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Time Monitoring Observations of SiO J = 2-1 and J = 3-2 Maser Emission toward Late-Type Stars
We present the results of simultaneous time monitoring observations ofSiO J=2-1 and J=3-2 maser emission for 10 late-type stars (8 Miravariables, 1 OH/IR star, and 1 supergiant) with the 14 m radio telescopeat Taeduk Radio Astronomy Observatory from 1999 January to 2001February. The SiO v=1, J=2-1 and J=3-2 maser emission was detected atalmost all observational epochs. The SiO v=2, J=2-1 maser was detectedfrom 4 late-type stars (VY CMa, R Cas, χ Cyg, R Leo) and the v=2,J=3-2 maser was detected from 7 stars (R Aqr, TX Cam, R Cas, χ Cyg,W Hya, R Leo, IK Tau). The v=3, J=2-1 and J=3-2 masers were alsodetected from χ Cyg and TX Cam, respectively. Based on theseobservational data, line profile and peak velocity variations withrespect to stellar velocity, antenna temperatures, and their ratiovariations as a function of optical phase of central star wereinvestigated. As main results, the line profile and the peak velocityvariation of the v=1, J=3-2 maser with pulsation phase was found todiffer from the v=1, J=2-1 transition. Similarly, the J=2-1 and J=3-2transitions also differ between rovibrational transitions at a givenpulsation phase. However, it is difficult to find significantcorrelations between the peak velocity variation relative to the stellarvelocity of either the J=3-2 or J=2-1 transitions over pulsation phase,due to limited time sampling in these data. The peak and integratedantenna temperature (PT and IT) ratios among rotational ladders andvibrational states are investigated. These ratios between rotationalladders of the v=1, J=2-1, and J=3-2 masers are averaged to be the peakantenna temperature ratio, PT(v=1, J=3-2)/PT(v=1, J=2-1)~0.29, and theintegrated antenna temperature ratio, IT(v=1, J=3-2)/IT(v=1,J=2-1)~0.21, respectively. In the v=2 state, these ratios are PT(v=2,J=3-2)/PT(v=2, J=2-1)~7.94 and IT(v=2, J=3-2)/IT(v=2, J=2-1)~8.50,respectively. The peak and integrated antenna temperature ratios betweenvibrational states are also averaged to be PT(v=2, J=3-2)/PT(v=1,J=3-2)~1.29, IT(v=2, J=3-2)/IT(v=1, J=3-2)~1.02, PT(v=2, J=2-1)/PT(v=1,J=2-1)~0.06, and IT(v=2, J=2-1)/IT(v=1, J=2-1)~0.05, respectively. Theseintensity ratios for the v=2, J=2-1 and v=2, J=3-2 masers suggest thatline overlaps operating in the v=2, J=2-1 transition do not similarlyaffect the v=2, J=3-2 transition.
| The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
The Boston University-Five College Radio Astronomy Observatory GalacticRing Survey is a new survey of Galactic 13CO J=1-->0emission. The survey used the SEQUOIA multipixel array on the FiveCollege Radio Astronomy Observatory 14 m telescope to cover a longituderange of l=18deg-55.7d and a latitude range of|b|<1deg, a total of 75.4 deg2. Using bothposition-switching and On-The-Fly mapping modes, we achieved an angularsampling of 22", better than half of the telescope's 46" angularresolution. The survey's velocity coverage is -5 to 135 kms-1 for Galactic longitudes l<=40deg and -5 to85 km s-1 for Galactic longitudes l>40deg. Atthe velocity resolution of 0.21 km s-1, the typical rmssensitivity is σ(T*A)~0.13 K. The surveycomprises a total of 1,993,522 spectra. We show integrated intensityimages (zeroth moment maps), channel maps, position-velocity diagrams,and an average spectrum of the completed survey data set. We alsodiscuss the telescope and instrumental parameters, the observing modes,the data reduction processes, and the emission and noise characteristicsof the data set. The Galactic Ring Survey data are available to thecommunity online or in DVD form by request.
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| Phase Lags in the Optical-Infrared Light Curves of Asymptotic Giant Branch Stars
To search for phase lags in the optical-infrared light curves ofasymptotic giant branch stars, we have compared infrared data from theCOBE DIRBE satellite with optical light curves from the AAVSO and othersources. We found 17 examples of phase lags between the times ofinfrared and optical maximum, and 4 stars with no observed lags. Thereis a clear difference between the Mira variables and the semiregularvariables in the sample, with the maximum in the optical preceding thatin the near-infrared in the Miras, while in most of the semiregularvariables no lags are observed. Comparison to published theoreticalmodels indicates that the phase lags in the Miras are due to strongtitanium oxide absorption in the visual at stellar maximum, and suggeststhat Miras pulsate in the fundamental mode, while at least somesemiregular variables are first-overtone pulsators. There is a clearoptical-near-infrared phase lag in the carbon-rich Mira V CrB; this islikely due to C2 and CN absorption variations in the optical.
| 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.
| CO line emission from circumstellar envelopes
Aims.We present the results of a multi-transition CO observationalprogram conducted on a sample of AGB and post-AGB stars envelopes. Wehave collected maps and single pointing observations of these envelopesin 5 rotational transitions ranging from J = 1-0 to J = 6-5, includingin particular new observations of the CO line at 691 GHz at the CSO. Theuse of such a set of mm and submm CO line on stellar envelopes is rareand limited to the work of some authors on IRC+10216. Methods:.Using a model for the CO emission of an AGB circumstellar envelope, incombination with a standard LVG approach, we have conducted a systematicmodelling analysis using the whole set of CO data collected for a sampleof 12 sources. We simultaneously fit all five transitions, taking intoaccount the spatial information provided by the maps. Results: .Wefind mass-loss rates in the range 1 × 10-7 to 4 ×10-4 M_ȯ/yr, and envelope temperatures ranging from 20 Kto 1000 K at a radius of 1016 cm. There seem to be a generalanti-correlation between mass loss rates and temperature, the high massloss rate AGBs having low temperatures, and vice versa. We show thatmost AGB data can be fitted using a constant mass loss rate, at leastwithin the calibration uncertainties associated with the data collectedat different frequencies. For some cases though (e.g. CIT 6, R Hya,χ Cyg), a change in the mass loss rate history needs to be invokedto reconcile data at low- and high-J, a scenario already mentioned byseveral authors to explain observations of WX Psc.
| An Excess Due to Small Grains around the Nearby K0 V Star HD 69830: Asteroid or Cometary Debris?
Spitzer photometry and spectroscopy of the star HD 69830 reveal anexcess of emission relative to the stellar photosphere between 8 and 35μm dominated by strong features attributable to crystalline silicateswith an emitting surface area more than 1000 times that of our zodiacalcloud. The spectrum closely resembles that of the comet C/1995 O1(Hale-Bopp). Since no excess is detected at 70 μm, the emittingmaterial must be quite warm, be confined within a few AU of the star,and originate in grains with low, long-wavelength emissivity, i.e.,grains much smaller than 70μm/2π~10 μm. The strongmineralogical features are evidence for even smaller, possiblysubmicron-sized grains. This small grain size is in direct contrast tothe 10-100 μm grains that dominate the relatively featureless spectraof our zodiacal dust cloud and most other main-sequence stars withexcesses. The upper limit at 70 μm also implies that any Kuiper Beltanalog must be either very cold or less massive than ~5 times our ownKuiper Belt. With collisional and Poynting-Robertson drag times of lessthan 1000 yr for small grains, the emitting material must either (1) becreated through continual grinding down of material in a dense asteroidbelt, or (2) originate in cometary debris arising from either a single``supercomet'' or a very large number of individual comets arriving froma distant reservoir. In the case of a cometary origin for the emission,the mass requirements for continuous generation by many individualcomets are unreasonable, and we favor the capture of a single supercomet into a 0.5-1 AU orbit, where it can evolve a large number of smallgrains over a 2 Myr period.
| 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.
| W Hya through the eye of Odin. Satellite observations of circumstellar submillimetre H2O line emission
We present Odin observations of the AGB star W Hya in the ground-statetransition of ortho-H{2}O, 1{10}-101, at 557 GHz. The line isclearly of circumstellar origin. Radiative transfer modelling of thewater lines observed by Odin and ISO results in a mass-loss rate of(2.5±0.5)×10-7 Mȯyr-1, and a circumstellar H{2}O abundance of(2.0±1.0)×10-3. The inferred mass-loss rate isconsistent with that obtained from modelling the circumstellar CO radioline emission, and also with that obtained from the dust emissionmodelling combined with a dynamical model for the outflow. The very highwater abundance, higher than the cosmic oxygen abundance, can beexplained by invoking an injection of excess water from evaporating icybodies in the system. The required extra mass of water is quite small,on the order of 0.1 Moplus.
| 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.
| First VLBI mapping of circumstellar 29SiO maser emission
We report the first VLBI map of the v=0 J= 1-0 maser line of29SiO in the long-period variable star IRC +10011. We havefound that this maser emission is composed of multiple spots distributedin an incomplete ring, suggesting that this maser is also amplifiedtangentially, as already proposed in other SiO circumstellar masers. Wepresent also VLBI maps for the 7 mm 28SiO v=1 and 2 J= 1-0and the 3 mm v=1 J= 2-1 lines. The 29SiO masing regionappears to be located in a layer in between the 28SiO v=1 J=1-0 and 28SiO v=1 J= 2-1 lines. In addition, we confirm thatthe 86 GHz maser v=1 J= 2-1 forms in an outer region of thecircumstellar envelope compared to the other 28SiO masersstudied. Finally, we discuss the possible implications of theobservational results on the SiO maser pumping theory.
| SiO masers in TX Cam. Simultaneous VLBA observations of two 43 GHz masers at four epochs
We present the results of simultaneous high resolution observations of v= 1 and v = 2, J = 1-0 SiO masers toward TX Cam at four epochs coveringa stellar cycle. We used a new observing technique to determine therelative positions of the two maser maps. Near maser maximum (Epochs IIIand IV), the individual components of both masers are distributed inring-like structures but the ring is severely disrupted near stellarmaser minimum (Epochs I and II). In Epochs III and IV there is a largeoverlap between the radii at which the two maser transitions occur.However in both epochs the average radius of the v = 2 maser ring issmaller than for the v = 1 maser ring, the difference being larger forEpoch IV. The observed relative ring radii in the two transitions, andthe trends on the ring thickness, are close to those predicted by themodel of Humphreys et al. ([CITE], A&A, 386, 256). In manyindividual features there is an almost exact overlap in space andvelocity of emission from the two transitions, arguing against pureradiative pumping. At both Epochs III and IV in many spectral featuresonly 50% of the flux density is recovered in our images, implyingsignificant smooth maser structure. For both transitions we find thatred- and blue-shifted masers occur in all parts of the rings, withrelatively few masers at the systemic velocity. Thus there is noevidence for rotation, although the blue-shifted masers are somewhatmore prominent to the west. At all four epochs red-shifted componentsare generally brighter than blue-shifted ones. Blue-shifted masersbecome very weak at some stellar phases but never completely disappear.At Epochs III and IV, we see many filamentary or spoke-like features inboth v = 1 and v = 2 masers, especially in the red-shifted gas. Thesespokes show systematic velocity gradients consistent with a deceleratingoutward flow with increasing radius. We outline a possible model toexplain why, given the presence of these spokes, there is a deficit ofmaser features at the systemic velocity. The breaking of sphericalsymmetry by spoke-like features may explain the high-velocity wings seenin SiO maser single dish spectra.
| 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
| A study of bright Southern long period variables
In this paper we present radial velocity curves of AGB variables thatexhibit various kinds of anomalies: semiregular variables (SRVs) withtypical mira periods, SRVs exceeding the mira 2.5 mag amplitude limit,miras with secondary maxima in their light curves, and a SRV with a longsecondary period. The stars with reliable Hipparcos parallaxes from thisand from previous studies are plotted in a log P-MK-diagram.Our objects nicely follow the log P-MK-relations determinedfor the LMC. This allows the pulsation mode to be identified. While allmiras fall on the fundamental mode sequence, the SRVs fall on both thefirst overtone and fundamental mode sequences. The SRVs on thefundamental mode sequence occur at both high and low luminosities, someof them being more luminous than larger amplitude miras. Thisdemonstrates observationally that some parameter other than luminosityaffects the stability of long period variables, probably mass. Firstovertone pulsators all show velocity amplitudes around 4 kms-1. For the fundamental mode pulsators, the velocityamplitude shows a correlation with light amplitude. The two miras R Cenand R Nor, known for their double-peaked light curves, have velocitycurves that are quite different. The R Nor velocity curve shows noevidence of the double peaks, meaning that the true pulsation period isthe time between alternate minima or maxima. There is slight evidencefor a double bump in the R Cen velocity curve. It is suggested thatthese stars are relatively massive (3-5 Mȯ).
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} 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/430/165}
| 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.
| Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud - II. Multiplicity of the period-luminosity relation
Data for 4.4 million stars from the Microlensing Observations inAstrophysics (MOA) project are compared with the near-infrared data ofthe Deep Near Infrared Southern Sky Survey (DENIS). More than 4000 starsobserved in both projects show a quite periodic light curve. Among them,a number of stars are likely eclipsing variables, and the others seem tobe pulsating stars. The KS magnitudes of these red variablesare in the range 10-12.5 but a minor clump at KS~ 12.2 mag isalso found. The multiplicity of the period-luminosity relation isconfirmed, but most of the regular, large-amplitude variables are foundon the relation established for the Mira stars. We study the propertiesof the variables on the colour-magnitude diagram constructed with theMOA red band Rm and KS of DENIS. Multiplicity ofthe period-luminosity relation is briefly discussed in relation to theexcitation mechanism of red pulsating variables.
| 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.
| Interferometric Observation of the Highly Polarized SiO Maser Emission from the v = 1, J = 5-4 Transition Associated with VY Canis Majoris
We used the Submillimeter Array to image the SiO maser emission in thev=1, J=5-4 transition associated with the peculiar red supergiant VYCanis Majoris. We identified seven maser components and measured theirrelative positions and linear polarization properties. Five of the masercomponents are coincident to within about 150 mas (~200 AU at thedistance of 1.5 kpc); most of them may originate in the circumstellarenvelope at a radius of about 50 mas from the star along with the SiOmasers in the lowest rotational transitions. Our measurements show thattwo of the maser components may be offset from the inner stellarenvelope (at the 3 σ level of significance) and may be part of alarger bipolar outflow associated with VY CMa identified by Shinnaga etal. The strongest maser feature at a velocity of 35.9 km s-1has a 60% linear polarization, and its polarization direction is alignedwith the bipolar axis. Such a high degree of polarization suggests thatmaser inversion is due to radiative pumping. Five of the other maserfeatures have significant linear polarization.
| Ground-State SiO Maser Emission toward Evolved Stars
We have made the first unambiguous detection of vibrational ground-statemaser emission from 28SiO toward six evolved stars. Using theVery Large Array (VLA), we simultaneously observed the v=0,J=1-0, 43.4GHz ground-state transitions and the v=1,J=1-0, 43.1 GHz firstexcited-state transitions of 28SiO toward the oxygen-richevolved stars IRC +10011, o Ceti, W Hya, RX Boo, NML Cyg, and R Cas andthe S-type star χ Cyg. We detected at least one v=0 SiO maserfeature from six of the seven stars observed, with peak maser brightnesstemperatures ranging from 10,000 to 108,800 K. In fact, four of theseven v=0 spectra show multiple maser peaks, a phenomenon that has notbeen previously observed. Ground-state thermal emission was detected forone of the stars, RX Boo, with a peak brightness temperature of 200 K.Comparing the v=0 and v=1 transitions, we find that the ground-statemasers are much weaker, with spectral characteristics different fromthose of the first excited-state masers. For four of the seven stars,the velocity dispersion is smaller for the v=0 emission than for the v=1emission; for one star, the dispersions are roughly equivalent; and fortwo stars (one of which is RX Boo), the velocity spread of the v=0emission is larger. In most cases, the peak flux density in the v=0emission spectrum does not coincide with the v=1 maser peak. Althoughthe angular resolution of these VLA observations was insufficient tocompletely resolve the spatial structure of the SiO emission, the SiOspot maps produced from the interferometric image cubes suggest that thev=0 masers are more extended than their v=1 counterparts.
| Using VLBI to Probe the Orion KL Outflow on AU Scales
We present the first contemporaneous 43 and 86 GHz VLBI images of thev=1, J=2-->1 and J=1-->0 SiO masers in the Orion KL nebula. Bothmaser species exhibit the same general morphology of earlier J=1-->0maser images, which appear to trace the edges of a bipolar conicaloutflow. Surprisingly, the J=2-->1 masers form farther from thecentral protostar than the J=1-->0 masers, a fact not readilyexplained by current SiO maser pumping models. The average magnitude ofoffsets between corresponding regions of the two masing transitions isapproximately 14% of the total radial extent of the SiO maser emission.This offset indicates that each transition must trace different physicalconditions.
| 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
| 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.
| Infrared investigation from earth and space on the evolutionary state of a sample of LPV
We selected a sample of highly reddened AGB stars among the sourcesobserved with the SWS instrument on the ISO satellite. These SWS dataallow us to compute the source's photometry in the mid-IR filters of thecamera TIRCAM at the TIRGO telescope. Our photometric data, supplementedwith other measurements taken from the literature, permit to select thecarbon-rich sources in the sample. For these stars, a linear relationholds between dust mass loss and the color index [8.8]-[12.5]. One maythen, from photometric data alone, evaluate the total mass loss (forwhich we used the estimate of \citet{loup}, based on radio data). Theoxygen-rich sources, on the other hand, are distributed in two branches,of which the upper one appears superimposed with carbon stars; the starsin this group have both high luminosity and high wind velocity andtherefore higher masses. Finally S stars lie between the carbon-starbranch and the low-mass oxygen-rich stars, in agreement with theirintermediate evolutionary status.
| Beobachtungsegebnisse Bundesdeutsche Arbeitsgemainschaft fur Veranderliche Sterne e.V.
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| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| R Cassiopeiae: Relative Strengths of SiO Masers at 43 and 86 GHz
We present simultaneous VLBI maps of the J=1-->0 (43.1 GHz) andJ=2-->1 (86.2 GHz) silicon monoxide (SiO) v=1 masers in thecircumstellar envelope of the asymptotic giant branch star R Cassiopeiaethat have been registered to submilliarcsecond precision. Our imagesallow specific comparisons of maser strengths from volumes that areabout 1/10 AU in projected size. Bright 86.2 GHz v=1 masers in two casesoperate from the same gas volumes as the 43.1 GHz v=1 masers. Therelative strengths of the 43.1 and 86.2 GHz masers vary from roughlyunity to lower limits of several hundred to 1. The low strength ofJ=2-->1 masers relative to J=1-->0 might arise from neutraldensities of H2<=4×109 cm-3 at1.5-2 times the photospheric radius from R Cas. The SiO masers of R Casat this epoch are not consistent with emission models that employpulsation-driven shocks unless the phasing of such models can beshifted.
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