Metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium.
This word-use is distinct from the conventional chemical or physical definition of a metal as an electrically conducting solid.
In 1802, William Hyde Wollaston[1] noted the appearance of a number of dark features in the solar spectrum.
[8] Their observations[9] were in the visible range where the strongest lines come from metals such as sodium, potassium, and iron.
[11]: Part 2 In contemporary usage in astronomy all the extra elements beyond just hydrogen and helium are termed metallic.
The presence of heavier elements results from stellar nucleosynthesis, where the majority of elements heavier than hydrogen and helium in the Universe (metals, hereafter) are formed in the cores of stars as they evolve.
Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were later attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars.
Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest.
Some methods include determining the fraction of mass that is attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun.
In most stars, nebulae, HII regions, and other astronomical sources, hydrogen and helium are the two dominant elements.
The overall stellar metallicity is conventionally defined using the total hydrogen content, since its abundance is considered to be relatively constant in the Universe, or the iron content of the star, which has an abundance that is generally linearly increasing in time in the Universe.
Iron is relatively easy to measure with spectral observations in the star's spectrum given the large number of iron lines in the star's spectra (even though oxygen is the most abundant heavy element – see metallicities in HII regions below).
[19] By this formulation, stars with a higher metallicity than the Sun have a positive common logarithm, whereas those more dominated by hydrogen have a corresponding negative value.
Primordial population III stars are estimated to have metallicity less than −6, a millionth of the abundance of iron in the Sun.
[21][22] The same notation is used to express variations in abundances between other individual elements as compared to solar proportions.
represents the difference in the logarithm of the star's oxygen abundance versus its iron content compared to that of the Sun.
In general, a given stellar nucleosynthetic process alters the proportions of only a few elements or isotopes, so a star or gas sample with certain
[24][25][26] The UV excess, δ(U−B), is defined as the difference between a star's U and B band magnitudes, compared to the difference between U and B band magnitudes of metal-rich stars in the Hyades cluster.
To help mitigate this degeneracy, a star's B−V color index can be used as an indicator for temperature.
Furthermore, the UV excess and B−V index can be corrected to relate the δ(U−B) value to iron abundances.
Measurements have demonstrated the connection between a star's metallicity and gas giant planets, like Jupiter and Saturn.
Current models show that the metallicity along with the correct planetary system temperature and distance from the star are key to planet and planetesimal formation.
[41][42][43][44][45] Young, massive and hot stars (typically of spectral types O and B) in HII regions emit UV photons that ionize ground-state hydrogen atoms, knocking electrons free; this process is known as photoionization.
The free electrons can strike other atoms nearby, exciting bound metallic electrons into a metastable state, which eventually decay back into a ground state, emitting photons with energies that correspond to forbidden lines.
[48][49][50][51] Theoretically, to determine the total abundance of a single element in an HII region, all transition lines should be observed and summed.
Oxygen has some of the stronger, more abundant lines in HII regions, making it a main target for metallicity estimates within these objects.
To calculate metal abundances in HII regions using oxygen flux measurements, astronomers often use the R23 method, in which
is the sum of the fluxes from oxygen emission lines measured at the rest frame λ = (3727, 4959 and 5007) Å wavelengths, divided by the flux from the Balmer series Hβ emission line at the rest frame λ = 4861 Å wavelength.
Metal abundances within HII regions are typically less than 1%, with the percentage decreasing on average with distance from the Galactic Center.
