Dynamics and radiation of young type-Ia supernova remnants: Important physical processes

Astronomy Letters - We analyze the physical processes that should be taken into account when modeling young type-Ia supernova remnants (SNRs) with ages of several hundred years in which forward...     

Spatial correlation of unidentified gamma-ray sources with flaring and Wolf-Rayet stars

The spatial positions of unidentified gamma-ray sources ( E>100 MeV) were analyzed. This analysis shows that gamma-ray sources with fluxes >5×10^?7 cm^?2 s^?1 correlate with Wolf-Rayet stars, while gamma-ray sources with fluxes <5×10^?7 cm^?2 s^?1 may be associated with flaring stars.     

Long-Term Changes of Polar Activity of the Sun

We discuss long-time changes of polar activity of the Sun using the new observational data sets in the optical range during 1872–2001. A study of the secular and cycle variations of the magnetic activity at the high-latitude regions is the main goal that includes polar magnetic field reversals during 1872–2001 and secular changes of the duration of polar activity cycles. The secular increase of the area of polar zones during the minimum activity in the last 120 years and as consequence a decrease of coronal temperature of the Sun in the high-latitude zones during the last 50 years. Correlation between the polar cycles of Caii-K bright points with the Wolf sunspot numbers cycles, W(t), and the 22-year polar magnetic cycles of Caii-K bright points at the high latitudes during 1905–1995 is discussed.     

An Upper Limit on Sunspot Activity During the Maunder Minimum

We have estimated the upper and lower limits of sunspot activity, in terms of active day fraction during the Maunder minimum (1645–1710), using raw information on individual daily observations (Hoyt and Schatten, 1998). Establishing the relation between the sunspot activity and active day fraction after 1850, we evaluate the upper limit of annual group sunspot number during the deep Maunder minimum (1645–1700) which does not exceed 4. The earlier finding of a dominant 22-year periodicity during the Maunder minimum is verified and shown to be robust. Also we confirm that the start of the Maunder minimum was very abrupt.     

Numerical simulation of seasonal dynamics of water vapor in the Martian atmosphere

The seasonal evolution of the H₂O snow in the Martian polar caps and the dynamics of water vapor in the Martian atmosphere are studied. It is concluded that the variations of the H₂O mass in the polar caps of Mars are determined by the soil thermal regime in the polar regions of the planet. The atmosphere affects water condensation and evaporation in the polar caps mainly by transferring water between the polar caps. The stability of the system implies the presence of a source of water vapor that compensates for the removal of water from the atmosphere due to permanent vapor condensation in the polar residual caps. The evaporation of the water ice that is present in the surface soil layers in the polar regions of the planet is considered as such a source. The annual growth of the water-ice mass in the residual polar caps is estimated. The latitudinal pattern of the seasonal distribution of water vapor in the atmosphere is obtained for the stable regime.Translated from Astronomicheskii Vestnik, Vol. 38, No. 6, 2004, pp. 497–503.Original Russian Text Copyright © 2004 by Aleshin.     

TOWARDS AN INTEGRATED SCIENTIFIC AND SOCIAL CASE FOR HUMAN SPACE EXPLORATION

I will argue that an ambitious programme of human space exploration, involving a return to the Moon, and eventually human missions to Mars, will add greatly to human knowledge. Gathering such knowledge is the primary aim of science, but science’s compartmentalisation into isolated academic disciplines tends to obscure the overall strength of the scientific case. Any consideration of the scientific arguments for human space exploration must therefore take a holistic view, and integrate the potential benefits over the entire spectrum of human knowledge. Moreover, science is only one thread in a much larger overall case for human space exploration. Other threads include economic, industrial, educational, geopolitical and cultural benefits. Any responsibly formulated public space policy must weigh all of these factors before deciding whether or not an investment in human space activities is scientifically and socially desirable.     

Search for short cosmic gamma-ray bursts using data from the APEX and LILAS experiments onboard the Phobos-2 interplanetary spacecraft

A statistical analysis of the spectral and temporal parameters for 546 triggering events on the APEX gamma-ray detector onboard the Phobos-2 spacecraft has revealed a group of 28 events that are probably short cosmic gamma-ray bursts (GRBs). The distribution of the full group of 74 events of the APEX experiment in duration parameter is bimodal in shape, which is in good agreement with the bimodal shape of the BATSE GRB distribution. A search for the detected group of short events using data from the LILAS X-ray and soft gamma-ray detector onboard the same spacecraft has yielded no positive result. A comparison of the APEX and LILAS data has led us to conclude that the short GRBs have a significantly reduced soft gamma-ray flux at energies <100 keV relative to the power law dN/dE=CE^?α with the average index α=2.62.     

Observations of radio emission from the cosmic gamma-ray burst GRB 030329

We present the radio observations of the afterglow from the intense cosmic gamma-ray burst GRB 030329 performed with the radio telescopes of the Institute of Applied Astronomy, Russian Academy of Sciences, at the Svetloe (λ=3.5 cm) and Zelenchuk (λ=6 cm) Observatories. The difference between the fluxes measured in two different polarization modes suggests the existence of a circular polarization in the radio afterglow from GRB 030329. However, since the measurement errors of the fluxes with different circular polarizations are large, we cannot draw a firm conclusion about its detection; we can only set an upper limit on its value. An analysis of the possible generation mechanisms for the circular polarization of the relativistic jet suggests that there is a helical magnetic field in the jet. The existence of significant flux densities at various wavelengths during a long (≥10 days) period leads us to conclude that the hydrodynamic evolution of the relativistic bow shock takes place in the stellar wind, not in the interstellar medium. We have estimated the total GRB energy (E=10⁵¹ erg) (under the assumption of isotropic radiation) and the plasma density of the stellar wind from the presupernova (n=3 cm^?3). The magnetic-field strength in the relativistic jet can be estimated as B≈100 G.     

Flare-related changes in the profiles of six photospheric spectral lines

The profiles of six photospheric absorption spectral lines (Fei 5250 Å, Fei 5324 Å, Fei 5576 Å, Cai 5590 Å, Cai 6103 Å and Fei 6165 Å), measured in the kernel of a 2N solar flare and in a quiet-Sun area, were compared. The observations were carried out with an echelle spectrograph of the Crimean Astrophysical Observatory. It was shown that, compared to the quiet-Sun profiles, the flare profiles are shallower in the line core and are less steep in the wings. Therefore, measurements of the longitudinal magnetic field made with a magnetograph system which uses the Cai 6103 Å  spectral line, can be underestimated by 18–25% in areas of bright H ribbons of a moderate solar flare. Modeling of the solar photosphere performed by using a synthesis method showed that, in a solar flare, the enhanced core emission seems to be related to heating of the photosphere by the flare, whereas the decrease of the slope of the wings was presumably caused by the inhomogeneity of the photospheric magnetic field.     

Impacts of Large Planetesimals on the Early Earth

We considered the impacts of very large cosmic bodies (with radii in the range 100–200 to 1000–2000 km) on the early Earth, whose mass, radius and density distribution are close to the current values. The impacts of such bodies were possible during the first hundreds of million years after the formation of the Earth and the Moon. We present and analyze the results of a numerical simulation of the impact of a planetesimal, the size of which is equal to that of the contemporary Moon (1700 km). In three-dimensional computations, the velocity (15 and 30 km/s) and the angle (45°, 60°, and 90°) of the impact are varied. We determined the mass losses and traced the evolution of the shape of the Earth's surface, taking into account the self-consistent gravitational forces that arise in the ejected and remaining materials in accordance with the real, time-dependent mass distribution. Shock waves reflected from the core are shown to propagate from the impact site deep into the Earth. The core undergoes strong, gradually damped oscillations. Although motions in the Earth's mantle gradually decline, they have enough time to put the Earth in a rotational motion. As a result, a wave travels over the Earth's surface, whose amplitude, in the case of an oblique impact, depends on the direction of the wave propagation. The maximum height of this wave is tremendous—it attains several hundred kilometers. Some portion of the ejected material (up to 40% of the impactor mass) falls back onto Earth under the action of gravity. This portion is equivalent to the layer of a condensed material with a thickness on the order of ten kilometers. The appearance of this hot layer should result in a global melting of near-surface layers, which can limit the age of terrestrial rocks by the time of the impact under consideration. For lesser-sized impactors, say, for impactors with radii of about 160 km, the qualitative picture resembles that described above but the amplitude of disturbances is considerably smaller. This amplitude, however, is sufficient to cause a crustal disruption (if such a crust has already formed) and intense volcanic activity.