From: LARRY KLAES (ljk4_at_msn.com)
Date: Wed Mar 28 2007 - 12:06:44 PDT
Astrophysics, abstract
astro-ph/0703616
From: Ilaria Pascucci [view email]
Date: Fri, 23 Mar 2007 16:49:50 GMT (85kb)
Detection of [Ne II] Emission from Young Circumstellar Disks
Authors: I. Pascucci, D. Hollenbach, J. Najita, J. Muzerolle, U. Gorti, G.
J. Herczeg, L. A. Hillenbrand, J. S. Kim, J. M. Carpenter, M. R. Meyer, E.
E. Mamajek, J. Bouwman
Comments: accepted for publication to The Astrophysical Journal
We report the detection of [Ne II] emission at 12.81 micron in four out of
the six optically thick dust disks observed as part of the FEPS Spitzer
Legacy program. In addition, we detect a H I(7-6) emission line at 12.37
micron from the source RXJ1852.3-3700. Detections of [Ne II] lines are
favored by low mid-infrared excess emission. Both stellar X-rays and extreme
UV (EUV) photons can sufficiently ionize the disk surface to reproduce the
observed line fluxes, suggesting that emission from Ne+ originates in the
hot disk atmosphere. On the other hand, the H I(7-6) line is not associated
with the gas in the disk surface and magnetospheric accretion flows can
account only for at most ~30% of the observed flux. We conclude that
accretion shock regions and/or the stellar corona could contribute to most
of the H I(7-6)emission. Finally, we discuss the observations necessary to
identify whether stellar X-rays or EUV photons are the dominant ionization
mechanism for Ne atoms.
Because the observed [Ne II] emission probes very small amounts of gas in
the disk surface (~10^{-6} Jupiter masses) we suggest using this gas line to
determine the presence or absence of gas in more evolved circumstellar
disks.
http://arxiv.org/abs/astro-ph/0703616
Astrophysics, abstract
astro-ph/0703608
From: Mark Wyatt [view email]
Date: Fri, 23 Mar 2007 10:55:40 GMT (430kb)
Steady-state evolution of debris disks around A stars
Authors: M. C. Wyatt, R. Smith, K. Y. L. Su, G. H. Rieke, J. S. Greaves, C.
A. Beichman, G. Bryden
Comments: Accepted by ApJ
In this paper a simple analytical model for the steady-state evolution of
debris disks due to collisions is confronted with Spitzer observations of
main sequence A stars. All stars are assumed to have planetesimal belts with
a distribution of initial masses and radii. In the model disk mass is
constant until the largest planetesimals reach collisional equilibrium
whereupon the mass falls off oc 1/t. We find that the detection statistics
and trends seen at both 24 and 70um can be fitted well by the model. While
there is no need to invoke stochastic evolution or delayed stirring to
explain the statistics, a moderate rate of stochastic events is not ruled
out. Potentially anomalous systems are identified by a high dust luminosity
compared with the maximum permissible in the model (HD3003, HD38678,
HD115892, HD172555). Their planetesimals may have unusual properties (high
strength or low eccentricity) or this dust could be transient. While
transient phenomena are also favored for a few systems in the literature,
the overall success of our model, which assumes planetesimals in all belts
have the same strength, eccentricity and maximum size, suggests a large
degree of uniformity in the outcome of planet formation. The distribution of
planetesimal belt radii, once corrected for detection bias, follows N(r) oc
r^{-0.8+-0.3} for 3-120AU. Since the inner edge is often attributed to an
unseen planet, this provides a unique constraint on the planetary systems of
A stars. It is also shown that P-R drag may sculpt the inner edges of A star
disks close to the Spitzer detection threshold (HD2262, HD19356, HD106591,
HD115892). This model can be readily applied to the interpretation of future
surveys, and predictions are made for the upcoming SCUBA-2 survey, including
that >17% of A stars should be detectable at 850um.
http://arxiv.org/abs/astro-ph/0703608
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