Protostellar Variability

Today, a new paper has just come out from the JCMT Transient survey, which is using the James Clerk Maxwell Telescope (JCMT) to look for variations of the brightness of protostars at submillimetre wavelengths. This work is interesting because such emission largely comes from vast envelopes of dust that immediately surround the protostars. The dust is heated primarily from the young protostar, and the submillimetre emission is the soft thermal glow of the (still admittedly pretty cold) dust. Hence, changes in the submillimetre brightness of these envelopes are indicative of changes in the brightness of the protostars themselves – indeed this may be the only way to track such changes because the protostars are buried so deeply in their dust envelope they can’t be seen directly at other wavelengths. In the new paper, the team, which includes our Doug Johnstone, looked at a protostar known as HOPS 373, which is actually a binary pair of young protostars that are located in the Orion molecular cloud about 1400 lightyears away. In this study, the team noted an uptick in the submillimetre wavelength brightness by about a factor of 3, then a decline, then a rebrightening that has lasted so far by about a year.

What a mystery! Changes in the brightness of the protostar can arise when the accretion of gas through a protoplanetary disk and onto the protostar changes, an increase in brightness coming from an increase in the accretion rate. This rate could change, for example, if there’s a buildup of gas in the disk that then rapidly drains onto the protostar, releasing a lot of energy. In this paper, the team investigated the situation over a wide range of wavelengths, from near-infrared data taken in a Gemini North fast turnaround project out to high-resolution data from ALMA. The data show a complex picture, with the southwestern protostar likely being responsible for the variability. The ALMA and Gemini/GNIRS data show a small outflow, one that may have been invigorated by recent accretion activity. (Some of that disk gas is rerouted into a jet that drives an outflow.) Interestingly, the GNIRS spectra show a rich collection of features that are indicative of shock heating along the walls of the outflow, as the fast moving gas slams into the walls as it passes alongside them, more than just the scattered light from a suddenly heated disk close to the protostar. These data show how complicated it can be to diagnose what’s going with young, highly obscured stars but lots of data from a diverse set of observatories, preferably taken around the same time, can be very, very welcome. Here’s a link to the arXiv preprint for those who’d like to see this impressive set of data for themselves before the paper is published soon in The Astrophysical Journal. Thanks for the heads-up, Doug! I’m always happy to highlight local results with everyone in these “pages.”