3,915 research outputs found
Senior Reflections
Wow, I made it! It’s hard to believe that only four years ago I was moving into a place where I didn’t know anybody and had no idea what to expect. I thought seniors were old and graduation was a very distant goal, but now I’m a graduated senior wondering where those four years went
Estimating Column Density in Molecular Clouds with FIR and Sub-mm Emission Maps
We have used a numerical simulation of a turbulent cloud to synthesize maps
of the thermal emission from dust at a variety of far-IR and sub-mm
wavelengths. The average column density and external radiation field in the
simulation is well matched to clouds such as Perseus and Ophiuchus. We use
pairs of single-wavelength emission maps to derive the dust color temperature
and column density, and we compare the derived column densities with the true
column density. We demonstrate that longer wavelength emission maps yield less
biased estimates of column density than maps made towards the peak of the dust
emission spectrum. We compare the scatter in the derived column density with
the observed scatter in Perseus and Ophiuchus. We find that while in Perseus
all of the observed scatter in the emission-derived versus the
extinction-derived column density can be attributed to the flawed assumption of
isothermal dust along each line of sight, in Ophiuchus there is additional
scatter above what can be explained by the isothermal assumption. Our results
imply that variations in dust emission properties within a molecular cloud are
not necessarily a major source of uncertainty in column density measurements.Comment: Accepted to ApJ Letter
VEISHEA 2002
VEISHEA 2002 brought with it a lot of success for the forestry club. This year’s co-chairs were T.J. Mathis and Brooke Arp, but we couldn’t have pulled it off without the help of many people within the university
High-frequency gate manipulation of a bilayer graphene quantum dot
We report transport data obtained for a double-gated bilayer graphene quantum
dot. In Coulomb blockade measurements, the gate dielectric Cytop(TM) is found
to provide remarkable electronic stability even at cryogenic temperatures.
Moreover, we demonstrate gate manipulation with square shaped voltage pulses at
frequencies up to 100 MHz and show that the signal amplitude is not affected by
the presence of the capacitively coupled back gate
CP and related phenomena in the context of Stellar Evolution
We review the interaction in intermediate and high mass stars between their
evolution and magnetic and chemical properties. We describe the theory of
Ap-star `fossil' fields, before touching on the expected secular diffusive
processes which give rise to evolution of the field. We then present recent
results from a spectropolarimetric survey of Herbig Ae/Be stars, showing that
magnetic fields of the kind seen on the main-sequence already exist during the
pre-main sequence phase, in agreement with fossil field theory, and that the
origin of the slow rotation of Ap/Bp stars also lies early in the pre-main
sequence evolution; we also present results confirming a lack of stars with
fields below a few hundred gauss. We then seek which macroscopic motions
compete with atomic diffusion in determining the surface abundances of AmFm
stars. While turbulent transport and mass loss, in competition with atomic
diffusion, are both able to explain observed surface abundances, the interior
abundance distribution is different enough to potentially lead to a test using
asterosismology. Finally we review progress on the turbulence-driving and
mixing processes in stellar radiative zones.Comment: Proceedings of IAU GA in Rio, JD4 on Ap stars; 10 pages, 7 figure
Angular momentum redistribution by mixed modes in evolved low-mass stars. I. Theoretical formalism
Seismic observations by the space-borne mission \emph{Kepler} have shown that
the core of red giant stars slows down while evolving, requiring an efficient
physical mechanism to extract angular momentum from the inner layers. Current
stellar evolution codes fail to reproduce the observed rotation rates by
several orders of magnitude, and predict a drastic spin-up of red giant cores
instead. New efficient mechanisms of angular momentum transport are thus
required.
In this framework, our aim is to investigate the possibility that mixed modes
extract angular momentum from the inner radiative regions of evolved low-mass
stars. To this end, we consider the Transformed Eulerian Mean (TEM) formalism,
introduced by Andrews \& McIntyre (1978), that allows us to consider the
combined effect of both the wave momentum flux in the mean angular momentum
equation and the wave heat flux in the mean entropy equation as well as their
interplay with the meridional circulation.
In radiative layers of evolved low-mass stars, the quasi-adiabatic
approximation, the limit of slow rotation, and the asymptotic regime can be
applied for mixed modes and enable us to establish a prescription for the wave
fluxes in the mean equations. The formalism is finally applied to a benchmark model, representative of observed CoRoT and \emph{Kepler}
oscillating evolved stars.
We show that the influence of the wave heat flux on the mean angular momentum
is not negligible and that the overall effect of mixed modes is to extract
angular momentum from the innermost region of the star. A quantitative and
accurate estimate requires realistic values of mode amplitudes. This is
provided in a companion paper.Comment: Accepted in A&A, 11 pages, and 6 figure
Angular momentum redistribution by mixed modes in evolved low-mass stars. II. Spin-down of the core of red giants induced by mixed modes
The detection of mixed modes in subgiants and red giants by the CoRoT and
\emph{Kepler} space-borne missions allows us to investigate the internal
structure of evolved low-mass stars. In particular, the measurement of the mean
core rotation rate as a function of the evolution places stringent constraints
on the physical mechanisms responsible for the angular momentum redistribution
in stars. It showed that the current stellar evolution codes including the
modelling of rotation fail to reproduce the observations. An additional
physical process that efficiently extracts angular momentum from the core is
thus necessary.
Our aim is to assess the ability of mixed modes to do this. To this end, we
developed a formalism that provides a modelling of the wave fluxes in both the
mean angular momentum and the mean energy equations in a companion paper. In
this article, mode amplitudes are modelled based on recent asteroseismic
observations, and a quantitative estimate of the angular momentum transfer is
obtained. This is performed for a benchmark model of 1.3 at three
evolutionary stages, representative of the evolved pulsating stars observed by
CoRoT and Kepler.
We show that mixed modes extract angular momentum from the innermost regions
of subgiants and red giants. However, this transport of angular momentum from
the core is unlikely to counterbalance the effect of the core contraction in
subgiants and early red giants. In contrast, for more evolved red giants, mixed
modes are found efficient enough to balance and exceed the effect of the core
contraction, in particular in the hydrogen-burning shell. Our results thus
indicate that mixed modes are a promising candidate to explain the observed
spin-down of the core of evolved red giants, but that an other mechanism is to
be invoked for subgiants and early red giants.Comment: Accepted in A&A, 7 pages, 8 figure
Polarization of Thermal Emission from Aligned Dust Grains Under an Anisotropic Radiation Field
If aspherical dust grains are immersed in an anisotropic radiation field,
their temperature depends on the cross-sections projected in the direction of
the anisotropy.It was shown that the temperature difference produces polarized
thermal emission even without alignment, if the observer looks at the grains
from a direction different from the anisotropic radiation. When the dust grains
are aligned, the anisotropy in the radiation makes various effects on the
polarization of the thermal emission, depending on the relative angle between
the anisotropy and alignment directions. If the both directions are parallel,
the anisotropy produces a steep increase in the polarization degree at short
wavelengths. If they are perpendicular, the polarization reversal occurs at a
wavelength shorter than the emission peak. The effect of the anisotropic
radiation will make a change of more than a few % in the polarization degree
for short wavelengths and the effect must be taken into account in the
interpretation of the polarization in the thermal emission. The anisotropy in
the radiation field produces a strong spectral dependence of the polarization
degree and position angle, which is not seen under isotropic radiation. The
dependence changes with the grain shape to a detectable level and thus it will
provide a new tool to investigate the shape of dust grains. This paper presents
examples of numerical calculations of the effects and demonstrates the
importance of anisotropic radiation field on the polarized thermal emission.Comment: 13pages, 7figure
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