The Nature of Class I Sources: Periodic Variables in Orion

TL;DR

This paper argues that some Class I sources are low-mass pre-main-sequence stars with extended dusty envelopes, based on their observed infrared variability and modeled envelope properties.

Contribution

It provides an empirical and modeling-based argument that certain Class I sources are low-mass stars with envelopes, challenging traditional views of their evolutionary status.

Findings

Class I sources can have envelopes with modest optical depth at 3.6 μm.

Envelope masses are typically less than 0.1 solar masses.

Periodic infrared variability matches stellar rotation rates.

Abstract

We present a quantitative, empirically based argument that at least some Class I sources are low-mass, 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-main-sequence 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 ${\mu}$m, typically ${\tau_{3.6}}$ ${\approx}$ 0.6. The total envelope mass, based on this limited study, is at most about 0.1 $\text{M}_{\odot}$, 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.