PACS and SPIRE range spectroscopy of cool, evolved stars

Context: At the end of their lives AGB stars are prolific producers of dust and gas. The details of this mass-loss process are still not understood very well. Herschel PACS and SPIRE spectra offer a unique way of investigating properties of AGB stars in general and the mass-loss process in particular. Methods: The HIPE software with the latest calibration is used to process the available PACS and SPIRE spectra of 40 evolved stars. The spectra are convolved with the response curves of the PACS and SPIRE bolometers and compared to the fluxes measured in imaging data of these sources. Custom software is used to identify lines in the spectra, and to determine the central wavelengths and line intensities. Standard molecular line databases are used to associate the observed lines. Because of the limited spectral resolution of the spectrometers several known lines are typically potential counterparts to any observed line. To help identifications the relative contributions in line intensity of the potential counterpart lines are listed for three characteristic temperatures based on LTE calculations and assuming optically thin emission. Result: The following data products are released: the reduced spectra, the lines that are measured in the spectra with wavelength, intensity, potential identifications, and the continuum spectra, i.e. the full spectra with all identified lines removed. As simple examples of how this data can be used in future studies we have fitted the continuum spectra with three power laws and find that the few OH/IR stars seem to have significantly steeper slopes than the other oxygen- and carbon-rich objects in the sample. As another example we constructed rotational diagrams for CO and fitted a two-component model to derive rotational temperatures.Comment: A&A accepte

Classifying the secondary component of the binary star W Aquilae

AIMS: The object W Aql is an asymptotic giant branch (AGB) star with a faint companion. By determining more carefully the properties of the companion, we hope to better constrain the properties of the AGB star. METHODS: We present new spectral observations of the binary star W Aql at minimum and maximum brightness and new photometric observations of W Aql at minimum brightness. RESULTS: The composite spectrum near minimum light is predominantly from the companion at wavelengths $\lambda$ < 6000 $\AA$. This spectrum can be classified as F8 to G0, and the brightness of the companion is that of a dwarf star. Therefore, it can be concluded that the companion is a main sequence star. From this, we are able to constrain the mass of the AGB component to 1.04 - 3 $M_\odot$ and the mass of the W Aql system to 2.1 - 4.1 $M_\odot$ . Our photometric results are broadly consistent with this classification and suggest that the main sequence component suffers from approximately 2 mag of extinction in the V band primarily due to the dust surrounding the AGB component.Comment: 5 pages, 1 figure, research not

Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide

Through a survey of (sub-)millimetre emission lines of various sulphur-bearing molecules, we aim to determine which molecules are the primary carriers of sulphur in different types of AGB stars. In this paper, the first in a series, we investigate the occurrence of H$_2$S in AGB circumstellar envelopes and determine its abundance, where possible. We have surveyed 20 AGB stars with a range of mass-loss rates and of different chemical types using the APEX telescope to search for rotational transition lines of five key sulphur-bearing molecules: CS, SiS, SO, SO$_2$ and H$_2$S. Here we present our results for H$_2$S, including detections, non-detections and detailed radiative transfer modelling of the detected lines. We compare results based on different descriptions of the molecular excitation of H$_2$S and different abundance distributions, including those derived from chemical modelling results. We detected H$_2$S towards five AGB stars, all of which have high mass-loss rates of $\dot{M}\geq 5\times 10^{-6}M_\odot$ yr$^{-1}$ and are oxygen-rich. H$_2$S was not detected towards the carbon or S-type stars that fall in a similar mass-loss range. For the stars in our sample with detections, we find peak o-H$_2$S abundances relative to H$_2$ between $4\times 10^{-7}$ and $2.5\times 10^{-5}$. Overall, we conclude that H$_2$S can play a significant role in oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a key role in stars of other chemical types or the lower mass-loss rate oxygen-rich stars. For two sources, V1300 Aql and GX Mon, H$_2$S is most likely the dominant sulphur-bearing molecule in the circumstellar envelope.Comment: 14 pages, 7 figures, accepted in A&

Detailed modelling of the circumstellar molecular line emission of the S-type AGB star W Aquilae

S-type AGB stars have a C/O ratio which suggests that they are transition objects between oxygen-rich M-type stars and carbon-rich C-type stars. As such, their circumstellar compositions of gas and dust are thought to be sensitive to their precise C/O ratio, and it is therefore of particular interest to examine their circumstellar properties. We present new Herschel HIFI and PACS sub-millimetre and far-infrared line observations of several molecular species towards the S-type AGB star W Aql. We use these observations, which probe a wide range of gas temperatures, to constrain the circumstellar properties of W Aql, including mass-loss rate and molecular abundances. We used radiative transfer codes to model the circumstellar dust and molecular line emission to determine circumstellar properties and molecular abundances. We assumed a spherically symmetric envelope formed by a constant mass-loss rate driven by an accelerating wind. Our model includes fully integrated H2O line cooling as part of the solution of the energy balance. We detect circumstellar molecular lines from CO, H2O, SiO, HCN, and, for the first time in an S-type AGB star, NH3. The radiative transfer calculations result in an estimated mass-loss rate for W Aql of 4.0e-6 Msol yr-1 based on the 12CO lines. The estimated 12CO/13CO ratio is 29, which is in line with ratios previously derived for S-type AGB stars. We find an H2O abundance of 1.5e-5, which is intermediate to the abundances expected for M and C stars, and an ortho/para ratio for H2O that is consistent with formation at warm temperatures. We find an HCN abundance of 3e-6, and, although no CN lines are detected using HIFI, we are able to put some constraints on the abundance, 6e-6, and distribution of CN in W Aql's circumstellar envelope using ground-based data. We find an SiO abundance of 3e-6, and an NH3 abundance of 1.7e-5, confined to a small envelope.Comment: 17 pages, 15 figure

Chemical modelling of dust–gas chemistry within AGB outflows – II. Effect of the dust-grain size distribution

Asymptotic giant branch (AGB) stars are, together with supernovae, the main contributors of stellar dust to the interstellar medium (ISM). Dust grains formed by AGB stars are thought to be large. However, as dust nucleation and growth within their outflows are still not understood, the dust-grain size distribution (GSD) is unknown. This is an important uncertainty regarding our knowledge of the chemical and physical history of interstellar dust, as AGB dust forms ∼70 per cent∼70 per cent of the starting point of its evolution. We expand on our chemical kinetics model, which uniquely includes a comprehensive dust–gas chemistry. The GSD is now allowed to deviate from the commonly assumed canonical Mathis, Rumpl & Nordsieck distribution. We find that the specific GSD can significantly influence the dust–gas chemistry within the outflow. Our results show that the level of depletion of gas-phase species depends on the average grain surface area of the GSD. Gas-phase abundance profiles and their possible depletions can be retrieved from observations of molecular emission lines when using a range of transitions. Because of degeneracies within the prescription of GSD, specific parameters cannot be retrieved, only (a lower limit to) the average grain surface area. None the less, this can discriminate between dust composed of predominantly large or small grains. We show that when combined with other observables such as the spectral energy distribution and polarized light, depletion levels from molecular gas-phase abundance profiles can constrain the elusive GSD of the dust delivered to the ISM by AGB outflows

An ALMA view of CS and SiS around oxygen-rich AGB stars

We aim to determine the distributions of molecular SiS and CS in the circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how these distributions differ between stars that lose mass at different rates. In this study we analyse ALMA observations of SiS and CS emission lines for three oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate of $5\times10^{-6}$M$_\odot$ yr$^{-1}$, and W Hya and R Dor with low mass loss rates of $\sim1\times10^{-7}$M$_\odot$ yr$^{-1}$. These molecules are usually more abundant in carbon stars but the high sensitivity of ALMA allows us to detect their faint emission in the low mass-loss rate AGB stars. The high spatial resolution of ALMA also allows us to precisely determine the spatial distribution of these molecules in the circumstellar envelopes. We run radiative transfer models to calculate the molecular abundances and abundance distributions for each star. We find a spread of peak SiS abundances with $\sim10^{-8}$ for R Dor, $\sim10^{-7}$ for W Hya, and $\sim3\times10^{-6}$ for IK Tau relative to H$_2$. We find lower peak CS abundances of $\sim7\times10^{-9}$ for R Dor, $\sim7\times10^{-8}$ for W Hya and $\sim4\times10^{-7}$ for IK Tau, with some stratifications in the abundance distributions. For IK Tau we also calculate abundances for the detected isotopologues: C$^{34}$S, $^{29}$SiS, $^{30}$SiS, Si$^{33}$S, Si$^{34}$S, $^{29}$Si$^{34}$S, and $^{30}$Si$^{34}$S. Overall the isotopic ratios we derive for IK Tau suggest a lower metallicity than solar.Comment: 16 page

The unusual 3D distribution of NaCl around the AGB star IK Tau

NaCl is a diatomic molecule with a large dipole moment, which allows for its detection even at relatively small abundances. It has been detected towards several evolved stars, among which is the AGB star IK Tau, around which it is distributed in several clumps that lie off-center from the star. We aim to study the three-dimensional distribution of NaCl around the AGB star IK Tau, and to obtain the abundance of NaCl relative to H$_2$ for each of the clumps. First, a new value for the maximum expansion velocity is determined. The observed ALMA channel maps are then deprojected to create a three-dimensional model of the distribution of NaCl. This model is then used as input for the radiative transfer modelling code magritte, which is used to obtain the NaCl abundances of each of the clumps by comparing the observations with the results of the magritte simulations. Additionally, the rotational temperature of the clumps is determined using population diagrams. We derive an updated value for the maximum expansion velocity of IK Tau $\upsilon_\mathrm{exp}$ = 28.4 km/s. A spiral-like shape can be discerned in our three-dimensional distribution model of the NaCl. This spiral lies more or less in the plane of the sky. The distribution is also flatter in the line-of-sight direction than in the plane of the sky. We find clump abundances between $9 \times 10^{-9}$ and $5 \times 10^{-8}$ relative to H$_2$, where the relative abundance is typically lower for clumps closer to the star. For the first time, we used deprojection to understand the three-dimensional environment of an AGB star and calculated the fractional abundance of NaCl in clumps surrounding the star.Comment: Accepted for publication in A&

The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit

The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql has been observed at subarcsecond resolution using ALMA. The aim of this paper is to investigate the wind properties of the AGB star and to analyse how the known companion has shaped the CSE. The average mass-loss rate during the creation of the detected CSE is estimated through modelling, using the ALMA brightness distribution and previously published single-dish measurements as observational constraints. The ALMA observations are presented and compared to the results from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with the same properties as the W Aql system and with two different orbital eccentricities. Three-dimensional radiative transfer modelling is performed and the response of the interferometer is modelled and discussed. The estimated average mass-loss rate of W~Aql agrees with previous results. The size of the emitting region is consistent with photodissociation models. The CO(J=3-2) emission is dominated by a smooth component overlayed with two weak arc patterns with different separations. The larger pattern is predicted by the binary interaction model with separations of 10" and therefore likely due to the known companion. It is consistent with a binary orbit with low eccentricity. The smaller separation pattern is asymmetric and coincides with the dust distribution, but the separation timescale (200 yrs) is not consistent with any known process of the system. The separation of the known companions of the system is large enough to not have a very strong effect on the circumstellar morphology. The density contrast across the envelope of a binary with an even larger separation will not be easily detectable, even with ALMA, unless the orbit is strongly asymmetric or the AGB star has a much larger mass-loss rate.Comment: 10 pages, 8 figure

Sulphur-bearing molecules in AGB stars II: Abundances and distributions of CS and SiS

We surveyed 20 AGB stars of different chemical types using the APEX telescope, and combined this with an IRAM 30 m and APEX survey of CS and SiS emission towards over 30 S-type stars. For those stars with detections, we performed radiative transfer modelling to determine abundances and abundance distributions. We detect CS towards all the surveyed carbon stars, some S-type stars, and the highest mass-loss rate oxygen-rich stars ($> 5\times 10^{-6}$ Msol yr$^{-1}$). SiS is detected towards the highest mass-loss rate sources of all chemical types ($> 8\times 10^{-7}$ Msol yr$^{-1}$). We find CS peak fractional abundances ranging from ~ $4\times 10^{-7}$ to ~ $2\times 10^{-5}$ for the carbon stars, from ~ $3\times 10^{-8}$ to ~ $1\times 10^{-7}$ for the oxygen-rich stars and from ~ $1\times 10^{-7}$ to ~ $8\times 10^{-6}$ for the S-type stars. We find SiS peak fractional abundances ranging from ~ $9\times 10^{-6}$ to ~ $2\times 10^{-5}$ for the carbon stars, from ~ $5\times 10^{-7}$ to ~ $2\times 10^{-6}$ for the oxygen-rich stars, and from ~ $2\times 10^{-7}$ to ~ $2\times 10^{-6}$ for the S-type stars. We derived Si$^{32}$S/Si$^{34}$S = 11.4 for AI Vol, the only star for which we had a reliable isotopologue detection. Overall, we find that wind density plays an important role in determining the chemical composition of AGB CSEs. It is seen that for oxygen-rich AGB stars both CS and SiS are detected only in the highest density circumstellar envelopes and their abundances are generally lower than for carbon-rich AGB stars by around an order of magnitude. For carbon-rich and S-type stars SiS was also only detected in the highest density circumstellar envelopes, while CS was detected consistently in all surveyed carbon stars and sporadically among the S-type stars