1,246 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
Multivariate characterization of hydrogen Balmer emission in cataclysmic variables
The ratios of hydrogen Balmer emission line intensities in cataclysmic
variables are signatures of the physical processes that produce them. To
quantify those signatures relative to classifications of cataclysmic variable
types, we applied the multivariate statistical analysis methods of principal
components analysis and discriminant function analysis to the spectroscopic
emission data set of Williams (1983). The two analysis methods reveal two
different sources of variation in the ratios of the emission lines. The source
of variation seen in the principal components analysis was shown to be
correlated with the binary orbital period. The source of variation seen in the
discriminant function analysis was shown to be correlated with the equivalent
width of the H line. Comparison of the data scatterplot with
scatterplots of theoretical models shows that Balmer line emission from T CrB
systems is consistent with the photoionization of a surrounding nebula.
Otherwise, models that we considered do not reproduce the wide range of Balmer
decrements, including "inverted" decrements, seen in the data.Comment: Accepted by PAS
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
Timing analysis of the isolated neutron star RX J0720.4-3125
We present a combined analysis of XMM-Newton, Chandra and Rosat observations
of the isolated neutron star RXJ0720.4-3125, spanning a total period of \sim 7
years. We develop a maximum likelihood periodogramme for our analysis based on
the \Delta C-statistic and the maximum likelihood method, which are appropriate
for the treatment of sparse event lists. Our results have been checked "a
posteriori" by folding a further BeppoSAX dataset with the period predicted at
the time of that observation: the phase is found to be consistent. The study of
the spin history and the measure of the spin-down rate is of extreme importance
in discriminating between the possible mechanisms suggested for the nature of
the X-ray emission. The value of \dot P, here measured for the first time, is
\approx 10^{-14} s/s. This value can not be explained in terms of torque from a
fossil disk. When interpreted in terms of dipolar losses, it gives a magnetic
field of B \approx 10^{13} G, making also implausible that the source is
accreting from the underdense surroundings. On the other hand, we also find
unlikely that the field decayed from a much larger value (B\approx 10^{15} G,
as expected for a magnetar powered by dissipation of a superstrong field) since
this scenario predicts a source age of \approx 10^4 yrs, too young to match the
observed X-ray luminosity. The observed properties are more compatible with a
scenario in which the source is \approx 10^6 yrs old, and its magnetic field
has not changed substantially over the lifetime.Comment: 11 Pages, 6 Figures. Accepted for publication in MNRA
First XMM-Newton observations of strongly magnetic cataclysmic variables - II. Timing studies of DP Leo and WW Hor
XMM-Newton was used to observe two eclipsing, magnetic cataclysmic variables,
DP Leo and WW Hor, continuously for three orbital cycles each. Both systems
were in an intermediate state of accretion. For WW Hor we also obtained optical
light curves with the XMM-Newton Optical Monitor and from ground-based
observations. Our analysis of the X-ray and optical light curves allows us to
constrain physical and geometrical parameters of the accretion regions and
derive orbital parameters and eclipse ephemerides of the systems. For WW Hor we
directly measure horizontal and vertical temperature variations in the
accretion column. From comparisons with previous observations we find that
changes in the accretion spot longitude are correlated with the accretion rate.
For DP Leo the shape of the hard X-ray light curve is not as expected for
optically thin emission, showing the importance of optical depth effects in the
post-shock region. We find that the spin period of the white dwarf is slightly
shorter than the orbital period and that the orbital period is decreasing
faster than expected for energy loss by gravitational radiation alone.Comment: Accepted for publication in MNRAS, 12 pages, 6 figure
The emergence of quantum capacitance in epitaxial graphene
We found an intrinsic redistribution of charge arises between epitaxial
graphene, which has intrinsically n-type doping, and an undoped substrate. In
particular, we studied in detail epitaxial graphene layers thermally elaborated
on C-terminated - (- ()). We have investigated
the charge distribution in graphene-substrate systems using Raman spectroscopy.
The influence of the substrate plasmons on the longitudinal optical phonons of
the substrates has been detected. The associated charge redistribution
reveals the formation of a capacitance between the graphene and the substrate.
Thus, we give for the first time direct evidence that the excess negative
charge in epitaxial monolayer graphene could be self-compensated by the
substrate without initial doping. This induced a previously unseen
redistribution of the charge-carrier density at the substrate-graphene
interface. There a quantum capacitor appears, without resorting to any
intentional external doping, as is fundamentally required for epitaxial
graphene. Although we have determined the electric field existing inside the
capacitor and revealed the presence of a minigap () for
epitaxial graphene on - face terminated carbon, it remains small in
comparison to that obtained for graphene on face terminated . The
fundamental electronic properties found here in graphene on substrates
may be important for developing the next generation of quantum technologies and
electronic/plasmonic devices.Comment: 26 pages, 8 figures, available online as uncorrected proof, Journal
of Materials Chemistry C (2016
Climate Change Across The Macaronesian Geographical Region, 1850 - 2100
The Macaronesian geographical zone extends from 10-40°N, 325-355°E and primarily includes the island chains of the Azores, Madeira, the Canary Islands and Cape Verde. This thesis presents a wide-ranging analysis of the physical climate and oceanography of the region back to 1850, in order to place recent climate change within a historical context. Subsequently, this thesis presents the most complete documentation of the physical climate of Macaronesia in the English language literature. One of the main outputs of this thesis is the creation of a long-term, monthly surface air temperature record for each island chain (from 1865 for the Azores and Madeira, 1885 for the Canary Islands and 1895 for Cape Verde). These temperature records exhibit generally coherent patterns of variability, and a post-1976 increase in temperature - most probably reflecting an anthropogenic climate signal - is the most ubiquitous, significant rise (or fall) in the record. Precipitation variability is also analysed, although only trends from Cape Verde are particularly significant, where a slight precipitation recovery - after the turn of the Twenty-first Century since drought conditions in the mid-late Twentieth Century - is apparent. Climatological extreme indices, based on calculations that assimilate daily temperature and precipitation data, were also analysed for the recent past (1979-2011) and point towards warmer conditions. An assessment of potential future changes in the mean state and extreme indices of climate across the islands by the end of the Twenty-first Century is provided. Warming magnitudes for the 2071-2100 period range between 0.8-3.0°C above the 1976-2005 mean temperature. Precipitation is expected to decrease across the Canary Islands and Madeira, whereas the Azores is expected to experience more extreme precipitation events and precipitation changes across Cape Verde are uncertain.
In addition to the analysis of temperature and precipitation changes, a daily North Atlantic Oscillation index extending back to 1850 using historical sea-level pressure data from the Azores was constructed. The temporal length of this newly created index exceeds the length of any previously available long-running, daily-resolution series by a hundred years and should be of great value to researchers across multiple disciplines. The spatial and temporal variability of the North Atlantic Oscillation was analysed, finding an increase in post-2004 winter variability, alongside a post-1991 negative summer trend. A novel method to characterise the strength of the Trade Winds by using data from the Azores and Cape Verde was also developed. The newly-defined Trade Wind index has been steadily increasing since 1973.
An additional analysis was a comprehensive overview and reconciliation of multiple data sources to answer the question of whether coastal upwelling has been increasing across the Canary Upwelling Ecosystem along the northwest African coastline. This analysis determined that the Bakun upwelling intensification hypothesis developed in 1990 appears to be realised in the summertime coastal upwelling indices. The North Atlantic Oscillation was discovered to be strongly related to upwelling magnitudes for all seasons except summer, in addition to exerting a strong control on temperatures and precipitation across the three northernmost Macaronesian island chains. The small-scale features affecting island climates and the large-scale modes of variability that influence the Macaronesian region are also discussed
UBVRI photopolarimetry of the long period eclipsing AM Herculis binary V1309
We report simultaneous UBVRI photo-polarimetric observations of the long
period (7.98 h) AM Her binary V1309 Ori. The length and shape of the eclipse
ingress and egress varies from night to night. We suggest this is due to the
variation in the brightness of the accretion stream. By comparing the phases of
circular polarization zero-crossovers with previous observations, we confirm
that V1309 Ori is well synchronized, and find an upper limit of 0.002 percent
for the difference between the spin and orbital periods. We model the
polarimetry data using a model consisting of two cyclotron emission regions at
almost diametrically opposite locations, and centered at colatitude 35 (deg)
and 145 (deg) on the surface of the white dwarf. We also present archive X-ray
observations which show that the negatively polarised accretion region is X-ray
bright.Comment: 11 pages, 12 figures (2 colour), Fig1 and Fig 4 are in lower
resolution than in original paper, accepted for publication in Monthly
Notices of the Royal Astronomical Societ
The isolated neutron star X-ray pulsars RX J0420.0â5022 and RX J0806.4â4123 : new X-ray and optical observations
We report on the analysis of new X-ray data obtained with XMM-Newton and Chandra from two ROSAT-discovered X-ray dim isolated neutron stars (XDINs). RX J0806.4â4123 was observed with XMM-Newton in April 2003, 2.5 years after
the first observation. The EPIC-pn data confirm that this object is an X-ray pulsar with 11.371 s neutron star spin period. The X-ray spectrum is consistent with absorbed black-body emission with a temperature kT = 96 eV and N H = 4 Ă 10 19 cm â2 without significant changes between the two observations. Four XMM-Newton observations of RX
J0420.0â5022 between December 2002 and July 2003 did not confirm the 22.7 s pulsations originally indicated in ROSAT data, but clearly reveal a 3.453 s period. A fit to the X-ray spectrum using an absorbed black-body model yields kT = 45 eV, the lowest value found from the small group of XDINs and N H = 1.0 Ă 10 20 cm â2. Including a broad absorption line improves the quality of the spectral fits considerably for both objects and may indicate the presence of absorption features similar to those reported from RBS1223, RX J1605.3+3249 and RX J0720.4â3125. For both targets we derive accurate X-ray positions from the Chandra data and present an optical counterpart candidate for RX J0420.0â5022 with B = 26.6 ± 0.3 mag from VLT imaging
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