Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
Searches for OH masers in the circumstellar envelopes of red giant irregular variable stars
Irregular variables of type Lb share many characteristics withasymptotic giant branch (AGB) stars such as Miras and semiregulars(SRs). Despite their large number, irregulars are poorly understood.Using the large Nançay radio telescope, we searched for 18-cm OHmaser emission from these stars. We selected two samples of Lbs thatpresent a noticeable circumstellar envelope. In the first one, 44high-latitude objects with infrared colours of dusty mass-losing starswere observed in a wide-bandwidth velocity-search mode. In addition, 16objects (including six from the first sample), in which othercircumstellar molecular lines had recently been found, were observedwith higher resolution and sensitivity. We did not detect any new OHmaser emission from these samples down to 3σ sensitivity limitsranging from 30 to 200 mJy. We derived upper limits for the efficiencyof a typical pump using far-infrared photons. When compared with knownOH masers in AGB stars, we show that a significant gap is leftunexplored below the threshold that we determined for Lbs. Lowerefficiencies exist in some OH Miras and in the OH SRs. We cannot discardthe possibility of faint OH emission from certain Lbs, based on the sameprocesses as encountered in those sources. We briefly discuss physicalconditions allowing OH emission in the envelope, and the evolutionarystage of objects classified as Lb variables with respect to the AGB.
| 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.
| 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
| Velocity variability of semiregular and irregular variables
We compare velocities from near infrared lines with center-of-massvelocities for a sample of semiregular and irregular variables to searchfor indications for a convective blueshift. It is shown that a generalblueshift is present but that the light variability is obviously notdominated by convective cells but rather by stellar pulsation. All starsof our sample show a similar shape and amplitude in the velocityvariations. Long secondary periods are a common feature in these objectsand strongly influence the measured velocity shifts. The star V366 Aqlis found to be the first SRV showing line doubling.
| Mass loss rates of a sample of irregular and semiregular M-type AGB-variables
We have determined mass loss rates and gas expansion velocities for asample of 69 M-type irregular (IRV 22 objects) and semiregular (SRV; 47objects) AGB-variables using a radiative transfer code to model theircircumstellar CO radio line emission. We believe that this sample isrepresentative for the mass losing stars of this type. The (molecularhydrogen) mass loss rate distribution has a median value of 2.0 x10-7 Msun yr-1, and a minimum of 2.0 x10-8 Msun yr-1 and a maximum of 8 x10-7 Msun yr-1. M-type IRVs and SRVswith a mass loss rate in excess of 5 x 10-7 Msunyr-1 must be very rare, and among these mass losing stars thenumber of sources with mass loss rates below a few 10-8Msun yr-1 must be small. We find no significantdifference between the IRVs and the SRVs in terms of their mass losscharacteristics. Among the SRVs the mass loss rate shows no dependenceon the period. Likewise the mass loss rate shows no correlation with thestellar temperature. The gas expansion velocity distribution has amedian of 7.0 km s-1, and a minimum of 2.2 km s-1and a maximum of 14.4 km s-1. No doubt, these objects samplethe low gas expansion velocity end of AGB winds. The fraction of objectswith low gas expansion velocities is very high, about 30% havevelocities lower than 5 km s-1, and there are objects withvelocities lower than 3 km s-1: V584 Aql,T Ari, BI Car, RXLac, and L2 Pup. The mass loss rate and thegas expansion velocity correlate well, a result in line with theoreticalpredictions for an optically thin, dust-driven wind. In general, themodel produces line profiles which acceptably fit the observed ones. Anexceptional case is R Dor, where the high-quality,observed line profiles are essentially flat-topped, while the model onesare sharply double-peaked. The sample contains four sources withdistinctly double-component CO line profiles, i.e., a narrow featurecentered on a broader feature: EP Aqr, RVBoo, X Her, and SV Psc.We have modelled the two components separately for each star and derivemass loss rates and gas expansion velocities. We have compared theresults of this M-star sample with a similar C-star sample analysed inthe same way. The mass loss rate characteristics are very similar forthe two samples. On the contrary, the gas expansion velocitydistributions are clearly different. In particular, the number oflow-velocity sources is much higher in the M-star sample. We found noexample of the sharply double-peaked CO line profile, which is evidenceof a large, detached CO-shell, among the M-stars. About 10% of theC-stars show this phenomenon.
| Monitoring of LPVs with an automatic telescope.. II: A comparison of APT data and visual observations
We discuss the possibilities of investigating the semiregular andirregular light change found in evolved late type giant stars withphotometric and visually obtained (amateur) data. Period analysis hasbeen done on light curve data from both sources for a sample ofsemiregular variables. The results are compared to test the ability ofvisual data to show the frequency contents present in these variables.We find that both sources of data complete each other, as fitting thefine structure visible only in the photometric data needs long-termvisual monitoring to guarantee the uniqueness of the fit.
| Long period variable stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used tocalibrate both infrared luminosities and kinematical parameters of LongPeriod Variable stars (LPVs). Individual absolute K and IRAS 12 and 25luminosities of 800 LPVs are determined and made available in electronicform. The estimated mean kinematics is analyzed in terms of galacticpopulations. LPVs are found to belong to galactic populations rangingfrom the thin disk to the extended disk. An age range and a lower limitof the initial mass is given for stars of each population. A differenceof 1.3 mag in K for the upper limit of the Asymptotic Giant Branch isfound between the disk and old disk galactic populations, confirming itsdependence on the mass in the main sequence. LPVs with a thin envelopeare distinguished using the estimated mean IRAS luminosities. The levelof attraction (in the classification sense) of each group for the usualclassifying parameters of LPVs (variability and spectral types) isexamined. Table only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/374/968 or via ASTRIDdatabase (http://astrid.graal.univ-montp2.fr).
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Oxygen-rich semiregular and irregular variables. A catalogue of circumstellar CO observations
Using the SEST, the Onsala 20 m telescope, the JCMT, and the IRAM 30 mtelescope we have carried out a survey of circumstellar CO(J=1-0, 2-1,3-2, and 4-3) emission on a large sample of oxygen-rich semiregular (SRaand SRb) and irregular variables (Lb). A total of 109 stars wereobserved in at least one CO line: 66 were shown to have circumstellar COline emission (7 SRa, 36 SRb, and 23 Lb variables), ~ 60% of thesemiregulars and all but one of the irregulars were detected for thefirst time. Most stars were observed in at least two transitions. Thereis a total of 138 detected CO lines. For twelve stars stronginterference from interstellar CO emission precluded detection. Wepresent here a catalogue of all observational data and the spectra ofall detections, as well as brief discussions on detection statistics(including its dependence on variability type, period, IRAS-colour, IRASLRS-class, and M-subclass), line profiles (including line shapeasymmetry, multi-component line shapes, and line intensity ratios), gasexpansion velocity distributions, and correlations between CO line andIR continuum fluxes (including implications for the mass-lossmechanism). Based on observations collected using at the EuropeanSouthern Observatory, La Silla, Chile, the Onsala Space Observatory,Chalmers Tekniska Högskola, Sweden, the James Clerk MaxwellTelescope, Hawaii and the IRAM 30~m telescope, Pico Veleta, Spain.
| 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.
| Irregular variables of type Lb. Circumstellar CO emission of an oxygen-rich sample
We report on the first systematic study of the mass-loss properties ofO-rich irregular variables of type Lb. (12) CO (J=1-0, 2-1, and 3-2)line observations were carried out using the SEST, the Onsala 20 mTELESCOPE, the JCMT and the IRAM 30 m ANTENNA. In total 20 stars weredetected among the 31 objects observed (all are new detections; in threecases detections are precluded because of strong interference frominterstellar CO emission). Hence, the detection rate is very high forthis sample selected on the basis of the 60 mu m flux strength. Themajority of the detected objects are weak in CO, and the inferredmass-loss rates are low (la 10(-7) M_ȯ/yr). They have, with only afew exceptions, envelopes with small gas expansion velocities (the meanvalue is ~8 km/s). We find that their mass-loss properties are verysimilar to those of bright O-rich semiregular (SRa or SRb) and Miravariables, i.e., at least in this mass-loss rate range the stellarmass-loss properties are not strongly influenced by the pulsationalbehaviour of the star. Based on observations collected at the EuropeanSouthern Observatory, La Silla, Chile, the Onsala Space Observatory,Chalmers Tekniska Högskola, Sweden, the James Clerk MaxwellTelescope, Hawaii and the IRAM 30\,m telescope, Pico Veleta, Spain.
| Circumstellar Shells of the Mass-Losing Asymptotic Giant Branch Stars: Limits for the Dust-Driven Winds
We investigated whether the radiation pressure on dust grains alone mayaccount for driving winds in semiregular, irregular and non-variable AGBstars. A simple theoretical model is employed to calculate the limitsfor the radiatively driven winds and to compare theoretical predictionswith observations. Present analysis indicates, that for most of theobjects in the studied sample of 67 oxygen-rich and 40 carbon-richmass-losing AGB stars radiation pressure on dust grains alone is noteffective enough to drive the observed circumstellar outflows.
| 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.
| H_2_O maser emission from irregular variables.
We have performed a search for the 22 GHz water maser line among 72optically identified irregular and semiregular red variables. Newdetections were made of five stars, while only four of nine objectspreviously known as maser sources were redetected. The probability forthe detection of H_2_O maser emission increases with V light amplitude,and with H-K and K-[12] colours just as in regular Mira and semiregularvariables of SRa- and SRb-types. The detection rate of water masers isabout 25% for nearby Lb objects (D<400 pc) in the sample, comparableto that observed in the SRa and SRb stars. No masers were detected inobjects with mass loss rates <=4x10^-8^Msun_/yr. Maserluminosities are 10^41^-10^43^photons/s similar to that of the bluestMiras and typical SRa and SRb stars showing water maser emission. Acomparison of our data on irregular stars with those previously obtainedon SRa and SRb variables suggests that most radio and infraredproperties are indistinguishable among both classes of objects.
| Irregular variables of type Lb. New JHKL'M-photometry for 160 stars.
This paper presents new near infrared observations of 160 Irregularvariables of type Lb in the JHKL'M filter bands. These measurements aresupplemented by data for additional 56 stars taken from the literature.In total 220 datasets are available because of some multipleobservations. From our sample, 216 stars have near infrared (NIR)photometry now. Our sample of visually bright Lb-variables displays verysimilar infrared properties when compared with SRa- and SRb-variables.Derived from NIR-two colour diagrams the oxygen-rich Lbs seem to haveintermediate atmospheric conditions between Miras and normal giants.There may be a sligthly larger "contamination" with non AGB-giants thanin the case of the semiregulars. Using only our IR-colours the S- andthe Carbon-stars among the Lbs again are undistinguishable fromSR-variables of the same chemistry.
| Circumstellar dust envelopes of oxygen-rich AGB stars.
Using IRAS observational data, the best-fitting models are searched foroxygen-rich AGB stars in various evolutionary stages from opticallyvisible red giant stars to OH/IR stars with heavy mass loss. The modelinfrared spectral energy distributions of the red giant stars withcircumstellar dust envelopes are calculated with the use of a radiativetransfer code. It is found that the oxygen-rich IRAS AGB starssurrounded by circumstellar dust envelopes are divided into threegroups, which can be clearly distinguished by their mass loss rate andthe grain temperature at the inner boundary of the circumstellar dustenvelopes. One group corresponds to Mira type objects with a moderatemass loss rate and a grain formation temperature of about 500 K, thesecond to OH/IR star type objects with a large mass loss rate and a highgrain formation temperature, and the third to M stars with a very coldgrain temperature at the inner boundary, indicating that the moderatemass loss stopped a considerable time ago in these objects. The highergrain formation temperature of the OH/IR star type objects indicates agrain formation point closer to the stellar surface, suggesting aneffective acceleration of dust grains by radiation pressure. Such aneffective acceleration can play an important role in the mass lossphenomena of OH/IR stars with large mass loss rates. From the modelfitting results, it is suggested that an AGB star undergoes severaldozen phases of moderate mass loss of˙(M)~10^-7^-10^-6^Msun_/yr for a period of10^3^-10^4^years with interruptions of 10^3^-10^4^years during theordinary Mira variable phase before becoming an OH/IR star with a massloss rate exceeding 10^-5^Msun_/yr and a lifetime of about10^3^-10^4^years.
| 86 GHz SiO, v=1, J=2--> 1 survey of southern IRAS point sources. II. Detection of 74 new maser sources
The detection of 74 new southern stellar SiO, v=1, J=2->1 masersassociated with IRAS point sources is reported. 57 of these were foundin an IRAS based survey of oxygen rich stellar envelopes. The detectionrate in this survey was 45%. 17 further new maser sources were detectedduring a search for strong pointing sources for the Swedish-ESOSubmillimeter Telescope (SEST). The distribution of the IRAS lowresolution spectral (LRS) classes of all the SiO masers (this paper andliterature) peaks sharply at class 15. The distribution of the LRSclasses of the maser sources from 21 to 29 is similar to thedistribution of these classes of all the point sources in the pointsource catalogue except for class 25. Only few maser sources of thisspectral class are known. This is possibly explained by a selectioneffect.
| Circumstellar shells resolved in the IRAS survey data. I - Data processing procedure, results, and confidence tests
We have examined the IRAS 60 and 100 micron survey data covering 512evolved stars and young planetary nebulae for evidence of spatiallyresolved structure. A simple model, consisting of a central unresolvedsource surrounded by a resolved isothermal shell, was fitted to the datafor each star. Seventy-six stars were found to be resolved in the 60micron data. Tests have been performed to verify that the extendedstructure seen is not an artifact of the data-processing algorithm.
| IRAS low-resolution spectral observations of the 10 and 18 micron silicate emission feature
The IRAS LRS Atlas contains 8-23 micron spectra of 1816 stars withsilicate emission features. In order to study the shapes of the silicatedust features, the 117 stars whose emission features appear to beoptically thin and which have the best signal-to-noise ratio at thelongest wavelengths were analyzed. Simple spherical dust shell modelswere calculated in both the optically thin and the slightly opticalthick approximations. From the comparison of the predicted spectra ofthe dust and stellar continuum, the emissivity function Kappa(lambda)was derived. In the different stars, the shape of the 10-micron featureis either narrow or broad and it is peaked either at about 9.7 micronsor at 10 microns. Either particle size effects (particles above 0.75microns in radius) or optical depth effects (central optical depth about1) could broaden the 10-micron feature. Chemical composition differencesno doubt are also important, particularly as regards the position of thepeak of the 10-micron feature. The stars with the peak at 10 microns aremore closely confined to the Galactic plane than the stars with the peakat 9.7 microns. The shape of the 18-micron feature is essentially thesame in all stars, and can be used to extend the interstellar extinctioncurve past 13 to 22 microns.
| New SiO masers associated with bright southern IRAS sources
The discovery of 31 new SiO v = 1, J = 1 - 0 (43.122 GHz) masersassociated with southern IRAS sources is reported. The candidate listfor the search was produced systematically, and the high detectionsuccess rate (about 50 percent) led to the consideration of differencesbetween stars associated with masers and those showing maser emission. Aset of conditions is proposed which virtually guarantee the presence ofdetectable stellar SiO maser. One of the most important requirements islarge optical variability (2.5 mag or greater), a condition associatedwith high mass-loss rate. In addition, it is found that themaser-to-infrared flux ratio is approximately proportional to themass-loss rate. New polarization observations of the SiO maserassociated with R Aqr, one of the two symbiotic stars known to show anytype of maser emission, are also presented.
| The circumstellar dust envelopes of red giant stars. I - M giant stars with the 10-micron silicate emission band
Spherical dust envelope models of red giant stars are constructed bysolving the radiative transfer equations of the generalized two-streamEddington approximation. The IRAS observations of M giant stars whichshow the 10-micron silicate emission band in IRAS LRS spectra areexplained by the models with the dirty silicate grains with Kproportional to lambda exp -1.5 for lambda greather than 28 microns.Under the assumption of steady mass flow in the envelope, this modelanalysis gives the following conclusions: (1) the strength of thesilicate emission peak at 10 microns is a good indicator of the massloss rate of the star, (2) no stars with the 10-microns silicateemission feature are observed in the range of mass loss rate smallerthan 7 x 10 to the -8th solar mass/yr, and (3) the characteristic timeof the mass loss process of M stars does not exceed a few 10,000 years.
| Infrared spectra and circumstellar emission of late-type stars
NIR photometry has been carried out for 63 M-type stars in eightphotometric bands. Four bands were the usual J, H, K, and L bands ofbroad-band photometry, and the other four were defined with relativelynarrow filters at 3.1, 3.3, 3.4, and 3.7 microns to measure the strengthof a stellar H2O absortion band centered at about 2.7 microns. Theeffects of circumstellar emission due to silicate dust on the energyspectra of oxygen-rich stars are extracted on the basis of the presentdata and IRAS photometric data. Peculiar carbon stars which exhibit asilicatelike emission feature at 10 microns were searched for from about1800 carbon stars. Eleven new candidate carbon stars were found.
| On the contribution of interstellar extinction to the 10 micron dust feature in OH/IR stars
The IRAS Low Resolution Spectra of 467 sources with the 10 micron dustfeature are analyzed. The strengths of the dust feature are determinedby the ratio of the flux at 9.7 micron to the fitted continuum level.Color temperatures are derived from the fluxes of the four IRASphotometric bands after correcting for the effect of the 10 micronfeature on the fluxes of the 12 micron band. A definite correlationbetween the strength of the feature and the color temperature of thecontinuum is found, implying that the 10 micron dust feature is largelycircumstellar in origin. A reexamination of the strength of the silicatefeature for seven of the OH/IR star used by Gehrs and colleagues in 1985has failed to reproduce the optical depth-distance relationship found bythese authors. It is concluded that interstellar extinction does notplay a major role in the formation of the 10 micron absorption feature.
| IRAS catalogues and atlases - Atlas of low-resolution spectra
Plots of all 5425 spectra in the IRAS catalogue of low-resolutionspectra are presented. The catalogue contains the average spectra ofmost IRAS poiont sources with 12 micron flux densities above 10 Jy.
| The stellar component of the galaxy as seen by the AFGL infrared sky survey
The noise-limited magnitudes for the Air Force Geophysical Laboratory(AFGL) Infrared Sky Survey have been estimated by direct comparison withground-based observations. Using these limiting magnitudes, 'pruned'versions of the AFGL catalog have been generated. Infrared observationsof all the stellar objects seen at 11, 20, or 27 microns and astatistical sample of the stars seen only at 4 microns are reported.Analysis of the observations leads to estimates of the absolute 4 and 10microns magnitudes and space densities for the two clases of objects.The expected results from the Infrared Astronomical Satellite arereexamined.
| Broad-band 20-micron photometry of 76 stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1973ApJ...186..193M&db_key=AST
| 316 neue Veränderlilche
Not Available
| Sixty-four new variable stars.
Not Available
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
| Constelación: | Hidra |
| Ascensión Recta: | 08h24m27.85s |
| Declinación: | -08°31'12.7" |
| Magnitud Aparente: | 7.64 |
| Distancia: | 387.597 parsecs |
| Movimiento Propio en Ascensión Recta: | -30.1 |
| Movimiento Propio en Declinación: | 19.8 |
| B-T magnitude: | 9.316 |
| V-T magnitude: | 7.779 |
Catálogos y designaciones:
|
