Astronomy Letters-A Journal of Astronomy and Space Astrophysics最新文献

Consistent stellar evolution and nonlinear radial stellar pulsation calculations were carried out for models of asymptotic giant branch stars with initial masses (1.5leq M_{textrm{ZAMS}}leq 3;M_{odot}) and initial metal abundance (Z=0.006). All the models are shown to be either the fundamental mode or the first overtone pulsators. The lower limit of the first overtone period increases with increasing mass of the Mira model from (Pi_{1,textrm{min}}approx 80) days for (M=1.3;M_{odot}) to (Pi_{1,textrm{min}}approx 120) days for (M=2.6;M_{odot}). The upper limit of the first overtone period and lower limit of the fundamental mode period depend on the stellar structure during mode switching and range from (Pi_{1,textrm{max}}=130) days, (Pi_{0,textrm{min}}=190) days for (M=0.96;M_{odot}) to (Pi_{1,textrm{max}}=210) days, (Pi_{0,textrm{min}}=430) days for (M=2.2;M_{odot}). The slope of the theoretical period–luminosity relation of Mira variables perceptibly increases with decreasing (Z). Fourier spectra of the kinetic energy of twelve hydrodynamic models show a split of the fundamental mode maximum into several equidistant components. Frequency intervals between split components fall within the range (0.03leqDeltanu/nu_{0}leq 0.1). The superposition of radial oscillations with the fundamental mode splitting leads to the long–term amplitude variations with the cycle length from 10 to 30 times longer than the fundamental mode period. A more thorough analysis of hydrodynamic models is required for understanding the origin of the principal pulsation mode splitting.

We investigate the possibility of experimentally determining the PPN (gamma) parameter by measuring the gravitational frequency shift of signals exchanged by two satellites in heliocentric orbits. The satellites are supposed to be equipped with highly stable atomic clocks and the Gravity Probe A non-relativistic Doppler compensation system. We demonstrate that the Doppler compensation system significantly lowers the requirements to the satellite velocity determination accuracy but, at the same time, cancels out the leading contribution, of (O(c^{-3})), to the frequency shift due to (gamma). We derive an equation for the Doppler-compensated frequency shift due to (gamma) in the next-to-leading order, (O(c^{-4})), and show that it is greatly enhanced by numerical factors that rapidly grow in magnitude for signals that propagate close to the gravitational field source. Due to these ‘‘enhanced’’ factors, the accuracy of the proposed experiment with the best of the currently available clocks, such as the JILA SrI, can reach (1.7times 10^{-7}) after 5 yr of data accumulation, if performed in the optimal orbital configuration. This is an order of magnitude worse than our earlier estimate for the accuracy of a similar experiment that does not rely on the Doppler compensation system but 2 orders of magnitude better than the current best result obtained with the Cassini interplanetary probe. Finally, we discuss aspects of the practical realization of the proposed experiment, including prospects for realizing it as part of a multi-science mission that also targets other kinds of gravitational experiments.

The models of force-free magnetic flux ropes differ in their internal current structure: strong electric currents are concentrated on the rope axis in one case and in a thin peripheral shell in the other one. In this paper we perform a comparative analysis of three new models of the second type. All force-free magnetic flux ropes have one common physical property leading to flare energy release: as the top of the loop rope exits into the solar chromosphere and corona, the external pressure keeping the rope from lateral expansion drops steadily; at some critical decrease in this pressure the longitudinal magnetic field of the rope approaches zero on the current inversion surface. At the same time, the azimuthal current (j_{varphi}(r)) and the force-free parameter (alpha(r)), while approaching the discontinuity on this surface, begin to increase indefinitely near it. This leads to the excitation of plasma ion-sound instability, a sharp decrease in the plasma conductivity, rapid magnetic energy dissipation in the rope, and the generation of super-Dreicer electric fields. The set of such processes in combination with the Parker effect, i.e., torque equalization along the rope axis with the Alfv’en speed, describes well the main manifestations of a solar flare.

Based on low-resolution ((Rapprox 3000)) spectra, we have determined some parameters of the star Gaia DR3 5877303483506681472 ((G=19.4) mag), the closest object to the X-ray source [GV2003]N toward the supernova remnant G315.4–2.30: its effective temperature (T_{textrm{eff}}=4830) K, surface gravity log (g=4.3), and metallicity (textrm{[Fe/H]}=-0.05). We have analyzed its radial velocity for variability and found no variations within the error limits ((sigma V_{textrm{rad}}approx 5) km s({}^{-1})). An overabundance of the elements Ca, Ti, V, and Mn has been revealed. The color excess of the star has been estimated from the intensity of diffuse interstellar bands to be (E(B-V)=0.68pm 0.08). Our estimate of the photometric distance (d=2.5pm 0.9) kpc points to a possible connection with the supernova remnant.

The rotational modulation of the radial velocities of narrow emission lines in four classical T Tauri Stars has been studied. The previously declared shift in the mean velocity of neutral and ionized helium lines relative to the mean velocity of the star has been found to be not related to the inflow of accreting gas into the spot, since the radial velocity curves for lines with different velocity shifts must experience phase shifts relative to one another, whereas the observed phase shifts are absent within the uncertainties and do not correspond to the observed line velocity shifts. This means that the line shifts are not caused by the real gas motion. In the case of neutral helium lines, the shifts can be explained by a large optical depth of the lines and the Stark effect for plasma parameters that correspond to the expected ones at the base of the accretion column in T Tauri stars.

The Gnevyshev–Ohl rule is considered for a new series of sunspot areas on a four-century time scale (Nagovitsyn and Osipova 2021). The hypothesis of Usoskin et al. (2001) about the existence of an additional small cycle on the descending branch of Zürich cycle 4 is accepted. This leads to a change in the parity of the cycles earlier than 5. Eleven-year cycles in the 17th century have been distinguished. Their average duration, from minimum to minimum, is (T=8.9pm 1.4) years. New data and approaches have led us to conclude that the Gnevyshev–Ohl rule holds for the 410-year interval as a whole without the exclusion of the pair of Zürich cycles 4 and 5 adopted in the rule previously.

A new approach to calculating the electrostatic energy of solid interiors of degenerate stars has been developed. It is applicable to systems with any ionic composition. The proposed method is used to study ordered Coulomb crystals formed by two types of ions with charge numbers (Z_{1}) and (Z_{2}). Eight different body-centered cubic lattices are considered at (x_{1}geq 1/2), where (x_{1}) is the relative number of ions with charge number (Z_{1}). Six of them are new, while for (x_{1}=1/2) and (1/4) the results for electrostatic energy coincide with those known earlier. All obtained results are approximated by an expression convenient for practical use.

We studied the model of the Galaxy with a bar which reproduces well the distributions of the observed radial, (V_{R}), and azimuthal, (V_{T}), velocities derived from the Gaia DR3 data along the Galactocentric distance (R). The model profiles of the distributions of the velocity (V_{R}) demonstrate a periodic increase and the formation of a hump (elevation) in the distance range of 6–7 kpc. The average amplitude and period of variations in the velocity (V_{R}) are (A=1.76pm 0.15) km s({}^{-1}) and (P=2.1pm 0.1) Gyr. We calculated angles (theta_{01}), (theta_{02}), and (theta_{03}) which determine orientations of orbits relative to the major axis of the bar at the time intervals: 0–1, 1–2, and 2–3 Gyr from the start of simulation. Stars whose orbits change orientations as follows: (0^{circ}<theta_{01}<45^{circ}), (-45^{circ}<theta_{02}<0^{circ}), and (0^{circ}<theta_{03}<45^{circ}), make a significant contribution to the hump formation. The fraction of orbits trapped into libration among orbits lying both inside and outside the outer Lindblad resonance (OLR) is 28(%). The median period of oscillations of librating orbits is 2.0 Gyr. The median period (P) of long-term variations in the angular momentum and total energy of stars increases as the Jacobi energy approaches the values typical for the OLR but then sharply drops. The distribution of model stars over the period (P) has two maxima located at (P=0.6) and (1.9) Gyr. Stars with orbits lying both inside and outside the corotation radius (CR) concentrate to the first maximum. The distribution of stars whose orbits lie both inside and outside the OLR depends on their orientation: orbits elongated perpendicular to the bar concentrate to the first maximum but those stretched parallel to the bar concentrate to the second maximum. The fact that the observed profile of the (V_{R})-velocity distribution derived from the Gaia DR3 data does not show a hump suggests that the age of the Galactic bar, counted from the moment of reaching its full power, must lie near one of two values: (2.0pm 0.3) or (4.0pm 0.5) Gyr.

The observational effects of the circumstellar gas around the superluminous supernova SN 2018ibb are investigated. The narrow Fe II absorption lines are reproduced in the model of a fragmented cold dense shell between the forward and reverse shocks. The unusual selective absorption in the [O I] emission doublet is explained by the scattering of radiation in the Si II doublet lines in the supernova envelope. It is shown that the [O III] emission doublet at (t_{mathrm{max}}+565) days can be radiated by the supernova envelope, with the asymmetry of the [O III] doublet being explained by the formation of dust in the supernova. Supernova–circumstellar gas interaction modeling in combination with observational data leads to an estimate of the circumstellar shell mass (({sim}0.14 M_{odot})).

The absorption of a gamma-ray burst (GRB) afterglow in the X-ray wavelength range in a dense molecular cloud is investigated. We present the results of our numerical simulations of the propagation of GRB radiation in the cloud for various gas densities, metallicities, and distances from the GRB progenitor star to the cloud. We consider a sample of 45 GRBs with known redshifts in which the isotropic-equivalent gamma-ray energy corresponds to the value adopted in our numerical simulations. For these GRBs we have analyzed the Swift/XRT energy spectra of their afterglows at late times, (tgeq 4times 10^{3}) s. We show that the hydrogen column densities estimated using the absorption model in which the ionization of metal ions is ignored and solar metallicity is used are a factor of 1–3 lower than the actual values if the molecular cloud is near the GRB progenitor star. If the gas cloud is located at a distance (Rgeq 10) pc from the GRB source or the gas metallicity ([M/H]leq-1), then the influence of the could ionization structure on the afterglow absorption is minor.