869 research outputs found
Gravitational instability of Einstein-Gauss-Bonnet black holes under tensor mode perturbations
We analyze the tensor mode perturbations of static, spherically symmetric
solutions of the Einstein equations with a quadratic Gauss-Bonnet term in
dimension . We show that the evolution equations for this type of
perturbations can be cast in a Regge-Wheeler-Zerilli form, and obtain the exact
potential for the corresponding Schr\"odinger-like stability equation. As an
immediate application we prove that for and , the sign
choice for the Gauss-Bonnet coefficient suggested by string theory, all
positive mass black holes of this type are stable. In the exceptional case , we find a range of parameters where positive mass asymptotically flat
black holes, with regular horizon, are unstable. This feature is found also in
general for .Comment: 7 pages, 1 figure, minor corrections, references adde
Imaging of the Host Galaxies of Three X-Ray Selected BL Lacertae Objects
Hubble Space Telescope (HST) WFPC-2 I-band (F814W) images of three X-ray
selected BL Lacertae objects (MS1221.8+2452, MS1407.9+5954, & MS2143.4+0704)
reveal that each of these BL Lac objects is well-centered in an extended
nebulosity that is consistent in brightness and morphology with being light
from an elliptical galaxy at the previously reported redshifts of these BL Lac
objects. Each of the detected host galaxies have radial surface brightness
profiles that are well fit by a DeVaucouleurs' law with effective radii of
between 3 to 12 kpc (H_0=50 km s^{-1} Mpc^{-1}, q_0 = 0). The absolute
magnitudes of the host galaxies fall in the range -24.7 < M_I < -23.5, in the
range of luminosities determined for other BL Lacertae object host galaxies. In
addition to allowing the measurement of the host galaxy magnitudes and radial
surface brightness profiles, the HST images allow a search for substructure in
the host galaxies and the presence of close companion galaxies at spatial
resolutions not yet achievable from the ground. While no evidence was found for
any ``bars'' or spiral arms, ``boxy'' isophotes are present in the host galaxy
of at least one of the three objects observed as part of this study
(MS2143.4+0704). The apparent magnitudes and image properties of the companions
of the BL Lac objects are catalogued as part of this work. The three BL Lacs
appear to occur in diverse environments, from being fairly isolated
(MS1221.8+2452) to possibly being a member of a rich group of galaxies
(MS1407.9+5954).Comment: Latex, 5 postscript figures, 4 jpeg figures, To appear in The
Astrophysical Journal Nov 20, 199
The UV Properties of the Narrow Line Quasar I Zwicky 1
I Zw 1 is the prototype narrow line quasar. We report here the results of our
study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained
with the HST FOS. The following main new results are obtained: 1. The Mg II and
Al III doublets are partially/fully resolved. The measured doublet ratios
verify theoretical predictions that the lines are thermalized in the BLR. 2. A
weak associated UV absorption system is detected in N~V, and possibly also in C
IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity
dispersion <300 km/s. 3. Lines from ions of increasing ionization level show
increasing excess blue wing flux, and an increasing line peak velocity shift,
reaching a maximum blueshift of about 2000 km/s for He II 1640. This may
indicate an out-flowing component in the BLR, where the ionization level
increases with velocity, and which is visible only in the approaching
direction. The highest velocity part of this outflow may produce the associated
UV absorption system. 4. The small C III] 1909 EW, and the small C III]
1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR
density of 10^11, i.e. about an order of magnitude larger than implied by C
III] 1909 in most quasars. A BLR component of a higher density is implied by
the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191
emission is seen, together with weaker line emission at 1294 and 1871 A. These
three features have been proposed as evidence for significant Lya pumping of
the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe
III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV
50, and UV 68 may also be present. (Shortened version)Comment: 28 pages, 1 table and 7 figures included. Uses aas2pp4.sty. Scheduled
for the Astrophysical Journal November 10, 1997 issue, Vol. 48
Dynamics and Selection of Giant Spirals in Rayleigh-Benard Convection
For Rayleigh-Benard convection of a fluid with Prandtl number \sigma \approx
1, we report experimental and theoretical results on a pattern selection
mechanism for cell-filling, giant, rotating spirals. We show that the pattern
selection in a certain limit can be explained quantitatively by a
phase-diffusion mechanism. This mechanism for pattern selection is very
different from that for spirals in excitable media
Evaluation of Methods to Assess in vivo Activity of Engineered Genome-Editing Nucleases in Protoplasts
Genome-editing is being implemented in increasing number of plant species using engineered sequence specific nucleases (SSNs) such as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated systems (CRISPR/Cas9), Transcription activator like effector nucleases (TALENs), and more recently CRISPR/Cas12a. As the tissue culture and regeneration procedures to generate gene-edited events are time consuming, large-scale screening methodologies that rapidly facilitate validation of genome-editing reagents are critical. Plant protoplast cells provide a rapid platform to validate genome-editing reagents. Protoplast transfection with plasmids expressing genome-editing reagents represents an efficient and cost-effective method to screen for in vivo activity of genome-editing constructs and resulting targeted mutagenesis. In this study, we compared three existing methods for detection of editing activity, the T7 endonuclease I assay (T7EI), PCR/restriction enzyme (PCR/RE) digestion, and amplicon-sequencing, with an alternative method which involves tagging a double-stranded oligodeoxynucleotide (dsODN) into the SSN-induced double stranded break and detection of on-target activity of gene-editing reagents by PCR and agarose gel electrophoresis. To validate these methods, multiple reagents including TALENs, CRISPR/Cas9 and Cas9 variants, eCas9(1.1) (enhanced specificity) and Cas9-HF1 (high-fidelity1) were engineered for targeted mutagenesis of Acetolactate synthase1 (ALS1), 5-Enolpyruvylshikimate- 3-phosphate synthase1 (EPSPS1) and their paralogs in potato. While all methods detected editing activity, the PCR detection of dsODN integration provided the most straightforward and easiest method to assess on-target activity of the SSN as well as a method for initial qualitative evaluation of the functionality of genome-editing constructs. Quantitative data on mutagenesis frequencies obtained by amplicon-sequencing of ALS1 revealed that the mutagenesis frequency of CRISPR/Cas9 reagents is better than TALENs. Context-based choice of method for evaluation of gene-editing reagents in protoplast systems, along with advantages and limitations associated with each method, are discussed
The Chandra XBootes Survey - III: Optical and Near-IR Counterparts
The XBootes Survey is a 5-ks Chandra survey of the Bootes Field of the NOAO
Deep Wide-Field Survey (NDWFS). This survey is unique in that it is the largest
(9.3 deg^2), contiguous region imaged in X-ray with complementary deep optical
and near-IR observations. We present a catalog of the optical counterparts to
the 3,213 X-ray point sources detected in the XBootes survey. Using a Bayesian
identification scheme, we successfully identified optical counterparts for 98%
of the X-ray point sources. The optical colors suggest that the optically
detected galaxies are a combination of z<1 massive early-type galaxies and
bluer star-forming galaxies whose optical AGN emission is faint or obscured,
whereas the majority of the optically detected point sources are likely quasars
over a large redshift range. Our large area, X-ray bright, optically deep
survey enables us to select a large sub-sample of sources (773) with high X-ray
to optical flux ratios (f_x/f_o>10). These objects are likely high redshift
and/or dust obscured AGN. These sources have generally harder X-ray spectra
than sources with 0.1<f_x/f_o<10. Of the 73 X-ray sources with no optical
counterpart in the NDWFS catalog, 47 are truly optically blank down to R~25.5
(the average 50% completeness limit of the NDWFS R-band catalogs). These
sources are also likely to be high redshift and/or dust obscured AGN.Comment: 19 pages, 13 figures, ApJ accepted. Catalog can be found at:
http://www.noao.edu/noao/noaodeep or
ftp://archive.noao.edu/pub/catalogs/xbootes
Young stars and non-stellar emission in the aligned radio galaxy 3C 256
We present ground-based images of the z=1.824 radio galaxy 3C 256 in the
standard BVRIJHK filters and an interference filter centered at 8800A, a Hubble
Space Telescope image in a filter dominated by Ly-alpha emission (F336W), and
spectra covering rest-frame wavelengths from Ly-alpha to [O III] 5007. Together
with published polarimetry observations, we use these to decompose the overall
spectral energy distribution into nebular continuum emission, scattered quasar
light, and stellar emission. The nebular continuum and scattered light together
comprise half (one third) of the V-band (K-band) light within a 4-arcsec
aperture, and are responsible for the strong alignment between the
optical/near-infrared light and the radio emission. The stellar emission is
dominated by a population estimated to be 100-200 Myr old (assuming a Salpeter
IMF), and formed in a short burst with a peak star formation rate of 1-4x10^3
Msun/yr. The total stellar mass is estimated to be no more than 2x10^{11} Msun,
which is far less than other luminous radio galaxies at similar redshifts, and
suggests that 3C 256 will undergo further star formation or mergers.Comment: 35 pages including 10 figures; to appear in Nov 10 Ap
The Spitzer Space Telescope First-Look Survey: KPNO MOSAIC-1 R-band Images and Source Catalogs
(Abridged) We present R-band images covering more than 11 square degrees of
sky obtained with the KPNO 4-m telescope in preparation for the Spitzer Space
Telescope First Look Survey. The FLS was designed to characterize the
mid-infrared sky at depths 2 orders of magnitude deeper than previous surveys.
The extragalactic component is the first cosmological survey done with Spitzer.
Two relatively large regions of the sky were observed: the main FLS extra
galactic field (17h18m+59d30m) and ELAIS-N1 field (16h10m+54d30m).
The overall quality of the images is high. The relative astrometric accuracy
is better than 0.1'' and the typical seeing is 1.1''. Images are relatively
deep since they reach a median 5-sigma depth limiting magnitude of R=25.5
(Vega). Catalogs have been extracted using SExtractor using thresholds in area
and flux for which the number of false detections is below 1% at R=25. Only
sources with S/N greater than 3 have been retained in the final catalogs.
Comparing the galaxy number counts from our images with those of deeper R-band
surveys, we estimate that our observations are 50% complete at R=24.5. These
limits in depth are sufficient to identify a substantial fraction of the
infrared sources which will be detected by Spitzer.Comment: 21 pages, 15 Postscript figures, uses aastex, AJ accepted. Images and
catalogs at http://ssc.spitzer.caltech.edu/fls/extragal/noaor_data.htm
On the Origin of Planetary Nebula K648 in Globular Cluster M15
We examine two scenarios for formation of the planetary nebula K648: a prompt
scenario where the planetary nebula is ejected and formed immediately after a
helium shell flash and a delayed scenario where a third dredge up occurs and
the envelope is ejected during the following interpulse phase. We present
models of both scenarios and find that each can produce K648-like systems. We
suggest that the prompt scenario is more favorable but cannot rule out the
delayed scenario.Comment: 19 pages, 2 figures, latex, 2 files containing postscript figures.
Paper to appear in Astrophysical Journal, 483, 837 (1997
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