Digging into the Interior of Hot Cores with ALMA (DIHCA). VII. Disk candidates around high-mass stars and evidence of anisotropic infall

TL;DR

The study tackles how high-mass stars accrete material by probing the inner disk and envelope kinematics with ALMA at ~230 au resolution across 30 high-mass star-forming fields. Using CH3OH and CH3CN (with HNCO and cis-HCOOH as supplements), the authors identify 32 disk candidates by fitting power-law edges to PV maps, finding Keplerian-like rotation with a median PV slope of about $\alpha\approx-0.7$ and central masses $M_c\sin^2 i$ between 7 and 45 $M_\odot$. The disks are compact ($R\lesssim200$ au) and typically gravitationally unstable (median Toomre $Q\approx0.5$), suggesting ongoing accretion through anisotropic inflows or streamers that feed the disk and resist feedback. The results demonstrate a statistically significant separation of disk and envelope motions in hot cores and support a scenario of anisotropic collapse feeding high-mass stars, with implications for disk stability, multiplicity, and mass growth. Future work will involve detailed 2-D kinematic modeling to disentangle disk versus envelope contributions and higher-sensitivity observations to expand the sample and refine the inference of central masses and stability.

Abstract

We study the kinematics of condensations in 30 fields forming high-mass stars with ALMA at a high-resolution of ~0.08'' on average (~230 au). The presence of disks is important for feeding high-mass stars without feedback halting growth as their masses increase. In the search for velocity gradients resembling rotation that can reveal the presence of disks, we analyze the emission of gas tracers in 49 objects using CH$_3$OH, CH$_3$CN, and tentative detections of HNCO and cis-HCOOH. Most of the velocity distributions show velocity gradients indicative of rotation. We reveal a total of 32 disk candidates, the largest sample to date that has been uniformly analyzed at a few hundred au scales in the high-mass regime. Their position-velocity maps are generally asymmetric with one side brighter than the opposite. We successfully fit a power law to the position-velocity maps of the disk candidates and find indices between -0.5 (Keplerian rotation) and -1 (rotation under specific angular momentum conservation) with a median of -0.7. Under Keplerian rotation assumption, we estimate central masses, uncorrected for inclination, ranging between 7 to 45 M$_\odot$. Excluding outliers, the disk candidates are relatively more compact (<200 au) and less massive (<5 M$_\odot$) than previous results at coarser angular resolution. We calculate an average Toomre-$Q$ parameter and find that most are gravitationally unstable (median of 0.5). We conclude that these observations offer the first opportunity to separate the disk and envelope components of hot cores on a statistically significant sample, and confirm that anisotropic collapse plays an role in feeding high-mass (proto)stars.

Figures (10)

• Figure 1: First order moment maps from the labeled molecule emission (color scale) and continuum emission (contours) of the detected condensations. The contour levels are 5, 10, 20, ... $\times \sigma$ where $\sigma$ is the noise level estimated as the standard deviation from the median absolute deviation of the continuum maps. The black crosses mark the position of the condensations in Table \ref{['tab:props']}, while the pink arrows show the direction of the velocity gradient used to calculate the position-velocity maps in Figure \ref{['fig:pvmaps']}. The green dashed lines show the direction of the (tentative) outflows. The synthesized beam of the continuum (gray ellipse) and the moment maps (green ellipse; see Tables \ref{['tab:obsprops:ch3oh']}-\ref{['tab:obsprops:add']}) are shown in the bottom left corner. The complete figure set (48 images) is available in the online journal.
• Figure 2: Position velocity maps along (left) and orthogonal (right) to the velocity gradient direction for each condensation with edge calculation. The molecular line emission used for the maps is listed in Table \ref{['tab:props']} (see also Figure \ref{['fig:mom1']}). Left: The black dots mark the edge points, and the continuous colored line and dashed red line show the boundless and Keplerian power law fits to the edge points, respectively (see §\ref{['subsec:kinematics']}). The colored lines and labels group sources in those with $\alpha>-0.7$ (purple), $-0.7\ge\alpha\ge-0.8$ (green), $\alpha\le-0.8$ (blue) and outliers ($\alpha<-2$, black). Right: The red dashed lines correspond to a free-fall velocity profile, i.e., $\sqrt{2}$ times the Keplerian power law in the left panel. Note that these are plotted in the four quadrants because the location of the closest half of the disk is unknown along the PV slit. Profiles resembling infall (blue skewed or inverse p-Cygni) or outflows (e.g., Hubble-like expansion) are labeled. The dotted vertical and dashed horizontal gray lines indicate the zero position offset and the systemic velocity (Table \ref{['tab:props']}), respectively. The complete figure set (32 images) is available in the online journal.
• Figure 3: Position velocity maps along and orthogonal to the velocity gradient direction for each condensation where the edge could not be determined. The cyan triangles indicate the position of the velocity extrema determined from the edge points. The dotted vertical and dashed horizontal gray lines indicate the zero position offset and the systemic velocity (Table \ref{['tab:props']}), respectively. The complete figure set (17 images) is available in the online journal.
• Figure 4: Histograms of (a) power law indices $\alpha$ values, (b) condensation masses, (c) condensation radii and (d) Toomre-$Q$ parameter. Outliers with $\alpha<-2$ are not considered in (a), while the outliers for the other parameters are defined in §\ref{['subsec:relations']}. A further outlier source in (b) is located at 40 ${\rm M}_\sun$, while two additional sources in (d) are located at 3.1 and 12 (see Table \ref{['tab:condprops']}). Note that in (a) the total include three sources studied in other lines than CH$_3$OH and CH$_3$CN.
• Figure 5: Correlations between selected source properties: distance $d$, power law index $\alpha$, central source mass $M_c\sin^2i$, disk candidate mass $M_g$, and disk candidate radius $R$. Spearman correlation coefficients and p-values (in parentheses) are calculated excluding outliers (see §\ref{['subsec:relations']} for details).