359 research outputs found
The acoustic cut-off frequency of the Sun and the solar magnetic activity cycle
The acoustic cut-off frequency -the highest frequency for acoustic solar
eigenmodes- is an important parameter of the solar atmosphere as it determines
the upper boundary of the p-mode resonant cavities. At frequencies beyond this
value, acoustic disturbances are no longer trapped but traveling waves.
Interference amongst them give rise to higher-frequency peaks -the pseudomodes-
in the solar acoustic spectrum. The pseudomodes are shifted slightly in
frequency with respect to p modes making possible the use of pseudomodes to
determine the acoustic cut-off frequency. Using data from GOLF and VIRGO
instruments on board the SOHO spacecraft, we calculate the acoustic cut-off
frequency using the coherence function between both the velocity and intensity
sets of data. By using data gathered by these instruments during the entire
lifetime of the mission (1996 till the present), a variation in the acoustic
cut-off frequency with the solar magnetic activity cycle is found.Comment: Paper accepted in ApJ. 26 Pages, 9 figure
Analysis of the solar cycle and core rotation using 15 years of Mark-I observations:1984-1999. I. The solar cycle
High quality observations of the low-degree acoustic modes (p-modes) exist
for almost two complete solar cycles using the solar spectrophotometer Mark-I,
located at the Observatorio del Teide (Tenerife, Spain) and operating now as
part of the Birmingham Solar Oscillations Network (BiSON). We have performed a
Fourier analysis of 30 calibrated time-series of one year duration covering a
total period of 15 years between 1984 and 1999. Applying different techniques
to the resulting power spectra, we study the signature of the solar activity
changes on the low-degree p-modes. We show that the variation of the central
frequencies and the total velocity power (TVP) changes. A new method of
simultaneous fit is developed and a special effort has been made to study the
frequency-dependence of the frequency shift. The results confirm a variation of
the central frequencies of acoustic modes of about 450 nHz, peak-to-peak, on
average for low degree modes between 2.5 and 3.7 mHz. The TVP is
anti-correlated with the common activity indices with a decrease of about 20%
between the minimum and the maximum of solar cycle 22. The results are compared
with those obtained for intermediate degrees, using the LOWL data. The
frequency shift is found to increase with the degree with a weak l-dependence
similar to that of the inverse mode mass. This verifies earlier suggestions
that near surface effects are predominant.Comment: Accepted by A&A October 3 200
Magnetic activities on two single-lined RS Canum Venaticorum binaries IM Pegasi and Geminorum
We present the study on continuous high-resolution spectroscopic observations
of two long-period single-lined RS Canum Venaticorum (RS CVn) binary stars IM
Pegasi (IM Peg) and Geminorum ( Gem), obtained with the
Hertzsprung SONG telescope during the 2015-2016 season. Chromospheric activity
indicators H, Na I D, D doublet, He I D, and
H lines have been analyzed by using the spectral subtraction
technique. The expected chromospheric emission features in the H, Na
I D, D doublet, and H lines confirm that both of two
stars are very active systems. In the spectra, the He I D line had been
always detected in absorption feature. Although the behavior of chromospheric
activity indicators is very similar for both stars, the activity level of IM
Peg is much stronger than that of Gem. Moreover, the EW variations of
the H, He I D, and H line subtractions correlate
well and show different behavior among different orbital cycles, which
indicates the presence and evolution of activity longitudes over the surface of
two stars. Furthermore, the subtracted H line profile is usually
asymmetric. The red-shifted excess absorption features could be interpreted as
a strong down-flow of cool absorbing material, while the blue-shifted emission
component is probably caused by up-flow of hot materials through microflare
events
The onset of solar cycle 24: What global acoustic modes are telling us
We study the response of the low-degree, solar p-mode frequencies to the
unusually extended minimum of solar surface activity since 2007. A total of
4768 days of observations collected by the space-based, Sun-as-a-star
helioseismic GOLF instrument are analyzed. A multi-step iterative
maximum-likelihood fitting method is applied to subseries of 365 days and 91.25
days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2
p-mode frequencies are then obtained from April 1996 to May 2009. While the
p-mode frequency shifts are closely correlated with solar surface activity
proxies during the past solar cycles, the frequency shifts of the l=0 and l=2
modes show an increase from the second half of 2007, when no significant
surface activity is observable. On the other hand, the l=1 modes follow the
general decreasing trend of the solar surface activity. The different
behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted
as different geometrical responses to the spatial distribution of the solar
magnetic field beneath the surface of the Sun. The analysis of the low-degree,
solar p-mode frequency shifts indicates that the solar activity cycle 24
started late 2007, despite the absence of activity on the solar surface.Comment: To be accepted by A&A (with minor revisions), 4 pages, 3 figures, 1
tabl
Atmospheric extinction coefficients in the band for several major international observatories: Results from the BiSON telescopes, 1984 to 2016
Over 30 years of solar data have been acquired by the Birmingham Solar
Oscillations Network (BiSON), an international network of telescopes used to
study oscillations of the Sun. Five of the six BiSON telescopes are located at
major observatories. The observational sites are, in order of increasing
longitude: Mount Wilson (Hale) Observatory (MWO), California, USA; Las Campanas
Observatory (LCO), Chile; Observatorio del Teide, Iza\~{n}a, Tenerife, Canary
Islands; the South African Astronomical Observatory (SAAO), Sutherland, South
Africa; Carnarvon, Western Australia; and the Paul Wild Observatory, Narrabri,
New South Wales, Australia. The BiSON data may be used to measure atmospheric
extinction coefficients in the band (approximately 700-900 nm),
and presented here are the derived atmospheric extinction coefficients from
each site over the years 1984 to 2016.Comment: 15 pages, 10 figures, 4 tables. Accepted by Astronomical Journal:
2017 July 2
Acoustic oscillations and dynamo action in the G8 sub-giant EK Eri
We present further evidence of the presence of acoustic oscillations on the
slowly-rotating, over-active G8 sub-giant EK Eri. This star was observed with
the 1-m Hertzsprung SONG telescope, at the Observatorio del Teide for two
different runs of 8 and 13 nights, respectively, and separated by about a year.
We determined a significant excess of power around Hz in the first observing run and we were able to determine the large
separation Hz. No significant excess of power
was instead detected in a subsequent SONG observing season, as also supported
by our analysis of the simultaneous TESS photometric observations. We propose a
new amplitude-luminosity relation in order to account for the missing power in
the power spectrum. Based on the evolutionary stage of this object we argue
that standard dynamo cannot be excluded as a possible origin
for the observed magnetic field.Comment: 8 pages, 8 figures, accepted for publication in A&
Gravitational Helioseismology?
The magnitudes of the external gravitational perturbations associated with
the normal modes of the Sun are evaluated to determine whether these solar
oscillations could be observed with the proposed Laser Interferometer Space
Antenna (LISA), a network of satellites designed to detect gravitational
radiation. The modes of relevance to LISA---the , low-order , and
-modes---have not been conclusively observed to date. We find that the
energy in these modes must be greater than about in order
to be observable above the LISA detector noise. These mode energies are larger
than generally expected, but are much smaller than the current observational
upper limits. LISA may be confusion-limited at the relevant frequencies due to
the galactic background from short-period white dwarf binaries. Present
estimates of the number of these binaries would require the solar modes to have
energies above about to be observable by LISA.Comment: 8 pages; prepared with REVTEX 3.0 LaTeX macro
A temperature inversion with atomic iron in the ultra-hot dayside atmosphere of WASP-189b
Temperature inversion layers are predicted to be present in ultra-hot giant
planet atmospheres. Although such inversion layers have recently been observed
in several ultra-hot Jupiters, the chemical species responsible for creating
the inversion remain unidentified. Here, we present observations of the thermal
emission spectrum of an ultra-hot Jupiter, WASP-189b, at high spectral
resolution using the HARPS-N spectrograph. Using the cross-correlation
technique, we detect a strong Fe I signal. The detected Fe I spectral lines are
found in emission, which is direct evidence of a temperature inversion in the
planetary atmosphere. We further performed a retrieval on the observed spectrum
using a forward model with an MCMC approach. When assuming a solar metallicity,
the best-fit result returns a temperature of K at the top
of the inversion, which is significantly hotter than the planetary equilibrium
temperature (2641 K). The temperature at the bottom of the inversion is
determined as K. Such a strong temperature inversion is
probably created by the absorption of atomic species like Fe I.Comment: 9 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysics, in pres
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