4,177 research outputs found
Probing Red Giant Atmospheres with Gravitational Microlensing
Gravitational microlensing provides a new technique for studying the surfaces
of distant stars. Microlensing events are detected in real time and can be
followed up with precision photometry and spectroscopy. This method is
particularly adequate for studying red giants in the Galactic bulge. Recently
we developed an efficient method capable of computing the lensing effect for
thousands of frequencies in a high-resolution stellar spectrum. Here we
demonstrate the effects of microlensing on synthesized optical spectra of red
giant model atmospheres. We show that different properties of the stellar
surface can be recovered from time-dependent photometry and spectroscopy of a
point-mass microlensing event with a small impact parameter. In this study we
concentrate on center-to-limb variation of spectral features. Measuring such
variations can reveal the depth structure of the atmosphere of the source star.Comment: 23 pages with 11 Postscript figures, submitted to ApJ; Section 2
expanded, references added, text revise
On the heavenly equation hierarchy and its reductions
Second heavenly equation hierarchy is considered using the framework of
hyper-K\"ahler hierarchy developed by Takasaki. Generating equations for the
hierarchy are introduced, they are used to construct generating equations for
reduced hierarchies. General -reductions, logarithmic reduction and rational
reduction for one of the Lax-Sato functions are discussed. It is demonstrated
that rational reduction is equivalent to the symmetry constraint.Comment: 13 pages, LaTeX, minor misprints corrected, references adde
Limb-Darkening of a K Giant in the Galactic Bulge: PLANET Photometry of MACHO 97-BLG-28
We present the PLANET photometric dataset for the binary-lens microlensing
event MACHO 97-BLG-28 consisting of 696 I and V-band measurements, and analyze
it to determine the radial surface brightness profile of the Galactic bulge
source star. The microlensed source, demonstrated to be a K giant by our
independent spectroscopy, crossed the central isolated cusp of the lensing
binary, generating a sharp peak in the light curve that was well-resolved by
dense (3 - 30 minute) and continuous monitoring from PLANET sites in Chile,
South Africa, and Australia. Our modeling of these data has produced stellar
profiles for the source star in the I and V bands that are in excellent
agreement with those predicted by stellar atmospheric models for K giants. The
limb-darkening coefficients presented here are the first derived from
microlensing, among the first for normal giants by any technique, and the first
for any star as distant as the Galactic bulge. Modeling indicates that the
lensing binary has a mass ratio q = 0.23 and an (instantaneous) separation in
units of the angular Einstein ring radius of d = 0.69 . For a lens in the
Galactic bulge, this corresponds to a typical stellar binary with a projected
separation between 1 and 2 AU. If the lens lies closer, the separation is
smaller, and one or both of the lens objects is in the brown dwarf regime.
Assuming that the source is a bulge K2 giant at 8 kpc, the relative lens-source
proper motion is mu = 19.4 +/- 2.6 km/s /kpc, consistent with a disk or bulge
lens. If the non-lensed blended light is due to a single star, it is likely to
be a young white dwarf in the bulge, consistent with the blended light coming
from the lens itself.Comment: 32 Pages, including 1 table and 9 postscript figures. (Revised
version has slightly modified text, corrected typo, and 1 new figure.)
Accepted for publication in 1999 Astrophysical Journal; data are now
available at http://www.astro.rug.nl/~plane
Grassmannians Gr(N-1,N+1), closed differential N-1 forms and N-dimensional integrable systems
Integrable flows on the Grassmannians Gr(N-1,N+1) are defined by the
requirement of closedness of the differential N-1 forms of rank
N-1 naturally associated with Gr(N-1,N+1). Gauge-invariant parts of these
flows, given by the systems of the N-1 quasi-linear differential equations,
describe coisotropic deformations of (N-1)-dimensional linear subspaces. For
the class of solutions which are Laurent polynomials in one variable these
systems coincide with N-dimensional integrable systems such as Liouville
equation (N=2), dispersionless Kadomtsev-Petviashvili equation (N=3),
dispersionless Toda equation (N=3), Plebanski second heavenly equation (N=4)
and others. Gauge invariant part of the forms provides us with
the compact form of the corresponding hierarchies. Dual quasi-linear systems
associated with the projectively dual Grassmannians Gr(2,N+1) are defined via
the requirement of the closedness of the dual forms . It
is shown that at N=3 the self-dual quasi-linear system, which is associated
with the harmonic (closed and co-closed) form , coincides with the
Maxwell equations for orthogonal electric and magnetic fields.Comment: 26 pages, references adde
Chiral Skyrmionic matter in non-centrosymmetric magnets
Axisymmetric magnetic strings with a fixed sense of rotation and nanometer
sizes (chiral magnetic vortices or Skyrmions) have been predicted to exist in a
large group of non-centrosymmetric crystals more than two decades ago. Recently
these extraordinary magnetic states have been directly observed in thin layers
of cubic helimagnet (Fe,Co)Si. In this report we apply our earlier theoretical
findings to review main properties of chiral Skyrmions, to elucidate their
physical nature, and to analyse these recent experimental results on
magnetic-field-driven evolution of Skyrmions and helicoids in chiral
helimagnets.Comment: 13 pages, 7 figures, invited talk - JEMS-2010 ( 23-28 August, Krakow,
Poland
Detecting Stellar Spots by Gravitational Microlensing
During microlensing events with a small impact parameter, the amplification
of the source flux is sensitive to the surface brightness distribution of the
source star. Such events provide a means for studying the surface structure of
target stars in the ongoing microlensing surveys, most efficiently for giants
in the Galactic bulge. In this work we demonstrate the sensitivity of
point-mass microlensing to small spots with radii source
radii. We compute the amplification deviation from the light curve of a
spotless source and explore its dependence on lensing and spot parameters.
During source-transit events spots can cause deviations larger than 2%, and
thus be in principle detectable. Maximum relative deviation usually occurs when
the lens directly crosses the spot. Its numerical value for a dark spot with
sufficient contrast is found to be roughly equal to the fractional radius of
the spot, i.e., up to 20% in this study. Spots can also be efficiently detected
by the changes in sensitive spectral lines during the event. Notably, the
presence of a spot can mimic the effect of a low-mass companion of the lens in
some events.Comment: 18 pages with 7 Postscript figures, to appear in ApJ, January 2000;
discussion expanded, references added, minor revisions in tex
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