2,628 research outputs found
Particle decay branching ratios for states of astrophysical importance in 19Ne
We have measured proton and alpha-particle branching ratios of excited states
in 19Ne formed using the 19F(3He,t) reaction at a beam energy of 25 MeV. These
ratios have a large impact on the astrophysical reaction rates of
15O(alpha,gamma), 18F(p,gamma) and 18F(p,alpha), which are of interest in
understanding energy generation in x-ray bursts and in interpreting anticipated
gamma-ray observations of novae. We detect decay protons and alpha-particles
using a silicon detector array in coincidence with tritons measured in the
focal plane detector of our Enge split-pole spectrograph. The silicon array
consists of five strip detectors of the type used in the Louvain-Edinburgh
Detector Array, subtending angles from 130 degrees to 165 degrees with
approximately 14% lab efficiency. The correlation angular distributions give
additional confidence in some prior spin-parity assignments that were based on
gamma branchings. We measure Gamma_p/Gamma=0.387+-0.016 for the 665 keV proton
resonance, which agrees well with the direct measurement of Bardayan et al.Comment: 5 pages, 2 figures, 3 tables. Prepared using RevTex 4 and BibTex.
Further minor revisions, incl. fig. 1 font size increase, 1 table removal,
and minor changes to the tex
The rapidly oscillating Ap star HD 99563 and its distorted dipole pulsation mode
We undertook a time-series photometric multi-site campaign for the rapidly
oscillating Ap star HD 99563 and also acquired mean light observations over two
seasons. The pulsations of the star, that show flatter light maxima than
minima, can be described with a frequency quintuplet centred on 1557.653
microHertz and some first harmonics of these. The amplitude of the pulsation is
modulated with the rotation period of the star that we determine with 2.91179
+/- 0.00007 d from the analysis of the stellar pulsation spectrum and of the
mean light data. We break the distorted oscillation mode up into its pure
spherical harmonic components and find it is dominated by the l=1 pulsation,
and also has a notable l=3 contribution, with weak l=0 and 2 components. The
geometrical configuration of the star allows one to see both pulsation poles
for about the same amount of time; HD 99563 is only the fourth roAp star for
which both pulsation poles are seen and only the third where the distortion of
the pulsation modes was modelled. We point out that HD 99563 is very similar to
the well-studied roAp star HR 3831. Finally, we note that the visual companion
of HD 99563 is located in the Delta Scuti instability strip and may thus show
pulsation. We show that if the companion was physical, the roAp star would be a
2.03 solar mass object, seen at a rotational inclination of 44 degrees, which
then predicts a magnetic obliquity of 86.4 degrees.Comment: 10 pages, 6 figures, accepted for publication by MNRA
SMEI observations of previously unseen pulsation frequencies in Îł Doradus
Aims. As g-mode pulsators, gamma-Doradus-class stars may naĂŻvely be expected to show a large number of modes. Taking advantage of the long photometric time-series generated by the solar mass ejection imager (SMEI) instrument, we have studied the star gamma Doradus to determine whether any other modes than the three already known are present at observable amplitude.
Methods. High-precision photometric data from SMEI taken between April 2003 and March 2006 were subjected to periodogram analysis with the PERIOD04 package.
Results. We confidently determine three additional frequencies at 1.39, 1.87, and 2.743 dâ1. These are above and beyond the known frequencies of 1.320, 1.364, and 1.47 dâ1.
Conclusions. Two of the new frequencies, at 1.39 and 1.87 dâ1, are speculated to be additional modes of oscillation, with the third frequency at 2.743â1 a possible combination frequency
Studying the photometric and spectroscopic variability of the magnetic hot supergiant Orionis Aa
Massive stars play a significant role in the chemical and dynamical evolution
of galaxies. However, much of their variability, particularly during their
evolved supergiant stage, is poorly understood. To understand the variability
of evolved massive stars in more detail, we present a study of the O9.2Ib
supergiant Ori Aa, the only currently confirmed supergiant to host a
magnetic field. We have obtained two-color space-based BRIght Target Explorer
photometry (BRITE) for Ori Aa during two observing campaigns, as well
as simultaneous ground-based, high-resolution optical CHIRON spectroscopy. We
perform a detailed frequency analysis to detect and characterize the star's
periodic variability. We detect two significant, independent frequencies, their
higher harmonics, and combination frequencies: the stellar rotation period
d, most likely related to the presence of the
stable magnetic poles, and a variation with a period of d
attributed to circumstellar environment, also detected in the H and
several He I lines, yet absent in the purely photospheric lines. We confirm the
variability with /4, likely caused by surface
inhomogeneities, being the possible photospheric drivers of the discrete
absorption components. No stellar pulsations were detected in the data. The
level of circumstellar activity clearly differs between the two BRITE observing
campaigns. We demonstrate that Ori Aa is a highly variable star with
both periodic and non-periodic variations, as well as episodic events. The
rotation period we determined agrees well with the spectropolarimetric value
from the literature. The changing activity level observed with BRITE could
explain why the rotational modulation of the magnetic measurements was not
clearly detected at all epochs.Comment: 20 pages, 5 tables, 12 figures, accepted for publication in A&
Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni
The BRITE Constellation of nanosatellites obtained mmag photometry of 28
Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection
Imager in 2003-2010 and 118 H line profiles were added.
For decades, 28 Cyg has exhibited four large-amplitude frequencies: two
closely spaced frequencies of spectroscopically confirmed modes near 1.5
c/d, one slightly lower exophotospheric (Stefl) frequency, and at 0.05 c/d the
difference frequency between the two g modes. This top-level framework is
indistinguishable from eta Cen (Paper I), which is also very similar in
spectral type, rotation rate, and viewing angle. The Stefl frequency is the
only one that does not seem to be affected by the difference frequency. The
amplitude of the latter undergoes large variations; around maximum the amount
of near-circumstellar matter is increased, and the amplitude of the Stefl
frequency grows by some factor. During such brightenings dozens of transient
spikes appear in the frequency spectrum, concentrated in three groups. Only
eleven frequencies were common to all years of BRITE observations.
Be stars seem to be controlled by several coupled clocks, most of which are
not very regular on timescales of weeks to months but function for decades. The
combination of g modes to the low difference frequency and/or the atmospheric
response to it appears significantly nonlinear. Like in eta Cen, the
difference-frequency variability seems the main responsible for the modulation
of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference
frequencies may reach the longest timescales known of the Be phenomenon.Comment: 17 pages, 21 figures, submitted to Astronomy & Astrophysic
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