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proj-stellarspectra.html
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<!DOCTYPE HTML>
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<h2 class="major">Empirical models of stellar spectra</h2>
<p>Stellar spectra are integral to nearly all aspects of modern astrophysics, as they allow us to learn
about a star's physical properties as well as its radial velocity and evolutionary stage. The
following animations show a generative data-driven model that predicts what a stellar spectrum
should look like for a given set of stellar properties (e.g., temperature, surface gravity, and
elemental abundances). The model (2nd-order polynomial in the 5 labels) was trained on data
from the <a href="https://arxiv.org/abs/1509.05420" target=_blank>APOGEE Survey</a> at the wavelength-pixel
level, closely following the procedure of <a href="https://arxiv.org/abs/1501.07604" target=_blank>Ness et al. 2015</a>.
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<p>The first movie shows how the spectrum changes as a star evolves off the main sequence and ascends
the red giant branch ("RGB"). The metallicity is fixed to solar metallicity, while logg varies
from 3.5 to 0.5 and Teff simultaneously varies such that the star moves along an isochrone. The
second movie shows how the same region of the spectrum changes with metallicity at fixed Teff and
logg. Clearly, both spectra show deeper absorption lines as the metallicity increases (with all
other parameters fixed) or as the star moves up the RGB at fixed composition. So how can one tell
the difference between a cool, low-logg star and a warmer, higher-logg star that is more metal-rich?
The key is to look at the <i>relative</i> strengths of the absorption lines. As the metallicity at
fixed stellar parameters increases, so does the optical depth of the stellar atmosphere, which
subsequently increases the strength of all the absorption lines somewhat uniformly. However, when
a star ascends the RGB, the physical mechanism for changing absorption line strength is the decrease
in effective temperature, which alters the ionization state of the atmosphere (as described
by the <a href="https://en.wikipedia.org/wiki/Saha_ionization_equation", target="_blank">Saha Equation</a> ).
This sets the the atomic line strengths — at higher temperatures, more atoms can become ionized due to
thermal collisions. When this happens these atoms can no longer absorb photons, thus decreasing the
strength of absorption lines. Therefore, decreasing Teff increases the strength of absorption lines,
but not all lines are affected equally because different atoms will have different ionization states
at a given temperature.
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