399 research outputs found
Asteroseismic stellar activity relations
In asteroseismology an important diagnostic of the evolutionary status of a
star is the small frequency separation which is sensitive to the gradient of
the mean molecular weight in the stellar interior. It is thus interesting to
discuss the classical age-activity relations in terms of this quantity.
Moreover, as the photospheric magnetic field tends to suppress the amplitudes
of acoustic oscillations, it is important to quantify the importance of this
effect by considering various activity indicators. We propose a new class of
age-activity relations that connects the Mt. Wilson index and the average
scatter in the light curve with the small frequency separation and the
amplitude of the p-mode oscillations. We used a Bayesian inference to compute
the posterior probability of various empirical laws for a sample of 19
solar-like active stars observed by the Kepler telescope. We demonstrate the
presence of a clear correlation between the Mt. Wilson index and the
relative age of the stars as indicated by the small frequency separation, as
well as an anti-correlation between the index and the oscillation
amplitudes. We argue that the average activity level of the stars shows a
stronger correlation with the small frequency separation than with the absolute
age that is often considered in the literature. The phenomenological laws
discovered in this paper have the potential to become new important diagnostics
to link stellar evolution theory with the dynamics of global magnetic fields.
In particular we argue that the relation between the Mt. Wilson index and
the oscillation amplitudes is in good agreement with the findings of direct
numerical simulations of magneto-convection.Comment: 5 pages, 4 figures, 2 tables. Accepted for publication in A&
Sounding stellar cycles with Kepler - preliminary results from ground-based chromospheric activity measurements
Due to its unique long-term coverage and high photometric precision,
observations from the Kepler asteroseismic investigation will provide us with
the possibility to sound stellar cycles in a number of solar-type stars with
asteroseismology. By comparing these measurements with conventional
ground-based chromospheric activity measurements we might be able to increase
our understanding of the relation between the chromospheric changes and the
changes in the eigenmodes.
In parallel with the Kepler observations we have therefore started a
programme at the Nordic Optical Telescope to observe and monitor chromospheric
activity in the stars that are most likely to be selected for observations for
the whole satellite mission. The ground-based observations presented here can
be used both to guide the selection of the special Kepler targets and as the
first step in a monitoring programme for stellar cycles. Also, the
chromospheric activity measurements obtained from the ground-based observations
can be compared with stellar parameters such as ages and rotation in order to
improve stellar evolution models.Comment: submitted to the proceedings of the IAU symposium No. 264, 200
The lost sunspot cycle: New support from Be10 measurements
It has been suggested that the deficit in the number of spots on the surface
of the Sun between 1790 and 1830, known as the Dalton minimum, contained an
extra cycle that was not identified in the original sunspot record by Wolf.
Though this cycle would be shorter and weaker than the average solar cycle, it
would shift the magnetic parity of the solar magnetic field of the earlier
cycles. This extra cycle is sometimes referred to as the 'lost solar cycle' or
'cycle 4b'. Here we reanalyse Be10 measurements with annual resolution from the
NGRIP ice core in Greenland in order to investigate if the hypothesis regarding
a lost sunspot cycle is supported by these measurements. Specifically, we make
use of the fact that the Galactic cosmic rays, responsible for forming Be10 in
the Earth's atmosphere, are affected differently by the open solar magnetic
field during even and odd solar cycles. This fact enables us to evaluate if the
numbering of cycles earlier than cycle 5 is correct. For the evaluation, we use
Bayesian analysis, which reveals that the lost sunspot cycle hypothesis is
likely to be correct. We also discuss if this cycle 4b is a real cycle, or a
phase catastrophe, and what implications this has for our understanding of
stellar activity cycles in general.Comment: accepted for publication in A&
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