Second-epoch ALMA Observations of 321 GHz Water Maser Emission in NGC 4945 and the Circinus Galaxy

We present the results of second-epoch ALMA observations of 321 GHz H2O emission toward two nearby active galactic nuclei, NGC 4945 and the Circinus galaxy, together with Tidbinbilla 70 m monitoring of their 22 GHz H2O masers. The two-epoch ALMA observations show that the strengths of the 321 GHz emission are variable by a factor of at least a few, confirming a maser origin. In the second epoch, 321 GHz maser emission from NGC 4945 was not detected, while for the Circinus galaxy the flux density significantly increased and the velocity gradient and dispersion have been measured. With the velocity gradient spanning ∼110 km s−1, we calculate the disk radius to be ∼28 pc, assuming disk rotation around the nucleus. We also estimate the dynamical mass within the central 28 pc to be 4.3 × 108 M ☉, which is significantly larger than the larger-scale dynamical mass, suggesting the velocity gradient does not trace circular motions on that scale. The overall direction of the velocity gradient and velocity range of the blueshifted features are largely consistent with those of the 22 GHz maser emission in a thin disk with smaller radii of 0.1–0.4 pc and molecular outflows within ∼1 pc from the central engine of the galaxy, implying that the 321 GHz masers could trace part of the circumnuclear disk or the nuclear outflows.

VLA Observations of Nine Extended Green Objects in the Milky Way: Ubiquitous Weak, Compact Continuum Emission, and Multi-epoch Emission from Methanol, Water, and Ammonia Masers

We have observed a sample of nine Extended Green Objects (EGOs) at 1.3 and 5 cm with the Very Large Array (VLA) with subarcsecond resolution and ∼7–14 μJy beam−1-sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find an EGO-associated continu um emission—within 1″ of the extended 4.5 μm emission—in every field, which is typically faint (order 101–102 μJy) and compact (unresolved at 0″.3–0″.5). The derived spectral indices of our 36 total detections are consistent with a wide array of physical processes, including both non-thermal (19% of detections) and thermal free–free processes (e.g., ionized jets and compact H ii regions, 78% of sample) and warm dust (1 source). We also find an EGO-associated 6.7 GHz CH3OH and 22 GHz H2O maser emission in 100% of the sample and a NH3 (3,3) masers in ∼45%; we do not detect any NH3 (6,6) masers at ∼5.6 mJy beam−1 sensitivity. We find statistically-significant correlations between L radio and L bol at two physical scales and three frequencies, consistent with thermal emission from ionized jets, but no correlation between L H 2 O and L radio for our sample. From these data, we conclude that EGOs likely host multiple different centimeter continuum-producing processes simultaneously. Additionally, at our ∼1000 au resolution, we find that all EGOs except G18.89−0.47 contain 1 ∼ 2 massive sources based on the presence of CH3OH maser groups, which is consistent with our previous work suggesting that these are typical massive protoclusters, in which only one to a few of the young stellar objects are massive.

Maser emission from the CO envelope of the asymptotic giant branch star W Hydrae

Context. Observation of CO emission around asymptotic giant branch (AGB) stars is the primary method to determine gas mass-loss rates. While radiative transfer models have shown that molecular levels of CO can become mildly inverted, causing maser emission, CO maser emission has yet to be confirmed observationally. Aims. High-resolution observations of the CO emission around AGB stars now have the brightness temperature sensitivity to detect possible weak CO maser emission. Methods. We used high angular resolution observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the small-scale structure of CO J = 3−2 emission around the oxygen-rich AGB star W Hya. Results. We find CO maser emission amplifying the stellar continuum with an optical depth τ ≈−0.55. The maser predominantly amplifies the limb of the star because CO J = 3−2 absorption from the extended stellar atmosphere is strongest towards the centre of the star. Conclusions. The CO maser velocity corresponds to a previously observed variable component of high-frequency H2O masers and with the OH maser that was identified as the amplified stellar image. This implies that the maser originates beyond the acceleration region and constrains the velocity profile since we find the population inversion primarily in the inner circumstellar envelope. We find that inversion can be explained by the radiation field at 4.6 μm and that the existence of CO maser emission is consistent with the estimated mass-loss rates for W Hya. However, the pumping mechanism requires a complex interplay between absorption and emission lines in the extended atmosphere. Excess from dust in the circumstellar envelope of W Hya is not sufficient to contribute significantly to the required radiation field at 4.6 μm. The interplay between molecular lines that cause the pumping can be constrained by future multi-level CO observations.

Identifying circumstellar dust around oxygen-rich mira variables with maser emission via continuum elimination

Stars between about 0.8 and 8 times the mass of the Sun will eventually evolve, becoming asymptotic giant branch (AGB) stars, where they pulsate and eject mass from their atmospheres, forming dust shells in the space around them. Evolved low- and intermediate-mass stars with carbon-to-oxygen ratios (C/O) below unity are known as oxygen-rich stars. O-rich stars are surrounded by dust shells containing mineral species dominated by silicate dust grains. In this dissertation, I examine whether dust grains around evolved, oxygen-rich AGB stars have any correlation with maser emission, and to understand the connection, if any, between specific types of maser emission and dust spectral features. I have investigated several methods of continuum elimination using spectroscopy data for the archetypal dusty AGB star, Mira. I have investigated the ~10[mu]m and ~18[mu]m spectral features in the continuum-eliminated spectrum including peak position, barycenter, and full width half maxima (FWHM). The positions and widthved spectral features were compared with those seen in laboratory spectra. I then looked for a correlation between maser emission and dust spectral features in a sample of Mira variables. The types of masers have been identified, and peak positions, barycenter positions, and FWHM have been measured for the sample spectra. The results show that the method of continuum elimination matters for correct identification of dust minerals, while varying the temperature and precise continuum shapes do not have a major effect on the positions of spectral features. Observed astronomical silicate features are complex and indicate the need for different compositions of minerals. Finally, there does not appear to be a correlation between the presence of a maser and dust spectral features based on the information available for analysis.

ALMA detection of millimetre 183 GHz H2O maser emission in the Superantennae galaxy at z ∼ 0.06

ABSTRACT We present the results of Atacama Large Millimeter/submillimeter Array (ALMA) band-5 (∼170 GHz) observations of the merging ultraluminous infrared galaxy, the ‘Superantennae’ (IRAS 19254−7245), at z = 0.0617, which has been diagnosed as containing a luminous obscured active galactic nucleus (AGN). In addition to dense molecular line emission (HCNJ = 2–1, HCO+J = 2–1, and HNC J = 2–1), we detect a highly luminous (∼6 × 10$^{4}\, \mathrm{L}_{\odot }$) 183 GHz H2O 31,3–22,0 emission line. We interpret the strong H2O emission as largely originating in maser amplification in AGN-illuminated dense and warm molecular gas, based on (1) the spatially compact (≲220 pc) nature of the H2O emission, unlike spatially resolved (≳500 pc) dense molecular emission, and (2) a strikingly different velocity profile from, and (3) significantly elevated flux ratio relative to, dense molecular emission lines. H2O maser emission, other than the widely studied 22 GHz 61,6–52,3 line, has been expected to provide important information on the physical properties of gas in the vicinity of a central mass-accreting supermassive black hole (SMBH), because of different excitation energy. We here demonstrate that with highly sensitive ALMA, millimetre 183 GHz H2O maser detection is feasible out to >270 Mpc, opening a new window to scrutinize molecular gas properties around a mass-accreting SMBH far beyond the immediately local Universe.

The magnetic field in the dense photodissociation region of DR 21

Measuring interstellar magnetic fields is extremely important for understanding their role in different evolutionary stages of interstellar clouds and of star formation. However, detecting the weak field is observationally challenging. We present measurements of the Zeeman effect in the 1665 and 1667 MHz (18 cm) lines of the hydroxyl radical (OH) lines toward the dense photodissociation region (PDR) associated with the compact H ii region DR 21 (Main). From the OH 18 cm absorption, observed with the Karl G. Jansky Very Large Array, we find that the line of sight magnetic field in this region is ∼0.13 mG. The same transitions in maser emission toward the neighbouring DR 21(OH) and W 75S-FR1 regions also exhibit the Zeeman splitting. Along with the OH data, we use [C ii] 158 μm line and hydrogen radio recombination line data to constrain the physical conditions and the kinematics of the region. We find the OH column density to be ∼3.6 × 1016(Tex/25 K) cm−2, and that the 1665 and 1667 MHz absorption lines are originating from the gas where OH and C+ are co-existing in the PDR. Under reasonable assumptions, we find the measured magnetic field strength for the PDR to be lower than the value expected from the commonly discussed density–magnetic field relation while the field strength values estimated from the maser emission are roughly consistent with the same. Finally, we compare the magnetic field energy density with the overall energetics of DR 21’s PDR and find that, in its current evolutionary stage, the magnetic field is not dynamically important.

Water vapour masers in long-period variable stars

Context. Water masers emitting at a radiofrequency of 22 GHz are often found in the circumstellar envelopes of evolved stars. We monitored the H2O maser emission of a larger sample of evolved stars of different types to study the maser properties as a function of stellar type. Aims. We wish to understand the origin and evolution of the H2O masers in circumstellar envelopes. In this paper, we take a closer look at R Crt and RT Vir, two nearby (<250 pc) semi-regular variable stars. The findings complement our monitoring results for RX Boo and SV Peg, two other semi-regular variable stars that we have discussed in a previous paper. Methods. Within the framework of the Medicina/Effelsberg H2O maser monitoring programme, we observed the maser emission of R Crt and RT Vir for more than two decades with single-dish telescopes. To get insights into the distribution of maser spots in the circumstellar envelopes at different times, to get an idea of their longevity, and, where possible, to be able to link the phenomena seen in our observations to maser locations within the envelopes, we collected interferometric data for these stars, taken within the same period, from the literature. Results. The H2O masers in R Crt and RT Vir exhibit brightness variations on a variety of timescales. We confirm short-time variations of individual features on timescales of months to up to 1.5 yr, as seen by previous monitoring programmes. Also decade-long variations of the general brightness level, independent from individual features, were seen in both stars. These long-term variations are attributed to brightness variations occurring independently from each other in selected velocity ranges and they are independent of the optical light curve of the stars. Expected drifts in velocity of individual features are usually masked by the blending of other features with similar velocities. However, in RT Vir, we found the exceptional case of a single feature with a constant velocity over 7.5 yr (<0.06 km s−1 yr−1). Conclusions. We attribute the long-term brightness variations to the presence of regions with higher-than-average density in the stellar wind and hosting several clouds which emit maser radiation on short timescales. These regions typically need ~20 yr to cross the H2O maser shell, where the right conditions for exciting H2O masers are present. Different clouds contained in such a region all move within a narrow range of velocities, and so does their maser emission. This sometimes gives the impression of longer-living features in single-dish spectra, in spite of the short lifetimes of the individual components that lie at their origin, thus, naturally explaining the longer timescales observed. The constant velocity feature (11 km s−1) is likely to come from a single maser cloud, which moved through about half of RT Vir’s H2O maser shell without changing its velocity. From this, we infer that its path was located in the outer part of the H2O maser shell, where RT Vir’s stellar wind has, apparently, already reached its terminal outflow velocity. This conclusion is independently corroborated by the observation that the highest H2O maser outflow velocity in RT Vir approaches the terminal outflow velocity, as given by OH and CO observations. This is generally not observed in other semi-regular variable stars. All four stars in our study are of optical variability type SRb, indicating the absence of periodic large-amplitude variations. Therefore, any likely responses of the maser brightness to variations of the optical emission are masked by the strong short-term maser fluctuations.