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Circumstellar molecular line emission from S-type AGB stars: mass-loss rates and SiO abundances
Aims: The main aim is to derive reliable mass-loss rates andcircumstellar SiO abundances for a sample of 40 S-type AGB stars basedon new multi-transitional CO and SiO radio line observations. Inaddition, the results are compared to previous results for M-type AGBstars and carbon stars to look for trends with chemical type. Methods: The circumstellar envelopes are assumed to be sphericallysymmetric and formed by a constant mass-loss rate. The mass-loss ratesare estimated from fitting the CO observations using a non-local,non-LTE radiative transfer code based on the Monte Carlo method. In theexcitation analysis, the energy balance equation is solvedself-consistently simultaneously as the radiative transfer and thetemperature structure of the gas is derived. Effects of dust grains arealso included in the molecular excitation analysis. Once the physicalproperties of the circumstellar envelopes are determined, the sameradiative transfer code is used to model the observed SiO lines in orderto derive circumstellar abundances and the sizes of the SiOline-emitting regions. Results: We have estimated mass-loss ratesof 40 S-type AGB stars and find that the derived mass-loss rates have adistribution that resembles those previously derived for similar samplesof M-type AGB stars and carbon stars. The estimated mass-loss rates alsocorrelate well with the corresponding expansion velocity of theenvelope, in accordance with results for M-type AGB stars and carbonstars. In all, this indicates that the mass loss is driven by the samemechanism in all three chemical types of AGB stars. In addition, we haveestimated the circumstellar fractional abundance of SiO relative to H{2}in 26 of the sample S-type AGB stars. The derived SiO abundances are, onaverage, about an order of magnitude higher than predicted by stellaratmosphere thermal equilibrium chemistry, indicating thatnon-equilibrium chemical processes determines the abundance of SiO inthe circumstellar envelope. Moreover, a comparison with the results forM-type AGB stars and carbon stars show that for a certain mass-lossrate, the circumstellar SiO abundance seems independent (although with alarge scatter) of the C/O-ratio. Conclusions: In our comparisonof S-type AGB stars with carbon stars and M-type AGB stars, we find nolarge differences in circumstellar physical properties or SiO abundancesdepending on the chemical type of the star.Appendices are only available in electronic form at http://www.aanda.org
| Infrared photometry and evolution of mass-losing AGB stars. II. Luminosity and colors of MS and S stars
Context: Asymptotic giant branch (AGB) phases mark the end of theevolution for low- and intermediate-mass stars. Our understanding of themechanisms through which they eject the envelope and our assessment oftheir contribution to the mass return to the interstellar medium and tothe chemical evolution of Galaxies are hampered by poor knowledge oftheir luminosities and mass loss rates, both for C-rich and for O-richsources. Aims: We plan to establish criteria permitting a morequantitative determination of luminosities (and subsequently of massloss rates) for the various types of AGB stars on the basis of infraredfluxes. In this paper, in particular, we concentrate on O-rich ands-element-rich MS, S stars and include a small sample of SC stars. Methods: We reanalyze the absolute bolometric magnitudes and colors ofMS, S, SC stars on the basis of a sample of intrinsic (single) andextrinsic (binary) long period variables. We derive bolometriccorrections as a function of near- and mid-infrared colors, adopting asreferences a group of stars for which the spectral energy distributioncould be reconstructed in detail over a large wavelength range. Wedetermine the absolute HR diagrams, and compare luminosities and colorsof S-type giants with those, previously derived, of C-rich AGB stars.Luminosity estimates are also verified on the basis of existingperiod-luminosity relations valid for O-rich Miras. Results: S starbolometric luminosities are almost indistinguishable from those ofC-rich AGB stars. On the contrary, their circumstellar envelopes arethinner and less opaque. Despite this last property the IR wavelengthsremain dominant, with the bluest stars having their maximum emission inthe H or K(short) bands. Near-to-mid infrared color differences are inany case smaller than for C stars. Based on period-luminosity relationsfor O-rich Miras and on Magnitude-color relations for the same variableswe show how approximate distances (hence intrinsic parameters) forsources of so far unknown parallax can be inferred. We argue that mostof the sources have a rather small mass (<2 Mȯ);dredge-up might then be not effective enough to let the C/O ratio exceedunity.Tables [see full textsee full textsee full textsee full textsee fulltextsee full text]-[see full textsee full textsee full textsee fulltextsee full textsee full text] are only available in electronic form athttp://www.aanda.org Tables 11 and 12 are 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/488/675
| Mass-loss properties of S-stars on the AGB
We have used a detailed non-LTE radiative transfer code to model newAPEX CO(J = 3 → 2) data, and existing CO radio line data, on asample of 40 AGB S-stars. The derived mass-loss-rate distribution has amedian value of 2 × 10-7~Mȯyr-1, and resembles values obtained for similar samples ofM-stars and carbon stars. Possibly, there is a scarcity ofhigh-mass-loss-rate (≥10-5~Mȯyr-1) S-stars. The distribution of envelope gas expansionvelocities is similar to that of the M-stars, the median is 7.5 kms-1, while the carbon stars, in general, have higher gasexpansion velocities. The mass-loss rate correlates well with the gasexpansion velocity, in accordance with results for M-stars and carbonstars.
| 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
| 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).
| 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.
| 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.
| Circumstellar shells and mass loss rates: Clues to the evolution of S stars
It is the purpose of this paper to rediscuss the circumstellarproperties of S stars and to put these properties in perspective withour current understanding of the evolutionary status of S stars, inparticular the intrinsic/extrinsic dichotomy. This dichotomy states thatonly Tc-rich (``intrinsic'') S stars are genuine thermally-pulsingasymptotic giant branch stars, possibly involved in the M-S-Cevolutionary sequence. Tc-poor S stars are referred to as ``extrinsic''S stars, because they are the cooler analogs of barium stars, and likethem, owe their chemical peculiarities to mass transfer across theirbinary system. Accordingly, an extensive data set probing thecircumstellar environment of S stars (IRAS flux densities, maseremission, CO rotational lines) has been collected and criticallyevaluated. This data set combines new observations (9 stars have beenobserved in the CO J=2-1 line and 3 in the CO J=3-2 line, with four newdetections) with existing material (all CO and maser observations of Sstars published in the literature). The IRAS flux densities of S starshave been re-evaluated by co-adding the individual scans, in order tobetter handle the intrinsic variability of these stars in the IRASbands, and possible contamination by Galactic cirrus. In the (K - [12],[25] - [60]) color-color diagram, S stars segregate into five distinctregions according to their Tc content and ZrO/TiO, C/O and IR spectralindices. Stars with photospheric colors (populating ``Region A'') may beidentified with extrinsic S stars. For the other regions characterizedby different excess levels in the 12, 25 and 60 mu m bands, severaldiagnostics (like the IRAS spectral class, maser emission, and shape ofCO rotational lines) have been collected to infer the physicalproperties of the dust shell. A simple radiative-transfer code has alsobeen used to infer the chemical nature (carbonaceous or silicate) of thedust grains from the observed IR colors. S stars with large K - [12]excesses and moderate [25] - [60] excesses (populating Regions B and C)exhibit the signatures of oxygen-rich shells (9.7 mu m silicate emissionand SiO maser emission). The situation is less clear for S stars withsmall K - [12] and moderate [25]-[60] indices (populating Regions D andE). Their IR colors are consistent with carbonaceous grains (as is theirfeatureless IRAS spectrum, and absence of silicate or SiO maseremission), but these features may equally well be explained by adetached shell. For many of these stars with a large 60 mu m excess, theshell is indeed resolved by the IRAS beam at 60 mu m. The prototypicalSC star FU Mon is among these. Since SC stars are believed to be in avery short-lived evolutionary phase where C/O = 1 within 1%, FU Mon maybe a good candidate for the ``interrupted mass-loss'' scenario advocatedby \cite[Willems & de Jong (1988).]{Wi88} The CO line profile of FUMon is also peculiar in being quite narrow (V_e = 2.8 km ssp {-1}),suggesting that the mass loss has just resumed in this star. Mass lossrates or upper limits have been derived for all S stars observed in theCO rotational lines, and range from < 2;10(-8) Msb ȯ y(-1) forextrinsic S stars to 1;10(-5) Msb ȯ y(-1) (the Mira S star W Aql).These mass-loss rates correlate well with the K - [12] color index,which probes the dust loss rate, provided that mathaccent 95 M ga 10({-)8} Msb ȯ y(-1) . Small mass-loss rates are found for extrinsic Sstars, consistent with their not being so evolved (RGB or Early-AGB) asthe Tc-rich S stars. This result does not support the claim often madein relation with symbiotic stars that binarity strongly enhances themass-loss rate. Based on observations carried out at the CaltechSubmillimeter Observatory (Mauna Kea, Hawaii).
| 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.
| H_2_O maser emission from irregular variables.
We have performed a search for the 22 GHz water maser line among 72optically identified irregular and semiregular red variables. Newdetections were made of five stars, while only four of nine objectspreviously known as maser sources were redetected. The probability forthe detection of H_2_O maser emission increases with V light amplitude,and with H-K and K-[12] colours just as in regular Mira and semiregularvariables of SRa- and SRb-types. The detection rate of water masers isabout 25% for nearby Lb objects (D<400 pc) in the sample, comparableto that observed in the SRa and SRb stars. No masers were detected inobjects with mass loss rates <=4x10^-8^Msun_/yr. Maserluminosities are 10^41^-10^43^photons/s similar to that of the bluestMiras and typical SRa and SRb stars showing water maser emission. Acomparison of our data on irregular stars with those previously obtainedon SRa and SRb variables suggests that most radio and infraredproperties are indistinguishable among both classes of objects.
| Circumstellar CO emission in S stars. I. Mass-loss with little or no dust.
47 S stars have been searched for circumstellar CO (J=1-0 and/or 2-1)emission, and 29 have been detected, including 4 which show no evidenceof dust in their IRAS LRS spectra, and one with possibly no Tc (andtherefore not an AGB star). Six stars show anomalous features in theirprofiles, showing the presence of more than one kinematic component inthe expanding outflow. Two stars may have detached-shell envelopes. Theexpansion velocity distribution for S stars envelopes is different thanthat for C-rich stars, with the former having a slightly lower meanexpansion velocity, and a significantly higher fraction of objects withvery low expansion velocities (<~5.5km/s). In most S stars, themass-loss rates are >2x10^-7^Msun_/yr and the gas-to-dustratios are >1000. Our detection of CO in S stars with little or nodetectable dust implies substantial mass-loss in these objects. Theexpansion velocities and mass-loss rates of the relatively dust-freestars show a much steeper dependence on the the far-infrared excess({DELTA}IR_e_), as compared to the more dusty stars. This suggests thatwhen the amount of dust becomes small, mass-loss may be partially drivenby a different mechanism than radiation pressure on grains, whichprobably dominates in the dusty envelopes.
| Circumstellar dust envelopes of oxygen-rich AGB stars.
Using IRAS observational data, the best-fitting models are searched foroxygen-rich AGB stars in various evolutionary stages from opticallyvisible red giant stars to OH/IR stars with heavy mass loss. The modelinfrared spectral energy distributions of the red giant stars withcircumstellar dust envelopes are calculated with the use of a radiativetransfer code. It is found that the oxygen-rich IRAS AGB starssurrounded by circumstellar dust envelopes are divided into threegroups, which can be clearly distinguished by their mass loss rate andthe grain temperature at the inner boundary of the circumstellar dustenvelopes. One group corresponds to Mira type objects with a moderatemass loss rate and a grain formation temperature of about 500 K, thesecond to OH/IR star type objects with a large mass loss rate and a highgrain formation temperature, and the third to M stars with a very coldgrain temperature at the inner boundary, indicating that the moderatemass loss stopped a considerable time ago in these objects. The highergrain formation temperature of the OH/IR star type objects indicates agrain formation point closer to the stellar surface, suggesting aneffective acceleration of dust grains by radiation pressure. Such aneffective acceleration can play an important role in the mass lossphenomena of OH/IR stars with large mass loss rates. From the modelfitting results, it is suggested that an AGB star undergoes severaldozen phases of moderate mass loss of˙(M)~10^-7^-10^-6^Msun_/yr for a period of10^3^-10^4^years with interruptions of 10^3^-10^4^years during theordinary Mira variable phase before becoming an OH/IR star with a massloss rate exceeding 10^-5^Msun_/yr and a lifetime of about10^3^-10^4^years.
| A millimeter-wavelength survey of S stars for mass loss and chemistry
We report the results of a survey of 27 S stars within 1 kpc of the Sunfor emission from the J = 1 goes to 0 and 2 goes to 1 lines of CO. Forthose stars with strong detections in CO, we also searched for emissionfrom the J = 1 goes to 0 line of HCN and the J = 2 goes to 1, nu = 0emission lines of SiO. In addition, we searched for SiO J = 2 goes to 1,nu = 1 maser emission in a total of 14 stars in our sample. We derivemass-loss rates for the S stars detected in CO and compare with otherestimates. Our mass loss rates from the CO J = 1 goes to 0 line are wellcorrelated with the rates derived by Jura (1988) from IR flux densitiesand a dust model, but our values are systematically higher than Jura'sby about a factor of 2. The derived gas mass-loss rates for our sampleare very similar to those found for carbon stars. We find, however, thatthe average dust/gas ratio of our sample of S stars is lower than thatfor carbon stars, suggesting that S stars are less efficient atproducing grains, per unit of ejected mass, than are carbon stars. Fourof the eight stars surveyed show definite detections of HCN in theircircumstellar envelopes. Five of seven stars have thermal (nu = 0) SiOemission, while eight of the 14 surveyed show maser (nu = 1) SiOemission. We use our results to estimate HCN and SiO abundances andcompare these with the results of thermodynamic equilibrium models forthe formation of these species under photospheric conditions with C/Onearly equal to unity. The HCN and SiO abundances indicate that theformation of these species must occur at temperatures low enough forgrain formation to be important, which may serve to enhance productionof HCN. Our results show no evidence that S stars experience a cessationof mass loss, contrary to recent suggestions.
| Circumstellar shells resolved in the IRAS survey data. I - Data processing procedure, results, and confidence tests
We have examined the IRAS 60 and 100 micron survey data covering 512evolved stars and young planetary nebulae for evidence of spatiallyresolved structure. A simple model, consisting of a central unresolvedsource surrounded by a resolved isothermal shell, was fitted to the datafor each star. Seventy-six stars were found to be resolved in the 60micron data. Tests have been performed to verify that the extendedstructure seen is not an artifact of the data-processing algorithm.
| CO and HCN observations of circumstellar envelopes. A catalogue - Mass loss rates and distributions
We have searched the literature for all observations of the (C-12)O(1-0), (C-12)O (2-1), and HCN (1-0) lines in circumstellar envelopes oflate type stars published between January 1985 and September 1992. Wereport data for 1361 observations (stellar velocity, expansion velocity,peak intensity, integrated area, noise level). This CO-HCN sample nowcontains 444 sources; 184 are identified as O-rich, 205 as C-rich, andthere are 9 S stars. About 85 percent of the sources are AGB stars.There are 32 planetary nebulae and about thirty post-AGB starscandidates. Besides results of mm-observations, we also listidentifications, coordinates, IRAS data, and chemical and spectral typesfor every source. For AGB stars, we have estimated (or compiled)bolometric fluxes and distances for 349 sources, and mass loss ratesdeduced from CO results for 324 sources, taking into account theinfluence of the CO photodissociation radius. We also list mass lossrates derived from detailed models of CO emission in the literature.
| Characterization and proportion of very cold C-rich circumstellar envelopes
The paper develops a method for identification of very cold C-richcircumstellar envelopes (CSEs) in IR regions colder than region VII(i.e., larger value of IRAS flux ratio S25/S12), by reviewing thedifferent criteria and by identifying C-rich CSEs in a sample of 228bright IRAS sources. By using the results of OH surveys and catalogstogether with the results of additional HCN/CO millimeter observationsperformed on critical classes of objects, a C/O classification isproposed for 94 percent of the sources with a good probability. It isshown that the low-resolution spectra by themselves are a reasonable andeasy way to identify C-rich late AGB CSEs. For post-AGB object with coldCSEs, HCN and OH observations or visible/near IR spectroscopy arenecessary.
| A survey of circumstellar CO emission from a sample of IRAS point sources
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
| A survey for infrared excesses among high galactic latitude SAO stars
This project involves extending the previous analysis of infraredexcesses among a volume-limited sample of 134 nearby A-K main-sequencestars to a magnitude-limited sample of stars, culled from the SAOCatalog, with excesses determined from the IRAS Point Source Catalogflux density ratios. This new sample includes 5706 B-M type stars, 379of which have infrared excesses. The objective involved use of astatistically complete survey of objects in a standard catalog in orderto assess the frequency with which different physical processes canaffect the infrared output of stars. These processes include, but arenot limited to, orbiting cold particle clouds and the onset of rapidmass loss. It is concluded that cold disks are consistent with theinfrared excesses found among A-G dwarfs and G-K giants in the sample.
| Emission features in IRAS low-resolution spectra of MS, S and SC stars
A progression of emission features due to dust grains in the 8-22-micronregion of IRAS low-resolution spectra has been detected which parallelstheir increasing C:O ratio and s-process enhancement. Strong S starstypically have a 9-14-micron emission feature which peaks around 10.8microns, while mild S or MS stars reveal a variety of features such as athree-component feature and a 10-micron silicate emission. The infraredexcesses in the 8-22-micron region are found to correlate with theperiod of Mira S, MS, and SC variables and to increase sharply for Miravariables with periods of between 380 and 400 days.
| Mass loss from S stars
The mass-loss process in S stars is studied using 65 S stars from thelisting of Wing and Yorka (1977). The role of pulsations in themass-loss process is examined. It is detected that stars with largermass-loss rates have a greater amplitude of pulsations. The dust-to-gasratio for the S stars is estimated as 0.002 and the average mass-lossrate is about 6 x 10 to the -8th solar masses/yr. Some of the propertiesof the S stars, such as scale height, surface density, and lifetime, aremeasured. It is determined that scale height is 200 pc; the totalduration of the S star phase is greater than or equal to 30,000 yr; andthe stars inject 3 x 10 to the -6th solar masses/sq kpc yr into theinterstellar medium.
| IRAS catalogues and atlases - Atlas of low-resolution spectra
Plots of all 5425 spectra in the IRAS catalogue of low-resolutionspectra are presented. The catalogue contains the average spectra ofmost IRAS poiont sources with 12 micron flux densities above 10 Jy.
| The brightest high-latitude 12-micron IRAS sources
The Infrared Astronomical Satellite (IRAS) Point Source catalog wassearched for sources brighter than 28 Jy (0 mag) at 12 microns withabsolute galactic latitude greater than 30 deg excluding the LargeMagellanic Cloud. The search resulted in 269 sources, two of which arethe galaxies NGC 1068 and M82. The remaining 267 sources are identifiedwith, or have infrared color indices consistent with late-type starssome of which show evidence of circumstellar dust shells. Seven sourcesare previously uncataloged stars. K and M stars without circumstellardust shells, M stars with circumstellar dust shells, and carbon starsoccupy well-defined regions of infrared color-color diagrams.
| The brightest S-type stars in the infrared
Lists of the brightest stars of type S at infrared wavelengths, based onsearches in the IRC and AFCRL catalogs, are presented in the hope ofstimulating further spectroscopic observations of S stars in theinfrared; the importance of such observations is stressed. Anexamination of the K magnitudes of the brightest stars having variousdegrees of S characteristics shows that the marginal MS stars are morecommon than stars with strong S characteristics, although they are alsomore difficult to recognize. It is found that the number ratio of coolcarbon stars to S stars, when selected in the same way according toinfrared magnitude and region of the sky, is about 3:1.
| Characteristics of OH emission from infrared stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1972A&A....17..385W&db_key=AST
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