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Masers: Precision Probes of Molecular Gas Available online at www.sciencedirect.com ScienceDirect Advances in Space Research 65 (2020) 780–789 www.elsevier.com/locate/asr Masers: Precision probes of molecular gas A.M.S. Richards a,⇑, A. Sobolev b, A. Baudry c, F. Herpin c, L. Decin d, M.D. Gray a S. Etoka a, E.M.L. Humphreys e, W. Vlemmings f a JBCA, School of Physics and Astronomy, University of Manchester, M13 9PL, UK b Astronomical Observatory, Ural Federal University, Ekaterinburg 620083, Russia c Laboratoire d’astrophysique de Bordeaux, Universite´ de Bordeaux, 33615 Pessac, France d Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, 3001 Leuven, Belgium e ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany f Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden Received 18 December 2018; received in revised form 7 May 2019; accepted 15 May 2019 Available online 11 June 2019 Abstract Maser emission from water, methanol, silicon monoxide and other molecules can reach brightness temperatures 1010 K. Such observations can achieve sub-pc precision for discs around black holes or sub-au scale interactions in protostellar discs and the regions where evolved star winds reach escape velocity. Ultra-high resolution maser observations also provide photon statistics, for fundamental physics experiments. RadioAstron has shown the success – and limitations – of cm-wave maser observations on scales 1 mas with sparse baseline coverage. ALMA, APEX and earlier single dish searches have found a wealth of mm and sub-mm masers, some of which probably also attain high brightness temperatures. Masers are ideal for high-resolution observations throughout the radio regime and we need to consider the current lessons for the best observational strategies to meet specific science cases. Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Masers; Stars: AGB; Stars: supergiant; Stars: mass loss; Planetary systems: protoplanetary disks; Star formation 1. Masers: highest spatial and velocity resolution required, and other benefits of observing from above the atmosphere. The exponential nature of stimulated emission means Masers are one of the few natural phenomena which that masers appear spectrally and spatially narrowed as have spatial profiles which are usually genuinely close to well as highly amplified. Other authors in these Proceedings Gaussian, so that fitting 2-D Gaussian components gives including Brunthaler, van Langevelde, Impellizzeri, Mat- very accurate results, effective resolution limited by (beam thews, Moran, Kutz and Shakhvorostova have introduced size)/(S/N) where S/N is the signal to noise ratio. This gives evolved stars and masers and covered applications such as the beamed size of a maser spot, but if the spectral channels astrometry and extra-galactic masers. This contribution are much narrower than the velocity dispersion (thermal or will give references for some recent Galactic maser research gradient) in an emitting cloud, say 0.1 km sÀ1 sampling 1– areas which would benefit from ultra-high resolution. We 2kmsÀ1, the positions of spots in adjacent channels traces will consider the angular resolution and sensitivity the true angular extent of the underlying cloud. This can be used to measure the beaming angle directly (Richards et al., 2011) and distinguish between emitting clumps which are ⇑ approximately spherical and quiescently outflowing (which Corresponding author. are very tightly beamed), v. shocked slabs, which can E-mail address: [email protected] (A.M.S. Richards). https://doi.org/10.1016/j.asr.2019.05.052 0273-1177/Ó 2019 COSPAR. Published by Elsevier Ltd. All rights reserved. A.M.S. Richards et al. / Advances in Space Research 65 (2020) 780–789 781 produce amplified emission of angular size approaching The 22-GHz maser proper motions from solitary stars that of the emission region even in a single channel. are predominantly in accelerating expansion, and individ- A list of telescope acronyms and links is given at the end. ual clouds appear to survive about a year (5 yr – decades) around AGB (RSG), which is puzzling as they are both 2. Mass loss from the stellar surface warmer and denser than the average of the outflow; a frozen-in magnetic field is a possible explanation. Fig. 2 Asymptotic Giant Branch (AGB) and Red Supergiant shows masers observed 5 years apart. In some cases, the (RSG) stars have diameters of 1–10 au or even greater, patterns of individual spots within a feature are recognis- and surface temperatures in the range of about 2200– able, such as the inset region A, which appears to have 3500 K. Matthews covered observations of their radio pho- rotated about 60 anticlockwise during 5 years. The appar- tospheres. These stars play a major role in enriching the ent speed is about 2–15 km sÀ1, the uncertainty mainly due interstellar medium with dust and molecules containing to the alignment of the epochs, but it appears to be at least the products of stellar nucleosynthesis. It is not well- partly supersonic. This could be any or all of bulk rotation, understood how the wind crosses the first few RH (stellar internal turbulence, or a pattern speed due to shocks/Alf- radii). These stars pulsate (with periods from a few months ve`n wave propagation. Turbulence is discussed further in to several years) and the pulsations levitate the surface lay- Section 3. ers, but something else must contribute to overcoming the SiO maser clumps, found within 5RH of AGB stars, À1 pull of the star’s gravity. Within a few RH, sufficiently have individual motions up to 10 km s but recent absorbent oxygen-rich grains would be destroyed by heat- proper motion analysis by Assaf (2018) (using the VLBA, ing; scattering might play a role in using the stellar radia- with a resolution of 0:04 mas) shows that the net outflow tion to drive the wind but requires special grain from R Cas is 0:4kmsÀ1, taking 50–70 yr to cross a dis- composition (Ho¨fner et al., 2016). It has been suggested tance of a few RH (Fig. 1). This slow transport could favour that mass loss and dust formation might be enhanced dust formation. It is likely that the clumps are differenti- above convection cells and that the magnetic field can ated in density and thus temperature, degree of ionisation, shape if not drive the wind (Vlemmings et al., 2005). Out- magnetic and even chemical properties. These are crucial side 5RH, where copious dust has formed, there is ample parameters in determining dust formation. ALMA pro- evidence for radiation pressure on dust driving a radially vides the most precise images of precursors like AlO (e.g. accelerating wind (e.g. review by Ho¨fner and Olofsson, Kamin´ski et al., 2016; Decin et al., 2017) but the relatively 2018), reaching escape velocity a little further out. The low brightness temperature of thermal lines makes it winds are known to be clumpy (e.g. Richards et al. impractical to map them at resolutions below 10–20 mas. (2012) and references therein); in the 22 GHz H2O maser VLTI (e.g. Wittkowski et al., 2007; Karovicova et al., shell from 5–50RH the maser clouds have a filling factor 2013) can give an indication of the distribution of the first 1% but contain 10 to > 50% of the mass. The cloud size molecules to form in a warm layer inside the surface of is roughly proportional to the stellar diameter, and, scaled AGB and RSG stars (known as the MOLsphere) and the back to the stellar surface (assuming radial expansion), is hot dust distribution but only masers can provide sub- consistent with the postulated size of convection cells mas structure and kinematics as matter leaves the surface. (Freytag et al., 2017; Chiavassa et al., 2009). The possibility of observing multiple transitions, along Fig. 1. left R Cas (at 176 pc): 23 epochs of 43 GHz SiO emission superimposed (each series of nearby spots in successive channels forms a feature). right The trajectories of maser features which could be matched in successive epochs as a function of stellar phase. 782 A.M.S. Richards et al. / Advances in Space Research 65 (2020) 780–789 with sophisticated models, now opens up the possibility of in H2O maser beaming was only seen in the thinnest-shelled using masers to measure physical conditions as well. The Miras which also showed other evidence for flares. masing rotational transitions of SiO occur at frequencies Mutiple mm and sub-mm H2O masers have already around J upperÂ43 GHz for the upper level in J = 1–0, been mapped with ALMA (Vlemmings et al., 2017; J = 2–1 etc., shifted slightly according to the isotopic com- Richards et al., 2014), Fig. 3 but not yet using the full position and vibrational state; v = 1 and v = 2 are usually 16-km baselines. These results broadly support the models strongest but v = 0–5 have been detected. Sub-mm SiO of Neufeld and Melnick (1991),Gray et al. (2016) etc. masers have been detected using APEX (De Beck and Over 50 H2O maser transitions are known or predicted Olofsson, 2018; Humphreys et al., 2017) and mapped at between 22 and 960 GHz in bands accessible from Earth, mm wavelengths up to J = 3–2, v = 1 at 129 GHz using with more only detectable from space or at THz frequen- the KVN (Cragg et al., 2005a). All resolved images so far cies. These transitions have energy levels from 200 to show SiO masers are confined to the inner 7RH, as tem- 7000 K, with pumping requirements in number density peratures > 1500 K are required. SiO models and the cor- and temperature ranges 105 < n < 1011 cmÀ3, relations between SiO maser velocity, intensity and stellar 300 < T K < 3000 K.
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