999 research outputs found
Do Proto-Jovian Planets Drive Outflows?
We discuss the possibility that gaseous giant planets drive strong outflows
during early phases of their formation. We consider the range of parameters
appropriate for magneto-centrifugally driven stellar and disk outflow models
and find that if the proto-Jovian planet or accretion disk had a magnetic field
of >~ 10 Gauss and moderate mass inflow rates through the disk of less than
10^-7 M_J/yr that it is possible to drive an outflow. Estimates based both on
scaling from empirical laws observed in proto-stellar outflows and the
magneto-centrigugal disk and stellar+disk wind models suggest that winds with
mass outflow rates of 10^-8 M_J/yr and velocities of order ~ 20 km/s could be
driven from proto-Jovian planets. Prospects for detection and some implications
for the formation of the solar system are briefly discussed.Comment: AAS Latex, accepted for Ap
Cloud Chamber Investigation of Anomalous θ^0 Particles
Eighteen anomalous θ^0, (θ^0_(anom)), decay events observed in the California Institute of Technology magnet cloud chambers have been analyzed. Many of these decays are dynamically inconsistent with the τ^0→π^++π^−+π^0 scheme, but most are consistent with the decay processes: θ^0_(anom)→π^++π^−+γ, π^±+μ^∓+ν, and π^±+e^∓+ν. However, at least one event is inconsistent with each decay scheme. From the locations of the decays in the cloud chamber, the lifetime is found to be significantly longer than that of the normal θ^0 particle, called here the θ^0_(π2) particle. Other differences in the behavior of the θ0anom and θ^0_(π2) particles were also observed in the (a) momentum distributions, (b) origin locations, (c) relative numbers of θ^0_(anom) and θ^0_(π2) particles traveling upward, and (d) the types of V particles produced in association with the θ^0_(anom) and θ^0_(π2). It is concluded that not all the θ^0_(anom) decays can result from alternate decay modes of the θ^0_(π2). Moreover, many decays can be neither τ^0 decays nor alternate decays of the θ^0_(π2).
The characteristics of the θ^0_2 particle proposed by Gell-Mann and Pais are consistent with those of the θ^0_(anom) particle, with the possible exception of the observed types of associations. An estimate was made of the relative number of θ^0_(anom) to θ^0_(π2) particles observed to decay in the cloud chamber. If all θ^0_(anom) decays are assumed to arise from decays of the θ^0_2 particle, then a lower limit for the θ^0_2 lifetime is found to be about 10^(−9) sec
Spitzer/MIPS Limits on Asteroidal Dust in the Pulsar Planetary System PSR B1257+1
With the MIPS camera on Spitzer, we have searched for far-infrared emission
from dust in the planetary system orbiting pulsar PSR 1257+12. With accuracies
of 0.05 mJy at 24 um and 1.5 mJy at 70 um, photometric measurements find no
evidence for emission at these wavelengths. These observations place new upper
limits on the luminosity of dust with temperatures between 20 and 1000 K. They
are particularly sensitive to dust temperatures of 100-200 K, for which they
limit the dust luminosity to below of the pulsar's spin-down
luminosity, three orders of magnitude better than previous limits. Despite
these improved constraints on dust emission, an asteroid belt similar to the
Solar System's cannot be ruled out
ExploreNEOs VIII: Dormant Short-Period Comets in the Near-Earth Asteroid Population
We perform a search for dormant comets, asteroidal objects of cometary
origin, in the near-Earth asteroid (NEA) population based on dynamical and
physical considerations. Our study is based on albedos derived within the
ExploreNEOs program and is extended by adding data from NEOWISE and the Akari
asteroid catalog. We use a statistical approach to identify asteroids on orbits
that resemble those of short-period near-Earth comets using the Tisserand
parameter with respect to Jupiter, the aphelion distance, and the minimum
orbital intersection distance with respect to Jupiter. From the sample of NEAs
on comet-like orbits, we select those with a geometric albedo
as dormant comet candidates, and find that only 50% of NEAs on comet-like
orbits also have comet-like albedos. We identify a total of 23 NEAs from our
sample that are likely to be dormant short-period near-Earth comets and, based
on a de-biasing procedure applied to the cryogenic NEOWISE survey, estimate
both magnitude-limited and size-limited fractions of the NEA population that
are dormant short-period comets. We find that 0.3-3.3% of the NEA population
with , and % of the population with diameters km, are dormant short-period near-Earth comets.Comment: 23 pages, 2 figures, 2 tables; accepted for publication in A
IRS Spectra of Solar-Type Stars: \break A Search for Asteroid Belt Analogs
We report the results of a spectroscopic search for debris disks surrounding
41 nearby solar type stars, including 8 planet-bearing stars, using the {\it
Spitzer Space Telescope}. With accurate relative photometry using the Infrared
Spectrometer (IRS) between 7-34 \micron we are able to look for excesses as
small as 2% of photospheric levels with particular sensitivity to weak
spectral features. For stars with no excess, the upper limit in a
band at 30-34 m corresponds to 75 times the brightness of our
zodiacal dust cloud. Comparable limits at 8.5-13 m correspond to
1,400 times the brightness of our zodiacal dust cloud. These limits correspond
to material located within the 1 to 5 AU region that, in our solar
system, originates from debris associated with the asteroid belt. We find
excess emission longward of 25 m from five stars of which four also
show excess emission at 70 m. This emitting dust must be located around
5-10 AU. One star has 70 micron emission but no IRS excess. In this case, the
emitting region must begin outside 10 AU; this star has a known radial velocity
planet. Only two stars of the five show emission shortward of 25 \micron
where spectral features reveal the presence of a population of small, hot dust
grains emitting in the 7-20 m band. The data presented here strengthen the
results of previous studies to show that excesses at 25 \micron and shorter
are rare: only 1 star out of 40 stars older than 1 Gyr or % shows an
excess. Asteroid belts 10-30 times more massive than our own appear are rare
among mature, solar-type stars
Planetary Science Goals for the Spitzer Warm Era
The overarching goal of planetary astronomy is to deduce how the present collection of objects found in our Solar System were formed from the original material present in the proto-solar nebula. As over two hundred exo-planetary systems are now known, and multitudes more are expected, the Solar System represents the closest and best system which we can study, and the only one in which we can clearly resolve individual bodies other than planets. In this White Paper we demonstrate how to use Spitzer Space Telescope InfraRed Array Camera Channels 1 and 2 (3.6 and 4.5 µm) imaging photometry with large dedicated surveys to advance our knowledge of Solar System formation and evolution. There are a number of vital, key projects to be pursued using dedicated large programs that have not been pursued during the five years of Spitzer cold operations. We present a number of the largest and most important projects here; more will certainly be proposed once the warm era has begun, including important observations of newly discovered objects
ExploreNEOs I: Description and first results from the Warm Spitzer NEO Survey
We have begun the ExploreNEOs project in which we observe some 700 Near Earth
Objects (NEOs) at 3.6 and 4.5 microns with the Spitzer Space Telescope in its
Warm Spitzer mode. From these measurements and catalog optical photometry we
derive albedos and diameters of the observed targets. The overall goal of our
ExploreNEOs program is to study the history of near-Earth space by deriving the
physical properties of a large number of NEOs. In this paper we describe both
the scientific and technical construction of our ExploreNEOs program. We
present our observational, photometric, and thermal modeling techniques. We
present results from the first 101 targets observed in this program. We find
that the distribution of albedos in this first sample is quite broad, probably
indicating a wide range of compositions within the NEO population. Many objects
smaller than one kilometer have high albedos (>0.35), but few objects larger
than one kilometer have high albedos. This result is consistent with the idea
that these larger objects are collisionally older, and therefore possess
surfaces that are more space weathered and therefore darker, or are not subject
to other surface rejuvenating events as frequently as smaller NEOs.Comment: AJ in pres
Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets
We study a solid protoplanetary core of 1-10 earth masses migrating through a
disk. We suppose the core luminosity is generated as a result of planetesimal
accretion and calculate the structure of the gaseous envelope assuming
equilibrium. This is a good approximation when the core mass is less than the
critical value, M_{crit}, above which rapid gas accretion begins. We model the
structure of the protoplanetary nebula as an accretion disk with constant
\alpha. We present analytic fits for the steady state relation between disk
surface density and mass accretion rate as a function of radius r. We calculate
M_{crit} as a function of r, gas accretion rate through the disk, and
planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M},
M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M}
onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to
prevent the attainment of M_{crit} during the migration process. Only at small
radii where planetesimals no longer exist can M_{crit} be attained. At small
radii, the runaway gas accretion phase may become longer than the disk lifetime
if the core mass is too small. However, massive cores can be built-up through
the merger of additional incoming cores on a timescale shorter than for in situ
formation. Therefore, feeding zone depletion in the neighborhood of a fixed
orbit may be avoided. Accordingly, we suggest that giant planets may begin to
form early in the life of the protostellar disk at small radii, on a timescale
that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be
published in ApJ, also available at http://www.ucolick.org/~ct/home.htm
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