Work

A breath of fresh air: Oxygen enhancement from silicate sublimation during early protostellar evolution

Year: 2026

Type: article

Abstract: Recently, an analog to the t=0 moment of our Solar System's meteoritic record was identified for the first time around the Class I protostar HOPS-315, using the James Webb Space Telescope (JWST). Crystalline silicates were found to recondense from SiO gas that cools as it advects upward from a hot thermostat region in the inner midplane. A depletion of iron and silicon in the protostellar jet suggests this t=0 moment may be associated with the formation of the first planetesimals in this disk, similar to the association of refractory inclusions and iron-meteorite parent bodies in the Solar System. In this work, we explored whether HOPS-68, a Class I protostar with similar properties and system geometries, is engaged in such refractory recondensation as well. A detection of cooler gas-phase SiO in absorption above a hot disk midplane would allow us to achieve this goal, thanks to a previous detection of crystalline silicates. Spitzer HOPS-68 was observed with the JWST Mid-Infrared Instrument and Near Infrared Spectrograph, showing multiple bands of molecular absorption similar to HOPS-315. We used local thermal equilibrium slab models to measure the temperature, column density, velocity, and radial location of these bands. The molecular absorption bands of gaseous SiO, trace a warm molecular inversion layer (∼ 300 - 500, ) at mid-altitudes above the hot disk midplane from 0.3 - 16 AU, which is consistently blueshifted with respect to the system velocity by 33-53 rm and rm ^ 13 CO K -1 . Atacama Large Millimeter/submillimeter Array (ALMA) observations of HOPS-68 kinematically resolve an SiO-rich jet and wind. We confirm that HOPS-68, like HOPS-315, is undergoing an epoch analogous to the t=0 moment in the Solar System's planet formation timeline. The detected warm molecular layer traces the base of a disk wind, which carries sublimated material from the thermostat region near 2 AU up into the outer disk. As for HOPS-315, the molecular layer in the inner few AU of HOPS-68 is enhanced in bulk oxygen as a result of this sublimation. Since HOPS-68 is less evolved than HOPS-315, this may indicate a prolonged epoch in protostellar evolution in which such oxygen enhancement occurs, perhaps associated with the expected radial drift of icy pebbles, supplemented by the drift of ice-free silicates into the thermostat region. We also find that the outer disk gas of HOPS-68, unlike HOPS-315, is unexpectedly carbon-rich, which will be discussed in detail in a companion paper.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202661414