Ka Tat Wong

Silicon monoxide (SiO) traces the physical conditions and dynamics in the circumstellar envelopes (CSEs) of AGB stars. We present high-resolution ALMA Band 6 observations of highly excited SiO emission in 14 oxygen-rich AGB stars. We cover transitions from υ = 0 to υ = 8, including first detections of ²⁸SiO υ = 3, 4, 8, J = 6 − 5, ²⁹SiO υ = 6, J = 6 − 5, and ³⁰SiO υ = 4, 5, J = 6 − 5, some of which are masers. The υ = 8 transition is the highest υ-state observed in an AGB star yet. Masers in υ = 0 are detected clearly in V PsA and IRC+10011 and tentatively in T Mic. R Hya exhibits the richest SiO spectrum. SiO J = 6 − 5 absorption is seen in R Aql, R Hya, S Pav, and T Mic, with features indicative of both infalls and outflows, and tentative detection of ²⁸SiO υ = 8, J = 6 − 5 absorption is found towards S Pav and R Aql. Highly excited SiO emission is often distributed in arcs or clumps with velocity gradients; components in R Hya and U Her align with predicted shock fronts. Detection rates show no significant difference between low and high mass-loss rate stars, although line overlap may affect some intensities. Maser detections appear uncorrelated with pulsation period or phase. The radius enclosing 90 per cent of compact SiO emission shows a tentative correlation with mass-loss rate. These results highlight the role of mass loss and CSE geometry in shaping high-excitation SiO emission.

Complex organic molecules (COMs) are considered essential precursors to prebiotic species in the interstellar and circumstellar medium. Despite their astrobiological relevance, many aspects of the formation of COMs remain unclear, particularly the role of ultraviolet (UV) radiation. While COMs were once expected to be efficiently destroyed under UV-irradiated conditions, detections in photodissociation regions (PDRs) have challenged this view. However, the mechanisms by which UV radiation contributes to their formation are still uncertain. Here we present moderately resolved maps of simple and complex organic molecules at the UV-illuminated edge of the Horsehead nebula, obtained by combining Atacama Large Millimeter/submillimeter Array (ALMA) and IRAM 30 m single-dish observations at ~15″ resolution. For the first time in this PDR environment, we analyzed the spatial distribution of species such as C ¹⁷ O, CH ₂ CO, CH ₃ CHO, HNCO, CH ₃ CN, and HC ₃ N. By incorporating previous C ¹⁷ O and C ¹⁸ O single-dish data as well as Plateau de Bure Interferometer (PdBI) maps of H ₂ CO and CH ₃ OH, we derived profiles of gas density, temperature, thermal pressure, and column densities of the organic species as a function of distance from the UV source. Our results show that most organic species – particularly H ₂ CO, CH ₂ CO, CH ₃ CHO, HNCO, and CH ₃ CN – exhibit enhanced column densities at the UV-illuminated edge compared to cloud interiors, possibly indicating efficient dust-grain surface chemistry driven by the diffusion of atomic C and radicals produced via photodissociation of CO and CH ₃ OH, as supported by recent laboratory experiments. The exceptions, HC ₃ N and CH ₃ OH, can be attributed to inefficient formation on dust grains and ineffective nonthermal desorption into the gas phase, respectively. Additionally, contributions from gas-phase hydrocarbon photochemistry, possibly seeded by grain-surface products, cannot be ruled out. Further chemical modeling is needed to confirm the efficiency of these pathways for the studied species, which could have important implications for other cold UV-irradiated environments such as protoplanetary disks.

Context. Low- and intermediate-mass stars on the asymptotic giant branch (AGB) account for a significant portion of the dust and chemical enrichment in their host galaxy. Understanding the dust formation process of these stars and their more massive counterparts, the red supergiants, is essential for quantifying galactic chemical evolution. Aims. To improve our understanding of the dust nucleation and growth process, we aim to better constrain stellar properties at millimetre wavelengths. To characterise how this process varies with the mass-loss rate and pulsation period, we studied a sample of oxygen-rich and S-type evolved stars. Methods. Here we present ALMA observations of the continuum emission around a sample of 17 stars from the ATOMIUM survey. We analysed the stellar parameters at 1.24 mm and the dust distributions at high angular resolutions. Results. From our analysis of the stellar contributions to the continuum flux, we find that the semi-regular variables all have smaller physical radii and fainter monochromatic luminosities than the Mira variables. Comparing these properties with pulsation periods, we find a positive trend between the stellar radius and period only for the Mira variables with periods of more than 300 days, and we find and a positive trend between the period and the monochromatic luminosity only for the red supergiants and the most extreme AGB stars with periods of more than 500 days. We find that the continuum emission at 1.24 mm can be classified into four groups; (i) ‘featureless’ continuum emission is confined to the (unresolved) regions close to the star for five stars in our sample, (ii) relatively uniform extended flux is seen for four stars, (iii) tentative elongated features are seen for three stars, and (iv) the remaining five stars have unique or unusual morphological features in their continuum maps. These features can be explained by the fact that 10 of the 14 AGB stars in our sample have binary companions. Conclusions. Based on our results, we conclude that there are two modes of dust formation: well-established pulsation-enhanced dust formation and our newly proposed companion-enhanced dust formation. If the companion is located close to the AGB star, in the wind acceleration region, then additional dust formed in the wake of the companion can increase the amount of mass lost through the dust-driven wind. This explains the different dust morphologies seen around our stars and partly accounts for the large scatter in literature mass-loss rates, especially among semi-regular stars with small pulsation periods.

Aims . We present high-angular-resolution imaging of the asymptotic giant branch star W Hya with the Atacama Large Millimeter/submillimeter Array (ALMA) to probe the dynamics and chemistry in the atmosphere and inner wind. Methods . W Hya was observed with the longest baselines of ALMA at 250–268 GHz with an angular resolution of ~17×20 mas. Results . ALMA’s high angular resolution allowed us to resolve the stellar disk of W Hya along with clumpy, irregularly shaped emission extending to ~100 mas. This emission includes a plume in the north-northwest, a tail in the south-southwest, and the extended atmosphere elongated in the east-northeast-west-southwest direction, with semimajor and semiminor axes of ~70 and 40 mas (~3.4 and 1.9 R _⋆ ), respectively. We identified 57 lines, which include SiO, H ₂ O, SO ₂ , SO, HCN, AlO, AlOH, TiO, TiO ₂ , OH, and some of their isotopologues, with about half of them being in vibrationally excited states. The molecular line images show spatially inhomogeneous molecular formation. Our ALMA data taken at phase 0.53 (minimum light) indicate global, accelerating infall within ~75 mas (3.6 R _⋆ ) but also outflow at up to ~10 km s ⁻¹ in deeper layers. While 38 of the detected lines appear in absorption against the continuum stellar disk as expected, we detect nonthermal emission on top of the continuum over the stellar disk in 19 lines, including SiO, H ₂ O, SO ₂ , and AlO. The emission of the SiO, AlO, TiO, TiO ₂ , SO, and SO ₂ lines coincides well with the clumpy dust cloud distribution obtained from contemporaneous visible polarimetric imaging in addition to H ₂ O reported in our previous work. This lends support to the idea that SiO, H ₂ O, and AlO are directly involved in grain nucleation. The overlap of SO/SO ₂ (possibly also TiO/TiO ₂ ) with the dust clouds suggests the formation of these molecules and dust behind shocks induced by pulsation and/or convection. We detect HCN emission close to the star, down to ~30 mas (~1.4 R _⋆ ), which is consistent with shock-induced chemistry.

Close companions influence stellar evolution through tidal interactions, mass transfer, and mass loss effects. While such companions are detected around young stellar objects, main-sequence stars, red giants, and compact objects, direct observational evidence of close-in companions around asymptotic giant branch (AGB) stars has remained elusive. Here, we present (sub)millimeter time-domain imaging spectroscopy revealing the Keplerian motion of a close-in companion around the AGB star pi1 Gruis. The companion, slightly more massive than the AGB star, is likely a main-sequence star. Unlike more evolved stars with companions at comparable distances, pi1 Gru's companion follows a circular orbit, suggesting an eccentricity-generating mechanism late- or post-AGB. Our analysis suggests that model-predicted circularization rates may be underestimated. Our results highlight the potential of multi-epoch (sub)millimeter interferometry in detecting the Keplerian motion of close companions to giant stars and open avenues for our understanding of tidal interaction physics and binary evolution.

Context. At the end of their lives, low- to intermediate-mass stars reach the asymptotic giant branch (AGB), during which their photospheres expand by up to several hundred times and strong stellar winds develop. These changes lead to various interactions with celestial bodies in their close circumstellar environments, including mass- and angular-momentum transfer. Aims. We aim to characterize the properties of the inner companion of the S-type AGB star π¹ Gru and to identify plausible future evolutionary scenarios for this triple system. Methods. We observed π¹ Gru with the Atacama Large Millimeter/sub-millimeter Array (ALMA) and the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument of the Very Large Telescope (VLT), collected archival photometric data, and used the HIPPARCOS-Gaia proper motion anomaly. We derived the best orbital parameters using Bayesian inference. Results. In June-July 2019, the inner companion, π¹ Gru C, was located at 37.4±2.0 mas from the primary (a projected separation of 6.05±0.55 au at 161.7±11.7 pc). The best orbital solution yields a companion mass of 0.86^+0.22_−0.20 M_⊙ (using the derived mass of the primary) and a semi-major axis of 7.05_−0.57^+0.54 au, corresponding to an orbital period of 11.0_−1.5^+1.7 yr. The preferred solution is an elliptical orbit with eccentricity e = 0.35_−0.17^+0.18, although a circular orbit cannot be fully excluded. The close companion could be either a K1V_K7V^F9.5V star or a white dwarf (WD). Ultraviolet and millimeter continuum photometry are consistent with the presence of an accretion disk around the close companion. The ultraviolet emission may originate from hot spots in an overall cooler disk, or from a hot disk if the companion is a WD. Conclusions. Although the close companion and the AGB star are interacting and an accretion disk is observed around the companion, the mass-accretion rate is too low to trigger a Type Ia supernova, but it could produce novæevery ≈900 yr. Short-wavelength, spatially resolved observations are required to further constrain the nature of the C companion. Searches for close-in companions similar to this system will improve our understanding of the physics of mass and angular momentum transfer, as well as orbital evolution during late evolutionary stages.

Context. Strong laser emission from hydrogen cyanide (HCN) at 805 and 891 GHz has been discovered towards carbon-rich (C-rich) asymptotic giant branch (AGB) stars. Both lines belong to the Coriolis-coupled system between the (1,1^1e,0) and (0,4⁰,0) vibrational states, which has been extensively studied in early molecular spectroscopy in the laboratory. However, the other lines in this system with frequencies above ∼900 GHz, which are challenging to observe with ground-based telescopes, have remained unexplored in astronomical contexts. Aims. We aim to (1) search for new HCN transitions that show laser activity in the (0,4⁰,0), J = 10−9 line at 894 GHz, the (1,1^1e,0)−(0,4⁰,0), J = 11−10 line at 964 GHz, the (1,1^1e,0), J = 11−10 at 968 GHz, and the (1,1^1e,0), J = 12−11 line at 1055 GHz towards C-rich AGB stars; (2) study the variability of multiple HCN laser lines, including the two known lasers at 805 and 891 GHz; and (3) construct a complete excitation scenario to the Coriolis-coupled system. Methods. We conducted SOFIA/4GREAT observations and combined our data with Herschel/HIFI archival data to construct a sample of eight C-rich AGB stars, covering six HCN transitions (i.e. the 805, 891, 894, 964, 968, and 1055 GHz lines) in the Coriolis-coupled system. Results. We report the discovery of HCN lasers at 964, 968, and 1055 GHz towards C-rich AGB stars. Laser emission in the 805, 891, and 964 GHz HCN lines was detected in seven C-rich stars, while the 968 GHz laser was detected in six stars and the 1055 GHz laser in five stars. Notably, the 894 GHz line emission was not detected in any of the targets. Among the detected lasers, the emission of the cross-ladder line at 891 GHz is always the strongest, with typical luminosities of a few 10⁴⁴ photons s⁻¹. The cross vibrational state 964 GHz laser emission, which is like a twin of the 891 GHz line, is the second strongest. The 1055 GHz laser emission always has a stronger 968 GHz counterpart. Towards IRC+10216, all five HCN laser transitions were observed in six to eight epochs and exhibited significant variations in line profiles and intensities. The 891 and 964 GHz lines exhibit similar variations, and their intensity changes do not follow the near-infrared light curve (i.e. they have non-periodic variations). In contrast, the variations in the 805, 968, and 1055 GHz lines appear to be quasi-periodic, with a phase lag of 0.1–0.2 relative to the near-infrared light curve. A comparative analysis indicates that these HCN lasers may be seen as analogues to vibrationally excited SiO and H₂O masers in oxygen-rich stars. Conclusions. We suggest that chemical pumping and radiative pumping could play an important role in the production of the cross-ladder HCN lasers, while the quasi-periodic behaviour of the rotational HCN laser lines may be modulated by additional collisional and radiative pumping driven by periodic shocks and variations in infrared luminosity.

Aims. We present contemporaneous high-angular-resolution millimeter imaging and visible polarimetric imaging of the nearby asymptotic giant branch (AGB) star W Hya to better understand the dynamics and dust formation within a few stellar radii. Methods. The star W Hya was observed in two vibrationally excited H₂O lines at 268 and 251 GHz with Atacama Large Millimeter/submillimeter Array (ALMA) at a spatial resolution of 16 × 20 mas and at 748 and 820 nm at a resolution of 26 × 27 mas with the Very Large Telescope (VLT)/Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE)-Zurich Imaging Polarimeter (ZIMPOL). Results. ALMA’s high spatial resolution allowed us to image strong emission of the vibrationally excited H₂O line at 268 GHz (v₂ = 2, JK_a, K_c = 6_5, 2–7_4, 3) over the stellar surface instead of absorption against the continuum, which is expected for thermal excitation. Strong, spotty emission was also detected along and just outside the stellar disk limb at an angular distance of ∼40 mas (∼1.9 R_⋆), extending to ∼60 mas (∼2.9 R_⋆). Another H₂O line (v₂ = 2, JK_a, K_c = 9_2, 8–8_3, 5) at 251 GHz with a similar upper-level energy was tentatively identified, which shows absorption over the stellar surface. This suggests that the emission over the surface seen in the 268 GHz H₂O line is suprathermal or even maser emission. The estimated gas temperature and H₂O density are consistent with the radiatively pumped masers. The 268 GHz H₂O line reveals global infall at up to ∼15 km s⁻¹ within 2–3 R_⋆, but outflows at up to ∼8 km s⁻¹ are also present. The polarized intensity maps obtained in the visible reveal clumpy dust clouds forming within ∼40 mas (∼1.9 R_⋆) with a particularly prominent cloud in the SW quadrant and a weaker cloud in the east. The 268 GHz H₂O emission overlaps very well with the visible polarized intensity maps, which suggests that both the nonthermal and likely maser H₂O emission and the dust originate from dense, cool pockets in the inhomogeneous atmosphere within ∼2–3 R_⋆.

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: POv= 1, SO₂v₁= 1 andv₂= 2, and several high energy H₂O transitions. We also find several first detections in S-type AGB stars: vibrationally excited HCNv₂= 2,3 and SiSv= 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, SO₂and SO, and SO₂and 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 high²⁹Si/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.