G. M. Rudnitskij

Evolved star water maser cloud size determined by star size

Cool, evolved stars undergo copious mass loss but the details of how the matter is returned to the ISM are still under debate. We investigated the structure and evolution of the wind at 5 to 50 stellar radii from Asymptotic Giant Branch and Red Supergiant stars. 22-GHz water masers around seven evolved stars were imaged using MERLIN, at sub-AU resolution. Each source was observed at between 2 and 7 epochs (several stellar periods). We compared our results with long-term Pushchino single dish monitoring. The 22-GHz emission is located in ~spherical, thick, unevenly filled shells. The outflow velocity doubles between the inner and outer shell limits. Water maser clumps could be matched at successive epochs separated by <2 years for AGB stars, or at least 5 years for RSG. This is much shorter than the decades taken for the wind to cross the maser shell, and comparison with spectral monitoring shows that some features fade and reappear. In 5 sources, most of the matched features brighten or dim in concert from one epoch to the next. One cloud in W Hya was caught in the act of passing in front of a background cloud leading to 50-fold, transient amplification. The masing clouds are 1-2 orders of magnitude denser than the wind average and contain a substantial fraction of the mass loss in this region, with a filling factor <1%. The RSG clouds are ~10x bigger than those round the AGB stars. Proper motions are dominated by expansion, with no systematic rotation. The maser clouds survive for decades (the shell crossing time) but the masers are not always beamed in our direction. Radiative effects cause changes in flux density throughout the maser shells on short timescales. Cloud size is proportional to parent star size; clouds have a similar radius to the star in the 22-GHz maser shell. Stellar properties such as convection cells must determine the clumping scale.

Time variation of the water maser in ON2

Context. The results of monitoring of the water-vapour maser in the 1.35-cm line toward ON2 in 1995-2004 are reported. Aims. The main goal was to study variations of the H 2 O maser emission on a long (10 years) time interval, in particular, velocity drifts and correlation between fluxes in various spectral features. Methods. The observations were carried out on the RT-22 radio telescope of Pushchino Radio Astronomy Observatory. Results. The emission of the northern component (ON2 N) took place in a broad interval of radial velocities, from -12 to 9 km s -1 . The total H 2 O emission has two variability components: slow and flaring. The period of the former component is most likely between 25 and 30 years. The flaring component has a cyclic character with a period from 1.1 to 2.6 years. The alternation of the activity cycles was accompanied by changes in the velocity structure of the H 2 O spectra. There is good correlation between the variations in the integrated flux and velocity centroid, as well as between the emissions in various segments of the H 2 O spectrum. We suggest that long-term variations and oscillations of the water-maser emission can be related to the non-stationarity and anisotropy of bipolar molecular outflow from the B-type star in the ultracompact H II region.

H2O maser emission of the M-type supergiant VX Sgr

Abstract We present the results of observations of the M-type semiregular supergiant VX Sgr in the water-vapour line at λ = 1.35 cm. The observations were carried out on the 22-metre radio telescope RT-22 in Pushchino in 1981–1996. The profiles of the H2O line have a complex structure typical for H2O masers associated with late-type supergiants. We reveal some features persistent during the last 15 years; we suggest that they arise from masering H2O molecules located in the circumstellar disk and the bipolar outflows from the disk poles. The parameters of the outflow are estimated.

A study of the asymmetry in the H2O maser line at λ =1 .35 cm on the base of the hyperfine structure

The results of analyzing asymmetry in the H2O maser lines at λ = 1.35 cm are presented. We investigated in detail the known mechanism of the asymmetry origin due to the hyperfine structure of the 616 → 523 transition and different (not equidistant) positions of the corresponding triplet components. The triplet of the lines is characterized by the sum of extinction factors that are continuously transformed into a single-peaked asymmetric extinction coefficient, with the increase in the Doppler widths (characterizing by effective Doppler velocity uD). A noticeable asymmetry corresponds to uD ≤ 0.5− 1k m s −1 . For higher values of uD the extinction factor approaches a Gaussian shape with the decreasing magnitude of the profile asymmetry. An appreciable asymmetry in unsaturated maser lines can only arise at uD ≤ 0. 5k m s −1 , if there are very narrow linewidths (Δu ≈ 0.2−0. 4k m s −1 ). But observed saturated masers possess a fairly high degree of the line asymmetry, 10−40%. Most probably, this asymmetry is a result of rebroadening of the linewidth up to the value that is comparable to the width of the asymmetric extinction coefficient. Long-term monitoring of the H2O maser emission sources at λ = 1.35 cm in active star-forming regions has shown the existence of separate lines (emission features) with various linewidths and shapes. The linewidths of the emission from such single features can be from 0.35 to 0.9 km s −1 . The linewidth is determined by thermal and small-scale turbulent motions of H2O molecules in a maser spot. The line shape is determined by possible superposition of some components with similar radial velocities. In the case of a single spectral line, which is emitted by a separate feature, the line shape is basically approximated by the usual Gaussian curve. The monitoring has shown that there are single lines with non-Gaussian shapes, and these lines mostly have an asymmetric shape.

OH and H 2 O maser variations in W33B

Context. The active star-forming region W33B is a source of OH and H2O maser emission located in distinct zones around the central object. Aims. The aim was to obtain the complete Stokes pattern of polarised OH maser emission and to trace its variability and to investigate flares and long-term variability of the H2O maser and evolution of individual emission features. Methods. Observations in the OH lines at a wavelength of 18 cm were carried out on the Nancay radio telescope (France) at a number of epochs in 2008–2014; H2O line observations (long-term monitoring) at λ = 1.35 cm were performed on the 22-metre radio telescope of the Pushchino Radio Astronomy Observatory (Russia) between 1981 and 2014. Results. We have observed strong variability of the emission features in the main 1665- and 1667-MHz OH lines as well as in the 1612-MHz satellite line. Zeeman splitting has been detected in the 1665-MHz OH line at 62 km s −1 and in the 1667-MHz line at 62 and 64 km s −1 . The magnetic field intensity was estimated to be from 2 to 3 mG. The H2O emission features form filaments, chains with radial-velocity gradients, or more complicated structures including large-scale ones. Conclusions. Long-term observations of the hydroxyl maser in the W33B region have revealed narrowband polarised emission in the 1612-MHz line with a double-peak profile characteristic of Type IIb circumstellar masers. The 30-year monitoring of the water-vapour maser in W33B showed several strong flares of the H2O line. The observed radial-velocity drift of the H2O emission features suggests propagation of an excitation wave in the masering medium with a gradient of radial velocities. In OH and H2O masers some turbulent motions of material are inferred.

The results of ultra‐rapid flux fluctuations monitoring in 2002–2009 for galactic sources of water‐vapor maser emission at a wavelength of 1.35 cm

Results of a search for ultra rapid flux fluctuations of galactic sources of maser emission in the water‐vapor line at a wavelength of 1.35 cm. An observational technique of a search for ultra rapid flux fluctuations has been developed. From 2002 to 2009 a number of observational sessions for about 40 maser sources have been carried out. Several of them have demonstrated such a variability. Possible mechanisms of the variability are discussed, among them internal processes within the sources themselves as well as external ones, including the theoretical possibility of a resonant effect of gravitational waves from galactic objects on the maser region. Clear evidence for the presence of such a variability has been found in several sources (Cep A, W43M3, W49N, W33B). This variability appears not permanently, but only certain states of particular H2O masers. In some sources (Ori A, W3 OH, W49N) demonstrated any variations by linear polarization of signal.

Twenty-six-year monitoring of water masers

Since 1980 variability of a sample of H 2 O maser sources has been monitored on the 22-metre radio telescope in Pushchino, Russia. The interval between successive observational sessions is 1–2 months. The sample includes 125 maser sources in star-forming regions (SFR) and late-type variable stars. Twenty-six-year time series of H 2 O line profiles have detected flares and velocity drift of spectral features. Very fast variations in the H 2 O maser flux (Δ t ≲ 1 hour) have been detected in several SFR sources, in particular, W33B. Variations of circumstellar H 2 O masers in late-type stars correlate with visual light curves with a time lag of 0.3-0.4 P ( P is the stars period). Exceptionally strong H 2 O maser flares were recorded in SFR sources (Sgr B2 and others) and in the stars W Hya, R Cas and U Ori. Models for H 2 O maser variability are reviewed. For stellar masers shock-wave excitation of H 2 O line variability is discussed.

Water masers in red supergiants

Results of long-term monitoring of circumstellar water maser sources in red supergiants are reviewed. The observations were carried out in 1980–2006 on the RT-22 radio telescope at Pushchino Radio Astronomy Observatory. We discuss the results for the semiregular variable M-supergiant VX Sgr and non-variable M-supergiant IRC–10414. In addition to our single-dish data, very-long-baseline interferometry results are invoked. VX Sgr and IRC–10414 display a characteristic water line profile, which suggests the presence of a rotating circumstellar disc and a bipolar outflow.

Investigation of a Sample of Long-Period Variable Stars Possessing Maser Emission

An investigation of parameters of a sample of more than two hundred late-type long-period variable stars (LPVs) has been carried out. In more detail were studied 13 giants (Mira Ceti-type variables R Aql, RR Aql, RT Aql, R Leo, U Ori, U Her, R Cas, R Tau, Z Cyg, R Peg, U Aur and semiregular variables RT Vir and RX Boo) and 2 supergiants (S Per, PZ Cas). A considerable fraction of the sample stars (about one third) possess circumstellar maser emission in molecular spectral lines (OH, H 2 0, SiO). Our aim was to elucidate the particularities of photometric characteristics of maser stars, such as period P, amplitude A, light curve asymmetry f = (M-m)/P, and, in prospect, to determine their status in course of their evolution on the asymptotic giant branch. An extensive comparison with non-maser LPVs was made.

Influence of turbulence on the shape of a spectral line: the analytical approach

We consider the propagation of a spectral-line radiation in a correlated turbulent atmosphere. The ensembles of realizations of turbulent velocities u(r,t) and optical depth τν are assumed to be Gaussian. We investigate the explicit analytical solution of the stochastic radiative transfer equation for the intensity Iν of radiation. The scattering term is not taken into account. It is shown that, in addition to the usual Doppler broadening of the spectral line, correlated turbulent motions of atoms and molecules give rise to considerable changes in the shape of a spectral line. It was found for the first time that the mean intensity I (0) ν (Iν = I (0) ν +I � ν , � I � ν � = 0) obeys the usual radiative transfer equation with renormalized extinction factor α eff ν if the correlation length R0 of the turbulence is small as compared to a photon free path. A simple analytical expression for α eff ν is given. This expression integrally depends on the two-point correlation function of the turbulent velocity field. We also discuss the problem how to obtain the main turbulence parameters from the analysis of the shape of a spectral line.