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Register a new userFormation of SiC Grains in Pulsation-Enhanced Dust-Driven Wind Around Carbon Rich Asymptotic Giant Branch Stars
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We investigate the formation of silicon carbide (SiC) grains in the framework of dust-driven wind around pulsating carbon-rich Asymptotic Giant Branch (C-rich AGB) stars in order to reveal not only the amount but also the size distribution. Two cases are considered for the nucleation process; one is the LTE case where the vibration temperature of SiC clusters $T_{rm v}$ is equal to the gas temperature as usual, and another is the non-LTE case in which $T_{rm v}$ is assumed to be the same as the temperature of small SiC grains. The results of hydrodynamical calculations for a model with stellar parameters of mass $M_{ast}$=1.0 $M_{odot}$, luminosity $L_{ast}$=10$^{4}$ $L_{odot}$, effective temperature $T_{rm eff}$=2600 K, C/O ratio=1.4, and pulsation period $P$=650 days show the followings: In the LTE case, SiC grains condense in accelerated outflowing gas after the formation of carbon grains and the resulting averaged mass ratio of SiC to carbon grains of $sim$ 10$^{-8}$ is too small to reproduce the value of 0.01-0.3 inferred from the radiative transfer models. On the other hand, in the non-LTE case, the formation region of SiC grains is inner than and/or almost identical to that of carbon grains due to the so-called inverse greenhouse effect. The mass ratio of SiC to carbon grains averaged at the outer boundary ranges from 0.098 to 0.23 for the sticking probability $alpha_{rm s}$=0.1-1.0. The size distributions with the peak at $sim$ 0.2-0.3 $rm{mu}$m in radius cover the range of size derived from the analysis of presolar SiC grains. Thus the difference between temperatures of small cluster and gas plays a crucial role in the formation process of SiC grains around C-rich AGB stars, and this aspect should be explored for the formation process of dust grains in astrophysical environments.
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Eleven nearby (<300 pc), short-period (50-130 days) asymptotic giant branch (AGB) stars were observed in the CO J = (2-1) line. Detections were made towards objects that have evidence for dust production (Ks-[22] >~ 0.55 mag; AK Hya, V744 Cen, RU Crt, alpha Her). Stars below this limit were not detected (BQ Gem, eps Oct, NU Pav, II Hya, CL Hyi, ET Vir, SX Pav). Ks-[22] colour is found to trace mass-loss rate to well within an order of magnitude. This confirms existing results, indicating a factor of 100 increase in AGB-star mass-loss rates at a pulsation period of ~60 days, similar to the known superwind trigger at ~300 days. Between ~60 and ~300 days, an approximately constant mass-loss rate and wind velocity of ~3.7 x 10^-7 solar masses per year and ~8 km/s is found. While this has not been corrected for observational biases, this rapid increase in mass-loss rate suggests a need to recalibrate the treatment of AGB mass loss in stellar evolution models. The comparative lack of correlation between mass-loss rate and luminosity (for L <~ 6300 solar luminosities) suggests that the mass-loss rates of low-luminosity AGB-star winds are set predominantly by pulsations, not radiation pressure on dust, which sets only the outflow velocity. We predict that mass-loss rates from low-luminosity AGB stars, which exhibit optically thin winds, should be largely independent of metallicity, but may be strongly dependent on stellar mass.
Stars evolving along the Asymptotic Giant Branch can become Carbon-rich in the final part of their evolution. They replenish the inter-stellar medium with nuclear processed material via strong radiative stellar winds. The determination of the luminosity function of these stars, even if far from being conclusive, is extremely important to test the reliability of theoretical models. In particular, strong constraints on the mixing treatment and the mass-loss rate can be derived. We present an updated Luminosity Function of Galactic Carbon Stars obtained from a re-analysis of available data already published in previous papers. Starting from available near- and mid-infrared photometric data, we re-determine the selection criteria. Moreover, we take advantage from updated distance estimates and Period-Luminosity relations and we adopt a new formulation for the computation of Bolometric Corrections. This leads us to collect an improved sample of carbon-rich sources from which we construct an updated Luminosity Function. The Luminosity Function of Galactic Carbon Stars peaks at magnitudes around -4.9, confirming the results obtained in a previous work. Nevertheless, the Luminosity Function presents two symmetrical tails instead of the larger high luminosity tail characterizing the former Luminosity Function. The derived Luminosity Function of Galactic Carbon Stars matches the indications coming from recent theoretical evolutionary Asymptotic Giant Branch models, thus confirming the validity of the choices of mixing treatment and mass-loss history. Moreover, we compare our new Luminosity Function with its counterpart in the Large Magellanic Cloud finding that the two distributions are very similar for dust-enshrouded sources, as expected from stellar evolutionary models. Finally, we derive a new fitting formula aimed to better determine Bolometric Corrections for C-stars.
We explore the detailed and broad properties of carbon burning in Super Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models. The location of first carbon ignition, quenching location of the carbon burning flames and flashes, angular frequency of the carbon core, and carbon core mass are studied as a function of the ZAMS mass, initial rotation rate, and mixing parameters such as convective overshoot, semiconvection, thermohaline and angular momentum transport. In general terms, we find these properties of carbon burning in SAGB models are not a strong function of the initial rotation profile, but are a sensitive function of the overshoot parameter. We quasi-analytically derive an approximate ignition density, $rho_{ign} approx 2.1 times 10^6$ g cm$^{-3}$, to predict the location of first carbon ignition in models that ignite carbon off-center. We also find that overshoot moves the ZAMS mass boundaries where off-center carbon ignition occurs at a nearly uniform rate of $Delta M_{rm ZAMS}$/$Delta f_{rm{ov}}approx$ 1.6 $M_{odot}$. For zero overshoot, $f_{rm{ov}}$=0.0, our models in the ZAMS mass range $approx$ 8.9 to 11 $M_{odot}$ show off-center carbon ignition. For canonical amounts of overshooting, $f_{rm{ov}}$=0.016, the off-center carbon ignition range shifts to $approx$ 7.2 to 8.8 $M_{odot}$. Only systems with $f_{rm{ov}}$ $geq 0.01$ and ZAMS mass $approx$ 7.2-8.0 $M_{odot}$ show carbon burning is quenched a significant distance from the center. These results suggest a careful assessment of overshoot modeling approximations on claims that carbon burning quenches an appreciable distance from the center of the carbon core.
Infrared spectra of carbon-rich objects which have evolved off the asymptotic giant branch reveal a range of dust properties, including fullerenes, polycyclic aromatic hydrocarbons (PAHs), aliphatic hydrocarbons, and several unidentified features, including the 21 um emission feature. To test for the presence of fullerenes, we used the position and width of the feature at 18.7-18.9 um and examined other features at 17.4 and 6-9 um. This method adds three new fullerene sources to the known sample, but it also calls into question three previous identifications. We confirm that the strong 11 um features seen in some sources arise primarily from SiC, which may exist as a coating around carbonaceous cores and result from photo-processing. Spectra showing the 21 um feature usually show the newly defined Class D PAH profile at 7-9 um. These spectra exhibit unusual PAH profiles at 11-14 um, with weak contributions at 12.7 um, which we define as Class D1, or show features shifted to ~11.4, 12.4, and 13.2 um, which we define as Class D2. Alkyne hydrocarbons match the 15.8 um feature associated with 21 um emission. Sources showing fullerene emission but no PAHs have blue colors in the optical, suggesting a clear line of sight to the central source. Spectra with 21 um features and Class D2 PAH emission also show photometric evidence for a relatively clear line of sight to the central source. The multiple associations of the 21 um feature to aliphatic hydrocarbons suggest that the carrier is related to this material in some way.
A long debated issue concerning the nucleosynthesis of neutron-rich elements in Asymptotic Giant Branch (AGB) stars is the identification of the neutron source. We report intermediate-mass (4 to 8 solar masses) AGB stars in our Galaxy that are rubidium-rich owing to overproduction of the long-lived radioactive isotope 87Rb, as predicted theoretically 40 years ago. This represents a direct observational evidence that the 22Ne(alpha,n)25Mg reaction must be the dominant neutron source in these stars. These stars then challenge our understanding of the late stages of the evolution of intermediate-mass stars and would promote a highly variable Rb/Sr environment in the early solar nebula.
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