Browsing by Author "Hajdu, Gergely"

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    Continued Search of RR Lyrae Binary Systems Towards the Galactic Bulge
    (2018) Hajdu, Gergely; Catelan, Márcio; Jurcsik, Johanna; Thompson, Ian B.
    Binary systems provide the most straightforward way of measuring the mass of individual stars, and through it, constraining the models of stellar evolution. Unfortunately, very few binary candidates containing RR Lyrae variables have been found previously. We continue our search for such systems in the photometric data of the OGLE survey towards the Galactic bulge, finding more than 30 such candidates. Furthermore, we show that our ongoing radial velocity follow-up observations have the potential to confirm or refute some of our candidates in the near future....
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    Establishing the galactic centre distance using VVV bulge RR lyrae variables
    (2018) Majaess, D.; Dékány, I.; Hajdu, Gergely; Minniti, D.; Turner, D.; Gieren Waiblinger, Wolfgang Paul
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    Into the Darkness: Classical and Type II Cepheids in the Zona Galactica Incognita
    (2019) Dekany, Istvan; Hajdu, Gergely; Grebel, E.K.; Catelan, Márcio
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    New RR Lyrae variables in binary systems
    (2015) Hajdu, Gergely; Catelan, Márcio; Dekany, Istvan
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    Properties of RR Lyrae variables in binary systems: towards a first-time mass measurement
    (2022) Hajdu, Gergely; Pietrzyński, Grzegorz; Catelan, Marcio; Jurcsik, Johanna; Karczmarek, Paulina; Pilecki, Bogumił; Thompson, Ian
    Despite their general usefulness as distance indicators and tracers of old stellar populations, several properties of RR Lyrae variable stars remain poorly understood. Crucially, the most basic stellar parameter, the stellar mass, has never been measured for any member of this class. We performed a new search for RR Lyrae variables in binary systems, looking for the light travel-time effect in the long-term light curves obtained by the OGLE project toward the Galactic bulge, finding 87 high-confidence candidates. The distributions of the inferred properties of these binary systems are quite peculiar: no system has a binary period below 1000d, and their frequency peaks between 3000 and 4000d; their eccentricities are strongly concentrated around 0.27; and their mass functions are apparently trimodal. Supposing an isotropic inclination distribution, and a mass of 0.65 times solar for a typical RR Lyrae, these three modes point to typical companion masses of ~0.6, ~0.2 and ~0.067 solar masses. These can be attributed to a combination of main sequence and white dwarf, red dwarf, and brown dwarf companions, respectively. As RR Lyrae variables are evolved from (close to) solar mass, Population II main sequence stars, their binary parameters provide a unique glimpse into the binary properties of the old stellar population. These properties seem to be wildly different from those of Population I binaries. The difference in the distribution of binary periods can be explained tentatively by binary stellar evolution and selection effects. However, the distribution of eccentricities, and especially the trimodal companion mass distribution is in stark contrast to Population I binary parameters of solar-type stars, requiring further observations to confirm, and theoretical efforts to explain them. Finally, the observed binary properties inform us on the prospects of both finding and RR Lyrae variable in an eclipsing binary system, as well as on the possibility of obtaining an astrometric mass measurement using observations of the Gaia satellite....