24 research outputs found
Characterization and Improvement of the Image Quality of the Data Taken with the Infrared Camera (IRC) Mid-Infrared Channels onboard AKARI
Mid-infrared images frequently suffer artifacts and extended point spread
functions (PSFs). We investigate the characteristics of the artifacts and the
PSFs in images obtained with the Infrared Camera (IRC) onboard AKARI at four
mid-infrared bands of the S7 (7{\mu}m), S11 (11{\mu}m), L15 (15{\mu}m), and L24
(24 {\mu}m). Removal of the artifacts significantly improves the reliability of
the ref- erence data for flat-fielding at the L15 and L24 bands. A set of
models of the IRC PSFs is also constructed from on-orbit data. These PSFs have
extended components that come from diffraction and scattering within the
detector arrays. We estimate the aperture correction factors for point sources
and the surface brightness correction factors for diffuse sources. We conclude
that the surface brightness correction factors range from 0.95 to 0.8, taking
account of the extended component of the PSFs. To correct for the extended PSF
effects for the study of faint structures, we also develop an image
reconstruction method, which consists of the deconvolution with the PSF and the
convolution with an appropriate Gaussian. The appropriate removal of the
artifacts, improved flat-fielding, and image reconstruction with the extended
PSFs enable us to investigate de- tailed structures of extended sources in IRC
mid-infrared images.Comment: 35 pages, 15 figures, accepted for publication in PAS
AKARI Observations of Brown Dwarfs I.: CO and CO_2 Bands in the Near-Infrared Spectra
Near-infrared medium-resolution spectra of seven bright brown dwarfs are
presented. The spectra were obtained with the Infrared Camera (IRC) on board
the infrared astronomical satellite AKARI, covering 2.5--5.0 um with a spectral
resolution of approximately 120. The spectral types of the objects range from
L5 to T8, and enable us to study the spectral evolution of brown dwarfs. The
observed spectra are in general consistent with the predictions from the
previous observations and photospheric models. We find that the CO fundamental
band around 4.6 um is clearly seen even in the T8 dwarf 2MASS J041519-0935,
confirming the presence of non-equilibrium chemical state in the atmosphere. We
also identify the CO_2 fundamental stretching-mode band at 4.2 um for the first
time in the spectra of late-L and T-type brown dwarfs.
We analyze the observed spectra by comparing with the predicted ones based on
the Unified Cloudy Model (UCM). Although overall spectral energy distributions
(SEDs) can be reasonably fitted with the UCM, observed CO and CO_2 bands in
late-L and T-dwarfs are unexpectedly stronger than the model predictions
assuming local thermodynamical equilibrium (LTE). We examine the vertical
mixing model and find that this model explains the CO band at least partly in
the T-dwarfs 2MASS J041519-0935 and 2MASS J055919-1404. The CO fundamental band
also shows excess absorption against the predicted one in the L9 dwarf SDSS
J083008+4828. Since CO is already highly abundant in the upper photospheres of
late-L dwarfs, the extra CO by vertical mixing has little effect on the CO band
strengths, and the vertical mixing model cannot be applied to this L-dwarf. A
more serious problem is that the significant enhancement of the CO_2 4.2 um
band in both the late-L and T dwarfs cannot be explained at all by the vertical
mixing model. The enhancement of the CO_2 band remains puzzling.Comment: 29 pages, 13 figures. To be published in ApJ October 2010 issu
Interstellar Extinction Law toward the Galactic Center III: J, H, Ks bands in the 2MASS and the MKO systems, and 3.6, 4.5, 5.8, 8.0 micron in the Spitzer/IRAC system
We have determined interstellar extinction law toward the Galactic center
(GC) at the wavelength from 1.2 to 8.0 micron, using point sources detected in
the IRSF/SIRIUS near-infrared survey and those in the 2MASS and
Spitzer/IRAC/GLIMPSE II catalogs. The central region |l| < 3deg and |b| < 1deg
has been surveyed in the J, H and Ks bands with the IRSF telescope and the
SIRIUS camera whose filters are similar to the Mauna Kea Observatories (MKO)
near-infrared photometric system. Combined with the GLIMPSE II point source
catalog, we made Ks versus (Ks - lambda) color-magnitude diagrams where lambda
= 3.6, 4.5, 5.8, and 8.0 micron. The Ks magnitudes of bulge red clump stars and
the (Ks - lambda) colors of red giant branches are used as a tracer of the
reddening vector in the color-magnitude diagrams. From these magnitudes and
colors, we have obtained the ratios of total to selective extinction
A(Ks)/E(Ks-lambda) for the four IRAC bands. Combined with A(lambda)/A(Ks) for
the J and H bands derived by Nishiyama et al., we obtain
A(J):A(H):A(Ks):A([3.6]):A([4.5]):A([5.8]):A([8.0])=3.02:1.73:1:0.50:0.39:0.36:0.43
for the line of sight toward the GC. This confirms the flattening of the
extinction curve at lambda > 3 micron from a simple extrapolation of the
power-law extinction at shorter wavelengths, in accordance with recent studies.
The extinction law in the 2MASS JHKs bands has also been calculated, and a good
agreement with that in the MKO system is found. In nearby molecular clouds and
diffuse interstellar medium, the lack of reliable measurements of the total to
selective extinction ratios hampers unambiguous determination of the extinction
law; however, observational results toward these lines of sight cannot be
reconciled with a single extinction law.Comment: 27 pages, 10 figures, Accepted for publication in Ap
AGB Stars in the Fornax Dwarf Spheroidal Galaxy
We report on a multi-epoch study of the Fornax dwarf spheroidal galaxy, made
with the Infrared Survey Facility, over an area of about 42'x42'. The
colour-magnitude diagram shows a broad well-populated giant branch with a tip
that slopes down-wards from red to blue, as might be expected given Fornax's
known range of age and metallicity. The extensive AGB includes seven Mira
variables and ten periodic semi-regular variables. Five of the seven Miras are
known to be carbon rich. Their pulsation periods range from 215 to 470 days,
indicating a range of initial masses. Three of the Fornax Miras are redder than
typical LMC Miras of similar period, probably indicating particularly heavy
mass-loss rates. Many, but not all, of the characteristics of the AGB are
reproduced by isochrones from Marigo et al. for a 2 Gyr population with a
metallicity of Z=0.0025.
An application of the Mira period-luminosity relation to these stars yields a
distance modulus for Fornax of 20.69+/-0.04 (internal), +/-0.08 (total) (on a
scale that puts the LMC at 18.39 mag) in good agreement with other
determinations. Various estimates of the distance to Fornax are reviewed.Comment: 15 pages, 13 figures, accepted for publication in MNRA
A Distinct Structure Inside the Galactic Bar
We present the result of a near-infrared (J H Ks) survey along the Galactic
plane, -10.5deg < l < +10.5deg and b=+1.0deg, with the IRSF 1.4m telescope and
the SIRIUS camera. Ks vs. H-Ks color-magnitude diagrams reveal a well-defined
population of red clump (RC) stars whose apparent magnitude peak changes
continuously along the Galactic plane, from Ks=13.4 at l=-10deg to Ks=12.2 at
l=+10deg after dereddening. This variation can be explained by the bar-like
structure found in previous studies, but we find an additional inner structure
at |l| < 4deg, where the longitude - apparent magnitude relation is distinct
from the outer bar, and the apparent magnitude peak changes by only 0.1 mag
over the central 8deg. The exact nature of this inner structure is as yet
uncertain.Comment: 8 pages, 4 figures. accepted by ApJ
Variable stars in the Magellanic Clouds: II. The data and infrared properties
The data of 8,852 and 2,927 variable stars detected by OGLE survey in the
Large and Small Magellanic Clouds are presented. They are cross-identified with
the SIRIUS JHK survey data, and their infrared properties are discussed.
Variable red giants are well separated on the period-J - K plane, suggesting
that it could be a good tool to distinguish their pulsation mode and type.Comment: Accepted for publication in MNRAS. High resolution version is
available at:
http://www.ir.isas.jaxa.jp/%7Eyita/scr/astro/papers/Refereed/yitaMD1266.ps.g