An Evolved Protostellar System Still Growing Fast

Released: 18th. February, 2022, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan

Outflows and jets in protostars are the most fascinating characteristics of stellar accretion. Jets carry away excess angular momentum from the inner-disk around protostar and allow material to fall into the central core, hence the central core grows up to become a true star like our Sun. The Atacama Large Millimeter/submillimeter Array (ALMA) observations by a team of astronomers led by Postdoc Fellow Somnath Dutta and Research Fellow Chin-Fei Lee in ASIAA have detected the oldest SiO jet from a protostar to date, at an unprecedentedly evolved stage than found by most observations. They observed G205.46-14.56S3 (HOPS 315) dense core, an isolated system located in Orion molecular cloud, as a part of "ALMA Survey of Orion Planck Galactic Cold Clumps (ALMASOP)". The core is evolved to a Class I system and exhibits low-density wide-angle outflow. The high-velocity jet is not only detected in CO but also in SiO molecular transition. The source is confirmed as Class I by comparing sub-mm flux and luminosity with other literature samples.

Young protostars (e.g., early Class 0) have a high rate of mass accretion/ejection, which produce a high density of outflowing material, providing high critical density to detect them with a high-density molecular tracer like SiO. Additionally, lower core luminosity at the early phase can not dissociate molecules via photodissociation, hence jets can be detected with SiO emission. In an evolved phase like Class I, the surrounding material reduces significantly so the high-mass accretion as well as high ejection could not be maintained. Higher luminosity in the evolved phase also dissociates molecules, and the jets are predominantly detected in ionized emission.

As Chin-Fei Lee concludes, "it is quite surprising to see an evolved system is still ejecting heavily. Perhaps, it is still accreting actively from the surroundings, and growing fast."

ALMA SiO map of G205.46-14.56S3 jet, shown in color, with CO contours overplotted for blueshifted (blue) and redshifted (red) lobes. Credit: ALMA (ESO/NAOJ/NRAO)/Dutta et al.

Flux vs bolometric luminosity for Class 0 (red circle) and Class I (black square), for the ALMASOP sources from Dutta et al. (2020). The star mark indicates G205.46-14.56S3 source. The solid black line is the best fit for the Class I and brown line is the best fit for Class 0 sources. Credit: Dutta et al.

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