51 research outputs found
Formation and Evolution of Planetary Systems (FEPS): Properties of Debris Dust around Solar-type Stars
We present Spitzer photometric (IRAC and MIPS) and spectroscopic (IRS low
resolution) observations for 314 stars in the Formation and Evolution of
Planetary Systems (FEPS) Legacy program. These data are used to investigate the
properties and evolution of circumstellar dust around solar-type stars spanning
ages from approximately 3 Myr to 3 Gyr. We identify 46 sources that exhibit
excess infrared emission above the stellar photosphere at 24um, and 21 sources
with excesses at 70um. Five sources with an infrared excess have
characteristics of optically thick primordial disks, while the remaining
sources have properties akin to debris systems. The fraction of systems
exhibiting a 24um excess greater than 10.2% above the photosphere is 15% for
ages < 300 Myr and declines to 2.7% for older ages. The upper envelope to the
70um fractional luminosity appears to decline over a similar age range. The
characteristic temperature of the debris inferred from the IRS spectra range
between 60 and 180 K, with evidence for the presence of cooler dust to account
for the strength of the 70um excess emission. No strong correlation is found
between dust temperature and stellar age. Comparison of the observational data
with disk models containing a power-law distribution of silicate grains suggest
that the typical inner disk radius is > 10 AU. Although the interpretation is
not unique, the lack of excess emission shortwards of 16um and the relatively
flat distribution of the 24um excess for ages <300~Myr is consistent with
steady-state collisional models.Comment: 85 pages, 18 figures, 4 tables; accepted for publication in ApJ
Dusty Planetary Systems
Extensive photometric stellar surveys show that many main sequence stars show
emission at infrared and longer wavelengths that is in excess of the stellar
photosphere; this emission is thought to arise from circumstellar dust. The
presence of dust disks is confirmed by spatially resolved imaging at infrared
to millimeter wavelengths (tracing the dust thermal emission), and at optical
to near infrared wavelengths (tracing the dust scattered light). Because the
expected lifetime of these dust particles is much shorter than the age of the
stars (>10 Myr), it is inferred that this solid material not primordial, i.e.
the remaining from the placental cloud of gas and dust where the star was born,
but instead is replenished by dust-producing planetesimals. These planetesimals
are analogous to the asteroids, comets and Kuiper Belt objects (KBOs) in our
Solar system that produce the interplanetary dust that gives rise to the
zodiacal light (tracing the inner component of the Solar system debris disk).
The presence of these "debris disks" around stars with a wide range of masses,
luminosities, and metallicities, with and without binary companions, is
evidence that planetesimal formation is a robust process that can take place
under a wide range of conditions. This chapter is divided in two parts. Part I
discusses how the study of the Solar system debris disk and the study of debris
disks around other stars can help us learn about the formation, evolution and
diversity of planetary systems by shedding light on the frequency and timing of
planetesimal formation, the location and physical properties of the
planetesimals, the presence of long-period planets, and the dynamical and
collisional evolution of the system. Part II reviews the physical processes
that affect dust particles in the gas-free environment of a debris disk and
their effect on the dust particle size and spatial distribution.Comment: 68 pages, 25 figures. To be published in "Solar and Planetary
Systems" (P. Kalas and L. French, Eds.), Volume 3 of the series "Planets,
Stars and Stellar Systems" (T.D. Oswalt, Editor-in-chief), Springer 201
The Kuiper Belt and Other Debris Disks
We discuss the current knowledge of the Solar system, focusing on bodies in
the outer regions, on the information they provide concerning Solar system
formation, and on the possible relationships that may exist between our system
and the debris disks of other stars. Beyond the domains of the Terrestrial and
giant planets, the comets in the Kuiper belt and the Oort cloud preserve some
of our most pristine materials. The Kuiper belt, in particular, is a
collisional dust source and a scientific bridge to the dusty "debris disks"
observed around many nearby main-sequence stars. Study of the Solar system
provides a level of detail that we cannot discern in the distant disks while
observations of the disks may help to set the Solar system in proper context.Comment: 50 pages, 25 Figures. To appear in conference proceedings book
"Astrophysics in the Next Decade
Unlocking the Climate Record Stored within Marsâ Polar Layered Deposits
In the icy beds of its polar layered deposits (PLD), Mars likely possesses a record of its recent climate history, analogous to terrestrial ice sheets that contain records of Earth's past climate. Both northern and southern PLDs store information on the climatic and atmospheric state during the
deposition of each layer (WPs: Becerra et al.; Smith et al). Reading the climate record stored in these layers requires detailed measurements of layer composition, thickness, isotope variability, and near-surface atmospheric measurements. We identify four fundamental questions that must be answered in order to interpret this climate record and decipher the recent climatic history of Mars:
1. Fluxes: What are the present and past fluxes of volatiles, dust, and other materials into and
out of the polar regions?
2. Forcings: How do orbital/axial forcing and exchange with other reservoirs affect those fluxes?
3. Layer Processes: What chemical and physical processes form and modify layers?
4. Record: What is the timespan, completeness, and temporal resolution of the climate history
recorded in the PLD?
In a peer reviewed report (1), we detailed a sequence of missions, instruments, and architecture needed to answer these questions. Here, we present the science drivers and a mission concept for a polar lander that would enable a future reading of the past few million years of the Martian
climate record. The mission addresses as-yet-unachieved science goals of the current Decadal Survey and of MEPAG for obtaining a record of Mars climate and has parallel goals to the NEXSAG and ICE-SAG reports
The 1997 CMU Sphinx-3 English Broadcast News Transcription System
This paper describes the 1997 Hub-4 Broadcast News Sphinx3 speech recognition system. This year's system includes fullbandwidth acoustic models trained on Broadcast News and Wall Street Journal acoustic training data, an expanded vocabulary, and a 4-gram language model for N-best list rescoring. The system structure, acoustic and language models, and adaptation components are described in detail, and results are presented to establish the contributions of multiple recognition passes. Additionally, experimental results are presented for several different acoustic and language model configurations. 1. INTRODUCTION This year's Hub-4 task consisted of transcribing broadcast news shows in a completely unpartitioned manner, meaning that the broadcast news audio was not accompanied by any types of markers indicating speaker or show changes. Recognition systems had to rely on completely automatic methods of segmenting the audio into manageable pieces. Additionally, no information was provide..
The 1996 Hub-4 Sphinx-3 System
This paper describes the CMU Sphinx-3 system, and the configuration we used for the 1996 DARPA (Hub-4) evaluation. The model structure, acoustic modeling, language modeling, lexical modeling, and system structure are summarized. We also discuss the experimental results obtained with this system on the most recent DARPA evaluation, and some subsequent results are also discussed. Motivation Past efforts on speech recognition have focusedon clean, good quality speech in friendly environments, and DARPA evaluations in past years have followed this agenda. While one must walk before one can run, we have, as a community, developed our technology to the point where we can handle large vocabulary dictation well, and spontaneous speech fairly well. The evaluations have tracked this, with the introduction last year of so-called "found" speech, recorded off the air from commercial broadcasts. For the 1996 Hub-4 evaluation we have continued in this vein, widening our horizons to include the addi..
Comparison of serial electrocardiographic and vectorcardiographic changes during recovery from status asthmaticus.
Vergleich der kombinatorischen Problemlösefahigkeit von PrimĂ€r- und SekundarstufenschĂŒlern im Hinblick auf Lerneffekte wĂ€hrend eines Interviews
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