Carlos Lázaro Hernando, Vikram Dhillon, Montserrat Armas Padilla, Manuel Pérez Torres, Ignacio González Martínez-País, María Jesús Arévalo Morales, Tariq Shahbaz, Paula Izquierdo Sánchez, Jorge Casares Velázquez, Teodoro Muñoz Darias
R. Iglesias Marzoa (ULL); M. López Morales (Harvard-Smithsonian Center for Astrophysics, EEUU); P. Abraham (Konkoly, Hungría; D. Steeghs, B.T Gänsicke, T.R. Marsh, E. Breedt (Univ. Warwick, RU); L. Schmidtobreick, J. M. Corral-Santana (ESO, Chile); S. B. Potter (SAAO, Suráfrica); W. Skidmore (Thirty Meter Telescope, Caltech, EEUU); P. Hakala, A. Somero (Univ. Helsinki, Finlandia); C. Tappert (Univ. Católica de Chile); K. Long (Space Telescope Science Institute, EEUU); V. Rana (Caltech, EEUU); M. Hernanz, G. Sala, a. Rebassa-Mansergas (Institut d'Etudis Espaciales de Cataluña); A. Schwope (Leibniz-Institut für Astrophysik Potsdam, Alemania), A. Nebot Gómez-Morán (Observatoire Astronomique de Strasbourg, Francia), J.R. Thorstensen (Dartmouth College, EEUU)
The study of binary stars is an essential part of the stellar astrophysics. It is believed that a big number of stars in the Galaxy formed and evolved as binary systems. Thus, their study is essential for understanding stellar and galactic evolution. In many cases when the orbital periods are short and the separation between components comparable to the radius of the stars, the evolution is influenced by their mutual interaction. This leads to physical processes and evolution traces different from those observed in isolated stars. Particularly relevant is the effect of mass transfer between stars, as occurs in Algol-type binaries and cataclysmic variables, some types of system studied in this project. Still, the best source of precise mass and radius measurements are binary stars.
We have different aims:
– The determination of absolute parameters of Algol-type eclipsing binaries, based on optical and infrared, complemented with Strömgren photometric and spectroscopic observations. Such binaries are of great interest to study the effects of external radiation on stellar atmospheres.
In 2006 we started a new project devoted to deriving accurate masses and radii of low-mass eclipsing binaries of spectral type M, by measuring their optical (VRI) and near-infrared (JK) light curves, and radial velocity curves.
– The Binary Star Group is also focused on the study of the structure and evolution of cataclysmic variables (CVs). Among the main goals we highlight a fundamental test of their current evolution theory, which will observationally prove or discard several of its predictions. To do so, population studies on both the observed minimum orbital period region (~ 80 min) and the upper boundary of the period gap (in the 3-4.5 hour orbital period range) are being performed. Along with a discordant minimum period of ~ 65 min the theory predicts that more than 90% of the total CV population should be found clustered around that minimum value. In order to confirm or reject this prediction the orbital periods of all new CVs identified by the Sloan Digital Sky Survey are being measured in a huge observational effort. On the other hand, our research has demonstrated the existence of a huge pile-up of intrinsically very luminous CVs (with the highest mass transfer rates known), collectively known as the SW Sextantis stars, with orbital periods just above 3 hours, in apparent contradiction with theoretical expectations. The masses of these systems are being determined using 8-m class telescopes, as VLT and Gemini, and the 10.4-m GTC, in the "Target of Opportunity" mode when a system hits a so-called low state characterised by the quenching of the mass transfer from the donor star to the white dwarf.
In 2010 a new research line started within our group: the study of central binary stars of planetary nebulae. One plausible effect leading to bipolar shapes in planetary nebulae is the presence of a progenitor binary star in their centres, containing at least one white dwarf. Therefore, the new project is aimed at finding a significant bipolarity-binarity correlation within bipolar planetary nebulae. A sample of spherical planetary nebulae will also be studied for comparison. Further, the history of the mass ejection episodes will give clues to the common-envelope stage, still poorly understood.
With the beginning of Paula Izquierdo Sánchez's PhD thesis (FPI) in mid-2017, another line of research has been opened in the field of white dwarfs: the study of post-planetary remnants around white dwarfs, which can e.g. greatly benefit from the studies of accretion discs in cataclysmic variables.
- With a measured recurrence period of 351 +/- 13 days and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become one of the main type-Ia supernova progenitor candidates. An international campaign has been set for the early detection of the 2017 explosion, which triggered monitoring programs in terrestrial and space telescopes, including visible-to-near-infrared photometry and visible spectroscopy, and observations in the ultraviolet and in X-rays from the Swift observatory. The explosion of 2017 took place on December 31. Early spectroscopic and photometric observations were conducted, including ToO observations in GTC led by our group.
- Using data from the extended Kepler mission in K2 Campaign 10, we identify two eclipsing binaries containing white dwarfs with cool companions that have extremely short orbital periods of only 71.2 min (SDSS J1205-0242) and 72.5 min (SDSS J1231+0041). Despite their short periods, both systems are detached with small, low-mass companions, in one case a brown dwarf and in the other case either a brown dwarf or a low-mass star. Preliminary estimates on the physical and binary parameters have been placed. SDSS J1205-0242 is composed of a 0.39 ± 0.02 M⊙ helium-core white dwarf that is totally eclipsed by a 0.049 ± 0.006 M⊙ (51 ± 6MJ) brown-dwarf companion, while SDSS J1231+0041 is composed of a 0.56 ± 0.07 M⊙ white dwarf that is partially eclipsed by a companion of mass ≲ 0.095 M⊙.
- The mechanism producing the flux increase and the time scale contraction of the variability in blazars is a question is still under debate. The proposed mechanisms include injection, acceleration and cooling of particles, with possible intervention of shock waves or turbulence. The results of the multi-frequency monitoring of blazar CTA 102 have been published in Nature. They show that the long-term evolution of the flux and the spectral variability are explained by an inhomogeneous, curved jet that undergoes changes in orientation over time, altering the relative Doppler factors in different regions of the jet. In particular, the extreme eruption detected in 2016-2017 (brightness increase of 6 mag in the visible) occurred when the corresponding emitting region presented a small angle with the line of sight. The agreement between observations and theoretical predictions can be seen as further validation of the relativistic beaming theory.