Pierre Mourier

Accepted in ApJL

ABSTRACT The admitted, conventional scenario to explain the complex spectral evolution of brown dwarfs (BDs) since their first detection 20 years ago has always been the key role played by micron-size condensates, called “dust” or “clouds,” in their atmosphere. This scenario, however, faces major problems, in particular the J-band brightening and the resurgence of FeH absorption at the L to T transition, and a physical first-principle understanding of this transition is lacking. In this Letter, we propose a new, completely different explanation for BD and extrasolar giant planet (EGP) spectral evolution, without the need to invoke clouds. We show that, due to the slowness of the CO/CH 4 and N 2 /NH 3 chemical reactions, brown dwarf (L and T, respectively) and EGP atmospheres are subject to a thermo-chemical instability similar in nature to the fingering or chemical convective instability present in Earth oceans and at the Earth core/mantle boundary. The induced small-scale turbulent energy transport reduces the temperature gradient in the atmosphere, explaining the observed increase in near-infrared J – H and J – K colors of L dwarfs and hot EGPs, while a warming up of the deep atmosphere along the L to T transition, as the CO/CH 4 instability vanishes, naturally solves the two aforementioned puzzles, and provides a physical explanation of the L to T transition. This new picture leads to a drastic revision of our understanding of BD and EGP atmospheres and their evolution.

We present an analysis of <it>Spitzer</it>/Infrared Array Camera primary transit and secondary eclipse light curves measured for HD 209458b, using Gaussian process models to marginalize over the intrapixel sensitivity variations in the 3.6 and 4.5 μm channels and the ramp effect in the 5.8 and 8.0 μm channels. The main advantage of this approach is that we can account for a broad range of degeneracies between the planet signal and systematics without actually having to specify a deterministic functional form for the latter. Our results do not confirm a previous claim of water absorption in transmission. Instead, our results are more consistent with a featureless transmission spectrum, possibly due to a cloud deck obscuring molecular absorption bands. For the emission data, our values are not consistent with the thermal inversion in the dayside atmosphere that was originally inferred from these data. Instead, we agree with another re-analysis of these same data, which concluded a non-inverted atmosphere provides a better fit. We find that a solar-abundance clear-atmosphere model without a thermal inversion underpredicts the measured emission in the 4.5 μm channel, which may suggest the atmosphere is depleted in carbon monoxide. An acceptable fit to the emission data can be achieved by assuming that the planet radiates as an isothermal blackbody with a temperature of 1484 ± 18 K.

It is expected that all astrophysical black holes in equilibrium are well described by the Kerr solution. Moreover, any black hole far away from equilibrium, such as one initially formed in a compact binary merger or by the collapse of a massive star, will eventually reach a final equilibrium Kerr state. At sufficiently late times in this process of reaching equilibrium, we expect that the black hole is modeled as a perturbation around the final state. The emitted gravitational waves will then be damped sinusoids with frequencies and damping times given by the quasinormal mode spectrum of the final Kerr black hole. An observational test of this scenario, often referred to as black hole spectroscopy, is one of the major goals of gravitational wave astronomy. It was recently suggested that the quasinormal mode description including the higher overtones might hold even right after the remnant black hole is first formed. At these times, the black hole is expected to be highly dynamical and nonlinear effects are likely to be important. In this paper we investigate this remarkable scenario in terms of the horizon dynamics. Working with high accuracy simulations of a simple configuration, namely the head-on collision of two nonspinning black holes with unequal masses, we study the dynamics of the final common horizon in terms of its shear and its multipole moments. We show that they are indeed well described by a superposition of ringdown modes as long as a sufficiently large number of higher overtones are included. This description holds even for the highly dynamical final black hole shortly after its formation. We discuss the implications and caveats of this result for black hole spectroscopy and for our understanding of the approach to equilibrium.

In general relativity, the remnant object originating from an uncharged black hole merger is a Kerr black hole. This final state is reached through the emission of a late train of radiation known as the black hole ringdown. In linear perturbation theory around the final state, the ringdown morphology is described by a countably infinite set of damped sinusoids---the quasinormal modes---whose complex frequencies are solely determined by the final black hole's mass and spin. Recent results advocate that ringdown waveforms from numerical relativity can be fully described from the peak of the strain onwards if quasinormal mode models with ${N}_{\mathrm{max}}=7$ overtones (beyond the fundamental mode) are used. In this work we extend this analysis to models with ${N}_{\mathrm{max}}\ensuremath{\ge}7$ up to ${N}_{\mathrm{max}}=16$ overtones by exploring the parameter bias on the final mass and spin obtained by fitting the nonprecessing binary black hole simulations from the SXS catalog. To this aim, we have computed the spin weight $\ensuremath{-}2$ Kerr quasinormal mode frequencies and angular separation constants for the $(l=m=2,n=8,9)$ co- and counter-rotating overtones, which all approach a Schwarzschild algebraically special mode at low spins. We provide tables of the values obtained for these modes, which are in agreement with previous results. From the systematic variable-${N}_{\mathrm{max}}$ analysis, we find that ${N}_{\mathrm{max}}\ensuremath{\sim}6$ overtones are on average sufficient to model the ringdown from the peak of the strain, although about 21% of the cases studied require at least ${N}_{\mathrm{max}}\ensuremath{\sim}12$ overtones to reach a comparable accuracy on the final state parameters. Considering the waveforms from an earlier or later point in time, we find that a very similar maximum accuracy can be reached in each case, occurring at a different number of overtones ${N}_{\mathrm{max}}$. We also provide new error estimates for the SXS waveforms based on the extrapolation and the resolution uncertainties of the gravitational wave strain, which dominate over the errors obtained from the quasilocal measures of the final mass and spin. Finally, we observe substantial instabilities on the best-fit amplitudes of the tones beyond the fundamental mode and the first overtone, that, nevertheless, do not impact significantly the mass and spin estimates.