294 research outputs found
Internal magnetic fields in 13 red giants detected by asteroseismology
While surface fields have been measured for stars across the HR diagram,
internal magnetic fields remain largely unknown. The recent seismic detection
of magnetic fields in the cores of several Kepler red giants has opened a new
avenue to understand better the origin of magnetic fields and their impact on
stellar structure and evolution. We aim to use asteroseismology to
systematically search for internal magnetic fields in red giant stars and to
determine the strengths and geometries of these fields. Magnetic fields are
known to break the symmetry of rotational multiplets. In red giants,
oscillation modes are mixed, behaving as pressure modes in the envelope and as
gravity modes in the core. Magnetism-induced asymmetries are expected to be
stronger for g-dominated modes than for p-dominated modes and to decrease with
frequency. After collecting a sample of 2500 Kepler red giant stars with clear
mixed-mode patterns, we specifically searched for targets among 1200 stars with
dipole triplets. We identified 13 stars exhibiting clear asymmetric multiplets
and measured their parameters, especially the asymmetry parameter and the
magnetic frequency shift. By combining these estimates with best-fitting
stellar models, we measured average core magnetic fields ranging from 20 to
150kG, corresponding to 5% to 30% of the critical field strengths. We showed
that the detected core fields have various horizontal geometries, some of which
significantly differ from a dipolar configuration. We found that the field
strengths decrease with stellar evolution, despite the fact that the cores of
these stars are contracting. Even though these stars have strong internal
magnetic fields, they display normal core rotation rates, suggesting no
significantly different histories of angular momentum transport compared to
other red giant stars. We also discuss the possible origin of the detected
fields.Comment: Accepted for publication in A&A. Long appendi
Asteroseismic Modeling of 1,153 Kepler Red Giant Branch Stars: Improved Stellar Parameters with Gravity-Mode Period Spacings and Luminosity Constraints
This paper reports estimated stellar parameters of 1,153 Kepler red giant
branch stars determined with asteroseismic modeling. We use radial-mode
oscillation frequencies, gravity-mode period spacings, Gaia luminosities, and
spectroscopic data to characterize these stars. Compared with previous studies,
we find that the two additional observed constraints, i.e., the gravity-mode
period spacing and luminosity, significantly improve the precision of
fundamental stellar parameters. The typical uncertainties are 2.9% for the
mass, 11% for the age, 1.0% for the radius, 0.0039 dex for the surface gravity,
and 0.5\% for the helium core mass, making this the best-characterized large
sample of red-giant stars available to date. With better characterizations for
these red giants, we recalibrate the seismic scaling relations and study the
surface term on the red-giant branch. We confirm that the surface term depends
on the surface gravity and effective temperature, but there is no significant
correlation with metallicity.Comment: Accepted by Ap
Solar-type Stars Observed by LAMOST and Kepler
Obtaining measurements of chromospheric and photometric activity of stars
with near-solar fundamental parameters and rotation periods is important for a
better understanding of solar-stellar connection. We select a sample of 2603
stars with near-solar fundamental parameters from the Large Sky Area
Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST
spectra to measure their chromospheric activity and Kepler light curves to
measure their photospheric activity (i.e., the amplitude of the photometric
variability). While the rotation periods of 1556 of these stars could not be
measured due to the low amplitude of the photometric variability and highly
irregular temporal profile of light curves, 254 stars were further identified
as having near-solar rotation periods. We show that stars with near-solar
rotation periods have chromospheric activities that are systematically higher
than stars with undetected rotation periods. Furthermore, while the solar level
of photospheric and chromospheric activity appears to be typical for stars with
undetected rotation periods, the Sun appears to be less active than most stars
with near-solar rotation periods (both in terms of photospheric and
chromospheric activity).Comment: 7 pages, 6 figure
- …