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

We investigate the possibility of experimentally determining the value of the PPN (gamma) parameter, which characterizes possible deviations from general relativity, by measuring the gravitational frequency shift of signals exchanged by two satellites in geo- and heliocentric orbits. We demonstrate that with modern optical clocks the accuracy of an experiment based on our concept can reach an accuracy of at least (1.4times 10^{-8}), which is 3 orders of magnitude better than the current best result achieved with the Cassini interplanetary probe.

In 2015–2021 we carried out observations of the Seyfert 1 galaxy (SyG 1) NGC 7469 in Bessell (UBVRI) filters with the 1.5-m telescope of the Maidanak Observatory, which have continued the monitoring of NGC 7469 since 1990. We took (UBVRI) CCD frames and performed standard frame processing, including bias correction, sky background subtraction, flat fielding, cosmic-ray particle removal, etc. All our data were obtained on CCD cameras with deep cooling and subarcsecond seeing. We carried out the calibrations using several standard stars in the same frame as the galaxy. The results of our photometry in a 13.5 arcsec aperture are presented graphically in the visibility windows of the observed period. A slow (S) flare was detected on the constructed variability curves in the (UBVRI) filters; a slow increase in brightness is observed since 2015, reaching a maximum in 2019. The amplitude of the S flare is 0.5 mag in the (U) filter and decreases to zero in the (I) filter. On the color–color ((U-B)/(B-V)) diagrams the colors in a 5 arcsec aperture are bluer than those in a 30 arcsec aperture.

The optical characteristics of a magnetocentrifugal disk wind from T Tauri stars that we calculated based on the MHD models of Safier (1993a, 1993b) are presented. For an outflow rate in the range (10^{-10}{-}10^{-7} M_{odot}) yr({}^{-1}) we calculated the critical angles at which the wind becomes opaque in the optical, ultraviolet, and X-ray parts of the spectrum. The illumination of the outer regions of protoplanetary disks involved in creating a photoevaporating wind by the star and the conditions for observing young stars in different wavelength ranges depend on these angles. We show that at the early evolutionary stages of T Tauri stars the disk wind is capable of completely shielding the star and preventing the direct illumination of the peripheral disk regions in both optical and X-ray ranges. By absorbing the bulk of the stellar radiation, the disk wind itself becomes a radiation source capable of heating the disk. We show that at an outflow rate ({geq}10^{-9} M_{odot}) yr({}^{-1}) the fraction of the radiation absorbed by the wind can reach (60%). Hot accretion spots can contribute significantly to the absorbed radiation. This allows the disk wind to be considered as an important infrared radiation source of T Tauri stars.

We have investigated the formation of optical radiation and refined the set of parameters for the young pre-cataclysmic binary with an sdB-subdwarf V1828 Aql. We have obtained moderate-resolution spectra with the 6-m BTA telescope and multiband light curves with the Zeiss-1000 telescope at the Special Astrophysical Observatory of the Russian Academy of Sciences. The method of model stellar atmospheres with an accurate allowance for the gravitational and radiative interaction of the components has been used to calculate the optical radiation from the system. We have found that, despite the high temperature of the primary component, (T_{textrm{eff}}=41,200pm 800) K, the amplitude of the photometric reflection effects limited by the small sizes of the secondary component does not exceed (Delta m_{V}=0overset{m}{.}16). The emission features in the HI lines forming on the irradiated part of the surface of the cool star, when added with the absorption lines in the spectrum of the sdB-subdwarfs, weaken them by 10–12(%) and shift them by (Delta V_{r}<10) km s({}^{-1}) in a narrow range of phases adjacent to the time of the secondary eclipse. As a result, the distortions of the observed radial velocity curves under the action of reflection effects turn out to be much smaller than the measurement errors and cannot be used to determine the component mass ratio from a model analysis. Therefore, to determine the set of parameters for V1828 Aql, we have used the assumption that the mass of the primary component is close to (M_{1}=0.47 M_{odot}) found previously for a large group of sdB-subdwarfs. Based on our model analysis of the observed spectra and light curves for V1828 Aql, we have estimated the atmospheric parameters of the subdwarf and the fundamental parameters of both components. We have found that using the method of model atmospheres in analyzing the reflection effects reduces the estimate of the radius of the cool star by 20(%) compared to the calculations in the blackbody approximation.

Based on moderate-resolution spectra in the range 4200–5200 Å, we have studied late O–early B stars in the very young open star cluster IC 1805. The temperatures of the stars under study have been derived by the differential method in which simple parameters were chosen for a number of spectral lines and were then compared with the analogous parameters from an extensive sample of O–B stars. Accurate (T_{textrm{eff}}) estimates have been obtained for them from model atmospheres. Applying this method for the sample of objects under study has allowed the cluster age (t=2pm 0.5) Myr and distance modulus ((v-V)_{0}=11.5^{m}) to be determined with confidence. We have studied the spectroscopic variability of the emission spectrum for the sole Be star of early spectral type B MWC 50 that is a member of the cluster. We show that it is a classical Be star probably seen pole-on and has no long-term variability of the profiles typical for many classical Be stars.

The kinematics of about 40 single stars belonging to the (beta) Pictoris moving group is studied. The age of the (beta) Pictoris moving group is estimated from these stars with ground-based line-of-sight velocity determinations by two methods. Both estimates are kinematic. In the first method we considered the traceback trajectories of the stars, giving an estimate of (t=13.2pm 1.4) Myr. In the second method, by analyzing the instantaneous velocities of the stars, we show that there is an expansion of the stellar system occurring in the Galactic (xy) plane. Based on this effect, we find the time interval elapsed from the beginning of the expansion of the (beta) Pictoris moving group to the present day, (t=20pm 2) Myr.

We consider the M1.1-class solar flare occurred on July 5, 2012, at (06{:}49) UT. This event is unique in that a helioseismic perturbation was detected in it despite its low hard X-ray flux in the 25–50 keV energy band and its very soft hard X-ray spectrum. As a rule, most of the known sunquakes have been detected in solar flares with large hard X-ray fluxes at high energies (at least up to 100–300 keV). The event under consideration contradicts the popular hypothesis about the generation of sunquakes by beams of accelerated high-energy electrons. An analysis of the available RHESSI X-ray spectra shows that they can be explained in two ways. The X-ray spectrum in the 25–50 keV energy band is explained by a power-law distribution of accelerated electrons with an power-law index of 7–9 or by the presence of a superhot plasma with a temperature (Tsim 30)–60 MK. In both cases, we are dealing with electrons of relatively low energies that either were responsible for the sunquake generation or should be considered as a secondary (accompanying) phenomenon with respect to the true cause of the photospheric perturbation. The results of a joint analysis of the X-ray and microwave spectra are presented for the first time for a helioseismically active solar flare. Our analysis shows that the spectra in both ranges can be well explained by the emission of a superhot magnetized plasma and not by accelerated electrons with a soft spectrum. However, the explanation of the spectra when considering partially magnetically trapped accelerated electrons is also possible. We have estimated the parameters of the thermal plasma, accelerated electrons, and energy fluxes of various types. We analyze the dynamics of ultraviolet and X-ray emission sources. We also present an analysis of the magnetic field structure based on vector magnetograms and the nonlinear force-free coronal magnetic field extrapolation. We discuss the mechanisms for the generation of the helioseismic perturbation during the solar flare under consideration. An eruptive process could probably be both primary and secondary causes of the sunquake. The appearance of a superhot plasma in the corona could give rise to propagating thermal fronts into the lower layers of the solar atmosphere, where helioseismic waves are excited. Our analysis does not allow the possibility of the sunquake generation by accelerated electrons with a soft spectrum to be ruled out either.

The origin of the diffuse gamma-ray background in the range from hundreds keV to several MeV is not known conclusively. From current models and observations it is believed that, at least partially, this background is formed by blazars and remnants of supernovae (SN) of type Ia in distant galaxies. However, these contributions are not sufficient to reproduce the observed level of the signal. In this work we propose another source which could contribute to this background, namely the jets of active galactic nuclei (AGN). The composition of jets is not known, but there are observational hints that the fraction of positrons there is substantial. Positrons are partially evacuated to the intergalactic medium and partially mix with the circumgalactic medium and annihilate there comparatively quickly. Using the AGN luminosity function, we estimated the positron production rate and the contribution of the positron annihilation to the cosmic background below 511 keV. We also estimated the analogous contribution from positron annihilation within SN Ia remnants in distant galaxies. The contribution of AGNs is estimated to be a factor of 5–10 smaller than the observed background intensity, and the contribution from SNe is yet smaller by one order of magnitude. Nevertheless, the contribution of AGNs appeared to be larger than the contribution of blazars estimated from Swift-BAT and Fermi-LAT observations. The main uncertainty in our model is the fraction of positrons remaining in the circumgalactic medium which makes our estimation an upper limit.