533 research outputs found
Theoretical Examination of the Lithium Depletion Boundary
We explore the sensitivity in open cluster ages obtained by the lithium
depletion boundary (LDB) technique to the stellar model input physics. The LDB
age technique is limited to open clusters with ages ranging from 20 to 200 Myr.
Effective 1-sig errors in the LDB technique due to uncertain input physics are
roughly 3% at the oldest age increasing to 8% at the youngest age. Bolometric
correction uncertainties add an additional 10 to 6% error to the LDB age
technique for old and young clusters, respectively. Rotation rates matching the
observed fastest rotators in the Pleiades affect LDB ages by less than 2%. The
range of rotation rates in an open cluster are expected to ``smear'' the LDB
location by only 0.02 mag for a Pleiades age cluster increasing to 0.06 mag for
a 20 Myr cluster. Thus, the observational error of locating the LDB (~7-10%)
and the bolometric correction uncertainty currently dominate the error in LDB
ages. For our base case, we formally derive a LDB age of 148 +- 19 Myr for the
Pleiades, where the error includes 8, 3, and 9% contributions from
observational, theoretical, and bolometric correction sources, respectively. A
maximally plausible 0.3 magnitude shift in the I-band bolometric correction to
reconcile main sequence isochrone fits with the observed (V-I) color for the
low mass Pleiades members results in an age of 126 +- 11 Myr, where the error
includes observational and theoretical errors only. Upper main-sequence-fitting
ages that do not include convective core overshoot for the Pleiades (~75 Myr)
are ruled out by the LDB age technique.Comment: 35 pages, 9 figures, accepted Ap
Stellar Rotation in Young Clusters. I. Evolution of Projected Rotational Velocity Distributions
Open clusters offer us the means to study stellar properties in samples with
well-defined ages and initial chemical composition. Here we present a survey of
projected rotational velocities for a large sample of mainly B-type stars in
young clusters to study the time evolution of the rotational properties of
massive stars. The survey is based upon moderate resolution spectra made with
the WIYN 3.5 m and CTIO 4 m telescopes and Hydra multi-object spectrographs,
and the target stars are members of 19 young open clusters with an age range of
approximately 6 to 73 Myr. We made fits of the observed lines He I 4026, 4387,
4471 and Mg II 4481 using model theoretical profiles to find projected
rotational velocities for a total of 496 OB stars. We find that there are fewer
slow rotators among the cluster B-type stars relative to nearby B stars in the
field. We present evidence consistent with the idea that the more massive B
stars (M > 9 solar masses) spin down during their main sequence phase. However,
we also find that the rotational velocity distribution appears to show an
increase in the numbers of rapid rotators among clusters with ages of 10 Myr
and higher. These rapid rotators appear to be distributed between the zero age
and terminal age main sequence locations in the Hertzsprung-Russell diagram,
and thus only a minority of them can be explained as the result of a spin up at
the terminal age main sequence due to core contraction. We suggest instead that
some of these rapid rotators may have been spun up through mass transfer in
close binary systems.Comment: 33 pages, 11 figures, accepted by Ap
The influence of mixing international and domestic students on competency learning in small groups in undergraduate medical education
BACKGROUND: Medical curricula are increasingly internationalized, with international students being mixed with domestic students in small group learning. Small group learning is known to foster competency learning in undergraduate medical education, specifically Communication, Collaboration, Leadership, and Professionalism. However, it is unclear what happens with the learning of competencies when international students are introduced in small groups. This study explores if students in international small groups master the competencies Collaboration, Leadership and Professionalism at the same level as students in domestic groups in an undergraduate medical curriculum. METHOD: In total, 1215 Students of three academic year cohorts participated in the study. They were divided into four learning communities (LCs), per year cohort, in which tutor groups were the main instructional format. The tutorials of two learning communities were taught in English, with a mix of international and Dutch students. The tutorials of the other two learning communities were taught in Dutch with almost all domestic students. Trained tutors assessed three competencies (Collaboration, Leadership, Professionalism) twice per semester, as 'Not-on-track', 'On-track', or 'Fast-on-track'. By using Chi-square tests, we compared students' competencies performance twice per semester between the four LCs in the first two undergraduate years. RESULTS: The passing rate ('On-track' plus 'Fast-on-track') for the minimum level of competencies did not differ between the mixed and domestic groups. However, students in the mixed groups received more excellent performance evaluations ('Fast-on-track') than the students in the homogenous groups of Dutch students. This higher performance was true for both international and Dutch students of the mixed groups. Prior knowledge, age, gender, and nationality did not explain this phenomenon. The effect could also not be explained by a bias of the tutors. CONCLUSION: When students are educated in mixed groups of international and Dutch students, they can obtain the same basic competency levels, no matter what mix of students is made. However, students in the mixed international groups outperformed the students in the homogenous Dutch groups in achieving excellent performance scores. Future research should explore if these findings can be explained from differences in motivation, perceived grading or social network interactions
Mass and Angular Momentum Transfer in the Massive Algol Binary RY Persei
We present an investigation of H-alpha emission line variations observed in
the massive Algol binary, RY Per. We give new radial velocity data for the
secondary based upon our optical spectra and for the primary based upon high
dispersion UV spectra. We present revised orbital elements and an estimate of
the primary's projected rotational velocity (which indicates that the primary
is rotating 7 times faster than synchronous). We use a Doppler tomography
algorithm to reconstruct the individual primary and secondary spectra in the
region of H-alpha, and we subtract the latter from each of our observations to
obtain profiles of the primary and its disk alone. Our H-alpha observations of
RY Per show that the mass gaining primary is surrounded by a persistent but
time variable accretion disk. The profile that is observed outside-of-eclipse
has weak, double-peaked emission flanking a deep central absorption, and we
find that these properties can be reproduced by a disk model that includes the
absorption of photospheric light by the band of the disk seen in projection
against the face of the star. We developed a new method to reconstruct the disk
surface density distribution from the ensemble of H-alpha profiles observed
around the orbit, and this method accounts for the effects of disk occultation
by the stellar components, the obscuration of the primary by the disk, and flux
contributions from optically thick disk elements. The resulting surface density
distribution is elongated along the axis joining the stars, in the same way as
seen in hydrodynamical simulations of gas flows that strike the mass gainer
near trailing edge of the star. This type of gas stream configuration is
optimal for the transfer of angular momentum, and we show that rapid rotation
is found in other Algols that have passed through a similar stage.Comment: 39 pages, 12 figures, ApJ in press, 2004 June 20 issu
Spectroscopic abundance analysis of dwarfs in young open cluster IC 4665
We report a detailed spectroscopic abundance analysis for a sample of 18 F-K
dwarfs of the young open cluster IC 4665. Stellar parameters and element
abundances of Li, O, Mg, Si, Ca, Ti, Cr, Fe and Ni have been derived using the
spectroscopic synthesis tool SME (Spectroscopy Made Easy). Within the
measurement uncertainties the iron abundance is uniform with a standard
deviation of 0.04 dex. No correlation is found between the iron abundance and
the mass of the stellar convective zone, and between the Li abundance and the
Fe abundance. In other words, our results do not reveal any signature of
accretion and therefore do not support the scenario that stars with planets
(SWPs) acquire their on the average higher metallicity compared to field stars
via accretion of metal-rich planetary material. Instead the higher metallicity
of SWPs may simply reflect the fact that planet formation is more efficient in
high metallicity environs. However, since that many details of the planet
system formation processes remain poorly understood, further studies are needed
for a final settlement of the problem of the high metallicity of SWPs.
The standard deviation of [Fe/H] deduced from our observations, taken as an
upper limit on the metallicity dispersion amongst the IC 4665 member stars, has
been used to constrain proto-planetary disk evolution, terrestrial and giant
planets formation and evolution processes. Our results do not support the
possibility that the migration of gas giants and the circularization of
terrestrial planets' orbits are regulated by their interaction with a residual
population of planetesimals and dust particles.Comment: 18 pages, 6 figures, accepted for publication in Ap
Twin Binaries: Studies of Stability, Mass Transfer, and Coalescence
Motivated by suggestions that binaries with almost equal-mass components
("twins") play an important role in the formation of double neutron stars and
may be rather abundant among binaries, we study the stability of synchronized
close and contact binaries with identical components in circular orbits. In
particular, we investigate the dependency of the innermost stable circular
orbit on the core mass, and we study the coalescence of the binary that occurs
at smaller separations. For twin binaries composed of convective main-sequence
stars, subgiants, or giants with low mass cores (M_c <~0.15M, where M is the
mass of a component), a secular instability is reached during the contact
phase, accompanied by a dynamical mass transfer instability at the same or at a
slightly smaller orbital separation. Binaries that come inside this instability
limit transfer mass gradually from one component to the other and then coalesce
quickly as mass is lost through the outer Lagrangian points. For twin giant
binaries with moderate to massive cores (M_c >~0.15M), we find that stable
contact configurations exist at all separations down to the Roche limit, when
mass shedding through the outer Lagrangian points triggers a coalescence of the
envelopes and leaves the cores orbiting in a central tight binary. In addition
to the formation of binary neutron stars, we also discuss the implications of
our results for the production of planetary nebulae with double degenerate
central binaries.Comment: 17 pages, accepted to ApJ, final version includes discussion of
planetary nebulae with central binaries and a new figure about shock heating,
visualizations at http://webpub.allegheny.edu/employee/j/jalombar/movies
Observational Tests and Predictive Stellar Evolution
We compare eighteen binary systems with precisely determined radii and masses
from 23 to 1.1 M_sol, and stellar evolution models produced with our newly
revised code TYCHO. ``Overshooting'' and rotational mixing were suppressed in
order to establish a baseline for isolating these and other hydrodynamic
effects. Acceptable coeval fits are found for sixteen pairs without optimizing
for heavy element or helium abundance. The precision of these tests is limited
by the accuracies of the observed effective temperatures. High dispersion
spectra and detailed atmospheric modeling should give more accurate effective
temperatures and heavy element abundances. PV Cas, a peculiar early A system,
EK Cep B, a known post-T Tauri star, and RS Cha, a member of a young OB
association, are matched by pre-main sequence models. Predicted mass loss
agrees with upper limits from IUE for CW Cep A and B. Relatively poor fits are
obtained for binaries having at least one component in the mass range 1.7 <
M/M_sol <2.6, whose evolution is sensitive to mixing. These discrepancies are
robust and consistent with additional mixing in real stars. The predicted
apsidal motion implies that massive star models are systematically less
centrally condensed than the real stars. If these effects are due to
overshooting, then the overshooting parameter alpha_OV increases with stellar
mass. The apsidal motion constants are controlled by radiative opacity under
conditions close to those directly measured in laser experiments, making this
test more stringent than possible before.Comment: 38 pages, 9 figures (color versions of figures 1,2,3,4, and 9 are
available separately). Accepted for publication in the Astrophysical Journa
Hubble Space Telescope Photometry of Hodge 301: An "Old" Star Cluster in 30 Doradus
We present Hubble Space Telescope Planetary Camera UVI data for the
little-studied cluster Hodge 301 3' northwest of 30 Doradus' central ionizing
cluster R136. The average reddening of Hodge 301 is found to be =
(0.28+-0.05) mag from published infrared and ultraviolet photometry. Using two
different sets of evolutionary models, we derive an age of ~ 20-25 Myr for
Hodge 301, which makes it roughly 10 times as old as R136. Hodge 301 is the
most prominent representative of the oldest population in the 30 Dor starburst
region; a region that has undergone multiple star formation events. This range
of ages is an important consideration for the modelling of starburst regions.
Hodge 301 shows a widened upper main sequence largely caused by Be stars. We
present a list of Be star candidates. The slope of the initial mass function
for intermediate-mass main sequence stars ranging from 10 to 1.3 solar masses
is found to be -1.4+-0.1 in good agreement with a Salpeter law. There is no
indication for a truncation or change of slope of the IMF within this mass
range. In accordance with the age of Hodge 301 no obvious pre-main-sequence
stars are seen down to 1 solar mass. We estimate that up to 41+-7 stars with
more than 12 solar masses may have turned into supernovae since the formation
of the cluster. Multiple supernova explosions are the most likely origin of the
extremely violent gas motions and the diffuse X-ray emission observed in the
cluster surroundings.Comment: To appear in the Astronomical Journal (Feb 2000 issue). 16 pages in
two-column style. 9 separate figures, in part in significantly reduced
resolution for space reasons (bitmapped postscript or jpg
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