AGB outflows as tests of chemical kinetics and radiative transfer models

2019 ◽  
Vol 15 (S350) ◽  
pp. 253-256
Author(s):  
M. Van de Sande ◽  
T. Danilovich ◽  
L. Decin
Keyword(s):  
Radiative Transfer ◽  
Laboratory Experiments ◽  
Reaction Rates ◽  
Chemical Processes ◽  
Asymptotic Giant Branch ◽  
Laboratory Astrophysics ◽  
Agb Stars ◽  
New Science ◽  
Chemical Models ◽  
Molecular Abundances

AbstractThe outflows of asymptotic giant branch (AGB) stars are important astrochemical laboratories, rich in molecular material and host to various chemical processes, including dust formation. Since the different chemistries are relatively easily probed, AGB outflows are ideal testbeds within the wider astrochemical community. Recent observations are pushing the limits of both our current chemical models and radiative transfer routines. Current chemical models are restricted by the completeness of their chemical networks and the accuracy of the reaction rates. The molecular abundances retrieved by radiative transfer routines are strongly dependent on collisional rates, which are often not measured or calculated for molecules of interest. To further our understanding of the chemistry within the outflow, collaboration with the laboratory astrophysics community is essential. This collaboration is mutually beneficial, as it in turn provides new science questions for laboratory experiments and computations.

scholarly journals Preliminary results for the 19F(ρ,α)16O reaction cross section measured at INFN-LNS

2020 ◽  
Vol 227 ◽  
pp. 02009
Author(s):  
T Petruse ◽  
G. L. Guardo ◽  
M. La Cognata ◽  
D. Lattuada ◽  
C. Spitalieri ◽  
...  
Keyword(s):  
Nuclear Reaction ◽  
Cross Section ◽  
Theoretical Models ◽  
Calibration Procedure ◽  
Reaction Rates ◽  
Reaction Cross ◽  
Asymptotic Giant Branch ◽  
Angular Distributions ◽  
Agb Stars ◽  
Indirect Measurements

The 19F(ρ,α)16O reaction is an important fluorine destruction chan- nel in the proton-rich outer layers of asymptotic giant branch (AGB) stars and it might also play a role in hydrogen-deficient post-AGB star nucleosynthesis. At present, theoretical models overproduce F abundances in AGB stars with re-spect to the observed values, thus calling for further investigation of the nuclear reaction rates involved in the production and destruction of fluorine. In the last years, new direct and indirect measurements improved significantly the knowl- edge of 19F(ρ,α)16O cross section at deeply sub-Coulomb energies (below 0.8 MeV). However, those data are larger by a factor of 1.4 with respect the previ- ous data reported in the NACRE compilation in the energy region 0.6-0.8 MeV. Using the Large High resolution Array of Silicons for Astrophysics (LHASA), we performed a new direct measurement of the 19F(ρ,α)16O. The goal of this experiment is to reduce the uncertainties in the nuclear reaction rate of the 19F(ρ,α)16O reaction. Here, experimental details, the calibration procedure and angular distributions are presented.

scholarly journals Probing chemical processes in AGB stars

2008 ◽  
Vol 4 (S251) ◽  
pp. 201-206 ◽  
Author(s):  
Fredrik L. Schöier ◽  
Hans Olofsson
Keyword(s):  
Radiative Transfer ◽  
Mass Loss ◽  
Line Emission ◽  
Chemical Processes ◽  
Agb Stars ◽  
Molecular Line ◽  
Large Sample ◽  
Preliminary Results ◽  
Transfer Modelling ◽  
Non Equilibrium

AbstractWe are conducting multi-transition observations of circumstellar line emission from common molecules such as HCN, SiO, CS, SiS and CN for a large sample of AGB stars with varying photospheric C/O-ratios and mass-loss charachteristics. Our recently published results for SiO and SiS clearly show that major constraints on the relative roles of non-equilibrium chemistry, dust condensation, and photodissociation can be obtained from the study of circumstellar molecular line emission. Presented here are also preliminary results based on detailed radiative transfer modelling of HCN line emission.

scholarly journals Luminosities and mass-loss rates of Local Group AGB stars and red supergiants

2018 ◽  
Vol 609 ◽  
pp. A114 ◽  
Author(s):  
M. A. T. Groenewegen ◽  
G. C. Sloan
Keyword(s):  
Radiative Transfer ◽  
Mass Loss ◽  
Local Group ◽  
Mass Loss Rate ◽  
Large Fraction ◽  
Large Magellanic Cloud ◽  
Asymptotic Giant Branch ◽  
Spectral Energy ◽  
Agb Stars ◽  
Evolved Stars

Context. Mass loss is one of the fundamental properties of asymptotic giant branch (AGB) stars, and through the enrichment of the interstellar medium, AGB stars are key players in the life cycle of dust and gas in the universe. However, a quantitative understanding of the mass-loss process is still largely lacking. Aims. We aim to investigate mass loss and luminosity in a large sample of evolved stars in several Local Group galaxies with a variety of metalliticies and star-formation histories: the Small and Large Magellanic Cloud, and the Fornax, Carina, and Sculptor dwarf spheroidal galaxies (dSphs). Methods. Dust radiative transfer models are presented for 225 carbon stars and 171 oxygen-rich evolved stars in several Local Group galaxies for which spectra from the Infrared Spectrograph on Spitzer are available. The spectra are complemented with available optical and infrared photometry to construct spectral energy distributions. A minimization procedure was used to determine luminosity and mass-loss rate (MLR). Pulsation periods were derived for a large fraction of the sample based on a re-analysis of existing data. Results. New deep K-band photometry from the VMC survey and multi-epoch data from IRAC (at 4.5 μm) and AllWISE and NEOWISE have allowed us to derive pulsation periods longer than 1000 days for some of the most heavily obscured and reddened objects. We derive (dust) MLRs and luminosities for the entire sample. The estimated MLRs can differ significantly from estimates for the same objects in the literature due to differences in adopted optical constants (up to factors of several) and details in the radiative transfer modelling. Updated parameters for the super-AGB candidate MSX SMC 055 (IRAS 00483−7347) are presented. Its current mass is estimated to be 8.5 ± 1.6 M⊙, suggesting an initial mass well above 8 M⊙ in agreement with estimates based on its large Rubidium abundance. Using synthetic photometry, we present and discuss colour-colour and colour-magnitude diagrams which can be expected from the James Webb Space Telescope.

scholarly journals Dynamical Atmospheres and Winds of AGB Stars

2003 ◽  
Vol 210 ◽  
pp. 353-365 ◽  
Author(s):  
S. Höfner ◽  
C. Sandin ◽  
B. Aringer ◽  
A. C. Andersen ◽  
U.G. Jørgensen ◽  
...  
Keyword(s):  
Radiative Transfer ◽  
Asymptotic Giant Branch ◽  
Current Status ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Synthetic Spectra ◽  
Wind Characteristics ◽  
Dust Grain ◽  
Grain Properties ◽  
Giant Branch Stars

We summarize the current status of our latest generation of model atmospheres for pulsating asymptotic giant branch stars, discussing effects of non-grey radiative transfer, dust grain properties and drift between gas and dust on the atmospheric structures and wind characteristics. In addition, we give an overview of the resulting synthetic spectra and how they compare with observations.

scholarly journals The chemistry of carbon dust formation

1999 ◽  
Vol 191 ◽  
pp. 251-260 ◽  
Author(s):  
Isabelle Cherchneff ◽  
Piero Cau
Keyword(s):  
Silicon Carbide ◽  
Polycyclic Aromatic Hydrocarbons ◽  
Aromatic Hydrocarbons ◽  
Laboratory Experiments ◽  
Carbonaceous Material ◽  
Dust Formation ◽  
Agb Stars ◽  
Metal Carbides ◽  
Chemical Models ◽  
Polycyclic Aromatic

We shall review the various types of chemistry involved in the formation of carbonaceous material present in carbon-rich AGB stars, mainly amorphous carbon, silicon carbide and other metal carbides discovered in pre-solar Stardust extracted from meteorites. The chemistry is discussed in the context of laboratory experiments and their application to circumstellar AGB winds. Emphasis is put on polycyclic aromatic hydrocarbons (PAHs), titanium carbide clusters and silicon carbide grains. Attempt to explain the condensation sequences derived from the study of pre-solar grains of meteoretical origin is made on the basis of physio-chemical models which describe the periodically shocked gas close to the photosphere of AGB stars.

scholarly journals The surprisingly carbon-rich environment of the S-type star W Aql,

2020 ◽  
Vol 642 ◽  
pp. A20
Author(s):  
E. De Beck ◽  
H. Olofsson
Keyword(s):  
Line Emission ◽  
Abundance Ratio ◽  
Asymptotic Giant Branch ◽  
Elemental Abundance ◽  
Agb Stars ◽  
Trace Species ◽  
Abundance Estimates ◽  
Chemical Content ◽  
Molecular Abundances ◽  
Circumstellar Environment

Context. W Aql is an asymptotic giant branch (AGB) star with an atmospheric elemental abundance ratio C/O ≈ 0.98. It has previously been reported to have circumstellar molecular abundances intermediate between those of M-type and C-type AGB stars, which respectively have C/O < 1 and C/O > 1. This intermediate status is considered typical for S-type stars, although our understanding of the chemical content of their circumstellar envelopes is currently rather limited. Aims. We aim to assess the reported intermediate status of W Aql by analysing the line emission of molecules that have never before been observed towards this star. Methods. We performed observations in the frequency range 159−268 GHz with the SEPIA/B5 and PI230 instruments on the APEX telescope. We made abundance estimates through direct comparison to available spectra towards a number of well-studied AGB stars and based on rotational diagram analysis in the case of one molecule. Results. From a compilation of our abundance estimates and those found in the literature for two M-type (R Dor, IK Tau), two S-type (χ Cyg, W Aql), and two C-type stars (V Aql, IRC +10 216), we conclude that the circumstellar environment of W Aql appears considerably closer to that of a C-type AGB star than to that of an M-type AGB star. In particular, we detect emission from C2H, SiC2, SiN, and HC3N, molecules previously only detected towards the circumstellar environment of C-type stars. This conclusion, based on the chemistry of the gaseous component of the circumstellar environment, is further supported by reports in the literature on the presence of atmospheric molecular bands and spectral features of dust species which are typical for C-type AGB stars. Although our observations mainly trace species in the outer regions of the circumstellar environment, our conclusion matches closely that based on recent chemical equilibrium models for the inner wind of S-type stars: the atmospheric and circumstellar chemistry of S-type stars likely resembles that of C-type AGB stars much more closely than that of M-type AGB stars. Conclusions. Further observational investigation of the gaseous circumstellar chemistry of S-type stars is required to characterise its dependence on the atmospheric C/O. Non-equilibrium chemical models of the circumstellar environment of AGB stars need to address the particular class of S-type stars and the chemical variety that is induced by the range in atmospheric C/O.

scholarly journals On the fluorine nucleosynthesis in AGB stars in the light of the 19F(p,α)16O and 19F(α,p)22Ne reaction rate measured via THM

2019 ◽  
Vol 49 ◽  
pp. 1960011
Author(s):  
S. Palmerini ◽  
G. D’Agata ◽  
M. La Cognata ◽  
R. G. Pizzone ◽  
I. Indelicato ◽  
...  
Keyword(s):  
Experimental Data ◽  
Reaction Rate ◽  
State Of The Art ◽  
Reaction Rates ◽  
Asymptotic Giant Branch ◽  
Asymptotic Giant Branch Stars ◽  
Agb Stars ◽  
Stellar Nucleosynthesis ◽  
Trojan Horse Method ◽  
Giant Branch Stars

In the last years the [Formula: see text]O and the [Formula: see text]F([Formula: see text],p)[Formula: see text]Ne reactions have been studied via the Trojan Horse Method in the energy range of interest for astrophysics. These are the first experimental data available for the main channels of [Formula: see text]F destruction that entirely cover the energy regions typical of the stellar H- and He- burning. In both cases the reaction rates are significantly larger than the previous estimations available in the literature. We present here a re-analysis of the fluorine nucleosynthesis in Asymptotic Giant Branch stars by employing in state-of-the-art models of stellar nucleosynthesis the THM reaction rates for [Formula: see text]F destruction.

The Atmospheres of AGB Stars: Opacities, Radiative Transfer and Models

2008 ◽  
Vol 28 ◽  
pp. 67-74 ◽  
Author(s):  
B. Aringer ◽  
W. Nowotny ◽  
S. Höfner
Keyword(s):  
Radiative Transfer ◽  
Agb Stars

scholarly journals Dust Production around Carbon-Rich Stars: The Role of Metallicity

Universe ◽  
2021 ◽  
Vol 7 (7) ◽  
pp. 233
Author(s):  
Ambra Nanni ◽  
Sergio Cristallo ◽  
Jacco Th. van Loon ◽  
Martin A. T. Groenewegen
Keyword(s):  
Wind Speed ◽  
Galactic Halo ◽  
Magellanic Clouds ◽  
Asymptotic Giant Branch ◽  
Agb Stars ◽  
Circumstellar Envelopes ◽  
Chemical Enrichment ◽  
Wide Range ◽  
The Universe

Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended circumstellar envelopes. Among these stars, carbon-rich TP-AGB stars (C-stars) are particularly relevant for the chemical enrichment of galaxies. We here investigated the role of the metallicity in the dust formation process from a theoretical viewpoint. Methods: We coupled an up-to-date description of dust growth and dust-driven wind, which included the time-averaged effect of shocks, with FRUITY stellar evolutionary tracks. We compared our predictions with observations of C-stars in our Galaxy, in the Magellanic Clouds (LMC and SMC) and in the Galactic Halo, characterised by metallicity between solar and 1/10 of solar. Results: Our models explained the variation of the gas and dust content around C-stars derived from the IRS Spitzer spectra. The wind speed of the C-stars at varying metallicity was well reproduced by our description. We predicted the wind speed at metallicity down to 1/10 of solar in a wide range of mass-loss rates.

Rubidium in Barium Stars

2020 ◽  
Author(s):  
M P Roriz ◽  
M Lugaro ◽  
C B Pereira ◽  
N A Drake ◽  
S Junqueira ◽  
...  
Keyword(s):  
Neutron Source ◽  
Slow Neutron ◽  
Asymptotic Giant Branch ◽  
Neutron Density ◽  
Data Sets ◽  
Agb Stars ◽  
Branching Points ◽  
Theoretical Predictions ◽  
Low Mass ◽  
Chemically Peculiar Stars

Abstract Barium (Ba) stars are chemically peculiar stars that display in their atmospheres the signature of the slow neutron-capture (the s-process) mechanism that occurs in asymptotic giant branch (AGB) stars, a main contributor to the cosmic abundances. The observed chemical peculiarity in these objects is not due to self-enrichment, but to mass transfer between the components of a binary system. The atmospheres of Ba stars are therefore excellent astrophysical laboratories providing strong constraints for the nucleosynthesis of the s-process in AGB stars. In particular, rubidium (Rb) is a key element for the s-process diagnostic because it is sensitive to the neutron density and therefore its abundance can reveal the main neutron source for the s-process in AGB stars. We present Rb abundances for a large sample of 180 Ba stars from high resolution spectra (R = 48000), and we compare the observed [Rb/Zr] ratios with theoretical predictions from AGB s-process nucleosynthesis models. The target Ba stars in this study display [Rb/Zr] &lt;0, showing that Rb was not efficiently produced by the activation of branching points. Model predictions from the Monash and FRUITY data sets of low-mass (≲ 4 M⊙) AGB stars are able to cover the Rb abundances observed in the target Ba stars. These observations indicate that the 13C(α,n)16O reaction is the main neutron source of the s-process in the low-mass AGB companions of the observed Ba stars. We have not found in the present study candidate companion for IR/OH massive AGB stars.

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