114 research outputs found
A Survey of Chromospheric Activity in the Solar-Type Stars in the Open Cluster M67
We present the results of a spectroscopic survey of the Ca II H & K core
strengths in a sample of 60 solar-type stars that are members of the solar-age
and solar-metallicity open cluster M67. We adopt the HK index, defined as the
summed H+K core strengths in 0.1 nm bandpasses centered on the H and K lines,
respectively, as a measure of the chromospheric activity that is present. We
compare the distribution of mean HK index values for the M67 solar-type stars
with the variation of this index as measured for the Sun during the
contemporary solar cycle. We find that the stellar distribution in our HK index
is broader than that for the solar cycle. Approximately 17% of the M67 sun-like
stars exhibit average HK indices that are less than solar minimum. About 7%-12%
are characterized by relatively high activity in excess of solar maximum values
while 72%-80% of the solar analogs exhibit Ca II H+K strengths within the range
of the modern solar cycle. The ranges given reflect uncertainties in the most
representative value of the maximum in the HK index to adopt for the solar
cycle variations observed during the period A.D. 1976--2004. Thus, ~ 20% - 30%
of our homogeneous sample of sun-like stars have mean chromospheric H+K
strengths that are outside the range of the contemporary solar cycle. Any
cycle-like variability that is present in the M67 solar-type stars appears to
be characterized by periods greater than ~ 6 years. Finally, we estimate a mean
chromospheric age for M67 in the range of 3.8--4.3 Gyr.Comment: Accepted in The Astrophysical Journa
Dairy Farm Business Summary: Central Valleys Region 1997
E.B. 98-12Dairy farm managers throughout New York State have been participating in Cornell Cooperative Extension's farm business summary and analysis program since the early 1950's. Managers of each participating farm business receive a comprehensive summary and analysis of their farm business. The information in this report represents averages of the data submitted from dairy farms in the Central Valleys Region for 1997
From the Provost
Welcome back to spring semester 2015 - enrollment updates -- Review of student learning outcomes - 300/400-level courses -- Startup of Diversity Action Committee -- Toward UA-wide minimum baccalaureate admission standards -- Completing adjunct faculty self-assessments -- Academic program review update
Limits of ultra-high-precision optical astrometry: Stellar surface structures
We investigate the astrometric effects of stellar surface structures as a
practical limitation to ultra-high-precision astrometry, e.g. in the context of
exoplanet searches, and to quantify the expected effects in different regions
of the HR-diagram. Stellar surface structures are likely to produce
fluctuations in the integrated flux and radial velocity of the star, as well as
a variation of the observed photocentre, i.e. astrometric jitter, and closure
phase. We use theoretical considerations supported by Monte Carlo simulations
to derive statistical relations between the corresponding astrometric,
photometric, and radial-velocity effects. For most stellar types the
astrometric jitter due to stellar surface structures is expected to be of order
10 micro-AU or greater. This is more than the astrometric displacement
typically caused by an Earth-size exoplanet in the habitable zone, which is
about 1-4 micro-AU for long-lived main-sequence stars. Only for stars with
extremely low photometric variability (<0.5 mmag) and low magnetic activity,
comparable to that of the Sun, will the astrometric jitter be of order 1
micro-AU, suffcient to allow the astrometric detection of an Earth-sized planet
in the habitable zone. While stellar surface structure may thus seriously
impair the astrometric detection of small exoplanets, it has in general
negligible impact on the detection of large (Jupiter-size) planets and on the
determination of stellar parallax and proper motion. From the starspot model we
also conclude that the commonly used spot filling factor is not the most
relevant parameter for quantifying the spottiness in terms of the resulting
astrometric, photometric and radial-velocity variations.Comment: 12 pages, 4 figures, submitted to A&
Patterns of photometric and chromospheric variation among Sun-like stars: A 20-year perspective
We examine patterns of variation of 32 primarily main sequence stars,
extending our previous 7-12 year time series to 13-20 years by combining b, y
data from Lowell Observatory with similar data from Fairborn Observatory.
Parallel chromospheric Ca II H and K emission data from the Mount Wilson
Observatory span the entire interval. The extended data strengthen the
relationship between chromospheric and photometric variation derived
previously. Twenty-seven stars are deemed variable. On a year-to-year timescale
young active stars become fainter when their Ca II emission increases while
older less active stars such as the Sun become brighter when their Ca II
emission increases. The Sun's total irradiance variation, scaled to the b and y
filter photometry, still appears to be somewhat smaller than stars in our
limited sample with similar mean chromospheric activity, but we now regard this
discrepancy as probably due mainly to our limited stellar sampl
RACE-OC Project: Rotation and variability in the open cluster M11 (NGC6705)
Rotation and magnetic activity are intimately linked in main-sequence stars
of G or later spectral types. The presence and level of magnetic activity
depend on stellar rotation, and rotation itself is strongly influenced by
strength and topology of the magnetic fields. Open clusters represent
especially useful targets to investigate the rotation/activity/age connection.
The open cluster M11 has been studied as a part of the RACE-OC project
(Rotation and ACtivity Evolution in Open Clusters), which is aimed at exploring
the evolution of rotation and magnetic activity in the late-type members of
open clusters with different ages. Photometric observations of the open cluster
M11 were carried out in June 2004 using LOAO 1m telescope. The rotation periods
of the cluster members are determined by Fourier analysis of photometric data
time series. We further investigated the relations between the surface
activity, characterized by the light curve amplitude, and rotation. We have
discovered a total of 75 periodic variables in the M11 FoV, of which 38 are
candidate cluster members. Specifically, among cluster members we discovered 6
early-type, 2 eclipsing binaries and 30 bona-fide single periodic late-type
variables. Considering the rotation periods of 16 G-type members of the almost
coeval 200-Myr M34 cluster, we could determine the rotation period distribution
from a more numerous sample of 46 single G stars at an age of about 200-230 Myr
and determine a median rotation period P=4.8d. A comparison with the younger
M35 cluster (~150 Myr) and with the older M37 cluster (~550 Myr) shows that G
stars rotate slower than younger M35 stars and faster than older M37 stars. The
measured variation of the median rotation period is consistent with the
scenario of rotational braking of main-sequence spotted stars as they age.Comment: Accepted by Astronomy and Astrophysics on Dec 15, 200
Rotation of planet-harbouring stars
The rotation rate of a star has important implications for the detectability,
characterisation and stability of any planets that may be orbiting it. This
chapter gives a brief overview of stellar rotation before describing the
methods used to measure the rotation periods of planet host stars, the factors
affecting the evolution of a star's rotation rate, stellar age estimates based
on rotation, and an overview of the observed trends in the rotation properties
of stars with planets.Comment: 16 pages, 4 figures: Invited review to appear in 'Handbook of
Exoplanets', Springer Reference Works, edited by Hans J. Deeg and Juan
Antonio Belmont
The rotation of field stars from CoRoT data
We present period measurements of a large sample of field stars in the solar
neighbourhood, observed by CoRoT in two different directions of the Galaxy. The
presence of a period was detected using the Scargle Lomb Normalized Periodogram
technique and the autocorrelation analysis. The assessment of the results has
been performed through a consistency verification supported by the folded light
curve analysis. The data analysis procedure has discarded a non-negligible
fraction of light curves due to instrumental artifacts, however it has allowed
us to identify pulsators and binaries among a large number of field stars. We
measure a wide range of periods, from 0.25 to 100 days, most of which are
rotation periods. The final catalogue includes 1978 periods, with 1727 of them
identified as rotational periods, 169 are classified as pulsations and 82 as
orbital periods of binary systems. Our sample suffers from selection biases not
easily corrected for, thus we do not use the distribution of rotation periods
to derive the age distribution of the main-sequence population. Nevertheless,
using rotation as a proxy for age, we can identify a sample of young stars (<
600 Myr), that will constitute a valuable sample, supported by further
spectroscopic observations, to study the recent star formation history in the
solar neighborhood.Comment: 13 pages, 11 figure
Kepler Mission Stellar and Instrument Noise Properties
Kepler Mission results are rapidly contributing to fundamentally new
discoveries in both the exoplanet and asteroseismology fields. The data
returned from Kepler are unique in terms of the number of stars observed,
precision of photometry for time series observations, and the temporal extent
of high duty cycle observations. As the first mission to provide extensive time
series measurements on thousands of stars over months to years at a level
hitherto possible only for the Sun, the results from Kepler will vastly
increase our knowledge of stellar variability for quiet solar-type stars. Here
we report on the stellar noise inferred on the timescale of a few hours of most
interest for detection of exoplanets via transits. By design the data from
moderately bright Kepler stars are expected to have roughly comparable levels
of noise intrinsic to the stars and arising from a combination of fundamental
limitations such as Poisson statistics and any instrument noise. The noise
levels attained by Kepler on-orbit exceed by some 50% the target levels for
solar-type, quiet stars. We provide a decomposition of observed noise for an
ensemble of 12th magnitude stars arising from fundamental terms (Poisson and
readout noise), added noise due to the instrument and that intrinsic to the
stars. The largest factor in the modestly higher than anticipated noise follows
from intrinsic stellar noise. We show that using stellar parameters from
galactic stellar synthesis models, and projections to stellar rotation,
activity and hence noise levels reproduces the primary intrinsic stellar noise
features.Comment: Accepted by ApJ; 26 pages, 20 figure
Suppressed Far-UV stellar activity and low planetary mass-loss in the WASP-18 system
WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (RâČHK activity parameter lies slightly below the basal level; there is no significant time-variability in the log RâČHK value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of HST aimed at explaining this anomaly. From the starâs spectral energy distribution, we infer the extinction (E(B â V) â 0.01mag) and then the ISM column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (>5Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg sâ1 cmâ2. We employ the rescaled XUV solar fluxes to model of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10â20MJ Gyrâ1. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape
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