Examinando por Autor "Schreiber, Matthias"

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  • Miniatura
    Ítem
    Magnetic fields in close white dwarf binaries
    (Universidad de Valparaíso, 2020-09) Contreras Pastén, Erick; Schreiber, Matthias
    White dwarfs are the degenerate remnants of stars that are born with initial masses 8 − 10 Mο (e.g., Smartt et al. 2009 [63], Cummings et al.2019 [16]). Whereas the possibility that some of these stellar remnants possess strong magnetic fields was explored already in 1947 (Blackett et al. 1947 [9]), the observational confirmation occurred much later but still already 50 years ago (Kemp et al. 1970 [34]). It is now firmly established that a small fraction (2 − 10 per cent) of single white dwarfs exhibit magnetic fields of B :2 1 MG (Hollands et al. 2015, Ferrario et al. 2015 [18], Kawka et al. 2010 [65]), and there is evidence that weaker fields are equally or possibly even more common (Landstreet & Bagnulo 2019 [39]). Interestingly, the fraction of magnetic white dwarfs is different for white dwarfs in binary stars and seems to depends on the binary configuration. A relatively large fraction (∼ 30 per cent) of cataclysmic variables, i.e. mass transferring systems, contains a strongly magnetic white dwarf, while they are nearly absent (less than one per cent) among detached white dwarf plus M dwarf binaries. The origin of magnetic fields in white dwarfs is still debated, with current hypotheses including fossil fields (Angel et al. 1981 [4], Braithwaite & Spruit 2004 [10]), binary interactions either during a common envelope phase (Tout et al. 2008 [66]) or mergers (Garc´ıa-Berro et al. 2012 [24]), and processes internal to the white dwarf (Isern et al. 2017 [29]). In this thesis I investigate magnetic white dwarfs in binaries observationally as well as theoretically. Weakly magnetic white dwarfs in close binaries may produce synchrotron emission which should be detectable with ALMA. I analysed ALMA observations of two close white dwarf binary stars finding evidence for synchrotron emission in one of them, while the interpretation of the observations of the other ob- ject remains unclear. I combine my observational results with observations from the literature and theoretically investigate whether a crystallisation and rotation driven dynamo could be responsible for at least some of the detected magnetic fields in close binaries. I found that indeed crystallisation might be important for understanding magnetic fields in close white dwarf binary stars and can most likely produce stronger fields than was previously thought. However, the rotation and crystallisation driven dynamo cannot explain the entire observed population of magnetic white dwarfs. For single white dwarfs, the merger scenario is most likely much more important.
  • Miniatura
    Ítem
    The origin of brown dwarfs
    (Universidad de Valparaíso, 2019-09) Santamaría Miranda, Alejandro; Schreiber, Matthias
    In the present thesis we study the problem of the formation of brown dwarfs, objects whose masses are in the range between planets and stars. There are four main for- mation theories for these substellar objects: turbulent fragmentation, disk fragmen- tation, ejection from filaments and photoevaporation. All these formation channels can form brown dwarfs but it remains an open question which is the one preferred by nature. Revealing this dominant formation mechanism was the main motivation of this thesis. In particular, we investigated if mass loss or accretion processes are a scaled-down versions of the same phenomena in the stellar regime. We used several astronomical techniques such as interferometry and spectroscopy at different wavelengths to study the formation of brown dwarfs. For our observational surveys of forming brown dwarfs the star forming region of Lupus and ρ Ophiuchus were chosen as their close distance and their youth enhance the possibilities of de- tecting and analysing faint objects. For objects in the earliest stages of their evolution we used ALMA for studying the cold dust envelopes which emit the released energy in the (sub)mm regime. ALMA was also used for the somewhat more evolved Class 0 and I sources, complemented with optical and infrared archival data. In one Class I/II source (Par-Lup3-4) we have used the emission of several molecular transitions to study the mass loss of a molecular outflow. We combined the ALMA emission line and continuum data with optical/infrared archival data. The further evolved Class II sources were studied using spectroscopic observations ranging from near ultraviolet to near infrared wavelengths as well as continuum emission in the (sub)mm and optical/infrared photometry. The most notable observational result is the discovery of eleven pre-substellar core can- didates that are in gravitational contraction. We found an interesting Class 0/I can- didate and resolved its structure. We measured the disk mass of the Class II sources and their properties are consistent with being scaled down versions of forming low mass stars. We detected for the first time the base of a bipolar molecular outflow in a very low mass star whose properties are a scaled-down version of the outflow properties ob- served in the stellar regime. We measured the accretion rate of the substellar companion to the stellar binary (SR 12) using several accretion indicators commonly used in the stellar regime. The companion’s radial velocity and its separation within the binary system point out that it is not very probable that this source was formed by disk fragmentation, while the accretion rate is in agreement with an extension of the relation obtained for more massive objects. Based on the mentioned scientific evidences (accretion rate, mass loss, the presence of isolated pre-substellar cores in gravitational contraction, and the relation between disk mass and central object) we conclude that it is highly probable that the main for- mation mechism of brown dwarfs is turbulent fragmentation (also known as scaled- down version of star formation).