Modeling and Analysis of a Spectrum of the Globular Cluster NGC 2419

NGC 2419 is the most distant massive globular cluster in the outer Galactic halo. It is unusual also due to the chemical peculiarities found in its red giant stars in recent years. We study the stellar population of this unusual object using spectra obtained at the 1.93-m telescope of the Haute-Provence Observatory. At variance with commonly used methods of high-resolution spectroscopy applicable only to bright stars, we employ spectroscopic information on the integrated light of the cluster. We carry out population synthesis modeling of medium-resolution spectra using synthetic stellar atmosphere models based on a theoretical isochrone corresponding accurately to the observed color-magnitude diagram. We study the influence of non-Local Thermodynamic Equilibrium for some chemical elements on our results. The derived age (12.6 Gyr), [Fe/H]=-2.25 dex, helium content Y=0.25, and abundances of 12 other chemical elements are in a good qualitative agreement with published high-resolution spectroscopy estimates for red giant members in the cluster. On the other hand, the derived element abundance, [alpha/Fe]=0.13 dex (the mean of [O/Fe], [Mg/Fe] and [Ca/Fe]), differs from the published one ([alpha/Fe] =0.4 dex) for selected red giants in the cluster and may be explained by a large dispersion in the alpha-element abundances recently discovered in NGC2419. We suggest that studies of the {\it integrated} light in the cluster using high-resolution spectrographs in different wavelength regions will help to understand the nature of these chemical anomalies.Comment: 19 pages, 6 figures, accepted for publication in the journal "Astronomy Reports". This work was presented in a poster at IAU General Assembly XXVIII, Beijing 2012 (Special Session 1 "Origin and Complexity of Massive Star Clusters"). Four sentences were added thanks comments of Th. H. Puzi

On the influence of multiple stellar populations in globular clusters on their medium-resolution integrated-light spectra

We take a closer look at our published results of determination of ages, metallicities, helium mass fractions and abundances of chemical elements in Galactic globular clusters in order to find possible signatures of the phenomenon of multiple stellar populations in these data. Our analysis reveals that carbon abundances in the atmospheres of stars in the studied clusters change gradually during their evolution. The changes of the helium mass fraction and C, O, Mg and Na abundance anomalies caused by the effect of multiple stellar populations on the analyzed integrated-light spectra are detected through the comparison of our results with models of chemical evolution and literature data for Galactic field stars.Comment: 4 pages, 2 tables, to be published in the proceedings of the conference 'Ground-Based Astronomy in Russia. 21st Century', Nizhnii Arkhyz, Russia, 21-25 Sep 202

Gemini spectroscopy of the outer disk star cluster BH176

BH176 is an old metal-rich star cluster. It is spatially and kinematically consistent with belonging to the Monoceros Ring. It is larger in size and more distant from the Galactic plane than typical open clusters, and it does not belong to the Galactic bulge. Our aim is to determine the origin of this unique object by accurately determining its distance, metallicity, and age. The best way to reach this goal is to combine spectroscopic and photometric methods. We present medium-resolution observations of red clump and red giant branch stars in BH176 obtained with the Gemini South Multi-Object Spectrograph.We derive radial velocities, metallicities, effective temperatures, and surface gravities of the observed stars and use these parameters to distinguish member stars from field objects. We determine the following parameters for BH176: $V_h= 0\pm 15$ km/s, $[Fe/H]=-0.1\pm 0.1$, age $7\pm 0.5$ Gyr, $E(V-I)=0.79\pm 0.03$, distance $15.2\pm 0.2$ kpc, $\alpha$-element abundance $[\alpha/Fe] \sim 0.25$ dex (the mean of [Mg/Fe], and [Ca/Fe]). BH176 is a member of old Galactic open clusters that presumably belong to the thick disk. It may have originated as a massive star cluster after the encounter of the forming thin disk with a high-velocity gas cloud or as a satellite dwarf galaxy.Comment: 15 pages, 7 fufures, Accepted for publication in Astronomy & Astrophysic

1RXS J180834.7+101041 is a new cataclysmic variable with non-uniform disc

Results of photometric and spectroscopic investigations of the recently discovered disc cataclysmic variable star 1RXS J180834.7+101041 are presented. Emission spectra of the system show broad double peaked hydrogen and helium emission lines. Doppler maps for the hydrogen lines demonstrate strongly non-uniform emissivity distribution in the disc, similar to that found in IP Peg. It means that the system is a new cataclysmic variable with a spiral density wave in the disc. Masses of the components (M_WD = 0.8 +/- 0.22 M_sun and M_RD = 0.14 +/- 0.02 M_sun), and the orbit inclination (i = 78 +/- 1.5 deg) were estimated using the various well-known relations for cataclysmic variables.Comment: 4 pages, 3 figures, conference "European White Dwarf Workshop, 2010", Tuebingen, German

Non-LTE effects for Na I lines in X-ray illuminated stellar atmospheres

The formation of Na I lines in X-ray illuminated atmospheres is investigated by abandoning the assumption of local thermodynamic equilibrium (LTE). Calculations are performed on the basis of a 21-level Na I model atom for the LTE model atmospheres of irradiated F-G stars obtained with allowance for a reflection effect in the first approximation. The state of extreme "overrecombination" is shown to exist for the populations of all Na I levels in the case of external illumination. Absorption features in the profiles of "cool" and "normal" Na I lines have been found to be enhanced compared to the LTE approximation. Effects of the angle of incidence and intensity of the external radiation on the formation of level populations and line profiles when abandoning LTE are analyzed. The existence of overrecombination for Na I is explained by the small X-ray heating function and the large optical cooling function. Na I level populations are shown to depend weakly on the presence of "overionization" for Na II in the atmospheres of irradiated stars. © 2000 MAIK "Nauka/Interperiodica"