210 research outputs found
Evidence for a Very Large-Scale Fractal Structure in the Universe from Cobe Measurements
In this work, we analyse the temperature fluctuations of the cosmic microwave
background radiation observed by COBE and show that the distribution can be
fitted by a fractal distribution with a fractal dimension .
This value is in close agreement with the fractal dimension obtained by Coleman
and Pietronero (1992) and Luo and Schramm (1992) from galaxy-galaxy and
cluster-cluster correlations up to . The fact that the
observed temperature fluctuations correspond to scales much larger than and are signatures of the primordial density fluctuations at the
recombination layer suggests that the structure of the matter at the early
universe was already fractal and thus non-homogeneous on those scales. This
result may have important consequences for the theoretical framework that
describes the universe.Comment: 11 pages, postscript file, 2 figures available upon request. To
appear in ApJ Letter
Probing the anomalous extinction of four young star clusters: the use of colour-excess, main sequence fitting and fractal analysis
Four young star clusters were studied in order to characterize their
anomalous extinction or variable reddening that could be due to a possible
contamination by dense clouds or circumstellar effects. The extinction law (Rv)
was evaluated by adopting two methods: (i) the use of theoretical expressions
based on the colour-excess of stars with known spectral type, and (ii) the
analysis of two-colour diagrams, where the slope of observed colours
distribution is compared to the normal distribution. An algorithm to reproduce
the zero age main sequence (ZAMS) reddened colours was developed in order to
derive the average visual extinction (Av) that provides the best fitting of the
observational data. The structure of the clouds was evaluated by means of
statistical fractal analysis, aiming to compare their geometric structure with
the spatial distribution of the cluster members. The cluster NGC 6530 is the
only object of our sample showing anomalous extinction. In average, the other
clusters are suffering normal extinction, but several of their members, mainly
in NGC 2264, seem to have high Rv, probably due to circumstellar effects. The
ZAMS fitting provides Av values that are in good agreement with those found in
the literature. The fractal analysis shows that NGC 6530 has a centrally
concentrated distribution of stars that is different of the sub-structures
found in the density distribution of the cloud projected in the Av map,
suggesting that the original cloud has been changed with the cluster formation.
On the other hand, the fractal dimension and the statistical parameters of
Berkeley 86, NGC 2244, and NGC 2264 indicate a good cloud-cluster correlation,
when compared to other works based on artificial distribution of points.Comment: 13 pages, 7 figure
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
Polarimetry of Li-rich giants
Protoplanetary nebulae typically present non-spherical envelopes. The origin
of such geometry is still controversial. There are indications that it may be
carried over from an earlier phase of stellar evolution, such as the AGB phase.
But how early in the star's evolution does the non-spherical envelope appear?
Li-rich giants show dusty circumstellar envelopes that can help answer that
question. We study a sample of fourteen Li-rich giants using optical
polarimetry in order to detect non-spherical envelopes around them. We used the
IAGPOL imaging polarimeter to obtain optical linear polarization measurements
in V band. Foreground polarization was estimated using the field stars in each
CCD frame. After foreground polarization was removed, seven objects presented
low intrinsic polarization (0.19 - 0.34)% and two (V859 Aql and GCSS 557)
showed high intrinsic polarization values (0.87 - 1.16)%. This intrinsic
polarization suggests that Li-rich giants present a non-spherical distribution
of circumstellar dust. The intrinsic polarization level is probably related to
the viewing angle of the envelope, with higher levels indicating objects viewed
closer to edge-on. The correlation of the observed polarization with optical
color excess gives additional support to the circumstellar origin of the
intrinsic polarization in Li-rich giants. The intrinsic polarization correlates
even better with the IRAS 25 microns far infrared emission. Analysis of
spectral energy distributions for the sample show dust temperatures for the
envelopes tend to be between 190 and 260 K. We suggest that dust scattering is
indeed responsible for the optical intrinsic polarization in Li-rich giants.
Our findings indicate that non-spherical envelopes may appear as early as the
red giant phase of stellar evolution.Comment: to be published in A&A, 15 pages, 10 figures. Fig. 3 is available in
ftp://astroweb.iag.usp.br/pub/antonio/4270/4270.fig3.pd
The alignment of the polarization of HAe/Be stars with the interstellar magnetic field
We present a study of the correlation between the direction of the symmetry
axis of the circumstellar material around intermediate mass young stellar
objects and that of the interstellar magnetic field. We use CCD polarimetric
data on 100 Herbig Ae/Be stars. A large number of them shows intrinsic
polarization, which indicates that their circumstellar envelopes are not
spherical. The interstellar magnetic field direction is estimated from the
polarization of field stars. There is an alignment between the position angle
of the Herbig Ae/Be star polarization and that of the field stars for the most
polarized objects. This may be an evidence that the ambient interstellar
magnetic field plays a role in shaping the circumstellar material around young
stars of intermediate mass and/or in defining their angular momentum axis.Comment: ApJ accepte
Getting to the core: Internal body temperatures help reveal the ecological function and thermal implications of the lions’ mane
It has been proposed that there is a thermal cost of the mane to male lions, potentially leading to increased body surface temperatures (Ts), increased sperm abnormalities, and to lower food intake during hot summer months. To test whether a mane imposes thermal costs on males, we measured core body temperature (Tb) continuously for approximately 1 year in 18 free-living lions. There was no difference in the 24-hr maximum Tb of males (n = 12) and females (n = 6), and males had a 24-hr mean Tb that was 0.2 ± 0.1°C lower than females after correcting for seasonal effects. Although feeding on a particular day increased 24-hr mean and 24-hr maximum Tb, this phenomenon was true of both male and female lions, and females had higher 24-hr mean and 24-hr maximum Tb than males, on both days when lions did not feed, and on days when lions did feed. Twenty-four-hour Tb was not influenced by mane length or color, and 24-hr mean Tb was negatively correlated with mane length. These data contradict the suggestion that there exists a thermal cost to male lions in possessing a long dark mane, but do not preclude the possibility that males compensate for a mane with increased heat loss. The increased insulation caused by a mane does not necessarily have to impair heat loss by males, which in hot environments is primarily through respiratory evaporative cooling, nor does in necessarily lead to increased heat gain, as lions are nocturnal and seek shade during the day. The mane may even act as a heat shield by increasing insulation. However, dominant male lions frequent water points more than twice as often as females, raising the possibility that male lions are increasing water uptake to facilitate increased evaporative cooling. The question of whether male lions with manes compensate for a thermal cost to the mane remains unresolved, but male lions with access to water do not have higher Tb than females or males with smaller manes.NCS201
Body water conservation through selective brain cooling by the carotid rete: a physiological feature for surviving climate change?
Some mammals have the ability to lower their hypothalamic temperature below that of carotid arterial blood temperature, a process termed selective brain cooling. Although the requisite anatomical structure that facilitates this physiological process, the carotid rete, is present in members of the Cetartiodactyla, Felidae and Canidae, the carotid rete is particularly well developed in the artiodactyls, e.g. antelopes, cattle, sheep and goats. First described in the domestic cat, the seemingly obvious function initially attributed to selective brain cooling was that of protecting the brain from thermal damage. However, hyperthermia is not a prerequisite for selective brain cooling, and selective brain cooling can be exhibited at all times of the day, even when carotid arterial blood temperature is relatively low. More recently, it has been shown that selective brain cooling functions primarily as a water-conservation mechanism, allowing artiodactyls to save more than half of their daily water requirements. Here, we argue that the evolutionary success of the artiodactyls may, in part, be attributed to the evolution of the carotid rete and the resulting ability to conserve body water during past environmental conditions, and we suggest that this group of mammals may therefore have a selective advantage in the hotter and drier conditions associated with current anthropogenic climate change. A better understanding of how selective brain cooling provides physiological plasticity to mammals in changing environments will improve our ability to predict their responses and to implement appropriate conservation measures.EM201
Multifractal Scaling, Geometrical Diversity, and Hierarchical Structure in the Cool Interstellar Medium
Multifractal scaling (MFS) refers to structures that can be described as a
collection of interwoven fractal subsets which exhibit power-law spatial
scaling behavior with a range of scaling exponents (concentration, or
singularity, strengths) and dimensions. The existence of MFS implies an
underlying multiplicative (or hierarchical, or cascade) process. Panoramic
column density images of several nearby star- forming cloud complexes,
constructed from IRAS data and justified in an appendix, are shown to exhibit
such multifractal scaling, which we interpret as indirect but quantitative
evidence for nested hierarchical structure. The relation between the dimensions
of the subsets and their concentration strengths (the "multifractal spectrum'')
appears to satisfactorily order the observed regions in terms of the mixture of
geometries present: strong point-like concentrations, line- like filaments or
fronts, and space-filling diffuse structures. This multifractal spectrum is a
global property of the regions studied, and does not rely on any operational
definition of "clouds.'' The range of forms of the multifractal spectrum among
the regions studied implies that the column density structures do not form a
universality class, in contrast to indications for velocity and passive scalar
fields in incompressible turbulence, providing another indication that the
physics of highly compressible interstellar gas dynamics differs fundamentally
from incompressible turbulence. (Abstract truncated)Comment: 27 pages, (LaTeX), 13 figures, 1 table, submitted to Astrophysical
Journa
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