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A survey for nanodiamond features in the 3 micron spectra of Herbig Ae/Be stars
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3micron wavelength region in search for the rare spectral features at3.43 and 3.53 micron. These features have been attributed to thepresence of large, hot, hydrogen-terminated nanodiamonds. Only twoHerbig Ae/Be stars, HD 97048 and Elias3-1 are known to display both these features. Methods:.We have obtained medium-resolution spectra (R 2500) with the ESOnear-IR instrument ISAAC in the 3.15-3.65 micron range. Results:.In our sample, no new examples of sources with prominent nanodiamondfeatures in their 3 micron spectra were discovered. Less than 4% of theHerbig targets show the prominent emission features at 3.43 and/or 3.53μm. Both features are detected in our spectrum of HD 97048. Weconfirm the detection of the 3.53 μm feature and the non-detection ofthe 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53μm detections in V921 Sco, HD 163296 and T CrA. The sources whichdisplay the nanodiamond features are not exceptional in the group ofHerbig stars with respect to disk properties, stellar characteristics,or disk and stellar activity. Moreover, the nanodiamond sources are verydifferent from each other in terms of these parameters. We do not findevidence for a recent supernova in the vicinity of any of thenanodiamond sources. We have analyzed the PAH 3.3 μm feature and thePfund δ hydrogen emission line, two other spectral features whichoccur in the 3 micron wavelength range. We reinforce the conclusion ofprevious authors that flared-disk systems display significantly more PAHemission than self-shadowed-disk sources. The Pf δ line detectionrate is higher in self-shadowed-disk sources than in the flared-disksystems. Conclusions: . We discuss the possible origin and paucityof the (nano)diamond features in Herbig stars. Different creationmechanisms have been proposed in the literature, amongst others in-situand supernova-induced formation. Our data set is inconclusive in provingor disproving either formation mechanism.
| Spectroscopic observations of the rapid rotating post-AGB star IRAS 05381+1012
We report on the high-resolution stellar parameters and abundanceanalysis of the rapidly rotating post-AGB star IRAS 05381+1012. Analysisof high-resolution spectra shows that IRAS 05381+1012 has an effectivetemperature of T_eff=5200± 100 K and a surface gravity of logg=1.0± 0.5 corresponding to a spectral type G(2-3)I. Theseparameters result in an estimated luminosity of 970 Lȯand a distance of 2700 pc. We also show that IRAS 05381+1012 has aprojected rotational velocity vsin i=40± 10 km s-1.The abundance analysis based on a few available lines reveals that thisstar is an iron-deficient object with [Fe/H]=-0.8. We also analyze theabundance pattern and compare it to other classes of stars with similarstellar parameters.
| 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.
| Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
We present the first mid-IR long baseline interferometric observationsof the circumstellar matter around binary post-AGB stars. Two objects,SX Cen and HD 52961, were observedusing the VLTI/MIDI instrument during Science Demonstration Time. Bothobjects are known binaries for which a stable circumbinary disc isproposed to explain the SED characteristics. This is corroborated by ourN-band spectrum showing a crystallinity fraction of more than 50% forboth objects, pointing to a stable environment where dust processing canoccur. Surprisingly, the dust surrounding SX Cen is not resolved in theinterferometric observations providing an upper limit of 11 mas (or 18AU at the distance of this object) on the diameter of the dust emission.This confirms the very compact nature of its circumstellar environment.The dust emission around HD 52961 originates from avery small but resolved region, estimated to be ~35 mas at 8 μm and~55 mas at 13 μm. These results confirm the disc interpretation ofthe SED of both stars. In HD 52961, the dust is nothomogeneous in its chemical composition: the crystallinity is clearlyconcentrated in the hotter inner region. Whether this is a result of theformation process of the disc, or due to annealing during the longstorage time in the disc is not clear.
| 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.
| Recent astrophysical results from the VLTI.
Not Available
| Nanodiamond Dust and the Far-Ultraviolet Quasar Break
We explore the possibility that the steepening observed shortward of1000 Å in the energy distribution of quasars may result fromabsorption by dust, being either intrinsic to the quasar environment orintergalactic. We find that a dust extinction curve consisting ofnanodiamonds, composed of terrestrial cubic diamonds or with surfaceimpurities as found in carbonaceous chondrite meteorites, such asAllende, is successful in reproducing the sharp break observed. Theintergalactic dust model is partially successful in explaining the shapeof the composite energy distribution but must be discarded in the end,as the amount of crystalline dust required is unreasonable and wouldimply an improbable fine-tuning among the dust formation processes. Thealternative intrinsic dust model requires a mixture of both cubicdiamonds and Allende nanodiamonds and provides a better fit of the UVbreak. The gas column densities implied are of the order 1020cm-2, assuming solar metallicity for carbon and fulldepletion of carbon into dust. The absorption only occurs in theultraviolet and is totally negligible in the visible. The minimum dustmass required is of the order ~0.003r2pcMsolar, where rpc is the distance in parsecsbetween the dust screen and the continuum source. The intrinsic dustmodel reproduces the flux rise observed around 660 Å in key quasarspectra quite well. We present indirect evidence of a shallow continuumbreak near 670 Å (18.5 eV), which would be intrinsic to the quasarcontinuum.
| Abundance Analyses of Field RV Tauri Stars. VI. An Extended Sample
An abundance analysis is presented and discussed for a sample of 14 RVTauri stars. The present abundance data and those from our previouspapers and by other workers are combined in an attempt to furtherunderstanding of the dust-gas separation process that afflicts many RVTauri variables. We propose that a star's intrinsic (i.e., initial)metallicity is given by the photospheric zinc abundance. Variableswarmer than about 5000 K and with an initial metallicity [Fe/H]>=-1are affected by dust-gas separation. Variables of all metallicities andcooler than about Teff~=5000 K are unaffected by dust-gasseparation. The RV Tauri variables show a spread in their C abundances,with the lower boundary of the points in the C versus Zn plane fallingclose to the predicted trend for giants after the first dredge-up. Theupper boundary is inhabited by a few stars that are carbon-rich. The Oabundances in the mean follow the predicted trend from unevolved stars,in line with the expectation that photospheric O abundance is unaffectedby the first dredge-up. An evolutionary scenario involving mass loss bya first-ascent or early-AGB red giant, the primary star of a binary, issketched.
| An Extremely Large Excess of 18O in the Hydrogen-deficient Carbon Star HD 137613
We report the discovery of a uniquely large excess of 18O inthe hydrogen-deficient carbon (HdC) star HD 137613 based on a spectrumof the first-overtone bands of CO at 2.3-2.4 μm in which three strongabsorption bands of 12C18O are clearly present.Bands of 12C16O also are present, but no bands of13C16O or 12C17O are seen.We estimate an isotopic ratio 16O/18O <~ 1. Thesolar value of this ratio is ~500. Neither He-core burning nor He-shellflash burning can produce the isotopic ratios of oxygen and carbonobserved in HD 137613. However, a remarkable similarity exists betweenthe observed abundances and those found in the outer layers of the broadHe shell of early-asymptotic giant branch (AGB) stars, soon after theend of He-core burning. It is not known how the outer envelope down tothe He shell could be lost, but some mechanism of enhanced mass lossmust be involved. HD 137613 may be a post-early-AGB star with the outerlayers of the former He-burning shell as its photosphere. The unusualelemental abundances of the HdC stars resemble those of the R CoronaeBorealis (RCB) stars, but HdC stars do not produce clouds of dust thatproduce declines in brightness. None of the other RCB or HdC starsobserved show significant 18O.
| 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.
| Chemical composition of evolved stars of high galactic latitude
We have carried out abundance analysis for a sample of high galacticlatitude supergiants in search of evolved stars.We find that HD 27381 has atmospheric parameters and an abundancepatternvery similar to that of the post-AGB star HD 107369.HD 10285 and HD 25291 are moderately metal-poor andshow the influence of mixing that has brought the productsof NeNa cycle to the surface.The high galactic latitude B supergiant HD 137569 shows selectivedepletion of refractory elements normally seen in post-AGB stars.We find that the high velocity B typestar HD 172324 shows moderate deficiency of Fe group elements butthe CNO abundances are verysimilar to that of disk B supergiants. The observed variations inthe radial velocities, transient appearance of emission componentsin hydrogen line profiles and doublingof O I lines at 7774 Å support the possibility of this star beinga pulsating variable or a binary star.
| Strong dust processing in circumstellar discs around 6 RV Tauri stars. Are dusty RV Tauri stars all binaries?
We present extended Spectral Energy Distributions (SEDs) of sevenclassical RV Tauri stars, using newly obtained submillimetre continuummeasurements and Geneva optical photometry supplemented with literaturedata. The broad-band SEDs show a large IR excess with a black-body slopeat long wavelengths in six of the seven stars, R Sct being thenoticeable exception. This long wavelength slope is best explainedassuming the presence of a dust component of large grains in thecircumstellar material. We show that the most likely distribution of thecircumstellar dust around the six systems is that the dust resides in adisc. Moreover, very small outflow velocities are needed to explain thepresence of dust near the sublimation temperature and we speculate thatthe discs are Keplerian. The structure and evolution of these compactdiscs are as yet not understood but a likely prerequisite for theirformation is that the dusty RV Tauri stars are binaries.
| First results from the ESO VLTI calibrators program
The ESO Very Large Telescope Interferometer (VLTI) is one of the leadinginterferometric facilities. It is equipped with several 8.2 and 1.8 mtelescopes, a large number of baselines up to 200 m, and with severalsubsystems designed to enable high quality measurements and to improvesignificantly the limits of sensitivities currently available tolong-baseline interferometry. The full scientific potential of the VLTIcan be exploited only if a consistent set of good quality calibrators isavailable. For this, a large number of observations of potentialcalibrators have been obtained during the commissioning phase of theVLTI. These data are publicly available. We briefly describe theinterferometer, the VINCI instrument used for the observations, the dataflow from acquisition to processed results, and we present and commenton the volume of observations gathered and scrutinized. The result is alist of 191 calibrator candidates, for which a total of 12 066observations can be deemed of satisfactory quality. We present a generalstatistical analysis of this sample, using as a starting point theangular diameters previously available in the literature. We derive thegeneral characteristics of the VLTI transfer function, and its trendwith time in the period 2001 through mid-2004. A second paper will bedevoted to a detailed investigation of a selected sample, aimed atestablishing a VLTI-based homogeneous system of calibrators.
| Mid-IR observations of circumstellar disks. II. Vega-type stars and a post-main sequence object
We present spectral energy distributions and new N-band photometry andspectroscopy for a sample of six main sequence stars and one post-MSobject using the ESO TIMMI2 camera at La Silla observatory (Chile). Allobjects are thought to possess circumstellar material and for themajority of the targets this is their first N-band spectroscopicobservation. The emission spectra (observed in three cases), modelledwith a mixture of silicates consisting of different grain sizes andcomposition, confirm the suspected presence of disks around thesetargets. The most important discovery is that HD 113766, a youngVega-type star, is host to highly processed dust which is probablysecond generation. It is the first time a Vega-type star with suchhighly evolved dust has been observed. Silicate emission of basicallyunevolved dust is seen in case of the post-MS object HD 41511 and theVega-type star HD 172555. In addition, to study the cold dust, weobserved a subsample at 1200 μm with the bolometer array SIMBA at theSEST in La Silla but we only got upper limits for those five objects.This shows that these Vega-type stars have a smaller amount of dust thantheir precursors, the T Tauri and Herbig Ae/Be stars.Based on observations collected at the European Southern Observatory, LaSilla, Chile (70.C-0468, 71.C-0001).
| VINCI-VLTI measurements of HR 4049: The physical size of the circumbinary envelope
We present the first detection of the envelope which surrounds thepost-AGB binary source HR 4049. VINCI-VLTI K-band interferometricobservations of this source imply the existence of a large structurewith a Gaussian angular FWHM 22.4 ± 1.4 mas or uniform diskdiameter of 34.9 ± 1.9 mas. With the Hipparcos parallax of 1.50± 0.64 mas these values correspond to a physical size of14.9+11.1-4.4 AU and23.3+17.3-7.0 AU, respectively. Our measurements,covering an azimuth range of 60 ° for the sky-projectedbaseline, provide information on the geometry of the emitting region andshow that there is only a slight variation of the measured angularvalues along the different directions sampled. Thus, our results areconsistent with a spherical geometry of the envelope. However, we cannotcompletely rule out the existence of an asymmetric envelope (like thecircumbinary disk envisaged by some recent models) because of thelimited spatial frequency and azimuth range covered by the observations.Based on public shared risk science program data released from the ESOVLTI: http://www.eso.org/projects/vlti/instru/vinci/vinci_data_sets.html
| Short-Period Variable Be Stars Discovered or Confirmed through Self-Correlation Analysis of Hipparcos Epoch Photometry
We have surveyed 277 bright Be stars for short-period (0.2-2 day)photometric variability using self-correlation analysis of Hipparcosepoch photometry. This extends the work of Percy et al.; the methods areas described there. We report the discovery, timescale, and amplitude ofshort-period variability in: HD 7636 (V764 Cas), HD 11606 (V777 Cas), HD13661 (V549 Per), HD 34921, HD 36408, HD 40978, HD 58343 (FW CMa), HD63460 (o Pup), HD 88195 (17 Sex), HD 89353 (AG Ant, HR 4049), HD 129954(CO Cir), HD 158220 (V862 Ara), HD 173219 (V947 Sct), and HD 187567(V1339 Aql). We report the confirmation of short-period variabilityusing an independent form of analysis in HD 52918 (19 Mon), HD 105382(V863 Cen), HD 137387 (κ-1 Aps), HD 157832 (V750 Ara), and HD163868 (V3984 Sgr). These include a β Cephei star (19 Mon), aVega-type star (17 Sex), and a post-asymptotic giant branch B star (HR4049). We find that short-period variability is more prevalent inearly-B stars than in late-B stars. We have found no β Cephei typevariables in our sample of Be stars, other than 19 Mon, which isapparently not a Be star.
| Binary post-AGB stars
An overview is given on the observational characteristics of someindividual post-AGB stars in binary systems, which inspired us to starta systematic search for binarity in a sample of post-AGB stars. In thiscontribution a preliminary report is presented of this search. One ofthe fundamental ingredients in the evolution of the systems discussedhere is the creation of a circumbinary dusty disc, probably during theAGB evolutionary phase. There is observational evidence that the discsare Keplerian and that they have important impact on the systems'(chemical) evolution. We discuss briefly the relation to other evolvedbinary classes, in which one of the components went through the AGBevolution.
| The Profiles of the 3-12 Micron Polycyclic Aromatic Hydrocarbon Features
We present spectra of the 3.3 μm and 11.2 μm polycyclic aromatichydrocarbon (PAH) features of a large number of stellar sources,planetary nebulae, reflection nebulae, H II regions, and galaxies,obtained with Infrared Space Observatory Short Wavelength Spectrometer.Clear variations are present in the profiles of these features. Most ofthe sources show a symmetric 3.3 μm feature peaking at ~3.290 μm,while only very few show an asymmetric 3.3 μm feature peaking at aslightly longer wavelength. The profiles of the 11.2 μm feature aredistinctly asymmetric. The majority of the sources has a 11.2 μmfeature peaking between 11.20 and 11.24 μm, with a very steep bluerise and a low tail-to-top ratio. A few sources show a 11.2 μmfeature with a peak position of ~11.25 μm, a less steep blue rise,and a high tail-to-top ratio. The sources are classified independentlyon the basis of the 3.3 and 11.2 μm feature profiles and peakpositions. Correlations between these classes and those based on the 6-9μm features (Peeters et al.) are found. In particular, sources withthe most common profiles in the 6-9 μm region also show the mostcommon 3.3 and 11.2 μm feature profiles. However, the uncommonprofiles do not correlate with each other. Also, these classificationsdepend on the type of object. In general, H II regions, nonisolatedHerbig AeBe stars and young stellar objects show the same profiles forall 3-12 μm features. Many planetary nebulae and post-asymptoticgiant branch stars show uncommon feature profiles. The three galaxies inour sample show the same profiles as the H II regions for all but the11.2 μm feature, being similar to that of evolved stars. The observedpronounced contrast in the spectral variations for the CH modes (3.3 and11.2 μm bands) versus the CC modes (6.2, 7.7, and 8.6 μm bands) isstriking: the peak wavelengths of the features attributed to CC modesvary by ~15-80 cm-1, while for the CH modes the variationsare ~4-6.5 cm-1. We summarize existing laboratory data andtheoretical calculations of the modes emitting in the 3-12 μm regionof PAH molecules and complexes. In contrast to the 6.2 and 7.7 μmcomponents, which are attributed to PAH cations, the 3.3 μm featureappears to originate in neutral and/or negatively charged PAHs. Weattribute the variations in peak position and profile of these IRemission features to the composition of the PAH family. The variationsin FWHM of the 3.3 μm feature remains an enigma, while those of the11.2 μm can be explained by anharmonicity and molecular structure.The possible origin of the observed contrast in profile variationsbetween the CH modes and the CC modes is highlighted.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of ISASand NASA.
| Does the 3.3 Micron Polycyclic Aromatic Hydrocarbon Emission Feature Require Ultraviolet Excitation?
Unidentified infrared bands (UIBs) have been observed in virtually everydusty astrophysical environment investigated. The UIB carrier must beabundant and ubiquitous. Strong evidence points to polycyclic aromatichydrocarbons as likely candidates, but the identification is notcomplete. Additional diagnostics are needed to further constrain the UIBcarrier, such as probing excitation sources ranging from UV-strong toUV-weak to determine the ``band gap'' of the UIB carrier. Observationsand models suggest that the UIBs can be found in sources with weak UVfields. To that end, we present new results of observing the 3.3 μmspectral region in six stars embedded in reflection nebulae and in sixVega-like stars. These objects have effective temperatures ranging from3500 to 12,000 K. Their environments include dust that should berelatively unprocessed (reflection nebulae) and dust that has mostlikely undergone significant processing (Vega-like) by the embeddedillumination source. Together with data from the literature, we have asample of 27 sight lines. Our analysis suggests that neither thestrength of the UV field impinging on the dust nor the effectivetemperature of the star is the determining factor in whether the 3.3μm UIB emission is present in an object. We found three detections ofthe 3.3 μm emission band, one in a Vega-type object, one in a HerbigAe/Be object, and one in a reflection nebula, and all with disks. Therole of disk geometry is likely to be important in revealing orobscuring the photodissociation regions from which the UIB emissionarises.
| An optical absorption study of the Helix planetary nebula. Na I and Ca II lines, and a search for diffuse bands
We present the first results of an optical absorption study ofNGC 7293, the Helix planetary nebula (PN), designedto search definitively for diffuse band (DIB) absorptions perhapsarising in the carbon-rich circumstellar matter of the PN. We used theVLT-UVES spectrograph to achieve high resolution (R=50 000) spectra of 8stars located angularly close to and behind the Helix. These targetswere selected through their photometric UBV or 2MASS properties,permitting derivation of spectral types (mainly F-G dwarfs) and distancethat place several of them far (700-1500 pc) beyond the Helix (210 pc).Through a detailed analysis of the Na I and Ca II lines to the 8targets, we find that two lines of sight situated close to the nebula asmapped in CO and H I exhibit very strong Na I absorption. Thisabsorption is unlikely to arise in the foreground or backgroundinterstellar medium which has a relatively low column density, becausethe Helix is at high galactic latitude b = -57°. It ismuch more probable that it is due to the PN neutral or molecularmaterial. This circumstellar origin is reinforced by the fact that nocorresponding Ca II line is observed (as it would usually be from theISM), which is in agreement with the very high Ca depletion oftenobserved in PNs. No trace of any DIB features was found in these twocircumstellar lines of sight, nor to the other targets. The twocircumstellar sightlines discovered in this work open the way to searchfor molecular species such as C2 which are not observable atradio wavelengths, and to obtain more information on the rich neutraland molecular content of the Helix.Based on observations carried out with the European Southern ObservatoryVLT/UVES, program 70.C-0100.
| λ Bootis stars with composite spectra
We examine the large sample of λ Boo candidates collected inTable 1 of Gerbaldi et al. (\cite{Gerbaldi2003}) to see how many of themshow composite spectra. Of the 132 λ Boo candidates we identify22 which definitely show composite spectra and 15 more for which thereare good reasons to suspect a composite spectrum. The percentage ofλ Boo candidates with composite spectra is therefore >17% andpossibly considerably higher. For such stars the λ Booclassification should be reconsidered taking into account the fact thattheir spectra are composite. We argue that some of the underabundancesreported in the literature may simply be the result of the failure toconsider the composite nature of the spectra. This leads to thelegitimate suspicion that some, if not all, the λ Boo candidatesare not chemically peculiar at all. A thorough analysis of even a singleone of the λ Boo candidates with composite spectra, in which thecomposite nature of the spectrum is duly considered, which woulddemonstrate that the chemical peculiarities persist, would clear thedoubt we presently have that the stars with composite spectra may not beλ Boo stars at all.Based on observations collected at ESO (Echelec spectrograph) and at TBL(Telescope Bernard Lyot) of the Pic du Midi Observatory (France).
| Optical data of meteoritic nano-diamonds from far-ultraviolet to far-infrared wavelengths
We have used different spectroscopic techniques to obtain a consistentquantitative absorption spectrum of a sample of meteoritic nano-diamondsin the wavelength range from the vacuum ultraviolet (0.12 μm) to thefar infrared (100 μm). The nano-diamonds have been isolated by achemical treatment from the Allende meteorite (Braatz et al. 2000,Meteorit. Planet. Sci., 35, 75). Electron energy loss spectroscopy(EELS) extends the optical measurements to higher energies and allowsthe derivation of the optical constants (n & k) by Kramers-Kroniganalysis. The results can be used to restrain observations and toimprove current models of the environment where the nano-diamonds areexpected to have formed. We also show that the amount of nano-diamondwhich can be present in space is higher than previously estimated byLewis et al. (1989, Nature, 339, 117).Table \ref{n&k} is also 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/423/983
| 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.
| Surface C-H stretching features on meteoritic nanodiamonds
Nanometre-sized diamonds (nanodiamonds) are to date the most abundantpresolar grains in primitive meteorites. They are therefore presumed tobe an abundant component of the dust in the interstellar medium. Whatthen are the expected spectroscopic signatures of these grains in theinterstellar medium? In order to answer this question we have examinedthe infrared spectroscopic properties of the nanodiamonds extracted fromthe Orgueil meteorite. The nanodiamonds were surface-cleaned andhydrogenated under vacuum. The spectra of the surface C-H stretchingfeatures in the 3-5 μm region were then taken. Comparison with largersynthetic nanodiamonds shows that the spectra are size-dependent. Theobserved meteoritic nanodiamond C-H stretching features are verydifferent from the features seen on the surfaces of larger diamonds(sizes ≥50 nm). Less-processed Orgueil nanodiamonds appear toprovide an intriguing similarity to the class B infrared emission bandspectra in the 3.3-3.7 μm wavelength region. The spectra of thenanodiamond C-H stretching features can be used as a template in thesearch for interstellar nanodiamonds in the infrared spectra ofastronomical objects. In addition the size-dependence of the nanodiamondsurface C-H features can be used to place rigid and robust constraintsupon the sizes of these particles in circumstellar media and in the ISM.Appendix A is only available in electronic form athttp://www.edpsciences.org
| Toward an adequate method to isolate spectroscopic families of diffuse interstellar bands
We divide some of the observed diffuse interstellar bands (DIBs) intofamilies that appear to have the spectral structure of single species.Three different methods are applied to separate such families, exploringthe best approach for future investigations of this type. Starting witha statistical treatment of the data, we found that statistical methodsby themselves give insufficient results. Two other methods of dataanalysis (`averaging equivalent widths' and `investigating the figureswith arranged spectrograms') were found to be more useful as tools forfinding the spectroscopic families of DIBs. On the basis of thesemethods, we suggest some candidates as `relatives' of 5780- and5797-Å bands.
| 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.
| Locally Aromatic Polycyclic Hydrocarbons as Potential Carriers of Infrared Emission Features
We report B-LYP/6-31G* and B3-LYP/6-31G* density functional theorycalculations on a set of polycyclic hydrocarbons, ranging in size fromC19H22 to C36H32, combiningaromatic (unsaturated) and aliphatic (saturated,sp3-hybridized carbon) ring systems. These locally aromaticpolycyclic hydrocarbons (LAPHs), generally exhibiting large deviationsfrom planarity, may be considered as intermediate structures betweenpolycyclic aromatic hydrocarbons (PAHs) and nanodiamonds. Calculatedinfrared vibrational frequencies are found to be similar to thoseobserved experimentally in spectra of hydrogenated amorphous carbon(HAC) and other carbonaceous solids. In the C-H stretching region(~3.1-3.6 μm) these species are characterized by strongabsorption/emission within both the aliphatic and aromatic C-H bands.They also show spectral features associated with tertiary C-H. Similarfeatures are evident in calculated spectra of the corresponding ions,which we have characterized in some cases. Ionization results in theparticular enhancement of a spectral feature typically seen at ~6.4μm, in the aromatic C-C stretching region. In keeping with previousexperimental and theoretical studies on the spectra of neutral andcationic PAHs, we find that the influence of ionization on the relativeintensities of C-C and C-H stretching features is much greater than theinfluence of molecular structure. We suggest that LAPHs may besignificant contributors to emission in Type B unidentified infraredemission sources.
| Laboratory Infrared Spectroscopy of Cationic Polycyclic Aromatic Hydrocarbon Molecules
Infrared spectroscopy of a variety of interstellar sources shows strongmid-IR emission bands, which are generally attributed to emission fromhighly vibrationally excited polycyclic aromatic hydrocarbon molecules(PAHs) in the neutral and, particularly, cationic states. Over the pastdecade, various experimental methods have been developed to record theinfrared spectra of cationic PAHs in the laboratory. In this paper, wediscuss available experimental spectra obtained with matrix isolationspectroscopy (MIS), infrared multiple-photon dissociation of trappedions (MPD), dissociation spectroscopy of ionic PAH van der Waalsclusters (VDW), and infrared emission (IRE). Moreover, we compare theseexperimental spectra to density functional theory (DFT) calculations.The main body of experimental data relies on MIS and MPD spectra, and wepresent a detailed comparison of results from these methods, providinglinear and multiple-photon absorption data, respectively. The effects ofmultiple-photon absorption, as encountered in MPD, and multiple-photonemission, occurring in interstellar spectra, are carefully assessed withthe use of mathematical models, which include the effects of vibrationalanharmonicity. We show that an analysis of the multiple-photon andlinear data can provide important information on the anharmonicityparameters, which is otherwise difficult to attain. This is illustratedwith a detailed comparison of the linear and multiple-photon absorptionspectra of the naphthalene cation, yielding experimental anharmonicityparameters for the IR-active modes in the 500-1700 cm-1range.
| The heterogeneous class of lambda Bootis stars
We demonstrate that it is arduous to define the lambda Boo stars as aclass of objects exhibiting uniform abundance peculiarities which wouldbe generated by a mechanism altering the structure of their atmosphericlayers. We collected the stars classified as lambda Boo up to now anddiscuss their properties, in particular the important percentage ofconfirmed binaries producing composite spectra (including our adaptiveoptics observations) and of misclassified objects. The unexplained RVvariables (and thus suspected binaries), the known SB for which we lackinformation on the companion, the stars with an UV flux inconsistentwith their classification, and the fast rotating stars for which noaccurate abundance analysis can be performed, are also reviewed.Partly based on observations collected at the CFH Telescope (Hawaii) andat TBL of the Pic du Midi Observatory (France).Table \ref{tab5} is only available in electronic form athttp://www.edpsciences.org
| 13C isotope effects on infrared bands of quenched carbonaceous composite (QCC)
We investigate carbon isotope effects on the infrared bands of alaboratory analogue of carbonaceous dust, the quenched carbonaceouscomposite (QCC), synthesized from a plasma gas of methane with various12C/13C ratios. Peak shifts to longer wavelengthsdue to the substitution of 12C by 13C are clearlyobserved in several absorption bands. The shifts are almost linearlyproportional to the 13C fraction. New features associatedwith 13C are not seen, indicating that the infrared bands inthe QCC are not very localized vibration modes but come from vibrationsassociated with rather large carbon structures. An appreciable peakshift (Delta lambda ~ 0.23-0.26 mu m per 13C fraction) isdetected in the 6.2 mu m band, which is attributed to a carbon-carbonvibration. A peak shift (Delta lambda ~ 0.16-0.18 mu m per13C fraction) in an out-of-plane bending mode of aromatic C-Hat 11.4 mu m is also observed, while only a small shift (Delta lambda< 0.015 mu m per 13C fraction) is detected in the 3.3 mu mband, which arises from a C-H stretching mode. The present experimentsuggests that peak shifts in the unidentified infrared (UIR) bands,particularly in the 6.2 mu m band, should be detectable in celestialobjects with low 12C/13C ratios (<10). Theisotopic shifts seen in the QCC are discussed in relation to thevariations in the UIR band peaks observed in post-asymptotic giantbranch stars and planetary nebulae. The observed peak shift pattern ofthe UIR bands is qualitatively in agreement with the isotopic shifts inthe QCC except for the 7.7 mu m band complex although the observedshifts in the UIR bands are larger than those inferred from derivedisotope ratios for individual objects. The poor quantitative agreementmay be attributed partly to large uncertainties in the derived12C/13C, to possible spatial variations of theisotope abundance within the object, and to combinations of othereffects, such as hetero-atom substitutions. The present investigationsuggests that part of the observed variations in the UIR band peaks maycome from the isotopic effects.
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