Nevin N. Weinberg

Recent NICER observations of photospheric radius expansion X-ray bursts reveal absorption features consistent with photospheres enriched in intermediate-mass elements. These features may arise from radiation-driven winds that eject freshly synthesized nuclear ashes, offering a new probe of X-ray bursts and neutron star properties. Motivated by these observations, we use the MESA stellar evolution code to simulate PRE bursts from accretion through the hydrodynamic wind phase. We model a range of ignition depths for both pure helium and mixed hydrogen/helium accretion and explore several prescriptions for convection during burst rise. We find that the wind abundances depend sensitively on both ignition depth and convective treatment, including the efficiency of semiconvective mixing and the prescription used to define convective boundaries. Bursts igniting at column depths ≳5 × 10 ⁸ g cm ⁻² produce ash-enriched winds, with ejecta ranging from intermediate-mass to iron-peak elements depending on ignition depth, accretion composition, and the treatment of convection.

The Roche limit, or the threshold separation within which a celestial object (the donor) M cannot remain in a stable configuration due to a companion’s tidal field, has been well established when M is in hydrostatic equilibrium and has synchronous rotation in a circular orbit. However, limited analyses exist considering corrections to the Roche limit due to hydrodynamical effects. We fill in the gap by providing a general theoretical framework involving nonlinear hydrodynamics. We consider both exact nonlinear equations derived from an affine model describing incompressible ellipsoids and series-expanded ones that can be calculated for realistic stars and planets. Our formulation addresses the Roche problem in generic orbits and synchronization levels of M , and fully accounts for the history-dependent hydrodynamical effects. We show that, as the orbital eccentricity increases, fluid instability is more likely to develop at the pericenter due to the increased dynamical tide that accumulates over multiple orbits. When M moves in a highly eccentric orbit (with eccentricity around 0.9) and the damping of the fluid is small, the threshold pericenter separation at which mass loss from M can occur can be at least 30% higher than the value predicted for a circular orbit with hydrostatic equilibrium. If only a single passage is considered, however, the threshold separation is 20% smaller than the static limit. The nonlinear interaction at each pericenter passage can also trigger a diffusive fluid evolution inside M even with moderate eccentricities, complementing previous studies of diffusive tides caused by random propagation phases. Our work has broad implications for interacting binaries in eccentric orbits, including migrating gaseous exoplanets, repeated partial tidal disruption events, and more.

Earth-like planets in the habitable zone (HZ) of M dwarfs have recently been targeted in the search for exomoons. We study the stability and lifetime of large (Luna-like) moons, accounting for the effects of three-body interactions and tidal forces using the N -body simulator rebound and its extension library reboundx . We find that those moons have a notably different likelihood of existence (and, by implication, observability). Large moons orbiting Earth-like planets in the HZs of M4 and M2 dwarfs become unstable well before 10 ⁷ and 10 ⁸ yr, respectively, and in most cases, those orbiting M0 dwarfs become unstable in much less than 10 ⁹ yr. We conclude that HZ planets orbiting M dwarfs are unlikely to harbor large moons, thus affecting the total number of possible moons in our galaxy and the Universe at large. Since moons may help enhance the habitability of their host planet, besides being possibly habitable themselves, these results may have notable implications for exolife, and should also be considered when seeking solutions to the Drake equation and the Fermi paradox.

K2-18b closely orbits a nearby M3 dwarf within its habitable zone, where this planet could be either a super-Earth or a mini-Neptune. Recent studies using transit spectroscopy suggest that it is Hycean in nature, but this classification is currently controversial. We use the N-body integrator rebound and its extension library reboundx to investigate the possibility of exomoons around K2-18b. Due to tidal interactions that induce outward migration, we find that any moons would be extremely unlikely. If formed, their lifetimes would be relatively short, not exceeding 10 Myr assuming Earth-like or Neptune-like tidal parameters for K2-18b. Recent studies estimate the stellar (and system) lifetime as 3 Gyr, which is significantly longer than the tidal migration timescale. We show that exomoons are unlikely to survive around K2-18b due to rapid tidal-driven migration, casting doubt on moon-based habitability scenarios for short-period M-dwarf planets in general.

K2-18b closely orbits a nearby M3 dwarf within its habitable zone, where this planet could be either a super-Earth or a mini-Neptune. Recent studies using transit spectroscopy suggest that it is Hycean in nature, but this classification is currently controversial. We use the N-body integrator rebound and its extension library reboundx to investigate the possibility of exomoons around K2-18b. Due to tidal interactions that induce outward migration, we find that any moons would be extremely unlikely. If formed, their lifetimes would be relatively short, not exceeding 10 Myr assuming Earth-like or Neptune-like tidal parameters for K2-18b. Recent studies estimate the stellar (and system) lifetime as 3 Gyr, which is significantly longer than the tidal migration timescale. We show that exomoons are unlikely to survive around K2-18b due to rapid tidal-driven migration, casting doubt on moon-based habitability scenarios for short-period M-dwarf planets in general.

Recent observations of δ Scuti stars find evidence of nonlinear three-mode coupling in their oscillation spectra. There are two types of three-mode coupling likely to be important in δ Scuti stars: (i) direct coupling, in which two linearly unstable modes (driven by the κ-mechanism) excite a linearly stable mode, and (ii) parametric coupling, in which one linearly unstable mode excites two linearly stable modes. M. Breger & M. H. Montgomery find especially strong evidence of direct coupling in the δ Scuti star KIC 8054146. However, direct coupling is inherently unstable and cannot be the mechanism by which the modes saturate and achieve nonlinear equilibrium. By integrating the amplitude equations of small mode networks, we show that the modes can achieve equilibrium if parametric coupling operates in tandem with direct coupling. Using mode parameters calculated from a δ Scuti model, we also find that parametric and direct coupling are likely to be simultaneously active. Importantly, parametric coupling does not necessarily disrupt the correlations found in KIC 8054146 between the amplitudes and phases of the directly coupled modes. We conclude that δ Scuti stars are likely impacted by both parametric and direct coupling, and that accounting for both in future large-mode-network calculations may help explain the complicated mode dynamics observed in many δ Scuti stars.

Context. The Neutron star Interior Composition ExploreR (NICER) is the instrument of choice for the spectral analysis of type I X-ray bursts, as it provides high throughput at the X-ray CCD resolution down to 0.3 keV. Aims. Triggered by the detection of absorption and emission lines in the first four photospheric radius expansion (PRE) bursts detected by NICER, we wish to test the dependence of the absorption line energies on the inferred blackbody radius because it was reported that the absorption line energies were positively correlated with the inferred blackbody radius. This was tentatively explained by a combination of a weaker gravitational redshift and higher blueshifts in a burst with a larger blackbody radius. Methods. We thus reanalysed these four bursts and analysed another eight bursts from 4U 1820-303, for which we report evidence for PRE. We first followed the spectral evolution of the burst on the shortest possible timescales (tenth of a second). We adopted two parallel continuum descriptions to characterise the photospheric expansion and line evolution. Using the accretion-enhanced model, in which the burst emission is modelled as the sum of a blackbody and a component describing the persistent emission recorded prior to the burst and multiplied by a constant (f_a), we inferred maximum equivalent blackbody radii up to ∼900 km. The peak bolometric (0.1–20 keV) luminosity reached between 4 and 7 × 10³⁸ erg s⁻¹ (and even higher when absorption from a putative photoionised absorber is accounted for) in our sample of bursts. This exceeds the Eddington luminosity of a helium accretor. In individual bursts, we detected absorption lines and assessed their significance through extensive Monte Carlo simulations. To characterise the spectral lines, we used dedicated plasma codes available within SPEX with a phenomenological continuum. A deep search throughout the temperature–velocity parameter space was run to explore Doppler shifts and minimise the chance of becoming stuck in local minima. Results. We detected several significant (> 99.9 % significance) absorption lines, including the 2.97 keV line that was previously reported. We do not confirm the correlation between the line energies and the inferred blackbody radius, but for some bursts with larger radii, up to four lines are reported, and the line strength is higher. From the modelling of the feature lines, a photoionised or collisionally ionised slightly redshifted (almost rest-frame) gas in emission is suggested in most cases. In particular for the burst presenting the greatest PRE, a combination of photoionisation plasma in emission and absorption is preferred, however.

We examine the apparent angular diameter of F2 to M5-type main-sequence stars as viewed from planets situated in their respective habitable zones. It is found that M-dwarfs appear largest, whereas F-type stars appear smallest, a result considered possibly counterintuitive. This outcome is unaffected by the adopted stellar models.

F-type star–planet systems represent an intriguing case for habitability studies. Although F-type stars spend considerably less time on the main sequence (MS) than G-, K-, and M-type stars, they still offer a unique set of features, allowing for the principal possibility of exolife. Examples of these features include the increased widths of stellar habitable zones as well as the presence of enhanced UV flux, which in moderation may have added to the origin of life in the Universe. In this study, we pursue a detailed statistical analysis of the currently known planet-hosting F-type stars by making use of the NASA Exoplanet Archive. After disregarding systems with little or no information on the planet(s), we identify 206 systems of interest. We also evaluate whether the stars are on the MS based on various criteria. In one approach, we use the stellar evolution code MESA. Depending on the adopted criterion, about 60–80 stars have been identified as MS stars. In 18 systems, the planet spends at least part of its orbit within the stellar habitable zone. In one case, i.e., HD 111998, known as 38 Vir, the planet is situated in the habitable zone at all times. Our work may serve as a basis for future studies, including studies on the existence of Earth-mass planets in F-type systems, as well as investigations of possibly habitable exomoons hosted by exo-Jupiters, as the lowest-mass habitable zone planet currently identified has a mass estimate of 143 Earth masses.