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T UMi Keeps Mysterious
The well known long-period variable T UMi of Mira type has ranked amongthe most observed stars becouse of its strange behaviour. 25 years ago,the rapid decrease of period started - from original 301d long period totodays 200 - 220 days. Such behaviour can be explained as He-burning inshell surrounding the stelar nucleus. Author presents his CCD multicolorphotometry of T UMi as well.
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| Phase Lags in the Optical-Infrared Light Curves of Asymptotic Giant Branch Stars
To search for phase lags in the optical-infrared light curves ofasymptotic giant branch stars, we have compared infrared data from theCOBE DIRBE satellite with optical light curves from the AAVSO and othersources. We found 17 examples of phase lags between the times ofinfrared and optical maximum, and 4 stars with no observed lags. Thereis a clear difference between the Mira variables and the semiregularvariables in the sample, with the maximum in the optical preceding thatin the near-infrared in the Miras, while in most of the semiregularvariables no lags are observed. Comparison to published theoreticalmodels indicates that the phase lags in the Miras are due to strongtitanium oxide absorption in the visual at stellar maximum, and suggeststhat Miras pulsate in the fundamental mode, while at least somesemiregular variables are first-overtone pulsators. There is a clearoptical-near-infrared phase lag in the carbon-rich Mira V CrB; this islikely due to C2 and CN absorption variations in the optical.
| Full polarization study of SiO masers at 86 GHz
Aims.We study the polarization of the SiO maser emission in arepresentative sample of evolved stars in order to derive an estimate ofthe strength of the magnetic field, and thus determine the influence ofthis magnetic field on evolved stars. Methods: .We madesimultaneous spectroscopic measurements of the 4 Stokes parameters, fromwhich we derived the circular and linear polarization levels. Theobservations were made with the IF polarimeter installed at the IRAM 30m telescope. Results: . A discussion of the existing SiO masermodels is developed in the light of our observations. Under the Zeemansplitting hypothesis, we derive an estimate of the strength of themagnetic field. The averaged magnetic field varies between 0 and 20Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughoutthe circumstellar envelope. As a consequence, the magnetic field mayplay the role of a shaping, or perhaps collimating, agent of thecircumstellar envelopes in evolved objects.
| CO line emission from circumstellar envelopes
Aims.We present the results of a multi-transition CO observationalprogram conducted on a sample of AGB and post-AGB stars envelopes. Wehave collected maps and single pointing observations of these envelopesin 5 rotational transitions ranging from J = 1-0 to J = 6-5, includingin particular new observations of the CO line at 691 GHz at the CSO. Theuse of such a set of mm and submm CO line on stellar envelopes is rareand limited to the work of some authors on IRC+10216. Methods:.Using a model for the CO emission of an AGB circumstellar envelope, incombination with a standard LVG approach, we have conducted a systematicmodelling analysis using the whole set of CO data collected for a sampleof 12 sources. We simultaneously fit all five transitions, taking intoaccount the spatial information provided by the maps. Results: .Wefind mass-loss rates in the range 1 × 10-7 to 4 ×10-4 M_ȯ/yr, and envelope temperatures ranging from 20 Kto 1000 K at a radius of 1016 cm. There seem to be a generalanti-correlation between mass loss rates and temperature, the high massloss rate AGBs having low temperatures, and vice versa. We show thatmost AGB data can be fitted using a constant mass loss rate, at leastwithin the calibration uncertainties associated with the data collectedat different frequencies. For some cases though (e.g. CIT 6, R Hya,χ Cyg), a change in the mass loss rate history needs to be invokedto reconcile data at low- and high-J, a scenario already mentioned byseveral authors to explain observations of WX Psc.
| Diameters of Mira Stars Measured Simultaneously in the J, H, and K' Near-Infrared Bands
We present the first spatially resolved observations of a sample of 23Mira stars simultaneously measured in the near-infrared J, H, and K'bands. The technique used was optical long-baseline interferometry, andwe present for each star visibility amplitude measurements as a functionof wavelength. We also present characteristic sizes at each spectralband, obtained by fitting the measured visibilities to a simple uniformdisk model. This approach reveals the general relation J diameter < Hdiameter < K' diameter.
| 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.
| Oxygen-rich AGB stars with optically thin dust envelopes
The dust composition and dynamics of the circumstellar envelopes ofoxygen-rich AGB stars with low mass-loss rates (5 ×10-8-10-5 Mȯ yr-1) havebeen investigated. We have analyzed the ISO-SWS spectra of twenty-eightoxygen-rich AGB stars with optically thin shells, and modelled theobservations with the radiative transfer code DUSTY using the opticalconstants from laboratory dust analogues. This has allowed us todetermine the composition of the dust and the physical conditions at theinner edge of the shell. Moreover, by comparing with CO observationsavailable in the literature, we have determined the gas-to-dust massratios and the mass-loss rates of these sources, and analyzed thewind-driving mechanism. The results show that the small amounts of dustpresent in these envelopes, characterized by visual optical depths inthe 0.03-0.6 range, are enough to drive the wind by radiation pressureon the grains. In some sources there are indications of circumstellardust that does not contribute to the wind-driving, and that maydistributed in a disk or clumps. Other sources show signs of variablemass-loss rates. A grain mixture in the shell consisting of aluminiumoxide, melilite, olivine, spinel and Mg{0.1}Fe{0.9}O fit the observedspectra well. From these species, only melilite is required to have afractional abundance greater than 25% in all cases. Although spinelreproduces the 13 μm feature, the absence of the 16.8 μm peak inour SWS spectra casts doubts on this identification. The outcome of themodelling reveals that the olivine content in these CSEs increases withpressure and temperature at the inner edge. Moreover, the aluminiumoxide percentage in the dust of the envelopes shows a positivecorrelation with the gas-to-dust mass ratio. These results, togetherwith the derived dust compositions, are consistent with thethermodynamic dust condensation sequence scenario and its freezing-outdue to kinetics. However, the temperatures at the inner edge of theshell are substantially lower than those predicted by theory.
| Properties of detached shells around carbon stars. Evidence of interacting winds
The nature of the mechanism responsible for producing the spectacular,geometrically thin, spherical shells found around some carbon stars hasbeen an enigma for some time. Based on extensive radiative transfermodelling of both CO line emission and dust continuum radiation for allobjects with known detached molecular shells, we present compellingevidence that these shells show clear signs of interaction with asurrounding medium. The derived masses of the shells increase withradial distance from the central star while their velocities decrease. Asimple model for interacting winds indicates that the mass-loss rateproducing the faster moving wind has to be almost two orders ofmagnitudes higher (~10-5 Mȯ yr-1)than the slower AGB wind (a few 10-7 Mȯyr-1) preceding this violent event. At the same time, thepresent-day mass-loss rates are very low indicating that the epoch ofhigh mass-loss rate was relatively short, on the order of a few hundredyears. This, together with the number of sources exhibiting thisphenomenon, suggests a connection with He-shell flashes (thermalpulses). We report the detection of a detached molecular shell aroundthe carbon star DR Ser, as revealed from newsingle-dish CO (sub-)millimetre line observations. The properties of theshell are similar to those characterising the young shell aroundU Cam.
| 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
| Secular Evolution in Mira and Semiregular Variable Pulsations (poster)
Not Available
| Variable Star Network: World Center for Transient Object Astronomy and Variable Stars
Variable Star Network (VSNET) is a global professional-amateur networkof researchers in variable stars and related objects, particularly intransient objects, such as cataclysmic variables, black-hole binaries,supernovae, and gamma-ray bursts. The VSNET has been playing apioneering role in establishing the field of transient object astronomy,by effectively incorporating modern advances in observational astronomyand global electronic networks, as well as collaborative progress intheoretical astronomy and astronomical computing. The VSNET is now oneof the best-featured global networks in this field of astronomy. Wereview the historical progress, design concept, associated technology,and a wealth of scientific achievements powered by VSNET.
| 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.
| 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.
| 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.
| Correlation of Successive Maxima in Mira, Chi Cygni, R Leonis, and R Hydrae
An anaylsis was done of the maxima of Mira, Chi Cygni, R Leonis, and RHydrae, four Mira-type variable stars chosen from among those studied byArgelander in 1839. A negative correlation between the luminosity ofconsecuitve maxima was found, except for R Hya, M(i+1) - M(i) vs. M(i)yields a correlation index of r2 = 0.6 for Mira and Chi Cyg,r2 = 0.44 for R Leo, and r2 = 0.25 for R Hya. Dataanalyzed range from the discovery of Mira (1596) to the present time;they include AAVSO data, and there are from 90( RHya) to 179 (Mira Ceti)pairs of consecutive maxima.
| High-Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer
We present first results of an experiment to combine data from Keckaperture masking and the Infrared-Optical Telescope Array to image thecircumstellar environments of evolved stars with ~20 mas resolution. Theunique combination of excellent Fourier coverage at short baselines andhigh-quality long-baseline fringe data allows us to determine thelocation and clumpiness of the innermost hot dust in the envelopes andto measure the diameters of the underlying stars themselves. We findevidence for large-scale inhomogeneities in some dust shells and alsosignificant deviations from uniform brightness for the photospheres ofthe most evolved M stars. Deviations from spherically symmetric massloss in the red supergiant NML Cyg could be related to recent evidencefor dynamically important magnetic fields and/or stellar rotation. Wepoint out that dust shell asymmetries, like those observed here, canqualitatively explain the difficulty recent workers have had insimultaneously fitting the broadband spectral energy distributions andhigh-resolution spatial information, without invoking unusual dustproperties or multiple distinct shells (from hypothetical``superwinds''). This paper is the first to combine opticalinterferometry data from multiple facilities for imaging, and we discussthe challenges and potential for the future of this method, givencurrent calibration and software limitations.
| Further Results of TiO-Band Observations of Starspots
We present measurements of starspot parameters (temperature and fillingfactor) on five highly active stars, using absorption bands of TiO, fromobservations made between 1998 March and 2001 December. We determinedstarspot parameters by fitting TiO bands using spectra of inactive G andK stars as proxies for the unspotted photospheres of the active starsand spectra of M stars as proxies for the spots. For three evolved RSCVn systems, we find spot filling factors between 0.28 and 0.42 for DMUMa, 0.22 and 0.40 for IN Vir, and 0.31 and 0.35 for XX Tri; thesevalues are similar to those found by other investigators usingphotometry and Doppler imaging. Among active dwarfs, we measured a lowerspot temperature (3350 K) for EQ Vir than found in a previous study ofTiO bands, and for EK Dra a lower spot temperature (~3800 K) than foundthrough photometry. For all active stars but XX Tri, we achieved goodphase coverage through a stellar rotational period. We also present ourfinal, extensive grid of spot and nonspot proxy stars.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| Astrophysics in 2002
This has been the Year of the Baryon. Some low temperature ones wereseen at high redshift, some high temperature ones were seen at lowredshift, and some cooling ones were (probably) reheated. Astronomerssaw the back of the Sun (which is also made of baryons), a possiblesolution to the problem of ejection of material by Type II supernovae(in which neutrinos push out baryons), the production of R CoronaeBorealis stars (previously-owned baryons), and perhaps found the missingsatellite galaxies (whose failing is that they have no baryons). A fewquestions were left unanswered for next year, and an attempt is made todiscuss these as well.
| 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.
| Annual report of the director for fiscal year 2001-2002.
Not Available
| On the Period of the Mira Variable LX Cygni
The period changes of the SC-type Mira variable LX Cygni are analyzed.
| Really Cool Stars and the Star Formation History at the Galactic Center
We present λ/Δλ=550-1200 near-infrared H and Kspectra for a magnitude-limited sample of 79 asymptotic giant branch andcool supergiant stars in the central ~5 pc (diameter) of the Galaxy. Weuse a set of similar spectra obtained for solar neighborhood stars withknown Teff and Mbol that is in the same range asthe Galactic center (GC) sample to derive Teff andMbol for the GC sample. We then construct the H-R diagram forthe GC sample. Using an automated maximum likelihood routine, we derivea coarse star formation history of the GC. We find that (1) roughly 75%of the stars formed in the central few parsecs are older than 5 Gyr; (2)the star formation rate (SFR) is variable over time, with a roughly 4times higher SFR in the last 100 Myr compared to the average SFR; (3)our model can match dynamical limits on the total mass of stars formedonly by limiting the initial mass function to masses above 0.7Msolar (this could be a signature of mass segregation or ofthe bias toward massive star formation from the unique star formationconditions in the GC); (4) blue supergiants account for 12% of the totalsample observed, and the ratio of red to blue supergiants is roughly1.5; and (5) models with isochrones with [Fe/H]=0.0 over all ages fitthe stars in our H-R diagram better than models with lower [Fe/H] in theoldest age bins, consistent with the finding of Ramírez et al.that stars with ages between 10 Myr and 1 Gyr have solar [Fe/H].
| 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).
| A Polarization Survey of SiO Maser Variability in Evolved Stars
We have monitored the SiO (v=1, J=2-->1) maser polarization in 17variable stars (Miras, OH-IR stars, and supergiants) to investigate thelong-term persistence of masers. The eight epochs of observations span2.5 yr, thereby sampling multiple cycles for these stars with typicalperiods of ~1 yr. The average polarization was 23% with a typicaldispersion of 7%, although the variability differed substantially fromstar to star. In the Stokes q and u spectra of individual stars, a fewstrong maser features tended to dominate the polarization, with themaser features persisting for less than one stellar cycle for somestars, and for multiple cycles for a few stars. Because individualmasers are not resolved in our beam-averaged total intensity spectra, wecorrelated the polarization spectra between epochs to measure thecharacteristic lifetimes of the features, rather than attempting totrace the evolution of separate line components. We found thatindividual maser feature lifetimes ranged from a few months or less tomore than 2 yr. These data indicate that for the sample of stars as awhole, the masers are not reset at a particular stellar phase.
| Intensities of E2 Spectral Lines in the Astrophysically Important Ion Mg II
Relativistic quantum defect orbital (RQDO) formalism is applied to thestudy of forbidden transitions in the astrophysically important ion MgII. The RQDO calculations have been performed in the LSJ coupling schemeon E2 inter- and intraconfiguration transitions. Line strengths,oscillator strengths, and Einstein coefficients are reported andcompared with the data found in the literature. A good agreement withthe comparative data as well as the expected systematic behavior of theRQDO line strengths along the spectral series confirms earlierconclusions about the quality of the RQDO wave functions. The E2spectrum of Mg II is graphically shown.
| 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.
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