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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.
| Near-infrared polarimetry and modelling of the dusty young planetary nebula IRAS 19306+1407
We present near-infrared polarimetric images of the dusty circumstellarenvelope (CSE) of IRAS 19306+1407, acquired at the United KingdomInfrared Telescope (UKIRT) using the UKIRT 1-5 μm Imager Spectrometer(UIST) in conjunction with the half-waveplate module IRPOL2. We presentadditional 450- and 850-μm photometry data obtained with theSubmillimetre Common-User Bolometer Array (SCUBA) at the James ClerkMaxwell Telescope (JCMT), as well as archived Hubble Space Telescope(HST) F606W- and F814W-filter images. The CSE structure in polarizedflux at J and K bands shows an elongation north of north-east and southof south-west with two bright scattering shoulders north-west andsouth-east. These features are not perpendicular to each other and couldsignify a recent `twist' in the outflow axis. We model the CSE using anaxisymmetric light scattering (ALS) code to investigate the polarizationproduced by the CSE, and an axisymmetric radiation transport (DART) codeto fit the spectral energy distribution. A good fit was achieved withthe ALS and DART models using silicate grains, 0.1-0.4 μm with apower-law size distribution of a-3.5, and an axisymmetricshell geometry with an equator-to-pole ratio of 7:1. The spectral typeof the central star is determined to be B1I supporting previoussuggestions that the object is an early planetary nebula. We haveconstrained the CSE and interstellar extinction as 2.0 and 4.2 mag,respectively, and have estimated a distance of 2.7 kpc. At thisdistance, the stellar luminosity is ~4500Lsolar and the massof the CSE is ~0.2Msolar. We also determine that the massloss lasted for ~5300 yr with a mass-loss rate of ~3.4 ×10-5Msolaryr-1.
| 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.
| Shaping Bipolar Planetary Nebulae: How Mass Loss Leads to Waistline Development
Asymptotic giant branch (AGB) stars generally have spherically symmetricenvelopes, whereas most post-AGB stars and planetary nebulae (PNe) showaxisymmetric circumstellar envelopes. While various mechanisms foraxisymmetric circumstellar structures may explain the shapes of PNe,they do not address how the shape of the circumstellar shell evolves.Here we address the temporal changes in the axisymmetry of AGB starenvelopes, and in particular the development of the torus required inthe generalized interacting stellar winds (GISW) model. Assuming that(1) an AGB star rotates with sufficient angular speed at the start ofthe AGB phase, and (2) the rotational angular momentum of the AGB staris conserved, we demonstrate that some very important observationalfeatures of AGB star axisymmetry evolution can be reproduced. We findthat, compared to the star's increasing luminosity and decreasingeffective temperature, the decreasing mass of the star primarily affectsthe axisymmetry of the envelope. When a representative mass-loss historyis adopted in which most of the mass is lost near the end of the AGBphase, the envelope's axisymmetry increases over time, with thestrongest increase occurring near the end of the AGB phase. This maynaturally explain why most AGB stars have spherically symmetricenvelopes, while axisymmetry seems commonplace in the post-AGB/PNephase. The degree of axisymmetry at the end of the AGB phase is found toincrease with increasing main-sequence mass, and the onset ofaxisymmetry occurs only after the onset of the superwind phase, in goodagreement with the observations.
| The Dust Envelope of the Preplanetary Nebula IRAS 19475+3119
We present the spectral energy distribution (SED) of the preplanetarynebula, IRAS 19475+3119 (hereafter I 19475), from the optical to thefar-infrared. We identify emission features due to crystalline silicatesin the ISO SWS spectra of the star. We have fitted the SED of I 19475using a one-dimensional radiative transfer code and find that a shellwith inner and outer radii of 8.8×1016 and4.4×1017 cm and dust temperatures ranging from about 94to 46 K provides the best fit. The mass of this shell is >~1[34cm2 g-1/κ(100 μm)](δ/200)Msolar, where κ(100 μm) is the 100 μm dust massabsorption coefficient (per unit dust mass) and δ is thegas-to-dust ratio. In agreement with results from optical imaging andmillimeter-wave observations of CO emission of I 19475, our model fitssupport an r-3 density law for its dust shell, with importantimplications for the interaction process between the fast collimatedpost-AGB winds and the dense AGB envelopes that results in the observedshapes of PPNs and PNs. We find that the observed JCMT flux atsubmillimeter wavelengths (850 μm) is a factor ~2 larger than ourmodel flux, suggesting the presence of large dust grains in the dustshell of I 19475 that are not accounted for by our adopted standard MRNgrain size distribution.
| 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.
| 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.
| 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.
| 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.
| Water ice growth around evolved stars. II. Modeling infrared spectra
We present combined radiative transfer and H2O ice formationcalculations for the dusty envelopes of oxygen-rich evolved stars. Westudy the effects of various (circum-)stellar parameters on the spectralenergy distribution of these stars, their infrared spectral water icefeatures at 3 μm and in the 30-100 μm region, and the propertiesof (water ice on) the grains in their envelopes. We also study the iceformation process as a function of stellar evolution for a star with aninitial mass of 5 Mȯ, which is followed during the AGB,post-AGB and planetary nebula (PN) phase. We find that its water icefeatures probe its evolution. Both crystalline and amorphous water iceform in our models. The 43 and 62 μm crystalline water ice featuresare most prominent during the post-AGB phase, and only modestly or notpresent during the AGB and PN phase, in agreement with observations. Thestrength of the 3, 43 and 62 μm water ice features decreases withdecreasing initial mass of the star. The total amount of ice predicted(a few percent of the total dust mass) also agrees with observations,but the crystalline ice mass fraction is consistently about two ordersof magnitude lower. This is mainly due to efficient amorphization byinterstellar UV photons, and leads to weaker 43 and 62 μm crystallinewater ice features than observed. The intensity of the interstellar UVradiation field strongly influences the strength of these features. Wediscuss several means to increase the crystalline water ice mass, andhence their strength. The strength of the features increasesdramatically when the mass-loss rate over luminosity ratio of the star,dot{M}/L, is large in the AGB phase. In case of the post-AGB star HD161796 we demonstrate that this indeed leads to the correct crystallineice mass fraction and feature strengths. Also, the formation of clumpsin the AGB wind provides high densities to stimulate the formation of(crystalline) ice. For stars with high initial masses, it additionallyprovides sufficient shielding from interstellar UV radiation to keep icecrystalline during the post-AGB and PN phase. Axisymmetric mass loss onthe AGB provides favorable conditions for the formation and preservationof water ice, and crystalline water ice in particular, as well. Incontrast we find that post-AGB crystallization of AGB produced amorphousice is unimportant for increasing the crystalline water ice mass around5 Mȯ stars.
| Detection of spectral variability of the optical component of the IR source IRAS 20508+2011
Our high-resolution spectral observations have revealed variability ofthe optical spectrum of the cool star identified with the IR source IRAS20508+2011. We measured the equivalent widths of numerous absorptionlines of neutral atoms and ions at wavelengths 4300 7930 Å, alongwith the corresponding radial velocities. Over the four years of ourobservations, the radial velocity derived from photospheric absorptionlines varied in the interval V rȯ = 15 30 km/s. In the same period,the Hα profile varied from being an intense bell-shaped emissionline with a small amount of core absorption to displaying two-peakedemission with a central absorption feature below the continuum level. Atall but one epoch, the positions of the metallic photospheric lines weresystematically shifted relative to the Hα emission: ΔV r = Vr(met) - V r(Hα, emis) ≈ -23 km/s. The Na D doublet displayed acomplex profile with broad (half-width ≈ 120 km/s) emission andphotospheric absorption, as well as an interstellar component. We usedmodel atmospheres to determine the physical parameters and chemicalcomposition of the star’s atmosphere: T eff = 4800 K, log g = 1.5,ξt = 4.0 km/s. The metallicity of the star differs little from thesolar value: [Fe/H]ȯ = -0.36. We detected overabundances of oxygen[O/Fe]ȯ = +1.79 (with the ratio [C/O] ≈ -0.9), andα-process elements, as well as a deficit of heavy metals. Theentire set of the star’s parameters suggests that the opticalcomponent of IRAS 20508+2011 is an “O-rich” AGB star withluminosity M v ≈ -3m that is close to its evolutionary transition tothe post-AGB stage.
| 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.
| 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.
| 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.
| 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.
| The evolutionary status of the bright high-latitude supergiant HD 190390
Despite its mean apparent magnitude of mV = 6.39, theevolutionary status of HD 190390 (HR7671), a luminous F-type supergiant at high galactic latitude,is still not very clear, but in most papers a post-AGB classification isassumed. New observational material has been obtained with fourdifferent instruments and is presented here. An extensive abundanceanalysis based on high resolution, high signal-to-noise NTT+EMMI spectraconfirms the metal deficiency of this object ([Fe/H] = -1.6), togetherwith a high lithium content (log ɛ(Li) = 1.9). A variabilityanalysis based on Geneva photometry over seven years reveals beatingwith a period of ~3000 days. It is, however, not clear whether thisbeating is caused by a stable triplet, or it is the consequence of smallchanges in the main frequency. More recent data obtained with theHIPPARCOS satellite and the Mercator telescope not only confirm the mainperiod, but also support the presence of a second periodicity of 11days, which was also found in the Geneva photometry. A conclusiveevolutionary status of this object is not given, but alternative to theUU Her (i.e. post-AGB) status, a W Vir classification is discussed.Based on observations collected at the European Southern Observatory, LaSilla, Chile (programme 61.E-0426), and at the Observatorio del Roque delos Muchachos, La Palma, Spain.
| Axisymmetry in protoplanetary nebulae - II. A near-infrared imaging polarimetric survey
In an imaging polarimetry survey of candidate post-AGB stars, scatteringenvelopes are detected around 20 objects. These envelopes represent thefinal mass-loss phases at the end of the AGB. In all cases, evidence foraxisymmetry in the dust density is seen, suggesting that the presence ofan axisymmetric outflow may be a ubiquitous phenomenon of the AGB topost-AGB transition. We use the polarized flux images to classify theobjects into detached shell, bipolar and unresolved targets. Modellingbased on a simple axisymmetric shell geometry supports the contentionthat post-AGB objects fall into one of two classes that differ in theamount of dust in the circumstellar environment: the detached shellscorrespond to stars with an optically thin expanding circumstellarenvelope (CSE) whereas the bipolar and unresolved targets have opticallythick dust structures, probably in the form of discs, which remain boundto the star, rather than partaking in the expansion of the AGB CSE. Itis suggested that this bifurcation in morphology is rooted in thepresence or absence of a binary companion, which determines whether ornot a disc forms. Because the detached shell objects also appearaxisymmetric, an additional mechanism for generating the axisymmetry,such as a magnetically shaped outflow, is needed if they do indeed havesingle star progenitors.
| Hunting post-AGB/RSG objects using Virtual Observatories and other internet-based technology.
After the Asymptotic Giant Branch or Red Supergiant stage of evolution,stars evolve rapidly from cool M-type to F-type and hotter before theirfinal fates. We describe the use of the European Virtual Observatory toderive a set of properties based on published data which identifypost-AGB transition objects. We then apply these selection criteria tofind new candidate transition objects and investigate their images andspectra in more detail. Heterogenous data collections from all over theworld can be tracked down, manipulated and compared using the internet.The internet is itself part of the telescope in a separate development,real-time correlation of E-VLBI observations of hydroxyl masers from thepost-RSG star IRC+10420.
| Mid-infrared imaging and modelling of the dust shell around post-AGB star HD 187885 (IRAS 19500-1709)
We present 10- and 20-μm images of IRAS 19500-1709 taken with themid-infrared camera, OSCIR, mounted on the Gemini North Telescope. Anextended circumstellar envelope is detected, with the N-band imageindicating an elongation in a north-east-south-west direction. We use adust radiation transport code to fit the spectral energy distributionfrom ultraviolet to submillimetre wavelengths, with a detached dustshell model. A good fit is achieved using dust composed of amorphouscarbon, silicon carbide and magnesium sulphide. We derive estimates forthe inner and outer radius, density and mass of the dust in the shell.The inner radius is not resolved in our OSCIR imaging, giving an upperlimit of 0.4 arcsec. With this constraint, we conclude that IRAS19500-1709 must be at least 4 kpc away in order to have the minimumluminosity consistent with a post-asymptotic giant branch status.
| 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.
| Polarimetric observations of OH masers in proto-planetary nebulae
The 1612 and 1667 MHz OH maser lines have been measured in all fourStokes parameters in 47 proto-planetary nebula (PPN) candidates. Out of42 objects detected, 40 and 34 are 1612 and 1667 MHz emitters,respectively. The spectral extent of the 1667 MHz line overshoots thatof the 1612 MHz line in about 80% of the targets. 52% and 26% of the1612 and 1667 MHz sources, respectively, show linear polarization in atleast some features. Circular polarization is more frequent, occurringin 78% and 32% of sources of the respective OH lines. The percentagepolarization is usually small (<15%) reaching up to 50-80% in a fewsources. Features of linearly polarized emission are usually weak (0.5-4Jy) and narrow (0.3-0.5 km s-1). The strength of the magneticfield inferred from likely Zeeman pairs in two sources of a few mG isconsistent with values reported elsewhere for those classes of objects.An upper limit of the electron density in the envelope of OH17.7-2.0derived from the difference in the position angle of polarizationvectors for the two OH lines is about 1 cm-3. Distinctprofiles of polarization position angle at 1612 and 1667 MHz are seen inabout one third of the sources and strongly suggest that the envelopesare permeated by structured magnetic fields. The geometry of themagnetic field is implicated as an important cause of the depolarizationfound in some PPN candidates. For the subset of targets which showaxisymmetric shells in the optical or radio images we found a dominanceof magnetic field components which are orthogonal to the long axis ofthe nebulae. This finding supports the hypothesis that such bipolarlobes are shaped by the magnetic field.Figures in Appendix are only available in electronic form athttp://www.edpsciences.org
| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| The phase of H2O ice and the librational band in OH231.8+4.2: new interpretations
The phase of H2O ice and the librational ice band detectedtoward the post-asymptotic giant branch (post-AGB) star OH231.8+4.2 havebeen examined using radiative transfer modelling. The results indicatethat the ice is largely crystalline and not amorphous as previouslyreported in the literature. The earlier result is shown to be due to theassumption of unrealistically thick ice mantles. It has also been shownthat, in comparison with radiative transfer modelling, Mie theorymodelling of the 3-μm ice band detected towards moderately opticallythick shells leads to an underestimate of the ice mantle thickness. Themodelling results also suggest that the mid-infrared (MIR) featurepreviously identified with the librational band of H2O icecould be the result of amorphous Al2O3. This dustcomponent can also account for the observed enhanced opacity between the10- and 18-μm silicate bands.
| Kinematics of the envelope of the post-AGB star V510 PupNucleus of a future planetary nebula
We have carried out a detailed identification of lines in the opticalspectrum of the post-AGB star V510 Pup associated with the infraredsource IRAS 08005-2356 based on observations with high spectralresolution. Absorption lines of the ions FeII, TiII, CrII, and YII arepresent at wavelengths from 4549 to 8546 Å. The absorption by YIIand other s-process elements is anomalously strong, and the absorptionis also strong in the circumstellar C2 Swan bands. The profiles of mostof the lines (of hydrogen and metals) display P Cygniabsorption-emission profiles. All the absorption lines are shiftedtoward the blue, suggesting an outflow of stellar material. Theexpansion velocity of the envelope derived from the Swan bands arisingthere is V exp =42 km/s. The highest wind velocity determined from theabsorption wings of the FeII(42) P Cygni profiles reaches 240 km/s.Based on the star’s kinematic characteristics and the amount ofinterstellar absorption, it is at a distance of d≈3-4 kpc, whichcorresponds to an absolute magnitude of M v≈-6m.
| 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).
| The nature of Hen 3-1312: A post-AGB star in a binary system
This work reports the low- and high-resolution spectroscopic diagnosticdiagrams, radial velocity, stellar parameters and abundance analysis ofthe planetary nebula Hen 3-1312. The low- and high-resolution spectrareveal that Hen 3-1312 is in fact a very-low-excitation object, in abinary system with a supergiant as a cool central star. The analysis ofthe high-resolution spectrum shows the cool stellar component to have aneffective temperature of Teff=6500±100 K and a surfacegravity of log g=0.8±0.2 corresponding to a spectral type ofF(6-7)I. These parameters result in an estimated primary luminosity of4100 Lȯ, implying a distance of 4400 pc which is inagreement with previous determinations. The abundance analysis revealsHen 3-1312 to be a metal-poor object having [Fe/H]=-1.1. The meanabundances of carbon, nitrogen and oxygen are found to be solar, howeverthe α-elements (Mg, Si and Ca) are underabundant relative to theSun. The abundance profile of Hen 3-1312 is analyzed and compared withother classes of stars with similar atmospheric parameters.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).Table \ref{tab4} is only available in electronic form athttp://www.edpsciences.org
| The disappearance of the 1667-MHz OH maser in IRAS 17436+5003 (HD 161796)
We report the diminution of the 1667-MHz OH maser in the post-asymptoticgiant branch star IRAS 17436+5003, by a factor of >~17 over a periodof <~12 yr, from observations with MERLIN. This circumstellar maserwas detected by Likkel in 1987, at the 13σ level of herobservations with the Green Bank Telescope. We discuss a number ofpossible reasons for this phenomenon and conclude that it is most likelydue to turbulence arising from interacting stellar winds.
| The Dust Ring of Luminous Blue Variable Candidate HD 168625: Infrared Observations and Model Calculations
We present a 2.218 μm image from the Hubble Space Telescope/NearInfrared Camera and Multi-Object Spectrometer (NICMOS) and a 55 μmimage from ISOPHOT of the dust ring surrounding the luminous bluevariable (LBV) candidate HD 168625, together with new temperature andoptical depth maps derived from mid-IR images. The shell is detachedfrom the star in the near-IR, and substructure in the overall toroidalshell is visible. The far-IR image constrains the extent of the dustshell to ~25" in diameter, providing an upper radius limit for modeling.The temperature maps and the NICMOS image show evidence for very smalltransiently heated dust grains in the shell. The opacity maps showhigher optical depth in the limbs, consistent with interpretation of thedust shell as an equatorially enhanced torus inclined ~60° withrespect to the observer. An overall trend in the dust emission locationwith wavelength is observed and interpreted as a variation with respectto location in the nebula of either the dust grain size distribution orgas-to-dust mass ratio. Radiative transfer calculations using 2-DUSTindicate that a mass-loss event occurred ~5700 yr ago with a rate of(1.9+/-0.1)×10-4Msolaryr-1,creating a dust torus that currently has a τV~0.22 in theequatorial plane and a dust mass of(2.5+/-0.1)×10-3Msolar. Using publishedvalues for the gas mass, we find a gas-to-dust mass ratio of 840, whichis ~4 times higher than current estimates for the interstellar medium.In addition to a high equator-to-pole density ratio (~31) torus, anelliptical midshell is needed to reproduce the appearance and spectralenergy distribution of the dust. Therefore, HD 168625 is an excellentexample of proposed models of LBV nebulae in which a stellar windinteracts with a preexisting density contrast and creates a blowout inthe polar direction perpendicular to the equatorial ring. Thecircumstellar shell is much lower in mass than that of LBV ηCarinae, suggesting that HD 168625 had a lower mass progenitor.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555 these observations are associatedwith proposal 7898. Based also on observations with ISO, an ESA projectwith instruments funded by ESA member states (especially the PIcountries: France, Germany, the Netherlands, and the United Kingdom) andwith the participation of ISAS and NASA.
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