On the Class II methanol maser periodic variability due to the rotating spiral shocks in the gaps of discs around young binary stars

Abstract

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.

Discovery of four periodic methanol masers and updated light curve for a further one

“…The same difficulty applies to the scenario where periodic variations of the dust temperature are caused by variability in the accretion rate in a binary system (Araya et al 2010). The illumination of the molecular disc by the bow shock hot and dense material of the spiral shock in the circumbinary disc central gap (Parfenov & Sobolev 2014) could explain the periodic maser flare of some part of maser regions, in a particular condition when the maser emission arises in a disc seen edge-on.…”

“…Such widely different patterns of the features cannot be explained by changes in the background free-free emission postulated in the simple CWB model (van der Walt 2011). An alternative explanation proposes a model of the circumbinary accretion disc (Parfenov & Sobolev 2014) in which the pattern of the maser flare follows that of the gas column density along the line between the central star and the maser region. For the specific structure of the disc (Sytov et al 2009), the column density changes with the inclination angle of the binary system, so the flare profile with R rd <1.0 may be generated in the molecular disc seen edge-on (i=90 • ).…”

“…In another model, the periodic maser variation is related to the flux of infrared emission that could be regulated by periodic accretion of material from the circumbinary disc onto a protostar or accretion disc (Araya et al 2010). Recently, Parfenov & Sobolev (2014) proposed that the dust temperature variations in the circumbinary accretion disc are caused by rotation of hot and dense material of the spiral shock wave in the disc central gap. The maser brightness depends on the density of the shocked material along the line between the central star and the maser region.…”

The shortest periodic and flaring flux variability of a methanol maser emission at 6.7 GHz in G 014.23−00.50

“…Another issue noted by van der Walt et al (2016) was that the luminosity of the spiral shock might be too low to play a role as the extra heating source. This issue was predicted from densities of gases in the gap region and of postshocked gases in the spiral shock at least five orders of magnitude lower than that used in the spiral shock model, which were estimated from a hydrogen density at the inner edge of the circumbinary disk assumed by Parfenov & Sobolev (2014). The same issue occurs even in the case of an improved model by replacing with the shock by the CWB (van der .…”

“…Four models have been proposed for interpretations of the periodic flux variability, in the point of view that variations of (multiple) spectral components are synchronized with 1-14 days' delay in some sources, possibly caused by global variation on a central engine: a colliding-wind binary (CWB: van der ; van der Walt 2011), a stellar pulsation (Inayoshi et al 2013;Sanna et al 2015), a circumbinary accretion disk (Araya et al 2010), and a rotation of spiral shocks within a gap region in a circumbinary disk (Parfenov & Sobolev 2014). The first model is based on changes in the flux of seed photons, while the remaining three ones are based on changes in the temperature of dust grains at the masing regions.…”