European VLBI Network imaging of 6.7 GHz methanol masers

Abstract

Context. Methanol masers at 6.7 GHz are well known tracers of high-mass star-forming regions. However, their origin is still not clearly understood. Aims. We aimed to determine the morphology and velocity structure for a large sample of the maser emission with generally lower peak flux densities than those in previous surveys. Methods. Using the European VLBI Network (EVN) we imaged the remaining sources from a sample of sources that were selected from the unbiased survey using the Torun 32 m dish. In this paper we report the results for 17 targets. Together they form a database of a total of 63 source images with high sensitivity (3σ rms = 15−30 mJy beam −1 ), milliarcsecond angular resolution (6−10 mas) and very good spectral resolution (0.09 km s −1 or 0.18 km s −1 ) for detailed studies. Results. We studied in detail the properties of the maser clouds and calculated the mean and median values of the projected size (17.4 ± 1.2 au and 5.5 au, respectively) as well as the FWHM of the line (0.373 ± 0.011 km s −1 and 0.315 km s −1 for the mean and median values, respectively), testing whether it was consistent with Gaussian profile. We also found maser clouds with velocity gradients (71%) that ranged from 0.005 km s −1 au −1 to 0.210 km s −1 au −1 . We tested the kinematic models to explain the observed structures of the 6.7 GHz emission. There were targets where the morphology supported the scenario of a rotating and expanding disk or a bipolar outflow. Comparing the interferometric and single-dish spectra we found that, typically, 50-70% of the flux was missing. This phenomena is not strongly related to the distance of the source. Conclusions. The EVN imaging reveals that in the complete sample of 63 sources the ring-like morphology appeared in 17% of sources, arcs were seen in a further 8%, and the structures were complex in 46% cases. The ultra-compact (UC) H II regions coincide in position in the sky for 13% of the sources. They are related both to extremely high and low luminosity masers from the sample.

The evolutionary status of protostellar clumps hosting class II methanol masers

“…However, VLBI observations have shown ring-like structures at a range of inclination angles (e.g. Moscadelli et al 2010;Bartkiewicz et al 2016;Sanna et al 2017). Therefore it is unlikely that there is a consistent preferred beaming direction within most maser sites and we consequently assume isotropic emission.…”

New evidence for Dicke’s superradiance in the 6.7 GHz methanol spectral line in the interstellar medium

“…There are two other nearby methanol sources, G24.494-0.039 and G24.790+0.083, which together with G24.329+0.144 form an expanding ring-like structure (Caswell & Green 2011). The morphology of this source was studied using the European VLBI Network (EVN) in June 2009 (Bartkiewicz, Szymczak & van Langevelde 2016) and a weak emission at 6.7 GHz line was reported, corresponding to a maser brightness temperature < 2 × 10 7 K. The comparison to detected flux levels from previous observations with the EVLA suggested strong variability in this source. Szymczak et al (2018a) monitored G24.329+0.144 between MJD 55011 to MJD 56386, and detected a synchronized outburst in all spectral features.…”

On the existence of accretion-driven bursts in massive star formation

“…Additionally, a luminosity outburst of the massive (≈ 20 M⊙) young star S255IR-NIRS3 was reported in Fujisawa et al (2015) by means of 6.7 GHz methanol maser emission. This emission line has been discovered by Menten (1991) and constitutes today a well known-tracer of high-mass star forming regions (see Bartkiewicz et al 2016, and references therein). Recent observation of the same object show brightness variations that resemble strongly FU-Orionis-type outbursts (Stecklum et al 2016).…”