Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars
We have measured nonzero closure phases for about 29% of our sample of56 nearby asymptotic giant branch (AGB) stars, using the three-telescopeInfrared Optical Telescope Array (IOTA) interferometer at near-infraredwavelengths (H band) and with angular resolutions in the range 5-10 mas.These nonzero closure phases can only be generated by asymmetricbrightness distributions of the target stars or their surroundings. Wediscuss how these results were obtained and how they might beinterpreted in terms of structures on or near the target stars. We alsoreport measured angular sizes and hypothesize that most Mira stars wouldshow detectable asymmetry if observed with adequate angular resolution.
| Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch (AGB)stars undergo a series of short thermal pulses that significantly changetheir luminosity and mass on timescales of hundreds to thousands ofyears. These pulses are confirmed observationally by the existence ofthe short-lived radioisotope technetium in the spectra of some of thesestars, but other observational consequences of thermal pulses are subtleand may only be detected over many years of observations. Secularchanges in these stars resulting from thermal pulses can be detected asmeasurable changes in period if the star is undergoing Mira pulsations.It is known that a small fraction of Mira variables exhibit largesecular period changes, and the detection of these changes among alarger sample of stars could therefore be useful in evolutionary studiesof these stars. The American Association of Variable Star Observers(AAVSO) International Database currently contains visual data for over1500 Mira variables. Light curves for these stars span nearly a centuryin some cases, making it possible to study the secular evolution of thepulsation behavior on these timescales. In this paper we present theresults of our study of period change in 547 Mira variables using datafrom the AAVSO. We use wavelet analysis to measure the period changes inindividual Mira stars over the span of available data. By making linearfits to the period versus time measurements, we determine the averagerates of period change, dlnP/dt, for each of these stars. We findnonzero dlnP/dt at the 2 σ significance level in 57 of the 547stars, at the 3 σ level in 21 stars, and at the level of 6 σor greater in eight stars. The latter eight stars have been previouslynoted in the literature, and our derived rates of period change largelyagree with published values. The largest and most statisticallysignificant dlnP/dt are consistent with the rates of period changeexpected during thermal pulses on the AGB. A number of other starsexhibit nonmonotonic period change on decades-long timescales, the causeof which is not yet known. In the majority of stars, the periodvariations are smaller than our detection threshold, meaning theavailable data are not sufficient to unambiguously measure slowevolutionary changes in the pulsation period. It is unlikely that morestars with large period changes will be found among heretoforewell-observed Mira stars in the short term, but continued monitoring ofthese and other Mira stars may reveal new and serendipitous candidatesin the future.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| Mass-loss from dusty, low outflow-velocity AGB stars. I. Wind structure and mass-loss rates
We present the first results of a CO(2-1), (1-0), and 86 GHz SiO masersurvey of AGB stars, selected by their weak near-infrared excess. Amongthe 65 sources of the present sample we find 10 objects with low COoutflow velocities, vexp <~ 5 km s-1.Typically, these sources show (much) wider SiO maser features.Additionally, we get 5 sources with composite CO line profiles, i.e. anarrow feature is superimposed on a broader one, where both componentsare centered at the same stellar velocity. The gas mass-loss rates,outflow velocities and velocity structures suggested by these lineprofiles are compared with the results of hydrodynamical modelcalculations for dust forming molecular winds of pulsating AGB stars.The observations presented here give support to our recent lowoutflow-velocity models, in which only small amounts of dust are formed.Therefore, the wind generation in these models is dominated by stellarpulsation. We interpret the composite line profiles in terms ofsuccessive winds with different characteristics. Our hydrodynamicalmodels, which show that the wind properties may be extremely sensitiveto the stellar parameters, support such a scenario.Based on observations obtained at the European Southern Observatory, LaSilla, Chile and at the IRAM, Pico Veleta, Spain.
| 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
| Angular Size Measurements of Mira Variable Stars at 2.2 Microns. II.
We present angular size measurements of 22 oxygen-rich Mira variablestars. These data are part of a long-term observational program usingthe Infrared Optical Telescope Array to characterize the observablebehavior of these stars. Complementing the infrared angular sizemeasurements, values for variable star phase, spectral type, bolometricflux, and distance were established for stars in the sample; flux anddistance led to values for effective temperature (Teff) andlinear radius, respectively. Additionally, values for the K-[12] colorexcess were established for these stars, which is indicative of dustymass loss. Stars with higher color excess are shown to be systematically120 Rsolar larger than their low color excess counterparts,regardless of period. This analysis appears to present a solution to along-standing question presented by the evidence that some Mira angulardiameters are indicative of first-overtone pulsation, while otherdiameters are more consistent with fundamental pulsation. A simpleexamination of the resultant sizes of these stars in the context ofpulsation mode is consistent with at least some of these objectspulsating in the fundamental mode.
| 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.
| Infrared colours for Mira-like long-period variables found in the (Mȯ<~10-7 Msolar yr-1) Hipparcos Catalogue
Near-infrared, JHKL, photometry is presented for 193 Mira andsemi-regular variables that were observed by Hipparcos; periods,bolometric magnitudes and amplitudes are derived for 92 of them. Becauseof the way in which the Hipparcos targets were selected, this group ofstars provides a useful data base of Miras with low mass-loss rates(Mȯ<~10-7Msolaryr-1).Various period-colour relationships are discussed in detail. The colour,particularly BCK = 10.86 - 38.10 K (J - K)0 +64.16(J - K)20 - 50.72(J -K)30 + 19, K-L, at a given period is found todepend on the pulsation amplitude of the star. A comparison with modelssuggests that this is a consequence of atmospheric extension, in thesense that large-amplitude pulsators have very extended atmospheres andredder Mȯ<10-7Msolaryr-1, K-L and H-K but bluerJ-H than their lower amplitude counterparts. The stars with veryextended atmospheres also have higher values of K-[12] and hence highermass-loss rates. This finding provides further evidence for the causalconnection between pulsation and mass loss. Two sequences are identifiedin the Hp-K versus logP diagram (where Hp is the Hipparcos broad-bandmagnitude) at short periods (logP<2.35). At a given period these twogroups have, on average, the same pulsation amplitude, but differentJHKL colours and spectral types. The short-period stars in the bluersequence have similar near-infrared colours to the Miras found inglobular clusters. Long-term trends in the infrared light curves arediscussed for stars that have sufficient data.
| Mira kinematics from Hipparcos data: a Galactic bar to beyond the Solar circle
The space motions of Mira variables are derived from radial velocities,Hipparcos proper motions and a period-luminosity relation. Thepreviously known dependence of Mira kinematics on the period ofpulsation is confirmed and refined. In addition, it is found that Miraswith periods in the range 145-200d in the general Solar neighbourhoodhave a net radial outward motion from the Galactic Centre of75+/-18kms-1. This, together with a lag behind the circularvelocity of Galactic rotation of 98+/-19kms-1, is interpretedas evidence for an elongation of their orbits, with their major axesaligned at an angle of ~17° with the Sun-Galactic Centre line,towards positive Galactic longitudes. This concentration seems to be acontinuation to the Solar circle and beyond of the bar-like structure ofthe Galactic bulge, with the orbits of some local Miras probablypenetrating into the bulge. These conclusions are not sensitive to thedistance scale adopted. A further analysis is given of the short-period(SP) red group of Miras discussed in companion papers in this series. InAppendix A the mean radial velocities and other data for 842 oxygen-richMira-like variables are tabulated. These velocities were derived frompublished optical and radio observations.
| 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
| 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.
| 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
| Semiregular variables of types SRa and SRb. Silicate dust emission features.
We have analysed the IRAS-LRS spectra of representative samples ofO-rich Semiregular (SR) variables of types SRa and SRb and of Miravariables. The silicate features were extracted by fitting the energydistribution with two blackbodies, approximating the continuous emissionfrom the photosphere and the circumstellar dust. The shape and strengthof the silicate features in the LRS range were then studied by computingthe residual fluxes in 5 selected wavelength regions covering the whole10μm and 18μm features and parts of the 10μm feature assignedto emission from olivine and possibly corundum. We compare our approachwith previous investigations and argue that a quantitative study ofdetails in the feature shape requires subtraction of the stellar and thedust continuum and the use of flux ratios rather than a discreteclassification system. The Miras form an extension of the SRb's towardslower stellar temperatures and higher dust shell opacities and they haveslightly higher average dust temperatures. The SRa's seem to be moresimilar to the Miras in their dust shell properties. The average 10μmfeature shapes of the three groups of variables agree, but taking intoaccount the photospheric and dust shell parameters, systematicdifferences show up. For stars hotter than about 2900K, the 10μmfeature width shows a wide range of values but no clear trend with thestellar temperature or the optical depth of the dust shell. These starsare generally SRb variables and have the thinnest dust envelopes. Atcooler stellar temperatures, where mostly Miras are found, the opticaldepth of the dust shell determines the feature width in the sense thatthicker shells have narrower features. It appears that the 13μmfeature is obvious only in a narrow range of effective temperature andoptical depth of the dust shell. We discuss our results in terms ofradiative transfer effects, differences in the average grain size,annealing and hydration of amorphous silicates and contributions fromother dust components. Of these possibilities the last one seems to bemost plausible with regard to the behavior of the 10μm feature width.The observations can be interpreted in terms of changing contributionsfrom olivine and corundum possibly caused by an increasing amount ofdust processing (Miras) and the influence of the atmospheric structureon the formation of these dust components (SRb's).
| 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.
| Mainline OH detection rates from blue circumstellar shells.
We identified 240 accessible circumstellar shells with water and/or SiOmasers, and used the Arecibo and Nancay radio telescopes to search formainline OH emission from them. Our targets are often Mira variableswithout previously known mainline masers. This search results in 89detections, of which 77 are new. The probability of detecting a maser islarger once a water maser is known, and becomes progressively larger thethicker and redder the shell. Nevertheless, almost all of our examplesof solitary 1665MHz masers, rather than the joint occurrence of both1665 and 1667MHz masers, are in the bluest shells. The IRAS lowresolution spectral type is the strongest factor correlating with themainline detection rate. We find that 67% of objects with a silicateemission feature exhibit masers, whereas only 27% of objects with acomparatively featureless 1n type do. These rates are colourinsensitive. We ascribe this clearcut difference to differing UVextinction properties of the two grain types, which is likely to resultfrom differing grain-size distributions. The IR colour sensitivity ofthe overall mainline detection rate is thus almost entirely anincidental artifact of the changing proportion of the two grain typeswith colour. Inferentially, since 90% of the sample exhibit watermasers, and the proportion of blue sources with silicate features issubstantially larger than an unbiased selection from the IRAS PointSource Catalog would give, the incidence of water masers is similarlysensitive to spectral type.
| 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.
| The origin and evolution of short-period Miras in the solar neighborhood: Constraints on the life cycle of old stars
The spatial distribution of the short-period (P less than 300 days)oxygen-rich Miras in the solar neighborhood can be fitted with anexponential scale height above the Galactic plane of about 600 pc. Usingthe Gliese catalog of local main-sequence stars, we estimate that thedensity of suitable G-type progenitor dwarfs within 20 pc of the Sun forthese short-period Miras is 6 x 10-4/cu pc. The portion ofthe H-R diagram near the main-sequence turnoff of thesevelocity-selected Gliese stars is intermediate between that of the oldopen cluster NGC 188 and that of the metal-rich globular cluster, 47Tuc. We infer that the main-sequence progenitors of the short-periodMiras have masses near 1.0 solar mass, and we estimate that these Mirashave ages approximately 9 x 109. We also identify a few olddisk red giants in the neighborhood of the Sun. On the basis of verylimited information, we estimate that the total amount of mass lost fromthese stars during their first ascent up the red giant branch is lessthan or equal to 0.1 solar mass. We derive a duration of theshort-period Mira phase of close to 5 x 105 yr. This estimatefor the duration of the short period Mira phase is longer than ourestimate of 2 x 105 yr for the duration of the Mira phase forstars with periods longer than 300 days. From their infrared colors, weestimate a typical mass-loss rate from the short-period Miras ofapproximately 1 x 10-7 solar mass/yr.
| 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.
| Near-infrared photometry of a sample of IRAS point sources
This paper presents the J, H, K, L, M photometry of 516 sourcespertaining to a sample of 787 sources which has been extracted from theIRAS Point Source Catalog in order to study the late stages of stellarevolution and the concomitant phenomena of mass loss. Three differentclssifications of these sources based on broad-band photometry and IRASlow-resolution spectra are given, and the distributions of the sourcesin terms of these classifications are presented. A subsample of peculiarsources, believed to have recently undergone a helium flash, has beenisolated using the K - L, (12-micron) color diagram. Some objectsmeriting further study are also mentioned.
| 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.
| Short- and intermediate-period oxygen-rich Miras
Available IR photometry and the period-IR luminosity relationship areused to analyze a nearly complete sample of oxygen-rich Miras atGalactic latitudes with /b/ greater than 30 deg. In agreement withprevious kinematic studies, a marked difference is found in the spatialdistributions between the stars with periods less than and those withperiods greater than 300 d. For stars with periods in the range 300-400d, the exponential scale height from the Galactic plane is close to 240pc, the projected surface density is about 100/sq kpc, and the localspace density is about 210/cu kpc. The progenitors of the Miras withperiods greater than 300 d appear to be disk dwarfs with typicalmain-sequence masses of between 1.0 and 1.2 solar masses. The masses ofthe main-sequence progenitors of the short-period Miras are 1.1 solarmasses or less. The duration of the Mira phase for theintermediate-period stars is estimated to be about 20,000 yr, somewhatlonger than other recent estimates. Both short- and intermediate-periodoxygen-rich Miras characteristically lose mass at about 10 exp -7 solarmass/yr.
| Observations of 43-GHz SiO masers associated with bright IRAS sources
A survey of potential SiO maser sources has been made using the Parkesradio telescope operating at 43 GHz. The candidate objects were allbright southern IRAS sources and were selected using criteria developedfor use in an earlier study. A further 29 masers were discovered in thepresent survey and, for most of these sources, data in both v = 1 and v= 2 (J = 1-0) transitions were obtained. Although many of the candidateIRAS sources have not been observed at visual wavelengths, there arepublished optical variability data for nine objects. In these cases therelationship between SiO maser emission and visual amplitude, proposedpreviously, is not violated. It is suggested that, in the main, the newmasers are associated with optically obscured Mira variables. This paperalso presents the latest 43-GHz maser polarization spectra for thenearest symbiotic binary, R Aqr, and for GL 5552, the most intensesource discovered in an earlier survey.
| Emission features in IRAS LRS spectra of M Mira variables
A total of 291 M Mira variables with IRAS low-resolution spectrometer(LRS) spectra that have 8-micron fluxes greater than 15 Jy have beenanalyzed. After subtraction of a 2500 K blackbody energy distributionfrom the spectrum, the remaining difference spectra can be classifiedinto seven groups, depending on the shape of their spectral emissionfeatures around 10 microns: Sil, Sil(+) Sil(2+), S, three-component,'broad', and no feature. The spectral emission features are interpretedas being produced primarily by amorphous silicates with differingamounts of crystalline olivine. A weak broad feature at 9-13 microns ispossibly produced by aluminum oxide and/or a size distribution ofsilicate particles that favors larger grains. The 8-22-micron IR excess,defined as the fraction of energy above the 2500 K continuum, shows onlya slight tendency to correlate with period and no tendency to correlatewith mass loss rate. Maser emission from OH and H2O is much morefrequently detected in stars that show the Sil or Sil(+) emissionfeature, but SiO maser emission is detected with the same percentage inall emission feature classes with the exception of the S feature. Starsshowing the S feature appear to be closely related to the MS and Sstars.
| On the contribution of interstellar extinction to the 10 micron dust feature in OH/IR stars
The IRAS Low Resolution Spectra of 467 sources with the 10 micron dustfeature are analyzed. The strengths of the dust feature are determinedby the ratio of the flux at 9.7 micron to the fitted continuum level.Color temperatures are derived from the fluxes of the four IRASphotometric bands after correcting for the effect of the 10 micronfeature on the fluxes of the 12 micron band. A definite correlationbetween the strength of the feature and the color temperature of thecontinuum is found, implying that the 10 micron dust feature is largelycircumstellar in origin. A reexamination of the strength of the silicatefeature for seven of the OH/IR star used by Gehrs and colleagues in 1985has failed to reproduce the optical depth-distance relationship found bythese authors. It is concluded that interstellar extinction does notplay a major role in the formation of the 10 micron absorption feature.
| 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.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Serpiente |
| Ascensión Recta: | 15h43m36.78s |
| Declinación: | -01°42'37.7" |
| Magnitud Aparente: | 10.405 |
| Movimiento Propio en Ascensión Recta: | 4.5 |
| Movimiento Propio en Declinación: | 10.2 |
| B-T magnitude: | 11.978 |
| V-T magnitude: | 10.535 |
Catálogos y designaciones:
|
INHABITED SKY
