Terahertz Water Masers: II. Further SOFIA/GREAT Detections toward Circumstellar Outflows, and a Multitransition Analysis

Abstract

Following up on our discovery of terahertz water masers, reported in 2017, we report two further detections of water maser emission at frequencies above 1 THz. Using the GREAT instrument on SOFIA, we have detected emission in the 1.296411 THz $8_{27}-7_{34}$ transition of water toward two additional oxygen-rich evolved stars, omicron Ceti (Mira) and R Crateris, and obtained an upper limit on the 1.296 THz line emission from U Orionis. Toward these three sources, and toward the red supergiant star VY Canis Majorae from which 1.296 THz line emission was reported previously, we have also observed several lower-frequency (sub)millimeter water maser transitions using the APEX 12-m telescope along with the 22 GHz transition using the Effelsberg 100-m telescope. We have used a simple model to analyse the multi-transition data thereby obtained. Adopting, as a prior, independent literature estimates of the mass-loss-rates in these four sources and in W Hydrae, we infer water abundances in a remarkably narrow range: $n({\mathrm{H}}_2\mathrm{O})/n({\mathrm{H}}_2)=1.4-2.5\times 10^{-4}$. For o Cet, VY CMa, and W Hya, the model is successful in predicting the maser line fluxes to within a typical factor $\sim 1.6-3$. For R Crt and U Ori, the model is less successful, with typical line flux predictions lying an order of magnitude above or below the observations; such discrepancies are perhaps unsurprising given the exponential nature of maser amplification.

arXiv comment37 pages, including 10 figures. Accepted for publication in the Astrophysical Journal. Two typos corrected: in two instances the ratio n(H2O)/n(H2) had erroneously been inverted in version 1

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• ATOMIUM: Probing the inner wind of evolved O-rich stars with new, highly excited H${}_2$O and OH lines
• Molecules and Chemistry in Red Supergiants