162,512 research outputs found
A method for exploiting domain information in astrophysical parameter estimation
I outline a method for estimating astrophysical parameters (APs) from
multidimensional data. It is a supervised method based on matching observed
data (e.g. a spectrum) to a grid of pre-labelled templates. However, unlike
standard machine learning methods such as ANNs, SVMs or k-nn, this algorithm
explicitly uses domain information to better weight each data dimension in the
estimation. Specifically, it uses the sensitivity of each measured variable to
each AP to perform a local, iterative interpolation of the grid. It avoids both
the non-uniqueness problem of global regression as well as the grid resolution
limitation of nearest neighbours.Comment: Proceedings of ADASS17 (September 2007, London). 4 pages. To appear
in ASP Conf. Pro
Microarcsecond astrometry with Gaia: the solar system, the Galaxy and beyond
Gaia is an all sky, high precision astrometric and photometric satellite of
the European Space Agency (ESA) due for launch in 2010-2011. Its primary
mission is to study the composition, formation and evolution of our Galaxy.
Gaia will measure parallaxes and proper motions of every object in the sky
brighter than V=20, amounting to a billion stars, galaxies, quasars and solar
system objects. It will achieve an astrometric accuracy of 10muas at V=15 -
corresponding to a distance accuracy of 1% at 1kpc. With Gaia, tens of millions
of stars will have their distances measured to a few percent or better. This is
an improvement over Hipparcos by several orders of magnitude in the number of
objects, accuracy and limiting magnitude. Gaia will also measure radial
velocities for source brighter than V~17. To characterize the objects, each
object is observed in 15 medium and broad photometric bands with an onboard CCD
camera. With these capabilities, Gaia will make significant advances in a wide
range of astrophysical topics. These include a detailed kinematical map of
stellar populations, stellar structure and evolution, the discovery and
characterization of thousands of exoplanetary systems and General Relativity on
large scales. I give an overview of the mission, its operating principles and
its expected scientific contributions. For the latter I provide a quick look in
five areas on increasing scale size in the universe: the solar system, exosolar
planets, stellar clusters and associations, Galactic structure and
extragalactic astronomy.Comment: (Errors corrected) Invited paper at IAU Colloquium 196, "Transit of
Venus: New Views of the Solar System and Galaxy". 14 pages, 6 figures.
Version with higher resolution figures available from
http://www.mpia-hd.mpg.de/homes/calj/gaia_venus2004.htm
A Bayesian method for the analysis of deterministic and stochastic time series
I introduce a general, Bayesian method for modelling univariate time series
data assumed to be drawn from a continuous, stochastic process. The method
accommodates arbitrary temporal sampling, and takes into account measurement
uncertainties for arbitrary error models (not just Gaussian) on both the time
and signal variables. Any model for the deterministic component of the
variation of the signal with time is supported, as is any model of the
stochastic component on the signal and time variables. Models illustrated here
are constant and sinusoidal models for the signal mean combined with a Gaussian
stochastic component, as well as a purely stochastic model, the
Ornstein-Uhlenbeck process. The posterior probability distribution over model
parameters is determined via Monte Carlo sampling. Models are compared using
the "cross-validation likelihood", in which the posterior-averaged likelihood
for different partitions of the data are combined. In principle this is more
robust to changes in the prior than is the evidence (the prior-averaged
likelihood). The method is demonstrated by applying it to the light curves of
11 ultra cool dwarf stars, claimed by a previous study to show statistically
significant variability. This is reassessed here by calculating the
cross-validation likelihood for various time series models, including a null
hypothesis of no variability beyond the error bars. 10 of 11 light curves are
confirmed as being significantly variable, and one of these seems to be
periodic, with two plausible periods identified. Another object is best
described by the Ornstein-Uhlenbeck process, a conclusion which is obviously
limited to the set of models actually tested.Comment: Published in A&A as free access article. Software and additional
information available from http://www.mpia.de/~calj/ctsmod.htm
A spectroscopic ruler for intermediate-zone FRET measurements
It is well known that Fluorescence Resonance Energy Transfer (FRET), the most common mechanism for electronic energy to migrate between molecular chromophores, has a predominantly inverse sixth power dependence on the rate of transfer as a function of the distance R between the chromophores. However, the unified theory of electronic energy transfer, derived from quantum electrodynamics, predicts an additional contribution with an R-4 dependence on distance. This intermediate-zone term becomes especially important when the chromophore spacing is similar in magnitude to the reduced wavelength (ƛ= λ 2π ) associated with the mediated energy. In previous theoretical studies we have suggested that inclusion of the intermediate term, through rate equation and quantum dynamical calculations, may be important for describing the exciton diffusion process in some circumstances, and in particular when the distance between the chromophores exceeds 5 nm. In this paper, we focus of the role of the intermediate-zone contribution to distance measurements between chromophores made through the application of spectroscopic ruler techniques. One of the major assumptions made in employing these experimental techniques is that the R−6dependence is valid. In this work, we reformulate the spectroscopic ruler principles for intermediate distances to include the inverse fourth power rate component, and compare the results of this reformulation to experimental FRET results from the literature. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only
Vibrational transfer functions for complex structures
Evaluation of effects of vibrational multiple frequency forcing functions is discussed. Computer program for developing vibrational transfer functions is described. Possible applications of computer program are enumerated
- …