תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
Beobachtungssergebnisse Bundesdeutsche Arbeitsgemeinschaft fuer Veraenderliche Sterne e.V.
Not Available
| Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch (AGB)stars undergo a series of short thermal pulses that significantly changetheir luminosity and mass on timescales of hundreds to thousands ofyears. These pulses are confirmed observationally by the existence ofthe short-lived radioisotope technetium in the spectra of some of thesestars, but other observational consequences of thermal pulses are subtleand may only be detected over many years of observations. Secularchanges in these stars resulting from thermal pulses can be detected asmeasurable changes in period if the star is undergoing Mira pulsations.It is known that a small fraction of Mira variables exhibit largesecular period changes, and the detection of these changes among alarger sample of stars could therefore be useful in evolutionary studiesof these stars. The American Association of Variable Star Observers(AAVSO) International Database currently contains visual data for over1500 Mira variables. Light curves for these stars span nearly a centuryin some cases, making it possible to study the secular evolution of thepulsation behavior on these timescales. In this paper we present theresults of our study of period change in 547 Mira variables using datafrom the AAVSO. We use wavelet analysis to measure the period changes inindividual Mira stars over the span of available data. By making linearfits to the period versus time measurements, we determine the averagerates of period change, dlnP/dt, for each of these stars. We findnonzero dlnP/dt at the 2 σ significance level in 57 of the 547stars, at the 3 σ level in 21 stars, and at the level of 6 σor greater in eight stars. The latter eight stars have been previouslynoted in the literature, and our derived rates of period change largelyagree with published values. The largest and most statisticallysignificant dlnP/dt are consistent with the rates of period changeexpected during thermal pulses on the AGB. A number of other starsexhibit nonmonotonic period change on decades-long timescales, the causeof which is not yet known. In the majority of stars, the periodvariations are smaller than our detection threshold, meaning theavailable data are not sufficient to unambiguously measure slowevolutionary changes in the pulsation period. It is unlikely that morestars with large period changes will be found among heretoforewell-observed Mira stars in the short term, but continued monitoring ofthese and other Mira stars may reveal new and serendipitous candidatesin the future.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Time-Series Analysis of Variable Star Data
Time-series analysis is a rich field of mathematical and statisticalanalysis, in which physical understanding of a time-varying system canbe gained through the analysis of time-series measurements. There areseveral different techniques of time-series analysis that can beusefully applied to variable star data sets. Some of these techniquesare particularly useful for data found in the AAVSO InternationalDatabase. In this paper, I give a broad overview of time-series analysistechniques useful for variable star data, along with some practicalsuggestions for the application of different techniques to differenttypes of variables. Included are elementary discussions of traditionalFourier methods, along with wavelet and autocorrelation analysis.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderlichen Serne e.V.
Not Available
| Beobachtungsegebnisse Bundesdeutsche Arbeitsgemainschaft fur Veranderliche Sterne e.V.
Not Available
| 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
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| Angular Size Measurements of Mira Variable Stars at 2.2 Microns. II.
We present angular size measurements of 22 oxygen-rich Mira variablestars. These data are part of a long-term observational program usingthe Infrared Optical Telescope Array to characterize the observablebehavior of these stars. Complementing the infrared angular sizemeasurements, values for variable star phase, spectral type, bolometricflux, and distance were established for stars in the sample; flux anddistance led to values for effective temperature (Teff) andlinear radius, respectively. Additionally, values for the K-[12] colorexcess were established for these stars, which is indicative of dustymass loss. Stars with higher color excess are shown to be systematically120 Rsolar larger than their low color excess counterparts,regardless of period. This analysis appears to present a solution to along-standing question presented by the evidence that some Mira angulardiameters are indicative of first-overtone pulsation, while otherdiameters are more consistent with fundamental pulsation. A simpleexamination of the resultant sizes of these stars in the context ofpulsation mode is consistent with at least some of these objectspulsating in the fundamental mode.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is 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/386/492, and from theauthors on CD-Rom.
| Snapshot VLBI Observations of SiO v=1 and v=2 (J=1 - 0) Maser Sources
We have made snapshot VLBI observations of SiO masers of the J=1-0transition at two vibrational levels v=1 and v=2 using theKashima-Nobeyama Interferometer (KNIFE). Out of 37 sources observed, wedetected both of the lines from 16 sources with a typical detectionlimit of 10 Jy. Most of the SiO masers were significantly spatiallyresolved with a KNIFE baseline (200 km), indicating extended/complicatedstructures of the SiO masers with a typical angular size of a fewmilliarcseconds. Similar fringe-phase profiles along a radial velocityspanning larger than 1.0km s-1 between the two lines indicatea similar spatial structure along the velocity spacings. This tendency,shown from 13 SiO maser sources except Orion KL, supports spatialcoincidence of the two lines found previously for M-type stars VY CMaand W Hya. The spatial coincidence should enable us to measuregroup-delays of the SiO masers for stellar astrometry.
| Testing the bivariate series of Mira light curve rise and fall times for changes
The American Association of Variable Star Observers supplies theastronomical community with a large data base of times of light maximaand minima of Mira (long-period pulsating) starsQ7. Period changestudies using these data invariably use either times between maxima, orthose between minima. A statistical analysis based on the two-componenttime series of light curve rise and fall times is developed. Theresults, which enable one to detect changes in the shapes of lightcurves, are applied to observations of seven long-period variables.
| The analysis of indexed astronomical time series -VII. Simultaneous use of times of maxima and minima to test for period changes in long-period variables
A statistical model is formulated that enables one to analyse jointlythe times between maxima and minima in the light curves of monoperiodicpulsating stars. It is shown that the combination of both sets of datainto one leads to analyses that are more sensitive. Illustrativeapplications to the American Association of Variable Star ObserversQ2data for a number of long-period variables demonstrate the usefulness ofthe approach.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| 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.
| 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 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.
| Vanadium Oxide in the Spectra of Mira Variables
As a preliminary step in deducing Teff and log(g) of Miravariables as a function of phase, a comparison is made between spectrasynthesized from LTE stellar atmosphere models and observed spectra. Theobserved spectra show obvious vanadium oxide (VO) absorption bands.However, the molecular line list used to produce the synthetic spectradoes not include the bound-bound VO opacities. The wavenumbers, lineoscillator strengths, and lowest energy levels are needed to calculatethese opacities. The equations, constants, and experimentally determinedfactors required to calculate the line oscillator strengths and lowestenergy levels from experimentally determined wavenumbers are presented.The effect of including the wavenumbers, line oscillator strengths, andlowest energy levels of the VO B-X (0, 0) band are calculated and showthe expected absorption features observed in the spectra of Miravariables. In the VO B-X (0, 0) band the line oscillator strengths rangefrom about 0.05 to 3.
| Phase-dependent Spectroscopy of Mira Variable Stars
Spectroscopic measurements of Mira variable stars as a function of phaseprobe the stellar atmospheres and underlying pulsation mechanisms. Forexample, measuring variations in TiO, VO, and ZrO with phase can be usedto help determine whether these molecular species are produced in anextended region above the layers where Balmer line emission occurs orbelow this shocked region. Using the same methods, the Balmer lineincrement, where the strongest Balmer line at phase zero is Hδ andnot Hα, can be measured and explanations tested, along withanother peculiarity, the absence of the Hɛ line in the spectra ofMira variables when the other Balmer lines are strong. We present newspectra covering the spectral range from 6200 to 9000 Å of 20 Miravariables. A relationship between variations in the Ca II IR triplet andHα as a function of phase support the hypothesis that Hɛ'sobservational characteristics result from an interaction of Hɛphotons with the Ca II H line. New periods and epochs of variability arealso presented for each star.
| Surface imaging of long-period variable stars
The photospheric surfaces of five long-period variables (o Cet, R Leo, WHya, chi Cyg and R Cas) have been imaged in the optical/near-IR with the4.2-m William Herschel Telescope. All exhibit departures from circularsymmetry. Our data are consistent with the presence of bright compactfeatures, individually contributing between 5 and 20 per cent of thetotal stellar flux, on most of the stars. The characteristics andbehaviour of the features are similar to those reported on the surfacesof late-type supergiants. However, the data presented here areinadequate to identify a common physical origin.
| Speckle Interferometry of New and Problem HIPPARCOS Binaries
The ESA Hipparcos satellite made measurements of over 12,000 doublestars and discovered 3406 new systems. In addition to these, 4706entries in the Hipparcos Catalogue correspond to double star solutionsthat did not provide the classical parameters of separation and positionangle (rho,theta) but were the so-called problem stars, flagged ``G,''``O,'' ``V,'' or ``X'' (field H59 of the main catalog). An additionalsubset of 6981 entries were treated as single objects but classified byHipparcos as ``suspected nonsingle'' (flag ``S'' in field H61), thusyielding a total of 11,687 ``problem stars.'' Of the many ground-basedtechniques for the study of double stars, probably the one with thegreatest potential for exploration of these new and problem Hipparcosbinaries is speckle interferometry. Results are presented from aninspection of 848 new and problem Hipparcos binaries, using botharchival and new speckle observations obtained with the USNO and CHARAspeckle cameras.
| 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
| Near-infrared photometry and analysis of SiO maser stars
Near-infrared photometry of 35 SiO maser stars (v = 1, J = 1 - 0) ofvarious types is presented in this paper. Combining the JHK fluxdensities from IRAS, the distributions of the stars on two color-colordiagrams are obtained. The spectral slopes, equivalent black-bodytemperatures, maser luminosities and mass-loss rates of the centralstars are calculated from the infrared or radio data. These parametersare then analysed together with the integrated SiO fluxes. The resultsshow that SiO masers are common in AGB stages. The integrated SiO maserflux is only weakly correlated with the mass-loss rate, and increaseslowly along the sequence from Mira stars to OH/IR stars. It is moreclosely correlated with the near-infrared colors and the equivalentblack-body temperatures. Also, its upper limit is correlated with12μm and 25μm fluxes. We also briefly discusses the generalfunction and specific role of SiO maser in the evolution of AGB stars.
| The Infrared Spectral Classification of Oxygen-rich Dust Shells
This paper presents infrared spectral classifications for a flux-limitedsample of 635 optically identified oxygen-rich variables includingsupergiants and sources on the asymptotic giant branch (AGB). Severalclasses of spectra from oxygen-rich dust exist, and these can bearranged in a smoothly varying sequence of spectral shapes known as thesilicate dust sequence. Classification based on this sequence revealsseveral dependencies of the dust emission on the properties of thecentral star. Nearly all S stars show broad emission features fromalumina dust, while most of the supergiants exhibit classic featuresfrom amorphous silicate dust. Mira variables with symmetric light curvesgenerally show broad alumina emission, while those with more asymmetriclight curves show classic silicate emission. These differences may arisefrom differences in the photospheric C/O ratio.
| Near-infrared photometry and analysis of SiO maser stars of different types.
Not Available
| Classification and Identification of IRAS Sources with Low-Resolution Spectra
IRAS low-resolution spectra were extracted for 11,224 IRAS sources.These spectra were classified into astrophysical classes, based on thepresence of emission and absorption features and on the shape of thecontinuum. Counterparts of these IRAS sources in existing optical andinfrared catalogs are identified, and their optical spectral types arelisted if they are known. The correlations between thephotospheric/optical and circumstellar/infrared classification arediscussed.
| Aluminum Oxide and the Opacity of Oxygen-rich Circumstellar Dust in the 12--17 Micron Range
Amorphous alumina (Al2O3) was produced by a sol-gel technique in orderto make available its optical constants for possible astrophysicalapplications. Gradual annealing showed that the X-ray amorphousness ofalumina ended somewhere between 723 and 873 K. Above this transitionpoint, the structure changes into disordered gamma -Al2O3. At T >1273 K, crystalline alpha -Al2O3 (corundum) is formed. Mie calculationsshow that amorphous alumina exhibits a wide Al-O vibrational band,peaking at 11.5--11.8 mu m and having a steep "blue" and an extended"red" wing. It may be an important contributor to the continuous opacitybetween the silicate bands in oxygen-rich circumstellar envelopes,whereas it is ruled out for the explanation of the 13 mu m band. Anaverage 13 mu m band profile was derived from 51 IRAS low-resolutionspectra of bright Mira stars and semiregular variables. Its shape, whichis satisfactorily represented by a Lorentz profile, can be reproduced byMie calculations with the data of alpha -Al2O3, but not with those ofgamma -Al2O3. The calculations show that the 13 mu m band profile ofalpha -Al2O3 is sensitive to grain shape. If alpha -Al2O3 is theabsorber, a second band should be present at 21 mu m. A closecorrelation was found between the strengths of the 13 mu m band and the10 mu m silicate band. It suggests that the 13 mu m band carrier couldalso be somehow connected with silicate dust. Experimental argumentssupporting this attribution are presented.
| Spectroscopy of Mira Variables at Different Phases.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1584C&db_key=AST
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
קטלוגים וכינוים:
|
INHABITED SKY
