32,441 research outputs found
Constraining the Sub-AU-Scale Distribution of Hydrogen and Carbon Monoxide Gas around Young Stars with the Keck Interferometer
We present Keck Interferometer observations of T Tauri and Herbig Ae/Be stars
with a spatial resolution of a few milliarcseconds and a spectral resolution of
~2000. Our observations span the K-band, and include the Br gamma transition of
Hydrogen and the v=2-0 and v=3-1 transitions of carbon monoxide. For several
targets we also present data from Keck/NIRSPEC that provide higher spectral
resolution, but a seeing-limited spatial resolution, of the same spectral
features. We analyze the Br gamma emission in the context of both disk and
infall/outflow models, and conclude that the Br gamma emission traces gas at
very small stellocentric radii, consistent with the magnetospheric scale.
However some Br gamma-emitting gas also seems to be located at radii of >0.1
AU, perhaps tracing the inner regions of magnetically launched outflows. CO
emission is detected from several objects, and we generate disk models that
reproduce both the KI and NIRSPEC data well. We infer the CO spatial
distribution to be coincident with the distribution of continuum emission in
most cases. Furthermore the Br gamma emission in these objects is roughly
coincident with both the CO and continuum emission. We present potential
explanations for the spatial coincidence of continuum, Br gamma, and CO
overtone emission, and explore the implications for the low occurrence rate of
CO overtone emission in young stars. Finally, we provide additional discussion
of V1685 Cyg, which is unusual among our sample in showing large differences in
emitting region size and spatial position as a function of wavelength.Comment: Accepted for publication in MNRA
(Never) Mind your p's and q's: Von Neumann versus Jordan on the Foundations of Quantum Theory
In two papers entitled "On a new foundation [Neue Begr\"undung] of quantum
mechanics," Pascual Jordan (1927b,g) presented his version of what came to be
known as the Dirac-Jordan statistical transformation theory. As an alternative
that avoids the mathematical difficulties facing the approach of Jordan and
Paul A. M. Dirac (1927), John von Neumann (1927a) developed the modern Hilbert
space formalism of quantum mechanics. In this paper, we focus on Jordan and von
Neumann. Central to the formalisms of both are expressions for conditional
probabilities of finding some value for one quantity given the value of
another. Beyond that Jordan and von Neumann had very different views about the
appropriate formulation of problems in quantum mechanics. For Jordan, unable to
let go of the analogy to classical mechanics, the solution of such problems
required the identication of sets of canonically conjugate variables, i.e., p's
and q's. For von Neumann, not constrained by the analogy to classical
mechanics, it required only the identication of a maximal set of commuting
operators with simultaneous eigenstates. He had no need for p's and q's. Jordan
and von Neumann also stated the characteristic new rules for probabilities in
quantum mechanics somewhat differently. Jordan (1927b) was the first to state
those rules in full generality. Von Neumann (1927a) rephrased them and, in a
subsequent paper (von Neumann, 1927b), sought to derive them from more basic
considerations. In this paper we reconstruct the central arguments of these
1927 papers by Jordan and von Neumann and of a paper on Jordan's approach by
Hilbert, von Neumann, and Nordheim (1928). We highlight those elements in these
papers that bring out the gradual loosening of the ties between the new quantum
formalism and classical mechanics.Comment: New version. The main difference with the old version is that the
introduction has been rewritten. Sec. 1 (pp. 2-12) in the old version has
been replaced by Secs. 1.1-1.4 (pp. 2-31) in the new version. The paper has
been accepted for publication in European Physical Journal
Migration as Adaptation? Exploring The Scope for Co-ordinating Environmental and Migration Policies in the European Union
Active Learning with Statistical Models
For many types of machine learning algorithms, one can compute the
statistically `optimal' way to select training data. In this paper, we review
how optimal data selection techniques have been used with feedforward neural
networks. We then show how the same principles may be used to select data for
two alternative, statistically-based learning architectures: mixtures of
Gaussians and locally weighted regression. While the techniques for neural
networks are computationally expensive and approximate, the techniques for
mixtures of Gaussians and locally weighted regression are both efficient and
accurate. Empirically, we observe that the optimality criterion sharply
decreases the number of training examples the learner needs in order to achieve
good performance.Comment: See http://www.jair.org/ for any accompanying file
Phase-resolved far-ultraviolet HST spectroscopy of the peculiar magnetic white dwarf RE J0317-853
We present phase resolved FUV HST FOS spectra of the rapidly rotating, highly
magnetic white dwarf RE J0317-853. Using these data, we construct a new model
for the magnetic field morphology across the stellar surface. From an expansion
into spherical harmonics, we find the range of magnetic field strengths present
is 180-800MG. For the first time we could identify an absorption feature
present at certain phases at 1160A as a ``forbidden'' 1s_0 -> 2s_0 component,
due to the combined presence of an electric and magnetic field.Comment: 15 pages including 4 figures. Accepted for publication in ApJ Letter
The structure of chromospheres around late-type giants and supergiants
Observations alpha Tau (K5III) and beta Gru (M2II) made at high resolution are used to confirm line identifications of features blended at low resolution. The high resolution spectra allow selected pairs of lines to be used to find the electron density and the opacity. Methods for determining these factors and the usual emission measure are presented. The electron density and opacity can be used together with the emission measure to place constraints on the structure of the atmosphere. The line formation processes are briefly discussed. Photo-excitation by strong lines appears to be important in these late type atmospheres
Universal thermodynamic bounds on nonequilibrium response with biochemical applications
Diverse physical systems are characterized by their response to small
perturbations. Near thermodynamic equilibrium, the fluctuation-dissipation
theorem provides a powerful theoretical and experimental tool to determine the
nature of response by observing spontaneous equilibrium fluctuations. In this
spirit, we derive here a collection of equalities and inequalities valid
arbitrarily far from equilibrium that constrain the response of nonequilibrium
steady states in terms of the strength of nonequilibrium driving. Our work
opens new avenues for characterizing nonequilibrium response. As illustrations,
we show how our results rationalize the energetic requirements of two common
biochemical motifs.Comment: 21 pages, 15 figure
- …