Ministerio de Economía y Competitividad Gobierno de Canarias Universidad de La Laguna CSIC Centro de Excelencia Severo Ochoa

Astrophysics Research Projects

Physic of Stars, Planetary Systems and the Interstellar Medium

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Binary Stars (P/308807)

C LÁZARO

Sergio Palomo Nevado, Teodoro Muñoz Darias, Vikram Dhillon, Montserrat Armas Padilla, Manuel Pérez Torres, María Jesús Arévalo Morales, Ignacio González Martínez-País, Pablo Rodríguez Gil, Tariq Shahbaz, Jorge Casares Velázquez

R. Iglesias Marzoa (ULL, Spain); M. López Morales (Harvard-Smithsonian Center for Astrophysics); P. Abraham (Konkoly,Hungary); D. Steeghs, B.T Gänsicke, T.R. Marsh, E. Breedt (Univ. Warwick); L. Schmidtobreick, J. M. Corral-Santana (ESO, Chile); S. B. Potter (SAAO, Sudáfrica); W. Skidmore (Thirty Meter Telescope, Caltech); P. Hakala, A. Somero (Univ. Helsinki, Finland); C. Tappert (Univ. Católica de Chile); K. Long (Space Telescope Science Institute); V. Rana (Caltech); M. Hernanz, G. Sala, a. Rebassa-Mansergas (Institut d'Etudis Espaciales de Cataluña, Spain); A. Schwope (Leibniz-Institut für Astrophysik Potsdam), A. Nebot Gómez-Morán (Observatoire Astronomique de Strasbourg), J.R. Thorstensen (Dartmouth College, USA)

Introduction

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.

Milestones

- We found an eclipsing dwarf nova (DN) inside the peculiar, bilobed nebula Te 11. Modelling of high-speed photometry of the eclipse finds the accreting white dwarf to have a mass 1.18 M and a temperature of 13,000 K. The donor spectral type of M2.5 results in a distance of 330 pc, colocated with Barnard's loop at the edge of the Orion-Eridanus superbubble. We match the DN to the historic nova of 483 CE in Orion and postulate that the nebula is the remnant of this eruption. This connection supports the millennia time-scale of the post-nova transition from high to low mass-transfer rates. Te 11 constitutes an important benchmark system for CV and nova studies as the only eclipsing binary out of just three DNe with nova shells.

- The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption 10 times, including yearly eruptions from 2008 to 2014. With a measured recurrence period of 351 +/- 13 days (we believe the true value to be half of this) and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate. We initiated a campaign to ensure early detection of the predicted 2015 eruption, which triggered ambitious ground- and space-based follow-up programs, including visible to near-infrared photometry and visible spectroscopy, and ultraviolet and X-ray observations from the Swift observatory. After the discovery of the 2015 eruption by the LCOGT 2-m telescope (Hawaii), early spectroscopic observations were conducted, which revealed short-lived emission from material with velocities ˜13,000 km s-1, possibly collimated outflows. Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor. The updated recurrence period based on all known eruptions is 174 +/- 10 days, and we expected the next eruption of M31N 2008-12a to occur around 2016 mid-September. The 2016 eruption actually took place on December 12, but most of the observations (GTC, Gemini, etc.) were weathered out due to terrible winter conditions in the northern hemisphere. A 38-page paper on this amazing object was published in The Astrophysical Journal.

- Following the discovery by Vanderburg et al. (2015) of transits in the K2 light curve of the white dwarf WD 1145+017, which also shows a large infrared excess and significant metal pollution in its photosphere, we performed high-time resolution photometry of WD 1145+017 during four weeks. Multiple transit events with different durations (3-12 min) and depths (10%-60%) were observed. The shortest-duration transits indicate that the cloud of debris has a few times the size of the white dwarf. The transits evolve on timescales of days. We were able to track several transits on multiple nights, with recurrence periods of ≃4.49 h. This indicates multiple planetary debris fragments on nearly identical orbits, whose origin is still unknown.

- The finding of a white dwarf/cool star binary exhibiting emission from X-ray to radio wavelengths has been reported in Nature. AR Scorpii, initially misclassified as a δ-Scuti star in the 1970s, is revealed to be a 3.56-h period close binary showing brightness pulses on a period of 1.97 min. This pulses reflect the rapid rotation of a magnetic white dwarf, which is slowing down on a 107-yr timescale. Although the pulses are driven by the rotation of the white dwarf, they have their origin on the cool, companion star. The broadband spectrum of AR Sco is characteristic of synchrotron radiation, which requires electrons moving at relativistic speeds. Therefore, they must either originate near the white dwarf or at the companion star through direct interaction with the magnetosphere of the white dwarf.

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