Authors: R. Rodrigues da Silva, B. L. Canto Martins, and J. R. De Medeiros
First Author’s Institution: Department of Theoretical and Experimental Physics, Federal University of Rio Grande do Norte
Status of paper: Published in ApJ
Nothing sits still in our Universe. Everything is always on the move. Like planets, stars rotate. Really, they are the most obsessive ballet dancers, perpetually doing spins (or fouetté, if you will) until they die. The authors of this paper found certain types of stars unexpectedly display rapid rotations when they are not supposed to.
Astronomers like — really like — to categorize things. Stars are categorized according to their spectral features (ie, the presence of certain elements in the spectrum) and can be one of the following spectral types: O, B, A, F, G, K, or M (“Oh Be A Fine Girl Kiss Me”). Temperature decreases from spectral type O to spectral type M, with O stars being the hottest and M stars being the coolest. However, because stars of the same spectral type can have widely different luminosities (and so different radii by the Stefan-Boltzmann Law, which relates luminosity, radius, and surface temperature of a star), a second classification by luminosity is added, where stars are assigned Roman numerals I-IV. The paper today focuses on evolved stars of spectral type G and K and luminosity class IV (subgiants), III (normal giants), II (bright giants), and Ib (supergiants). Supergiants are the brightest and largest, followed by bright giants, normal giants, and finally subgiants.
Humans wind down over the years, and stars do too. Stars spin down as they age, induced by loss of angular momentum through outflows of gas particles ejected from stellar atmospheres, also known as stellar winds. Therefore we expect evolved stars to spin slower than young stars. Evolved G- and K-stars are known to be slow-rotators (rotating at a few km/s), with rotation decreasing gradually from early-G stars to late-K stars. However, as it always the case in astronomy, there are always counter examples. As far back as four decades ago, astronomers found rapidly rotating G and K giant stars (luminosity class III) spinning as fast as 80 km/s. How and why these stars are able to spin this fast is still a puzzle, with theories ranging from coalescing binary stars, sudden dredge-up of angular momentum from the stellar interiors, and engulfment of hot Jupiters (Jupiter-sized exoplanets that orbit very close to their parent stars, hence the name “hot Jupiters”) by giant stars causing a spin-up.
Using a set of criteria, the authors of this paper hunted for single rapidly-rotating G- and K-stars in the Bright Star Catalog and catalog of G- and K- stars compiled by Egret (1980). Out of 2010 stars, they uncovered a total of 30 new rapidly-rotating stars among subgiants, giants, bright giants, and supergiants. To date, rapid rotators have only been found among giant stars; this work reports for the first time the presence of such rapid rotators among subgiants, bright giants, and supergiants. In fact, these objects make up more than half of the number of rapid rotators in their sample. Figure 1 shows the velocities along line of the sight (v sin i) versus effective temperatures for their sample of evolved rapid rotators, compared with G and K binaries (ie, binary star systems consisting of G- and K-stars). The similarity between the two populations implies a similar synchronization mechanism between the rotation of single evolved stars and orbital motion of the binary systems. That interesting relation aside, the main point to note from the plot is the large observed velocities of the rapid rotators compared to the mean rotational velocities of G- and K-stars.
The rapidly-rotating stars are analyzed for far-IR excess emission, which may indicate the presence of warm dust surrounding the stars (warm dust emits radiation in the mid- to far-IR regime). Looking at figure 2, a trend of far-IR excess emission is clearly seen for almost all of the 23 stars they analyzed. The origin of dust close to to these stars are not well understood; some attributed it to stellar winds driven by magnetic activity, while others hypothesized that it comes from collisions of planetary companions around these stars. In any case, any theory that tries to explain the nature of rotation in these single systems needs to account for the presence of warm dust.
The authors proposed that the coalescence scenario between a star and a low-mass stellar or a substellar companion (ie, a brown dwarf) or the tidal interaction in planetary systems with hot Jupiters to be plausible scenarios that can explain singly rapidly-rotating evolved stars. Because each scenario should produce different chemical abundances, the authors suggested searching for changes in specific abundance ratios, such as the relative enhancement of refractory over volatile elements, in these stars to differentiate between the various possible scenarios above.
Spinning stars are cool. Even cooler are rapidly rotating giant-like stars that spin of the clutches of theoretical predictions. While in the past rapid rotators among evolved giant stars can be explained away using small-number statistics, the authors of this paper added an order of magnitude more items to the list, forcing stellar astrophysicists to come face-to-face with the question of the nature of these rapid rotations.