Ay219 Week 5:
Measurements Elemental Abundances from
Stellar Spectra Using Line Strengths
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Line strength vs. abundance
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Equivalent width
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Continuum normalization
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Line profiles
Turning equivalent widths into abundances
1
Curve of growth
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Curve of growth
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Ingredients for abundance measurements
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Equivalent widths or raw spectrum
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Line list
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Model atmosphere
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Abundance code
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Assumptions
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One-dimensional atmospheres
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Does not capture convection
Local thermodynamic equilibrium (LTE)
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i.e., an isotropic radiation field at all points in the
atmosphere
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Good assumption for some lines, bad for other
Hydrostatic equilibrium
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i.e., pressure balances gravity
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Good assumption for most stars, but not variable
stars
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Equivalent width
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Spectral continuum normalization
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Spectral continuum normalization
1) Linear fit across one line
2) Polynomial fit:
3) Spline fit: (just a combination of polynomials)
4) Fitting a model:
5) Sigma clipping
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Equivalent width
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Line profiles
Gaussian:
Lorentzian:
Voigt:
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Voigt profile
Observed spectrum of the Sun, Mg b 5172
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Measuring equivalent width
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Measuring equivalent width
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Measuring equivalent width
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Measuring equivalent width
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Measuring equivalent width
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Spectrophotometric index
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Example line list
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Atomic and molecular data
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NIST
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VALD
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http://www.physics.nist.gov/PhysRefData/ASD/lines
_form.html
http://vald.astro.uu.se/
Kurucz
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http://www.pmp.uni-hannover.de/cgi-bin/ssi/test/kuru
cz/sekur.html
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Model atmosphere
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Example model atmosphere
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Where to find model atmospheres
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ATLAS
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http://www.stsci.edu/hst/observatory/crds/castelli_ku
rucz_atlas.html
Castelli & Kurucz 2004, arXiv:astro-ph/0405087
MARCS
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http://marcs.astro.uu.se/
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Gustafsson et al. 2008, A&A, 486, 951
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Codes that measure abundances
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MOOG
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http://www.as.utexas.edu/~chris/moog.html
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Sneden 1973, Ph.D. thesis, UT Austin
Turbospectrum
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http://www.pages-perso-bertrand-plez.univ-montp2.f
r/Turbospectrum-v14.1.tar.gz
Alvarez & Plez 1998, A&A, 330, 1109
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Plez 2012, ascl:1205.004
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Spectroscopy Made Easy (SME)
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http://www.stsci.edu/~valenti/sme.html
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Valenti & Piskunov 1996, A&AS, 118, 595
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Measuring effective temperature (Teff)
Raise temperature → decrease slope
Lower temperature → increase slope
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Measuring surface gravity (log g)
Abundance from Fe I lines
=
Abudance from Fe II lines
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Measuring microturbulent velocity (vt or ξ)
Raise ξ → decrease slope
Lower ξ → increase slope
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