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First Detection of the SiO (v = 3, J = 2-1) Maser Emission from χ Cygni
From the simultaneous observations of SiO v=3, J=2-1 and J=3-2 masers,SiO v=3, J=2-1 maser emission was detected for the first time toward theS-type Mira variable χ Cyg. The line was a single spike and wasredshifted with respect to the stellar velocity. The SiO v=3, J=3-2maser emission was not detected from this star. However, for the O-richMira variable TX Cam, SiO v=3, J=3-2 maser emission was detected, whilev=3, J=2-1 maser was not detected. Two possible line overlaps wereadopted as an explanation of these observational results.
| Circumstellar Atomic Hydrogen in Evolved Stars
We present new results of a spectroscopic survey of circumstellar H I inthe direction of evolved stars made with the NançayRadiotelescope. The H I line at 21 cm has been detected in thecircumstellar shells of a variety of evolved stars: asymptotic giantbranch stars, oxygen-rich and carbon-rich stars, semiregular and Miravariables, and planetary nebulae. The emissions are generally spatiallyresolved, i.e., larger than 4', indicating shell sizes on the order of 1pc, which opens the possibility of tracing the history of mass loss overthe past ~104-105 yr. The line profiles aresometimes composite. The individual components generally have aquasi-Gaussian shape; in particular, they seldom show the double-hornprofile that would be expected from the spatially resolved opticallythin emission of a uniformly expanding shell. This probably implies thatthe expansion velocity decreases outward in the external shells (0.1-1pc) of these evolved stars. The H I line profiles do not necessarilymatch those of the CO rotational lines. Furthermore, the centroidvelocities do not always agree with those measured in the CO linesand/or the stellar radial velocities. The H I emissions may also beshifted in position with respect to the central stars. Without excludingthe possibility of asymmetric mass ejection, we suggest that these twoeffects could also be related to a nonisotropic interaction with thelocal interstellar medium. H I was detected in emission toward severalsources (ρ Per, α Her, δ2 Lyr, U CMi) thatotherwise have not been detected in any radio lines. Conversely, it wasnot detected in the two oxygen-rich stars with substantial mass-lossrate, NML Tau and WX Psc, possibly because these sources are young, withhydrogen in molecular form, and/or because the temperature of thecircumstellar H I gas is very low (<5 K).This paper is dedicated to the memory of Marie-Odile Mennessier(1940-2004).
| Massive young stellar objects in the Large Magellanic Cloud: water masers and ESO-VLT 3-4 μm spectroscopy
We investigate the conditions of star formation in the Large MagellanicCloud (LMC). We have conducted a survey for water maser emission arisingfrom massive young stellar objects in the 30 Doradus region (N157) andseveral other HII regions in the LMC (N105A, N113 and N160A). We haveidentified a new maser source in 30Dor at the systemic velocity of theLMC. We have obtained 3-4 μm spectra, with the European SouthernObservatory (ESO)-Very Large Telescope (VLT), of two candidate youngstellar objects. N105AIRS1 shows H recombination line emission, and itsSpectral Energy Distribution (SED) and mid-infrared colours areconsistent with a massive young star ionizing the molecular cloud.N157BIRS1 is identified as an embedded young object, based on its SEDand a tentative detection of water ice. The data on these four HIIregions are combined with mid-infrared archival images from the SpitzerSpace Telescope to study the location and nature of the embedded massiveyoung stellar objects and signatures of stellar feedback. Our analysisof 30Dor, N113 and N160A confirms the picture that the feedback from themassive O- and B-type stars, which creates the HII regions, alsotriggers further star formation on the interfaces of the ionized gas andthe surrounding molecular cloud. Although in the dense cloud N105A starformation seems to occur without evidence of massive star feedback, thegeneral conditions in the LMC seem favourable for sequential starformation as a result of feedback. In an Appendix, we present watermaser observations of the galactic red giants RDoradus and WHydrae.
| Submillimeter vibrationally excited water emission from the peculiar red supergiant VY Canis Majoris
Context: .Vibrationally excited emission from the SiO and H2O moleculesprobes the innermost circumstellar envelopes of oxygen-rich red giantand supergiant stars. VY CMa is the most prolific known emission sourcein these molecules. Aims: .Observations were made to search forrotational lines in the lowest vibrationally excited state of H2O.Methods: .The APEX telescope was used for observations of H2O lines atfrequencies around 300 GHz. Results: .Two vibrationally excitedH2O lines were detected, a third one could not be found. In one of thelines we find evidence for weak maser action, similar to known(sub)millimeter ν2 = 1 lines. We find that the otherline's intensity is consistent with thermal excitation by thecircumstellar infrared radiation field. Several SiO lines were detectedtogether with the H2O lines.
| 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.
| Changes in Apparent Size of Giant Stars with Wavelength due to Electron-Hydrogen Collisions
Interferometric measurements of stellar sizes in frequency bands rangingfrom the near-infrared to longer wavelengths give different results.Various explanations have been proposed to account for these variationsin apparent size with wavelength, but none have been entirelyconsistent. We propose that thermal ionization in the stellar atmosphereand resulting opacity, primarily due to free-free electron-hydrogencollisions, play a significant role. Such an opacity has a quadraticdependence on photon wavelength and produces variations in the opacityof the atmosphere with wavelength, consistent with pertinentmeasurements. This may be particularly important for Mira-type stars,and two examples, o Ceti and W Hya, are analyzed as examples. For starsthat are much smaller or with more concentrated mass, it is not likelyto be significant.
| 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.
| 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.
| The 10 μm Feature of M-Type Stars in the Large Magellanic Cloud and the Dust Condensation Sequence
We present 7-14 μm Infrared Space Observatory (ISO) spectroscopy of12 M-type evolved stars in the Large Magellanic Cloud (LMC), in order tostudy the dust mineralogy and condensation process around these stars.The sample stars show a broad dust feature in the 7-14 μm region,which is seen in either emission or (self-) absorption. The shape of thefeature changes with increasing mass-loss rate, M˙, suggesting achange in dust mineralogy as the central star evolves. At low mass-lossrates amorphous alumina and amorphous silicates are observed, while athigh mass-loss rates only amorphous silicates are seen, in agreementwith the classical condensation sequence expected for these materials.We find a clear correlation between M˙ and the peak wavelengthposition of the broad dust feature. Our data suggest a strong dependenceof the dust mineralogy on the temperature at the dust condensationradius.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands and the UK) and with the participation of ISAS and NASA.
| 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.
| Oxygen-rich AGB stars with optically thin dust envelopes
The dust composition and dynamics of the circumstellar envelopes ofoxygen-rich AGB stars with low mass-loss rates (5 ×10-8-10-5 Mȯ yr-1) havebeen investigated. We have analyzed the ISO-SWS spectra of twenty-eightoxygen-rich AGB stars with optically thin shells, and modelled theobservations with the radiative transfer code DUSTY using the opticalconstants from laboratory dust analogues. This has allowed us todetermine the composition of the dust and the physical conditions at theinner edge of the shell. Moreover, by comparing with CO observationsavailable in the literature, we have determined the gas-to-dust massratios and the mass-loss rates of these sources, and analyzed thewind-driving mechanism. The results show that the small amounts of dustpresent in these envelopes, characterized by visual optical depths inthe 0.03-0.6 range, are enough to drive the wind by radiation pressureon the grains. In some sources there are indications of circumstellardust that does not contribute to the wind-driving, and that maydistributed in a disk or clumps. Other sources show signs of variablemass-loss rates. A grain mixture in the shell consisting of aluminiumoxide, melilite, olivine, spinel and Mg{0.1}Fe{0.9}O fit the observedspectra well. From these species, only melilite is required to have afractional abundance greater than 25% in all cases. Although spinelreproduces the 13 μm feature, the absence of the 16.8 μm peak inour SWS spectra casts doubts on this identification. The outcome of themodelling reveals that the olivine content in these CSEs increases withpressure and temperature at the inner edge. Moreover, the aluminiumoxide percentage in the dust of the envelopes shows a positivecorrelation with the gas-to-dust mass ratio. These results, togetherwith the derived dust compositions, are consistent with thethermodynamic dust condensation sequence scenario and its freezing-outdue to kinetics. However, the temperatures at the inner edge of theshell are substantially lower than those predicted by theory.
| 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.
| Diameters of Mira Stars Measured Simultaneously in the J, H, and K' Near-Infrared Bands
We present the first spatially resolved observations of a sample of 23Mira stars simultaneously measured in the near-infrared J, H, and K'bands. The technique used was optical long-baseline interferometry, andwe present for each star visibility amplitude measurements as a functionof wavelength. We also present characteristic sizes at each spectralband, obtained by fitting the measured visibilities to a simple uniformdisk model. This approach reveals the general relation J diameter < Hdiameter < K' diameter.
| 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).
| Chandra Spatially Resolved Spectroscopic Study and Multiwavelength Imaging of the Supernova Remnant 3C 397 (G41.1-0.3)
We present a Chandra observation of the supernova remnant (SNR) 3C 397(G41.1-0.3) obtained with the Advanced CCD Imaging Spectrometer(ACIS-S). Previous studies of this SNR have shown that the remnantharbors a central X-ray ``hot spot'' suggestive of a compact objectassociated with 3C 397. With the Chandra data, we can rule out thenature of the hot spot as a pulsar or a pulsar wind nebula and put anupper limit on the flux of a hidden compact object ofFX(0.5-10keV)~6×10-13 ergs cm-2s-1. We found two point sources in the observed Chandrafield. We argue that neither of them is associated with 3C 397 and thatthe hard source, CXO J190741.2+070650, which is characterized by aheavily absorbed spectrum with a strong Fe line, is a newly discoveredactive galactic nucleus. The Chandra image reveals arcsecond-scaleclumps and knots that are strongly correlated with the radio VLA image,except for the X-ray hot spot. Our Chandra spatially resolvedspectroscopic study shows that one-component models are inadequate andthat at least two nonequilibrium ionization thermal components areneeded to fit the spectra of each selected region. The derived averagespectral parameters are consistent with the previous global ASCA fitsperformed by Safi-Harb and coworkers. However, the hard componentrequires a high abundance of Fe indicating the presence of hot Feejecta. When comparing the eastern with the western lobe, we find thatthe column density, the brightness, and the ionization timescales aregenerally higher for the western side. This result, combined with ourstudy of the 3C 397 environs at millimeter wavelengths, indicates adenser medium to the west of the SNR. Our multiwavelength imaging andspectral study favors the scenario in which 3C 397 is a ~5300 year oldSNR expanding in a medium with a marked density gradient and is likelyto be encountering a molecular cloud on the western side. We proposethat 3C 397 will evolve into a mixed-morphology SNR.
| Beam Size, Shape and Efficiencies for the ATNF Mopra Radio Telescope at 86-115 GHz
We present data characterising the performance of the Mopra RadioTelescope during the period 2000-2004, including measurements of thebeam size and shape, as well as the overall beam efficiency of thetelescope. In 2004 the full width half maximum of the beam was measuredto be 36+/-3'' at 86GHz, falling to 33+/-2'' at 115GHz. Based on ourobservations of Jupiter we measured the beam efficiency of the Gaussianmain beam to be 0.49+/-0.03 at 86GHz and 0.42+/-0.02 at 115GHz. Sourceswith angular sizes of ~80'' couple well to the main beam, while sourceswith angular sizes between ~80'' and ~160'' couple to the both the mainbeam and inner error beam. Measurements indicate that the inner errorbeam contains approximately one-third the power of the main beam. Wealso compare efficiency corrected spectra to measurements made atsimilar facilities and present standard spectra taken towards themolecular clouds Orion-KL and M17-SW.
| 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.
| The COBE DIRBE Point Source Catalog
We present the COBE DIRBE Point Source Catalog, an all-sky catalogcontaining infrared photometry in 10 infrared bands from 1.25 to 240μm for 11,788 of the brightest near and mid-infrared point sources inthe sky. Since DIRBE had excellent temporal coverage (100-1900independent measurements per object during the 10 month cryogenicmission), the Catalog also contains information about variability ateach wavelength, including amplitudes of variation observed during themission. Since the DIRBE spatial resolution is relatively poor (0.7d),we have carefully investigated the question of confusion and haveflagged sources with infrared-bright companions within the DIRBE beam.In addition, we filtered the DIRBE light curves for data points affectedby companions outside of the main DIRBE beam but within the ``sky''portion of the scan. At high Galactic latitudes(|b|>5deg), the Catalog contains essentially all of theunconfused sources with flux densities greater than 90, 60, 60, 50, 90,and 165 Jy at 1.25, 2.2, 3.5, 4.9, 12, and 25 μm, respectively,corresponding to magnitude limits of approximately 3.1, 2.6, 1.7, 1.3,-1.3, and -3.5. At longer wavelengths and in the Galactic plane, thecompleteness is less certain because of the large DIRBE beam andpossible contributions from extended emission. The Catalog also containsthe names of the sources in other catalogs, their spectral types,variability types, and whether or not the sources are known OH/IR stars.We discuss a few remarkable objects in the Catalog, including theextremely red object OH 231.8+4.2 (QX Pup), an asymptotic giant branchstar in transition to a proto-planetary nebula, which has a DIRBE 25μm amplitude of 0.29 +/- 0.07 mag.
| 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
| Infrared Irradiance Calibration
Infrared astronomical measurements are calibrated against referencesources, usually primary standard stars that are, in turn, calibratedeither by direct or indirect means. A direct calibration compares thestar with a certified source, typically a blackbody. Indirect methodsextrapolate a direct measurement of the flux at one wavelength to theflux at another. Historically, α Lyr (Vega) has been used as theprimary standard as it is bright, easily accessible from the northernhemisphere, and is well calibrated in the visual. Until recently, thedirect absolute infrared calibrations of α Lyr and those derivedfrom the absolute solar flux scaled to the observed spectral energydistributions of solar type stars increasingly diverged with wavelengthfrom those obtained using a model atmosphere to extrapolate the absolutevisual flux of Vega into the infrared. The exception is the directcalibration by the 1996/97 Midcourse Space Experiment of the absolutefluxes for a number of the commonly used infrared standard stars,including Vega.
| Probing the inner wind of AGB stars: Interferometric observations of SiO millimetre line emission from the oxygen-rich stars R Dor and L2Pup
High angular resolution Australia Telescope Compact Array (ATCA)observations of SiO ``thermal'' millimetre line emission towards the twooxygen-rich, low mass loss rate AGB stars R Dor andL2 Pup are presented. In both cases the emission isresolved with an overall spherical symmetry. Detailed radiative transfermodelling of the SiO line emission has been performed, and thecomparison between observations and models are conducted in thevisibility plane, maximizing the sensitivity. The excitation analysissuggests that the abundance of SiO is as high as 4×10-5 in the inner part of the wind, close to the predictedvalues from stellar atmosphere models. Beyond a radius of ≈ 1×1015 cm the SiO abundance is significantly lower, about3× 10-6, until it decreases strongly at a radius ofabout 3× 1015 cm. This is consistent with a scenariowhere SiO first freezes out onto dust grains, and then eventuallybecomes photodissociated by the interstellar UV-radiation field. Inthese low expansion velocity sources the turbulent broadening of thelines plays an important role in the line formation. Micro-turbulentvelocity widths in the range 1.1-1.5 km s-1 result in a verygood reproduction of the observed line shapes even if the gas expansionvelocity is kept constant. This, combined with the fact that the SiO andCO lines are well fitted using the same gas expansion velocity (towithin 5-10%), suggest that the envelope acceleration occurs close tothe stellar photosphere, within 20-30 stellar radii.
| Observations of Water Vapor Outflow from NML Cygnus
We report new observations of the far-infrared and submillimeter watervapor emission of NML Cygnus based on data gathered with the InfraredSpace Observatory and the Submillimeter Wave Astronomy Satellite. Wecompare the emission from NML Cyg to that previously published for VYCMa and W Hya in an attempt to establish the validity of recentlyproposed models for the outflow from evolved stars. The data obtainedsupport the contention by Ivezić & Elitzur that theatmospheres of evolved stars obey a set of scaling laws in which theoptical depth of the outflow is the single most significant scalingparameter, affecting both the radiative transfer and the dynamics of theoutflow. Specifically, we provide observations comparing the water vaporemission from NML Cyg, VY CMa, and W Hya and find, to the extentpermitted by the quality of our data, that the results are in reasonableagreement with a model developed by Zubko & Elitzur. Using thismodel we derive a mass loss based on the dust opacities, spectral linefluxes, and outflow velocities of water vapor observed in theatmospheres of these oxygen-rich giants. For VY CMa and NML Cyg, we alsoobtain an estimate of the stellar mass.
| 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.
| 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.
| Multiwavelength diameters of nearby Miras and semiregular variables
We have used optical interferometry to obtain multiwavelength visibilitycurves for eight red giants over the wavelength range 650-1000 nm. Theobservations consist of wavelength-dispersed fringes recorded withMAPPIT (Masked Aperture-Plane Interference Telescope) at the 3.9-mAnglo-Australian Telescope. We present results for four Miras (R Car, oCet, R Hya and R Leo) and four semiregular variables (R Dor, W Hya,L2 Pup and γ Cru). All stars except γ Cru showstrong variations of angular size with wavelength. A uniform-disc modelwas found to be a poor fit in most cases, with Gaussian (or other moretapered) profiles preferred. This, together with the fact that moststars showed a systematic increase in apparent size towards the blue anda larger-than-expected linear size, even in the red, all point towardssignificant scattering by dust in the inner circumstellar environment.Some stars showed evidence for asymmetric brightness profiles, whileL2 Pup required a two-component model, indicating anasymmetrical circumstellar dust shell.
| 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.
| 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.
| High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer
We present first results of an experiment to combine data from Keckaperture masking and the Infrared-Optical Telescope Array to image thecircumstellar environments of evolved stars with ~20 mas resolution. Theunique combination of excellent Fourier coverage at short baselines andhigh-quality long-baseline fringe data allows us to determine thelocation and clumpiness of the innermost hot dust in the envelopes andto measure the diameters of the underlying stars themselves. We findevidence for large-scale inhomogeneities in some dust shells and alsosignificant deviations from uniform brightness for the photospheres ofthe most evolved M stars. Deviations from spherically symmetric massloss in the red supergiant NML Cyg could be related to recent evidencefor dynamically important magnetic fields and/or stellar rotation. Wepoint out that dust shell asymmetries, like those observed here, canqualitatively explain the difficulty recent workers have had insimultaneously fitting the broadband spectral energy distributions andhigh-resolution spatial information, without invoking unusual dustproperties or multiple distinct shells (from hypothetical``superwinds''). This paper is the first to combine opticalinterferometry data from multiple facilities for imaging, and we discussthe challenges and potential for the future of this method, givencurrent calibration and software limitations.
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