772 research outputs found
A Census of Object Types and Redshift Estimates in the SDSS Photometric Catalog from a Trained Decision-Tree Classifier
We have applied ClassX, an oblique decision tree classifier optimized for
astronomical analysis, to the homogeneous multicolor imaging data base of the
Sloan Digital Sky Survey (SDSS), training the software on subsets of SDSS
objects whose nature is precisely known via spectroscopy. We find that the
software, using photometric data only, correctly classifies a very large
fraction of the objects with existing SDSS spectra, both stellar and
extragalactic. ClassX also accurately predicts the redshifts of both normal and
active galaxies in SDSS. To illustrate ClassX applications in SDSS research, we
(a) derive the object content of the SDSS DR2 photometric catalog and (b)
provide a sample catalog of resolved SDSS objects that contains a large number
of candidate AGN galaxies, 27,000, along with 63,000 candidate normal galaxies
at magnitudes substantially fainter than typical magnitudes of SDSS
spectroscopic objects. The surface density of AGN selected by ClassX to i~19 is
in agreement with that quoted by SDSS. When ClassX is applied to the
photometric data fainter than the SDSS spectroscopic limit, the inferred
surface density of AGN rises sharply, as expected. The ability of the
classifier to accurately constrain the redshifts of huge numbers (ultimately ~
10^7) of active galaxies in the photometric data base promises new insights
into fundamental issues of AGN research, such as the evolution of the AGN
luminosity function with cosmic time, the starburst--AGN connection, and
AGN--galactic morphology relationships.Comment: Accepted for publication in The Astronomical Journal, Vol. 130, 2005;
33 pages, 10 figures, 5 tables, AASTeX v5.0. Table 5 will be electronic in
the published journal, but available now at
http://www-int.stsci.edu/~margon/table5.ascii and
http://www-int.stsci.edu/~margon/table5.ascii.g
Evidence for Prolonged Main Sequence Stellar Evolution of F Stars in close binaries
Binary F stars exhibit large brightness anomaly, which is defined here as the
difference between the absolute magnitude from the uvby photometry and the
actual absolute magnitude of the star. We have found that the anomaly inversely
correlates with the binary components separation. There is evidence that the
correlation reflects actual population differences between close and wide
binary pairs, in which case it indicates that the anomaly is somehow associated
with the interaction of binary's components. The anomaly has also been found to
correlate with both kinematics and metallicity. The sense of the correlations
implies that the anomaly increases as the star evolves, suggesting a peculiar
evolution of a primary F star in a tight binary pair. This conclusion has
further been supported by the study of the age-velocity relation (AVR) of F
stars that are cataloged in the HIPPARCOS as single. Among these stars, those
with brightness anomaly were previously shown to be most likely unidentified
close binaries. We have found that the AVR of these binary candidates is
different from that of the ``truly single'' F stars. The discrepancy between
the two AVRs indicates that the putative binaries are, on average, older than
similar normal single F stars at the same effective temperature and luminosity,
which is consistent with the inferred peculiar evolution in close binaries. It
appears that this peculiarity is caused by the impact of the components
interaction in a tight pair on stellar evolution, which results in the
prolonged main sequence lifetime of the primary F star.Comment: 8 pages, 7 figures, accepted by Astronomy and Astrophysic
Whispering gallery modes in optical fibers based on reflectionless potentials
We consider an optical fiber with nanoscale variation of the effective fiber
radius supporting whispering gallery modes slowly propagating along the fiber,
and reveal that the radius variation can be designed to support reflectionless
propagation of these modes. We show that reflectionless modulations can realize
control of transmission amplitude and temporal delay, while enabling close
packing due to the absence of cross-talk, in contrast to conventional
potentials.Comment: 4 pages, 3 figure
The Case for a "Strategic Pause": Russia and the United States in a New Era
U.S.-Russia relations are no longer central to international relations, but they still occupy center stage when it comes to global security. While these relations are usually analyzed based on problem areas between the two countries, this article argues that the key issue between Russia and the United States at the moment is not the poor state of the relationship, but rather its changed nature, since the two pillars that used to shape the relationship - principles and agenda - have evolved. The essay also argues that Moscow and Washington need a "strategic pause" to critically assess the value of relations for each party and that it may take some time - and a few election cycles in the US and a change of power in the Kremlin - to produce a situation that is qualitatively different from what we are observing today
Wave scattering on a domain wall in a chain of PT-symmetric couplers
We study wave propagation in linear arrays composed of pairs of conjugate
waveguides with balanced gain and loss, i.e. arrays of the PT-symmetric
couplers, where the linear spectrum is known to feature high-frequency and
low-frequency branches. We introduce a domain wall by switching the gain and
loss in a half of the array, and analyze the scattering of linear waves on this
defect. The analysis reveals two major effects: amplification of both reflected
and transmitted waves, and excitation of the reflected and transmitted
low-frequency and high-frequency waves by the incident high-frequency and
low-frequency waves, respectively.Comment: 7 pages, 9 figures, Physical Review A, in pres
Open questions in the study of population III star formation
The first stars were key drivers of early cosmic evolution. We review the
main physical elements of the current consensus view, positing that the first
stars were predominantly very massive. We continue with a discussion of
important open questions that confront the standard model. Among them are
uncertainties in the atomic and molecular physics of the hydrogen and helium
gas, the multiplicity of stars that form in minihalos, and the possible
existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxie
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