15,627 research outputs found
The connection between stellar granulation and oscillation as seen by the Kepler mission
The long and almost continuous observations by Kepler show clear evidence of
a granulation background signal in a large sample of stars, which is
interpreted as the surface manifestation of convection. It has been shown that
its characteristic timescale and rms intensity fluctuation scale with the peak
frequency (\nu_{max}) of the solar-like oscillations. Various attempts have
been made to quantify the observed signal, to determine scaling relations, and
to compare them to theoretical predictions. We use a probabilistic method to
compare different approaches to extracting the granulation signal. We fit the
power density spectra of a large set of Kepler targets, determine the
granulation and global oscillation parameter, and quantify scaling relations
between them. We establish that a depression in power at about \nu_{max}/2,
known from the Sun and a few other main-sequence stars, is also statistically
significant in red giants and that a super-Lorentzian function with two
components is best suited to reproducing the granulation signal in the broader
vicinity of the pulsation power excess. We also establish that the specific
choice of the background model can affect the determination of \nu_{max},
introducing systematic uncertainties that can significantly exceed the random
uncertainties. We find the characteristic background frequency and amplitude to
tightly scale with \nu_{max} for a wide variety of stars, and quantify a mass
dependency of the latter. To enable comparison with theoretical predictions, we
computed effective timescales and intensity fluctuations and found them to
approximately scale as \tau_{eff} \propto g^{-0.85}\,T^{-0.4} and A_{gran}
\propto (g^2M)^{-1/4}, respectively. Similarly, the bolometric pulsation
amplitude scales approximately as A_{puls} \propto (g^2M)^{-1/3}, which
implicitly verifies a separate mass and luminosity dependence of A_{puls}.Comment: 18 pages, 12 figures, accepted for A&
Evidence for Granulation and Oscillations in Procyon from Photometry with the WIRE satellite
We report evidence for the granulation signal in the star Procyon A, based on
two photometric time series from the star tracker on the WIRE satellite. The
power spectra show evidence of excess power around 1 milliHz, consistent with
the detection of p-modes reported from radial velocity measurements. We see a
significant increase in the noise level below 3 milliHz, which we interpret as
the granulation signal. We have made a large set of numerical simulations to
constrain the amplitude and timescale of the granulation signal and the
amplitude of the oscillations. We find that the timescale for granulation is
T(gran) = 750(200) s, the granulation amplitude is 1.8(0.3) times solar, and
the amplitude of the p-modes is 8(3) ppm. We found the distribution of peak
heights in the observed power spectra to be consistent with that expected from
p-mode oscillations. However, the quality of the data is not sufficient to
measure the large separation or detect a comb-like structure, as seen in the
p-modes of the Sun. Comparison with the recent negative result from the MOST
satellite reveal that the MOST data must have an additional noise source that
prevented the detection of oscillations.Comment: 23 pages, 12 figures, submitted to ApJ; v2 revisions: one reference
corrected and a comment in Figure 7 correcte
Evidence for granulation in early A-type stars
Stars with spectral types earlier than about F0 on (or close) to the main
sequence have long been believed to lack observable surface convection,
although evolutionary models of A-type stars do predict very thin surface
convective zones. We present evidence for granulation in two delta Scuti stars
of spectral type A2: HD174936 and HD50844. Recent analyses of space-based CoRoT
(Convection, Rotation, and planetary Transits) data revealed up to some 1000
frequencies in the photometry of these stars. The frequencies were interpreted
as individual pulsation modes. If true, there must be large numbers of
nonradial modes of very high degree l which should suffer cancellation effects
in disk-integrated photometry (even of high space-based precision). The p-mode
interpretation of all the frequencies in HD174936 and HD50844 depends on the
assumption of white (frequency independent) noise. Our independent analyses of
the data provide an alternative explanation: most of the peaks in the Fourier
spectra are the signature of non-white granulation background noise, and less
than about 100 of the frequencies are actual stellar p-modes in each star. We
find granulation time scales which are consistent with scaling relations that
describe cooler stars with known surface convection. If the granulation
interpretation is correct, the hundreds of low-amplitude Fourier peaks reported
in recent studies are falsely interpreted as independent pulsation modes and a
significantly lower number of frequencies are associated with pulsation,
consistent with only modes of low degree.Comment: accepted by ApJ
Temporal variations in the acoustic signal from faculae
The integrated brightness of the Sun shows variability on time-scales from
minutes to decades. This variability is mainly caused by pressure mode
oscillations, by granulation and by dark spots and bright faculae on the
surface of the Sun. By analyzing the frequency spectrum of the integrated
brightness we can obtain greater knowledge about these phenomena. It is shown
how the frequency spectrum of the integrated brightness of the Sun in the
frequency range from 0.1 to 3.2 mHz shows clear signs of both granulation,
faculae and p-mode oscillations and that the measured characteristic
time-scales and amplitudes of the acoustic signals from granulation and faculae
are consistent with high-resolution observations of the solar surface. Using 13
years of observations of the Sun's integrated brightness from the VIRGO
instrument on the SOHO satellite it is shown that the significance of the
facular component varies with time and that it has a significance above 0.99
around half the time. Furthermore, an analysis of the temporal variability in
the measured amplitudes of both the granulation, faculae and p-mode oscillation
components in the frequency spectrum reveals that the amplitude of the p-mode
oscillation component shows variability that follows the solar cycles, while
the amplitudes of the granulation and facular components show signs of
quasi-annual and quasi-biennial variability, respectively.Comment: Accepted for publication in MNRA
Hydrodynamical simulations of convection-related stellar micro-variability. II. The enigmatic granulation background of the COROT target HD49933
Local-box hydrodynamical model atmospheres provide statistical information
about a star's emergent radiation field which allows one to predict the level
of its granulation-related micro-variability. Space-based photometry is now
sufficiently accurate to test model predictions. We aim to model the
photometric granulation background of HD49933 as well as the Sun, and compare
the predictions to the measurements obtained by the COROT and SOHO satellite
missions. We construct hydrodynamical model atmospheres representing HD49933
and the Sun, and use a previously developed scaling technique to obtain the
observable disk-integrated brightness fluctuations. We further performed
exploratory magneto-hydrodynamical simulations to gauge the impact of small
scale magnetic fields on the synthetic light-curves. We find that the
granulation-related brightness fluctuations depend on metallicity. We obtain a
satisfactory correspondence between prediction and observation for the Sun,
validating our approach. For HD49933, we arrive at a significant
over-estimation by a factor of two to three in total power. Locally generated
magnetic fields are unlikely to be responsible, otherwise existing fields would
need to be rather strong to sufficiently suppress the granulation signal.
Presently suggested updates on the fundamental stellar parameters do not
improve the correspondence; however, an ad-hoc increase of the HD49933 surface
gravity by about 0.2dex would eliminate most of the discrepancy. We diagnose a
puzzling discrepancy between the predicted and observed granulation background
in HD49933, with only rather ad-hoc ideas for remedies at hand.Comment: 7 pages, 5 figures, accepted for publication in A&
Dynamics of Magnetic Flux Elements in the Solar Photosphere
The interaction of magnetic fields and convection is investigated in the
context of the coronal heating problem. We study the motions of photospheric
magnetic elements using filtergrams obtained at the Swedish Vacuum Solar
Telescope at La Palma. We use potential-field modeling to extrapolate the
magnetic and velocity fields to larger height. We find that the velocity in the
chromosphere can be locally enhanced at the separatrix surfaces between
neighboring flux tubes. The predicted velocities are several km/s,
significantly larger than those of the photospheric flux tubes, which may have
important implications for coronal heating. sComment: submitted to ApJ, 21 pages, 10 figure
Granulation in K-type Dwarf Stars. II. Hydrodynamic simulations and 3D spectrum synthesis
We construct a 3D radiative-hydrodynamic model atmosphere of parameters Teff
= 4820 K, log g = 4.5, and solar chemical composition. The theoretical line
profiles computed with this model are asymmetric, with their bisectors having a
characteristic C-shape and their core wavelengths shifted with respect to their
laboratory values. The line bisectors span from about 10 to 250 m/s, depending
on line strength, with the stronger features showing larger span. The
corresponding core wavelength shifts range from about -200 m/s for the weak Fe
I lines to almost +100 m/s in the strong Fe I features. Based on observational
results for the Sun, we argue that there should be no core wavelength shift for
Fe I lines of EW > 100 mA. The cores of the strongest lines show contributions
from the uncertain top layers of the model, where non-LTE effects and the
presence of the chromosphere, which are important in real stars, are not
accounted for. The comparison of model predictions to observed Fe I line
bisectors and core wavelength shifts for a reference star, HIP86400, shows
excellent agreement, with the exception of the core wavelength shifts of the
strongest features, for which we suspect inaccurate theoretical values. Since
this limitation does not affect the predicted line equivalent widths
significantly, we consider our 3D model validated for photospheric abundance
work.Comment: A&A, in pres
Stellar granulation and interferometry
Stars are not smooth. Their photosphere is covered by a granulation pattern
associated with the heat transport by convection. The convection-related
surface structures have different size, depth, and temporal variations with
respect to the stellar type. The related activity (in addition to other
phenomena such as magnetic spots, rotation, dust, etc.) potentially causes bias
in stellar parameters determination, radial velocity, chemical abundances
determinations, and exoplanet transit detections.
The role of long-baseline interferometric observations in this astrophysical
context is crucial to characterize the stellar surface dynamics and correct the
potential biases. In this Chapter, we present how the granulation pattern is
expected for different kind of stellar types ranging from main sequence to
extremely evolved stars of different masses and how interferometric techniques
help to study their photospheric dynamics.Comment: To appear in the Book of the VLTI School 2013, held 9-21 Sep 2013
Barcelonnette (France), "What the highest angular resolution can bring to
stellar astrophysics?", Ed. Millour, Chiavassa, Bigot, Chesneau, Meilland,
Stee, EAS Publications Series (2015
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