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Maser Studies in Evolved Stars

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 نشر من قبل Francisco Colomer
 تاريخ النشر 2009
  مجال البحث فيزياء
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 تأليف Francisco Colomer




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High resolution maps of maser emission provide very detailed information on processes occurring in circumstellar envelopes of late-type stars. A particularly detailed picture of the innermost shells around AGB stars is provided by SiO masers. Considerable progress is being made to provide astrometrically aligned multi-transition simultaneous observations of these masers, which are needed to better constrain the models. In view of the large amount of high quality data available, models should now be developed to fully explain all maser characteristics together (spatial distribution, variability, etc). New generation instruments (VERA, VSOP-2), new observational techniques (frequency-phase transfer), and new models promise important improvements of our knowledge on this topic.



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Context. HCN is a major constituent of the circumstellar envelopes of carbon-rich evolved stars, and rotational lines from within its vibrationally excited states probe parts of these regions closest to the stellar surface. A number of such lines are known to show maser action. Historically, in one of them, the 177 GHz $J=2rightarrow1$ line in the $l$-doubled bending mode has been found to show relatively strong maser action, with results only published for a single object, the archetypical high-mass loss asymptotic giant branch (AGB) star IRC+10216. Aims. To examine how common 177 GHz HCN maser emission is, we conducted an exploratory survey for this line toward a select sample of carbon-rich asymptotic giant branch stars that are observable from the southern hemisphere. Methods. We used the Atacama Pathfinder Experiment 12 meter submillimeter Telescope (APEX) equipped with a new receiver to simultaneously observe three $J=2rightarrow1$ HCN rotational transitions, the $(0,1^{{1}_{rm c}},0)$ and $(0,1^{{1}_{rm d}},0)$ $l$-doublet components, and the line from the (0,0,0) ground state. Results. The $(0,1^{{1}_{rm c}},0)$ maser line is detected toward 11 of 13 observed sources, which all show emission in the (0,0,0) transition. In most of the sources, the peak intensity of the $(0,1^{{1}_{rm c}},0)$ line rivals that of the (0,0,0) line; in two sources, it is even stronger. Except for the object with the highest mass-loss rate, IRC+10216, the $(0,1^{{1}_{rm c}},0)$ line covers a smaller velocity range than the (0,0,0) line. Conclusions. Maser emission in the 177 GHz $J=2rightarrow1$ $(0,1^{{1}_{rm c}},0)$ line of HCN appears to be common in carbon-rich AGB stars. (Abbreviated)
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In this Letter, we report detections of SiO v=3 J=1--0 maser emission in very long baseline interferometric (VLBI) observations towards 4 out of 12 long-period variable stars: WX Psc, R Leo, W Hya, and T Cep. The detections towards WX Psc and T Cep a re new ones. We also present successful astrometric observations of SiO v=2 and v=3 J=1--0 maser emissions associated with two stars: WX Psc and W Hya and their position-reference continuum sources: J010746.0+131205 and J135146.8-291218 with the VLBI Exploration of Radio Astrometry (VERA). The relative coordinates of the position-reference continuum source and SiO v=3 maser spots were measured with respect to those of an SiO v=2 maser spot adopted as fringe-phase reference. Thus the faint continuum sources were inversely phase-referenced to the bright maser sources. It implies possible registration of multiple SiO maser line maps onto a common coordinate system with 10 microarcsecond-level accuracy.
We investigate the use of 183 GHz H2O masers for characterization of the physical conditions and mass loss process in the circumstellar envelopes of evolved stars. We used APEX SEPIA Band 5 to observe the 183 GHz H2O line towards 2 Red Supergiant and 3 Asymptotic Giant Branch stars. Simultaneously, we observed lines in 28SiO v0, 1, 2 and 3, and for 29SiO v0 and 1. We detected the 183 GHz H2O line towards all the stars with peak flux densities greater than 100 Jy, including a new detection from VY CMa. Towards all 5 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. 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 H2O maser however, fits with the expectation for a non-saturated maser. 183 GHz H2O 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.
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