The Nature of Class I Sources: Periodic Variables in Orion

Abstract

We present a quantitative, empirically based argument that at least some Class I sources are lowmass, pre-main-sequence stars surrounded by spatially extended envelopes of dusty gas. The source luminosity arises principally from stellar gravitational contraction, as in optically visible pre-mainsequence stars that lack such envelopes. We base our argument on the fact that some Class I sources in Orion and other star-forming regions have been observed by Spitzer to be periodic variables in the mid-infrared, and with periods consistent with T Tauri rotation rates. Using a radiative transfer code, we construct a variety of dust envelopes surrounding rotating, spotted stars, to see if an envelope that produces a Class I SED at least broadly matches the observed modulations in luminosity. Acceptable envelopes can either be spherical or flattened, and may or may not have polar cavities. The key requirement is that they have a modest equatorial optical depth at the Spitzer waveband of 3.6 µm, typically τ 3.6 ≈ 0.6. The total envelope mass, based on this limited study, is at most about 0.1 M ⊙ , less than a typical stellar mass. Future studies should focus on the dynamics of the envelope, to determine whether material is actually falling onto the circumstellar disk.

A protostellar system fed by a streamer of 10,500 au length

“…In fact, this stellar system has undergone at least one accretion burst of at least a factor of 10x in the last 10,000 yrs [19,20] . This further suggests that accretion events from the outer envelope (>3,000 au) [32] could drive variability of young sources [33] .…”