Simultaneous VLBA polarimetric observations of the v =   J = 1-0 and v =1, J = 2-1 SiO maser emission towards VY CMa: maser morphology and pumping

Abstract

This paper presents a component-level comparison of the polarized v=1 J =1-0, v=2 J=1-0 and v=1 J=2-1 SiO maser emission towards the supergiant star VY CMa at milliarcsecond-scale, as observed using the VLBA at λ = 7mm and λ = 3mm. An earlier paper considered overall maser morphology and constraints on SiO maser excitation and pumping derived from these data. The goal of the current paper is to use the measured polarization properties of individual co-spatial components detected across multiple transitions to provide constraints on several competing theories for the transport of polarized maser emission. This approach minimizes the significant effects of spatial blending. We present several diagnostic tests designed to distinguish key features of competing theoretical models for maser polarization. The number of coincident features is limited by sensitivity however, particularly in the v=1 J=2-1 transition at 86 GHz, and deeper observations are needed. Preliminary conclusions based on the current data provide some support for: i) spin-independent solutions for linear polarization; ii) the influence of geometry on the distribution of fractional linear polarization with intensity; and, iii) π/2 rotations in linear polarization position angle arising from transitions across the Van Vleck angle (sin 2 θ = 2/3) between the maser line-of-sight and magnetic field. There is weaker evidence for several enumerated non-Zeeman explanations for circular polarization. The expected 2:1 ratio in circular polarization between J=1-0 and J=2-1 predicted by standard Zeeman theory cannot unfortunately be tested conclusively due to insufficient coincident components.

Laboratory astrophysics and astrochemistry in the Herschel/ALMA era

“…It is therefore expected that the emitting volumes change from one maser to another. This is confirmed by interferometry at low v-states (e. g. Gonidakis et al 2010;Kamiński et al 2013;Richter et al 2013). Such restrictive physical conditions are met through different mechanisms (radiative pumping, collisional pumping, overlaps), as explained in Sect.…”

ALMA sub-mm maser and dust distribution of VY Canis Majoris

“…Exponential maser amplification exaggerates small differences in conditions, so single-epoch data are interpreted with caution. The 658 GHz masers have a complex distribution close to the star, but mostly outside the typical SiO outer rim, taken from Richter et al (2013), as shown in Fig. 6 (although found a more extended bipolar outflow).…”

“…The 658 GHz masers have a complex distribution close to the star, but mostly outside the typical SiO outer rim, taken from Richter et al (2013), as shown in Fig. 6 (although Shinnaga et al (2004) found a more extended bipolar outflow).…”

SiO MASERS AROUND WX PSC MAPPED WITH THE KVN AND VERA ARRAY (KaVA)

“…Consequently, our results strongly imply that the spokelike features of the v=1 and v=2 J=1→0 SiO masers are kinematically coupled. Nevertheless, with only this result, we cannot argue which pumping mechanism is dominant in the v=1 and v=2 J=1→0 SiO masers because boththe radiative and the collisional pumping models predict the strong physical links between these SiO masers (Humphreys et al 2002;Soria-Ruiz et al 2004;Gray et al 2009;Richter et al 2013;Desmurs et al 2014). …”