From AAS NOVA : “Gaia Identifies a Stellar Gap”

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From AAS NOVA

11 July 2018
Susanna Kohler

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New measurements of hundreds of thousands of stars have revealed a surprising gap in main-sequence stars on a Hertzsprung-Russell diagram. [NASA, ESA, and Hubble Heritage Team]

Sometimes more-precise measurements are all we need to make new discoveries in old structures! In a new study, data from the Gaia mission has revealed a surprise hidden among main-sequence stars.

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An observational Hertzsprung–Russell diagram with 22,000 stars plotted from the Hipparcos Catalogue and 1,000 from the Gliese Catalogue of nearby stars. Stars tend to fall only into certain regions of the diagram. The most prominent is the diagonal, going from the upper-left (hot and bright) to the lower-right (cooler and less bright), called the main sequence. In the lower-left is where white dwarfs are found, and above the main sequence are the subgiants, giants and supergiants. The Sun is found on the main sequence at luminosity 1 (absolute magnitude 4.8) and B−V color index 0.66 (temperature 5780 K, spectral type G2V).

Old Diagram with a New Feature

If you’ve ever taken an introductory astronomy class, you’ve probably encountered the Hertzsprung-Russell (HR) diagram: a diagram on which stellar luminosities are plotted against their colors, which serve as a proxy for their effective temperatures. The resulting positions of stars on the HR diagram reveal distinct stellar evolutionary stages — and perhaps the most striking population is the swath of main-sequence stars that cuts diagonally across the diagram.

Though we’ve constructed HR diagrams for nearby stars for more than a century, they continue to change as our data for these stars improve. In particular, today’s era of precision astrometry has significantly improved the distance measurements for the stars that surround us, allowing them to be placed more accurately on the diagram. The recent second data release from the Gaia mission presented precise astrometry measurements for billions of stars, covering virtually all types on the HR diagram — including M dwarfs, which were sparsely sampled in the past.

A team of scientists led by Wei-Chun Jao (Georgia State University) has now explored this data and discovered a surprise: there’s a gap in the HR diagram at mid-M dwarfs.

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Portion of the HR diagram for stars within 100 pc in the Gaia DR2 data set. A narrow low-density gap is visible cutting through the main sequence between the two dashed lines. [Jao et al. 2018]

Mind the Gap

Jao and collaborators plotted a total of nearly 250,000 stars from the Gaia archive on an HR diagram. The new data and improved measurements revealed a previously unseen feature: a narrow, diagonal slice through the main sequence that is underpopulated. The missing stars seem to lie in the middle of the M-dwarf region.

The authors cross-match the stars against the 2MASS catalog, finding that the gap exists in other data and color bands as well — which means it’s not just a weird quirk of Gaia’s photometry. They then check whether the gap exists only in stars at a specific distance. Another no: it’s visible similarly in various populations spanning distances up to 425 light-years.

Transitioning Convection

So what’s causing this unexpected feature? The authors argue that the presence and persistence of the gap suggest that it’s due to some underlying physics that we haven’t yet thought of — which is always an exciting prospect!

In particular, Jao and collaborators suggest that the gap may be related to a known transition in mid-M dwarfs, from larger stars that are mostly convective with a thin radiative layer, to smaller stars that are fully convective. The authors propose that the missing stars in the gap may be due to subtle changes of structure that occur at this transition between partial and full convection in these M dwarfs.

In the future, the authors propose gathering more data — like dynamical masses, radii, metallicities, rotational periods, and magnetic properties — on stars in and near the gap, to better understand population trends. In the meantime, we can be excited to know that there are still some surprises left for us to discover in old structures, if we just keep improving our data.
Citation

Wei-Chun Jao et al 2018 ApJL 861 L11. http://iopscience.iop.org/article/10.3847/2041-8213/aacdf6/meta

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