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 (<8 M _⊙ ) and substellar (≳5 M _Jup ) regimes. Comparing our resulting initial mass function to previous measurements shows good agreement above 0.8 M _⊙ and a divergence at lower masses. Our 20 pc space densities are best fit with a quadripartite power law, $\xi (M)={dN}/{dM}\propto {M}^{-\alpha }$ , with long-established values of α = 2.3 at high masses (0.55 < M < 8.00 M _⊙ ), and α = 1.3 at intermediate masses (0.22 < M < 0.55 M _⊙ ), but at lower masses, we find α = 0.25 for 0.05 < M < 0.22 M _⊙ , and α = 0.6 for 0.01 < M < 0.05 M _⊙ . 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 _⊙ .

Keywords

• Initial mass function
• Stellar mass functions
• Brown dwarfs
• Trigonometric parallax
• Solar neighborhood
• Binary stars