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Das helle Maximum von chi Cyg im Sommer 2006.
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| Einstieg in die Veraenderlichenbeobachtung von Mirasternen.
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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.
| 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.
| Mass-loss properties of S-stars on the AGB
We have used a detailed non-LTE radiative transfer code to model newAPEX CO(J = 3 → 2) data, and existing CO radio line data, on asample of 40 AGB S-stars. The derived mass-loss-rate distribution has amedian value of 2 × 10-7~Mȯyr-1, and resembles values obtained for similar samples ofM-stars and carbon stars. Possibly, there is a scarcity ofhigh-mass-loss-rate (≥10-5~Mȯyr-1) S-stars. The distribution of envelope gas expansionvelocities is similar to that of the M-stars, the median is 7.5 kms-1, while the carbon stars, in general, have higher gasexpansion velocities. The mass-loss rate correlates well with the gasexpansion velocity, in accordance with results for M-stars and carbonstars.
| 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.
| Comparison of dynamical model atmospheres of Mira variables with mid-infrared interferometric and spectroscopic observations
We present a comparison of dynamical model atmospheres with mid-infrared(~11 μm) interferometric and spectroscopic observations of the Miravariable o Cet. The dynamical model atmospheres of Mira variablespulsating in the fundamental mode can fairly explain, without assumingad-hoc components, the seemingly contradictory mid-infraredspectroscopic and interferometric observations of o Cet: the 11 μmsizes measured in the bandpass without any salient spectral features areabout twice as large as those measured in the near-infrared. Ourcalculations of synthetic spectra show that the strong absorption due toa number of optically thick H2O lines is filled in by the emission ofthese H2O lines originating in the geometrically extended layers,providing a possible physical explanation for the picture proposed byOhnaka (2004a) based on a semi-empirical modeling. This filling-ineffect results in rather featureless, continuum-like spectra in roughagreement with the observed high-resolution 11 μm spectra, althoughthe models still predict the H2O lines to be more pronounced than theobservations. The inverse P-Cyg profiles of some strong H2O linesobserved in the 11 μm spectra can also be reasonably reproduced byour dynamical model atmospheres. The presence of the extended H2O layersmanifests itself as mid-infrared angular diameters much larger than thecontinuum diameter. The 11 μm uniform-disk diameters predicted by ourdynamical model atmospheres are in fair agreement with those observedwith the Infrared Spatial Interferometer (ISI), but still somewhatsmaller than the observed diameters. We discuss possible reasons forthis discrepancy and problems with the current dynamical modelatmospheres of Mira variables.
| Forty Years of Spectroscopic Stellar Astrophysics in Japan
The development of Japanese spectroscopic stellar astrophysics in therecent 40 years is reviewed from an observational point of view. In thisarticle, the research activities are provisionally divided into fourfields: hot stars, hot emission-line (Be) stars, cool stars, and otherstars. Historical developments of the observational facilities atOkayama Astrophysical Observatory (spectrographs and detectors) are alsosummarized in connection with the progress in scientific researchactivities.
| Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.
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| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
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| 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.
| No Methane Here. The HCN Puzzle: Searching for CH3OH and C2H in Oxygen-rich Stars
I present results from a sensitive imaging search for CH3OHand C2H in two oxygen-rich stars (IK Taurus and TXCamelopardalis) carried out with the Owens Valley Millimeter Array.Neither CH3OH nor C2H are detected in these stars,in agreement with single-dish observations by Charnley and Latter. Theobservations place upper limits on the fractional abundance ofC2H of 9.7×10-9 in IK Tau and3.2×10-8 in TX Cam and also place upper limits on thefractional abundance of CH3OH of 4.5×10-9 inIK Tau and 1.1×10-8 in TX Cam. A chemical modeldeveloped by Charnley and coworkers explaining the presence of HCN inthese objects is clearly ruled out by these observations. The HCNdistribution in these oxygen-rich stars has been mapped with aresolution of ~3". The HCN emission is marginally resolved, centrallyconcentrated, and does not appear to be well fitted by a thin-shellmodel. Continuum fluxes for the sources have been measured. At 96.7 GHz,the observed continuum fluxes were 6.1+/-0.4 mJy for IK Tau and3.7+/-0.4 mJy for TX Cam. The continuum emission is unresolved at the~3" level, as expected from a model for the emission mechanism proposedby Reid and Menten.
| Atmospheric dynamics in carbon-rich Miras
Originating in different depths of the very extended atmospheres of AGBstars, various molecular spectral lines observable in the near-infraredshow diverse behaviours and can be used to probe atmospheric dynamicsthroughout the outer layers of these pulsating red giants. In Nowotny etal. (2005b, A&A, 437, 273, Paper I) time series of synthetichigh-resolution spectra were presented, computed from a dynamic modelatmosphere for a typical carbon-rich Mira. In this work, line profileshapes, their variations during the lightcycle and radial velocitiesderived from wavelength shifts are analyzed and compared with resultsfrom observed FTS spectra of the C-rich Mira S Cep and other Miras. Itis found that the global velocity structure of the model is inqualitative agreement with observations. Radial velocities of molecularlines sampling different layers behave comparably, although somedifferences are apparant concerning absolute values. A correction factorof p ≈ 1.36 between measured RVs and actual gas velocities is derivedfor CO Δ v=3 lines. It is shown that dynamic model atmospheres arecapable of reproducing Mira spectra without introducing an additional"static layer" proposed by several authors.
| Atmospheric dynamics in carbon-rich Miras
Atmospheres of evolved AGB stars are heavily affected by pulsation, dustformation and mass loss, and they can become very extended. Time seriesof observed high-resolution spectra proved to be a useful tool to studyatmospheric dynamics throughout the outer layers of these pulsating redgiants. Originating at various depths, different molecular spectrallines observed in the near-infrared can be used to probe gas velocitiesthere for different phases during the lightcycle. Dynamic modelatmospheres are needed to represent the complicated structures of Miravariables properly. An important aspect which should be reproduced bythe models is the variation of line profiles due to the influence of gasvelocities. Based on a dynamic model, synthetic spectra (containing COand CN lines) were calculated, using an LTE radiative transfer code thatincludes velocity effects. It is shown that profiles of lines thatsample different depths qualitatively reproduce the behaviour expectedfrom observations.
| 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
| Mid-infrared interferometry of the Mira variable RR Sco with the VLTI MIDI instrument
We present the results of the first mid-infrared interferometricobservations of the Mira variable RR Sco with the MID-infraredInterferometer (MIDI) coupled to the European Southern Observatory's(ESO) Very Large Telescope Interferometer (VLTI), together with K-bandobservations using VLTI VINCI. The observations were carried out in June2003, when the variability phase of the object was 0.6, using two unittelescopes (UT1 and UT3), as part of the Science Demonstration Time(SDT) program of the instrument. Projected baseline lengths ranged from73 to 102 m, and a spectral resolution of 30 was employed in theobservations, which enabled us to obtain the wavelength dependence ofthe visibility in the region between 8 and 13 μm. The uniform-diskdiameter was found to be 18 mas between 8 and 10 μm, while itgradually increases at wavelengths longer than 10 μm to reach 24 masat 13 μm. The uniform-disk diameter between 8 and 13 μm issignificantly larger than the K-band uniform-disk diameter of 10.2± 0.5 mas measured using VLTI VINCI with projected baselinelengths of 15-16 m, three weeks after the MIDI observations. Our modelcalculations show that optically thick emission from a warm molecularenvelope consisting of H2O and SiO can cause the apparent mid-infrared diameter to be much larger than the continuum diameter. Wefind that the warm molecular envelope model extending to 2.3R\star with a temperature of 1400 K and column densitiesof H2O and SiO of 3 × 1021 cm-2and 1 × 1020 cm-2, respectively, canreproduce the observed uniform-disk diameters between 8 and 10 μm.The observed increase of the uniform-disk diameter longward of 10 μmcan be explained by an optically thin dust shell consisting of silicateand corundum grains. The inner radius of the optically thin dust shellis derived to be 7-8 R\star with a temperature of 700 K,and the optical depth at 10 μm is found to be 0.025.Based on observations made with the Very Large Telescope Interferometerof the European Southern Observatory.
| Pulsation of M-type Mira variables with moderately different mass: search for observable mass effects
Models of M-type Miras with masses of 1 and 1.2 Msolar, i.e.with envelope masses of about 0.4 and 0.6 Msolar, have beenconstructed, and a comparison has been made of their observableproperties. Geometric pulsation of continuum-forming layers is found tobe little affected by the mass difference. The influence of molecularcontamination of near-infrared continuum bandpasses uponinterferometrically measured fit diameters ranges from undetectable toquite significant. Some pulsation cycles of the lower-mass model Mirashow substantially stronger contamination than that found in any cycleof the higher-mass star. Observations which sample pulsation phase welland continuously are crucial for avoiding misinterpretations, becausethe assignment of absolute pulsation phases is inherently uncertain byat least 0.1 cycles, diameter changes may be strongly phase-dependent,and cycle-to-cycle variations may be substantial. In accord withexpectations, we find that cycle-to-cycle variations that show up inlight curves and in near-continuum diameters tend to be larger and morecommon in the low-mass models, leading to one possible way todiscriminate mass. Two other methods, based on high-precisionmeasurements of the pulsation amplitude and on derivation of pre-maximumeffective temperatures from diameter measurements, are also discussed.High-layer features that may be strongly affected by mass are not welldescribed by present dust-free models.
| On the observability of geometric pulsation of M-type Mira variables
The geometric pulsation of an M-type Mira variable as a function ofphase is studied by means of a series of complete self-excited modelsthat follows two successive cycles with time intervals of about 0.1cycles. Pulsation of the optical-depth diameter in a real-continuumbandpass in the near-infrared is predicted to be a simplesinusoidal-like motion. Pulsation of interferometrically measurablediameters is close to this motion only in the case of very smallmolecular-band contamination of the continuum in the bandpass ofobservation. Contamination depends on phase and on cycle and tends to besmallest just before and after maximum visual phase. The observed phasedependence of near-continuum diameters quoted in the literature resultsfrom superposition of effects of geometric pulsation and molecularabsorption. Measurements of such molecule-affected diameters are a goodtool for probing the time-dependent structure of the upper Miraatmosphere where molecules form.
| 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.
| Letter to the Editor: Brighter Maxima of 30 Selected Mira-Type Variable Stars for the Period 1978-1977
Letter to the Editor
| Correlation of Successive Maxima in Mira, Chi Cygni, R Leonis, and R Hydrae
An anaylsis was done of the maxima of Mira, Chi Cygni, R Leonis, and RHydrae, four Mira-type variable stars chosen from among those studied byArgelander in 1839. A negative correlation between the luminosity ofconsecuitve maxima was found, except for R Hya, M(i+1) - M(i) vs. M(i)yields a correlation index of r2 = 0.6 for Mira and Chi Cyg,r2 = 0.44 for R Leo, and r2 = 0.25 for R Hya. Dataanalyzed range from the discovery of Mira (1596) to the present time;they include AAVSO data, and there are from 90( RHya) to 179 (Mira Ceti)pairs of consecutive maxima.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
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| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 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.
| 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.
| 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
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Observation and Astrometry data
| Constellation: | Schwan |
| Right ascension: | 19h50m33.90s |
| Declination: | +32°54'51.0" |
| Apparent magnitude: | 4.23 |
| Distance: | 106.045 parsecs |
| Proper motion RA: | -22.5 |
| Proper motion Dec: | -39.8 |
| B-T magnitude: | 9.648 |
| V-T magnitude: | 8.128 |
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