ALMA Band 9 CO(6--5) Reveals a Warm Ring Structure Associated with the Embedded Protostar in the Cold Dense Core MC 27/L1521F

TL;DR

ALMA Band 9 CO(6-5) observations reveal a warm, dense ring structure around a protostar, providing new insights into early star formation processes through high-J CO line analysis.

Contribution

First high-resolution detection of a warm, dense ring in a cold core using high-J CO lines, highlighting the role of shock heating in early protostellar evolution.

Findings

Detected a 1000 au ring structure not visible in low-J CO data.

CO(6-5) emission indicates warm (≥20 K), dense (≥10^5 cm^-3) gas.

Suggests shock heating linked to magnetic interactions in early star formation.

Abstract

Infall and outflows, coupled with magnetic fields, rapidly structure the gas around newborn protostars. Shocks from interacting components encode the temperature and density distribution, offering a direct probe of the earliest evolution history. However, interferometric observations characterizing warm envelopes using high-excitation lines remain scarce. We present ALMA Band 9 observations of the Taurus dense core MC 27/L1521F, which hosts a Class 0 protostar, targeting the CO($J$=6-5) line at an angular resolution of $\sim$2\arcsec\ ($\approx$300 au). We detect an off-centered ring-like structure with a diameter of $\sim$1000 au that was not identifiable in previous low-$J$ CO data, where emission close to the systemic velocity is strongly affected by optical depth. The ring shows a typical peak brightness temperature of $\sim$3 K at our resolution. Excitation considerations indicate that the detected CO($J$=6-5) emission likely arises from relatively warm ($T \gtrsim 20$ K) and dense ($n({\rm H_2}) \gtrsim 10^{5}$ cm$^{-3}$) gas embedded within the surrounding cold, dense core. The morphology and kinematics suggest an energetic and localized shock-heating event, potentially linked to dynamical gas--magnetic-field interactions in the earliest protostellar phase. Our results demonstrate that high-$J$ CO observations provide a powerful new window on warm and dense gas components, enabling a more direct view of the physical processes operating at the onset of star formation.