No Arabic abstract
The ease of interstellar rocket travel is an issue with implications for the long term fate of our own and other civilizations and for the much-debated number of technological civilizations in the Galaxy. We show that the physical barrier to interstellar travel can be greatly reduced if voyagers are patient, and wait for the close passage of another star. For a representative time of $sim$1 Gyr, characteristic of the remaining time that Earth will remain habitable, one anticipates a passage of another star within $sim 1500$~AU. This lowers the travel time for interstellar migration by $sim$ two orders of magnitude compared with calculated travel times based on distances comparable to average interstellar separations (i.e., $sim$1 pc) in the solar vicinity. We consider the implications for how long-lived civilizations may respond to stellar evolution, including the case of stars in wide binaries, and the difficulties of identifying systems currently undergoing a relevant close encounter. Assuming that life originates only around G-type stars, but migrates primarily to lower mass hosts when the original system becomes uninhabitable, the fraction of extant technological civilizations that exist as diaspora can be comparable to the fraction that still orbit their original host stars.
We study an interstellar signaling scheme which was originally proposed by Seto (2019) and efficiently links intentional transmitters to ETI searchers through a conspicuous astronomical burst, without prior communication. Based on the geometrical and game theoretic viewpoints, the scheme can be refined so that intentional signals can be sent and received after observing a reference burst, in contrast to the original proposal (before observing a burst). Given this inverted temporal structure, Galactic supernovae recorded in the past 2000 years can be regarded as interesting guideposts for an ETI search. While the best use period of SN 393 has presumably passed $sim$100 years ago, some of the historical supernovae might allow us to compactify the ETI survey regions down to less than one present of $4pi$, around two rings in the sky.
Many stars do not live alone, but instead have one or more stellar companions. Observations show that these binaries, triples and higher-order multiples are common. Whereas the evolution of single stars and binaries have been studied extensively, the same is not true for the evolution of stellar triples. To fill this gap in our general understanding of stellar lives, we aim to systematically explore the long-term evolution of triples and to map out the most common evolutionary pathways that triples go through. We quantitatively study how triples evolve, which processes are most relevant, and how this differs from binary evolution. We simulate the evolution of several large populations of triples with a population synthesis approach. We make use of the triple evolution code TRES to simulate the evolution of each triple in a consistent way; including three-body dynamics (based on the secular approach), stellar evolution and their mutual influences. We simulate the evolution of the system up until mass transfer starts, the system becomes dynamically unstable, or a Hubble time has passed. We find that stellar interactions are common in triples. Compared to a binary population, we find that the fraction of systems that can undergo mass transfer is about 2 to 3 times larger in triples. Moreover, whereas in binaries the orbits typically reach circularisation before Roche-lobe overflow, this is not true anymore in triples. In our simulations, about 40% of systems retain an eccentric orbit. Additionally, we discuss various channels of triple evolution in detail such as those where the secondary or the tertiary is the first star to initiate a mass transfer event.
We use a new interdisciplinary approach to study the UV surface habitability of Proxima $b$ under quiescent and flaring stellar conditions. We assumed planetary atmospheric compositions based on CO$_2$ and N$_2$ and surface pressures from 100 to 5000 mbar. Our results show that the combination of these atmospheric compositions and pressures provide enough shielding from the most damaging UV wavelengths, expanding the UV-protective planetary atmospheric compositions beyond ozone. Additionally, we show that the UV radiation reaching the surface of Proxima $b$ during quiescent conditions would be negligible from the biological point of view, even without an atmosphere. Given that high UV fluxes could challenge the existence of life, then, we experimentally tested the effect that flares would have on microorganisms in a worst-case scenario (no UV-shielding). Our results show the impact that a typical flare and a superflare would have on life: when microorganisms receive very high fluences of UVC, such as those expected to reach the surface of Proxima $b$ after a typical flare or a superflare, a fraction of the population is able to survive. Our study suggests that life could cope with highly UV irradiated environments in exoplanets under conditions that cannot be found on Earth.
Dense stellar clusters are natural sites for the origin and evolution of exotic objects such as relativistic binaries (potential gravitational wave sources), blue stragglers, etc. We investigate the secular dynamics of a binary system driven by the global tidal field of an axisymmetric stellar cluster in which the binary orbits. In a companion paper (Hamilton & Rafikov 2019a) we developed a general Hamiltonian framework describing such systems. The effective (doubly-averaged) Hamiltonian derived there encapsulates all information about the tidal potential experienced by the binary in its orbit around the cluster in a single parameter $Gamma$. Here we provide a thorough exploration of the phase-space of the corresponding secular problem as $Gamma$ is varied. We find that for $Gamma > 1/5$ the phase-space structure and the evolution of binary orbital element are qualitatively similar to the Lidov-Kozai problem. However, this is only one of four possible regimes, because the dynamics are qualitatively changed by bifurcations at $Gamma = 1/5,0,-1/5$. We show how the dynamics are altered in each regime and calculate characteristics such as secular evolution timescale, maximum possible eccentricity, etc. We verify the predictions of our doubly-averaged formalism numerically and find it to be very accurate when its underlying assumptions are fulfilled, typically meaning that the secular timescale should exceed the period of the binary around the cluster by $gtrsim 10-10^2$ (depending on the cluster potential and binary orbit). Our results may be relevant for understanding the nature of a variety of exotic systems harboured by stellar clusters.
Polluted white dwarfs are generally accreting terrestrial-like material that may originate from a debris belt like the asteroid belt in the solar system. The fraction of white dwarfs that are polluted drops off significantly for white dwarfs with masses $M_{rm WD}gtrsim 0.8,rm M_odot$. This implies that asteroid belts and planetary systems around main-sequence stars with mass $M_{rm MS}gtrsim 3,rm M_odot$ may not form because of the intense radiation from the star. This is in agreement with current debris disc and exoplanet observations. The fraction of white dwarfs that show pollution also drops off significantly for low mass white dwarfs $(M_{rm WD}lesssim 0.55,rm M_odot)$. However, the low-mass white dwarfs that do show pollution are not currently accreting but have accreted in the past. We suggest that asteroid belts around main sequence stars with masses $M_{rm MS}lesssim 2,rm M_odot$ are not likely to survive the stellar evolution process. The destruction likely occurs during the AGB phase and could be the result of interactions of the asteroids with the stellar wind, the high radiation or, for the lowest mass stars that have an unusually close-in asteroid belt, scattering during the tidal orbital decay of the inner planetary system.