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.

International audienceWe trace the disk of HD 169142 (A8 Ve) from 0.57" to 1.4" (~80-200 AU projected distance) in 1.1 μm scattered light with HST NICMOS coronagraphy. The azimuthally symmetric disk has a peak azimuthally medianed surface brightness (SB) of ~5 mJy arcsec-2 at 0.57" from the star, and drops ~r-3. This radial SB profile is consistent with the presence of spatially resolved PAH emission and a Meeus group I IR SED only if the inner disk is either substantially flatter than the outer disk or partially devoid of material. Analysis of new HST ACS FUV imagery in tandem with archival IUE data indicates M˙acc<=10-9 Msolar yr-1. We estimate the age of HD 169142 to be 6+6-3 Myr by identifying 2MASS 18242929-2946559, located 9.3" to the southwest, as a 130 mas separation weak-line T Tauri binary that is comoving with HD 169142 at the 4 σ confidence level. We find no evidence for any additional stellar companion in either the ACS or Chandra ACIS-S data at r<=1''. HD 169142 has previously been interpreted as a slowly rotating, chemically peculiar star. However, by combining the disk inclination and vsini from the literature, we find that the star has vequatorial~240 km s-1, making it a rapid rotator, similar to Altair or Vega. The UV data for HD 169142 are consistent with gravity darkening, while the X-ray luminosity and spectrum resembles early F stars at the age of the β Pictoris moving group, rather than mid-A stars. In this context, spectral features previously interpreted as evidence for chemical peculiarity are more likely to reflect the presence of a strong photospheric latitudinal temperature gradient. With such a gradient, HD 169142 should closely resemble Vega at the epoch of central disk clearing

The inner 100 AU of HD 100546 is studied via far-ultraviolet long-slit spectroscopy with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS). The star is surrounded by reflection nebulosity, which can be traced 100 AU in the continuum, and by emission from H i Ly, N i, Si ii, and fluorescent H 2 transitions. The Ly emission can be traced up to 200 AU along the system semimajor axis and 300 AU along the semiminor axis. The radial surface brightness profile and the presence of both reflection nebulosity and molecular gas suggest that we have detected the flared surface of the disk predicted from analysis of the IR spectral energy distribution. When corrected for the r À2 falloff in illumination from the Herbig Be star, the reflection nebulosity, neutral atomic gas, and H 2 emission all reveal the presence of a central cavity extending 0B13 AE 0B025 (13 AU ) from the star, more than 20 times larger in radius than would be expected from dust sublimation alone. The reflection nebulosity within the cavity is centered on a location 0B05 (5 AU ) to the southeast of the star along the system semimajor axis. The pericenter asymmetry in the cavity is inconsistent with cavity formation by the combined effects of ice sublimation, radiation pressure blowout on small grains, or other disk chemistry that should produce azimuthally symmetric features. The STIS data are also consistent with a current accretion rate onto the Herbig Be star no higher than a few times 10 À9 M yr À1 , an order of magnitude lower than seen in 5-8 Myr old Herbig Ae stars. The low accretion rate, large cavity, pericenter asymmetry, and deficit of warm dust grain emission observed over 2-8 m are all consistent with dynamical sculpting of the disk by one or more bodies within the disk. An upper limit to the flux from any chromospherically active, low-mass companion is a factor of 5-10 fainter than AU Mic (M1 Ve; t ¼ 12 Myr) at the distance of HD 100546. This upper limit firmly excludes a low-mass stellar companion within the cavity but does not exclude a less active and/or luminous brown dwarf. The absence of similar central clearing in younger Herbig Ae stars suggests that any companion must become externally detectable late in the evolution of the disk, favoring a giant planet as the source of the structure in the HD 100546 disk.

The nebular circumstellar environments of cool evolved stars are known to harbour a rich morphological complexity of gaseous structures on different length scales. A large part of these density structures are thought to be brought about by the interaction of the stellar wind with a close companion. The S-type asymptotic giant branch (AGB) star π1Gruis, which has a known companion at ∼440 au and is thought to harbour a second, closer-by (< 10 au) companion, was observed with the Atacama Large Millimeter/submillimeter Array as part of the ATOMIUM Large programme. In this work, the brightest CO, SiO, and HCN molecular line transitions are analysed. The continuum map shows two maxima, separated by 0.04″ (6 au). The CO data unambiguously reveal that π1Gru’s circumstellar environment harbours an inclined, radially outflowing, equatorial density enhancement. It contains a spiral structure at an angle of ∼38 ± 3° with the line-of-sight. The HCN emission in the inner wind reveals a clockwise spiral, with a dynamical crossing time of the spiral arms consistent with a companion at a distance of 0.04″ from the AGB star, which is in agreement with the position of the secondary continuum peak. The inner wind dynamics imply a large acceleration region, consistent with a beta-law power of ∼6. The CO emission suggests that the spiral is approximately Archimedean within 5″, beyond which this trend breaks down as the succession of the spiral arms becomes less periodic. The SiO emission at scales smaller than 0.5″ exhibits signatures of gas in rotation, which is found to fit the expected behaviour of gas in the wind-companion interaction zone. An investigation of SiO maser emission reveals what could be a stream of gas accelerating from the surface of the AGB star to the companion. Using these dynamics, we have tentatively derived an upper limit on the companion mass to be ∼1.1 M⊙.

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 ∼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 ∼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 ∼115°. 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 ∼70° in the plane of the sky. We tentatively estimate that a lower limit on the mass of the nearby companion is ∼0.65 M⊙ on the basis of the highest measured speeds in the disc and the location of its inner rim at ∼6 au from the AGB star.

Aims. An anomalous effect in the abundances of oxygen isotopes in the most refractory calcium-aluminum-rich inclusions (CAIs) was discovered some thirty years ago. The origin of these oxygen isotopic anomalies has hitherto remained unexplained. The origin is neither nuclear, nor has the recent photochemical self-shielding explanation been proven to be valid. We discuss a possible chemical mechanism to resolve these observed effects. Methods. By uniting the most recent laboratory observations of nanoclusters of silicates in beams and the first principles theoretical studies of their structure and properties with a major dynamical constraint recently described as the surface non-RRKM effect during SiO 2 formation on the growing grain, we show that the origin of the anomalous isotopic effect in high-temperature minerals in CAIs is chemical and strictly mass-independent. Results. We report that the surface non-RRKM effect would represent a major process in the formation of our own solar system and observable protoplanetary accretion disks, and the mass-independent isotope effects are directly associated with the formation of primary grains in the high temperature nebular environment. We expect that this chemical reaction mechanistic approach combined with future time-resolved studies on the kinetics of growth of silicates and a precise knowledge of the oxygen isotopic abundances of the sun would provide a very detailed understanding of the origins of formation of our solar system.

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