A Novel Survey for Young Substellar Objects with the W-band Filter. VI. Spectroscopic Census of Substellar Members and the IMF of the σ Orionis Cluster

Abstract

Low-mass stars and substellar objects are essential in tracing the initial mass function (IMF). We study the nearby young σ Orionis cluster (d ∼ 408 pc, age ∼ 1.8 Myr) using deep near-infrared (NIR) photometric data in the J, W, and H bands from WIRCam on the Canada–France–Hawaii Telescope. We use the water absorption feature to select brown dwarfs photometrically and confirm their nature spectroscopically with IRTF-SpeX. Additionally we select candidate low-mass stars for spectroscopy and analyze their membership and those of literature sources using astrometry from Gaia DR3. We obtain NIR spectra for 28 very-low-mass stars and brown dwarfs and estimate their spectral type between M3 and M8.5 (masses ranging between 0.3 and 0.01 M ⊙). Apart from these, we also identify five new planetary-mass candidates which require further spectroscopic confirmation of youth. We compile a comprehensive catalog of 170 spectroscopically confirmed members in the central region of the cluster, for a wide mass range of ∼19–0.004 M ☉. We estimate the star-to-brown-dwarf ratio to be ∼4, within the range reported for other nearby star-forming regions. With the updated catalog of members we trace the IMF down to 4 M Jup and we find that a two-segment power law fits the substellar IMF better than a log-normal distribution.

A Volume-limited Sample of Ultracool Dwarfs. II. The Substellar Age and Mass Functions in the Solar Neighborhood

“…However, since objects at the faint end of the luminosity range we used for our population synthesis (M bol = 21 mag) have higher masses at ages 100 Myr-as much as 0.026 M e at 10 Gyr-our analysis is not sensitive to the low-mass cutoff. At present, the best available constraint is that the minimum brown dwarf mass cannot be greater than the ≈5 M Jup of the known freefloating planetary-mass brown dwarfs in nearby star-forming regions (e.g., Luhman et al 2009;Best et al 2017;Zhang et al 2021;Damian et al 2023).…”

An HST Study of the Substellar Population of NGC 2024

“…This is reflected in the original broken power law of Kroupa (2001), which has an α exponent with large uncertainty, i.e., α = 0.3 ± 0.7 in the mass range 0.01 m/M e 0.08. While studies of different clusters in the solar vicinity, e.g., the Orion Nebula Cluster (ONC; Gennaro & Robberto 2020), NGC 2244, (Almendros-Abad et al 2023, Chamaeleon I (Luhman 2007), λ Ori (Bayo et al 2011), σ Ori (Peña et al 2012;Damian et al 2023), ρ Ophiuchi (Oliveira et al 2012), Upper Scorpius (Lodieu 2013), NGC 1333 and IC 348 (Scholz et al 2013), Lupus 3 (Mužić et al 2015), and 25 Ori (Downes et al 2014;Suárez et al 2019), report IMFs generally consistent with the canonical laws, a number of studies (Weidner et al 2013;Dib 2014;Dib et al 2017) provide statistically significant evidence of variations of IMF between clusters. A spread of IMF parameters may actually explain the integrated IMF of galaxies of different metallicity (Dib 2022).…”