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The Dusty AGB Star RS CrB: First Mid-Infrared Interferometric Observations with the Keck Telescopes
We report interferometric observations of the semiregular variable starRS CrB, a red giant with strong silicate emission features. The datawere among the first long-baseline mid-infrared stellar fringes obtainedbetween the Keck telescopes, using parts of the new nulling beamcombiner. The light was dispersed by a low-resolution spectrometer,allowing simultaneous measurement of the source visibility and intensityspectra from 8 to 12 μm. The interferometric observations allow anonambiguous determination of the dust shell spatial scale and relativeflux contribution. Using a simple spherically symmetric model, in whicha geometrically thin shell surrounds the stellar photosphere, we findthat ~30% to ~70% of the overall mid-infrared flux-depending on thewavelength-originates from 7-8 stellar radii. The derived shell opacityprofile shows a broad peak around 11 μm (τ~=0.06), characteristicof Mg-rich silicate dust particles.
| 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).
| Technetium and the third dredge up in AGB stars. I. Field stars
We searched for Tc in a sample of long period variables selected bystellar luminosity derived from Hipparcos parallaxes. Tc, as an unstables-process element, is a good indicator for the evolutionary status ofstars on the asymptotic giant branch (AGB). In this paper we study theoccurrence of Tc as a function of luminosity to provide constraints onthe minimum luminosity for the third dredge up as estimated from recentstellar evolution models.A large number of AGB stars above the estimated theoretical limit forthe third dredge up are found not to show Tc. We confirm previousfindings that only a small fraction of the semiregular variables show Tclines in their spectra. Contrary to earlier results by Little et al.(\cite{llmb87}) we find also a significant number of Miras without Tc.The presence and absence of Tc is discussed in relation to the massdistribution of AGB stars. We find that a large fraction of the stars ofour sample must have current masses of less than 1.5 Msun .Combining our findings with stellar evolution scenarios we conclude thatthe fraction of time a star is observed as a SRV or a Mira is dependenton its mass.Partly based on observations collected at the European SouthernObservatory, Paranal, Chile (ESO-Programme 65.L-0317(A)).
| 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.
| Velocity Observations of Multiple-Mode Asymptotic Giant Branch Variable Stars
Numerous infrared spectroscopic observations were obtained of nineasymptotic giant branch (AGB) field M giants that have multiple periodsof light variability. Each star has a short period of several months,which is typical of low amplitude pulsation for stars on the AGB, aswell as a long period of 1-3 yr, which is significantly longer than thepredicted fundamental-mode pulsations for these stars. The location ofthese stars in the AGB period-luminosity relation is discussed. For sixof the nine giants we found radial-velocity periods that confirm thelong-period light variability. Although we considered the possibilitythat the velocity variations result from orbital motion, we concludethat the long-period velocity changes in most, if not all of the samplestars, likely result from a currently unknown type of pulsation ratherthan duplicity.
| SiO maser survey of AGB stars in the North Galactic Cap
A SiO maser survey in the J=1-0, v=1 and 2 transitions has been made forIRAS sources in the North Galactic Cap b > 30o) with theNobeyama 45 m radio telescope. The sources were selected on the basis oftheir IRAS 12/25-μm and 25/60-μm flux ratios as likely oxygen-richAGB candidates. SiO masers were detected from 24 out of the 97 selectedsources, where 17 were new detections. The distances and heights abovethe Galactic plane are calculated. The Galactic distribution of detectedand undetected stars indicates that metallicity is likely to govern thedetection rate. The Galactocentric angular velocities of the subsampledstars are derived and their variation with the Galactic height isdiscussed. Based on observations at the Nobeyama Radio Observatory(NRO). NRO is a branch of the National Astronomical Observatory, aninter-university research institute, operated by the Ministry ofEducation, Science and Culture, Japan.
| 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).
| Multiperiodicities from the Hipparcos epoch photometry and possible pulsation in early A-type stars
A selection criterion based on the relative strength of the largestpeaks in the amplitude spectra, and an information criterion are used incombination to search for multiperiodicities in Hipparcos epochphotometry. The method is applied to all stars which have beenclassified as variable in the Hipparcos catalogue: periodic, unsolvedand microvariables. Results are assessed critically: although there aremany problems arising from aliasing, there are also a number ofinteresting frequency combinations which deserve further investigation.One such result is the possible occurrence of multiple periods of theorder of a day in a few early A-type stars. The Hipparcos catalogue alsocontains a number of these stars with single periodicities: such starswith no obvious variability classifications are listed, and informationabout their properties (e.g., radial velocity variations) discussed.These stars may constitute a new class of pulsators.
| 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.
| 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
| Multiperiodicity in semiregular variables. I. General properties
We present a detailed period analysis for 93 red semiregular variablesby means of Fourier and wavelet analyses of long-term visualobservations carried out by amateur astronomers. The results of thisanalysis yield insights into the mode structure of semiregular variablesand help to clarify the relationship between them and Mira variables.After collecting all available data from various international databases(AFOEV, VSOLJ, HAA/VSS and AAVSO) we test the accuracy and reliabilityof data. We compare the averaged and noise-filtered visual light curveswith simultaneous photoelectric V-measurements, the effect of the lengthversus the relatively low signal-to-noise ratio is illustrated by periodanalysis of artificial data, while binning effects are tested bycomparing results of frequency analyses of the unbinned and averagedlight curves. The overwhelming majority of the stars studied showmultiperiodic behaviour. We found two significant periods in 44variables, while there are definite signs of three periods in 12 stars.29 stars turned out to be monoperiodic with small instabilities in theperiod. Since this study deals with the general trends, we wanted tofind only the most dominant periods. The distribution of periods andperiod ratios is examined through the use of the (log P_0, log P_1) and(log P_1, log P_0/P_1) plots. Three significant and two less obvioussequences are present which could be explained as the direct consequenceof different pulsational modes. This hypothesis is supported by theresults for multiperiodic variables with three periods. Finally, thesespace methods are illustrated by several interesting case studies thatshow the best examples of different special phenomena such as long-termamplitude modulation, amplitude decrease and mode switching.
| 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.
| 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.
| 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.
| A Renewed Search for Water Maser Emission from Mira Variables.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1602L&db_key=AST
| Semiregular variables of types SRa and SRb. Silicate dust emission features.
We have analysed the IRAS-LRS spectra of representative samples ofO-rich Semiregular (SR) variables of types SRa and SRb and of Miravariables. The silicate features were extracted by fitting the energydistribution with two blackbodies, approximating the continuous emissionfrom the photosphere and the circumstellar dust. The shape and strengthof the silicate features in the LRS range were then studied by computingthe residual fluxes in 5 selected wavelength regions covering the whole10μm and 18μm features and parts of the 10μm feature assignedto emission from olivine and possibly corundum. We compare our approachwith previous investigations and argue that a quantitative study ofdetails in the feature shape requires subtraction of the stellar and thedust continuum and the use of flux ratios rather than a discreteclassification system. The Miras form an extension of the SRb's towardslower stellar temperatures and higher dust shell opacities and they haveslightly higher average dust temperatures. The SRa's seem to be moresimilar to the Miras in their dust shell properties. The average 10μmfeature shapes of the three groups of variables agree, but taking intoaccount the photospheric and dust shell parameters, systematicdifferences show up. For stars hotter than about 2900K, the 10μmfeature width shows a wide range of values but no clear trend with thestellar temperature or the optical depth of the dust shell. These starsare generally SRb variables and have the thinnest dust envelopes. Atcooler stellar temperatures, where mostly Miras are found, the opticaldepth of the dust shell determines the feature width in the sense thatthicker shells have narrower features. It appears that the 13μmfeature is obvious only in a narrow range of effective temperature andoptical depth of the dust shell. We discuss our results in terms ofradiative transfer effects, differences in the average grain size,annealing and hydration of amorphous silicates and contributions fromother dust components. Of these possibilities the last one seems to bemost plausible with regard to the behavior of the 10μm feature width.The observations can be interpreted in terms of changing contributionsfrom olivine and corundum possibly caused by an increasing amount ofdust processing (Miras) and the influence of the atmospheric structureon the formation of these dust components (SRb's).
| 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.
| Semiregular variables of types SRa and SRb. New JHKL'M-photometry for 200 stars.
This paper presents new JHKL'M observations of 200 Semiregular variables(SRVs) of types SRa and SRb. The sample was defined in Kerschbaum &Hron (1992a, Paper I) by means of a certain limit in bolometricalmagnitude. From the sample of 350 objects, 260 now have near infrared(NIR) photometry - for 60 of these stars data from the literature areused. In total 290 datasets are available because of some multipleobservations. We briefly compare the photometry obtained at differentobservatories. Small but significant differences are found. A firstanalysis of the photometry supports one of the main findings of Paper I.The, in many aspects inhomogeneous, O-rich semiregular variables oftypes SRa and SRb can be successfully split in two subgroups called the`blue' and `red'/`Mira' SRVs. A separation of the `red' SRVs fromintrinsic Miras additionally requires variability information.
| 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.
| OH/IR star color mimics
IR color criteria are used to select potential OH/IR stars from the IRASPoint Source Catalog. These OH/IR star color mimics, despite oftenhaving thick and demonstrably O-rich dust shells to shield theirmolecules against interstellar UV, have no 1612 MHz masers. The mostlikely reason for this is that these stars have degenerate companions,which collect an accretion disk from the red giant wind, which in turnprovides them with a local source of UV to dissociate molecules fromwithin their dust shells. In some cases this self-generated UV issufficient to excise all molecules from a shell, as happens withsymbiotic novae; in some cases it merely reduces their number and theability of a shell to support a maser. It is suggested that D-typesymbiotic stars can be identified among sources with thick opaque dustshells by a persistent absence of appropriate masers: these are theOH/IR color mimics.
| A survey of circumstellar CO emission from a sample of IRAS point sources
The first results from a survey of circumstellar CO(1-0) emission arepresented. The sources were selected from the IRAS point source catalogaccording to the IRAS color criteria described in van der Veen andHabing (1988). The sources have good quality fluxes at 12, 25, and 60microns, flux densities larger than 20 Jy at 25 microns, and aresituated more than 5 deg away from the Galactic plane. The survey isundertaken to study the relationship between mass loss rates, dustproperties, and the evolution along the AGB. The sample consists of 787sources and contains both oxygen and carbon-rich stars, including Miravariables, OH/IR objects, protoplanetary nebulae, planetary nebulae, and60-micron excess sources. So far, 519 objects, situated on both thenorthern and the southern sky, have been observed; 163 sources werefound to have circumstellar CO emission, and in 58 of these CO emissionhas not previously been detected.
| Short- and intermediate-period oxygen-rich Miras
Available IR photometry and the period-IR luminosity relationship areused to analyze a nearly complete sample of oxygen-rich Miras atGalactic latitudes with /b/ greater than 30 deg. In agreement withprevious kinematic studies, a marked difference is found in the spatialdistributions between the stars with periods less than and those withperiods greater than 300 d. For stars with periods in the range 300-400d, the exponential scale height from the Galactic plane is close to 240pc, the projected surface density is about 100/sq kpc, and the localspace density is about 210/cu kpc. The progenitors of the Miras withperiods greater than 300 d appear to be disk dwarfs with typicalmain-sequence masses of between 1.0 and 1.2 solar masses. The masses ofthe main-sequence progenitors of the short-period Miras are 1.1 solarmasses or less. The duration of the Mira phase for theintermediate-period stars is estimated to be about 20,000 yr, somewhatlonger than other recent estimates. Both short- and intermediate-periodoxygen-rich Miras characteristically lose mass at about 10 exp -7 solarmass/yr.
| 1612 MHz OH survey of IRAS point sources. I - Observations made at Dwingeloo, Effelsberg and Parkes
The data from a large sky survey are presented including a northernpilot study and a detailed southern study in which detections are biasedtoward the most evolved sources and distant sources. Both areinvestigated at the 1612-MHz transition of OH to take advantage of thestrongest line for AGB stars with optically thick dust shells. The IRASsources are chosen by considering their IR colors related to fluxes at12, 25, and 60 microns. Observations are reported for 2703 IRAS pointsources at the 1612-MHz transition, and 738 OH/IR stars are detected.The survey identifies 597 of the sources as previously unidentified, and95 percent of the OH profiles observed have twin-peak masercharacteristics which are related to emission from expandingcircumstellar shells. The other 5 percent of the sources are concludedto be transition objects between OH/IR stars and planetary nebulae.
| A survey for infrared excesses among high galactic latitude SAO stars
This project involves extending the previous analysis of infraredexcesses among a volume-limited sample of 134 nearby A-K main-sequencestars to a magnitude-limited sample of stars, culled from the SAOCatalog, with excesses determined from the IRAS Point Source Catalogflux density ratios. This new sample includes 5706 B-M type stars, 379of which have infrared excesses. The objective involved use of astatistically complete survey of objects in a standard catalog in orderto assess the frequency with which different physical processes canaffect the infrared output of stars. These processes include, but arenot limited to, orbiting cold particle clouds and the onset of rapidmass loss. It is concluded that cold disks are consistent with theinfrared excesses found among A-G dwarfs and G-K giants in the sample.
| Photometry of masing and nonmasing OH/IR stars
Broadband infrared photometry performed on a sample of 60 stars withIRAS colors in the range displayed by Type II OH/IR stars is reported.All of the stars have been tested for 1612 MHz OH maser emission byEder, Lewis, and Terzian (1988), but emission has been detected in only27 of them. A morphological comparison is made between the masing andnonmasing groups to discern whether masering is correlated with near-and middle-infrared properties. Both groups have the same scale heightperpendicular to the galactic plane and share many characteristics incommon with the local Mira population. Where the IRAS colors of the twogroups overlap significantly, no statistically significant difference inthe K - L color distribution is found. Using theoretical models relatingthis color and the strength of the 9.7 micron silicate feature to theoptical depth of the circumstellar dust shell, it is found that redgiant dust shells need a minimum optical depth of tau(9.7) about 0.1 tosupport maser emission. Stars with a silicate feature in absorption onlyoccur at low galactic latitudes, whereas stars with the feature inemission are found at all latitudes. It is concluded that interstellarextinction is partly responsible for this effect.
| 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.
| OH/IR stars without OH masers - Nondetection statistics
Potential OH/IR stars are readily recognized from their far-infraredcolors. These are confirmed as OH/IR stars by detecting their 1612 MHzmasers. Color selection from the sources listed in the IRAS Point SourceCatalog therefore provides an efficient means of identifying newobjects. However, this approach is not so efficient with weak sources(S25 less than 2 Jy). Color-selected sources with S25 greater than 2 Jythat are not associated with OH masers have the same color-color range,the same IR flux range, and a similar galactic distribution to thedetections. These sources are explicable as normal OH/IR stars in whichthe OH maser is being suppressed. This would occur when a star with acircumstellar envelope also has a companion star that is able to disruptthe velocity coherence of the normal masing shell. The detection tonondetection ratio then provides a tool for studying the duplicity ofthese stars over the whole Galaxy.
| 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.
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