High-resolution infrared spectroscopy in the 2.3-4.6 micron region is reported for the peculiar A supergiant, single-lined spectroscopic binary HR 4049. Lines from the CO fundamental and first overtone, OH fundamental, and several H2O vibration-rotation transitions have been observed in the near-infrared spectrum. The spectrum of HR 4049 appears principally in emission through the 3 and 4.6 micron region and in absorption in the 2 micron region. The 4.6 micron spectrum shows a rich 'forest' of emission lines. All the spectral lines observed in the 2.3-4.6 micron spectrum are shown to be circumbinary in origin. The presence of OH and H2O lines confirm the oxygen-rich nature of the circumbinary gas which is in contrast to the previously detected carbon-rich material. The emission and absorption line profiles show that the circumbinary gas is located in a thin, rotating layer near the dust disk. The properties of the dust and gas circumbinary disk and the spectroscopic orbit yield masses for the individual stars, M_AI~0.58 Msolar and M_MV~0.34 Msolar. Gas in the disk also has an outward flow with a velocity of $\gtrsim$ 1 km/s. The severe depletion of refractory elements but near-solar abundances of volatile elements observed in HR 4049 results from abundance winnowing. The separation of the volatiles from the grains in the disk and the subsequent accretion by the star are discussed. Contrary to prior reports, the HR 4049 carbon and oxygen isotopic abundances are typical AGB values: 12C/13C=6^{+9}_{-4} and 16O/17O>200.
In this work we present the results of a systematic search for diffuse bands (DBs, hereafter) in the circumstellar envelopes of a carefully selected sample of post-AGB stars. We concentrated on the analysis of 9 of the DBs most commonly found in the interstellar medium. The strength of these features is determined using high resolution optical spectroscopy and the results obtained are compared with literature data on field stars affected only by interstellar reddening. Based on the weak features observed in the subsample of post-AGB stars dominated by circumstellar reddening we conclude that t…
AGB stars, the precursors of Planetary Nebulae, exhibit high rates of mass loss and eject material in the form of a slow (10-20 km/s), dusty molecular wind. The general belief that the dust component of AGB circumstellar envelopes have smooth density profiles and spherical symmetry have recently been shaken by new high resolution images, showing that clumpy and asymmetric structures, analogous to the ones observed in Planetary Nebulae, can be present even before the end of the AGB. To illustrate how and when these structures appear, and possibly to address the question of how they may shape th…
We report the results of an exploratory program to image the extended circumstellar envelopes of asymptotic giant branch (AGB) stars in dust-scattered galactic light. The goal is to characterize the morphology of the envelopes as a probe of the mass-loss process. The observations consist of short exposures with the VLT and longer exposures with 1-2m telescopes, augmented with archival images from the Hubble Space Telescope. We observed 12 AGB stars and detected the circumstellar envelopes in 7. The detected envelopes have mass loss rates more than about 5 10E-6 solar mass per year, and they ca…
Recent optical spectra of post-AGB stars show the presence of C2, CN, and CH+ originating in the circumstellar shell. We present here new, higher resolution spectra which provide constraints on the physical parameters and information on the line profiles. An empirical curve of growth for the C2 Phillips and CN Red system lines in the spectrum of HD 56126 yields b = 0.50 (+0.59, -0.23) km/s. CH+ (0,0) emission lines in the spectrum of the Red Rectangle have been resolved with a FWHM approx 8.5 pm 0.8 km/s. The circumstellar CN lines of IRAS 08005--2356 are resolved into two separate components …
The S-stars have been suggested to be a brief transitional phase as stars evolve from oxygen-rich M-type stars into carbon stars, through the dredge up of carbon from He-shell burning. As possible transition objects, S-stars might help achieve a deeper understanding of the chemical evolution as a star ascends the AGB, as well as shed more light on the mass-loss mechanism. We have initiated a large survey of 40 S-stars to observe line emission in common molecules such as CO, SiO, HCN, CS and SiS. Detailed radiative transfer modelling of multi-transition CO radio line observations towards a samp…
The circumstellar envelopes of carbon-rich AGB stars show a chemical complexity that is exemplified by the prototypical object IRC +10216, in which about 60 different molecules have been detected to date. Most of these species are carbon chains of the type CnH, CnH2, CnN, HCnN. We present the detection of new species (CH2CHCN, CH2CN, H2CS, CH3CCH and C3O) achieved thanks to the systematic observation of the full 3 mm window with the IRAM 30m telescope plus some ARO 12m observations. All these species, known to exist in the interstellar medium, are detected for the first time in a circumstellar…
Using a radiative transfer code (DUSTY) parameters of the circumstellar dust shells of 15 hot post-AGB stars have been derived. Combining the optical, near and far-infrared (ISO, IRAS) data of the stars, we have reconstructed their spectral energy distributions (SEDs) and estimated the dust temperatures, mass loss rates, angular radii of the inner boundary of the dust envelopes and the distances to these stars. The mass loss rates (10$^{-6}-10^{-5}$M$_{\odot}$yr$^{-1}$) are intermediate between stars at the tip of the AGB and the PN phase. We have also studied the ISO spectra of 7 of these sta…
We present circular polarization measurements of circumstellar water masers around 2 Mira variables and 4 supergaint stars. Both an LTE and non-LTE method are used to analyze the circular polarization and total intensity spectra and to obtain the magnetic field strength. The non-LTE method is found to be able to reproduce the observations best. Using this, we find fields from a few hundred milliGauss up to a few Gauss, indicating a solar-type r^(-2) dependence of the magnetic field on the distance, which leads to stellar surface magnetic fields of up to several hundred Gauss. No linear polariz…
We present the results of a VLA search for HI emission in the circumstellar envelopes of five nearby AGB stars: RS Cnc, IRC+10216, EP Aqr, R Cas, and R Aqr. We have detected emission coincident in both position and velocity with RS Cnc, implying that the emission arises from its extended envelope. For R Cas, we detected weak emission that peaks at the stellar systemic velocity and overlaps with the location of its circumstellar dust shell and thus is probably related to the star. Toward IRC+10216 and EP Aqr, we detected multiple, arcminute-scale HI emission features at velocities consistent wi…
We report the detection of the HI line at 21 cm from the circumstellar shell around the AGB star X Her using the position-switching technique with the Nancay Radio Telescope. At the star position the line shows 2 components: (i) a broad one (FWHM ~ 13 km/s) centered at -72.2 km/s, and (ii) a narrow one (FWHM \~ 4 km/s) centered at ~ -70.6 km/s. Our map shows that the source associated to the broad component is asymmetric with material flowing preferentially towards the North-East. This source extends to ~ 10 arcmin. (~ 0.4 pc) from the star in that direction. On the other hand, the narrow comp…
We have made the first detection of circumstellar absorption lines of the 12C13C (Phillips) system 1-0 band and the 12C16O first-overtone 2-0 band in the spectrum of the post-AGB star HD56126 (IRAS07134+1005). The rotational temperatures are lower for molecules with a higher permanent dipole moment. Derived relative column densities ratios are 12C12C/12C13C=36+-13, 12C16O/(12C12C+12C13C)=606+-230, and 12C16O/(12C14N+13C14N)=475+-175. The isotopic exchange reaction for 12C12C is too slow to significantly alter the 12C12C/12C13C ratio and the 12C12C to 12C13C ratio a good measure of half the car…
New SiO multi-transition millimetre line observations of a sample of carbon stars, including J=8-7 observations with the APEX telescope, are used to probe the role of non-equilibrium chemistry and the influence of grains in circumstellar envelopes of carbon stars. A detailed radiative transfer modelling, including the effect of dust emission in the excitation analysis, of the observed SiO line emission is performed. A combination of low- and high-energy lines are important in constraining the abundance distribution. It is found that the fractional abundance of SiO in these C-rich environments …
We present a list of post-AGB stars showing molecular line absorption and emission in the optical spectrum. Two objects show CH+, one in emission and one in absorption, and ten stars show C2 and CN in absorption. The Doppler velocities of the C2 lines and the rotational temperatures indicate that the line forming region is the AGB remnant. An analysis of the post-AGB stars of which CO millimeter data is available suggests that the C2} expansion velocity is of the same order as the CO expansion velocity. HD56126 has been studied in detail and we find a mass-loss rate of Mdot=2.8e-4 Msol/yr, fC2…
We present optical high-resolution spectra of a sample of sixteen post-AGB stars and IRC +10216. Of the post-AGB stars, ten show C2 Phillips and Swan and CN Red System absorption, one CH+ emission, one CH+ absorption, and four without any molecules. We find typically Trot=43-399, 155-202, and 18-50 K, log N = 14.90-15.57, 14.35, and 15.03-16.47 cm-2 for C2, CH+, and CN respectively, and 0.6<N(CN)/N(C2)<11.2. We did not detect isotopic lines, which places a lower limit on the isotope ratio of 12C/13C>20. The presence of C2 and CN absorption is correlated with cold dust (Tdust<300K) and the pres…
New SiS multi-transition (sub-)millimetre line observations of a sample of AGB stars with varying photospheric C/O-ratios and mass-loss rates are presented. A combination of low- and high-energy lines are important in constraining the circumstellar distribution of SiS molecules. A detailed radiative transfer modelling of the observed SiS line emission is performed, including the effect of thermal dust grains in the excitation analysis. We find that the circumstellar fractional abundance of SiS in these environments has a strong dependence on the photospheric C/O-ratio as expected from chemical…
We have detected circumstellar absorption lines of the $^{12}$CN and $^{13}$CN Violet and Red System in the spectrum of the post-AGB star HD~56126. From a synthetic spectrum analysis, we derive a Doppler broadening parameter of $b=0.51\pm0.04$ km~s$^{-1}$, $^{12}$CN/$^{13}$CN$=38\pm2$, and a lower limit of $2000$ on $^{12}$CN/$^{14}$CN and $^{12}$C$^{14}$N/$^{12}$C$^{15}$N. A simple chemical model has been computed of the circumstellar shell surrounding HD~56126 that takes into account the gas-phase ion-molecule reaction between CN and C$^{+}$. From this we infer that this reaction leads to is…
We have investigated the dynamical interaction of low- and-intermediate mass stars (from 1 to 5 Msun) with their interstellar medium (ISM). In this first paper, we examine the structures generated by the stellar winds during the Asymptotic Giant Branch (AGB) phase, using a numerical code and the wind history predicted by stellar evolution. The influence of the external ISM is also taken into account. We find that the wind variations associated with the thermal pulses lead to the formation of transient shells with an average lifetime of 20,000 yr, and consequently do not remain recorded in the …
Aims: Spectrally resolved circumstellar H2O(1_10 - 1_01) lines have been obtained towards three M-type AGB stars using the Odin satellite. This provides additional strong constrains on the properties of circumstellar H2O and the circumstellar envelope. Methods: ISO and Odin satellite H2O line data are used as constraints for radiative transfer models. Special consideration is taken to the spectrally resolved Odin line profiles, and the effect of excitation to the first excited vibrational states of the stretching modes (nu1=1 and nu3=1) on the derived abundances is estimated. A non-local, radi…
Circumstellar shells around AGB stars are built over long periods of time that may reach several million years. They may therefore be extended over large sizes (~1 pc, possibly more), and different complementary tracers are needed to describe their global properties. In the present work, we combined 21-cm HI and CO rotational line data obtained on an oxygen-rich semi-regular variable, RX Lep, to describe the global properties of its circumstellar environment. With the SEST, we detected the CO(2-1) rotational line from RX Lep. The line profile is parabolic and implies an expansion velocity of ~…
The goal is to build up a simple dynamical model for the out-flowing circumstellar envelope around AGB stars in a wide binary system to explore the parameter dependence of the geometrical characteristics of column density patterns. An AGB star in a wide binary system is considered as a 3-D piston model that can induce a 3-D quasi-spherical density structure in the circumstellar envelope by orbital motion of the AGB star. The column density pattern only depends on two parameters: eccentricity of the orbit e and the terminal outflow velocity to mean orbital velocity ratio gamma. When viewed perp…
The main-line OH masers around 4 AGB stars have been observed with the NRAO Very Long Baseline Array (VLBA) at 8 epochs over a period of 2.5 years. Using a phase referencing technique, the position of the most compact maser spot of each star was monitored with respect to two extragalactic reference sources. For U Her and W Hya, we observe the most blue-shifted maser spot, while for R Cas and S CrB we only detect a compact red-shifted maser spot. We managed to determine an accurate proper motion and parallax for U Her, R Cas and S CrB, while additional motion of the compact blue-shifted maser o…
The yellow hypergiant stars (YHGs) are extremely luminous and massive objects whose general properties are poorly known. Only two of this kind of star show massive circumstellar envelopes, IRC+10420 and AFGL2343. We aim to study the chemistry of the circumstellar envelopes around these two sources, by comparison with well known AGB stars and protoplanetary nebulae. We also estimate the abundances of the observed molecular species. We have performed single-dish observations of different transitions for twelve molecular species. We have compared the ratio of the intensities of the molecular tran…
(Abridged) We examine the effects of cool bottom processing (CBP) on several isotopic ratios in the convective envelope during the TP-AGB phase of evolution in a 1.5 M_sun initial-mass star of solar initial composition. We use a parametric model which treats extra mixing by introducing mass flow between the convective envelope and the underlying radiative zone. The parameters of this model are the mass circulation rate (Mdot) and the maximum temperature (T_P) experienced by the circulating material. The effects of nuclear reactions in the flowing matter were calculated using a set of structure…
Analysis of the low resolution UV(IUE) spectra of 15 hot post-AGB candidates is presented. The UV(IUE) spectra of 10 stars suggest partial obscuration of the hot stars due to circumstellar dust. The reddened continua of these 10 stars were used to model and estimate the circumstellar extinction. The circumstellar extinction law was found to be linear in $λ^{-1}$ in the case of IRAS13266-5551 (CPD-55 5588), IRAS14331-6435 (Hen3-1013), IRAS16206-5956 (SAO 243756), IRAS17074-1845 (Hen3-1347), IRAS17311-4924 (Hen3-1428), IRAS18023-3409 (LSS 4634), IRAS18062+2410 (SAO 85766), IRAS18371-3159 (LSE 63…
Aims: By performing a detailed radiative transfer analysis, we determine fractional abundances of circumstellar H2O in the envelopes around six M-type asymptotic giant branch stars. The models are also used to predict H2O spectral line emission for the upcoming Herschel/HIFI mission. Methods: We use Infrared space observatory long wavelength spectrometer spectra to constrain the circumstellar fractional abundance distribution of ortho-H2O, using a non-local thermal equilibrium, and non-local, radiative transfer code based on the accelerated lambda iteration formalism. The mass-loss rates and k…
In the radio regime the mass-loss rate of AGB stars is best probed using molecular (and atomic) line emission arising in the CSE formed by the stellar wind. The numerical modelling of the circumstellar emission where intricate interplays between physical and chemical processes take place, is a challenge. The derived mass-loss rates depend crucially on the assumptions in the circumstellar model, of which some can be constrained if enough observational data exist. Therefore, a reliable mass-loss-rate determination for an individual star requires, in addition to a detailed radiative transfer anal…
An extensive radiative transfer analysis of circumstellar SiO `thermal' radio line emission from a large sample of M-type AGB stars has been performed. The sample contains 18 irregulars of type Lb (IRV), 7 and 34 semiregulars of type SRa and SRb (SRV), respectively, and 12 Miras. New observational material, which contains data in several SiO rotational lines in the ground vibrational state, is presented for the IRV/SRVs. The detection rate was about 60%. SiO fractional abundances have been determined through radiative transfer modelling. The abundance distribution of the IRV/SRV sample has a m…
We have observed with the 30-m IRAM telescope, the CSO telescope and the ISOsatellite (Infrared Space Observatory) the rotational lines of CO at millimeter, submillimeter and far infrared wavelengths in the direction of C-rich stellar objects at different stages of evolution : CRL 2688 (a very young Proto-Planetary Nebula), CRL 618 (a Proto-Planetary Nebula), and NGC 7027 (a young Planetary Nebula). Several changes in the longwave emission of CO and other molecules are discussed here in relation with the degree of evolution of the objects. In the early stages, represented by CRL 2688, the long…
We performed a millimetric survey of a sample of 24 post-AGB stars aimed to search for emission from circumstellar matter, in order to investigate the physical properties of the outer parts of the envelopes. The observations were conducted using the 37-channel Max-Planck Millimeter Bolometer array at the 30-meter IRAM telescope. The continuum emission toward the detected sources was used to quantify the mass of the emitting dust. We combined our observations with data available in literature to construct the spectral energy distribution (SED) of the sources. When the observational data cover a…
We present observations of 86 post-Asymptotic Giant Branch (post-AGB) stars of OH maser transitions, taken with the Parkes Telescope between September 2002 and August 2003. Post-AGB stars are the precursors of planetary nebulae, which have a wide range of morphologies that are not well explained. By studying the circumstellar envelopes of post-AGB stars through the masers produced in them, we hope to shed light on the origin of planetary nebula morphologies.
We argue that the presence of water vapor in the circumstellar outflow of a carbon-rich AGB star is potentially a distinctive signature of extra-solar cometary systems. Detailed models show that at suitable distances from the star, water ice can survive well into the carbon-rich AGB phase; water vapor abundances as large as 10^-6 could result from the vaporization of a collection of orbiting icy bodies with a total mass comparable to what might have been originally present in the solar system's Kuiper Belt. In particular, the recently-reported detection by the Submillimeter Wave Astronomy Sate…
We present a detailed study of the circumstellar envelope of the post-asymptotic giant branch ``21 micron object'' HD 56126. We build a detailed dust radiative transfer model of the circumstellar envelope in order to derive the dust composition and mass, and the mass-loss history of the star. To model the emission of the dust we use amorphous carbon, hydrogenated amorphous carbon, magnesium sulfide and titanium carbide. We present a detailed parametrisation of the optical properties of hydrogenated amorphous carbon as a function of H/C content. The mid-infrared imaging and spectroscopy is best…
We analyse public domain time-series photometric observations of 30 known and candidate binary post-AGB stars for measuring pulsation and orbital periods. We derive periodicities for 17 stars for the first time in the literature. Besides identifying five new RV Tauri type pulsating variables (three with the RVb phenomenon, i.e. long-term changes of the mean brightness), we find multiply periodic (or possibly irregular) post-AGB stars on the two edges of the instability strip. The temperature dependence of the peak-to-peak light curve amplitudes clearly indicates the changes in excitation as po…
We present the first VLBI images of SiO masers in the circumstellar envelope of an S-type star obtained with the newly developed capabilities of the Very Long Baseline Array (VLBA) at 86 GHz. We combine these data with those obtained quasy simultaneously at 43 GHz. These observations provide information on the structure and dynamics of the innermost circumstellar shells, where the return of large quantities of stellar material to the interstellar medium starts. Despite of the fact that these are prelimininary results, we report that the maser emission of the $v$=1 J=1--0 and J=2--1 present a v…
The 21micron and 30micron bands are the strongest dust emission features detected in evolved low- and intermediate-mass C-rich stars (i.e. asymptotic giant branch [AGB] stars, proto-planetary nebulae [PPN], and planetary nebulae [PN]). While the 21micron feature is rare and exists only in the transient PPN phase, the 30micron feature is more common and seen in the entire late stage of stellar evolution, from AGB to PPN and PN phases, as well as in the low-metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The carriers of these features remain unidentif…
Atomic carbon has been detected in the envelopes of three carbon-rich evolved stars: HD 44179 (=AFGL 915, the `Red Rectangle'); HD 56126; and, tentatively, the carbon star V Hya. This brings to seven the number of evolved star envelopes in which CI has been detected. Upper limits were found for several other stars, including R CrB. CI was not detected in several oxygen rich post asymptotic giant branch (AGB) stars (OH231.8+4.2, for example), although it is detected in their carbon-rich analogues. Two trends are evident in the data. First, circumstellar envelopes with detectable CI are overwhel…
We have observed a sample of OH 1612-MHz masing objects in all four OH ground--state transitions with the Australia Telescope Compact Array. One likely post-AGB object is found to emit in the 1612-MHz, 1665-MHz and 1720-MHz transitions. We discuss the evidence that this object may be an early post-AGB object and the possibility for such a circumstellar envelope to harbour a 1720-MHz maser. We argue that during a very brief period, just after the star has left the thermally-pulsing phase of the AGB and the wind velocity starts to increase, post-AGB objects might show \sath emission. The best ob…
We show here that the absorptive H-alpha polarized line profile previously seen in many Herbig Ae/Be (HAeBe) stars is a nearly ubiquitous feature of other types of embedded or obscured stars. This characteristic 1% linear polarization variation across the absorptive part of the H-alpha line is seen in Post-AGB stars as well as RV-Tau, Delta-Scuti, and other types. Each of these stars shows evidence of obscuration by intervening circumstellar hydrogen gas and the polarization effect is in the absorptive component, consistent with an optical pumping model. We present ESPaDOnS spectropolarimetric…
We present ground-based mid-infrared imaging for 27 M-, S- and C-type Asymptotic Giant Branch (AGB) stars. The data are compared with those of the database available thanks to the IRAS, ISO, MSX and 2MASS catalogues. Our goal is to establish relations between the IR colors, the effective temperature $T_{eff}$, the luminosity $L$ and the mass loss rate $\dot M$, for improving the effectiveness of AGB modelling. Bolometric (absolute) magnitudes are obtained through distance compilations, and by applying previously-derived bolometric corrections; the variability is also studied, using data accumu…
Intermediate mass stars (1-8 solar masses) evolve along the Asymptotic Giant Branch after completion of hydrogen and helium core burning. At the tip they lose for several ten to hundred thousand years copious amounts of mass and exhibit various forms of variability, as for example large-amplitude variations with periods of one to five years. In the oxygen-rich circumstellar envelopes powerful OH, H2O and SiO masers may operate. Part of the AGB population is converted to carbon stars. During the latest phases of AGB evolution the mass loss rates approach several 10e-5 solar masses/year, so that…
We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50 % for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in …
The specific characteristic of the SED of serendipitously discovered post-AGB binaries, allowed us to launch a very extensive multi-wavelength study of evolved objects, selected on the basis of very specific selection criteria. Those criteria were tuned to discover more stars with circumstellar dusty discs. The observational study includes radial velocity monitoring, high spectral resolution optical studies, infrared spectral dust studies, sub-mm bolometric observations and high spatial resolution interferometric experiments with the VLTI. In this contribution, we will review the preliminary r…
We present in this paper a study on the peculiar supergiant HD172481. Its spectral type (F2Ia), high galactic latitude (b=-10.37), circumstellar dust, high radial velocity and moderate metal deficiency ([Fe/H]=-0.55) confirm the post-AGB character of this object. A detailed chemical analysis shows slight but real s-process overabundances, however no CNO-enhancement was detected. Furthermore, the spectral energy distribution and the TiO bands in the red part of the spectrum reveal a red luminous companion. The luminosity ratio of the hot F type component and this cool M type companion L(F)/L(M)…
We investigate the infrared properties of the IRAS sources not confirmed as PNe in Van de Steene & Pottasch (1993). We observed 28 objects in the N-band of which 20 were detected and 5 were resolved. We obtained medium resolution Br_gamma spectra and we took high resolution JHKL images of these 20 objects. We critically assessed the identification of the IRAS counterpart in the images. Of sixteen positively identified objects, seven show Br_gamma in absorption, another six objects show Br_gamma in emission on top of a continuum. In another three objects, no clear Br_gamma absorption or emissio…
We present optical and near-infrared finding charts taken from the DSS and 2MASS surveys of 94 IRAS sources selected from the GLMP catalogue (Garc\'ıa-Lario 1992), and accurate astrometry (~0.2") for most of them. Selection criteria were very red IRAS colours representative for OH/IR stars with optically thick circumstellar shells and the presence of variability according to the IRAS variability index (VAR>50). The main photometric properties of the stars in this `GLMP sample' are presented, discussed and compared with the correspondent properties of the `Arecibo sample' of OH/IR stars studied…
The high resolution spectra covering the wavelength range 4900 A to 8250 A of the hot post-AGB stars IRAS13266-5551 (CPD-55 5588) and IRAS17311-4924 (Hen3-1428) reveal absorption lines of CII, NII, OII, AlIII, SiIII and FeIII and a rich emission line spectrum consisting of HI, HeI, CII, NI, OI, MgII, AlII, SiII, VI, MnI, FeIII, [FeII] and [CrII]. The presence of [NII] and [OI] lines and absence of [OIII] indicate low excitation nebulae around these stars. The components of NaI absorption lines indicate the presence of neutral circumstellar envelopes in addition to the low excitation nebulae ar…
On this paper, we present simultaneous VLBA observations of the J=1-0 and J=2-1 rotational lines in the v=1 and v=2 vibrationally excited states of SiO (at 7 and 3 mm wavelengths). We have mapped these four maser lines in the circumstellar envelopes of three AGB stars: IRC +10011, Chi Cyg and TX Cam. We study the relative spatial distribution between these maser lines. In particular, for the two v=1 transitions, we found that the J=1-0 and J=2-1 maser spot distributions are unalike, challenging all the current theoretical pumping mechanisms for SiO masers (which predict quite similar distribut…
We present in this paper a homogeneous photospheric abundance study, on the basis of the analysis of high resolution optical spectra, of six post-AGB objects displaying a 21mic circumstellar dust feature in their IR spectrum. The F-G spectral type of the 21mic stars make that a large range of elements including a wide variety of s-process elements, can be studied by their atomic lines. The high C/O-ratios together with the large overabundance of s-process elements prove that the objects are descendants of field carbon stars. We discuss in detail the s-process abundance distribution displayed b…
Radiative transfer modelling of AGB circumstellar envelopes is applied to a sample of AGB stars previously observed with the mid-IR imaging camera TIRCAM (Busso et al. 1996: Paper I). We present the results of our simulations, aimed at deriving the physical parameters of the envelope, such as the optical depth and the radial thermal structure, the mass loss and the dust-to-gas mass ratio. The chemical composition of the dust in the observed envelopes is discussed. The ability of different sets of dust opacities to fit the mid-infrared spectra is evaluated. The hypothesis of dust grain aging an…
We present 10 and 20 micron images of IRAS 19500-1709 taken with the mid-infrared camera, OSCIR, mounted on the Gemini North Telescope. An extended circumstellar envelope is detected, with the N band image indicating an elongation in a NE-SW direction. We use a dust radiation transport code to fit the spectral energy distribution from UV to sub-mm wavelengths, with a detached dust shell model. A good fit is achieved using dust composed of amorphous carbon, silicon carbide and magnesium sulphide. We derive estimates for the inner and outer radius, density and mass of the dust in the shell. The …
