Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
The mass loss of C-rich giants
The mass loss rates, expansion velocities and dust-to-gas density ratiosfrom millimetric observations of 119 carbon-rich giants are compared, asfunctions of stellar parameters, to the predictions of recenthydrodynamical models. Distances and luminosities previously estimatedfrom HIPPARCOS data, masses from pulsations and C/O abundance ratiosfrom spectroscopy, and effective temperatures from a new homogeneousscale, are used. Predicted and observed mass loss rates agree fairlywell, as functions of effective temperature. The signature of the massrange M≤4 Mȯ of most carbon-rich AGB stars is seenas a flat portion in the diagram of mass loss rate vs. effectivetemperature. It is flanked by two regions of mass loss rates increasingwith decreasing effective temperature at nearly constant stellar mass.Four stars with detached shells, i.e. episodic strong mass loss, andfive cool infrared carbon-rich stars with optically-thick dust shells,have mass loss rates much larger than predicted values. The latter(including CW Leo) could be stars of smaller masses (M≃ 1.5-2.5Mȯ) while M≃ 4 Mȯ is indicated formost of the coolest objects. Among the carbon stars with detachedshells, R Scl returned to a predicted level (16 times lower) accordingto recent measurements of the central source. The observed expansionvelocities are in agreement with the predicted velocities at infinity ina diagram of velocities vs. effective temperature, provided the carbonto oxygen abundance ratio is 1≤ɛ C/ɛO≤2, i.e. the range deduced from spectra and modelatmospheres of those cool variables. Five stars with detached shellsdisplay expansion velocities about twice that predicted at theireffective temperature. Miras and non-Miras do populate the same locus inboth diagrams at the present accuracy. The predicted dust-to-gas densityratios are however about 2.2 times smaller than the values estimatedfrom observations. Recent drift models can contribute to minimize thediscrepancy since they include more dust. Simple approximate formulaeare proposed.This research has made use of the Simbad database operated at CDS.Partially based on data from the ESA HIPPARCOS astrometry satellite.Table 3 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/235
| Fibonacci Sequences and the Multiperiodicity of the Variable Star UW Herculis
We present an application of the methods recently developed for thestudy of quasicrystal structures to the analysis of multiperiodicity ofsemiregular variables. A light curve analysis of UW Her showsfrequencies that can be included within the general schemecharacterizing the Fourier spectra of Fibonacci quasiperiodic sequences.The analysed data come from the BAA Variable Star Section computerisedarchive.
| Spectral Classification of Faint Carbon Stars
R--N classification of 187 faint carbon stars is based on the classicalcriteria adjusted to the yellow-red spectral region, with two newcriteria added -- the ratios of the red CN bands 6206/6332 (Å) and6478/6631 (Å).
| Reprocessing the Hipparcos data of evolved stars. III. Revised Hipparcos period-luminosity relationship for galactic long-period variable stars
We analyze the K band luminosities of a sample of galactic long-periodvariables using parallaxes measured by the Hipparcos mission. Theparallaxes are in most cases re-computed from the Hipparcos IntermediateAstrometric Data using improved astrometric fits and chromaticitycorrections. The K band magnitudes are taken from the literature andfrom measurements by COBE, and are corrected for interstellar andcircumstellar extinction. The sample contains stars of several spectraltypes: M, S and C, and of several variability classes: Mira, semiregularSRa, and SRb. We find that the distribution of stars in theperiod-luminosity plane is independent of circumstellar chemistry, butthat the different variability types have different P-L distributions.Both the Mira variables and the SRb variables have reasonablywell-defined period-luminosity relationships, but with very differentslopes. The SRa variables are distributed between the two classes,suggesting that they are a mixture of Miras and SRb, rather than aseparate class of stars. New period-luminosity relationships are derivedbased on our revised Hipparcos parallaxes. The Miras show a similarperiod-luminosity relationship to that found for Large Magellanic CloudMiras by Feast et al. (\cite{Feast-1989:a}). The maximum absolute Kmagnitude of the sample is about -8.2 for both Miras and semi-regularstars, only slightly fainter than the expected AGB limit. We show thatthe stars with the longest periods (P>400 d) have high mass lossrates and are almost all Mira variables.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA \cite{Hipparcos}).Table \ref{Tab:data1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/403/993
| How many Hipparcos Variability-Induced Movers are genuine binaries?
Hipparcos observations of some variable stars, and especially oflong-period (e.g. Mira) variables, reveal a motion of the photocentercorrelated with the brightness variation (variability-induced mover -VIM), suggesting the presence of a binary companion. A re-analysis ofthe Hipparcos photometric and astrometric data does not confirm the VIMsolution for 62 among the 288 VIM objects (21%) in the Hipparcoscatalogue. Most of these 288 VIMs are long-period (e.g. Mira) variables(LPV). The effect of a revised chromaticity correction, which accountsfor the color variations along the light cycle, was then investigated.It is based on ``instantaneous'' V-I color indices derived fromHipparcos and Tycho-2 epoch photometry. Among the 188 LPVs flagged asVIM in the Hipparcos catalogue, 89 (47%) are not confirmed as VIM afterthis improved chromaticity correction is applied. This dramatic decreasein the number of VIM solutions is not surprising, since the chromaticitycorrection applied by the Hipparcos reduction consortia was based on afixed V-I color. Astrophysical considerations lead us to adopt a morestringent criterion for accepting a VIM solution (first-kind risk of0.27% instead of 10% as in the Hipparcos catalogue). With this moresevere criterion, only 27 LPV stars remain VIM, thus rejecting 161 ofthe 188 (86%) of the LPVs defined as VIMs in the Hipparcos catalogue.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).Table 1 is also available in electronic form at the CDS, via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/399/1167
| Hipparcos red stars in the HpV_T2 and V I_C systems
For Hipparcos M, S, and C spectral type stars, we provide calibratedinstantaneous (epoch) Cousins V - I color indices using newly derivedHpV_T2 photometry. Three new sets of ground-based Cousins V I data havebeen obtained for more than 170 carbon and red M giants. These datasetsin combination with the published sources of V I photometry served toobtain the calibration curves linking Hipparcos/Tycho Hp-V_T2 with theCousins V - I index. In total, 321 carbon stars and 4464 M- and S-typestars have new V - I indices. The standard error of the mean V - I isabout 0.1 mag or better down to Hp~9 although it deteriorates rapidly atfainter magnitudes. These V - I indices can be used to verify thepublished Hipparcos V - I color indices. Thus, we have identified ahandful of new cases where, instead of the real target, a random fieldstar has been observed. A considerable fraction of the DMSA/C and DMSA/Vsolutions for red stars appear not to be warranted. Most likely suchspurious solutions may originate from usage of a heavily biased color inthe astrometric processing.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997).}\fnmsep\thanks{Table 7 is onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/397/997
| High Angular Resolution Observations of Late-Type Stars
This paper presents speckle observations of Mira (o Cet) and late-typestars with the PISCO speckle camera of Pic du Midi during the period1995-1998. A survey for binarity among a sample of late-type stars wasperformed, which led to seven positive detections out of 36 objects.Photometric and color variations of the companion of Mira were searchedfor, but no significant brightness variations could be found over atimescale of ~5-10 minutes. The position and photometry measurements,the restored images with high angular resolution of the binary systemMira A-B (ADS 1778) are in full agreement with Hubble Space Telescopedata obtained at the same epoch. A new orbit has been derived for MiraA-B.
| Carbon-rich giants in the HR diagram and their luminosity function
The luminosity function (LF) of nearly 300 Galactic carbon giants isderived. Adding BaII giants and various related objects, about 370objects are located in the RGB and AGB portions of the theoretical HRdiagram. As intermediate steps, (1) bolometric corrections arecalibrated against selected intrinsic color indices; (2) the diagram ofphotometric coefficients 1/2 vs. astrometric trueparallaxes varpi are interpreted in terms of ranges of photosphericradii for every photometric group; (3) coefficients CR andCL for bias-free evaluation of mean photospheric radii andmean luminosities are computed. The LF of Galactic carbon giantsexhibits two maxima corresponding to the HC-stars of the thick disk andto the CV-stars of the old thin disk respectively. It is discussed andcompared to those of carbon stars in the Magellanic Clouds and Galacticbulge. The HC-part is similar to the LF of the Galactic bulge,reinforcing the idea that the Bulge and the thick disk are part of thesame dynamical component. The CV-part looks similar to the LF of theLarge Magellanic Cloud (LMC), but the former is wider due to thesubstantial errors on HIPPARCOS parallaxes. The obtained meanluminosities increase with increasing radii and decreasing effectivetemperatures, along the HC-CV sequence of photometric groups, except forHC0, the earliest one. This trend illustrates the RGB- and AGB-tracks oflow- and intermediate-mass stars for a range in metallicities. From acomparison with theoretical tracks in the HR diagram, the initial massesMi range from about 0.8 to 4.0 Msun for carbongiants, with possibly larger masses for a few extreme objects. A largerange of metallicities is likely, from metal-poor HC-stars classified asCH stars on the grounds of their spectra (a spheroidal component), tonear-solar compositions of many CV-stars. Technetium-rich carbon giantsare brighter than the lower limit Mbol =~ -3.6+/- 0.4 andcentered at =~-4.7+0.6-0.9 at about =~(2935+/-200) K or CV3-CV4 in our classification. Much like the resultsof Van Eck et al. (\cite{vaneck98}) for S stars, this confirms theTDU-model of those TP-AGB stars. This is not the case of the HC-stars inthe thick disk, with >~ 3400 K and>~ -3.4. The faint HC1 and HC2-stars( =~ -1.1+0.7-1.0) arefound slightly brighter than the BaII giants ( =~-0.3+/-1.3) on average. Most RCB variables and HdC stars range fromMbol =~ -1 to -4 against -0.2 to -2.4 for those of the threepopulation II Cepheids in the sample. The former stars show the largestluminosities ( <~ -4 at the highest effectivetemperatures (6500-7500 K), close to the Mbol =~ -5 value forthe hot LMC RCB-stars (W Men and HV 5637). A full discussion of theresults is postponed to a companion paper on pulsation modes andpulsation masses of carbon-rich long period variables (LPVs; Paper IV,present issue). This research has made use of the Simbad databaseoperated at CDS, Strasbourg, France. Partially based on data from theESA HIPPARCOS astrometry satellite. Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/967
| Amplitude and phase changes in the light curves of long-period variables
We describe the derivation of periodicities from visual data forlong-period variables, and give expected errors for the derivedamplitude, phase and frequency. We extend this analysis by using amoving window to deduce variation in phase and amplitude over time. Themethod is demonstrated on the Mira-type variable T Cas, and showsevidence that the phase of the first harmonic (period 222.4d) variesrelative to the fundamental (period 444.8d) with a period ofapproximately 3000d. Our analysis parallels, but is completelyindependent of, the work of Szatmáry, Gál & Kiss andBedding et al., who both use an approach based on wavelet analysis.
| The semiregular variable UU Aurigae ( analysis in adversity?
BAAVSS data for UU Aurigae from 1971 to 1998 are analysed to deriveperiodicities and to determine the behaviour of the periodic variationsin phase and amplitude. Despite the difficulties associated withobserving this object, distinct pulsational periods of 439.4 and 233.1days are identified, and a long-standing phase and amplitude stabilityis demonstrated.
| ST Camelopardalis: A doubly periodic semiregular variable star
The BAAVSS data for the semiregular variable ST Camelopardalis isanalysed showing periods of 201.0 and 368.6 days (ratio of approximately1.8). This star was not previously known to be doubly periodic. Theamplitudes of the periods tend to vary inversely, the variate itselfhaving a period of about 2200 days. Pulsational mode switching isdiscussed as a possible mechanism for these effects.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| General Catalog of Galactic Carbon Stars by C. B. Stephenson. Third Edition
The catalog is an updated and revised version of Stephenson's Catalogueof Galactic Cool Carbon Stars (2nd edition). It includes 6891 entries.For each star the following information is given: equatorial (2000.0)and galactic coordinates, blue, visual and infrared magnitudes, spectralclassification, references, designations in the most significantcatalogs and coordinate precision classes. The main catalog issupplemented by remarks containing information for which there was noplace in entries of the main part, as well as some occasional notesabout the peculiarities of specific stars.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| The effective temperatures of carbon-rich stars
We evaluate effective temperatures of 390 carbon-rich stars. Theinterstellar extinction on their lines of sights was determined andcircumstellar contributions derived. The intrinsic (dereddened) spectralenergy distributions (SEDs) are classified into 14 photometric groups(HCi, CVj and SCV with i=0,5 and j=1,7). The newscale of effective temperatures proposed here is calibrated on the 54angular diameters (measured on 52 stars) available at present from lunaroccultations and interferometry. The brightness distribution on stellardiscs and its influence on diameter evaluations are discussed. Theeffective temperatures directly deduced from those diameters correlatewith the classification into photometric groups, despite the large errorbars on diameters. The main parameter of our photometric classificationis thus effective temperature. Our photometric < k right >1/2 coefficients are shown to be angular diameters on arelative scale for a given photometric group, (more precisely for agiven effective temperature). The angular diameters are consistent withthe photometric data previously shown to be consistent with the trueparallaxes from HIPPARCOS observations (Knapik, et al. \cite{knapik98},Sect. 6). Provisional effective temperatures, as constrained by asuccessful comparison of dereddened SEDs from observations to modelatmosphere predictions, are in good agreement with the values directlycalculated from the observed angular diameters and with those deducedfrom five selected intrinsic color indices. These three approaches wereused to calibrate a reference angular diameter Phi 0 and theassociated coefficient CT_eff. The effective temperatureproposed for each star is the arithmetic mean of two estimates, one(``bolometric'') from a reference integrated flux F0, theother (``spectral'') from calibrated color indices which arerepresentative of SED shapes. Effective temperatures for about 390carbon stars are provided on this new homogeneous scale, together withvalues for some stars classified with oxygen-type SEDs with a total of438 SEDs (410 stars) studied. Apparent bolometric magnitudes are given.Objects with strong infrared excesses and optically thick circumstellardust shells are discussed separately. The new effective temperaturescale is shown to be compatible and (statistically) consistent with thesample of direct values from the observed angular diameters. Theeffective temperatures are confirmed to be higher than the mean colortemperatures (from 140 to 440 K). They are in good agreement with thepublished estimates from the infrared flux method forTeff>= 3170 K, while an increasing discrepancy is observedtoward lower temperatures. As an illustration of the efficiency of thephotometric classification and effective temperature scale, the C/Oratios and the Merrill-Sanford (M-S) band intensities are investigated.It is shown that the maximum value, mean value and dispersion of C/Oincrease along the photometric CV-sequence, i.e. with decreasingeffective temperature. The M-S bands of SiC2 are shown tohave a transition from ``none'' to ``strong'' at Teff =~(2800+/- 150right ) K. Simultaneously, with decreasing effectivetemperature, the mean C/O ratio increases from 1.04 to 1.36, thetransition in SiC2 strength occurring while 1.07<= C/O<= 1.18. This research has made use of the Simbad database operatedat CDS, Strasbourg, France. Table 10 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)}or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/178
| Models of circumstellar molecular radio line emission. Mass loss rates for a sample of bright carbon stars
Using a detailed radiative transfer analysis, combined with an energybalance equation for the gas, we have performed extensive modelling ofcircumstellar CO radio line emission from a large sample of opticallybright carbon stars, originally observed by Olofsson et al. (ApJS, 87,267). Some new observational results are presented here. We determinesome of the basic parameters that characterize circumstellar envelopes(CSEs), e.g., the stellar mass loss rate, the gas expansion velocity,and the kinetic temperature structure of the gas. Assuming a sphericallysymmetric CSE with a smooth gas density distribution, created by acontinuous mass loss, which expands with a constant velocity we are ableto model reasonably well 61 of our 69 sample stars. The derived massloss rates depend crucially on the assumptions in the circumstellarmodel, of which some can be constrained if enough observational dataexist. Therefore, a reliable mass loss rate determination for anindividual star requires, in addition to a detailed radiative transferanalysis, good observational constraints in the form of multi-lineobservations and radial brightness distributions. In our analysis we usethe results of a model for the photodissociation of circumstellar CO byMamon et al. (1988). This leads to model fits to observed radialbrightness profiles that are, in general, very good, but there are alsoa few cases with clear deviations, which suggest departures from asimple r-2 density law. The derived mass loss rates spanalmost four orders of magnitude, from ~ 5 10-9Msun yr-1 up to ~ 2 10-5Msun yr-1, with the median mass loss rate being ~3 10-7 Msun yr-1. We estimate that themass loss rates are typically accurate to ~ 50% within the adoptedcircumstellar model. The physical conditions prevailing in the CSEs varyconsiderably over such a large range of mass loss rates. Among otherthings, it appears that the dust-to-gas mass ratio and/or the dustproperties change with the mass loss rate. We find that the mass lossrate and the gas expansion velocity are well correlated, and that bothof them clearly depend on the pulsational period and (with largerscatter) the stellar luminosity. Moreover, the mass loss rate correlatesweakly with the stellar effective temperature, in the sense that thecooler stars tend to have higher mass loss rates, but there seems to beno correlation with the stellar C/O-ratio. We conclude that the massloss rate increases with increased regular pulsation and/or luminosity,and that the expansion velocity increases as an effect of increasingmass loss rate (for low mass loss rates) and luminosity. Five, of theremaining eight, sample stars have detached CSEs in the form ofgeometrically thin CO shells. The present mass loss rates and shellmasses of these sources are estimated. Finally, in three cases weencounter problems using our model. For two of these sources there areindications of significant departures from overall spherical symmetry ofthe CSEs. Carbon stars on the AGB are probably important in returningprocessed gas to the ISM. We estimate that carbon stars of the typeconsidered here annually return ~ 0.05 Msun of gas to theGalaxy, but more extreme carbon stars may contribute an order ofmagnitude more. However, as for the total carbon budget of the Galaxy,carbon stars appear to be of only minor importance. Presented in thispaper is observational data collected using the Swedish-ESOsubmillimetre telescope, La Silla, Chile, the 20\,m telescope at OnsalaSpace Observatory, Chalmers Tekniska Högskola, Sweden, and the NRAO12\,m telescope located at Kitt Peak, USA.}
| Modeling of C stars with core/mantle grains: Amorphous carbon + SiC
A set of 45 dust envelopes of carbon stars has been modeled. Among them,34 were selected according to their dust envelope class (as suggested bySloan et al. \cite{Sloan98}) and 11 are extreme carbon stars. The modelswere performed using a code that describes the radiative transfer indust envelopes considering core/mantle grains composed by an alpha -SiCcore and an amorphous carbon (A.C.) mantle. In addition, we have alsocomputed models with a code that considers two kinds of grains - alpha-SiC and A.C. - simultaneously. Core-mantle grains seem to fit dustenvelopes of evolved carbon stars, while two homogeneous grains are moreable to reproduce thinner dust envelopes. Our results suggest that thereexists an evolution of dust grains in the carbon star sequence. In thebeginning of the sequence, grains are mainly composed of SiC andamorphous carbon; with dust envelope evolution, carbon grains are coatedin SiC. This phenomena could perhaps explain the small quantity of SiCgrains observed in the interstellar medium. However, in this work weconsider only alpha -SiC grains, and the inclusion of beta -SiC grainscan perhaps change some of these results.
| Circumstellar Gas, Dust Emission, and Mass Loss from Evolved Carbon Stars
A model to calculate the circumstellar dust emission of an evolvedcarbon star is developed, and the relations between the infrared fluxdensities at 2.2, 12, 25, and 60 μm and the dust properties aredescribed. The model is combined with a preexisting model for COemission, keeping physical quantities consistent between the two models.The results are used to analyze the infrared and CO data of a sample of17 evolved carbon stars. The dust-shell opacity determined from the 60μm flux density correlates well with that determined from the[25]-[2.2], [60]-[12], and [25]-[12] colors over a range of almost 4orders of magnitude within the sample. The scaled mass-loss rate M isrevealed through the CO line profile observational parameterz†=(M/1.5x10-6 Msolaryr-1){([CO]/[H2])/6.4x10-4}[(L*/Lsolar)/104]-0.5, which ranges from 0.2 to 30 and averages 4.3. The envelopedust-to-gas mass ratio, f, is found to be <~10-3 on theassumption of usual values of [CO]/[H2] and dust absorptionefficiency. Typical grain radii a are found to be very small, with ageometric mean of <~2 nm. It does not appear that the dustproperties, such as f and a, have a direct, strong influence on themass-loss rate, but the selected sample is, admittedly, small.
| On the distance and mass-loss rate of carbon stars showing the silicon carbide emission feature
The distances and the mass-loss rates of carbon stars are in generalvery poorly known. The various estimates of the distances, taken fromthe general literature, show considerable discrepancies, while theevaluations of the mass-loss rates can be in error by more than an orderof magnitude. In this work we have evaluated these two important stellarparameters for a previously selected sample of 55 carbon stars showingthe 11.3 mu m band, commonly attributed to silicon carbide (SiC) grains.To perform the calculation we have used the values of geometrical andphysical parameters of these sources obtained from the best fits oftheir observed spectra. Using the distance values derived in this wayand the 11.3 mu m band intensity, we have evaluated the absolute bandstrength and we have found that, in agreement with other authors, thereis a correlation between this quantity and the mass-loss rate. Thiscorrelation can be very useful to determine the mass-loss rate of othercarbon stars not included in our sample, by means of the intensity ofthe SiC band, without using the usual technique based on COobservations. The same procedure can be conveniently applied to the sameas well as to other carbon stars, whose spectra will be available to thecommunity in the next future (i.e. the infrared spectra of sourcesobserved by the Infrared Satellite Observatory, ISO).
| Distance Determination of Mass-Losing Stars
Based on the Principal Component Analysis on IRAS colors and the radiodata, the distances to 183 mass-losing red giant stars were determinedusing the radial velocity and Oort's galactic rotation model for azero-point calibration in the distance modulus. Also, based on therequirement of higher accuracy of the distance determination, themass-losing red giant stars were divided into two groups by means of thefirst-principal component representing an intrinsic photometric propertyof the expanding shell; then, the distances were estimated to be log{d(kpc)}=0.458 p_2+0.09+/-0.13 for group 1 and log {d(kpc)}=0.325p_2+0.45+/-0.15 for group 2, where p_2 is the principal componentcorresponding to the distance, as obtained from the IRAS flux, which wasassumed to be inversely proportional to the square of the distance.Thus,these two groups differ from each other not only by theirphotometric properties, but also by their average distances, by a factorof about 2. Systematic differences exist between the two groups in theirpopulation characteristics and in their evolutionary stages.
| Multiperiodicity in semiregular variables
We present a detailed period analysis for 98 red semiregular variablesby means of Fourier and wavelet analysis of long-term visualobservations carried out by amateur astronomers. The overwhelmingmajority of the studied stars show multiperiodic behaviour. We found twosignificant periods in 62 variables, while there are definite signs ofthree periods in 13 stars. 20 stars turned out to be monoperiodic withsmall instabilities in the period. Since this study deals with thegeneral trends, we want to find only the most dominant periods. Thedistribution of periods and period ratios is examined in the (logP1, log P0 / P1) plots. Threesignificant and two less obvious sequences are present which can beexplained as the straight consequence of different pulsational modes.This hypothesis is supported by the multiperiodic variables with threeperiods. A clear distinction between C-rich and O-rich stars has beenfound in these diagrams suggesting a connection between the chemistryand pulsational characteristics.
| Multiperiodicity in semiregular variables. I. General properties
We present a detailed period analysis for 93 red semiregular variablesby means of Fourier and wavelet analyses of long-term visualobservations carried out by amateur astronomers. The results of thisanalysis yield insights into the mode structure of semiregular variablesand help to clarify the relationship between them and Mira variables.After collecting all available data from various international databases(AFOEV, VSOLJ, HAA/VSS and AAVSO) we test the accuracy and reliabilityof data. We compare the averaged and noise-filtered visual light curveswith simultaneous photoelectric V-measurements, the effect of the lengthversus the relatively low signal-to-noise ratio is illustrated by periodanalysis of artificial data, while binning effects are tested bycomparing results of frequency analyses of the unbinned and averagedlight curves. The overwhelming majority of the stars studied showmultiperiodic behaviour. We found two significant periods in 44variables, while there are definite signs of three periods in 12 stars.29 stars turned out to be monoperiodic with small instabilities in theperiod. Since this study deals with the general trends, we wanted tofind only the most dominant periods. The distribution of periods andperiod ratios is examined through the use of the (log P_0, log P_1) and(log P_1, log P_0/P_1) plots. Three significant and two less obvioussequences are present which could be explained as the direct consequenceof different pulsational modes. This hypothesis is supported by theresults for multiperiodic variables with three periods. Finally, thesespace methods are illustrated by several interesting case studies thatshow the best examples of different special phenomena such as long-termamplitude modulation, amplitude decrease and mode switching.
| Baldone Schmidt Telescope Plate Archive and Catalogue
The article presents information on the archive and catalogue of theastrophotos taken with the Schmidt telescope of the Institute ofAstronomy of the University of Latvia (until July 1, 1997 --Radioastrophysical Observatory of the Latvian Academy of Sciences) inthe period 1967--1998. The archive and catalogue contain more than 22000direct and 2300 spectral photos of various sky regions. Information onthe types of photo materials and color filters used as well as on mostfrequently photographed sky fields or objects is given. The catalogue isavailable in a computer readable form at the Institute of Astronomy ofthe University of Latvia and at the Astrophysical Observatory in Baldone(Riekstukalns, Baldone, LV-2125, Latvia), e-mail: astra@latnet.lv.
| The carbon-rich dust sequence - Infrared spectral classification of carbon stars
We have developed a classification system for the infrared spectralemission from carbon stars using a sample of 96 bright carbon-richvariables associated with the asymptotic giant branch. In addition tothe stellar contribution, most spectra include the 11.2 micron emissionfeature from SiC and either a smooth, cool continuum from amorphouscarbon or a secondary emission feature at 9.0 microns. We haveidentified a carbon-rich dust sequence along which the amorphous carboncomponent grows while the 9.0 micron feature declines in strength. Alongthis spectral sequence, the proportion of Mira variables increases, asdoes the period of variability, the mass-loss rate, and the thickness ofthe circumstellar shell. Thus the carbon-rich dust sequence appears tobe an evolutionary sequence. One class of spectra shows a particularlystrong 9.0 micron feature, enhanced C/O ratio, and several other unusualproperties that suggest a different sequence, perhaps related to Jstars.
| The PL relation of galactic carbon LPVs. The distance modulus to LMC
We present a period-luminosity (PL) diagram of 115 galactic carbon-richlong period variables (LPVs) observed by the HIPPARCOS satellite, in theform of the (MK,log P) relation. Our plot is compared to thediagram of carbon variables observed in the Large Magellanic Cloud(LMC). Both diagrams are found very similar and three samples aredelineated: long period variables close to the PL relation of Feast etal. (1989), short period-overluminous variables and a few underluminousLPVs, respectively Samples 1, 2 and 3. The used data were deduced fromexpectations of true parallaxes (Knapik et al. 1997) which arestatistically free of the Lutz-Kelker effect. The remaining bias due tothe non-gaussian distribution of absolute magnitudes is avoided: anon-linear parametric method is applied in Sect. 4 to the analysis ofthe PL relation for Sample 1 (72 LPVs). We obtainMK=(-3.99+/-0.13)log P+(2.07+/-0.15), in good agreement withthe slope found for LMC variables by Reid et al. (1995). The LMCdistance modulus then derived is mu =18.50+/-0.17. A well-defined upperlimit (ul) for long period stars in Sample 1 is found, with similarslopes in both the Galaxy (-4.85) and LMC (-4.72). No correction formetallicity was applied to the results. This research has made use ofthe Simbad database operated at CDS, Strasbourg, France.
| Circumstellar emission from dust envelopes around carbon stars showing the silicon carbide feature
Spectroscopic and photometric data relative to a sample of 55 carbonstars showing the 11.3 mu m feature have been fitted in the wavelengthrange between 0.4 and 100 mu m by means of a radiative transfer modelusing the laboratory extinction spectra of amorphous carbon and siliconcarbide (SiC) grains. The transfer code allows to determine in aself-consistent way the grain equilibrium temperature of the variousspecies at different distances from the central star and gives all therelevant circumstellar parameters which can be very important for theevolutionary study of carbon stars. In order to get meaningfulinformation on the nature and physical properties of the dust grainsresponsible for the 11.3 mu m feature and the underlying continuum, thefitting procedure of the spectra has been applied individually to everysingle source. For this reason it has been possible to take into accountany variation in position and shape of the band from source to source.Our analysis show that all the sources, in addition to the amorphouscarbon grains accounting for the continuum emission, need always thepresence of alpha -SiC particles while some of them require also beta-SiC. Moreover, the presence of one or both types of SiC particles seemsnot correlated neither with the total optical thickness nor with anyother physical and geometrical parameters of the circumstellar envelope.The best-fit parameters found in this work have been used to calculatethe mass-loss rate from the central stars. The clear correlation, thatwe find between the strength of the SiC feature and the total massloss-rate, confirms the results already found by other authors for thesame kind of sources and derived from the observed CO emission lines.
| Circumstellar molecular radio line intensity ratios
We have observed a sample of 61 AGB--stars (39 M--stars and 22 C--stars)in circumstellar CO, CS, HCN, SiO, SiS, and SO radio line emission. Themain results presented are based on the use of line intensity ratios, awell defined observational quantity that can be used to infer importantconclusions as well as to provide constraints on models. Taken togetherthe data are fully consistent with the facts that for this sample thecircumstellar envelopes have the same basic chemistry (i.e., C/O<1 or>1) as the central stars, and that the mass loss rates have notchanged drastically over periods between 10(2) --10(3) years. TheHCN({\jtra10})/SiO({\jtra21}) intensity ratio discriminatesunambiguously between {``}normal{''} circumstellar envelopes withC/O<1 (O--CSEs) and >1 (C--CSEs), while the CS({\jtra21}),HCN({\jtra10}), SiO({\jtra21}), and SiS({\jtra54}) intensity ratios withrespect to CO({\jtra10}) are not perfect for this purpose, and neitheris the SiS({\jtra54})/SiO({\jtra21}) intensity ratio. The data furthershows that SO and the C-bearing molecule HCN are ubiquitously present inO--CSEs, and that their line intensities in O--CSEs are qualitativelyconsistent with the fact that the molecules are formed in aphoto--induced circumstellar chemistry in a quantity that depends on themass loss rate. Hence, both species can in principle be used to estimatethe mass loss rate, and the tight relation between the SO(J_K=3_2->2_1) and CO({\jtra10}) intensities in O--CSEs shows that SO lineemission may even be a good mass loss rate estimator. On the contrary,the SiO({\jtra21}) luminosity appears to be essentially independent ofthe mass loss rate in O--CSEs, possibly due to a larger influence frommolecular adhesion onto grains. These results explain why theHCN({\jtra10})/SiO({\jtra21}) intensity ratio increases with the massloss rate in O--CSEs, and there is no need to invoke e.g. a spread inC/O--ratios for the M--stars to explain the large range of this ratio.Maser emission is very likely present in the HCN({\jtra10}) line inC--CSEs, and it seems to be sensitively dependent on the mass loss rate,i.e., it appears only for dot M la 5x 10(-7) M_sunpyr. Based on timemonitoring of this emission towards the C--stars W Ori and X TrA, wesuggest that the strongest maser features are due to radialamplification in the {\ftra21} transition. The predominance ofredshifted maser emission could be caused by an additional amplificationin the {\ftra11} transition. We find no evidence for a similar maser inO--CSEs.
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Complementary Trace Element Abundances in Meteoritic SiC Grains and Carbon Star Atmospheres
Equilibrium condensation calculations successfully explain thecomplementary trace element abundance patterns observed in carbon staratmospheres and circumstellar SiC grains found in meteorites. Fractionaltrace element condensation into SiC depletes the gas in refractory traceelements, while more volatile elements remain in the gas. The observedcomplementary patterns imply that dust forms relatively close to thestar, possibly during the minimum light phase in stellar variabilitycycles. Once the gas falls back onto the star during stellarcontraction, photospheric abundances become relatively enriched in morevolatile elements. The complementary trace element abundances linkcircumstellar SiC grains from meteorites to carbon star atmospheres.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Жираф |
| Прямое восхождение: | 04h51m13.35s |
| Склонение: | +68°10'07.6" |
| Видимая звёздная величина: | 6.696 |
| Расстояние: | 2777.778 парсек |
| Собственное движение RA: | -1.5 |
| Собственное движение Dec: | -4.6 |
| B-T magnitude: | 11.028 |
| V-T magnitude: | 7.054 |
Каталоги и обозначения:
|
