118 research outputs found
Quasi-Biennial variations in helioseismic frequencies: Can the source of the variation be localized?
We investigate the spherical harmonic degree (l) dependence of the "seismic"
quasi-biennial oscillation (QBO) observed in low-degree solar p-mode
frequencies, using Sun-as-a-star Birmingham Solar Oscillations Network (BiSON)
data. The amplitude of the seismic QBO is modulated by the 11-yr solar cycle,
with the amplitude of the signal being largest at solar maximum. The amplitude
of the signal is noticeably larger for the l=2 and 3 modes than for the l=0 and
1 modes. The seismic QBO shows some frequency dependence but this dependence is
not as strong as observed in the 11-yr solar cycle. These results are
consistent with the seismic QBO having its origins in shallow layers of the
interior (one possibility being the bottom of the shear layer extending 5per
cent below the solar surface). Under this scenario the magnetic flux
responsible for the seismic QBO is brought to the surface (where its influence
on the p modes is stronger) by buoyant flux from the 11-yr cycle, the strong
component of which is observed at predominantly low-latitudes. As the l=2 and 3
modes are much more sensitive to equatorial latitudes than the l=0 and 1 modes
the influence of the 11-yr cycle on the seismic QBO is more visible in l=2 and
3 mode frequencies. Our results imply that close to solar maximum the main
influence of the seismic QBO occurs at low latitudes (<45 degrees), which is
where the strong component of the 11-yr solar cycle resides. To isolate the
latitudinal dependence of the seismic QBO from the 11-yr solar cycle we must
consider epochs when the 11-yr solar cycle is weak. However, away from solar
maximum, the amplitude of the seismic QBO is weak making the latitudinal
dependence hard to constrain.Comment: 10 pages, 6 figures, accepted for publication in MNRA
The Octave (Birmingham - Sheffield Hallam) automated pipeline for extracting oscillation parameters of solar-like main-sequence stars
The number of main-sequence stars for which we can observe solar-like
oscillations is expected to increase considerably with the short-cadence
high-precision photometric observations from the NASA Kepler satellite. Because
of this increase in number of stars, automated tools are needed to analyse
these data in a reasonable amount of time. In the framework of the asteroFLAG
consortium, we present an automated pipeline which extracts frequencies and
other parameters of solar-like oscillations in main-sequence and subgiant
stars. The pipeline uses only the timeseries data as input and does not require
any other input information. Tests on 353 artificial stars reveal that we can
obtain accurate frequencies and oscillation parameters for about three quarters
of the stars. We conclude that our methods are well suited for the analysis of
main-sequence stars, which show mainly p-mode oscillations.Comment: accepted by MNRA
Planetary detection limits taking into account stellar noise. II. Effect of stellar spot groups on radial-velocities
The detection of small mass planets with the radial-velocity technique is now
confronted with the interference of stellar noise. HARPS can now reach a
precision below the meter-per-second, which corresponds to the amplitudes of
different stellar perturbations, such as oscillation, granulation, and
activity. Solar spot groups induced by activity produce a radial-velocity noise
of a few meter-per-second. The aim of this paper is to simulate this activity
and calculate detection limits according to different observational strategies.
Based on Sun observations, we reproduce the evolution of spot groups on the
surface of a rotating star. We then calculate the radial-velocity effect
induced by these spot groups as a function of time. Taking into account
oscillation, granulation, activity, and a HARPS instrumental error of 80 cm/s,
we simulate the effect of different observational strategies in order to
efficiently reduce all sources of noise. Applying three measurements per night
of 10 minutes every three days, 10 nights a month seems the best tested
strategy. Depending on the level of activity considered, from log(R'_HK)= -5 to
-4.75, this strategy would allow us to find planets of 2.5 to 3.5 Earth masses
in the habitable zone of a K1V dwarf. Using Bern's model of planetary
formation, we estimate that for the same range of activity level, 15 to 35 % of
the planets between 1 and 5 Earth masses and with a period between 100 and 200
days should be found with HARPS. A comparison between the performance of HARPS
and ESPRESSO is also emphasized by our simulations. Using the same optimized
strategy, ESPRESSO could find 1.3 Earth mass planets in the habitable zone of
early-K dwarfs. In addition, 80 % of planets with mass between 1 and 5 Earth
masses and with a period between 100 and 200 days could be detected.Comment: 11 pages, 11 figures, accepted for publication in A&
The radius and mass of the close solar twin 18 Sco derived from asteroseismology and interferometry
The growing interest in solar twins is motivated by the possibility of
comparing them directly to the Sun. To carry on this kind of analysis, we need
to know their physical characteristics with precision. Our first objective is
to use asteroseismology and interferometry on the brightest of them: 18 Sco. We
observed the star during 12 nights with HARPS for seismology and used the PAVO
beam-combiner at CHARA for interferometry. An average large frequency
separation Hz and angular and linear radiuses of mas and R were estimated. We used these
values to derive the mass of the star, M.Comment: 5 pages, 5 figure
Estimating the p-mode frequencies of the solar twin 18 Sco
Solar twins have been a focus of attention for more than a decade, because
their structure is extremely close to that of the Sun. Today, thanks to
high-precision spectrometers, it is possible to use asteroseismology to probe
their interiors. Our goal is to use time series obtained from the HARPS
spectrometer to extract the oscillation frequencies of 18 Sco, the brightest
solar twin. We used the tools of spectral analysis to estimate these
quantities. We estimate 52 frequencies using an MCMC algorithm. After
examination of their probability densities and comparison with results from
direct MAP optimization, we obtain a minimal set of 21 reliable modes. The
identification of each pulsation mode is straightforwardly accomplished by
comparing to the well-established solar pulsation modes. We also derived some
basic seismic indicators using these values. These results offer a good basis
to start a detailed seismic analysis of 18 Sco using stellar models.Comment: 12 pages, 6 figures, to be published in A&
Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars
Red giants are evolved stars that have exhausted the supply of hydrogen in
their cores and instead burn hydrogen in a surrounding shell. Once a red giant
is sufficiently evolved, the helium in the core also undergoes fusion.
Outstanding issues in our understanding of red giants include uncertainties in
the amount of mass lost at the surface before helium ignition and the amount of
internal mixing from rotation and other processes. Progress is hampered by our
inability to distinguish between red giants burning helium in the core and
those still only burning hydrogen in a shell. Asteroseismology offers a way
forward, being a powerful tool for probing the internal structures of stars
using their natural oscillation frequencies. Here we report observations of
gravity-mode period spacings in red giants that permit a distinction between
evolutionary stages to be made. We use high-precision photometry obtained with
the Kepler spacecraft over more than a year to measure oscillations in several
hundred red giants. We find many stars whose dipole modes show sequences with
approximately regular period spacings. These stars fall into two clear groups,
allowing us to distinguish unambiguously between hydrogen-shell-burning stars
(period spacing mostly about 50 seconds) and those that are also burning helium
(period spacing about 100 to 300 seconds).Comment: to appear as a Letter to Natur
Solar-like oscillations in low-luminosity red giants: first results from Kepler
We have measured solar-like oscillations in red giants using time-series
photometry from the first 34 days of science operations of the Kepler Mission.
The light curves, obtained with 30-minute sampling, reveal clear oscillations
in a large sample of G and K giants, extending in luminosity from the red clump
down to the bottom of the giant branch. We confirm a strong correlation between
the large separation of the oscillations (Delta nu) and the frequency of
maximum power (nu_max). We focus on a sample of 50 low-luminosity stars (nu_max
> 100 muHz, L <~ 30 L_sun) having high signal-to-noise ratios and showing the
unambiguous signature of solar-like oscillations. These are H-shell-burning
stars, whose oscillations should be valuable for testing models of stellar
evolution and for constraining the star-formation rate in the local disk. We
use a new technique to compare stars on a single echelle diagram by scaling
their frequencies and find well-defined ridges corresponding to radial and
non-radial oscillations, including clear evidence for modes with angular degree
l=3. Measuring the small separation between l=0 and l=2 allows us to plot the
so-called C-D diagram of delta nu_02 versus Delta nu. The small separation
delta nu_01 of l=1 from the midpoint of adjacent l=0 modes is negative,
contrary to the Sun and solar-type stars. The ridge for l=1 is notably
broadened, which we attribute to mixed modes, confirming theoretical
predictions for low-luminosity giants. Overall, the results demonstrate the
tremendous potential of Kepler data for asteroseismology of red giants.Comment: accepted by ApJ Letters, to appear in special Kepler issue. Updated
reference
The quest for the solar g modes
Solar gravity modes (or g modes) -- oscillations of the solar interior for
which buoyancy acts as the restoring force -- have the potential to provide
unprecedented inference on the structure and dynamics of the solar core,
inference that is not possible with the well observed acoustic modes (or p
modes). The high amplitude of the g-mode eigenfunctions in the core and the
evanesence of the modes in the convection zone make the modes particularly
sensitive to the physical and dynamical conditions in the core. Owing to the
existence of the convection zone, the g modes have very low amplitudes at
photospheric levels, which makes the modes extremely hard to detect. In this
paper, we review the current state of play regarding attempts to detect g
modes. We review the theory of g modes, including theoretical estimation of the
g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the
techniques that have been used to try to detect g modes. We review results in
the literature, and finish by looking to the future, and the potential advances
that can be made -- from both data and data-analysis perspectives -- to give
unambiguous detections of individual g modes. The review ends by concluding
that, at the time of writing, there is indeed a consensus amongst the authors
that there is currently no undisputed detection of solar g modes.Comment: 71 pages, 18 figures, accepted by Astronomy and Astrophysics Revie
A chemical survey of exoplanets with ARIEL
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planetâs birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25â7.8 ÎŒm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10â100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed â using conservative estimates of mission performance and a full model of all significant noise sources in the measurement â using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL â in line with the stated mission objectives â will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio
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