Determining the effect of a non-uniform AGB outflow on its chemistry

2017 ◽  
Vol 13 (S332) ◽  
pp. 43-46
Author(s):  
M. Van de Sande ◽  
J. O. Sundqvist ◽  
T. J. Millar ◽  
L. Decin
Keyword(s):  
Thermal Equilibrium ◽  
Elemental Carbon ◽  
Abundance Ratio ◽  
Molecular Composition ◽  
Spherically Symmetric ◽  
Agb Stars ◽  
Stellar Outflows ◽  
Uv Photons ◽  
Inner Region ◽  
Innermost Region

AbstractThe molecular composition of the stellar outflows of AGB stars is determined by the stellar elemental carbon-to-oxygen abundance ratio, together with the physical circumstances in the innermost region of the outflow. Near the stellar surface, thermal equilibrium (TE) can be assumed. This leads to a certain molecular composition with a O- or C-rich signature. However, several molecular species have been detected that are not expected to be present in the inner region under the assumption of TE chemistry. As a solution to explain the presence of these unexpected species, non-equilibrium chemistry in the inner region of the outflow has been proposed. The outflows of AGB stars are generally not spherically symmetric or homogeneous, which influences the penetration of interstellar UV photons throughout the outflow. We investigate the effect of a clumpy, non-homogeneous outflow on the composition of the inner region by introducing a simple porosity formalism in our chemical model.

scholarly journals Search for high-mass protostars with ALMA revealed up to kilo-parsec scales (SPARKS)

2019 ◽  
Vol 632 ◽  
pp. A57 ◽  
Author(s):  
T. Csengeri ◽  
A. Belloche ◽  
S. Bontemps ◽  
F. Wyrowski ◽  
K. M. Menten ◽  
...  
Keyword(s):  
Accretion Disk ◽  
Moderate Temperature ◽  
Molecular Composition ◽  
High Mass ◽  
Central Position ◽  
Molecular Emission ◽  
Physical Location ◽  
Inner Region ◽  
Inner Envelope ◽  
Accretion Shocks

Context. Classical hot cores are rich in molecular emission, and they show a high abundance of complex organic molecules (COMs). The emergence of molecular complexity that is represented by COMs, in particular, is poorly constrained in the early evolution of hot cores. Aims. We put observational constraints on the physical location of COMs in a resolved high-mass protostellar envelope associated with the G328.2551−0.5321 clump. The protostar is single down to ~400 au scales and we resolved the envelope structure down to this scale. Methods. High angular resolution observations using the Atacama Large Millimeter Array allowed us to resolve the structure of the inner envelope and pin down the emission region of COMs. We use local thermodynamic equilibrium modelling of the available 7.5 GHz bandwidth around ~345 GHz to identify the COMs towards two accretion shocks and a selected position representing the bulk emission of the inner envelope. We quantitatively discuss the derived molecular column densities and abundances towards these positions, and use our line identification to qualitatively compare this to the emission of COMs seen towards the central position, corresponding to the protostar and its accretion disk. Results. We detect emission from 10 COMs, and identify a line of deuterated water (HDO). In addition to methanol (CH3OH), methyl formate (CH3OCHO) and formamide (HC(O)NH2) have the most extended emission. Together with HDO, these molecules are found to be associated with both the accretion shocks and the inner envelope, which has a moderate temperature of Tkin ~ 110 K. We find a significant difference in the distribution of COMs. O-bearing COMs, such as ethanol, acetone, and ethylene glycol are almost exclusively found and show a higher abundance towards the accretion shocks with Tkin ~ 180 K. Whereas N-bearing COMs with a CN group, such as vinyl and ethyl cyanide peak on the central position, thus the protostar and the accretion disk. The molecular composition is similar towards the two shock positions, while it is significantly different towards the inner envelope, suggesting an increase in abundance of O-bearing COMs towards the accretion shocks. Conclusions. We present the first observational evidence for a large column density of COMs seen towards accretion shocks at the centrifugal barrier at the inner envelope. The overall molecular emission shows increased molecular abundances of COMs towards the accretion shocks compared to the inner envelope. The bulk of the gas from the inner envelope is still at a moderate temperature of Tkin ~ 110 K, and we find that the radiatively heated inner region is very compact (<1000 au). Since the molecular composition is dominated by that of the accretion shocks and the radiatively heated hot inner region is very compact, we propose this source to be a precursor to a classical, radiatively heated hot core. By imaging the physical location of HDO, we find that it is consistent with an origin within the moderately heated inner envelope, suggesting that it originates from sublimation of ice from the grain surface and its destruction in the vicinity of the heating source has not been efficient yet.

scholarly journals A detailed look at chemical abundances in the Magellanic Clouds

2011 ◽  
Vol 7 (S283) ◽  
pp. 502-503
Author(s):  
Richard A. Shaw ◽  
Ting-Hui Lee ◽  
Letizia Stanghellini ◽  
James E. Davies ◽  
D. Anibal García-Hernández ◽  
...  
Keyword(s):  
Direct Methods ◽  
Abundance Ratio ◽  
Type I ◽  
Large Set ◽  
Agb Stars ◽  
Correction Factors ◽  
Chemical Abundances ◽  
Elemental Abundances ◽  
Low Metallicity ◽  
Low Mass

AbstractWe determine elemental abundances of He, N, O, Ne, S, and Ar in Magellanic Cloud planetary nebulae (PNe) using direct methods and a large set of observed ions, minimizing the need for ionization correction factors. In contrast to prior studies, we find a clear separation between Type I and non-Type I PNe in these low-metallicity environments, and no evidence that the O-N nucleosynthesis cycle is active in low-mass progenitors. We find that the S/O abundance ratio is anomalously low compared to H ii regions, confirming the “sulfur anomaly” found for Galactic PNe. We also found that Ne/O is elevated in some cases, raising the possibility that Ne yields in low-mass AGB stars may be enhanced at low metallicity.

scholarly journals A new model for dark matter fluid sphere

2020 ◽  
Vol 35 (34) ◽  
pp. 2050280
Author(s):  
Shyam Das ◽  
Nayan Sarkar ◽  
Monimala Mondal ◽  
Farook Rahaman
Keyword(s):  
Dark Matter ◽  
Compact Star ◽  
Outer Region ◽  
Fluid Sphere ◽  
Constant Density ◽  
Spherically Symmetric ◽  
Causality Condition ◽  
New Model ◽  
Inner Region ◽  
Tov Equation

We develop a new model for a spherically symmetric dark matter fluid sphere containing two regions: (i) Isotropic inner region with constant density and (ii) Anisotropic outer region. We solve the system of field equation by assuming a particular density profile along with a linear equation of state. The obtained solutions are well-behaved and physically acceptable which represent equilibrium and stable matter configuration by satisfying the Tolman–Oppenheimer–Volkoff (TOV) equation and causality condition, condition on adiabatic index, Harrison–Zeldovich–Novikov criterion, respectively. We consider the compact star EXO 1785-248 (Mass [Formula: see text] and radius R[Formula: see text]8.8 km) to analyze our solutions by graphical demonstrations.

Separation of gas and dust in the winds of AGB stars

2018 ◽  
Vol 14 (S343) ◽  
pp. 462-463
Author(s):  
Lars Mattsson ◽  
Christer Sandin ◽  
Paolo Ventura
Keyword(s):  
High Resolution ◽  
Asymptotic Giant Branch ◽  
Radiation Hydrodynamics ◽  
Symmetric Model ◽  
Spherically Symmetric ◽  
Agb Stars ◽  
First Results ◽  
Forced Turbulence ◽  
Spherically Symmetric Model ◽  
Entire Picture

AbstractWe present first results from a project aiming at a better understanding of how gas and dust interact in dust-driven winds from Asymptotic Giant Branch (AGB) stars. We are at the final stage of developing a new parallelised radiation-hydrodynamics (RHD) code for AGB-wind modelling including a new generalised implementation of drift. We also discuss first results from high-resolution box simulations of forced turbulence intended to give quantitative “3D corrections” to dust-driven winds from AGB stars. It is argued that modelling of dust-driven winds of AGB stars is a problem that may need to be treated in a less holistic way, where some parts of the problem are treated separately in detailed simulations and are parameterised back into a less detailed (1D spherically symmetric) model describing the entire picture.

DYNAMICS AND EMISSION OF MILDLY RELATIVISTIC PLASMA

2012 ◽  
Vol 12 ◽  
pp. 1-9
Author(s):  
A. G. AKSENOV ◽  
R. RUFFINI ◽  
I. A. SIUTSOU ◽  
G. V. VERESHCHAGIN
Keyword(s):  
Compton Scattering ◽  
Thermal Equilibrium ◽  
Nonequilibrium Plasma ◽  
Distribution Functions ◽  
Photon Pair ◽  
Accelerated Expansion ◽  
Spherically Symmetric ◽  
Two Photon ◽  
Electron Positron ◽  
Pair Annihilation

Initially optically thick (with τ = 3⋅107) spherically symmetric outflow consisting of electron-positron pairs and photons is considered. We do not assume thermal equilibrium, and include the two-body processes that occur in such plasma: Möller and Bhabha scattering of pairs, Compton scattering, two-photon pair annihilation, two-photon pair production, together with their radiative three-body variants: bremsstrahlung, double Compton scattering, and three-photon pair annihilation, with their inverse processes. We solve numerically the relativistic Boltzmann equations in spherically symmetric case for distribution functions of pairs and photons. Three epochs are considered in details: a) the thermalization, which brings initially nonequilibrium plasma to thermal equilibrium; b) the self-accelerated expansion, which we find in agreement with previous hydrodynamic studies and c) decoupling of photons from the expanding electron-positron plasma. Photon spectra are computed, and appear to be non thermal near the peak of the luminosity. In particular, the low energy part of the spectrum contain more power with respect to the black body one.

Higher-dimensional gravitational collapse of perfect fluid spherically symmetric spacetime in f(R, T) gravity

2018 ◽  
Vol 33 (12) ◽  
pp. 1850065 ◽  
Author(s):  
Suhail Khan ◽  
Muhammad Shoaib Khan ◽  
Amjad Ali
Keyword(s):  
Perfect Fluid ◽  
Outer Region ◽  
Energy Tensor ◽  
Spherically Symmetric ◽  
Field Equations ◽  
Higher Dimensional ◽  
Stress Energy ◽  
Inner Region ◽  
Symmetric Spacetime ◽  
Spherically Symmetric Spacetime

In this paper, our aim is to study (n + 2)-dimensional collapse of perfect fluid spherically symmetric spacetime in the context of f(R, T) gravity. The matching conditions are acquired by considering a spherically symmetric non-static (n + 2)-dimensional metric in the inner region and Schwarzschild (n + 2)-dimensional metric in the outer region of the star. To solve the field equations for above settings in f(R, T) gravity, we choose the stress–energy tensor trace and the Ricci scalar as constants. It is observed that two physical horizons, namely, cosmological and black hole horizons appear as a consequence of this collapse. A singularity is also formed after the birth of both the horizons. It is also observed that the term f(R0, T0) slows down the collapsing process.

scholarly journals Selected aspects of the analysis of molecular line observations of edge-on circumbinary disks

2020 ◽  
Vol 642 ◽  
pp. A127
Author(s):  
R. Avramenko ◽  
S. Wolf ◽  
T. F. Illenseer ◽  
S. Rehberg
Keyword(s):  
Density Distribution ◽  
Line Emission ◽  
Density Waves ◽  
Molecular Line ◽  
Hydrodynamic Simulations ◽  
Spatially Resolved ◽  
Transition Area ◽  
Multi Wavelength ◽  
Inner Region ◽  
Innermost Region

Context. Inner cavities, accretion arms, and density waves are characteristic structures in the density distribution of circumbinary disks. They are the result of the tidal interaction of the non-axisymmetric gravitational forces of the central binary with the surrounding disk and are most prominent in the inner region, where the asymmetry is most pronounced. Aims. The goal of this study is to test the feasibility of reconstructing the gas density distribution and quantifying properties of structures in the inner regions of edge-on circumbinary disks using multiple molecular line observations. Methods. The density distribution in circumbinary disks is calculated with 2D hydrodynamic simulations. Subsequently, molecular line emission maps are generated with 3D radiative transfer simulations. Based on these, we investigate the observability of characteristic circumbinary structures located in the innermost region for spatially resolved and unresolved disks. Results. We find that it is possible to reconstruct the inner cavity, accretion arms, and density waves from spatially resolved multi-wavelength molecular line observations of circumbinary disks seen edge-on. For the spatially unresolved observations only, an estimate can be derived for the density gradient in the transition area between the cavity and the disk’s inner rim.

scholarly journals A spatio-kinematic model for jets in post-AGB stars,

2019 ◽  
Vol 631 ◽  
pp. A53 ◽  
Author(s):  
D. Bollen ◽  
D. Kamath ◽  
H. Van Winckel ◽  
O. De Marco
Keyword(s):  
Binary Systems ◽  
Velocity Structure ◽  
Kinematic Model ◽  
Asymptotic Giant Branch ◽  
Opening Angle ◽  
Agb Stars ◽  
Line Profiles ◽  
Initial Spectrum ◽  
Orbital Phase ◽  
Inner Region

Aims. We aim to determine the geometry, density gradient, and velocity structure of jets in post-asymptotic giant branch (post-AGB) binaries. Methods. Our high cadence time series of high-resolution optical spectra of jet-creating post-AGB binary systems provide us with a unique tomography of the jet. We determine the spatio-kinematic structure of the jets based on these data by fitting the synthetic spectral line profiles created by our model to the observed, orbital phase-resolved, Hα-line profiles of these systems. The fitting routine is provided with an initial spectrum and is allowed to test three configurations, derived from three specific jet launching models: a stellar jet launched by the star, an X-wind, and a disk wind configuration. We apply a Markov-chain Monte Carlo routine in order to fit our model to the observations. Our fitting code is tested on the post-AGB binary IRAS 19135+3937. Results. We find that a model using the stellar jet configuration gives a marginally better fit to our observations. The jet has a wide half-opening angle of about 76° and reaches velocities up to 870 km s−1. Conclusions. Our methodology is successful in determining some parameters for jets in post-AGB binaries. The model for IRAS 19135+3937 includes a transparent, low density inner region (for a half-opening angle < 40°). The source feeding the accretion disk around the companion is most likely the circumbinary disk. We will apply this jet fitting routine to other jet-creating post-AGB stars in order to provide a more complete description of these objects.

scholarly journals Tidal Stripping and Disruption of Globular Clusters

1985 ◽  
Vol 113 ◽  
pp. 423-425
Author(s):  
T.S. van Albada ◽  
T. R. Bontekoe
Keyword(s):  
Globular Clusters ◽  
Spherically Symmetric ◽  
Low Density ◽  
Circular Velocity ◽  
Galactic Potential ◽  
The Galaxy ◽  
Inner Region

Tidal stripping and disruption of globular clusters may be responsible for the absence of low density clusters in the inner region of the Galaxy. We have studied these processes by integrating orbits of stars while the cluster is moving through a spherically symmetric galactic potential with constant circular velocity (Vcir =220 km/s). The response of the cluster to the tidal field of the galaxy is calculated in a selfconsistent manner with a collisionless N-body code with N=5000 (van Albada 1982, van Albada and Bontekoe in preparation).

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.

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