Kepler Watches Stellar Dancers in the Pleiades Cluster

[bystander]

Kepler Watches Stellar Dancers in the Pleiades Cluster
NASA | JPL-Caltech | Ames Research Center | Kepler | 2016 Aug 12

[img3="Pleiades Cluster as seen by WISE - Credit: NASA/JPL-Caltech/UCLA"]http://www.jpl.nasa.gov/spaceimages/ima ... _hires.jpg[/img3][hr][/hr]

Like cosmic ballet dancers, the stars of the Pleiades cluster are spinning. But these celestial dancers are all twirling at different speeds. Astronomers have long wondered what determines the rotation rates of these stars.

By watching these stellar dancers, NASA's Kepler space telescope during its K2 mission has helped amass the most complete catalog of rotation periods for stars in a cluster. This information can help astronomers gain insight into where and how planets form around these stars, and how such stars evolve. ...

The Pleiades star cluster is one of the closest and most easily seen star clusters, residing just 445 light-years away from Earth, on average. At about 125 million years old, these stars -- known individually as Pleiads -- have reached stellar "young adulthood." In this stage of their lives, the stars are likely spinning the fastest they ever will.

As a typical star moves further along into adulthood, it loses some zip due to the copious emission of charged particles known as a stellar wind (in our solar system, we call this the solar wind). The charged particles are carried along the star's magnetic fields, which overall exerts a braking effect on the rotation rate of the star. ...

Rotation in the Pleiades with K2: I. Data and First Results - L. M. Rebull et al

• arXiv.org > astro-ph > arXiv:1606.00052 > 31 May 2016

Rotation in the Pleiades with K2: II. Multi-Period Stars - L. M. Rebull et al

• arXiv.org > astro-ph > arXiv:1606.00055 > 31 May 2016

Rotation in the Pleiades with K2: III. Speculations on Origins and Evolution - J. R. Stauffer et al

• arXiv.org > astro-ph > arXiv:1606.00057 > 31 May 2016

[Ann]

The study of the rotation of the stars in the Pleiades seems conclude that the massive stars in this ≈125 million year old cluster spin slowly, whereas the light-weight ones spin fast.

Click to view full size image

All stars to true relative scale. Diagram by Roen Kelly/Astronomy.
Used by permission of Astronomy.

But some hot massive stars with ages at least somewhat comparable to that of the Pleiades spin very rapidly.

Ken Croswell wrote:

McAlister's team observed Regulus on 22 nights in March and April 2004. The interferometric observations, together with spectroscopic data, indicate the star's equatorial diameter is 32 percent greater than its polar diameter: the former is 1.65 milliarcseconds and the latter 1.25 milliarcseconds. (One milliarcsecond is 1/3,600,000 of a degree--the apparent size of a car on Earth as seen from the Moon.) At Regulus' distance, these numbers translate into actual equatorial and polar diameters of 4.16 and 3.14 solar diameters...

McAlister and his colleagues say the star owes its odd shape to its rapid rotation. The spin distorts both the star and its spectral lines. As the star spins, one limb approaches us and the other recedes, smearing out all spectral lines because the approaching limb is blueshifted and the receding limb is redshifted. By studying the spectral lines, the astronomers find the equatorial rotation velocity, as seen from Earth, is a whopping 317 kilometers per second, or 709,000 miles per hour--versus the Sun's 2 kilometers per second (4,470 miles per hour). Regulus spins once every 15.9 hours, whereas the Sun takes a month to do the same.

If Regulus spun much faster, it would fly apart. The astronomers estimate the star is rotating at 86 percent of its break-up speed.

From their models, the team also determined the star's mass and temperature. Regulus is 3.4 times more massive than the Sun and has an average temperature of 12,900 Kelvin. However, the poles are hotter and the equator is cooler. The polar temperature is 15,400 K and the equatorial temperature only 10,300 K. For comparison, the Sun's temperature is 5,780 K...

Regulus isn't the first bright star found to be oblate. In 2001, astronomers reported that fast-spinning Altair, a white star of spectral type A7, has an equatorial diameter 14 percent greater than its polar diameter, less extreme than Regulus. And in 2003, astronomers discovered the equatorial diameter of the blue B3 main-sequence star Achernar is 56 percent greater than its polar diameter. In fact, Achernar is still the flattest star known.

Ann