Formation of close binaries by triple dynamics in the context of supernovae la progenitors



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Universidad de Valparaíso



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Instituto de Fisica y Astronomia




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The unique capabilities of Type Ia Supernovae (SN Ia), with emission bright and uniform enough to serve as yardsticks on cosmological distance scales, has resulted in them becoming some of the most important objects in the universe, and have led to the discovery of its accelerating expansion and eventually to the award of the 2011 Nobel Prize in Physics. Although it is well established that SN Ia are related to the thermonuclear ignition of a white dwarf (WD) that surpassed the Chandrasekhar mass limit, there is not yet a general consensus on the pathways leading to the explosion. While it is clear that the progenitors of SN Ia are close binaries which contain at least one WD, how these close binaries form and the detailed nature of the second stellar component remain one of the largest unsolved problems in astronomy. The two main progenitor channels that have been proposed are the single degenerate channel in which the WD accretes from a non-degenerate companion, and the double degenerate channel, which explains SN Ia explosions as the merger of two WDs. However, whether nature has a strong preference for one of these channels, or whether a combination of several evolutionary channels contributes to the observed SN Ia rate, remains an open question. The fact that we haven’t solved yet this important issue has two main reasons. First, the evolution of initial main sequence binary stars into close WD binary stars is a very complicated process and current theories are unable to simulate it in detail. Binary population models therefore rely on rather simple empirical relations with often completely unconstrained parameters, which makes it virtually impossible to make reliable predictions on SN Ia rates produced by any of the proposed channels. On top of that, it might even be that triple star dynamics produce significant numbers of close WD binaries, which is usually entirely ignored. Second, despite some significant recent progress, we still haven’t been able to provide decisive observational constraints on WD binary pathways towards SN Ia. This is largely because the direct progenitors of SN Ia explosions are either short-lived and potentially highly obscured super soft X-ray sources and/or faint and hard to detect close double WD binaries. The difficulty of providing clear constraints from surveys of the direct progenitors of SN Ia motivated us to go one step back in the evolution of the proposed progenitor systems. Before a double WD system is formed and before a WD accretes from a non-degenerate companion, these systems must have been detached binary stars consisting of a WD and an intermediate mass star (typically FGK spectral types) companion, formed in most cases after a common envelope (CE) phase. Characterizing a large sample of such detached WD plus main-sequence FGK star systems, that both classic SN Ia progenitors originally descended from, can provide crucial constraints on close WD binary evolution in general and the SN Ia progenitor problem in particular. The compact binary star group at the University of Valparaiso runs a large scale observational project aiming at identifying a large number of detached WD+FGK binary stars. Several close binaries have already been identified and some early results have been published. Interestingly, about 33 ± 12% of the identified close binaries are in eccentric orbits, which can not be explained by the main formation channel of close WD+FGK systems, the CE phase. Based on previous studies that involve hierarchical triple systems and the effect of a distant extra component perturbing the binary, we propose that those observed eccentric binaries are in fact triples systems, where the third star alters the orbital properties like the eccentricity in the so called Kozai-lidov Mechanisms (KLMs), and together with tidal forces can produce close binaries with eccentric orbits. As most of the observed systems only have spectroscopic measurements of the main sequence star component of the close binary, we have two possible configurations that locate the WD either orbiting this sun-like star or being a distant companion to an inner binary consisting of the sun-like star and an unseen low-mass main sequence star. In this thesis we estimate the amount of close WD+FGK binaries that evolved through KLMs (i.e., the binary as part of the triple systems) instead of via the CE phase, and the fraction of triple systems where the WD is either part of the binary or is the distant companion itself. To do this, we use the statistical research of hierarchical multiple stars of Tokovinin [2014b] to generate the initial conditions of a population of binary and triple systems that will be evolved using the Binary Star Evolution code (BSE). As the BSE algorithm only includes the evolution of single and binary stars, we evolve the distant companion as an isolated stars. Four simulations (assuming different eccentricity distribution) show that on average 23% of binaries of the observed sample could be potential hierarchical triple systems that evolved via KLMs, where about 79% correspond to systems where the WD is the distant companion, in agreement with the observations. Finally, we observationally study one of the binaries with eccentric orbit, and find the third component to be the white dwarf. We estimate that most likely the KLMs were active before the WD formed.


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