The Initial Mass Function Based on the Full-sky 20 pc Census of ∼3600 Stars and Brown Dwarfs
Tsai, Chao-Wei; Mamajek, Eric; Gonzales, Eileen C.; Kiman, Rocio; Meisner, Aaron M.; Raddi, Roberto; Smart, R. L.; Kirkpatrick, J. Davy; Gelino, Christopher R.; Burgasser, Adam J.; Eisenhardt, Peter R.; Connor, Thomas; Stern, Daniel; Schneider, Adam C.; Marocco, Federico; Faherty, Jacqueline K.; Caselden, Dan; Kuchner, Marc J.; Gagné, Jonathan; Theissen, Christopher; Gerasimov, Roman; Aganze, Christian; Bardalez Gagliuffi, Daniella C.; Stevnbak Andersen, Nikolaj; Gramaize, Léopold; Martin, David W.; Sainio, Arttu; Rothermich, Austin; Calamari, Emily; Kabatnik, Martin; Pendrill, William; Tanner, Christopher; Zurek, David; Vos, Johanna M.; Bickle, Thomas P.; Suárez, Genaro; The Backyard Worlds: Planet 9 Collaboration; Casewell, S. L.; Brooks, Hunter; Popinchalk, Mark; Hsu, Chih-Chun; Costa, Edgardo; Durantini Luca, Hugo A.; Hyogo, Michiharu; Sun, Guoyou; Allers, Katelyn N.; Doll, Katharina; Beaulieu, Paul; Colin, Guillaume; Hamlet, Leslie K.; Jonkeren, Alexander; Pumphrey, Benjamin; Stenner, Andres; Thévenot, Melina; Voloshin, Nikita V.; Walla, Jim; Wędracki, Zbigniew; Li, Guodong; Kiwy, Frank; Apps, Kevin; Rees, Jon M.; Schurr, Steven D.; Raghu, Yadukrishna; Higashimura, Hiro; Dereveanco, Alexandru; Sanchez, John; Schümann, Jörg; Kesseli, Aurora; Antonini, Edoardo; Bilsing, Martin; Chieng, Raymond; Deen, Sam; Frazer, Anya; Gantier, Jean Marc; Grant, Kristin; Jałowiczor, Peter A.; Michaels, Marianne N.; Pessanha Machado, Celso; Russwurm, Rebekah; Sapelkin-Tambling, Fyodor Theo; Selg-Mann, Karl; Singh, Harshdeep; Ventura, Maurizio; Adorno, Jose I.; Low, Ryan; Pantoja, Blake M.
United States, United Kingdom, Italy, Spain, Denmark, Canada, Japan, China, Chile, Ireland
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, $\xi (M)={dN}/{dM}\propto {M}^{-\alpha }$ , 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 ⊙.