2,019 research outputs found
The Fractal Dimension of Projected Clouds
The interstellar medium seems to have an underlying fractal structure which
can be characterized through its fractal dimension. However, interstellar
clouds are observed as projected two-dimensional images, and the projection of
a tri-dimensional fractal distorts its measured properties. Here we use
simulated fractal clouds to study the relationship between the tri-dimensional
fractal dimension (D_f) of modeled clouds and the dimension resulting from
their projected images. We analyze different fractal dimension estimators: the
correlation and mass dimensions of the clouds, and the perimeter-based
dimension of their boundaries (D_per). We find the functional forms relating
D_f with the projected fractal dimensions, as well as the dependence on the
image resolution, which allow to estimatethe "real" D_f value of a cloud from
its projection. The application of these results to Orion A indicates in a
self-consistent way that 2.5 < D_f < 2.7 for this molecular cloud, a value
higher than the result D_per+1 = 2.3 some times assumed in literature for
interstellar clouds.Comment: 27 pages, 13 figures, 1 table. Accepted for publication in ApJ. Minor
change
On the properties of fractal cloud complexes
We study the physical properties derived from interstellar cloud complexes
having a fractal structure. We first generate fractal clouds with a given
fractal dimension and associate each clump with a maximum in the resulting
density field. Then, we discuss the effect that different criteria for clump
selection has on the derived global properties. We calculate the masses, sizes
and average densities of the clumps as a function of the fractal dimension
(D_f) and the fraction of the total mass in the form of clumps (epsilon). In
general, clump mass does not fulfill a simple power law with size of the type
M_cl ~ (R_cl)**(gamma), instead the power changes, from gamma ~ 3 at small
sizes to gamma<3 at larger sizes. The number of clumps per logarithmic mass
interval can be fitted to a power law N_cl ~ (M_cl)**(-alpha_M) in the range of
relatively large masses, and the corresponding size distribution is N_cl ~
(R_cl)**(-alpha_R) at large sizes. When all the mass is forming clumps
(epsilon=1) we obtain that as D_f increases from 2 to 3 alpha_M increases from
~0.3 to ~0.6 and alpha_R increases from ~1.0 to ~2.1. Comparison with
observations suggests that D_f ~ 2.6 is roughly consistent with the average
properties of the ISM. On the other hand, as the fraction of mass in clumps
decreases (epsilon<1) alpha_M increases and alpha_R decreases. When only ~10%
of the complex mass is in the form of dense clumps we obtain alpha_M ~ 1.2 for
D_f=2.6 (not very different from the Salpeter value 1.35), suggesting this a
likely link between the stellar initial mass function and the internal
structure of molecular cloud complexes.Comment: 32 pages, 13 figures, 1 table. Accepted for publication in Ap
Fractal dimension of interstellar clouds: opacity and noise effects
There exists observational evidence that the interstellar medium has a
fractal structure in a wide range of spatial scales. The measurement of the
fractal dimension (Df) of interstellar clouds is a simple way to characterize
this fractal structure, but several factors, both intrinsic to the clouds and
to the observations, may contribute to affect the values obtained. In this work
we study the effects that opacity and noise have on the determination of Df. We
focus on two different fractal dimension estimators: the perimeter-area based
dimension (Dper) and the mass-size dimension (Dm). We first use simulated
fractal clouds to show that opacity does not affect the estimation of Dper.
However, Dm tends to increase as opacity increases and this estimator fails
when applied to optically thick regions. In addition, very noisy maps can
seriously affect the estimation of both Dper and Dm, decreasing the final
estimation of Df. We apply these methods to emission maps of Ophiuchus, Perseus
and Orion molecular clouds in different molecular lines and we obtain that the
fractal dimension is always in the range 2.6 < Df < 2.8 for these regions.
These results support the idea of a relatively high (> 2.3) average fractal
dimension for the interstellar medium, as traced by different chemical species.Comment: 17 pages including 6 figures and 1 table. Accepted for publication in
Ap
Ionized and neutral gas in the peculiar star/cluster complex in NGC 6946
The characteristics of ionized and HI gas in the peculiar star/cluster
complex in NGC 6946, obtained with the 6-m telescope (BTA) SAO RAS, the Gemini
North telescope, and the Westerbork Synthesis Radio Telescope (WSRT), are
presented. The complex is unusual as hosting a super star cluster, the most
massive known in an apparently non-interacting giant galaxy. It contains a
number of smaller clusters and is bordered by a sharp C-shaped rim. We found
that the complex is additionally unusual in having peculiar gas kinematics. The
velocity field of the ionized gas reveals a deep oval minimum, ~300 pc in size,
centered 7" east of the supercluster. The Vr of the ionized gas in the dip
center is 100 km/s lower than in its surroundings, and emission lines within
the dip appear to be shock excited. This dip is near the center of an HI hole
and a semi-ring of HII regions. The HI (and less certainly, HII) velocity
fields reveal expansion, with the velocity reaching ~30 km/s at a distance
about 300 pc from the center of expansion, which is near the deep minimum
position. The super star cluster is at the western rim of the minimum. The
sharp western rim of the whole complex is plausibly a manifestation of a
regular dust arc along the complex edge. Different hypotheses about the complex
and the Vr depression origins are discussed, including a HVC/dark mini-halo
impact, a BCD galaxy merging, and a gas outflow due to release of energy from
the supercluster stars.Comment: MN RAS, accepte
First report of Tomato torrado virus on weed hosts in Spain
Alfaro Fernández, AO.; Córdoba-Sellés, C.; Cebrián, M.; Herrera-Vásquez, J.; Sanchez Navarro, JA.; Juárez, M.; Espino, A.... (2088). First report of Tomato torrado virus on weed hosts in Spain. Plant Disease. 92(5):831-831. https://doi.org/10.1094/pdis-92-5-0831b83183192
Probing quantum gravity using photons from a flare of the active galactic nucleus Markarian 501 observed by the MAGIC telescope
We analyze the timing of photons observed by the MAGIC telescope during a
flare of the active galactic nucleus Mkn 501 for a possible correlation with
energy, as suggested by some models of quantum gravity (QG), which predict a
vacuum refractive index \simeq 1 + (E/M_{QGn})^n, n = 1,2. Parametrizing the
delay between gamma-rays of different energies as \Delta t =\pm\tau_l E or
\Delta t =\pm\tau_q E^2, we find \tau_l=(0.030\pm0.012) s/GeV at the 2.5-sigma
level, and \tau_q=(3.71\pm2.57)x10^{-6} s/GeV^2, respectively. We use these
results to establish lower limits M_{QG1} > 0.21x10^{18} GeV and M_{QG2} >
0.26x10^{11} GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC
sensitivity to propagation effects at these levels. Thermal plasma effects in
the source are negligible, but we cannot exclude the importance of some other
source effect.Comment: 12 pages, 3 figures, Phys. Lett. B, reflects published versio
Charge separation relative to the reaction plane in Pb-Pb collisions at TeV
Measurements of charge dependent azimuthal correlations with the ALICE
detector at the LHC are reported for Pb-Pb collisions at TeV. Two- and three-particle charge-dependent azimuthal correlations in
the pseudo-rapidity range are presented as a function of the
collision centrality, particle separation in pseudo-rapidity, and transverse
momentum. A clear signal compatible with a charge-dependent separation relative
to the reaction plane is observed, which shows little or no collision energy
dependence when compared to measurements at RHIC energies. This provides a new
insight for understanding the nature of the charge dependent azimuthal
correlations observed at RHIC and LHC energies.Comment: 12 pages, 3 captioned figures, authors from page 2 to 6, published
version, figures at http://aliceinfo.cern.ch/ArtSubmission/node/286
A note on comonotonicity and positivity of the control components of decoupled quadratic FBSDE
In this small note we are concerned with the solution of Forward-Backward
Stochastic Differential Equations (FBSDE) with drivers that grow quadratically
in the control component (quadratic growth FBSDE or qgFBSDE). The main theorem
is a comparison result that allows comparing componentwise the signs of the
control processes of two different qgFBSDE. As a byproduct one obtains
conditions that allow establishing the positivity of the control process.Comment: accepted for publicatio
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