91 research outputs found
Planetary Formation Scenarios Revistied: Core-Accretion Versus Disk Instability
The core-accretion and disk instability models have so far been used to
explain planetary formation. These models have different conditions, such as
planet mass, disk mass, and metallicity for formation of gas giants. The
core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the
case of G-type stars), and the mass of planets formed is less than 6 times that
of the Jupiter mass MJ. On the other hand, the disk instability model does not
have the metallicity condition, but requires the disk to be 15 times more
massive compared to the minimum mass solar nebulae model. The mass of planets
formed is more than 2MJ. These results are compared to the 161 detected planets
for each spectral type of the central stars. The results show that 90% of the
detected planets are consistent with the core-accretion model regardless of the
spectral type. The remaining 10% are not in the region explained by the
core-accretion model, but are explained by the disk instability model. We
derived the metallicity dependence of the formation probability of gas giants
for the core-accretion model. Comparing the result with the observed fraction
having gas giants, they are found to be consistent. On the other hand, the
observation cannot be explained by the disk instability model, because the
condition for gas giant formation is independent of the metallicity.
Consequently, most of planets detected so far are thought to have been formed
by the core-accretion process, and the rest by the disk instability process.Comment: accepted for publication in The Astrophysical Journa
A likely detection of a local interplanetary dust cloud passing near the Earth in the AKARI mid-infrared all-sky map
Context. We are creating the AKARI mid-infrared all-sky diffuse maps. Through
a foreground removal of the zodiacal emission, we serendipitously detected a
bright residual component whose angular size is about 50 x 20 deg. at a
wavelength of 9 micron. Aims. We investigate the origin and the physical
properties of the residual component. Methods. We measured the surface
brightness of the residual component in the AKARI mid-infrared all-sky maps.
Results. The residual component was significantly detected only in 2007
January, even though the same region was observed in 2006 July and 2007 July,
which shows that it is not due to the Galactic emission. We suggest that this
may be a small cloud passing near the Earth. By comparing the observed
intensity ratio of I_9um/I_18um with the expected intensity ratio assuming
thermal equilibrium of dust grains at 1 AU for various dust compositions and
sizes, we find that dust grains in the moving cloud are likely to be much
smaller than typical grains that produce the bulk of the zodiacal light.
Conclusions. Considering the observed date and position, it is likely that it
originates in the solar coronal mass ejection (CME) which took place on 2007
January 25.Comment: 5 pages, 4 figures, accepted by Astronomy and Astrophysic
Physical Properties of Asteroid (308635) 2005 YU55 derived from multi-instrument infrared observations during a very close Earth-Approach
The near-Earth asteroid (308635) 2005 YU55 is a potentially hazardous
asteroid which was discovered in 2005 and passed Earth on November 8th 2011 at
0.85 lunar distances. This was the closest known approach by an asteroid of
several hundred metre diameter since 1976 when a similar size object passed at
0.5 lunar distances. We observed 2005 YU55 from ground with a recently
developed mid-IR camera (miniTAO/MAX38) in N- and Q-band and with the
Submillimeter Array (SMA) at 1.3 mm. In addition, we obtained space
observations with Herschel/PACS at 70, 100, and 160 micron. Our thermal
measurements cover a wide range of wavelengths from 8.9 micron to 1.3 mm and
were taken after opposition at phase angles between -97 deg and -18 deg. We
performed a radiometric analysis via a thermophysical model and combined our
derived properties with results from radar, adaptive optics, lightcurve
observations, speckle and auxiliary thermal data. We find that (308635) 2005
YU55 has an almost spherical shape with an effective diameter of 300 to 312 m
and a geometric albedo pV of 0.055 to 0.075. Its spin-axis is oriented towards
celestial directions (lam_ecl, beta_ecl) = (60 deg +/- 30deg, -60 deg +/- 15
deg), which means it has a retrograde sense of rotation. The analysis of all
available data combined revealed a discrepancy with the radar-derived size. Our
radiometric analysis of the thermal data together with the problem to find a
unique rotation period might be connected to a non-principal axis rotation. A
low to intermediate level of surface roughness (r.m.s. of surface slopes in the
range 0.1 - 0.3) is required to explain the available thermal measurements. We
found a thermal inertia in the range 350-800 Jm^-2s^-0.5K^-1, very similar to
the rubble-pile asteroid (25143) Itokawa and indicating a mixture of low
conductivity fine regolith with larger rocks and boulders of high thermal
inertia on the surface.Comment: Accepted for publication in Astronomy & Astrophysics, 12 pages, 10
figure
The Infrared Camera (IRC) for AKARI - Design and Imaging Performance
The Infrared Camera (IRC) is one of two focal-plane instruments on the AKARI
satellite. It is designed for wide-field deep imaging and low-resolution
spectroscopy in the near- to mid-infrared (1.8--26.5um) in the pointed
observation mode of AKARI. IRC is also operated in the survey mode to make an
all-sky survey at 9 and 18um. It comprises three channels. The NIR channel
(1.8--5.5um) employs a 512 x 412 InSb array, whereas both the MIR-S
(4.6--13.4um) and MIR-L (12.6--26.5um) channels use 256 x 256 Si:As impurity
band conduction arrays. Each of the three channels has a field-of-view of about
10' x 10' and are operated simultaneously. The NIR and MIR-S share the same
field-of-view by virtue of a beam splitter. The MIR-L observes the sky about
$25' away from the NIR/MIR-S field-of-view. IRC gives us deep insights into the
formation and evolution of galaxies, the evolution of planetary disks, the
process of star-formation, the properties of interstellar matter under various
physical conditions, and the nature and evolution of solar system objects. The
in-flight performance of IRC has been confirmed to be in agreement with the
pre-flight expectation. This paper summarizes the design and the in-flight
operation and imaging performance of IRC.Comment: Publications of the Astronomical Society of Japan, in pres
Properties of active galactic star-forming regions probed by imaging spectroscopy with the Fourier transform spectrometer (FTS) onboard AKARI
We investigate the structure of the interstellar medium (ISM) and identify
the location of possible embedded excitation sources from far-infrared (FIR)
line and mid-infrared continuum emission maps. We carried out imaging
spectroscopic observations of four giant Galactic star-forming regions with the
Fourier Transform Spectrometer (FTS) onboard AKARI. We obtained [OIII] 88
micron and [CII] 158 micron line intensity maps of all the regions:
G3.270-0.101, G333.6-0.2, NGC3603, and M17. For G3.270-0.101, we obtained
high-spatial-resolution [OIII] 88 micron line-emission maps and a FIR continuum
map for the first time, which imply that [OIII] 88 micron emission identifies
the excitation sources more clearly than the radio continuum emission. In
G333.6-0.2, we found a local [OIII] 88 micron emission peak, which is
indicative of an excitation source. This is supported by the 18 micron
continuum emission, which is considered to trace the hot dust distribution. For
all regions, the [CII] 158 micron emission is distributed widely as suggested
by previous observations of star-forming regions. We conclude that [OIII] 88
micron emission traces the excitation sources more accurately than the radio
continuum emission, especially where there is a high density and/or column
density gradient. The FIR spectroscopy provides a promising means of
understanding the nature of star-forming regions.Comment: 14 pages with 15 figures, accepted for publication in Astronomy and
Astrophysic
AKARI/IRC 18 μm survey of warm debris disks
Context. Little is known about the properties of the warm (Tdust ≳ 150 K) debris disk material located close to the central star, which has a more direct link to the formation of terrestrial planets than does the low-temperature debris dust that has been detected to date.
Aims: To discover new warm debris disk candidates that show large 18 μm excess and estimate the fraction of stars with excess based on the AKARI/IRC Mid-Infrared All-Sky Survey data.
Methods: We searched for point sources detected in the AKARI/IRC All-Sky Survey, which show a positional match with A-M dwarf stars in the Tycho-2 Spectral Type Catalogue and exhibit excess emission at 18 μm compared to what is expected from the KS magnitude in the 2MASS catalogue.
Results: We find 24 warm debris candidates including 8 new candidates among A-K stars. The apparent debris disk frequency is estimated to be 2.8 ± 0.6%. We also find that A stars and solar-type FGK stars have different characteristics of the inner component of the identified debris disk candidates. While debris disks around A stars are cooler and consistent with steady-state evolutionary model of debris disks, those around FGK stars tend to be warmer and cannot be explained by the steady-state model
The Main Belt Comets and ice in the Solar System
We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies
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