E. De Beck

In-orbit performance of Herschel-HIFI

Aims. In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described. Methods. The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources. Results. The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies.

Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. II. CO line survey of evolved stars: derivation of mass-loss rate formulae

Context. The evolution of intermediate and low-mass stars on the asymptotic giant branch is dominated by their strong dust-driven winds. More massive stars evolve into red supergiants with a similar envelope structure and strong wind. These stellar winds are a prime source for the chemical enrichment of the interstellar medium. Aims: We aim to (1) set up simple and general analytical expressions to estimate mass-loss rates of evolved stars, and (2) from those calculate estimates for the mass-loss rates of the asymptotic giant branch, red supergiant, and yellow hypergiant stars in our galactic sample. Methods: The rotationally excited lines of carbon monoxide (CO) are a classic and very robust diagnostic in the study of circumstellar envelopes. When sampling different layers of the circumstellar envelope, observations of these molecular lines lead to detailed profiles of kinetic temperature, expansion velocity, and density. A state-of-the-art, nonlocal thermal equilibrium, and co-moving frame radiative transfer code that predicts CO line intensities in the circumstellar envelopes of late-type stars is used in deriving relations between stellar and molecular-line parameters, on the one hand, and mass-loss rate, on the other. These expressions are applied to our extensive CO data set to estimate the mass-loss rates of 47 sample stars. Results: We present analytical expressions for estimating the mass-loss rates of evolved stellar objects for 8 rotational transitions of the CO molecule and thencompare our results to those of previous studies. Our expressions account for line saturation and resolving of the envelope, thereby allowing accurate determination of very high mass-loss rates. We argue that, for estimates based on a single rotational line, the CO(2-1) transition provides the most reliable mass-loss rate. The mass-loss rates calculated for the asympotic giant branch stars range from 4 \times 10^-8 M_E¯ yr^-1 up to 8 \times 10^-5 M_E¯ yr^-1. For red supergiants they reach values between 2 \times 10^-7 M_E¯ yr^-1 and 3 \times 10^-4 M_E¯ yr^-1. The estimates for the set of CO transitions allow time variability to be identified in the mass-loss rate. Possible mass-loss-rate variability is traced for 7 of the sample stars. We find a clear relation between the pulsation periods of the asympotic giant branch stars and their derived mass-loss rates, with a levelling off at ~3 \times 10^-5 M_E¯ yr^-1 for periods exceeding 850 days. Conclusions: Appendices are only available in electronic form at http://www.aanda.org

Warm water vapour in the sooty outflow from a luminous carbon star

The detection of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star, grain surface reactions, and photochemistry in the outer circumstellar envelope. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite. This includes some high-excitation lines with energies corresponding to ∼1,000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances are much higher than hitherto predicted.

Circumstellar molecular composition of the oxygen-rich AGB star IK Tauri II. In-depth non-LTE chemical abundance analysis

Context. The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflow from these stars creates a circumstellar envelope in which a rich gas-phase chemistry takes place. Complex shock-induced nonequilibrium chemistry takes place in the inner wind envelope, dust-gas reactions and ion-molecule reactions alter the abundances in the intermediate wind zone, and the penetration of cosmic rays and ultraviolet photons dissociates the molecules in the outer wind region. Aims. Little observational information exists on the circumstellar molecular abundance stratifications of many molecules. Furthermore, our knowledge of oxygen-rich envelopes is not as profound as for the carbon-rich counterparts. The aim of this paper is therefore to study the circumstellar chemical abundance pattern of 11 molecules and isotopologs ( 12 CO, 13 CO, SiS, 28 SiO, 29 SiO, 30 SiO, HCN, CN, CS, SO, SO2) in the oxygen-rich evolved star IK Tau. Methods. We have performed an in-depth analysis of a large number of molecular emission lines excited in the circumstellar envelope around IK Tau. The analysis is done based on a non-local thermodynamic equilibrium (non-LTE) radiative transfer analysis, which calculates the temperature and velocity structure in a self-consistent way. The chemical abundance pattern is coupled to theoretical outer wind model predictions including photodestruction and cosmic ray ionization. Not only the integrated line intensities, but also the line shapes are used as diagnostic tool to study the envelope structure. Results. The deduced wind acceleration is much slower than predicted from classical theories. SiO and SiS are depleted in the envelope, possibly due to the adsorption onto dust grains. For HCN and CS a clear difference with respect to inner wind non-equilibrium predictions is found, either indicating uncertainties in the inner wind theoretical modeling or the possibility that HCN and CS (or the radical CN) participate in the dust formation. The low signal-to-noise profiles of SO and CN prohibit an accurate abundance determination; the modeling of high-excitation SO2 lines is cumbersome, possibly related to line misidentifications or problems with the collisional rates. The SiO isotopic ratios ( 29 SiO/ 28 SiO and 30 SiO/ 28 SiO) point toward an enhancement in 28 SiO compared to results of classical stellar evolution codes. Predictions for H2O emission lines in the spectral range of the Herschel/HIFI mission are performed.

A high-resolution line survey of IRC+10216 with Herschel/HIFI. First results: Detection of warm silicon dicarbide (SiC2)

We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5 − 636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal–to– noise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC2). We identified 55 SiC2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC2 is produced in the inner dust formation zone, with an abundance of ∼2×10−7 relative to molecular hydrogen. These SiC2 lines have been observed for the first time in space and have been used to derive an SiC2 rotational temperature of ∼204 K and a source-averaged column density of ∼6.4×1015 cm−2. Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC2.We present the first results of a high-spectral-resolution survey of the carbon-rich evolved star IRC+10216 that was carried out with the HIFI spectrometer onboard Herschel. This survey covers all HIFI bands, with a spectral range from 488 to 1901 GHz. In this letter we focus on the band-1b spectrum, in a spectral range 554.5−636.5 GHz, where we identified 130 spectral features with intensities above 0.03 K and a signal-tonoise ratio >5. Detected lines arise from HCN, SiO, SiS, CS, CO, metal-bearing species and, surprisingly, silicon dicarbide (SiC2). We identified 55 SiC2 transitions involving energy levels between 300 and 900 K. By analysing these rotational lines, we conclude that SiC2 is produced in the inner dust formation zone, with an abundance of ∼2 × 10 −7 relative to molecular hydrogen. These SiC2 lines have been observed for the first time in space and have been used to derive an SiC2 rotational temperature of ∼204 K and a source-averaged column density of ∼6.4 × 10 15 cm −2 . Furthermore, the high quality of the HIFI data set was used to improve the spectroscopic rotational constants of SiC2.

On the physical structure of IRC +10216 - Ground-based and Herschel observations of CO and C2H

Context. The carbon-rich asymptotic giant branch star IRC +10 216 undergoes strong mass loss, and quasi-periodic enhancements of the density of the circumstellar matter have previously been reported. The star’s circumstellar environment is a well-studied and complex astrochemical laboratory, in which many molecular species have been proved to be present. CO is ubiquitous in the circumstellar envelope, while emission from the ethynyl (C2H) radical is detected in a spatially confined shell around IRC +10 216. We recently detected unexpectedly strong emission from the N = 4−3, 6−5, 7−6, 8−7, and 9−8 transitions of C2H with the IRAM 30 m telescope and with Herschel/HIFI, which challenges the available chemical and physical models. Aims. We aim to constrain the physical properties of the circumstellar envelope of IRC +10 216, including the effect of episodic mass loss on the observed emission lines. In particular, we aim to determine the excitation region and conditions of C2H to explain the recent detections and to reconcile them with interferometric maps of the N = 1−0 transition of C2H. Methods. Using radiative-transfer modelling, we provide a physical description of the circumstellar envelope of IRC +10 216, constrained by the spectral-energy distribution and a sample of 20 high-resolution and 29 low-resolution CO lines – to date, the largest modelled range of CO lines towards an evolved star. We furthermore present the most detailed radiative-transfer analysis of C2 Ht hat has been done so far. Results. Assuming a distance of 150 pc to IRC +10 216, the spectral-energy distribution was modelled with a stellar luminosity of 1

Water content and wind acceleration in the envelope around the oxygen-rich AGB star IK Tauri as seen by Herschel/HIFI

During their asymptotic giant branch evolution, low-mass stars lose a significant fraction of their mass through an intense wind, enriching the interstellar medium with products of nucleosynthesis. We observed the nearby oxygen-rich asymptotic giant branch star IK Tau using the high-resolution HIFI spectrometer onboard Herschel. We report on the first detection of (H2O)-O-16 and the rarer isotopologues (H2O)-O-17 and (H2O)-O-18 in both the ortho and para states. We deduce a total water content (relative to molecular hydrogen) of 6.6 x 10(-5), and an ortho-to-para ratio of 3:1. These results are consistent with the formation of H2O in thermodynamical chemical equilibrium at photospheric temperatures, and does not require pulsationally induced non-equilibrium chemistry, vaporization of icy bodies or grain surface reactions. High-excitation lines of (CO)-C-12, (CO)-C-13, (SiO)-Si-28, (SiO)-Si-29, (SiO)-Si-30, HCN, and SO have also been detected. From the observed line widths, the acceleration region in the inner wind zone can be characterized, and we show that the wind acceleration is slower than hitherto anticipated.

Probing the dust formation region in IRC +10216 with the high vibrational states of hydrogen cyanide ,

We report the detection in IRC +10216 of 63 rotational transitions of HCN, most of them with quantum numbers J = 3–2, pertaining to 28 different vibrational states with energies up to 10 700 K (ν1+3ν2+ν3). Some of the transitions were also observed for the rare isotopologue H 13 CN. The observations were carried out with the IRAM 30-m telescope. The HCN lines with level energies above 5000 K arise within 1.5 stellar radius from the photosphere. Their intensities imply a vibrational temperature of Tvib � 2400 K and a fractional HCN abundance relative to H2, x(HCN) = 5–7 × 10 −5 . These high-energy levels are mainly populated by photospheric radiation, and their vibrational temperature yields a direct measurement of the stellar photospheric temperature. The lines with energy levels between 2000 and 5000 K yield Tvib � 1300 K and x(HCN) � 0.8–2 × 10 −5 . They are radiatively excited and arise from a shell extending between 1.5 and 5 stellar radii. Finally, the lines from the low-energy vibrational states (<2000 K) trace a larger region of the envelope (5–28 stellar radii) withTvib = 400–600 K and x(HCN) = 4–6× 10 −6 . They are strongly affected by mid and near-infrared pumping. The line widths increase from 5 km s −1 near the 2400 K photosphere, to 19 km s −1 in the warm 400 K shell. This provides a unique insight into the physical conditions of the gas acceleration and dust formation region.

Detection of anhydrous hydrochloric acid, HCl, in IRC +10216 with the Herschel SPIRE and PACS spectrometers - detection of HCl in IRC +10216

We report on the detection of anhydrous hydrochloric acid (hydrogen chlorine, HCl) in the carbon-rich star IRC+10216 using the spectroscopic facilities onboard the Herschel satellite. Lines from J = 1-0 up to J = 7-6 have been detected. From the observed intensities, we conclude that HCl is produced in the innermost layers of the circumstellar envelope with an abundance relative to H-2 of 5 x 10(-8) and extends until the molecules reach its photodissociation zone. Upper limits to the column densities of AlH, MgH, CaH, CuH, KH, NaH, FeH, and other diatomic hydrides have also been obtained.

Study of the inner dust envelope and stellar photosphere of the AGB star R Doradus using SPHERE/ZIMPOL

We use high-angular-resolution images obtained with SPHERE/ZIMPOL to study the photosphere, the warm molecular layer, and the inner wind of the close-by oxygen-rich AGB star R Doradus. We present observations in filters V, cntH