Strmömgren zones of type II supernovae

The emission measures EM in the directions of supernova remnants and pulsars are considered as functions of their ages t. The resulting plot has a well-defined lower boundary, which can be approximated by the expression EM_min∝1/t. The quantity EM_min increases with decreasing age t and does not level off or reach a maximum until t?500 yr. It is concluded that the bulk of the radiative energy that goes into ionizing and heating the interstellar gas is released at early stages of the supernova remnant’s evolution. We suggest that most of the kinetic energy of the supernova shell is converted into thermal energy and radiated at remnant ages t<100 yr, when the supernova shell, which is expanding at an enormous speed (about 10⁴ km/s), overtakes the shell produced by the presupernova in the supergiant stage. We have estimated the ionization energy E?10⁵¹ erg, diameter L?60 pc, and electron density N_e?7 cm^?3 of the HII regions around the supernovae (the supernova Strömgren zones). A list of objects that can be reliably identified as Strömgren zones of type II supernovae is presented. The plot of pulsar pulse broadening τ as a function of the pulsar age t also has a well-defined lower boundary, for which τ∝t^?2 when t≥1000 yr. This suggests that turbulence develops during the first thousand years after the supernova outburst. It is also concluded that turbulence plays an important role in the formation and evolution of the Strömgren zones of type II supernovae.     

A correlation between the distributions of pulsars and emission measures in the galaxy

A correlation has been detected between the volume density of pulsars and the density of interstellar ionized gas on scales of more than 500 pc in Galactic longitude and 200 pc in Galactic latitude. On smaller scales, the correlation is present only for pulsars with ages less than 60000 years, which are located predominantly near supernova remnants and H II regions. This all indicates that pulsars are born in regions with high concentrations of interstellar gas. The minimum emission measures observed in the directions toward pulsars are inversely proportional to the pulsar ages. It is concluded that the ionized gas in the vicinities of a number of pulsars was formed during supernova explosions, and corresponds to Strömgren zones. The ionization of the gas in these zones requires a radiation energy on the order of 10⁵⁰–10⁵¹ erg.     

Restructuring and destruction of hydrocarbon dust in the interstellar medium

A model describing the main processes determining the evolution of hydrocarbon dust grains of arbitrary size under astrophysical conditions corresponding to regions of ionized hydrogen (HII regions) and supernova remnants is presented. The processes considered include aromatization and photodestruction, sputtering by electrons and ions, and shattering during collisions between grains. The model can be used to calculate the size distribution of the grains and the degree of aromatization during the evolution of HII regions and supernova remnants for a specified radiation field, relative velocity between the gas and dust, etc. The contribution of various processes to the evolution of hydrocarbon dust grains for parameters typical for the interstellar medium of our Galaxy is considered. Small grains (with fewer than 50 carbon atoms) should be fully aromatized in the interstellar medium. If larger grains initially have an aliphatic structure, this is preserved to a substantial extent. Variation in the size distribution of the grains due to collisions between grains depend appreciably on the adopted initial size distribution. With an initial distribution corresponding to that of Mathis et al. (1977), the mass fraction contributed by smaller grains tends to increase with time, while, with an initial distribution corresponding to that of Jones et al. (2013), in which the fraction of small grains is initially high, there is a general decrease in the number of grains of various sizes with time.     

Luminosity calibration for OB stars from Hipparcos trigonometric parallaxes

Hipparcos trigonometric parallaxes and observational data for open clusters are used to determine the absolute magnitudes M _V of O and B stars of various spectral subtypes and luminosity classes, including those with broadened lines and hydrogen emission lines. The same data are used to derive a relation between M _V and the index β of the Strömgren photometric system. The resulting calibrations and the spectroscopic-parallax method are used to determine the distances to young open clusters.     

Determination of the physical parameters, Lyman continua, and chemical compositions of HII regions in blue compact dwarf galaxies

A new method for determining the physical parameters, ionizing Lyman continua (Lyc) and chemical compositions of HII regions in blue compact dwarf galaxies is developed. We propose our modified NLEHII method, which is independent on the initial mass function (IMF) for the determination of the Lyc spectra of the ionizing nuclei of HII regions. This method is based on the assumptions of ionization-recombination and thermal equilibria in the HII regions. It is used to calculate the Lyc spectra for optimization photoionization models (OPhMs) of HII regions in an iterative way, since the Lyc spectrum depends on parameters that can be found from photoionization modeling. We apply this method to determine the chemical composition of an HII region in the blue compact dwarf galaxy SBS 0940+544 and, in particular, the helium mass fraction Y. Published in Russian in Astronomicheskiĭ Zhurnal. 2008, Vol. 85, No. 3, pp. 213–229. The article was translated by the authors.     

The pulsar J1852+0040 in Kes 79 and the nature of central compact objects in supernova remnants

A possible model for the pulsar PSR J1852+0040 associated with the supernova remnant Kes 79 and detected in place of a central compact object in this remnant is discussed. The main observational properties of the pulsar can be understood as consequences of its weak surface magnetic field (B _s < 3 × 10¹¹ G) and short rotational period (P ～ 0.1 s). Its X-ray emission is thermal, and is generated in a small region near the surface of the neutron star due to cooling of the surface as the surface accretes matter from a relict disk surrounding the pulsar. The radio emission is generated in the outer layers of the pulsar magnetosphere by the synchrotron (cyclotron) mechanism. The optical luminosity of J1852+0040 is estimated to be L _opt < 10²⁸ erg/s. If the spectral features in another central compact object, 1E 1207.4+5209, are interpreted as electron cyclotron lines, this provides evidence for a weak surface magnetic field for this neutron star as well (B < 6 × 10¹⁰ G). The hypothesis that all central compact objects have weak surface fields makes it possible to explain the number of detected central compact objects, the absence of pulsar-wind nebulae associated with these objects, and the fact that no pulsar has yet been detected at the position of SN 1987a. We suggest that, after the supernova remnant has dissipated, the central compact object becomes a weak X-ray source (XDINS), whose weak emission is also due to the weakness of its magnetic field.     

Search for main-line OH emission toward high-latitude IRAS sources in the southern hemisphere

We present the results of a search for the emission in the main OH lines toward high-latitude IRAS sources using the Australia Telescope Compact Array (ATCA) in Narrabri. A sample of 239 sources from the IRAS catalog selected with the color criterion for ultracompact HII regions and with no restriction on flux was studied. No maser or thermal emission or absorption was detected in these sources. The reason for this could be the low sensitivity of the ATCA to faint high-latitude objects. However, we have mapped the regions of eight known masers in star-forming regions in the plane of the Galaxy. The results are given in the form of spectra and maps.     

Multi-frequency studies of the non-stationary radiation of the blazar 3C 454.3

Long-term monitoring data at five radio frequencies from 4.8 to 37 GHz obtained at the Crimean Astrophysical Observatory, Metsahovi Radio Observatory of Aalto University, and the University of Michigan Radio Astronomy Observatory are used to analyze variations of the flux of the Active Galactic Nucleus (AGN) 3C 454.3. The dynamical characteristics of the three latest powerful flares from 2004 to 2010 are analyzed in detail. Observations in the gamma-ray (0.1–300 GeV), X-ray (2–10 kev, 15–50 keV), and optical are also used. Delays in the development of flares at different frequencies are derived. An empirical frequency dependence for the delays of flares from the gamma-ray to the radio is determined, which can be fit using a logarithmic low and remains the same from flare to flare. The physical characteristics of the central region of the AGN 3C 454.3 are used to estimate the size of its Strömgren sphere, taking into account the relevant mechanisms for heating and cooling the medium, as well as the adopted laws for the variation of the density and temperature with distance from the source of ionization. A model for the location of the radiation regions in the jet at various frequency ranges during the development of flares is proposed.     

Search for class I methanol maser emission in various types of objects in the interstellar medium

Observations of various types of objects in the northern sky were obtained at 44 GHz in the 7₀-6₁ A ⁺ methanol line on the 20-m Onsala radio telescope (Sweden), in order to search for Class I methanol maser emission in the interstellar medium: regions of formation of high-mass stars, dust rings around HII regions, and protostellar candidates associated with powerful molecular outflows and Galactic HII regions. Seven new Class Imethanolmasers have been discovered toward regions of formation of highmass stars, and the existence of two previously observed masers confirmed. The following conclusions are drawn: (1) neither the association of a bipolar outflow manifest in the wings of CO lines with a highmass protostellar object (HMPO) nor the presence of thermal emission in lines of complex molecules are sufficient conditions for the detection of Class I methanol emission; no association with HMPOs radiating at 44 GHz was found for EGOs (a new class of object tracing bipolar outflows); (2) the existence of H₂O masers and Class II methanol masers in the region of aHMPOenhances the probability of detecting Class I methanol emission toward the HMPO; Class II methanol masers with stronger line fluxes are associated with Class I methanol masers.     

Circumstellar nebulae of WR and OB stars with strong winds: Simultaneous excitation by stellar radiation and shocks

We use a two-phase model for the structure of the circumstellar nebulae of hot stars to analyze the radiative cooling of a dense, compact cloud behind the shock produced by the compression of the cloud by hot gas from the stellar wind, taking into account ionization and heating by radiation from the central star. We can distinguish three stages of the evolution of the cloud during its compression. In the first stage, relevant for the entire cloud before compression and the gas ahead of the shock front, the state of the gas is determined purely by ionization by the stellar radiation. The next stage is characterized by the simultaneous action of two gas excitation mechanisms—photoionization by the stellar radiation and shock heating. In this stage, the gas intensively radiates thermal energy received at the shock front. After radiative cooling, in the final stage, ionization and heating of the gas are again determined mainly by the star. To compute the spectrum of the cloud radiation, we solved for the propagation of a plane-parallel, homogeneous flux through the shock front in the radiation field of the hot star. The computations show that a combination of two excitation mechanisms considerably enriches the theoretical spectrum. The relative intensities of emission lines of a single cloud may resemble either those for an HII region or of a supernova remnant.     

Search for class I methanol maser emission toward several supernova remnants

Observations at 44 GHz in the 7₀−6₁ A ⁺ methanol line have been carried out on the 20-m telescope of the Onsala Space Observatory (Sweden) in the directions of the poorly studied region G27.4–0.2 and of several supernova remnants, at the coordinates of the OH(1720) maser satellite emission, with the aim of searching for Class I methanol maser emission in these sources. The region G27.4–0.2 has beenmapped, and contains maser sources and two supernova remnants with similar coordinates and radial velocities, which may accelerate condensation of the ambient gas-dust medium. This may play a role in enhancing the probability of methanol formation and maser emission. This is the first detection of 44 GHz maser emission in this source, and this maser is among the 10% of the strongest Class I methanol masers, within the uncertainties in the integrated flux (of a total of 198 currently knownmasers). A 27′ × 27′ region around the maser has been mapped at 44 GHz in steps of 1′. The 44-GHz emission forms only within the previously known maser region. Further studies in water lines are needed to estimate the influence of shocks from supernovae. No 44-GHz Class I methanol maser emission was detected at the 3σ level at the coordinates of the OH(1720) satellite emission in six supernova remnants; i.e., the presence of OH(1720) emission is not a sufficient condition for the detection of Class I methanol masers.     

Masses of protostars with methanol protoplanetary disks

The masses of 12 protostars assumed to be the central bodies in circumstellar protoplanetary disks are estimated based on analysis of their methanol maser spectra and fine spatial structure. The calculations are based on the hypothesis that the class II methanol maser lines are formed in an edge-on Keplerian disk, while the thermal methanol emission and CS lines are formed in a cocoon around the protostar. This provides information about the velocities of the protostar and the methanol maser condensations relative to the center. In most of the star-forming regions studied, the derived masses are within limits admissible for disks around massive OB stars. The masses are in good agreement with the calculations of other authors based on models of the velocity gradients of the maser features. It is suggested that the methanol spectra display a triplet structure in which the two lateral features are class II methanol lines and the central component is a class I methanol maser line or thermal methanol line. This is consistent with the fact that the correlation of regions of maser emission with regions of emission of dense molecular gas in the CS line is about twice as strong(about 100%) as the correlation with ultracompact HII regions (about 50%). This should be taken into account when modeling protoplanetary disks and star-forming regions.     

Distribution of the relative plasma turbulence level in the galaxy

The statistical dependence of τ/(DM)² (the ratio of the broadening of a pulsar pulse due to scattering in the interstellar medium to the square of the pulsar’s dispersion measure) on the pulsar’s dispersionmeasure, Galactic coordinates, age, and the angular distance to the nearest supernova remnant are studied. This parameter describes the relative level of electron density fluctuations in the turbulent interstellar plasma. It is shown that the interstellar plasma turbulence level is three orders of magnitude higher in the spiral arms of the Galaxy than outside the arms. The plasma turbulence level is approximately an order of magnitude higher in the Galactic arms, in regions within ?0.3° of supernova remnants, than outside these regions. We conclude that the source of energy for the turbulence in the Galactic arms is supernova explosions in the denser medium there.     

The chemical composition of HII regions in blue compact dwarf galaxies

A set of empirical relationships for the ionization correction factors used by various authors to determine the chemical compositions of the gas in nebular objects is tested. New expressions for the ionization correction factors are used to find the nebular-gas compositions in HII regions in blue compact dwarf galaxies. The abundances of He, N, O, Ne, S, and Ar in 41 HII regions are determined. The derived elemental abundances are compared with the results of other studies. The Y-O/H, Y-N/H, and Y-Z dependences are analyzed in detail. The primordial helium abundance Y _p and its enrichment dY/dZ are also determined.     

Compact radio sources and interstellar scattering near the galactic center

Using literature data on approximately 400 compact radio sources detected with the Very Large Array and located in the direction of the Galactic center within 2° of the compact source Sgr A*, 69 sources whose angular sizes are determined by scattering on electron density inhomogeneities were distinguished. Fifty-five of these are extragalactic, two are supercompact HII regions, ten are sources of maser emission, and two are variable Galactic sources. The excess of the apparent angular sizes of maser sources within 2° of the Galactic center above the mean size of objects of this class in other parts of the Galaxy found in many studies cannot be explained purely by the effect of scattering of their radio emission on interstellar plasma inhomogeneities. The angular sizes of these objects are increased due to scattering only within Galactic longitudes of about 0.4° and Galactic latitudes less than 0.1°. The turbulent medium responsible for scattering of radio emission of compact sources in the immediate vicinity of the Galactic center is strongly concentrated toward the compact source Sgr A* at the Galactic center. No extragalactic sources are observed within 0.4° in longitude and 0.2° in latitude of the Galactic center, because of their low brightness due to the superstrong scattering in this region. Data on scatter broadening can be used to study the distribution of turbulent plasma near the Galactic center.     

http://dx.doi.org/10.1016/j.gsf.2016.06.010

Galactic cosmic rays and solar protons ionize the present terrestrial atmosphere,and the air showers are simulated by well-tested Monte-Carlo simulations,such as PHITS code.We use the latest version of PHITS to evaluate the possible ionization of protoplanetary disks by galactic cosmic rays(GCRs),solar protons,and by supernova remnants.The attenuation length of GCR ionization is updated as 118 g cm~(-2),which is approximately 20% larger than the popular value.Hard and soft possible spectra of solar protons give comparable and 20% smaller attenuation lengths compared with those from standard GCR spectra,respectively,while the attenuation length is approximately 10% larger for supernova remnants.Further,all of the attenuation lengths become 10% larger in the compound gas of cosmic abundance,e.g.128 g cm~(-2) for GCRs,which can affect the minimum estimate of the size of dead zones in protoplanetary disks when the incident flux is unusually high.     

Meteoritic Fe-Ni alloys: A review of 57Fe Mössbauer spectroscopy studies

Discovered by Rudolph L. Mössbauer in 1957, the Mössbauer effect (i.e. gamma-resonance spectroscopy) is the phenomenon of the emission or absorption of a gamma ray without loss of energy due to recoil of the nucleus and without thermal broadening. This technique has been applied to many science fields (e.g., physics, chemistry, geology, biology), since it provides information about the nuclear and electronic properties of materials. In this paper, a review of works focusing on the application of ⁵⁷Fe Mössbauer spectroscopy study of the meteoritic Fe-Ni system will be reported.     

The Spectral Type of the Ionizing Stars and the Infrared Fluxes of HII Regions

The 20-cm radio continuum fluxes of 91 HII regions in a previously compiled catalog have been determined. The spectral types of the ionizing stars in 42 regions with known distances are estimated. These spectral types range from B0.5 to O7, corresponding to effective temperatures of 29 000–37 000 K. The dependences of the infrared (IR) fluxes at 8, 24, and 160 μm on the 20-cm flux are considered. The IR fluxes are used as a diagnostic of heating of the matter, and the radio fluxes as measurements of the number of ionizing photons. It is established that the IR fluxes grow approximately linearly with the radio flux. This growth of the IR fluxes probably indicates a growth of the mass of heated material in the envelope surrounding the HII region with increasing effective temperature of the star.     

Gamma-Ray Emission During the Accretion of Matter from a Supernova Envelope onto a Compact Remnant

The aim of this study is to investigate the accretion of matter onto a compact gravitating remnant (neutron star) in the central region of the expanding shell of a Type II supernova. Computations of an explosion with the energetics of a Type II supernova have been performed to derive the structure of matter in the vicinity of the neutron star. The energy of the expanding shell and the parameters of the presupernova correspond to the known values for SN 1987A. This accretion leads to the formation of a layer of fairly dense and hot gas at the surface of the compact remnant, providing the conditions for nucleosynthesis reactions. Thus, one result of the study is to demonstrate the importance of the r and rbc processes, or explosive nucleosynthesis, in the compact envelope of a neutron star. A second result is the production of emission lines from unstable elements formed in the central part of the neutron-star envelope.

     

The evolution of H<Subscript>2</Subscript>O maser spots in star-forming regions

Strong flares of the H₂O maser emission in sources associated with active star-forming regions are analyzed. The main characteristics of 13 flares in nine sources selected using special criteria are presented. The observed phenomena are explained as flares in double emission features. The approach of two emission features in the spectrum with increasing flux and their recession with decreasing flux is explained using a model with two physically related clumps of material that are partially superposed in the line of sight. Calculations have shown that, in this type of model, exponential amplification (unsaturated maser emission) in the overlapping parts of the clumps can produce the observed line narrowing with increasing flux. In most cases, the maser spots are inhomogeneous. During the evolution of some flares, the maser condensations may split into separate fragments. A less catastrophic evolutionary path may be an initial stage of formation of chainlike structures, which are fairly widespread in envelopes around ultracompact HII regions.