97 research outputs found
The incidence of mid-infrared excesses in G and K giants
Using photometric data from the 2MASS and GLIMPSE catalogues, I investigate
the incidence of mid-infrared excesses (~10 microns) of G and K stars of
luminosity class III. In order to obtain a large sample size, stars are
selected using a near-IR colour-magnitude diagram. Sources which are candidates
for showing mid-IR excess are carefully examined and modelled to determined
whether they are likely to be G/K giants. It is found that mid-IR excesses are
present at a level of (1.8 +/- 0.4) x 10^-3. While the origin of these excesses
remains uncertain, it is plausible that they arise from debris discs around
these stars. I note that the measured incidence is consistent with a scenario
in which dust lifetimes in debris discs are determined by Poynting-Robertson
drag rather than by collisions.Comment: Accepted for publication in MNRAS. 13 pages, 5 figures, 2 tables (1
landscape table
Controller time and delay costs - a trade-off analysis
Air traffic controller shortages remain a significant challenge in European ATM. Comparing different rules, we quantify the cost effectiveness of adding controller hours to Area Control Centre regulations to avert the delay cost impact on airlines. Typically, adding controller hours results in a net benefit. Distributions of delay duration and aircraft weight play an important role in determining the total cost of a regulation. Errors are likely to be incurred when analysing performance based on average delay values, particularly at the disaggregate level
Building a Holistic ATM Model for Future KPI Trade-Offs
We present the model developed within the Vista project, studying the future evolution of trade-offs between Key Performance Indicators. The model has a very broad scope and aims to simulate the changes that business and regulatory forces have at a strategic, pre-tactical and tactical level. The relevant factors that will affect the air transportation system are presented, as well as the scenarios to be simulated. The overall architecture of the model is described and a more detailed presentation of the economic component of the model is given. Some preliminary results of this part of the model illustrate its main mechanisms and capabilities
Images of unclassified and supergiant B[e] stars disks with interferometry
B[e] stars are among the most peculiar objects in the sky. This spectral
type, characterised by allowed and forbidden emission lines, and a large
infrared excess, does not represent an homogenous class of objects, but
instead, a mix of stellar bodies seen in all evolutionary status. Among them,
one can find Herbig stars, planetary nebulae central stars, interacting
binaries, supermassive stars, and even "unclassified" B[e] stars: systems
sharing properties of several of the above. Interferometry, by resolving the
innermost regions of these stellar systems, enables us to reveal the true
nature of these peculiar stars among the peculiar B[e] stars.Comment: Proceeding submitted to the editors, to be published in the
conference proceedin
Debris Disks in NGC 2547
We have surveyed the 30 Myr-old cluster NGC 2547 for planetary debris disks
using Spitzer. At 4.5-8 um we are sensitive to the photospheric level down to
mid-M stars (0.2 Msol) and at 24 um to early-G stars (1.2 Msol). We find only
two to four stars with excesses at 8 um out of ~400-500 cluster members,
resulting in an excess fraction <~1 percent at this wavelength. By contrast,
the excess fraction at 24 um is ~40 percent (for B-F types). Out of four
late-type stars with excesses at 8 um two marginal ones are consistent with
asteroid-like debris disks. Among stars with strong 8 um excesses one is
possibly from a transitional disk, while another one can be a result of a
catastrophic collision. Our survey demonstrates that the inner 0.1-1 AU parts
of disks around solar-type stars clear out very thoroughly by 30 Myrs of age.
Comparing with the much slower decay of excesses at 24 and 70 um, disks clear
from the inside out, of order 10 Myr for the inner zones probed at 8 um
compared with a hundred or more Myr for those probed with the two longer
wavelengths.Comment: Accepted to ApJ, 29 pages, 13 figs. A Note in Proof concerning
cluster's age was added in the original submission of 2007 July 19. Full
Tables 1 and 2 in the electronic form together with the article with full
resolution figures are available at
http://www.astro.ufl.edu/~ngorlova/disksNGC2547
Spitzer 24 micron Survey of Debris Disks in the Pleiades
We performed a 24 micron 2 Deg X 1 Deg survey of the Pleiades cluster, using
the MIPS instrument on Spitzer. Fifty four members ranging in spectral type
from B8 to K6 show 24 micron fluxes consistent with bare photospheres. All Be
stars show excesses attributed to free-free emission in their gaseous
envelopes. Five early-type stars and four solar-type stars show excesses
indicative of debris disks. We find a debris disk fraction of 25 % for B-A
members and 10 % for F-K3 ones. These fractions appear intermediate between
those for younger clusters and for the older field stars. They indicate a decay
with age of the frequency of the dust-production events inside the planetary
zone, with similar time scales for solar-mass stars as have been found
previously for A-stars.Comment: accepted to Ap
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