23,408 research outputs found
Hedging Options in a GARCH Environment: Testing the Term Structure of Stochastic Volatility Models
This paper develops a methodology for testing the term structure of volatility forecasts derived from stochastic volatility models, and implements it to analyze models of S&P 500 index volatility. Volatility models are compared by their ability to hedge options positions sensitive to the term structure of volatility. Overall, the most effective hedge is a Black-Scholes (BS) delta-gamma hedge, while the BS delta-vega hedge is the least effective. The most successful volatility hedge is GARCH components delta-gamma, suggesting that the GARCH components estimate of the term structure of volatility is most accurate. The success of the BS delta-gamma hedge may be due to mispricing in the options market over the sample period.
Random mixtures of polycyclic aromatic hydrocarbon spectra match interstellar infrared emission
The mid-infrared (IR; 5-15~m) spectrum of a wide variety of astronomical
objects exhibits a set of broad emission features at 6.2, 7.7, 8.6, 11.3 and
12.7 m. About 30 years ago it was proposed that these signatures are due
to emission from a family of UV heated nanometer-sized carbonaceous molecules
known as polycyclic aromatic hydrocarbons (PAHs), causing them to be referred
to as aromatic IR bands (AIBs). Today, the acceptance of the PAH model is far
from settled, as the identification of a single PAH in space has not yet been
successful and physically relevant theoretical models involving ``true'' PAH
cross sections do not reproduce the AIBs in detail. In this paper, we use the
NASA Ames PAH IR Spectroscopic Database, which contains over 500
quantum-computed spectra, in conjunction with a simple emission model, to show
that the spectrum produced by any random mixture of at least 30 PAHs converges
to the same 'kernel'-spectrum. This kernel-spectrum captures the essence of the
PAH emission spectrum and is highly correlated with observations of AIBs,
strongly supporting PAHs as their source. Also, the fact that a large number of
molecules are required implies that spectroscopic signatures of the individual
PAHs contributing to the AIBs spanning the visible, near-infrared, and far
infrared spectral regions are weak, explaining why they have not yet been
detected. An improved effort, joining laboratory, theoretical, and
observational studies of the PAH emission process, will support the use of PAH
features as a probe of physical and chemical conditions in the nearby and
distant Universe
Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers A method to determine molecular gas properties in luminous galaxies
We present fluxes in both neutral carbon [CI] lines at the centers of 76
galaxies with FIR luminosities between 10^{9} and 10^{12} L(o) obtained with
Herschel-SPIRE and with ground-based facilities, along with the J=7-6, J=4-3,
J=2-1 12CO and J=2-1 13CO line fluxes. We investigate whether these lines can
be used to characterize the molecular ISM of the parent galaxies in simple ways
and how the molecular gas properties define the model results. In most
starburst galaxies, the [CI]/13CO flux ratio is much higher than in Galactic
star-forming regions, and it is correlated to the total FIR luminosity. The
[CI](1-0)/CO(4-3), the [CI](2-1) (2-1)/CO(7-6), and the [CI] (2-1)/(1-0) flux
ratios are also correlated, and trace the excitation of the molecular gas. In
the most luminous infrared galaxies (LIRGs), the ISM is fully dominated by
dense and moderately warm gas clouds that appear to have low [C]/[CO] and
[13CO]/[12CO] abundances. In less luminous galaxies, emission from gas clouds
at lower densities becomes progressively more important, and a multiple-phase
analysis is required to determine consistent physical characteristics. Neither
the CO nor the [CI] velocity-integrated line fluxes are good predictors of H2
column densities in individual galaxies, and X(CI) conversion factors are not
superior to X(CO) factors. The methods and diagnostic diagrams outlined in this
paper also provide a new and relatively straightforward means of deriving the
physical characteristics of molecular gas in high-redshift galaxies up to z=5,
which are otherwise hard to determine
The excitation of near-infrared H2 emission in NGC 253
Because of its large angular size and proximity to the Milky Way, NGC 253, an
archetypal starburst galaxy, provides an excellent laboratory to study the
intricacies of this intense episode of star formation. We aim to characterize
the excitation mechanisms driving the emission in NGC 253. Specifically we aim
to distinguish between shock excitation and UV excitation as the dominant
driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line
tracers. Using SINFONI observations, we create linemaps of Br\gamma,
[FeII]_{1.64}, and all detected H_2 transitions. By using symmetry arguments of
the gas and stellar gas velocity field, we find a kinematic center in agreement
with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H_2
transitions can be used as a diagnostic to discriminate between shock and
fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as
several other H_2 line ratios and the morphological comparison between H_2 and
Br\gamma and [FeII], we find that excitation from UV photons is the dominant
excitation mechanisms throughout NGC 253. We employ a diagnostic energy level
diagram to quantitatively differentiate between mechanisms. We compare the
observed energy level diagrams to PDR and shock models and find that in most
regions and over the galaxy as a whole, fluorescent excitation is the dominant
mechanism exciting the H_2 gas. We also place an upper limit of the percentage
of shock excited H_2 at 29%. We find that UV radiation is the dominant
excitation mechanism for the H_2 emission. The H_2 emission does not correlate
well with Br\gamma but closely traces the PAH emission, showing that not only
is H_2 fluorescently excited, but it is predominately excited by slightly lower
mass stars than O stars which excite Br\gamma, such as B stars
Research on processes for utilization of lunar resources quarterly report, 16 jul. - 15 oct. 1964
Lunar resource utilization - silicate reduction unit and carbon monoxide reduction reacto
A survey of the television viewing habits of students in Somerville High School, Somerville, Massachusetts
Thesis (Ed.M.)--Boston UniversityVast amounts of published and unpublished
materials concerning the implications of television in the field of
education are available. A glimpse into the literature indicates a great
deal of controversy over the uses, purposes, advantages, and disadvantages
of television and its role in the lives of school children. The purposes
of this survey are to find out the television viewing habits of a group of
high school students, and the impact of television on some of the
educational activities of these students. [TRUNCATED
- …