5,784 research outputs found
Asteroseismic Signatures of Stellar Magnetic Activity Cycles
Observations of stellar activity cycles provide an opportunity to study
magnetic dynamos under many different physical conditions. Space-based
asteroseismology missions will soon yield useful constraints on the interior
conditions that nurture such magnetic cycles, and will be sensitive enough to
detect shifts in the oscillation frequencies due to the magnetic variations. We
derive a method for predicting these shifts from changes in the Mg II activity
index by scaling from solar data. We demonstrate this technique on the
solar-type subgiant beta Hyi, using archival International Ultraviolet Explorer
spectra and two epochs of ground-based asteroseismic observations. We find
qualitative evidence of the expected frequency shifts and predict the optimal
timing for future asteroseismic observations of this star.Comment: 5 pages including 3 figures and 1 table, MNRAS Letters accepte
Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey
We report preliminary results of a deep near-infrared search for
methane-absorbing brown dwarfs; almost five years after the discovery of Gl
229b, there are only a few confirmed examples of this type of object. New J
band, wide-field images, combined with pre-existing R band observations, allow
efficient identification of candidates by their extreme (R-J) colours.
Follow-up measurements with custom filters can then confirm objects with
methane absorption. To date, we have surveyed a total of 11.4 square degrees to
J~20.5 and R~25. Follow-up CH_4 filter observations of promising candidates in
1/4 of these fields have turned up no methane absorbing brown dwarfs. With 90%
confidence, this implies that the space density of objects similar to Gl 229b
is less than 0.012 per cubic parsec. These calculations account for the
vertical structure of the Galaxy, which can be important for sensitive
measurements. Combining published theoretical atmospheric models with our
observations sets an upper limit of alpha <= 0.8 for the exponent of the
initial mass function power law in this domain.Comment: 11 pages + 2 figures To be published in Astrophysical Journal Letter
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
Lessons for Asteroseismology from White Dwarf Stars
The interpretation of pulsation data for Sun-like stars is currently facing
challenges quite similar to those faced by white dwarf modelers ten years ago.
The observational requirements for uninterrupted long-term monitoring are
beginning to be satisfied by successful multi-site campaigns and dedicated
satellite missions. But exploration of the most important physical parameters
in theoretical models has been fairly limited, making it difficult to establish
a detailed best-fit model for a particular set of oscillation frequencies. I
review the past development and the current state of white dwarf
asteroseismology, with an emphasis on what this can tell us about the road to
success for asteroseismology of other types of stars.Comment: 10 pgs, 4 figs, Internat'l Workshop on Asteroseismology (Nainital
Caltech Faint Field Galaxy Redshift Survey IX: Source detection and photometry in the Hubble Deep Field Region
Detection and photometry of sources in the U_n, G, R, and K_s bands in a 9x9
arcmin^2 region of the sky, centered on the Hubble Deep Field, are described.
The data permit construction of complete photometric catalogs to roughly
U_n=25, G=26, R=25.5 and K_s=20 mag, and significant photometric measurements
somewhat fainter. The galaxy number density is 1.3x10^5 deg^{-2} to R=25.0 mag.
Galaxy number counts have slopes dlog N/dm=0.42, 0.33, 0.27 and 0.31 in the
U_n, G, R and K_s bands, consistent with previous studies and the trend that
fainter galaxies are, on average, bluer. Galaxy catalogs selected in the R and
K_s bands are presented, containing 3607 and 488 sources, in field areas of
74.8 and 59.4 arcmin^2, to R=25.5 and and K_s=20 mag.Comment: Accepted for publication in ApJS; some tables and slightly nicer
figures available at http://www.sns.ias.edu/~hogg/deep
Sounding stellar cycles with Kepler - II. Ground-based observations
We have monitored 20 Sun-like stars in the Kepler field-of-view for excess
flux with the FIES spectrograph on the Nordic Optical Telescope since the
launch of Kepler spacecraft in 2009. These 20 stars were selected based on
their asteroseismic properties to sample the parameter space (effective
temperature, surface gravity, activity level etc.) around the Sun. Though the
ultimate goal is to improve stellar dynamo models, we focus the present paper
on the combination of space-based and ground-based observations can be used to
test the age-rotation-activity relations.
In this paper we describe the considerations behind the selection of these 20
Sun-like stars and present an initial asteroseismic analysis, which includes
stellar age estimates. We also describe the observations from the Nordic
Optical Telescope and present mean values of measured excess fluxes. These
measurements are combined with estimates of the rotation periods obtained from
a simple analysis of the modulation in photometric observations from Kepler
caused by starspots, and asteroseismic determinations of stellar ages, to test
relations between between age, rotation and activity.Comment: Accepted for publication in MNRA
Sand stirred by chaotic advection
We study the spatial structure of a granular material, N particles subject to
inelastic mutual collisions, when it is stirred by a bidimensional smooth
chaotic flow. A simple dynamical model is introduced where four different time
scales are explicitly considered: i) the Stokes time, accounting for the
inertia of the particles, ii) the mean collision time among the grains, iii)
the typical time scale of the flow, and iv) the inverse of the Lyapunov
exponent of the chaotic flow, which gives a typical time for the separation of
two initially close parcels of fluid. Depending on the relative values of these
different times a complex scenario appears for the long-time steady spatial
distribution of particles, where clusters of particles may or not appear.Comment: 4 pages, 3 figure
Measuring the Angular Correlation Function for Faint Galaxies in High Galactic Latitude Fields
A photometric survey of faint galaxies in three high Galactic latitude fields
(each ) with sub-arcsecond seeing is used to study the
clustering properties of the faint galaxy population. Multi-color photometry of
the galaxies has been obtained to magnitude limits of , and
. Angular correlation analysis is applied to magnitude-limited and
color-selected samples of galaxies from the three fields for angular
separations ranging from . General agreement is obtained with other
recent studies which show that the amplitude of the angular correlation
function, , is smoothly decreasing as a function of limiting
magnitude. The observed decline of rules out the viability of
``maximal merger'' galaxy evolution models. Using redshift distributions
extrapolated to faint magnitude limits, models of galaxy clustering evolution
are calculated and compared to the observed I-band . Faint
galaxies are determined to have correlation lengths and clustering evolution
parameters of either and ;
and ; or and
, assuming and with . The latter case is for clustering fixed in co-moving
coordinates and is probably unrealistic since most local galaxies are observed
to be more strongly clustered. No significant variations in the clustering
amplitude as a function of color are detected, for all the color-selected
galaxy samples considered. (Abridged)Comment: LaTeX (aaspp4.sty), 54 pages including 15 postscript figures; 3
additional uuencoded, gzipped postscript files (~300 kb each) of Figs. 1, 2
and 3 available at ftp://ftp.astro.ubc.ca/pub/woods ; To be published in the
Nov. 20, 1997 issue of The Astrophysical Journa
Test-Retest reliability of a commercial linear position transducer (GymAware PowerTool) to measure velocity and power in the Back Squat and Bench Press
This study examined the test-retest reliability of the GymAware PowerTool (GYM) to measure velocity and power in the free-weight back squat and bench press. Twenty-nine academy rugby league players (age: 17.6 ± 1.0 years; body mass: 87.3 ± 20.8 kg) completed 2 test-retest sessions for the back squat followed by 2 test-retest sessions for the bench press. GYM measured mean velocity (MV), peak velocity (PV), mean power (MP), and peak power at 20, 40, 60, 80, and 90% of 1 repetition maximum (1RM). GYM showed good reliability (intraclass correlation coefficient [ICC] and standard error of measurement percentage, respectively) for the measurement of MV at loads of 40 (0.77, 3.9%), 60 (0.83, 4.8%), 80 (0.83, 5.8%), and 90% (0.79, 7.9%) of 1RM in the back squat. In the bench press, good reliability was evident for PV at 40 (0.82, 3.9%), 60 (0.81, 5.1%), and 80% (0.77, 8.4%) of 1RM, and for MV at 80 (0.78, 7.9%) and 90% (0.87, 9.9%) of 1RM. The measurement of MP showed good to excellent levels of reliability across all relative loads (ICC ≥0.75). In conclusion, GYM provides practitioners with reliable kinematic information in the back squat and bench press, at least with loads of 40–90% of 1RM. This suggests that strength and conditioning coaches can use the velocity data to regulate training load according to daily readiness and target specific components of the force-velocity curve. However, caution should be taken when measuring movement velocity at load
Asteroseismic determination of obliquities of the exoplanet systems Kepler-50 and Kepler-65
Results on the obliquity of exoplanet host stars -- the angle between the
stellar spin axis and the planetary orbital axis -- provide important
diagnostic information for theories describing planetary formation. Here we
present the first application of asteroseismology to the problem of stellar
obliquity determination in systems with transiting planets and Sun-like host
stars. We consider two systems observed by the NASA Kepler Mission which have
multiple transiting small (super-Earth sized) planets: the previously reported
Kepler-50 and a new system, Kepler-65, whose planets we validate in this paper.
Both stars show rich spectra of solar-like oscillations. From the asteroseismic
analysis we find that each host has its rotation axis nearly perpendicular to
the line of sight with the sines of the angles constrained at the 1-sigma level
to lie above 0.97 and 0.91, respectively. We use statistical arguments to show
that coplanar orbits are favoured in both systems, and that the orientations of
the planetary orbits and the stellar rotation axis are correlated.Comment: Accepted for publication in ApJ; 46 pages, 11 figure
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