2,274 research outputs found
The Impact of Replacing Principals on Student Achievement in DC Public Schools
In 2007, the District of Columbia (D.C.) passed the Public Education Reform Amendment Act, which established mayoral control of D.C. Public Schools (DCPS) and led to the appointment of Michelle Rhee as school chancellor. In an effort to boost student achievement, Chancellor Rhee replaced many school principals as one of her first reforms. For the 2008 -- 2009 school year, 39 percent of the principals in the school district -- 51 individuals -- did not return, and more were replaced in the following years. We measured whether students in a school with a new principal performed better on standardized tests than they would have if the original principal had been retained. To do so, we analyzed the changes in student achievement that occurred when principals who left at the end of each of the school years from 2007 -- 2008 through 2010 -- 2011 were replaced. We compared the achievement of students in DCPS schools before and after a change in school leadership, and then compared this change to the change in the achievement of students from a sample of comparison schools within DCPS that kept the same principal
The Interior Structure Constants as an Age Diagnostic for Low-Mass, Pre-Main Sequence Detached Eclipsing Binary Stars
We propose a novel method for determining the ages of low-mass, pre-main
sequence stellar systems using the apsidal motion of low-mass detached
eclipsing binaries. The apsidal motion of a binary system with an eccentric
orbit provides information regarding the interior structure constants of the
individual stars. These constants are related to the normalized stellar
interior density distribution and can be extracted from the predictions of
stellar evolution models. We demonstrate that low-mass, pre-main sequence stars
undergoing radiative core contraction display rapidly changing interior
structure constants (greater than 5% per 10 Myr) that, when combined with
observational determinations of the interior structure constants (with 5 -- 10%
precision), allow for a robust age estimate. This age estimate, unlike those
based on surface quantities, is largely insensitive to the surface layer where
effects of magnetic activity are likely to be most pronounced. On the main
sequence, where age sensitivity is minimal, the interior structure constants
provide a valuable test of the physics used in stellar structure models of
low-mass stars. There are currently no known systems where this technique is
applicable. Nevertheless, the emphasis on time domain astronomy with current
missions, such as Kepler, and future missions, such as LSST, has the potential
to discover systems where the proposed method will be observationally feasible.Comment: Accepted for publication in ApJ, 8 pages, 3 figure
Horizontal Branch Morphology and Mass Loss in Globular Clusters
The connection between mass loss on the red giant branch (RGB) and horizontal
branch (HB) morphology in globular clusters (GCs) has long been acknowledged
but the mechanisms governing mass loss remains poorly understood from a
theoretical perspective. The present study uses synthetic HB models to
demonstrate for the first time that alpha-enhancement and a simple relation
between mass loss and metallicity can explain the entire range of HB morphology
(characterized by the HB type index) observed in old, coeval GCs. The mass
loss-metallicity relation accounts naturally for the fact that the most metal
poor GCs ([Fe/H] < -2) have redder HBs than is typical of GCs with -2 < [Fe/H]
< -1.5 without invoking younger ages. These results may prove useful in
studying the contribution of HB stars to integrated light via stellar
population synthesis.Comment: 13 pages, 5 figures, to appear in ApJ Letters (figure 2 may not
display correctly in some PDF viewers
Capella (alpha Aurigae) revisited: New binary orbit, physical properties, and evolutionary state
Knowledge of the chemical composition and absolute masses of Capella are key
to understanding the evolutionary state of this benchmark binary system
comprising two giant stars. Previous efforts, including our own 2009 study,
have largely failed to reach an acceptable agreement between the observations
and current stellar evolution models, preventing us from assessing the status
of the primary. Here we report a revision of the physical properties of the
components incorporating recently published high-precision radial velocity
measurements, and a new detailed chemical analysis providing abundances for
more than 20 elements in both stars. We obtain highly precise (to about 0.3%)
masses of 2.5687 +/- 0.0074 and 2.4828 +/- 0.0067 solar masses, radii of 11.98
+/- 0.57 and 8.83 +/- 0.33 solar radii, effective temperatures of 4970 +/- 50 K
and 5730 +/- 60 K, and independently measured luminosities based on the orbital
parallax (78.7 +/- 4.2 and 72.7 +/- 3.6 solar luminosities). We find an
excellent match to stellar evolution models at the measured composition of
[Fe/H] = -0.04 +/- 0.06. Three different sets of models place the primary star
firmly at the end of the core helium-burning phase (clump), while the secondary
is known to be evolving rapidly across the Hertzprung gap. The measured lithium
abundance, the C/N ratio, and the 12C/13C isotopic carbon abundance ratio,
which change rapidly in the giant phase, are broadly in agreement with
expectations from models. Predictions from tidal theory for the spin rates,
spin-orbit alignment, and other properties do not fare as well, requiring a
40-fold increase in the efficiency of the dissipation mechanisms in order to
match the observations.Comment: 15 pages in emulateapj format, including figures and tables, accepted
for publication in The Astrophysical Journa
Photometric Metallicities in Bootes I
We present new Stromgren and Washington data sets for the Bootes I dwarf
galaxy, and combine them with the available SDSS photometry. The goal of this
project is to refine a ground-based, practical, accurate method to determine
age and metallicity for individual stars in Bootes I that can be selected in an
unbiased imaging survey, without having to take spectra. We produce photometric
metallicities from Stromgren and Washington photometry, for stellar systems
with a range of . To avoid the decrease in sensitivity of the
Stromgren metallicity index on the lower red-giant branch, we replace the
Stromgren v-filter with the broader Washington C-filter; we find that
is the most successful filter combination, for individual stars with
, to maintain ~0.2 dex -resolution over the whole
red-giant branch. We demonstrate that we can break the isochrones'
age-metallicity degeneracy with these filters, using stars with log g=2.5-3.0,
which have less than a 2% change in their -colour due to age, over a
range of 11-14 Gyr.Comment: 24 pages, 18 figures, accepted by MNRA
On the Estimation of Systematic Uncertainties of Star Formation Histories
In most star formation history (SFH) measurements, the reported uncertainties
are those due to effects whose sizes can be readily measured: Poisson noise,
adopted distance and extinction, and binning choices in the solution itself.
However, the largest source of error, systematics in the adopted isochrones, is
usually ignored and very rarely explicitly incorporated into the uncertainties.
I propose a process by which estimates of the uncertainties due to evolutionary
models can be incorporated into the SFH uncertainties. This process relies on
application of shifts in temperature and luminosity, the sizes of which must be
calibrated for the data being analyzed. While there are inherent limitations,
the ability to estimate the effect of systematic errors and include them in the
overall uncertainty is significant. Effects of this are most notable in the
case of shallow photometry, with which SFH measurements rely on evolved stars.Comment: 28 pages, 9 figures, ApJ in pres
Deep 2MASS Photometry of M67 and Calibration of the Main Sequence J-Ks Color Difference as an Age Indicator
We present an analysis of Two Micron All Sky Survey (2MASS) calibration
photometry of the old open cluster M67 (NGC 2682). The proper motion-cleaned
color-magnitude diagram (CMD) resulting from these data extends ~3 magnitudes
deeper than one based on data from the point source catalog. The CMD extends
from above the helium-burning red clump to a faint limit that is more than 7
magnitudes below the main sequence turnoff in the Ks band. After adopting a
reddening of E(B-V) = 0.041 +/- 0.004 and a metal abundance of [Fe/H] = -0.009
+/- 0.009 based on a survey of published values, we fit the unevolved main
sequence of M67 to field main sequence stars with 2MASS photometry and
Hipparcos parallaxes. This analysis yields distance moduli of (m-M)Ks = 9.72
+/- 0.05 and (m-M)o = 9.70 +/- 0.05, which are consistent with published
values. We compare the theoretical isochrones of Girardi et al. and Dotter et
al. to the CMD of M67 and comment on the relative merits of each set of models.
These comparisons suggest an age between 3.5 and 4.0 Gyr for M67. The depth of
the M67 data make them ideal for the calibration of a new age indicator that
has recently been devised by Calamida et al.- the difference in (J-Ks) color
between the main sequence turnoff (TO) and the point on the lower main sequence
where it turns down (TD) and becomes nearly vertical [D(J-Ks)]. Coupled with
deep 2MASS photometry for three other open clusters, NGC 2516, M44, and NGC
6791, we calibrate D(J-Ks) in terms of age and find D(J-Ks) = (3.017 +/- 0.347)
- (0.259 +/- 0.037)*Log Age (yrs).Comment: 19 pages, 9 figures, accepted for publication in The Astrophysical
Journa
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