ALMA spectral line and imaging survey of a low and a high mass-loss rate AGB star between 335 and 362 GHz

Abstract

Context. Low and intermediate mass stars are known to power strong stellar winds when evolving through the asymptotic giant branch (AGB) phase. Initial mass, luminosity, temperature, and composition determine the pulsation characteristics of the star and the dust species formed in the pulsating photospheric layers. Radiation pressure on these grains triggers the onset of a stellar wind. However, as of today, we still cannot predict the wind mass-loss rates and wind velocities from first principles neither do we know which species are the first to condense in the upper atmospheric regions. Aims. We aim to characterise the dominant physical, dynamical, and chemical processes in the inner wind region of two archetypical oxygen-rich (C/O < 1) AGB stars, that is, the low mass-loss rate AGB star R Dor (Ṁ ~ 1 × 10−7 M⊙ yr−1) and the high mass-loss rate AGB star IK Tau (Ṁ ~ 5 × 10−6 M⊙ yr−1). The purpose of this study is to observe the key molecular species contributing to the formation of dust grains and to cross-link the observed line brightnesses of several species to the global and local properties of the star and its wind. Methods. A spectral line and imaging survey of IK Tau and R Dor was made with ALMA between 335 and 362 GHz (band 7) at a spatial resolution of ~150 mas, which corresponds to the locus of the main dust formation region of both targets. Results. Some two hundred spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states (up to v = 5 for SiO). Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2. We present a spectral atlas for both stars and the parameters of all detected spectral features. A clear dichotomy for the sulphur chemistry is seen: while CS, SiS, SO, and SO2 are abundantly present in IK Tau, only SO and SO2 are detected in R Dor. Also other species such as NaCl, NS, AlO, and AlOH display a completely different behaviour. From some selected species, the minor isotopologues can be used to assess the isotopic ratios. The channel maps of many species prove that both large and small-scale inhomogeneities persist in the inner wind of both stars in the form of blobs, arcs, and/or a disk. The high sensitivity of ALMA allows us to spot the impact of these correlated density structures in the spectral line profiles. The spectral lines often display a half width at zero intensity much larger than expected from the terminal velocity, v∞, previously derived for both objects (36 km s−1 versus v∞~ 17.7 km s−1 for IK Tau and 23 km s−1 versus v∞~ 5.5 km s−1 for R Dor). Both a more complex 3D morphology and a more forceful wind acceleration of the (underlying) isotropic wind can explain this trend. The formation of fractal grains in the region beyond ~400 mas can potentially account for the latter scenario. From the continuum map, we deduce a dust mass of ~3.7 × 10−7 M⊙ and ~2 × 10−8 M⊙ for IK Tau and R Dor, respectively. Conclusions. The observations presented here provide important constraints on the properties of these two oxygen-dominated AGB stellar winds. In particular, the ALMA data prove that both the dynamical and chemical properties are vastly different for this high mass-loss rate (IK Tau) and low mass-loss rate (R Dor) star.

The nascent wind of AGB star R doradus: evidence for a recent episode of enhanced mass loss

“…The position angle of the approximate symmetry axis, ∼ 140 • , differs clearly from the position angle of the rotation axis in the inner region, ∼20 • , suggesting that outflows and inner rotation are unrelated. It matches instead the position angle of the blue-shifted high Doppler velocity component, whichDecin et al (2018) andHoman et al (2018) call the "blue-blob", probably an accidental coincidence.…”

“…In the present paper, we study instead the slower wind, exclusive of higher Doppler velocities. Earlier studies by Khouri et al (2016), Danilovich et al (2016), De Beck & Olofsson (2018), Van de Sande et al (2018), Decin et al (2018), Homan et al (2018) and Vlemmings et al (2018) have contributed a considerable amount of detailed information of relevance to the physicochemistry and dynamics of both dust and gas in this slower wind, however without attempting a detailed description of the morpho-kinematics, which is the aim of the present paper.…”

Observation of narrow polar jets in the nascent wind of oxygen-rich AGB star EP Aqr

“…It is often interpreted as resulting from star pulsations and is meant to be confined to the close neighbourhood of the star (Winters et al 2003;McDonald & Zijlstra 2016). Recently, Decin et al (2018) have observed the presence of wind velocities much larger than terminal in the inner regions of the circumstellar envelopes of oxygen-rich AGB stars IK Tau and R Dor and have discussed possible interpretations in terms of features other than narrow jets. The question then arises of how general, or how exceptional, is the presence of narrow polar jets in the nascent wind of AGB stars.…”

Molecular Data Needs for Modelling AGB Stellar Winds and Other Molecular Environments

“…Historically, vibrationally excited rotational transitions of SiO [27,28] and HCN [29][30][31] have been frequently detected, often thanks to maser activity. These are now joined by a plethora of other molecules in vibrationally excited states such as CO [32], H 2 O [33,34], SiS, SO, SO 2 [35], and others. Of course, the identification of these vibrationally excited lines requires their presence in line lists, which is true of the molecules listed above.…”

“…Of course, the identification of these vibrationally excited lines requires their presence in line lists, which is true of the molecules listed above. It is difficult to gauge to what extent unidentified lines seen in various observations [35][36][37] are due to not yet calculated vibrationally excited rotational transitions of "known" molecules rather than transitions of molecules or isotopologues for which no line lists have been calculated yet.…”