Detection of the YORP Effect on the contact-binary (68346) 2001 KZ66 from combined radar and optical observations

The YORP effect is a small thermal-radiation torque experienced by small asteroids, and is considered to be crucial in their physical and dynamical evolution. It is important to understand this effect by providing measurements of YORP for a range of asteroid types to facilitate the development of a theoretical framework. We are conducting a long-term observational study on a selection of near-Earth asteroids to support this. We focus here on (68346) 2001 KZ66, for which we obtained both optical and radar observations spanning a decade. This allowed us to perform a comprehensive analysis of the asteroid’s rotational evolution. Furthermore, radar observations from the Arecibo Observatory enabled us to generate a detailed shape model. We determined that (68346) is a retrograde rotator with its pole near the southern ecliptic pole, within a 15○ radius of longitude 170○ and latitude −85○. By combining our radar-derived shape model with the optical light curves we developed a refined solution to fit all available data, which required a YORP strength of $(8.43\pm 0.69)\times 10^{-8} \rm ~rad ~day^{-2}$. (68346) has a distinct bifurcated shape comprising a large ellipsoidal component joined by a sharp neckline to a smaller non-ellipsoidal component. This object likely formed from either the gentle merging of a binary system, or from the deformation of a rubble pile due to YORP spin-up. The shape exists in a stable configuration close to its minimum in topographic variation, where regolith is unlikely to migrate from areas of higher potential.

The Sun Through Time

Magnetic activity of stars like the Sun evolves in time because of spin-down owing to angular momentum removal by a magnetized stellar wind. These magnetic fields are generated by an internal dynamo driven by convection and differential rotation. Spin-down therefore converges at an age of about 700 Myr for solar-mass stars to values uniquely determined by the stellar mass and age. Before that time, however, rotation periods and their evolution depend on the initial rotation period of a star after it has lost its protostellar/protoplanetary disk. This non-unique rotational evolution implies similar non-unique evolutions for stellar winds and for the stellar high-energy output. I present a summary of evolutionary trends for stellar rotation, stellar wind mass loss and stellar high-energy output based on observations and models.

Magnetic torques on T Tauri stars: Accreting versus non-accreting systems

Context. Classical T Tauri stars (CTTs) magnetically interact with their surrounding disks, a process that is thought to regulate their rotational evolution. Aims. We compute torques acting on the stellar surface of CTTs that arise from different accreting (accretion funnels) and ejecting (stellar winds and magnetospheric ejections) flow components. Furthermore, we compare the magnetic braking due to stellar winds in two different systems: isolated (i.e., weak-line T Tauri and main-sequence) and accreting (i.e., classical T Tauri) stars. Methods. We use 2.5D magnetohydrodynamic, time-dependent, axisymmetric simulations that were computed with the PLUTO code. For both systems, the stellar wind is thermally driven. In the star-disk-interaction (SDI) simulations, the accretion disk is Keplerian, viscous, and resistive, and is modeled with an alpha prescription. Two series of simulations are presented, one for each system (i.e., isolated and accreting stars). Results. In classical T Tauri systems, the presence of magnetospheric ejections confines the stellar-wind expansion, resulting in an hourglass-shaped geometry of the outflow, and the formation of the accretion columns modifies the amount of open magnetic flux exploited by the stellar wind. These effects have a strong impact on the stellar-wind properties, and we show that the stellar-wind braking is more efficient in the SDI systems than in the isolated ones. We further derive torque scalings over a wide range of magnetic field strengths for each flow component in an SDI system (i.e., magnetospheric accretion and ejections, and stellar winds), which directly applies a torque on the stellar surface. Conclusions. In all the performed SDI simulations, the stellar wind extracts less than 2% of the mass accretion rate and the disk is truncated by up to 66% of the corotation radius. All simulations show a net spin-up torque. We conclude that in order to achieve a stellar-spin equilibrium, we need either more massive stellar winds or disks that are truncated closer to the corotation radius, which increases the torque efficiency of the magnetospheric ejections.

Evidence for metallicity-dependent spin evolution in the Kepler field

ABSTRACT A curious rotation period distribution in the colour–magnitude–period diagram (CMPD) of the Kepler field was recently revealed, thanks to data from Gaia and Kepler spacecraft. It was found that redder and brighter stars are spinning slower than the rest of the main sequence. On the theoretical side, it was demonstrated that metallicity should affect the rotational evolution of stars as well as their evolution in the Hertzprung–Rüssel or colour–magnitude diagram. In this work, we combine this data set with medium- and high-resolution spectroscopic metallicities and carefully select main-sequence single stars in a given mass range. We show that the structure seen in the CMPD also corresponds to a broad correlation between metallicity and rotation, such that stars with higher metallicity rotate, on average, more slowly than those with low metallicity. We compare this sample to theoretical rotational evolution models that include a range of different metallicities. They predict a correlation between rotation rate and metallicity that is in the same direction and of about the same magnitude as that observed. Therefore, metallicity appears to be a key parameter to explain the observed rotation period distributions. We also discuss a few different ways in which metallicity can affect the observed distribution of rotation period, due to observational biases and age distributions, as well as the effect on stellar wind torques.

Escape and evolution of Titan’s N2 atmosphere constrained by 14N/15N isotope ratios

ABSTRACT We apply a 1D upper atmosphere model to study thermal escape of nitrogen over Titan’s history. Significant thermal escape should have occurred very early for solar extreme ultraviolet (EUV) fluxes 100–400 times higher than today with escape rates as high as ≈1.5 × 1028 s−1 and ≈4.5 × 1029 s−1, respectively, while today it is ≈7.5 × 1017 s−1. Depending on whether the Sun originated as a slow, moderate, or fast rotator, thermal escape was the dominant escape process for the first 100–1000 Myr after the formation of the Solar system. If Titan’s atmosphere originated that early, it could have lost between $\approx0.5\,\, \mathrm{ and}\,\, 16$ times its present atmospheric mass depending on the Sun’s rotational evolution. We also investigated the mass-balance parameter space for an outgassing of Titan’s nitrogen through decomposition of NH3-ices in its deep interior. Our study indicates that, if Titan’s atmosphere originated at the beginning, it could have only survived until today if the Sun was a slow rotator. In other cases, the escape would have been too strong for the degassed nitrogen to survive until present day, implying later outgassing or an additional nitrogen source. An endogenic origin of Titan’s nitrogen partially through NH3-ices is consistent with its initial fractionation of 14N/15N ≈ 166–172, or lower if photochemical removal was relevant for longer than the last ≈ 1000 Myr. Since this ratio is slightly above the ratio of cometary ammonia, some of Titan’s nitrogen might have originated from refractory organics.

Activity evolution for F, G, K, and M dwarfs and the importance for planetary atmospheres

<p>The magnetic activity of stars is a crucially important factor influencing planet formation processes and the subsequent evolution of planetary atmospheres. Understanding how stellar activity evolves for stars with different masses is crucially important for understanding the effects of stellar winds and radiation at X-ray and ultraviolet wavelengths on the erosion of circumstellar disks and planetary atmospheres. I will present a new and comprehensive description of the rotational evolution of stars and the resulting evolution of X-ray and ultraviolet emission for F, G, K, and M dwarfs. I will demonstrate the importance of the star's initial rotation rate on the subsequent activity evolution and clarify common misunderstandings regarding the dependence on stellar mass, including the common belief that M dwarfs are more XUV active than G dwarfs. I will show why these results are important for the evolution of planetary atmospheres.</p>

Turbulent, pinned superfluids in neutron stars and pulsar glitch recoveries

ABSTRACT Pulsar glitches offer an insight into the dynamics of superfluids in the high-density interior of a neutron star. To model these phenomena, however, one needs to have an understanding of the dynamics of a turbulent array of superfluid vortices moving through a pinning lattice. In this paper, we develop a theoretical approach to describe vortex-mediated mutual friction in a pinned, turbulent and rotating superfluid. Our model is then applied to the study of the post-glitch rotational evolution in the Vela pulsar and in PSR J0537-6910. We show that in both cases a turbulent model fits the evolution of the spin frequency derivative better than a laminar one. We also predict that the second derivative of the frequency after a glitch should be correlated with the waiting time since the previous glitch, which we find to be consistent with observational data for these pulsars. The main conclusion of this paper is that in the post-glitch rotational evolution of these two pulsars we are most likely observing the response to the glitch of a pinned turbulent region of the star (possibly the crust) and not the laminar response of a regular straight vortex array.