Evidence for a bloated massive protostar in IRAS 20126+4104

INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

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Received: 5 March 2026
Accepted: 13 April 2026

Abstract

Context. Variability is a well-known phenomenon in low-mass young stellar objects, but in recent years the monitoring of methanol masers and IR continuum emission has permitted the detection of both burst-like episodes and periodic variations in high-mass (proto)stars. Multi-epoch studies on large samples of these objects have become possible thanks to the NEOWISE database, which surveyed the sky in the mid-IR for about a decade.

Aims. Our goal is to analyse mid-IR emission from the well-studied massive protostar IRAS 20126+4104 and confirm the hypothesis that such emission is periodic, as proposed in previous studies.

Methods. We used the NEOWISE, ALLWISE, and Spitzer databases to obtain 24 images of 3.4 µm IRAS 20126+4104 emission spanning 19 years, with ~6-month sampling over a decade. With these data we created a light curve for each lobe of the bipolar nebulosity and outflow associated with the protostar.

Results. Our results confirm that the IR emission from IRAS 20126+4104 varies regularly with a period of ~6.8 yr. The period is the same for both lobes, but their emissions are anti-correlated with a phase difference of ~2.5 yr. This variation is consistent with that found in previous studies for the 6 GHz CH₃OH masers and the near-IR emission from the lobes.

Conclusions. After discussing four possible ‘clocks’ that could determine the observed periodicity, we rule out all but a model involving stellar rotation with a spot obscuring ~20% of the stellar surface. The long rotation period implies that the 12 M_⊙ protostar is bloated, with a radius of ~200 R_⊙.