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Search Results: 1 - 10 of 87784 matches for " Zlatan I. Tsvetanov "
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The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data
SDSS Collaboration,Zlatan I. Tsvetanov
Physics , 2000, DOI: 10.1086/312515
Abstract: We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J134646.45-003150.4 (SDSS 1346-00), was selected because of its very red color and stellar appearance. Its spectrum between 0.8-2.5 mic is dominated by strong absorption bands of H_2O and CH_4 and closely mimics those of Gliese 229B and SDSS 162414.37+002915.6 (SDSS 1624+00), two other known methane brown dwarfs. SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that it lies about 11 pc from the sun. The ratio of flux at 2.1 mic to that at 1.27 mic is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others. Based on a search area of 130 sq. deg. and a detection limit of z* = 19.8, we estimate a space density of 0.05 pc^-3 for methane brown dwarfs with T_eff ~ 1000 K in the 40 pc^3 volume of our search. This estimate is based on small-sample statistics and should be treated with appropriate caution.
The Color Distributions of Globular Clusters in Virgo Elliptical Galaxies
Eric H. Neilsen, Jr.,Zlatan I. Tsvetanov
Physics , 1999, DOI: 10.1086/311965
Abstract: This Letter presents the color distributions of the globular cluster (GC) systems of 12 Virgo elliptical galaxies, measured using data from the Hubble Space Telescope. Bright galaxies with large numbers of detected GC's show two distinct cluster populations with mean V-I colors near 1.01 and 1.26. The GC population of M86 is a clear exception; its color distribution shows a single sharp peak near V-I=1.03. The absence of the red population in this galaxy, and the consistency of the peak colors in the others, may be indications of the origins of the two populations found in most bright elliptical galaxies.
The Surface Brightness Fluctuations and Globular Cluster Populations of M87 and its Companions
Eric H. Neilsen, Jr.,Zlatan I. Tsvetanov,Holland C. Ford
Physics , 1997, DOI: 10.1086/304267
Abstract: Using the surface brightness fluctuations in HST WFPC-2 images, we determine that M87, NGC 4486B, and NGC 4478 are all at a distance of ~16 Mpc, while NGC 4476 lies in the background at ~21 Mpc. We also examine the globular clusters of M87 using archived HST fields. We detect the bimodal color distribution, and find that the amplitude of the red peak relative to the blue peak is greatest near the center. This feature is in good agreement with the merger model of elliptical galaxy formation, where some of the clusters originated in progenitor galaxies while other formed during mergers.
Physical Conditions in the Seyfert Galaxy NGC 2992
Mark G. Allen,Michael A. Dopita,Zlatan I. Tsvetanov,Ralph S. Sutherland
Physics , 1998, DOI: 10.1086/306718
Abstract: This paper presents long slit spectral maps of the bi-cone shaped extended narrow line region (ENLR) in the Seyfert galaxy NGC 2992. We investigate the physical properties of the ENLR via emission line diagnostics, and compare the observations to shock and photoionization models for the excitation mechanism of the gas. The line ratios vary as a function of position in the ENLR, and the loci of the observed points on line ratio diagrams are shown to be most consistent with shock+precursor model grids. We consider the energetics of a nuclear ionizing source for the ENLR, and perform the q-test in which the rate of ionizing photons from the nucleus is inferred from measurements of the density and ionization parameter. The q-test is shown to be invalid in the case of NGC 2992 because of the limitations of the [S II]6717/6731 density diagnostic. The excitation of the gas is shown to be broadly consistent with the kinematics, with higher [N II]6583/H-alpha present in the more dynamically active region. We also show that the pressure associated with the X-ray emitting plasma may provide a large fraction of the pressure required to power the ENLR via shocks.
The Nuclear Disk in M87: A Review
Holland Ford,Zlatan Tsvetanov
Physics , 1998, DOI: 10.1007/BFb0106439
Abstract: Three sets of HST+COSTAR FOS and FOC observations provide strong evidence that the nuclear disk in M87 is in Keplerian rotation around a black hole with a mass of (2 - 3)x10^9 M_sol. A deep (6 orbits), high resolution H-alpha+[NII] PC2 HST image shows a trailing, three arm spiral superposed on the underlying nuclear disk. Several of the extended filaments appear to connect directly to the disk. The filaments extending to the NW appear to be twisted, as in NGC 4258. Earlier arguments that the NW filaments are flowing from the nucleus are supported by the presence of blue shifted non-Keplerian components within 20 pc of the nucleus. The gas in the blue and red shifted non-Keplerian components has negative energy and will fall back into the nucleus. The morphological and kinematical observations can be explained by assuming that the filaments originate in a bidirectional wind from the disk. Such a wind will carry away angular momentum, enabling gas in the disk to move toward the black hole. Small (r ~ 1"; r ~ 100-200 pc), well-defined dusty (D-type) and ionized (I-type) "nuclear" disks are common in elliptical galaxies. We suggest that the size of the black hole's radius of influence R_BH relative to the radius of the nuclear disk R_disk determines whether the disk will be a D-type or I-type. I-type disks (M87 and M81) occur when R_BH > R_disk. Differential Keplerian rotation throughout the disk may then generate turbulence and shocks that ionize the gas. D-type disks (NGC 4261 and NGC 6251) occur when R_BH << R_disk. The regions of a disk that are exterior to R_BH will rotate at approximately constant angular velocity in the galaxy's potential, shocks will be suppressed, and the gas will remain cold and dusty. Intermediate D/I types (3C264) may occur when R_BH is a significant fraction of the disk's radius.
M87: A Misaligned BL LAC?
Zlatan I. Tsvetanov,George F. Hartig,Holland C. Ford,Michael A. Dopita,Gerard A. Kriss,Yichuan C. Pei,Linda L. Dressel,Richard J. Harms
Physics , 1997,
Abstract: The nuclear region of M87 was observed with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) at 6 epochs, spanning 18 months, after the HST image quality was improved with the deployment of the corrective optics (COSTAR) in December 1993. From the FOS target acquisition data, we have established that the flux from the optical nucleus of M87 varies by a factor ~2 on time scales of ~2.5 months and by as much as 25% over 3 weeks, and remains unchanged (<= 2.5%) on time scales of ~1 day. The changes occur in an unresolved central region <= 5 pc in diameter, with the physical size of the emitting region limited by the observed time scales to a few hundred gravitational radii. The featureless continuum spectrum becomes bluer as it brightens while emission lines remain unchanged. This variability combined with the observations of the continuum spectral shape, strong relativistic boosting and the detection of significant superluminal motions in the jet, strongly suggest that M87 belongs to the class of BL Lac objects but is viewed at an angle too large to reveal the classical BL Lac properties.
Broad Absorption Line Quasars in the Early Data Release from the Sloan Digital Sky Survey
Alin Tolea,Julian H. Krolik,Zlatan Tsvetanov
Physics , 2002, DOI: 10.1086/344563
Abstract: A new broad absorption line quasar (BAL) sample is derived from the first data released by the Sloan Digital Sky Survey. With 116 objects, it is the largest BAL sample yet assembled. Over the redshift range 1.8 < z < 3.8, the crude fraction with broad absorption in the CIV line is roughly 15%. This fraction may be subject to small selection-efficiency adjustments. There are also hints of redshift-dependence in the BAL fraction. The sample is large enough to permit the first estimate of the distribution of "balnicity index": subject to certain arbitrary parameters in the definition of this quantity, it is very broad, with (roughly) equal numbers of objects per logarithmic interval of balnicity. BAL quasars are also found to be redder on average than non-BAL quasars. The fraction of radio-loud BAL quasars is (weakly) consistent with the fraction of radio-loud ordinary quasars.
Kepler-413b: a slightly misaligned, Neptune-size transiting circumbinary planet
Veselin B. Kostov,Peter R. McCullough,Joshua A. Carter,Magali Deleuil,Rodrigo F. Diaz,Daniel C. Fabrycky,Guillaume Hebrard,Tobias C. Hinse,Tsevi Mazeh,Jerome A. Orosz,Zlatan I. Tsvetanov,William F. Welsh
Physics , 2014, DOI: 10.1088/0004-637X/784/1/14
Abstract: We report the discovery of a transiting, Rp = 4.347+/-0.099REarth, circumbinary planet (CBP) orbiting the Kepler K+M Eclipsing Binary (EB) system KIC 12351927 (Kepler-413) every ~66 days on an eccentric orbit with ap = 0.355+/-0.002AU, ep = 0.118+/-0.002. The two stars, with MA = 0.820+/-0.015MSun, RA = 0.776+/-0.009RSun and MB = 0.542+/-0.008MSun, RB = 0.484+/-0.024RSun respectively revolve around each other every 10.11615+/-0.00001 days on a nearly circular (eEB = 0.037+/-0.002) orbit. The orbital plane of the EB is slightly inclined to the line of sight (iEB = 87.33+/-0.06 degrees) while that of the planet is inclined by ~2.5 degrees to the binary plane at the reference epoch. Orbital precession with a period of ~11 years causes the inclination of the latter to the sky plane to continuously change. As a result, the planet often fails to transit the primary star at inferior conjunction, causing stretches of hundreds of days with no transits (corresponding to multiple planetary orbital periods). We predict that the next transit will not occur until 2020. The orbital configuration of the system places the planet slightly closer to its host stars than the inner edge of the extended habitable zone. Additionally, the orbital configuration of the system is such that the CBP may experience Cassini-States dynamics under the influence of the EB, in which the planet's obliquity precesses with a rate comparable to its orbital precession. Depending on the angular precession frequency of the CBP, it could potentially undergo obliquity fluctuations of dozens of degrees (and complex seasonal cycles) on precession timescales.
Optimal multihump filter for photometric redshifts
Tamas Budavari,Alexander S. Szalay,Istvan Csabai,Andrew J. Connolly,Zlatan Tsvetanov
Physics , 2001, DOI: 10.1086/321068
Abstract: We propose a novel type filter for multicolor imaging to improve on the photometric redshift estimation of galaxies. An extra filter - specific to a certain photometric system - may be utilized with high efficiency. We present a case study of the Hubble Space Telescope's Advanced Camera for Surveys and show that one extra exposure could cut down the mean square error on photometric redshifts by 34% over the z<1.3 redshift range.
Discovery of Two Distant Type Ia Supernovae in the Hubble Deep Field North with the Advanced Camera for Surveys
John P. Blakeslee,Zlatan I. Tsvetanov,Adam G. Riess,Holland C. Ford,Garth D. Illingworth,Daniel Magee,John L. Tonry,N. Benitez,M. Clampin,G. F. Hartig,G. R. Meurer,M. Sirianni,D. R. Ardila,F. Bartko,R. Bouwens,T. Broadhurst,N. Cross,P. D. Feldman,M. Franx,D. A. Golimowski,C. Gronwall,R. Kimble,J. Krist,A. R. Martel,F. Menanteau,G. Miley,M. Postman,P. Rosati,W. Sparks,L. -G. Strolger,H. D. Tran,R. L. White,W. Zheng
Physics , 2003, DOI: 10.1086/374797
Abstract: We present observations of the first two supernovae discovered with the recently installed Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The supernovae were found in Wide Field Camera images of the Hubble Deep Field North taken with the F775W, F850LP, and G800L optical elements as part of the ACS guaranteed time observation program. Spectra extracted from the ACS G800L grism exposures confirm that the objects are Type Ia supernovae (SNe Ia) at redshifts z=0.47 and z=0.95. Follow-up HST observations have been conducted with ACS in F775W and F850LP and with NICMOS in the near-infrared F110W bandpass, yielding a total of 9 flux measurements in the 3 bandpasses over a period of 50 days in the observed frame. We discuss many of the important issues in doing accurate photometry with the ACS. We analyze the multi-band light curves using two different fitting methods to calibrate the supernovae luminosities and place them on the SNe Ia Hubble diagram. The resulting distances are consistent with the redshift-distance relation of the accelerating universe model, although evolving intergalactic grey dust remains as a less likely possibility. The relative ease with which these SNe Ia were found, confirmed, and monitored demonstrates the potential ACS holds for revolutionizing the field of high-redshift SNe Ia, and therefore of testing the accelerating universe cosmology and constraining the "epoch of deceleration".
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