Fr.: métallicité critique
The → metallicity of a → star-forming → molecular cloud when → cooling → rates by → metals dominate the → gravitational → heating during → protostellar collapse. The minimum → Jeans mass achieved by gravitational → fragmentation depends on the presence/absence of → coolants in the cloud. Since cooling rate in metal lines is more efficient than in primordial molecular lines (H2 and HD), metals favor fragmentation in gas and formation of → low-mass stars.
Fr.: métallicité de gaz
The metallicity derived from observations of the gas component of a galaxy. It is mainly measured from optical → emission lines using primarily oxygen abundances. The gas → metallicity is one of the most important tools to investigate the evolutionary history of galaxies. The reason is that the gas metallicity of galaxies is basically determined by their star-formation history. Recent observational studies has allowed the investigation of the gas metallicity even in → high redshift beyond z = 1, such as → Lyman break galaxies, submillimeter-selected high-z galaxies, and so on. Such observational insights on the metallicity evolution of galaxies provide constraints on the theoretical understandings of the formation and the evolution of galaxies.
Fr.: environnement faible en métaux
A medium in which chemical elements have abundances smaller than the solar values.
mass-metallicity relation (MZR)
Fr.: relation masse-métallicité
A correlation between the → stellar mass (or → luminosity) and the → gas metallicity of → star-forming galaxies (Lequeux et al. 1979) according to which massive galaxies have higher gas metallicities. Several large galaxy surveys, such as the → Sloan Digital Sky Survey (SDSS), have confirmed that galaxies at all → redshifts with higher stellar masses retain more metals than galaxies with lower stellar masses. Besides the dependence on stellar mass, other studies have found further dependences of gas metallicity on other physical properties at a given mass, such as → specific star formation rate, → star formation rate, and stellar age. These higher dimensional relations could provide additional constraints to the processes that regulate the metal enrichment in galaxies. In addition to gas metallicity, also the → stellar metallicity of galaxies is found to correlate with the stellar mass, suggesting the mass-metallicity relation already existed at early epochs of galaxy evolution (Lian et al., 2017, MNRAS 474, 1143, and references therein).
In a star, nebula, or galaxy, the proportion of the material that is made up of
→ metals, that is elements heavier than → helium.
It is generally denoted by Z.
The term "metallicity" is a misnomer used in astrophysics.
metallicity distribution function (MDF)
karyâ-ye vâbâžeš-e felezigi
Fr.: fonction de distribution de métallicité
Fr.: gradient de métallicité
The decrease in the → abundances of → heavy elements in a → disk galaxy as a function of distance from the center. Radial metallicity gradients are observed in many galaxies, including the → Milky Way and other galaxies of the → Local Group. In the case of the Milky Way, several objects can be used to determine the gradients: → H II regions, → B stars, → Cepheids, → open clusters, and → planetary nebulae. The main diagnostic elements are oxygen, sulphur, neon, and argon in photoionized nebulae, and iron and other elements in Cepheids, open clusters, and stars. Cepheids are probably the most accurate indicators of abundance gradients in the Milky Way. They are bright enough to be observed at large distances, so that accurate distances and spectroscopic abundances of several elements can be obtained. Average abundance gradients are generally between -0.03 → dex/kpc and -0.10 dex/kpc, with a a flattening out of the gradients at large galactocentric distances (≥ 10 kpc). The existence of these gradients offers the opportunity to test models of → chemical evolution of galaxies and stellar → nucleosynthesis.
Fr.: métallicité solaire
The proportion of the solar matter made up of → chemical elements heavier than → helium. It is denoted by Z, which represents the sum of all elements heavier than → helium, in mass fraction. The most recent determination of the solar Z gives a value of 0.0134 (Asplund et al. 2009, ARAA 47, 481), corresponding to the present-day photospheric composition.
Fr.: métallicité stellaire
The metallicity derived from observations of stars in galaxies. It is mainly based on spectral → absorption lines in → ultraviolet (UV) and optical ranges. Stellar metallicity is a direct measure of the amount of metals in a galaxy, since large part of heavy elements lies in its stars.