Abstract
Abstract We present a Hubble Space Telescope/Wide-Field Camera 3 near-infrared spectrum of the archetype Y dwarf WISEP 182831.08+265037.8. The spectrum covers the 0.9–1.7 μm wavelength range at a resolving power of λ/Δλ ≈ 180 and is a significant improvement over the previously published spectrum because it covers a broader wavelength range and is uncontaminated by light from a background star. The spectrum is unique for a cool brown dwarf in that the flux peaks in the Y, J, and H bands are of near equal intensity in units of f λ . We fail to detect any absorption bands of NH3 in the spectrum, in contrast to the predictions of chemical equilibrium models, but tentatively identify CH4 as the carrier of an unknown absorption feature centered at 1.015 μm. Using previously published ground- and spaced-based photometry, and using a Rayleigh–Jeans tail to account for flux emerging longward of 4.5 μm, we compute a bolometric luminosity of log ( L bol / ⊙ N ) = − 6.50 ± 0.02 , which is significantly lower than previously published results. Finally, we compare the spectrum and photometry to two sets of atmospheric models and find that the best overall match to the observed properties of WISE 1828+2650 is a ∼1 Gyr old binary composed of two T eff ∼ 325 K, ∼5 M Jup brown dwarfs with subsolar [C/O] ratios.
Highlights
The search for cool brown dwarfs with effective temperatures (Teff) less than 700 K was one of the driving science goals of wide-area, redoptical, and infrared surveys such as the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS; Lawrence et al 2007), the Canada France Brown Dwarf Survey (CFBDS; Delorme et al 2008b) and its counterpart in the near-infrared CFBDSIR (Delorme et al 2010), and the Wide-field Infrared Survey Explorer (WISE; Wright et al 2010)
With masses less than ∼20 MJup at the typical age of the ultracool dwarf field population of 3 Gyr (Dahn et al 2002), such cool brown dwarfs are most likely the least massive products of star formation (e.g., Kirkpatrick et al 2021). While such cool brown dwarfs have been discovered using each of these surveys (e.g., Burningham et al 2008; Delorme et al 2008a, 2010; Cushing et al 2011; Kirkpatrick et al 2011; Liu et al 2011) the vast majority of brown dwarfs with effective temperatures less than 500 K have been discovered using WISE data
The near-infrared spectrum of one such brown dwarf, WISEP J182831.08+265037.8, was distinct enough from that of the late-type T dwarfs that Cushing et al (2011) identified it has the archetype of the new Y spectral class
Summary
The search for cool brown dwarfs with effective temperatures (Teff) less than 700 K (approximately a spectral type of T8) was one of the driving science goals of wide-area, redoptical, and infrared surveys such as the United Kingdom Infrared Telescope Infrared Deep Sky Survey (UKIDSS; Lawrence et al 2007), the Canada France Brown Dwarf Survey (CFBDS; Delorme et al 2008b) and its counterpart in the near-infrared CFBDSIR (Delorme et al 2010), and the Wide-field Infrared Survey Explorer (WISE; Wright et al 2010). The near-infrared spectrum of one such brown dwarf, WISEP J182831.08+265037.8 (hereafter WISE 1828+2650), was distinct enough from that of the late-type T dwarfs that Cushing et al (2011) identified it has the archetype of the new Y spectral class. At the time of its discovery, WISE 1828+2650 had the reddest near- to mid-infrared color of any brown dwarf known at J−W2 = 9.29 ± 0.35 (mag) Atmospheric models indicated such a color corresponded to an effective temperature of less than ∼300 K.
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