Josef Hron

Dynamic model atmospheres of AGB stars - IV. A comparison of synthetic carbon star spectra with observations

We have calculated synthetic opacity sampling spectra for carbon-rich Asymptotic Giant Branch (AGB) stars based on dynamic model atmospheres which couple time-dependent dynamics and frequency-dependent radiative transfer, as pre- sented in the third paper of this series. We include the molecules CO, CH, CN, C2, CS, HCN, C2H2 and C3 in our calculations, both when computing the atmospheric structures, and the synthetic spectra. A comparison of the synthetic spectra with various observed colours and spectra in the wavelength range between 0.5 and 25 µm of TX Psc, WZ Cas, V460 Cyg, T Lyr and S Cep is presented. We obtain good agreement between observations gathered at different phases and synthetic spectra of one single hydrodynamical model for each star in the wavelength region between 0.5 and 5 µm. At longer wavelengths our models show- ing mass loss offer a first self-consistent qualitative explanation of why a strong feature around 14 µm, which is predicted by all hydrostatic models as well as dynamical models showing no mass loss, is missing in observed AGB carbon star spectra.

Technetium and the third dredge up in AGB stars. I. Field stars

Context. We searched for Technetium (Tc) in a sample of bright oxygenrich asymptotic giant branch (AGB) stars located in the oute r galactic bulge. Tc is an unstable element synthesised via the s-proce ss in deep layers of AGB stars, thus it is a reliable indicator of both recent s-process activity and third dredge-up. Aims. We aim to test theoretical predictions on the luminosity lim it for the onset of third dredge-up. Methods. Using high resolution optical spectra obtained with the UVE S spectrograph at ESO’s VLT we search for resonance lines of n eutral Tc in the blue spectral region of our sample stars. These meas ur ments allow us to improve the procedure of classification of stars with respect to their Tc content by using flux ratios. Synthetic spectra ba sed on MARCS atmospheric models are presented and compared t o the observed spectra around three lines of Tc. Bolometric magnitudes are c lculated based on near infrared photometry of the objects . Results. Among the sample of 27 long period bulge variables four were f ound to definitely contain Tc in their atmospheres. Conclusions. The luminosity of the Tc rich stars is in agreement with predi ctions from AGB evolutionary models on the minimum luminosi ty at the time when third dredge-up sets in. However, AGB evolut i nary models and a bulge consisting of a single old populati on cannot be brought into agreement. This probably means that a younger p opulation is present in the bulge, as suggested by various au thors, which contains the Tc-rich stars here identified.

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Context. We study the circumstellar environment of the carbon-rich star R Sculptoris using the near- and mid-infrared high spatial resolution observations from the ESO-VLTI focal instruments VINCI and MIDI, respectively. Aims. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Methods. We first compare the spectro-interferometric measurements of the star at different epochs to detect the dynamic signatures of the circumstellar structures at different spatial and spectral scales. We then interpret these data using a self-consistent dynamic model atmosphere to discuss the dynamic picture deduced from the observations. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort is first applied to determining these parameters. Results. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 μm. The model agrees well with the time-dependent flux data at 8.5 μm, whereas it is too faint at 11.3 and 12.5 μm. The VINCI visibility measurements are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C2H2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Conclusions. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The transition from windless atmospheres to models with considerable mass-loss rates occurs in a very narrow range of stellar parameters, especially for the effective temperature, the C/O ratio, and the pulsation amplitude. A denser sampling of such critical regions of the parameter space with additional models might lead to a better representation of the extended structures of low mass-loss carbon stars like R Sculptoris. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data.

TIMMI2: a new multimode mid-infrared instrument for the ESO 3.6-m telescope

TIMMI2 is a focal reducer with variable magnification using a reflective collimator and various camera lenses from Silicon, CdTe, Germanium and KRS-5. The primary operating wavelength is 8-24 microns with limited access also to the 3-5 micron region. Longslit and Echelle spectroscopy up to a resolving power of 1000 are implemented with grisms. A cryogenic wire grid polarizer allows for imaging polarimetry. TIMMI2 uses a 240 X 320 As:Si detector array and is cooled by a 2 stage Gifford-McMahon cooler. Maximum field is 72 X 96 arcsec. TIMMI2 has 5 internal cryogenic functions and one external wheel holding calibration targets. TIMMI2 will be interfaced to the 3.6m telescope with a special IR adapter allowing wheel holding registration of the IR images with respect to astrometric reference frames. The instrument design, electronics and results from laboratory test will be presented. Sensitivity estimates as well as an outlook on possible astronomical programs will be given.

Interferometric properties of pulsating C-rich AGB stars - Intensity profiles and uniform disc diameters of dynamic model atmospheres

Aims. On the basis of a set of dynamic model atmospheres of C-rich AGB stars, we present the first theoretical study of centre-to-limb variation (CLV) properties and relative radius interpretation on narrow and broad-band filters. We computed visibility profiles and the equivalent uniform disc (UD) radii to investigate the dependence of these quantities on the wavelength and pulsation phase. Methods. After an accurate morphological analysis of the visibility and intensity profiles determined in narrow and broad-band filters, we fitted our visibility profiles with a UD function simulating the observational approach. The UD-radii were computed using three different fitting-methods to investigate the influence of the visibility sampling profile: single point, two points and a least squares method. Results. The intensity and visibility profiles of models characterises by mass loss show a behaviour very different from a UD. We found that UD-radii are wavelength dependent and that this dependence is stronger if mass loss is present. Strong opacity contributions from C2H2 affect all radius measurements at 3 μm and in the N-band, resulting in higher values for the UD-radii. In the case of models with mass loss the predicted behaviour of UD-radii versus phase is complicated, while the radial changes are almost sinusoidal for models without mass loss. Compared to the M-type stars, for the C-stars no windows are available for measuring the pure continuum.

The PTI Carbon Star Angular Size Survey: Effective Temperatures and Non-sphericity

We report new interferometric angular diameter observations of 41 carbon stars observed with the Palomar Testbed Interferometer. Two of these stars are CH carbon stars and represent the first such measurements of this subtype. Of these, 39 have Yamashita spectral classes and are of sufficiently high quality that we can determine the dependence of effective temperature on spectral type. We find that there is a tendency for the effective temperature to increase with increasing temperature index by ~120 K per step, starting at T_(EFF) ≃ 2500 K for C3, y, although there is a large amount of scatter in this relationship. Overall, the median effective temperature of the carbon star sample is 2800 ± 270 K and the median linear radius is 360 ± 100 R_☉. We also find agreement, on average within 15 K, with the T_(EFF) determinations of Bergeat et al. and a refinement of the carbon star angular size prediction based on V & K magnitudes is presented that is good to an rms of 12%. A subsample of our stars have sufficient {u, v} coverage to permit non-spherical modeling of their photospheres, and a general tendency for detection of statistically significant departures from sphericity with increasing interferometric signal-to-noise is seen. The implications of most—and potentially all—carbon stars being non-spherical is considered in the context of surface inhomogeneities and a rotation-mass-loss connection.

The geometry of the close environment of SV Piscium as probed by VLTI/MIDI

Context. SVPsc is an asymptotic giant branch (AGB) star surrounded by an oxygen-rich dust envelope. The mm-CO line profile of the objects outflow shows a clear double-component structure. Because of the high angular resolution, mid-IR interferometry may give strong constraints on the origin of this composite profile. Aims. The aim of this work is to investigate the morphology of the environment around SVPsc using high-angular resolution interferometry observations in the mid-IR with the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI). Methods. Interferometric data in the N-band taken at different baseline lengths (ranging from 32-64 m) and position angles (73-142 degrees) allow a study of the morphology of the circumstellar environment close to the star. The data are interpreted on the basis of 2-dimensional, chromatic geometrical models using the fitting software tool GEM-FIND developed for this purpose. Results. The results favor two scenarios: (i) the presence of a highly inclined, optically thin, dusty disk surrounding the central star; (ii) the presence of an unresolved binary companion at a separation of 13.7(-4.8)(+4.2) AU and a position angle of 121.8 degrees(-24.5)degrees(+15.4)degrees. NE. The derived orbital period of the binary is 38.1(-22.6)(+20.4) yr. This detection is in good agreement with hydrodynamic simulations showing that a close companion could be responsible for the entrainment of the gas and dust into a circumbinary structure.

Catching the fish – Constraining stellar parameters for TX Piscium using spectro-interferometric observations

Context. Stellar parameter determination is a challenging task when dealing with galactic giant stars. The combination of different investigation techniques has proven to be a promising approach. Aims. We analyse archive spectra obtained with the Short Wavelength Spectrometer (SWS) onboard ISO, and new interferometric observations from the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI) of a very well studied carbon-rich giant: TX Psc. The aim of this work is to determine stellar parameters using spectroscopy and interferometry. Methods. The observations are used to constrain the model atmosphere, and eventually the stellar evolutionary model in the region where the tracks map the beginning of the carbon star sequence. Two different approaches are used to determine stellar parameters: (i) the “classic” interferometric approach where the effective temperature is fixed by using the angular diameter in the N-band (from interferometry) and the apparent bolometric magnitude; (ii) parameters are obtained by fitting a grid of state-of-the-art hydrostatic models to spectroscopic and interferometric observations. Results. We find good agreement between the parameters of the two methods. The effective temperature and luminosity clearly place TX Psc in the carbon-rich AGB star domain in the H-R-diagram. Current evolutionary tracks suggest that TX Psc became a C-star just recently, which means that the star is still in a “quiet” phase compared to the subsequent strong-wind regime. This agrees with the C/O ratio being only slightly greater than one.

Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI/MIDI ,

Aims. We present a study of the envelope morphology of the carbon Mira R For with VLTI/MIDI. This object is one of the few asymptotic giant branch (AGB) stars that underwent a dust-obscuration event. The cause of such events is still a matter of discussion. Several symmetric and asymmetric scenarios have been suggested in the literature. Methods. Mid-infrared interferometric observations were obtained separated by two years. The observations probe different depths of the atmosphere and cover different pulsation phases. The visibilities and the differential phases were interpreted using GEM-FIND, a tool for fitting spectrally dispersed interferometric observations with the help of wavelength-dependent geometric models. Results. We report the detection of an asymmetric structure revealed through the MIDI differential phase. This asymmetry is observed at the same baseline and position angle two years later. The observations are best simulated with a model that includes a uniform-disc plus a Gaussian envelope plus a point-source. The geometric model can reproduce both the visibilities and the differential phase signatures. Conclusions. Our MIDI data favour explanations of the R For obscuration event that are based on an asymmetric geometry. We clearly detect a photocentre shift between the star and the strongly resolved dust component. This might be caused by a dust clump or a substellar companion. However, the available observations do not allow us to distinguish between the two options. The finding has strong implications for future studies of the geometry of the envelope of AGB stars: if this is a binary, are all AGB stars that show an obscuration event binaries as well? Or are we looking at asymmetric mass-loss processes (i.e. dusty clumps) in the inner part of a carbon-rich Mira?

Determination of the stellar parameters of C-rich hydrostatic stars from spectro-interferometric observations

Context. Giant stars, and especially C-rich giants, contribute sign ificantly to the chemical enrichment of galaxies. The determ ination of precise parameters for these stars is a necessary prerequ isite for a proper implementation of this evolutionary phase in the models of galaxies. Infrared interferometry opened new horizons in the study of the stellar parameters of giant stars, and provid ed new important constraints for the atmospheric and evolutionary models. Aims. We aim to determine which stellar parameters can be constrained by using infrared interferometry and spectroscopy, in the case of C-stars what is the precision which can be achieved and what are the limitations. Methods. For this purpose we obtained new infrared spectra and combined them with unpublished interferometric measurements for five mildly variable carbon-rich asymptotic giant branch st ars. The observations were compared with a large grid of hydrostatic model atmospheres and with new isochrones which include the predictions of the thermally pulsing phase. Results. For the very first time we are able to reproduce spectra in the r ange between 0.9 and 4µm, and K broad band interferometry with hydrostatic model atmospheres. Temperature, mass, log(g), C/O and a reasonable range for the distance were derived for all the objects of our study. All our targets have at least one combination of best-fitting parameters which lays in the region of the HRdiagram where C-stars are predicted. Conclusions. We confirm that low resolution spectroscopy is not sensitive to the mass and log(g) determination. For hydrostatic objects the 3µm feature is very sensitive to temperature variations there fore it is a very powerful tool for accurate temperature determinations. Interferometry can constrain mass, radius and log(g) but a distance has to be assumed. The large uncertainty in the distance measurements available for C-rich stars remains a major problem.