199 research outputs found
Absolute differential positronium-formation cross sections
The first absolute experimental determinations of the differential cross-sections for the formation of ground-state positronium are presented for He, Ar, H2 and CO2 near 0â—‹. Results are compared with available theories. The ratio of the differential and integrated cross-sections for the targets exposes the higher propensity for forward-emission of positronium formed from He and H2
The HERMES Solar Atlas and the spectroscopic analysis of the seismic solar analogue KIC3241581
Solar-analog stars provide an excellent opportunity to study the Sun's
evolution, i.e. the changes with time in stellar structure, activity, or
rotation for solar-like stars. The unparalleled photometric data from the NASA
space telescope Kepler allows us to study and characterise solar-like stars
through asteroseismology. We aim to spectroscopically investigate the
fundamental parameter and chromospheric activity of solar analogues and twins,
based on observations obtained with the HERMES spectrograph and combine them
with asteroseismology. Therefore, we need to build a solar atlas for the
spectrograph, to provide accurate calibrations of the spectroscopically
determined abundances of solar and late type stars observed with this
instrument and thus perform differential spectral comparisons. We acquire
high-resolution and high signal-to-noise spectroscopy to construct three solar
reference spectra by observing the reflected light of Vesta and Victoria
asteroids and Europa (100<S/N<450) with the \Hermes spectrograph. We then
observe the Kepler solar analog KIC3241581 (S/N~170). We constructed three
solar spectrum atlases from 385 to 900 nm obtained with the Hermes spectrograph
from observations of two bright asteroids and Europa. A comparison between our
solar spectra atlas to the Kurucz and HARPS solar spectrum shows an excellent
agreement. KIC3241581 was found to be a long-periodic binary system. The
fundamental parameter for the stellar primary component are Teff=5689+/-11K,
logg=4.385+/-0.005, [Fe/H]=+0.22+/-0.01, being in agreement with the published
global seismic values confirming its status of solar analogue. KIC 3241581 is a
metal rich solar analogue with a solar-like activity level in a binary system
of unknown period. The chromospheric activity level is compatible to the solar
magnetic activity.Comment: 12 pages, 8 figures, accepted for publication in A&
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Deep Minima and Vortices for Positronium Formation in Low-Energy Positron-Hydrogen Collisions
Article using the two-channel Kohn inverse and Kohn variational methods to investigate ground-state positronium (Ps) formation in positron-hydrogen collisions in the Ore gap. This is the Accepted Manuscript Version of the article
Gravity-mode period spacings as seismic diagnostic for a sample of gamma Doradus stars from Kepler space photometry and high-resolution ground-based spectroscopy
Gamma Doradus stars (hereafter gamma Dor stars) are gravity-mode pulsators of
spectral type A or F. Such modes probe the deep stellar interior, offering a
detailed fingerprint of their structure. Four-year high-precision space-based
Kepler photometry of gamma Dor stars has become available, allowing us to study
these stars with unprecedented detail. We selected, analysed, and characterized
a sample of 67 gamma Dor stars for which we have Kepler observations available.
For all the targets in the sample we assembled high-resolution spectroscopy to
confirm their F-type nature. We found fourteen binaries, among which four
single-lined binaries, five double-lined binaries, two triple systems and three
binaries with no detected radial velocity variations. We estimated the orbital
parameters whenever possible. For the single stars and the single-lined
binaries, fundamental parameter values were determined from spectroscopy. We
searched for period spacing patterns in the photometric data and identified
this diagnostic for 50 of the stars in the sample, 46 of which are single stars
or single-lined binaries. We found a strong correlation between the
spectroscopic vsini and the period spacing values, confirming the influence of
rotation on gamma Dor-type pulsations as predicted by theory. We also found
relations between the dominant g-mode frequency, the longest pulsation period
detected in series of prograde modes, vsini, and log Teff.Comment: 61 pages, 61 figures, 6 tables, accepted for publication in ApJ
Detecting non-uniform period spacings in the Kepler photometry of gamma Doradus stars: methodology and case studies
Context. The analysis of stellar oscillations is one of the most reliable
ways to probe stellar interiors. Recent space missions such as Kepler have
provided us with an opportunity to study these oscillations with unprecedented
detail. For many multi-periodic pulsators such as {\gamma} Doradus stars, this
led to the detection of dozens to hundreds of oscillation frequencies that
could not be found from ground-based observations. Aims. We aim to detect
non-uniform period spacings in the Fourier spectra of a sample of {\gamma}
Doradus stars observed by Kepler. Such detection is complicated by both the
large number of significant frequencies in the space photometry and by
overlapping non-equidistant rotationally split multiplets. Methods. Guided by
theoretical properties of gravity-mode oscillation of {\gamma} Doradus stars,
we developed a period-spacing detection method and applied it to Kepler
observations of a few stars, after having tested the performance from
simulations. Results. The application of the technique resulted in the clear
detection of non-uniform period spacing series for three out of the five
treated Kepler targets. Disadvantages of the technique are also discussed, and
include the disability to distinguish between different values of the spherical
degree and azimuthal order of the oscillation modes without additional
theoretical modelling. Conclusions. Despite the shortcomings, the method is
shown to allow solid detections of period spacings for {\gamma} Doradus stars,
which will allow future asteroseismic analyses of these stars.Comment: 10 pages, 13 figures, 2 tables. Accepted for publication in Astronomy
& Astrophysic
Many-body theory of gamma spectra from positron-atom annihilation
A many-body theory approach to the calculation of gamma spectra of positron
annihilation on many-electron atoms is developed. We evaluate the first-order
correlation correction to the annihilation vertex and perform numerical
calculations for the noble gas atoms. Extrapolation with respect to the maximal
orbital momentum of the intermediate electron and positron states is used to
achieve convergence. The inclusion of correlation corrections improves
agreement with experimental gamma spectra.Comment: 25 pages, 9 figures, submitted to J. Phys.
Discovery of binarity, spectroscopic frequency analysis, and mode identification of the delta Sct star 4CVn
More than 40 years of ground-based photometric observations of the delta Sct
star 4CVn revealed 18 independent oscillation frequencies, including radial as
well as non-radial p-modes of low spherical degree l<=2. From 2008 to 2011,
more than 2000 spectra were obtained at the 2.1-m Otto-Struve telescope at the
McDonald Observatory. We present the analysis of the line-profile variations,
based on the Fourier-parameter fit method, detected in the absorption lines of
4CVn, which carry clear signatures of the pulsations. From a non-sinusoidal,
periodic variation of the radial velocities, we discovered that 4CVn is an
eccentric binary system, with an orbital period Porb = 124.44 +/- 0.03 d and an
eccentricity e = 0.311 +/- 0.003. We firmly detect 20 oscillation frequencies,
9 of which are previously unseen in photometric data, and attempt mode
identification for the two dominant modes, f1 = 7.3764 c/d and f2 = 5.8496 c/d,
and determine the prograde or retrograde nature of 7 of the modes. The
projected rotational velocity of the star, vsini ~ 106.7 km/s, translates to a
rotation rate of veq/vcrit >= 33%. This relatively high rotation rate hampers
unique mode identification, since higher-order effects of rotation are not
included in the current methodology. We conclude that, in order to achieve
unambiguous mode identification for 4CVn, a complete description of rotation
and the use of blended lines have to be included in mode-identification
techniques.Comment: 18 pages, 18 figures (including Appendices), accepted for publication
in A&
Unresolved Rossby and gravity modes in 214 A and F stars showing rotational modulation
Here we report an ensemble study of 214 A- and F-type stars observed by
\textit{Kepler}, exhibiting the so-called \textit{hump and spike} periodic
signal, explained by Rossby modes (r~modes) -- the \textit{hump} -- and
magnetic stellar spots or overstable convective (OsC) modes -- the
\textit{spike} -- respectively. We determine the power confined in the
non-resolved hump features and find additional gravity~modes (g~modes) humps
always occurring at higher frequencies than the spike. Furthermore, we derive
projected rotational velocities from FIES, SONG and HERMES spectra for 28 stars
and the stellar inclination angle for 89 stars. We find a strong correlation
between the spike amplitude and the power in the r and g~modes, which suggests
that both types of oscillations are mechanically excited by either stellar
spots or OsC modes. Our analysis suggests that stars with a higher power in
r~modes humps are more likely to also exhibit humps at higher azimuthal
orders ( = 2, 3, or 4). Interestingly, all stars that show g~modes humps are
hotter and more luminous than the observed red edge of the Scuti
instability strip, suggesting that either magnetic fields or convection in the
outer layers could play an important role.Comment: 18 pages, 19 figure
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