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Published April 2024 | Version Published
Journal Article Open

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

Creators

  • 1. ROR icon California Institute of Technology
  • 2. ROR icon Jet Propulsion Lab

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 (≳5MJup) 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⊙.

Copyright and License

© 2024. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

D.K., F.M., and C.G. acknowledge support from grant No. 80NSSC20K0452 awarded for proposal 18-2ADAP18-0175 under the NASA Astrophysics Data Analysis Program. Data presented here are based on observations obtained at the Hale Telescope, Palomar Observatory as part of a continuing collaboration between the California Institute of Technology, NASA/JPL, Yale University, and the National Astronomical Observatories of China. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. The first author would like to thank Patrick Shopbell at Caltech for resurrecting an Exabyte drive that successfully read raw CTIO data from 1997. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). He would also like to thank Mike Cushing for discussion of evolved star loci in the 20 pc color–magnitude diagrams.

One observation reported in this paper was obtained with the Southern African Large Telescope (SALT) under program 2021-2-SCI-027 (PI: Faherty). R.R. acknowledges support from grant RYC2021-030837-I funded by MCIN/AEI/ 10.13039/501100011033 and by "European Union NextGenerationEU/PRTR," and partial support by the AGAUR/Generalitat de Catalunya grant SGR-386/2021 and by the Spanish MINECO grant PID2020-117252GB-I00. E.G. acknowledges support from the Heising-Simons Foundation through a 51 Pegasi b Fellowship. This publication makes use of VOSA, developed under the Spanish Virtual Observatory (https://svo.cab.inta-csic.es) project funded by MCIN/AEI/10.13039/501100011033/ through grant PID2020-112949GB-I00. This research made use of the Montreal Open Clusters and Associations (MOCA) database, operated at the Montréal Planétarium (J. Gagné et al. 2024, in preparation). Backyard Worlds research was supported by NASA grant 2017-ADAP17-0067 and by the NSF under grants AST- 2007068, AST-2009177, and AST-2009136. J.V. acknowledges support from a Royal Society—Science Foundation Ireland University Research Fellowship (URF\1\221932). The authors would like to thank the Near-Earth Object Surveyor mission for covering the page charges for this paper through contract No. 80MSFC20C0043.

Facilities

WISE, Gaia, Spitzer (IRAC), Hale (WIRC, DBSP, TSpec), Keck:I (MOSFIRE, NIRES), IRTF (SpeX), Gemini:South (FLAMINGOS-2), Blanco (RCSPec, ARCoIRIS), NTT (SOFI), Magellan:Baade (FIRE), Shane (Kast), SALT (RSS), SOAR (OSIRIS), ARC (TSpec)

Software References

WiseView (Caselden et al. 2018), Spextool (Vacca et al. 2003; Cushing et al. 2004), MOPEX/APEX (Makovoz & Khan 2005; Makovoz & Marleau 2005), IRAF (Tody 1986, 1993), kastredux (A. J. Burgasser et al. 2024, in preparation), FIREHOSE/MASE (Bochanski et al. 2009), crowdsource (Schlafly et al. 2018)

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Additional details

Identifiers

ISSN
1538-4365

Funding

National Aeronautics and Space Administration
80NSSC20K0452
National Aeronautics and Space Administration
18-2ADAP18-0175
W. M. Keck Foundation
National Aeronautics and Space Administration
80NM0018D0004
Agencia Estatal de Investigación
Ramón y Cajal program RYC2021-030837-I
Agencia Estatal de Investigación
MCIN/AEI/ 10.13039/501100011033
Government of Catalonia
SGR-386/2021
Ministerio de Ciencia, Innovación y Universidades
PID2020-117252GB-I00
Heising-Simons Foundation
51 Pegasi b Postdoctoral Fellowship
Agencia Estatal de Investigación
MCIN/AEI/10.13039/501100011033
Agencia Estatal de Investigación
PID2020-112949GB-I00
National Aeronautics and Space Administration
2017-ADAP17-0067
National Science Foundation
AST-2007068
National Science Foundation
AST-2009177
National Science Foundation
AST-2009136
Royal Society
Science Foundation Ireland
URF\1\221932
National Aeronautics and Space Administration
80MSFC20C0043

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Caltech groups
Infrared Processing and Analysis Center (IPAC)