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The Three-Dimensional Morphology of VY Canis Majoris. I. The Kinematics of the Ejecta
Images of the complex circumstellar nebula associated with the famousred supergiant VY CMa show evidence for multiple and asymmetricmass-loss events over the past 1000 yr. Doppler velocities of the arcsand knots in the ejecta show that they are not only spatially distinctbut also kinematically separate from the surrounding diffuse material.In this paper we describe second-epoch HST WFPC2 images to measure thetransverse motions, which when combined with the radial motions providea complete picture of the kinematics of the ejecta, including the totalspace motions and directions of the outflows. Our results show that thearcs and clumps of knots are moving at different velocities, indifferent directions, and at different angles relative to the plane ofthe sky and to the star, confirming their origin from eruptions atdifferent times and from physically separate regions on the star. Weconclude that the morphology and kinematics of the arcs and knots areconsistent with a history of mass ejections not aligned with anypresumed axis of symmetry. The arcs and clumps represent relativelymassive outflows and ejections of gas very likely associated withlarge-scale convective activity and magnetic fields.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Lithium and zirconium abundances in massive Galactic O-rich AGB stars
Lithium and zirconium abundances (the latter taken as representative ofs-process enrichment) are determined for a large sample of massiveGalactic O-rich AGB stars, for which high-resolution opticalspectroscopy has been obtained (R˜ 40 000{-}50 000). This was doneby computing synthetic spectra based on classical hydrostatic modelatmospheres for cool stars and using extensive line lists. The resultsare discussed in the framework of "hot bottom burning" (HBB) andnucleosynthesis models. The complete sample is studied for variousobservational properties such as the position of the stars in the IRAStwo-colour diagram ([ 12] - [25] vs. [ 25] - [60] ), Galacticdistribution, expansion velocity (derived from the OH maser emission),and period of variability (when available). We conclude that aconsiderable fraction of these sources are actually massive AGB stars(M>3{-}4 Mȯ) experiencing HBB, as deduced from thestrong Li overabundances we found. A comparison of our results withsimilar studies carried out in the past for the Magellanic Clouds (MCs)reveals that, in contrast to MC AGB stars, our Galactic sample does notshow any indication of s-process element enrichment. The differencesobserved are explained as a consequence of metallicity effects. Finally,we discuss the results obtained in the framework of stellar evolution bycomparing our results with the data available in the literature forGalactic post-AGB stars and PNe.Based on observations at the 4.2 m William Herschel Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de Los Muchachos of the Instituto de Astrofisicade Canarias. Also based on observations with the ESO 3.6 m telescope atLa Silla Observatory (Chile). Tables [see full textsee full text]-[seefull textsee full text] are only available in electronic form athttp://www.aanda.org
| The Long-Term Behavior of the Semiregular M Supergiant Variable BC Cygni
The long-term brightness changes in the M supergiant, type C semiregularvariable, Type II supernova progenitor BC Cyg from 1890 to the presentare examined using archival data obtained from photographic plates inthe collections of Harvard College Observatory and SternbergAstronomical Institute, in conjunction with AAVSO observations. BC Cygpulsates with a period close to 700 days but underwent a 0.5 magincrease in brightness between 1900 and 2000, in conjunction with adecrease in pulsation period from 699 to 687 days. Such changes appearto be evolutionary in origin, representing a small contraction of thestar associated with a slight decrease in luminosity. Its evolutionarymass is estimated to be ~19 Msolar from its membership inBerkeley 87, a feature that also permits it to be used to test the Msupergiant period-luminosity relation.
| Variability in red supergiant stars: pulsations, long secondary periods and convection noise
We study the brightness variations of galactic red supergiant starsusing long-term visual light curves collected by the AmericanAssociation of Variable Star Observers over the last century. The fullsample contains 48 red semiregular or irregular variable stars, with amean time-span of observations of 61 yr. We determine periods and periodvariability from analyses of power density spectra and time-frequencydistributions. We find two significant periods in 18 stars. Most ofthese periods fall into two distinct groups, ranging from a few hundredto a few thousand days. Theoretical models imply fundamental, first andpossibly second overtone mode pulsations for the shorter periods.Periods greater than 1000 d form a parallel period-luminosity relationthat is similar to the long secondary periods of the asymptotic giantbranch stars. A number of individual power spectra shows a single moderesolved into multiple peaks under a Lorentzian envelope, which weinterpret as evidence for stochastic oscillations, presumably caused bythe interplay of convection and pulsations. We find a strong 1/f noisecomponent in the power spectra that is remarkably similar in almost allstars of the sample. This behaviour fits the picture of irregularphotometric variability caused by large convection cells, analogous tothe granulation background seen in the Sun.
| Changes in the Velocity Centroid of SiO Maser Emission
The v=1, J=1-0 silicon monoxide maser emission from 85 stars wasresurveyed in 2005 and 2006. The emission from these stars hadpreviously been measured in 1979 and/or 1983. For the detected stars thevelocity centroids were determined and compared with the previousvalues. If supergiant stars, known binary stars, and S Virginis wereremoved from the data set, 76 sources remained. Seven of these sourceswere not of sufficient flux density to be claimed as detections. Themean difference in the velocity centroids of the remaining sources was0.065 km s-1, and the standard deviation was 2.00 km s-1. It is important to determine the standard deviation of SiOmaser time variations in order to identify the level of the timevariability of the sources, possible binary stars, unusual emissionpatterns, or perhaps planets. The velocity centroid of S Virginisdiffered by 13.5 km s-1 from its 1983 value. This differenceindicates that S Virginis is probably part of a multiple-star system. Anaive analysis of the present data combined with earlier publishedobservations indicates a revolution period of 14.8 yr.
| Discovery of an Extraordinarily Massive Cluster of Red Supergiants
We report the discovery of an extraordinarily massive young cluster ofstars in the Galaxy, having an inferred total initial cluster masscomparable to the most massive young clusters in the Galaxy. UsingIRMOS, 2MASS, and Spitzer observations, we conclude that there are 14red supergiants in the cluster, compared with five, in what waspreviously thought to be the richest Galactic cluster of such stars. Weinfer spectral types from near-infrared spectra that reveal deep CObandhead absorption that can only be fit by red supergiants. We identifya gap of ΔKs~4 mag between the stars and the bulk ofthe other stars in the region that can only be fit by models if thebrightest stars in the cluster are red supergiants. We estimate adistance of 5.8 kpc to the cluster by associating an OH maser with theenvelope of one of the stars. We also identify a ``yellow'' supergiantof G6 I type in the cluster. Assuming a Salpeter IMF, we infer aninitial cluster mass of 20,000-40,000 Msolar for cluster agesof 7-12 Myr. Continuing with these assumptions, we find that 80% of theinitial mass and 99% of the number of stars remain at the present time.We associate the cluster with an X-ray source (detected by ASCA andEinstein), a recently discovered very high energy γ-ray source(detected by INTEGRAL and HESS), and several nonthermal radio sources,finding that these objects are likely related to recent supernovae inthe cluster. In particular, we claim that the cluster has produced atleast one recent supernova remnant with properties similar to the CrabNebula.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| 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.
| Inward Motions of the Compact SiO Masers around VX Sagittarii
We report Very Long Baseline Array (VLBA) observations of 43 GHz v=1,J=1-0 SiO masers in the circumstellar envelope of the M-type semiregularvariable star VX Sgr at three epochs during 1999 April-May. Thesehigh-resolution VLBA images reveal a persistent ringlike distribution ofSiO masers with a projected radius of ~3 stellar radii. The typicalangular size of 0.5 mas for individual maser features was estimated fromtwo-point correlation function analysis of maser spots. We found thatthe apparent size scale of maser features was distinctly smaller thanthat observed in the previous observations by comparing their fractionsof the total power imaged. This change in the size scale of maseremission may be related to stellar activity that caused a large SiOflare during our observations. Our observations confirmed the asymmetricdistribution of maser emission, but the overall morphology has changedsignificantly, with the majority of the masers clustering to thenortheast of the star compared to the majority of the masers lying inthe southwest in 1992. By identifying 42 matched maser featuresappearing in all three epochs, we determined the contraction of an SiOmaser shell toward VX Sgr at a proper motion of -0.507+/-0.069 masyr-1, corresponding to a velocity of about 4 kms-1 at a distance of 1.7 kpc to VX Sgr. Such a velocity is onthe order of the sound speed and can be easily explained by thegravitational infall of material from the circumstellar dust shell.
| Thermal SiO and H13CO+ Line Observations of the Dense Molecular Cloud G0.11-0.11 in the Galactic Center Region
We obtained the first view in H13CO+ J=1-0 and ahigh-resolution map in thermal SiO lines of G0.11-0.11, which is amolecular cloud situated between the Galactic Center radio arc and SgrA. From a comparison with previous line observations, we found that theH13CO+ J=1-0 line is optically thin, whereas thethermal SiO lines are optically thick. The line intensity inH13CO+ J=1-0 shows that the cloud has a largecolumn density, up to N(H2)=(6-7)×1023cm-2, which corresponds to about 640-740 mag in AVor 10-12 mag in A25μm. The estimated column density is thelargest known of any even in the Galactic center region. We concludefrom the intensity ratio of SiO J=1-0 to CS J=1-0 that emitting gas ishighly inhomogeneous for SiO abundance on a scale smaller than the beamwidth ~35".This work was carried out under the common-use observation program atNobeyama Radio Observatory (NRO).
| The Circumstellar Environments of NML Cygni and the Cool Hypergiants
We present high-resolution HST WFPC2 images of compact nebulositysurrounding the cool M-type hypergiants NML Cyg, VX Sgr, and S Per. Thepowerful OH/IR source NML Cyg exhibits a peculiar bean-shaped asymmetricnebula that is coincident with the distribution of its H2Ovapor masers. We show that NML Cyg's circumstellar envelope is likelyshaped by photodissociation from the powerful, nearby association CygOB2 inside the Cygnus X superbubble. The OH/IR sources VX Sgr and S Perhave marginally resolved envelopes. S Per's circumstellar nebula appearselongated in a northeast/southwest orientation similar to that for itsOH and H2O masers, while VX Sgr is embedded in a spheroidalenvelope. We find no evidence for circumstellar nebulosity around theintermediate-type hypergiants ρ Cas, HR 8752, and HR 5171a, noraround the normal M-type supergiant μ Cep. We conclude that there isno evidence for high mass loss events prior to 500-1000 years ago forthese four stars.
| Structure of Solar-Wind Streams at the Maximum of Solar Cycle 23
We study the formation of solar-wind streams in the years of maximumsolar activity 2000 2002. We use observations of the scattering of radioemission by solar-wind streams at distances of ˜4 60R S from theSun, data on the magnetic field structure and strength in the sourceregion (R ˜ 2.5R S), and observations with the LASCO coronagraphonboard the SOHO spacecraft. Analysis of these data allowed us toinvestigate the changes in the structure of circumsolar plasma streamsduring the solar maximum. We constructed radio maps of the solar-windtransition, transonic region in which the heliolatitudinal streamstructure is compared with the structure of the white-light corona. Weshow that the heliolatitudinal structure of the white-light coronalargely determines the structure of the solar-wind transition region. Weanalyze the correlation between the location of the inner boundary ofthe transition region R in and the magnetic field strength on the sourcesurface |B R|. We discuss the peculiarities of the R in = F(|B R|)correlation diagrams that distinguish them from similar diagrams atprevious phases of the solar cycle.
| An empirical formula for the mass-loss rates of dust-enshrouded red supergiants and oxygen-rich Asymptotic Giant Branch stars
We present an empirical determination of the mass-loss rate as afunction of stellar luminosity and effective temperature, foroxygen-rich dust-enshrouded Asymptotic Giant Branch stars and redsupergiants. To this aim we obtained optical spectra of a sample ofdust-enshrouded red giants in the Large Magellanic Cloud, which wecomplemented with spectroscopic and infrared photometric data from theliterature. Two of these turned out to be hot emission-line stars, ofwhich one is a definite B[e] star. The mass-loss rates were measuredthrough modelling of the spectral energy distributions. We thus obtainthe mass-loss rate formula log dot{M} = -5.65 + 1.05 log ( L / 10 000{L}_ȯ ) -6.3 log ( T_eff / 3500 K ), valid for dust-enshrouded redsupergiants and oxygen-rich AGB stars. Despite the low metallicity ofthe LMC, both AGB stars and red supergiants are found at late spectraltypes. A comparison with galactic AGB stars and red supergiants showsexcellent agreement between the mass-loss rate as predicted by ourformula and that derived from the 60 μm flux density fordust-enshrouded objects, but not for optically bright objects. Wediscuss the possible implications of this for the mass-loss mechanism.
| 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.
| Three-micron spectra of AGB stars and supergiants in nearby galaxies
The dependence of stellar molecular bands on the metallicity is studiedusing infrared L-band spectra of AGB stars (both carbon-rich andoxygen-rich) and M-type supergiants in the Large and Small MagellanicClouds (LMC and SMC) and in the Sagittarius Dwarf Spheroidal Galaxy. Thespectra cover SiO bands for oxygen-rich stars, and acetylene (C2H2), CHand HCN bands for carbon-rich AGB stars. The equivalent width ofacetylene is found to be high even at low metallicity. The high C2H2abundance can be explained with a high carbon-to-oxygen (C/O) ratio forlower metallicity carbon stars. In contrast, the HCN equivalent width islow: fewer than half of the extra-galactic carbon stars show the 3.5μm HCN band, and only a few LMC stars show high HCN equivalent width.HCN abundances are limited by both nitrogen and carbon elementalabundances. The amount of synthesized nitrogen depends on the initialmass, and stars with high luminosity (i.e. high initial mass) could havea high HCN abundance. CH bands are found in both the extra-galactic andGalactic carbon stars. One SMC post-AGB star, SMC-S2, shows the 3.3μm PAH band. This first detection of a PAH band from an SMC post-AGBstar confirms PAHs can form in these low-metallicity stars. None of theoxygen-rich LMC stars show SiO bands, except one possible detection in alow quality spectrum. The limits on the equivalent widths of the SiObands are below the expectation of up to 30 Å for LMC metallicity.Several possible explanations are discussed, mostly based on the effectof pulsation and circumstellar dust. The observations imply that LMC andSMC carbon stars could reach mass-loss rates as high as their Galacticcounterparts, because there are more carbon atoms available and morecarbonaceous dust can be formed. On the other hand, the lack of SiOsuggests less dust and lower mass-loss rates in low-metallicityoxygen-rich stars. The effect on the ISM dust enrichment is discussed.
| Time Variation of SiO Masers in VX Sagittarii over an Optically Quiescent Phase
The time variation of SiO masers in a semi-regular variable, VX Sgr, wasinvestigated in the period between 1994 and 2004 when the optical lightcurve exhibited an ˜6-yr quiescent phase intercepting a regularlypulsating era. The quiescent period occurred with a delay of severalyears after a decrease in the SiO maser flux. VLBA observations of SiOmasers made during this period showed no drastic spatial variationexcept for emission features being shifted from south-west tonorth-east. The SiO maser flux decrease, and a succeeding opticalquiescent phase, may indicate that the stellar mass-loss rate diminishedover a few years around 1994. A SiO maser flare occurring in 1999 may bea reminiscence of a final gas blow, which resulted in the opticallyquiescent period.
| 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.
| 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.
| VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure
Studies of the winds from single K and early M evolved stars indicatethat these flows typically reach a significant fraction of theirterminal velocity within the first couple of stellar radii. The mostdetailed spatially resolved information of the extended atmospheres ofthese spectral types comes from the ζ Aur eclipsing binaries.However, the wind acceleration inferred for the evolved primaries inthese systems appears significantly slower than for stars of similarspectral type. Since there are no successful theories for mass loss fromK and early M evolved stars, it is important to place strong empiricalconstraints on potential models and determine whether this difference inacceleration is real or an artifact of the analyses. We have undertakena radio continuum monitoring study of ζ Aurigae (K4 Ib + B5 V)using the Very Large Array to test the wind density model of Baade etal. that is based on Hubble Space Telescope (HST) Goddard HighResolution Spectrograph ultraviolet spectra. ζ Aur was monitored atcentimeter wavelengths over a complete orbital cycle, and fluxvariations during the orbit are found to be of similar magnitude tovariations at similar orbital phases in the adjacent orbit. Duringeclipse, the flux does not decrease, showing that the radio emissionoriginates from a volume substantially larger thanR3K~(150Rsolar)3 surroundingthe B star. Using the one-dimensional density model of the K4 Ibprimary's wind derived from HST spectral line profile modeling andelectron temperature estimates from previous optical and new HSTstudies, we find that the predicted radio fluxes are consistent withthose observed. Three-dimensional hydrodynamic simulations indicate thatthe accretion flow perturbations near the B star do not contributesignificantly to the total radio flux from the system, consistent withthe radio eclipse observations. Our radio observations confirm the slowwind acceleration for the evolved K4 Ib component. ζ Aur's velocitystructure does not appear to be typical of single stars with similarspectral types. This highlights the need for more comprehensivemultiwavelength studies for both single stars, which have been sadlyneglected, and other ζ Aur systems to determine if its windproperties are typical.
| 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
| Joint VLBA/VLTI Observations of the Mira Variable S Orionis
We present the first coordinated Very Long Baseline Array (VLBA) / VeryLarge Telescope Interferometer (VLTI) measurements of the stellardiameter and circumstellar atmosphere of a Mira variable star.Observations of the v=1, J=1-0 (43.1 GHz) and v=2, J=1-0 (42.8 GHz) SiOmaser emission toward the Mira variable S Ori were conducted using theVLBA. Coordinated near-infrared K-band measurements of the stellardiameter were performed using VLTI-VINCI closely spaced in time to theVLBA observations. Analysis of the SiO maser data recorded at a visualvariability phase 0.73 shows the average distance of the masers from thecenter of the distribution to be 9.4 mas for the v=1 masers and 8.8 masfor the v=2 masers. The velocity structure of the SiO masers appears tobe random, with no significant indication of global expansion/infall orrotation. The determined near-infrared, K-band, uniform disk (UD)diameters decreased from ~10.5 mas at phase 0.80 to ~10.2 mas at phase0.95. For the epoch of our VLBA measurements, an extrapolated UDdiameter of ΘKUD=10.8+/-0.3 mas wasobtained, corresponding to a linear radius ofRKUD=2.3+/-0.5 AU orRKUD=490+/-115 Rsolar. Our coordinatedVLBA/VLTI measurements show that the masers lie relatively close to thestellar photosphere at a distance of ~2 photospheric radii, consistentwith model estimates. This result is virtually free of the usualuncertainty inherent in comparing observations of variable stars widelyseparated in time and stellar phase.
| High-Resolution, Long-Slit Spectroscopy of VY Canis Majoris: The Evidence for Localized High Mass Loss Events
High spatial and spectral resolution spectroscopy of the OH/IRsupergiant VY CMa and its circumstellar ejecta reveals evidence for highmass loss events from localized regions on the star occurring over thepast 1000 yr. The reflected absorption lines and the extremely strong KI emission lines show a complex pattern of velocities in the ejecta. Weshow that the large, dusty northwest arc, expanding at ~50 kms-1 with respect to the embedded star, is kinematicallydistinct from the surrounding nebulosity and was ejected about 400 yrago. Other large, more filamentary loops were probably expelled as muchas 800-1000 yr ago, whereas knots and small arcs close to the starresulted from more recent events 100-200 yr ago. The more diffuse,uniformly distributed gas and dust is surprisingly stationary, withlittle or no velocity relative to the star. This is not what we wouldexpect for the circumstellar material from an evolved red supergiantwith a long history of mass loss. We therefore suggest that the highmass loss rate for VY CMa is a measure of the mass carried out by thesespecific ejections accompanied by streams or flows of gas throughlow-density regions in the dust envelope. VY CMa may thus be our mostextreme example of stellar activity, but our results also bring intoquestion the evolutionary state of this famous star. In a separateappendix, we discuss the origin of the very strong K I and other rareemission lines in its spectrum.
| 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.
| Asymptotic giant branch superwind speed at low metallicity
We present the results of a survey for OH maser emission at 1612 MHzfrom dust-enshrouded asymptotic giant branch (AGB) stars and supergiantsin the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud(SMC), with the Parkes radio telescope, aimed at deriving the speed ofthe superwind from the double-peaked OH maser profiles. Out of eighttargets in the LMC we detected five, of which three are new detections -no maser emission was detected in the two SMC targets. We detected forthe first time the redshifted components of the OH maser profile in theextreme red supergiant IRAS 04553-6825, confirming the suspicion thatits wind speed had been severely underestimated. Despite a much improvedspectrum for IRAS 04407-7000, which was known to exhibit a single-peakedOH maser, no complementary peak could be detected. The new detection inIRAS 05003-6712 was also single-peaked, but for two other newdetections, IRAS 04498-6842 and IRAS 05558-7000, wind speeds could bedetermined from their double-peaked maser profiles. The complete sampleof known OH/IR stars in the LMC is compared with a sample of OH/IR starsin the galactic centre. The LMC sources generally show a pronouncedasymmetry between the bright blueshifted maser emission and weakerredshifted emission, which we attribute to the greater contribution ofamplification of radiation coming directly from the star itself, as theLMC sources are both more luminous and less dusty than their galacticcentre counterparts. We confirm that the OH maser strength is a measureof the dust (rather than gas) mass-loss rate. At a given luminosity orpulsation period, the wind speed in LMC sources is lower than ingalactic centre sources, and the observed trends confirm simpleradiation-driven wind theory if the dust-to-gas ratio is approximatelyproportional to the metallicity.
| 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.
| High resolution observations of SiO masers: Comparing the spatial distribution at 43 and 86 GHz
We present sub-milliarcsecond observations of SiO masers in thelate-type stars IRC +10011 and χ Cyg. We have used the NRAO VeryLong Baseline Array (VLBA) to map the 43 GHz (v=1, 2 J=1-0) and the 86GHz (v=1, 2 J=2-1) SiO masers. All the transitions have been imagedexcept the v=2 J=2-1 in IRC +10011. We report the first VLBI map of thev=1 J=2-1 28SiO maser in IRC +10011 as well as the first VLBAimages of SiO masers in an S-type Mira variable, χ Cyg. In thispaper we have focused on the study of the relative spatial distributionof the different observed lines. We have found that in some cases theobservational results are not reproduced by the current theoreticalpumping models, either radiative or collisional. In particular, for IRC+10011, the v=1 J=1-0 and J=2-1 28SiO lines have differentspatial distributions and emitting region sizes, the J=2-1 emissionbeing located in an outer region of the envelope. For χ Cyg, thedistributions also differ, but the sizes of the masing regions arecomparable. We suggest that the line overlaps between ro-vibrationaltransitions of two abundant molecular species, H2O and28SiO, is a possible explanation for the discrepancies foundbetween the observations and the theoretical predictions. We haveintroduced this overlapping process in the calculations of theexcitation of the SiO molecule. We conclude that the line overlaps canstrongly affect the excitation of SiO and may reproduce the unexpectedobservational results for the two sources studied.
| 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.
| 28SiO v = 1 and v = 2, J = 1-0 maser variability in evolved stars. Eleven years of short spaced monitoring
This paper presents and discusses the final data set of a long-term andshort-spaced monitoring of 21 SiO maser sources, mostly evolved stars,carried out in two SiO maser lines at 43 GHz with the ObservatorioAstronómico Nacional 13.7 m telescope at the CentroAstronómico de Yebes (Guadalajara, Spain). In most objects, morethan 80 spectra per transition over a period of 11 years have beenrecorded. The new data presented here, previously unpublished, representnearly 50% of the total SiO data collected in the project. In addition,the availability of optical light curves from the AAVSO for most of theobjects during the whole period of the SiO monitoring, ground-basednear-IR data for four sources overlapping with 3 to 5 observed SiOperiods, and DIRBE near-IR data covering a significant portion of an SiOperiod in 10 sources, make this data set a unique reference forcomparing optical, NIR and SiO variability in order to elucidate thephysical mechanisms that pump SiO masers in evolved stars. The basis forthe conclusions obtained in this work comes from a numerical time seriesanalysis of the suitable SiO, optical and NIR light curves in regularvariables to obtain precise values of the periods and phase lags betweenthe different curves. This analysis shows evidence that in regularvariable evolved stars the three types of emission have the same periodand that the SiO maxima happen in phase with NIR maxima and with a phaselag typically between 0.05 and 0.20 with respect to optical maxima. Weconclude that in these objects the observational evidence presented inthis work favors the radiative pumping of SiO masers against thecollisional pumping.Figures 1-4 and 11-21 are only available in electronic form athttp://www.edpsciences.org
| QX Pup (OH 231-8+4.2) - a Post-AGB/PPN or a Symbiotic Mira?
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