100 research outputs found
Observations of Cool-Star Magnetic Fields
Cool stars like the Sun harbor convection zones capable of producing
substantial surface magnetic fields leading to stellar magnetic activity. The
influence of stellar parameters like rotation, radius, and age on cool-star
magnetism, and the importance of the shear layer between a radiative core and
the convective envelope for the generation of magnetic fields are keys for our
understanding of low-mass stellar dynamos, the solar dynamo, and also for other
large-scale and planetary dynamos. Our observational picture of cool-star
magnetic fields has improved tremendously over the last years. Sophisticated
methods were developed to search for the subtle effects of magnetism, which are
difficult to detect particularly in cool stars. With an emphasis on the
assumptions and capabilities of modern methods used to measure magnetism in
cool stars, I review the different techniques available for magnetic field
measurements. I collect the analyses on cool-star magnetic fields and try to
compare results from different methods, and I review empirical evidence that
led to our current picture of magnetic fields and their generation in cool
stars and brown dwarfs.Comment: Published version at http://www.livingreviews.org/lrsp-2012-
Rotation- and temperature-dependence of stellar latitudinal differential rotation
More than 600 high resolution spectra of stars with spectral type F and later
were obtained in order to search for signatures of differential rotation in
line profiles. In 147 stars, the rotation law could be measured, 28 of them are
found to be differentially rotating. Comparison to rotation laws in stars of
spectral type A reveals that differential rotation sets in at the convection
boundary in the HR-diagram; no star that is significantly hotter than the
convection boundary exhibits the signatures of differential rotation. Four late
A-/early F-type stars close to the convection boundary and at vsini~100 km/s
show extraordinarily strong absolute shear at short rotation periods around one
day. It is suggested that this is due to their small convection zone depth and
that it is connected to a narrow range in surface velocity. Detection
frequencies of differential rotation were analyzed in stars with varying
temperature and rotation velocity. Measurable differential rotation is more
frequent in late-type stars and slow rotators. The strength of absolute shear
and differential rotation are examined as functions of the stellar effective
temperature and rotation period. The strongest shear is found at rotation
periods between two and three days. In slower rotators, the strongest shear at
a given rotation rate is given approximately by DOmega_max ~ P^{-1}. In faster
rotators, alpha_max and DOmega_max diminish less rapidly. A comparison with
differential rotation measurements in stars of later spectral type shows that
F-stars exhibit stronger shear than cooler stars do, the upper boundary in
absolute shear DOmega with temperature is consistent with the temperature
scaling law found in Doppler Imaging measurements.Comment: 15 pages, accepted for publication in A&A, typos correcte
A fast and reliable method to measure stellar differential rotation from photometric data
Co-rotating spots at different latitudes on the stellar surface generate
periodic photometric variability and can be useful proxies to detect
Differential Rotation (DR). DR is a major ingredient of the solar dynamo but
observations of stellar DR are rather sparse. In view of the Kepler space
telescope we are interested in the detection of DR using photometric
information of the star, and to develop a fast method to determine stellar DR
from photometric data. We ran a large Monte-Carlo simulation of differentially
rotating spotted stars with very different properties to investigate the
detectability of DR. For different noise levels the resulting light curves are
prewhitened using Lomb-Scargle periodograms to derive parameters for a global
sine fit to detect periodicities. We show under what conditions DR can
successfully be detected from photometric data, and in which cases the light
curve provides insufficient or even misleading information on the stellar
rotation law. In our simulations, the most significant period P1_{out} could be
detected in 96.2% of all light curves. Detection of a second period close to
P1_{out} is the signature of DR in our model. For the noise-free case, in 64.2%
of all stars such a period was found. Calculating the measured latitudinal
shear of two distinct spots \alpha_{out}, and comparing it to the known
original spot rotation rates shows that the real value is on average 3.2%
lower. Comparing the total equator-to-pole shear to we
find that is underestimated by 8.8%, esp. the detection of DR for
stars with < 6% is challenging. Finally, we apply our method to four
differentially rotating Kepler stars and find close agreement with results from
detailed modeling. Our method is capable of measuring stellar rotation periods
and detecting DR with relatively high accuracy and is suitable for large data
sets.Comment: accepted by A&
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