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Formation of Polycyclic Aromatic Hydrocarbons and Carbonaceous Solids in Gas-Phase Condensation Experiments
Carbonaceous grains represent a major component of cosmic dust. In orderto understand their formation pathways, they have been prepared in thelaboratory by gas-phase condensation reactions such as laser pyrolysisand laser ablation. Our studies demonstrate that the temperature in thecondensation zone determines the formation pathway of carbonaceousparticles. At temperatures lower than 1700 K, the condensationby-products are mainly polycyclic aromatic hydrocarbons (PAHs) that arealso the precursors or building blocks for the condensing soot grains.The low-temperature condensates contain PAH mixtures that are mainlycomposed of volatile three to five ring systems. At condensationtemperatures higher than 3500 K, fullerene-like carbon grains andfullerene compounds are formed. Fullerene fragments or completefullerenes equip the nucleating particles. Fullerenes can be identifiedas soluble components. Consequently, condensation products in cool andhot astrophysical environments such as cool and hot asymptotic giantbranch stars or Wolf-Rayet stars should be different and should havedistinct spectral properties.
| Spectroscopic binaries among Hipparcos M giants. III. The eccentricity - period diagram and mass-transfer signatures
Context: This paper is the third one in a series devoted to studying theproperties of binaries involving M giants. Aims: We use a new set oforbits to construct the first (e - log P) diagram of an extensive sampleof M giant binaries, to obtain their mass-function distribution, and toderive evolutionary constraints for this class of binaries and relatedsystems. Methods: The orbital properties of binaries involving M giantswere analysed and compared with those of related families of binaries (Kgiants, post-AGB stars, barium stars, Tc-poor S stars). Results: Theorbital elements of post-AGB stars and M giants are not very different,which may indicate that, for the considered sample of post-AGB binaries,the post-AGB star left the AGB at quite an early stage (M4 or so).Neither are the orbital elements of post-mass-transfer binaries likebarium stars very different from those of M giants, suggesting that themass transfer did not alter the orbital elements much, contrary tocurrent belief. Finally, we show that binary systems with e < 0.4 logP - 1 (with periods expressed in days) are predominantlypost-mass-transfer systems, because (i) the vast majority of barium andS systems match this condition; and (ii) these systems have companionmasses peaking around 0.6 Mȯ, as expected for whitedwarfs. The latter property has been shown to hold as well foropen-cluster binaries involving K giants, for which a lower bound on thecompanion mass may easily be set.Based on observations carried out at the Swiss telescope installed atthe Observatoire de Haute Provence (OHP, France), and at the 1.93-m OHPtelescope.
| SPITZER survey of dust grain processing in stable discs around binary post-AGB stars
Aims: We investigate the mineralogy and dust processing in thecircumbinary discs of binary post-AGB stars using high-resolution TIMMI2and SPITZER infrared spectra. Methods: We perform a full spectralfitting to the infrared spectra using the most recent opacities ofamorphous and crystalline dust species. This allows for theidentification of the carriers of the different emission bands. Our fitsalso constrain the physical properties of different dust species andgrain sizes responsible for the observed emission features. Results: In all stars the dust is oxygen-rich: amorphous andcrystalline silicate dust species prevail and no features of acarbon-rich component can be found, the exception being EP Lyr, where amixed chemistry of both oxygen- and carbon-rich species is found. Ourfull spectral fitting indicates a high degree of dust grain processing.The mineralogy of our sample stars shows that the dust is constituted ofirregularly shaped and relatively large grains, with typical grain sizeslarger than 2 ?m. The spectra of nearly all stars show a high degreeof crystallinity, where magnesium-rich end members of olivine andpyroxene silicates dominate. Other dust features of e.g. silica oralumina are not present at detectable levels. Temperature estimates fromour fitting routine show that a significant fraction of grains must becool, significantly cooler than the glass temperature. This shows thatradial mixing is very efficient is these discs and/or indicatesdifferent thermal conditions at grain formation. Our results show thatstrong grain processing is not limited to young stellar objects and thatthe physical processes occurring in the discs are very similar to thosein protoplanetary discs.Based on observations obtained at the European Southern Observatory(ESO), La Silla, observing program 072.D-0263, on observations made withthe 1.2 m Flemish Mercator telescope at Roque de los Muchachos, Spain,the 1.2 m Swiss Euler telescope at La Silla, Chile and on observationsmade with the SPITZER Space Telescope (program id 3274), which isoperated by the Jet Propulsion Laboratory, California Institute ofTechnology under a contract with NASA.Appendices A and B are only available in electronic form athttp://www.aanda.org
| Infrared spectroscopic study of a selection of AGB and post-AGB stars
We present here near-infrared spectroscopy in the H and K bands of aselection of nearly 80 stars that belong to various asymptotic giantbranch (AGB) types, namely S type, M type and SR type. This sample alsoincludes 16 post-AGB (PAGB) stars. From these spectra, we seekcorrelations between the equivalent widths of some important spectralsignatures and the infrared colours that are indicative of mass-loss.Repeated spectroscopic observations were made on some PAGB stars to lookfor spectral variations. We also analyse archival Spitzer mid-infraredspectra on a few PAGB stars to identify spectral features due topolycyclic aromatic hydrocarbon (PAH) molecules providing confirmationof the advanced stage of their evolution. Further, we model the spectralenergy distributions (SEDs) of the stars (compiled from archival data)and compare circumstellar dust parameters and mass-loss rates indifferent types.Our near-infrared spectra show that in the case of M- and S type stars,the equivalent widths of the CO(3-0) band are moderately correlated withinfrared colours, suggesting a possible relationship with mass-lossprocesses. A few PAGB stars revealed short-term variability in theirspectra, indicating episodic mass-loss: the cooler stars showed in COfirst overtone bands and the hotter ones showed in HI Brackett lines.Our spectra on IRAS 19399+2312 suggest that it is a transition object.From the Spitzer spectra, there seems to be a dependence between thespectral type of the PAGB stars and the strength of the PAH features.Modelling of SEDs showed among the M and PAGB stars that the higher themass-loss rates, the higher the [K - 12] colour in our sample.
| A search for diffuse bands in the circumstellar envelopes of post-AGB stars
In this work we present the results of a systematic search for diffusebands (DBs, hereafter) in the circumstellar envelopes of a carefullyselected sample of post-AGB stars. We concentrated on analyzing 9 of theDBs most commonly found in the interstellar medium. The strength ofthese features is determined using high-resolution optical spectroscopy,and the results obtained are compared with literature data on fieldstars affected only by interstellar reddening. Based on the weakfeatures observed in the subsample of post-AGB stars dominated bycircumstellar reddening, we conclude that the carrier(s) of these DBsmust not be present in the circumstellar environment of these sources,or at least not under the excitation conditions in which DBs are formed.This conclusion is applicable to all the post-AGB stars studied,irrespective of the dominant chemistry or the spectral type of the starconsidered. A detailed radial velocity analysis of the features observedin individual sources confirms this result, as the Doppler shiftsmeasured are found to be consistent with an interstellar origin.
| Variability and evolution in various classes of post-AGB stars
We aim to compare properties of early-type post-asymptotic giant-branch(post-AGB) stars, including normal first-time B-type post-AGB stars, andextreme helium stars (EHes). Hipparcos photometry for 12 post-AGB starsand 7 EHe stars has been analyzed; 5 post-AGB stars are clearlyvariable. The Hipparcos data are not sufficiently sensitive to detectvariability in any of the EHes.
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/799
| Progenitors of type Ia supernovae: Binary stars with white dwarf companions
Type Ia SNe (SNe Ia) are thought to come from carbon oxygen white dwarfsthat accrete mass from binary companions until they approach theChandrasekhar limit, ignite carbon, and undergo complete thermonucleardisruption. A survey of the observed types of binaries that containwhite dwarfs is presented. We propose that certain systems that seemmost promising as SN Ia progenitors should be more intensively observedand modeled, to determine whether the white dwarfs in these systems willbe able to reach the Chandrasekhar limit. In view of the number ofpromising single-degenerate systems and the dearth of promisingdouble-degenerate systems, we suspect that single-degenerates producemost or perhaps all SNe Ia, while double-degenerates produce some orperhaps none.
| Defining the instability strip of pulsating post-AGB binary stars from ASAS and NSVS photometry
We analyse public domain time-series photometric observations of 30known and candidate binary post-AGB stars for measuring pulsation andorbital periods. We derive periodicities for 17 stars for the first timein the literature. Besides identifying five new RV Tauri type pulsatingvariables (three with the RVb phenomenon, i.e. long-term changes of themean brightness), we find multiply periodic (or possibly irregular)post-AGB stars on the two edges of the instability strip. Thetemperature dependence of the peak-to-peak light-curve amplitudesclearly indicates the changes in excitation as post-AGB stars evolvethrough the strip. One object, the peculiar Type II Cepheid ST Pup,showed a period increase from 18.5 to 19.2 d, which is consistent withthe known period fluctuations in the past. In HD 44179, the central starof the Red Rectangle nebula, we see very similar asymmetric light curvethan was measured 10-15 yr ago, suggesting a very stable circumstellarenvironment. In contrast to this, HD 213985 shows coherent but highlynon-repetitive brightness modulation, indicating changes in thecircumstellar cloud on a similar time-scale to the orbital period.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| 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.
| Keplerian discs around post-AGB stars: a common phenomenon?
Aims.We aim at showing that the broad-band SED characteristics of oursample of post-AGB stars are best interpreted, assuming thecircumstellar dust is stored in Keplerian rotating passivediscs.Methods.We present a homogeneous and systematic study of theSpectral Energy Distributions (SEDs) of a sample of 51 post-AGB objects.The selection criteria to define the whole sample were tuned to coverthe broad-band characteristics of known binary post-AGB stars. The wholesample includes 20 dusty RV Tauri stars from the General Catalogue ofVariable Stars (GCVS). We supplemented our own Geneva optical photometrywith literature data to cover a broad range of fluxes from the UV to thefar-IR.Results.All the SEDs display very similar characteristics: alarge IR excess with a dust excess starting near the sublimationtemperature, irrespective of the effective temperature of the centralstar. Moreover, when available, the long wavelength fluxes show ablack-body slope indicative of the presence of a component of large mmsized grains.Conclusions.We argue that in all systems, gravitationallybound dusty discs are present. The discs must be puffed-up to cover alarge opening angle for the central star and we argue that the discshave some similarity with the passive discs detected around youngstellar objects. We interpret the presence of a disc to be a signaturefor binarity of the central object, but this will need confirmation bylong-term monitoring of the radial velocities. We argue that dusty RVTauri stars are those binaries which happen to be in the Population IIinstability strip.
| Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211
We present low-resolution VLT spectroscopy of the afterglow of thegamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB991216 is the only optical spectrum for this afterglow. It shows twoprobable absorption systems at z=0.80 and z=1.02, where the highestredshift most likely reflects the distance to the host galaxy. A thirdsystem may be detected at z=0.77. HST imaging of the field, obtained 4months after the burst, has resulted in the detection of two amorphousregions of emission, one at the projected afterglow position, and theother 0.primeprime6 away. The spectrum shows a depression influx in between 4000 Å and 5500 Å. This could be the resultof a 2175 Å-type extinction feature in the host of GRB 991216, butat a rather red wavelength of 2360 Å. If this interpretation iscorrect, it is the first time the extinction feature is seen in a GRBafterglow spectrum. It is centered at a wavelength similar to that ofthe ultra-violet (UV) bumps inferred from observations of a fewUV-strong, hydrogen-poor stars in the Galaxy. All significant absorptionlines (except for one) detected in the spectrum of GRB 011211 areidentified with lines originating in a single absorption system atz=2.142±0.002, the redshift of the GRB 011211 host galaxy. Wealso detect the Lyα absorption line in the host, to which we fit aneutral hydrogen column density of log N(ion{H}{i}) = 20.4±0.2,which indicates that it is a damped Lyα system. Using acurve-of-growth analysis, we estimate the Si, Fe and Al metallicity atthe GRB 011211 redshift to be [Si/H] =-0.9+0.6-0.4, [Fe/H]=-1.3±0.3, and [Al/H] =-1.0+0.5-0.3. For GRB 021211, we detect a singleemission line in a spectrum obtained tens of days after the burst, whichwe identify as [ion{O}{ii}] λ3727 at z=1.006. The correspondingunobscured [ion{O}{ii}] star-formation rate is 1.4 Mȯyr-1.
| 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).
| GSC 2139 2190 is a slowly changing Be variable and not a hot low mass post asymptotic giant branch star
Formerly presented as a Population II post AGB star, GSC 2139 2190 isactually a variable Population I Be star of (GCVS) type BE.
| The dust disk of HR 4049. Another brick in the wall
We present the Spectral Energy Distribution of HR 4049 based onliterature data and new continuum measurements at 850 ?m . The SEDshows variable absorption in the UV, and a large IR excess, both causedby circumstellar dust. The shape of the IR excess from 1 ?m all theway down to 850 ?m can be nearly perfectly fitted with a singleblackbody function at T ~ 1150 K or alternatively with a sum ofblackbodies in a narrow temperature range. The energy emitted in this IRcontinuum radiation is about one-third of the stellar luminosity. Weshow that this blackbody radiation must be due to the presence of acircumbinary disk with a large height. This disk must also be gas-rich,in agreement with the observations of molecular bands in the ISO-SWSspectrum. We present two possible scenario's for explaining the shapeand the intensity of the IR excess. The first scenario involves largegrains (a >=1 mm) that each radiate like a blackbody. The secondscenario argues that the blackbody radiation is due to a very opticallythick circumbinary disk. We investigate if such a disk would indeedproduce blackbody radiation by presenting results from radiativetransfer calculations. We further quantify the properties of such a diskand its stability in the framework of (hydro)dynamics, grain settling,radiation pressure and grain drift. The virtues and shortcomings of bothmodels for the origin of the IR blackbody are discussed by contrastingthem with other observations and assessing them in the framework of(binary) (post-)AGB evolution.
| 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.
| VLT/UVES and WHT/UES absorption spectroscopy of the circumstellar envelope of IRC + 10o 216 using background stars. First results and a search for DIBs
A unique and novel set of observations has been undertaken to probe thecircumstellar envelope (CSE) of the nearby (130 pc) carbon star IRC +10degr 216 using optical absorption spectroscopy towards background starslying beyond the envelope. The primary aim of the observations is tosearch for diffuse band (DIB) carriers in the CSE, for which themass-losing envelopes of carbon stars are a likely place of origin. Ourprincipal target is a V=16 G-type star located 37 arcsec from IRC +10degr 216 and was observed with VLT/UVES. A detailed model atmosphere andabundance analysis shows that it is somewhat metal-poor and hasconfirmed that it lies far beyond IRC +10 degr 216. The circumstellarH+2H2 column density expected along the line of sight towardsthis target is relatively high, ~ 2*E21 cm-2, andis large compared to that derived from the small interstellar extinctionestimated in the zone of IRC +10 degr 216 at b=+43degr ,EB-V< 0.03 mag. The CSE is certainly detected in the K Iresonance lines, which are centred at the heliocentric velocity of IRC+10 degr 216 and have FWHM ~ 30 km s-1, consistent with twicethe terminal expansion velocity of the circumstellar gas. The data showalso that circumstellar Na I is very probably detected, as seen towardstwo background stars. The strongest DIB (6284 Å) present in theUVES wavelength coverage is detected but very probably arises in theforeground ISM. No DIB is detected at 6614 Å, or elsewhere.Overall, the data suggest that the DIB carriers, if present in the CSE,have a low abundance relative to H in the C-rich envelope of IRC +10degr 216, in comparison with this ratio in the ISM.
| RU Cen and SX Cen: Two strongly depleted RV Tauri stars in binary systems. The RV Tauri photometric b phenomenon and binarity
We present a chemical abundance analysis on the basis of highsignal-to-noise and high-resolution (lambda /Delta lambda ~ 48 000)optical spectra of two RV Tauri stars RU Cen and SX Cen. With an [Fe/H]= -1.9 and a [Zn/Fe] = +0.9 for RU Cen and a [Fe/H] = -1.1 and a [Zn/Fe]= +0.6 for SX Cen, both stars of spectroscopic class B display strongdepletion of refractory elements in their photospheres. Our CORALIEradial velocity measurements prove the stars to be members of binarysystems and a detailed construction of the spectral energy distributionindicate the presence of a large amount of hot circumstellar dust.Moreover, the orbital period of SX Cen of around 600 days is similar tothe published period of mean magnitude variation in the light curve (RVTauri phomometric class b phenomenon). All these observations indicatethe presence of a stable circumbinary disk in the objects and strengthenthe model that this is a necessary condition for the depletion processto take place. Based on observations collected at the European SouthernObservatory in Chile (64.L-0117(A), 67.D-0054(A)), on the 1.2 m SwissEuler telescope on La Silla and on the 70 cm Swiss photometric telescopealso on La Silla.
| Why every bipolar planetary nebula is `unique'
We present the many evolutionary routes that progenitors of bipolarplanetary nebulae (BPNe) can take. Overall, there are about a hundredqualitatively different evolutionary routes, hence about a hundredqualitatively different types of BPNe. Within each type there arequantitative differences as well. Adding the dependence of theappearance on inclination, we find that the number of different apparentstructures of BPNe is about equal to, or even larger than, the number ofknown BPNe and proto-BPNe. Accordingly we argue that every BPN is a`unique' object in its appearance, but all can be explained within thebinary model paradigm. Therefore, we request a stop to the attaching ofadjectives such as `unique', `peculiar', and `unusual' to BPNe andproto-BPNe, thereby removing the need to invoke a new model for almostevery `unusual' BPN. As a case study we try to build a binary model forthe proto-BPN OH 231.8+4.2. In our preferred model the AGB Mira-typestar has a main sequence companion of mass ~1Msolar, orbitalperiod of ~5yr, and eccentricity of >~0.1.
| IUE and ISO observations of the bipolar proto-planetary nebula Hen 401 (IRAS 10178-5958)
We present ultraviolet (IUE) and infrared (ISO) observations of thebipolar proto-planetary nebula Hen 401 which, combined with previouslyavailable optical and near infrared data, are used to reconstruct theoverall spectral energy distribution from 1150 Å to 100 mu m. TheISO spectrum is dominated by strong PAH emission superimposed on a verycold continuum which is interpreted as thermal emission originating inthe C-rich cool dust ( ~ 106 K) present in the circumstellar envelope,the remnant of the previous AGB phase. In addition, a second, hottercomponent detected in the near infrared is attributed to thermalemission from hot dust ( ~ 640 K), suggesting that mass loss and dustgrain formation is still on-going during the current post-AGB phase. Theultraviolet (IUE) spectrum shows a stellar continuum in the wavelengthinterval 2400 Å to 3200 Å which corresponds to a moderatelyreddened B8-type central star. Unexpectedly, the UV flux in thewavelength interval 1150 Å to 1900 Å is very weak or absentwith no evidence of a hotter binary companion which could explain thedetection of the nebular emission lines observed in the availableground-based optical spectra of Hen 401. HST WFPC2 high resolutionimages also show no indication of a hot companion to the B8-type centralstar observed both in the optical and in the UV. The evolutionaryimplications of a possible single nature for the central star of Hen 401are discussed. Based on observations obtained with the InternationalUltraviolet Explorer, retrieved from the INES Archive at VILSPA, Madrid,Spain, and observations made with ISO, an ESA project with instrumentsfunded by ESA Member States (especially the PI countries: France,Germany, The Netherlands and the UK) with the participation of ISAS andNASA.
| Binary (Post) AGB evolution
In this contribution the observational characteristics of F to G typepost-AGB stars, showing both O-rich and C-rich circumstellar chemistry,are reviewed. It turns out that binarity and the presence of a stablecircumbinary dusty disc are fundamental properties of these objects. Thepossible common origin of the mixed chemistry observed in these systemsand in IR-bright [WC] stars is discussed.
| Binary "Post-AGB" Stars
Not Available
| Optical Spectrum of the Infrared Source IRAS 23304+6147
Based on CCD spectra obtained with the PFES echelle spectrometer of the6-m telescope, we have determined for the first time the effectivetemperature T_eff = 5900 K, surface gravity log g = 0.0, and detailedchemical composition of the faint star identified with the infraredsource IRAS 23304+6147 by the model-atmosphere method. Its metallicityindicates that the object belongs to the old Galactic disk (the meanabundance of the iron-group elements V, Cr, and Fe for IRAS 23304+6147is [X/H] = -0.61 dex). The stellar atmosphere exhibits an enhancement ofcarbon and nitrogen, [C/Fe] = +0.98, [N/Fe] = +1.36, and [C/O] > 1.Significant overabundances of lanthanides were detected: the mean [X/Fe]= +1.04 for La, Ce, Pr, Nd, and Eu. The elemental abundances suggestthat the atmospheric chemical composition of IRAS 23304+6147 wasmodified mainly by nucleosynthesis followed by mixing. By modeling theobject's spectrum, we revealed absorption features at the positions ofwell-known absorption diffuse interstellar bands (DIBs). An analysis ofradial-velocity and intensity measurements for these DIBs leads us toconclude that, for IRAS 23304+6147, the DIBs originate mostly in itscircumstellar dust envelope expanding at a velocity of about 20 km/s.Molecular C_2 Swan emission bands were detected in the object'sspectrum, which also originate in the circumstellar envelope. There is aclose match between the object's atmospheric effective temperaturesdetermined independently by the model-atmosphere method and by modelingits optical and infrared energy distribution, within the accuracy of themethods.
| The RV\ Tauri phenomenon and binarity
We present accurate radial velocity measurements on the pulsatingextremely iron-deficient post-AGB object HD 52961 and the RV Tauri starEN TrA (HD 131356) proving them to be binaries. Our long-termphotometric monitoring campaign shows that the RV Tauri photometricclass ``b'' phenomenon in HD 52961 is due to variable circumstellarextinction during orbital motion. By comparing carefully theobservational characteristics of RV Tauri stars and the class ofextremely iron-deficient post-AGB objects we conclude that binarity is awidespread phenomenon in the RV Tauri class of objects. The observedchemical depletion patterns, weak circumstellar CO emission, peculiarspectral energy distribution and the difference in photospheric class ofthe RV Tauri objects can all be naturally explained by assuming that thecircumstellar material is not freely expanding, but trapped in thebinary system. Based on observations collected at the European SouthernObservatory (proposals codes 51.7-0052; 51.7-0053; 52.7-0048; 58.E-0462;59.E-0432; 61.E-0426); with the Swiss telescopes at ESO and OHP and theAPT telescope at Mt. Hopkins
| The laboratory approach to the UV extinction.
Not Available
| Fitting the unusual UV extinction curve of V348 SGR
We have produced new types of carbonaceous condensates from the ejectaof a hydrocarbon plasma. These condensates are named QuenchedCarbonaceous Composites (QCC). There are various types of QCC formedaround the center of the ejecta that show an absorption feature in thespectral range 220-250 nm. The QCC with an absorption feature at 220 nmis composed of onion-like spherules; that with an absorption feature at230-240 nm are composed of polyhedral particles; and that with anabsorption feature at 250 nm is composed of ribbons with bent graphiticlayers. The 240-250 nm absorption feature of the hot evolved star V348Sgr can be matched by the ribbon-like QCC, suggesting that this type ofQCC may be present in the circumstellar shell of this star.
| Spectral variability of the binary HR 4049
The C I, Na I D, and H? lines of the post-AGB binary HR 4049 havebeen studied. Na I D variability results from a photospheric absorptioncomponent ([Na/H]=-1.6+/-0.2) which follows the velocity of the primaryand a stationary, non-photospheric component. An emission component isattributed to the circumbinary disc, and an absorption component tomass-loss from the system with a velocity of 5.3+/-0.5 km s(-1) . TheH? profile varies with the orbital period. The two strong shelltype emission peaks are identified as from one single broad emissionfeature with an absorption centered around -7.5 km s(-1) . The intensityvariations are largely attributed to a differential amount of reddeningtowards the H? emitting region and the stellar continuum. Theradial velocities suggest that the H? emission moves in phase withthe primary, but with a slightly lower velocity amplitude. From this weinfer that the H? emission comes from outside the orbit of theprimary, but still gravitationally bound to the primary. H? alsoshows a weak emission feature at -21.3+/-3.5 km s(-1) , which originatesfrom the circumbinary disc and a weak absorption feature at -7.5+/-1.6km s(-1) due to absorption by the circumbinary disc. We propose twocompeting models that could account for the observed velocity andintensity variations of the H? profile. Model I: light from theprimary reflects on a localized spot near the inner radius of thecircumbinary disc which is closest to the primary. Model II: H?emission originates in the outer layers of the extended atmosphere ofthe primary due to activity. These activities are locked to the positionof the primary in its orbit. We discuss the similarities of variabilityand shape of the H? emission of HR 4049 with those of early typeT-Tauri stars (e.g SU Aur). Based on observations obtained at theMcDonald, ESO, CTIO, and La Palma observatories.
| Discovery of Far-IR Pure Rotational Transitions of CH + in NGC 7027
We report the discovery of the rotational spectrum of CH+ in theInfrared Space Observatory Long Wavelength Spectrometer (LWS) spectrumof the planetary nebula NGC 7027. The identification relies on a 1996reanalysis of the LWS spectrum by Liu et al. and on new LWS data. Thestrong line at 179.62 mu m (coinciding with the 212--101 transition ofwater vapor) and the lines at 119.90 and 90.03 mu m (reported asunidentified by Liu et al.), whose frequencies are in the harmonicrelation 2?:?3:4, are shown to arise from the J = 2--1,3--2, and 4--3 rotational transitions of CH+. This identification isstrengthened by the new LWS spectra of NGC 7027, which clearly show thenext two rotational lines of CH+ at 72.140 and 60.247 mu m. This is thefirst time that the pure rotational spectrum of CH+ has been observed.This discovery opens the possibility of probing the densest and warmestzones of photodissociation regions. We derive a rotational temperaturefor the CH+ lines of 150 +/- 20 K and a CH+/CO abundance ratio of 2--6 x10-4.
| Circumstellar Molecular Spectra Towards Evolved Stars
In this paper we discuss the relevance, and possible scientific gains,which can be acquired from studying circumstellar molecular spectratoward evolved stars. Where can we expect circumstellar molecularspectra, why would we want to study these spectra, which molecules mightbe present, and what can we learn from these studies? We present anoverview of reported detections, and discuss some of the results.
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