Mid-infrared followup of cold brown dwarfs: Diversity in age, mass and metallicity

¹ Gemini Observatory, 670 N. A’ohoku Place, Hilo, HI 96720, USA
² Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK
³ Los Alamos National Laboratory, PO Box 1663, MS F663, Los Alamos, NM 87545, USA
⁴ NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035, USA
⁵ Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK
⁶ INAF/Osservatrio Astronomico di Torino, Strada Osservatrio 20, 10025 Pino Torinese, Italy

^a e-mail: sleggett@gemini.edu

Abstract

We use Spitzer IRAC 3.6–8.0 μm photometry of late-type T dwarfs to investigate various trends which can aid the planning and interpretation of infrared (IR) surveys for the coldest T or Y dwarfs. Brown dwarfs with effective temperature (T_eff)<700 K emit>50% of their flux at λ>3 μm, and the ratio of the mid-IR to the near-IR flux becomes very sensitive to T_eff. The color H − [4.5] is a good indicator of T_eff with a weak dependence on metallicity ([m/H]) and gravity (g) while H −K and [4.5] − [5.8] are sensitive to [m/H] and g. Thus T_eff and g can be constrained and mass and age can then be determined from evolutionary models. There are 12 dwarfs known with H − [4.5] > 3.0 and 500 ≲ T_eff K ≲ 800, which we examine in detail. The ages of these dwarfs range from very young (0.1–1.0 Gyr) to old (3–12 Gyr). The mass range is possibly as low as 5 M_Jup to 70 M_Jup, and [m/H] also spans a large range of ~ −0.3 to ~ +0.3. The T8–T9 dwarfs found so far in the UKIRT IR Deep Sky Survey are unexpectedly young and low-mass. Extensions to the warm Spitzer and WISE space missions are needed to obtain mid-IR data for cold brown dwarfs, and to discover more of these rare objects.