4,638 research outputs found
Semiclassical Theory of Elastic Perturbation
Transition probabilities formulation to explain perturbation effects in elastic differential scattering cross section measurement
Extrasolar planet taxonomy: a new statistical approach
In this paper we present the guidelines for an extrasolar planet taxonomy.
The discovery of an increasing number of extrasolar planets showing a vast
variety of planetary parameters, like Keplerian orbital elements and
environmental parameters, like stellar masses, spectral types, metallicity
etc., prompts the development of a planetary taxonomy. In this work via
principal component analysis followed by hierarchical clustering analysis, we
report the definition of five robust groups of planets. We also discuss the
physical relevance of such analysis, which may provide a valid basis for
disentangling the role of the several physical parameters involved in the
processes of planet formation and subsequent evolution. For instance, we were
able to divide the hot Jupiters into two main groups on the basis of their
stellar masses and metallicities. Moreover, for some groups, we find strong
correlations between metallicity, semi-major axis and eccentricity. The
implications of these findings are discussed.Comment: accepted for publication on Ap
I. Collisional evolution and reddening of asteroid surfaces: The problem of conflicting timescales and the role of size-dependent effects
Space weathering is the generic term used for processes that modify the
optical properties of surfaces of atmosphereless rocky bodies under exposure to
the space environment. The general agreement about the relevance of the effects
of space weathering on the spectral properties of S-complex asteroids fails
when some basic quantitative estimates are attempted. In particular, there is
severe disagreement regarding the typical timescales for significant spectral
reddening to occur, ranging from 1 Myr to 1 Gyr. Generally speaking, the
spectral reddening of an individual object can be considered as the sum of
three terms, one (which is relevant for statistical analyses) depending on the
exposure of the object to space weathering during its lifetime, a second one
due to the original surface composition, and a third one (a "noise" term) due
to the combination of poorly constrained effects (e.g., structure and texture
of the surface). The surface of an asteroid is usually covered by regolith, and
its presence and properties presumably play a critical role in the weathering
processes. In this paper we discuss the role played by collisional evolution in
affecting the spectral properties of asteroids and refreshing the surfaces due
to the formation of ejecta, and the necessity of a simultaneous modeling of
collisions and weathering processes. We introduce a new idea, based on the
possibility of a sort of saturation of the refreshing process whenever a
massive reaccumulation of the impact ejecta takes place. In this case, a
dependence of the overall reddening on the asteroid size should naturally come
out. We show that this conclusion is indeed supported by available main belt
asteroid spectroscopic data.Comment: Accepted by MNRA
The stellar content of the infalling molecular clump G286.21+0.17
The early evolution during massive star cluster formation is still uncertain.
Observing embedded clusters at their earliest stages of formation can provide
insight into the spatial and temporal distribution of the stars and thus probe
different star cluster formation models. We present near-infrared imaging of an
8'*13'(5.4pc*8.7pc) region around the massive infalling clump G286.21+0.17(also
known as BYF73). The stellar content across the field is determined and
photometry is derived in order to { obtain} stellar parameters for the cluster
members. We find evidence for some sub-structure (on scales less than a pc
diameter) within the region with apparently at least three different
sub-clusters associated with the molecular clump based on differences in
extinction and disk fractions. At the center of the clump we identify a deeply
embedded sub-cluster. Near-infrared excess is detected for 39-44% in the two
sub-clusters associated with molecular material and 27% for the exposed
cluster. Using the disk excess as a proxy for age this suggests the clusters
are very young. The current total stellar mass is estimated to be at least 200
Msun. The molecular core hosts a rich population of pre-main sequence stars.
There is evidence for multiple events of star formation both in terms of the
spatial distribution within the star forming region and possibly from the disk
frequency.Comment: Submitted to A
Measles in Italy: Viral strains and crossing borders.
In 2017, Italy experienced one of the largest outbreaks of measles in recent years, with 5404 notified cases and 4347 confirmed cases. A further 2029 cases were notified during the first 6 months of 2018, and 1516 of them were laboratory-confirmed. The B3 and D8 genotypes were identified as those responsible for the outbreak. Possible transmission routes can be established by monitoring the circulating measles virus strains in support of the national health authorities to warn people and travellers
Heating of near-Earth objects and meteoroids due to close approaches to the Sun
It is known that near-Earth objects (NEOs) during their orbital evolution may
often undergo close approaches to the Sun. Indeed it is estimated that up to
~70% of them end their orbital evolution colliding with the Sun. Starting from
the present orbital properties, it is possible to compute the most likely past
evolution for every NEO, and to trace its distance from the Sun. We find that a
large fraction of the population may have experienced in the past frequent
close approaches, and thus, as a consequence, a considerable Sun-driven
heating, not trivially correlated to the present orbits. The detailed dynamical
behaviour, the rotational and the thermal properties of NEOs determine the
exact amount of the resulting heating due to the Sun. In the present paper we
discuss the general features of the process, providing estimates of the surface
temperature reached by NEOs during their evolution. Moreover, we investigate
the effects of this process on meteor-size bodies, analyzing possible
differences with the NEO population. We also discuss some possible effects of
the heating which can be observed through remote sensing by ground-based
surveys or space missions.Comment: 8 pages, 5 figures, accepted by MNRA
The Cratering History of Asteroid (2867) Steins
The cratering history of main belt asteroid (2867) Steins has been
investigated using OSIRIS imagery acquired during the Rosetta flyby that took
place on the 5th of September 2008. For this purpose, we applied current models
describing the formation and evolution of main belt asteroids, that provide the
rate and velocity distributions of impactors. These models coupled with
appropriate crater scaling laws, allow the cratering history to be estimated.
Hence, we derive Steins' cratering retention age, namely the time lapsed since
its formation or global surface reset. We also investigate the influence of
various factors -like bulk structure and crater erasing- on the estimated age,
which spans from a few hundred Myrs to more than 1Gyr, depending on the adopted
scaling law and asteroid physical parameters. Moreover, a marked lack of
craters smaller than about 0.6km has been found and interpreted as a result of
a peculiar evolution of Steins cratering record, possibly related either to the
formation of the 2.1km wide impact crater near the south pole or to YORP
reshaping.Comment: Accepted by Planetary and Space Scienc
Mass Segregation in NGC 2298: limits on the presence of an Intermediate Mass Black Hole
[abridged] Theoretical investigations have suggested the presence of
Intermediate Mass Black Holes (IMBHs, with masses in the 100-10000 Msun range)
in the cores of some Globular Clusters (GCs). In this paper we present the
first application of a new technique to determine the presence or absence of a
central IMBH in globular clusters that have reached energy equipartition via
two-body relaxation. The method is based on the measurement of the radial
profile for the average mass of stars in the system, using the fact that a
quenching of mass segregation is expected when an IMBH is present. Here we
measure the radial profile of mass segregation using main-sequence stars for
the globular cluster NGC 2298 from resolved source photometry based on HST-ACS
data. The observations are compared to expectations from direct N-body
simulations of the dynamics of star clusters with and without an IMBH. The mass
segregation profile for NGC 2298 is quantitatively matched to that inferred
from simulations without a central massive object over all the radial range
probed by the observations, that is from the center to about two half-mass
radii. Profiles from simulations containing an IMBH more massive than ~ 300-500
Msun (depending on the assumed total mass of NGC 2298) are instead inconsistent
with the data at about 3 sigma confidence, irrespective of the IMF and binary
fraction chosen for these runs. While providing a null result in the quest of
detecting a central black hole in globular clusters, the data-model comparison
carried out here demonstrates the feasibility of the method which can also be
applied to other globular clusters with resolved photometry in their cores.Comment: 21 pages, 3 figures, ApJ accepte
WIYN Open Cluster Study 1: Deep Photometry of NGC 188
We have employed precise V and I photometry of NGC 188 at WIYN to explore the
cluster luminosity function (LF) and study the cluster white dwarfs (WDs). Our
photometry is offset by V = 0.052 (fainter) from Sandage (1962) and Eggen &
Sandage (1969). All published photometry for the past three decades have been
based on these two calibrations, which are in error by 0.05 +- 0.01. We employ
the Pinsonneault etal (1998) fiducial main sequence to derive a cluster
distance modulus of 11.43 +- 0.08. We report observations that are >= 50%
complete to V = 24.6 and find that the cluster central-field LF peaks at M_I ~
3 to 4. This is unlike the solar neighborhood LF and unlike the LFs of
dynamically unevolved portions of open and globular clusters, which rise
continuously until M_I ~ 9.5. Although we find that >= 50% of the unresolved
cluster objects are multiple systems, their presence cannot account for the
shape of the NGC 188 LF. For theoretical reasons (Terlevich 1987; Vesperini &
Heggie 1997) having to do with the survivability of NGC 188 we believe the
cluster is highly dynamically evolved and that the missing low luminosity stars
are either in the cluster outskirts or have left the cluster altogether. We
identify nine candidate WDs, of which we expect three to six are bona fide
cluster WDs. The luminosities of the faintest likely WD indicates an age
(Bergeron, Wesemael, & Beauchamp 1995) of 1.14 +- 0.09 Gyrs. This is a lower
limit to the cluster age and observations probing to V = 27 or 28 will be
necessary to find the faintest cluster WDs and independently determine the
cluster age. While our age limit is not surprising for this ~6 Gyr old cluster,
our result demonstrates the value of the WD age technique with its very low
internal errors. (abridged)Comment: 26 pages, uuencoded gunzip'ed latex + 16 postscrip figures, to be
published in A
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