443 research outputs found
Characterizing the Variability of Stars with Early-release Kepler Data
We present a variability analysis of the early-release first quarter of data publicly released by the Kepler project. Using the stellar parameters from the Kepler Input Catalog, we have separated the sample into 129,000 dwarfs and 17,000 giants and further sub-divided the luminosity classes into temperature bins corresponding approximately to the spectral classes A, F, G, K, and M. Utilizing the inherent sampling and time baseline of the public data set (30 minute sampling and 33.5 day baseline), we have explored the variability of the stellar sample. The overall variability rate of the dwarfs is 25% for the entire sample, but can reach 100% for the brightest groups of stars in the sample. G dwarfs are found to be the most stable with a dispersion floor of σ ~ 0.04 mmag. At the precision of Kepler, >95% of the giant stars are variable with a noise floor of ~0.1 mmag, 0.3 mmag, and 10 mmag for the G giants, K giants, and M giants, respectively. The photometric dispersion of the giants is consistent with acoustic variations of the photosphere; the photometrically derived predicted radial velocity distribution for the K giants is in agreement with the measured radial velocity distribution. We have also briefly explored the variability fraction as a function of data set baseline (1-33 days), at the native 30 minute sampling of the public Kepler data. To within the limitations of the data, we find that the overall variability fractions increase as the data set baseline is increased from 1 day to 33 days, in particular for the most variable stars. The lower mass M dwarf, K dwarf, and G dwarf stars increase their variability more significantly than the higher mass F dwarf and A dwarf stars as the time baseline is increased, indicating that the variability of the lower mass stars is mostly characterized by timescales of weeks while the variability of the higher mass stars is mostly characterized by timescales of days. A study of the distribution of the variability as a function of galactic latitude suggests that sources closer to the galactic plane are more variable. This may be the result of sampling differing populations (i.e., ages) as a function of latitude or may be the result of higher background contamination that is inflating the variability fractions at lower latitudes. A comparison of the M dwarf statistics to the variability of 29 known bright M dwarfs indicates that the M dwarfs are primarily variable on timescales of weeks or longer presumably dominated by spots and binarity. On shorter timescales of hours, which are relevant for planetary transit detection, the stars are significantly less variable, with ~80% having 12 hr dispersions of 0.5 mmag or less
The Cowl - v.83 - n.3 - Sep 20, 2018
The Cowl - student newspaper of Providence College. Vol 83 - No. 3 - September 20, 2018. 24 pages
Early Dynamical Evolution of the Solar System: Pinning Down the Initial Condition of the Nice Model
In the recent years, the "Nice" model of solar system formation has attained
an unprecedented level of success in reproducing much of the observed orbital
architecture of the solar system by evolving the planets to their current
locations from a more compact configuration. Within the context of this model,
the formation of the classical Kuiper belt requires a phase during which the
ice giants have a high eccentricity. An outstanding question of this model is
the initial configuration from which the Solar System started out. Recent work
has shown that multi-resonant initial conditions can serve as good candidates,
as they naturally prevent vigorous type-II migration. In this paper, we use
analytical arguments, as well as self-consistent numerical N-body simulations
to identify fully-resonant initial conditions, whose dynamical evolution is
characterized by an eccentric phase of the ice-giants, as well as planetary
scattering. We find a total of eight such initial conditions. Four of these
primordial states are compatible with the canonical "Nice" model, while the
others imply slightly different evolutions. The results presented here should
prove useful in further development of a comprehensive model for solar system
formation.Comment: 10 pages, 8 figures, 4 tables. Accepted to the Astrophysical Journal
Giant Gravitons - with Strings Attached (III)
We develop techniques to compute the one-loop anomalous dimensions of
operators in the super Yang-Mills theory that are dual to open
strings ending on boundstates of sphere giant gravitons. Our results, which are
applicable to excitations involving an arbitrary number of open strings,
generalize the single string results of hep-th/0701067. The open strings we
consider carry angular momentum on an S embedded in the S of the
AdSS background. The problem of computing the one loop anomalous
dimensions is replaced with the problem of diagonalizing an interacting Cuntz
oscillator Hamiltonian. Our Cuntz oscillator dynamics illustrates how the
Chan-Paton factors for open strings propagating on multiple branes can arise
dynamically.Comment: 66 pages; v2: improved presentatio
Period-luminosity relations of pulsating M giants in the solar neighbourhood and the Magellanic Clouds
We analyse the results of a 5.5-yr photometric campaign that monitored 247
southern, semi-regular variables with relatively precise Hipparcos parallaxes
to demonstrate an unambiguous detection of Red Giant Branch (RGB) pulsations in
the solar neighbourhood. We show that Sequence A' contains a mixture of AGB and
RGB stars, as indicated by a temperature related shift at the TRGB. Large
Magellanic Cloud (LMC) and Galactic sequences are compared in several ways to
show that the P-L sequence zero-points have a negligible metallicity
dependence. We describe a new method to determine absolute magnitudes from
pulsation periods and calibrate the LMC distance modulus using Hipparcos
parallaxes to find \mu (LMC) = 18.54 +- 0.03 mag. Several sources of systematic
error are discussed to explain discrepancies between the MACHO and OGLE
sequences in the LMC. We derive a relative distance modulus of the Small
Magellanic Cloud (SMC) relative to the LMC of \Delta \mu = 0.41 +- 0.02 mag. A
comparison of other pulsation properties, including period-amplitude and
luminosity-amplitude relations, confirms that RGB pulsation properties are
consistent and universal, indicating that the RGB sequences are suitable as
high-precision distance indicators. The M giants with the shortest periods
bridge the gap between G and K giant solar-like oscillations and M-giant
pulsation, revealing a smooth continuity as we ascend the giant branch.Comment: 12 pages, 17 figures, 1 table. Accepted for publication in MNRA
Education Policy in Pakistan: National Challenges, Global Commitments
This article explores the main issues facing education policy in Pakistan, a relatively new country, still seeking to find its feet in terms of educating its future generations. The article highlights Pakistan’s intentions reflected in the commitments it makes on the world stage and the challenges of implementation at nation state level. A disparity between these two levels highlights why education in Pakistan remains sporadic and poor quality, against a backdrop of international interest in terms of the ‘war on terror’ and the perceived rising of madrassas and extremism. The article concludes with some practical recommendations of how global commitments could be used to address national challenges by shaping policy and importantly its implementation at ground level. One of these is to focus on educational research, within and by Pakistanis themselves, to understand their own educational needs more fully and consequently be able to construct policy more reflective of national challenges and feed into global commitments
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