ATOMIUM: Inner circumstellar envelopes of oxygen-rich AGB stars as revealed by highly excited SiO lines

TL;DR

This study uses high-resolution ALMA observations to analyze highly excited SiO lines in 14 oxygen-rich AGB stars, revealing new maser detections, complex emission structures, and insights into mass loss and circumstellar envelope dynamics.

Contribution

First detection of high-v SiO masers up to v=8 in AGB stars, providing new insights into circumstellar conditions and shock fronts.

Findings

Detected highly excited SiO lines, including first observations of v=8 transitions.

Observed SiO emission in arcs or clumps with velocity gradients.

Found no significant difference in detection rates between stars with different mass-loss rates.

Abstract

Silicon monoxide (SiO) traces the physical conditions and dynamics in the circumstellar envelopes (CSEs) of AGB stars. We present high-resolution ALMA Band 6 observations of highly excited SiO emission in 14 oxygen-rich AGB stars. We cover transitions from v = 0 to v = 8, including first detections of 28SiO v = 3, 4, 8, J = 6-5, 29SiO v = 6, J = 6-5, and 30SiO v = 4, 5, J = 6-5, some of which are masers. The v = 8 transition is the highest v-state observed in an AGB star yet. Masers in v = 0 are detected clearly in V PsA and IRC+10011 and tentatively in T Mic. R Hya exhibits the richest SiO spectrum. SiO J = 6-5 absorption is seen in R Aql, R Hya, S Pav, and T Mic, with features indicative of both infalls and outflows, and tentative detection of 28SiO v = 8, J = 6-5 absorption is found towards S Pav and R Aql. Highly excited SiO emission is often distributed in arcs or clumps with velocity gradients; components in R Hya and U Her align with predicted shock fronts. Detection rates show no significant difference between low and high mass-loss rate stars, although line overlap may affect some intensities. Maser detections appear uncorrelated with pulsation period or phase. The radius enclosing 90 per cent of compact SiO emission shows a tentative correlation with mass-loss rate. These results highlight the role of mass loss and CSE geometry in shaping high-excitation SiO emission.