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Period evolution in very long period variables: the semiregulars. .
The analysis of semiregulars (SRs) light curves using the wavelet is noteasy due to the nature of these stars. But we were able to noticesimilarities with the Mira-type stars for the same range of periodevolution (greater than 450 days): the LPV-SRs are unstable, moreovermeandering and continuous changes have been detected. Nevertheless thepresence of a sudden change is quite ambiguous. All these variations inthe period evolution of the semiregulars may be the sign of the presenceof physical processes such thermal pulses or convection for example. Therelated mass loss phenomenon could then be better investigated.
| 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).
| Evolution from AGB to planetary nebula in the MSX survey
We investigate the evolution of oxygen- and carbon-rich AGB stars,post-AGB objects, and planetary nebulae using data collected mainly fromthe MSX catalogue. Magnitudes and colour indices are compared with thosecalculated from a grid of synthetic spectra that describe the post-AGBevolution beginning at the onset of the superwind. We find that carbonstars and OH/IR objects form two distinct sequences in the (K-[8.3])×([8.3]-[14.7]) MSX colour diagram. OH/IR objects are distributedin two groups: the bluest ones are crowded near [14.7]-[21.3]≃ 1and [8.3]-[14.7]≃ 2, and a second, redder group is spread over alarge area in the diagram, where post-AGB objects and planetary nebulaeare also found. High mass-loss rate OH/IR objects, post-AGB stars, andplanetary nebulae share the same region in the (K-[8.3])×([8.3]-[14.7]) and [14.7]-[21.3]×([8.3]-[14.7]) colour-colourdiagrams. This region in the diagram is clearly separated from a bluerone where most OH/IR stars are found. We use a grid of models ofpost-AGB evolution, which are compared with the data. The gap in thecolour-colour diagrams is interpreted as the result of the rapidtrajectory in the diagram of the stars that have just left the AGB.Based on results obtained by the MSX survey.Tables 1 to 3 are 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/431/565
| A W Roberts: the observations (paper 2).
Not Available
| The analysis of indexed astronomical time series - IX. A period change test
A frequency domain test statistic is applied to the full time series ofbrightness maxima and minima in order to test for changes in the meanpulsation periods of 382 Mira stars. The test statistic depends on twoparameters that are related to the intrinsic cycle-to-cycle scatter inthe period, and to the measurement error, respectively. It is shown thatthe former is strongly related to the mean pulsation period of the star.
| Period and chemical evolution of SC stars
The SC and CS stars are thermal-pulsing asymptotic giant branch starswith a C/O ratio close to unity. Within this small group, the Miravariable BH Cru recently evolved from spectral type SC (showing ZrObands) to CS (showing weak C2). Wavelet analysis shows thatthe spectral evolution was accompanied by a dramatic period increase,from 420 to 540 d, indicating an expanding radius. The pulsationamplitude also increased. Old photographic plates are used to establishthat the period before 1940 was around 490 d. Chemical models indicatethat the spectral changes were caused by a decrease in stellartemperature, related to the increasing radius. There is no evidence fora change in C/O ratio. The evolution in BH Cru is unlikely to be relatedto an ongoing thermal pulse. Periods of the other SC and CS stars,including nine new periods, are determined. A second SC star, LX Cyg,also shows evidence for a large increase in period, and one further starshows a period inconsistent with a previous determination. Mira periodsmay be intrinsically unstable for C/O ~ 1; possibly because of afeedback between the molecular opacities, pulsation amplitude, andperiod. LRS spectra of 6 SC stars suggest a feature at λ > 15μm, which resembles one recently attributed to the iron-sulphidetroilite. Chemical models predict a large abundance of FeS in SC stars,in agreement with the proposed association.
| The study of the carbon star IRC+10216
The study of IRC+10216 will be the basis of our work on thecomprehension of the mass loss phenemenon in evolved stars. Ourhypothesis is that the annular structures we can see around thisparticular object are caused by variations in the stellar wind. Wepresent the process which will allow us to test our hypothesis and itsfuture improvements.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| A weakly non-adiabatic one-zone model of stellar pulsations: application to Mira stars
There is growing observational evidence that the irregular changes inthe light curves of certain variable stars might be due to deterministicchaos. Supporting these conclusions, several simple models of non-linearoscillators have been shown to be capable of reproducing the observedcomplex behaviour. In this paper, we introduce a non-linear,non-adiabatic one-zone model intended to reveal the factors leading toirregular luminosity variations in some pulsating stars. We have studiedand characterized the dynamical behaviour of the oscillator as the inputparameters are varied. The parametric study implied values correspondingto stellar models in the family of long period variables and inparticular of Mira-type stars. We draw attention to certain solutionsthat reproduce with reasonable accuracy the observed behaviour of somepeculiar Mira variables.
| Infrared stellar populations in the central parts of the Milky Way galaxy
Near- and mid-IR survey data from DENIS and ISOGAL are used toinvestigate the structure and formation history of the inner 10°(1.4 kpc) of the Milky Way galaxy. Synthetic bolometric corrections andextinction coefficients in the near- and mid-infrared (mid-IR) arederived for stars of different spectral types, to allow thetransformation of theoretical isochrones into observablecolour-magnitude diagrams. The observed IR colour-magnitude diagrams areused to derive the extinction, metallicity and age for individual stars.The inner galaxy is dominated by an old population (>~7 Gyr). Inaddition, an intermediate-age population (~200 Myr-7 Gyr) is detected,which is consistent with the presence of a few hundred asymptotic giantbranch stars with heavy mass loss. Furthermore, young stars (<~200Myr) are found across the inner bulge. The metallicities of thesestellar population components are discussed. These results can beinterpreted in terms of an early epoch of intense star formation andchemical enrichment that shaped the bulk of the bulge and nucleus, and amore continuous star formation history that gradually shaped the discfrom the accretion of subsolar metallicity gas from the halo. A possibleincrease in star formation ~200 Myr ago might have been triggered by aminor merger. Ever since the formation of the first stars, mechanismshave been at play that mix the populations from the nucleus, bulge anddisc. Luminosity functions across the inner Galactic plane indicate thepresence of an inclined (bar) structure at >~1 kpc from the GalacticCentre, near the inner Lindblad resonance. The innermost part of thebulge, within ~1 kpc from the Galactic Centre, seems azimuthallysymmetric.
| Guilt by Association: The 13 Micron Dust Emission Feature and Its Correlation to Other Gas and Dust Features
A study of all full-scan spectra of optically thin oxygen-richcircumstellar dust shells in the database produced by the ShortWavelength Spectrometer on ISO reveals that the strength of severalinfrared spectral features correlates with the strength of the 13 μmdust feature. These correlated features include dust features at 19.8and 28.1 μm and the bands produced by warm carbon dioxide molecules(the strongest of which are at 13.9, 15.0, and 16.2 μm). The databasedoes not provide any evidence for a correlation of the 13 μm featurewith a dust feature at 32 μm, and it is more likely that a weakemission feature at 16.8 μm arises from carbon dioxide gas ratherthan dust. The correlated dust features at 13, 20, and 28 μm tend tobe stronger with respect to the total dust emission in semiregular andirregular variables associated with the asymptotic giant branch than inMira variables or supergiants. This family of dust features also tendsto be stronger in systems with lower infrared excesses and thus lowermass-loss rates. We hypothesize that the dust features arise fromcrystalline forms of alumina (13 μm) and silicates (20 and 28 μm).Based on observations with the ISO, a European Space Agency (ESA)project with instruments funded by ESA member states (especially thePrincipal Investigator countries: France, Germany, the Netherlands, andthe United Kingdom) and with the participation of the Institute of Spaceand Astronautical Science (ISAS) and the National Aeronautics and SpaceAdministration (NASA).
| 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
| The He-shell flash in action: T Ursae Minoris revisited
We present an updated and improved description of the light curvebehaviour of T Ursae Minoris, which is a Mira star with the strongestperiod change (the present rate is an amazing -3.8+/-0.4 days/yearcorresponding to a relative decrease of about 1% per cycle). Ninetyyears of visual data were collected from all available databases and theresulting, almost uninterrupted light curve was analysed with the O-Cdiagram, Fourier analysis and various time-frequency methods. TheChoi-Williams and Zhao-Atlas-Marks distributions gave the clearest imageof frequency and light curve shape variations. A decrease of theintensity average of the light curve was also found, which is inaccordance with a period-luminosity relation for Mira stars. We predictthe star will finish its period decrease in the meaningfully near future(c.c. 5 to 30 years) and strongly suggest to closely follow the star'svariations (photometric, as well as spectroscopic) during this period.
| 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 evolution of the Mira variable R Hydrae
The Mira variable R Hydrae is well known for its declining period, whichWood & Zarro attributed to a possible recent thermal pulse. Here weinvestigate the long-term period evolution, covering 340 years, goingback to its discovery in AD 1662. The data include photometricmonitoring by amateur and other astronomers over the last century, andrecorded dates of maximum for earlier times. Wavelets are used todetermine both the period and the semi-amplitude. We show that theperiod decreased linearly between 1770 and 1950; since 1950 the periodhas stabilized at 385d. The semi-amplitude is shown to follow the periodevolution closely. Analysis of the oldest data shows that before 1770the period was about 495d. We find no evidence for an increasing periodduring this time as found by Wood & Zarro. We discuss the mass-losshistory of R Hya: the IRAS data show that the mass loss droppeddramatically around AD 1750. The evolution of the mass loss as functionof period agrees with the mass-loss formalism from Vassiliadis &Wood; it is much larger than predicted by the Blöcker law. An outerdetached IRAS shell suggests that R Hya has experienced mass-lossinterruptions before. The period evolution can be explained by twomodels: a thermal pulse occurring around AD 1600, or a non-linearinstability leading to an internal relaxation of the stellar structure.The elapsed time between the mass-loss decline giving rise to the outerdetached shell and the recent event, of approximately 5000yr, suggeststhat only one of these events could be due to a thermal pulse. Furthermonitoring of R Hya is recommended, as both models make strongpredictions for the future period evolution. We argue that R Hya-typeevents could provide part of the explanation for the rings seen aroundsome asymptotic giant branch (AGB) and post-AGB stars. Changes in Miraproperties were already known on a cycle-to-cycle basis, and on thethermal pulse time-scale of ~104yr. R Hya shows thatsignificant evolution can also occur on intermediate time-scales of theorder of 102-103yr.
| 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.
| S Orionis: A Mira-type variable with a marked period decrease
We studied the pulsational period of the Mira star S Orionis based onvisual observations that cover a total of 71 years. We found that theperiod decreased markedly from around 445 days to 397 days inapproximately 16 years, between JD 2438000 and JD 2444000. The rate ofthis period variation was of the order of 0.007 day/day, too fast forthe usual variations observed in most Mira variables. This result is ingood agreement with the theoretical models that suggest a helium-shellflash as the cause of these large-period variations. In particular, thevariation of the period and luminosity indicates that this Mira star maynow be in an immediate post-primary helium-shell flash state.
| Mira kinematics in the post-Hipparcos era
The complete data set of Mira variables from the 4th edition of theGeneral Catalog of Variable Stars was analyzed and supplemented by theproper motions and radial velocities presently available for Miras. Theresulting sample of 724 Miras with periods between 78 and 612 dayscontains proper motions reduced to the Hipparcos system, radialvelocities and V magnitudes. For each of 10 subgroups of Miras dividedaccording to their periods and spectral types, statistical parallaxeswere determined by application of five different methods. The meanabsolute magnitudes, the spatial velocities and their dispersions aswell as the elements of the Galactic orbits were computed as functionsof the periods. The (Mbol-< ~ g P) relation obtained wasfound to be considerably steeper than the PLR usually assumed for LMCMiras. For Miras in the period range 145-200 days, no significant netmotion radially outwards in the Galaxy as suggested by Feast &Whitelock (2000) was found. The predicted frequency of Miras wascomputed as a function of the visual magnitude.
| R Centauri: An Unusual Mira Variable in a He-Shell Flash
We present an analysis of AAVSO visual observations of the Mira variableR Cen from 1918 to 2000. The period of the dominant mode has beensteadily decreasing from 550 days at JD 2,434,000 (1951) to its presentvalue of 505-510 days. In the same interval, the pulsational amplitudehas decreased by 3 mag, from 5.5-11.8 V to 6.3-9.1 V. We suggest thatboth are caused by a He-shell flash, as the period decrease is similarto that of other He-shell flash stars such as R Hya, R Aql, and T UMi.The period change is consistent with the luminosity drop expectedimmediately after the flash, as predicted by He-shell flash models forstars of 2-3 Msolar or less. The light curve shows thefamiliar pattern of alternating deep and shallow minima, giving theappearance of double maxima. While the amplitude of the main mode hasdecreased 3 mag in the last 50 years, the amplitude of the secondarymode near 274 days has remained almost constant, so that the doublemaxima have nearly vanished from the light curve in recent years. Thepower spectrum between 1930 and 1966 shows harmonics up to 8 times themain frequency at 1/548 cycle day-1. The most likelyexplanation for the double-peaked light curve is a resonance between twomodes.
| R Centauri at Millennium's End
We have analyzed ~20,000 visual measures of the double-maximum Mira starR Centauri made by the AAVSO and The RASNZ VSS during the past 72years,. Recently there has been a marked change in the amplitude ofvariability and the shape of the light curve. The period is alsoshortening at an increasingly fast rate. V and B-V measures indicatethat the first maximum is comparable to the maximum in normal,single-peaked Mira stars.
| 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.
| 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.
| 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.
| 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).
| Infrared optical properties of spinels. A study of the carrier of the 13, 17 and 32 mu m emission features observed in ISO-SWS spectra of oxygen-rich AGB stars
In a previous paper, we have proposed magnesium aluminium spinel to bethe carrier of the 13 and 17 mu m band features observed in the ISOspectra of some red giants. The IR optical properties of spinel stronglydepend on its chemistry and its internal structure. To study thedependence of spinel's IR-spectra on its aluminium content, we havesynthesized a number of crystals with different Al/Mg-ratios.Additionally, we performed an annealing experiment to investigate thephase transition between ordered and partially disordered spinel takingplace at about 1200 K. We derived sets of optical constants of ournatural, annealed and synthetic spinels in order to calculate theabsorption efficiencies of small (sub-mu m-sized) spherical particles.Thereby, it turned out that natural as well as near-stoichiometricsynthetic spinel can indeed be considered as a suitable candidate forthe carrier of the 13 mu m feature observed in the spectra of someoxygen rich circumstellar shells. To illustrate this, we reinvestigatedthe mean profile of the residual dust emission in the 12-18 mu mwavelength range (i.e., in the so-called trough region between the twosilicate bands). The reality of the emission feature at 16.8 mu m isconfirmed by our new investigation. We demonstrated that it is not aninstrumental artifact since it is not present in ISO spectra of K-stars.In the course of our laboratory work, we found a third prominentemissivity maximum of spinel at 32 mu m. This feature could also bedetected in the spectra of the brightest 13 mu m band emission sources;we derived its mean band profile, too. The new sets of optical constantshere presented have been made available for public access in theelectronic database http://www.astro.uni-jena.de. Based on observationswith ISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, The Netherlands and theUK) and with the participation of ISAS and NASA.
| 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.
| On the Variability of K5-M Stars
I investigate the Hipparcos Satellite photometry of K5-M stars to seethe pattern of activity of these stars. A few stars for which furtherstudy is desirable are identified.
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