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Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| The OH 1612 MHz maser pump rates of stellar, interstellar and post-AGB OH masers
(Pseudo) radiative pumprate of OH 1612 MHz masers is defined for asample of 44 OH/IR sources (infrared sources with OH 1612 MHz maser),irrespective of the real maser pumping mechanisms. The correlationbetween the (pseudo) maser pumprates and the evolutionary status of themaser sources reveals that the radiative pumprates of stellar OH masersare nearly fixed, which agrees with the theoretical prediction forradiatively pumped OH maser. The (pseudo) radiative pumprates ofinterstellar OH masers are not only very small but also varying broadlyover two orders of magnitude, which is argued to be the manifestation ofvarying number of quiet absorbing OH cloudlets and/or various OH maserpumping mechanisms and/or competitive gain between mainline and 1612 MHzOH masers and/or anisotropy of the maser emission. The radiativepumprates of post-AGB OH masers very possibly decrease with increasingIRAS C32 color indices and distribute in an interim regionbetween the stellar and interstellar OH masers in the pumprate-colordiagram.
| Search for radiative pumping lines of OH masers. I. The 34.6 m absorption line towards 1612 MHz OH maser sources
The 1612 MHz hydroxyl maser in circumstellar envelopes has long beenthought to be pumped by 34.6 μm photons. Only recently, the InfraredSpace Observatory has made possible spectroscopic observations whichenable the direct confirmation of this pumping mechanism in a few cases.To look for the presence of this pumping line, we have searched theInfrared Space Observatory Data Archive and found 178 spectra with dataaround 34.6 μm for 87 galactic 1612 MHz masers. The analysisperformed showed that the noise level and the spectral resolution of thespectra are the most important factors affecting the detection of the34.6 μm absorption line. Only 5 objects from the sample (3 redsupergiants and 2 galactic center sources) are found to show clear 34.6μm absorption (all of them already known) while two additionalobjects only tentatively show this line. The 3 supergiants show similarpump rates and their masers might be purely radiatively pumped. The pumprates of OH masers in late type stars are found to be about 0.05, only1/5 of the theoretical value of 0.25 derived by Elitzur (1992). We havealso found 16 maser sources which, according to the analysis assumingElitzur's pump rate, should show the 34.6 μm absorption line but donot. These non-detections can be tentatively explained by far-infraredphoton pumping, clumpy nature of the OH masing region or a limb-fillingemission effect in the OH shell.
| First mm-VLBI Observations between the TRAO 14-m and the NRO 45-m Telescopes: Observations of 86 GHz SiO Masers in VY Canis Majoris
We have made VLBI observations at 86GHz using a 1000-km baseline betweenKorea and Japan with successful detections of SiO v = 1, J = 2 ‑ 1maser emissions from VY CMa and Orion KL in 2001 June. This was thefirst VLBI result for this baseline and the first astronomical VLBIobservation for the Korean telescope. Since then, we observed SiO v = 1,J = 2 ‑ 1 maser emission in VY CMa in 2002 January and 2003February and derived the distributions of the maser emissions. Ourresults show that the maser emissions extend over 2-4 stellar radii, andwere within the inner radius of the dust shell. We observed other SiOmaser sources and continuum sources, and 86-GHz continuum emissions weredetected from three continuum sources. It was verified that thisbaseline has a performance comparable to the most sensitive baseline inthe VLBA and the CMVA, and is capable of investigating the propermotions of maser features in circumstellar envelopes using monitoringobservations.
| Searching for radiative pumping lines of OH masers: II. The 53.3 μm absorption line towards 1612 MHz OH maser sources
This is the second paper in a series aiming at searching for infraredpumping lines for galactic 1612 MHz OH masers. Our paper I is devoted tothe 34.6 μm absorption lines in ISO SWS spectra towards a largesample of galactic OH/IR sources. This paper analyzes the 53.3 μmline in the ISO LWS spectra towards a similar sample of OH/IR sources. Asearch with position radius of 1 arcmin in ISO Data Archive (IDA)results in 137 LWS spectra covering 53.3 μm associated with 47galactic OH/IR sources and 4 ones associated with megamasers Arp 220 andNGC 253. (These two magamasers are included for comparison purposeonly.) Ten of these galactic OH/IR sources are found to show and another5 ones tentatively show the 53.3 μm absorption while another 7sources (our group U1 and U2 sources) highly probably do not show thisline. The source class is found to be correlated with the type ofspectral profile: red supergiants (RSGs) and AGB stars tend to showstrong blue-shifted filling emission in their 53.3 μm absorption lineprofiles while H II regions tend to show a weak red-shifted fillingemission in the line profile. GC sources and megamasers mainly showsymmetrical profile in the line core while megamasers tend to show anadditional absorption tail on the blue side of the line profile. It isargued that the filling emission might be the manifestation of anunresolved half emission half absorption profile of the 53.3 μmdoublet which might be produced by the transitions among the two levels:2Π1/2(J=3/2) and2Π1/2(J=5/2) and their closely related levels.The 53.3 to 34.6 μm equivalent width (EW) ratio is close to unity forRSGs but much larger than unity for GC sources and megamasers while H IIregions only show the 53.3 μm line. The pump rate defined as maser toIR photon flux ratio is approximately 5% for RSGs. The pump rates of GCsources are three order of magnitude smaller. Both the large 53.3 to34.6 μm EW ratio and the small pump rate of the GC OH masers reflectthat the two detected `pumping lines' in these sources are actually ofinterstellar origin. The pump rate of Arp 220 is 32% - much larger thanthat of RSGs, which indicates that the contribution of other pumpingmechanisms to this megamaser is important. A handful of non-detectionsof the 34.6 or 53.3 μm line or both can be explained partly by thegenuinely weakness of the OH masers and partly by some other mechanismsweakening the IR pumping lines, such as clumpy OH shell or limb fillingemission.
| MERLIN polarimetry of the OH masers in IRAS 20406+2953
We present the third in a series of results from our programme of thedetection and measurement of magnetic fields in protoplanetary nebulae(PPN), using full-polarization MERLIN observations of OH masers. Theseare the first maps of the OH 1612- and 1667-MHz masing emission fromIRAS 20406+2953. We have identified a Zeeman pair in the 1612-MHz data;this result heralds the second magnetic field strength measured in aPPN. We show that the field, of strength -3.1 mG, is sufficiently strongto be shaping the outflowing mass from the star. By considering thespatial distribution of the masers and the structure of the linearpolarization data, we suggest a field configuration that is toroidal.The observed change in polarization position angle between the peak red-and blueshifted components of the maser features, if attributed toFaraday rotation within the circumstellar envelope, gives an electrondensity of ~1 cm-3. We show that this is sufficient to freezethe field into the outflowing wind.
| Detection of OH toward the Extreme Carbon Star IRC +10216
We report the detection of the 1665 and 1667 MHz main lines of OH(hydroxyl) and upper limits on the 1612 MHz satellite line of OH towardthe carbon-rich AGB star IRC +10216. We find a beam-averaged fractionalabundance x(OH)~4×10-8. This detection supports theidentification by Melnick et al. of the 110-101transition of water vapor with a 556.936 GHz rest-frequency emissionfeature detected toward IRC +10216, since OH is the expectedphotodissociation product of water vapor. The shape of the OH lines,however, differs significantly from the shape expected on the basis ofthe observations of Melnick et al. Possible explanations for theanomalous shapes of the 1665 and 1667 MHz lines are discussed. The mostlikely explanations for the unexpected OH line shapes are either masingor an asymmetric distribution of OH molecules around IRC +10216.
| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima.
| ``Thermal'' SiO radio line emission towards M-type AGB stars: A probe of circumstellar dust formation and dynamics
An extensive radiative transfer analysis of circumstellar SiO``thermal'' radio line emission from a large sample of M-type AGB starshas been performed. The sample contains 18 irregulars of type Lb (IRV),7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12Miras. New observational data, which contain spectra of several groundvibrational state SiO rotational lines, are presented. The detectionrate was about 60% (44% for the IRVs, and 68% for the SRVs). SiOfractional abundances have been determined through radiative transfermodelling. The abundance distribution of the IRV/SRV sample has a medianvalue of 6*E-6, and a minimum of 2*E-6 and amaximum of 5*E-5. The high mass-loss rate Miras have a muchlower median abundance, la 10-6. The derived SiO abundancesare in all cases well below the abundance expected from stellaratmosphere equilibrium chemistry, on average by a factor of ten. Inaddition, there is a trend of decreasing SiO abundance with increasingmass-loss rate. This is interpreted in terms of depletion of SiOmolecules by the formation of silicate grains in the circumstellarenvelopes, with an efficiency which is high already at low mass-lossrates and which increases with the mass-loss rate. The high mass-lossrate Miras appear to have a bimodal SiO abundance distribution, a lowabundance group (on average 4*E-7) and a high abundance group(on average 5*E-6). The estimated SiO envelope sizes agreewell with the estimated SiO photodissociation radii using an unshieldedphotodissociation rate of 2.5*E-10 s-1. The SiOand CO radio line profiles differ in shape. In general, the SiO lineprofiles are narrower than the CO line profiles, but they havelow-intensity wings which cover the full velocity range of the CO lineprofile. This is interpreted as partly an effect of selfabsorption inthe SiO lines, and partly (as has been done also by others) as due tothe influence of gas acceleration in the region which produces asignificant fraction of the SiO line emission. Finally, a number ofsources which have peculiar CO line profiles are discussed from thepoint of view of their SiO line properties.Based on observations using the SEST at La Silla, Chile, the 20 mtelescope at Onsala Space Observatory, Sweden, the JCMT on Hawaii, andthe IRAM 30 m telescope at Pico Veleta, Spain.
| VLBI astrometry of circumstellar OH masers: Proper motions and parallaxes of four AGB stars
The main-line OH masers around 4 AGB stars have been observed with theNRAO Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5yrs. Using a phase referencing technique, the position of the mostcompact maser spot of each star was monitored with respect to twoextragalactic reference sources. For U Her and W Hya, we observe themost blue-shifted maser spot, while for R Cas and S CrB we only detect acompact red-shifted maser spot. We managed to determine an accurateproper motion and parallax for U Her, R Cas and S CrB, while additionalmotion of the compact blue-shifted maser of W Hya is shown to possiblybe related to the stellar pulsation. The motion and radio position arecompared with the stellar trajectory and absolute optical positiondetermined by the Hipparcos satellite. For U Her and W Hya, the mostblue-shifted maser is consistent with the amplified stellar image. Thenew distances are compared with several published P-L relations, and inthis respect the VLBI distances seem an improvement upon the Hipparcosdistances.
| Detection of 1612 MHz OH emission in the semiregular variable stars RT Vir, R Crt and W Hya
We present evidence of 1612 MHz emission in SR variable stars. The twoSRb, RT Vir and R Crt, as well as the SRa W Hya have been monitored withthe upgraded Nançay radio telescope since February 2001. Allthree objects have shown a weak 1612 MHz emission occuring in thevelocity range of the strongest emission observed in the main-lines.Such a detection is the second observational evidence for emission inthe 1612 MHz OH maser satellite line from SRb stars. It also confirmsthe presence of 1612 MHz emission in the SRa W Hya discovered by Etokaet al. (\cite{etoka01}). Such a finding strongly suggests that the shellproperties of those three objects are quite similar to those of the Mirastars with similar IR characteristics.
| How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on ``instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also available in electronic form at the CDS, via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1167
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| Detection of Water Maser Flare in the Seyfert/LINER, NGC 6240
We report on a flare-up of water-vapor maser emission in a prototypicalultraluminous FIR galaxy with an AGN, NGC6240. The flux density of thedetected maser line was ≈ 0.1 Jy with FWHM ≈ 5 kms-1, indicating an apparent isotropic luminosity of ≈100 Lsolar at a distance of 100 Mpc, which is typical forwater megamasers. The maser line was redshifted by ≈ 200 kms-1 relative to the systemic velocity of the galaxy, while nosystemic-velocity feature was detected. The previous possible detectionor observed upper limits were at a level lower than the flux density inthe present paper, indicating that the water maser in NGC6240 flared upby more than a factor of 5 in 2001.
| Magnetic activity of the cool component in symbiotic systems
I argue that cool giant components in most symbiotic binary systemspossess magnetic activity on a much higher level than isolated coolgiants or those in wide binary systems. Based on the behaviour ofmain-sequence stars, I assume that magnetic activity and X-rayluminosity increase with rotation velocity. I then show that the coolcomponents in symbiotic systems are likely to rotate much faster thanisolated, or in wide binary systems, cool giants. The magnetic activityof the cool giant may be observed as a global axisymmetrical mass-lossgeometry of the cool giant (before the hot companion influences theoutflow), a stochastic mass-loss process, i.e. a variation of mass-lossrate with time and location on the surface of the giant and inrelatively strong X-ray emission. The variation in the mass-loss processfrom the cool giant may cause variation in the properties of jets blownby the hot compact companion. I conclude that symbiotic systems shouldbe high-priority X-ray targets.
| A high-sensitivity OH 5-cm line survey in late-type stars
We have undertaken a comprehensive search for 5-cm excited OH maseremission from evolved stars representative of various stages of latestellar evolution. Observed sources were selected from known 18-cm OHsources. This survey was conducted with the 100-m Effelsberg telescopeto achieve high signal to noise ratio observations and a sensitivitylimit of about 0.05 to 0.1 Jy. A total of 65 stellar sources weresearched for both main line and satellite line emission. We confirm theprevious detection of 5 cm OH in Vy 2-2, do not confirm emission fromNML-Cyg and do not report any other new detection within the abovesensitivity limit. Implications of these results on the pumpingmechanism of the OH radical in circumstellar envelopes are brieflydiscussed. Table \ref{table1} is also available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/394/975
| The molecular environment of NGC 3603. I. Spatial distribution and kinematic structure
We present CS 2-1 and CS 3-2 observations of the molecular gasassociated with the Galactic starburst template NGC 3603, over an areaof 5farcm 8 x 16farcm 7, with the OB cluster at the center. Totalvelocity integrated maps and channel maps give insight into the spatialdistribution and the kinematic structure of the dense gas in the giantmolecular cloud from which the starburst cluster originated. We identify13 molecular clumps with radii less than 0.8 pc and derive upper limitsfor their virial masses as well as lower limits for their H2column densities: <≈ (1.0 +/- 0.6) x103 Msolar and < N(H2)> >≈(0.4 +/- 0.2) x 1023 cm-2. One of the clumps, MM11, clearly stands out with a mass and column density 4 times higherthan average. The CS 3-2 / CS 2-1 map shows higher intensity ratios tothe south of the OB cluster than to the north (0.80 +/- 0.08 versus 0.32+/- 0.11), which indicates a substantial difference in the physicalconditions (either opacities or excitation temperatures) of themolecular gas. From the average of the line peak velocities, 14.2 +/-1.6 km s-1, we deduce a kinematic distance of 7.7 +/- 0.2 kpcfor NGC 3603. We estimate the star formation efficiency (>≈ 30%)of the central part of the NGC 3603 H II ~region. If we assume the ageof the OB cluster to be less than 3 Myr and the star formation rate tobe larger than 1.3 x 10-3 Msolar yr-1,the derived timescale for gas removal (tau ~ 6 Myr) can explain why thestarburst cluster itself is nearly void of interstellar material. Theremnant clump MM 1 appears to constitute the head of a prominent pillarwhich still becomes dispersed by ionizing radiation and stellar windsoriginating from the massive stars of the cluster. Because some of themolecular clumps are associated with near and mid infrared sources aswell as OH, H2O and CH3OH maser sources weconclude that star formation is still going on within NGC 3603. Based ondata sets obtained at the European Southern Observatory, La Silla, Chileunder proposal numbers 56.C-0670, 58.C-0662 and 60.C-0489.
| Velocity variability of semiregular and irregular variables
We compare velocities from near infrared lines with center-of-massvelocities for a sample of semiregular and irregular variables to searchfor indications for a convective blueshift. It is shown that a generalblueshift is present but that the light variability is obviously notdominated by convective cells but rather by stellar pulsation. All starsof our sample show a similar shape and amplitude in the velocityvariations. Long secondary periods are a common feature in these objectsand strongly influence the measured velocity shifts. The star V366 Aqlis found to be the first SRV showing line doubling.
| Mass loss rates of a sample of irregular and semiregular M-type AGB-variables
We have determined mass loss rates and gas expansion velocities for asample of 69 M-type irregular (IRV 22 objects) and semiregular (SRV; 47objects) AGB-variables using a radiative transfer code to model theircircumstellar CO radio line emission. We believe that this sample isrepresentative for the mass losing stars of this type. The (molecularhydrogen) mass loss rate distribution has a median value of 2.0 x10-7 Msun yr-1, and a minimum of 2.0 x10-8 Msun yr-1 and a maximum of 8 x10-7 Msun yr-1. M-type IRVs and SRVswith a mass loss rate in excess of 5 x 10-7 Msunyr-1 must be very rare, and among these mass losing stars thenumber of sources with mass loss rates below a few 10-8Msun yr-1 must be small. We find no significantdifference between the IRVs and the SRVs in terms of their mass losscharacteristics. Among the SRVs the mass loss rate shows no dependenceon the period. Likewise the mass loss rate shows no correlation with thestellar temperature. The gas expansion velocity distribution has amedian of 7.0 km s-1, and a minimum of 2.2 km s-1and a maximum of 14.4 km s-1. No doubt, these objects samplethe low gas expansion velocity end of AGB winds. The fraction of objectswith low gas expansion velocities is very high, about 30% havevelocities lower than 5 km s-1, and there are objects withvelocities lower than 3 km s-1: V584 Aql,T Ari, BI Car, RXLac, and L2 Pup. The mass loss rate and thegas expansion velocity correlate well, a result in line with theoreticalpredictions for an optically thin, dust-driven wind. In general, themodel produces line profiles which acceptably fit the observed ones. Anexceptional case is R Dor, where the high-quality,observed line profiles are essentially flat-topped, while the model onesare sharply double-peaked. The sample contains four sources withdistinctly double-component CO line profiles, i.e., a narrow featurecentered on a broader feature: EP Aqr, RVBoo, X Her, and SV Psc.We have modelled the two components separately for each star and derivemass loss rates and gas expansion velocities. We have compared theresults of this M-star sample with a similar C-star sample analysed inthe same way. The mass loss rate characteristics are very similar forthe two samples. On the contrary, the gas expansion velocitydistributions are clearly different. In particular, the number oflow-velocity sources is much higher in the M-star sample. We found noexample of the sharply double-peaked CO line profile, which is evidenceof a large, detached CO-shell, among the M-stars. About 10% of theC-stars show this phenomenon.
| VLBI Monitoring Observations of Water Masers around the Semi-Regular Variable Star R Crateris
We monitored water-vapor masers around the semi-regular variable star RCrateris with the Japanese VLBI Network (J-Net) at the 22 GHz bandduring four epochs with intervals of one month. The relative propermotions and Doppler-velocity drifts of twelve maser features weremeasured. Most of them existed for longer than 80 days. The 3-Dkinematics of the features indicates a bipolar expanding flow. The majoraxis of the asymmetric flow was estimated to be at P.A. = 136°. Theexistence of a bipolar outflow suggests that a Mira variable star hadalready formed a bipolar outflow. The water masers are in a region ofapparent minimum radii of 1.3 × 1012 m and maximumradii of 2.6 × 1012 m, between which the expansionvelocity ranges from 4.3 to 7.4 km s-1. These values suggestthat the water masers are radially accelerated, but stillgravitationally bound, in the water-maser region. The most positive andnegative velocity-drifting features were found relatively close to thesystemic velocity of the star. We found that the blue-shifted featuresare apparently accelerated and the red-shifted apparently decelerated.The acceleration of only the blue-shifted features seems to beconsistent with that of the expanding flow from the star.
| On the Transience of High-Latitude OH/IR Stars. II. Thermal Pulse Link
High-latitude, |b|>=10deg, asymptotic (AGB) stars in theirsuperwind phase have distinctive IR colors, which make them easy toidentify and count. But the expansion velocity (Ve) versuscolor plots of these stars are a puzzle, with features implying (1) astep increase in dM/dt and (2) short-term changes in the circumstellarshell that are enhanced by increased metallicity. The net duration oftheir superwind phase is ~3700 yr when estimated from the relativefrequency of associated proto-planetary nebulae and the ~103 yrexpansion age of one, IRAS 18095+2704. Since these stars lose on average~0.04 Msolar during the superwind phase and have progenitormasses circa 1 Msolar, they must lose more than 0.3Msolar prior to the AGB. A single 3700 yr superwind episode,however, cannot explain the 2-18 km s-1 range of expansionvelocities exhibited by these stars. This range is best understood asresulting from a cyclical superwind, wherein stars lose most of theirenvelope mass in three to four discrete episodes of ever largerVe, with each episode being triggered by a sudden onset inthe coupling of photon momentum by dust to gas near a thermal pulse. Theresulting newly accelerated shell thereafter quickly expands beyond theprotective dust shroud of the prior shell, which allows interstellar UVto degrade its molecules. This turns the shell into an OH/IR star colormimic (a shell with the dM/dt of an OH/IR star without 1612 MHz masers),until enough dust is again in place beyond the shell to allow moleculesthe longevity needed to support masers. Two by-products of this modelare the natural explanation it offers for why Ve(CO)>Ve(OH), as well as for the occurrence and integration ofmimics into the normal development of a shell. The circumstellar shellsof high-latitude OH/IR stars are thus cyclically transient phenomena.
| Polarization and Extent of Maser Emission from Late-Type Stars: Support for a Plasma Turbulence Model of Maser Production
The integrated spectrum of OH emission from late-type stars is oftencircularly polarized, by as much as 50% in some cases. While the spectraare partially polarized, the individual maser components revealed byVLBI are much more so. Using VLBI observations of late-type stars fromthe literature, we show that the difference in circular polarizationbetween main lines correlates with a difference in angular extent for agiven object. This is a natural result if turbulent magnetic fields arecausing the masers to be polarized via the Cook mechanism and mightserve as a good diagnostic for determining which objects should beinvestigated in the search for magnetic fields around evolved stars.
| On the Difference between Type E and Type A OH/IR Stars
The observed spectral energy distributions of a sample of 60 OH/IR starsare fitted using a radiative transfer model of a dusty envelope. Amongthe whole sample, 21 stars have reliable phase-lag distances, while theothers have less accurate distances. L*-P, M-P, andM-L* relations have been plotted for these stars. It is foundthat type E (with an emission feature at 10 μm) and type A (with anabsorption feature at 10 μm) OH/IR stars have differentL*-P and M-L* relations, while both follow asingle M-P relation. The type E stars are shown to be located in an areawithout large-scale dense interstellar medium, while the type A starsare probably located in dense interstellar medium. It is argued herethat this may indicate that the two types of OH/IR stars do not sharethe same chemical composition or zero-age main-sequence mass and soevolve in different ways. This conclusion has reinforced the argument byChen et al., who reached a similar conclusion from the Galacticdistribution of about 1000 OH/IR stars, based on IRAS low-resolutionspectrometer spectra.
| Variations of polarisation properties of OH maser emission from three semiregular variables
We present an analysis of polarimetric observations of the threesemiregular variables RT Vir, R Crt and W Hya obtained in the 1665 and1667 MHz OH maser lines. Circular polarisation data were taken atintervals over a period of 10-14 years. During the two last years of themonitoring program the targets were observed at intervals of 3-4 weeksin circular and linear polarisation. Circular polarisation was dominant,whereas linear polarisation, if detected, was weak. The degree ofcircular polarisation varied considerably across the maser profiles. Itdecreased at velocities where strong OH total flux density was observed,most likely due to blending effects. Individual circularly polarisedfeatures exhibited various types of changes; some features weretransient or showed significant variations on timescales of a few weeksto few months. Other features varied slowly over the period ofobservations of about 5000 days and polarisation reversal occurred. Netcircular polarised emission was detected in the all three stars.Variations of net circular polarisation and the degree of circularpolarisation of the maser features of RT Vir implies a well-alignedcircumstellar magnetic field. Linearly polarised features preferentiallyappeared at blue-shifted velocities where they are not suppressed byFaraday rotation. In the blue-shifted 1667 MHz features of RT Vir weobserved a systematic increase of degree of linear polarisationassociated with a gradual decrease in the degree of circularpolarisation. This behaviour was possibly linked to variations in theelectron density or propagation effect in the maser regions. It issuggested that changes in the observational characteristics of thepolarised maser emission of the studied stars can be caused byturbulence effects in the circumstellar magnetic field and by globalmagnetic field reversal.
| Monitoring of long term behaviour of OH masers in semiregular variables: R Crt, W Hya and RT Vir
We present and interpret the results of a long-term OH variability studyof three semiregular stars, one type a (SRa), W Hya, and two type b(SRb), R Crt and RT Vir. The 1665 and 1667 MHz OH masers of the threesemiregulars were observed at intervals in the period 1982 January-1995December using the Nançay radio telescope, and we searched for1612 MHz emission. The OH maser profiles of the studied starssignificantly deviated from a standard double-peaked profile. Thetimescale of profile changes in the two SRb stars R Crt and RT Vir wasas short as a month. The OH profiles of the SRa star W Hya were muchmore stable but since November 1986 a very blue-shifted feature appearedat 1667 MHz. Our phase-lag measurements suggest that this feature comesfrom a detached OH shell of radius ~ 3x 1016 cm. Faint 1612MHz emission was found in W Hya only. Weak emission at velocities veryclose to the systemic velocity usually appeared during some intervals ofhigh maser activity in R Crt and RT Vir and was almost always present inW Hya. For R Crt we estimated that this tangential emission disappearedwhen the kinetic temperature in the OH maser regions dropped below150-200 K. For a few features, line narrowing and re-broadening wereobserved on timescales of 90-200 days. The linewidth was inverselyproportional to the peak flux density, suggesting unsaturatedamplification. Cyclic variations in the integrated flux density wereobserved in all the three stars. The OH variability curves weregenerally characterised by large amplitude (4-6m) variationsover 400-800 days superimposed with 100-300-day variations of0.2-2m. Only the measured OH period of W Hya, of 362+/- 7days, was in agreement with the optical period. The two SRb starsexhibited multi-periodic OH variability including with two statisticallysignificant periods. The behaviour of their red- and blue-shiftedemission was less correlated than in W Hya. The ratios of the fluxdensities at 1667 MHz to that at 1665 MHz in all the three stars wereabout 2 at epochs of high OH activity and usually increased during weakmaser emission. Long term behaviour of the OH masers from W Hyaresembled that of standard OH Miras, while that of R Crt and RT Virsuggested thin and clumpy envelopes where unsaturated emission wassustained in some clouds. Figures A1-A3 are only available in electronicform at http://www.edpsciences.org
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| Mira kinematics from Hipparcos data: a Galactic bar to beyond the Solar circle
The space motions of Mira variables are derived from radial velocities,Hipparcos proper motions and a period-luminosity relation. Thepreviously known dependence of Mira kinematics on the period ofpulsation is confirmed and refined. In addition, it is found that Miraswith periods in the range 145-200d in the general Solar neighbourhoodhave a net radial outward motion from the Galactic Centre of75+/-18kms-1. This, together with a lag behind the circularvelocity of Galactic rotation of 98+/-19kms-1, is interpretedas evidence for an elongation of their orbits, with their major axesaligned at an angle of ~17° with the Sun-Galactic Centre line,towards positive Galactic longitudes. This concentration seems to be acontinuation to the Solar circle and beyond of the bar-like structure ofthe Galactic bulge, with the orbits of some local Miras probablypenetrating into the bulge. These conclusions are not sensitive to thedistance scale adopted. A further analysis is given of the short-period(SP) red group of Miras discussed in companion papers in this series. InAppendix A the mean radial velocities and other data for 842 oxygen-richMira-like variables are tabulated. These velocities were derived frompublished optical and radio observations.
| Modelling of oxygen-rich envelopes using corundum and silicate grains
A set of 31 oxygen-rich stars has been modelled using corundum andsilicate grains. These stars were selected according to theirdust-envelope class, as suggested by Little-Marenin and Little in 1990.Then 16 stars classified as Sil were modelled using silicate grains; 10Broad class stars using corundum (Al2O3) grains;and 5 `Intermediate' class stars using two kinds of grainsimultaneously: corundum and silicate. The temperature of the centralstars and some characteristics of their circumstellar envelopes such astheir extinction opacities and extensions were determined by fitting theflux curves. The corundum/silicate ratios as well as the energydistributions and temperature laws have been obtained. Based on theauthors' results they suggest the existence of chemical and structuralevolution of the modelled circumstellar dust shells. The temperature ofthe central stars and the temperature of the hottest grains decreasefrom Broad to Intermediate to Sil classes, while the inner radii andoptical depths increase in this sequence.
| ISO-SWS Observations of CO_2 and H_2O in R Cassiopeiae
We present ISO-SWS spectra of the O-rich Mira variable R Cas, showingCO_2 in absorption and emission, and H_2O in absorption. The CO_2absorption feature is the 0110-0000 ro-vibrationalband at 14.97 μm. The emission features are the1000-0110 and 1110-0220ro-vibrational transitions at 13.87 and 13.48 μm respectively. Thewater absorption spectrum shows the nu_1 and nu_3 ro-vibrational bandsin the 2.75-3 μm region. Using LTE models, we derive physicalparameters for the features. We find the CO_2 emission temperature to be~ 1100 K. We discuss the nature of the CO_2 feature at 15 μm and showthat it can be modeled as an emission/absorption band by deviating fromthermal equilibrium for the population of the 0110vibrational level. The H_2O absorption spectrum is shown to arise fromgas at different temperatures, but can be fit reasonably well with twocomponents at T=950 K and T=250 K. The CO_2 emission and hot H_2Oabsorption temperatures are similar, suggesting that these featuresprobe the same region of the inner envelope. We discuss the innerenvelope chemistry using molecular equilibrium calculations and recentmodeling work by Duari et al. (1999), and find our observationsconsistent with the results. We also report the detection of the CO_20110-0000 ro-vibrational band in absorptiontowards another oxygen-rich Mira, IRC+10011. Based on observations withISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, the Netherlands and theUnited Kingdom) with the participation of ISAS and NASA. The SWS is ajoint project of SRON and MPE.
| The spatial and velocity structure of circumstellar water masers
We report observations with the VLA of 22 GHz H_2O maser emission in thecircumstellar envelopes of 11 late-type stars. We model the maseremission towards six of them (IRC +60169, R Crt, U Her, RX Boo, R LMi,and R Cas) with a computer program we have developed, that fitsthree-dimensional Gaussian sources (in position and velocity) to themeasured intensity distribution. This approach to the problem issuperior to the standard procedure of fitting two-dimensional Gaussianmodels to images for each frequency channel, and it is shown to be apowerful method of identifying blended and/or weak maser features. As aresult, we find that the H_2O maser emission is located in a thin shellexpanding from the star in four cases, despite of the fact that theemission is clumpy and the shells appear incomplete. Moreover, our mapsindicate that the H_2O maser emission comes from inner parts of thecircumstellar envelope, which are comparable in extent to the regions inwhich dust grains form and in which the expanding envelope has notreached its terminal velocity. Our analysis also provides the spectrallinewidth of each maser feature, which contains information about thephysical conditions in the emitting region. In particular, it provides atool to discriminate whether or not these masers are saturated as, forunsaturated masers, theory predicts that a correlation between thestrongest features and the narrowest linewidths should exist. Thisbehaviour is found in at least U Her, RX Boo, and R Cas.
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