56 research outputs found
Gyrochronology and its usage for main sequence cool star ages
The construction of all age indicators consists of certain basic steps which
lead to the identification of the properties desirable for stellar age
indicators. Prior age indicators for main sequence field stars possess only
some of these properties. The measured rotation periods of cool stars are
particularly useful in this respect because they have well-defined dependencies
that allow stellar ages to be determined with ~20% errors. This method, called
gyrochronology, is explained informally in this talk, shown to have the desired
properties, compared to prior methods, and used to derive ages for samples of
main sequence field stars.Comment: 10 pages, 6 figures, presented at IAU258, Ages of Star
A connection between the morphology of the X-ray emission and rotation for solar-type stars in open clusters
It is suggested that the three-segmented morphology of the soft X-ray
emission from cluster and field stars may be understood in terms of the recent
classification of rotating stars into three kinds: those lying on the
convective sequence, on the interface sequence, or in the gap between them.Comment: 7 pages, 1 (color) figure, accepted by ApJ Letter
Evidence of New Magnetic Transitions in Late-Type Dwarfs from Gaia DR2
The second Gaia data release contains the identification of 147 535 low-mass
() rotational modulation variable candidates on (or close
to) the main sequence, together with their rotation period and modulation
amplitude. The richness, the period and amplitude range, and the photometric
precision of this sample make it possible to unveil, for the first time,
signatures of different surface inhomogeneity regimes in the amplitude-period
density diagram. The modulation amplitude distribution shows a clear
bimodality, with an evident gap at periods d. The low amplitude
branch, in turn, shows a period bimodality with a main clustering at periods 5 - 10 d and a secondary clustering of ultra-fast rotators at d. The amplitude-period multimodality is correlated with the position in
the period-absolute magnitude (or period-color) diagram, with the low- and
high-amplitude stars occupying different preferential locations. Here we argue
that such a multimodality represents a further evidence of the existence of
different regimes of surface inhomogeneities in young and middle-age low-mass
stars and we lay out possible scenarios for their evolution, which manifestly
include rapid transitions from one regime to another. In particular, the data
indicate that stars spinning up close to break-up velocity undergo a very rapid
change in their surface inhomogeneities configuration, which is revealed here
for the first time. The multimodality can be exploited to identify field stars
of age 100 -- 600 Myr belonging to the slow-rotator low-amplitude
sequence, for which age can be estimated from the rotation period via
gyrochronology relationships.Comment: 15 pages, 6 figures, Accepted by Ap
Rotation periods for cool stars in the open cluster Ruprecht 147 (NGC 6774): Implications for gyrochronology
Gyrochronology allows the derivation of ages for cool main sequence stars
based on their observed rotation periods and masses, or a suitable proxy
thereof. It is increasingly well-explored for FGK stars, but requires further
measurements for older ages and K-M-type stars. We study the nearby, 3 Gyr-old
open cluster Ruprecht 147 to compare it with the previously-studied, but far
more distant, NGC 6819 cluster, and especially to measure cooler stars than was
previously possible there. We constructed an inclusive list of 102 cluster
members from prior work, including Gaia DR2, and for which light curves were
also obtained during Campaign 7 of the Kepler/K2 space mission. [...] Periodic
signals are found for 32 stars, 21 of which are considered to be both highly
reliable and to represent single, or effectively single, Ru147 stars. These
stars cover the spectral types from late-F to mid-M stars, and they have
periods ranging from 6d-32d, allowing for a comparison of Ruprecht 147 to both
of the other open clusters and to models of rotational spindown. The derived
rotation periods connect reasonably to, overlap with, and extend to lower
masses the known rotation period distribution of the 2.5 Gyr-old cluster NGC
6819. The data confirm that cool stars lie on a single surface in rotation
period-mass-age space, and they simultaneously challenge its commonly assumed
shape. The shape at the low mass region of the color-period diagram at the age
of Ru147 favors a recently-proposed model, which requires a third
mass-dependent timescale in addition to the two timescales required by a former
model, suggesting that a third physical process is required to model rotating
stars effectively.Comment: 40 pages (17 pages + Appendix), 51 figures, Accepted for publication
in A&
Wide binaries demonstrate the consistency of rotational evolution between open cluster and field stars
Gyrochronology enables the derivation of ages of late-type main sequence
stars based on their rotation periods and a mass proxy, such as color. It has
been explored in open clusters, but a connection to field stars has yet to be
successfully established. We explore the rotation rates of wide binaries,
representing enlightening intermediaries between clusters and field stars, and
their overlap with those of open cluster stars. We investigated a recently
created catalog of wide binaries, matched the cataloged binaries to
observations by the Kepler mission (and its K2 extension), validated or
re-derived their rotation periods, identified 283 systems where both stars are
on the main sequence and have vetted rotation periods, and compared the systems
with open cluster data. We find that the vast majority of these wide binaries
(236) line up directly along the curvilinear ribs defined by open clusters in
color-period diagrams or along the equivalent interstitial gaps between
successive open clusters. The parallelism in shape is remarkable. Twelve
additional systems are clearly rotationally older. The deviant systems, a
minority, are mostly demonstrably hierarchical. Furthermore, the position of
the evolved component in the color-magnitude diagram for the additional wide
binary systems that contain one is consistent with the main sequence
component's rotational age. We conclude that wide binaries, despite their
diversity, follow the same spindown relationship as observed in open clusters,
and we find that rotation-based age estimates yield the same ages for both
components in a wide binary. This suggests that cluster and field stars spin
down in the same way and that gyrochronology can be applied to field stars to
determine their ages, provided that they are sufficiently distant from any
companions to be considered effectively single.Comment: 23 pages, 19 figures, published in A&A 675, A180 (2023
Stellar rotation, binarity, and lithium in the open cluster IC4756
An important aspect in the evolutionary scenario of cool stars is their
rotation and the rotationally induced magnetic activity and interior mixing.
Stars in open clusters are particularly useful tracers for these aspects
because of their known ages. We aim to characterize the open cluster IC4756 and
measure stellar rotation periods and surface differential rotation for a sample
of its member stars. Thirty-seven cluster stars were observed continuously with
the CoRoT satellite for 78 days in 2010. Follow-up high-resolution spectroscopy
of the CoRoT targets and deep Str\"omgren and H photometry
of the entire cluster were obtained with our robotic STELLA facility and its
echelle spectrograph and wide-field imager, respectively. We determined
high-precision photometric periods for 27 of the 37 CoRoT targets and found
values between 0.155 and 11.4 days. Twenty of these are rotation periods.
Twelve targets are spectroscopic binaries of which 11 were previously unknown;
orbits are given for six of them. Six targets were found that show evidence of
differential rotation with in the range 0.04-0.15. Five
stars are non-radially pulsating stars with fundamental periods of below 1d,
two stars are semi-contact binaries, and one target is a micro-flaring star
that also shows rotational modulation. Nine stars in total were not considered
members because of much redder color(s) and deviant radial velocities with
respect to the cluster mean. H photometry indicates that the cluster
ensemble does not contain magnetically over-active stars. The cluster average
metallicity is -0.080.06 (rms) and its logarithmic lithium abundance for
12 G-dwarf stars is 2.390.17 (rms). [...]Comment: A&A, in pres
Angular Momentum Loss from Cool Stars: An Empirical Expression and Connection to Stellar Activity
We show here that the rotation period data in open clusters allow the
empirical determination of an expression for the rate of loss of angular
momentum from cool stars on the main sequence. One significant component of the
expression, the dependence on rotation rate, persists from prior work; others
do not. The expression has a bifurcation, as before, that corresponds to an
observed bifurcation in the rotation periods of coeval open cluster stars. The
dual dependencies of this loss rate on stellar mass are captured by two
functions, and , that can be determined from the rotation
period observations. Equivalent masses and other [UBVRIJHK] colors are provided
in Table 1. Dimensional considerations, and a comparison with appropriate
calculated quantities suggest interpretations for and , both of which
appear to be related closely (but differently) to the calculated convective
turnover timescale, , in cool stars. This identification enables us to
write down symmetrical expressions for the angular momentum loss rate and the
deceleration of cool stars, and also to revive the convective turnover
timescale as a vital connection between stellar rotation and stellar activity
physics.Comment: 20 pages, 9 color figures; this version includes corrections listed
in the associated journal erratu
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