79 research outputs found
Spectroscopic Survey of G and K Dwarfs in the Hipparcos Catalog. I. Comparison between the Hipparcos and Photometric Parallaxes
The tension between the Hipparcos parallax of the Pleiades and other independent distance estimates continues even after the new reduction of the Hipparcos astrometric data and the development of a new geometric distance measurement for the cluster. A short Pleiades distance from the Hipparcos parallax predicts a number of stars in the solar neighborhood that are sub-luminous at a given photospheric abundance. We test this hypothesis using the spectroscopic abundances for a subset of stars in the Hipparcos catalog, which occupy the same region as the Pleiades in the color–magnitude diagram. We derive stellar parameters for 170 nearby G- and K-type field dwarfs in the Hipparcos catalog based on high-resolution spectra obtained using KPNO 4 m echelle spectrograph. Our analysis shows that, when the Hipparcos parallaxes are adopted, most of our sample stars follow empirical color–magnitude relations. A small fraction of stars are too faint compared to main-sequence fitting relations by ΔM_V ≳ 0.3 mag, but the differences are marginal at a 2σ level, partly due to relatively large parallax errors. On the other hand, we find that the photometric distances of stars showing signatures of youth as determined from lithium absorption line strengths and R_(HK)^' chromospheric activity indices are consistent with the Hipparcos parallaxes. Our result is contradictory to a suggestion that the Pleiades distance from main-sequence fitting is significantly altered by stellar activity and/or the young age of its stars, and provides an additional supporting evidence for the long-distance scale of the Pleiades
Environmental Variation and Cohort Effects in an Antarctic Predator
Understanding the potential influence of environmental variation experienced by animals during early stages of development on their subsequent demographic performance can contribute to our understanding of population processes and aid in predicting impacts of global climate change on ecosystem functioning. Using data from 4,178 tagged female Weddell seal pups born into 20 different cohorts, and 30 years of observations of the tagged seals, we evaluated the hypothesis that environmental conditions experienced by young seals, either indirectly through maternal effects and/or directly during the initial period of juvenile nutritional independence, have long-term effects on individual demographic performance. We documented an approximately 3-fold difference in the proportion of each cohort that returned to the pupping colonies and produced a pup within the first 10 years after birth. We found only weak evidence for a correlation between annual environmental conditions during the juvenile-independence period and cohort recruitment probability. Instead, the data strongly supported an association between cohort recruitment probability and the regional extent of sea ice experienced by the mother during the winter the pup was in utero. We suggest that inter-annual variation in winter sea-ice extent influences the foraging success of pregnant seals by moderating the regional abundance of competing predators that cannot occupy areas of consolidated sea ice, and by directly influencing the abundance of mid-trophic prey species that are sea-ice obligates. We hypothesize that this environmentally-induced variation in maternal nutrition dictates the extent of maternal energetic investment in offspring, resulting in cohort variation in mean size of pups at weaning which, in turn, contributes to an individual?s phenotype and its ultimate fitness. These linkages between sea ice and trophic dynamics, combined with demonstrated and predicted changes in the duration and extent of sea ice associated with climate change, suggest significant alterations in Antarctic marine ecosystems in the future
Rotational velocities of low-mass stars in the Pleiades and Hyades
We have obtained high-resolution spectra of 89 M dwarf members of the
Pleiades and Hyades and have derived radial velocities, H-alpha equivalent
widths, and spectroscopic rotational velocities for these stars. Typical masses
of the newly-observed Pleiades and Hyades stars are ~ 0.4 M_{\sun} and ~ 0.2
M_{\sun}, respectively. We combine our new observations with previously
published data to explore the rotational evolution of young stars with M < 0.4
M_\sun. The average rotation rate in the Hyades (age 600 Myr) is about 0.4 that
of the Pleiades (110 Myr), and the mean equivalent widths of H-alpha are also
lower. As found in previous studies, the correlation between rotation and
chromospheric activity is identical in both clusters, implying that the lower
activity in the Hyades is a result of the lower rotation rates. We show that a
simple scaling of the Pleiades rotational distribution for M \leq 0.4 M_{\sun},
corrected for the effects of structural evolution, matches that of the Hyades
if the average angular momentum loss from the Pleiades to the Hyades age is
factor of \approx 6. This suggests that the distribution of initial angular
momenta and disk-locking lifetimes for the lowest mass stars was similar in
both clusters. We argue that this result provides further evidence for a
saturation of the angular momentum loss rate at high rotational velocities.Comment: 22 pages, 11 figures, accepted for publication in The Astronomical
Journal, tentatively scheduled for March 200
The Ursinus Weekly, December 4, 1950
Gillespie to play at Friday night senior formal • Men students cast 107-71 vote against dorm amendment • Music organizations to present Messiah in Bomberger chapel Thursday night • Y Commissions to meet; PAC plans Xmas party • Response to WSSF is disappointing; Receipts total $350 • Critic hails Angel Street as vehicle for superb thespian dramatic acting • Hungarian to address Tuesday night Forum • Twelve to become Rosicrucians • Yule traditions dominate ensuing campus activities • Bloodmobile to be at Trinity church Thursday morning • 26 are accepted by local chapter of Pi Gamma Mu • Cafe Pigalle to return to gym Saturday night • Mary MacPherson chosen May Queen; Marge Paynter named pageant manager • Editorial: Dynamic force • WSGA notes • Delta Pi Sigma welcomes ten off-campus men • English Club admits members • Revived rec center attracts many • 45 future teachers approach termination of tribulation • IRC hears attorney speak on problems of western nations • Ruby schedules photos, pushes subscriptions • Pigskin parade • Bears top textile 64-50 in court season inaugural • Six close careers on soccer squad • Derr deadlocks Albright 6-6 • Four senior girls play hockey finale • Grid player scans all-state selections • Ursinus grid aggregation suffers loss of twelve graduating upper classmen • Penn triumphs 3-1 over Ursinus girls • Reid Watson became football manager when injury benched former grid star • Messiah reputation stems from mastery of simple techniques • Eight teams compete in debate tournament • Chess team loses • Kershner does dialect in fourth lit readinghttps://digitalcommons.ursinus.edu/weekly/1554/thumbnail.jp
The Distances to Open Clusters from Main-Sequence Fitting. I. New Models and a Comparison to the Properties of the Hyades Eclipsing Binary vB 22
In the first of a new series of papers on open cluster distances, we use
updated stellar evolution models to construct an isochrone appropriate for the
Hyades, and compare it with the Hyades eclipsing binary system vB 22. We find
that the absolute and relative luminosities of the two stars are in good
agreement with the model, but the radii do not match the values inferred from
eclipse data. We present evidence that there is a consistency problem with the
flux ratios and the inferred radii, and discuss possible theoretical effects
that could be responsible for the mismatch in the radii. We derive a helium
abundance for the Hyades of Y = 0.271 \pm 0.006, which is equal within the
errors to the Sun's initial helium abundance even though the Hyades is
considerably more metal-rich.Comment: Accepted for publication in Astrophysical Journa
Why are the K dwarfs in the Pleiades so Blue?
The K dwarfs in the Pleiades fall nearly one half magnitude below a main
sequence isochrone when plotted in a color-magnitude diagram utilizing V
magnitude as the luminosity index and B-V as the color index. This peculiarity
has been known for forty years but has gone unexplained and mostly ignored.
When compared to Praesepe members, the Pleiades K dwarfs again are subluminous
(or blue) in a color-magnitude diagram using B-V as the color index. However,
using V-I as the color index, stars in the two clusters are coincident to M_V ~
10; using V-K as the color index, Pleiades late K and M stars fall above the
main sequence locus defined by Praesepe members. We believe that the anomalous
spectral energy distributions for the Pleiades K dwarfs, as compared to older
clusters, are a consequence of rapid stellar rotation and may be primarily due
to spottedness. If so, the required areal filling factor for the cool component
has to be very large (=> 50%). Weak-lined T Tauri stars have similar color
anomalies, and we suspect this is a common feature of all very young K dwarfs
(sp. type > K3). The peculiar spectral energy distribution needs to be
considered in deriving accurate pre-main sequence isochrone-fitting ages for
clusters like the Pleiades since the age derived will depend on the temperature
index used.Comment: 41 pages, 15 figures, AASTeX5.0. Accepted 05 May 2003; Scheduled for
publication in the Astronomical Journal (August 2003
Keck Spectra of Brown Dwarf Candidates and a Precise Determination of the Lithium Depletion Boundary in the Alpha Persei Open Cluster
We have identified twenty-seven candidate very low mass members of the
relatively young Alpha Persei open cluster from a six square degree CCD imaging
survey. Based on their I magnitudes and the nominal age and distance to the
cluster, these objects should have masses less than 0.1 Msunif they are cluster
members. We have subsequently obtained intermediate resolution spectra of
seventeen of these objects using the Keck II telescope and LRIS spectrograph.
We have also obtained near-IR photometry for many of the stars. Our primary
goal was to determine the location of the "lithium depletion boundary" and
hence to derive a precise age for the cluster. We detect lithium with
equivalent widths greater than or equal to 0.4 \AA in five of the program
objects. We have constructed a color-magnitude diagram for the faint end of the
Alpha Persei main sequence. These data allow us to accurately determine the
Alpha Persei single-star lithium depletion boundary at M(I) = 11.47, M(Bol)
= 11.42, (R-I) = 2.12, spectral type M6.0. By reference to theoretical
evolutionary models, this converts fairly directly into an age for the Alpha
Persei cluster of 90 10 Myr. At this age, the two faintest of our
spectroscopically confirmed members should be sub-stellar (i.e., brown dwarfs)
according to theoretical models.Comment: Accepted Ap
Rotation and activity in the solar-metallicity open cluster NGC2516
We report new measures of radial velocities and rotation rates (v sin i) for
51 F and early-G stars in the open cluster NGC2516, and combine these with
previously published data. From high signal-to-noise spectra of two stars, we
show that NGC2516 has a relative iron abundance with respect to the Pleiades of
delta([Fe/H])= +0.04 +/- 0.07 at the canonical reddening of E(B - V) = 0.12, in
contrast to previous photometric studies that placed the cluster 0.2 to 0.4 dex
below solar. We construct a color-magnitude diagram based on radial velocity
members, and explore the sensitivity of photometric determinations of the
metallicity and distance to assumed values of the reddening. For a metal
abundance near solar, the Hipparcos distance to NGC2516 is probably
underestimated. Finally, we show that the distribution of rotation rates and
X-ray emission does not differ greatly from that of the Pleiades, when
allowance is made for the somewhat older age of NGC2516.Comment: Accepted for publication in the Astrophysical Journal. 35 pages
including 7 figure
The Baryon Oscillation Spectroscopic Survey of SDSS-III
The Baryon Oscillation Spectroscopic Survey (BOSS) is designed to measure the
scale of baryon acoustic oscillations (BAO) in the clustering of matter over a
larger volume than the combined efforts of all previous spectroscopic surveys
of large scale structure. BOSS uses 1.5 million luminous galaxies as faint as
i=19.9 over 10,000 square degrees to measure BAO to redshifts z<0.7.
Observations of neutral hydrogen in the Lyman alpha forest in more than 150,000
quasar spectra (g<22) will constrain BAO over the redshift range 2.15<z<3.5.
Early results from BOSS include the first detection of the large-scale
three-dimensional clustering of the Lyman alpha forest and a strong detection
from the Data Release 9 data set of the BAO in the clustering of massive
galaxies at an effective redshift z = 0.57. We project that BOSS will yield
measurements of the angular diameter distance D_A to an accuracy of 1.0% at
redshifts z=0.3 and z=0.57 and measurements of H(z) to 1.8% and 1.7% at the
same redshifts. Forecasts for Lyman alpha forest constraints predict a
measurement of an overall dilation factor that scales the highly degenerate
D_A(z) and H^{-1}(z) parameters to an accuracy of 1.9% at z~2.5 when the survey
is complete. Here, we provide an overview of the selection of spectroscopic
targets, planning of observations, and analysis of data and data quality of
BOSS.Comment: 49 pages, 16 figures, accepted by A
The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones
We continue our series of papers on open cluster distances with a critical
assessment of the accuracy of main-sequence fitting using isochrones that
employ empirical corrections to the color-temperature relations. We use four
nearby open clusters with multicolor photometry and accurate metallicities and
present a new metallicity for Praesepe ([Fe/H] = +0.11 +/- 0.03) from
high-resolution spectra. The internal precision of distance estimates is about
a factor of 5 better than the case without the color calibrations. After taking
into account all major systematic errors, we obtain distances accurate to about
2%-3% when there exists a good metallicity estimate. Metallicities accurate to
better than 0.1 dex may be obtained from BVIcKs photometry alone. We also
derive a helium abundance for the Pleiades of Y = 0.279 +/- 0.015, which is
equal within the errors to the Sun's initial helium abundance and that of the
Hyades. Our best estimates of distances are (m - M)_0 = 6.33 +/- 0.04, 8.03 +/-
0.04, and 9.61 +/- 0.03 to Praesepe, NGC 2516, and M67, respectively. Our
Pleiades distance at the spectroscopic metallicity, (m - M)_0 = 5.66 +/- 0.01
(internal) +/- 0.05 (systematic), is in excellent agreement with several
geometric distance measurements. We have made calibrated isochrones for -0.3 <=
[Fe/H] <= +0.2 available at http://www.astronomy.ohio-state.edu/iso/ .Comment: 28 pages, 23 figures; accepted for publication in the Ap
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