Gas-giant planets that form via core accretion might have very different characteristics from those that form via disk-instability. Disk-instability objects are typically thought to have higher entropies, larger radii, and (generally) higher effective temperatures than core-accretion objects. In this paper, we provide a large set of models exploring the observational consequences of highentropy (hot) and low-entropy (cold) initial conditions, in the hope that this will ultimately help to distinguish between different physical mechanisms of planet formation. However, the exact entropies and radii of newly-formed planets due to these two modes of formation cannot, at present, be precisely predicted. It is possible that the distribution of properties of coreaccretion-formed planets and the distribution of properties of disk-instability-formed planets overlap. We, therefore, introduce a broad range of "Warm Start" gas-giant planet models. Between the hottest and the coldest models that we consider, differences in radii, temperatures, luminosities, and spectra persist for only a few million to a few tens of millions of years for planets that are a few times Jupiter's mass or less. For planets that are ∼five times Jupiter's mass or more, significant differences between hottest-start and coldest-start models persist for on the order of 100 Myrs. We find that out of the standard infrared bands (J, H, K, L ′ , M , N ) the K and H bands are the most diagnostic of the initial conditions. A hottest-start model can be from ∼4.5 magnitudes brighter (at Jupiter's mass) to ∼9 magnitudes brighter (at ten times Jupiter's mass) than a coldest-start model in the first few million years. In more massive objects, these large differences in luminosity and spectrum persist for much longer than in less massive objects. Finally, we consider the influence of atmospheric conditions on spectra, and find that the presence or absence of clouds, and the metallicity of an atmosphere, can affect an object's apparent brightness in different bands by up to several magnitudes.

We present IAS15, a 15th-order integrator to simulate gravitational dynamics. The integrator is based on a Gauß-Radau quadrature and can handle conservative as well as non-conservative forces. We develop a step-size control that can automatically choose an optimal timestep. The algorithm can handle close encounters and high-eccentricity orbits. The systematic errors are kept well below machine precision and long-term orbit integrations over 10 9 orbits show that IAS15 is optimal in the sense that it follows Brouwer's law, i.e. the energy error behaves like a random walk. Our tests show that IAS15 is superior to a mixed-variable symplectic integrator (MVS) and other popular integrators, including high-order ones, in both speed and accuracy. In fact, IAS15 preserves the symplecticity of Hamiltonian systems better than the commonly-used nominally symplectic integrators to which we compared it.We provide an open-source implementation of IAS15. The package comes with several easy-to-extend examples involving resonant planetary systems, Kozai-Lidov cycles, close encounters, radiation pressure, quadrupole moment, and generic damping functions that can, among other things, be used to simulate planet-disc interactions. Other non-conservative forces can be added easily.

With their long mean free paths and efficient heating of the intergalactic medium (IGM), X-rays could have a dramatic impact on the thermal and ionization history of the Universe. Here we run several semi-numeric simulations of the Dark Ages and the Epoch of Reionization (EoR), including both X-rays and UV radiation fields, attempting to provide an intuitive framework for interpreting upcoming observations. We explore the impact of X-rays on various signals: (i) Reionization history: including X-rays results in an earlier, slightly more extended EoR. However, efficient thermal feedback from X-ray heating could yield an extended epoch in which the Universe was ≈ 10% ionized. (ii) Reionization morphology: a sizable (∼10%) contribution of X-rays to reionization results in a more uniform morphology, though the impact is modest when compared at the same global neutral fraction,x HI . Specifically, X-rays produce a dearth of fully neutral regions and a suppression of smallscale (k ∼ > 0.1Mpc −1 ) ionization power by a factor of ∼ < 2. However, these changes in morphology cannot be countered by increasing the bias of the ionizing sources, making them a robust indicator of an X-ray contribution to the EoR. (iii) The kinetic Sunyaev-Zel'dovich (kSZ) effect: at a fixed reionization history, X-rays decrease the kSZ power at l = 3000 by ≈ 0.5µK 2 . Our extreme model in which X-rays entirely drive reionization is the only one which is marginally consistent with the recent upper limits on this signal from the South Pole Telescope, assuming no thermal Sunyaev-Zel'dovich (tSZ) -dusty galaxy cross-correlation. Since this extreme model is unlikely, we conclude that there should be a sizable tSZ-dusty galaxy cross-correlation. (iv) The redshifted 21cm signal: the impact of X-rays on the 21cm power spectrum during the advanced stages of reionization (x HI ∼ < 0.7) is modest, except in extreme, X-ray dominated models. The largest impact of X-rays is to govern the timing and duration of IGM heating. In fact, unless thermal feedback is efficient, the epoch of X-ray heating likely overlaps with the beginning of reionization. This results in a 21cm power spectrum which is ∼10-100 times higher atx HI ∼ > 0.9 than obtained from naive estimates ignoring this overlap. On the other hand, if thermal feedback is efficient, the resulting extended epoch between X-ray heating and reionization could provide a clean probe of the matter power spectrum in emission, at redshifts more accessible than the Dark Ages.