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The Unusual Silicate Dust around HV 2310, an Evolved Star in the Large Magellanic Cloud
The spectrum of HV 2310, an evolved star in the Large Magellanic Cloud,taken with the Infrared Spectrograph (IRS) on the Spitzer SpaceTelescope, reveals the presence of an optically thin shell of silicatedust with unusual spectral structure in the 10 μm feature: anemission peak at 9.7 μm, a saddle at 10.4 μm, and an extendedshoulder to 11.2 μm. This structure is similar to spectra fromcrystalline silicate grains, and of the available optical constants,forsterite provides the best fit. The spectrum also shows structure at14 μm that may arise from an unidentified dust feature.
| 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.
| A self-consistent model of a 22 GHz water maser in a dusty environment near late-type stars
We study the conditions for operation of the 22 GHz ortho-water maser ina dusty medium near late-type stars. The main physical processes, suchas exchange of energy between dust and gas in the radiation field of astar, radiative cooling by water molecules and pumping of water masersare described self-consistently. We show that the presence of dustgrains of various types (or of one type with size distribution) stronglyaffects the maser action. The pumping mechanism based on the presence ofthe dust of different optical properties is able to explain water masersin the silicate carbon star V778 Cyg. However, the masers in the windsfrom asymptotic giant branch stars require an additional source ofheating, for instance due to the dust drift through the gas.
| Magnetic fields around late-type stars using H_2O maser observations .
We present the analysis of the circular polarization, due to Zeemansplitting, of the H_2O masers around a sample of late-type stars todetermine the magnetic fields in their circumstellar envelopes (CSEs).The magnetic field strengths in the H_2O maser regions around the Miravariable stars U Ori and U Her are shown to be several Gauss while thoseof the supergiants S Per, NML Cyg and VY CMa are several hundred mG. Wealso show that large scale magnetic fields permeate the CSE of anevolved star; the polarization of the H_2O masers around VX Sgr revealsa dipole field structure. We shortly discuss the coupling of themagnetic field with the stellar outflow, as such fields could possiblybe the cause of distinctly aspherical mass-loss and the resultingaspherical planetary nebulae.
| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
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| Angular diameters and effective temperatures of 19 evolved stars by lunar occultations.
Not Available
| Evidence of asymmetry in Mira variable U Ori
Near simultaneous, two high angular resolution observations by lunaroccultation technique at the same wavelength (2.2mu m) but at differentposition angles (PA) result in two unique Uniform Disk (UD) angulardiameters of Mira variable U Ori. UD angular diameter obtained fromobservations at Mt. Abu observatory is 11.9(+/-)0.3 milliarcsecond (mas)at PA 136o while from observations at TIRGO observatory UDvalue obtained is 15.14(+/-)0.05 mas at PA 75o. The sourcebrightness profile derived from a model independent analysis shows anasymmetric spatial structure in both cases. Asymmetric structure of thesource at higher spatial scale was also reported by several authors fromOH and H2O maser distribution at radio wavelengths; thesource is more extended at PA of 30 - 60o. Furthermore,moderate level maximum optical intrinsic polarization of ~1-2% at PAapprox 20o - 40o is also detected. All theevidences bring out the spatial asymmetry in U Ori.
| Lunar occultations in the near infrared: achievements and new challenges
A brief review of the lunar occultation program in the near infrared forhigh angular resolution study of bright IR sources carried out at PRL inthe last decade is presented. The development of the two channel Fast IRphotometer is described. Major results pertaining to circumstellar duststructures surrounding occulted objects like IRC+10216 and WR104 areoutlined. The challenges for the future in observing lunar occultationsin the L band and in the use of IR arrays for occultation work arediscussed.
| Diameters of Mira Stars Measured Simultaneously in the J, H, and K' Near-Infrared Bands
We present the first spatially resolved observations of a sample of 23Mira stars simultaneously measured in the near-infrared J, H, and K'bands. The technique used was optical long-baseline interferometry, andwe present for each star visibility amplitude measurements as a functionof wavelength. We also present characteristic sizes at each spectralband, obtained by fitting the measured visibilities to a simple uniformdisk model. This approach reveals the general relation J diameter < Hdiameter < K' diameter.
| Angular Diameter Measurements of Evolved Variables by Lunar Occultations at 2.2 and 3.8 Microns
We report the angular diameters of two Mira variables (U Ari and Z Sco),three semiregular (SR) and irregular variables (SW Vir, η Gem, andμ Gem), and a supergiant SR variable (TV Gem) by lunar occultationobservations in the near-IR broad K band (2.2 μm). Lunar occultationsof η Gem and μ Gem were also observed for the first timesimultaneously in both the K and L' bands, yielding angular diameters at2.2 and 3.8 μm. Effective temperatures and linear radii are alsoderived for all the observed sources and compared with earliermeasurements. The mode of pulsation of both Mira and SR sources in oursample is discussed.
| Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch (AGB)stars undergo a series of short thermal pulses that significantly changetheir luminosity and mass on timescales of hundreds to thousands ofyears. These pulses are confirmed observationally by the existence ofthe short-lived radioisotope technetium in the spectra of some of thesestars, but other observational consequences of thermal pulses are subtleand may only be detected over many years of observations. Secularchanges in these stars resulting from thermal pulses can be detected asmeasurable changes in period if the star is undergoing Mira pulsations.It is known that a small fraction of Mira variables exhibit largesecular period changes, and the detection of these changes among alarger sample of stars could therefore be useful in evolutionary studiesof these stars. The American Association of Variable Star Observers(AAVSO) International Database currently contains visual data for over1500 Mira variables. Light curves for these stars span nearly a centuryin some cases, making it possible to study the secular evolution of thepulsation behavior on these timescales. In this paper we present theresults of our study of period change in 547 Mira variables using datafrom the AAVSO. We use wavelet analysis to measure the period changes inindividual Mira stars over the span of available data. By making linearfits to the period versus time measurements, we determine the averagerates of period change, dlnP/dt, for each of these stars. We findnonzero dlnP/dt at the 2 σ significance level in 57 of the 547stars, at the 3 σ level in 21 stars, and at the level of 6 σor greater in eight stars. The latter eight stars have been previouslynoted in the literature, and our derived rates of period change largelyagree with published values. The largest and most statisticallysignificant dlnP/dt are consistent with the rates of period changeexpected during thermal pulses on the AGB. A number of other starsexhibit nonmonotonic period change on decades-long timescales, the causeof which is not yet known. In the majority of stars, the periodvariations are smaller than our detection threshold, meaning theavailable data are not sufficient to unambiguously measure slowevolutionary changes in the pulsation period. It is unlikely that morestars with large period changes will be found among heretoforewell-observed Mira stars in the short term, but continued monitoring ofthese and other Mira stars may reveal new and serendipitous candidatesin the future.
| Variability of the H2O maser associated with the M-supergiant S Persei
We present the results from observing the circumstellar maser emissionof the M-type supergiant S Per in the 6{16}-5{23} water-vapour line at1.35 cm. The observations were carried out in 1981-2002 (JD=2 444 900-2452 480) on the RT-22 radio telescope of the Pushchino Radio AstronomyObservatory, Astrospace Center of the Lebedev Institute of Physics,Russian Academy of Sciences. The H2O spectra obtained represent anunprecedented long, uniform dataset on this star. We discuss theproperties of the optical and maser variations of S Per, together withparticulars of the available VLBI maps. The close relation between maserand optical variations favors a model in which mass-loss is episodic.Changes observed in the total H2O line flux follow the visual lightcurve with a delay of 0.01 to 0.5P, where P≈ 800 d is the mean lightcycle for S Per. The feature at VLSR=-44 km s-1flared in July 1988, which seemed to be the response of the maser to anunusually bright optical maximum. The position of the -44-kms-1 feature on the VLBI maps coincides with the directiontoward the optical stellar disc, which can be explained by amplificationof enhanced stellar continuum by the H2O line.
| The magnetic field around late-type stars revealed by the circumstellar H_2O masers
Through polarization observations, circumstellar masers are excellentprobes of the magnetic field in the envelopes of late-type stars.Whereas observations of the polarization of the SiO masers close to thestar and on the OH masers much further out were fairly commonplace,observations of the magnetic field strength in the intermediate densityand temperature region where the 22 GHz H2O masers occur have onlyrecently become possible. Here we present the analysis of the circularpolarization, due to Zeeman splitting, of the H2O masers around the Miravariable stars U Her and U Ori and the supergiant VX Sgr. We present anupper limit of the field around U Her that is lower but consistent withprevious measurements, reflecting possible changes in the circumstellarenvelope. The field strengths around U Ori and VX Sgr are shown to be ofthe order of several Gauss. Moreover, we show for the first time thatlarge scale magnetic fields permeate the circumstellar envelopes of anevolved star; the polarization of the H2O masers around VX Sgr reveals adipole field structure. We discuss the coupling of the magnetic fieldwith the stellar outflow, as such fields could possibly be the cause ofdistinctly aspherical mass-loss.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| 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.
| Evidence of asymmetric structure in the atmosphere of Mira variable U Orionis from lunar occultation observations in the near-infrared
We present the infrared angular diameter of Mira variable U Ori,obtained from lunar occultation observations at 2.2 μm. The uniformdisc (UD) angular diameter is determined to be 11.9 +/- 0.3 mas atvariability phase 0.28. The source brightness profile derived from amodel-independent analysis shows an asymmetric spatial structure. Thedispersion in UD angular diameter measurements in comparison with othersimilar measurements at the same phase in the near-infrared can beexplained by a spatial asymmetry of the source, being elongated in thedirection northeast - southwest, at position angle of 50°-70°.Several corollary evidences for the spatial asymmetry of the source arepresented.
| Letter to the Editor: Brighter Maxima of 30 Selected Mira-Type Variable Stars for the Period 1978-1977
Letter to the Editor
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
Not Available
| Observations of Late-Type Variable Stars in the Water Vapor Radio Line. The Long-Period Variable R Cassiopeia
Observations of circumstellar maser emission from the long-periodvariable R Cas in the 1.35-cm water-vapor line are reported. Theobservations were carried out on the 22-m radio telescope of thePushchino Radio Astronomy Observatory in 1980 2003 (JD=2444409 2452724).Over the 23 years of observations, strong flares in the H2O line profilewere recorded in 1982 (with a peak flux density up to 400 Jy) and 19861989 (up to 750 Jy). Subsequently, from 1990 to March 2003, the H2O lineflux was usually below the detection threshold of the radio telescope(<5 10 Jy). Episodic small increases of the emission with peak fluxdensities of 20 60 Jy were observed. The variations of the H2O line fluxF are correlated with variations in the visual brightness of the star.The phase delay Δγ of the F variations relative to theoptical light curve of R Cas ranged from 0.2 0.3P during theobservations (P=430.46d is the star's period). A model for thevariability of the H2O maser in R Cas is discussed. If the variationsare due to periodic impacts by shock waves driven by the stellarpulsations, the time for the shock to travel from the photosphere to theinner boundary of the H2O-masing shell may reach 2 4P. The flares couldbe due to transient episodes of enhanced mass loss by the star or to thepropagation of an exceptionally strong shock from the stellar surface.
| Mid-Infrared Interferometry on Dust Shells around Four Late-Type Stars
The spatial distributions of dust around four late-type stars measuredwith the University of California, Berkeley, Infrared SpatialInterferometer (ISI) located at Mount Wilson, California, are described.Used as a heterodyne interferometer at 11.15 μm, the ISI was able toresolve the dust shells around late-type stars U Orionis, χ Cygni, WAquilae, and IRC +10011 (CIT 3), including their inner radii. Models fordust distribution around these stars have been obtained by fitting thevisibility data for both maximum and minimum luminosities and alsoavailable mid-infrared spectra of each star. Dust grains are modeled asa mixture of silicates and graphite, and since only two telescopes wereused for these observations, the shells are assumed to be sphericallysymmetric. Visibility curves are shown to change with the luminosityphase of the star, with a larger fraction of the total 11 μm fluxemitted from the dust near the star at the maximum than at the minimumphase. Mass-loss rates and estimates of the composition of the dustshell are provided for each star, and dust particle size is determinedfor IRC +10011 (CIT 3) by comparison of near- and mid-infraredvisibilities.
| Unveiling Mira stars behind the molecules. Confirmation of the molecular layer model with narrow band near-infrared interferometry
We have observed Mira stars with the FLUOR beamcombiner on the IOTAinterferometer in narrow bands around 2.2 μm wavelength. We findsystematically larger diameters in bands contaminated by water vapor andCO. The visibility measurements can be interpreted with a modelcomprising a photosphere surrounded by a thin spherical molecular layer.The high quality of the fits we obtain demonstrates that this simplemodel accounts for most of the star's spatial structure. For each starand each period we were able to derive the radius and temperature of thestar and of the molecular layer as well as the optical depth of thelayer in absorption and continuum bands. The typical radius of themolecular layer is 2.2 R* with a temperature ranging between1500 and 2100 K. The photospheric temperatures we find are in agreementwith spectral types of Mira stars. Our photospheric diameters are foundsmaller than in previous studies by several tens of percent. We believeprevious diameters were biased by the use of unsuited geometrical modelsto explain visibilities. The conclusions of this work are various.First, we offer a consistent view of Mira stars over a wide range ofwavelengths. Second, the parameters of the molecular layer we find areconsistent with spectroscopic studies. Third, from our diametermeasurements we deduce that all Mira stars are fundamental modepulsators and that previous studies leading to the conclusion of thefirst-overtone mode were biased by too large diameter estimates.Based on observations collected at the IOTA interferometer, WhippleObservatory, Mount Hopkins, Arizona.Table 3 is only available in electronic form athttp://www.edpsciences.org
| VLBA observations of SiO masers towards Mira variable stars
We present new total intensity and linear polarization VLBA observationsof the ν=2 and ν=1 J=1-0 maser transitions of SiO at 42.8 and 43.1GHz in a number of Mira variable stars over a substantial fraction oftheir pulsation periods. These observations were part of an observingprogram that also includes interferometric measurements at 2.2 and 3.6micron \citep{Mennesson2002}; comparison of the results from differentwavelengths allows studying the envelope independently of the poorlyknown distances to these stars. Nine stars were observed at from one tofour epochs during 2001. The SiO emission is largely confined to ringswhich are smaller than the inner radius of the dust shells reported by\citet{Danchi1994}. Two stars (U Orionis, R Aquarii) have maser ringswith diameters corresponding to the size of the hot molecular layer asmeasured at 3.6 micron; in the other cases, the SiO rings aresubstantially larger. Variations of ring diameter for most, but not allstars, had an rms amplitude in agreement with the models of\citet{Humphreys2002} although the expected relationship between thediameter and pulsation phase was not seen. The ring diameter in UOrionis shows remarkably small variation. A correlation between the2.2/3.6 μm diameter ratio with that of the SiO/3.6 μm diameterratio is likely due to differences in the opacities at 2.2 and 3.6 μmin a molecular layer. A further correlation with the inner size of thedust shell reported by \citet{Danchi1994} suggest some differences inthe temperature structure. Clear evidence is seen in R Aquarii for anequatorial disk similar to that reported by \citet{Hollis2001}; rotationis possibly also detected in S Coronae Boralis.
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| Sub-au imaging of water vapour clouds around four asymptotic giant branch stars
We present MERLIN maps of the 22-GHz H2O masers around fourlow-mass late-type stars (IK Tau, U Ori, RT Vir and U Her), made with anangular resolution of ~15 milliarcsec and a velocity resolution of 0.1km s-1. The H2O masers are found in thickexpanding shells with inner radii ~6 to 16 au and outer radii four timeslarger. The expansion velocity increases radially through theH2O maser regions, with logarithmic velocity gradients of0.5-0.9. IK Tau and RT Vir have well-filled H2O maser shellswith a spatial offset between the near and far sides of the shell, whichsuggests that the masers are distributed in oblate spheroids inclined tothe line of sight. U Ori and U Her have elongated poorly filled shellswith indications that the masers at the inner edge have been compressedby shocks; these stars also show OH maser flares. MERLIN resolvesindividual maser clouds, which have diameters of 2-4 au and fillingfactors of only ~0.01 with respect to the whole H2O masershells. The circumstellar envelope velocity structure gives additionalevidence the maser clouds are density-bounded. Masing clouds can beidentified over a similar time-scale to their sound crossing time (~2yr) but not longer. The sizes and observed lifetimes of these clouds arean order of magnitude smaller than for those around red supergiants,similar to the ratio of low-mass:high-mass stellar masses and sizes.This suggests that cloud size is determined by stellar properties, notlocal physical phenomena in the wind.
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993
| Infrared high angular resolution measurements of stellar sources. VI. Accurate angular diameters of X Cnc, U Ori and Eta Gem
We present the results of near-IR lunar occultation measurements ofthree well-known, large angular diameter, cool giant stars, namelyX Cnc (CII), U Ori (M8III) andeta Gem (M3III). Their 2.2 mu m angular diameters,derived under the assumption of a uniform circular disk, are8.26+/-0.04, 15.14+/-0.05 and 12.57+/-0.04 milliarcsec, respectively. Inthe case of U Ori, we find a significant deviation ofthe brightness profile from circular symmetry, as well as a faintsignature of extended emission. The asymmetry of the brightness profilecannot be explained by scintillation effects in the lunar occultationlightcurve, and could have contributed to the important differences inprevious determinations of the angular diameter of UOri in the near-IR. Also in the case of etaGem, significant differences exist among the available angulardiameter determinations.Based on observations collected at TIRGO (Gornergrat, Switzerland).TIRGO is operated by CNR - CAISMI Arcetri, Italy.
| 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
| Morphology and Galactic Distribution of PNe: a New Scenario
We review recent works on morphology and Galactic distribution ofplanetary nebulae (PNe), as well as recent advances in MHD modeling ofPNe. We arrive at a tentative explanation for the connection betweenmorphological classes and Galactic distribution.
| Stellar masers: a review
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