The Initial Mass Function Based on the Full-sky 20 pc Census of ∼3600 Stars and Brown Dwarfs

Abstract

A complete accounting of nearby objects—from the highest-mass white dwarf progenitors down to low-mass brown dwarfs—is now possible, thanks to an almost complete set of trigonometric parallax determinations from Gaia, ground-based surveys, and Spitzer follow-up. We create a census of objects within a Sun-centered sphere of 20 pc radius and check published literature to decompose each binary or higher-order system into its separate components. The result is a volume-limited census of ∼3600 individual star formation products useful in measuring the initial mass function across the stellar (<8M ⊙) and substellar (≳5M Jup) regimes. Comparing our resulting initial mass function to previous measurements shows good agreement above 0.8M ⊙ and a divergence at lower masses. Our 20 pc space densities are best fit with a quadripartite power law, ξ ( M ) = dN / dM ∝ M − α , with long-established values of α = 2.3 at high masses (0.55 < M < 8.00M ⊙), and α = 1.3 at intermediate masses (0.22 < M < 0.55M ⊙), but at lower masses, we find α = 0.25 for 0.05 < M < 0.22M ⊙, and α = 0.6 for 0.01 < M < 0.05M ⊙. This implies that the rate of production as a function of decreasing mass diminishes in the low-mass star/high-mass brown dwarf regime before increasing again in the low-mass brown dwarf regime. Correcting for completeness, we find a star to brown dwarf number ratio of, currently, 4:1, and an average mass per object of 0.41 M ⊙.

The Low-mass Stellar Initial Mass Function in Nearby Ultrafaint Dwarf Galaxies

“…The Chabrier and Kroupa IMFs are generally valid above the hydrogen burning limit (∼0.08M e ), and the IMF of brown dwarfs is usually considered separately. The IMF of such extremely low-mass objects is generally thought to be shallower than the IMF above the hydrogen burning limit (for example, the analysis of the local disk in Kirkpatrick et al 2024; or a young cluster in Mužić et al 2017), although there remains uncertainty as to exactly at what mass the slope changes. Further, the very faint nature of brown dwarfs tends to limit analyses to relatively small samples, which in turn limits the precision with which the IMF can be determined, as discussed below.…”

The Variation in the Galaxy-wide Initial Mass Function for Low-mass Stars: Modeling and Observational Insights

“…Here, Z MW represents the mean metal mass fraction of field stars in the solar neighborhood, which served as the basis for canonical IMF measurements, that is, within about 20 pc (Kroupa et al 1993;Bochanski et al 2010;Kirkpatrick et al 2024). Consequently, the According to the aforementioned postulations, with parameters summarized in Table 1, the IMF would exhibit a bottom-heavier trend, forming a relatively greater number of lower-mass stars as metallicity increases.…”

Discovery of the Remarkably Red L/T Transition Object VHS J183135.58-551355.9

“…When the age of an object is inferred, evolutionary models can be used to estimate physical properties such as mass and effective temperature (e.g., Filippazzo et al 2015;Suárez et al 2021). Such objects are vital benchmarks for studying brown dwarf evolution, and at young ages and low masses can provide important empirical constraints to the initial mass function (Gagné et al 2017;Kirkpatrick et al 2021Kirkpatrick et al , 2024.…”