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Spectroscopic binaries among Hipparcos M giants. III. The eccentricity - period diagram and mass-transfer signatures
Context: This paper is the third one in a series devoted to studying theproperties of binaries involving M giants. Aims: We use a new set oforbits to construct the first (e - log P) diagram of an extensive sampleof M giant binaries, to obtain their mass-function distribution, and toderive evolutionary constraints for this class of binaries and relatedsystems. Methods: The orbital properties of binaries involving M giantswere analysed and compared with those of related families of binaries (Kgiants, post-AGB stars, barium stars, Tc-poor S stars). Results: Theorbital elements of post-AGB stars and M giants are not very different,which may indicate that, for the considered sample of post-AGB binaries,the post-AGB star left the AGB at quite an early stage (M4 or so).Neither are the orbital elements of post-mass-transfer binaries likebarium stars very different from those of M giants, suggesting that themass transfer did not alter the orbital elements much, contrary tocurrent belief. Finally, we show that binary systems with e < 0.4 logP - 1 (with periods expressed in days) are predominantlypost-mass-transfer systems, because (i) the vast majority of barium andS systems match this condition; and (ii) these systems have companionmasses peaking around 0.6 Mȯ, as expected for whitedwarfs. The latter property has been shown to hold as well foropen-cluster binaries involving K giants, for which a lower bound on thecompanion mass may easily be set.Based on observations carried out at the Swiss telescope installed atthe Observatoire de Haute Provence (OHP, France), and at the 1.93-m OHPtelescope.
| Hubble Space Telescope Snapshot Survey of Post-AGB Objects
The results of a Hubble Space Telescope (HST) snapshot survey ofpost-AGB objects are shown. The aim of the survey is to complementexisting HST images of protoplanetary nebulae and to connect varioustypes of nebulosities with the physical and chemical properties of theircentral stars. Nebulosities are detected in 15 of 33 sources. Images andphotometric and geometric measurements are presented. For sources withnebulosities we see a morphological bifurcation into two groups, DUPLEXand SOLE, as previous studies have found. We find further support forthe previous results, suggesting that this dichotomy is caused by adifference in the optical thickness of the dust shell. The remaining 18sources are classified as stellar post-AGB objects, because ourobservations indicate a lack of nebulosity. We show that some stellarsources may in fact be DUPLEX or SOLE objects based on their infraredcolors. The causes of the differences among the groups are investigated.We discuss some evidence suggesting that high progenitor mass AGB starstend to become DUPLEX post-AGB objects and intermediate progenitor massAGB stars tend to become SOLE post-AGB objects. Most of the stellarsources probably have low-mass progenitors and do not seem to developnebulosities during the post-AGB phase; therefore, they do not becomeplanetary nebulae.
| Preplanetary Nebulae: A Hubble Space Telescope Imaging Survey and a New Morphological Classification System
Using the Hubble Space Telescope (HST), we have carried out a survey ofcandidate preplanetary nebulae (PPNs). We report here our discoveries ofobjects having well-resolved geometric structures, and we use the largesample of PPNs now imaged with HST (including previously studied objectsin this class) to devise a comprehensive morphological classificationsystem for this category of objects. The wide variety of asphericalmorphologies which we have found for PPNs are qualitatively similar tothose found for young planetary nebulae (PNs) in previous surveys. Wealso find prominent halos surrounding the central aspherical shapes inmany of our objects; these are direct signatures of the undisturbedcircumstellar envelopes of the progenitor AGB stars. Although themajority of these have surface brightness distributions consistent witha constant mass-loss rate with a constant expansion velocity, there arealso examples of objects with varying mass-loss rates. As in our surveysof young PNs, we find no round PPNs. The similarities in morphologiesbetween our survey objects and young PNs supports the view that theformer are the progenitors of aspherical PNs. This suggests that theprimary shaping of a PN does not occur during the PN phase via the fastradiative wind of the hot central star, but significantly earlier in itsevolution.
| Galactic distributions and statistics of the HD stars in the michigan spectral catalogue.
Not Available
| An evolutionary catalogue of galactic post-AGB and related objects
Aims.With the ongoing AKARI infrared sky survey, of much greatersensitivity than IRAS, a wealth of post-AGB objects may be discovered.It is thus time to organize our present knowledge of known post-AGBstars in the galaxy with a view to using it to search for new post-AGBobjects among AKARI sources. Methods: We searched the literatureavailable on the NASA Astrophysics Data System up to 1 October 2006, anddefined criteria for classifying sources into three categories: verylikely, possible and disqualified post-AGB objects. The category of verylikely post-AGB objects is made up of several classes. Results: We havecreated an evolutionary, on-line catalogue of Galactic post-AGB objects,to be referred to as the Toruń catalogue of Galactic post-AGB andrelated objects. The present version of the catalogue contains 326 verylikely, 107 possible and 64 disqualified objects. For the very likelypost-AGB objects, the catalogue gives the available optical and infraredphotometry, infrared spectroscopy and spectral types, and links tofinding charts and bibliography.A stable version of the catalogue is available at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/799
| Optical spectrum of the post-AGB Star HD56126 in the wavelength interval 4010-8790 Å Å
The optical spectrum of the post-AGB star HD56126 identified with theinfrared source IRAS07134+1005 is studied in detail using high spectralresolution observations (R = 25000 and 60000) performed with the echellespectrographs of the 6-m telescope. A total of about one and a halfthousand absorptions of neutral atoms and ions, absorption bands ofC2, CN, and CH molecules, and interstellar bands (DIBs) areidentified in the 4012 to 8790 Å Å wavelength interval, andthe depths and radial velocities of these spectral features aremeasured. Differences are revealed between the variations of the radialvelocities measured from spectral features of different excitation. Inaddition to the well-known variability of the H α profile, wefound variations in the profiles of a number of FeII, YII, and BaIIlines. We also produce an atlas of the spectrum of HD56126 and itscomparison star α Per. The full version of the Atlas is availablein electronic form from: http://www.sao.ru/hq/ssl/Atlas/Atlas.html.
| H2O Maser Observations of Candidate Post-AGB Stars and Discovery of Three High-Velocity Water Sources
We present the results of 22 GHz H2O maser observations of asample of 85 post-asymptotic giant branch (post-AGB) candidate stars,selected on the basis of their OH 1612 MHz maser and far-infraredproperties. All sources were observed with the Tidbinbilla 70 m radiotelescope, and 21 detections were made; 86 GHz SiO Mopra observations ofa subset of the sample are also presented. Of the 21 H2Odetections, 15 are from sources that are likely to be massive AGB starsand most of these show typical, regular H2O maser profiles.In contrast, nearly all the detections of more evolved stars exhibitedhigh-velocity H2O maser emission. Of the five sources seen,v223 (W43A, IRAS 18450-0148) is a well-known ``water-fountain'' sourcethat belongs to a small group of post-AGB stars with highly collimated,high-velocity H2O maser emission. A second source in oursample, v270 (IRAS 18596+0315), is also known to have high-velocityemission. We report the discovery of similar emission from a furtherthree sources, d46 (IRAS 15445-5449), d62 (IRAS 15544-5332), and b292(IRAS 18043-2116). The source d46 is an evolved post-AGB star withhighly unusual maser properties. The H2O maser emission fromd62 is probably associated with a massive star. The source b292 is ayoung post-AGB star that is highly likely to be a water-fountain source,with masers detected over a velocity range of 210 km s-1.
| Molecules in (proto)planetary Nebulae
This contribution reviews results on the structure and dynamics ofplanetary and protoplanetary nebulae from observations of molecular lineemission. Molecular line data have been particularly useful to study thevery fast evolution of these nebulae. Our knowledge on the molecularcontent of these objects, i.e., the molecule abundances, is alsosummarized.
| Low-temperature single crystal reflection spectra of forsterite
The infrared reflectivities of crystalline forsterite(Mg2SiO4) were measured for the temperature range295-50 K for each crystal axis, between wavenumber 5000 and 100cm-1. The reflection spectra show clear dependence oftemperature; most of the bands become more intense, sharper and theirpeak positions shift to higher wavenumber with decreasing temperature.Reflection spectra were fitted with dispersion formula of dampedoscillator model of the dielectric constants and the oscillatorparameters in the model were derived. The absorption spectra offorsterite particle are calculated with the derived dielectric constantsto show that the forsterite features are good thermal indicator for coldtemperature range below 295 K.
| 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.
| 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.
| 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.
| The Post-AGB Star HD 101584
High resolution spectra of a post-AGB star HD 101584 were analyzed.Profiles and radial velocities of various emission and absorption lineswere measured. The primary star of the system hasTeff=8500±1000 K and logg=2.0+0.5-1.0. The abundances of C, N ,O, Na andMg are close to the solar values but Si is underabundant. The dustshell surrounding the system is modeled.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| 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 chemistry of protoplanetary nebulae
We describe the chemistry occurring in a dense slab of outwardlyexpanding gas in the post-AGB phase of stellar evolution. Despite thehigh flux of UV photons from the central star, the intrinsic dustextinction in the slab prevents rapid photodissociation and allowschemical reactions to make a variety of gas-phase species, some complex.We find that chemical evolution ends when the intrinsic extinction fallsbelow about 10 mag. At this point, rapid photodissociation occurs. As aparticular example of our model, we discuss chemical synthesis in theprotoplanetary nebula, CRL 618, and find that formany species we obtain good agreement between the model and observation,despite the complexity of the source.
| 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.
| Classification of 2.4-45.2 Micron Spectra from the Infrared Space Observatory Short Wavelength Spectrometer
The Infrared Space Observatory observed over 900 objects with the ShortWavelength Spectrometer in full-grating scan mode (2.4-45.2 μm). Wehave developed a comprehensive system of spectral classification usingthese data. Sources are assigned to groups based on the overall shape ofthe spectral energy distribution (SED). The groups include naked stars,dusty stars, warm dust shells, cool dust shells, very red sources, andsources with emission lines but no detected continuum. These groups arefurther divided into subgroups based on spectral features that shape theSED such as silicate or carbon-rich dust emission, silicate absorption,ice absorption, and fine-structure or recombination lines. Caveatsregarding the data and data reduction, as well as biases intrinsic tothe database, are discussed. We also examine how the subgroups relate tothe evolution of sources to and from the main sequence and how thisclassification scheme relates to previous systems. Based on observationswith the Infrared Space Observatory (ISO), a European Space Agency (ESA)project with instruments funded by ESA Member States (especially thePrinciple Investigator countries: France, Germany, Netherlands, andUnited Kingdom) and with the participation of the Institute of Space andAstronautical Science and the National Aeronautics and SpaceAdministration (NASA).
| Water vapor masers in stars departing from the AGB
The 22 GHz water vapor masers of four ``non-variable'' OH/IR starsconsidered to be young Proto-Planetary Nebulae (PPNs) were monitored formore than 10 years. In two of them, OH 15.7+0.8 andOH 17.7-2.0 having Mira-like line profiles, the maserflux decreased and the maser finally disappeared. The decrease of themaser brightness is spectacular for OH 17.7-2.0 in particular. The maserhad a flux density of several hundred Jansky in the eighties and is nowabsent since 1990. The masers of OH 15.7+0.8 and OH 17.7-2.0 probablyemerged in the ``dying AGB wind'' during the mass loss reduction processaccompanying the departure from the AGB. The other two, OH12.8-0.9 and OH 37.1-0.8, havedouble-peaked profiles with a considerably larger velocity spread thanthe OH masers in these sources. The velocities are incompatible with thespherical symmetric mass loss process on the AGB and give evidence forthe presence of probably bipolar outflows with a projected velocity ofve~28 km s-1 during the very early PPN phase. Thegross structure of their profiles was rather stable during the monitorperiod while individual maser lines appeared and disappeared. Anincrease of the velocity range was found in both sources implying thatthe outflow is accelerating. Analysis of the profile variations of allfour objects suggests a lifetime of individual maser components of 1-3years. There is no evidence for ordered motion traced by the masers. TheH2O maser properties of all these objects are stronglydifferent from those of ordinary OH/IR stars, allowing to find new youngPPNs among optically still hidden objects by looking for irregular maserprofiles. In contrast to current definitions in the literature``non-variable'' OH/IR stars are proposed to be classified as (young)PPNs. It is argued that beyond the end of the AGB, maser propertiescannot be used to infer the evolutionary stage of the objects intransition to Planetary Nebulae.
| The 69-μm forsterite band as a dust temperature indicator
A band of pure crystalline forsterite (100 per centMg2SiO4) occurs at 69.67μm at room temperature(295K) for olivines with >~10 per cent Fe the corresponding featureis at >~73μm. The Mg-rich forsterite feature is observed in avariety of ISO LWS spectra, but the corresponding Fe-rich olivinefeature is not. For the 10 astronomical sources in our sample, theforsterite band peaks in the 68.9-69.3μm range and narrows withdecreasing peak wavelength. This is consistent with the shortwardsshifting of the peak observed when laboratory samples are cooled to 77K(69.07μm) and 3.5K (68.84μm). The shifted peak is produced bylattice contraction and the sharpening is due to a decrease in phonondensity at lower temperatures. However, the astronomical bands arenarrower than those of the laboratory samples. By comparing thelaboratory and astronomical peak wavelengths, we deduce characteristicforsterite 69-μm band temperatures that are in the 27-84K range forthe eight post-main-sequence objects in our sample. These values areshown to be consistent with the local continuum temperatures derivedusing a β=1.5 dust emissivity index, similar to derivedinterstellar values of the opacity index. For the pre-mainsequence-objects HD 100546 and MWC 922, the characteristic 69-μmforsterite band temperatures (127+/-18 and 139+/-10K, respectively) aresignificantly higher than those of the post-main-sequence objects andare more than twice as high as their local continuum temperaturesdeduced using β=1.5. The assumption of large grains (β=0) canproduce agreement between the derived 69-μm and continuumtemperatures for one of these objects but not for the other - a spatialseparation between the forsterite and continuum-emitting grains maytherefore be implied for it. We conclude that observations of the peakwavelength and FWHM of the 69-μm forsterite band show great promiseas a new diagnostic of characteristic grain temperatures.
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| Radiative Transfer Models of Dust Shells Around Post-AGB Stars
We present a radiative transfer analysis of circumstellar dust shellsaround the Post-AGB stars HD 179821, HD 56126, HD 101584 and early Rstar HD 100764, using the code DUSTY. Parameters like mass-loss, shellinner radius, dust temperature, outflow velocity etc., are derived forHD 179821 and HD 56126 whose observed SED could be reproduced by ourmodels.
| Mass, linear momentum and kinetic energy of bipolar flows in protoplanetary nebulae
We have studied the CO emission from protoplanetary nebulae (PPNe). Oursample is composed of 37 objects and includes, we think, all wellidentified PPNe detected in CO, together with the two yellow hypergiantsemitting in CO and one young PN. We present a summary of the existing COdata, including accurate new observations of the 12CO and13CO J=1-0 and J=2-1 lines in 16 objects. We identify in thenebulae a slowly expanding shell (represented in the spectra by acentral core) and a fast outflow (corresponding to the line wings), thatin the well studied PPNe is known to be bipolar. Excluding poor data, weend up with a sample of 32 sources (including the 16 observed by us);fast flows are detected in 28 of these nebulae, being absent in only 4.We present a method to estimate from these data the mass, ``scalar''momentum and kinetic energy of the different components of the molecularoutflows. We argue that the uncertainties of our method can hardly leadto significant overestimates of these parameters, althoughunderestimates may be present in not well studied objects. The totalnebular mass is often as high as ~1 Msun, and the mass-lossrate, that (presumably during the last stages of the AGB phase)originated the nebula, had typical values ~10-4Msun yr-1. The momentum corresponding to this massejection process in most studied nebulae is accurately coincident withthe maximum momentum that radiation pressure, acting through absorptionby dust grains, is able to supply (under expected conditions). Weestimate that this high-efficiency process lasts about 1000-10 000 yr,after which the star has ejected a good fraction of its mass and the AGBphase ends. On the other hand, the fast molecular outflows, that haveprobably been accelerated by shock interaction with axial post-AGB jets,carry a significant fraction of the nebular mass, with a very highmomentum (in most cases between 1037 and 1040 g cms-1) and very high kinetic energy (usually between1044 and 1047 erg). In general, yellow hypergiantsand post-AGB objects with low initial mass show nebular masses andmomenta that are, respectively, higher and lower than these values. Wecompare the momenta of the fast outflows with those that can be suppliedby radiation pressure, taking into account the expected shortacceleration times and some effects that can increase the momentumtransfer. We find that in about 80% of PPNe, the fast molecular flowshave too high momenta to be powered by radiation pressure. In some casesthe momentum of the outflow is ~1000 larger than that carried byradiation pressure; such high factors are difficult to explain evenunder exceptional conditions. Wind interaction is the basic phenomenonin the PN shaping from the former AGB envelopes; we conclude that thisinteraction systematically takes place along a dominant direction andthat this process is not powered by radiation pressure. Due to the lackof theoretical studies, the possible momentum source remains a matter ofspeculation.
| Bipolar outflows in OH/IR stars
We investigate the development of bipolar outflows during the earlypost-AGB evolution. A sample of 10 OH/IR stars with irregular OH spectraand unusually large expansion velocities is observed at high angularresolution. The sample includes bipolar nebulae (e.g., OH231.8+4.2),bright post-AGB stars (HD 101584) and reflection nebulae (e.g., Roberts22). The IRAS colour-colour diagram separates the sample into differenttypes of objects. One group may contain the immediate progenitors to the(few) extreme bipolar planetary nebulae. Two objects show colours andchemistry very similar to the planetary nebulae with late IR-[WC] stars.One object is a confirmed close binary. A model is presented consistingof an outer AGB wind which is swept up by a faster post-AGB wind, witheither the AGB or post-AGB wind being non-spherically symmetric. Theinterface of the two winds is shown to exhibit a linear relation betweenvelocity and distance from the star, giving the impression of anaccelerating outflow. The OH data confirm the predicted linear velocitygradients, and also reveal torus-like, uniformly expanding components.All sources are discussed in detail using optical/HST images whereavailable. ISO data for Roberts 22 reveal a chemical dichotomy, withboth crystalline silicates and PAH features being present. IRAS16342-3814 shows a dense torus with mass 0.1Msolar anddensity of 108cm-3 HST data show four point-likesources located symmetrically around the nebula, near the outer edge ofthe dense torus. Lifetimes for the bipolar OH/IR stars are shown to bein excess of 104yr, longer than normal post-AGB time-scales.This suggests that the toruses or discs are near-stationary. We suggestthat accretion from such a disc slows down the post-AGB evolution. Sucha process could explain the link between the long-lived bipolar nebulargeometry and the retarded star.
| Dust Grains and Circumstellar Motion: The Problem of the Hen and the Egg
In the last ten years, the number of models of circumstellar envelopeshas exponentially increased, both concerning evolved stars and formingstars with their planetary systems. The main progress was intoduction ofchemistry and coupling with dynamics. However, paradoxically, thenucleation theories did not evolve at the same rate; moreover, dynamicsand nucleation are rarely analysed at the same degree of accuracy: oneis of them is always mostly drawing attention. At the same timeobservations showed the presence of dust grains at a lot of sites whereit could not be expected from usual quasi equilibrium theories. Selfconsistent systems are omnipresent. Thus, it is important to distinguishcauses and effects, even if the phenomena are are strongly coupled.
| VLBI observations of 6.7 and 12.2 GHz methanol masers toward high mass star-forming regions. I. Observational results: protostellar disks or outflows?
We present VLBI observations of 6.7 and 12.2 GHz methanol masers in 14star-forming regions, namely NGC 7538, S 252, W 75N, W 48, G 31.28+0.06,S 231, S 255, S 269, Mon R2, G 9.62+0.20, Cep A, W 51, G 59.78+0.06 andG 29.95-0.02. In 10 of these sources, the methanol masers show elongatedmorphologies in the VLBI map and exhibit linear velocity gradients.These linear structures have lengths of 50 to 1300 AU. We considerwhether these structures are consistent with circumstellar disks.Assuming that we see the whole diameter of a Keplerian disk then wederive sub-solar central masses with the exception of W 48. However, itis possible that we only see masers in a fraction of the disk which liesin front of a young massive star. Assuming that the Keplerian disks havediameters of 1000 AU, then the derived enclosed masses vary from 1 to 75Msun. We suggest that other models such as acceleratingoutflows could explain the linear structures; proper motions are neededin order to discriminate between the possible models.
| On the Variability of A6 to F9 Supergiants
Hipparcos Satellite photometry of A6 to F9 supergiants reveals theiractivity. A few are especially quiescient. A0-A5II stars are examinedto help connect this study with that of earlier supergiants.
| The Etched Hourglass Nebula MYCN 18. I. HUBBLE SPACE TELESCOPE Observations
We have obtained emission-line and continuum images of the youngplanetary nebula MyCn 18 with the Wide Field Planetary Camera 2 on theHubble Space Telescope (HST). Although from the ground MyCn 18 appearedto have a triple-ring structure similar to SN 1987A, the HST images showthat MyCn 18 has an overall hourglass shape. A series of arcs appear tobe etched on the walls of the hourglass near its rims. In the complexcentral region of the nebula we find a small, inner hourglass structureand two rings. Ring 1 is a bright elliptical ring, and ring 2 a smaller,higher excitation ring. The outer and inner hourglass, and ring 1 andring 2, all have different centers, and none are coincident with thecentral star. The hourglass shape of the main nebula is consistent withthe predictions of the generalized interacting-winds hypothesis forplanetary nebula formation. However, the complex inner nebular structureof MyCn 18 and the offset of the central star from the center of thenebula remain a mystery. We discuss several mechanisms for producing theoffset of the central star. Although none are found to be completelysatisfactory, those involving a binary central star probably offer thebest hope of successful explanation.
| The circumstellar molecular envelope of HD 101584
CO radio line observations reveal a molecular gas envelope around thepeculiar star HD101584 with characteristics very similar to those ofwellknown young post-AGB objects. We estimate that there is at least 0.1M_sun of molecular gas, very likely remnant gas from a formerAGB-envelope. This gas has been efficiently accelerated to very highvelocities (>50 {km s(-1) , and a significant fraction to >100 {kms(-1) ). There is evidence for an expanding disk-like structure seenclose to edge-on, and a high-velocity bipolar outflow. In the latter theexpansion velocity increases linearly with distance from the star,suggesting either a brief period of ejection or a fast wind interactingwith a slower wind. A significant fraction of the high-velocity gas hasreached a welldefined terminal velocity. Momentum well above theavailable radiation momentum has been transferred to the gas. There arealso indications of high-density, low-velocity molecular gas, possiblyin a disk close to the star. The (12) CO/(13) CO-ratio in the envelopeis uncertain, but probably quite low ( ~ 10). The systemic heliocentricvelocity is 50+/-2 {km s(-1) .
| Spectroscopy of the post-AGB star HD 101584 (IRAS 11385-5517)
From an analysis of the spectrum (4000 Ä to 8800 Ä) of HD101584 it is found that most of the neutral and single ionized metalliclines are in emission. The forbidden emission lines of [OI] 6300 Äand 6363 Ä and [CI] 8727 Ä are detected, which indicate thepresence of a very low excitation nebula. The Hα , FeII 6383Ä, NaI D1, D2 lines and the CaII IR tripletlines show P-Cygni profiles indicating a mass outflow. The Hα lineshows many velocity components in the profile. The FeII 6383 Ä alsohas almost the same line profile as the Hα line indicating thatthey are formed in the same region. From the spectrum synthesis analysiswe find the atmospheric parameters to be T_eff=8500 K, log g=1.5,V_turb=13 km s(-1) and [Fe/H] = 0.0. From an analysis of the absorptionlines the photospheric abundances of some of the elements are derived.Carbon and nitrogen are found to be overabundant. From the analysis ofFe emission lines we derived T_exi=6100 K +/- 200 for the emission lineregion. Based on observations obtained at the European SouthernObservatory (ESO), Chile and the Vainu Bappu Observatory, Kavalur,India.
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| קבוצת-כוכבים: | קנטאור |
| התרוממות ימנית: | 11h40m58.81s |
| סירוב: | -55°34'25.8" |
| גודל גלוי: | 7.011 |
| מרחק: | 813.008 פארסק |
| תנועה נכונה: | -7.7 |
| תנועה נכונה: | -0.7 |
| B-T magnitude: | 7.396 |
| V-T magnitude: | 7.043 |
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