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

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.

💡 Research Summary

This paper presents high‑resolution ALMA Band 9 observations of the Taurus dense core MC 27/L1521F, which harbors a very low‑luminosity Class 0 protostar (MMS‑1). By targeting the CO J = 6–5 transition at a frequency of 691 GHz with an angular resolution of ~2″ (≈300 au), the authors reveal a warm, dense gas component that has been invisible in previous low‑J CO studies due to severe optical depth and self‑absorption near the systemic velocity (V_sys ≈ 6.5 km s⁻¹).

The CO J = 6–5 data, obtained with the ACA 7‑m array, achieve an rms noise of 0.14 K per 0.12 km s⁻¹ channel and a synthesized beam of 1.8″ × 1.3″. Comparison with single‑dish CSO spectra indicates that missing flux in the interferometric data is modest (≤ 30 % of the total line flux), confirming that the main morphological features are robust.

A striking result is the detection of an off‑centered, roughly circular ring of CO J = 6–5 emission with a diameter of ~1000 au, centered slightly south‑west of the protostar. The ring is most prominent in velocity channels close to the systemic velocity (6.3–6.7 km s⁻¹) and exhibits a peak brightness temperature of ~3 K at the native resolution. The ring’s limb‑brightened appearance, together with faint interior emission, suggests that the line is optically thin and that the gas temperature is modestly elevated relative to the surrounding cold core (≈ 10 K).

Kinematic analysis using moment 0 and moment 1 maps shows a clear velocity gradient across the ring: the southern side is blueshifted (≈ 5 km s⁻¹) while the northern side is redshifted (≈ 7 km s⁻¹). Moreover, the eastern portion of the ring is systematically more redshifted than the western side, indicating an asymmetric velocity field that cannot be explained by simple spherical expansion. This asymmetry, together with the ring’s off‑centered position, points to a localized dynamical event rather than a globally symmetric outflow or infall.

The authors estimate the physical conditions of the ring by comparing its emission to that of higher‑density tracers such as H¹³CO⁺ J = 3–2, which trace gas at n(H₂) ≈ 10⁶–10⁷ cm⁻³ in nearby condensations (MMS‑2, MMS‑3). Given that the critical density of CO J = 6–5 is ≈ 10⁵–10⁶ cm⁻³ for kinetic temperatures of 10–100 K, the ring likely has n(H₂) ≈ 10⁵–10⁶ cm⁻³ and a kinetic temperature T ≳ 20 K. These values are significantly higher than the bulk of the core but lower than the compact condensations, placing the ring in an intermediate regime of warm, moderately dense gas.

Two plausible mechanisms are discussed for the origin of the ring. First, a nascent protostellar jet or wind could be colliding with the dense material surrounding MMS‑2, generating a localized shock that heats the gas and creates the observed ring‑like morphology. Second, magnetic‑field–gas interactions, such as rapid magnetic flux transport or reconnection events, could produce a shock front that propagates asymmetrically, consistent with the observed velocity gradient. Both scenarios are compatible with the presence of an energetic, localized heating event inferred from the data.

The detection of the CO J = 6–5 ring demonstrates the power of high‑J CO observations to probe warm, dense gas in the earliest stages of star formation, where low‑J lines fail due to optical depth effects. The study also highlights that even in a core previously thought to be uniformly cold (≈ 10 K), small‑scale dynamical processes can create pockets of significantly warmer gas.

Future work suggested by the authors includes observations of even higher‑J CO transitions (e.g., J = 7–6, 8–7) to better constrain the temperature structure, as well as polarized dust or molecular line observations to map the magnetic field geometry. Combining such data with detailed magnetohydrodynamic simulations will enable a more quantitative assessment of the relative roles of shocks, outflows, and magnetic fields in shaping the immediate environment of the youngest protostars.