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.
A copy of the paper can be found at this link
Citation is: The Astrophysical Journal