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Metals in Low Mass Stars

The low mass M stars are the most abundant type of stars in the universe. These stars are relatively cool, less than 3900 K, and as a result their spectra are very complicated due to the plethora of spectral lines resulting from molecules that form in their atmospheres. As a result of this confusion of spectral lines, it is often very difficult to accurately measure various characteristics of these stars.

One property of stars that has become increasingly of interest is the relative abundance of elements heavier than Helium, that is present in a star. This property is called the metallicity of a star. There appears to be a strong correlation between a star's metallicity and the likelihood that planets will form around that star. This is not really a surprise. A star and its planets all form out of the same material (an interstellar gas cloud), but through somewhat different processes. A star forms when the gravity in a region of an interstellar cloud overwhelms the other forces that are holding the gas up. The main thing that is important here is the total mass of the cloud, but not what the material is made of. Once the young star forms, some of the material in the cloud forms a disk surrounding the star. It is here that planets will form. The seeds that will eventually form the planets start to build up when heavy element particles called dust grains by astronomers start to collide and stick together in the disk. If the material has a higher metallicity to begin with, this process is easier to get started. Measuring the metallicity of a star tells us what the metallicity of the cloud it formed out of was. Thus, by measuring the metallicity of a star, we may be able to determine how likely it is that that star will have planets around it. Since the M stars are the most abundant stars in the universe, there is a greater potential of finding planets (including nearby ones) around these stars.

In this paper we develop a new technique for measuring the metallicity of M dwarfs. The paper presents this technique and tests it by measuring the metallicity of M dwarfs which are in visual double-star systems with more massive and hotter companion stars. It is relatively easy to measure the metallicity of the hotter star because it has no molecules in its atmosphere. Since the two stars formed out of the same gas cloud, they should have the same metallicity, so the hotter star in each binary pair serves as a check for what the metallicity of the M star should be. We show that are new technique does indeed accurately measure the metallicity of the low mass M stars.

GJ876b
An artist's conception of the planetary system around the M dwarf Gliese 876. Currently, 3 planets are known to be orbiting this low mass M star, which is only 15.3 lightyears away. How many other M stars have planetary systems?

A copy of the paper can be found at this link

Citation is: The Astrophysical Journal