On and On They Spin

Title: On the Nature of Rapidly Rotating Single Evolved Stars

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.


FIG 1 – Projected rotational velocity along line of sight, v sin i, vs. effective temperature Teff for rapidly rotating single G- and K-stars (filled circles) and rapidly rotating G and K binary systems (open circles). The rectangular zones at the bottom of the figure represent the mean rotational velocities for G- and K-stars that are subgiants (solid line), normal giants (dashed line), and supergiants (dashed–dotted line).


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.


FIG 2 – Far-IR colors for 23 G and K single and evolved rapid rotators. The left plot is V-[12] color, where V refers to optical V-band and [12] refers to IRAS‘ 12 µ band) while the right plot is V-[25], where [25] is IRAS’ 25 µ band. The rapid rotators are the red points while dashed, solid, and dotted lines are far-IR colors for normal-behaving G-and K-stars compiled by three different studies. The large offsets of the red points from the lines are evidence of far-IR excess emission.


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.


About Suk Sien Tie

I am a third year PhD student at the Department of Astronomy at The Ohio State University. I am currently working on quantitative analyses of various quasar selection methods using the Dark Energy Survey (DES) and quasar variability via microlensing.

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  1. Thanks for the article! This is very interesting. Could convection contribute to rapid rotation? If convection redistributes the star’s angular momentum toward the surface, we would observe higher surface rotation rates. Is there any way these rapid rotators are experiencing more convection?

  2. Very nice study! Regarding the question of why stars rotate, can the gravitational collapse of a cloud be responsible for this rotation?

  3. How much of a “kick” would stars get from engulfing or interacting with a hot Jupiter? And how long might its effects be noticeable? It seems like engulfment would be a relatively rare event, so unless its effects last a long time would we really expect to see such a high number of observable stars rotating faster due to it?

  4. I’d be interested to know how they determined the rotation velocity of these rapidly rotating stars

  5. Interesting post! Are there any other theories other than the two mentioned that could be plausible explanations for this phenomenon?

  6. Thanks for this post! I’m really interested to see how future theories/research explain the rapid rotation of these giant stars.

  7. I’m curious about this warm dust. As you say, it seems to be present in all the stars analyzed. Is it just a cloud of dust around the star or is it some type of disc? Have we detected a similar thing anywhere else in different conditions?

  8. I want to thank you for the style of your writing! It is sooo much easier to understand when the writing is really engaging and I wish more people wrote like you. I think if scientists could write like this more often we’d have a public opinion more in tune with scientific community.

  9. Loved the style of this article! What causes stars to rapidly rotate? Is it due to mass distribution and angular momentum? Or do they just start out spinning faster than normal stars?

  10. Is there any other plausible way to reproduce an IR excess besides dust? Could a companion object account for some of the observed IR excess?

  11. In the general case where stars slow their rotations as they age, and “evolved G- and K-stars are known to be slow-rotators”, is there a physical explanation for why G and K stars specifically are the slow ones? I could’ve seen that larger stars have more massive outflows, but I then would’ve expected M dwarfs to be ranked among the slowest rotating stars.

  12. What is the slowest measured rotation of a star? Can the rotation of a star slow down to the point that it affects the disk around it?

  13. Aren’t rapidly spinning stars bad in some sense? When doing RV calculations for Exoplanets they must contribute a significant noise.

  14. Fascinating post! I’m really looking forward to your findings on the answer to why these stars are spinning so unusually fast.


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