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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.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| A non-LTE abundance analysis of the post-AGB star ROA5701
An analysis of high-resolution Anglo-Australian Telescope(AAT)/University College London Échelle Spectrograph(UCLÉS) optical spectra for the ultraviolet (UV)-bright starROA5701 in the globular cluster ω Cen (NGC5139) is performed,using non-local thermodynamic equilibrium (non-LTE) model atmospheres toestimate stellar atmospheric parameters and chemical composition.Abundances are derived for C, N, O, Mg, Si and S, and compared withthose found previously by Moehler et al. We find a general metalunderabundance relative to young B-type stars, consistent with theaverage metallicity of the cluster. Our results indicate that ROA5701has not undergone a gas-dust separation scenario as previouslysuggested. However, its abundance pattern does imply that ROA5701 hasevolved off the asymptotic giant branch (AGB) prior to the onset of thethird dredge-up.
| The 78th Name-List of Variable Stars
We present the next regular Name-List of variable stars containinginformation on 1706 variable stars recently designated in the system ofthe General Catalogue of Variable Stars.
| Optical Spectroscopy of the Post-AGB Carbon Star Cgcs 6857 = IRAS 20000+3239
Using the echelle spectrograph of the 6 m telescope, we obtainedmoderate resolution spectra of the post-AGB star CGCS 6857 associatedwith an IR-source IRAS 20000 +3239 having an IR-feature at 21 μ m inemission. In the optical spectrum we revealed bands of C2 andCN molecules, but no C3. The model atmospheric parametersadopted are: effective temperature T eff=5000 ± 200 K,logarithmic gravity log g=0.0 ± 0.3 and microturbulent velocityξ t=9.0±1.0 km s-1. We obtained lowmetallicity [Fe/H]=-1.4, a C/O abundance ratio close to 1 together withlarge overabundances of the s-process elements Y, Zr, Ba, La, Ce, Pr andSm, with an average value [El/Fe] = +1.4. The heliocentric radialvelocity from metallic lines is equal to v rad(met) =-15± 2 km s-1.
| Study of the IR-excess supergiant HD 331319
We report the discovery of brightness variability in the IR-excess earlyF supergiant HD 331319, a candidate post-AGB star. Over three years ofsystematic U BV observations, the star showed low-amplitude (up to in V)quasi-periodic brightness variations on a time scale of 45 days. Apreliminary analysis of our photometry indicates that HD 331319 andother typical post-AGBF supergiants have a similar pattern ofvariability. A study of the extinction toward HD 331319 leads us toconclude that the fraction of the circumstellar extinction is small forthis star. We present low-resolution spectroscopy for HD 331319 anddiscuss the spectral classification of post-AGB F supergiants using HD331319, HD 161796, HD 187885, and HD 56126 as examples.
| 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.
| Interferometric CO J = 2-1 Emission Mapping of the Protoplanetary Nebula IRAS 19475+3119
We present ~2'' resolution interferometric maps of the12CO J=2-1 emission in the PPN IRAS 19475+3119 obtained withOVRO. These data probe two distinct molecular components, namely, aslowly expanding shell and a fast bipolar outflow.We have used aspatiokinematic model of the 12CO J=2-1 emission to constrainthe properties of these two components. The shell has inner and outerradii of Rin~6.5×1016 cm andRout~2×1017 cm and expands atVexp~11 km s-1. The 12CO J=2-1 linewing emission arises in a bipolar structure that emerges from twodiametrically opposite holes in the slow shell. The bipolar outflow isaligned with one of the two lobe pairs of the quadrupolar optical nebula(at P.A.~80deg). Both the holes and the bipolar outflow aremost likely the result of the interaction of fast, collimated post-AGBwinds with the shell. The quadrupolar morphology of the optical nebulaindicates two distinct bipolar post-AGB winds ejected in two differentdirections. The elongation of the optical counterpart of the shell (atP.A.~-45deg) and two similarly aligned CO clumps suggest thatthe slow shell has also been affected by the wind interaction. Theexpansion velocity in the bipolar outflow increases linearly with thedistance from the nebula center and reaches Vexp=30 kms-1 (projected) at the tips of the lobes. This velocitygradient yields a relatively long kinematical age of ~1900 yr, assumingan outflow inclination of i=30deg with respect to the planeof the sky; this age is comparable with the post-AGB lifetime estimatedfrom the shell expansion velocity and inner radius. We derive a meankinetic temperature of ~14 K and a total mass of ~0.4 Msolar.The collimation and linear momentum (P~4×1038 g cms-1) of the outflow are unlikely to result from radiationpressure on dust grains.
| 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.
| Subarcsecond Mid-Infrared Imaging of Dust in the Bipolar Nebula Hen 3-401
We present high-resolution, nearly diffraction-limited narrow- andbroadband mid-IR images of bipolar reflection nebula Hen 3-401. Thedeconvolved images yield a pixel-limited spatial resolution of 0.09",demonstrating the superior imaging quality of the Gemini telescope. Theinfrared image of Hen 3-401 consists of a prominent core region of size1" and extended emissions along the walls of the bipolar lobes. We findthat the distribution of aromatic infrared band (AIB) emission isdifferent from that of the continuum emission and that the star hasundergone multiple ejection of the AIB emission feature carrier at theend stages of its evolution. From the observed temperature gradient inthe core, we suggest that the core has a flared disk geometry.This paper is based on observations obtained at the Gemini Observatory,which is operated by the Association of Universities for Research inAstronomy, Inc., under a cooperative agreement with the NSF on behalf ofthe Gemini partnership: the National Science Foundation (US), theParticle Physics and Astronomy Research Council (UK), the NationalResearch Council (Canada), CONICYT (Chile), the Australian ResearchCouncil (Australia), CNPq (Brazil), and CONICET (Argentina).
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| 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.
| Infrared photometry and evolution of mass-losing AGB stars. I. Carbon stars revisited
As part of a reanalysis of galactic Asymptotic Giant Branch (AGB) starsat infrared (IR) wavelengths, we discuss a sample (357) of carbon starsfor which mass loss rates, near-IR photometry and distance estimatesexist. For 252 sources we collected mid-IR fluxes from the MSX (6C) andthe ISO-SWS catalogues. Most stars have spectral energy distributions upto 21 μm, and some (1/3) up to 45 μm. This wide wavelengthcoverage allows us to obtain reliable bolometric magnitudes. Theproperties of our sample are discussed with emphasis on ~70 stars withastrometric distances. We show that mid-IR fluxes are crucial toestimate the magnitude of stars with dusty envelopes. We construct HRdiagrams and show that the luminosities agree fairly well with modelpredictions based on the Schwarzschild's criterion, contrary to what iswidely argued in the literature. A problem with the brightness of Cstars does not appear to exist. From the relative number of Mira andSemiregular C-variables, we argue that the switch between these classesis unlikely to be connected to thermal pulses. The relevance of the twopopulations varies with the evolution, with Miras dominating the finalstages. We also analyze mass loss rates, which increase for increasingluminosity, but with a spread that probably results from a dependence ona number of parameters (like e.g. different stellar masses and differentmechanisms powering stellar winds). Instead, mass loss rates are wellmonitored by IR colours, especially if extended to 20 μm and beyond,where AGB envelopes behave like black bodies. From these colours theevolutionary status of various classes of C stars is discussed.
| Is the 21 Micron Feature Observed in Some Post-AGB Stars Caused by the Interaction between Ti Atoms and Fullerenes?
Recent measurements of fullerenes and Ti atoms recorded in ourlaboratory have demonstrated the presence of an infrared feature near 21μm. The feature observed has nearly the same shape and position as isobserved for one of the most enigmatic features in post-asymptotic giantbranch (AGB) stars. In our experimental system, large-cage carbonparticles, such as large fullerenes, were produced from CO gas by theBoudouard reaction. Large-cage carbon particles intermixed with Ti atomswere produced by the evaporation of a Ti-metal-wrapped carbon electrodein CO gas. The infrared spectra of large fullerenes interacting with Tiatoms show a characteristic feature at 20.3 μm that closelycorresponds to the 20.1 μm feature observed in post-AGB stars. Boththe laboratory and stellar spectra also show a small but significantpeak at 19.0 μm, which is attributed to fullerenes. Here we proposethat the interaction between fullerenes and Ti atoms may be a plausibleexplanation for the 21 μm feature seen in some post-AGB stars.
| On Silicon Carbide Grains as the Carrier of the 21 μm Emission Feature in Post-Asymptotic Giant Branch Stars
The mysterious 21 μm emission feature seen in 12 proto-planetarynebulae remains unidentified since its first detection in 1989. Over adozen candidate materials have been proposed within the past decade, butnone of them have received general acceptance. Very recently, siliconcarbide (SiC) grains with impurities were suggested to be the carrier ofthis enigmatic feature, based on recent laboratory data that doped SiCgrains exhibit a resonance at ~21 μm. This proposal gains strengthfrom the fact that SiC is a common dust species in carbon-richcircumstellar envelopes. However, SiC dust has a strong vibrational bandat ~11.3 μm. We show in this Letter that in order to be consistentwith the observed flux ratios of the 11.3 μm feature to the 21 μmfeature, the band strength of the 21 μm resonance has to be verystrong, too strong to be consistent with current laboratorymeasurements. But this does not yet readily rule out the SiC hypothesissince recent experimental results have demonstrated that the 21 μmresonance of doped SiC becomes stronger as the C impurity increases.Further laboratory measurements of SiC dust with high fractions of Cimpurity are urgently needed to test the hypothesis of SiC as thecarrier of the 21 μm feature.
| 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.
| CO J = 2-1 and 4-3 Observations of Proto-planetary Nebulae: Time-variable Mass Loss
Observations made with the Heinrich Hertz Telescope of CO millimeter andsubmillimeter emission toward a sample of 22 proto-planetary nebula(PPN) candidates resulted in detections of 12 sources in the CO J=2-1line. Of these 12, seven sources were also detected in the J=4-3 line.These 4-3 transitions are the highest yet observed in all but one ofthese PPNs. Statistical equilibrium/radiative transfer models werecalculated for the CO emission in the circumstellar envelopes (CSEs),assuming various power-law density distributions. These models werecompared with the intensity and profile shape of the observed spectra.For the region of the CSE probed by CO emission, the density laws mustbe steeper than inverse squared and are consistent with power lawsbetween ρ~r-3 and r-4. These radial densitydistributions imply that the mass loss was not constant but increasedduring the last part of the asymptotic giant branch (AGB) phase.Mass-loss rates at the end of the AGB for the three best-constrainedsources are found to be 7.7×10-5 Msolaryr-1 (IRAS 22272+5435), 2.3×10-5Msolar yr-1 (IRAS 07134+1005), and1.3×10-5 Msolar yr-1 (IRAS17436+5003) for the case of ρ~r-3. These time-varyingmass-loss rates can be integrated to calculate the enclosed envelopemasses ejected in the past ~10,000 yr. The ejected envelope masses closeto the star lie in the range 0.02-0.30 Msolar these valuesare consistent with theoretical models, which indicate that <20% ofthe stellar mass loss occurs in the last 10,000 years of the AGB. Theseresults are in contrast to some recent dust studies based on infraredemission, however, in which much higher envelope masses are determined.The density laws, mass-loss rates, and enclosed envelope masses that wederive furnish important constraints for evolutionary models of stars inthe late AGB and during the transition to the planetary nebula phase.
| A Possible Hidden Population of Spherical Planetary Nebulae
We argue that relative to nonspherical planetary nebulae (PNs),spherical PNs are about an order of magnitude less likely to be detectedat distances of several kiloparsecs. Noting the structure similarity ofhalos around nonspherical PNs to that of observed spherical PNs, weassume that most unobserved spherical PNs are also similar in structureto the spherical halos around nonspherical PNs. The fraction ofnonspherical PNs with detected spherical halos around them, taken from arecent study, leads us to the claim of a large (relative to that ofnonspherical PNs), hidden population of spherical PNs in the visibleband. Building a toy model for the luminosity evolution of PNs, we showthat the claimed detection fraction of spherical PNs based on halosaround nonspherical PNs is compatible with observational sensitivities.We use this result to update earlier studies on the different PN-shapingroutes in the binary model. We estimate that ~30% of all PNs arespherical, namely, that their progenitors did not interact with anybinary companion. This fraction is to be compared with the ~3% fractionof observed spherical PNs among all observed PNs. From all PNs, ~15% owetheir moderate elliptical shape to the interaction of their progenitorswith planets, while ~55% of all PNs owe their elliptical or bipolarshapes to the interaction of their progenitors with stellar companions.
| 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.
| Hubble Space Telescope NICMOS Imaging Polarimetry of Proto-Planetary Nebulae: Probing the Dust Shell Structure via Polarized Light
Using NICMOS on the Hubble Space Telescope, we have performed imagingpolarimetry of proto-planetary nebulae (PPNs). Our objective is to studythe structure of optically thin circumstellar shells of post-asymptoticgiant branch (AGB) stars by separating dust-scattered, linearlypolarized starlight from unpolarized direct starlight. This uniquetechnique allows us to probe faint reflection nebulae around the brightcentral star, which can be buried under the point-spread function of thecentral star in conventional imaging. Our observations and archivalsearch have yielded polarimetric images for five sources: IRAS07134+1005 (HD 56126), 06530-0213, 04296+3429, (Z)02229+6208, and16594-4656. These images have revealed the circumstellar dustdistribution in unprecedented detail via polarized intensity maps,providing a basis for understanding the three-dimensional structure ofthese dust shells. We have observationally confirmed the presence of theinner cavity caused by the cessation of AGB mass loss, and the internalshell structures, which are strongly tied to the progenitor star'smass-loss history on the AGB. We have also found that equatorialenhancement in these circumstellar shells comes with various degrees ofcontrast, suggesting a range of optical depths in these optically thinshells. Our data support the interpretation that the dichotomy of PPNmorphologies is due primarily to differences in optical depth andsecondarily to the inclination effect. The polarization maps reveal arange of inclination angles for these optically thin reflection nebulae,dispelling the notion that elliptical nebulae are pole-on bipolarnebulae.
| 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.
| The evolution of planetary nebulae. II. Circumstellar environment and expansion properties
We investigate and discuss the expansion properties of planetary nebulaeby means of 1D radiation-hydrodynamics models computed for differentinitial envelope configurations and central star evolutionary tracks. Inparticular, we study how the expansion depends on the initial densitygradient of the circumstellar envelope and show that it is possible toderive information on the very last mass-loss episodes during the star'sfinal evolution along and off the asymptotic giant branch. To facilitatethe comparison of the models with real objects, we have also computedobservable quantities like surface brightness and emission-lineprofiles. With the help of newly acquired high-resolution emission-lineprofiles for a sample of planetary nebulae we show that models withinitial envelopes based on the assumption of a stationary wind outflowfail to explain the observed expansion speeds of virtually all of theobserved planetary nebulae. Instead it must be assumed that during thevery last phase of evolution along the final asymptotic giant branchevolution the mass-loss rate increases in strength, resulting in a muchsteeper slope of the circumstellar radial density distribution. Underthese conditions, the expansion properties of the nebular gas differconsiderably from the self-similar solutions found for isothermalconditions. Furthermore, the mass loss must remain at a rather highlevel until the stellar remnant begins to evolve quickly towards thecentral star regime. Current theoretical computations of dust-drivenmass-loss which are restricted to rather low temperatures cannot beapplied during the star's departure from the asymptotic giant branch.Based on observations obtained at the 3.5 μm NTT and the 1.2 μmCAT telescope of the European Southern Observatory, La Silla, and at the2.6 μm NOT telescope operated on the island of La Palma by NOTSA, inthe Spanish Observatorio del Roque de Los Muchachos of the InstitutodeAstrofísica de Canarias.Dedicated to Prof. V. Weidemann on the occasion of his 80th birthday,October 3, 2004.
| Molecules in bipolar proto-planetary nebulae
Two bipolar proto-planetary nebulae, IRAS 16594-4656and IRAS 17150-3224, have been detected in variousmolecular lines, namely CO, 13CO, HCN and CN, and remainundetected in several other species. CO(J=2-1) and CO(J=3-2) lineprofiles are compared to new spectra of similar PPN candidates,previously undetected in CO(J=2-1): CPD-53°5736,IRAS 17106-3046, IRAS 17245-3951and IRAS 17441-2411. CO(J=2-1) maps of IRAS16594-4656 and IRAS 17150-3224 show thatboth PPNe have little separation between blue, centre and red-shiftedemission, and also that the CO(J=2-1) emission is of a similar size tothe telescope beam. Fractional abundances of all detected molecules(except CO) are calculated using the results of CO line modelling and asimple photodissociation model. For those species not detected, upperlimits are derived. Comparisons between these fractional abundances andthose of other PPNe show that IRAS 16594-4656 andIRAS 17150-3224 are quite under-abundant whencompared to molecule-rich sources like CRL618,CRL2688 and OH231.8+4.2. As areason for this deficit, the difference in circumstellar envelope/torusdensity between the molecule-rich sources and the molecule-poor sourcesis proposed, and supported by a chemical model which follows thetransition of a circumstellar envelope through the AGB phase and intothe PPN phase of evolution. The model includes the effects of UVradiation, cosmic rays and also X-rays. Finally, the post-AGB ages ofthese two objects (200-400 yr) are estimated using CN/HCN and HCN/COratios and both ages are found to be in agreement with previous figurescited in the literature, IRAS 17150-3224 being theyounger of the two PPNe.
| Post-AGB s-processed stars and theoretical interpretation
Post-AGB stars show the s-process signature and the carbon enhancementtypical of their parent AGB stars at the end of the TP-AGB phase. Theless luminous protoplanetary nebulae not showing C nor s-enhancement areinterpreted as descendants of AGB stars of M < 1.2 Mȯ, whose envelope mass was too low for the third dredge up to occur.
| Reconsidering the Origin of the 21 Micron Feature: Oxides in Carbon-rich Protoplanetary Nebulae?
The origin of the so-called 21 μm feature that is especiallyprominent in the spectra of some carbon-rich protoplanetary nebulae(PPNs) is the matter of a lively debate. A large number of potentialband carriers have been presented and discarded within the past decade.This paper gives an overview of the problems related to the hithertoproposed feature identifications, including the recently suggestedcandidate carrier silicon carbide. We also discuss the case forspectroscopically promising oxides. SiC is shown to produce a strongresonance band at 20-21 μm if coated by a layer of silicon dioxide.At low temperatures, core-mantle particles composed of SiC and amorphousSiO2 indeed have their strongest spectral signature at aposition of 20.1 μm, which coincides with the position of the 21μm emission band. The optical constants of another candidate carrierthat has been relatively neglected so far-iron monoxide-are proven topermit a fairly accurate reproduction of the 21 μm feature profile aswell, especially when low-temperature measurements of the infraredproperties of FeO are taken into account. As candidate carrier of the 21μm emission band, FeO has the advantage of being stable againstfurther oxidation and reduction only in a narrow range of chemical andphysical conditions, coinciding with the fact that this feature, too, isdetected in a small group of objects only. However, it is unclear howFeO would form or survive, particularly in carbon-rich PPNs.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.
| The Molecular and Dust Envelope of HD 56126
We present millimeter interferometry images of the CO J=1-0 lineemission arising in the circumstellar envelope of HD 56126 (=IRAS07134+1005), which is one of the best-studied 21 μm proto-planetarynebulae (PPNs). The CO emission extends from 1.2" to 7" in radius fromthe central star and appears consistent with a simple expandingenvelope, as expected for a post-AGB star. The CO envelope is veryclumpy with no apparent fast wind to explain these microstructures thatmust have arisen during the AGB mass loss. We quantitatively model themolecular envelope using a radiative transfer code that we have modifiedfor detached shells. Our best-fit model reveals that two sequentialwinds created the circumstellar envelope of HD 56126: an AGB wind thatlasted 6500 yr with a mass-loss rate of 5.1×10-6Msolar yr-1 and a more intense superwind thatlasted 840 yr with a mass-loss rate of 3×10-5Msolar yr-1 and that ended the star's life on theAGB 1240 yr ago. The total mass of this envelope is 0.059Msolar, which indicates a lower limit progenitor mass for thesystem of 0.66 Msolar, quite reasonable for thislow-metallicity star that probably resides in the thick disk of theGalaxy. Comparison with images of the dust emission reveals a structuresimilar to that of the gas in the inner regions. Using 2-D UST, we modelthe dust emission of this source so that the model is consistent withthe CO emission model and find a total dust mass of7.8×10-4 Msolar, a superwind-dust mass-lossrate of 1.9×10-7 Msolar yr-1 andan AGB-dust mass-loss rate of 9.6×10-8Msolar yr-1. We derive an average gas-to-dust massratio of 75, which is consistent with ISM values but low for what mostconsider for carbon stars. Our results indicate that TiC nanocrystalsare probably not the carrier of the 21 μm feature.
| Processing of Presolar Grains around Post-Asymptotic Giant Branch Stars: Silicon Carbide as the Carrier of the 21 Micron Feature
Some proto-planetary nebulae (PPNs) exhibit an enigmatic feature intheir infrared spectra at ~21 μm. This feature is not seen in thespectra of either the precursors to PPNs, the asymptotic giant branch(AGB) stars, or the successors of PPNs, ``normal'' planetary nebulae(PNs). However, the 21 μm feature has been seen in the spectra of PNswith Wolf-Rayet central stars. Therefore, the carrier of this feature isunlikely to be a transient species that only exists in the PPN phase.This feature has been attributed to various molecular and solid-statespecies, none of which satisfy all constraints, although titaniumcarbide (TiC) and polycyclic aromatic hydrocarbons (PAHs) have seemedthe most viable. We present new laboratory data for silicon carbide(SiC) and show that it has a spectral feature that is a good candidatefor the carrier of the 21 μm feature. The SiC spectral featureappears at approximately the same wavelength (depending on thepolytype/grain size) and has the same asymmetric profile as the observedastronomical feature. We suggest that processing and cooling of the SiCgrains known to exist around carbon-rich AGB stars are responsible forthe emergence of the enigmatic 21 μm feature. The emergence of thisfeature in the spectra of post-AGB stars demonstrates the processing ofdust due to the changing physical environments around evolving stars.
| CO J=2-1 and 4-3 Observations of Proto-Planetary Nebulae: Time-Variable Mass Loss
Observations made with the Heinrich Hertz Telescope of CO millimeter andsubmillimeter emission toward a sample of 22 proto-planetary nebulae(PPNe) candidates resulted in detections of 12 sources in the CO J=2-1line. Of these 12, 7 sources were also detected in the J=4-3 line. These4-3 transitions are the highest yet observed in all but one of thesePPNe. Statistical equilibrium/radiative transfer models were calculatedfor the CO emission in the circumstellar envelopes (CSEs), assumingvarious power-law density distributions. These models were compared withthe intensity and profile shape of the observed spectra. For the regionof the CSE probed by CO emission, the density laws must be steeper thaninverse-squared, and are consistent with power laws between ρ ∝r-3 and ρ ∝ r-4. These radial densitydistributions imply that the mass loss was not constant but increasedduring the last part of the AGB phase. Mass loss rates at the end of theAGB for the three best-constrained sources are found to be 7.7 x10-5 Mȯ yr-1 (IRAS 22272+5435),2.3 x 10-5 Mȯ yr-1 (IRAS07134+1005), and 1.3 x 10-5 Mȯ yr-1 (IRAS 17436+5003), for the case of ρ ∝r-3. These time-varying mass-loss rates can be integrated tocalculate the enclosed envelope masses ejected in the past 10,000yr. The ejected envelope masses close to the star lie in the range0.02-0.30 Mȯ ; these values are consistent withtheoretical models which indicate that <20% of the stellar mass lossoccurs in the last 10,000 years of the AGB. These results are incontrast to some recent dust studies based on infrared emission,however, in which much higher envelope masses are determined. Thedensity laws, mass loss rates, and enclosed envelope masses which wederive furnish important constraints for evolutionary models of stars inthe late AGB and during the transition to the planetary nebula phase.The research was funded by grants from the NSF.
| A study of the s-process in the carbon-rich post-AGB stars IRAS 06530-0213 and IRAS 08143-4406 on the basis of VLT-UVES spectra
In an effort to extend the still limited sample of s-process enrichedpost-AGB stars, high-resolution, high signal-to-noise VLT+UVES spectraof the optical counterparts of the infrared sources IRAS06530-0213 and IRAS 08143-4406 wereanalysed. The objects are moderately metal deficient by [Fe/H]=-0.5 and-0.4 respectively, carbon-rich and, above all, heavily s-processenhanced with a [ls/Fe] of 1.8 and 1.5 respectively. Especially thespectrum of IRAS 06530-0213 is dominated bytransitions of s-process species, and therefore resembling the spectrumof IRAS 05341+0852, the most s-process enrichedobject known so far. The two objects are chemically very similar to the21 μm objects discussed in \citet{vanwinckel00}. A homogeneouscomparison with the results of these objects reveals that the relationbetween the third dredge-up efficiency and the neutron nucleosynthesisefficiency found for the 21 μm objects, is further strengthened. Onthe other hand, a detailed comparison with the predictions of the latestAGB models indicates that the observed spread in nucleosynthesisefficiency is certainly intrinsic, and proves that different13C pockets are needed for stars with comparable mass andmetallicity to explain their abundances.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO Programme 66.D-0171).Table \ref{tab:equivalentwdths} is only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/417/269
| MgS in detached shells around carbon stars. Mining the mass-loss history
We investigate the dust composition of detached shells around carbonstars, with a focus to understand the origin of the coolmagnesium-sulfide (MgS) material around warm carbon stars, which hasbeen detected around several of these objects\citep{2002A&A...390..533H}. We build a radiative transfer model ofa carbon star surrounded by an expanding detached shell of dust. Theshell contains amorphous carbon grains and MgS grains. We find that asmall fraction of MgS dust (2% of the dust mass) can give a significantcontribution to the IRAS 25 μm flux. However, the presence of MgS inthe detached shell cannot be inferred from the IRAS broadband photometryalone but requires infrared spectroscopy.We apply the model to the detached-shell sources R Scl and U Cam, bothexhibiting a cool MgS feature in their ISO/SWS spectra. We use the shellparameters derived for the molecular shell, using the CO submillimetremaps \citep{1999A&A...351L...1L,2001A&A...368..969S}. Themodels, with MgS grains located in the detached shell, explain the MgSgrain temperature, as derived from their ISO spectra, very well. Thisdemonstrates that the MgS grains are located at the distance of thedetached shell, which is a direct indication that these shells originatefrom a time when the stellar photosphere was already carbon-rich. In thecase of R Scl, the IRAS photometry is simultaneously explained by thesingle shell model. In the case of U Cam, the IRAS photometry is underpredicted, pointing to a contribution from cooler dust located evenfarther away from the star than the molecular shell.We present a simple diagnostic to constrain the distance of the shellusing the profile of the MgS emission feature. The emission featureshifts to longer wavelength with decreasing grain temperature. One cantherefore infer a temperature and a corresponding distance to the starfrom the observed profile. Such a diagnostic might prove useful forfuture studies of such systems with SIRTF or SOFIA.based on observations obtained with ISO, an ESA project with instrumentsfunded by ESA member states (especially the PI countries: France,Germany, the Netherlands and UK) with the participation of ISAS andNASA.}
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| Сазвежђа: | Мали пас |
| Ректацензија: | 07h16m10.26s |
| Deклинација: | +09°59'47.9" |
| Apparent магнитуда: | 8.331 |
| Proper motion RA: | 0.9 |
| Proper motion Dec: | -0.9 |
| B-T magnitude: | 9.419 |
| V-T magnitude: | 8.421 |
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