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

This study uses ALMA Band 9 CO($J$=6--5) observations of MC 27/L1521F to map warm, dense gas around a very young protostar. The authors identify a ~1000 au off-centered ring at velocities near the systemic value and analyze its excitation and kinematics, arguing that magnetic-flux redistribution combined with localized shocks best explains the structure rather than a simple outflow geometry. They infer gas with $n({\rm H_2})\sim10^{5}$--$10^{6}$ cm$^{-3}$ and $T_{kin}\gtrsim20$ K, and find velocity patterns consistent with expansion along the ring's major axis and slight contraction along the minor axis, compatible with an interchange-instability–driven, magnetically regulated scenario. The results highlight high-$J$ CO as a powerful diagnostic for warm, dense gas in the earliest stages of star formation and suggest magnetic-field processes play a key role in shaping protostellar envelopes.

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.

Figures (5)

• Figure 1: Top: Mean CO($J$=6--5) spectra extracted from a circular aperture of radius $5\arcsec$ centered at (ICRS) $\alpha=04^{\rm h}28^{\rm m}39\fs97$, $\delta=+26^\circ51^\prime21\farcs5$. The systemic velocity of the core, $V_{\rm sys}=6.5$ km s$^{-1}$, is indicated by the vertical dotted line. Bottom: Mean CO($J$=3--2) spectrum obtained with the previous study Tokuda_2018, averaged within a circular aperture of radius $10\arcsec$ centered at the same position. The black histogram shows the Total Power (TP) spectrum, while the blue histogram shows the ALMA 12 m array spectrum. Both spectra are displayed at a velocity resolution of 0.85 km s$^{-1}$.
• Figure 2: Velocity-binned intensity maps comparing the previously obtained CO($J$=3--2) and HCO$^{+}$($J$=3--2) emission with new ALMA Band 9 CO($J$=6--5) and continuum data. (a--c) CO($J$=3--2) intensity maps Tokuda_2018 shown in color, with white contours showing the CO($J$=6--5) averaged over the same velocity intervals. The velocity ranges are indicated at the top of each panel. The middle-velocity panel (b) corresponds to the range where the CO($J$=3--2) emission is strongly suppressed by self-absorption; the blueshifted and redshifted components relative to this interval are shown in panels (a) and (c), respectively. The CO($J$=6--5) contour levels are [0.5, 1.0, 2.0, 3.0, 4.0] K. (d--f) CO($J$=6--5) intensity maps are shown in color. White contours show the HCO$^{+}$($J$=3--2) emission Tokuda_2014, while yellow contours in panel (e) indicate the ALMA Band 9 continuum emission. The velocity ranges are shown at the top of each panel; the intermediate velocity interval in panel (e) is defined based on the HCO$^{+}$ data. The HCO$^{+}$ contour levels are [0.5, 2.0, 3.0] K, and the Band 9 continuum contour levels are [11, 22, 33] mJy beam$^{-1}$. In all panels, the cross marks the protostellar position. The synthesized beam in the color-scale data is shown in the lower-left corner of the relevant panels. The reduced ALMA Band 9 continuum 2D image and the CO($J$=6--5) data cube used to generate the results shown here are available as Data behind the Figure in the FITS format.
• Figure 3: Channel maps of the CO($J$=6--5) emission at the native spectral resolution ($\Delta v=0.12~{\rm km~s^{-1}}$), shown as a $3\times3$ panel. All panels are displayed in brightness temperature units (K). The central velocity channel is shown in the upper right corners of each panel in km s$^{-1}$ unit. The beam size of $1\farcs8 \times 1\farcs3$ (P.A.$=-6\fdg1$) is shown in the ellipse at the lower left corner in the lower left panel. The red cross shows the position of the protostar.
• Figure 4: (a) Integrated-intensity (moment 0) map of CO($J$=6--5) integrated over $V_{\rm LSRK}=4.4$--$8.6~{\rm km~s^{-1}}$. The beam size ellipse is shown in the lower left corner. The cross mark shows the protostar position. (b) Intensity-weighted mean velocity (moment 1) map computed over the same velocity range. (c) Same as panel (a), but for the integrated velocity range of $V_{\rm LSRK}=6.3$--$6.7~{\rm km~s^{-1}}$. (d) Same as panel (b), but the velocity range as same as in panel (c).
• Figure 5: (a) The background image is constructed from Spitzer bands with $4.5~\mu$m shown in green and $3.6~\mu$m shown in blue Bourke06. Orange color shows the integrated intensity (moment 0) of the CO($J$=6--5) emission integrated over $V_{\rm LSRK}=6.2$--$6.9~{\rm km~s^{-1}}$. (b) The colored ring indicates the qualitative line-of-sight velocity pattern, with red/magenta and blue/cyan denoting redshifted and blueshifted emission, respectively. The star marks the position of the protostar, which is associated with a spike-like protostellar disk structure Tokuda_2024. The arrows highlight the characteristic sense of motion discussed in Section \ref{['subsec:ring_kin']}. The black solid lines indicate the adopted orientation of the magnetic-field direction on the plane of the sky, guided by single-dish dust polarization measurements Fukaya23.