Heidi Thiemann
We have identified a set of candidate stars that appear to be long-period examples of near-contact eclipsing binary stars, with orbital periods of up to a month or more. To be in contact, or near contact, at such long periods requires the stellar components to be giants. Such objects have been proposed as the progenitors of red novae, but none have been conclusively identified pre-nova. The outbursts are believed to be due to stellar mergers, but only one progenitor of such an event has ever been studied, V1309 Sco (Tylenda et al. 2011), and that was only recognised retrospectively, after the merger occurred.
We have identified our candidates from a search of the SuperWASP periodicity catalogue (Norton, A. J. 2018) and ASAS-SN Catalogue of Variable Stars. Each target has a SuperWASP lightcurve showing the same period and shape, characteristic of a contact, or near-contact eclipsing system.
We are coming to the end of a 2 year long programme of multi-colour photometry of these targets using the LCO robotic telescopes and The Open University's own PIRATE observatory. We have taken spectroscopy of northern candidates with the Liverpool Telescope, and southern candidates using the SAAO 74-inch telescope and SALT. By combining the multi-colour photometry with radial velocity spectroscopy we have been able to model the parameters (masses, radii, temperatures, etc.) of the stellar components using the Wilson-Devinney code, implemented in the PHOEBE modelling package. We are also currently working on modelling the evolution of the binaries using MESA and binary_c.
We have now confirmed that at least 10 of the 27 candidates are long-period near-contact giant eclipsing binaries. By studying these objects, we have an unrivalled opportunity to identify and characterise binary mergers before the merger event itself, and advance our understanding of the formation of red novae.
Title: Red novae candidates: An investigation of long-period near-contact giant eclipsing binaries
Authors: Heidi Thiemann, Supervisors: Andrew Norton, Ulrich Kolb
University: School of Physical Sciences, The Open University
We have identified 27 unusually long period red giant near-contact eclipsing binary stars.
Background
A contact binary star is a system with two close stellar components. The stars orbit so closely that they have started to merge and share a gaseous envelope. Contact binary stars typically have a short orbital period of less than one day.
We have identified a set of candidate stars that appear to be long-period examples of near-contact eclipsing binaries, with orbital periods of up to a month or more.
To be in contact, or near contact, at such long periods requires the both stars in the binary to be giants. This is a new and unusual configuration of binary stars.
Contact binary star mergers are thought to be the progenitors of red novae, but none have been identified pre-nova. Red novae are smaller, redder, dustier, and more mysterious cousins of supernovae. There are only approximately 16 known red novae.
Image: A diagram of the light curve of a contact binary star.
Image: A Hubble photo showing a red star in the centre, surrounded by expanding clouds of light brown dust.
Slide 2
Methods
We identified 27 candidate binaries from a search of the SuperWASP Periodicity Catalogue, the ASAS-SN Catalogue of Variable Stars, and through our Zooniverse citizen science project, SuperWASP Variable Stars.
We conducted a 2-year long follow-up campaign of multi-colour photometry and spectroscopy of the 27 targets. We used observational data for modelling the binary star system.
Image: A photo of the SuperWASP telescope, which is made up of a U-shape arm holding 8 cameras.
Image: The author standing underneath a large, orange telescope.
Slide 3
Results
Using PHOEBE, a binary star modelling programme, we have modelled the stellar components of the targets.
For one system in particular, each star has a mass of approximately 3 solar masses, and a radius of approximately 20 to 25 solar radii.
We have shown that 12 out of 27 candidate binary stars are real near-contact red giant eclipsing binaries. The other 19 are likely to be Cepheids. 9 binaries are semi-detached systems, where one star fills or overflows its Roche lobe. 3 are W UMa type systems, surrounded by a common envelope. These binaries are all made up of late-type giants, with low temperatures, relatively low separations and low masses.
Image: A black and white graph of the light curve of a contact binary.
Image: A graph of the radial velocity curve. It shows two overlapping sine waves.
Image: A black and white model of a binary star. It shows a larger star next to a smaller star.
Slide 4
Ongoing and future work
We are currently modelling the evolution of these binary systems using MESA and binary_c, 1D stellar evolution codes. By studying these binaries, we have an opportunity to identify and characterise binary mergers before the merger event itself, and advance our understanding of the formation of red novae.
Image: Possible stellar evolutionary pathways.The stars start far apart, then merge, and could either become red novae or a white dwarf binary.
Image: A screenshot of the SuperWASP Variable Stars Zooniverse project. The background is a density map of stars in the night sky, and the text in front reads “help us discover the weirdest variable stars”.