56 research outputs found
Are the gyro-ages of field stars underestimated?
By using the current photometric rotational data on eight galactic open
clusters, we show that the evolutionary stellar model (isochrone) ages of these
clusters are tightly correlated with the period shifts applied to the (B-V)_0 -
P_rot ridges that optimally align these ridges to the one defined by Praesepe
and the Hyades. On the other hand, when the traditional Skumanich-type
multiplicative transformation is used, the ridges become far less aligned due
to the age-dependent slope change introduced by the period multiplication.
Therefore, we employ our simple additive gyro-age calibration on various
datasets of Galactic field stars to test its applicability. We show that, in
the overall sense, the gyro-ages are systematically greater than the isochrone
ages. The difference could exceed several giga years, depending on the stellar
parameters. Although the age overlap between the open clusters used in the
calibration and the field star samples is only partial, the systematic
difference indicates the limitation of the currently available gyro-age methods
and suggests that the rotation of field stars slows down with a considerably
lower speed than we would expect from the simple extrapolation of the stellar
rotation rates in open clusters.Comment: 10 pages, 15 figures; accepted for publication in Astronomy &
Astrophysic
Signature of non-isotropic distribution of stellar rotation inclination angles in the Praesepe cluster
The distribution of the stellar rotation axes of 113 main sequence stars in
the open cluster Praesepe are examined by using current photometric rotation
periods, spectroscopic rotation velocities, and estimated stellar radii. Three
different samples of stellar rotation data on spotted stars from the Galactic
field and two independent samples of planetary hosts are used as control
samples to support the consistency of the analysis. Considering the high
completeness of the Praesepe sample and the behavior of the control samples, we
find that the main sequence F - K stars in this cluster are susceptible to
rotational axis alignment. Using a cone model, the most likely inclination
angle is 76+/-14 degrees with a half opening angle of 47+/-24 degrees.
Non-isotropic distribution of the inclination angles is preferred over the
isotropic distribution, except if the rotation velocities used in this work are
systematically overestimated. We found no indication of this being the case on
the basis of the currently available data.Comment: Accepted for publication in Astronomy & Astrophysics: 5 pages with 4
figure
Synergies between Exoplanet Surveys and Variable Star Research
With the discovery of the first transiting extrasolar planetary system back
to 1999, a great number of projects started to hunt for other similar systems.
Because of the incidence rate of such systems was unknown and the length of the
shallow transit events is only a few percent of the orbital period, the goal
was to monitor continuously as many stars as possible for at least a period of
a few months. Small aperture, large field of view automated telescope systems
have been installed with a parallel development of new data reduction and
analysis methods, leading to better than 1% per data point precision for
thousands of stars. With the successful launch of the photometric satellites
CoRot and Kepler, the precision increased further by one-two orders of
magnitude. Millions of stars have been analyzed and searched for transits. In
the history of variable star astronomy this is the biggest undertaking so far,
resulting in photometric time series inventories immensely valuable for the
whole field. In this review we briefly discuss the methods of data analysis
that were inspired by the main science driver of these surveys and highlight
some of the most interesting variable star results that impact the field of
variable star astronomy.Comment: This is a review presented at "Wide-field variability surveys: a
21st-century perspective" - 22nd Los Alamos Stellar Pulsation Conference
Series Meeting, held in: San Pedro de Atacama, Chile, Nov. 28-Dec. 2, 2016.
To appear in Web of Conferences Journal: 13 pages, 8 figure
Are all RR Lyrae stars modulated?
We analyzed 151 variables previously classified as fundamental mode RR Lyrae
stars from Campaigns 01-04 of the Kepler two wheel (K2) archive. By employing a
method based on the application of systematics filtering with the aid of
co-trending light curves in the presence of the large amplitude signal
component, we searched for additional Fourier signals in the close neighborhood
of the fundamental period. We found only 13 stars without such components,
yielding the highest rate of 91% of modulated (Blazhko) stars detected so far.
A detection efficiency test suggests that this occurrence rate likely implies a
100% underlying rate. Furthermore, the same test performed on a subset of the
Large Magellanic Cloud RR Lyrae stars from the MACHO archive shows that the
conjecture of high true occurrence rate fits well to the low observed rate
derived from this database.Comment: Submitted to Astronomy & Astrophysics (6 pages, 8 figures
More planetary candidates from K2 Campaign 5 by tran_k2
CONTEXT. The exquisite precision of the space-based photometric surveys and
the unavoidable presence of instrumental systematics and intrinsic stellar
variability call for the development of sophisticated methods that separate
these signal components from those caused by planetary transits. AIMS. Here we
introduce tran_k2 a stand-alone Fortran code to search for planetary transits
under the colored noise of stellar variability and instrumental effects. With
this code we perform a survey for new candidates. METHODS. Stellar variability
is represented by a Fourier series, and, if needed, by an autoregressive model
to avoid excessive Gibbs overshoots at the edges. For the treatment of
systematics, cotrending and external parameter decorrelation are employed by
using cotrending stars with low stellar variability, the chip position and the
background flux level at the target. The filtering is made within the framework
of the standard weighted least squares, where the weights are determined
iteratively, to allow robust fit and separate the transit signal from stellar
variability and systematics. Once the periods of the transit components are
determined from the filtered data by the box-fitting least squares method, we
reconstruct the full signal and determine the transit parameters with a higher
accuracy. This step greatly reduces the excessive attenuation of the transit
depths and minimizes shape deformation. RESULTS. The code was tested on the
field of Campaign 5 of the K2 mission. We detected 98% of the systems with all
their candidate planets reported earlier by other authors, surveyed the whole
field and discovered 15 new systems. Additional 3 planets were found in 3
multiplanetary systems and 2 more planets were found in a previously known
single planet system.Comment: 1.15 Mb, 18 pages, after the 1st referee report from A&
Toward more accurate RR Lyrae metallicities
By using a large sample of published spectroscopic iron abundances, we point
out the importance of gravity correction in deriving more accurate metal
abundances for RR Lyrae stars. For the 197 stars with multiple spectra we find
overall [Fe/H] standard deviations of 0.167 (as published), 0.145 (shifted by
data source zero points) and 0.121 (both zero point-shifted and
gravity-corrected). These improvements are significant at the ~2 sigma level at
each correction step, leading to a clearly significant improvement after both
corrections applied. The higher quality of the gravity-corrected metallicities
is strongly supported also by the tighter correlation with the metallicities
predicted from the period and Fourier phase phi_31. This work underlines the
need for using some external estimates of the temporal gravity in the chemical
abundance analysis rather than relying on a full-fetched spectrum fit that
leads to large correlated errors in the estimated parameters.Comment: Submitted to A&A. After the 2nd (less negative/almost positive)
referee report. 1.2Mb, 10 pages (7+3 with appendices). Aesthetic
old-fashioned references with clickable ADS link
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