Quartet of Stars May Unlock Secrets of Stellar Evolution
by Evgenya Shkolnik
Artist's view of the gaseous disk that may have once engulfed and maneuvered the quadruple stellar system into its unusually small orbit. Art by Karen Teramura.
Using telescopes on Mauna Kea, IfA astronomer Michael Liu, I. Neill Reid of the Space Telescope Science Institute in Baltimore, and I found an extremely rare quartet of stars that orbit each other within a region smaller than Jupiter's orbit around the Sun. The quartet, called "BD -22°5866," appears as a speck of light even when viewed with the world's most powerful telescopes, but its spectrum reveals not one, but four, distinct stars arranged in two pairs. We are now struggling to work out whether they could have been born that way, or were forced together by a dense disk of gas in their youth.
We were monitoring several hundred nearby low-mass stars when one observation caught our attention because it was unlike anything we had seen before. At the time of the observations, two of the stars were orbiting each other at 300,000 mph (133 km/s), a speed that would get you from Honolulu to New York in less than a minute, while the second pair moved at a more modest speed of 120,000 mph (52 km/s). Using these speeds and the stars' masses, we were able to determine the maximum sizes of their oddly tight orbits. Less than one in 2,000 stars observed might be in such tightly bound systems, making this quadruple stellar system extremely rare.
Our velocity measurements set an upper limit on the orbital distances and periods. We determined that the first pair orbits each other in less than 5 days with an orbital radius at most 0.06 AU (astronomical unit, the distance from Earth to the Sun). A Scottish group of planet-hunters has informed us that this pair actually eclipses each other and has measured the true period to be much smaller, 2.2 days. The second pair orbits in less than 55 days, and its orbit has a maximum radius of 0.26 AU. The two pairs are orbiting each other with a maximum radius of only 5.8 AU, about the same as Jupiter's distance from the Sun. We estimate that the orbital period—the time it takes the two pairs to go around once—is less than nine years. (Jupiter orbits the Sun in 12 years.) The mass of each star is about half that of our Sun.
A to-scale schematic of the quadruple stellar system overlaid for comparison with a diagram of the solar system's planetary orbits. Art by Karen Teramura.
The extraordinarily tight configuration of this stellar system tells us that there may have been a single gaseous disk that forced them into such small orbits within the first 100,000 years of their evolution, as the stars could not have formed so close to one another. This is the first evidence of a disk completely encompassing four stars. It is remarkable how much a single stellar spectrum can tell us about both the present and the past of these stars.
We observed the quartet on the Keck I 10-m (33-foot) telescope and on the Canada-France-Hawaii 3.6-m (12-foot) telescope, both located on the summit of Mauna Kea. Each telescope is equipped with a high-resolution spectrograph, an instrument capable of breaking up the star's light into different colors (or wavelengths), known as a spectrum (commonly seen in a rainbow).
The stellar system is 166 light-years away from the Sun and lies just south of the constellation Aquarius (The Water Bearer). Though BD -22°5866 cannot be seen without a telescope, it is relatively bright and will be carefully monitored to map the orbits in more detail. Since most stars form as part of a binary- or multiple-star system, the enormous potential of this quadruple system to give us previously unavailable physical information makes it a key to unlocking a few mysteries of stellar evolution.
Evgenya Shkolnik is a postdoctoral fellow who studies extrasolar planets, star-planet interactions, and young low-mass stars.