130 research outputs found
A survey of T Tauri stars with AKARI toward the Taurus-Auriga region
Aims: We search new T Tauri star (TTS) candidates with the mid-infrared (MIR)
part of the AKARI All-Sky Survey at 9 and 18 um wavelengths. Methods: We used
the point source catalogue (PSC), obtained by the Infrared Camera (IRC) on
board AKARI. We combined the 2MASS PSC and the 3rd version of the USNO CCD
Astrograph Catalogue (UCAC) with the AKARI IRC-PSC, and surveyed 517 known TTSs
over a 1800-square-degree part of the Taurus-Auriga region to find criteria to
extract TTSs. We considered asymptotic giant branch (AGB) stars, post-AGB
stars, Planetary Nebulae (PNe), and galaxies, which have similar MIR colours,
to separate TTSs from these sources. Results: Of the 517 known TTSs, we
detected 133 sources with AKARI. Based on the colour-colour and
colour-magnitude diagrams made from the AKARI, 2MASS, and UCAC surveys, we
propose the criteria to extract TTS candidates from the AKARI All-Sky data. On
the basis of our criteria, we selected 176/14725 AKARI sources as TTS
candidates which are located around the Taurus-Auriga region. Comparing these
sources with SIMBAD, there are 148 previously identified sources including 115
Young Stellar Objects (YSOs), and 28 unidentified sources. Conclusions: Based
on SIMBAD identifications, we take the TTS-identification probability using our
criteria to be ~75 %. We find 28 TTS candidates, of which we expect 21 to be
confirmed once follow-up observations can be obtained. Although the probability
of ~75 % is not so high, it is affected by the completeness of the SIMBAD
database, and we can search for TTSs over the whole sky, over all star forming
regions.Comment: 12 pages, 9 figures, accepted for publication in A&
AKARI Mission Program: Excavating Mass Loss History in Extended Dust Shells of Evolved Stars (MLHES) I. Far-IR Photometry
We performed a far-IR imaging survey of the circumstellar dust shells of 144
evolved stars as a mission programme of the AKARI infrared astronomical
satellite using the Far-Infrared Surveyor (FIS) instrument. With this survey,
we deliver far-IR surface brightness distributions of roughly 10' x 40' or 10'
x 20' areas of the sky around the target evolved stars in the four FIS bands at
65, 90, 140, and 160 microns. Our objectives are to characterize the far-IR
surface brightness distributions of the cold dust component in the
circumstellar dust shells, from which we derive the amount of cold dust grains
as low as 20 K and empirically establish the history of the early mass loss
history. In this first installment of the series, we introduce the project and
its aims, describe the observations, data reduction, and surface brightness
correction process, and present the entire data set along with the results of
integrated photometry measurements (i.e., the central source and circumstellar
dust shell altogether). We find that (1) far-IR emission is detected from all
but one object at the spatial resolution about 30" - 50" in the corresponding
bands, (2) roughly 60 - 70 % of the target sources show some extension, (3)
previously unresolved nearby objects in the far-IR are now resolved around 28
target sources, (4) the results of photometry measurements are reasonable with
respect to the entries in the AKARI/FIS Bright Source Catalogue, despite the
fact that the targets are assumed to be point-sources when catalogue flux
densities were computed, and (5) an IR two-color diagram would place the target
sources in a roughly linear distribution that may correlate with the age of the
circumstellar dust shell and can potentially be used to identify which targets
are more extended than others.Comment: To be published in PASJ AKARI Special Issue: 25 pages, 5 figures, 5
tables (and 28 supplementary figures available only in PASJ on-line
Predicting dust extinction properties of star-forming galaxies from H-alpha/UV ratio
Using star-forming galaxies sample in the nearby Universe (0.02<z<0.10)
selected from the SDSS (DR7) and GALEX all-sky survey (GR5), we present a new
empirical calibration for predicting dust extinction of galaxies from
H-alpha-to-FUV flux ratio. We find that the H-alpha dust extinction (A(Ha))
derived with H-alpha/H-beta ratio (Balmer decrement) increases with increasing
H-alpha/UV ratio as expected, but there remains a considerable scatter around
the relation, which is largely dependent on stellar mass and/or H-alpha
equivalent width (EW(Ha)). At fixed H-alpha/UV ratio, galaxies with higher
stellar mass (or galaxies with lower EW(Ha)) tend to be more highly obscured by
dust. We quantify this trend and establish an empirical calibration for
predicting A(Ha) with a combination of H-alpha/UV ratio, stellar mass and
EW(Ha), with which we can successfully reduce the systematic uncertainties
accompanying the simple H-alpha/UV approach by ~15-30%. The new recipes
proposed in this study will provide a convenient tool for predicting dust
extinction level of galaxies particularly when Balmer decrement is not
available. By comparing A(Ha) (derived with Balmer decrement) and A(UV)
(derived with IR/UV luminosity ratio) for a subsample of galaxies for which
AKARI FIR photometry is available, we demonstrate that more massive galaxies
tend to have higher extra extinction towards the nebular regions compared to
the stellar continuum light. Considering recent studies reporting smaller extra
extinction towards nebular regions for high-redshift galaxies, we argue that
the dust geometry within high-redshift galaxies resemble more like low-mass
galaxies in the nearby Universe.Comment: 14 pages, 14 figures, Accepted for publication in MNRA
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