Planetary surfaces

Abstract:

We report the discovery of 16 detached M dwarf eclipsing binaries with J < 16 mag and provide a detailed characterization of three of them, using high-precision infrared light curves from the WFCAM Transit Survey (WTS). Such systems provide the most accurate and model-independent method for measuring the fundamental parameters of these poorly understood yet numerous stars, which currently lack sufficient observations to precisely calibrate stellar evolution models. We fully solve for the masses and radii of three of the systems, finding orbital periods in the range 1.5 < P < 4.9 d, with masses spanning 0.35–0.50 M⊙ and radii between 0.38 and 0.50 R⊙, with uncertainties of ∼3.5–6.4 per cent in mass and ∼2.7–5.5 per cent in radius. Close companions in short-period binaries are expected to be tidally locked into fast rotational velocities, resulting in high levels of magnetic activity. This is predicted to inflate their radii by inhibiting convective flow and increasing starspot coverage. The radii of the WTS systems are inflated above model predictions by ∼3–12 per cent, in agreement with the observed trend, despite an expected lower systematic contribution from starspot signals at infrared wavelengths. We searched for correlation between the orbital period and radius inflation by combining our results with all existing M dwarf radius measurements of comparable precision, but we found no statistically significant evidence for a decrease in radius inflation for longer period, less active systems. Radius inflation continues to exists in non-synchronized systems, indicating that the problem remains even for very low activity M dwarfs. Resolving this issue is vital not only for understanding the most populous stars in the Universe, but also for characterizing their planetary companions, which hold the best prospects for finding Earth-like planets in the traditional habitable zone.

Abstract:

New line data describing the absorption of CH 4 and CH 3D from 1.26 to 1.71μm (Campargue, A., Wang, L., Mondelain, D., Kassi, S., Bézard, B., Lellouch, E., Coustenis, A., de Bergh, C., Hirtzig, M., Drossart, P. [2012]. Icarus 219, 110-128), building upon previous papers by Campargue et al. (Campargue, A., Wang, L., Kassi, S., Masat, M., Votava, O. [2010]. J. Quant. Spectrosc. Radiat. Transfer 111, 1141-1151; Wang, L., Kassi, S., Campargue, A. [2010]. J. Quant. Spectrosc. Radiat. Transfer 111, 1130-1140; Wang, L., Kassi, S., Liu, A.W., Hu, S.M., Campargue, A. [2011]. J. Quant. Spectrosc. Radiat. Transfer 112, 937-951)) have been applied to the analysis of Gemini-N/NIFS observations of Uranus made in 2010 and compared with earlier disc-averaged observations made by KPNO/FTS in 1982. The new line data are found to improve greatly the fit to the observed spectra and present a huge advance over previous methane absorption tables by allowing us to determine the CH 3D/CH 4 ratio and also start to break the degeneracy between methane abundance and cloud top height. The best fits are obtained if the cloud particles in the main cloud deck at the 2-3bar level become less scattering with wavelength across the 1.4-1.6μm region and we have modelled this variation here by varying the extinction cross-section and single-scattering albedo of the particles.Applying the new line data to the NIFS spectra of Uranus, we determine a new estimate of the CH 3D/CH 4 ratio of 2.9-0.5+0.9×10-4, which is consistent with the estimate of de Bergh et al. (de Bergh, C., Lutz, B.L., Owen, T., Brault, J., Chauville, J. [1986]. Astrophys. J. 311, 501-510) of 3.6-2.8+3.6×10-4, made by fitting a disc-averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement.Using k-tables fitted to the new line data, the central meridian observations of Uranus' H-band spectrum (1.49-1.64μm) made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new methane absorption coefficients and the modified scattering properties of the cloud particles in the main cloud deck appears to break the degeneracy between cloud height and methane abundance immediately above it in this spectral region and we find that both vary with latitude across Uranus' disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45°N and 45°S. At the same time, using the 'D' temperature pressure profile of Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. [1987]. J. Geophys. Res. 92, 14987-15001) and a deep methane abundance of 1.6% (Baines, K.H., Mickelson, M.E., Larson, L.E., Ferguson, D.W. [1995]. Icarus 144, 328-340) we find that the relative humidity of methane is high near the equator (~60%) and decreases sharply towards the poles, except near the circumpolar zone at 45°N, which has brightened steadily since 2007, and where there is a local maximum in methane relative humidity. In tests conducted with the warmer 'F1' profile of Sromovsky et al. (2011) we find a similar variation of methane abundance above the main cloud, although for this warmer temperature profile this abundance is dependent mostly on the fitted deep methane mole fraction. © 2012 Elsevier Inc.

Abstract:

A far-infrared emission band attributed to volatile or refractory haze in Titan's stratosphere has been decreasing in intensity since Cassini's arrival in 2004. The 220cm-1 feature, first seen by the Voyager Infrared Interferometer Spectrometer, has only been found in Titan's winter polar region. The emission peaks at about 140km altitude near the winter stratospheric temperature minimum. Observations recorded over the period 2004-2012 by the Composite Infrared Spectrometer on Cassini show a decrease in the intensity of this feature by about a factor of four. Possible seasonal causes of this decline are an increase in photolytic destruction of source chemicals at high altitude, a lessening of condensation as solar heating increased, or a weakening of downwelling of vapors. As of early 2012, the 220cm-1 haze has not yet been detected in the south. The haze composition is unknown, but its decrease is similar to that of HC3N gas in Titan's polar stratosphere, pointing to a nitrile origin. © 2012. The American Astronomical Society. All rights reserved.