Keywords

Synonyms

Solar vicinity

Definition

The solar neighborhood is the space associated with a cylinder centered at the Sun and perpendicular to the Milky Way disk. This solar cylinder is located at ∼8 kpc from the galactic center, with a radius of 1 kpc. Hence, the solar vicinity includes diverse physical entities that belong to the galactic disk and halo, such as planets; stars of different masses, ages, and evolutionary stages; interstellar medium; and HII regions, among others.

Overview

The solar neighborhood contains mostly baryonic matter (stars and gas); the amount of dark matter is negligible inside it. The total mass surface density is roughly 50 M⊙ pc2, of which the stars take 80 % and 20 % is in gaseous material.

The solar cylinder contains two main structures: first, a disk component where approximately 95 % of the total mass is found in Population I stars embedded in metal-rich gas. They are located between 1.5 kpc above and below the central plane of the disk, and secondly, a halo component formed by the older Population II stars and no gas.

The first component (disk) in turn shows two subcomponents: (1) a thin disk of 300 pc formed by interstellar gas with 1.5 times the solar metallicity (Z⊙) and a young stellar population with an average age of 5 Gyr and a average metal content of 0.8 Z⊙. (2) A thick disk of ∼1,000 pc thickness and formed by an older stellar population with an average age of 10 Gyr and a metallicity of 0.2 Z⊙. The stellar mass of the thick disk is ∼10 % of the mass in the thin disk.

The solar neighborhood stars show an age-metallicity relation, such that low age correlates with high metallicity. That relation has been basic to understanding the chemical evolution of galaxies. However, this relation presents a spread in ages for a fixed value of Z. The main cause for this is not clear yet.

Those stars show another important evolutionary relation, involving α/Fe versus Fe/H, where α represents an alpha chemical element (e.g., O, Mg, Si, S, Ca, and Ti). This relation has led to elucidating several questions, such as the chemical contribution in time by massive stars (the main producers of α elements) and by thermonuclear supernovae (the main producers of Fe), the star formation efficiency, and the different scenarios for galactic formation. The thick disk stars, just like the stars of the galactic bulge, exhibit overabundances of α elements, compared to the thin disk stars with the same Fe abundance.

The solar neighborhood is the astronomical laboratory for studying the properties of the stellar populations and of the interstellar medium.

Due to the current methods for exoplanet detection, most of the extrasolar planets have been found in stars at distances from the Sun closer than 0.5 kpc, that is, in the disk component of the solar vicinity.

There are other definitions of the solar neighborhood in the literature, for instance, (1) the region within 0.2 pc, where the planets, minor bodies, and the Oort cloud of the solar system are; (2) the region up to 100 pc from the Sun, where the closest stars are; and (3) the region up to 20 Mpc from the Milky Way, where the galaxies of the local cluster are.

See Also