目录
图像
上传图像
DSS Images Other Images
相关文章
Observable effects of dust formation in dynamic atmospheres of M-type Mira variables
The formation of dust with temperature-dependent non-grey opacity isconsidered in a series of self-consistent model atmospheres at differentphases of an O-rich Mira variable of mass 1.2Msolar.Photometric and interferometric properties of these models are predictedunder different physical assumptions regarding the dust formation. Theiron content of the initial silicate that forms and the availability ofgrain nuclei are found to be critical parameters that affect theobservable properties. For certain plausible parameter values, dustwould form at 2-3 times the average continuum photospheric radius. Thiswork provides a consistent physical explanation for the larger apparentsize of Mira variables at wavelengths shorter than 1μm than thatpredicted by dust-free fundamental-mode pulsation models.
| Spaeter Einstieg in die Veraenderlichenbeobachtung.
Not Available
| 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.
| Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars
In the light of the recent results of the stellar interferometry, weexamine the nature of the extra molecular layer outside the photosphereof red supergiant stars, so far studied mostly with the use of theinfrared spectra. Although the visibility data are more direct probes ofthe spatial structure of the outer atmosphere, it is essential that theyare analyzed in combination with the spectral data. In the case of theM2 supergiant μ Cephei, several sets of data, both spectra andvisibilities, strongly suggested the presence of an extra molecularlayer (which we referred to as ``MOLsphere'' for simplicity), and thebasic parameters of the MOLsphere are estimated to be excitationtemperature Tex~1600 K, column densities of CO andH2O molecules Ncol~3.0×1020cm-2, and located at about one stellar radius above thephotosphere or Rin~2.0R*. The result showsreasonable agreement with the one based on the infrared spectra alone,and the model inferred from the spectra is now fully supported with therecent visibility data. In the case of the M2 supergiant αOrionis, the infrared spectra and visibilities show a consistent picturein that its MOLsphere is closer to the photosphere(Rin~1.3R*) with higher gas temperature(Tex~2250 K) and lower gas column density(Ncol~1020 cm-2), compared with that ofμ Cep. Some controversy on the interpretation of the mid-infrareddata of α Orionis can be reconciled. Given that the presence ofthe extra molecular layer is reasonably well established, the majorunsolved problem is how to understand the origin of such a rather warmand dense layer in the outer atmosphere.
| 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.
| Speckle Measurements and Differential Photometry of Visual Binaries with the 6 Meter Telescope of the Special Astrophysical Observatory
Results of speckle measurements and differential photometry carried outwith the 6 m telescope of the Special Astrophysical Observatory (Russia)at the end of 2004 are presented. One new (COU 1569) and four improvedorbits (ADS 440AC, ADS 1729, ADS 5726, and COU 1897), as well as theirdynamical mass estimates, are reported.
| 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.
| 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.
| Recent astrophysical results from the VLTI.
Not Available
| Dust scattering in the Miras R Car and RR Sco resolved by optical interferometric polarimetry
We present optical interferometric polarimetry measurements of theMira-like variables R Car and RR Sco, using the Sydney UniversityStellar Interferometer. By making visibility measurements in twoperpendicular polarizations, the relatively low-surface brightness lightscattered by atmospheric dust could be spatially separated from thebright Mira photospheric flux. This is the first reported successful useof long-baseline optical interferometric polarimetry. Observations wereable to place constraints on the distribution of circumstellar materialin R Car and RR Sco. The inner radius of dust formation for both starswas found to be less than 3 stellar radii: much closer than the expectedinnermost stable location for commonly assumed astrophysical `dirtysilicate' dust in these systems (silicate dust with a significant ironcontent). A model with the dust distributed over a shell which isgeometrically thin compared to the stellar radius was preferred over anoutflow. We propose dust components whose chemistry and opacityproperties enable survival at these extreme inner radii.
| Ubernahme der AFOEV Daten in die Einzelbeobachtungsdatenbank der BAV.
Not Available
| Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
Not Available
| Interpretation of angular diameter measurements of Mira variables: role of water
For stars with atmospheres as complex as those of Miras, determinationof fundamental stellar parameters remains a challenge. Occurrence ofwater "shells" and their implication on the interpretation of theangular diameter measurements will be discussed in this talk. The roleof a spectrophotometric index of water in estimating the continuumdiameter will be highlighted.
| Lunar occultations in the near infrared: achievements and new challenges
A brief review of the lunar occultation program in the near infrared forhigh angular resolution study of bright IR sources carried out at PRL inthe last decade is presented. The development of the two channel Fast IRphotometer is described. Major results pertaining to circumstellar duststructures surrounding occulted objects like IRC+10216 and WR104 areoutlined. The challenges for the future in observing lunar occultationsin the L band and in the use of IR arrays for occultation work arediscussed.
| Near-Infrared Water Lines in V838 Monocerotis
V838 Monocerotis had an intriguing, nova-like outburst in 2002 Januarythat has subsequently led to several studies of the object. It is nowrecognized that the outburst of V838 Mon and its evolution are differentfrom those of a classical nova or other classes of well-known eruptivevariables. V838 Mon, along with two other objects that have analogousproperties, appears to make up a new class of eruptive variables. Thereare limited infrared studies of V838 Mon. Here we present near-infraredH-band (1.5-1.75 μm) spectra of V838 Mon from late 2002 to the end of2004. The principal new result from our work is the detection of severalrotation-vibration lines of water in the H-band spectra. The observedwater lines have been modeled to first establish that they are indeeddue to water. Subsequently the temperature and column densities of theabsorbing material, from where the water absorption features originate,are derived. From our analysis, we find that the water features arisefrom a cool, ~750-900 K, region around V838 Mon that appears to begradually cooling with time.
| 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.
| 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.
| 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.
| 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.
| Variability of the H2O maser associated with the M-supergiant S Persei
We present the results from observing the circumstellar maser emissionof the M-type supergiant S Per in the 6{16}-5{23} water-vapour line at1.35 cm. The observations were carried out in 1981-2002 (JD=2 444 900-2452 480) on the RT-22 radio telescope of the Pushchino Radio AstronomyObservatory, Astrospace Center of the Lebedev Institute of Physics,Russian Academy of Sciences. The H2O spectra obtained represent anunprecedented long, uniform dataset on this star. We discuss theproperties of the optical and maser variations of S Per, together withparticulars of the available VLBI maps. The close relation between maserand optical variations favors a model in which mass-loss is episodic.Changes observed in the total H2O line flux follow the visual lightcurve with a delay of 0.01 to 0.5P, where P≈ 800 d is the mean lightcycle for S Per. The feature at VLSR=-44 km s-1flared in July 1988, which seemed to be the response of the maser to anunusually bright optical maximum. The position of the -44-kms-1 feature on the VLBI maps coincides with the directiontoward the optical stellar disc, which can be explained by amplificationof enhanced stellar continuum by the H2O line.
| The K-band intensity profile of R Leonis probed by VLTI/VINCI
We present near-infrared K-band interferometric measurements of the Mirastar R Leonis obtained in April 2001 and January 2002 with the VLTI, thecommissioning instrument VINCI, and the two test siderostats. Theseepochs correspond to near-maximum stellar variability phases ~0.08 and~1.02 (one cycle later), respectively. The April 2001 data cover a rangeof spatial frequencies (31 35 cycles/arcsecond) within the first lobe ofthe visibility function. These measurements indicate a center-to-limbintensity variation (CLV) that is clearly different from a uniform disk(UD) intensity profile. We show that these measured visibility valuesare consistent with predictions from recent self-excited dynamic Miramodel atmospheres that include molecular shells close tocontinuum-forming layers. We derive high-precision Rosseland diametersof 28.5 ± 0.4 mas and 26.2 ± 0.8 mas for the April 2001and January 2002 data, respectively. Together with literature estimatesof the distance and the bolometric flux, these values correspond tolinear radii of 350+50-40 R_ȯ and320+50-40 R_ȯ, and to effective temperaturesof 2930 ± 270 K and 3080 ± 310 K, respectively.Based on public commissioning data released from the ESO VLTI(www.eso.org/projects/vlti/instru/vinci/ vinci_data_sets.html)
| 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).
| 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.
| 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.
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
