4,276 research outputs found
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
Age spreads and the temperature dependence of age estimates in Upper Sco
Past estimates for the age of the Upper Sco Association are typically 11-13
Myr for intermediate-mass stars and 4-5 Myr for low-mass stars. In this study,
we simulate populations of young stars to investigate whether this apparent
dependence of estimated age on spectral type may be explained by the star
formation history of the association. Solar and intermediate mass stars begin
their pre-main sequence evolution on the Hayashi track, with fully convective
interiors and cool photospheres. Intermediate mass stars quickly heat up and
transition onto the radiative Henyey track. As a consequence, for clusters in
which star formation occurs on a similar timescale as the transition from a
convective to a radiative interior, discrepancies in ages will arise when ages
are calculated as a function of temperature instead of mass. Simple simulations
of a cluster with constant star formation over several Myr may explain about
half of the difference in inferred ages versus photospheric temperature;
speculative constructions that consist of a constant star formation followed by
a large supernova-driven burst could fully explain the differences, including
those between F and G stars where evolutionary tracks may be more accurate. The
age spreads of low-mass stars predicted from these prescriptions for star
formation are consistent with the observed luminosity spread of Upper Sco. The
conclusion that a lengthy star formation history will yield a temperature
dependence in ages is expected from the basic physics of pre-main sequence
evolution and is qualitatively robust to the large uncertainties in pre-main
sequence evolutionary models.Comment: 13 pages, accepted by Ap
An Enhanced Spectroscopic Census of the Orion Nebula Cluster
We report new spectral types or spectral classification constraints for over
600 stars in the Orion Nebula Cluster (ONC) based on medium resolution R~
1500-2000 red optical spectra acquired using the Palomar 200" and Kitt Peak
3.5m telescopes. Spectral types were initially estimated for F, G, and early K
stars from atomic line indices while for late K and M stars, constituting the
majority of our sample, indices involving TiO and VO bands were used. To ensure
proper classification, particularly for reddened, veiled, or
nebula-contaminated stars, all spectra were then visually examined for type
verification or refinement. We provide an updated spectral type table that
supersedes Hillenbrand (1997), increasing the percentage of optically visible
ONC stars with spectral type information from 68% to 90%. However, for many
objects, repeated observations have failed to yield spectral types primarily
due to the challenges of adequate sky subtraction against a bright and
spatially variable nebular background. The scatter between our new and our
previously determined spectral types is approximately 2 spectral sub-classes.
We also compare our grating spectroscopy results with classification based on
narrow-band TiO filter photometry from Da Rio et al. (2012, finding similar
scatter. While the challenges of working in the ONC may explain much of the
spread, we highlight several stars showing significant and unexplained bona
fide spectral variations in observations taken several years apart; these and
similar cases could be due to a combination of accretion and extinction
changes. Finally, nearly 20% of ONC stars exhibit obvious Ca II triplet
emission indicative of strong accretion.Comment: Accepted to the Astronomical Journal; 37 pages, including 11 Figures
and 3 Tables (one long table not reproduced here but available upon request
or from the journal
Efficient Aggregated Deliveries with Strong Guarantees in an Event-based Distributed System
A popular approach to designing large scale distributed systems is to follow an event-based approach. In an event-based approach, a set of software components interact by producing and consuming events. The event-based model allows for the decoupling of software components, allowing distributed systems to scale to a large number of components. Event correlation allows for higher order reasoning of events by constructing complex events from single, consumable events. In many cases, event correlation applications rely on centralized setups or broker overlay networks. In the case of centralized setups, the guarantees for complex event delivery are stronger, however, centralized setups create performance bottlenecks and single points of failure. With broker overlays, the performance and fault tolerance are improved but at the cost of weaker guarantees
Modeling the Economic Impacts of Large Deployments on Local Communities
The Global War on Terrorism has required the large scale deployment of active-duty troops to support operations in Afghanistan and Iraq. This thesis presents an economic analysis of the impacts these deployments have had on local communities as measured by changes in local retail sales. County level data from Colorado, Kansas, North Carolina, and Tennessee were combined with open source deployment information to construct panel models. Panel model analysis is a form of regression that combines cross-sectional and time-series dimensions. Seasonality and general economic conditions were also incorporated into the model. The goal of this research is to provide an empirical model to community leaders and federal agencies that addresses the potential effects of continued deployments. The results of this research have demonstrated that the deployment of brigade size units decrease the level of retail sales in a county by 0.3 percent, which leads to losses in local governmental revenue
A flow equation approach to periodically driven quantum systems
We present a theoretical method to generate a highly accurate {\em
time-independent} Hamiltonian governing the finite-time behavior of a
time-periodic system. The method exploits infinitesimal unitary transformation
steps, from which renormalization group-like flow equations are derived to
produce the effective Hamiltonian. Our tractable method has a range of validity
reaching into frequency regimes that are usually inaccessible via high
frequency expansions in the parameter , where is the
upper limit for the strength of local interactions. We demonstrate our approach
on both interacting and non-interacting many-body Hamiltonians where it offers
an improvement over the more well-known Magnus expansion and other high
frequency expansions. For the interacting models, we compare our approximate
results to those found via exact diagonalization. While the approximation
generally performs better globally than other high frequency approximations,
the improvement is especially pronounced in the regime of lower frequencies and
strong external driving. This regime is of special interest because of its
proximity to the resonant regime where the effect of a periodic drive is the
most dramatic. Our results open a new route towards identifying novel
non-equilibrium regimes and behaviors in driven quantum many-particle systems.Comment: 25 pages, 14 figure
- …