163 research outputs found
Stellar Flares Are Far-Ultraviolet Luminous
We identify 182 flares on 158 stars within 100 pc of the Sun in both the
near-ultraviolet (NUV: 1750-2750 \r{A}) and far-ultraviolet (FUV: 1350-1750
\r{A}) using high-cadence light curves from the Galaxy Evolution Explorer
(GALEX). Ultraviolet (UV) emission from stellar flares plays a crucial role in
determining the habitability of exoplanetary systems. However, whether such UV
emission promotes or threatens such life depends strongly on the energetics of
these flares. Most studies assessing the effect of flares on planetary
habitability assume a 9000 K blackbody spectral energy distribution that
produces more NUV flux than FUV flux (). Instead, we observe the opposite with the excess FUV
reaching , roughly times the expectation of a
9000 K blackbody. The ratio of FUV to NUV time-integrated flare energies is 3.0
times higher on average than would be predicted by a constant 9000 K blackbody
during the flare. Finally, we find that the FUV/NUV ratio at peak tentatively
correlates ( significance) both with total UV flare energy and
with the G - RP color of the host star. On average, we observe higher FUV/NUV
ratios at peak in erg flares and in flares on fully
convective stars.Comment: Submitted to MNRAS, comments welcom
Stellar Cruise Control: Weakened Magnetic Braking Leads to Sustained Rapid Rotation of Old Stars
Despite a growing sample of precisely measured stellar rotation periods and
ages, the strength of magnetic braking and the degree of departure from
standard (Skumanich-like) spindown have remained persistent questions,
particularly for stars more evolved than the Sun. Rotation periods can be
measured for stars older than the Sun by leveraging asteroseismology, enabling
models to be tested against a larger sample of old field stars. Because
asteroseismic measurements of rotation do not depend on starspot modulation,
they avoid potential biases introduced by the need for a stellar dynamo to
drive starspot production. Using a neural network trained on a grid of stellar
evolution models and a hierarchical model-fitting approach, we constrain the
onset of weakened magnetic braking. We find that a sample of stars with
asteroseismically-measured rotation periods and ages is consistent with models
that depart from standard spindown prior to reaching the evolutionary stage of
the Sun. We test our approach using neural networks trained on model grids
produced by separate stellar evolution codes with differing physical
assumptions and find that the choices of grid physics can influence the
inferred properties of the braking law. We identify the normalized critical
Rossby number as the
threshold for the departure from standard rotational evolution. This suggests
that weakened magnetic braking poses challenges to gyrochronology for roughly
half of the main sequence lifetime of sun-like stars.Comment: 26 pages, 10 figure
Characterizing two solar-type Kepler subgiants with asteroseismology: KIC10920273 and KIC11395018
Determining fundamental properties of stars through stellar modeling has
improved substantially due to recent advances in asteroseismology. Thanks to
the unprecedented data quality obtained by space missions, particularly CoRoT
and Kepler, invaluable information is extracted from the high-precision stellar
oscillation frequencies, which provide very strong constraints on possible
stellar models for a given set of classical observations. In this work, we have
characterized two relatively faint stars, KIC10920273 and KIC11395018, using
oscillation data from Kepler photometry and atmospheric constraints from
ground-based spectroscopy. Both stars have very similar atmospheric properties;
however, using the individual frequencies extracted from the Kepler data, we
have determined quite distinct global properties, with increased precision
compared to that of earlier results. We found that both stars have left the
main sequence and characterized them as follows: KIC10920273 is a
one-solar-mass star (M=1.00 +/- 0.04 M_sun), but much older than our Sun
(t=7.12 +/- 0.47 Gyr), while KIC11395018 is significantly more massive than the
Sun (M=1.27 +/- 0.04 M_sun) with an age close to that of the Sun (t=4.57 +/-
0.23 Gyr). We confirm that the high lithium abundance reported for these stars
should not be considered to represent young ages, as we precisely determined
them to be evolved subgiants. We discuss the use of surface lithium abundance,
rotation and activity relations as potential age diagnostics.Comment: 12 pages, 3 figures, 5 tables. Accepted by Ap
Poster CS20.5 - Weakened magnetic braking supported by asteroseismic rotation
Studies using asteroseismic ages and rotation rates from star-spot rotation have indicated that standard age-rotation relations may break down roughly half-way through the main sequence lifetime, a phenomenon referred to as weakened magnetic braking. While rotation rates from spots can be difficult to determine for older, less active stars, rotational splitting of asteroseismic oscillation frequencies can provide rotation rates for both active and quiescent stars, and so can confirm whether this effect really takes place on the main sequence. In this talk, I’ll show how we obtained asteroseismic rotation rates of 91 main sequence stars showing high signal-to-noise modes of oscillation. Using these new rotation rates, along with effective temperatures, metallicities and seismic masses and ages, we built a hierarchical Bayesian mixture model that showed that our new ensemble more closely agreed with weakened magnetic braking, over a standard rotational evolution scenario
Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars
A knowledge of stellar ages is crucial for our understanding of many
astrophysical phenomena, and yet ages can be difficult to determine. As they
become older, stars lose mass and angular momentum, resulting in an observed
slowdown in surface rotation. The technique of 'gyrochronology' uses the
rotation period of a star to calculate its age. However, stars of known age
must be used for calibration, and, until recently, the approach was untested
for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for
stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for
old field stars whose ages have been determined with asteroseismology. The data
for the cluster agree with previous period-age relations, but these relations
fail to describe the asteroseismic sample. Here we report stellar evolutionary
modelling, and confirm the presence of unexpectedly rapid rotation in stars
that are more evolved than the Sun. We demonstrate that models that incorporate
dramatically weakened magnetic braking for old stars can---unlike existing
models---reproduce both the asteroseismic and the cluster data. Our findings
might suggest a fundamental change in the nature of ageing stellar dynamos,
with the Sun being close to the critical transition to much weaker magnetized
winds. This weakened braking limits the diagnostic power of gyrochronology for
those stars that are more than halfway through their main-sequence lifetimes.Comment: 25 pages, 3 figures in main paper, 6 extended data figures, 1 table.
Published in Nature, January 2016. Please see https://youtu.be/O6HzYgP5uyc
for a video description of the resul
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