1612 MHz OH maser emission from axisymmetric circumstellar envelopes - Miras

1992 ◽  
Vol 390 ◽  
pp. 191 ◽  
Author(s):  
Alan J. Collison ◽  
John D. Fix
Keyword(s):  
Circumstellar Envelopes ◽  
Maser Emission

scholarly journals Water vapour masers in long-period variable stars

2020 ◽  
Vol 644 ◽  
pp. A45
Author(s):  
J. Brand ◽  
D. Engels ◽  
A. Winnberg
Keyword(s):  
Stellar Wind ◽  
Constant Velocity ◽  
Variable Stars ◽  
Circumstellar Envelopes ◽  
Maser Emission ◽  
Evolved Stars ◽  
Outflow Velocity ◽  
Regular Variable ◽  
Individual Features

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.

scholarly journals Maser emission during post-AGB evolution

2012 ◽  
Vol 8 (S287) ◽  
pp. 217-224 ◽  
Author(s):  
J.-F. Desmurs
Keyword(s):  
Planetary Nebulae ◽  
Transition Phase ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Circumstellar Envelopes ◽  
Maser Emission ◽  
Giant Branch Stars ◽  
Short Transition

AbstractThis contribution reviews recent observational results concerning astronomical masers toward post-AGB objects with a special attention to water fountain sources and the prototypical source OH 231.8+4.2. These sources represent a short transition phase in the evolution between circumstellar envelopes around asymptotic giant branch stars and planetary nebulae. The main masing species are considered and key results are summarized.

scholarly journals HCN, The First Strong Maser in Carbon-Rich Stars

1989 ◽  
Vol 131 ◽  
pp. 453-453
Author(s):  
A. Omont ◽  
S. Guilloteau ◽  
R. Lucas
Keyword(s):  
Ground State ◽  
Circumstellar Envelope ◽  
Circumstellar Envelopes ◽  
Maser Emission

Maser emission has been observed at the frequency of the (0,2,0) J - 1 - 0 line of HCN (89088 MHz, energy 2050 K above the ground state) in 7 C-rich circumstellar envelopes: CIT 6, S Cep, IRC+ 50096, IRC+30374, FX Ser, AFGL 2513 and IRAS 17581-1744. This is the second molecule showing strong maser emission in millimeter lines, and the first strong maser ever observed in a C-rich circumstellar envelope. The emission is particularly strong (70 Jy) in CIT 6 because of its proximity (Guilloteau, Omont, and Lucas 1987). The masers in the stars have luminosities and linewidths (FWHM about 1 km/s) similar to CIT 6; they are also blueshifted by a few km/s with respect to the velocity of the star given by the central velocity of the ground-state HCN emission (Lucas, Guilloteau, and Omont 1987). These properties are somewhat similar to those of SiO masers in O-rich stars, although the intensities of HCN masers are weaker.

scholarly journals OH maser emission in the circumstellar envelopes of U Herculis, R Cassiopeiae and W Hydrae

1994 ◽  
Vol 268 (2) ◽  
pp. 475-496 ◽  
Author(s):  
J. M. Chapman ◽  
P. Sivagnanam ◽  
R. J. Cohen ◽  
A. M. Le Squeren
Keyword(s):  
Circumstellar Envelopes ◽  
Maser Emission

Microwave OH maser emission in the circumstellar envelopes of late-type stars

Nature ◽  
1981 ◽  
Vol 291 (5814) ◽  
pp. 382-385 ◽  
Author(s):  
P. F. Bowers ◽  
K. J. Johnston ◽  
J. H. Spencer
Keyword(s):  
Late Type ◽  
Circumstellar Envelopes ◽  
Maser Emission

scholarly journals Submillimetre water masers at 437, 439, 471, and 474 GHz towards evolved stars

2020 ◽  
Vol 638 ◽  
pp. A19
Author(s):  
P. Bergman ◽  
E. M. L. Humphreys
Keyword(s):  
Radiative Transfer ◽  
Kinetic Temperature ◽  
Circumstellar Envelopes ◽  
Maser Emission ◽  
Evolved Stars ◽  
Mira Variables ◽  
Dust Temperature ◽  
Stellar Velocity ◽  
Maser Line ◽  
Water Masers

Aims. Here we aim to characterise submillimetre water masers at 437, 439, 471, and 474 GHz towards a sample of evolved stars. Methods. We used the Atacama Pathfinder Experiment (APEX1) to observe submillimetre water transitions and the CO (4–3) line towards 11 evolved stars. The sample included semi-regular and Mira variables, plus a red supergiant star. We performed radiative transfer modelling for the water masers. We also used the CO observations to determine mass loss rates for the stars. Results. From the sample of 11 evolved stars, 7 display one or more of the masers at 437, 439, 471, and 474 GHz. We therefore find that these masers are common in evolved star circumstellar envelopes. The fact that the maser lines are detected near the stellar velocity indicates that they are likely to originate from the inner circumstellar envelopes of our targets. We tentatively link the presence of masers to the degree of variability of the target star, that is, masers are more likely to be present in Mira variables than in semi-regular variables. We suggest that this indicates the importance of strong shocks in creating the necessary conditions for the masers. Typically, the 437 GHz line is the strongest maser line observed among those studied here. We cannot reproduce the above finding in our radiative transfer models. In general, we find that maser emission is very sensitive to dust temperature in the lines studied here. To produce strong maser emission, the dust temperature must be significantly lower than the gas kinetic temperature. In addition to running grids of models in order to determine the optimum physical conditions for strong masers in these lines, we performed smooth wind modelling for which we cannot reproduce the observed line shapes. This also suggests that the masers must originate predominantly from the inner envelopes.

scholarly journals Circumstellar envelopes of OH-IR sources

1987 ◽  
Vol 122 ◽  
pp. 229-239 ◽  
Author(s):  
R. J. Cohen
Keyword(s):  
Radio Interferometer ◽  
Circumstellar Envelope ◽  
Radio Lines ◽  
Circumstellar Envelopes ◽  
Radio Observations ◽  
Maser Emission ◽  
Ir Sources ◽  
Consistent Picture

This article reviews recent radio observations of maser emission from OH, H2O and SiO molecules in the circumstellar envelopes of OH-IR sources. The different radio lines require different conditions for their excitation, and each therefore probes different regions in the circumstellar envelope. For some stars radio interferometer maps of several maser lines are now available, and a consistent picture of the envelope structure is beginning to emerge.

scholarly journals Observational Constraints on Silicon Chemistry in the Circumstellar Envelopes of Red Giants

1987 ◽  
Vol 120 ◽  
pp. 341-342
Author(s):  
Raghvendra Sahai
Keyword(s):  
Mass Loss ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Observational Constraints ◽  
Red Giants ◽  
Circumstellar Envelopes ◽  
Maser Emission ◽  
Circumstellar Gas ◽  
Silicon Chemistry

We review observations to date of silicon-bearing molecules in circumstellar gas and dust around red giants. New constraints on circumstellar chemistry are provided by the discovery of (i) a systematic decrease in the [SiO]/[H2] ratio with mass loss rate, ii) SiO in carbon-rich CIT 6. The presence/absence of SiO maser emission in oxygen-rich/carbon-rich envelopes is linked to differences in grain composion and the grain-condensation radius.

scholarly journals Maser Observations of Cool Stars

2003 ◽  
Vol 210 ◽  
pp. 377-386
Author(s):  
E.M.L. Humphreys
Keyword(s):  
Mass Loss ◽  
Observational Data ◽  
Radio Interferometry ◽  
Circumstellar Envelopes ◽  
Maser Emission ◽  
Giant Stars ◽  
Cool Stars ◽  
Red Giant Stars ◽  
Red Giant

SiO, H2O and OH masers are ubiquitous in the circumstellar envelopes (CSEs) of oxygen-rich, red giant stars. Radio interferometry allows the imaging of stellar maser emission down to sub-milliarcsecond scales. Such observations reveal asymmetry, inhomogeneity and apparent clumpiness in the extended atmosphere and surrounding envelope of the star. The studies place constraints on processes which are seldom included in models. Here, I review the observational data on stellar masers and discuss their implications for our understanding of the mass-loss and evolution of red giant stars.

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