Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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).
| 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.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
Not Available
| ``Thermal'' SiO radio line emission towards M-type AGB stars: A probe of circumstellar dust formation and dynamics
An extensive radiative transfer analysis of circumstellar SiO``thermal'' radio line emission from a large sample of M-type AGB starshas been performed. The sample contains 18 irregulars of type Lb (IRV),7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12Miras. New observational data, which contain spectra of several groundvibrational state SiO rotational lines, are presented. The detectionrate was about 60% (44% for the IRVs, and 68% for the SRVs). SiOfractional abundances have been determined through radiative transfermodelling. The abundance distribution of the IRV/SRV sample has a medianvalue of 6*E-6, and a minimum of 2*E-6 and amaximum of 5*E-5. The high mass-loss rate Miras have a muchlower median abundance, la 10-6. The derived SiO abundancesare in all cases well below the abundance expected from stellaratmosphere equilibrium chemistry, on average by a factor of ten. Inaddition, there is a trend of decreasing SiO abundance with increasingmass-loss rate. This is interpreted in terms of depletion of SiOmolecules by the formation of silicate grains in the circumstellarenvelopes, with an efficiency which is high already at low mass-lossrates and which increases with the mass-loss rate. The high mass-lossrate Miras appear to have a bimodal SiO abundance distribution, a lowabundance group (on average 4*E-7) and a high abundance group(on average 5*E-6). The estimated SiO envelope sizes agreewell with the estimated SiO photodissociation radii using an unshieldedphotodissociation rate of 2.5*E-10 s-1. The SiOand CO radio line profiles differ in shape. In general, the SiO lineprofiles are narrower than the CO line profiles, but they havelow-intensity wings which cover the full velocity range of the CO lineprofile. This is interpreted as partly an effect of selfabsorption inthe SiO lines, and partly (as has been done also by others) as due tothe influence of gas acceleration in the region which produces asignificant fraction of the SiO line emission. Finally, a number ofsources which have peculiar CO line profiles are discussed from thepoint of view of their SiO line properties.Based on observations using the SEST at La Silla, Chile, the 20 mtelescope at Onsala Space Observatory, Sweden, the JCMT on Hawaii, andthe IRAM 30 m telescope at Pico Veleta, Spain.
| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima.
| 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
| Infrared investigation from earth and space on the evolutionary state of a sample of LPV
We selected a sample of highly reddened AGB stars among the sourcesobserved with the SWS instrument on the ISO satellite. These SWS dataallow us to compute the source's photometry in the mid-IR filters of thecamera TIRCAM at the TIRGO telescope. Our photometric data, supplementedwith other measurements taken from the literature, permit to select thecarbon-rich sources in the sample. For these stars, a linear relationholds between dust mass loss and the color index [8.8]-[12.5]. One maythen, from photometric data alone, evaluate the total mass loss (forwhich we used the estimate of \citet{loup}, based on radio data). Theoxygen-rich sources, on the other hand, are distributed in two branches,of which the upper one appears superimposed with carbon stars; the starsin this group have both high luminosity and high wind velocity andtherefore higher masses. Finally S stars lie between the carbon-starbranch and the low-mass oxygen-rich stars, in agreement with theirintermediate evolutionary status.
| H2O in stellar atmospheres. II. ISO spectra of cool red giants and hydrostatic models
We present 26 ISO-SWS spectra taken from a sample of 13 M-typeSemiregular, Lb and Mira variables and covering the wavelength rangebetween 2.36 and 5 mu m at a medium resolution. All of the studiedobjects show intense water bands producing a deep absorption dip around2.5 mu m. Features of CO, OH, SiO and CO2 are also visible.Using the new H2O linelist published in the first paper ofthis series and available opacity data for the other importantmolecules, we calculated a grid of hydrostatic MARCS atmospheres and thecorresponding synthetic ISO-SWS spectra. Based on the comparison withthese theoretical results the ISO observations can be divided into fourclasses. The first two groups include the spectra of the Semiregular(SRb) and Lb variables in our sample. For all of them the region between2.36 and 4.2 mu m can be quite well reproduced by our hydrostaticmodels. Only the predicted SiO bands above 4 mu m are in some cases toostrong which is due to known dynamical effects. Depending on thetemperature (above or below 3000 K) of the atmosphere, which mainlydetermines the intensity of the water depression at 2.5 mu m, thespectra of the Semiregular and Lb variables fall into the first orsecond class. The third group consists of observations of Mira starsobtained around maximum light where the range between 2.36 and 4.2 mu mcan be fitted with our MARCS models except for a strong emission bumpappearing in the ISO-SWS data in the region of the SiO features and theslope very close to the short wavelength border. Finally, the last typeof spectra corresponds to Mira variables during the phases around theminimum of their visual light curve. For this class the observed waterabsorption at 2.5 mu m is much more intense than in any hydrostaticatmosphere with a realistic choice of effective temperature and surfacegravity. Thus, we conclude that dynamical models are needed to explainthe ISO-SWS data of Mira stars. For all of the cooler objects from oursample the predicted CO2 bands between 4.2 and 4.6 mu m aretoo weak which may be due to the opacity data. Based on observationswith ISO, an ESA project with instruments funded by ESA Member Stateswith participation of ISAS and NASA. The SWS is a joint project of SRONand MPE.
| On the origin of the 19.5 μ m feature. Identifying circumstellar Mg-Fe-oxides
We report the detection of a broad, prominent 19.5 mu m dust emissionfeature in ISO-SWS spectra. It is especially conspicuous in the spectraof low mass-loss AGB stars belonging to the variability classes SRb andLb with relatively high photospheric temperatures. The feature carrieris proposed to be Mg0.1Fe0.9O, a solid solution ofMgO (periclase) and FeO (wustite). This dust species has cubic crystalsymmetry like spinel, the carrier of the 13, 16.8 and 31.8 mu m spectralfeatures, together with which the 19.5 mu m feature is being observed. Abroad emission plateau between 11 and 15 mu m, which is attributable toamorphous Al2O3, is also detected together withthe 19.5 mu m and the spinel features. As a consequence of ourdiscovery, we postulate the existence of a distinct class ofcircumstellar shells, dominated by oxide and not by silicate dust. Basedon observations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| Velocity variability of semiregular and irregular variables
We compare velocities from near infrared lines with center-of-massvelocities for a sample of semiregular and irregular variables to searchfor indications for a convective blueshift. It is shown that a generalblueshift is present but that the light variability is obviously notdominated by convective cells but rather by stellar pulsation. All starsof our sample show a similar shape and amplitude in the velocityvariations. Long secondary periods are a common feature in these objectsand strongly influence the measured velocity shifts. The star V366 Aqlis found to be the first SRV showing line doubling.
| Monitoring of LPVs with an automatic telescope.. II: A comparison of APT data and visual observations
We discuss the possibilities of investigating the semiregular andirregular light change found in evolved late type giant stars withphotometric and visually obtained (amateur) data. Period analysis hasbeen done on light curve data from both sources for a sample ofsemiregular variables. The results are compared to test the ability ofvisual data to show the frequency contents present in these variables.We find that both sources of data complete each other, as fitting thefine structure visible only in the photometric data needs long-termvisual monitoring to guarantee the uniqueness of the fit.
| Multi-colour light variation of AGB stars observed with ISO
New visual light curves and infrared multi-epoch photometry arepresented for a sample of AGB-stars spectroscopically observed with ISO.While the ISO work is or will be presented elsewhere, the aim of thispaper is to give an overview of the properties of the light change ofthe objects. This information is crucial for the interpretation of thevaluable ISO material. Using the University of Vienna Twin AutomaticPhotoelectric Telescope (APT) we monitored the stars of our sample inthe photometric bands V and I_C. We present the light curve of eachobject and derive the parameters of the current light change, such asperiod(s) and amplitude. Furthermore we give V-I_C colours and colourvariations for these objects. Our results allow us to derive somegeneral results on semiregular and irregular variables. Only in lessthan 50% of the cases could we confirm the GCVS period. Moreover, we didnot find any pronounced difference between SRb and Lb variables in theregularity of the light curve. The existence of rapid oscillationsindicated by Hipparcos data could not be confirmed. In addition to thevisual light changes we present new near infrared photometry data.Although typically only few data points are available, they can beviewed relative to the better-monitored visual light curves providinginformation on possible phase shifts and differences in amplitude indifferent parts of the spectrum. Furthermore, multi-epoch photometryallows us to derive mean colours for these objects. Based onobservations made with the Carlos Sánchez Telescope operated onthe island of Tenerife by the Instituto de Astrofísica deCanarias (IAC) in the Observatorio del Teide, Izaña. Tables B2 toB5 are only available 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/375/527
| 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 Surroundings of Disturbed, Active Galaxies
The brightest apparent magnitude examples of ultraluminous infraredgalaxies (ULIRGs) are studied here in X-ray, optical, infrared, andradio wavelengths. It is found that they have associated materialreaching out to apparent diameters of the order of 1° on the sky.Gas, dust, X-ray material, and quasars appear to be ejected from theactive nuclei with all objects nearer than their redshift distances.
| 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.
| On the origin of the 13 mu m feature. A study of ISO-SWS spectra of oxygen-rich AGB stars
We have derived a mean profile of the 13 mu m emission feature from 11ISO spectra of oxygen-rich AGB stars and present an overview of themineral species that could account for it. Our results can be summarizedas follows: i) Spherical particles of alpha -Al_2O_3 (corundum) have asharp emissivity maximum peaking at 12.7 mu m. The difference of 0.3 mum in the peak position compared to the mean observed band profile is aserious obstacle to assigning the 13 mu m dust feature to this dustspecies. ii) From a continuous distribution of ellipsoidal alpha-Al_2O_3-particles, a broad emissivity profile peaking at 13-14 mu memerges. It is difficult to reconcile the large width of this profilewith the width of the observed 13 mu m band. iii) The most prominentemissivity maximum of TiO_2 (rutile) is located at 13.5 mu m; its widthis larger and its strength three times smaller than the emissivitymaximum of corundum. iv) Core-mantle-grains composed of rutile andcorundum or of corundum and amorphous olivine can both produce, in acertain domain of core volume fractions, a spectral signature verysimilar to the observed 13 mu m feature. However, the necessity toassume rather artificial distribution functions of the core volumefractions makes this scenario improbable. v) We consider MgAl_2O_4(spinel) to be the most promising candidate for the carrier of the 13 mum feature since its emissivity peaks are located at 12.95 mu m and at16.8 mu m for spherical particles and since at both positions featuresare present in most of the spectra of our sample. In view of therelevance of this substance for circumstellar shells, laboratory spectraof spinel particles embedded in KBr are also presented. Based onobservations with ISO, an ESA project with instruments funded by ESAMember States (especially the PI countries: France, Germany, theNetherlands and the United Kingdom) and with the participation of ISASand NASA.}
| Spectral and luminosity classification for the cool components in symbiotic stars
The near infrared spectra of 12 S-type symbiotic stars and 78 comparisonstars have been observed with moderate dispersion in five runs from 1992to 1997, the resolving power being R= (lambda )/(Delta lambda )>2000,with a signal to noise ratio S/N>100. The triple-headed absorptionband of TiO (lambda lambda 8432, 8422 and 8452 Ä) emerges when astar is later than M2, and the depth of the TiO absorption band is verysensitive to the spectral type (ST) and insensitive to the luminosityclass of the star. We fit a curve of spectral type against the index ofthe absorption depth of this band with a standard deviation sigma =0.37of a subdivision of one spectral type. The IR CaII triplet (lambdalambda 8498, 8542, 8662 Ä ), Fe I 8689 Ä, and Fe I 8675 Äare good luminosity indicators although the equivalent widths (EWs) ofthese lines clearly decrease for a star later than M3. When the star isa supergiant, the lines have a smaller central residual intensity andbroader wings than in the case of a normal giant. The Ca II 8662 Ä/Fe I 8675 Ä and Fe I 8689 Ä /Fe I 8675 Ä ratios are alsogood luminosity indicators for K-type giants. The latter is particularlyuseful when there are abundance anomalies. The metal-poor symbiotic starAG Dra is classified as a Ib or II giant, as is TX CVn, on the basis ofFe I 8689 Ä /Fe I 8675 Ä. 9 other symbiotic stars containingM-type cool components are classified as giants by direct comparison andquantitative analysis. Due to there being no known good ratio indicatorof luminosity for M-type stars in the band studied and because there isno metal abundance data for the symbiotic stars studied by us except forAG Dra, the results for these 9 symbiotic stars are only preliminary.The infrared Ca II triplet of most symbiotic stars clearly variesbetween the different observing runs. The different luminosity classesgiven to the same symbiotic star are probably caused by the variabilityof the lines of ionized elements, while in some cases they are affectedby a low metal abundance.
| Period-Luminosity-Colour distribution and classification of Galactic oxygen-rich LPVs. I. Luminosity calibrations
The absolute K magnitudes and kinematic parameters of about 350oxygen-rich Long-Period Variable stars are calibrated, by means of anup-to-date maximum-likelihood method, using Hipparcos parallaxes andproper motions together with radial velocities and, as additional data,periods and V-K colour indices. Four groups, differing by theirkinematics and mean magnitudes, are found. For each of them, we alsoobtain the distributions of magnitude, period and de-reddened colour ofthe base population, as well as de-biased period-luminosity-colourrelations and their two-dimensional projections. The SRa semiregulars donot seem to constitute a separate class of LPVs. The SRb appear tobelong to two populations of different ages. In a PL diagram, theyconstitute two evolutionary sequences towards the Mira stage. The Mirasof the disk appear to pulsate on a lower-order mode. The slopes of theirde-biased PL and PC relations are found to be very different from theones of the Oxygen Miras of the LMC. This suggests that a significantnumber of so-called Miras of the LMC are misclassified. This alsosuggests that the Miras of the LMC do not constitute a homogeneousgroup, but include a significant proportion of metal-deficient stars,suggesting a relatively smooth star formation history. As a consequence,one may not trivially transpose the LMC period-luminosity relation fromone galaxy to the other Based on data from the Hipparcos astrometrysatellite. Appendix B 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/Abstract.html
| Oxygen-rich semiregular and irregular variables. A catalogue of circumstellar CO observations
Using the SEST, the Onsala 20 m telescope, the JCMT, and the IRAM 30 mtelescope we have carried out a survey of circumstellar CO(J=1-0, 2-1,3-2, and 4-3) emission on a large sample of oxygen-rich semiregular (SRaand SRb) and irregular variables (Lb). A total of 109 stars wereobserved in at least one CO line: 66 were shown to have circumstellar COline emission (7 SRa, 36 SRb, and 23 Lb variables), ~ 60% of thesemiregulars and all but one of the irregulars were detected for thefirst time. Most stars were observed in at least two transitions. Thereis a total of 138 detected CO lines. For twelve stars stronginterference from interstellar CO emission precluded detection. Wepresent here a catalogue of all observational data and the spectra ofall detections, as well as brief discussions on detection statistics(including its dependence on variability type, period, IRAS-colour, IRASLRS-class, and M-subclass), line profiles (including line shapeasymmetry, multi-component line shapes, and line intensity ratios), gasexpansion velocity distributions, and correlations between CO line andIR continuum fluxes (including implications for the mass-lossmechanism). Based on observations collected using at the EuropeanSouthern Observatory, La Silla, Chile, the Onsala Space Observatory,Chalmers Tekniska Högskola, Sweden, the James Clerk MaxwellTelescope, Hawaii and the IRAM 30~m telescope, Pico Veleta, Spain.
| 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.
| 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.
| The Infrared Spectral Classification of Oxygen-rich Dust Shells
This paper presents infrared spectral classifications for a flux-limitedsample of 635 optically identified oxygen-rich variables includingsupergiants and sources on the asymptotic giant branch (AGB). Severalclasses of spectra from oxygen-rich dust exist, and these can bearranged in a smoothly varying sequence of spectral shapes known as thesilicate dust sequence. Classification based on this sequence revealsseveral dependencies of the dust emission on the properties of thecentral star. Nearly all S stars show broad emission features fromalumina dust, while most of the supergiants exhibit classic featuresfrom amorphous silicate dust. Mira variables with symmetric light curvesgenerally show broad alumina emission, while those with more asymmetriclight curves show classic silicate emission. These differences may arisefrom differences in the photospheric C/O ratio.
| Multiple Molecular Winds in Evolved Stars. I. A Survey of CO (2-1) and CO (3-2) Emission from 45 Nearby AGB Stars
This paper describes observations of a new phenomenon in evolvedmass-losing asymptotic giant branch (AGB) stars: the presence of twowinds with different expansion velocities. CO(2-1) and CO(3-2) lineemission was observed for 45 AGB stars at high velocity resolution anddouble winds found in 20% of the sample. Highly asymmetric lines werefound in six other stars. The data tentatively suggest that double windsoccur when the star undergoes a change (pulsational mode, chemicalcomposition) and that the very narrow components represent the onset ofa new phase of mass loss.
| New detections of H_2O maser sources on the 13.7 M radio telescope of Purple Mountain Observatory
Observations of H_2O maser sources on the 13.7 m radio telescope ofPurple Mountain Observatory from 1990 Aug. to 1994 Jan. are summarized.For searching new water masers, the total number of search candidates isabout 360, with 110 objects detected. Among them are 96 new detections.A list of the new detections and their spectra are presented. Table 1and Fig. 1 are 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/Abstract.html
| HIPPARCOS Astrometry of Infrared-Selected Sources and the Connection Between Optical and Infrared Reference Frames
Astrometric data from the Hipparcos satellite are reported for theoptical counterparts of 87 bright infrared sources. These sources may beuseful in defining a reference frame for infrared observations. The dataare also useful in studying the locations of circumstellar SiO masers.(SECTION: Stars)
| 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.
| A Renewed Search for Water Maser Emission from Mira Variables.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1602L&db_key=AST
| Aluminum Oxide and the Opacity of Oxygen-rich Circumstellar Dust in the 12--17 Micron Range
Amorphous alumina (Al2O3) was produced by a sol-gel technique in orderto make available its optical constants for possible astrophysicalapplications. Gradual annealing showed that the X-ray amorphousness ofalumina ended somewhere between 723 and 873 K. Above this transitionpoint, the structure changes into disordered gamma -Al2O3. At T >1273 K, crystalline alpha -Al2O3 (corundum) is formed. Mie calculationsshow that amorphous alumina exhibits a wide Al-O vibrational band,peaking at 11.5--11.8 mu m and having a steep "blue" and an extended"red" wing. It may be an important contributor to the continuous opacitybetween the silicate bands in oxygen-rich circumstellar envelopes,whereas it is ruled out for the explanation of the 13 mu m band. Anaverage 13 mu m band profile was derived from 51 IRAS low-resolutionspectra of bright Mira stars and semiregular variables. Its shape, whichis satisfactorily represented by a Lorentz profile, can be reproduced byMie calculations with the data of alpha -Al2O3, but not with those ofgamma -Al2O3. The calculations show that the 13 mu m band profile ofalpha -Al2O3 is sensitive to grain shape. If alpha -Al2O3 is theabsorber, a second band should be present at 21 mu m. A closecorrelation was found between the strengths of the 13 mu m band and the10 mu m silicate band. It suggests that the 13 mu m band carrier couldalso be somehow connected with silicate dust. Experimental argumentssupporting this attribution are presented.
| Semiregular variables of types SRa and SRb. Circumstellar CO emission of an oxygen-rich sample.
O-rich Semiregular Variables (SRVs) of types SRa and SRb have beenobserved in the ^12^CO(J=1-0) and (J=2-1) lines using the SEST, LaSilla, Chile, and the 20-m telescope at Onsala, Sweden. In total 22detections (13 are new ones) and 1 tentative detection can be reportedamong the 48 stars observed. In 7 cases detections are precluded becauseof strong interference from interstellar CO emission. The majority ofthe detected objects, covering both small and longer periods, are weakin CO, i.e. they are low mass-loss rate objects(<=10^-7^Msun_/yr), and have, with only a few exceptions,envelopes with small expansion velocities (the mean value is =~8km/s).However, in this respect their properties are very similar to those ofbright O-rich Mira variables, whose pulsational periods are on theaverage 2-3 times higher. A comparison between stellar and circumstellarproperties shows that the gas expansion velocity does not depend on thestellar effective temperature, nor on the period. Likewise, we find nocorrelation between mass-loss rate and period, but there might be a weakdependence of the former on the stellar effective temperature.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Buyuk Ayi |
| Sað Açýklýk: | 12h40m21.28s |
| Yükselim: | +55°50'47.6" |
| Görünürdeki Parlaklýk: | 8.458 |
| Uzaklýk: | 312.5 parsek |
| özdevim Sað Açýklýk: | -2.7 |
| özdevim Yükselim: | 2.3 |
| B-T magnitude: | 10.01 |
| V-T magnitude: | 8.587 |
Kataloglar ve belirtme:
|
