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The young star β Pictoris is well known for its dusty debris disk produced through collisional grinding of planetesimals, kilometre-sized bodies in orbit around the star. In addition to dust, small amounts of gas are also known to orbit the star; this gas is likely the result of vaporisation of violently colliding dust grains. The disk is seen edge on and from previous absorption spectroscopy we know that the gas is very rich in carbon relative to other elements. The oxygen content has been more difficult to assess, however, with early estimates finding very little oxygen in the gas at a C/O ratio that is 20× higher than the cosmic value. A C/O ratio that high is difficult to explain and would have far-reaching consequences for planet formation. Here we report on observations by the far-infrared space telescope Herschel, using PACS, of emission lines from ionised carbon and neutral oxygen. The detected emission from C+ is consistent with that previously reported observed by the HIFI instrument on Herschel, while the emission from O is hard to explain without assuming a higher density region in the disk, perhaps in the shape of a clump or a dense torus required to sufficiently excite the O atoms. A possible scenario is that the C/O gas is produced by the same process responsible for the CO clump recently observed by the Atacama Large Millimeter/submillimeter Array in the disk and that the redistribution of the gas takes longer than previously assumed. A more detailed estimate of the C/O ratio and the mass of O will have to await better constraints on the C/O gas spatial distribution. [ABSTRACT FROM AUTHOR]

Context. Red giants are evolved stars that exhibit solar-like oscillations. Although a multitude of stars have been observed with space telescopes, only a handful of red giant stars were targets of spectroscopic asteroseismic observing projects. Aims. We search for solar-like oscillations in the two bright red giant stars γ Psc and θ¹ Tau from a time series of ground-based spectroscopy and determine the frequency of the excess of oscillation power ?max and the mean large frequency separation Δν for both stars. Seismic constraints on the stellar mass and radius will provide robust input for stellar modelling. Methods. The radial velocities of γ Psc and θ¹ Tau were monitored for 120 and 190 days, respectively. Nearly 9000 spectra were obtained. To reach accurate radial velocities, we used simultaneous thorium-argon and iodine-cell calibration of our optical spectra. In addition to the spectroscopy, we acquired interferometric observations of γ Psc for an independent estimate of the radius. We also analysed 22 days of observations of θ¹ Tau with the MOST satellite. Results. The frequency analysis of the radial velocity data of γ Psc revealed an excess of oscillation power around 32 µHz and a large frequency separation of 4.1 ± 0.1 µHz. θ¹ Tau exhibits oscillation power around 90 µHz, with a large frequency separation of 6.9 ± 0.2 µHz. Scaling relations indicate that γ Psc is a star of about 1 M⊙ and 10 R⊙. The object θ¹Tau appears to be a massive star of about 2.7 M⊙ and 10 R⊙. The radial velocities of both stars were found to be modulated on timescales much longer than the oscillation periods. Conclusions. The estimated radii from seismology are in agreement with interferometric observations and also with estimates based on photometric data. While the mass of θ¹ Tau is in agreement with results from dynamical parallaxes, we find a lower mass for γ Psc than is found in the literature. The long periodic variability agrees with the expected timescales of rotational modulation. [ABSTRACT FROM AUTHOR]

Context. Infrared spectroscopy has been extensively used to determine the mineralogy of circumstellar dust. The identification of dust species with featureless opacities, however, is still ambiguous. Here we present a method to lift the degeneracy using the combination of infrared spectroscopy and interferometry. Aims. The binary post-AGB star HR4049 is surrounded by a circumbinary disk viewed at a high inclination angle. Apart from gaseous emission lines and molecular emission bands of polycyclic aromatic hydrocarbons (PAH), diamonds, and fullerenes, the 2-25 μm infrared spectrum is featureless. The goal of the paper is to identify the dust species responsible for the smooth spectrum. Methods. We gathered high-angular-resolution measurements in the near- and mid-infrared with the VLTI interferometric instruments AMBER and MIDI. The data set is expanded with archival Geneva optical photometry, ISO-SWS and Spitzer-IRS infrared spectroscopy, and VISIR N-band images and spectroscopy. We computed a grid of radiative-transfer models of the circumbinary disk of HR4049 using the radiative-transfer code MCMax. We searched for models that provide good fits simultaneously to all available observations. Results. We find that the variable optical extinction towards the primary star is consistent with the presence of very small (0.01 μm) iron-bearing dust grains or amorphous carbon grains. The combination of the interferometric constraint on the disk extent and the shape of the infrared spectrum points to amorphous carbon as the dominant source of opacity in the circumbinary disk of HR4049. The disk is optically thick to the stellar radiation in the radial direction. At infrared wavelengths it is optically thin. The PAH emission is spatially resolved in the VISIR data and emanates from a region with an extent of several hundreds of AU, with a projected photocenter displacement of several tens of AU from the disk center. The PAHs most likely reside in a bipolar outflow. Conclusions. Dust species with featureless opacity curves, such as metallic iron and amorphous carbon, can be identified by combining infrared spectroscopy and high-angular-resolution measurements. In essence, this is because the temperatures of the dust species are notably different at the same physical distance to the star. [ABSTRACT FROM AUTHOR]

Context. The predicted orbital period histogram of a subdwarf B (sdB) population is bimodal with a peak at short (<10 days) and long (>250 days) periods. Observationally, however, there are many short-period sdB systems known, but only very few long-period sdB binaries are identified. As these predictions are based on poorly understood binary interaction processes, it is of prime importance to confront the predictions to well constrained observational data. We therefore initiated a monitoring program to find and characterize long-period sdB stars. Aims. In this contribution we aim to determine the absolute dimensions of the long-period binary system PG 1104+243 consisting of an sdB and a main-sequence (MS) component, and determine its evolution history. Methods. High-resolution spectroscopy time-series were obtained with HERMES at the Mercator telescope at La Palma, and analyzed to determine the radial velocities of both the sdB and MS components. Photometry from the literature was used to construct the spectral energy distribution (SED) of the binary. Atmosphere models were used to fit this SED and determine the surface gravity and temperature of both components. The gravitational redshift provided an independent confirmation of the surface gravity of the sdB component. Results. An orbital period of 753 ± 3 d and a mass ratio of q = 0.637 ± 0.015 were found for PG 1104+243 from the radial velocity curves. The sdB component has an effective temperature of Teff = 33   500 ± 1200 K and a surface gravity of log g = 5.84 ± 0.08 dex, while the cool companion is found to be a G-type star with Teff = 5930 ± 160 K and log g = 4.29 ± 0.05 dex. When a canonical mass of MsdB = 0.47 Mʘ is assumed, the MS component has a mass of MMS = 0.74 ± 0.07 Mʘ, and its temperature corresponds to what is expected for a terminal age main-sequence star with sub-solar metalicity. Conclusions. PG 1104+243 is the first long-period sdB binary in which accurate and consistent physical parameters of both components could be determined, and the first sdB binary in which the gravitational redshift is measured. Furthermore, PG 1104+243 is the first sdB+MS system that shows consistent evidence for being formed through stable Roche-lobe overflow. An analysis of a larger sample of long-period sdB binaries will allow for the refinement of several essential parameters in the current formation channels. [ABSTRACT FROM AUTHOR]

Herschel images in six photometric bands show the thermal emission of the debris disk surrounding β Pic. In the three PACS bands at 70 μm, 100 μm and 160 μm and in the 250 μm SPIRE band, the disk is well-resolved, and additional photometry is available in the SPIRE bands at 350 μm and 500 μm, where the disk is only marginally resolved. The SPIRE maps reveal a blob to the southwest of β Pic, coinciding with submillimetre detection of excess emission in the disk. We investigated the nature of this blob. Our comparison of the colours, spectral energy distribution and size of the blob, the disk and the background sources shows that the blob is most likely a background source with a redshift between z = 1.0 and z = 1.6. [ABSTRACT FROM AUTHOR]

Context. Our comprehension of stellar evolution on the AGB still faces many difficulties. To improve on this, a quantified understanding of large-amplitude pulsator atmospheres and interpretation in terms of their fundamental stellar parameters are essential. Aims. We wish to evaluate the effectiveness of the recently released CODEX dynamical model atmospheres in representing M-type Mira variables through a confrontation with the time-resolved spectro-photometric and interferometric PTI data set of TU And. Methods. We calibrated the interferometric K-band time series to high precision. This results in 50 nights of observations, covering 8 subsequent pulsation cycles. At each phase, the flux at 2.2 μm is obtained, along with the spectral shape and visibility points in 5 channels across the K-band. We compared the data set to the relevant dynamical, self-excited CODEX models. Results. Both spectrum and visibilities are consistently reproduced at visual minimum phases. Near maximum, our observations show that the current models predict a photosphere that is too compact and hot, and we find that the extended atmosphere lacks H2O opacity. Since coverage in model parameter space is currently poor, more models are needed to make firm conclusions on the cause of the discrepancies. We argue that for TU And, the discrepancy could be lifted by adopting a lower value of the mixing length parameter combined with an increase in the stellar mass and/or a decrease in metallicity, but this requires the release of an extended model grid. [ABSTRACT FROM AUTHOR]

Context. The study of the formation of massive stars is complicated because of the short times scales, large distances, and obscuring natal clouds. There are observational and theoretical indications that the circumstellar environment of Herbig Be (HBe) stars is substantially different from that of their lower mass counterparts, the T Tauri and Herbig Ae stars. Aims. We map the spatial distribution and mineralogy of the warm circumstellar dust of a sample of HBe stars. We compare our results to a sample of less massive Herbig Ae stars. Methods. We used literature photometry to obtain optical extinctions and stellar parameters of the targets. We obtained N-band imaging and long-slit spectroscopic data with the VISIR instrument at the VLT and we analyzed these data. We performed photometry of the images and extracted spatial information. We corrected the spectra for extinction and performed mineralogical fits. We fitted Gaussian profiles to characterize the spatial extent of the spectra along the VISIR slit. Results. We find that the mid-infrared (IR) emission of the HBe stars is typically characterized by a circumstellar disk that efficiently reprocesses a substantial portion of the stellar flux. The mid-IR flux levels, the spatial compactness, and the dust composition are quite similar to those of the Herbig Ae stars. We find upper limits to the full-width-at-half-maximum (FWHM) size of the mid-IR emission of ∼500AU. The main differences with the lower mass stars are the lower overall IR excess with a greater variety in shapes, the weaker PAH reprocessing power, and the lack of a silica-forsterite relation. The discrepancies between VISIR and IRAS photometry, the far-IR contributions and the large PAH sizes of HBe stars are attributed to natal clouds. Conclusions. Our results suggest that the Herbig Be disks are flatter than those around lower mass stars and they are likely truncated from the outside by photoevaporation. [ABSTRACT FROM AUTHOR]