Simultaneous 183 GHz H₂O maser and SiO observations towards evolved stars using APEX SEPIA Band 5

Abstract

Aims. The aim is to investigate the use of 183 GHz H 2 O masers for characterization of the physical conditions and mass loss process in the circumstellar envelopes of evolved stars. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to observe the 183 GHz H 2 O line towards two Red Supergiant (RSG) and three Asymptotic Giant Branch (AGB) stars. Simultaneously, we observed the J=4−3 line for 28 SiO v=0, 1, 2 and 3, and for 29 SiO v=0 and 1. We compared the results with simulations and radiative transfer models for H 2 O and SiO, and examined data for the individual linear orthogonal polarizations. Results. We detected the 183 GHz H 2 O line towards all the stars with peak flux densities >100 Jy, including a new detection from VY CMa. Towards all five targets, the water line had indications of being due to maser emission and had higher peak flux densities than for the SiO lines. The SiO lines appear to originate from both thermal and maser processes. Comparison with simulations and models indicate that 183 GHz maser emission is likely to extend to greater radii in the circumstellar envelopes than SiO maser emission and to similar or greater radii than water masers at 22, 321 and 325 GHz. We speculate that a prominent blue-shifted feature in the W Hya 183 GHz spectrum is amplifying the stellar continuum, and is located at a similar distance from the star as mainline OH maser emission. We note that the coupling of an SiO maser model to a hydrodynamical pulsating model of an AGB star yields qualitatively similar simulated results to the observations. From a comparison of the individual polarizations, we find that the SiO maser linear polarization fraction of several features exceeds the maximum fraction allowed under standard maser assumptions and requires strong anisotropic pumping of the maser transition and strongly saturated maser emission. The low polarization fraction of the H 2 O maser however, fits with the expectation for a non-saturated maser. Conclusions. 183 GHz H 2 O masers can provide strong probes of the mass loss process of evolved stars. Higher angular resolution observations of this line using ALMA Band 5 will enable detailed investigation of the emission location in circumstellar envelopes and can also provide information on magnetic field strength and structure.

THEZA: TeraHertz Exploration and Zooming-in for Astrophysics

“…Due to heavy atmospheric absorption the mm and sub-mm masers are much harder to observe from Earth than the well-studied 22 GHz transition. While bright water masers, at, e.g., 183 GHz and 325 GHz, can be observed by dedicated ground-based telescopes (see [54]), others have only been detected from above the main atmosphere layer by the Kuiper Airborne Observatory and SOFIA, e.g., [46]; [95]; [105] and references therein. There are over a hundred predicted water maser transitions from GHz-THz frequencies (a few tens have been detected), mostly excited by collisional pumping under distinctive combinations of temperature, number density and other parameters [38].…”

Maser polarization through anisotropic pumping

“…The 183 GHz maser is commonly associated with latetype stars (Yates et al 1995), protostars (Waters et al 1980), and it also occurs as a megamaser (Humphreys et al 2016;Pesce et al 2023). Humphreys et al (2017) find detectable differences in flux density between the two orthogonal polarization receivers of the SEPIA Band 5 receiver on APEX (full calibration of the linearly polarized Stokes parameters could not be achieved as the integration was not obtained over the necessary range of parallactic angles), when observing the 183 GHz H 2 O maser toward late-type stars, indicating that the maser is significantly polarized (lower limits of a few percents) while it is likely unsaturated. This is in agreement with our calculations, that indicate that this maser species is likely to be strongly linearly polarized ∼10%, and may therefore be an excellent tracer of the magnetic field morphology.…”

Stellar Evolution Studies Using Masers

“…Proper motion studies reveal outflow, infall and complex non-radial motions [51,52]. Higher-frequency SiO masers are common in evolved stars [e.g., [53][54][55]. Polarization of SiO masers has been studied in multiple publications [e.g., 56], see later for a discussion of derived magnetic field estimates.…”