SN 2025kg, linked to EP 250108a, is among the brightest broad-lined Type Ic supernova (SN) known, showing unique helium absorptions, a late-time broad H alpha, and an early bump. In this Letter, we propose a jet-cocoon origin to explain EP 250108a as off-axis cooling emission from a mildly relativistic inner cocoon viewed at similar to 45 degrees and the early bump of SN 2025kg as the outer cocoon cooling emission, both constraining an energy of similar to(1-2) x10(52) erg and a progenitor radius of similar to 5 R-circle dot. To explain SN 2025kg's exceptionally luminous peak, potential energy injection into the similar to 2.5 M-circle dot ejecta from a magnetar with initial period similar to 1.7 ms and magnetic field similar to 2 x10(15) G may be required, implying a rapidly rotating similar to 4 M-circle dot progenitor. Thus, the progenitor may be a low-mass helium star with an extended helium envelope, supported by helium absorption lines and an inferred weak pre-SN wind. Hydrogen-rich material may reside in the inner ejecta layers, as suggested by the late-time broad H alpha, possibly originating from main-sequence companion material evaporated by the magnetar wind. Since the observed near-solar metallicity challenges the popular quasi-chemically homogeneous evolution channel, the rapidly rotating helium star progenitor of EP 250108a/SN 2025kg might attain angular momentum by being tidally spun up by a main-sequence companion in a close binary formed through isolated binary evolution.
Using highly irregular agglomerated debris particles, we analyse the polarimetric observations of the second interstellar Comet 3I/ATLAS beyond the snow line. Unlike the vast majority of Solar system comets at shorter heliocentric distance, modelling of the polarization of the inner coma in Comet 3I/ATLAS necessarily requires water-ice particles (30-40 per cent by volume). Our modelling suggests the other two species to be Mg-rich silicate and organics/amorphous carbon, which are common for Solar system comets. The relative abundance of the Mg-rich silicates is lower than what was needed in modelling the polarization of Solar system comets. The modelling suggests that if Comet 3I/ATLAS were to keep its activity at heliocentric distances smaller than that of the snow line, it would be an extremely high P-max comet with P-max approximate to 36-40 per cent in red light.
We reanalysed the Atacama Large Millimeter/submillimeter Array (ALMA) observations of the [O III]lambda 88 mu m emission line in JADES-GS-z14-0, so one of the most distant spectroscopically confirmed galaxies at z = 14.18. Our analysis shows a tentative detection of a velocity gradient of [O III]lambda 88 mu m using three independent tests: (1) construction of moment maps; (2) extraction of integrated spectra from a grid of apertures; and (3) spectro-astrometry in both the image and uv planes, confirming the presence of the velocity gradient at 3 sigma significance. We performed kinematical fitting using the KINMS code and estimated a dynamical mass of log(10)( M-dyn /M-circle dot) = 9.4(-0.4)(+0.8), with the bulk of the uncertainties due to the degeneracy between dynamical mass and inclination. We measure an upper limit on the velocity dispersion (sigma(v)) of<40 km s(-1) which results in an estimate of V-rot /sigma> 2.5. This result, if confirmed with higher resolution observations, would imply that kinematically cold discs are already in place at z similar to 14. Comparison with mock observations from the SERRA cosmological simulations confirms that even low-resolution observations are capable of detecting a velocity gradient in z > 10 galaxies as compact as JADES-GS-z14-0. This Letter shows that deeper ALMA or JWST (James Webb Space Telescope)/Near-Infrared Spectrograph integral field spectroscopy observations with high spatial resolution will be able to estimate an accurate dynamical mass for JADES-GS-z14-0, providing an upper limit to the stellar mass of this overluminous galaxy.
We investigate scaling relations for stellar mass, radius, and age in stars exhibiting solar-like oscillations using two observational data sets comprising 110 stars. Using data set 1 (80 stars), we derive empirical scaling relations incorporating asteroseismic parameters - the frequency of maximum oscillation power, large, and small frequency separations - alongside effective temperature and metallicity. To account for non-Gaussian uncertainties in the stellar parameters, we employ an ensemble machine learning approach that combines Ridge, Huber, and Gradient Boosting regressors to predict stellar mass, radius, and age, along with deriving the corresponding coefficients for the scaling relations. We assess coefficient uncertainties by combining jackknife and bootstrap resampling techniques. Using the resulting scaling relations, we predict the mass, radius, and age of 30 stars in data set 2. These predictions closely match observational data, even when the parameters slightly differ from those in the training set. The predicted stellar ages range from 2.29 to 10.09 Gyr, consistent with Solar-like oscillators. Our findings show that combining machine learning regression with resampling-based uncertainty estimation provides an effective and reliable approach for deriving scaling relations, offering valuable insights for stellar characterization in asteroseismic research.
In the vicinity of a supermassive black hole, magnetohydrodynamics (MHD) simulations and models of hot accretion flow predicted a density of injected pairs divided by the Goldreich-Julian density - the pair multiplicity - in the range 1-10(3). When the reconnection mechanism in the magnetosphere is introduced, the multiplicity could reach 10(8) -10(9) and can explain the observed superluminal radio blobs. Here we show that the range 1-10(3) is also difficult to reconcile with observations at the Alfven point of the M87 jet on the polar axis at few tens of gravitational radii. An axisymmetric and frozen flux MHD approach allows to show that the pair multiplicity on the Alfven point only depends on the black hole mass, the fluid temperature and velocity, the magnetic field, and the isorotation frequency. Normalizing our calculations from observational constraints on M87 jet and with reasonable assumptions, we obtain pair multiplicity around 10(9). In conclusion, for a pure pair plasma, the velocity and magnetic field deduced from observations of superalfvenic jets lead to high values of the pair multiplicity at the Alfven point.
