9,270 research outputs found
Minimum-Cost Coverage of Point Sets by Disks
We consider a class of geometric facility location problems in which the goal
is to determine a set X of disks given by their centers (t_j) and radii (r_j)
that cover a given set of demand points Y in the plane at the smallest possible
cost. We consider cost functions of the form sum_j f(r_j), where f(r)=r^alpha
is the cost of transmission to radius r. Special cases arise for alpha=1 (sum
of radii) and alpha=2 (total area); power consumption models in wireless
network design often use an exponent alpha>2. Different scenarios arise
according to possible restrictions on the transmission centers t_j, which may
be constrained to belong to a given discrete set or to lie on a line, etc. We
obtain several new results, including (a) exact and approximation algorithms
for selecting transmission points t_j on a given line in order to cover demand
points Y in the plane; (b) approximation algorithms (and an algebraic
intractability result) for selecting an optimal line on which to place
transmission points to cover Y; (c) a proof of NP-hardness for a discrete set
of transmission points in the plane and any fixed alpha>1; and (d) a
polynomial-time approximation scheme for the problem of computing a minimum
cost covering tour (MCCT), in which the total cost is a linear combination of
the transmission cost for the set of disks and the length of a tour/path that
connects the centers of the disks.Comment: 10 pages, 4 figures, Latex, to appear in ACM Symposium on
Computational Geometry 200
Ruling out Stellar Companions and Resolving the Innermost Regions of Transitional Disks with the Keck Interferometer
With the Keck Interferometer, we have studied at 2 um the innermost regions
of several nearby, young, dust depleted "transitional" disks. Our observations
target five of the six clearest cases of transitional disks in the
Taurus/Auriga star-forming region (DM Tau, GM Aur, LkCa 15, UX Tau A, and RY
Tau) to explore the possibility that the depletion of optically thick dust from
the inner disks is caused by stellar companions rather than the more typical
planet-formation hypothesis. At the 99.7% confidence level, the observed
visibilities exclude binaries with flux ratios of at least 0.05 and separations
ranging from 2.5 to 30 mas (0.35 - 4 AU) over >= 94% of the area covered by our
measurements. All targets but DM Tau show near-infrared excess in their SED
higher than our companion flux ratio detection limits. While a companion has
previously been detected in the candidate transitional disk system CoKu Tau/4,
we can exclude similar mass companions as the typical origin for the clearing
of inner dust in transitional disks and of the near-infrared excess emission.
Unlike CoKu Tau/4, all our targets show some evidence of accretion. We find
that all but one of the targets are clearly spatially resolved, and UX Tau A is
marginally resolved. Our data is consistent with hot material on small scales
(0.1 AU) inside of and separated from the cooler outer disk, consistent with
the recent SED modeling. These observations support the notion that some
transitional disks have radial gaps in their optically thick material, which
could be an indication for planet formation in the habitable zone (~ a few AU)
of a protoplanetary disk.Comment: 36 pages, 7 figures. Accepted for publication in Ap
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