Benchmarks and Standards

I like to study benchmarks and standards, from velocities to wavelengths to stellar ages.

One of my interests is how binary stars and clusters help us set standards for steller astrophysics.  One of the difficulties for the lowest-mass stars (“M dwarfs”) is that their metallicity is very difficult to measure.  We normally measure the composition of stars (really, the relative abundance of “metals” meaning everything heavier than helium) compared to the Sun, or “metallicity”) by studying their spectra and asking how many metals it would required in what proportion to reproduce what we see.

But M dwarfs are cool enough that molecules form in their atmospheres, and molecules have very complex spectra that are very hard to model or measure in the lab.  This makes attempts to derive metallicity from M dwarf spectra very hard.

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The problem has been tackled from many angles, mostly by comparing binary systems where one star is a more massive, Sun-like star and the other is an M dwarf.  Since binaries presumably formed from the same cloud of gas, the metallicities should be the same, so we can measure the metallicity of the more massive star, then look to see what an M dwarf spectrum at that metallicity would look like.  This approach was followed by Barbara Ayala-Rojas for her dissertation work at Cornell while I was there (using K-band spectral indices), and also by Penn State’s own Ryan Terrien and Suvrath Mahadevan (using H-band indices).
Now, Ryan, Suvrath, and other members of their group have applied this method to an important benchmark M dwarf with a white dwarf companion,  CM Draconis.  Please hop on over to his blog and take a look at the great work they’ve done!  The bottom line is they have significantly revised the metallicity of this important system and that our best models of M dwarfs…  still don’t make any sense.
[Image Credit: P. Marenfeld and NOAO/AURA/NSF]