Stars with the mass of the sun or greater are usually accompanied by one or more companion stars in orbit. When gravity contracts the gas and dust of an interstellar cloud, clumps that are dense enough to coalesce into stars form. According to one model, when the cloud has a slight spin, multiple stellar systems form. This produces a disc, which then fragments to produce multiple stars.
Turbulence in the cloud causes clumps to fragment into multiple systems in a competing model. When the orbits of two binary stars coincide with our line of sight, the stars form an eclipsing binary. Days are typical periods for ordinary, roughly solar-mass-sized eclipsing binaries. Triplet stellar systems can be eclipsing as well, but because the third star in a typical triplet orbit is farther out (further out in order for the system to remain relatively stable and not eject one of the stars), its period is closer to a year, and longer monitoring is required to spot and study them. Although there are over a million known eclipsing binary systems, only twenty triply eclipsing systems have been published.
Triply eclipsing systems and binaries have the advantage of allowing astronomers to measure several otherwise degenerate physical characteristics of the systems, such as orbital inclinations and eccentricities, as well as stellar masses, radii, ages, temperatures, and chemical compositions when combined with other data (“metallicity”). Complex dynamical interactions occurring over short timescales can, however, be studied in eclipsing triples. Not to mention that the statistics of eclipsing triplets shed light on the formation mechanisms of these systems, which can be compared to simulations.
Willie Torres, a CfA astronomer, was part of a team that used transiting observations from TESS (the Transiting Exoplanet Survey Satellite) to identify approximately fifty new triply eclipsing systems, twenty of which had reliable orbits for all three stars. The team reports on six of these eclipsing triplets, for which additional data has allowed a more complete description of the stars’ personalities. All six stellar systems are about a billion years old. All six are nearly edge-on, with the inner binary occasionally eclipsing the outer tertiary star and vice versa.
All twelve stars in the inner binary system have masses ranging from.7 to 1.8 solar masses and are in the main-sequence phase of their lives; the six tertiary stars have masses ranging from 1.5 to 2.3 solar masses. The authors conclude by discussing the statistics of these systems, discovering that approximately 0.02 percent of close binaries host the third star in a flat configuration like their current set, implying that there are likely hundreds of thousands of them in our galaxy. They also mention possible connections between triplets and more complex stellar systems, such as “2 plus 2 compact quadruple systems.”
The findings were published in the Royal Astronomical Society’s Monthly Notices.