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Our technique for selecting AGB-dominated clusters in nearby galaxies promises to be effective for discriminating the uncertainties associated with AGB stars in intermediate-age populations that plague age and mass estimation in high-z galaxies. Meidt, Sharon E. The Astrophysical Journal Letters 2 : 6 pp.

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Faculty Scholarship. Astrophysics and Astronomy Commons. Advanced Search. Privacy Copyright. Skip to main content. Title The S4G perspective on circumstellar dust extinction of asymptotic giant branch stars in M Abstract We examine the effect of circumstellar dust extinction on the near-IR NIR contribution of asymptotic giant branch AGB stars in intermediate-age clusters throughout the disk of M All details are found in Marigo et al.

The most relevant to this study include: As we show below, the variation of the magnitude of the efficiency of the third dredge-up does not change the TP-AGB lifetimes in mass-loss prescriptions with a high T eff dependence. For a star to have a wind, there must be an outward force that provides momentum and energy input, accelerating the surface layers to velocities larger than the escape velocity.

This may be realized in various ways, including the scattering of UV radiation by resonance line opacity in hot stars, the generation of magneto-acoustic waves above the photosphere in red giants, or the absorption of photons by dust grains in the outer atmospheres of the coolest and most luminous stars e. Mass loss dominates an AGB star's evolution and fate. Despite the recent progress in the theory of AGB mass loss e.

Combining theoretical efforts and empirical evidence, a reasonable scenario takes form in which mass loss on the AGB can be divided into three regimes: In our scheme, the phase of pre-dust mass loss with rate is thought to apply to the early stages on the AGB in which either dust has not yet formed in the outermost atmospheric layers, or if present in some small amount, is unable to generate an outflow.

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In stellar evolutionary calculations a frequent choice to describe mass loss during the early phases is the classical Reimers law, a simple scaling relation of stellar parameters based on observations of few red giants and supergiants. A self-consistent and more detailed theoretical approach is developed to follow the generation of energy flux due to magnetohydrodynamic turbulence from subsurface convection zones to its eventual dissipation and escape through the stellar wind.

Following the pre-dust phase of mass loss, as the star climbs the AGB at increasing luminosity, suitable conditions can be met in the cool atmosphere for stellar winds to be generated through a different intervening mechanism.

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The most plausible hypothesis resides in the momentum input when the stellar radiation field is absorbed or scattered by dust grains and transferred to the gas via collisions. Within this framework, the mass-loss prescriptions adopted in the TP-AGB stellar models computed for this study are as follows. For the dust driven wind phase we adopt a formula similar to Bedijn , which predicts an exponential increase of mass loss dependent on stellar parameters derived from models of periodic shocked atmospheres.

Coefficients a and b are calibrated on a sample of Galactic Mira stars. This prescription is also discussed in Marigo et al. We keep the same prescriptions for and and vary only the. For the mass-loss rates before the onset of dust-driven winds we consider four options:.

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Following Marigo et al. In this context, the role of RGB mass loss and its possible influence on the subsequent AGB evolution of low-mass stars is postponed to a future work. However, there is no doubt that mass loss is the principal mechanism that controls the duration of this phase, which ends when almost all the stellar mantle is ejected into the interstellar medium. In this study we opt to analyze the significance of the early stages of AGB mass loss, since this regime may be particularly important for low-mass stars whose small envelopes may already be removed before the onset of the dust-driven wind cf.

This choice seems appropriate given the sample of galaxies under consideration, which are all characterized by a significant fraction of old stellar populations and thus will have TP-AGB populations dominated by lower mass stars Figure 2. While analyzing the impact of different laws for and is postponed to future works, it is worth mentioning that the prescriptions adopted here have already successfully passed a few observational tests, including the recovery of the expansion velocities of AGB circumstellar envelopes Nanni et al. The panels are organized from top to bottom following a sequence of progressively more efficient mass loss.

Red lines mark when the star becomes C-rich. The case corresponds to the longest duration of the TP-AGB phase, and also to the reddest excursion on the Hertzsprung—Russell H-R diagram, while the TP-AGB tracks computed with have the shortest lifetimes and exhibit a smaller displacement toward the coolest T eff region.

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In general, comparing the panels from top to bottom we obtain a sequence of decreasing lifetimes and cooler H-R tracks, a trend which is more pronounced at lower masses. Both factors combine to influence the mass-loss rates e. In general, at the transition to the C-star regime, TP-AGB models with variable molecular opacities predict a sudden cooling of the track that makes the mass-loss rates increase provided the adopted prescription is a sensitive function of T eff with consequent reduction of the lifetimes.

This point is fully discussed in Marigo The TP-AGB lifetime may also be sensitive to changes of the third dredge-up efficiency after the transition C-star phase. To test this possibility, we computed additional sets of TP-AGB tracks by varying the efficiency of the third dredge-up, while keeping the same mass-loss formalism.

We find that the predicted TP-AGB lifetimes barely change see Figure 5 , right panel , even when the total amount of dredged-up material varies by a factor of 3. The limited effect of the third dredge-up can be explained as a combination of two main factors. The first is due to the efficiency of the mass-loss prescription adopted here. Therefore, there simply is not enough time for the third dredge-up to produce dramatic effects, no matter how it is varied. In addition, as the star reaches the super-wind phase, the mass-loss rates settle to typical values that are little affected by variations of other stellar parameters.

In the low mass, low metallicity regime, TP-AGB lifetimes are more affected the pre-dust mass-loss prescription than the efficiency of the third dredge-up. The second factor is related to the sensitivity of the effective temperature as a function of the carbon excess in the atmosphere of carbon stars.

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In general, more carbon excess corresponds to lower effective temperature which causes higher rates of mass loss. As a consequence, the impact of the third dredge-up on the effective temperature becomes progressively weaker as more mixing events occur. This result is important, as it strengthens the robustness of the analysis described next, whose primary aim is to obtain a quantitative estimation of the TP-AGB lifetimes as a function of the initial stellar mass, in the low-metallicity regime.

However, we must also emphasize that although the TP-AGB lifetimes are found to be little influenced by the third dredge-up, the chemical composition of the ejecta is much affected by the properties of the mixing events. In particular, for the same amount of mass lost, the quantity of primary carbon, and hence of carbonaceous dust, that is injected in the interstellar medium does depend strongly on the efficiency of the third dredge-up.

Therefore, although it is outside the scope of this paper, future calibration of the third dredge-up process is an essential step toward a comprehensive description of the TP-AGB phase that includes not only the spectro-photometric but also the chemical role of TP-AGB stars in the context of galaxy evolution. First, the computations discussed here are based on a completely new release of stellar evolution models Bressan et al.

This leads, for instance, to produce Hayashi lines that are on average somewhat cooler than in G For these reasons, as discussed above, a new T eff scaling was in order, so we proceed with the model in lieu of. M giants now come from an extended database from B.


For C-type stars, we adopt the Aringer et al. Radiation reprocessing by circumstellar dust shells in mass-losing stars are taken into account as in Marigo et al. Finally, the synthetic CMDs are corrected for distance and extinction, A V using extinction coefficients from Girardi et al. We synthesize at least 50 stellar populations with SFHs that are randomly sampled within the uncertainties of the best fit SFH.