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A 1.2 mm MAMBO survey of post-AGB stars
Aims.We performed a millimetric survey of a sample of 24 post-AGB starsaimed at searching for emission from circumstellar matter, in order toinvestigate the physical properties of the outer parts of theenvelopes. Methods: .The observations were conducted using the37-channel Max-Planck Millimeter Bolometer array at the 30-meter IRAMtelescope. The continuum emission toward the detected sources was usedto quantify the mass of the emitting dust. We combined our observationswith data available in the literature to construct the spectral energydistribution (SED) of the sources. When the observational data covered aspectral range wide enough, some properties of circumstellar envelopeswere derived by comparison with spectra computed using a radiativetransfer code. Results: .Of the 24 objects in our sample, wedetected millimetric continuum emission toward 11 sources. Two othersources were detected at a flux level close to 3σ. The derivedcircumstellar dust masses range between 0.4 and 24 ×10-4 Mȯ, but these results are affected bythe uncertainty about the source distances. The parameters derived fromthe SED fits are consistent with the values characteristic of this kindof object. As confirmed from the flux density extrapolated in the firstlight channels of the Atacama Large Millimetric Array, these sourcescould be good targets for future high-resolution mapping with the ALMAfacility.
| Evolution of the Circumstellar Molecular Envelope. I. A BIMA CO Survey of Evolved Stars
This paper reports the results of a small imaging survey of eightevolved stars including two AGB stars (IRC +10216 and Mira), fiveproto-planetary nebula (PPN) candidates (AFGL 2688, IRAS 22272+5435, HD161796, 89 Her, and HD 179821), and a planetary nebula (PN, NGC 7027).We present high-resolution 12CO J=1-->0 maps of their fullmolecular envelopes made by combining BIMA Millimeter Array and NRAO 12m telescope observations. For the PPNe and PN, the neutral molecularenvelopes are compared with images taken at optical, near-IR, and mid-IRwavelengths. Drawing from the literature, we augmented our BIMA surveysample to 38 well-studied sources with CO emission maps. We classifiedthis sample of sources based on the kinematics and morphologies of theCO emission into three types: spherical/elliptical/shell sources, disksources, and structured outflow sources. Confirming previous studies, wefind strong evidence for the photodissociation of the molecular envelopeas an object evolves from the AGB to PN stages. While the spherical AGBstars follow theoretical expectations for mass-loss rate versus envelopesize, the post-AGB structured outflow sources have significantly highermass-loss rates than expected probably because of their recentsuperwinds. We find evidence that the structured outflows are clearlyyounger than the AGB wind. The disk sources have little correlationbetween mass-loss rate and envelope size because their properties aredetermined more by the properties of the central stars and diskevolution than by the mass-loss rate history that shapes the sphericaland structured-outflow sources.
| On the CO Near-Infrared Band and the Line-splitting Phenomenon in the Yellow Hypergiant ρ Cassiopeiae
We report on multiepoch optical and near-infrared spectroscopy aroundthe first-overtone rovibrational band of CO in the pulsating yellowhypergiant ρ Cas, one of the most massive stars in the Galaxy and acandidate SN II progenitor. We argue that the double cores of the COabsorption lines, which have previously been attributed to separatecircumstellar shells expelled during its recurrent outbursts, result infact from a superposition of a wide absorption line and a narrow centralemission line. The CO line doubling returns over subsequent pulsationcycles, where the superposed line emission assumes its largest intensitynear phases of maximum light. We find that the morphology and behaviorof the CO band closely resemble the remarkable ``line-splittingphenomenon'' also observed in optical low-excitation atomic lines. Basedon radiative transport calculations, we present a simplified model ofthe near-infrared CO emission emerging from cooler atmospheric layers inthe immediate vicinity of the photosphere. We speculate that the kinetictemperature minimum in our model results from a periodicpulsation-driven shock wave. We further discuss a number of alternativeexplanations for the origin of the ubiquitous emission-line spectrum,possibly due to a quasi-chromosphere or a steady shock wave at theinterface of a fast expanding wind and the interstellar medium. Wepresent a number of interesting spectroscopic similarities between ρCas and other types of cool variable supergiants, such as the RV Tau andR CrB stars. We further propose a possibly common mechanism for theenigmatic outburst behavior of these luminous pulsating cool stars.
| Revealing the Mid-Infrared Emission Structure of IRAS 16594-4656 and IRAS 07027-7934
TIMMI2 diffraction-limited mid-infrared images of a multipolarproto-planetary nebula IRAS 16594-4656 and a young [WC] ellipticalplanetary nebula IRAS 07027-7934 are presented. Their dust shells arefor the first time resolved (only marginally in the case of IRAS07027-7934) by applying the Lucy-Richardson deconvolution algorithm tothe data, taken under exceptionally good seeing conditions (<=0.5").IRAS 16594-4656 exhibits a two-peaked morphology at 8.6, 11.5, and 11.7μm, which is mainly attributed to emission from PAHs. Ourobservations suggest that the central star is surrounded by a toroidalstructure, observed edge-on, with a radius of 0.4" (~640 AU at anassumed distance of 1.6 kpc) and with its polar axis atP.A.~80deg, coincident with the orientation defined by onlyone of the bipolar outflows identified in the HST optical images. Wesuggest that the material expelled from the central source is currentlybeing collimated in this direction and that the multiple outflowformation has not been coeval. IRAS 07027-7934 shows a bright,marginally extended emission (FWHM=0.3") in the mid-infrared with aslightly elongated shape along the north-south direction, consistentwith the morphology detected by HST in the near-infrared. Themid-infrared emission is interpreted as the result of the combinedcontribution of small, highly ionized PAHs and relatively hot dustcontinuum. We propose that IRAS 07027-7934 may have recently experienceda thermal pulse (likely at the end of the AGB) which has produced aradical change in the chemistry of its central star.Based on observations collected at the European Southern Observatory (LaSilla, Chile), on observations made with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, Netherlands, and the United Kingdom) with theparticipation of ISAS and NASA, and on observations made with theNASA/ESA Hubble Space Telescope, obtained from the data archive at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy (AURA), Inc., under NASAcontract NAS5-26555.
| Post-AGB stars as testbeds of nucleosynthesis in AGB stars
We construct a data base of 125 post-AGB objects (including R CrB andextreme helium stars) with published photospheric parameters (effectivetemperature and gravity) and chemical composition. We estimate themasses of the post-AGB stars by comparing their position in the (logT{eff}, log g) plane with theoretical evolutionary tracks ofdifferent masses. We construct various diagrams, with the aim of findingclues to AGB nucleosynthesis. This is the first time that a large sampleof post-AGB stars has been used in a systematic way for such a purposeand we argue that, in several respects, post-AGB stars should be morepowerful than planetary nebulae to test AGB nucleosynthesis. Our mainfindings are that: the vast majority of objects which do not showevidence of N production from primary C have a low stellar mass(Mstar < 0.56 Mȯ); there is no evidencethat objects which did not experience 3rd dredge-up have a differentstellar mass distribution than objects that did; there is clear evidencethat 3rd dredge-up is more efficient at low metallicity. The sample ofknown post-AGB stars is likely to increase significantly in the nearfuture thanks to the ASTRO-F and follow-up observations, making theseobjects even more promising as testbeds for AGB nucleosynthesis.
| 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.
| The OH 1612 MHz maser pump rates of stellar, interstellar and post-AGB OH masers
(Pseudo) radiative pumprate of OH 1612 MHz masers is defined for asample of 44 OH/IR sources (infrared sources with OH 1612 MHz maser),irrespective of the real maser pumping mechanisms. The correlationbetween the (pseudo) maser pumprates and the evolutionary status of themaser sources reveals that the radiative pumprates of stellar OH masersare nearly fixed, which agrees with the theoretical prediction forradiatively pumped OH maser. The (pseudo) radiative pumprates ofinterstellar OH masers are not only very small but also varying broadlyover two orders of magnitude, which is argued to be the manifestation ofvarying number of quiet absorbing OH cloudlets and/or various OH maserpumping mechanisms and/or competitive gain between mainline and 1612 MHzOH masers and/or anisotropy of the maser emission. The radiativepumprates of post-AGB OH masers very possibly decrease with increasingIRAS C32 color indices and distribute in an interim regionbetween the stellar and interstellar OH masers in the pumprate-colordiagram.
| Axisymmetry in protoplanetary nebulae - II. A near-infrared imaging polarimetric survey
In an imaging polarimetry survey of candidate post-AGB stars, scatteringenvelopes are detected around 20 objects. These envelopes represent thefinal mass-loss phases at the end of the AGB. In all cases, evidence foraxisymmetry in the dust density is seen, suggesting that the presence ofan axisymmetric outflow may be a ubiquitous phenomenon of the AGB topost-AGB transition. We use the polarized flux images to classify theobjects into detached shell, bipolar and unresolved targets. Modellingbased on a simple axisymmetric shell geometry supports the contentionthat post-AGB objects fall into one of two classes that differ in theamount of dust in the circumstellar environment: the detached shellscorrespond to stars with an optically thin expanding circumstellarenvelope (CSE) whereas the bipolar and unresolved targets have opticallythick dust structures, probably in the form of discs, which remain boundto the star, rather than partaking in the expansion of the AGB CSE. Itis suggested that this bifurcation in morphology is rooted in thepresence or absence of a binary companion, which determines whether ornot a disc forms. Because the detached shell objects also appearaxisymmetric, an additional mechanism for generating the axisymmetry,such as a magnetically shaped outflow, is needed if they do indeed havesingle star progenitors.
| A 2 Micron H2 Spectral Survey of Proto-Planetary Nebulae
We measured 2.1-2.3 μm spectra for a mostly complete sample of knownproto-planetary nebulae (PPNe) at declinations north of -30°. Thisspectral range includes the H2 emission lines 1-0 S(1), 1-0S(0), 2-1 S(1), 2-1 S(2), and 3-2 S(3). We detected H2emission from 16 of the 51 objects in our survey. Twelve of these arenew detections, doubling the number of PPNe with detected H2emission. We find that H2 emission commences at a spectralclass of mid-G in PPNe with bipolar morphologies and optically thickequatorial regions; in fact, all bipolar F-G PPNe with an opticallythick equatorial region have collisionally excited H2emission. Radiative excitation becomes important in PPNe when thecentral star reaches an early B spectral type, just beforephotoionization of the nebula and the commencement of the planetarynebula (PN) phase. Almost all of the PPNe with B central stars showH2 emission, with either a purely radiative or a mixedcollisional plus radiative spectrum. Since H2 emission israre in nonbipolar PNe, the destruction of the H2 in thenonbipolars must be roughly coincident with the photoionization of thenebula. As with H2-emitting PNe, the bipolarH2-emitting PPNe are found at low Galactic latitudes.Optically thin H2-emitting nebulae are not similarlyrestricted to low latitudes. Brγ emission is detected in 15 of thesources, including all those of B spectral type.
| Polarization Measurements of Post-Asymptotic Giant Branch Candidates and Related Stars
We have obtained UBVRI polarization measurements of 26 post-asymptoticgiant branch (post-AGB) candidates and related stars. The extremelymetal-poor post-AGB star HR 4049 has been observed several times. Inmost cases we find the objects to be intrinsically polarized. Thepolarization measurements presented in this paper indicate asymmetriccircumstellar dust shells and disks around these stars. For some objectsthe steep percent polarization λ-dependence and large degree ofpolarization suggest that scattering by circumstellar dust grains may beresponsible for the observed polarizations in the blue.
| Search for radiative pumping lines of OH masers. I. The 34.6 m absorption line towards 1612 MHz OH maser sources
The 1612 MHz hydroxyl maser in circumstellar envelopes has long beenthought to be pumped by 34.6 μm photons. Only recently, the InfraredSpace Observatory has made possible spectroscopic observations whichenable the direct confirmation of this pumping mechanism in a few cases.To look for the presence of this pumping line, we have searched theInfrared Space Observatory Data Archive and found 178 spectra with dataaround 34.6 μm for 87 galactic 1612 MHz masers. The analysisperformed showed that the noise level and the spectral resolution of thespectra are the most important factors affecting the detection of the34.6 μm absorption line. Only 5 objects from the sample (3 redsupergiants and 2 galactic center sources) are found to show clear 34.6μm absorption (all of them already known) while two additionalobjects only tentatively show this line. The 3 supergiants show similarpump rates and their masers might be purely radiatively pumped. The pumprates of OH masers in late type stars are found to be about 0.05, only1/5 of the theoretical value of 0.25 derived by Elitzur (1992). We havealso found 16 maser sources which, according to the analysis assumingElitzur's pump rate, should show the 34.6 μm absorption line but donot. These non-detections can be tentatively explained by far-infraredphoton pumping, clumpy nature of the OH masing region or a limb-fillingemission effect in the OH shell.
| Searching for radiative pumping lines of OH masers: II. The 53.3 μm absorption line towards 1612 MHz OH maser sources
This is the second paper in a series aiming at searching for infraredpumping lines for galactic 1612 MHz OH masers. Our paper I is devoted tothe 34.6 μm absorption lines in ISO SWS spectra towards a largesample of galactic OH/IR sources. This paper analyzes the 53.3 μmline in the ISO LWS spectra towards a similar sample of OH/IR sources. Asearch with position radius of 1 arcmin in ISO Data Archive (IDA)results in 137 LWS spectra covering 53.3 μm associated with 47galactic OH/IR sources and 4 ones associated with megamasers Arp 220 andNGC 253. (These two magamasers are included for comparison purposeonly.) Ten of these galactic OH/IR sources are found to show and another5 ones tentatively show the 53.3 μm absorption while another 7sources (our group U1 and U2 sources) highly probably do not show thisline. The source class is found to be correlated with the type ofspectral profile: red supergiants (RSGs) and AGB stars tend to showstrong blue-shifted filling emission in their 53.3 μm absorption lineprofiles while H II regions tend to show a weak red-shifted fillingemission in the line profile. GC sources and megamasers mainly showsymmetrical profile in the line core while megamasers tend to show anadditional absorption tail on the blue side of the line profile. It isargued that the filling emission might be the manifestation of anunresolved half emission half absorption profile of the 53.3 μmdoublet which might be produced by the transitions among the two levels:2Π1/2(J=3/2) and2Π1/2(J=5/2) and their closely related levels.The 53.3 to 34.6 μm equivalent width (EW) ratio is close to unity forRSGs but much larger than unity for GC sources and megamasers while H IIregions only show the 53.3 μm line. The pump rate defined as maser toIR photon flux ratio is approximately 5% for RSGs. The pump rates of GCsources are three order of magnitude smaller. Both the large 53.3 to34.6 μm EW ratio and the small pump rate of the GC OH masers reflectthat the two detected `pumping lines' in these sources are actually ofinterstellar origin. The pump rate of Arp 220 is 32% - much larger thanthat of RSGs, which indicates that the contribution of other pumpingmechanisms to this megamaser is important. A handful of non-detectionsof the 34.6 or 53.3 μm line or both can be explained partly by thegenuinely weakness of the OH masers and partly by some other mechanismsweakening the IR pumping lines, such as clumpy OH shell or limb fillingemission.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Polarimetric observations of OH masers in proto-planetary nebulae
The 1612 and 1667 MHz OH maser lines have been measured in all fourStokes parameters in 47 proto-planetary nebula (PPN) candidates. Out of42 objects detected, 40 and 34 are 1612 and 1667 MHz emitters,respectively. The spectral extent of the 1667 MHz line overshoots thatof the 1612 MHz line in about 80% of the targets. 52% and 26% of the1612 and 1667 MHz sources, respectively, show linear polarization in atleast some features. Circular polarization is more frequent, occurringin 78% and 32% of sources of the respective OH lines. The percentagepolarization is usually small (<15%) reaching up to 50-80% in a fewsources. Features of linearly polarized emission are usually weak (0.5-4Jy) and narrow (0.3-0.5 km s-1). The strength of the magneticfield inferred from likely Zeeman pairs in two sources of a few mG isconsistent with values reported elsewhere for those classes of objects.An upper limit of the electron density in the envelope of OH17.7-2.0derived from the difference in the position angle of polarizationvectors for the two OH lines is about 1 cm-3. Distinctprofiles of polarization position angle at 1612 and 1667 MHz are seen inabout one third of the sources and strongly suggest that the envelopesare permeated by structured magnetic fields. The geometry of themagnetic field is implicated as an important cause of the depolarizationfound in some PPN candidates. For the subset of targets which showaxisymmetric shells in the optical or radio images we found a dominanceof magnetic field components which are orthogonal to the long axis ofthe nebulae. This finding supports the hypothesis that such bipolarlobes are shaped by the magnetic field.Figures in Appendix are only available in electronic form athttp://www.edpsciences.org
| Optical and infrared spectroscopic analysis of the post-AGB star candidate IRAS 19386+0155
This work reports high-resolution spectroscopic stellar parameters andabundance analysis of the post-AGB star IRAS 19386+0155. The analysis ofthe high resolution spectrum shows that it has an effective temperatureof Teff=6800±100 K and a surface gravity of log g =1.4±0.2. The abundance analysis reveals IRAS 19386+0155 to be ametal-poor object having [Fe/H]=-1.1. Carbon, oxygen and nitrogen arefound to be slight underabundant than solar, and so are theα-elements (Mg, Si and Ca). The analysis of ISO spectra revealedan oxygen-rich envelope and showed crystalline silicate features. Thedust envelope of this object was modelled using a Monte Carlo typenumerical method and amorphous and crystalline silicate grains wereconsidered. The photospheric temperature obtained through the opticalspectroscopy was used to constrain these models. The Spectral EnergyDistribuition (SED) of this object is broad and extensive suggesting thepresence of a dust disk around IRAS 19386+0155.Based on observations made with the 1.52 m telescope at the EuropeanSouthern Observatory (La Silla, Chile) under the agreement with theCNPq-Observatório Nacional (Brazil) Based on observations withISO, an ESA project with instruments funded by ESA Member States(especially the PI countries: France, Germany, the Netherlands and theUK) and with the participation of ISAS and NASA.
| Mid-infrared imaging of the dust shell around the post-asymptotic giant branch star HD 161796
We present mid-infrared (IR) images of HD 161796 (IRAS 17436+5003),taken with the OSCIR imager on the Gemini North Telescope, that resolvefor the first time the thermal emission structure of the dust shellaround this post-asymptotic giant branch (AGB) star. As well as a basicaxisymmetric structure, the observations show deviations fromaxisymmetry in the dust density and a twist in the symmetry axis.Modelling of the mid-IR images and of the spectral energy distributionfrom ultraviolet to submillimetre wavelengths reproduces all of theaxisymmetric features with an equator-to-pole density contrast of 6: 1and an inclination of the symmetry axis of 10° to the plane of thesky. We find that a model incorporating small (0.01μm) grains and asteep (~a-6) power-law size distribution can successfullyaccount for the thermal emission and for the observed degrees of near-IRpolarization. Assuming a distance of 1.2 kpc to HD 161796, the stellarluminosity is 3.4 × 103 Lsolar and the massof the shell is ~0.7 Msolar. This is consistent with a starof initial mass between 1 and 2 Msolar that has undergone anintensive (2.2 × 10-4 Msolaryr-1) phase of mass loss lasting about 3000 yr at the end ofthe AGB. A current stellar mass of 0.56 Msolar, as indicatedby the luminosity, suggests that HD 161796 is a few hundred years intoits post-AGB evolution and will take about 5000 yr to evolve from itspresent temperature of 7500 K to become the central star of an extendedelliptical planetary nebula.
| The Molecular Morphology of Evolved Stars
Not Available
| Post-Agb Stars
In this contribution, a review is presented on the ample data obtainedon post-AGB stars, both on the central stars and their circumstellarmaterial. The fast evolutionary phase is characterized by a rapid changein the properties of the objects, but the variety is so large that thereis yet no clear consensus on how the detailed studies of individualobjects are linked together by evolutionary channels. The absence ofstrong molecular veiling in the photospheres of the central stars,together with a spread in intrinsic metallicity make post-AGB stars veryuseful in constraining AGB chemical evolutionary models. We discuss thesurprisingly wide variety of chemical signatures observed. The onset inthe creation process of the panoply of structures and shapes observed inplanetary nebulae occurs during the short post-AGB evolution, but thephysical nature of the processes involved is still badly understood. Inthe rapidly growing field of circumstellar mineralogy, post-AGB starshave their story to tell and also the molecular envelope changessignificantly due to dilution and hardening of the stellar radiation.The real-time evolution of some objects suffering a late thermal flashis reviewed and their possible link to other hydrogen-deficient objectsis discussed. Any review on stellar evolution has a section on binariesand this contribution is no exception because binaries make up asignificant fraction of the post-AGB stars known to date.
| High-Resolution Spectroscopy of the Yellow Hypergiant ρ Cassiopeiae from 1993 through the Outburst of 2000-2001
We present an overview of the spectral variability of the peculiarF-type hypergiant ρ Cas, obtained from our long-term monitoringcampaigns over the past 8.5 yr with four spectrographs in the northernhemisphere. Between 2000 June and September an exceptional variabilityphase occurred when the V brightness dimmed by about a full magnitude.The star recovered from this deep minimum by 2001 April. It is the thirdoutburst of ρ Cas on record in the last century. We observe TiOabsorption bands in high-resolution near-IR spectra obtained with theUtrecht Echelle Spectrograph during the summer of 2000. TiO formation inthe outer atmosphere occurred before the deep brightness minimum.Atmospheric models reveal that the effective temperature decreases by atleast 3000 K, and the TiO shell is driven supersonically withM~=5.4×10-2 Msolar yr-1. Strongepisodic mass loss and TiO have also been observed during the outburstsof 1945-1947 and 1985-1986. A detailed analysis of the exceptionaloutburst spectra is provided, by comparing with high-resolution opticalspectra of the early M-type supergiants μ Cep (Ia) and Betelgeuse(Iab). During the outburst, central emission appears above the localcontinuum level in the split Na D lines. A prominent optical emissionline spectrum appears in variability phases of fast wind expansion. Theradial velocity curves of Hα and of photospheric metal absorptionlines signal a very extended and velocity-stratified dynamic atmosphere.The outburst spectra indicate the formation of a low-temperature,optically thick circumstellar gas shell of 3×10-2Msolar during 200 days, caused by dynamic instability of theupper atmosphere of this pulsating massive supergiant near the Eddingtonluminosity limit. We observe that the mass-loss rate during the outburstis of the same order of magnitude as has been proposed for the outburstsof η Carinae. We present calculations that correctly predict theoutburst timescale, whereby the shell ejection is driven by the releaseof hydrogen ionization recombination energy.
| Water ice growth around evolved stars
We present a model of the growth of water ice on silicate grains in thecircumstellar envelopes of Asymptotic Giant Branch (AGB) stars and RedSuper Giants. We consider the growth of ice by gas grain collisions, thethermal evaporation of ice from a grain, and sputtering. Our modelcontains several improvements compared to earlier models, including adetailed treatment of the effects of sputtering, a detailed calculationof the radiation pressure on the grain, and the treatment of subsonicdrift velocities. In terms of drift velocity between the grains and gasin the envelope, we find that the ice formation process can be dividedinto three regimes: (i) a sputtering dominated regime where ice growthis heavily suppressed, (ii) an intermediate regime with moderatelyefficient condensation and (iii) a thermally dominated regime where iceformation is highly efficient. Sputtering is the critical factor whichdetermines if ice formation can occur at all. We find that in RedSupergiants, ice formation is suppressed, while the winds of OH/IR starsallow for efficient condensation and can convert significant fractionsof the available water vapor (tens of percent) into ice mantles ongrains. Population II AGB stars hardly form ice due to their low dust togas ratios. We also modify an analytical equation describingcondensation and depletion (Jura & Morris\cite{1985ApJ...292..487J}) in order to give reasonable results for highand low drift velocities. Initially, ice will condense in crystallineform, but continuing condensation at low temperatures, and damage causedby interstellar UV photons favor the production of amorphous ice aswell. We predict that a significant fraction of the ice formed will beamorphous.
| Observational survey of the puzzling star HD 179821: Photometric variations and period analysis
From new photometric observations (UBVRI), we present the characteristicfeatures of the light variations of the evolved star HD 179821 (= SAO124414 = IRAS 19114+0002). Our data, collected through 1999 and 2000,have been combined together with previous photometric measurementsavailable in the literature. Thus, a long term V-light curve (gatheringmore than 10 years of observations for HD 179821) has been composed. Wehave analysed it with the Fourier transform method. Two main frequenciesare present in the resulting power spectrum, reflecting a dominantbimodal pulsator behavior. A long term phenomenon is also found, but itis not possible to decide whether it is periodic. The Fourier analysishas also been applied on two other filters (U and B) and confirms thedetected frequencies. On the basis of our period analysis, we discussthe nature of HD 179821: low-mass post-AGB star or high-mass star.based on observations carried out at the Observatoire de Haute Provence,France, operated by the Centre National de la Recherche Scientifique(CNRS).
| Submillimetre photometry of post-asymptotic giant branch stars
Stars in the post-asymptotic giant branch (post-AGB) phase of evolutionare surrounded by detached circumstellar envelopes containing dust whichemits thermally in the mid- and far-infrared. Here we present 850-μmSCUBA photometry of nine candidate post-AGB stars. All targets aredetected at 850μm and we use these fluxes to estimate the envelopedust masses and, by comparison with the 100-μm IRAS fluxes, the dustemissivity index.
| Classification of planetary nebulae by their departure from axisymmetry
We propose a scheme to classify planetary nebulae (PNe) according totheir departure from axisymmetric structure. We consider only departurealong and near the equatorial plane, i.e. between the two sidesperpendicular to the symmetry axis of the nebula. We consider six typesof departure from axisymmetry: (1) PNe where the central star is not atthe centre of the nebula; (2) PNe having one side brighter than theother; (3) PNe having unequal size or shape of the two sides; (4) PNewhere the symmetry axis is bent, e.g. the two lobes in a bipolar PN arebent toward the same side; (5) PNe where the main departure fromaxisymmetry is in the outer regions, e.g. an outer arc; and (6) PNe thatshow no departure from axisymmetry, i.e. any departure, if it exists, ison scales smaller than the scale of blobs, filaments and otherirregularities in the nebula. PNe that possess more than one type ofdeparture are classified by the most prominent type. We discuss theconnection between departure types and the physical mechanisms that maycause them, mainly resulting from the influence of a stellar binarycompanion. We find that ~50 per cent of all PNe in the analysed samplepossess large-scale departure from axisymmetry. This number is largerthan that expected from the influence of binary companions, namely~25-30 per cent. We argue that this discrepancy comes from many PNewhere the departure from axisymmetry, mainly unequal size, shape orintensity, results from the presence of long-lived and large (hot orcool) spots on the surface of their asymptotic giant branch progenitors.Such spots locally enhance the mass-loss rate, leading to a departurefrom axisymmetry, mainly near the equator, in the descendent PN.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Classification of 2.4-45.2 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer
The Infrared Space Observatory observed over 900 objects with the ShortWavelength Spectrometer in full-grating scan mode (2.4-45.2 μm). Wehave developed a comprehensive system of spectral classification usingthese data. Sources are assigned to groups based on the overall shape ofthe spectral energy distribution (SED). The groups include naked stars,dusty stars, warm dust shells, cool dust shells, very red sources, andsources with emission lines but no detected continuum. These groups arefurther divided into subgroups based on spectral features that shape theSED such as silicate or carbon-rich dust emission, silicate absorption,ice absorption, and fine-structure or recombination lines. Caveatsregarding the data and data reduction, as well as biases intrinsic tothe database, are discussed. We also examine how the subgroups relate tothe evolution of sources to and from the main sequence and how thisclassification scheme relates to previous systems. Based on observationswith the Infrared Space Observatory (ISO), a European Space Agency (ESA)project with instruments funded by ESA Member States (especially thePrinciple Investigator countries: France, Germany, Netherlands, andUnited Kingdom) and with the participation of the Institute of Space andAstronautical Science and the National Aeronautics and SpaceAdministration (NASA).
| The Massive Disk around OH 231.8+4.2
We have obtained 11.7 and 17.9 μm images at the Keck I telescope ofthe circumstellar dust emission from OH 231.8+4.2, an evolvedmass-losing red giant with a well-studied bipolar outflow. We detectboth a central unresolved point source with a diameter of less than 0.5"producing Fν(17.9μm)=60 Jy and emission extended morethan 1" away from the star, which is aligned with the bipolar outflowseen on larger scales. We find that the unresolved central source can beexplained by an opaque, flared disk with an outer radius of~5×1015 cm and an outer temperature of ~130 K. Onepossible model to explain this flaring is that the material in the diskis orbiting the central star and not simply undergoing a radialexpansion.
| Radiative Transfer Models of Dust Shells Around Post-AGB Stars
We present a radiative transfer analysis of circumstellar dust shellsaround the Post-AGB stars HD 179821, HD 56126, HD 101584 and early Rstar HD 100764, using the code DUSTY. Parameters like mass-loss, shellinner radius, dust temperature, outflow velocity etc., are derived forHD 179821 and HD 56126 whose observed SED could be reproduced by ourmodels.
| The ISO-SWS post-helium atlas of near-infrared stellar spectra
We present an atlas of near-infrared spectra (2.36 mu m-4.1 mu m) of ~300 stars at moderate resolution (lambda /delta lambda ~ 1500-2000). Thespectra were recorded using the Short-Wavelength Spectrometer aboard theInfrared Space Observatory (ISO-SWS). The bulk of the observations wereperformed during a dedicated observation campaign after the liquidhelium depletion of the ISO satellite, the so-called post-heliumprogramme. This programme was aimed at extending the MK-classificationto the near-infrared. Therefore the programme covers a large range ofspectral types and luminosity classes. The 2.36 mu m-4.05 mu m region isa valuable spectral probe for both hot and cool stars. H I lines(Bracket, Pfund and Humphreys series), He I and He II lines, atomiclines and molecular lines (CO, H2O, NH, OH, SiO, HCN,C2H2, ...) are sensitive to temperature, gravityand/or the nature of the outer layers of the stellar atmosphere(outflows, hot circumstellar discs, etc.). Another objective of theprogramme was to construct a homogeneous dataset of near-infraredstellar spectra that can be used for population synthesis studies ofgalaxies. At near-infrared wavelengths these objects emit the integratedlight of all stars in the system. In this paper we present the datasetof post-helium spectra completed with observations obtained during thenominal operations of the ISO-SWS. We discuss the calibration of the SWSdata obtained after the liquid helium boil-off and the data reduction.We also give a first qualitative overview of how the spectral featuresin this wavelength range change with spectral type. The dataset isscrutinised in two papers on the quantitative classification ofnear-infrared spectra of early-type stars ({Lenorzer} et al.\cite{lenorzer:2002a}) and late-type stars (Vandenbussche et al., inprep). Based on observations with ISO, an ESA project with instrumentsfunded by ESA Members States (especially the PI countries France,Germany, the Netherlands and the United Kingdom) and with theparticipation of ISAS and NASA. The full atlas is available inelectronic form at www.edpsciences.org Table 1 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/390/1033
| Multiple scattering of polarized radiation by non-spherical grains: First results
We present the first numerical radiative transfer simulation of multiplelight scattering in dust configurations containing aligned non-spherical(spheroidal) dust grains. Such models are especially important if onewants to explain the circular polarization of light, observed in avariety of astronomical objects. The optical properties of thespheroidal grains are calculated using the method of separation ofvariables developed by Voshchinnikov & Farafonov (\cite{vf93}). Theradiative transfer problem is solved on the basis of the Monte Carlomethod. Test simulations, confirming the correct numericalimplementation of the scattering mechanism, are presented. As a firstapplication, we investigate the linear and circular polarization oflight coming from a spherical circumstellar shell. This shell containsperfectly aligned prolate or oblate spheroidal grains. We investigatethe dependence of the results on the grain parameters (equivolumeradius, aspect ratio) and the shell parameters (inner/outer radius,optical thickness). The most remarkable features of the simulated linearpolarization maps are so-called polarization null points where thereversal of polarization occurs. They appear in the case when the grainalignment axis is perpendicular to the line of sight. The position ofthese points may be used for the estimation of grain shape andgeometrical structure of the shell. The origin of null points lies inthe physics of light scattering by non-spherical particles and is notrelated to the cancellation of polarization as was discussed in previousmodels. The maps of circular polarization have a sector-like structurewith maxima at the ends of lines inclined to the grain alignment axis by+/- 45degr .
| Crystalline silicate dust around evolved stars. III. A correlations study of crystalline silicate features
We have carried out a quantitative trend analysis of the crystallinesilicates observed in the ISO spectra of a sample of 14 stars withdifferent evolutionary backgrounds. We have modeled the spectra using asimple dust radiative transfer model and have correlated the resultswith other known parameters. We confirm the abundance difference of thecrystalline silicates in disk and in outflow sources, as found byMolster et al. (\cite{Molster1999a}). We found some evidence that theenstatite over forsterite abundance ratio differs, it is slightly higherin the outflow sources with respect to the disk sources. It is clearthat more data is required to fully test this hypothesis. We show thatthe 69.0 micron feature, attributed to forsterite, may be a verysuitable temperature indicator. We found that the enstatite is moreabundant than forsterite in almost all sources. The temperature of theenstatite grains is about equal to that of the forsterite grains in thedisk sources but slightly lower in the outflow sources. Crystallinesilicates are on average colder than amorphous silicates. This may bedue to the difference in Fe content of both materials. Finally we findan indication that the ratio of ortho to clino enstatite, which is about1:1 in disk sources, shifts towards ortho enstatite in the highluminosity (outflow) sources. Based on observations with ISO, an ESAproject with instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) and with theparticipation of ISAS and NASA.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Vízöntő |
| Rektaszcenzió: | 19h13m58.61s |
| Deklináció: | +00°07'31.9" |
| Vizuális fényesség: | 8.204 |
| Távolság: | 5555.556 parszek |
| RA sajátmozgás: | 1 |
| Dec sajátmozgás: | -3.1 |
| B-T magnitude: | 10.173 |
| V-T magnitude: | 8.367 |
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