LkH$\alpha$ 225 (V1318 Cyg) South in Outburst

TL;DR

LkHa 225 South is an episodically brightening young stellar object with periodic brightness variations, spectral features indicating accretion outbursts, and similarities to FU Ori stars and other outburst sources.

Contribution

This study provides detailed optical and near-infrared monitoring, spectral modeling, and analysis of LkHa 225 South's outburst behavior and accretion processes, highlighting its similarities to known FU Ori and outburst objects.

Findings

Periodic brightness oscillations every 43 days

Spectral features consistent with episodic accretion outbursts

Presence of strong wind/outflow signatures

Abstract

Magakian et al. (2019) called attention to the current bright state of LkHa 225 South, which over the past two decades has changed from $>20^m$ to $<13^m$. We present recent optical photometric monitoring that shows colorless, non-sinusoidal, periodic brightness variations. The oscillations occur every 43 days, and have amplitude $\sim$0.7 mag with some variation among cycles. We also present new flux-calibrated optical and near-infrared spectroscopy, which we model in terms of a keplerian disk. Additional high dispersion spectra demonstrate the similarity of the absorption line pattern to some categories of "mixed temperature" accretion outburst objects. At blue wavelengths, LkHa 225 South has a pure absorption spectrum and is a good spectral match to the FU Ori stars V1515 Cyg and V1057 Cyg. At red optical and infrared wavelengths, however, the spectrum is more similar to Gaia 19ajj, showing emission in TiO, CO, and metals. Sr II lines indicate a low surface gravity atmosphere. There are also signatures of a strong wind/outflow. LkHa 225 South was moderately bright in early 1950's as well as in late 1980's, with evidence for deep fades during intervening epochs. The body of evidence suggests that LkHa225 South is another case of a source with episodically enhanced accretion that causes brightening by orders of magnitude, and development of a hot absorption spectrum and warm wind. It is similar to Gaia 19ajj, but also reminiscent in its long brightening time and brightness oscillation near peak, to the embedded sources L1634 IRS7 and ESO Ha 99.