Evolutionary synthesis models for the emission from radioactive isotopes in H ii regions

We have extended our evolutionary synthesis models for Star Forming Regions to the γ-ray domain. Applying different yields to the resulting synthetic populations, we obtain lightcurves of γ-ray line emission due to the radioactive decay of stellar nucleosynthesis products, such as ²⁶Al and ⁶⁰Fe. We present here this time evolution and the application of the models to regions of star formation in our galaxy.     

The Contribution from Radio Quiet Quasars to γ-ray Background

The EGRET observations have confirmed and proposed the new isotropically distributed γ-ray background, but the known objects radiating γ-rays can not supply so much radiation. Meanwhile, EGRET also reveals a population of γ-ray sources with no radio counterparts which are isotropically distributed in the sky, indicating their possible cosmological origins. Wang et al.^[13] proposed a new γ-ray radiation process driven by the radiation feedback of AGNs. The energy of the radiation peaks around 1 GeV—0.1 TeV with the typical luminosity of 10⁴²—10⁴³ ergs · s^?1. This kind of radiation process in the radio quiet quasars make them the potential γ-ray radiation sources as well as the contributors to the γ-ray background. We consider two cases in which the seed photons in the inverse-Compton processes are from the accretion disks of quasars and cosmic microwave background (CMB), respectively. We find that the former contributes 78%—92% of the background radiation around 1 GeV, while the contribution from the latter is negligible. The radio quiet quasars are highly likely to become the objects which contribute the most energy to the γ-ray background around 1 GeV.     

Understanding 26Al Emission from Cygnus

The COMPTEL observations of the galactic 1.809 MeV emission attributed to the radioactive decay of ²⁶Al have confirmed the diffuse nature of this interstellar emission line. One of the most significant features of the reconstructed intensity pattern is a flux enhancement towards Cygnus. This region is fairly young and contains a wealth of massive stars, most of them grouped in the Cygnus OB associations. Multi-frequency model fitting strongly supports the hypothesis of massive stars and their descendent supernovae being the dominant sources of interstellar ²⁶Al as observed by COMPTEL. Massive stars and supernovae are known to impart a large amount of kinetic energy into their surroundings causing shock regions and large cavities in the ISM. In addition, a significant fraction of the electro-magnetic radiation of these stars is emitted in the EUV regime leading to photoionisation of the surrounding medium. We applied a population synthesis model in combination with an 1D model of expanding superbubbles to the Cygnus OB associations. Besides the expected 1.809 MeV flux and the γ-ray line intensity due to interstellar ⁶⁰Fe we compute the sizes and expansion parameters of the expected HI-structures and the free–free emission intensities due to the photoionizing radiation from massive stars within this region of the sky. We discuss our present understanding of the Cygnus region with respect to the massive star census. Our model assigns about 70% of the 1.809 MeV intensity to six known OB associations, about 20% to known isolated sources and roughly 10% to an unknown diffuse component.     

Cosmic gamma-ray emission and comet collisions with compact stars

The probability that γ-ray bursts may be generated by the infall of comet-like objects on the neutron stars, as recently proposed by Harwit and Salpeter (1973), is reexamined. Although hypothetical cometary clouds around the parent star may survive the supernova outburst virtually untouched, the frequency of γ-outbursts due to the comet impact on the neutron star or white dwarf is only about 10^?3 of the observed occurrence. A considerably higher rate of comets passing per year at critical periastron distance must be assumed if the γ-ray outbursts are to be due to the collision of coments with compact stars.     

The origin of cosmic gamma-ray bursts

The positions of 46 γ-ray burst sources on the sky are used to show that the majority of these objects either originate in very distant regions with redshifts ?30000 km s^?1 or within ?0.5β of the Sun; where β is the scale-height of the parent population perpendicular to the galactic disc. An origin of the majority of γ-ray bursts in the more distant parts of the galactic disc, the galactic nuclear bulge, the Virgo supercluster, in galaxies withm _pg<18 and rich in Abell clusters of distance classes 0 to 4 is excluded by the data.     

Lithium in black hole binaries: the case of X-ray Nova Muscae 1991

We report the detection of the LiI λ6708 resonance line in the K4V secondary of the black-hole binary Nova Muscae 1991. The LiI feature changes with orbital phase, being stronger (EW ∼ 400 mÅ) around φ = 0.0 (observing the dark side of the secondary) and weaker (EW ≤ 190 mÅ) around φ = 0.5 (observing the side facing the compact object). This effect is interpreted as LiI ionization induced by UV/X-ray illumination of the inner hemisphere of the companion star. From the strength of the LiI feature observed around φ = 0.0 we infer a photospheric lithium abundance of logN(Li) ∼ 3. This value should be considered in fact as a lower limit to the true Li abundance because we have not fully corrected for the LiI overionization expected from UV/X-ray illumination.High lithium abundances are becoming a common feature in the secondaries of black hole binaries. After the Li detections in V404 Cyg, A0620-00 and GS2000 + 25, our observations of Nova Muscae not only add a new case to the list, but also demonstrate the impact of irradiation on the formation of the LiI line for the first time. The LiI features observed in the other black hole binaries are probably also affected by irradiation, although not as strongly as in Nova Muscae, and their Li abundances are higher than previously thought.The most attractive scenario for explaining the origin of the high Li abundances in black hole binaries appears to be nucleosynthesis during the explosive accretion events that characterize these objects. We argue that our LiI detection in the secondary of X-ray Nova Muscae 1991, and the transient γ-ray narrow emission feature at 476 keV observed during the 1991 outburst, are both signatures of Li production around the black hole. We propose that the γ-ray emission line was due to ⁷Be electron capture which yielded excited ⁷Li atoms. This reaction is able to account for the central energy, narrow width and time scale of the observed γ-ray emission feature. The presence of Li on the surface of the secondary shows that Li atoms created during the outburst can escape and enrich the interstellar medium.Shrinivas Kulkarni     

KDG218, a nearby ultra-diffuse galaxy

We present properties of the low-surface-brightness galaxy KDG218 observed with the HST/ACS. The galaxy has a half-light (effective) diameter of a _e = 47″ and a central surface brightness of SB _V (0) = 24.^m4/□″. The galaxy remains unresolved with the HST/ACS, which implies its distance of D > 13.1 Mpc and linear effective diameter of A _e > 3.0 kpc. We notice that KDG218 is most likely associated with a galaxy group around the massive lenticular NGC4958 galaxy at approximately 22 Mpc, or with the Virgo Southern Extension filament at approximately 16.5 Mpc. At these distances, the galaxy is classified as an ultra-diffuse galaxy (UDG) similar to those found in the Virgo, Fornax, and Coma clusters. We also present a sample of 15 UDG candidates in the Local Volume. These sample galaxies have the following mean parameters: 〈D〉 = 5.1 Mpc, 〈A _e 〉 = 4.8 kpc, and 〈SB _B (e)〉 = 27.^m4/□″. All the local UDG candidates reside near massive galaxies located in the regions with the mean stellar mass density (within 1 Mpc) about 50 times greater than the average cosmic density. The local fraction of UDGs does not exceed 1.5% of the Local Volume population. We notice that the presented sample of local UDGs is a heterogeneous one containing irregular, transition, and tidal types, as well as objects consisting of an old stellar population.     

Variability in GRB afterglows and GRB 021004

We present general analytic expressions for GRB afterglow light curves arising from a variable external density profile and/or a variable energy in the blast wave. The former could arise from a clumpy ISM or a variable stellar wind; The latter could arise from refreshed shocks or from an angular dependent jet structure (patchy shell). Both scenarios would lead to a variable light curve. Our formalism enables us to invert the observed light curve and obtain possible density or energy profiles. The optical afterglow of GRB 021004 was detected 537 s AB (after the burst) [GCN (2002) 1564]. Extensive follow up observations revealed a significant temporal variability. We apply our formalism to the R-band light curve of GRB 021004 and we find that several models provide a good fit to the data. We consider the patchy shell model with p=2.2 as the most likely explanation. According to this model our line of sight was towards a ‘cold spot’ that has lead to a relativity low γ-ray flux and an initially weak afterglow (while the X-ray afterglow flux after a day was above average). Observations above the cooling frequency, ν_c, could provide the best way to distinguish between our different models.     

CDM, Feedback and the Hubble Sequence

We have performed TreeSPH simulations of galaxy formation in a standard ΛCDM cosmology, including effects of star formation, energetic stellar feedback processes and a meta-galactic UV field, and obtain a mix of disk, lenticular and elliptical galaxies. The disk galaxies are deficient in angular momentum by only about a factor of two compared to observed disk galaxies. The stellar disks have approximately exponential surface density profiles, and those of the bulges range from exponential to r ^1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations and likewise are their integrated B-V colours, which have been calculated using stellar population synthesis techniques. Furthermore, we can match the observed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is (M/L _I) ≃ 0.6–0.7. The ellipticals and lenticulars have approximately r ^1/4 stellar surface density profiles, are dominated by non-disklike kinematics and flattened due to non-isotropic stellar velocity distributions, again consistent with observations. This revised version was published online in September 2006 with corrections to the Cover Date.     

A model of the γ-ray burst of 1979 March 5 (paper II)

We consider that the cosmic γ-ray burst of 1979 March 5 may have originated in a binary system containing a neutron star. If the stellar wind from the companion star is suddenly increased for some reason, a great quantity of plasma will pile up at the magnetopause of the neutron star, forming a high density accreting ring. When the mass of the plasma exceeds a critical value, the Krukal-Schwarzschild instability will set in and the plasma will pour into the magnetosphere. When the plasma reaches the surface of the star at the magnetic poles γ-ray and hard x-ray bursts will occur. Results obtained in this paper agree roughly with the physical constraints of the model given in /1/.     

Progress in ultraviolet stellar spectrophotometry with S59

A progress report is given of investigations related to observations of stellar spectra obtained with the ultraviolet stellar spectrophotometer S59 aboard the ESRO TD-1A satellite. We describe first the processing of the observations: intensity and wavelength calibration, identification of lines, classification of spectra. Thereafter some important groups of lines are dealt with: non-LTE computation of the Mgii lines are presented: apart from the peculiar emission line and shell stars they compare well with the observations; intensity ratios, of Feii and Feiii lines are an important temperature classification criterion. Interstellar UV lines indicate large deficiencies of some metals, as compared to solar values. A new ultraviolet continuous extinction curve is determined. We discuss the strong outstreaming motions observed in α Cygni, and the consequent mass loss of this star (< 3 × 10^?10 \(\mathfrak{M}_ \odot \) yr^?1and finally we describe the composite spectrum of the (WC8+09I) bnary γ² Velorum; the ultraviolet continuous spectrum of the WC star is about one magnitude brighter than any theory predicts.     

Irradiation phenomena in young solar-type stars and the early solar system: X-ray observations and γ-ray constraints

The high levels of X-ray flaring activity observed in young stars and protostars suggest correspondingly high levels of low-energy particle irradiation of their circumstellar disks, and hence of early solar system material like meteorites. We first briefly review the latest X-ray observational results obtained by Chandra and XMM-Newton on two ‘typical’ star-forming regions, Orion and ρ Ophiuchi. We then discuss a new ‘accretion–ejection–irradiation–transport’ model for young stars which, when scaled to the X-ray fluxes, accounts simultaneously for four extinct radioactivity ratios, in particular the purely spallogenic ¹⁰Be/⁹Be ratio, and the ²⁶Al/²⁷Al ratio. We point out the importance of the environment in which nearby star formation is taking place today, namely the Gould Belt, and the possible connection between Comptel detections of ²⁶Al γ-ray line emission from these regions, and new constraints on the origin of the solar system.     

Strong spherical magnetogasdynamic shock with radiation near the surface of a star

The propagation of a shock-wave, originating in a stellar interior, is considered when it approaches the surface of the star. The flow behind the magnetogasdynamic shock wave is assumed to be spatially isothermal rather than adiabatic to stimulate the conditions of large radiative transfer near the stellar surface. The exact shock-propagation laws obtained by solving the equations in similarity variables, for different values of the parameter δ in the undisturbed density law, ρ₀ ∝ γ^δ     

Possible effects of positronium negative ion on 0.511 MeV ψ-ray line in astrophysical sources

The astrophysical importance of the negative positronium ion, detected recently in the laboratory, has been pointed out. It is found that the presence of Ps^– ions will contribute additionally to the width of 0.511 -ray line formed by pair annihilation. The formation of Ps^– ion from an aggregate of electrons, positrons and positronium results in a variable positron population in the 0.511 MeV -ray line source.     

The Correlation between γ-Ray and Radio Emissions for the Fermi Blazars

Based upon the Fermi blazars sample, the radio and γ-ray emissions are compiled for a sample of 74 γ-ray loud blazars to calculate the radio to γ-ray effective spectrum index α _{R γ}. The correlations between α _{R γ} and γ-ray luminosity, and between radio and γ-ray luminosity are also investigated.     

Equation of state under nuclear statistical equilibrium conditions

We investigate a three-parameter equation of state for stellar matter under nuclear statistical equilibrium conditions in the ranges of temperatures 3×10⁹–10¹¹ K and densities 10⁴–10¹³ g cm^?3 and for various ratios of the total number of neutrons to the total number of protons within the range 1–1.5. These conditions correspond to the initial stages of the gravitational collapse of iron stellar cores that are accompanied by nonequilibrium matter neutronization. We analyze the effect of the excited levels of atomic nuclei on the thermodynamic properties of the matter. We show that this effect is insignificant at low densities, ρ?10¹⁰ g cm^?3, but it leads to an expansion of the instability region, γ<4/3, at higher densities. The incorporated effects of the Fermi degeneracy of free nucleons prove to be insignificant, because their concentrations are low at low temperatures. In the future, we plan to investigate the effects of Coulomb interactions and neutron-rich nuclei on the thermodynamic properties of the matter.     

New in the optical spectrum and kinematic state of the atmosphere of the variable V1027 Cyg (=IRAS 20004+2955)

Based on our high-spectral-resolution observations performed with the NES echelle spectrograph of the 6-m telescope, we have studied the peculiarities of the spectrum and the velocity field in the atmosphere and envelope of the cool supergiant V1027 Cyg, the optical counterpart of the infrared source IRAS 20004+2955. A splitting of the cores of strong absorptions of metals and their ions (Si II, Ni I, Ti I, Ti II, Sc II, Cr I, Fe I, Fe II, BaII) has been detected in the stellar spectrum for the first time. The broad profile of these lines contains a stable weak emission in the core whose position may be considered as the systematic velocity V _sys = 5.5 km s^?1. Small radial velocity variations with an amplitude of 5–6 km s^?1 due to pulsations have been revealed by symmetric low- and moderate-intensity absorptions. A long-wavelength shift of the Hα profile due to line core distortion is observed in the stellar spectrum. Numerous weak CN molecular lines and the KI 7696 Å line with a P Cyg profile have been identified in the red spectral region. The coincidence of the radial velocities measured from symmetric metal absorptions and CN lines suggests that the CN spectrum is formed in the stellar atmosphere. We have identified numerous diffuse interstellar bands (DIBs) whose positions in the spectrum, V _r (DIBs) = ?12.0 km s^?1, correspond to the velocity of the interstellar medium in the Local Arm of the Galaxy.     

Internal absorption of gamma-rays in relativistic blobs of active galactic nuclei

We investigate the production of gamma-rays in the inverse Compton (IC) scattering process by leptons accelerated inside relativistic blobs in jets of active galactic nuclei. Leptons are injected homogeneously inside the spherical blob and initiate IC e ^± pair cascade in the synchrotron radiation (produced by the same population of leptons, SSC model), provided that the optical depth for gamma-rays is larger than unity. It is shown that for likely parameters internal absorption of gamma-rays has to be important. We suggest that new type of blazars might be discovered by the future simultaneous X-ray and γ-ray observations, showing peak emissions in the hard X-rays, and in the GeV γ-rays. Moreover, the considered scenario might be also responsible for the orphan X-ray flares recently reported from BL Lac type active galaxies.      

Radio-to-TeV γ-ray emission from PSR B1259–63

We discuss the implications of the recent X-ray and TeV γ-ray observations of the PSR B1259–63 system (a young rotation powered pulsar orbiting a Be star) for the theoretical models of interaction of pulsar and stellar winds. We show that previously considered models have problems to account for the observed behaviour of the system. We develop a model in which the broad band emission from the binary system is produced in result of collisions of GeV–TeV energy protons accelerated by the pulsar wind and interacting with the stellar disk. In this model the high energy γ-rays are produced in the decays of secondary neutral pions, while radio and X-ray emission are synchrotron and inverse Compton emission produced by low-energy (≤100 MeV) electrons from the decays of secondary charged π ^± mesons. This model can explain not only the observed energy spectra, but also the correlations between TeV, X-ray and radio emission components.      

Measurement of the 25Mg(p,γ)26Al reaction at stellar energies

Stellar nuclear reactions, generally occurring at energies well below the Coulomb barrier, have extremely low cross sections posing a very difficult task for their determination using the usual prompt γ-ray detection method. We describe here the application of the highly sensitive Accelerator Mass Spectrometry technique (AMS) as an alternative method for the determination of the resonance strengths of the ²⁵Mg(p,γ) reaction, by off-line counting of the produced ²⁶Al nuclei.