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Register a new userThe Contribution of Thermally-Pulsing Asymptotic Giant Branch and Red Super Giant Stars to the Luminosities of the Magellanic Clouds at 1-24 um
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J. Melbourne
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We present the near- through mid-infrared flux contribution of thermally-pulsing asymptotic giant branch (TP-AGB) and massive red super giant (RSG) stars to the luminosities of the Large and Small Magellanic Clouds (LMC and SMC, respectively). Combined, the peak contribution from these cool evolved stars occurs at ~3-4 um, where they produce 32% of the SMC light, and 25% of the LMC flux. The TP-AGB star contribution also peaks at ~3-4 um and amounts to 21% in both galaxies. The contribution from RSG stars peaks at shorter wavelengths, 2.2 um, where they provide 11% of the SMC flux, and 7% for the LMC. Both TP-AGB and RSG stars are short lived, and thus potentially impose a large stochastic scatter on the near-IR derived mass-to-light ratios of galaxies at rest-frame 1-4 um. To minimize their impact on stellar mass estimates, one can use the M/L ratio at shorter wavelengths (e.g. at 0.8 - 1 um). At longer wavelengths (>=8 um), emission from dust in the interstellar medium dominates the flux. In the LMC, which shows strong PAH emission at 8 um, TP-AGB and RSG contribute less than 4% of the 8 um flux. However, 19% of the SMC 8 um flux is from evolved stars, nearly half of which is produced by the rarest, dustiest, carbon-rich TP-AGB stars. Thus, star formation rates of galaxies, based on an 8 um flux (e.g. observed-frame 24 um at z=2), may be biased modestly high, especially for galaxies with little PAH emission.
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We discuss the dust chemistry and growth in the circumstellar envelopes (CSEs) of Thermally Pulsing Asymptotic Giant Branch (TP-AGB) star models computed with the COLIBRI code, at varying initial mass and metallicity (Z=0.001, 0.008, 0.02, 0.04, 0.06). A relevant result of our analysis deals with the silicate production in M-stars. We show that, in order to reproduce the observed trend between terminal velocities and mass-loss rates in Galactic M-giants, one has to significantly reduce the efficiency of chemisputtering by H2 molecules, usually considered as the most effective dust destruction mechanism. This indication is also in agreement with the most recent laboratory results, which show that silicates may condense already at T=1400 K, instead than at Tcond=1000 K, as obtained by models that include chemisputtering. From the analysis of the total dust ejecta, we find that the total dust-to-gas ejecta of intermediate-mass stars are much less dependent on metallicity than usually assumed. In a broader context, our results are suitable to study the dust enrichment of the interstellar medium provided by TP-AGB stars in both nearby and high redshift galaxies.
We present the dust ejecta of the new stellar models for the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase computed with the COLIBRI code. We use a formalism of dust growth coupled with a stationary wind for both M and C-stars. In the original version of this formalism, the most efficient destruction process of silicate dust in M-giants is chemisputtering by H2 molecules. For these stars we find that dust grains can only form at relatively large radial distances (r~5 R*), where they cannot be efficiently accelerated, in agreement with other investigations. In the light of recent laboratory results, we also consider the alternative case that the condensation temperature of silicates is determined only by the competition between growth and free evaporation processes (i.e. no chemisputtering). With this latter approach we obtain dust condensation temperatures that are significantly higher (up to Tcond~1400 K) than those found when chemisputtering is included (Tcond~900 K), and in better agreement with condensation experiments. As a consequence, silicate grains can remain stable in inner regions of the circumstellar envelopes (r~2 R*), where they can rapidly grow and can be efficiently accelerated. With this modification, our models nicely reproduce the observed trend between terminal velocities and mass loss rates of Galactic M-giants. For C-stars the formalism is based on the homogeneous growth scheme where the key role is played by the carbon over oxygen excess. The models reproduce fairly well the terminal velocities of Galactic stars and there is no need to invoke changes in the standard assumptions. At decreasing metallicity the carbon excess becomes more pronounced and the efficiency of dust formation increases. This trend could be in tension with recent observational evidence in favour of a decreasing efficiency, at decreasing metallicity.
We extend the formalism presented in our recent calculations of dust ejecta from the Thermally Pulsing Asymptotic Giant Branch (TP-AGB) phase, to the case of super-solar metallicity stars. The TP-AGB evolutionary models are computed with the COLIBRI code. We adopt our preferred scheme for dust growth. For M-giants, we neglect chemisputtering by H$_2$ molecules and, for C-stars we assume a homogeneous growth scheme which is primarily controlled by the carbon over oxygen excess. At super-solar metallicities, dust forms more efficiently and silicates tend to condense significantly closer to the photosphere (r~1.5 R$_*$) - and thus at higher temperatures and densities - than at solar and sub-solar metallicities (r~2-3 R$_*$). In such conditions, the hypothesis of thermal decoupling between gas and dust becomes questionable, while dust heating due to collisions plays an important role. The heating mechanism delays dust condensation to slightly outer regions in the circumstellar envelope. We find that the same mechanism is not significant at solar and sub-solar metallicities. The main dust products at super-solar metallicities are silicates. We calculate the total dust ejecta and dust-to-gas ejecta, for various values of the stellar initial masses and initial metallicities Z=0.04, 0.06. Merging these new calculations with those for lower metallicities it turns out that, contrary to what often assumed, the total dust-to-gas ejecta of intermediate-mass stars exhibit only a weak dependence on the initial metal content.
A zero point calibration of the Red Giant Branch Tip (TRGB) in the $I$-band is determined from OGLE photometry of the Magellanic Clouds (MCs). It is shown that TRGB measurements made in star-forming regions, with concomitantly high quantities of gas and dust, are less precise and biased to fainter magnitudes, as compared to the same measurements made in quiescent regions. Once these low accuracy fields are excluded from consideration, the TRGB can be used for the first time to constrain the three-dimensional plane geometry of the LMC. Composite CMDs are constructed for the SMC and LMC from only those fields with well-defined TRGB features, and the highest accuracy TRGB zero point calibration to date is presented. The $I$-band TRGB magnitude is measured to be flat over the color range $ 1.45 < (V-I)_0 < 1.95$ mag, with a modest slope introduced when including metal-rich (up to $(V-I)_0 = 2.2$ mag) Tip stars into the fit. Both the flat, blue zero point and the shallow slope calibration are consistent with the canonical value of $-4.05$ mag for the old, metal-poor TRGB, and would appear to resolve a recent debate in the literature over the methods absolute calibration.
The thermally-pulsing asymptotic giant branch (TP-AGB) experienced by low- and intermediate-mass stars is one of the most uncertain phases of stellar evolution and the models need to be calibrated with the aid of observations. To this purpose, we couple high-quality observations of resolved stars in the Small Magellanic Cloud (SMC) with detailed stellar population synthesis simulations computed with the TRILEGAL code. The strength of our approach relies on the detailed spatially-resolved star formation history of the SMC, derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds, as well as on the capability to quickly and accurately explore a wide variety of parameters and effects with the COLIBRI code for the TP-AGB evolution. Adopting a well-characterized set of observations -- star counts and luminosity functions -- we set up a calibration cycle along which we iteratively change a few key parameters of the TP-AGB models until we eventually reach a good fit to the observations. Our work leads to identify two best-fitting models that mainly differ in the efficiencies of the third dredge-up and mass loss in TP-AGB stars with initial masses larger than about 3 M$_{odot}$. On the basis of these calibrated models we provide a full characterization of the TP-AGB stellar population in the SMC in terms of stellar parameters (initial masses, C/O ratios, carbon excess, mass-loss rates). Extensive tables of isochrones including these improved models are publicly available.
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