1,175 research outputs found
Solving the m-mixing problem for the three-dimensional time-dependent Schr\"{o}dinger equation by rotations: application to strong-field ionization of H2+
We present a very efficient technique for solving the three-dimensional
time-dependent Schrodinger equation. Our method is applicable to a wide range
of problems where a fullly three-dimensional solution is required, i.e., to
cases where no symmetries exist that reduce the dimensionally of the problem.
Examples include arbitrarily oriented molecules in external fields and atoms
interacting with elliptically polarized light. We demonstrate that even in such
cases, the three-dimensional problem can be decomposed exactly into two
two-dimensional problems at the cost of introducing a trivial rotation
transformation. We supplement the theoretical framework with numerical results
on strong-field ionization of arbitrarily oriented H2+ molecules.Comment: 5 pages, 4 figure
Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules
The yield of strong-field ionization, by a linearly polarized probe pulse, is
studied experimentally and theoretically, as a function of the relative
orientation between the laser field and the molecule. Experimentally, carbonyl
sulfide, benzonitrile and naphthalene molecules are aligned in one or three
dimensions before being singly ionized by a 30 fs laser pulse centered at 800
nm. Theoretically, we address the behaviour of these three molecules. We
consider the degree of alignment and orientation and model the angular
dependence of the total ionization yield by molecular tunneling theory
accounting for the Stark shift of the energy level of the ionizing orbital. For
naphthalene and benzonitrile the orientational dependence of the ionization
yield agrees well with the calculated results, in particular the observation
that ionization is maximized when the probe laser is polarized along the most
polarizable axis. For OCS the observation of maximum ionization yield when the
probe is perpendicular to the internuclear axis contrasts the theoretical
results.Comment: 14 pages, 4 figure
Spectroscopy of the roAp star Alpha-Cir -- II. The bisector and equivalent-width of the H-alpha line
We present bisector measurements of the H-alpha line of the rapidly
oscillating Ap (roAp) star, Alpha-Cir, obtained from dual-site observations
with medium-dispersion spectrographs. The velocity amplitude and phase of the
principal pulsation mode vary significantly, depending on the height in the
H-alpha line, including a phase reversal between the core and the wings of the
line. This supports the theory, suggested in Paper I, of a radial pulsational
node in the atmosphere of the star. Blending with metal lines partially affects
the H-alpha bisector results but probably not enough to explain the phase
reversal.
We have also detected changes in the equivalent-width of the line during the
pulsation, and measured the oscillatory signal as a function of wavelength
across the H-alpha region.Comment: 13 pages, 14 figures, accepted by MNRA
Solar-like oscillations and magnetic activity of the slow rotator EK Eri
We aim to understand the interplay between non-radial oscillations and
stellar magnetic activity and test the feasibility of doing asteroseismology of
magnetically active stars. We analyze 30 years of photometric time-series data,
3 years of HARPS radial velocity monitoring, and 3 nights of high-cadence HARPS
asteroseismic data. We construct a high-S/N HARPS spectrum that we use to
determine atmospheric parameters and chemical composition. Spectra observed at
different rotation phases are analyzed to search for signs of temperature or
abundance variations. An upper limit on the projected rotational velocity is
derived from very high-resolution CES spectra. We detect oscillations in EK Eri
with a frequency of the maximum power of nu_max = 320+/-32 muHz, and we derive
a peak amplitude per radial mode of ~0.15 m/s, which is a factor of ~3 lower
than expected. We suggest that the magnetic field may act to suppress
low-degree modes. Individual frequencies can not be extracted from the
available data. We derive accurate atmospheric parameters, refining our
previous analysis. We confirm that the main light variation is due to cool
spots, but that other contributions may need to be taken into account. We
suggest that the rotation period is twice the photometric period, i.e., P_rot =
2 P_phot = 617.6 d. We conclude from our derived parameters that v sin i < 0.40
km/s. We also link the time series of direct magnetic field measurements
available in the literature to our newly derived photometric ephemeris.Comment: 11 pages, 10 figures. Accepted by A&
Oscillations in the Sun with SONG: Setting the scale for asteroseismic investigations
Context. We present the first high-cadence multi-wavelength radial-velocity
observations of the Sun-as-a-star, carried out during 57 consecutive days using
the stellar \'echelle spectrograph at the Hertzsprung SONG Telescope operating
at the Teide Observatory. Aims. The aim was to produce a high-quality data set
and reference values for the global helioseismic parameters {\nu_{max}}, and
{\Delta \nu} of the solar p-modes using the SONG instrument. The obtained data
set or the inferred values should then be used when the scaling relations are
applied to other stars showing solar-like oscillations which are observed with
SONG or similar instruments. Methods. We used different approaches to analyse
the power spectrum of the time series to determine {\nu_{max}}; simple Gaussian
fitting and heavy smoothing of the power spectrum. {\Delta\nu} was determined
using the method of autocorrelation of the power spectrum. The amplitude per
radial mode was determined using the method described in Kjeldsen et al.
(2008). Results. We found the following values for the solar oscillations using
the SONG spectrograph: {\nu_{max}} = 3141 {\pm} 12 {\mu}Hz, {\Delta\nu} =
134.98 {\pm} 0.04 {\mu}Hz and an average amplitude of the strongest radial
modes of 16.6 {\pm} 0.4 cm/s. These values are consistent with previous
measurements with other techniques.Comment: 5 pages, 5 figures, letter accepted for A&
Modeling Kepler Observations of Solar-like Oscillations in the Red-giant Star HD 186355
We have analysed oscillations of the red giant star HD 186355 observed by the
NASA Kepler satellite. The data consist of the first five quarters of science
operations of Kepler, which cover about 13 months. The high-precision
time-series data allow us to accurately extract the oscillation frequencies
from the power spectrum. We find the frequency of the maximum oscillation
power, {\nu}_max, and the mean large frequency separation, {\Delta}{\nu}, are
around 106 and 9.4 {\mu}Hz respectively. A regular pattern of radial and
non-radial oscillation modes is identified by stacking the power spectra in an
echelle diagram. We use the scaling relations of {\Delta}{\nu} and {\nu}_max to
estimate the preliminary asteroseismic mass, which is confirmed with the
modelling result (M = 1.45 \pm 0.05 M_sun) using the Yale Rotating stellar
Evolution Code (YREC7). In addition, we constrain the effective temperature,
luminosity and radius from comparisons between observational constraints and
models. A number of mixed l = 1 modes are also detected and taken into account
in our model comparisons. We find a mean observational period spacing for these
mixed modes of about 58 s, suggesting that this red giant branch star is in the
shell hydrogen-burning phase.Comment: 26 pages, 5 figures and 2 table
Stellar Oscillations Network Group
Stellar Oscillations Network Group (SONG) is an initiative aimed at designing
and building a network of 1m-class telescopes dedicated to asteroseismology and
planet hunting. SONG will have 8 identical telescope nodes each equipped with a
high-resolution spectrograph and an iodine cell for obtaining precision radial
velocities and a CCD camera for guiding and imaging purposes. The main
asteroseismology targets for the network are the brightest (V<6) stars. In
order to improve performance and reduce maintenance costs the instrumentation
will only have very few modes of operation. In this contribution we describe
the motivations for establishing a network, the basic outline of SONG and the
expected performance.Comment: Proc. Vienna Workshop on the Future of Asteroseismology, 20 - 22
September 2006. Comm. in Asteroseismology, Vol. 150, in the pres
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