805 research outputs found
Star clusters and the structure of the ISM. Tunnels and wakes in giant extragalactic HII regions
Several structures have been discovered embedded in regions of recent or
ongoing star formation, which point to the importance of the interaction
between fast moving wind-blowing stars and their environment. Using
hydrodynamic simulations, we investigate the passage through the interstellar
medium of a supersonic stellar wind source, and show how it can naturally lead
to the formation of tubes, channels and swamps of globules as interfaces are
crossed. The results are in excellent agreement with observation of 30 Doradus.Comment: 12 pages + 5 figures (GIF format) - Accepted for pub. in Astrophys.
J. Letter
Extension of Tycho catalog for low-extinction windows in the galactic bulge
We present in this work secondary catalogs up to based on
the Tycho reference frame (ESA, 1997) for 12 selected low-extinction fields
towards the galactic bulge. The observations have been performed with the
Askania-Zeiss Meridian Circle equiped with a CCD camera, located at the
Abrah\~ao de Moraes Observatory (Valinhos, Brazil) and operated by the
Institute of Astronomy and Geophysics, S\~ao Paulo University. The presented
catalog, though not complete, has been designed to help in intensive search
programmes (e.g. microlensing and variable searches) and therefore the selected
standards have a high astrometric and photometric ( band, approximately)
quality. The mean precisions obtained were in , 0.013'' in
, 0.030 for the standard deviation in magnitude and 0.0042 for the
magnitude when weighted with the error bars in each night (in the mean, 42
stars for the catalog of each window). Tables B.1 to B.12 are also available in
eletronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr
(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.Comment: 16 pages, 6 figures, A&A Latex style. Published in A&A
On the influence of statistics on the determination of the mean value of the depth of shower maximum for ultra high energy cosmic ray showers
The chemical composition of ultra high energy cosmic rays is still uncertain.
The latest results obtained by the Pierre Auger Observatory and the HiRes
Collaboration, concerning the measurement of the mean value and the
fluctuations of the atmospheric depth at which the showers reach the maximum
development, Xmax, are inconsistent. From comparison with air shower
simulations it can be seen that, while the Auger data may be interpreted as a
gradual transition to heavy nuclei for energies larger than ~ 2-3x10^18 eV, the
HiRes data are consistent with a composition dominated by protons. In Ref. [1]
it is suggested that a possible explanation of the observed deviation of the
mean value of Xmax from the proton expectation, observed by Auger, could
originate in a statistical bias arising from the approximated exponential shape
of the Xmax distribution, combined with the decrease of the number of events as
a function of primary energy. In this paper we consider a better description of
the Xmax distribution and show that the possible bias in the Auger data is at
least one order of magnitude smaller than the one obtained when assuming an
exponential distribution. Therefore, we conclude that the deviation of the
Auger data from the proton expectation is unlikely explained by such
statistical effect.Comment: To be published in Journal of Physics G: Nuclear and Particle Physic
Stellar variability in low-extinction regions towards the Galactic Bulge
Intensive monitoring of low-extinction windows towards the galactic bulge has
provided in the last years valuable information for studies about the dynamics,
kinematics and formation history of this part of the galaxy, mainly by
characterizing the bulge stellar populations (Paczy\'nski, 1996). Since 1997,
we have been conducting an intensive photometric-astrometric survey of the
galactic bulge, with the monitoring of about 120000 stars in 12 windows
uniformly distributed in galactic latitude and longitude (Blanco & Terndrup,
1989 e Blanco, 1988) never before submitted to this kind of survey. For this
purpose, we have used the IAG/USP CCD Meridian Circle of the Abrah\~ao de
Moraes Observatory. The main objective of this work is the identification and
classification of variable objects. In this work we present the set up and
development of the necessary tools for a project like this and the posterior
analysis of our data. We briefly describe the construction of a program to
organize and detect variables among the observed stars, including real time
alerts (for variations greater than 0.3 magnitudes). The preliminary analysis
after the processing of 76 nights of observation yielded 479 variable stars,
from which 96.7 % of them are new. We discuss the preliminary classification of
this variables, based on: a) the observed amplitude of variation; b) the shape
of light curve; c) the expected variable classes among our data and d) the
calculated periods, whenever possible. Finally, we discuss the future
perspectives for the project and for the applications and analysis of the
discovered variable stars.Comment: 10 pages, 14 figures. Accepted by A&A
Fast Synthetic Dataset for Kitchen Object Segmentation in Deep Learning
Object recognition has been widely investigated in computer vision for many years. Currently, this process is carried out through neural networks, but there are very few public datasets available with mask and class labels of the objects for the training process in usual applications. In this paper, we address the problem of fast generation of synthetic datasets to train neural models because creating a handcraft labeled dataset with object segmentation is a very tedious and time-consuming task. We propose an efficient method to generate a synthetic labeled dataset that adequately combines background images with foreground segmented objects. The synthetic images can be created automatically with random positioning of the objects or, alternatively, the method can produce realistic images by keeping the realism in the scales and positions of the objects. Then, we employ Mask-RCNN deep learning model, to detect and segment classes of kitchen objects using images. In the experimental evaluation, we study both synthetic datasets, automatic or realistic, and we compare the results. We analyze the performance with the most widely used indexes and check that the realistic synthetic dataset, quickly created through our method, can provide competitive results and accurately classify the different objects
Magnetically Driven Outflows in a Starburst Environment
We here investigate the possibility that the observed collimated outflows in
luminous infrared galaxies (LIGs) and some Seyfert galaxies can be produced in
a starburst (SB) environment. A nuclear disk can be quickly produced by gas
infall during star formation in a rotating, stellar cluster. We find that
massive nuclear SBs with core disk masses M_d \sim 10^8 - 10^9 M_{\odot}, and
supernova rates \nu_{SN} \simeq 5 \times 10^{-3} - 2 yr^{-1} (which are
consistent with the \nu_{SN} values inferred from the observed non-thermal
radio power in source candidates) may inject kinetic energies which are high
enough to blow out directed flows from the accreting disk surface, within the
SB lifetimes. In our models, the acceleration and collimation of the nuclear
outflow are provided by magnetic fields anchored into the rotating SB-disk. The
emerging outflow carries a kinetic power that is only a small fraction (a few
percent) of the supernovae energy rate produced in the SB. Based on conditions
determined from observed outflows and disks, we find that moderate disk
magnetic fields (\gtrsim 8 \times 10^{-4} G) are able to accelerate the
outflows up to the observed terminal velocities (\lesssim few 100 km s^{-1} in
the case of the Seyfert galaxies, and \sim 400 - 950 km s^{-1} in the case of
the LIGs). The outflow is produced within a wind zone in the disk of radius
\lesssim 100 pc in the LIGs, and \lesssim 10 pc in the Seyferts, with wind mass
loss to disk accretion rate ratios \dot M_w /\dot M_d \gtrsim 0.1 (where \dot
M_d \sim 100 M_{\odot} yr^{-1}). The observation of rotating nuclear disks of
gas within few 100 pc scales in source candidates like the LIG Arp 220, and
magnetized outflows provide observational support for the picture drawn here.Comment: 31 pages, Latex file, 1 Figure, accepted for publication in the
Astrophys. Journa
The AGASA/SUGAR Anisotropies and TeV Gamma Rays from the Galactic Center: A Possible Signature of Extremely High-energy Neutrons
Recent analysis of data sets from two extensive air shower cosmic ray
detectors shows tantalizing evidence of an anisotropic overabundance of cosmic
rays towards the Galactic Center (GC) that ``turns on'' around eV. We
demonstrate that the anisotropy could be due to neutrons created at the
Galactic Center through charge-exchange in proton-proton collisions, where the
incident, high energy protons obey an power law associated with
acceleration at a strong shock. We show that the normalization supplied by the
gamma-ray signal from EGRET GC source 3EG J1746-2851 -- ascribed to pp induced
neutral pion decay at GeV energies -- together with a very reasonable spectral
index of 2.2, predicts a neutron flux at eV fully consistent
with the extremely high energy cosmic ray data. Likewise, the normalization
supplied by the very recent GC data from the HESS air-Cerenkov telescope at
\~TeV energies is almost equally-well compatible with the eV
cosmic ray data. Interestingly, however, the EGRET and HESS data appear to be
themselves incompatible. We consider the implications of this discrepancy. We
discuss why the Galactic Center environment can allow diffusive shock
acceleration at strong shocks up to energies approaching the ankle in the
cosmic ray spectrum. Finally, we argue that the shock acceleration may be
occuring in the shell of Sagittarius A East, an unusual supernova remnant
located very close to the Galactic Center. If this connection between the
anisotropy and Sagittarius A East could be firmly established it would be the
first direct evidence for a particular Galactic source of cosmic rays up to
energies near the ankle.Comment: 57 pages, 2 figure
Nebular abundances of southern symbiotic stars
We have calculated relative elemental abundances for a sample of 43 symbiotic
stars. Helium abundances and the relative elemental abundances N/O, Ne/O, Ar/O
were derived from new spectra collected in the optical range through low
dispersion spectroscopy. The He ionic abundances were derived taking into
account self-absorption effects in Balmer lines. We found that the symbiotic
stars in the galactic bulge have heavy element abundances showing the same wide
distribution as other bulge objects. In the galactic disk, the symbiotic stars
follow the abundance gradient as derived from different kinds of objects.Comment: 12 pages, 6 figures, A&A - accepte
The clustering of ultra-high energy cosmic rays and their sources
The sky distribution of cosmic rays with energies above the 'GZK cutoff'
holds important clues to their origin. The AGASA data, although consistent with
isotropy, shows evidence for small-angle clustering, and it has been argued
that such clusters are aligned with BL Lacertae objects, implicating these as
sources. It has also been suggested that clusters can arise if the cosmic rays
come from the decays of very massive relic particles in the Galactic halo, due
to the expected clumping of cold dark matter. We examine these claims and show
that both are in fact not justified.Comment: 13 pages, 8 figures, version in press at Phys. Rev.
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