133 research outputs found
銀河系外縁部における星生成
学位の種別: 課程博士審査委員会委員 : (主査)東京大学教授 河野 孝太郎, 東京大学教授 田村 之秀, 東京大学准教授 片坐 宏一, 明星大学准教授 小野寺 幸子, 東京理科大准教授 大越 克也University of Tokyo(東京大学
Star Formation Activity Beyond the Outer Arm II: Distribution and Properties of Star Formation
The outer Galaxy beyond the Outer Arm represents a promising opportunity to
study star formation in an environment vastly different from the solar
neighborhood. In our previous study, we identified 788 candidate star-forming
regions in the outer Galaxy (at galactocentric radii 13.5
kpc) based on Wide-field Infrared Survey Explorer (WISE) mid-infrared (MIR)
all-sky survey. In this paper, we investigate the statistical properties of the
candidates and their parental molecular clouds derived from the Five College
Radio Astronomy Observatory (FCRAO) CO survey. We show that the molecular
clouds with candidates have a shallower slope of cloud mass function, a larger
fraction of clouds bound by self-gravity, and a larger density than the
molecular clouds without candidates. To investigate the star formation
efficiency (SFE) at different , we used two parameters: 1) the
fraction of molecular clouds with candidates and 2) the monochromatic MIR
luminosities of candidates per parental molecular cloud mass. We did not find
any clear correlation between SFE parameters and at of
13.5 kpc to 20.0 kpc, suggesting that the SFE is independent of environmental
parameters such as metallicity and gas surface density, which vary considerably
with . Previous studies reported that the SFE per year (SFE/yr)
derived from the star-formation rate surface density per total gas surface
density, HI plus H, decreases with increased . Our results might
suggest that the decreasing trend is due to a decrease in HI gas conversion to
H gas.Comment: 40 pages, 26 figures, 5 tables, accepted for publication in Ap
Poly(A)-Specific Ribonuclease Mediates 3′-End Trimming of Argonaute2-Cleaved Precursor MicroRNAs
SummaryMicroRNAs (miRNAs) are typically generated as ∼22-nucleotide double-stranded RNAs via the processing of precursor hairpins by the ribonuclease III enzyme Dicer, after which they are loaded into Argonaute (Ago) proteins to form an RNA-induced silencing complex (RISC). However, the biogenesis of miR-451, an erythropoietic miRNA conserved in vertebrates, occurs independently of Dicer and instead requires cleavage of the 3′ arm of the pre-miR-451 precursor hairpin by Ago2. The 3′ end of the Ago2-cleaved pre-miR-451 intermediate is then trimmed to the mature length by an unknown nuclease. Here, using a classical chromatographic approach, we identified poly(A)-specific ribonuclease (PARN) as the enzyme responsible for the 3′–5′ exonucleolytic trimming of Ago2-cleaved pre-miR-451. Surprisingly, our data show that trimming of Ago2-cleaved precursor miRNAs is not essential for target silencing, indicating that RISC is functional with miRNAs longer than the mature length. Our findings define the maturation step in the miRNA biogenesis pathway that depends on Ago2-mediated cleavage
Cosmic-ray-driven enhancement of the C/CO abundance ratio in W51C
We examine spatial variations of the C/CO abundance ratio
() in the vicinity of the -ray supernova remnant
W51C, based on [CI] (-), CO(1-0), and CO(1-0)
observations with the ASTE and Nobeyama 45-m telescopes. We find that
varies in a range of 0.02-0.16 (0.05 in median) inside the
molecular clouds of 100 mag, where photodissociation of CO by the
interstellar UV is negligible. Furthermore, is locally
enhanced up to by a factor of four near the W51C center, depending on the
projected distance from the W51C center. In high- molecular clouds,
is determined by the ratio of the cosmic-ray (CR)
ionization rate to the H density, and we find no clear spatial variation of
the H density against the projected distance. Hence, the high CR ionization
rate may locally enhance near the W51C center. We also find
that the observed spatial extent of the enhanced (17
pc) is consistent with the diffusion distance of CRs with the energy of 100
MeV. The fact suggests that the low-energy CRs accelerated in W51C enhance
. The CR ionization rate at the -enhanced
cloud is estimated to be 310 s on the basis of
time-dependent PDR simulations of , the value of which is 30
times higher than that in the standard Galactic environment. These results
demonstrate that [CI] is a powerful probe to investigate the interaction
between CRs and the interstellar medium for a wide area in the vicinity of
supernova remnants.Comment: 17 pages, 8 figures, accepted for publication in PAS
- …