Variability in water masers and possible accretion burst in NGC 2071-IR

Abstract

Although stars are ubiquitous in the Universe, the processes leading to their formation are still poorly understood. Developing a unified theory of star formation is hindered by the fact that stars in the different mass regimes, namely, low (M∗ < 2 M), intermediate (2 M <M∗ > 8 M), and high-mass (M∗ > 8 M) regimes have similar but differing forma-tion processes. Further compounding the challenge is that intermediate-mass protostars are more obscured than their low-mass conterparts and also do not affect their surround-ing environments like high-mass stars, making it difficult to establish a clear criterion for identifying intermediate-mass protostars observationally. One process common to star formation across all mass regimes is accretion, and episodic accretion events known as accretion bursts are reported to drive strong flares in water masers accompanied with an increase in luminosity. Interstellar masers serve as indirect indicators of ongoing star formation as well as probes of the physical conditions such as temperature and density in star-forming regions. In 2019, flaring of the 22 GHz water maser (hereafter H2O maser) in the NGC 2071-IR star forming region was reported by the Maser Monitoring Organisation (M2O). Single-dish spectra of the H2O maser from the Hartebeesthoek 26-m telescope covering a period from 2019 to 2022 were analysed, revealing high variability and a strong flare in November 2019 which reached a peak of 4722 ± 4 Jy. Observations conducted to detect the mainline 6.7 GHz methanol maser towards NGC 2071-IR yielded no detections. This study aims to investigate the variability observed in the H2O maser spectra and verify if the 2019 flare was a result of an accretion burst event. Signatures of accretion bursts such as flaring of multiple maser species and increase in infrared luminosity were probed. Interferometric imaging of (sub)millimetre dust continuum and spectral line (specifically carbon monox-ide (CO)) observations with the Atacama Large Millimetre/submillimetre Array and the Very Large Array, revealed the NGC 2071-IR region is populated with multiple protostars with small spatial separations, driving multiple outflows. Water maser positions from the literature superimposed over the continuum images showed the H2O masers trace disks and outflows. Ks-band observations made during the 2019 flare with Kanata/HONIR telescope revealed a 0.2 mag increase compared to the 2MASS magnitude.