1,796 research outputs found
Dual-Frequency VSOP Observations of AO 0235+164
AO 0235+164 is a very compact, flat spectrum radio source identified as a BL
Lac object at a redshift of z=0.94. It is one of the most violently variable
extragalactic objects at both optical and radio wavelengths. The radio
structure of the source revealed by various ground-based VLBI observations is
dominated by a nearly unresolved compact component at almost all available
frequencies.
Dual-frequency space VLBI observations of AO 0235+164 were made with the VSOP
mission in January-February 1999. The array of the Japanese HALCA satellite and
co-observing ground radio telescopes in Australia, Japan, China and South
Africa allowed us to study AO 0235+164 with an unprecedented angular resolution
at frequencies of 1.6 and 5 GHz. We report on the sub-milliarcsecond structural
properties of the source. The 5-GHz observations led to an estimate of T_B >
5.8 x 10^{13} K for the rest-frame brightness temperature of the core, which is
the highest value measured with VSOP to date.Comment: 8 pages, 8 figures, to appear in Publ. Astron. Soc. Japa
Giant Planet Occurrence in the Stellar Mass-Metallicity Plane
Correlations between stellar properties and the occurrence rate of exoplanets
can be used to inform the target selection of future planet search efforts and
provide valuable clues about the planet formation process. We analyze a sample
of 1194 stars drawn from the California Planet Survey targets to determine the
empirical functional form describing the likelihood of a star harboring a giant
planet as a function of its mass and metallicity. Our stellar sample ranges
from M dwarfs with masses as low as 0.2 Msun to intermediate-mass subgiants
with masses as high as 1.9 Msun. In agreement with previous studies, our sample
exhibits a planet-metallicity correlation at all stellar masses; the fraction
of stars that harbor giant planets scales as f \propto 10^{1.2 [Fe/H]}. We can
rule out a flat metallicity relationship among our evolved stars (at 98%
confidence), which argues that the high metallicities of stars with planets are
not likely due to convective envelope "pollution." Our data also rule out a
constant planet occurrence rate for [Fe/H]< 0, indicating that giant planets
continue to become rarer at sub-Solar metallicities. We also find that planet
occurrence increases with stellar mass (f \propto Mstar), characterized by a
rise from 3.5% around M dwarfs (0.5 Msun) to 14% around A stars (2 Msun), at
Solar metallicity. We argue that the correlation between stellar properties and
giant planet occurrence is strong supporting evidence of the core accretion
model of planet formation.Comment: Fixed minor typos, modified the last paragraph of Section
A multifrequency analysis of radio variability of blazars
We have carried out a multifrequency analysis of the radio variability of
blazars, exploiting the data obtained during the extensive monitoring programs
carried out at the University of Michigan Radio Astronomy Observatory (UMRAO,
at 4.8, 8, and 14.5 GHz) and at the Metsahovi Radio Observatory (22 and 37
GHz). Two different techniques detect, in the Metsahovi light curves, evidences
of periodicity at both frequencies for 5 sources (0224+671, 0945+408, 1226+023,
2200+420, and 2251+158). For the last three sources consistent periods are
found also at the three UMRAO frequencies and the Scargle (1982) method yields
an extremely low false-alarm probability. On the other hand, the 22 and 37 GHz
periodicities of 0224+671 and 0945+408 (which were less extensively monitored
at Metsahovi and for which we get a significant false-alarm probability) are
not confirmed by the UMRAO database, where some indications of ill-defined
periods about a factor of two longer are retrieved. We have also investigated
the variability index, the structure function, and the distribution of
intensity variations of the most extensively monitored sources. We find a
statistically significant difference in the distribution of the variability
index for BL Lac objects compared to flat-spectrum radio quasars (FSRQs), in
the sense that the former objects are more variable. For both populations the
variability index steadily increases with increasing frequency. The
distribution of intensity variations also broadens with increasing frequency,
and approaches a log-normal shape at the highest frequencies. We find that
variability enhances by 20-30% the high frequency counts of extragalactic
radio-sources at bright flux densities, such as those of the WMAP and Planck
surveys.Comment: A&A accepted. 12 pages, 16 figure
Doppler Boosting, Superluminal Motion, and the Kinematics of AGN Jets
We discuss results from a decade long program to study the fine-scale
structure and the kinematics of relativistic AGN jets with the aim of better
understanding the acceleration and collimation of the relativistic plasma
forming AGN jets. From the observed distribution of brightness temperature,
apparent velocity, flux density, time variability, and apparent luminosity, the
intrinsic properties of the jets including Lorentz factor, luminosity,
orientation, and brightness temperature are discussed. Special attention is
given to the jet in M87, which has been studied over a wide range of
wavelengths and which, due to its proximity, is observed with excellent spatial
resolution.
Most radio jets appear quite linear, but we also observe curved non-linear
jets and non-radial motions. Sometimes, different features in a given jet
appear to follow the same curved path but there is evidence for ballistic
trajectories as well. The data are best fit with a distribution of Lorentz
factors extending up to gamma ~30 and intrinsic luminosity up to ~10^26 W/Hz.
In general, gamma-ray quasars may have somewhat larger Lorentz factors than non
gamma-ray quasars. Initially the observed brightness temperature near the base
of the jet extend up to ~5x10^13 K which is well in excess of the inverse
Compton limit and corresponds to a large excess of particle energy over
magnetic energy. However, more typically, the observed brightness temperatures
are ~2x10^11 K, i.e., closer to equipartition.Comment: 10 pages, 12 color figures; proceedings of the 5th Stromlo Symposium:
Disks, Winds, and Jets - from Planets to Quasars; accepted in Astrophysics &
Space Scienc
MOJAVE: monitoring of jets in active galactic nuclei with VLBA experiments. V. Multi-epoch VLBA images
We present images from a long-term program (MOJAVE: Monitoring of Jets in active galactic nuclei (AGNs) with VLBA Experiments) to survey the structure and evolution of parsec-scale jet phenomena associated with bright radio-loud active galaxies in the northern sky. The observations consist of 2424 15 GHz Very Long Baseline Array (VLBA) images of a complete flux-density-limited sample of 135 AGNs above declination –20°, spanning the period 1994 August to 2007 September. These data were acquired as part of the MOJAVE and 2 cm Survey programs, and from the VLBA archive. The sample-selection criteria are based on multi-epoch parsec-scale (VLBA) flux density, and heavily favor highly variable and compact blazars. The sample includes nearly all the most prominent blazars in the northern sky, and is well suited for statistical analysis and comparison with studies at other wavelengths. Our multi-epoch and stacked-epoch images show 94% of the sample to have apparent one-sided jet morphologies, most likely due to the effects of relativistic beaming. Of the remaining sources, five have two-sided parsec-scale jets, and three are effectively unresolved by the VLBA at 15 GHz, with essentially all of the flux density contained within a few tenths of a milliarcsecond
An Economic Analysis of Carbon Sequestration for Wheat and Grain Sorghum Production in Kansas
This study examined the economic potential with and without carbon credit payments of two crop and tillage systems in South Central Kansas that could reduce carbon dioxide emissions and sequester carbon in the soil. Experiment station cropping practices, yield data, and soil carbon data for continuously cropped wheat and grain sorghum produced with conventional tillage and no-tillage from1986 to 1995 were used to determine soil carbon changes and to develop enterprise budgets to determine expected net returns for a typical dryland farm in South Central Kansas. No-till had lower net returns because of lower yields and higher overall costs. Both crops produced under no-till had higher annual soil C gains than under conventional tillage. Carbon credit payments may be critical to induce farm managers to use cropping practices, such as no-till, that sequester soil carbon. The carbon credit payments needed will be highly dependent on cropping system production costs, especially herbicide costs, which substitute for tillage as a means of weed control. The C values estimated in this study that would provide an incentive to adopt no-tillage range from 95.991ton/year, depending upon the assumption about herbicide costs. In addition, if producers were compensated for other environmental benefits associated with no-till, carbon credits could be reduced.carbon credit value, carbon sequestration, grain sorghum, no-tillage, wheat, Crop Production/Industries,
Derived Carbon Credit Values for Carbon Sequestration: Do CO2 Emissions From Production Inputs Matter?
Environmental Economics and Policy,
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