[C I] and [C II] emission in the circumstellar envelope of IRC +10216 I. Observational data and NLTE modeling of the [C I] emission

Aims: The study at hand aims to describe the distribution of atomic carbon, C0, throughout the envelope, in support of an improved understanding of its photo-chemistry. Additionally, we also briefly discuss the observation of [CII] emission towards the star. Methods: We obtain spectra of the [CI] $\mathrm{^3P_1} \rightarrow \mathrm{^3P_0}$ fine structure line at projected distances of up to 78" from the star. The line profiles are characterized by both direct fitting of Gaussian components, and by modeling the observed line of the [CI] triplet. We also report the detection of the $\mathrm{^2P_{3/2}} \rightarrow \mathrm{^2P_{1/2}}$ line from the C+ fine structure singlet at the central position and at 32" from the star. Results: The overall picture of the [CI] emission from IRC +10216 agrees with more limited previous studies. The satisfying agreement between the observed and modeled line profiles, with emission at the systemic velocity appearing beyond one beam from the star, rules out that the C0 is located in a thin shell. Given that the bond energy of CO falls only 0.1 eV below the ionization threshold of C0, the absence of observable [CII] emission from sightlines beyond a projected distance of $\sim 10^{17}$ cm from the star (adopting a distance of 130 pc) does not contradict a scenario where the bulk of C0 is located between that of CO and C+, as expected for an external FUV radiation field. This conjecture is also corroborated by a model in which the C0 shell is located farther outside, failing to reproduce the [CI] line profiles at intermediate sky-plane distances from the star. Comparing a photo-chemical model adopted from literature with the simplifying assumption of a constant C0 abundance with respect to the $\mathrm{H}_2$ density, we constrain the inner boundary of the [CI] emitting shell, located at $\sim 10^{16}$ cm from the star.Comment: 10 pages, 7 figures, accepted for publication in A&

Vibrationally excited HCN transitions in circumstellar envelopes of carbon-rich AGB stars

Context. The most abundant molecule after H2 and CO in the circumstellar envelopes (CSEs) of carbon-rich asymptotic giant branch (AGB) stars is HCN. Its rotational lines within vibrationally excited states are exceptional tracers of the innermost region of carbon-rich CSEs. Aims. We aim to constrain the physical conditions of CSEs of carbon-rich stars using thermal lines of the HCN molecule. Additionally, we also search for new HCN masers and probe the temporal variations for HCN masers, which should shed light on their pumping mechanisms. Methods. We observed 16 carbon-rich AGB stars in various HCN rotational transitions, within the ground and 12 vibrationally excited states, with the Atacama Pathfinder Experiment (APEX) 12-metre sub-millimetre telescope. Results. We detect 68 vibrationally excited HCN lines from 13 carbon-rich stars, including 39 thermal transitions and 29 maser lines, suggesting that vibrationally excited HCN lines are ubiquitous in carbon-rich stars. Population diagrams constructed for two objects from the sample, for thermal transitions from different vibrationally excited states, give excitation temperatures around 800–900 K, confirming that they arise from the hot innermost regions of CSEs (i.e. r 108 cm−3 are required to excite the HCN masers. In some ways, HCN masers in carbon-rich stars might be regarded as an analogy of SiO masers in oxygen-rich stars

ATOMIUM: molecular inventory of 17 oxygen-rich evolved stars observed with ALMA

Context. The dusty winds of cool evolved stars are a major contributor of the newly synthesised material enriching the Galaxy and future generations of stars. However, the details of the physics and chemistry behind dust formation and wind launching have yet to be pinpointed. Recent spatially resolved observations show the importance of gaining a more comprehensive view of the circumstellar chemistry, but a comparative study of the intricate interplay between chemistry and physics is still difficult because observational details such as frequencies and angular resolutions are rarely comparable.Aims. Aiming to overcome these deficiencies, ATOMIUM is an ALMA Large Programme to study the physics and chemistry of the circumstellar envelopes of a diverse set of oxygen-rich evolved stars under homogeneous observing conditions at three angular resolutions between ∼0.02′′−1.4′′.Here we summarize the molecular inventory of these sources, and the correlations between stellar parameters and molecular content.Methods. Seventeen oxygen-rich or S-type asymptotic giant branch (AGB) and red supergiant (RSG) stars have been observed in several tunings with ALMA Band 6, targeting a range of molecules to probe the circumstellar envelope and especially the chemistry of dust formation close to the star. We systematically assigned the molecular carriers of the spectral lines and measured their spectroscopic parameters and the angular extent of the emission of each line from integrated intensity maps.Results. Across the ATOMIUM sample, we detect 291 transitions of 24 different molecules and their isotopologues. This includes several first detections in oxygen-rich AGB/RSG stars: PO ν = 1, SO2 ν1 = 1 and ν2 = 2, and several high energy H2O transitions. We also find several first detections in S-type AGB stars: vibrationally excited HCN ν2 = 2, 3 and SiS ν = 4, 5, 6, as well as first detections of the molecules SiC, AlCl, and AlF in W Aql. Overall, we find strong correlations between the following molecular pairs: CS and SiS, CS and AlF, NaCl and KCl, AlO and SO, SO2 and SO, and SO2 and H2O; meaning both molecules tend to have more detected emission lines in the same sources. The measured isotopic ratios of Si and S are found to be consistent with previous measurements, except for an anomalously high 29Si/30Si ratio of 4 ± 1 in the RSG VX Sgr.Conclusions. This paper presents the overall molecular inventory and an initial analysis of the large ATOMIUM dataset, laying the groundwork for future work deriving molecular abundances and abundance profiles using radiative transfer modeling which will provide more rigorous tests for chemical models.<br/

ATOMIUM: The astounding complexity of the near circumstellar environment of the M-type AGB star R Hydrae: I. Morpho-kinematical interpretation of CO and SiO emission

Evolved low- to intermediate-mass stars are known to shed their gaseous envelope into a large, dusty, molecule-rich circumstellar nebula which typically develops a high degree of structural complexity. Most of the large-scale, spatially correlated structures in the nebula are thought to originate from the interaction of the stellar wind with a companion. As part of the ATOMIUM large programme, we observed the M-type asymptotic giant branch (AGB) star R Hydrae with the Atacama Large Millimeter/submillimeter Array. The morphology of the inner wind of R Hya, which has a known companion at similar to 3500 au, was determined from maps of CO and SiO obtained at high angular resolution. A map of the CO emission reveals a multi-layered structure consisting of a large elliptical feature at an angular scale of similar to 10 '' that is oriented along the north-south axis. The wind morphology within the elliptical feature is dominated by two hollow bubbles. The bubbles are on opposite sides of the AGB star and lie along an axis with a position angle of similar to 115 degrees. Both bubbles are offset from the central star, and their appearance in the SiO channel maps indicates that they might be shock waves travelling through the AGB wind. An estimate of the dynamical age of the bubbles yields an age of the order of 100 yr, which is in agreement with the previously proposed elapsed time since the star last underwent a thermal pulse. When the CO and SiO emission is examined on subarcsecond angular scales, there is evidence for an inclined, differentially rotating equatorial density enhancement, strongly suggesting the presence of a second nearby companion. The position angle of the major axis of this disc is similar to 70 degrees in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is similar to 0.65 M-circle dot on the basis of the highest measured speeds in the disc and the location of its inner rim at similar to 6 au from the AGB star