Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Observable effects of dust formation in dynamic atmospheres of M-type Mira variables
The formation of dust with temperature-dependent non-grey opacity isconsidered in a series of self-consistent model atmospheres at differentphases of an O-rich Mira variable of mass 1.2Msolar.Photometric and interferometric properties of these models are predictedunder different physical assumptions regarding the dust formation. Theiron content of the initial silicate that forms and the availability ofgrain nuclei are found to be critical parameters that affect theobservable properties. For certain plausible parameter values, dustwould form at 2-3 times the average continuum photospheric radius. Thiswork provides a consistent physical explanation for the larger apparentsize of Mira variables at wavelengths shorter than 1μm than thatpredicted by dust-free fundamental-mode pulsation models.
| First sky validation of an optical polarimetric interferometer
Aims.We present the first lab and sky validation of spectro-polarimetricequipment put at the combined focus of an optical long-baselineinterferometer. We tested the polarimetric mode designed for the visibleGI2T Interferometer to offer spectropolarimetric diagnosis at themilliarcsecond scale. Methods.We first checked the wholeinstrumental polarization in the lab with a fringe simulator, and thenwe observed α Cep and α Lyr as stellar calibrators ofdifferent declinations to tabulate the polarization effects throughoutthe GI2T declination range. Results.The difference between bothlinear polarizations is within the error bars and the visibilitiesrecorded in natural light (i.e. without the polarimeter) for calibrationpurposes are the same order of magnitude as the polarized ones. Wefollowed the α Cep visibility for 2 h after the transit andα Lyr for 1.5 h and detected no decrease with hour angle due tothe fringe pattern smearing by instrumental polarization. Conclusions.Differential celestial rotation due to the dissymetricCoudé trains of the GI2T is well-compensated by the fieldrotators, so the instrumental polarization is controlled over arelatively wide hour angle range (±2 h around the transit atleast). Such a polarimetric mode opens new opportunities especially forstudies of circumstellar environments and significantly enhances boththe potential of an optical array and its ability for accuratecalibration.
| Dust scattering in the Miras R Car and RR Sco resolved by optical interferometric polarimetry
We present optical interferometric polarimetry measurements of theMira-like variables R Car and RR Sco, using the Sydney UniversityStellar Interferometer. By making visibility measurements in twoperpendicular polarizations, the relatively low-surface brightness lightscattered by atmospheric dust could be spatially separated from thebright Mira photospheric flux. This is the first reported successful useof long-baseline optical interferometric polarimetry. Observations wereable to place constraints on the distribution of circumstellar materialin R Car and RR Sco. The inner radius of dust formation for both starswas found to be less than 3 stellar radii: much closer than the expectedinnermost stable location for commonly assumed astrophysical `dirtysilicate' dust in these systems (silicate dust with a significant ironcontent). A model with the dust distributed over a shell which isgeometrically thin compared to the stellar radius was preferred over anoutflow. We propose dust components whose chemistry and opacityproperties enable survival at these extreme inner radii.
| 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).
| A W Roberts: the observations (paper 2).
Not Available
| Multiwavelength diameters of nearby Miras and semiregular variables
We have used optical interferometry to obtain multiwavelength visibilitycurves for eight red giants over the wavelength range 650-1000 nm. Theobservations consist of wavelength-dispersed fringes recorded withMAPPIT (Masked Aperture-Plane Interference Telescope) at the 3.9-mAnglo-Australian Telescope. We present results for four Miras (R Car, oCet, R Hya and R Leo) and four semiregular variables (R Dor, W Hya,L2 Pup and γ Cru). All stars except γ Cru showstrong variations of angular size with wavelength. A uniform-disc modelwas found to be a poor fit in most cases, with Gaussian (or other moretapered) profiles preferred. This, together with the fact that moststars showed a systematic increase in apparent size towards the blue anda larger-than-expected linear size, even in the red, all point towardssignificant scattering by dust in the inner circumstellar environment.Some stars showed evidence for asymmetric brightness profiles, whileL2 Pup required a two-component model, indicating anasymmetrical circumstellar dust shell.
| Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud - II. Multiplicity of the period-luminosity relation
Data for 4.4 million stars from the Microlensing Observations inAstrophysics (MOA) project are compared with the near-infrared data ofthe Deep Near Infrared Southern Sky Survey (DENIS). More than 4000 starsobserved in both projects show a quite periodic light curve. Among them,a number of stars are likely eclipsing variables, and the others seem tobe pulsating stars. The KS magnitudes of these red variablesare in the range 10-12.5 but a minor clump at KS~ 12.2 mag isalso found. The multiplicity of the period-luminosity relation isconfirmed, but most of the regular, large-amplitude variables are foundon the relation established for the Mira stars. We study the propertiesof the variables on the colour-magnitude diagram constructed with theMOA red band Rm and KS of DENIS. Multiplicity ofthe period-luminosity relation is briefly discussed in relation to theexcitation mechanism of red pulsating variables.
| 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
| 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.
| From Amateur Astronomer to Observatory Director: The Curious Case of R. T. A. Innes
Robert Innes was one of a select band of amateur astronomers who madethe transition to professional ranks towards the end of the nineteenthcentury. Initially he had a passion for mathematical astronomy, butafter settling in Sydney he developed a taste for observationalastronomy, specialising in the search for new double stars. He quicklybecame known for his success in this field and for his publications onsolar system perturbations, and with John Tebbutt's patronage managed tosecure a clerical position at the Royal Observatory, Cape of Good Hope.Once there he continued to observe in his spare time and to publish,and, with strong support from Sir David Gill, was appointed foundingDirector of the Transvaal Observatory. By the time he died in 1933,Innes had received an honorary D.Sc. from Leiden University, and hadestablished an international reputation as a positional astronomer. Thispaper provides an interesting case study of a well-known`amateur-turned-professional', and an example of the ways in whichpatronage played a key role in nineteenth and early twentieth centuryAustralian and South African astronomy.
| 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.
| Metal Emission Lines as Diagnostic Tools for Shock Waves in Outer Atmospheres of M-type Mira Stars
One way to reveal the thermo- and hydrodynamical conditions in M-typeMira atmospheres is to study the various emission lines which areemitted behind a shock front and can be observed over a substantialportion of the pulsation period. Analysing a time-resolved series ofthese emission lines offers the possibility to determine theseconditions in different atmospheric layers influenced by the passingshock wave. In particular, the metal emission lines are a diagnostictool to probe the hydrodynamical conditions of the outer, dust-forminglayers of the atmosphere, because they appear late in the pulsationcycle when the shock wave has reached these layers. We presentquantitive data on radial velocities, shapes, widths and fluxes of metalemission lines obtained by spectral observations in the opticalwavelength region for a sample of six M-type Miras (periods 281-389days), namely R Aql, RR Sco, R Car, R Leo, S Scl and R Hya (cf. Richter& Wood 2001, A&A 369, 1027-1047). Because of the multiple phasecoverage of our observations, the data shows the history of the shock asit emerges through the deep photosphere and then moves out through theatmosphere. The observations are analysed and discussed with regard tothe atmospheric conditions.
| 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).
| On the shock-induced variability of emission lines in M-type Mira variables. I. Observational data
We present time-resolved observations of metallic emission lines of MgI, Mn I, Si I, Fe I and Fe II, including forbidden emission lines of [FeII], for the six M-type Mira variables RR Sco, R Aql, R Car, R Leo, SScl and R Hya, which range in period from 281 to 389 days. Data is alsopresented for the Balmer emission lines Hγ , Hdelta , Hzeta andHeta . The observations were carried out in the optical wavelengthregion 3600-5700 Å. Narrow-slit observations with dispersion of1.53 km s-1 pixel-1 and a 3-pixel resolution of4.6 km s-1 were made in some cases while most observationswere done with a wide slit corresponding to a resolution of 15.3 kms-1. The variation of line shape, flux and velocity withphase is discussed. The data presented in this paper will be used insubsequent papers for comparison with detailed models of the emissionfrom shock waves in the upper atmospheric layers of Mira variables. Itis in these important upper layers that dust formation occurs and massloss is initiated.
| 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.
| Interpreting the Mg IIh andk Line Profiles of Mira Variables
We use radiative transfer calculations to reproduce the basic appearanceof Mg II lines observed from Mira variables. These lines have centroidsthat are blueshifted by at least 30 km s-1 from the stellarrest frame. It is unlikely that flow velocities in the stellaratmospheres are this fast, so radiative transfer effects must beresponsible for this behavior. Published hydrodynamic models predict theexistence of cool, downflowing material above the shocked materialresponsible for the Mg II emission, and we demonstrate that scatteringin this layer can result in Mg II profiles as highly blueshifted asthose that are observed. However, our models also show that scatteringwithin the shock plays an equally strong role in shaping the Mg IIprofiles, and our calculations illustrate the importance of partialredistribution and the effects of being out of ionization equilibrium.
| Studying the Pulsation of Mira Variables in the Ultraviolet
We present results from an empirical study of the Mg II h and k emissionlines of selected Mira variable stars, using spectra from theInternational Ultraviolet Explorer (IUE). The stars all exhibit similarMg II behavior during the course of their pulsation cycles. The Mg IIflux always peaks after optical maximum near pulsation phaseφ=0.2-0.5, although the Mg II flux can vary greatly from one cycleto the next. The lines are highly blueshifted, with the magnitude of theblueshift decreasing with phase. The widths of the Mg II lines are alsophase dependent, decreasing from about 70 km s-1 to 40 kms-1 between φ=0.2 and φ=0.6. We also study other UVemission lines apparent in the IUE spectra, most of them Fe II lines.These lines are much narrower and not nearly as blueshifted as the Mg IIlines. They exhibit the same phase-dependent flux behavior as Mg II, butthey do not show similar velocity or width variations.
| The Variability of Emission Lines in Shocked M Mira Atmospheres
One of the outstanding problems concerning M Mira variables is the basicmass loss mechanism and its relation to stellar pulsation and dustformation. Theoretical works suggest that the substantial mass loss canbe explained by the pulsation of the star producing shock waves whichlevitate the outer atmosphere and thereby trigger the dust formation,which in turn can amplify the shock waves. One way to probe thehydrodynamical conditions in different layers of the atmosphereinfluenced by the passing shock wave is to analyse a time resolvedseries of emission lines profiles. In particular emission linesappearing late in phase are good candidates to study the hydrodynamicalconditions of the dust producing layers, since they appear when theshock wave has reached the outer atmosphere. A sample of six M Miras (RRSco, R Aql, R Car, R Leo, S Scl and R Hya), which range in period from281 to 389 days, have been observed from maximum to minimum visiblelight. We obtained time resolved, high resolution spectra in the opticalwavelength region (3600-5700 Å) of the Balmer emission linesHγ, Hδ, Hζ,Hη and several metal emission lines, namely Mg I, Mn I,Si I, Fe I, Fe II and [Fe II]. The variation with phase of the emissionline profiles, the velocity shifts and fluxes are analysed in detail.Emphasis is put on metal emission lines which appear around the minimumlight, in particular the [Fe II] lines, which are good candidates toprobe the outermost atmospheric layers.
| The Temperature Scale of Metal-rich M Giants Based on TIO Bands: Population Synthesis in the Near-Infrared
We have computed a grid of high-resolution synthetic spectra for coolstars (2500
| Millimeter and some near infra-red observations of short-period Miras and other AGB stars
Millimeter observations of 48 oxygen- and 20 carbon-rich AGB Miras withperiods shorter than 400 days are presented. In addition, observationsof 14 O-rich and 15 C-stars with longer, or no known, periods have alsobeen obtained. The detection statistics is as follows: in12CO J=1-0 and 2-1 we observed 97 stars, and detected 66 inat least one line. We find 24 new detections in the 1-0 line, 38 newdetections in the 2-1 line, and 29 stars have been detected for thefirst time in one or both lines. In 12CO J=3-2 we observed 14stars and detected 11, with 4 new detections. In 13CO J=2-1,3-2 we observed 2 stars and had one new detection. In HCN(1-0) weobserved 5 carbon stars and detected 3, one new. In SO(6_5-5_4) weobserved the same 5 stars and detected none. In CS(3-2) we observed 8carbon stars and detected 3, all new. In SiO(3-2, v=0) we observed 34O-rich stars and detected 25, all new except one. Near-infrared JHKphotometry is presented for seven stars. For four stars it is the firstNIR data published. The luminosity and dust mass loss rate are obtainedfor seven very red stars with unknown pulsation period from modellingthe spectral energy distribution (SED) and IRAS LRS spectra. Thereby, anew IR supergiant is confirmed (AFGL 2968). For the rest of the sample,luminosity and distance are obtained in a variety of ways: usinghipparcos parallaxes, period-luminosity and period-M_K-relationscombined with apparent K magnitudes, and kinematic distances. The dustmass loss rate is obtained from model fitting of the SED (either fromthe literature, or presented in the present paper), or from the observedIRAS 60 mu m flux, corrected for the photospheric contribution. The gasmass loss rate is derived from the observed CO line intensities, aspresented here, combined with existing literature data, if any. Thisallows the derivation of the dust-to-gas ratio. Our and literature CO J= 3-2 data has been used to calibrate the relation between mass lossrate and peak intensity of the CO(3-2) line. Diagrams showing mass lossrate, dust-to-gas ratio and expansion velocity versus pulsation periodare presented. Our observations confirm the existence of an upper limitfor the expansion velocity of C- and O-rich stars, and that this maximumis larger for C-stars, as predicted by the theory of radiation pressureon dust particles. The James Clerk Maxwell Telescope is operated by theObservatories on behalf of the Science and Engineering Council of theUK, the Netherlands Organization for Scientific Research, and theNational Research Council of Canada. Based on observations made with theCarlos Sanchez telescope operated on the island of Tenerife in theSpanish Observatorio del Teide of the Instituto de Astrof\'\i sica deCanarias. Based on observations collected at the European SouthernObservatory, La Silla, Chile within programs ESO 57.E-0105, 59.E-0198and 61.E-0254. Based on data from the ESA Hipparcos astrometrysatellite.
| 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
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Carena |
| Ascensione retta: | 09h32m14.60s |
| Declinazione: | -62°47'20.0" |
| Magnitudine apparente: | 6.1 |
| Distanza: | 127.551 parsec |
| Moto proprio RA: | -39.6 |
| Moto proprio Dec: | 19.4 |
| B-T magnitude: | 9.428 |
| V-T magnitude: | 8.564 |
Cataloghi e designazioni:
|
