Evidence of Accretion Burst: The Viscously Heated Inner Disk of the Embedded Protostar IRAS 16316-1540

TL;DR

This study presents high-resolution spectroscopic evidence indicating that the embedded protostar IRAS 16316-1540 is undergoing an accretion outburst, with a viscously heated inner disk detectable through detailed spectral line profiles.

Contribution

The paper demonstrates that high-resolution near-IR spectroscopy can distinguish outburst candidates by revealing disk-origin features, providing new evidence for ongoing accretion bursts in embedded protostars.

Findings

High-resolution spectra show boxy/double-peaked absorption lines indicating a viscously heated disk.

The projected disk rotation velocity is approximately 41 km/s, corresponding to about 0.1 AU.

IRAS 16316-1540 is identified as an ongoing outburst candidate based on spectral evidence.

Abstract

Outbursts of young stellar objects occur when the mass accretion rate suddenly increases. However, such outbursts are difficult to detect for deeply embedded protostars due to their thick envelope and the rarity of outbursts. The near-IR spectroscopy is a useful tool to identify ongoing outburst candidates by the characteristic absorption features that indicate a disk origin. However, without high-resolution spectroscopy, the spectra of outburst candidates can be confused with the late-type stars since they have similar spectral features. For the protostar IRAS 16316-1540, the near-IR spectrum has line equivalent widths that are consistent with M-dwarf photospheres. However, our high-resolution IGRINS spectra reveal that the absorption lines have boxy and/or double-peaked profiles, as expected from a disk and not the star. The continuum emission source is likely the hot, optically thick disk, heated by viscous accretion. The projected disk rotation velocity of 41$\pm$5 km s$^{-1}$ corresponds to $\sim 0.1$ AU. Based on the result, we suggest IRAS 16316-1540 as an ongoing outburst candidate. Viscous heating of disks is usually interpreted as evidence for ongoing bursts, which may be more common than previously estimated from low-resolution near-IR spectra.