We study the evolution of degenerate electron cores primarily composed of the carbon burning products 16O, 20Ne, and 24Mg (hereafter ONeMg cores) that are undergoing compression. Electron capture ...reactions on A = 20 and 24 isotopes reduce the electron fraction and heat the core. We develop and use a new capability of the Modules for Experiments in Stellar Astrophysics (mesa) stellar evolution code that provides a highly accurate implementation of these key reactions. These new accurate rates and the ability of mesa to perform extremely small spatial zoning demonstrates a thermal runaway in the core triggered by the temperature and density sensitivity of the 20Ne electron capture reactions. Both analytics and numerics show that this thermal runaway does not trigger core convection, but rather leads to a centrally concentrated (r < km) thermal runaway that will subsequently launch an oxygen deflagration wave from the centre of the star. We use mesa to perform a parameter study that quantifies the influence of the 24Mg mass fraction, the central temperature, the compression rate, and uncertainties in the electron capture reaction rates on the ONeMg core evolution. This allows us to establish a lower limit on the central density at which the oxygen deflagration wave initiates of ρc ≳ 8.5 × 109 g cm− 3. Based on previous work and order-of-magnitude calculations, we expect objects which ignite oxygen at or above these densities to collapse and form a neutron star. Calculations such as these are an important step in producing more realistic progenitor models for studies of the signature of accretion-induced collapse.
We present advances in modeling Type IIP supernovae (SNe IIP) using MESA for evolution to shock breakout coupled with STELLA for generating light and radial velocity curves. Explosion models and ...synthetic light curves can be used to translate observable properties of SNe (such as the luminosity at day 50 and the duration of the plateau, as well as the observable quantity ET, defined as the time-weighted integrated luminosity that would have been generated if there were no 56Ni in the ejecta) into families of explosions that produce the same light curve and velocities on the plateau. These predicted families of explosions provide a useful guide toward modeling observed SNe and can constrain explosion properties when coupled with other observational or theoretical constraints. For an observed SN with a measured 56Ni mass, breaking the degeneracies within these families of explosions (ejecta mass, explosion energy, and progenitor radius) requires independent knowledge of one parameter. We expect the most common case to be a progenitor radius measurement for a nearby SN. We show that ejecta velocities inferred from the Fe ii λ5169 line measured during the majority of the plateau phase provide little additional information about explosion characteristics. Only during the initial shock cooling phase can photospheric velocity measurements potentially aid in unraveling light-curve degeneracies.
ABSTRACT The light curves of some luminous supernovae are suspected to be powered by the spindown energy of a rapidly rotating magnetar. Here we describe a possible signature of the central engine: a ...burst of shock breakout emission occurring several days after the supernova explosion. The energy input from the magnetar inflates a high-pressure bubble that drives a shock through the pre-exploded supernova ejecta. If the magnetar is powerful enough, that shock will near the ejecta surface and become radiative. At the time of shock breakout, the ejecta will have expanded to a large radius ( ∼ 10 14 cm) so that the radiation released is at optical/ultraviolet wavelengths ( T eff 20,000 K) and lasts for several days. The luminosity and timescale of this magnetar-driven shock breakout are similar to the first peak observed recently in the double-peaked light curve of SN-LSQ14BDQ. However, for a large region of model parameter space, the breakout emission is predicted to be dimmer than the diffusive luminosity from direct magnetar heating. A distinct double-peaked light curve may therefore only be conspicuous if thermal heating from the magnetar is suppressed at early times. We describe how such a delay in heating may naturally result from inefficient dissipation and thermalization of the pulsar wind magnetic energy. Without such suppression, the breakout may only be noticeable as a small bump or kink in the early luminosity or color evolution, or as a small but abrupt rise in the photospheric velocity. A similar breakout signature may accompany other central engines in supernovae, such as a black hole accreting fallback material.
ABSTRACT Recent asteroseismic analyses indicate the presence of strong (B 105 G) magnetic fields in the cores of many red giant stars. Here, we examine the implications of these results for the ...evolution of stellar magnetic fields, and we make predictions for future observations. Those stars with suppressed dipole modes indicative of strong core fields should exhibit moderate but detectable quadrupole mode suppression. The long magnetic diffusion times within stellar cores ensure that dynamo-generated fields are confined to mass coordinates within the main-sequence (MS) convective core, and the observed sharp increase in dipole mode suppression rates above 1.5 M is likely explained by the larger convective core masses and faster rotation of these more massive stars. In clump stars, core fields of ∼105 G can suppress dipole modes, whose visibility should be equal to or less than the visibility of suppressed modes in ascending red giants. High dipole mode suppression rates in low-mass (M 2 M ) clump stars would indicate that magnetic fields generated during the MS can withstand subsequent convective phases and survive into the compact remnant phase. Finally, we discuss implications for observed magnetic fields in white dwarfs and neutron stars, as well as the effects of magnetic fields in various types of pulsating stars.
We update the capabilities of the software instrument Modules for Experiments in Stellar Astrophysics (MESA) and enhance its ease of use and availability. Our new approach to locating convective ...boundaries is consistent with the physics of convection, and yields reliable values of the convective-core mass during both hydrogen- and helium-burning phases. Stars with become white dwarfs and cool to the point where the electrons are degenerate and the ions are strongly coupled, a realm now available to study with MESA due to improved treatments of element diffusion, latent heat release, and blending of equations of state. Studies of the final fates of massive stars are extended in MESA by our addition of an approximate Riemann solver that captures shocks and conserves energy to high accuracy during dynamic epochs. We also introduce a 1D capability for modeling the effects of Rayleigh-Taylor instabilities that, in combination with the coupling to a public version of the radiation transfer instrument, creates new avenues for exploring Type II supernova properties. These capabilities are exhibited with exploratory models of pair-instability supernovae, pulsational pair-instability supernovae, and the formation of stellar-mass black holes. The applicability of MESA is now widened by the capability to import multidimensional hydrodynamic models into MESA. We close by introducing software modules for handling floating point exceptions and stellar model optimization, as well as four new software tools- , -Docker, , and mesastar.org-to enhance MESA's education and research impact.
ABSTRACT White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable helium burning that is a concern for many SNe Ia ...scenarios. We study binaries with helium stars of mass 1.25 M ≤ M He ≤ 1.8 M , which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods ( P orb ) of several hours with non-rotating 0.84 and 1.0 M C/O WDs. The helium stars fill their Roche lobes after exhaustion of central helium and donate helium on their thermal timescales ( ∼ 10 5 years). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass ( M Ch ) and a core carbon ignition. We show here, however, that many of these scenarios lead to an ignition of hot carbon ashes near the outer edge of the WD and an inward going carbon flame that does not cause an explosive outcome. For P orb = 3 hr, 1.0 M C/O WDs with donor masses M He 1.8 M experience a shell carbon ignition, while M He 1.3 M will fall below the steady helium burning range and undergo helium flashes before reaching core C ignition. Those with 1.3 M M He 1.7 M will experience a core C ignition. We also calculate the retention fraction of accreted helium when the accretion rate leads to recurrent weak helium flashes.
Asteroseismology of 1.0-2.0 M sub(middot in circle) red giants by the Kepler satellite has enabled the first definitive measurements of interior rotation in both first ascent red giant branch (RGB) ...stars and those on the helium burning clump. The inferred rotation rates are 10-30 days for the approximately 0.2 M sub(middot in circle) He degenerate cores on the RGB and 30-100 days for the He burning core in a clump star. Using the Modules for Experiments in Stellar Evolution code, we calculate state-of-the-art stellar evolution models of low mass rotating stars from the zero-age main sequence to the cooling white dwarf (WD) stage. We include transport of angular momentum due to rotationally induced instabilities and circulations, as well as magnetic fields in radiative zones (generated by the Tayler-Spruit dynamo). We find that all models fail to predict core rotation as slow as observed on the RGB and during core He burning, implying that an unmodeled angular momentum transport process must be operating on the early RGB of low mass stars. Later evolution of the star from the He burning clump to the cooling WD phase appears to be at nearly constant core angular momentum. We also incorporate the adiabatic pulsation code, ADIPLS, to explicitly highlight this shortfall when applied to a specific Kepler asteroseismic target, KIC8366239.
Using Modules for Experiments in Stellar Astrophysics (MESA)+STELLA, we show that very different physical models can adequately reproduce a specific observed Type II-Plateau supernova (SN). We ...consider SN2004A, SN2004et, SN2009ib, SN2017eaw, and SN2017gmr, nickel-rich ( ) events with bolometric lightcurves and a well-sampled decline from the plateau. These events also have constraints on the progenitor radius, via a progenitor image, or, in the case of SN2017gmr, a radius from fitting shock-cooling models. In general, many explosions spanning the parameter space of progenitors can yield excellent lightcurve and Fe-line velocity agreement, demonstrating the success of scaling laws in motivating models that match plateau properties for a given radius and highlighting the degeneracy between plateau luminosity and velocity in models and observed events, which can span over 50% in ejecta mass, radius, and explosion energy. This can help explain disagreements in explosion properties reported for the same event using different model calculations. Our calculations yield explosion properties when combined with pre-explosion progenitor radius measurements or a robust understanding of the outermost of material that quantifies the progenitor radius from SN observations a few days after explosion.
Internal stellar magnetic fields are inaccessible to direct observations, and little is known about their amplitude, geometry, and evolution. We demonstrate that strong magnetic fields in the cores ...of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, arising from a magnetic greenhouse effect that scatters and traps oscillation-mode energy within the core of the star. The Kepler satellite has observed a few dozen red giants with depressed dipole modes, which we interpret as stars with strongly magnetized cores. We find that field strengths larger than ∼105 gauss may produce the observed depression, and in one case we infer a minimum core field strength of ≈107 gauss.
Abstract
We study the evolution of accreting oxygen–neon (ONe) white dwarfs (WDs), with a particular emphasis on the effects of the presence of the carbon-burning products 23Na and 25Mg. These ...isotopes lead to substantial cooling of the WD via the 25Mg–25Na, 23Na–23Ne and 25Na–25Ne Urca pairs. We derive an analytic formula for the peak Urca-process cooling rate and use it to obtain a simple expression for the temperature to which the Urca process cools the WD. Our estimates are equally applicable to accreting carbon–oxygen WDs. We use the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to evolve a suite of models that confirm these analytic results and demonstrate that Urca-process cooling substantially modifies the thermal evolution of accreting ONe WDs. Most importantly, we show that MESA models with lower temperatures at the onset of the 24Mg and 24Na electron captures develop convectively unstable regions, even when using the Ledoux criterion. We discuss the difficulties that we encounter in modelling these convective regions and outline the potential effects of this convection on the subsequent WD evolution. For models in which we do not allow convection to operate, we find that oxygen ignites around a density of log(ρc/g cm−3) ≈ 9.95, very similar to the value without Urca cooling. Nonetheless, the inclusion of the effects of Urca-process cooling is an important step in producing progenitor models with more realistic temperature and composition profiles which are needed for the evolution of the subsequent oxygen deflagration and hence for studies of the signature of accretion-induced collapse.
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