The Field Substellar Mass Function Based on the Full-sky 20 pc Census of 525 L, T, and Y Dwarfs

Abstract

We present final Spitzer trigonometric parallaxes for 361 L, T, and Y dwarfs. We combine these with prior studies to build a list of 525 known L, T, and Y dwarfs within 20 pc of the Sun, 38 of which are presented here for the first time. Using published photometry and spectroscopy as well as our own follow-up, we present an array of colormagnitude and color-color diagrams to further characterize census members, and we provide polynomial fits to the bulk trends. Using these characterizations, we assign each object a T eff value and judge sample completeness over bins of T eff and spectral type. Except for types T8 and T eff < 600 K, our census is statistically complete to the 20 pc limit. We compare our measured space densities to simulated density distributions and find that the best fit is a power law ( µ a -dN dM M ) with α = 0.6 ± 0.1. We find that the evolutionary models of Saumon & Marley correctly predict the observed magnitude of the space density spike seen at 1200 K < T eff < 1350 K, believed to be caused by an increase in the cooling timescale across the L/T transition. Defining the low-mass terminus using this sample requires a more statistically robust and complete sample of dwarfs Y0.5 and with T eff < 400 K. We

The Enigmatic Brown Dwarf WISEA J153429.75-104303.3 (a.k.a. “The Accident”)

“…The lack of other discoveries as cold as WISE 0855−0714 may imply that it is not a typical member of the ultracold brown dwarf population. As discussed in Section 9.4 of Kirkpatrick et al (2021), objects at these coldest temperatures are expected to span a wider range of ages than warmer brown dwarfs, suggesting a wider range of metallicities and photometric properties.…”

“…Studies at other wavelengths are hampered by intrinsically faint magnitudes. For known Y dwarfs-the coldest brown dwarfs with effective temperatures below ∼450K (Cushing et al 2011)-absolute J-band magnitudes range from 19.4 to 28.2 mag (Kirkpatrick et al 2021).…”

“…Despite these difficulties, the number of known, very cold brown dwarfs is slowly growing, with 50 confirmed or suspected Y dwarfs now cataloged (Kirkpatrick et al 2021). Nonetheless, uncovering clear trends in colors and absolute magnitudes is proving elusive.…”

“…Despite estimates that -+ 15 11 20 such frigid objects (Wright et al 2014) have been imaged in data sets by the Wide-Field Infrared Survey Explorer (WISE; Wright et al 2010;Mainzer et al 2014), it is the only brown dwarf recognized to have T eff < 350 K (Kirkpatrick et al 2021), although two others may also fall in this temperature zone-WISEA J083011.95+283716.0 (hereafter, WISE 0830+2837; Bardalez Gagliuffi et al 2020) and WISEPA J182831.08+265037.8 (hereafter, WISE 1828+2650; Cushing et al 2011). WISE 1828+2650, in fact, stands out on colormagnitude plots as 1.5-2.0 magnitudes more luminous at J and ch2 than Y dwarfs of similar J − ch2 color (Kirkpatrick et al 2021), and it has a unique near-infrared spectrum (Cushing et al 2011(Cushing et al , 2021. WISE 1828+2650 may be hinting at the variety present among the coldest brown dwarfs, and this diversity is becoming more evident as we obtain detailed characterization of the larger Y dwarf sample.…”

“…Meisner et al (2020) showed that the J − ch2 color limit and presumed high velocity of WISE 1534−1043 placed it among a small set of mid-to late-T subdwarfs. Kirkpatrick et al (2021) used available WISE and Spitzer data to measure a parallax, albeit with a large (30%) uncertainty. Here we present two new data sets that produce a robust color and parallax reinforcing the uniqueness of this object.…”