538 research outputs found
Synchrotron and Inverse Compton Constraints on Lorentz Violations for Electrons
We present a method for constraining Lorentz violation in the electron
sector, based on observations of the photons emitted by high-energy
astrophysical sources. The most important Lorentz-violating operators at the
relevant energies are parameterized by a tensor c^{nu mu) with nine independent
components. If c is nonvanishing, then there may be either a maximum electron
velocity less than the speed of light or a maximum energy for subluminal
electrons; both these quantities will generally depend on the direction of an
electron's motion. From synchrotron radiation, we may infer a lower bound on
the maximum velocity, and from inverse Compton emission, a lower bound on the
maximum subluminal energy. With observational data for both these types of
emission from multiple celestial sources, we may then place bounds on all nine
of the coefficients that make up c. The most stringent bound, on a certain
combination of the coefficients, is at the 6 x 10^(-20) level, and bounds on
the coefficients individually range from the 7 x 10^(-15) level to the 2 x
10^(-17) level. For most of the coefficients, these are the most precise bounds
available, and with newly available data, we can already improve over previous
bounds obtained by the same methods.Comment: 28 page
Global survey of star clusters in the Milky Way IV. 63 new open clusters detected by proper motions
AIMS: In their 1st extension to the Milky Way Star Clusters (MWSC) survey,
Schmeja et al. applied photometric filters to the 2MASS to find new cluster
candidates that were subsequently confirmed or rejected by the MWSC pipeline.
To further extend the MWSC census, we aimed at discovering new clusters by
conducting an almost global search in proper motion catalogues as a starting
point. METHODS: We first selected high-quality samples from the PPMXL and UCAC4
for comparison and verification of the proper motions. For 441 circular proper
motion bins (radius 15 mas/yr) within 50 mas/yr, the sky outside a thin
Galactic plane zone (5) was binned in small areas ('sky
pixels') of 0.250.25 deg. Sky pixels with enhanced numbers of stars
with a certain common proper motion in both catalogues were considered as
cluster candidates. After visual inspection of the sky images, we built an
automated procedure that combined these representations of the sky for
neighbouring proper motion subsamples after a background correction. RESULTS:
About half of our 692 candidates overlapped with known clusters (46 globular
and 68 open clusters in the Galaxy, about 150 known clusters of galaxies) or
the Magellanic Clouds. About 10% of our candidates turned out to be 63 new open
clusters confirmed by the MWSC pipeline. They occupy predominantly the two
inner Galactic quadrants and have apparent sizes and numbers of high-probable
members slightly larger than those of the typically small MWSC clusters,
whereas their other parameters (ages, distances, tidal radii) fall in the
typical ranges. As our search aimed at finding compact clusters, we did not
find new very nearby (extended) clusters. (abridged)Comment: 14 pages, 14 figures, accepted for publication in Astronomy and
Astrophysic
Global survey of star clusters in the Milky Way II. The catalogue of basic parameters
Although they are the main constituents of the Galactic disk population, for
half of the open clusters in the Milky Way reported in the literature nothing
is known except the raw position and an approximate size. The main goal of this
study is to determine a full set of uniform spatial, structural, kinematic, and
astrophysical parameters for as many known open clusters as possible. On the
basis of stellar data from PPMXL and 2MASS, we used a dedicated data-processing
pipeline to determine kinematic and photometric membership probabilities for
stars in a cluster region. For an input list of 3784 targets from the
literature, we confirm that 3006 are real objects, the vast majority of them
are open clusters, but associations and globular clusters are also present. For
each confirmed object we determined the exact position of the cluster centre,
the apparent size, proper motion, distance, colour excess, and age. For about
1500 clusters, these basic astrophysical parameters have been determined for
the first time. For the bulk of the clusters we also derived the tidal radius.
We estimated additionally average radial velocities for more than 30% of the
confirmed clusters. The present sample (called MWSC) reaches both the central
parts of the Milky Way and its outer regions. It is almost complete up to 1.8
kpc from the Sun and also covers neighbouring spiral arms. However, for a small
subset of the oldest open clusters () we found some evidence
of incompleteness within about 1 kpc from the Sun.Comment: 8 pages, 5 figures, accepted for publication in Astronomy and
Astrophysic
Why Simple Stellar Population models do not reproduce the colours of Galactic open clusters
(...) We search for an explanation of the disagreement between the observed
integrated colours of 650 local Galactic clusters and the theoretical colours
of present-day SSP models. We check the hypothesis that the systematic offsets
between observed and theoretical colours, which are and
, are caused by neglecting the discrete nature of the
underlying mass function. Using Monte Carlo simulations, we construct
artificial clusters of coeval stars taken from a mass distribution defined by
an Salpeter initial mass function (IMF) and compare them with corresponding
"continuous-IMF" SSP models. If the discreteness of the IMF is taken into
account, the model fits the observations perfectly and is able to explain
naturally a number of red "outliers" observed in the empirical colour-age
relation. We find that the \textit{systematic} offset between the continuous-
and discrete-IMF colours reaches its maximum of about 0.5 in for a
cluster mass at ages , and diminishes
substantially but not completely to about one hundredth of a magnitude at at cluster masses . At younger ages, it is still
present even in massive clusters, and for it is
larger than 0.1 mag in . Only for very massive clusters () with ages is the offset small (of the order of 0.04
mag) and smaller than the typical observational error of colours of
extragalactic clusters.Comment: 4 pages, 3 figures, accepted for publication in Astronomy and
Astrophysics Letters, revised version after language editing and with an
additional reference to Cervino and Luridiana (2004
Unmasking the Active Galactic Nucleus in PKS J2310-437
PKS J2310-437 is an AGN with bright X-ray emission relative to its weak radio
emission and optical continuum. It is believed that its jet lies far enough
from the line of sight that it is not highly relativistically beamed. It thus
provides an extreme test of AGN models. We present new observations aimed at
refining the measurement of the source's properties. In optical photometry with
the NTT we measure a central excess with relatively steep spectrum lying above
the bright elliptical galaxy emission, and we associate the excess wholly or in
part with the AGN. A new full-track radio observation with the ATCA finds that
the core 8.64GHz emission has varied by about 20 per cent over 38 months, and
improves the mapping of the weak jet. With Chandra we measure a
well-constrained power-law spectral index for the X-ray core, uncontaminated by
extended emission from the cluster environment, with a negligible level of
intrinsic absorption. Weak X-ray emission from the resolved radio jet is also
measured. Our analysis suggests that the optical continuum in this radio galaxy
has varied by at least a factor of four over a timescale of about two years,
something that should be testable with further observations. We conclude that
the most likely explanation for the bright central X-ray emission is
synchrotron radiation from high-energy electrons.Comment: 7 pages, 12 figure
Open star clusters in the Milky Way - Comparison of photometric and trigonometric distance scales based on Gaia TGAS data
The global survey of star clusters in the Milky Way (MWSC) is a comprehensive
list of 3061 objects that provides, among other parameters, distances to
clusters based on isochrone fitting. The Tycho-Gaia Astrometric Solution (TGAS)
catalogue, which is a part of Gaia data release 1 (Gaia DR1), delivers accurate
trigonometric parallax measurements for more than 2 million stars, including
those in star clusters. We compare the open cluster photometric distance scale
with the measurements given by the trigonometric parallaxes from TGAS to
evaluate the consistency between these values. The average parallaxes of
probable cluster members available in TGAS provide the trigonometric distance
scale of open clusters, while the photometric scale is given by the distances
published in the MWSC. Sixty-four clusters are suited for comparison as they
have more than 16 probable members with parallax measurements in TGAS. We
computed the average parallaxes of the probable members and compared these to
the photometric parallaxes derived within the MWSC. We find a good agreement
between the trigonometric TGAS-based and the photometric MWSC-based distance
scales of open clusters, which for distances less than 2.3 kpc coincide at a
level of about 0.1 mas with no dependence on the distance. If at all, there is
a slight systematic offset along the Galactic equator between and
galactic longitude.Comment: Accepted for publication as a Letter in Astronomy and Astrophysics, 4
pages, 4 figure
Detection of Cherenkov light from air showers with Geiger-APDs
We have detected Cherenkov light from air showers with Geiger-mode APDs
(G-APDs). G-APDs are novel semiconductor photon-detectors, which offer several
advantages compared to conventional photomultiplier tubes in the field of
ground-based gamma-ray astronomy. In a field test with the MAGIC telescope we
have tested the efficiency of a G-APD / light catcher setup to detect Cherenkov
light from air showers. We estimate a detection efficiency, which is 60% higher
than the efficiency of a MAGIC camera pixel. Ambient temperature dark count
rates of the tested G-APDs are below the rates of the night sky light
background. According to these recent tests G-APDs promise a major progress in
ground-based gamma-ray astronomy.Comment: 4 pages, 5 figures, to appear in the proceedings of the 30th
International Cosmic Ray Conference, Merida, July 200
Global survey of star clusters in the Milky Way: III. 139 new open clusters at high Galactic latitudes
Context. An earlier analysis of the Milky Way Star Cluster (MWSC) catalogue revealed an apparent lack of old (t Ăą?€ 1 Gyr) open clusters in the solar neighbourhood (d Ăą?„ 1 kpc). Aims. To fill this gap we undertook a search for hitherto unknown star clusters, assuming that the missing old clusters reside at high Galactic latitudes | b | > 20Ă°. Methods. We were looking for stellar density enhancements using a star count algorithm on the 2MASS point source catalogue. To increase the contrast between potential clusters and the field, we applied filters in colour-magnitude space according to typical colour-magnitude diagrams of nearby old open clusters. The subsequent comparison with lists of known objects allowed us to select thus far unknown cluster candidates. For verification they were processed with the standard pipeline used within the MWSC survey for computing cluster membership probabilities and for determining structural, kinematic, and astrophysical parameters. Results. In total we discovered 782 density enhancements, 524 of which were classified as real objects. Among them 139 are new open clusters with ages 8.3 < log (t [yr]) < 9.7, distances d< 3 kpc, and distances from the Galactic plane 0.3 <Z< 1 kpc. This new sample has increased the total number of known high latitude open clusters by about 150%. Nevertheless, we still observe a lack of older nearby clusters up to 1 kpc from the Sun. This volume is expected to still contain about 60 unknown clusters that probably escaped our detection algorithm, which fails to detect sparse overdensities with large angular size
Global survey of star clusters in the Milky Way I. The pipeline and fundamental parameters in the second quadrant
Aims: On the basis of the PPMXL star catalogue we performed a survey of star
clusters in the second quadrant of the Milky Way. Methods: From the PPMXL
catalogue of positions and proper motions we took the subset of stars with
near-infrared photometry from 2MASS and added the remaining 2MASS stars without
proper motions (called 2MAst, i.e. 2MASS with astrometry). We developed a
data-processing pipeline including interactive human control of a standardised
set of multi-dimensional diagrams to determine kinematic and photometric
membership probabilities for stars in a cluster region. The pipeline
simultaneously produced the astrophysical parameters of a cluster. From
literature we compiled a target list of presently known open and globular
clusters, cluster candidates, associations, and moving groups. From established
member stars we derived spatial parameters (coordinates of centres and radii of
the main morphological parts of clusters) and cluster kinematics (average
proper motions and sometimes radial velocities). For distance, reddening, and
age determination we used specific sets of theoretical isochrones. Tidal
parameters were obtained by a fit of three-parameter King profiles to the
observed density distributions of members. Results: We investigated all 871
objects in the 2nd Galactic quadrant, of which we successfully treated 642 open
clusters, 2 globular clusters, and 8 stellar associations. The remaining 219
objects (24%) were recognised by us to be nonexistent clusters, duplicate
entries, or clusters too faint for 2MAst. We found that our sample is complete
in the 2nd quadrant up to a distance of 2 kpc, where the average surface
density is 94 clusters per kpc. Compared with literature values we found
good agreement in spatial and kinematic data, as well as for optical distances
and reddening. Small, but systematic offsets were detected in the age
determination.Comment: published in Astronomy and Astrophysics, 10 pages, 7 figures (plus 3
pages of appendices incl. 2 more figures), catalogues will be available at
the CDS, all the machine-readable online data described in appendices A, B,
and C are also available at:
http://www.aip.de/People/rdscholz/kharchenko_etal_2012
Predicting delay factors when chipping wood at forest roadside landings
Chipping of bulky biomass assortments at roadside landings is a common and costly step in the biomass-to-energy supply chain. This operation normally involves one chipping unit and one or several transport trucks working together for simultaneous chipping and chip transport to a terminal or end user. Reducing the delay factors in these operations is a relevant ambition for lowering supply costs. A method to estimate organizational delay based on: (1) the capacity ratio between the transport and the chipper, (2) the use of buffer storage, and (3) the number of transport units involved is suggested here. Other delays will also be present, and some of these may relate to the working conditions at the landing. A method to set a landing functionality index based on characteristics of the forest landing is also suggested. A total of 14 roadside chipping operations were assessed and the operators were interviewed to address the impact of machinery configuration and landing characteristics on machine utilization. At most sites, the chipper was the more productive part, and the chipper utilization was to a large extent limited by organizational delay. Still the utilization of the transport units varied between 37 and 97%, of which some 36% of the variation was explained by the landing functionality index. Knowledge from the work presented here should be a good starting point for improving biomass supply planning and supply chain configuration.acceptedVersio
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