MID-INFRARED PHOTOMETRY OF COLD BROWN DWARFS: DIVERSITY IN AGE, MASS, AND METALLICITY
- Gemini Observatory, Northern Operations Center, 670 North A'ohoku Place, Hilo, HI 96720 (United States)
- Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB (United Kingdom)
- Los Alamos National Laboratory, P.O. Box 1663, MS F663, Los Alamos, NM 87545 (United States)
- NASA Ames Research Center, Mail Stop 245-3, Moffett Field, CA 94035 (United States)
- Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ (United Kingdom)
- INAF/Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese (Italy)
- National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 (Japan)
We present new Spitzer Infrared Array Camera (IRAC) photometry of 12 very late-type T dwarfs: nine have [3.6], [4.5], [5.8], and [8.0] photometry and three have [3.6] and [4.5] photometry only. Combining this with previously published photometry, we investigate trends with type and color that are useful for both the planning and interpretation of infrared surveys designed to discover the coldest T or Y dwarfs. The online appendix provides a collation of MKO-system YJHKL'M' and IRAC photometry for a sample of M, L, and T dwarfs. Brown dwarfs with effective temperature (T{sub eff}) below 700 K emit more than half their flux at wavelengths longer than 3 {mu}m, and the ratio of the mid-infrared flux to the near-infrared flux becomes very sensitive to T{sub eff} at these low temperatures. We confirm that the color H (1.6 {mu}m) - [4.5] is a good indicator of T{sub eff} with a relatively weak dependence on metallicity and gravity. Conversely, the colors H - K (2.2 {mu}m) and [4.5] - [5.8] are sensitive to metallicity and gravity. Thus, near- and mid-infrared photometry provide useful indicators of the fundamental properties of brown dwarfs, and if temperature and gravity are known, then mass and age can be reliably determined from evolutionary models. There are 12 dwarfs currently known with H- [4.5] >3.0, and 500 K {approx}< T{sub eff} {approx}<800 K, which we examine in detail. The ages of the dwarfs in the sample range from very young (0.1-1.0 Gyr) to relatively old (3-12 Gyr). The mass range is possibly as low as 5 Jupiter masses to up to 70 Jupiter masses, i.e., near the hydrogen burning limit. The metallicities also span a large range, from [m/H] = -0.3 to [m/H] = +0.3. The small number of T8-T9 dwarfs found in the UK Infrared Telescope Infrared Deep Sky Survey to date appear to be predominantly young low-mass dwarfs. Accurate mid-infrared photometry of cold brown dwarfs is essentially impossible from the ground, and extensions to the mid-infrared space missions, warm-Spitzer and Wide-Field Infrared Survey Explorer, are desirable in order to obtain the vital mid-infrared data for cold brown dwarfs, and to discover more of these rare objects.
- OSTI ID:
- 21394408
- Journal Information:
- Astrophysical Journal, Vol. 710, Issue 2; Other Information: DOI: 10.1088/0004-637X/710/2/1627; ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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