Super-AGB Stars


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Contact: F.X.Timmes
my one page vitae,
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teaching statement.
Turbulent Chemical Diffusion In Convectively Bounded Carbon Flames (2016)

Here we describe an idealized model of convectively bounded carbon flames with 3D hydrodynamic simulations of the Boussinesq equations using the pseudospectral software instrument Dedalus.

On Carbon Burning in Super Asymptotic Giant Branch Stars (2015)

In this article we explore the detailed and broad properties of carbon burning in 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, ρign ≈ 1.3 × 106 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 Δ MZAMS/Δfov ≈ 1.6 M. For zero overshoot, fov=0.0, our models in the ZAMS mass range ≈ 8.9 to 11 M show off-center carbon ignition. For canonical amounts of overshooting, fov=0.016, the off-center carbon ignition range shifts to ≈7.2 to 8.8 M. Only systems with fov ≥ 0.01 and ZAMS mass ≈7.2 - 8.0 M 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 at an appreciable distance from the center of the carbon core.


SAGB illustration

3 slices, 2418 MESA models
kippenhahn 7.5 M
ignition point moves inwards
angular momentum evolution
ignition location in mass-over plane

Advanced burning stages and fate 8-10 M stars
The stellar mass range 8 ≲ M/M ≲ 12 corresponds to the most massive AGB stars and the most numerous massive stars. It is host to a variety of supernova progenitors and is therefore very important for galactic chemical evolution and stellar population studies. In this article, we study the transition from super-AGB star to massive star and find that a propagating neon-oxygen burning shell is common to both the most massive electron capture supernova (EC-SN) progenitors and the lowest mass iron-core collapse supernova (FeCCSN) progenitors.

Of the models that ignite neon burning off-center, the 9.5 M would evolve to an FeCCSN after the neon-burning shell propagates to the center, as in previous studies. The neon-burning shell in the 8.8 M, however, fails to reach the center as the URCA process and an extended (0.6 M) region of low Ye (0.48) in the outer part of the core begin to dominate the late evolution; the model evolves to an EC-SN. This is the first study to follow the most massive EC-SN progenitors to collapse, representing an evolutionary path to EC-SN in addition to that from SAGB stars undergoing thermal flashes. We also present models of an 8.75 M super-AGB star through its entire thermal pulse phase until electron captures on 20Ne begin at its center and of a 12 M star up to the iron core collapse. We discuss key uncertainties and how the different pathways to collapse affect the pre-supernova structure. Finally, we compare our results to the observed neutron star mass distribution.

HR diagram
Divergence after C burn
kippenhahn 8.8 M
kippenhahn 9.5 M
kippenhahn 12 M
flame temperature profiles 8.8 M