The class II masers of methanol are associated with the early stages of formation of high-mass stars. Modelling of these dense, dusty environments has demonstrated that pumping by infrared radiation can account for the observed masers. Collisions with other molecules in the ambient gas also play a significant role, but have not been well modelled in the past. Here we examine the effects on the maser models of newly available collision rate coefficients for methanol. The new collision data does not alter which transitions become masers in the models, but does influence their brightness and the conditions under which they switch on and off. At gas temperatures above 100 K the effects are broadly consistent with a reduction in the overall collision cross-section. This means, for example, that a slightly higher gas density than identified previously can account for most of the observed masers in W3(OH). We have also examined the effects of including more excited state energy levels in the models, and find that these play a significant role only at dust temperatures above 300 K. An updated list of class II methanol maser candidates is presented.Comment: 14 pages, 4 figures, Accepted for publication in MNRA

Class II methanol masers are found in close association with OH main‐line masers in many star‐forming regions, where both are believed to flag the early stages in the evolution of a massive star. We have studied the formation of masers in methanol and OH under identical model conditions for the first time. Infrared pumping by radiation from warm dust at temperatures >100 K can account for the known maser lines in both molecules, many of which develop simultaneously under a range of conditions. The masers form most readily in cooler gas (<100 K) of moderately high density (105–108 cm‐3), although higher gas temperatures and/or lower densities are also compatible with maser action. The agreement between the current model (developed for methanol) and the established OH maser trends is very encouraging, and we anticipate that further tuning of the model will further improve such agreement. We find the gas‐phase molecular abundance to be the key determinant of observable maser activity for both molecules. Sources exhibiting both 6668‐MHz methanol and 1665‐MHz OH masers have a typical flux density ratio of 16; our model suggests that this may be a consequence of maser saturation. We find that the 1665‐MHz maser approaches the saturated limit for OH abundances >10−7.3, while the 6668‐MHz maser requires a greater methanol abundance >10−6. OH‐favoured sources are likely to be less abundant in methanol, while methanol‐favoured sources may be less abundant in OH or experiencing warm (>125 K), dense (∼107 cm−3) conditions. These abundance requirements offer the possibility of tying the appearance of masers to the age of the new‐born star via models of gas‐phase chemical evolution following the evaporation of icy grain mantles.

We report the results of a search for class II methanol masers at 37.7, 38.3 and 38.5 GHz towards a sample of 70 high-mass star formation regions. We primarily searched towards regions known to show emission either from the 107 GHz class II methanol maser transition, or from the 6.035 GHz excited OH transition. We detected maser emission from 13 sources in the 37.7 GHz transition, eight of these being new detections. We detected maser emission from three sources in the 38 GHz transitions, one of which is a new detection. We find that 37.7 GHz methanol masers are only associated with the most luminous 6.7 and 12.2 GHz methanol maser sources, which in turn are hypothesised to be the oldest class II methanol sources. We suggest that the 37.7 GHz methanol masers are associated with a brief evolutionary phase (of 1000-4000 years) prior to the cessation of class II methanol maser activity in the associated high-mass star formation region.

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We argue that the periodic variability of Class II methanol masers can be explained by variations of the dust temperature in the accretion disk around proto-binary star with at least one massive component. The dust temperature variations are caused by rotation of hot and dense material of the spiral shock wave in the disk central gap. The aim of this work is to show how different can be the Class II methanol maser brightness in the disk during the M oment of M aximum I llumination by the S piral S hock material (hereafter MMISS) and the M oment when the disk is I lluminated by the S tars O nly (MISO). We used the code CLOUDY (v13.02) to estimate physical conditions in the flat disk in the MISO and the MMISS. Model physical parameters of the disk were then used to estimate the brightness of 6.7, 9.9, 12.1 and 107 GHz masers at different impact parameters p using LVG approximation. It was shown that the strong masers experience considerable brightness increase during the MMISS with respect to MISO. There can happen both flares and dips of the 107 GHz maser brightness under the MMISS conditions, depending on the properties of the system.

Aims. In order to test the nature of an (accretion) disk in the vicinity of Cepheus A HW2, we measured the three-dimensional velocity field of the CH 3 OH maser spots, which are projected within 1000 au of the HW2 object, with an accuracy of the order of 0.1 km s −1 . Methods. We made use of the European VLBI Network (EVN) to image the 6.7 GHz CH 3 OH maser emission towards Cepheus A HW2 with 4.5 milli-arcsecond resolution (3 au). We observed at three epochs spaced by one year between 2013 and 2015. During the last epoch, on mid-march 2015, we benefited from the new deployed Sardinia Radio Telescope. Results. We show that the CH 3 OH velocity vectors lie on a preferential plane for the gas motion with only small deviations of 12• ±9• away from the plane. This plane is oriented at a position angle of 134• east of north, and inclined by 26• with the line-of-sight, closely matching the orientation of the disk-like structure previously reported by Patel et al. (2005). Knowing the orientation of the equatorial plane, we can reconstruct a face-on view of the CH 3 OH gas kinematics onto the plane. CH 3 OH maser emission is detected within a radius of 900 au from HW2, and down to a radius of about 300 au, the latter coincident with the extent of the dust emission at 0.9 mm. The velocity field is dominated by an infall component of about 2 km s −1 down to a radius of 300 au, where a rotational component of 4 km s −1 becomes dominant. We discuss the nature of this velocity field and the implications for the enclosed mass. Conclusions. These findings bring direct support to the interpretation that the high-density gas and dust emission, surrounding Cepheus A HW2, trace an accretion disk.

We present (sub)millimeter imaging at 0. 5 resolution of the massive star-forming region G358.93−0.03 acquired in multiple epochs at 2 and 3 months following the recent flaring of its 6.7 GHz CH 3 OH maser emission. Using SMA and ALMA, we have discovered 14 new Class II CH 3 OH maser lines ranging in frequency from 199 to 361 GHz, which originate mostly from v t =1 torsionally-excited transitions and include one v t =2 transition. The latter detection provides the first observational evidence that Class II maser pumping involves levels in the v t =2 state. The masers are associated with the brightest continuum source (MM1), which hosts a line-rich hot core. The masers present a con-

The ATCA, MERLIN and VLA interferometers were used to measure the absolute positions of 35 6.7-GHz methanol masers to subarcsecond or higher accuracy. Our measurements represent essential preparatory data for Very Long Baseline Interferometry, which can provide accurate parallax and proper motion determinations of the star-forming regions harboring the masers. Our data also allow associations to be established with infrared sources at different wavelengths. Our findings support the view that the 6.7 GHz masers are associated with the earliest phases of high-mass star formation.