CCD photometry inVRI bands of the galactic cluster NGC 2818

bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M) ₀ = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1) × 10₈ years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor mass of the planetary nebula on the main sequence is ≥2.5M_⊙ Based on observations obtained with the Vainu Bappu Telescope.     

二维太阳光谱观测方法和技术

经典的太阳光谱观测是一维的，它有很大的局限性。从５０年代起，天文工作者采用多种方法开展二维太阳光谱观测，已经研制出一系列仪器，建立完整的资料归算程序，取得优良成果。在二维观测资料的基础上，用理论方法推出深度分布，可以得出三维的立体图像，这会成为太阳研究的主要方法之一。     

Flickering in the Magnetic Cv Star Am Herculis

In this study, we present a photometric study of AM Her, a prototype of a class of magnetic CVs. Optical photometry of AM Her was obtained using the Russian–Turkish 1.5 m telescope at TüBİTAK National Observatory (TUG) in August 2003. The R band light curve of the system shows two maxima and two minima during one orbital cycle. In both observing nights the star showed flickering at a significant level. The measured flickering time scale is about 5 min.     

The Flared Disc Project: RXTE and ASCA observations of X 1822−371

We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M_⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M_⊙) black hole and is in a transitional state of duration shortward of 10⁷ yr.     

Measurement of radiative lifetimes and determination of transition probabilities of astrophysical interest in Ho iii

Lifetimes of six levels belonging to the 4f¹⁰6p configuration of Ho  iii have been measured for the first time using the time-resolved laser-induced fluorescence method. They are compared with multiconfigurational pseudo-relativistic Hartree–Fock calculations. A very good agreement is found. Using the experimental lifetimes and theoretical branching fractions, a first set of transition probabilities of astrophysical interest has been obtained for this ion.     

Cosmological constraints from Chandra X-ray observations of galaxy clusters

Chandra X-ray observations of rich, dynamically relaxed galaxy clusters allow the properties of the X-ray gas and the total gravitating mass to be determined precisely. Here, we discuss how Chandra observations may be used as a powerful tool for cosmological studies. By combining Chandra X-ray results on the X-ray gas mass fractions in clusters with independent measurements of the Hubble constant and the mean baryonic matter density of the universe, we obtain a tight constraint on the mean total matter density of the universe, Ο_m, and an interesting constraint on the cosmological constant, Ο_Λ. Using these results, together with the observed local X-ray luminosity function of the most X-ray luminous galaxy clusters, a mass-luminosity relation determined from Chandra and ROSAT X-ray data and weak gravitational lensing observations, and the mass function predicted by numerical simulations, we obtain a precise constraint on the normalization of the power spectrum of density fluctuations in the nearby universe,σ₈. We compare our results with those obtained from other, independent methods. This revised version was published online in July 2006 with corrections to the Cover Date.