Title: The Apogee Podcast – Where Have all the Circumbinaries Gone?
Description: In this Apogee Podcast, Cosmic discusses the search for circumbinary planets orbiting eclipsing binary stars.
Background Music: ‘Binaural Journey’ by CosmicLettuce
Bio: Cosmic (aka Matt Cheselka) is an independent research astronomer and space musician.
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Hello! This is Cosmic, and welcome to the Apogee podcast! In these podcasts, I chronicle a single astronomical reference thread from the past to the present. Many threads are possible — I’ve chosen just one. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.
The apogee for this podcast happened back on 7 November 2019 at 08:38 UTC. The lunar distance at that time was 405,059 km, which is 842 km closer than last apogee on 10 October, and 614 km further away than the next apogee on 5 December.
If you have any topic suggestions for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.
In 1953, Ralph Elmer Wilson published the ‘General Catalog of stellar radial velocities’. There one set of measurements sat for 17 years, until Nelson and Young combined these with other measurements to conclude that for the first time a hot white dwarf was observered to be a member of an eclipsing binary system. Nelson and Young’s paper, entitled “A New Eclipsing Binary Containing a very hot white dwarf” appeared in the Publications of the Astronomical Society of the Pacific in 1970. They analzed the strange-looking light curve of this system they called “BD plus 16 degrees 516” over several nights in three different wavelength bands and found a period of 0.52118 days. They also used data taken from spectra to feed into models that told them that the system consisted of a K0 primary star of 0.8 solar masses, and a hot companion star (B-V of -0.2) of 0.6 solar masses and 1.3 times the radius of the Earth. The separation between the primary and secondary is estimated to be three solar radii. From our point of view, the K0 star completely eclipses the white dwarf for about 47 minutes.
It would be another 31 years until Guinan and Ribas looked at the accumulated photometric data and found periodic variations in the timings of the minima. It was announced in their 2001 paper “The Best Brown Dwarf Yet?: A Companion to the Hyades Eclipsing Binary V471 Tau” that this same binary star system, renamed, had in fact a third component: a 0.04 solar mass brown dwarf star. In a full reversal, currently it is believed that there actually is no brown dwarf in this system. Recent adaptive optics images show that no such object appears at the predicted location. So what’s going on here? The current theory is that for this particular object, we are observing sunspots on the K star (currently defined as a K2 V). But no one knows for sure. More about that later.
This work inspired many others to look at old photometric data, and collect new data, in order to look for period variations that might indicate the existence of an extrasolar planet or brown dwarf. This included Zorotovic and Schreiber who, in 2013, published a paper discussing the origin of these period variations and questioning whether planets are actually being detected in such systems (by which time several had claimed to have been). They found that it was very unlikely that gas giant planets formed when the binary system formed, but they also found that most binary systems had period variations. So something other than giant gas planets must be the cause of the period variations. They concluded that the only alterative explainations were either that 2nd generation planets formed after a common envelope ejection of the two stars, or that the shape of the magnetically active secondary star (the K star) was affecting the orbit of the system.
Work in this area continues today with a recently published paper by Faillace et al that came out a couple of weeks ago entitled “Eclipse time variations and the continued search for companions to short period eclipsing binary systems”.
In this paper, the authors conclude that there are four possible explanations for orbital period variations: (i) angular momentum loss through magnetic braking or the emission of gravitational waves; (ii) angular momentum redistribution through Applegate-type mechanisms (magnetic coupling); (iii) the apparent changing of the binary period through the presence of a circumbinary object; or (iv) apsidal motion. In their paper, they show that the predicted period variations using a circumbinary model do not match current observations.
They conclude that no one, yet, knows for sure what’s going on in these systems. More data and the passage of time will help us move closer to the truth.
This is a very interesting and complex area of astronomy. I like the fact that observations AND theory are being used to move our understanding forward. Those of you who like both stellar evolution and exoplanets will have a fun time exloring the threads laid out in these references which go back at least 50 years!
Thanks for listening! Until the next apogee, I bid you Peace.
End of podcast:
365 Days of Astronomy
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