We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). RSP is a new functionality in MESAstar that models the non-linear radial stellar pulsations that characterize RR Lyrae, Cepheids, and other classes of variable stars. We significantly enhance numerical energy conservation capabilities, including during mass changes. For example, this enables calculations through the He flash that conserve energy to better than 0.001%. To improve the modeling of rotating stars in MESA, we introduce a new approach to modifying the pressure and temperature equations of stellar structure, and a formulation of the projection effects of gravity darkening. A new scheme for tracking convective boundaries yields reliable values of the convective-core mass, and allows the natural emergence of adiabatic semiconvection regions during both core hydrogen-and helium-burning phases. We quantify the parallel performance of MESA on current generation multicore architectures and demonstrate improvements in the computational efficiency of radiative levitation. We report updates to the equation of state and nuclear reaction physics modules. We briefly discuss the current treatment of fallback in core-collapse supernova models and the thermodynamic evolution of supernova explosions. We close by discussing the new MESA Testhub software infrastructure to enhance source-code development.
We present the analysis of four first overtone RR Lyrae stars observed with the Kepler space telescope, based on data obtained over nearly 2.5 yr. All four stars are found to be multiperiodic. The strongest secondary mode with frequency f 2 has an amplitude of a few mmag, 20−45 times lower than the main radial mode with frequency f 1 . The two oscillations have a period ratio of P 2 /P 1 = 0.612 − 0.632 that cannot be reproduced by any two radial modes. Thus, the secondary mode is nonradial. Modes yielding similar period ratios have also recently been discovered in other variables of the RRc and RRd types. These objects form a homogenous group and constitute a new class of multimode RR Lyrae pulsators, analogous to a similar class of multimode classical Cepheids in the Magellanic Clouds. Because a secondary mode with P 2 /P 1 ∼ 0.61 is found in almost every RRc and RRd star observed from space, this form of multiperiodicity must be common. In all four Kepler RRc stars studied, we find subharmonics of f 2 at ∼ 1/2f 2 and at ∼ 3/2f 2 . This is a signature of period doubling of the secondary oscillation, and is the first detection of period doubling in RRc stars. The amplitudes and phases of f 2 and its subharmonics are variable on a timescale of 10 − 200 d. The dominant radial mode also shows variations on the same timescale, but with much smaller amplitude. In three Kepler RRc stars we detect additional periodicities, with amplitudes below 1 mmag, that must correspond to nonradial g-modes. Such modes never before have been observed in RR Lyrae variables.
Type II-plateau supernovae (SNe II-P) are the classic variety of core-collapse events that result from isolated, massive stars with thick hydrogen envelopes intact at the time of explosion. Their distances are now routinely estimated through two techniques: the expanding photosphere method (EPM), a primary distance-determining method, and the recently developed standard-candle method (SCM), a promising secondary technique. Using Cycle 10 Hubble Space Telescope (HST) observations, we identify 41 Cepheid variable stars in NGC 1637, the host galaxy of the most thoroughly studied SN II-P to date, SN 1999em. Remarkably, the Cepheid distance that we derive to NGC 1637, D ¼ 11:7 AE 1:0 Mpc, is nearly 50% larger than earlier EPM distance estimates to SN 1999em. This is the first direct comparison between these two primary distance-determining methods for a galaxy hosting a well-observed, spectroscopically and photometrically normal, SN II-P. Extensive consistency checks show strong evidence to support the Cepheid distance scale, so we are led to believe either that SN 1999em is in some heretofore unsuspected way an unusual SN II-P, or that the SN II-P distance scale must be revised. Assuming the latter, this one calibration yields H 0 ðEPMÞ ¼ 57 AE 15 km s À1 Mpc À1 and H 0 ðSCMÞ ¼ 59 AE 11 km s À1 Mpc À1 ; additional calibrating galaxies are clearly desirable in order to test the robustness of both determinations of H 0 . The HST observations of NGC 1637 also captured the fading SN 1999em 2 yr after explosion, providing the latest photometry ever obtained for an SN II-P. The nebularphase photometric behavior of SN 1999em closely follows that observed for SN 1987A at similar epochs. The V and I light curves are both declining at rates significantly greater than the decay slope of 56 Co predicts. This is likely due to an increasing transparency of the envelope to gamma rays and perhaps also to the formation of dust in the cooling atmosphere of the SN. The absolute V-band brightness of SN 1999em is $0.25 mag brighter than SN 1987A at the same epochs, which suggests that a slightly greater amount of radioactive 56 Ni, $0.09 M , was ejected by SN 1999em than was derived for SN 1987A (0.075 M ).
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