Context. We have analysed far–infrared spectra of 32 circumstellar disks around Herbig Ae/Be and T Tauri stars obtained within the Herschel key programme Dust, Ice and Gas in Time (DIGIT). The ...spectra were taken with the Photodetector Array Camera and Spectrometer (PACS) on board the Herschel Space Observatory. In this paper we focus on the detection and analysis of the 69 μm emission band of the crystalline silicate forsterite. Aims. This work aims at providing an overview of the 69 μm forsterite bands present in the DIGIT sample. We use characteristics of the emission band (peak position and FWHM) to derive the dust temperature and to constrain the iron content of the crystalline silicates. With this information, constraints can be placed on the spatial distribution of the forsterite in the disk and the formation history of the crystalline grains. Methods. The 69 μm forsterite emission feature is analysed in terms of position and shape to derive the temperature and composition of the dust by comparison to laboratory spectra of that band. The PACS spectra are combined with existing Spitzer IRS spectra and we compare the presence and strength of the 69 μm band to the forsterite bands at shorter wavelengths. Results. A total of 32 disk sources have been observed. Out of these 32, 8 sources show a 69 μm emission feature that can be attributed to forsterite. With the exception of the T Tauri star AS 205, all of the detections are for disks associated with Herbig Ae/Be stars. Most of the forsterite grains that give rise to the 69 μm bands are found to be warm (~100–200 K) and iron-poor (less than ~2% iron). AB Aur is the only source where the emission cannot be fitted with iron-free forsterite requiring approximately 3–4% of iron. Conclusions. Our findings support the hypothesis that the forsterite grains form through an equilibrium condensation process at high temperatures. The large width of the emission band in some sources may indicate the presence of forsterite reservoirs at different temperatures. The connection between the strength of the 69 and 33 μm bands shows that at least part of the emission in these two bands originates fom the same dust grains. We further find that any model that can explain the PACS and the Spitzer IRS observations must take the effects of a wavelength dependent optical depth into account. We find weak indications of a correlation of the detection rate of the 69 μm band with the spectral type of the host stars in our sample. However, the sample size is too small to obtain a definitive result.
Context. There exist few variability studies of stars in the region in the Hertzsprung-Russell diagram between the A and B-star pulsational instability strips. With the aid of the high precision ...continuous measurements of the CoRoT space satellite, low amplitudes are more easily detected, making a study of this neglected region worthwhile. Aims: We collected a small sample of B stars observed by CoRoT to determine the origin of the different types of variability observed. Methods: We combine literature photometry and spectroscopy to measure the fundamental parameters of the stars in the sample, and compare asteroseismic modelling of the light curves with (differentially rotating) spotted star models. Results: We found strong evidence for the existence of spots and differential rotation in HD 174648, and formulated hypotheses for their origin. We show that the distinction between pulsations and rotational modulation is difficult to make solely based on the light curve, especially in slowly rotating stars.
Some planetary systems harbour debris disks containing planetesimals such as asteroids and comets. Collisions between such bodies produce small dust particles, the spectral features of which reveal ...their composition and, hence, that of their parent bodies. A measurement of the composition of olivine crystals (Mg(2-2x)Fe(2x)SiO(4)) has been done for the protoplanetary disk HD 100546 (refs 3, 4) and for olivine crystals in the warm inner parts of planetary systems. The latter compares well with the iron-rich olivine in asteroids (x ≈ 0.29). In the cold outskirts of the β Pictoris system, an analogue to the young Solar System, olivine crystals were detected but their composition remained undetermined, leaving unknown how the composition of the bulk of Solar System cometary olivine grains compares with that of extrasolar comets. Here we report the detection of the 69-micrometre-wavelength band of olivine crystals in the spectrum of β Pictoris. Because the disk is optically thin, we can associate the crystals with an extrasolar proto-Kuiper belt a distance of 15-45 astronomical units from the star (one astronomical unit is the Sun-Earth distance), determine their magnesium-rich composition (x = 0.01 ± 0.001) and show that they make up 3.6 ± 1.0 per cent of the total dust mass. These values are strikingly similar to those for the dust emitted by the most primitive comets in the Solar System, even though β Pictoris is more massive and more luminous and has a different planetary system architecture.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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 ~500 AU. 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.
Aims.We have carried out a survey of 60 Herbig Ae/Be stars in the 3 micron wavelength region in search for the rare spectral features at 3.43 and 3.53 micron. These features have been attributed to ...the presence of large, hot, hydrogen-terminated nanodiamonds. Only two Herbig Ae/Be stars, HD 97048 and Elias 3-1 are known to display both these features. Methods.We have obtained medium-resolution spectra ($R \sim 2500$) with the ESO near-IR instrument ISAAC in the 3.15-3.65 micron range. Results.In our sample, no new examples of sources with prominent nanodiamond features in their 3 micron spectra were discovered. Less than 4% of the Herbig targets show the prominent emission features at 3.43 and/or 3.53 μm. Both features are detected in our spectrum of HD 97048. We confirm the detection of the 3.53 μm feature and the non-detection of the 3.43 μm feature in MWC 297. Furthermore, we report tentative 3.53 μm detections in V921 Sco, HD 163296 and T CrA. The sources which display the nanodiamond features are not exceptional in the group of Herbig stars with respect to disk properties, stellar characteristics, or disk and stellar activity. Moreover, the nanodiamond sources are very different from each other in terms of these parameters. We do not find evidence for a recent supernova in the vicinity of any of the nanodiamond sources. We have analyzed the PAH 3.3 μm feature and the Pfund δ hydrogen emission line, two other spectral features which occur in the 3 micron wavelength range. We reinforce the conclusion of previous authors that flared-disk systems display significantly more PAH emission than self-shadowed-disk sources. The Pf δ line detection rate is higher in self-shadowed-disk sources than in the flared-disk systems. Conclusions. We discuss the possible origin and paucity of the (nano)diamond features in Herbig stars. Different creation mechanisms have been proposed in the literature, amongst others in-situ and supernova-induced formation. Our data set is inconclusive in proving or disproving either formation mechanism.
Aims. We present the first direct comparison of the distribution of the gas, as traced by the O I 6300 Aa emission, and the dust, as traced by the 10 mum emission, in the planet-forming region of ...proto-planetary disks around three intermediate-mass stars: HD 101412, HD 135344 B and HD 179218. Methods. N-band visibilities were obtained with VLTI/MIDI. Simple geometrical models are used to compare the dust emission to high-resolution optical spectra in the 6300 Aa O I line of the same targets. Results. HD 101412 and HD 135344 B show compact (<2 AU) 10 mum emission while the O I brightness profile shows a double peaked structure. The inner peak is strongest and is consistent with the location of the dust, the outer peak is fainter and is located at 5-10 AU. In both systems, spatially extended PAH emission is found. HD 179218 shows a double ring-like 10 mum emission with the first ring peaking at similar to 1 AU and the second at similar to 20 AU. The O I emitting region is more compact, peaking between 3-6 AU. Conclusions. The disks around HD 101412 and HD 135344 B appear strongly flared in the gas, but self-shadowed in the dust beyond similar to 2 AU. The difference in the gas and dust vertical structure beyond 2 AU might be the first observational evidence of gas-dust decoupling in protoplanetary disks. The disk around HD 179218 is flared in the dust. The 10 mum emission emerges from the inner rim and from the flared surface of the disk at larger radii. No dust emission is detected between similar to 3-15 AU. The oxygen emission seems also to come from a flared structure, however, the bulk of this emission is produced between similar to 1-10 AU. This could indicate a lack of gas in the outer disk or could be due to chemical effects which reduce the abundance of OH-the parent molecule of the observed O I emission-further away from the star. It may also be a contrast effect if the O I emission is much stronger in the inner disk. We suggest that the three systems, HD 179218, HD 135344 B and HD 101412, may form an evolutionary sequence: the disk initially flared becomes flat under the combined action of gas-dust decoupling, grain growth and dust settling.
Context.For a large group of post-AGB binaries, the presence of a stable reservoir of dust is postulated. Although this reservoir will influence the final evolution stages of these objects ...significantly, its actual geometry and structure remains largely unknown. Aims.We aim at determining the dust morphology of a member of this group, IRAS 08544-4431. Methods.We use the interferometric capabilities of the AMBER and MIDI instruments, operating in the K and N-band respectively. The high spatial resolution measurements are used in conjunction with the broad band spectral characteristics to determine the dust geometry, based on self consistent 2D radiative transfer models. Results.We resolve the object in both K and N. Moreover, using the closure phase capabilities of AMBER, we measure in the K-band a large asymmetry of the dusty environment. The interferometric data are clearly incompatible with a spherical outflow. We model the dusty environment with a passive irradiated dusty disc model. Although this model is constrained mainly on the basis of the spectral energy distribution, it reproduces simultaneously the amplitude and closure phase of the visibilities, in both wavelength bands. Conclusions.Our model of a passive, irradiated disc in equilibrium gives an excellent fit to both the K and N-band visibilities and closure phase. The dust around this evolved binary star is indeed locked in a circumbinary disc with a significant scale height. Grain growth, settling, radial mixing and crystallization are efficient in such an environment. We conclude that the circumbinary disc of this evolved object, is governed by the same physical processes that govern the proto-planetary discs around young stellar objects.
Context. We present far-infrared spectroscopic observations, taken with the Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory, of the protoplanetary disk around the ...pre-main-sequence star HD 100546. These observations are the first within the DIGIT Herschel key program, which aims to follow the evolution of dust, ice, and gas from young stellar objects still embedded in their parental molecular cloud core, through the final pre-main-sequence phases when the circumstellar disks are dissipated. Aims. Our aim is to improve the constraints on temperature and chemical composition of the crystalline olivines in the disk of HD 100546 and to give an inventory of the gas lines present in its far-infrared spectrum. Methods. The 69 μm feature is analyzed in terms of position and shape to derive the dust temperature and composition. Furthermore, we detected 32 emission lines from five gaseous species and measured their line fluxes. Results. The 69 μm emission comes either from dust grains with ~70 K at radii larger than 50 AU, as suggested by blackbody fitting, or it arises from ~200 K dust at ~13 AU, close to the midplane, as supported by radiative transfer models. We also conclude that the forsterite crystals have few defects and contain at most a few percent iron by mass. Forbidden line emission from C ii at 157 μm and O i at 63 and 145 μm, most likely due to photodissociation by stellar photons, is detected. Furthermore, five H2O and several OH lines are detected. We also found high-J rotational transition lines of CO, with rotational temperatures of ~300 K for the transitions up to J = 22–21 and T ~ 800 K for higher transitions.
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. The binary post-AGB star HR4049 is surrounded by a circumbinary disk viewed at a high indination angle. Apart from gaseous emission lines and molecular emission bands of polycyclic aromatic hydrocarbons (PAH), diamonds, and fullerenes, the 2-25 mu m infrared spectrum is featureless. The goal of the paper is to identify the dust species responsible for the smooth spectrum. We gathered high-angular-resolution measurements in the near- and mid- infrared with the VLTI interftrometric instruments AMBER and MIDI. 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.
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.
PDF
