We model the synthesis of molecules and dust in the inner wind of the oxygen-rich Mira-type star IK Tau, by considering the effects of periodic shocks induced by the stellar pulsation on the gas, and by following the non-equilibrium chemistry in the shocked gas layers between 1 and 10 Rstar. We consider a complete set of molecules and dust clusters, and combine the nucleation phase of dust formation with the condensation of these clusters into dust grains. Our derived molecular abundances and dust properties are compared to the most recent observational data. The chemistry is described by using a chemical kinetic network of reactions and the condensation mechanism is described by a Brownian formalism. The shocks drive an active non-equilibrium chemistry in the dust formation zone of IK Tau where the collision destruction of CO in the post-shock gas triggers the formation of C-bearing species such as HCN and CS. Most of the modelled molecular abundances agree well with the latest values derived from Herschel data. Clusters of alumina are produced within 2 Rstar and lead to a population of alumina grains close to the stellar surface. Clusters of silicates form at larger radii (r > 3 Rstar), where their nucleation is triggered by the formation of HSiO and H2SiO. They efficiently condense and reach their final grain size distribution between ~ 6 and 8 Rstar, with a major population of medium size grains peaking at~ 0.02 microns. This two dust-shell configuration agrees with recent interferometric observations. The derived dust-to-gas mass ratio for IK Tau is in the range 1-6x10^-3 and agrees with values derived from observations of O-rich Mira-type stars. Our results confirm the importance of periodic shocks in chemically shaping the inner wind of AGB stars and providing gas conditions conducive to the efficient synthesis of molecules and dust by non-equilibrium processes.
Condensation of circumstellar dust begins with formation of molecular clusters close to the stellar photosphere. These clusters are predicted to act as condensation cores at lower temperatures and allow efficient dust formation farther away from the star. Recent observations of metal oxides, such as AlO, AlOH, TiO, and TiO$_2$, whose emission can be traced at high angular resolutions with ALMA, have allowed first observational studies of the condensation process in oxygen-rich stars. We are now in the era when depletion of gas-phase species into dust can be observed directly. I review the most…
The Mass-Loss Return from Evolved Stars to the Large Magellanic Cloud IV: Construction and Validation of a Grid of Models for Oxygen-Rich AGB Stars, Red Supergiants, and Extreme AGB Stars
Low- and intermediate-mass stars at the asymptotic giant branch (AGB) are known to be prevalent dust providers to galaxies. However, the mechanisms responsible for the formation and acceleration of dust in the cool extended atmospheres of AGB stars are still open to debate. We present visible polarimetric imaging observations of the oxygen-rich AGB star IK Tau obtained with SPHERE-ZIMPOL (phase 0.27) as well as interferometric observations with AMBER. IK Tau was observed with ZIMPOL at three wavelengths in the pseudo-continuum (645, 748, and 820 nm), in the H$α$ line at 656.3 nm, and in the Ti…
Previous determinations of the oxygen isotopic ratios in AGB carbon stars were at odds with the existing theoretical predictions. We aim to redetermine the oxygen ratios in these stars using new spectral analysis tools and further develop discussions on the carbon and nitrogen isotopic ratios in order to elucidate this problem. Oxygen isotopic ratios were derived from spectra in the K-band in a sample of galactic AGB carbon stars of different spectral types and near solar metallicity. Synthetic spectra calculated in LTE with spherical carbon-rich atmosphere models and updated molecular line li…
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 ALM…
We present the detection of neutral atomic carbon CI(3 P1 - 3 P0 ) line emission towards omi Cet. This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch (AGB) star. We also confirm the previously tentative CI detection around V Hya, a carbon-rich AGB star. As one of the main photodissociation products of parent species in the circumstellar envelope (CSE) around evolved stars, CI can be used to trace sources of ultraviolet (UV) radiation in CSEs. The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fra…
We report on our search for spectroscopic binaries among a sample of AGB stars. Observations were carried out in the framework of the monitoring of radial velocities of (candidate) binary stars performed at the Mercator 1.2m telescope, using the HERMES spectrograph. We found evidence for duplicity in UV Cam, TU Tau, BL Ori, VZ Per, T Dra, and V Hya. This short communication focus on V Hya, found to behave like RV Tau of the b subtype, which are binaries surrounded by a disc.
We have monitored over a ten-year interval the light variations of five evolved stars with very large mid-infrared excesses. All five objects appear to have oxygen-rich or mixed oxygen-rich and carbon-rich chemistries. They all vary in light: four over a small range of $\sim$0.2 mag and the fifth over a larger range of $\sim$0.7 mag. Spectral types range from G2 to B0. Periodic pulsations are found for the first time in the three cooler ones, IRAS 18075$-$0924 (123 days), 19207$+$2023 (96 days), and 20136$+$1309 (142 days). No significant periodicity is found in the hotter ones, but they appea…
Detections of SiO masers from the Bulge Asymmetries and Dynamical Evolution (BAaDE) survey more tightly define the region where Oxygen-rich (O) Asymptotic Giant Branch (AGB) stars reside in multiple infrared (IR) color-color diagrams. Using MSX and 2MASS data along with radio spectra from the BAaDE survey we find that three main populations were observed in the BAaDE survey: O-rich AGB stars of which about $73\%$ host SiO masers, Carbon-rich (C) AGB stars which do not host these masers, and a small contaminating set of possible Young Stellar Objects (YSOs). The distinction between YSOs and AGB…
The aim of this paper is to investigate the $^{17}$O/$^{18}$O ratio for a sample of AGB stars, containing M-, S- and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period. Circumstellar $^{13}$C$^{16}$O, $^{12}$C$^{17}$O and $^{12}$C$^{18}$O line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface $^{17}$O/$^{18}$O abundance ratio. Stellar evolution models predict the $^{17}$O/$^{18}$O …
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 …
Using ALMA, we observed the stellar wind of two oxygen-rich Asymptotic Giant Branch (AGB) stars, IK Tau and R Dor, between 335 and 362 GHz. One aim was to detect metal oxides and metal hydroxides (AlO, AlOH, FeO, MgO, MgOH), some of which are thought to be direct precursors of dust nucleation and growth. We report on the potential first detection of FeO (v=0, Omega=4, J=11-10) in RDor (mass-loss rate, Mdot, ~1e-7 Msun/yr). The presence of FeO in IK Tau (Mdot~5e-6 Msun/yr) cannot be confirmed due to a blend with 29SiS, a molecule that is absent in R Dor. The detection of AlO in R Dor and of AlO…
We performed a photometric and spectroscopic analysis of two RV Tauri stars GK Car and GZ Nor. Both objects are surrounded by hot circumstellar dust. Their pulsation periods, derived from ASAS photometric time series, have been used to derive their luminosities and distances via the PLC relation. In addition, for both objects, GAIA distances are available. The Gaia distances and luminosities are consistent with the values obtained from the PLC relationship. GK Car is at distance of 4.5$\pm{1.3}$ kpc and has a luminosity of 1520$\pm{840}$ L$_{\odot}$, while GZ Nor is at distance of 8.4$\pm{2.3}…
This review presents the latest advances in the nebular studies of post-AGB objects. Post-AGB stars are great tools to test nucleosynthesis and evolution models for stars of low and intermediate masses, and the evolution of dust in harsh environment. I will present the newly discovered class of post-RGB stars, formed via binary interaction on the RGB. Binary systems can also lead to the formation of two class of aspherical post-AGB, the Proto-Planetary Nebulae and the naked post-AGBs (dusty RV Taus , a.k.a. Van Winckel's stars).
Through a survey of (sub-)millimetre emission lines of various sulphur-bearing molecules, we aim to determine which molecules are the primary carriers of sulphur in different types of AGB stars. In this paper, the first in a series, we investigate the occurrence of H$_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…
We present 43 GHz VLA spectra for 51 AGB sources with the goal of verifying an infrared (IR) color cut intended to separate Carbon-rich (C) and Oxygen-rich (O) AGB sources throughout the Galaxy. The color cut is a simple line in the $[K_s]-[A]$ versus $[A]-[E]$ color-color diagram based on 2MASS and MSX photometry, and was originally derived from SiO detection rates in the Bulge Asymmetries and Dynamical Evolution (BAaDE) sample. The division is fully supported by the spectra presented here, which show that SiO maser detections lie on the O-rich side, and SiO nondetections and a single HC$_3$N…
ATOMIUM: Inner circumstellar envelopes of oxygen-rich AGB stars as revealed by highly excited SiO lines
National Astronomical Research Institute of Thailand (Public Organization)（泰国国家天文研究所）；Jodrell Bank Centre for Astrophysics, The University of Manchester（曼彻斯特大学乔德雷尔银行天体物理学中心）；Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS（波尔多大学CNRS波尔多天体物理实验室）；School of Physics & Astronomy, Monash University（蒙纳士大学物理与天文学院）；Institute of Astronomy, KU Leuven（荷鲁汶大学天文研究所）；School of Chemistry, University of Leeds（利兹大学化学学院）；Leiden Observatory, Leiden University（莱顿大学莱登天文台）；Departament de Física, EEBE, Universitat Politècnica de Catalunya（加泰罗尼亚理工大学EEBE物理系）；Harvard-Smithsonian Center for Astrophysics（哈佛-史密森尼天体物理中心）；Universität zu Köln, Astrophysik/I. Physikalisches Institut（科隆大学天体物理/第一物理研究所）；California Institute of Technology, Jet Propulsion Laboratory（加州理工学院喷气推进实验室）；Theoretical Astrophysics, Department of Physics and Astronomy, Uppsala University（乌普萨拉大学物理与天文学院理论天体物理）；Department of Computer Science, University College London（伦敦大学学院计算机科学系）
通过高分辨率 ALMA 观测，研究氧丰富晚生星的内 circumstellar 包层，发现高激发 SiO 线的分布特征及与质量损失的关系。
We present and analyse SO and SO$_2$, recently observed with high angular resolution and sensitivity in a spectral line survey with ALMA, for two oxygen-rich AGB stars: the low mass-loss rate R Dor and high mass-loss rate IK Tau. We analyse 8 lines of SO detected towards both stars, 78 lines of SO$_2$detected towards R Dor and 52 lines of SO$_2$ detected towards IK Tau. We detect several lines of $^{34}$SO, $^{33}$SO and $^{34}$SO$_2$ towards both stars, and tentatively S$^{18}$O towards R Dor, and hence derive isotopic ratios for these species. The spatially resolved observations show us that…
We conduct a population synthesis study of the common envelope evolution (CEE) of a white dwarf (WD) and an asymptotic giant branch (AGB) star and find that the potential number of type Ia supernovae (SNe Ia) from the core degenerate (CD) and from the double degenerate (DD) scenarios are of the same order of magnitude. For the CD scenario we consider cases where a carbon oxygen rich (CO) AGB core and a CO WD merge during the CEE and leave a WD remnant with a mass of >1.35Mo, and for the DD scenario we count surviving CO WD binary systems that merge within a time of 10 Gyr. When either the AGB …
C and O isotopic ratios are reported for a sample of 46 Mira and SRa-type variable AGB stars. Vibration-rotation 1st and 2nd overtone CO lines in 1.5 to 2.5 $μ$m spectra were measured to derive isotopic ratios for 12C/13C, 16O/17O, and 16O/18O. Comparisons with previous measurements for individual stars and with various samples of evolved stars are discussed. Models for solar composition AGB stars of different initial masses are used to interpret our results. We find that the majority of the M stars had main sequence masses < 2 Msun and have not experienced sizable third dredge-up episodes. Th…
Using ALMA observations of $^{12}$CO(2-1), $^{28}$SiO(5-4) and $^{32}$SO$_2$(16$_{6,10}$-17$_{5,13}$) emissions of the circumstellar envelope of AGB star EP Aqr, we describe the morpho-kinematics governing the nascent wind. Main results are: 1) Two narrow polar structures, referred to as jets, launched from less than 25 au away from the star, build up between $\sim$ 20 au and $\sim$ 100 au to a velocity of $\sim$ 20 \kms. They fade away at larger distances and are barely visible in CO data. 2) SO$_2$, SiO and CO emissions explore radial ranges reaching respectively $\sim$30 au, 250 au and 1000…
Aluminium monoxide, AlO, is likely efficiently depleted from the gas around oxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence in the extended atmospheres of evolved stars has been derived from optical spectroscopy. More recently, AlO gas was also detected at long wavelengths around the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB) star o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the IRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at studying other molecular species. We report here on observ…
The study of post asymptotic giant branch (post-AGB) stars is a valuable tool to study still poorly known aspects of the evolution of the stars through the AGB. This is due to the accurate determination of their surface chemical composition and to the peculiar shape of the SED: the emission from the central star can be easily disentangled from the contribution from the dusty shell, which can then be characterized. The goal of the present study is to reconstruct the dust formation process and more generally the late phases of the evolution of O-rich stars across the AGB phase. This is performed…
We present here the application of a model for a mass circulation mechanism in between the H-burning shell and the base of the convective envelope of low mass AGB stars, aimed at studying the isotopic composition of those presolar grains showing the most extreme levels of 18O depletion and high concentration of 26Mg from the decay of 26Al. The mixing scheme we present is based on a previously suggested magnetic-buoyancy process, already shown to account adequately for the formation of the main neutron source for slow neutron captures in AGB stars. We find that this scenario is also capable of …
We use archival ALMA observations of the CO(2-1) and SiO(5-4) molecular line emissions of AGB star R Hya to illustrate the relative contributions of rotation, expansion and line broadening to the morphology and kinematics of the circumstellar envelope (CSE) within some ~70 au (~0.5 arcsec) from the centre of the star. We give evidence for rotation and important line broadening to dominate the inner region, within ~14 au (~100 mas) from the centre of the star. The former is about an axis that projects a few degrees west of north and has a projected rotation velocity of a few km/s. The latter oc…
Based on their stellar parameters and the presence of a mid-IR excess due to circumstellar dust, RV Tauri stars have been classified as post-AGB stars. Our recent studies, however, reveal diverse SEDs among RV Tauri stars, suggesting they may occupy other evolutionary channels as well. The aim of this paper is to present the diverse SED characteristics of RV Tauri stars and investigate their evolutionary nature as a function of their SEDs. We carried out a systematic study of RV Tauri stars in the SMC and LMC because of their known distances and hence luminosities. Their SEDs were classified i…
Context. AGB stars go through a process of strong mass-loss that involves pulsations of the atmosphere, which extends to a region where the conditions are adequate for dust grains to form. Radiation pressure acts on these grains which, coupled to the gas, drive a massive outflow. The details of this process are not clear, including which molecules are involved in the condensation of dust grains. Aims. To study the role of the SiO molecule in the process of dust formation and mass-loss in M-type AGB stars. Methods. Using the IRAM NOEMA interferometer we observed the $^{28}$SiO and $^{29}$SiO $J…
Optically visible post-AGB/RGB stars and young stellar objects in the Small Magellanic Cloud: candidate selection, spectral energy distributions and spectroscopic examination
A spectral line and imaging survey of the low mass-loss rate AGB star R Dor (Mdot ~ 1e-7 Msun/yr) and the high mass-loss rate AGB star IK Tau (Mdot ~5e-6 Msun/yr) was made with ALMA between 335 and 362 GHz at a spatial resolution of ~150 mas, corresponding to the locus of the main dust formation region of both targets. Some 200 spectral features from 15 molecules (and their isotopologues) were observed, including rotational lines in both the ground and vibrationally excited states. Detected species include the gaseous precursors of dust grains such as SiO, AlO, AlOH, TiO, and TiO2. We present …
The late stellar evolutionary phases of low and intermediate-mass stars are strongly constrained by their mass-loss rates. The wind surrounding cool evolved stars frequently shows non-spherical features, thought to be due to an unseen companion orbiting the donor star. We study the morphology of the circumbinary envelope, in particular around oxygen-rich asymptotic giant branch (AGB) stars. We run a grid of 70 3D hydrodynamics simulations of a progressively accelerating wind propagating in the Roche potential formed by a mass-loosing evolved star in orbit with a main sequence companion. We res…
In the ONeMg cores of $8.8-9.5~{\rm M}_\odot$ stars, neon and oxygen burning is ignited off-center. Whether the neon-oxygen flame propagates to the center is critical to determine whether these stars undergo Fe core collapse or electron capture induced ONeMg core collapse. We present more details of stars that ignite neon and oxygen burning off-center. The neon flame is established in a similar manner to the carbon flame of super-AGB stars, albeit with a narrower flame width. The criteria for establishing a flame are able to be met if the strict Schwarzschild criterion for convective instabili…
