhat LkHα 225 South is another case of a source with episodically enhanced accretion that causes brightening byorders of magnitude, and development of a hot absorption spectrum and warm wind. It is similar to Gaia 19ajj, butalso reminiscent in its long brightening time and brightness oscillation near peak to the embedded sources L1634IRS7 and ESO Ha 99.