544 research outputs found
A reconnaissance space sensing investigation of crustal structure for a strip from the eastern Sierra Nevada to the Colorado Plateau
There are no author-identified significant results in this report. Research progress in applications of ERTS-1 MSS imagery in study of Basin-Range tectonics is summarized. Field reconnaissance of ERTS-1 image anomalies has resulted in recognition of previously unreported fault zones and regional structural control of volcanic and plutonic activity. NIMBUS, Apollo 9, X-15, U-2, and SLAR imagery are discussed with specific applications, and methods of image enhancement and analysis employed in the research are summarized. Areas studied and methods employed in geologic field work are outlined
Implications of flexible spacer rotational processes on the liquid crystal behavior of 4,5-dihydroisoxazole benzoate dimers
The synthesis of some novel non-symmetric liquid crystal dimers, {3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-(decyloxy)benzoates (5a–d) and 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-{[6-(octyloxy)naphthalen-2-yl]ethynyl}benzoate (9a–d), are reported. The liquid-crystalline properties, theoretical calculations based on the conformational aspects of the flexible alkyl spacer and X-ray experiments are discussed. The syntheses of the key intermediates, 2-{3-[4-(octyloxy)phenyl]-4,5-dihydroisoxazol-5-yl}alkanol (3a–d), presenting the flexible alkyl spacer were achieved through [3+2] cycloaddition reactions between nitrile oxides, which were generated in situ by oxidation of the respective aromatic oximes, and dipolarophile alkenols (CH2[double bond, length as m-dash]CH(CH2)nOH, n = 1, 2, 3, and 4). The benzoates 5a–d were synthesized through esterification of 3a–d and p-n-decyloxybenzoic acid (4). The esters 9a–d were synthesized through derivatization of isoxazolines 3a–d into 4-{3-[4-(octyloxyphenyl)]-4,5-dihydroisoxazol-5-yl}alkyl 4-bromobenzoate (7a–d) followed by a Sonogashira reaction with 2-ethynyl-6-octyloxynaphthalene (8). 5a and 5b showed a monotropic smectic C phase. 9a/c displayed a enantiotropic nematic (N) mesophase, whereas 9b/d showed a monotropic nematic mesophase. No mesophase was observed for 7a–d. An odd–even effect was observed for 5a–d and 9a–d associated with the crystal to isotropic phase transition and crystal to nematic phase, respectively, as the length of the spacer was increased from 1 to 4 carbon atoms. The transitional properties were higher for odd-numbered members (n = 1 and 3) for all of the series studied. The X-ray data of compounds 5a and 5b are in agreement with polarizing optical microscopy observations with the assignment of an SmC mesophase. Density functional theory calculations using the B3LYP hybrid functional with the level 6-311G(d,p) basis set were performed for molecules 5a–d to correlate the conformation of the flexible spacer and the transitional properties. The conformational analysis showed that the most stable conformation for 5a–d is one where all of the carbon atoms of the flexible spacer are orientated at 180° (antiperiplanar orientation) except for 5a because the spacer is too short. The odd-numbered members have a more bent shape and are less elongated molecules than the even-numbered members. Thus, mesomorphic behavior is dictated by the conformational constraint imposed by the flexible spacer on the mesogenic groups
The X-ray absorber of PKS2126-158
BeppoSAX observed the z=3.27 quasar PKS2126-158 on 1999 May 24-28 when its
2-10keV and 0.1-2.5keV fluxes were 1.1E-11 and 4.4E-12 cgs respectively, a
factor of 2 higher than in all previous ROSAT and ASCA observations and 40%
higher than in two more recent Chandra and XMM-Newton observations. The
shortest detected rest frame variability timescale is of a few months,
comparable to the causal timescale associated to an emission region of ~10
Schwarzschild radii around a few 1E10 solar masses black hole. The source is
detected with a signal to noise ratio S/N>=3 up to ~50 keV, 215 keV rest frame.
The BeppoSAX observations confirm the presence of low energy absorption along
the line of sight, independent on the continuum model adopted, at high
confidence level. Despite the limited spectral resolution of the BeppoSAX LECS
and MECS it is possible to put constraints on different absorption and
continuum models, but not to unambiguously determine the redshift of the
absorber. If the absorber is not significantly ionized the BeppoSAX data do
prefer an absorber at z<=2.7. Strong and complex metal line systems along the
line of sight to PKS2126-158 have been found at z=0.6631 and at 2.64<z<2.82.
They could well be associated to the X-ray absorption. Conversely, an ionized
(``warm'') absorber at the quasar redshift provides a good fit only if the iron
abundance is smaller than ~0.3 solar, while that of the other elements is fixed
to the solar value.Comment: A&A in pres
The structural properties and star formation history of Leo T from deep LBT photometry
We present deep, wide-field g and r photometry of the transition type dwarf
galaxy Leo T, obtained with the blue arm of the Large Binocular Telescope. The
data confirm the presence of both very young (5
Gyr) stars. We study the structural properties of the old and young stellar
populations by preferentially selecting either population based on their color
and magnitude. The young population is significantly more concentrated than the
old population, with half-light radii of 104+-8 and 148+-16 pc respectively,
and their centers are slightly offset. Approximately 10% of the total stellar
mass is estimated to be represented by the young stellar population. Comparison
of the color-magnitude diagram (CMD) with theoretical isochrones as well as
numerical CMD-fitting suggest that star formation began over 10 Gyr ago and
continued in recent times until at least a few hundred Myr ago. The CMD-fitting
results are indicative of two distinct star formation bursts, with a quiescent
period around 3 Gyr ago, albeit at low significance. The results are consistent
with no metallicity evolution and [Fe/H] ~ -1.5 over the entire age of the
system. Finally, the data show little if any sign of tidal distortion of Leo T.Comment: 8 pages, 9 figures, some small textual changes, accepted for
publication in the Astrophysical Journa
A deep Large Binocular Telescope view of the Canes Venatici I dwarf galaxy
We present the first deep color-magnitude diagram of the Canes Venatici I
(CVnI) dwarf galaxy from observations with the wide field Large Binocular
Camera on the Large Binocular Telescope. Reaching down to the main-sequence
turnoff of the oldest stars, it reveals a dichotomy in the stellar populations
of CVnI: it harbors an old (> 10 Gyr), metal-poor ([Fe/H] ~ -2.0) and spatially
extended population along with a much younger (~ 1.4-2.0 Gyr), 0.5 dex more
metal-rich, and spatially more concentrated population. These young stars are
also offset by 64_{-20}^{+40} pc to the East of the galaxy center. The data
suggest that this young population, which represent ~ 3-5 % of the stellar mass
of the galaxy within its half-light radius, should be identified with the
kinematically cold stellar component found by Ibata et al. (2006). CVnI
therefore follows the behavior of the other remote MW dwarf spheroidals which
all contain intermediate age and/or young populations: a complex star formation
history is possible in extremely low-mass galaxies.Comment: 4 pages, 3 figures, accepted for publication in ApJL. Minor changes,
conclusions unchange
XMM-Newton spectroscopy of high redshift quasars
We present XMM-Newton X-ray spectra and optical photometry of four high
redshift (z=2.96-3.77) quasars, [HB89] 0438-436, [HB89] 2000-330, [SP89]
1107+487 and RX J122135.6+280613; of these four objects the former two are
radio-loud, the latter two radio-quiet. Model fits require only a power law
with Galactic absorption in each case; additional intrinsic absorption is also
needed for [HB89] 0438-436 and RX J122135.6+280613. The spectra are hard (Gamma
\~1.7 for [HB89] 0438-436, [HB89] 2000-330 and ~1.4 for RX J122135.6+280613)
with the exception of [SP89]~1107+487 which is softer (Gamma ~2.0); the
combined Galactic and intrinsic absorption of lower energy X-rays in the latter
source is much less significant than in the other three. The two intrinsically
unabsorbed sources have greater optical fluxes relative to the X-ray
contributions at the observed energies. While there is no need to include
reflection or iron line components in the models, our derived upper limits (99%
confidence) on these parameters are not stringent; the absence of these
features, if confirmed, may be explained in terms of the high power law
contribution and/or a potentially lower albedo due to the low disc temperature.
However, we note that the power-law spectrum can be produced via mechanisms
other than the Comptonization of accretion disc emission by a corona; given
that all four of these quasars are radio sources at some level we should also
consider the possibility that the X-ray emission originates, at least
partially, in a jet.Comment: Accepted for publication in MNRA
The soft X-ray properties of quasars in the Sloan Digital Sky Survey
We use the ROSAT All Sky Survey (RASS) to study the soft X-ray properties of
a homogeneous sample of 46,420 quasars selected from the third data release of
the Sloan Digital Sky Survey (SDSS). Optical luminosities, both at rest-frame
2500\AA () and in [OIII] () span more than three
orders of magnitude, while redshifts range over . We detect 3366
quasars directly in the observed 0.1--2.4 keV band. Sub-samples of radio-loud
and radio-quiet objects (RLQs and RQQs) are obtained by cross-matching with the
FIRST catalogue. We study the distribution of X-ray luminosity as a function of
optical luminosity, redshift and radio power using both individual detections
and stacks of complete sets of similar quasars. At every optical luminosity and
redshift \log L_{2\kev} is, to a good approximation, normally distributed
with dispersion , at least brightwards of the median X-ray
luminosity. This median X-ray luminosity of quasars is a power law of optical
luminosity with index for and for
. RLQs are systematically brighter than RQQs by about a factor
of 2 at given optical luminosity. The zero-points of these relations increase
systematically with redshift, possibly in different ways for RLQs and RQQs.
Evolution is particularly strong at low redshift and if the optical luminosity
is characterised by . At low redshift and at given
the soft X-ray emission from type II AGN is more than 100
times weaker than that from type I AGN.Comment: 16 pages, 15 figures, 3 tables, replaced with final version accepted
by MNRA
A Uniform Analysis of the Ly-alpha forest at z = 0 - 5: II. Measuring the mean intensity of the extragalactic ionizing background using the proximity effect
A homogeneous sample of 99 moderate resolution QSO spectra at z > 1.7 were
presented in Paper I, including 39 previously unpublished spectra from the
Multiple Mirror Telescope. The statistics of the Lyman alpha forest were
discussed. In this analysis, we demonstrate that a proximity effect is present
in the data, ie. there exists a significant (5.5) deficit of lines at
. Within 1.5 Mpc of the QSO emission redshift,
the significance does depend on QSO luminosity, in accordance with the theory
that this effect is caused by enhanced ionization of hydrogen in the vicinity
of the QSO from UV photons from the QSO itself. The photoionization model of
Bajtlik, Duncan, and Ostriker (1988) permits an estimate of the mean intensity
of the extragalactic background radiation at the Lyman limit. We compare the
results of this standard analysis with those obtained using a maximum
likelihood technique. The best fit value for is
7.0 x 10 ergs/s/cm/Hz/sr, over the redshift range
1.7 < z < 3.8, using QSO redshifts based on narrow emission lines. The best fit
value for the HI ionization rate is 1.9 x 10 s,
in good agreement with models of the background which incorporate QSOs only.
This large absorption line sample and these techniques for measuring the
background and understanding the systematics involved allow us to place what we
believe are are the firmest limits on the background at these redshifts.Comment: revised figures 13 and 14, and other minor corrections, 42 Latex
pages, 23 encapsulated Postscript figures, uses emulateapj.sty, To appear in
the Sept. 2000 ApJ
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