1,435 research outputs found
Environmental effects on star formation in dwarf galaxies and star clusters
We develop a simple analytical criterion to investigate the role of the
environment on the onset of star formation. We will consider the main external
agents that influence the star formation (i.e. ram pressure, tidal interaction,
Rayleigh-Taylor and Kelvin-Helmholtz instabilities) in a spherical galaxy
moving through an external environment. The theoretical framework developed
here has direct applications to the cases of dwarf galaxies in galaxy clusters
and dwarf galaxies orbiting our Milky Way system, as well as any primordial
gas-rich cluster of stars orbiting within its host galaxy. We develop an
analytic formalism to solve the fluid dynamics equations in a non-inertial
reference frame mapped with spherical coordinates. The two-fluids instability
at the interface between a stellar system and its surrounding hotter and less
dense environment is related to the star formation processes through a set of
differential equations. The solution presented here is quite general, allowing
us to investigate most kinds of orbits allowed in a gravitationally bound
system of stars in interaction with a major massive companion. We present an
analytical criterion to elucidate the dependence of star formation in a
spherical stellar system (as a dwarf galaxy or a globular cluster) on its
surrounding environment useful in theoretical interpretations of numerical
results as well as observational applications. We show how spherical
coordinates naturally enlighten the interpretation of the two-fluids
instability in a geometry that directly applies to astrophysical case. This
criterion predicts the threshold value for the onset of star formation in a
mass vs. size space for any orbit of interest. Moreover, we show for the first
time the theoretical dependencies of the different instability phenomena acting
on a system in a fully analytical way.Comment: ACCEPTED in A&A the 09/09/2014. Changes from ver 1: the non-inertial
linear-response theory for gas instabilities in spherical coordinates is
moved to the Appenidx and will be available only on-lin
A Detection of an Anti-correlated Hard X-ray Lag in AM Herculis
Context {Earlier cross-correlation studies for AM Her were performed in
various energy range from optical to X-ray and suggested that it mostly shows a
high level of correlation but on occasion it shows a low level of correlation
or uncorrelation.} Aims {To investigate the degree of correlation between soft
(2-4 keV) and hard (9-20 keV) X-rays, we perform the cross-correlation study of
the X-ray data sets of AM Her obtained with {\it RXTE}.}
Methods {We cross-correlate the background-subtracted soft and hard X-ray
light curves using the XRONOS program crosscor and fit a model to the obtained
cross-correlation functions.}
Results {We detect a hard X-ray lag of s in a specific section of
energy-dependent light curve, where the soft X-ray (2-4 keV) intensity
decreases but the hard X-ray (9-20 keV) intensity increases. From a spectral
analysis, we find that the X-ray emission temperature increases during the
anti-correlated intensity variation. In two other observations, the
cross-correlation functions show a low level of correlation, which is
consistent with the earlier results performed in a different energy range.}
Conclusions {We report a detection of an anti-correlated hard X-ray lag of
190 s from the proto-type polar AM Her. The hard X-ray lag is detected
for the first time in the given energy range, and it is the longest lag among
those reported in magnetic cataclysmic variables. We discuss the implications
of our findings regarding the origin of the hard X-ray lag and the
anti-correlated intensity variation.}Comment: Accepted in A&A, 4 page
The Mind-Body Connection: The Association between Adolescent Locus of Control and Indicators of Physical Health
Locus of control (LOC) describes an individual’s generalized beliefs or expectancies that their reinforcements are under internal versus external control (1). An individual exhibits either an internal or external LOC. This study examines the link between LOC and selected health risk factors in adolescents. A convenience sample of 167 high school physical education students completed a 13-item LOC questionnaire based on Rotter’s 1966 instrument.
Various anthropometric measurements, blood pressure, body mass index (BMI), and body fat were recorded on all subjects. A subsample of 61 female students received blood chemistry analysis that included a lipid profile, hemoglobin A1c (HbA1c), Apo A-1, and Apo B. Mean body fat percents and LOC scores were significantly correlated (r=0.208; p=0.006), and mean body fat percent was significantly different between the internal and external LOC groups (p=0.039). Regression equations were developed to predict HbA1c from hip circumference and LOC, and to predict Apo A-1 from LOC. Predictability was 99.8% and 97% respectively.
We conclude that, with further research, LOC may prove to be a useful tool for screening large groups of adolescents for disease risk
Stokes imaging, Doppler mapping and Roche tomography of the AM Her system V834 Cen
We report on new simultaneous phase resolved spectroscopic and polarimetric
observations of the polar (AM Herculis star) V834 Cen during a high state of
accretion. Strong emission lines and high levels of variable circular and
linear polarization are observed over the orbital period. The polarization data
is modelled using the Stokes imaging technique of Potter et al. The
spectroscopic emission lines are investigated using the Doppler tomography
technique of Marsh and Horne and the Roche tomography technique of Dhillon and
Watson. Up to now all three techniques have been used separately to investigate
the geometry and accretion dynamics in Cataclysmic Variables. For the first
time, we apply all three techniques to simultaneous data for a single system.
This allows us to compare and test each of the techniques against each other
and hence derive a better understanding of the geometry, dynamics and system
parameters of V834 Cen.Comment: 10 pages, 8 figures; Accepted for publication in MNRA
Generalisation through negation and predicate invention
The ability to generalise from a small number of examples is a fundamental challenge in machine learning. To tackle this challenge, we introduce an inductive logic programming (ILP) approach that combines negation and predicate invention. Combining these two features allows an ILP system to generalise better by learning rules with universally quantified body-only variables. We implement our idea in NOPI, which can learn normal logic programs with predicate invention, including Datalog programs with stratified negation. Our experimental results on multiple domains show that our approach can improve predictive accuracies and learning times
Variability of the Accretion Stream in the Eclipsing Polar EP Dra
We present the first high time resolution light curves for six eclipses of
the magnetic cataclysmic variable EP Dra, taken using the superconducting
tunnel junction imager S-Cam2. The system shows a varying eclipse profile
between consecutive eclipses over the two nights of observation. We attribute
the variable stream eclipse after accretion region ingress to a variation in
the amount and location of bright material in the accretion stream. This
material creates an accretion curtain as it is threaded by many field lines
along the accretion stream trajectory. We identify this as the cause of
absorption evident in the light curves when the system is in a high accretion
state. We do not see direct evidence in the light curves for an accretion spot
on the white dwarf; however, the variation of the stream brightness with the
brightness of the rapid decline in flux at eclipse ingress indicates the
presence of some form of accretion region. This accretion region is most likely
located at high colatitude on the white dwarf surface, forming an arc shape at
the foot points of the many field lines channeling the accretion curtain.Comment: Accepted for publication in MNRAS (7 pages
ORFEUS II Far-UV Spectroscopy of AM Herculis
Six high-resolution (\lambda/\Delta\lambda ~ 3000) far-UV (\lambda\lambda =
910-1210 \AA) spectra of the magnetic cataclysmic variable AM Herculis were
acquired in 1996 November during the flight of the ORFEUS-SPAS II mission. AM
Her was in a high optical state at the time of the observations, and the
spectra reveal emission lines of O VI \lambda\lambda 1032, 1038, C III \lambda
977, \lambda 1176, and He II \lambda 1085 superposed on a nearly flat
continuum. Continuum flux variations can be described as per Gansicke et al. by
a ~ 20 kK white dwarf with a ~ 37 kK hot spot covering a fraction f~0.15 of the
surface of the white dwarf, but we caution that the expected Lyman absorption
lines are not detected. The O VI emission lines have narrow and broad component
structure similar to that of the optical emission lines, with radial velocities
consistent with an origin in the irradiated face of the secondary and the
accretion funnel, respectively. The density of the narrow- and broad-line
regions is n_{nlr} ~ 3\times 10^{10} cm^{-3} and n_{blr} ~ 1\times 10^{12}
cm^{-3}, respectively, yet the narrow-line region is optically thick in the O
VI line and the broad-line region is optically thin; apparently, the velocity
shear in the broad-line region allows the O VI photons to escape, rendering the
gas effectively optically thin. Unexplained are the orbital phase variations of
the emission-line fluxes.Comment: 15 pages, 6 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on April 17, 1998 for publication in The
Astrophysical Journa
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