71 research outputs found
Transition from spot to faculae domination -- An alternate explanation for the dearth of intermediate \textit{Kepler} rotation periods
The study of stellar activity cycles is crucial to understand the underlying
dynamo and how it causes activity signatures such as dark spots and bright
faculae. We study the appearance of activity signatures in contemporaneous
photometric and chromospheric time series. Lomb-Scargle periodograms are used
to search for cycle periods present in both time series. To emphasize the
signature of the activity cycle we account for rotation-induced scatter in both
data sets by fitting a quasi-periodic Gaussian process model to each observing
season. After subtracting the rotational variability, cycle amplitudes and the
phase difference between the two time series are obtained by fitting both time
series simultaneously using the same cycle period. We find cycle periods in 27
of the 30 stars in our sample. The phase difference between the two time series
reveals that the variability in fast rotating active stars is usually in
anti-phase, while the variability of slowly rotating inactive stars is in
phase. The photometric cycle amplitudes are on average six times larger for the
active stars. The phase and amplitude information demonstrates that active
stars are dominated by dark spots, whereas less active stars are dominated by
bright faculae. We find the transition from spot to faculae domination at the
Vaughan-Preston gap, and around a Rossby number equal to one. We conclude that
faculae are the dominant ingredient of stellar activity cycles at ages >2.55
Gyr. The data further suggest that the Vaughan-Preston gap can not explain the
previously detected dearth of Kepler rotation periods between 15-25 days.
Nevertheless, our results led us to propose an explanation for the rotation
period dearth to be due to the non-detection of periodicity caused by the
cancellation of dark spots and bright faculae at 800 Myr.Comment: 12+15 pages, 10+2 figures, accepted for publication in A&
Fast and Automated Peak Bagging with DIAMONDS (FAMED)
Stars of low and intermediate mass that exhibit oscillations may show tens of
detectable oscillation modes each. Oscillation modes are a powerful to
constrain the internal structure and rotational dynamics of the star, hence
tool allowing one to obtain an accurate stellar age. The tens of thousands of
solar-like oscillators that have been discovered thus far are representative of
the large diversity of fundamental stellar properties and evolutionary stages
available. Because of the wide range of oscillation features that can be
recognized in such stars, it is particularly challenging to properly
characterize the oscillation modes in detail, especially in light of large
stellar samples. Overcoming this issue requires an automated approach, which
has to be fast, reliable, and flexible at the same time. In addition, this
approach should not only be capable of extracting the oscillation mode
properties of frequency, linewidth, and amplitude from stars in different
evolutionary stages, but also able to assign a correct mode identification for
each of the modes extracted. Here we present the new freely available pipeline
FAMED (Fast and AutoMated pEak bagging with DIAMONDS), which is capable of
performing an automated and detailed asteroseismic analysis in stars ranging
from the main sequence up to the core-Helium-burning phase of stellar
evolution. This, therefore, includes subgiant stars, stars evolving along the
red giant branch (RGB), and stars likely evolving toward the early asymptotic
giant branch. In this paper, we additionally show how FAMED can detect rotation
from dipolar oscillation modes in main sequence, subgiant, low-luminosity RGB,
and core-Helium-burning stars. FAMED can be downloaded from its public GitHub
repository (https://github.com/EnricoCorsaro/FAMED).Comment: 46 pages, 19 figures, 4 tables. Accepted for publication in A&
KOI-3890: A high mass-ratio asteroseismic red-giantM-dwarf eclipsing binary undergoing heartbeat tidal interactions
KOI-3890 is a highly eccentric, 153-day period eclipsing, single-lined
spectroscopic binary system containing a red-giant star showing solar-like
oscillations alongside tidal interactions. The combination of transit
photometry, radial velocity observations, and asteroseismology have enabled the
detailed characterisation of both the red-giant primary and the M-dwarf
companion, along with the tidal interaction and the geometry of the system. The
stellar parameters of the red-giant primary are determined through the use of
asteroseismology and grid-based modelling to give a mass and radius of
and
respectively. When combined with
transit photometry the M-dwarf companion is found to have a mass and radius of
and
. Moreover, through
asteroseismology we constrain the age of the system through the red-giant
primary to be . This provides a constraint on
the age of the M-dwarf secondary, which is difficult to do for other M-dwarf
binary systems. In addition, the asteroseismic analysis yields an estimate of
the inclination angle of the rotation axis of the red-giant star of
degrees. The obliquity of the system\textemdash the
angle between the stellar rotation axis and the angle normal to the orbital
plane\textemdash is also derived to give degrees
showing that the system is consistent with alignment. We observe no radius
inflation in the M-dwarf companion when compared to current low-mass stellar
models.Comment: 11 pages, 5 figures, accepted for publication in MNRA
Bayesian hierarchical inference of asteroseismic inclination angles
The stellar inclination angle-the angle between the rotation axis of a star
and our line of sight-provides valuable information in many different areas,
from the characterisation of the geometry of exoplanetary and eclipsing binary
systems, to the formation and evolution of those systems. We propose a method
based on asteroseismology and a Bayesian hierarchical scheme for extracting the
inclination angle of a single star. This hierarchical method therefore provides
a means to both accurately and robustly extract inclination angles from red
giant stars. We successfully apply this technique to an artificial dataset with
an underlying isotropic inclination angle distribution to verify the method. We
also apply this technique to 123 red giant stars observed with
. We also show the need for a selection function to account
for possible population-level biases, that are not present in individual
star-by-star cases, in order to extend the hierarchical method towards
inferring underlying population inclination angle distributions.Comment: 20 pages, 12 figures, accepted for publication in MNRA
Detection of solar-like oscillations in relics of the Milky Way: asteroseismology of K giants in M4 using data from the NASA K2 mission
Asteroseismic constraints on K giants make it possible to infer radii, masses
and ages of tens of thousands of field stars. Tests against independent
estimates of these properties are however scarce, especially in the metal-poor
regime. Here, we report the detection of solar-like oscillations in 8 stars
belonging to the red-giant branch and red-horizontal branch of the globular
cluster M4. The detections were made in photometric observations from the K2
Mission during its Campaign 2. Making use of independent constraints on the
distance, we estimate masses of the 8 stars by utilising different combinations
of seismic and non-seismic inputs. When introducing a correction to the Delta
nu scaling relation as suggested by stellar models, for RGB stars we find
excellent agreement with the expected masses from isochrone fitting, and with a
distance modulus derived using independent methods. The offset with respect to
independent masses is lower, or comparable with, the uncertainties on the
average RGB mass (4-10%, depending on the combination of constraints used). Our
results lend confidence to asteroseismic masses in the metal poor regime. We
note that a larger sample will be needed to allow more stringent tests to be
made of systematic uncertainties in all the observables (both seismic and
non-seismic), and to explore the properties of RHB stars, and of different
populations in the cluster.Comment: 6 pages, 3 figures, accepted for publication in MNRA
The Asteroseismic Poltential of TESS: Exoplanet-Host Stars
New insights on stellar evolution and stellar interior physics are being made possible by asteroseismology. Throughout the course of the Kepler mission, asteroseismology has also played an important role in the characterization of exoplanet-host stars and their planetary systems. The upcoming NASA Transiting Exoplanet Survey Satellite (TESS) will be performing a near all-sky survey for planets that transit bright nearby stars. In addition, its excellent photometric precision, combined with its fine time sampling and long intervals of uninterrupted observations, will enable asteroseismology of solar-type and red-giant stars. Here we develop a simple test to estimate the detectability of solar-like oscillations in TESS photometry of any given star. Based on an all-sky stellar and planetary synthetic population, we go on to predict the asteroseismic yield of the TESS mission, placing emphasis on the yield of exoplanet-host stars for which we expect to detect solar-like oscillations. This is done for both the target stars (observed at a 2-minute cadence) and the full-frame-image stars (observed at a 30-minute cadence). A similar exercise is also conducted based on a compilation of known host stars. We predict that TESS will detect solar-like oscillations in a few dozen target hosts (mainly subgiant stars but also in a smaller number of F dwarfs), in up to 200 low-luminosity red-giant hosts, and in over 100 solar-type and red-giant known hosts, thereby leading to a threefold improvement in the asteroseismic yield of exoplanet-host stars when compared to Kepler's.Science and Technology Facilities Council (Great Britain
Kepler-432: a red giant interacting with one of its two long period giant planets
We report the discovery of Kepler-432b, a giant planet ()
transiting an evolved star with an orbital period of days. Radial velocities (RVs) reveal that
Kepler-432b orbits its parent star with an eccentricity of , which we also measure independently with
asterodensity profiling (AP; ), thereby confirming
the validity of AP on this particular evolved star. The well-determined
planetary properties and unusually large mass also make this planet an
important benchmark for theoretical models of super-Jupiter formation.
Long-term RV monitoring detected the presence of a non-transiting outer planet
(Kepler-432c; days), and adaptive optics imaging revealed a nearby
(0\farcs87), faint companion (Kepler-432B) that is a physically bound M dwarf.
The host star exhibits high signal-to-noise asteroseismic oscillations, which
enable precise measurements of the stellar mass, radius and age. Analysis of
the rotational splitting of the oscillation modes additionally reveals the
stellar spin axis to be nearly edge-on, which suggests that the stellar spin is
likely well-aligned with the orbit of the transiting planet. Despite its long
period, the obliquity of the 52.5-day orbit may have been shaped by star-planet
interaction in a manner similar to hot Jupiter systems, and we present
observational and theoretical evidence to support this scenario. Finally, as a
short-period outlier among giant planets orbiting giant stars, study of
Kepler-432b may help explain the distribution of massive planets orbiting giant
stars interior to 1 AU.Comment: 22 pages, 19 figures, 5 tables. Accepted to ApJ on Jan 24, 2015
(submitted Nov 11, 2014). Updated with minor changes to match published
versio
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