SiO-emitting condensations throughout the envelope of the yellow hypergiant IRC+10420

Abstract

IRC+10420 is a massive (> 20M☉), very luminous (> 〖10〗^6L☉) star that is in the rare phase of evolution from the red supergiant to the luminous blue variable or Wolf-Rayet phase. Previous observations reveal that the circumstellar envelope is rich in molecular gas, and can be detected out to a radius of about 8” = 6.0 X 〖10〗^17 cm. Observations in CO also reveal that the global massloss rate of IRC+10420 has changed dramatically over the last 6000 years, comprising two major episodes of mass loss lasting for about 1000 and 4000 years respectively separated by period of very low mass-loss rate lasting for about 1000 years. Surprising, previous observation in SiO(J = 2 - 1) revealed a ring-like enhancement at a radius of about 1” (7.5X 〖10〗^16 cm) from the star, contrary to the expectation that SiO molecules should be frozen onto dust grains very close to the star (within ～ 〖10〗^16cm). This ring-like enhancement has been attributed to a large-scale shock produced by interactions between faster and slower moving portions of the expanding envelope. In this thesis, we mapped the circumstellar envelope in SiO(J = 1 - 0) to better constrain the physical conditions (gas density, temperature and SiO abundance) in the SiO-emitting gas. We find a similar ring-like enhancement in SiO(J = 1 - 0) but located further out at a radius of about 2” (1.5 X 〖10〗^17 cm), and confirm that the SiO emission extends as far out as the CO envelope. The computed SiO(J = 2-1)/SiO(J = 1-0) line ratio significantly exceeds unity at radius out to about the location of the ring-like enhancement (2”), and drops to a value of about unity beyond this radius. From a one-dimensional non-local thermodynamic equilibrium model, we explore the physical conditions that can reproduce the observed brightness temperatures in both SiO(J = 10) and SiO(J = 2-1) as well as their line ratio as a function of radius. The SiO-emitting gas is required to have a density that is much higher (from a factor of a few to about two orders of magnitude) than has been inferred for the CO-emitting gas at the same radii. The required surface filling factor of the SiO-emitting gas depends on their unknown gas-phase SiO abundance; for an abundance of ～〖10〗^(-5), as inferred just above the photospheres of lowmass evolved stars, the surface filling factor of these condensations range from ～0.001 to ～0.1. Thus, the SiO emission from the envelope of IRC+10420 most likely originates from dense condensations that are immersed in more diffuse gas that produces the bulk of the observed CO emission. We reason that the SiO-emitting condensations correspond to the dust clumps detected in reflected light with the Hubble Space Telescope. These dust clumps are distributed from near the star out to a radius of 2”, spanning the same extent as the peaks of SiO- (and CO-) emitting envelope. We show that these dust clumps are expanding in every direction away from the stars at a velocity that is significantly higher than the CO-emitting gas, and anticipate that shocks thus generated heats up the dust clumps to release SiO into the gas phase.