Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
V4332 Sagittarii: a circumstellar disc obscuring the main object
Context. V4332 Sgr experienced an outburst in 1994 whose observationalcharacteristics in many respects resemble those of the eruption of V838Mon in 2002. It has been proposed that these objects erupted because ofa stellar-merger event. Aims: Our aim is to derive, fromobservational data, information on the present (10-15 yrs after theoutburst) nature and structure of the object. Methods: We presentand analyse a high-resolution (R ? 21 000) spectrum of V4332 Sgrobtained with the Subaru Telescope in June 2009. Various components(stellar-like continuum, atomic emission lines, molecular bands inemission) in the spectrum are analysed and discussed. We alsoinvestigate a global spectral energy distribution (SED) of the objectmostly derived from broadband optical and infrared photometry. Results: The observed continuum resembles that of an ~M 6 giant. Theemission features (atomic and molecular) are most probably produced byradiative pumping. The observed strengths of the emission featuresstrongly suggest that we only observe a small part of the radiation ofthe main object responsible for pumping the emission features. Aninfrared component seen in the observed SED, which can be roughlyapproximated by two blackbodies of ~950 and ~200 K, is ~50 timesbrighter than the M 6 stellar component seen in the optical. Thisfurther supports the idea that the main object is mostly obscured forus. Conclusions: The main object in V4332 Sgr, an ~M 6(super)giant, is surrounded by a circumstellar disc, which is seenalmost edge-on so the central star is obscured. The observed M 6spectrum probably results from scattering the central star spectrum ondust grains at the outer edge of the disc.Based in part on data collected at Subaru Telescope, which is operatedby the National Astronomical Observatory of Japan.
| 3.6 Years of DIRBE Near-infrared Stellar Light Curves
The weekly averaged near-infrared fluxes for 2652 stars were extractedfrom the cold and warm era all-sky maps of the Diffuse InfraredBackground Experiment (DIRBE). Since the DIRBE program only archived theindividual Calibrated Infrared Observations for the 10 month cold eramission, the weekly averaged fluxes were all that were available for thewarm era. The steps required to extract stellar fluxes are described asare the adjustments that were necessary to correct the results forseveral systematic effects. The observations are at a cadence of once aweek for 3.6 years (~1300 days), providing continuous sampling onvariable stars that span the entire period for the longest fundamentalpulsators. The stars are divided into three categories: those with largeamplitude of variability, smaller amplitude variables, and sources whosenear-infrared brightness do not vary according to our classificationcriteria. We show examples of the results and the value of the addedbaseline in determining the phase lag between the visible and infrared.
| Period Measurements of Variable Stars Using AAVSO Data
Not Available
| The Mass-loss Return from Evolved Stars to the Large Magellanic Cloud. II. Dust Properties for Oxygen-rich Asymptotic Giant Branch Stars
We model multi-wavelength broadband UBVIJHKs and Spitzer IRACand MIPS photometry and Infrared Spectrograph spectra from the SAGE andSAGE-Spectroscopy observing programs of two oxygen-rich asymptotic giantbranch (O-rich AGB) stars in the Large Magellanic Cloud (LMC) usingradiative transfer (RT) models of dust shells around stars. We chose astar from each of the bright and faint O-rich AGB populations found byearlier studies of the SAGE sample in order to derive a baseline set ofdust properties to be used in the construction of an extensive grid ofRT models of the O-rich AGB stars found in the SAGE surveys. From thebright O-rich AGB population, we chose HV 5715, and from the faintO-rich AGB population we chose SSTISAGE1C J052206.92-715017.6(SSTSAGE052206). We found the complex indices of refraction ofoxygen-deficient silicates from Ossenkopf et al. and a power law withexponential decay grain size distribution like what Kim et al. used butwith ? of -3.5, a min of 0.01 ?m, and a 0of 0.1 ?m to be reasonable dust properties for these models. There isa slight indication that the dust around the faint O-rich AGB may bemore silica-rich than that around the bright O-rich AGB. Simple modelsof gas emission suggest a relatively extended gas envelope for the faintO-rich AGB star modeled, consistent with the relatively large dust shellinner radius for the same model. Our models of the data require theluminosity of SSTSAGE052206 and HV 5715 to be ~5100 L sun and~36,000 L sun, respectively. This, combined with the stellareffective temperatures of 3700 K and 3500 K, respectively, that we findbest fit the optical and near-infrared data, suggests stellar masses of~3 M sun and ~7 M sun. This, in turn, suggeststhat HV 5715 is undergoing hot-bottom burning and that SSTSAGE052206 isnot. Our models of SSTSAGE052206 and HV 5715 require dust shells ofinner radius ~17 and ~52 times the stellar radius, respectively, withdust temperatures there of 900 K and 430 K, respectively, and withoptical depths at 10 ?m through the shells of 0.095 and 0.012,respectively. The models compute the dust mass-loss rates for the twostars to be 2.0 × 10-9 Msun yr-1and 2.3 × 10-9 Msun yr-1,respectively. When a dust-to-gas mass ratio of 0.002 is assumed forSSTSAGE052206 and HV 5715, the dust mass-loss rates imply totalmass-loss rates of 1.0 × 10-6 Msunyr-1 and 1.2 × 10-6 Msunyr-1, respectively. These properties of the dust shells andstars, as inferred from our models of the two stars, are found to beconsistent with properties observed or assumed by detailed studies ofother O-rich AGB stars in the LMC and elsewhere.
| AKARI's infrared view on nearby stars. Using AKARI infrared camera all-sky survey, 2MASS, and Hipparcos catalogs
Context. The AKARI, a Japanese infrared space mission, has performed anAll-Sky Survey in six infrared-bands from 9 to 180 ?m with higherspatial resolutions and better sensitivities than IRAS. Aims: Weinvestigate the mid-infrared (9 and 18 ?m) point source catalog (PSC)obtained with the infrared camera (IRC) onboard AKARI, in order tounderstand the infrared nature of the known objects and to identifypreviously unknown objects. Methods: Color-color diagramsand a color-magnitude diagram were plotted with the AKARI-IRC PSCand other available all-sky survey catalogs. We combined the Hipparcosastrometric catalog and the 2MASS all-sky survey catalog with theAKARI-IRC PSC. We furthermore searched literature and SIMBADastronomical database for object types, spectral types, and luminosityclasses. We identified the locations of representative stars and objectson the color-magnitude and color-color diagram schemes. Theproperties of unclassified sources can be inferred from their locationson these diagrams. Results: We found that the (B-V) vs.(V-S9W) color-color diagram is useful for identifying thestars with infrared excess emerged from circumstellar envelopes ordisks. Be stars with infrared excess are separated well from other typesof stars in this diagram. Whereas (J-L18W) vs. (S9W-L18W)diagram is a powerful tool for classifying several object types.Carbon-rich asymptotic giant branch (AGB) stars and OH/IR stars formdistinct sequences in this color-color diagram. Young stellarobjects (YSOs), pre-main sequence (PMS) stars, post-AGB stars, andplanetary nebulae (PNe) have the largest mid-infrared color excess andcan be identified in the infrared catalog. Finally, we plot the L18W vs.(S9W-L18W) color-magnitude diagram, using the AKARI data togetherwith Hipparcos parallaxes. This diagram can be used to identify low-massYSOs and AGB stars. We found that this diagram is comparable to the [24]vs. ([8.0]-[24]) diagram of Large Magellanic Cloud sources usingthe Spitzer Space Telescope data. Our understanding of Galactic objectswill be used to interpret color-magnitude diagram of stellar populationsin the nearby galaxies that Spitzer Space Telescope observed. Conclusions: Our study of the AKARI color-color andcolor-magnitude diagrams will be used to explore properties ofunknown objects in the future. In addition, our analysis highlights afuture key project to understand stellar evolution with a circumstellarenvelope, once the forthcoming astronometrical data with GAIA areavailable.Catalog (full Tables 3 and 4) are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/514/A2
| Correlation between technetium and lithium in a sample of oxygen-rich AGB variables
Context. The elements technetium and lithium are two importantindicators of internal nucleosynthesis and mixing in late-type stars.Studying their occurrence and abundance can give deep insight into thestructure and evolution in the late phases of the stellar life cycle. Aims: The aims of this paper are: 1) to revisit the Tc content ofa sample of oxygen-rich (M-type) asymptotic giant branch (AGB) variablesand 2) to increase the number of such stars for which the Li abundancehas been measured to provide constraints on the theoretical models ofextra-mixing processes. Methods: To this end, we analysedhigh-resolution spectra of 18 sample stars for the presence ofabsorption lines of Tc and Li. The abundance of the latter wasdetermined by comparing the observed spectra to hydrostatic MARCS modelspectra. Bolometric magnitudes were established from near-IR photometryand pulsation periods. Results: We correct the classificationwith respect to the presence of Tc for a number of stars. We reclassifythe star V441 Cyg as Tc-rich, and the unusual Mira star R Hya, as wellas W Eri, as Tc-poor. The abundance of Li, or an upper limit to it, wasdetermined for all of the sample stars. In all stars with Tc we alsodetected Li. Most of them have a Li content slightly below the solarphotospheric value, except for V441 Cyg, which is super-Li rich with1000 times the solar abundance. We also found that, similar to Tc, alower luminosity limit seems to exist for the presence of Li. Conclusions: We conclude that the higher Li abundance found in thecooler and higher luminosity objects could stem from a Li productionmechanism operating on the thermally pulsing AGB. The stellar mass mighthave a crucial influence on this (extra mixing) production mechanism.Our findings for R Hya and V441 Cyg have some important consequences forthese intriguing objects. It was speculated that the declining pulsationperiod of R Hya is caused by a recent thermal pulse (TP). While notdetecting Tc does not rule out a TP, it indicates that the TPs are notstrong enough to drive 3DUP in R Hya. The pieces of evidence concerningthe mass of V441 Cyg are not entirely conclusive. It could either be alow-mass, intrinsic S-star that produced its large amount of Li byextra-mixing processes, or an intermediate-mass star (M? 4{M}&sun;) undergoing Li production via hot bottom burning.Based on observations at the Very Large Telescope of the EuropeanSouthern Observatory, Cerro Paranal/Chile under Programme 65.L-0317(A),and at the Calar Alto Observatory Centro Astronómico HispanoAlemán, Calar Alto, Spain.
| 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.
| Technetium and the third dredge up in AGB stars. II. Bulge stars
Context: Technetium (Tc) is an unstable element synthesised via thes-process in deep layers of asymptotic giant branch (AGB) stars, makingit a reliable indicator of both recent s-process activity and thirddredge-up. Aims: We searched for Tc in a sample of brightoxygen-rich asymptotic giant branch (AGB) stars located in the outergalactic bulge. We test theoretical predictions of the luminosity limitfor the onset of third dredge-up. Methods: Using high-resolutionoptical spectra obtained with the UVES spectrograph at ESO's VLT, wesearched for resonance lines of neutral Tc in the blue spectral regionof our sample stars. These measurements allow us to improve theprocedure of classification of stars with respect to their Tc content byusing flux ratios. Synthetic spectra based on MARCS atmospheric modelsare presented and compared to the observed spectra around three lines ofTc. Bolometric magnitudes are calculated based on the near-infraredphotometry of the objects. Results: Among the sample of 27long-period bulge variables, four were found to definitely contain Tc intheir atmospheres. The luminosity of the Tc-rich stars agrees withpredictions from AGB evolutionary models on the minimum luminosity atthe time when third dredge-up sets in. However, AGB evolutionary modelsand a bulge consisting of a single old population cannot be brought intoagreement. This probably means that a younger population is present inthe bulge, as suggested by various authors, that contains the Tc-richstars identified here.Based on observations at the Very Large Telescope of the EuropeanSouthern Observatory, Cerro Paranal/Chile under Program 65.L-0317(A, B).
| The CO content of the Local Group dwarf irregular galaxies IC5152, UGCA438 and the Phoenix dwarf
We present a search for CO(1 -> 0) emission in three Local Groupdwarf irregular galaxies: IC5152, the Phoenix dwarf and UGCA 438, usingthe ATNF Mopra radio telescope. Our scans largely cover the opticalextent of the galaxies and the stripped HI cloud west of the Phoenixdwarf. Apart from a tentative but non-significant emission peak at oneposition in the Phoenix dwarf, no significant emission was detected inthe CO spectra of these galaxies. For a velocity width of 6kms-1, we derive 4σ upper limits of 0.03, 0.04 and0.06Kkm s-1 for IC5152, the Phoenix dwarf and UGCA438,respectively. This is an improvement of over a factor of 10 comparedwith previous observations of IC5152 the other two galaxies had not yetbeen observed at millimetre wavelengths. Assuming a GalacticCO-to-H2 conversion factor, we derive upper limits on themolecular gas mass of 6.2 × 104,3.7 ×103 and 1.4 × 105Msolar forIC5152, the Phoenix dwarf and UGCA438, respectively. We investigate twopossible causes for the lack of CO emission in these galaxies. On theone hand, there may be a genuine lack of molecular gas in these systems,in spite of the presence of large amounts of neutral gas. However, inthe case of IC5152 which is actively forming stars, molecular gas is atleast expected to be present in the star-forming regions. On the otherhand, there may be a large increase in the CO-to-H2conversion factor in very low-metallicity dwarfs (-2 <= [Fe/H] <=-1), making CO a poor tracer of the molecular gas content in dwarfgalaxies.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Phase Lags in the Optical-Infrared Light Curves of Asymptotic Giant Branch Stars
To search for phase lags in the optical-infrared light curves ofasymptotic giant branch stars, we have compared infrared data from theCOBE DIRBE satellite with optical light curves from the AAVSO and othersources. We found 17 examples of phase lags between the times ofinfrared and optical maximum, and 4 stars with no observed lags. Thereis a clear difference between the Mira variables and the semiregularvariables in the sample, with the maximum in the optical preceding thatin the near-infrared in the Miras, while in most of the semiregularvariables no lags are observed. Comparison to published theoreticalmodels indicates that the phase lags in the Miras are due to strongtitanium oxide absorption in the visual at stellar maximum, and suggeststhat Miras pulsate in the fundamental mode, while at least somesemiregular variables are first-overtone pulsators. There is a clearoptical-near-infrared phase lag in the carbon-rich Mira V CrB; this islikely due to C2 and CN absorption variations in the optical.
| Technetium and the third dredge-up in bulge AGB stars.
We searched for Technetium (Tc) in a sample of bright oxygen-richasymptotic giant branch (AGB) stars located in the galactic bulge. Tc isan unstable element synthesised via the s-process in deep layers of AGBstars, thus it is a reliable indicator of both, recent s-process andthird dredge-up. High resolution optical spectra obtained with the UVESspectrograph at ESO's VLT are used to search for resonance lines ofneutral Tc. We present a method to improve the classification of starswith respect to their Tc content by using flux ratios. With the attainedclassification theoretical predictions on the luminosity limit for theonset of the third dredge-up are tested. Among the sample of 27 longperiod variables four were found to definitely contain Tc in theiratmosphere, giving the first direct evidence for third dredge-up in thebulge. The results are shortly compared to what is found for fieldstars.
| 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.
| A study of bright Southern long period variables
In this paper we present radial velocity curves of AGB variables thatexhibit various kinds of anomalies: semiregular variables (SRVs) withtypical mira periods, SRVs exceeding the mira 2.5 mag amplitude limit,miras with secondary maxima in their light curves, and a SRV with a longsecondary period. The stars with reliable Hipparcos parallaxes from thisand from previous studies are plotted in a log P-MK-diagram.Our objects nicely follow the log P-MK-relations determinedfor the LMC. This allows the pulsation mode to be identified. While allmiras fall on the fundamental mode sequence, the SRVs fall on both thefirst overtone and fundamental mode sequences. The SRVs on thefundamental mode sequence occur at both high and low luminosities, someof them being more luminous than larger amplitude miras. Thisdemonstrates observationally that some parameter other than luminosityaffects the stability of long period variables, probably mass. Firstovertone pulsators all show velocity amplitudes around 4 kms-1. For the fundamental mode pulsators, the velocityamplitude shows a correlation with light amplitude. The two miras R Cenand R Nor, known for their double-peaked light curves, have velocitycurves that are quite different. The R Nor velocity curve shows noevidence of the double peaks, meaning that the true pulsation period isthe time between alternate minima or maxima. There is slight evidencefor a double bump in the R Cen velocity curve. It is suggested thatthese stars are relatively massive (3-5 Mȯ).
| Technetium and the third dredge up in AGB stars. I. Field stars
We searched for Tc in a sample of long period variables selected bystellar luminosity derived from Hipparcos parallaxes. Tc, as an unstables-process element, is a good indicator for the evolutionary status ofstars on the asymptotic giant branch (AGB). In this paper we study theoccurrence of Tc as a function of luminosity to provide constraints onthe minimum luminosity for the third dredge up as estimated from recentstellar evolution models.A large number of AGB stars above the estimated theoretical limit forthe third dredge up are found not to show Tc. We confirm previousfindings that only a small fraction of the semiregular variables show Tclines in their spectra. Contrary to earlier results by Little et al.(\cite{llmb87}) we find also a significant number of Miras without Tc.The presence and absence of Tc is discussed in relation to the massdistribution of AGB stars. We find that a large fraction of the stars ofour sample must have current masses of less than 1.5 Msun .Combining our findings with stellar evolution scenarios we conclude thatthe fraction of time a star is observed as a SRV or a Mira is dependenton its mass.Partly based on observations collected at the European SouthernObservatory, Paranal, Chile (ESO-Programme 65.L-0317(A)).
| Infrared Colors and Variability of Evolved Stars from COBE DIRBE Data
For a complete 12 μm flux-limited sample of 207 IRAS sources(F12>=150 Jy, |b|>=5deg), the majority ofwhich are AGB stars (~87%), we have extracted light curves in seveninfrared bands between 1.25 and 60 μm using the database of theDiffuse Infrared Background Experiment (DIRBE) instrument on the CosmicBackground Explorer (COBE) satellite. Using previous infrared surveys,we filtered these light curves to remove data points affected by nearbycompanions and obtained time-averaged flux densities and infraredcolors, as well as estimates of their variability at each wavelength. Inthe time-averaged DIRBE color-color plots, we find clear segregation ofsemiregulars, Mira variables, carbon stars, OH/IR stars, and red giantswithout circumstellar dust (i.e., V-[12]<5) and with little or novisual variation (ΔV<0.1 mag). The DIRBE 1.25-25 μm colorsbecome progressively redder and the variability in the DIRBE databaseincreases along the oxygen-rich sequence nondusty slightly varying redgiants-->SRb/Lb-->SRa-->Mira-->OH/IR and the carbon-richSRb/Lb-->Mira sequence. This supports previous assertions that theseare evolutionary sequences involving the continued production andejection of dust. The carbon stars are redder than their oxygen-richcounterparts for the same variability type, except in theF12/F25 ratio, where they are bluer. Of the 28sources in the sample not previous noted to be variable, 18 are clearlyvariable in the DIRBE data, with amplitudes of variation of ~0.9 mag at4.9 μm and ~0.6 mag at 12 μm, consistent with them being verydusty Mira-like variables. We also present individual DIRBE light curvesof a few selected stars. The DIRBE light curves of the semiregularvariable L2 Pup are particularly remarkable. The maxima at1.25, 2.2, and 3.5 μm occur 10-20 days before those at 4.9 and 12μm, and, at 4.9 and 12 μm, another maximum is seen between the twonear-infrared maxima.
| 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
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| Infrared Light Curves of Mira Variable Stars from COBE DIRBE Data
We have used the COBE DIRBE database to derive near- and mid-infraredlight curves for a well-defined sample of 38 infrared-bright Miravariable stars and compared with optical data from the AAVSO. In generalthe 3.5 and 4.9 μm DIRBE bandpasses provide the light curves with thebest signal-to-noise ratio (S/N), with S/N decreasing with wavelength atlonger wavelengths. At 25 μm good light curves are only available for~10% of our stars, and at wavelengths >=60 μm extracting highquality light curves is not possible. The amplitude of variability istypically less in the near-infrared than in the optical and less in themid-infrared than in the near-infrared, with decreasing amplitude withincreasing wavelength. On average there are 0.20+/-0.01 mag variation at1.25 μm and 0.14+/-0.01 mag variation at 4.9 μm for each magnitudevariation in V. The observed amplitudes are consistent with results ofrecent theoretical models of circumstellar dust shells around Miravariables. For a few stars in our sample we find clear evidence of timelags between the optical and near-infrared maxima of phase ~0.05-0.13,with no lags in the minima. For three stars mid-infrared maximum appearsto occur slightly before that in the near-infrared, but after opticalmaximum. We find three examples of secondary maxima in the risingportions of the DIRBE light curves, all of which have opticalcounterparts in the AAVSO data, supporting the hypothesis that they aredue to shocks rather than newly formed dust layers. We find noconclusive evidence for rapid (hours to days) variations in the infraredbrightnesses of these stars.
| 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.
| Lower limits on the maximum orbital frequency around rotating strange stars
Observations of kHz quasi-periodic oscillations (QPOs) in the X-rayfluxes of low-mass X-ray binaries (LMXBs) have been used in attempts toconstrain the external metric of the compact members of these binaries,as well as their masses and the equations of state of matter atsupranuclear denisties. We compute the maximum orbital frequency ofstable circular motion around uniformly rotating strange stars describedby the MIT bag model. The calculations are performed for both normal andsupramassive constant baryon mass sequences of strange stars rotating atall possible rates. We find the lower limits on the maximum orbitalfrequency and discuss them for a range of masses and for all rotationalfrequencies allowed in the model considered. We show that for slowly andmoderately rotating strange stars the maximum value of orbital frequencycan be a good indicator of the mass of the compact object. However, forrapidly rotating strange stars the same value of orbital frequency inthe innermost stable circular orbit is obtained for stars with massesranging from that of a planetoid to about three solar masses. Atsufficiently high rotation rates of the strange star, the rotationalperiod alone constrains the stellar mass to a surprisingly narrow range.
| Polarimetry of 167 Cool Variable Stars: Data
Multicolor photoelectric polarimetry is presented for 167 stars, most ofwhich are variable stars. The observations constitute a data set thatfor some stars covers a time span of 35 yr. Complex variations are foundover time and wavelength and in both the amount of polarization and itsposition angle, providing constraints for understanding the polarizingenvironments in and around these cool 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.
| Hipparcos parallaxes for Mira-like long-period variables
This paper concerns the calibration of the K period-luminosity relationfor Mira variables using Hipparcos parallaxes. K magnitudes areavailable for 255 Mira-like variables which were observed by Hipparcos.Period-luminosity zero-points are evaluated for various subgroups ofdata. The best solution for oxygen-rich Miras, which uses 180 stars,omitting the short-period red group (which had different kinematics fromthe short-period blue stars) and the low-amplitude variables, provides azero-point of σ2σ2π +(0.4605)2π2PL(K)σ2K + σ2PL(K),0.84+/-0.14mag, which implies a distance modulus for the LargeMagellanic Cloud of σK = 0.3ΔK√N,18.64+/-0.14mag, or perhaps slightly greater if a metallicity correctionis required, in good agreement with the value derived from Cepheids. Thezero-point of the period-luminosity relation for carbon stars is brieflydiscussed. Linear diameters are derived for red variables with measuredangular diameters and parallaxes, and are used to examine thelong-standing question of the pulsation mode(s) of these stars. Evidenceis presented to suggest that most of them are pulsating in the same modeand, if published model atmospheres are correct, this is probably thefirst overtone. Some discussion is given of sequences in theperiod-luminosity and period-colour diagrams and their bearing on thepulsation mode problem.
| 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.
| The Henize sample of S stars --- III. Uncovering the binary intruders
The properties of S stars are investigated thanks to a large observingprogram devoted to the well-defined Henize sample (205 S stars south ofδ = -25̂ and brighter than R = 10.5, covering allgalactic latitudes), in order to derive the respective properties of theintrinsic S stars (genuine thermally-pulsing AGB stars) and of theextrinsic S stars (post mass-transfer binaries). The stellar sample isfirst cleaned from a few stars misclassified as S thanks to UBV Genevaphotometry and low-resolution spectroscopy. These low-resolution spectraalso allow to successfully distinguish subclasses within the S starfamily. Dedicated Geneva photometry and high-resolution spectroscopyhave led to the discovery of two symbiotic S stars. The more stringentdifference between extrinsic and intrinsic stars is their technetiumcontent, but several other observational parameters are shown to beefficient to some extent in segregating intrinsic S stars from theirextrinsic masqueraders (UBV , JHKL and IRAS photometry, radial-velocitystandard deviation, shape of the CORAVEL cross-correlation dip,combination of band strength indices derived from low-resolutionspectra). Multivariate classification has been performed on the Henizedata sample in order to guarantee a classification as objective aspossible and handling at the same time a large number of parameters. Theresulting clusters separate efficiently extrinsic and intrinsic S stars,allowing to derive the respective properties of these two distinctstellar classes. The population difference between intrinsic andextrinsic S stars is for the first time clearly demonstrated, sinceintrinsic S stars are far more concentrated towards the galactic planethan extrinsic S stars (zint = 200 ∓ 100 pc andzext = 600 ∓ 100 pc), and are therefore believed tobelong to a younger, more massive population. The frequency of extrinsicand intrinsic S stars in the magnitude-limited Henize sample amounts to33% and 67%, respectively. In a volume-limited sample, this proportionis subject to large uncertainties mainly because of uncertainluminosities. There are probably as many as 40% extrinsic stars among Sstars in a volume-limited sample. Based on observations carried out atthe European Southern Observatory (ESO, La Silla, Chile) and at theSwiss 70 cm telescope (La Silla, Chile)
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Horologium |
| Ascensió Recta: | 02h53m52.77s |
| Declinació: | -49°53'22.7" |
| Magnitud Aparent: | 8.147 |
| Moviment propi RA: | 126.4 |
| Moviment propi Dec: | 40.9 |
| B-T magnitude: | 9.422 |
| V-T magnitude: | 8.253 |
Catàlegs i designacions:
|
