55 research outputs found
Axisymmetric smoothed particle hydrodynamics with self-gravity
The axisymmetric form of the hydrodynamic equations within the smoothed
particle hydrodynamics (SPH) formalism is presented and checked using idealized
scenarios taken from astrophysics (free fall collapse, implosion and further
pulsation of a sun-like star), gas dynamics (wall heating problem, collision of
two streams of gas) and inertial confinement fusion (ICF, -ablative implosion
of a small capsule-). New material concerning the standard SPH formalism is
given. That includes the numerical handling of those mass points which move
close to the singularity axis, more accurate expressions for the artificial
viscosity and the heat conduction term and an easy way to incorporate
self-gravity in the simulations. The algorithm developed to compute gravity
does not rely in any sort of grid, leading to a numerical scheme totally
compatible with the lagrangian nature of the SPH equations.Comment: 17 pages, 10 figures, 1 Table. Accepted for publication in MNRA
Photoionization Models of NGC 346
We present spherically symmetric and plane parallel photoionization models of
NGC 346, an HII region in the Small Magellanic Cloud. The models are based on
CLOUDY and on the observations of Peimbert, Peimbert, & Ruiz (2000). We find
that approximately 45% of the H ionization photons escape from the HII region
providing an important ionizing source for the low density interstellar medium
of the SMC. The predicted I(4363)/I(5007) value is smaller than that observed,
probably implying that there is an additional source of energy not taken into
account by the models. From the ionization structure of the best model and the
observed line intensities we determine the abundances of N, Ne, S, Ar, and Fe
relative to O.Comment: 23 pages, 2 figures, Accepted for publication in Ap
Shocked similarity collapses and flows in star formation processes
We propose self-similar shocked flow models for certain dynamical evolution
phases of young stellar objects (YSOs), `champagne flows' of H {\sevenrm II}
regions surrounding OB stars and shaping processes of planetary nebulae (PNe).
We analyze an isothermal fluid of spherical symmetry and construct families of
similarity shocked flow solutions featured by: 1. either a core expansion with
a finite central density or a core accretion at constant rate with a density
scaling ; 2. a shock moving outward at a constant speed; 3. a
preshock gas approaching a constant speed at large with a density scaling
. In addition to testing numerical codes, our models can
accommodate diverse shocked flows with or without a core collapse or outflow
and an envelope expansion or contraction. As an application, we introduce our
model analysis to observations of Bok globule B335.Comment: ApJL accepte
The effects of spatial resolution on Integral Field Spectrograph surveys at different redshifts. The CALIFA perspective
Over the past decade, 3D optical spectroscopy has become the preferred tool
for understanding the properties of galaxies and is now increasingly used to
carry out galaxy surveys. Low redshift surveys include SAURON, DiskMass,
ATLAS3D, PINGS and VENGA. At redshifts above 0.7, surveys such as MASSIV, SINS,
GLACE, and IMAGES have targeted the most luminous galaxies to study mainly
their kinematic properties. The on-going CALIFA survey () is the
first of a series of upcoming Integral Field Spectroscopy (IFS) surveys with
large samples representative of the entire population of galaxies. Others
include SAMI and MaNGA at lower redshift and the upcoming KMOS surveys at
higher redshift. Given the importance of spatial scales in IFS surveys, the
study of the effects of spatial resolution on the recovered parameters becomes
important. We explore the capability of the CALIFA survey and a hypothetical
higher redshift survey to reproduce the properties of a sample of objects
observed with better spatial resolution at lower redshift. Using a sample of
PINGS galaxies, we simulate observations at different redshifts. We then study
the behaviour of different parameters as the spatial resolution degrades with
increasing redshift.Comment: 20 pages, 16 figures. Accepted for publication in A&
Counting function fluctuations and extreme value threshold in multifractal patterns: the case study of an ideal noise
To understand the sample-to-sample fluctuations in disorder-generated
multifractal patterns we investigate analytically as well as numerically the
statistics of high values of the simplest model - the ideal periodic
Gaussian noise. By employing the thermodynamic formalism we predict the
characteristic scale and the precise scaling form of the distribution of number
of points above a given level. We demonstrate that the powerlaw forward tail of
the probability density, with exponent controlled by the level, results in an
important difference between the mean and the typical values of the counting
function. This can be further used to determine the typical threshold of
extreme values in the pattern which turns out to be given by
with . Such observation provides a
rather compelling explanation of the mechanism behind universality of .
Revealed mechanisms are conjectured to retain their qualitative validity for a
broad class of disorder-generated multifractal fields. In particular, we
predict that the typical value of the maximum of intensity is to be
given by , where is the
corresponding singularity spectrum vanishing at . For the
noise we also derive exact as well as well-controlled approximate
formulas for the mean and the variance of the counting function without
recourse to the thermodynamic formalism.Comment: 28 pages; 7 figures, published version with a few misprints
corrected, editing done and references adde
JINGLE – IV. Dust, H I gas and metal scaling laws in the local Universe
Scaling laws of dust, Hi gas and metal mass with stellar mass, specific star formation rate and metallicity are crucial to our understanding of the buildup of galaxies through their enrichment with metals and dust. In this work, we analyse how the dust and metal content varies with specific gas mass (MHI/M?) across a diverse sample of 423 nearby galaxies. The observed trends are interpreted with a set of Dust and Element evolUtion modelS (DEUS) – including stellar dust production, grain growth, and dust destruction – within a Bayesian framework to enable a rigorous search of the multi-dimensional parameter space. We find that these scaling laws for galaxies with −1.0 . logMHI/M? . 0 can be reproduced using closed-box models with high fractions (37-89%) of supernova dust surviving a reverse shock, relatively low grain growth efficiencies (=30-40), and long dust lifetimes (1-2Gyr). The models have present-day dust masses with similar contributions from stellar sources (50-80%) and grain growth (20-50%). Over the entire lifetime of these galaxies, the contribution from stardust (>90%) outweighs the fraction of dust grown in the interstellar medium (<10%). Our results provide an alternative for the chemical evolution models that require extremely low supernova dust production efficiencies and short grain growth timescales to reproduce local scaling laws, and could help solving the conundrum on whether or not grains can grow efficiently in the interstellar medium
Gas flows, star formation and galaxy evolution
In the first part of this article we show how observations of the chemical
evolution of the Galaxy: G- and K-dwarf numbers as functions of metallicity,
and abundances of the light elements, D, Li, Be and B, in both stars and the
interstellar medium (ISM), lead to the conclusion that metal poor HI gas has
been accreting to the Galactic disc during the whole of its lifetime, and is
accreting today at a measurable rate, ~2 Msun per year across the full disc.
Estimates of the local star formation rate (SFR) using methods based on stellar
activity, support this picture. The best fits to all these data are for models
where the accretion rate is constant, or slowly rising with epoch. We explain
here how this conclusion, for a galaxy in a small bound group, is not in
conflict with graphs such as the Madau plot, which show that the universal SFR
has declined steadily from z=1 to the present day. We also show that a model in
which disc galaxies in general evolve by accreting major clouds of low
metallicity gas from their surroundings can explain many observations, notably
that the SFR for whole galaxies tends to show obvious variability, and
fractionally more for early than for late types, and yields lower dark to
baryonic matter ratios for large disc galaxies than for dwarfs. In the second
part of the article we use NGC 1530 as a template object, showing from
Fabry-Perot observations of its Halpha emission how strong shear in this
strongly barred galaxy acts to inhibit star formation, while compression acts
to stimulate it.Comment: 20 pages, 10 figures, to be presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, Eds. D.L. Block, K.C. Freeman, I. Puerari, & R. Groes
Modeling Dust and Starlight in Galaxies Observed by Spitzer and Herschel: The KINGFISH Sample
Dust and starlight are modeled for the KINGFISH project galaxies. With data
from 3.6 micron to 500 micron, models are strongly constrained. For each pixel
in each galaxy we estimate (1) dust surface density; (2) q_PAH, the dust mass
fraction in PAHs; (3) distribution of starlight intensities heating the dust;
(4) luminosity emitted by the dust; and (5) dust luminosity from regions with
high starlight intensity. The models successfully reproduce both global and
resolved spectral energy distributions. We provide well-resolved maps for the
dust properties. As in previous studies, we find q_PAH to be an increasing
function of metallicity, above a threshold Z/Z_sol approx 0.15. Dust masses are
obtained by summing the dust mass over the map pixels; these "resolved" dust
masses are consistent with the masses inferred from model fits to the global
photometry. The global dust-to-gas ratios obtained from this study correlate
with galaxy metallicities. Systems with Z/Z_sol > 0.5 have most of their
refractory elements locked up in dust, whereas when Z/Z_sol < 0.3 most of these
elements tend to remain in the gas phase. Within galaxies, we find that q_PAH
is suppressed in regions with unusually warm dust with nu L_nu(70 um) >
0.4L_dust. With knowledge of one long-wavelength flux density ratio (e.g.,
f_{160}/f_{500}), the minimum starlight intensity heating the dust (U_min) can
be estimated to within ~50%. For the adopted dust model, dust masses can be
estimated to within ~0.07 dex accuracy using the 500 micron luminosity nu
L_nu(500) alone. There are additional systematic errors arising from the choice
of dust model, but these are hard to estimate. These calibrated prescriptions
may be useful for studies of high-redshift galaxies.Comment: ApJ, accepte
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