Examinando por Autor "Zorotovic, Mónica"

Mostrando 1 - 4 de 4
  • Miniatura
    Ítem
    Cataclysmic variable evolution and the white dwarf mass problem: A Review
    (Elsevier, 2020) Zorotovic, Mónica; Schreiber, Matthias R.
    Although the theory of cataclysmic variable (CV) evolution is able to explain several observational aspects, strong discrepancies have existed for decades between observations and theoretical predictions of the orbital period distribution, the location of the minimum period, and the space density of CVs. Moreover, it has been shown in the last decade that the average white dwarf (WD) mass observed in CVs is significantly higher than the average mass in single WDs or in detached progenitors of CVs, and that there is an absence of helium-core WDs in CVs which is not observed in their immediate detached progenitors. This highly motivated us to revise the theory of CV formation and evolution. A new empirical model for angular momentum loss in CVs was developed in order to explain the high average WD mass observed and the absence of systems with helium-core WDs. This model seems to help, at the same time, with all of the above mentioned disagreements between theory and observations. Moreover, it also provides us with a very likely explanation for the existence of low-mass WDs without a companion. Here we will review the standard model for CV evolution and the disagreements that have existed for decades between simulations and observations with their possible solutions and/or improvements. We will also summarize the recently confirmed disagreement related to the average WD mass and the fraction of helium-core WDs among CVs, as well as the development of an empirical model that allows us to solve all the disagreements, discussing the physics that could be involved.
  • Miniatura
    Ítem
    Evidence for reduced magnetic braking in polars from binary population models
    (Royal Astronomical Society, 2020) Belloni, Diogo; Schreiber, Matthias R.; Zorotovic, Mónica
    We present the first population synthesis of synchronous magnetic cataclysmic variables, called polars, taking into account the effect of the white dwarf (WD) magnetic field on angular momentum loss. We implemented the reduced magnetic braking (MB) model proposed by Li, Wu & Wickramasinghe into the Binary Stellar Evolution (BSE) code recently calibrated for cataclysmic variable (CV) evolution. We then compared separately our predictions for polars and non-magnetic CVs with a large and homogeneous sample of observed CVs from the Sloan Digital Sky Survey. We found that the predicted orbital period distributions and space densities agree with the observations if period bouncers are excluded. For polars, we also find agreement between predicted and observed mass transfer rates, while the mass transfer rates of non-magnetic CVs with periods ≳3 h drastically disagree with those derived from observations. Our results provide strong evidence that the reduced MB model for the evolution of highly magnetized accreting WDs can explain the observed properties of polars. The remaining main issues in our understanding of CV evolution are the origin of the large number of highly magnetic WDs, the large scatter of the observed mass transfer rates for non-magnetic systems with periods ≳3 h, and the absence of period bouncers in observed samples.
  • Miniatura
    Ítem
    The White Dwarf Binary Pathways Survey −III. Contamination from hierarchical triples containing a white dwarf
    (Royal Astronomical Society, 2020) Lagos, Felipe; Schreiber, Matthias R.; Hernández, M-H; Tappert, Claus; Zorotovic, Mónica; Cáceres, C.
    The White Dwarf Binary Pathways Survey aims at increasing the number of known detached A, F, G, and K main-sequence stars in close orbits with white dwarf companions (WD+AFGK binaries) to refine our understanding about compact binary evolution and the nature of Supernova Ia progenitors. These close WD+AFGK binary stars are expected to form through common envelope evolution, in which tidal forces tend to circularize the orbit. However, some of the identified WD+AFGK binary candidates show eccentric orbits, indicating that these systems are either formed through a different mechanism or perhaps they are not close WD+AFGK binaries. We observed one of these eccentric WD+AFGK binaries with SPHERE and find that the system TYC 7218-934-1 is in fact a triple system where the WD is a distant companion. The inner binary likely consists of the G-type star plus an unseen low-mass companion in an eccentric orbit. Based on this finding, we estimate the fraction of triple systems that could contaminate the WD+AFGK sample. We find that less than 15 per cent of our targets with orbital periods shorter than 100 d might be hierarchical triples.
  • Miniatura
    Ítem
    White dwarfs in binaries and hierarchical triple systems as a test for mass transfer models and close binary formation mechanisms
    (Universidad de Valparaíso, 2021) Lagos Vilches, Felipe; Zorotovic, Mónica; Schreiber, Matthias
    Most stars with masses above ≈ 1M8 will end their lives as white dwarfs. Given that a non negligible fraction of such stars are in binary or multiple star systems, the characteristics of some of these white dwarfs will are affected by interactions between its progenitor and a close companion. These interactions can continue after the formation of the white dwarf producing a large variety of interesting objects and astrophysical phenomena like cataclysmic variables, double white dwarf binaries, super soft X-ray sources or type Ia supernovae. Formation of white dwarfs affected by binary interactions, or its subsequent interaction with a close companion, can be also enhanced due to perturbations exerted by a third companion. Under certain conditions, inner binaries in triple systems are prone to experience long term changes in their eccentricities, allowing otherwise non-interacting binaries to experience tidal migration, mass trans- fer, common envelope evolution or even mergers/collisions. In addition, although it is generally assumed that the interacting companion is another star, recent studies indicate that planets can interact with their host star, survive the metamorphosis of the latter into a white dwarf and later potentially pollute its atmosphere with heavy elements. Therefore, white dwarfs that reside in binary and triple configurations provide invaluable astrophysical laboratories to study stellar and binary evolution, orbital dynamics, and planet formation/evolution. Despite remarkable progress made in the last forty years in modeling binary interactions and the secular dynamic evolution of triple systems, the physics behind several processes, in particular those involved in the former, is still not well under- stood. This is mainly due to the fact that most of the currently available modeling tools are based on analytical simplifications involving parameters that must be fit- ted through (extensive and detailed) observations. With the aim of reducing this gap in our knowledge, I present in this thesis three different cases in which binaries and hierarchical triple systems will allow to test and improve current models of binary interactions and to measure the impact of tertiary companions in the formation of tight binaries.