11,802 research outputs found
Following the Mobile Student: Can We Develop the Capacity for a Comprehensive Database to Assess Student Progression?
Presents a study of state-level databases on postsecondary student retention and completion rates and the feasibility of tracking students across state lines. Outlines challenges and recommendations, including establishing a common reporting standard
Fermionic bound states in Minkowski-space: Light-cone singularities and structure
The Bethe-Salpeter equation for two-body bound system with spin
constituent is addressed directly in the Minkowski space. In order to
accomplish this aim we use the Nakanishi integral representation of the
Bethe-Salpeter amplitude and exploit the formal tool represented by the exact
projection onto the null-plane. This formal step allows one i) to deal with
end-point singularities one meets and ii) to find stable results, up to
strongly relativistic regimes, that settles in strongly bound systems. We apply
this technique to obtain the numerical dependence of the binding energies upon
the coupling constants and the light-front amplitudes for a fermion-fermion
state with interaction kernels, in ladder approximation, corresponding to
scalar-, pseudoscalar- and vector boson exchanges, respectively. After
completing the numerical survey of the previous cases, we extend our approach
to a quark-antiquark system in state, taking both constituent-fermion and
exchanged boson masses, from lattice calculations. Interestingly, the
calculated light-front amplitudes for such a mock pion show peculiar signatures
of the spin degrees of freedom.Comment: 22 pages, 7 figures, bst file include
A grid of Synthetic Spectra for Hot DA White Dwarfs and Its Application in Stellar Population Synthesis
In this work we present a grid of LTE and non-LTE synthetic spectra of hot DA
white dwarfs (WDs). In addition to its usefulness for the determination of
fundamental stellar parameters of isolated WDs and in binaries, this grid will
be of interest for the construction of theoretical libraries for stellar
studies from integrated light. The spectral grid covers both a wide temperature
and gravity range, with 17,000 K <= T_eff <= 100,000 K and 7.0 <= log(g) <=
9.5. The stellar models are built for pure hydrogen and the spectra cover a
wavelength range from 900 A to 2.5 microns. Additionally, we derive synthetic
HST/ACS, HST/WFC3, Bessel UBVRI and SDSS magnitudes. The grid was also used to
model integrated spectral energy distributions of simple stellar populations
and our modeling suggests that DAs might be detectable in ultraviolet bands for
populations older than ~8 Gyr.Comment: to be published in The Astrophysical Journal Supplement Serie
The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)
We have used NIRSPEC on Keck II to obtain -band spectroscopy of the low
mass X-ray binary V616 Mon (= A062000). V616 Mon is the proto-typical soft
x-ray transient containing a black hole primary. As such it is important to
constrain the masses of the binary components. The modeling of the infrared
observations of ellipsoidal variations in this system lead to a derived mass of
11.0 M_{\sun} for the black hole. The validity of this derivation has been
called into question due to the possiblity that the secondary star's spectral
energy distribution is contaminated by accretion disk emission (acting to
dilute the variations). Our new -band spectrum of V616 Mon reveals a
late-type K dwarf secondary star, but one that has very weak CO
absorption features. Comparison of V616 Mon with SS Cyg leads us to estimate
that the accretion disk supplies only a small amount of -band flux, and the
ellipsoidal variations are not seriously contaminated. If true, the derived
orbital inclination of V616 Mon is not greatly altered, and the mass of the
black hole remains large. A preliminary stellar atmosphere model for the
-band spectrum of V616 Mon reveals that the carbon abundance is
approximately 50% of the solar value. We conclude that the secondary star in
V616 Mon has either suffered serious contamination from the accretion of
supernova ejecta that created the black hole primary, or it is the stripped
remains of a formerly more massive secondary star, one in which the CNO cycle
had been active.Comment: 20 pages, 5 figure
Full spectral fitting of Milky Way and M31 globular clusters: ages and metallicities
Context: The formation and evolution of disk galaxies are long standing
questions in Astronomy. Understanding the properties of globular cluster
systems can lead to important insights on the evolution of its host galaxy.
Aims: We aim to obtain the stellar population parameters - age and metallicity
- of a sample of M31 and Galactic globular clusters. Studying their globular
cluster systems is an important step towards understanding their formation and
evolution in a complete way. Methods: Our analysis employs a modern
pixel-to-pixel spectral fitting technique to fit observed integrated spectra to
updated stellar population models. By comparing observations to models we
obtain the ages and metallicities of their stellar populations. We apply this
technique to a sample of 38 globular clusters in M31 and to 41 Galactic
globular clusters, used as a control sample. Results: Our sample of M31
globular clusters spans ages from 150 Myr to the age of the Universe.
Metallicities [Fe/H] range from -2.2 dex to the solar value. The
age-metallicity relation obtained can be described as having two components: an
old population with a flat age-[Fe/H] relation, possibly associated with the
halo and/or bulge, and a second one with a roughly linear relation between age
and metallicity, higher metallicities corresponding to younger ages, possibly
associated with the M31 disk. While we recover the very well known Galactic GC
metallicity bimodality, our own analysis of M31's metallicity distribution
function (MDF) suggests that both GC systems cover basically the same [Fe/H]
range yet M31's MDF is not clearly bimodal. These results suggest that both
galaxies experienced different star formation and accretion histories.Comment: A&A, in pres
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