Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs

📝 Abstract

M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric CO is present at abundance levels far in excess of those predicted from chemical equilibrium. These new and unambiguous detections of CO, together with an earlier spectroscopic detection of CO in Gliese 229B and existing M band photometry of a large selection of T dwarfs, suggest that vertical mixing in the photosphere drives the CO abundance out of chemical equilibrium and is a common, and likely universal feature of mid-to-late type T dwarfs. The M band spectra allow determinations of the time scale of vertical mixing in the atmosphere of each object, the first such measurements of this important parameter in late T dwarfs. A detailed analysis of the spectral energy distribution of 2MASS J09373487+2931409 results in the following values for metallicity, temperature, surface gravity, and luminosity: [M/H]~-0.3, T_eff=925-975K, log g=5.20-5.47, log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range 45-69 M_Jup.

💡 Analysis

M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric CO is present at abundance levels far in excess of those predicted from chemical equilibrium. These new and unambiguous detections of CO, together with an earlier spectroscopic detection of CO in Gliese 229B and existing M band photometry of a large selection of T dwarfs, suggest that vertical mixing in the photosphere drives the CO abundance out of chemical equilibrium and is a common, and likely universal feature of mid-to-late type T dwarfs. The M band spectra allow determinations of the time scale of vertical mixing in the atmosphere of each object, the first such measurements of this important parameter in late T dwarfs. A detailed analysis of the spectral energy distribution of 2MASS J09373487+2931409 results in the following values for metallicity, temperature, surface gravity, and luminosity: [M/H]~-0.3, T_eff=925-975K, log g=5.20-5.47, log L/L_sun=-5.308 +/- 0.027. The age is 3-10 Gyr and the mass is in the range 45-69 M_Jup.

📄 Content

arXiv:0901.2134v1 [astro-ph.SR] 14 Jan 2009 submitted to ApJ on 24 June 2008 Spectroscopic Detection of Carbon Monoxide in Two Late-type T Dwarfs T. R. Geballe1, D. Saumon2, D. A. Golimowski3, S. K. Leggett1, M.S. Marley4, K. S. Noll5 ABSTRACT M band spectra of two late-type T dwarfs, 2MASS J09373487+2931409, and Gliese 570D, confirm evidence from photometry that photospheric CO is present at abundance levels far in excess of those predicted from chemical equilibrium. These new and unambiguous detections of CO, together with an earlier spectro- scopic detection of CO in Gliese 229B and existing M band photometry of a large selection of T dwarfs, suggest that vertical mixing in the photosphere drives the CO abundance out of chemical equilibrium and is a common, and likely universal feature of mid-to-late type T dwarfs. The M band spectra allow determinations of the time scale of vertical mixing in the atmosphere of each object, the first such measurements of this important parameter in late T dwarfs. A detailed analy- sis of the spectral energy distribution of 2MASS J09373487+2931409 results in the following values for metallicity, temperature, surface gravity, and luminosity: [M/H]∼−0.3, Teff= 925−975 K, log g = 5.20−5.47, log L/L⊙= −5.308±0.027. The age is 3–10 Gyr and the mass is in the range 45–69 MJupiter. Subject headings: stars: low-mass, brown dwarfs — infrared: general — stars: individual (Gliese 570D, 2MASS J09373487+2931409) 1Gemini Observatory, 670 N. A’ohoku Place, Hilo, HI 96720; tgeballe@gemini.edu 2Los Alamos National Laboratory, PO Box 1663, MS F663, Los Alamos, NM 87545 3Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218 4NASA Ames Research Center, MS245-3, Moffett Field, CA 94035 5Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 – 2 – 1. Introduction In the 1500–4000 K photospheres of late-type stars and early-mid L-type brown dwarfs carbon is predominantly locked up in carbon monoxide (CO), due to the much higher binding energy of CO than that of any other carbon-bearing molecule. At temperatures of ∼1500 K, however, methane (CH4) begins to be stable against collisional dissociation. As a brown dwarf cools below this temperature, the overwhelming abundance of hydrogen slowly drives the carbon that was in CO into CH4 (Fegley & Lodders 1996). This chemical change leads to a striking transformation of the infrared spectrum of the brown dwarf and marks the division between the L and T classifications (Kirkpatrick et al. 1999; Burgasser et al. 2002; Geballe et al. 2002; Burgasser et al. 2006). Absorption bands of CH4 together with those of H2O dominate the 1-2.5 µm spectra of T dwarfs, making the λ = 2.3–2.5 µm first overtone band of CO, which is easily observed in K to L dwarfs, virtually undetectable by spectral type T4 (Geballe et al. 2002), even if a significant amount of CO is still present. In contrast, the fundamental vibration-rotation band of CO is centered near 4.7µm, where methane is not an important absorber. Over a decade ago Noll, Geballe & Marley (1997) discovered evidence of CO at 4.7 µm at an abundance roughly three orders of magnitude larger than expected from chemical equilibrium, in the bright T7p brown dwarf Gliese 229B. The discovery was promptly confirmed by Oppenheimer et al. (1998). Noll, Geballe & Marley (1997) found that the CO abundance in Gl 229B matched that expected at a temperature of 1250 K, roughly 400 K higher than the photospheric temperature. They interpreted the detection as evidence for vertical transport of CO from hotter internal atmospheric layers to the surface, which was later supported by more detailed analysis (Saumon et al. 2000). This finding was not unexpected, as mixing is predicted to occur in the outer, ra- diative zones of brown dwarf atmospheres, and the reaction(s) moving carbon from CO to CH4 are slow (Fegley & Lodders 1994, 1996; Lodders & Fegley 2002). Recently, the M band fluxes of numerous late T dwarfs have been measured to be up to one magnitude fainter than predictions based on their brightnesses in other wavebands and the assumption of equilibrium chemistry (Saumon et al. 2003; Golimowski et al. 2004; Patten et al. 2006). This strongly indicates that the CO phenomenon seen in Gl 229B is common to most or all late T dwarfs. Here we describe spectroscopic observations of two late-type T dwarfs, 2MASS 09373487+2931409 (hereafter, 2MASS 0937) and Gliese 570D, whose physical prop- erties differ significantly from Gl 229B, in order to test this hypothesis spectroscopically. 2MASS 0937 and Gl 229B have nearly the same spectral classifications, T6p and T7.5, respectively (Burgasser et al. 2006). However, the K band flux of 2MASS 0937 is signifi- cantly depressed relative to Gl 229b and other T6-T7 dwarfs; e.g., its J −K color is 0.8 mag – 3 – bluer (Burgasser et al. 2002; Knapp et al. 2004). This is believed to be due to the effect of greatly enhanced collision-indu

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