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Title:The fates of binary companions in Type Ia supernovae within the single-degenerate scenario
Author(s):Pan, Kuo-Chuan
Director of Research:Ricker, Paul M.
Doctoral Committee Chair(s):Ricker, Paul M.
Doctoral Committee Member(s):Taam, Ronald E.; Webbink, Ronald F.; Chu, You-Hua; Gammie, Charles F.; Lamb, Susan A.
Department / Program:Astronomy
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Type Ia supernovae (SNe~Ia) are thought to be caused by thermonuclear explosions of carbon-oxygen white dwarfs in close binary systems. However, the nature of the progenitor systems is still uncertain. In the single-degenerate scenario (SDS), the companion star is non-degenerate and can be significantly affected by the explosion. We explore this interaction by means of multi-dimensional adaptive mesh refinement simulations using the FLASH code. We consider several different companion types, including main-sequence-like stars, red giants, and helium stars. In addition, we include the symmetry-breaking effects of orbital motion, rotation of the non-degenerate star, and Roche-lobe overflow. We find that the relationship between the unbound stellar mass and the initial binary separation can be fitted by a power law. The power-law index is found to be about $-3.5$ to $-3.7$ for the main-sequence star, $-2.9$ to $-3.4$ for the helium star, and $-4.0$ to $-4.2$ for the red giant. After the explosion, the companion receives a kick from the supernova ejecta. It is found that the kick velocity is also related to the binary separation by a power law, except for the red giant companion, due to the large numerical uncertainty created by the small kick in this case. The power-law index is found to be about $-1.7$ to $-1.9$ for the main-sequence star and $-2.6$ to $-2.8$ for the helium star. By using tracer particles in FLASH, the process leading to the unbinding of matter can be determined by analyzing the fluid elements in a time sequence. It is found that the process is dominated by ablation, which has usually been neglected in past analytical studies. The level of Ni/Fe contamination of the companion that results from the passage of the supernova ejecta is found to be $\sim 10^{-5} M_\odot$ for the main-sequence star, $\sim 10^{-4} M_\odot$ for the He star, and $\sim 10^{-8} M_\odot$ for the red giant, suggesting that the ratio of nickel/iron to hydrogen plus helium abundance in the remnant should be found to be larger than the solar ratio if the contamination is mixed only in the companion's envelope. A hole in the ejecta shadowed by the companion star is also found in the simulation, which is a possible source of the variation in SN~Ia light curves. The spinning main-sequence companion star loses about $48\%$ of its initial angular momentum during the impact, causing the rotational velocity to drop to $23\%$ of the original rotational velocity. One way to distinguish between the single- and double-degenerate scenarios is to search for the post-impact remnant star expected in the SDS. We explore the evolution of the post-impact remnants in our simulations for main sequence-like and helium-rich star binary companions using the stellar evolution code MESA. Our results show that the energy deposited in a main sequence companion's outer layers by the SN causes the outermost $\sim 5$\% of the star to expand on the local thermal timescale ($\sim 300-1000$~yr), making the star brighten to $\sim 10-100L_\odot$. In most cases the effective temperature also increases significantly. As this excess energy is radiated away, the outer layers slowly recollapse. The time evolution is sensitive not only to the amount of SN energy deposited but also the depth to which it penetrates. For Tycho G, the proposed remnant of Tycho's SN, we find that main sequence companions can match the observed effective temperature and rotation speed (assuming conservation of specific angular momentum post-impact), but they are typically $\sim 2\times$ too bright. We have also examined helium star models for both normal SNe~Ia and the newly classified subluminous ``Type Iax'' SNe and find that these cases follow very different Hertzsprung-Russell (H-R) diagram tracks than do main sequence models. Helium star companions brighten to $\sim 10^4 L_\odot$ within a decade, becoming hot blue subdwarfs (sdOs) and contracting over $> 10^6$~yr to become helium red giants. Because helium star companions start in more compact systems, their kick velocities and rotation speeds can be much higher than for main sequence companions. Our results show that, given the age of a supernova remnant, we can test the plausibility of candidate observed remnant companions, and based on such comparisons we can draw conclusions about the nature of the companions, if the SDS is valid. Moreover, the luminosity enhancements due to SN impact can be sufficient to make remnant companion stars detectable in the Magellanic Clouds or other nearby galaxies with distances $\lesssim 4$~Mpc.
Issue Date:2013-08-22
Rights Information:Copyright 2013 Kuo-Chuan Pan
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08

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