14,833 research outputs found
Star-Formation in Low Radio Luminosity AGN from the Sloan Digital Sky Survey
We investigate faint radio emission from low- to high-luminosity Active
Galactic Nuclei (AGN) selected from the Sloan Digital Sky Survey (SDSS). Their
radio properties are inferred by co-adding large ensembles of radio image
cut-outs from the FIRST survey, as almost all of the sources are individually
undetected. We correlate the median radio flux densities against a range of
other sample properties, including median values for redshift, [OIII]
luminosity, emission line ratios, and the strength of the 4000A break. We
detect a strong trend for sources that are actively undergoing star-formation
to have excess radio emission beyond the ~10^28 ergs/s/Hz level found for
sources without any discernible star-formation. Furthermore, this additional
radio emission correlates well with the strength of the 4000A break in the
optical spectrum, and may be used to assess the age of the star-forming
component. We examine two subsamples, one containing the systems with emission
line ratios most like star-forming systems, and one with the sources that have
characteristic AGN ratios. This division also separates the mechanism
responsible for the radio emission (star-formation vs. AGN). For both cases we
find a strong, almost identical, correlation between [OIII] and radio
luminosity, with the AGN sample extending toward lower, and the star-formation
sample toward higher luminosities. A clearer separation between the two
subsamples is seen as function of the central velocity dispersion of the host
galaxy. For systems with similar redshifts and velocity dispersions, the
star-formation subsample is brighter than the AGN in the radio by an order of
magnitude. This underlines the notion that the radio emission in star-forming
systems can dominate the emission associated with the AGN.Comment: Accepted for publication in Astronomical Journal; 15 pages, 8 color
figure
Matthew D. Holm, percussion
Martine Wesley-SmithJacob DruckmanChristopher DeaneBob Becke
Three-body Thomas-Ehrman shifts of analog states of Ne and N
The lowest-lying states of the Borromean nucleus Ne (O+ +
) and its mirror nucleus N (N+ + ) are compared by using
the hyperspheric adiabatic expansion. Three-body resonances are computed by use
of the complex scaling method. The measured size of O and the low-lying
resonances of F (O+) are first used as constraints to
determine both central and spin-dependent two-body interactions. The
interaction obtained reproduces relatively accurately both experimental
three-body spectra. The Thomas-Ehrman shifts, involving excitation energy
differences, are computed and found to be less than 3% of the total Coulomb
energy shift for all states.Comment: 9 pages, 3 postscript figures, revtex style. To be published in Phys.
Rev.
The evolved circumbinary disk of AC Her: a radiative transfer, interferometric and mineralogical study
We aim to constrain the structure of the circumstellar material around the
post-AGB binary and RV Tauri pulsator AC Her. We want to constrain the spatial
distribution of the amorphous as well as of the crystalline dust. We present
very high-quality mid-IR interferometric data that were obtained with
MIDI/VLTI. We analyse the MIDI data and the full SED, using the MCMax radiative
transfer code, to find a good structure model of AC Her's circumbinary disk. We
include a grain size distribution and midplane settling of dust
self-consistently. The spatial distribution of crystalline forsterite in the
disk is investigated with the mid-IR features, the 69~m band and the
11.3~m signatures in the interferometric data. All the data are well
fitted. The inclination and position angle of the disk are well determined at
i=50+-8 and PA=305+-10. We firmly establish that the inner disk radius is about
an order of magnitude larger than the dust sublimation radius. Significant
grain growth has occurred, with mm-sized grains being settled to the midplane
of the disk. A large dust mass is needed to fit the sub-mm fluxes. By assuming
{\alpha}=0.01, a good fit is obtained with a small grain size power law index
of 3.25, combined with a small gas/dust ratio <10. The resulting gas mass is
compatible with recent estimates employing direct gas diagnostics. The spatial
distribution of the forsterite is different from the amorphous dust, as more
warm forsterite is needed in the surface layers of the inner disk. The disk in
AC Her is very evolved, with its small gas/dust ratio and large inner hole.
Mid-IR interferometry offers unique constraints, complementary to mid-IR
features, for studying the mineralogy in disks. A better uv coverage is needed
to constrain in detail the distribution of the crystalline forsterite in AC
Her, but we find strong similarities with the protoplanetary disk HD100546.Comment: update with final version published in A&
A new limit on the Ultra-High-Energy Cosmic-Ray flux with the Westerbork Synthesis Radio Telescope
A particle cascade (shower) in a dielectric, for example as initiated by an
ultra-high energy cosmic ray, will have an excess of electrons which will emit
coherent \v{C}erenkov radiation, known as the Askaryan effect. In this work we
study the case in which such a particle shower occurs in a medium just below
its surface. We show, for the first time, that the radiation transmitted
through the surface is independent of the depth of the shower below the surface
when observed from far away, apart from trivial absorption effects. As a direct
application we use the recent results of the NuMoon project, where a limit on
the neutrino flux for energies above \,eV was set using the Westerbork
Synthesis Radio Telescope by measuring pulsed radio emission from the Moon, to
set a limit on the flux of ultra-high-energy cosmic rays.Comment: Accepted for publication in Phys. Rev.
Dust absorption and scattering in the silicon K-edge
The composition and properties of interstellar silicate dust are not well
understood. In X-rays, interstellar dust can be studied in detail by making use
of the fine structure features in the Si K-edge. The features in the Si K-edge
offer a range of possibilities to study silicon-bearing dust, such as
investigating the crystallinity, abundance, and the chemical composition along
a given line of sight. We present newly acquired laboratory measurements of the
silicon K-edge of several silicate-compounds that complement our measurements
from our earlier pilot study. The resulting dust extinction profiles serve as
templates for the interstellar extinction that we observe. The extinction
profiles were used to model the interstellar dust in the dense environments of
the Galaxy. The laboratory measurements, taken at the Soleil synchrotron
facility in Paris, were adapted for astrophysical data analysis and implemented
in the SPEX spectral fitting program. The models were used to fit the spectra
of nine low-mass X-ray binaries located in the Galactic center neighborhood in
order to determine the dust properties along those lines of sight. Most lines
of sight can be fit well by amorphous olivine. We also established upper limits
on the amount of crystalline material that the modeling allows. We obtained
values of the total silicon abundance, silicon dust abundance, and depletion
along each of the sightlines. We find a possible gradient of
dex/kpc for the total silicon abundance versus the Galactocentric distance. We
do not find a relation between the depletion and the extinction along the line
of sight.Comment: 18 pages, 16 figures. Accepted for publication in Astronomy and
Astrophysic
The problematically short superwind of OH/IR stars - Probing the outflow with the 69 {\mu}m spectral band of forsterite
Spectra of OH/IR stars show prominent spectral bands of crystalline olivine
(MgFeSiO). To learn more about the timescale of the
outflows of OH/IR stars, we study the spectral band of crystalline olivine at
69 {\mu}m. The 69 {\mu}m band is of interest because its width and peak
wavelength position are sensitive to the grain temperature and to the exact
composition of the crystalline olivine. With Herschel/PACS, we observed the 69
{\mu}m band in the outflow of 14 OH/IR stars. By comparing the crystalline
olivine features of our sample with those of model spectra, we determined the
size of the outflow and its crystalline olivine abundance.
The temperature indicated by the observed 69 {\mu}m bands can only be
reproduced by models with a geometrically compact superwind
( 2500 AU = 1400 R).This means that the superwind
started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The
small amount of mass lost in one superwind and the high progenitor mass of the
OH/IR stars introduce a mass loss and thus evolutionary problem for these
objects, which has not yet been understood.Comment: Accepted by A&
Investigating the interstellar dust through the Fe K-edge
The chemical and physical properties of interstellar dust in the densest
regions of the Galaxy are still not well understood. X-rays provide a powerful
probe since they can penetrate gas and dust over a wide range of column
densities (up to ). The interaction (scattering and
absorption) with the medium imprints spectral signatures that reflect the
individual atoms which constitute the gas, molecule, or solid. In this work we
investigate the ability of high resolution X-ray spectroscopy to probe the
properties of cosmic grains containing iron. Although iron is heavily depleted
into interstellar dust, the nature of the Fe-bearing grains is still largely
uncertain. In our analysis we use iron K-edge synchrotron data of minerals
likely present in the ISM dust taken at the European Synchrotron Radiation
Facility. We explore the prospects of determining the chemical composition and
the size of astrophysical dust in the Galactic centre and in molecular clouds
with future X-ray missions. The energy resolution and the effective area of the
present X-ray telescopes are not sufficient to detect and study the Fe K-edge,
even for bright X-ray sources. From the analysis of the extinction cross
sections of our dust models implemented in the spectral fitting program SPEX,
the Fe K-edge is promising for investigating both the chemistry and the size
distribution of the interstellar dust. We find that the chemical composition
regulates the X-ray absorption fine structures in the post edge region, whereas
the scattering feature in the pre-edge is sensitive to the mean grain size.
Finally, we note that the Fe K-edge is insensitive to other dust properties,
such as the porosity and the geometry of the dust.Comment: 11 pages, 10 figures. Accepted for publication in Astronomy and
Astrophysic
- …