1,521 research outputs found
The number of point-splitting circles
Let S be a set of 2n+1 points in the plane such that no three are collinear
and no four are concyclic. A circle will be called point-splitting if it has 3
points of S on its circumference, n-1 points in its interior and n-1 in its
exterior. We show the surprising property that S always has exactly n^2 point-
splitting circles, and prove a more general result.Comment: 12 pages, 4 figure
Tropical eigenwave and intermediate Jacobians
Tropical manifolds are polyhedral complexes enhanced with certain kind of
affine structure. This structure manifests itself through a particular
cohomology class which we call the eigenwave of a tropical manifold. Other wave
classes of similar type are responsible for deformations of the tropical
structure.
If a tropical manifold is approximable by a 1-parametric family of complex
manifolds then the eigenwave records the monodromy of the family around the
tropical limit. With the help of tropical homology and the eigenwave we define
tropical intermediate Jacobians which can be viewed as tropical analogs of
classical intermediate Jacobians.Comment: 38 pages, 8 figure
Extended Coronal Emission Lines in Active Galactic Nuclei
VLT and NTT spectra are used to examine the nuclear and extended coronal line
emission in a sample of well-known Seyfert 1 and 2 galaxies. The excellent
spatial resolution obtained with VLT allowed us to map [SiVI] 1.963 m and
[SiVII] 2.48 m on scales of up to 20 pc. Coronal line emission, extended
to distances of 100 pc, is detected in some of the lines analyzed,
particularly in [FeX] 6374\AA, [FeXI] 7891\AA, and [SiVII] 2.48m. Most
coronal lines are strongly asymmetric towards the blue and broader than
low-ionization lines. This result is particularly important for Circinus, where
previous observations had failed at detecting larger widths for high-ionization
lines. Photoionization models are used to investigate the physical conditions
and continuum luminosities necessary to produced the observed coronal emission.
We found that an ionization parameter U> 0.10 is necessary to reproduce the
observations, although the clouds should be located at distances < 30 pc.Comment: 4 pages, 6 figures, to appear in proceedings of IAU Symposium No.
222, The Interplay Among Black Holes, Stars and ISM in Galacti Nucle
A Consistent Model of the Accretion Shock Region in Classical T Tauri Stars
We develop a consistent model of the accretion shock region in Classical T Tauri Stars (CTTSs). The initial conditions of the post-shock flow are determined by the irradiated shock precursor and the ionization state is calculated without assuming ionization equilibrium. Comparison with observations of the C IV resonance lines (λλ 1550 Å) for CTTSs indicate that the post-shock emission predicted by the model is too large, for a reasonable range of parameters. If the model is to reproduce the observations, C IV emission from CTTSs has to be dominated by pre-shock emission, for stars with moderate to large accretion rates. For stars with low accretion rates, the observations suggest a comparable contribution between the pre- and post-shock regions. These conclusions are consistent with previous results indicating that the post-shock will be buried under the stellar photosphere for moderate to large accretion rates
The Narrow Line Region of Ark 564
The continuum and emission-line spectrum of the narrow-line Seyfert 1 galaxy
Ark 564 is used to investigate, for the first time, the physical conditions and
structure of its narrow line region (NLR). For this purpose, composite models,
accounting for the coupled effect of photoionization and shocks, are employed.
The emission-line spectrum of Ark 564, which ranges from the ultraviolet to the
near-infrared, shows a rich forbidden line spectrum. Strong emphasis is given
to the study of the coronal line region. The diversity of physical conditions
deduced from the observations requires multi-cloud models to reproduce the
observed lines and continuum. We find that a combination of high velocity (Vs =
1500 km/s) shock-dominated clouds as well as low velocity (Vs = 150 km/s)
radiation-dominated clouds explains the coronal lines, while the optical
low-ionization lines are mainly explained by shock-dominated clouds. The
results for Ark 564 are compared with those obtained for other Seyfert galaxies
previously analyzed such as NGC 5252, Circinus, NGC 4051 and NGC 4151. The
model results for the ultraviolet and optical permitted lines suggest that the
broad line region may contribute up to 80%, depending on the emission-line,
being of about 30% for Hbeta. The consistency of the multi-cloud model is
checked by comparing the predicted and observed continuum, from radio to X-ray,
and indicate that the dust-to-gas ratio in the clouds varies from 10^{-15} to
10^{-12}.Comment: 18 pages, 1 figure. Accepted in A&
Ysovar: The First Sensitive, Wide-area, Mid-infrared Photometric Monitoring of the Orion Nebula Cluster
We present initial results from time-series imaging at infrared wavelengths of 0.9 deg^2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I_c) and/or near-infrared (JK_s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs—YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs
- …