Molecular outflows offer clues to how massive young stars gather gas

Reading stellar growth through expelled gas

A forming star can gather material while an associated jet or wind pushes surrounding molecular gas outward. These molecular outflows therefore offer an indirect way to investigate accretion: the process by which a young star gains mass. They do not, however, measure the gas arriving at the stellar surface directly.

The SQUALO II preprint examines this connection in 13 massive star-forming clumps already selected for evidence of large-scale inward gas motions. The targets span a broad range of luminosity-to-mass ratios and evolutionary conditions. This selection makes them useful for comparing stellar growth with the surrounding gas supply, but they are not an unbiased sample of all massive clumps.

How the observations were analysed

The researchers observed the carbon monoxide CO rotational transition known as J = 2–1 with the Atacama Large Millimeter/submillimeter Array, or ALMA. They combined its 12-metre and 7-metre arrays with Total Power observations to recover both compact and extended emission.

Outflow candidates were identified through spatially coherent emission at velocities distinguishable from the surrounding cloud, exceeding five times the channel noise and plausibly associated with a compact dust-continuum source. Higher-resolution ALMAGAL continuum observations helped identify candidate driving cores in seven targets; all outflow measurements came from the SQUALO CO data.

The team identified 51 lobes belonging to 31 outflow systems in 12 clumps. One clump had no detected molecular outflow. The systems include bipolar structures, single detected lobes and more complex overlapping components. A missing counter-lobe need not mean that an outflow is intrinsically one-sided: sensitivity, cloud confusion and viewing angle can obscure it.

From CO emission to estimated accretion

Gas masses were calculated assuming local thermodynamic equilibrium, a specified CO abundance relative to molecular hydrogen, and fixed corrections for opacity. The authors calculated solutions at excitation temperatures of 20 and 80 kelvin, adopting their average as the fiducial estimate.

They estimated dynamical timescales by dividing projected lobe length by maximum observed velocity offset. Outflow momentum divided by this timescale gives momentum flux: the rate at which momentum is carried outward. Converting that quantity into an accretion estimate requires further assumptions, including a wind speed of 500 kilometres per second and a momentum-transfer efficiency of 0.25.

The authors report typical inferred accretion rates of approximately 0.00001–0.001 solar masses per year, consistent with active massive-star formation. These are approximate, time-averaged estimates, not measurements of instantaneous stellar growth. Combining contributions assigned to each candidate driving source, the study finds that one source accounts for more than 60% of the inferred accretion in ten of the twelve detected clumps.

Which relationships stand out?

For several comparisons, the researchers selected the outflow with the highest inferred accretion rate in each detected clump, yielding 12 independent clump-level measurements. Its momentum flux increases with clump surface density—the amount of mass per projected area—with a Pearson correlation coefficient of 0.627 and p = 0.029. This meets the authors’ nominal significance threshold of p below 0.05, but does not establish that surface density controls accretion.

The suggested decrease in accretion rate with increasing virial parameter, a measure used to compare internal motions with gravity, is not statistically significant: p = 0.201. Likewise, summed outflow-inferred accretion rises only tentatively with previously estimated clump-scale infall, with p = 0.064.

The reported summed accretion estimates remain below the clump infall estimates in every detected target. Their nominal ratios range from about 0.0007 to 0.32. This is not a measured efficiency of gas delivery to stars: the rates describe different spatial scales and timescales, and unresolved sources or temporarily stored gas may contribute to the mismatch.

Maximum observed outflow velocity shows no significant relationship with candidate core mass. A core’s gas reservoir is not the same as its embedded star’s mass, and projection further complicates the comparison.

Why the interpretation remains limited

Representative projected outflows are shorter at higher clump luminosity-to-mass ratios, a nominally significant association with p = 0.028. It does not demonstrate that outflows shorten as clumps age. That ratio also depends on the embedded stellar population, while measured lengths depend on viewing angle, sensitivity and map coverage. Eight lobes reach the mapped boundary.

No inclination corrections enter the main analysis. Quoted temperature-based uncertainties exclude other systematic errors, and using maximum velocity rather than the full gas-velocity distribution affects the derived quantities. Apparent alignments with infrared-dark filaments also cannot prove physical interaction.

The observations establish widespread outflow activity inside infalling clumps. They are consistent with a connection between large-scale gas supply and stellar growth, but do not directly trace material travelling from filaments to individual protostars or distinguish uniquely between competing formation models.