اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدIdentification of a Class of Low-Mass Asymptotic Giant Branch Stars Struggling to Become Carbon Stars in the Magellanic Clouds
197
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We have identified a new class of Asymptotic Giant Branch (AGB) stars in the Small and Large Magellanic Clouds (SMC/LMC) using optical to infrared photometry, light curves, and optical spectroscopy. The strong dust production and long-period pulsations of these stars indicate that they are at the very end of their AGB evolution. Period-mass-radius relations for the fundamental-mode pulsators give median current stellar masses of 1.14 M_sun in the LMC and 0.94 M_sun in the SMC (with dispersions of 0.21 and 0.18 M_sun, respectively), and models suggest initial masses of <1.5 M_sun and <1.25 M_sun, respectively. This new class of stars includes both O-rich and C-rich chemistries, placing the limit where dredge-up allows carbon star production below these masses. A high fraction of the brightest among them should show S star characteristics indicative of atmospheric C/O ~ 1, and many will form O-rich dust prior to their C-rich phase. These stars can be separated from their less-evolved counterparts by their characteristically red J-[8] colors.
قيم البحث
اقرأ أيضاً
We present a 2Dust model for the dust shell around a LMC long-period variable (LPV) previously studied as part of the OGLE survey. OGLE LMC LPV 28579 (SAGE J051306.40-690946.3) is a carbon-rich asymptotic giant branch (AGB) star for which we have pho
tometry and spectra from the Spitzer SAGE and SAGE-Spec programs along with UBVIJHK_s photometry. By modeling this source, we obtain a baseline set of dust properties to be used in the construction of a grid of models for carbon stars. We reproduce its spectral energy distribution using a mixture of AmC and SiC (15% by mass). The grain sizes are distributed according to the KMH model. The best-fit model has an optical depth of 0.28 for the shell at the peak of the SiC feature, with R_in~1430 R_sun or 4.4 R_star. The temperature at this inner radius is 1310 K. Assuming an expansion velocity of 10 km s^-1, we obtain a dust mass-loss rate of 2.5x10^-9 M_sun yr-1. We calculate a 15% variation in this rate by testing the fit sensitivity against variation in input parameters. We also present a simple model for the molecular gas in the extended atmosphere that could give rise to the 13.7 mu m feature seen in the spectrum. We find that a combination of CO and C_2H_2 gas at an excitation temperature of about 1000 K and column densities of 3x10^21 cm^-2 and 10^19 cm^-2 respectively are able to reproduce the observations. Given that the excitation temperature is close to T_dust(R_in), most of the molecular contribution probably arises from the inner shell region. The luminosity corresponding to the first epoch of SAGE observations is 6580 L_sun. For an effective temperature of about 3000 K, this implies a stellar mass of 1.5-2 M_sun and an age of 1-2.5 Gyr. For a gas:dust ratio of 200, we obtain a gas mass-loss rate of 5.0x10^-7 M_sun yr^-1, consistent with the gas mass-loss rates estimated from the period, color and 8 mu m flux of the source.
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.
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 luminos
ity 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.
This paper presents a summary of four invited and twelve contributed presentations on asymptotic giant branch stars and red supergiants, given over the course of two afternoon splinter sessions at the 19th Cool Stars Workshop. It highlights both rece
nt observations and recent theory, with some emphasis on high spatial resolution, over a wide range of wavelengths. Topics covered include 3D models, convection, binary interactions, mass loss, dust formation and magnetic fields.
We have searched for near-infrared stellar counterparts of IRAS point sources in the Large Magellanic Cloud (LMC), in J and K-bands. This resulted in the detection of 21 counterparts, of which 19 are new discoveries. Using colour-magnitude and colour
-colour diagrams, we identify 13 Asymptotic Giant Branch (AGB) stars with thick circumstellar dust envelopes, 7 possible early post-AGB stars or stars recovering from a thermal pulse, and 1 red supergiant or foreground star. For 10 of the IRAS targets we do not succeed in detecting and/or identifying a near-infrared counterpart. We serendipitously detect 14 other red sources, of which 2 are known Long Period Variables, and a few galaxies. The near-infrared and optical colours of the galaxies may indicate considerable interstellar extinction through the LMC, as much as A_V about 2-4 mag. The relative number of AGB carbon stars over oxygen stars is shown to decrease as the luminosity increases. Yet amongst the faintest mass-losing AGB stars oxygen-rich stars still exist, which puts constraints on current convection theories that predict the occurrence of third dredge-up and Hot Bottom Burning. We investigate the nature of some LMC stars that have infrared properties very similar to suspected Galactic post-AGB stars.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
