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Tashkent, Uzbekistan

Lim B.,Sejong University | Sung H.,Sejong University | Kim J.S.,University of Arizona | Bessell M.S.,Australian National University | Karimov R.,Ulugh Beg Astronomical Institute
Monthly Notices of the Royal Astronomical Society | Year: 2014

IC 1848 is one of the young open clusters in the giant star-formingCas OB6association. Several interesting aspects relating to star formation processes in giant star-forming regions attracted us to study the initial mass function (IMF), star formation mode and properties of pre-mainsequence (PMS) stars. A UBVI and Hα photometric study of the young open cluster IC 1848 was conducted as part of the 'Sejong Open Cluster Surve'y. We have selected 105 earlytype members from photometric diagrams. Their mean reddening is (E(B-V))=0.660±0.054 mag. Using the published photometric data with near-and mid-infrared archival data we confirmed the normal reddening law (RV = 3.1) towards the cluster (IC 1848). A careful zero-age main-sequence fitting gives a distance modulus of V0-MV = 11.7±0.2mag, equivalent to 2.2± 0.2 kpc. Hα photometry and the list of young stellar objects identified by Koenig et al. permitted us to select a large number of PMS stars comprising 196 Hα emission stars, 35 Hα emission candidates, 5 Class I, 368 Class II and 24 transition disc candidates. From the Hertzsprung-Russell diagram using stellar evolution models, we estimate an age of 5 Myr from several evolved stars and 3 Myr from the PMS stars. The IMF was derived from stars with mass larger than 3⊙, and the slope is slightly steeper (Γ = -1.6±0.2) than the Salpeter/Kroupa IMF. Finally, we estimated the mass accretion rate of PMS stars with an ultraviolet excess. The mean mass accretion rate is about 1.4×10-8M⊙yr-1 in the mass range of 0.5-2M⊙ whereas intermediate-mass stars (≥=2.5M⊙) exhibit a much higher accretion rate of M>10-6M⊙yr-1. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Melnikov S.,Thuringer Landessternwarte Tautenburg | Melnikov S.,Ulugh Beg Astronomical Institute | Eisloffel J.,Thuringer Landessternwarte Tautenburg
Astronomy and Astrophysics | Year: 2012

Context. Little is known about the population of the Coma Berenices open cluster (age ∼ 500 Myr) below 0.2 M⊙, and statistics show that there is a prominent deficit of very low-mass objects in this mass range compared to younger open clusters with ages of < 250 Myr. Aims. We search for very low-mass stars and substellar objects (brown dwarfs) in the Coma open cluster to derive the present-day cluster mass function below 0.2 M ⊙. Methods. An imaging survey of the Coma open cluster in the R-and I-bands was carried out with the 2k × 2k CCD Schmidt camera at the 2-m telescope in Tautenburg. We performed a photometric selection of the cluster member candidates by combining results of our survey with 2MASS JHKs photometry. We also analysed low-resolution optical spectroscopic observations of a sample of 14 of our stellar cluster member candidates to estimate their spectral types and luminosity. Results. We present a photometric survey covering 22.5 deg2 around the Coma open cluster centre with a completeness limit of 21m.5 in the R-band and 20m.5 in the I-band. Using optical/IR colour-magnitude diagrams, we identify 82 very low-mass cluster member candidates in the magnitude range 14m7 I 20m.1. Five of them have luminosities and colour indices consistent with brown dwarfs. We calculate a mass spectrum of the very low-mass end of the cluster under the assumption that all selected candidates are probable cluster members. The calculated present-day mass function (dN/dm m-α) can be divided into two parts with a slope of α = 0.6 in the mass interval 0.2 > Ṁ > 0.14 M⊙ and α ∼ 0 in 0.14 > Ṁ > 0.06 M⊙. Conclusions. Our analysis of the spectroscopy of 14 objects shows that they are very low-mass MV-dwarfs of spectral type and luminosity consistent with Coma open cluster membership. This suggests that the membership probability of our other candidates may be high. Our results suggest that the mass function of the Coma open cluster can be traced towards substellar objects, but comparison with a mass spectra of younger clusters indicates that the Coma open cluster has probably lost its lowest mass members by means of dynamical evolution. © 2012 ESO.

Podio L.,NOVA Kapteyn Astronomical Institute | Eisloffel J.,Thuringer Landessternwarte Tautenburg | Melnikov S.,Ulugh Beg Astronomical Institute | Hodapp K.W.,University of Hawaii at Hilo | Bacciotti F.,National institute for astrophysics
Astronomy and Astrophysics | Year: 2011

Context. Jets from young stars can be highly asymmetric and have multiple velocity components. Aims.To clarify the origin of jet asymmetries and constrain the launch mechanism, we study as a test case the physical and kinematical structure of the prototypical asymmetric flow emitted by DG Tau B. Methods. The analysis of deep, high spectral resolution observations taken with the KECK telescope allows us to infer the properties and the spatial distribution of the velocity components in the two jet lobes. From selected line ratios we derive the gas physical conditions (the electron and total density, ne and n H, the ionisation fraction, xe, and the temperature, Te), as a function of the distance from the source and the gas velocity. The presence of dust grains in the jet is investigated by estimating the gas-phase abundance of calcium with respect to its solar value. Results. The detected lines show broad velocity profiles at the base of the jet (up to ∼100 km s-1), where up to three velocity components are detected. At 5′ from the source, however, only the denser and more excited high-velocity components survive and the lines are narrower (∼10-30 km s-1). The jet is strongly asymmetric in the velocity and in its physical structure. The red lobe, which is slower (∼140 km s-1) and more collimated (opening angle: a ∼ 3-4.), presents low ionisation fractions (xe ∼ 0.1-0.4) and temperatures (Te > 5 × 103 K), while the total density is up to ∼2.5 × 104 cm-3. The blue lobe, faster (∼-320 km s-1) and less collimated (a ∼ 14.), is also less dense (nH > 104 cm-3), but highly excited (Te up to ∼5 × 104 K and xe up to 0.9). The estimated mass-loss rate turns out to be similar in the two lobes (∼6-8 × 10-9 M yr-1), while the flux of the linear momentum is three times higher in the blue one (∼2.5 × 10-7 M yr-1 km s-1). Calcium is strongly depleted with respect to its solar abundance, indicating that the jet contains dust grains. The depletion is lower for higher velocities, which is consistent with dust destruction by shocks. Conclusions. The similar mass-loss rate in the two lobes suggests that the ejection power is comparable on the two sides of the system, as expected from a magneto-centrifugal ejection mechanism, and that the observed asymmetries are caused by a different mass load and propagation properties in an inhomogeneous environment. The presence of dust grains implies that the jet is generated from a region of the disc extending beyond the dust sublimation radius. © 2011 ESO.

Jeon Y.,Seoul National University | Im M.,Seoul National University | Ibrahimov M.,Ulugh Beg Astronomical Institute | Lee H.M.,Seoul National University | And 3 more authors.
Astrophysical Journal, Supplement Series | Year: 2010

We present the source catalog and the properties of the B-, R-, and I-band images obtained to support the AKARI North Ecliptic Pole Wide (NEP-Wide) survey. The NEP-Wide is an AKARI infrared imaging survey of the north ecliptic pole covering a 5.8 deg2 area over 2.5-6μm wavelengths. The optical imaging data were obtained at the Maidanak Observatory in Uzbekistan using the Seoul National University 4k × 4k Camera on the 1.5 m telescope. These images cover 4.9 deg2 where no deep optical imaging data are available. Our B-, R-, and I-band data reach the depths of ∼23.4, ∼23.1, and ∼22.3mag (AB) at 5σ, respectively. The source catalog contains 96,460 objects in the R band, and the astrometric accuracy is about 0.″15 at 1σ in each R.A. and decl. direction. These photometric data will be useful for many studies including identification of optical counterparts of the infrared sources detected by AKARI, analysis of their spectral energy distributions from optical through infrared, and the selection of interesting objects to understand the obscured galaxy evolution. © 2010. The American Astronomical Society. All rights reserved.

Papnoi U.,Jamia Millia Islamia University | Atamurotov F.,Institute of Nuclear Physics of Uzbekistan | Ghosh S.G.,Jamia Millia Islamia University | Ghosh S.G.,University of South Africa | And 3 more authors.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2014

A black hole casts a shadow as an optical appearance because of its strong gravitational field. We study the shadow cast by the five-dimensional Myers-Perry black hole with equal rotation parameters. We demonstrate that the null geodesic equation can be integrated, which provides us an opportunity to investigate the shadow cast by a black hole. The shadow of a black hole is found to be a dark zone covered by a deformed circle. Interestingly, the shapes of the black hole shadow are more distorted and the size decreases for larger black hole spins. Interestingly, it turns out that, for fixed values of the rotation parameter, the shadow is slightly smaller and less deformed than for its four-dimensional Kerr black hole counterpart. Further, the shadow of the five-dimensional Kerr black hole is concentric deformed circles. The effect of the rotation parameter on the shape and size of a naked singularity shadow is also analyzed. © 2014 American Physical Society.

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