Keywords

We study the tidal disruption of binaries by a massive point mass (e.g., the black hole at the Galactic center), and we discuss how the ejection and capture preference between unequal-mass binary members depends on which orbit they approach the massive object. We show that the restricted three-body approximation provides a simple and clear description of the dynamics. The orbit of a binary with mass m around a massive object M should be almost parabolic with an eccentricity of |1 − e| ≲ (m/M)^1/3 ≪ 1 for a member to be captured, while the other is ejected. Indeed, the energy change of the members obtained for a parabolic orbit can be used to describe non-parabolic cases. If a binary has an encounter velocity much larger than (M/m)^1/3 times the binary rotation velocity, it would be abruptly disrupted, and the energy change at the encounter can be evaluated in a simple disruption model. We evaluate the probability distributions for the ejection and capture of circular binary members and for the final energies. In principle, for any hyperbolic (elliptic) orbit, the heavier member has more chance to be ejected (captured), because it carries a larger fraction of the orbital energy. However, if the orbital energy is close to zero, the difference between the two members becomes small, and there is practically no ejection and capture preferences. The preference becomes significant when the orbital energy is comparable to the typical energy change at the encounter. We discuss its implications to hypervelocity stars and irregular satellites around giant planets.

Triton is one of the few bodies in the solar system with observed cryo-volcanic activity, in the form of plumes at its south pole, which suggests large-scale surface volatile transport over time. Triton's large variations in obliquity have motivated prior predictions of changing atmospheric column densities of several orders of magnitude, driven by seasonal evaporation of surface volatiles. Using the Hubble Space Telescope, we directly imaged Triton's surface and have detected large-scale differences in increased and decreased reflectance when compared with Voyager data at UV, visual, and methane-band wavelengths. Our surface map shows regions of increased brightness at near-equatorial latitudes and near the Neptune-facing side, and darkened regions near longitudes of ±180°, indicating the presence of ongoing seasonal volatile transport.

Spectra of Triton between 1.8 and 5.5 μm, obtained in 2007 May and 2009 November, have been analyzed to determine the global surface composition. The spectra were acquired with the grism and the prism of the Infrared Camera on board AKARI with spectral resolutions of 135 and 22, respectively. The data from 4 to 5 μm are shown in this Letter and compared to the spectra of N₂, CO, and CO₂, i.e., all the known ices on this moon that have distinct bands in this previously unexplored wavelength range. We report the detection of a 4σ absorption band at 4.76 μm (2101 cm⁻¹), which we attribute tentatively to the presence of solid HCN. This is the sixth ice to be identified on Triton and an expected component of its surface because it is a precipitating photochemical product of Triton's thin N₂ and CH₄ atmosphere. It is also formed directly by irradiation of mixtures of N₂ and CH₄ ices. Here we consider only pure HCN, although it might be dissolved in N₂ on the surface of Triton because of the evaporation and recondensation of N₂ over its seasonal cycle. The AKARI spectrum of Triton also covers the wavelengths of the fundamental (1–0) band of β-phase N₂ ice (4.296 μm, 2328 cm⁻¹), which has never been detected in an astronomical body before, and whose presence is consistent with the overtone (2–0) band previously reported. Fundamental bands of CO and CO₂ ices are also present.

The study of the origin of irregular satellites remains important in planetary science because it provides constraints on the formation process of giant planets and probes the properties of a now-extinct planetesimal disk that existed at 5-30 AU early in solar system history. While several putative scenarios of irregular-satellite capture around giant planets have been developed, various uncertainties and the lack of an accurate model of the evolutionary history of the solar system usually prevent an assessment of their overall likelihood. Here we study a three-body interaction scenario in which irregular satellites are formed by dissociation of a planetesimal binary in the gravity field of a planet. Within the frame of the Nice model, we determine how many irregular satellites are expected to be formed about each of the giant planets. We pay special attention to a possible capture of Triton via this mechanism. We find that Triton could have been captured via a binary dissociation very soon after Neptune's formation when the planetesimal disk was still dynamically cold. Triton was most likely captured by a dissociation of a binary system where the more massive component was ∼2–5 times heavier than Triton. Our results suggest that Neptune, the formation of Triton's binary, and the capture of Triton around Neptune all occurred within the first ∼5–10 Myr of solar system formation when the gas disk was still present. This would rule out the late formation of ice giants. Our results also indicate that binary dissociation is a highly unlikely process for the origin of small irregular satellites for two reasons. First, the orbital distribution of the captured bodies is inconsistent with that of the observed irregular satellites. Second, the efficiency of the captures is too low to explain the numerous populations of small irregular satellites.