Tracking Protostellar Variability in Massive Protoclusters with ALMA: I. Insights from QUARKS and MaMMOtH

Millimeter/submillimeter variability is often attributed to dynamical disk-mediated accretion, yet detection is limited to low-mass protostars in nearby clouds. Recent observations have also revealed significant (sub)millimeter variability in high-mass protostars, but the confirmed cases are scarce and lack systematic monitoring. In this work, we analyzed multi-epoch Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 (1.3 mm) continuum observations of 22 massive protoclusters, with epoch separations ranging from a few hours to more than two years, while achieving a consistent angular resolution of approximately 0.3 arcsec. These data allow us to track variability of protostars across a broader mass range and in an environment markedly different from nearby clouds. Using a custom processing pipeline for data reduction, image alignment, and relative flux calibration, we achieve high-precision flux measurements and, for the first time, investigate millimeter variability in massive protoclusters based on interferometric data in a statistical manner. Applying the astrodendro algorithm, we identified 383 condensations and tracked their variations in peak intensities. Standard deviation analysis and difference maps reveal five variable sources, corresponding to a lower limit of 1.3% on the variable fraction. Among these, I13111-6228 stands out as it hosts a hypercompact H II region that exhibits a 68% increase in continuum peak intensity over one year, with an uncertainty of 2%. This supports the burst-mode accretion picture in massive star formation as a viable route for the formation of massive stars.

💡 Research Summary

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This paper presents the first systematic interferometric study of (sub)millimeter continuum variability in massive protoclusters using multi‑epoch ALMA Band 6 (1.3 mm) observations. The authors assembled a sample of 22 distant protoclusters drawn from two complementary ALMA surveys – QUARKS and MaMMOtH – each targeting regions associated with massive star formation and, in the case of MaMMOtH, with 6.7 GHz methanol masers. All data were re‑processed to a common angular resolution of ~0.3″, corresponding to linear scales of 400–1700 AU for the distances (1.4–11.6 kpc) of the sources.

The data‑reduction pipeline consists of (1) standard CASA calibration, (2) identification of line‑free channels and construction of continuum images, (3) self‑calibration to improve signal‑to‑noise, and (4) smoothing of every epoch to the same synthesized beam to guarantee that any flux change is not caused by resolution differences. Source extraction was performed with the astrodendro algorithm, which identified 383 compact condensations (cores, disks, hyper‑compact H II regions, etc.) above a 3σ threshold. A unified mask was applied across all epochs so that each condensation’s peak intensity could be measured consistently.

To achieve high‑precision relative photometry, the authors implemented a relative flux‑calibration step. They selected a set of apparently non‑variable condensations (those with <0.5 % variation) as reference, derived scaling factors for each epoch, and applied these to all sources. This procedure reduces systematic calibration errors to the few‑percent level, enabling detection of genuine variability.

Variability was assessed using two independent methods. First, the standard deviation of each source’s peak intensity time series was computed; sources whose σ exceeded the sample‑wide median (≈2 %) by a large margin were flagged as candidates. Second, difference maps between epochs were generated; any residual emission exceeding 5σ of the image noise was considered a sign of variability. Only sources identified by both techniques were retained as robust variables. Five condensations satisfied these criteria, implying a lower limit of 1.3 % (5/383) on the variable fraction in massive protoclusters.

The most striking variable is I13111‑6228, which hosts a hyper‑compact H II region. Its continuum peak intensity increased by 68 % over a one‑year interval, with an uncertainty of only 2 %. This dramatic brightening is comparable to the well‑known outbursts of massive young stellar objects such as S255IR‑NIRS3 and NGC 6334I‑MM1, which showed multi‑wavelength luminosity increases of factors 5–16. The authors interpret the I13111‑6228 event as a burst‑mode accretion episode, consistent with theoretical models where gravitational instability in massive disks produces clumps that migrate inward and trigger episodic accretion onto the central protostar. The other four variables display more modest changes (15–30 %) and are associated with dense cores or young disks, suggesting a spectrum of accretion variability amplitudes.

The paper discusses several limitations. The temporal sampling is irregular, ranging from a few hours to over two years, which hampers the ability to characterize variability timescales and to distinguish short‑lived flares from longer bursts. The relative calibration relies on the assumption that the reference condensations are truly stable; any hidden variability among them could bias the scaling factors. The 0.3″ resolution, while impressive for distant regions, cannot fully resolve structures below ~400 AU, potentially blending multiple protostars into a single dendrogram leaf and diluting variability signals. Moreover, the analysis focuses on peak intensities; integrated fluxes and morphological changes are not explored.

Future work outlined by the authors includes (1) higher‑resolution ALMA observations (≤0.05″) to isolate individual protostars, (2) longer‑term monitoring campaigns extending over several years to capture the full duty cycle of burst events, (3) coordinated multi‑wavelength campaigns (infrared, radio, maser monitoring) to link millimeter variability with changes in the far‑infrared radiation field and maser activity, and (4) comparison with numerical simulations of massive disk fragmentation and clump migration to quantify the expected burst frequency and amplitude.

In summary, this study demonstrates that millimeter‑wave variability, a hallmark of episodic accretion in low‑mass protostars, also occurs in massive protoclusters, albeit at a lower detectable fraction given current sensitivities and sampling. The detection of a 68 % brightening in a hyper‑compact H II region provides compelling observational support for burst‑mode accretion as a viable pathway for building up the mass of high‑mass stars. The methodology—precise relative calibration, dendrogram‑based source extraction, and dual statistical tests—sets a benchmark for future variability surveys with ALMA and other interferometers.