4,247 research outputs found
Source extraction and photometry for the far-infrared and sub-millimeter continuum in the presence of complex backgrounds
(Abridged) We present a new method for detecting and measuring compact
sources in conditions of intense, and highly variable, fore/background. While
all most commonly used packages carry out the source detection over the signal
image, our proposed method builds from the measured image a "curvature" image
by double-differentiation in four different directions. In this way point-like
as well as resolved, yet relatively compact, objects are easily revealed while
the slower varying fore/background is greatly diminished. Candidate sources are
then identified by looking for pixels where the curvature exceeds, in absolute
terms, a given threshold; the methodology easily allows us to pinpoint
breakpoints in the source brightness profile and then derive reliable guesses
for the sources extent. Identified peaks are fit with 2D elliptical Gaussians
plus an underlying planar inclined plateau, with mild constraints on size and
orientation. Mutually contaminating sources are fit with multiple Gaussians
simultaneously using flexible constraints. We ran our method on simulated
large-scale fields with 1000 sources of different peak flux overlaid on a
realistic realization of diffuse background. We find detection rates in excess
of 90% for sources with peak fluxes above the 3-sigma signal noise limit; for
about 80% of the sources the recovered peak fluxes are within 30% of their
input values.Comment: Accepted on A&
The luminosity function of cluster galaxies. II. Data reduction procedures applied to the cluster Abell 496
We initiated a large project aimed to estimate the Luminosity Function of
galaxies in clusters and to evaluate its relation to cluster morphology. With
this paper we deem necessary to outline the general procedures of the data
reduction and details of the data analysis. The cluster sample includes the
brightest southern ROSAT all-sky survey clusters with z < 0.1. These have been
observed in three colours g, r, i, and mapped up to a few core radii using a
mosaic of CCD frames. E/S0 galaxies in the cluster core are singled out both by
morphology (for the brightest galaxies), and by colour. The details of the data
reduction procedure are illustrated via the analysis of the cluster Abell 496,
which has been used as a pilot cluster for the whole program. The related
photometric catalogue consists of 2355 objects. The limiting magnitudes (the
reference Surface Brightness is given in parenthesis) in the various colours
are respectively g(25.5) = 24.14, r(25.5) = 24.46, i(25.0) = 23.75$. These
correspond to the limiting absolute magnitudes -12.28, -11.96 and -12.67
(H_0=50 km/sec/Mpc).Comment: 17 pages, 19 ps figures, aa.cl
A 100 pc Elliptical and Twisted Ring of Cold and Dense Molecular Clouds Revealed by Herschel Around the Galactic Center
Thermal images of cold dust in the Central Molecular Zone of the Milky Way, obtained with the far-infrared cameras on board the Herschel satellite, reveal a ~3 Ă 10^7 M_â ring of dense and cold clouds orbiting the Galactic center. Using a simple toy model, an elliptical shape having semi-major axes of 100 and 60 pc is deduced. The major axis of this 100 pc ring is inclined by about 40° with respect to the plane of the sky and is oriented perpendicular to the major axes of the Galactic Bar. The 100 pc ring appears to trace the system of stable x_2 orbits predicted for the barred Galactic potential. Sgr Aâ is displaced with respect to the geometrical center of symmetry of the ring. The ring is twisted and its morphology suggests a flattening ratio of 2 for the Galactic potential, which is in good agreement with the bulge flattening ratio derived from the 2MASS data
Bias-dependent Contact Resistance in Rubrene Single-Crystal Field-Effect Transistors
We report a systematic study of the bias-dependent contact resistance in
rubrene single-crystal field-effect transistors with Ni, Co, Cu, Au, and Pt
electrodes. We show that the reproducibility in the values of contact
resistance strongly depends on the metal, ranging from a factor of two for Ni
to more than three orders of magnitude for Au. Surprisingly, FETs with Ni, Co,
and Cu contacts exhibits an unexpected reproducibility of the bias-dependent
differential conductance of the contacts, once this has been normalized to the
value measured at zero bias. This reproducibility may enable the study of
microscopic carrier injection processes into organic semiconductors.Comment: 4 pages, 4 figure
Not a galaxy: IRAS 04186+5143, a new young stellar cluster in the outer Galaxy
We report the discovery of a new young stellar cluster in the outer Galaxy
located at the position of an IRAS PSC source that has been previously
mis-identified as an external galaxy. The cluster is seen in our near-infrared
imaging towards IRAS 04186+5143 and in archive Spitzer images confirming the
young stellar nature of the sources detected. There is also evidence of
sub-clustering seen in the spatial distributions of young stars and of gas and
dust.
Near- and mid-infrared photometry indicates that the stars exhibit colours
compatible with reddening by interstellar and circumstellar dust and are likely
to be low- and intermediate-mass YSOs with a large proportion of Class I YSOs.
Ammonia and CO lines were detected, with the CO emission well centred near
the position of the richest part of the cluster. The velocity of the CO and
NH lines indicates that the gas is Galactic and located at a distance of
about 5.5 kpc, in the outer Galaxy.
Herschel data of this region characterise the dust environment of this
molecular cloud core where the young cluster is embedded. We derive masses,
luminosities and temperatures of the molecular clumps where the young stars
reside and discuss their evolutionary stages.Comment: 14 pages, 15 figure
The existence of a critical length scale in regularised friction
We study a regularisation of Coulomb's friction law on the propagation of
local slip at an interface between a deformable and a rigid solid. This
regularisation, which was proposed based on experimental observations, smooths
the effect of a sudden jump in the contact pressure over a characteristic
length scale. We apply it in numerical simulations in order to analyse its
influence on the behaviour of local slip. We first show that mesh convergence
in dynamic simulations is achieved without any numerical damping in the bulk
and draw a convergence map with respect to the characteristic length of the
friction regularisation. By varying this length scale on the example of a given
slip event, we observe that there is a critical length below which the friction
regularisation does not affect anymore the propagation of the interface
rupture. A spectral analysis of the regularisation on a periodic variation of
Coulomb's friction is conducted to confirm the existence of this critical
length. The results indicate that if the characteristic length of the friction
regularisation is smaller than the critical length, a slip event behaves as if
it was governed by Coulomb's law. We therefore propose that there is a domain
of influence of the friction regularisation depending on its characteristic
length and on the frequency content of the local slip event. A byproduct of the
analysis is related to the existence of a physical length scale characterising
a given frictional interface. We establish that the experimental determination
of this interface property may be achieved by experimentally monitoring slip
pulses whose frequency content is rich enough.Comment: 21 pages, 7 figure
Shear Viscosity of Quark Matter
We consider the shear viscosity of a system of quarks and its ratio to the
entropy density above the critical temperature for deconfinement. Both
quantities are derived and computed for different modeling of the quark
self-energy, also allowing for a temperature dependence of the effective mass
and width. The behaviour of the viscosity and the entropy density is argued in
terms of the strength of the coupling and of the main characteristics of the
quark self-energy. A comparison with existing results is also discussed.Comment: 15 pages, 4 figure
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
Two massive star-forming regions at early evolutionary stages
We report sensitive ATCA radio-continuum observations toward IRAS 15596-5301
and 16272-4837, two luminous objects (> 2x10^4 Lsun) thought to represent
massive star-forming regions in early stages of evolution (due to previously
undetected radio emission at the 1-sigma level of 2 mJy per beam). Also
reported are 1.2-millimeter continuum and a series of molecular-line
observations made with the SEST telescope. For IRAS 15596-5301, the
observations reveal the presence of three distinct compact radio-continuum
sources associated with a dense molecular core. We suggest that this core
contains a cluster of B stars which are exciting compact HII regions that are
in pressure equilibrium with the dense molecular surroundings. No radio
continuum emission was detected from IRAS 16272-4837 (3-sigma limit of 0.2
mJy). However, a dense molecular core has been detected. The high luminosity
and lack of radio emission from this massive core suggests that it hosts an
embedded young massive protostar that is still undergoing an intense accretion
phase. This scenario is supported by the observed characteristics of the line
profiles and the presence of a bipolar outflow detected from observations of
the SiO emission. We suggest that IRAS 16272-4837 is a bona fide massive star-
forming region in a very early evolutionary stage, being the precursor of an
ultra compact HII region.Comment: 25 pages, 9 figures, accepted for publication in The Astrophysical
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