119 research outputs found
Factors Etc., Inc. v. Pro Arts, Inc.: Deference to Circuit Court Rulings on State Law, 15 J. Marshall L. Rev. 499 (1982)
The Highly Dynamic Behavior of the Innermost Dust and Gas in the Transition Disk Variable LRLL 31
We describe extensive synoptic multi-wavelength observations of the
transition disk LRLL 31 in the young cluster IC 348. We combined four epochs of
IRS spectra, nine epochs of MIPS photometry, seven epochs of cold-mission IRAC
photometry and 36 epochs of warm mission IRAC photometry along with multi-epoch
near-infrared spectra, optical spectra and polarimetry to explore the nature of
the rapid variability of this object. We find that the inner disk, as traced by
the 2-5micron excess stays at the dust sublimation radius while the strength of
the excess changes by a factor of 8 on weekly timescales, and the 3.6 and
4.5micron photometry shows a drop of 0.35 magnitudes in one week followed by a
slow 0.5 magnitude increase over the next three weeks. The accretion rate, as
measured by PaBeta and BrGamma emission lines, varies by a factor of five with
evidence for a correlation between the accretion rate and the infrared excess.
While the gas and dust in the inner disk are fluctuating the central star stays
relatively static. Our observations allow us to put constraints on the physical
mechanism responsible for the variability. The variabile accretion, and wind,
are unlikely to be causes of the variability, but both are effects of the same
physical process that disturbs the disk. The lack of periodicity in our
infrared monitoring indicates that it is unlikely that there is a companion
within ~0.4 AU that is perturbing the disk. The most likely explanation is
either a companion beyond ~0.4 AU or a dynamic interface between the stellar
magnetic field and the disk leading to a variable scale height and/or warping
of the inner disk.Comment: Accepted to ApJ. 10 pages of text, plus 11 tables and 13 figures at
the en
Protoplanetary and Transitional Disks in the Open Stellar Cluster IC 2395
We present new deep UBVRI images and high-resolution multi-object optical
spectroscopy of the young (~ 6 - 10 Myr old), relatively nearby (800 pc) open
cluster IC 2395. We identify nearly 300 cluster members and use the photometry
to estimate their spectral types, which extend from early B to middle M. We
also present an infrared imaging survey of the central region using the IRAC
and MIPS instruments on board the Spitzer Space Telescope, covering the
wavelength range from 3.6 to 24 microns. Our infrared observations allow us to
detect dust in circumstellar disks originating over a typical range of radii ~
0.1 to ~ 10AU from the central star. We identify 18 Class II, 8 transitional
disk, and 23 debris disk candidates, respectively 6.5%, 2.9%, and 8.3% of the
cluster members with appropriate data. We apply the same criteria for
transitional disk identification to 19 other stellar clusters and associations
spanning ages from ~ 1 to ~ 18 Myr. We find that the number of disks in the
transitional phase as a fraction of the total with strong 24 micron excesses
([8] - [24] > 1.5) increases from 8.4 +\- 1.3% at ~ 3 Myr to 46 +\- 5% at ~ 10
Myr. Alternative definitions of transitional disks will yield different
percentages but should show the same trend.Comment: accepted by the Astrophysical Journa
Savor the Cryosphere
This article provides concise documentation of the ongoing retreat of glaciers, along with the implications that the ice loss presents, as well as suggestions for geoscience educators to better convey this story to both students and citizens. We present the retreat of glaciers—the loss of ice—as emblematic of the recent, rapid contraction of the cryosphere. Satellites are useful for assessing the loss of ice across regions with the passage of time. Ground-based glaciology, particularly through the study of ice cores, can record the history of environmental conditions present during the existence of a glacier. Repeat photography vividly displays the rapid retreat of glaciers that is characteristic across the planet. This loss of ice has implications to rising sea level, greater susceptibility to dryness in places where people rely upon rivers delivering melt water resources, and to the destruction of natural environmental archives that were held within the ice. Warming of the atmosphere due to rising concentrations of greenhouse gases released by the combustion of fossil fuels is causing this retreat. We highlight multimedia productions that are useful for teaching this story effectively. As geoscience educators, we attempt to present the best scholarship as accurately and eloquently as we can, to address the core challenge of conveying the magnitude of anthropogenic impacts, while also encouraging optimistic determination on the part of students, coupled to an increasingly informed citizenry. We assert that understanding human perturbation of nature, then choosing to engage in thoughtful science-based decision-making, is a wise choice. This topic comprised “Savor the Cryosphere,” a Pardee Keynote Symposium at the 2015 Annual Meeting in Baltimore, Maryland, USA, for which the GSA recorded supporting interviews and a webinar
Spitzer Observations of Long-term Infrared Variability among Young Stellar Objects in Chamaeleon I
Infrared variability is common among young stellar objects, with surveys finding daily to weekly fluctuations of a few tenths of a magnitude. Space-based observations can produce highly sampled infrared light curves, but are often limited to total baselines of about 1 month due to the orientation of the spacecraft. Here we present observations of the Chameleon I cluster, whose low declination makes it observable by the Spitzer Space Telescope over a 200-day period. We observe 30 young stellar objects with a daily cadence to better sample variability on timescales of months. We find that such variability is common, occurring in ~80% of the detected cluster members. The change in [3.6]–[4.5] color over 200 days for many of the sources falls between that expected for extinction and fluctuations in disk emission. With our high cadence and long baseline we can derive power spectral density curves covering two orders of magnitude in frequency and find significant power at low frequencies, up to the boundaries of our 200-day survey. Such long timescales are difficult to explain with variations driven by the interaction between the disk and stellar magnetic field, which has a dynamical timescale of days to weeks. The most likely explanation is either structural or temperature fluctuations spread throughout the inner ~0.5 au of the disk, suggesting that the intrinsic dust structure is highly dynamic
Spitzer Imaging of the Nearby Rich Young Cluster, Cep OB3b
We map the full extent of a rich massive young cluster in the Cep OB3b
association with the IRAC and MIPS instruments aboard the {\it Spitzer} Space
Telescope and the ACIS instrument aboard the X-Ray Observatory.
At 700 pc, it is revealed to be the second nearest large ( member),
young ( Myr) cluster known. In contrast to the nearest large cluster, the
Orion Nebula Cluster, Cep OB3b is only lightly obscured and is mostly located
in a large cavity carved out of the surrounding molecular cloud. Our infrared
and X-ray datasets, as well as visible photometry from the literature, are used
to take a census of the young stars in Cep OB3b. We find that the young stars
within the cluster are concentrated in two sub-clusters; an eastern
sub-cluster, near the Cep B molecular clump, and a western sub-cluster, near
the Cep F molecular clump. Using our census of young stars, we examine the
fraction of young stars with infrared excesses indicative of circumstellar
disks. We create a map of the disk fraction throughout the cluster and find
that it is spatially variable. Due to these spatial variations, the two
sub-clusters exhibit substantially different average disk fractions from each
other: and . We discuss whether the discrepant disk
fractions are due to the photodestruction of disks by the high mass members of
the cluster or whether they result from differences in the ages of the
sub-clusters. We conclude that the discrepant disk fractions are most likely
due to differences in the ages.Comment: 48 Pages, 12 figures, 6 table
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