Contents
Images
Upload your image
DSS Images Other Images
Related articles
The mid-infrared diameter of W Hydrae
Aims: Asymptotic giant branch (AGB) stars are among the largestdistributors of dust into the interstellar medium, and it is thereforeimportant to understand the dust formation process and sequence in theirstrongly pulsating extended atmosphere. By monitoring the AGB star W Hyainterferometrically over a few pulsations cycles, the upper atmosphericlayers can be studied to obtain information on their chemical gas anddust composition and their intracycle and cycle-to-cycle behavior. Methods: Mid-infrared (8-13 ?m) interferometric data of W Hya wereobtained with MIDI/VLTI between April 2007 and September 2009, coveringnearly three pulsation cycles. The spectrally dispersed visibility dataof all 75 observations were analyzed by fitting a circular fullylimb-darkened disk (FDD) model to all data and individual pulsationphases. Asymmetries were studied with an elliptical FDD. Results:Modeling results in an apparent angular FDD diameter of W Hya of about(80 ± 1.2) mas (7.8 AU) between 8 and 10 ?m, which correspondsto an about 1.9 times larger diameter than the photospheric one. Thediameter gradually increases up to (105 ± 1.2) mas (10.3 AU) at12 ?m. In contrast, the FDD relative flux fraction decreases from(0.85 ± 0.02) to (0.77 ± 0.02), reflecting the increasedflux contribution from a fully resolved surrounding silicate dust shell.The asymmetric character of the extended structure could be confirmed.An elliptical FDD yields a position angle of (11 ± 20)° andan axis ratio of (0.87 ± 0.07). A weak pulsation dependency isrevealed with a diameter increase of (5.4 ± 1.8) mas betweenvisual minimum and maximum, while detected cycle-to-cycle variations aresmaller. Conclusions: W Hya's diameter shows a behavior that isvery similar to the Mira stars RR Sco and S Ori and can be described byan analogous model. The constant diameter part results from a partiallyresolved stellar disk, including a close molecular layer ofH2O, while the increase beyond 10 ?m can most likely beattributed to the contribution of a spatially resolved nearbyAl2O3 dust shell. Probably due to the lowmass-loss rate, close Fe-free silicate dust could not be detected. Theresults suggest that the formation of amorphousAl2O3 occurs mainly at visual minimum. A possibleclose Al2O3 dust shell has now been revealed in afew objects calling for self-consistent dynamic atmospheric modelsincluding dust formation close to the star. The asymmetry might beexplained by an enhanced dust concentration along an N-S axis.Based on observations made with the Very Large Telescope Interferometer(VLTI) at the Paranal Observatory under program IDs 079.D-0140,080.D-0005, 081.D-0198, 082.D-0641 and 083.D-0294.FITS files of thecalibrated visibilities are only available at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/530/A120
| Polarization observations of circumstellar OH masers
Results of observations of circumstellar OH masers in lines withwavelengths near 18 cm are reported. The observations were carried outon the radio telescope of the Nan cay Radio Astronomy Observatory(France). In 2007–2009, 70 late-type stars were observed(including Mira and semi-regular variables). For 53 of these, emissionwas detected in at least one of three OH lines (1612, 1665, or 1667MHz). Circular and linear polarization of the maser emission wasmeasured, yielding all four Stokes parameters. Polarized emissionfeatures were detected in the OH line spectra of 41 stars. A summary ofall the observations is given. The results obtained for T Lep, R LMi,and R Crt are discussed. Emission in the 1665 and 1667 MHz OH lines wasdetected in T Lep for the first time. Features probably due to Zeemansplitting were detected in the OH line profiles of all three stars.Estimates of the magnetic-field strengths in the maser sources wereobtained (0.46–2.32 mG). Variability of the polarizationcharacteristics of the maser emission of the stars on time intervals ofseveral months was found.
| Simultaneous Observations of SiO and H2O Masers Toward Known Stellar SiO and H2O Maser Sources. I.
We present the results of simultaneous observations of both SiO andH2O masers toward 166 known SiO and H2O masersources using the Korean VLBI Network Yonsei 21 m radio telescope during2009 June. Both SiO and H2O maser emission were detected from112 sources giving a detection rate of 67% at one epoch observation.SiO-only maser emission was detected from 42 sources, whileH2O-only maser emission was detected from four sources. Mostof the SiO masers appear around the stellar velocity, whileH2O masers show different characteristics compared with SiOmasers. There are more than 20 sources that show a one-way peak ordouble peaks with respect to the stellar velocity and SiO maser peakvelocity. The H2O maser peak and integrated intensity ratioswith respect to those of SiO (v = 1) show increasingly larger valuesfrom Mira variables, to OH/IR stars, to semi-regular variables. Inaddition, the IRAS two-color diagram of SiO and H2O maserobservational results is discussed.
| Statistical Properties of Stellar H2O Masers --- Results of Three-Year Single-Dish Observations with the VERA Iriki Telescope
We report on the results of monitoring observations of 242 stellarH2O masers, which have been made with the Iriki 20m telescopeof the VLBI Exploration of Radio Astrometry (VERA) from 2003 July to2006 November. The present paper mainly focuses on 85 stellarH2O masers that have been tightly observed with a timespacing of typically 1--2 months. In particular, 46 masers out of themhave been recognized concerning their periodic flux variation and havelight-curve data of stellar visual light. Thus, the present paper showssome statistical views of the observed time variability properties ofstellar H2O masers. We found a good correlation between atime delay of the variation in the H2O masers flux withrespect to that in the stellar visual light and the stellar pulsationperiod. The corresponding phase lags are mildly scattered, but aremainly concentrated in the range, 0.7 ≤ Δφ ≤ 1.5. Wealso measured line-of-sight velocity drifts of the individual spectralpeaks of H2O maser emission, which indicate radialacceleration of mass-loss outflows from the evolved stars. We discusspossible pulsation-driven shock waves that are enhanced near the stellarsurface, and are propagating outwards in the circumstellar envelope.
| AGB variables and the Mira period-luminosity relation
Published data for large-amplitude asymptotic giant branch variables inthe Large Magellanic Cloud (LMC) are re-analysed to establish theconstants for an infrared (K) period-luminosity relation of the formMK = ρ[logP - 2.38] + δ. A slope of ρ = -3.51+/- 0.20 and a zero-point of δ = -7.15 +/- 0.06 are found foroxygen-rich Miras (if a distance modulus of 18.39 +/- 0.05 is used forthe LMC). Assuming this slope is applicable to Galactic Miras we discussthe zero-point for these stars using the revised Hipparcos parallaxestogether with published very long baseline interferometry (VLBI)parallaxes for OH masers and Miras in globular clusters. These result ina mean zero-point of δ = -7.25 +/- 0.07 for O-rich Galactic Miras.The zero-point for Miras in the Galactic bulge is not significantlydifferent from this value.Carbon-rich stars are also discussed and provide results that areconsistent with the above numbers, but with higher uncertainties. Withinthe uncertainties there is no evidence for a significant differencebetween the period-luminosity relation zero-points for systems withdifferent metallicity.
| Lithium and zirconium abundances in massive Galactic O-rich AGB stars
Lithium and zirconium abundances (the latter taken as representative ofs-process enrichment) are determined for a large sample of massiveGalactic O-rich AGB stars, for which high-resolution opticalspectroscopy has been obtained (R˜ 40 000{-}50 000). This was doneby computing synthetic spectra based on classical hydrostatic modelatmospheres for cool stars and using extensive line lists. The resultsare discussed in the framework of "hot bottom burning" (HBB) andnucleosynthesis models. The complete sample is studied for variousobservational properties such as the position of the stars in the IRAStwo-colour diagram ([ 12] - [25] vs. [ 25] - [60] ), Galacticdistribution, expansion velocity (derived from the OH maser emission),and period of variability (when available). We conclude that aconsiderable fraction of these sources are actually massive AGB stars(M>3{-}4 Mȯ) experiencing HBB, as deduced from thestrong Li overabundances we found. A comparison of our results withsimilar studies carried out in the past for the Magellanic Clouds (MCs)reveals that, in contrast to MC AGB stars, our Galactic sample does notshow any indication of s-process element enrichment. The differencesobserved are explained as a consequence of metallicity effects. Finally,we discuss the results obtained in the framework of stellar evolution bycomparing our results with the data available in the literature forGalactic post-AGB stars and PNe.Based on observations at the 4.2 m William Herschel Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de Los Muchachos of the Instituto de Astrofisicade Canarias. Also based on observations with the ESO 3.6 m telescope atLa Silla Observatory (Chile). Tables [see full textsee full text]-[seefull textsee full text] are only available in electronic form athttp://www.aanda.org
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| 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.
| Searches for OH masers in the circumstellar envelopes of red giant irregular variable stars
Irregular variables of type Lb share many characteristics withasymptotic giant branch (AGB) stars such as Miras and semiregulars(SRs). Despite their large number, irregulars are poorly understood.Using the large Nançay radio telescope, we searched for 18-cm OHmaser emission from these stars. We selected two samples of Lbs thatpresent a noticeable circumstellar envelope. In the first one, 44high-latitude objects with infrared colours of dusty mass-losing starswere observed in a wide-bandwidth velocity-search mode. In addition, 16objects (including six from the first sample), in which othercircumstellar molecular lines had recently been found, were observedwith higher resolution and sensitivity. We did not detect any new OHmaser emission from these samples down to 3σ sensitivity limitsranging from 30 to 200 mJy. We derived upper limits for the efficiencyof a typical pump using far-infrared photons. When compared with knownOH masers in AGB stars, we show that a significant gap is leftunexplored below the threshold that we determined for Lbs. Lowerefficiencies exist in some OH Miras and in the OH SRs. We cannot discardthe possibility of faint OH emission from certain Lbs, based on the sameprocesses as encountered in those sources. We briefly discuss physicalconditions allowing OH emission in the envelope, and the evolutionarystage of objects classified as Lb variables with respect to the AGB.
| 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
| Ionization and Cooling of a Hot Plasma with Temperature Fluctuations
Cooling functions for a stationary plasma are calculated in a widetemperature range from 5·103 K to 108 K, both for a plasma withthe solar abundances of elements and for a plasma with an anomalouschemical composition typical of Wolf—Rayet stars. The HILYSproject is described, with the aim of calculating cross sections andrates of excitation by electron collision of atoms and ions with acharge Z 26 and principal electron quantum numbers n 10, needed tocalculate the ionization and thermal states of a plasma and thedevelopment of methods of calculating the plasma's spectrum in thevisible, UV, and x-ray ranges. The results of a calculation of crosssections and effective collision strengths obtained within the frameworkof the project are given. The influence of temperature fluctuations (T/T0.16) on the relative ion abundances and the total cooling function isstudied. It is shown that the presence of such fluctuations considerablyincreases the temperature range in which the abundances of ions of agiven degree of ionization are not negligible, while the coolingfunction can differ considerably from that calculated for aone-temperature plasma. The contribution of dielectronic recombinationto the total cooling function is investigated, and it proves to besignificant only for a plasma with high abundances of heavy elements.The x-ray spectrum of the bright supergiant Pup is analyzed.
| Stellar and circumstellar evolution of long period variable stars
In a first paper, HIPPARCOS astrometric and kinematic data were used tocalibrate both infrared K and IRAS luminosities at the same time askinematic parameters of Long Period Variable stars (LPVs). Individualestimated absolute magnitudes and a probabilistic assignation togalactic populations were deduced from these calibrations for each LPVof our sample. Here we propose a scenario of simultaneous stellar andcircumstellar evolution according to the galactic populations. Thetransitory states of S and Tc stars allow us to confirm the location ofthe first dredge-up at Mbol=-3.5. There is also evidencesuggesting that a previous enrichment in s-elements from a more evolvedcompanion may accelerate the evolution along the AGB. The possibleevolution to OH LPVs is included in this scenario, and any of thesestars may have a mass at the limit of the capability for a C enrichmentup to C/O > 1. A list of bright massive LPVs with peculiar envelopeand luminosity properties is proposed as Hot Bottom Burning candidates.The He-shell flash star, R Cen, is found to be exceptionally bright andcould become, before leaving the AGB, a C-rich LPV brighter than theusual luminosity limit of carbon stars.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| 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.
| Dust features in the 10-mu m infrared spectra of oxygen-rich evolved stars
We have analyzed the 8-13.5 mu m UKIRT CGS3 spectra of 142 M-type starsincluding 80 oxygen-rich AGB stars and 62 red supergiants, with a viewto understanding the differences and similarities between the dustfeatures of these stars. We have classified the spectra into groupsaccording to the observed appearance of the infrared features. In eachcase the normalized continuum-subtracted spectrum has been compared tothose of the other stars to find similarities and form groups. The dustfeatures of the AGB stars are classified into six groups: broad AGB,where the feature extends from 8 mu m to about 12.5 mu m with littlestructure; broad+sil AGB, which consists of a broad feature with anemerging 9.7 mu m silicate bump; and four silicate AGB groups in which a``classic'' 9.7 mu m silicate feature gets progressively narrower.Likewise, the supergiant spectra have also been classified into groups,however these do not all coincide with the AGB star groups. In thesupergiant case we again have six groups: featureless, where there islittle or no emission above the continuum; broad Super, where thefeature extends from about 9 mu m to about 13 mu m; and four silicateSuper groups, which again show a progression towards the narrowest``classic'' 9.7 mu m silicate feature. We compare the mean spectrum foreach group, which yields two main results. Firstly, while the``classic'' silicate feature is essentially identical for both AGB starsand red supergiants, the broad features observed for these two stellartypes are quite different. We suggest that the dust in these twoenvironments follows different evolutionary paths, with the dust aroundMira stars, whose broad feature spectra can be fit by a combination ofalumina (Al2O3) and magnesium silicate,progressing from this composition to dust dominated by magnesiumsilicate only, while the dust around supergiants, whose broad featurecan be fit by a combination of Ca-Al-rich silicate andAl2O3, progresses from this initial composition toone eventually also dominated by magnesium silicate. The reason for thedifference in the respective broad features is not clear as yet, butcould be influenced by lower C/O ratios and chromospheric UV radiationfields in supergiant outflow environments. The second result concernsthe 12.5 - 13.0 mu m feature discovered in IRAS LRS spectra and widelyattributed to Al2O3. This feature is seenpredominantly in the spectra of semiregular variables, sometime in Mirasand only once (so far) in supergiant spectra. We argue that it isunlikely that this feature is due to Al2O3 or, ashas more recently been suggested, spinel(MgAl2O4), but could be associated with silicondioxide or highly polymerized silicates (not pyroxenes or olivines).
| 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.
| Modelling of oxygen-rich envelopes using corundum and silicate grains
A set of 31 oxygen-rich stars has been modelled using corundum andsilicate grains. These stars were selected according to theirdust-envelope class, as suggested by Little-Marenin and Little in 1990.Then 16 stars classified as Sil were modelled using silicate grains; 10Broad class stars using corundum (Al2O3) grains;and 5 `Intermediate' class stars using two kinds of grainsimultaneously: corundum and silicate. The temperature of the centralstars and some characteristics of their circumstellar envelopes such astheir extinction opacities and extensions were determined by fitting theflux curves. The corundum/silicate ratios as well as the energydistributions and temperature laws have been obtained. Based on theauthors' results they suggest the existence of chemical and structuralevolution of the modelled circumstellar dust shells. The temperature ofthe central stars and the temperature of the hottest grains decreasefrom Broad to Intermediate to Sil classes, while the inner radii andoptical depths increase in this sequence.
| 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
| Early Radio Positions of Stars
Early radio positions for a sample of 100 Hipparcos stars, threeTycho-only stars, and nine radio stars with optical positions referredto the Hipparcos/ICRS frame are analyzed. The optical proper motions areused to compare the optical and radio positions. From an original sampleof 247 radio positions for the above 112 stars, a set of 220 showscoincidence between the optical and radio centers of emission closerthan 500 mas. This set is analyzed for systematic departures between theoptical and radio positions. A smaller subset of 136 early radiopositions for 72 stars show radio-minus-optical offsets smaller than 100mas and are useful for monitoring of the spin of the Hipparcos frame.
| 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.
| Infrared carbon stars: new identifications and their space distribution in the Galaxy
We present JHKL photometry of a sample of 150 IRAS stars in the thirdand fourth galactic quadrant with | b | <2(deg) selected according totheir IRAS colour (0.160<=[12-25]<=1.156). We identify 27 carbonstar candidates using the [12-25] vs. K-L two-colour diagram method.Among them, 10 are listed in the Stephenson's catalogue of carbon starsand 17 are new infrared carbon stars (IRCS) candidates. Their distancesare found to be larger than 3 kpc. These data are combined with previousdata to study the space distribution of IRCS. The number density seemsto be independent of the galactocentric distance (R) toward the GalacticCenter and to decrease exponentially toward the anticenter. At thepresent stage, it is not possible to disentangle effects such asincompleteness of the sample, different space distribution law anddependence of the luminosity on metallicity. In particular, we suggestthat the apparent scarcity of IRCS in the central direction of theGalaxy (R<5 kpc) might be due to a selection effect. From theavailable data, we have found no IRCS within 1 kpc of the GalacticCenter. Based on observations obtained at the European SouthernObservatory, Chile
| Detection of short-term variations in Mira-type variables from HIPPARCOS photometry
The Hipparcos photometric observations carried out over 37 months havebeen investigated in detail for ~ 250 Miras. This leads to an unexpectedby-product of the mission that we already suspected from ESOC real-timereductions: the first detection from space of photometric short-termbrightness variations in Mira-type variables. Altogether, 51 events in39 M-type Miras are detected and no similar variations are found for Sand C-type Miras. Their amplitude ranges from 0.23 mag up to 1.11 magand their duration extends from 2 hours up to almost 6 days. Theseevents seem to occur preferentially in late spectral types. We suggestthat they might be related to molecular opacity effects. Based on datafrom the \HIP astrometry ESA satellite
| 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.
| Mean light curves of long-period variables and discrimination between carbon- and oxygen-rich stars
Using 75 years of AAVSO data, mean light curve parameters of a sample of355 long period M, S, and C mira and semi-regular variable stars areinvestigated. We present a classification of the light curves of LPVsinto 6 distinct groups. Combining this classification with IRAS colorsmakes it possible to distinguish oxygen-rich from carbon-rich miras.Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The nature of the unusual source IRAS 18530+0817.
Infrared photometry and spectroscopy of the unassociated (infrared-only)IRAS source, IRAS 18530+0817, indicate that it is a highly unusualevolved star surrounded by a dense envelope of cool gas and dust. TheIRAS data reveal a large infrared excess and significant fluxvariability in the 12-25?m region, as well as a prominent, silicateemission feature at 8.5-11.5?m. The shape of the feature also appearsto vary; in 1993 it was apparently attenuated by an absorption with asharp edge at 9.5?m, which may be circumstellar or photospheric inorigin. At other times the silicate feature appears to be self-absorbedby varying amounts. Near-infrared spectra of IRAS 18530+0817 aredominated by steam absorptions, with the K-band spectrum showing only anarrow band of relatively unabsorbed continuum at 2.15-2.29?m. Theobject appears to possess the strongest stellar photospheric H_2_Oabsorptions yet observed. We tentatively conclude that the central starof IRAS 18530+0817 is a late-type, O-rich, AGB star (a Mira variable)surrounded by an envelope of O-rich dust.
| OH eruptive Mira stars.
We report the evolution of new OH flares detected from four Miras. Theseeruptive stars possess in common the fact that they have very thinenvelopes. Except for one of them, each eruptive feature has a velocitynear that of the star. Preferentially, the eruptive lines are at 1612and 1665MHz. It is likely that OH flares take place in a zone of theenvelope closer to the star than standard emission.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Poupe |
| Right ascension: | 07h32m38.07s |
| Declination: | -20°39'29.1" |
| Apparent magnitude: | 11.104 |
| Proper motion RA: | 1.8 |
| Proper motion Dec: | -2.8 |
| B-T magnitude: | 12.905 |
| V-T magnitude: | 11.253 |
Catalogs and designations:
|
INHABITED SKY
