35,457 research outputs found
Identification of flexible structures for robust control
Documentation is provided of the authors' experience with modeling and identification of an experimental flexible structure for the purpose of control design, with the primary aim being to motivate some important research directions in this area. A multi-input/multi-output (MIMO) model of the structure is generated using the finite element method. This model is inadequate for control design, due to its large variation from the experimental data. Chebyshev polynomials are employed to fit the data with single-input/multi-output (SIMO) transfer function models. Combining these SIMO models leads to a MIMO model with more modes than the original finite element model. To find a physically motivated model, an ad hoc model reduction technique which uses a priori knowledge of the structure is developed. The ad hoc approach is compared with balanced realization model reduction to determine its benefits. Descriptions of the errors between the model and experimental data are formulated for robust control design. Plots of select transfer function models and experimental data are included
Orbital structure of the GJ876 extrasolar planetary system, based on the latest Keck and HARPS radial velocity data
We use full available array of radial velocity data, including recently
published HARPS and Keck observatory sets, to characterize the orbital
configuration of the planetary system orbiting GJ876. First, we propose and
describe in detail a fast method to fit perturbed orbital configuration, based
on the integration of the sensitivity equations inferred by the equations of
the original -body problem. Further, we find that it is unsatisfactory to
treat the available radial velocity data for GJ876 in the traditional white
noise model, because the actual noise appears autocorrelated (and demonstrates
non-white frequency spectrum). The time scale of this correlation is about a
few days, and the contribution of the correlated noise is about 2 m/s (i.e.,
similar to the level of internal errors in the Keck data). We propose a
variation of the maximum-likelihood algorithm to estimate the orbital
configuration of the system, taking into account the red noise effects. We
show, in particular, that the non-zero orbital eccentricity of the innermost
planet \emph{d}, obtained in previous studies, is likely a result of
misinterpreted red noise in the data. In addition to offsets in some orbital
parameters, the red noise also makes the fit uncertainties systematically
underestimated (while they are treated in the traditional white noise model).
Also, we show that the orbital eccentricity of the outermost planet is actually
ill-determined, although bounded by . Finally, we investigate
possible orbital non-coplanarity of the system, and limit the mutual
inclination between the planets \emph{b} and \emph{c} orbits by
, depending on the angular position of the mutual orbital
nodes.Comment: 36 pages, 11 figures, 3 tables; Accepted to Celestial Mechanics and
Dynamical Astronom
Physical Conditions in Circumstellar Gas surrounding SN 1987A 12 Years After Outburst
Two-dimensional spectra of Supernova 1987A were obtained on 1998 November
14-15 (4282 days after outburst) with the Space Telescope Imaging Spectrograph
(STIS) on board the Hubble Space Telescope (HST). The slit sampled portions of
the inner circumstellar ring at the east and west ansae as well as small
sections of both the northern and southern outer rings. The temperature and
density at these locations are estimated by nebular analysis of [N II], [O
III], and [S II] emission line ratios, and with time-dependent
photoionization/recombination models. The results from these two methods are
mutually consistent. The electron density in the inner ring is ~ 4000 cm-3 for
S II, with progressively lower densities for N II and O III. The electron
temperatures determined from [N II] and [O III] line ratios are ~11,000 K and
\~22,000 K, respectively. These results are consistent with evolutionary trends
in the circumstellar gas from similar measurements at earlier epochs. We find
that emission lines from the outer rings come from gas of lower density (n_e
\la 2000 cm-3) than that which emits the same line in the inner ring. The N/O
ratio appears to be the same in all three rings. Our results also suggest that
the CNO abundances in the northern outer ring are the same as in the inner
ring, contrary to earlier results of Panagia et al. (1996). Physical conditions
in the southern outer ring are less certain because of poorer signal-to-noise
data. The STIS spectra also reveal a weak Ha emission redshifted by ~100 km s-1
at p.a. 103\arcdeg that coincides with the recently discovered new regions that
are brightening (Lawrence et al. 2000). This indicates that the shock
interaction in the SE section of the inner ring commenced over a year before it
became apparent in HST images.Comment: 25 pages, 6 figures, to appear in December 1, 2000 Astrophysical
Journa
Solutions for 10,000 Eclipsing Binaries in the Bulge Fields of OGLE II Using DEBiL
We have developed a fully-automated pipeline for systematically identifying
and analyzing eclipsing binaries within large datasets of light curves. The
pipeline is made up of multiple tiers which subject the light curves to
increasing levels of scrutiny. After each tier, light curves that did not
conform to a given criteria were filtered out of the pipeline, reducing the
load on the following, more computationally intensive tiers. As a central
component of the pipeline, we created the fully automated Detached Eclipsing
Binary Light curve fitter (DEBiL), which rapidly fits large numbers of light
curves to a simple model. Using the results of DEBiL, light curves of interest
can be flagged for follow-up analysis. As a test case, we analyzed the 218699
light curves within the bulge fields of the OGLE II survey and produced 10862
model fits. We point out a small number of extreme examples as well as
unexpected structure found in several of the population distributions. We
expect this approach to become increasingly important as light curve datasets
continue growing in both size and number.Comment: Accepted for publication in ApJ, 36 pages, 15 figures, 5 tables. See
http://cfa-www.harvard.edu/~jdevor/DEBiL.html for high-resolution figures and
further informatio
- …