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Post Info TOPIC: Asymptotic giant branch (AGB) variables
Blobrana


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RE: Asymptotic giant branch (AGB) variables
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Low Sodium Diet Key to Old Age for Stars

Astronomers expect that stars like the Sun will blow off much of their atmospheres into space near the ends of their lives. But new observations of a huge star cluster made using ESO's Very Large Telescope have shown - against all expectations - that a majority of the stars studied simply did not get to this stage in their lives at all. The international team found that the amount of sodium in the stars was a very strong predictor of how they ended their lives.
The way in which stars evolve and end their lives was for many years considered to be well understood. Detailed computer models predicted that stars of a similar mass to the Sun would have a period towards the ends of their lives - called the asymptotic giant branch, or AGB - when they undergo a final burst of nuclear burning and puff off a lot of their mass in the form of gas and dust.
The scientists detected the neutron star's abrupt slow-down with NASA's Swift observatory, a satellite with three telescopes whose science and flight operations are controlled by Penn State from the Mission Operations Centre on the University Park campus.
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Blobrana


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Posts: 131433
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Asymptotic Giant Branch stars
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Title: Heavy Element Nucleosynthesis in the Brightest Galactic Asymptotic Giant Branch stars
Authors: Amanda I. Karakas, D. A. García-Hernández, Maria Lugaro

We present updated calculations of stellar evolutionary sequences and detailed nucleosynthesis predictions for the brightest asymptotic giant branch (AGB) stars in the Galaxy with masses between 5 solar masses to 9 solar masses, with an initial metallicity of Z =0.02 ([Fe/H] = 0.14). In our previous studies we used the Vassiliadis & Wood (1993) mass-loss rate, which stays low until the pulsation period reaches 500 days after which point a superwind begins. Vassiliadis & Wood noted that for stars over 2.5 solar masses the superwind should be delayed until P ~ 750 days at 5 solar masses. We calculate evolutionary sequences where we delay the onset of the superwind to pulsation periods of P ~ 700-800 days in models of M = 5, 6, and 7 solar masses. Post-processing nucleosynthesis calculations show that the 6 and 7Msun models produce the most Rb, with [Rb/Fe] ~ 1 dex, close to the average of most of the Galactic Rb-rich stars ([Rb/Fe] ~ 1.4 plus or minus 0.8 dex). Changing the rate of the 22Ne + alpha reactions results in variations of [Rb/Fe] as large as 0.5 dex in models with a delayed superwind. The largest enrichment in heavy elements is found for models that adopt the NACRE rate of the 22Ne(a,n)25Mg reaction. Using this rate allows us to best match the composition of most of the Rb-rich stars. A synthetic evolution algorithm is then used to remove the remaining envelope resulting in final [Rb/Fe] of ~ 1.4 dex although with C/O ratios > 1. We conclude that delaying the superwind may account for the large Rb overabundances observed in the brightest metal-rich AGB stars.

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Blobrana


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Posts: 131433
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S-type AGB stars
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Title: The Spitzer Spectroscopic Survey of S-type Stars
Authors: K. Smolders, P. Neyskens, J. A. D. L. Blommaert, S. Hony, H. Van Winckel, L. Decin, S. Van Eck, G. C. Sloan, J. Cami, S. Uttenthaler, P. Degroote, D. Barry, M. Feast, M. A. T. Groenewegen, M. Matsuura, J. Menzies, R. Sahai, J. Th. van Loon, A. A. Zijlstra, B. Acke, S. Bloemen, N. Cox, P. de Cat, M. Desmet, K. Exter, D. Ladjal, R. Ostensen, S. Saesen, F. van Wyk, T. Verhoest, W. Zima

S-type AGB stars are thought to be in the transitional phase between M-type and C-type AGB stars. Because of their peculiar chemical composition, one may expect a strong influence of the stellar C/O ratio on the molecular chemistry and the mineralogy of the circumstellar dust. In this paper, we present a large sample of 87 intrinsic galactic S-type AGB stars, observed at infrared wavelengths with the Spitzer Space Telescope, and supplemented with ground-based optical data. On the one hand, we derive the stellar parameters from the optical spectroscopy and photometry, using a grid of model atmospheres. On the other, we decompose the infrared spectra to quantify the flux-contributions from the different dust species. Finally, we compare the independently determined stellar parameters and dust properties. For the stars without significant dust emission, we detect a strict relation between the presence of SiS absorption in the Spitzer spectra and the C/O ratio of the stellar atmosphere. These absorption bands can thus be used as an additional diagnostic for the C/O ratio. For stars with significant dust emission, we define three groups, based on the relative contribution of certain dust species to the infrared flux. We find a strong link between group-membership and C/O ratio. We show that these groups can be explained by assuming that the dust-condensation can be cut short before silicates are produced, while the remaining free atoms and molecules can then form the observed magnesium sulphides or the carriers of the unidentified 13 and 20 micron features. Finally, we present the detection of emission features attributed to molecules and dust characteristic to C-type stars, such as molecular SiS, hydrocarbons and magnesium sulphide grains. We show that we often detect magnesium sulphides together with molecular SiS and we propose that it is formed by a reaction of SiS molecules with Mg.

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Blobrana


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Posts: 131433
Date:
super-AGB stars
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Title: The "double final fate" of super-AGB stars and its possible consequences for some astrophysical issues
Authors: M. L. Pumo, P. Ventura, F. D'Antona, R. A. Zappala'

Super-AGB stars can conclude their evolution either as neon-oxygen white dwarfs or as electron-capture supernovae. We discuss the possible consequences of the existence of this "double final fate" in the self-enrichment of globular clusters and in the nucleosynthesis process of s-nuclei.

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Blobrana


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Posts: 131433
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M-type AGB stars
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Title: Circumstellar water vapour in M-type AGB stars: Constraints from H2O(1_10 - 1_01) lines obtained with Odin
Authors: M. Maercker, F.L. Schoeier, H. Olofsson, P. Bergman, U. Frisk, A Hjalmarson, K. Justtanont, S. Kwok, B. Larsson, M. Olberg, Aa Sandqvist

Aims: Spectrally resolved circumstellar H2O(1_10 - 1_01) lines have been obtained towards three M-type AGB stars using the Odin satellite. This provides additional strong constrains on the properties of circumstellar H2O and the circumstellar envelope. Methods: ISO and Odin satellite H2O line data are used as constraints for radiative transfer models. Special consideration is taken to the spectrally resolved Odin line profiles, and the effect of excitation to the first excited vibrational states of the stretching modes (nu1=1 and nu3=1) on the derived abundances is estimated. A non-local, radiative transfer code based on the ALI formalism is used. Results: The H2O abundance estimates are in agreement with previous estimates. The inclusion of the Odin data sets stronger constraints on the size of the H2O envelope. The H2O(1_10 - 1_01) line profiles require a significant reduction in expansion velocity compared to the terminal gas expansion velocity determined in models of CO radio line emission, indicating that the H2O emission lines probe a region where the wind is still being accelerated. Including the nu3=1 state significantly lowers the estimated abundances for the low-mass-loss-rate objects. This shows the importance of detailed modelling, in particular the details of the infrared spectrum in the range 3 to 6 micron, to estimate accurate circumstellar H2O abundances. Conclusions: Spectrally resolved circumstellar H2O emission lines are important probes of the physics and chemistry in the inner regions of circumstellar envelopes around asymptotic giant branch stars. Predictions for H2O emission lines in the spectral range of the upcoming Herschel/HIFI mission indicate that these observations will be very important in this context.

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Blobrana


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Posts: 131433
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RE: Asymptotic giant branch (AGB) variables
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Title: The Formation of Crystalline Dust in AGB Winds from Binary-induced Spiral Shocks
Authors: Richard G. Edgar, Jason Nordhaus, Eric G. Blackman, and Adam Frank

As stars evolve along the asymptotic giant branch (AGB), strong winds are driven from the outer envelope. These winds form a shell, which may ultimately become a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at the presence of a binary companion. Some post-AGB objects are surrounded by tori of crystalline dust, but there is no generally accepted mechanism for annealing the amorphous grains in the wind to crystals. Here, we show that the shaping of an AGB wind by a binary companion provides a possible mechanism for forming crystalline dust in the orbital plane.

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Blobrana


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Posts: 131433
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Title: AGB Variables and the Mira Period-Luminosity Relation
Authors: Patricia A. Whitelock, Michael W. Feast, Floor van Leeuwen

Published data for large amplitude asymptotic giant branch variables in the Large Magellanic Cloud are re-analysed to establish the constants for an infrared (K) period-luminosity relation of the form: Mk=rho[log P-2.38] + delta. A slope of rho=-3.51 ±0.20 and a zero point of delta=-7.15 ±0.06 are found for oxygen-rich Miras (if a distance modulus of 18.39 ±0.05 is used for the LMC). Assuming this slope is applicable to Galactic Miras we discuss the zero-point for these stars using the revised Hipparcos parallaxes together with published VLBI parallaxes for OH Masers and Miras in Globular Clusters. These result in a mean zero-point of delta=-7.25 ±0.07 for O-rich Galactic Miras. The zero-point for Miras in the Galactic Bulge is not significantly different from this value.
Carbon-rich stars are also discussed and provide results that are consistent with the above numbers, but with higher uncertainties. Within the uncertainties there is no evidence for a significant difference between the period-luminosity relation zero-points for systems with different metallicity.

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