346 research outputs found
Star Formation Efficiency in the Central 1 kpc Region of Early-Type Spiral Galaxies
It has been reported recently that there are some early-type spiral (Sa--Sab)
galaxies having evident star-forming regions which concentrate in their own
central 1-kpc. In such central region, is the mechanism of the star formation
distinct from that in disks of spiral galaxies? To reveal this, we estimate the
star formation efficiency (SFE) in this central 1-kpc star-forming region of
some early-type spiral galaxies, taking account of the condition for this 1-kpc
region to be self-gravitating. Using two indicators of present star formation
rate (H and infrared luminosity), we estimate the SFE to be a few
percents. This is equivalent to the observational SFE in the disks of late-type
spiral (Sb--) galaxies. This coincidence may support the universality of the
mean SFE of spiral galaxies reported in the recent studies. That is, we find no
evidence of distinct mechanism of the star formation in the central 1-kpc
region of early-type galaxies. Also, we examine the structure of the central
star-forming region, and discuss a method for estimating the mass of
star-forming regions.Comment: accepted by A
Numerical Investigations of Small Punch Tests for Determining Tensile Properties
Small punch tests were conducted for carbon steel specimens prepared with and without cold working. A digital image correlation technique was applied to measure displacement. Finite element analyses using the true stress-strain curves including post-necking strain could reproduce the load-displacement (LD) curve. Use of the nominal stress-strain curve obtained by the conventional tensile test was not enough to simulate the maximum load. It was found that the maximum load did not depend on the ultimate strength. Therefore, the ultimate strength prediction using the maximum load in the LD curve may not be valid for the general case. On the other hand, the yield strength correlated well with the deflection point in the LD curve. A new equation was proposed to estimate the yield strength that takes into consideration the influence of the specimen thickness
Shocked and Scorched: The Tail of a Tadpole in an Interstellar Pond
We report multi-wavelength observations of the far-infrared source IRAS
20324+4057, including high-resolution optical imaging with HST, and
ground-based near-infrared, millimeter-wave and radio observations. These data
show an extended, limb-brightened, tadpole-shaped nebula with a bright,
compact, cometary nebula located inside the tadpole head. Our molecular line
observations indicate that the Tadpole is predominantly molecular, with a total
gas mass exceeding 3.7 Msun. Our radio continuum imaging, and archival Spitzer
IRAC images, show the presence of additional tadpole-shaped objects in the
vicinity of IRAS 20324+4057 that share a common E-W head-tail orientation: we
propose that these structures are small, dense molecular cores that originated
in the Cygnus cloud and are now being (i) photoevaporated by the ultraviolet
radiation field of the Cyg OB2 No. 8 cluster located to the North-West, and
(ii) shaped by ram pressure of a distant wind source or sources located to the
West, blowing ablated and photoevaporated material from their heads eastwards.
The ripples in the tail of the Tadpole are interpreted in terms of
instabilities at the interface between the ambient wind and the dense medium of
the former.Comment: (accepted by the Astrophysical Journal
Is Thermal Instability Significant in Turbulent Galactic Gas?
We investigate numerically the role of thermal instability (TI) as a
generator of density structures in the interstellar medium (ISM), both by
itself and in the context of a globally turbulent medium. Simulations of the
instability alone show that the condenstion process which forms a dense phase
(``clouds'') is highly dynamical, and that the boundaries of the clouds are
accretion shocks, rather than static density discontinuities. The density
histograms (PDFs) of these runs exhibit either bimodal shapes or a single peak
at low densities plus a slope change at high densities. Final static situations
may be established, but the equilibrium is very fragile: small density
fluctuations in the warm phase require large variations in the density of the
cold phase, probably inducing shocks into the clouds. This result suggests that
such configurations are highly unlikely. Simulations including turbulent
forcing show that large- scale forcing is incapable of erasing the signature of
the TI in the density PDFs, but small-scale, stellar-like forcing causes
erasure of the signature of the instability. However, these simulations do not
reach stationary regimes, TI driving an ever-increasing star formation rate.
Simulations including magnetic fields, self-gravity and the Coriolis force show
no significant difference between the PDFs of stable and unstable cases, and
reach stationary regimes, suggesting that the combination of the stellar
forcing and the extra effective pressure provided by the magnetic field and the
Coriolis force overwhelm TI as a density-structure generator in the ISM. We
emphasize that a multi-modal temperature PDF is not necessarily an indication
of a multi-phase medium, which must contain clearly distinct thermal
equilibrium phases.Comment: 18 pages, 11 figures. Submitted to Ap
The Minimum Stellar Mass in Early Galaxies
The conditions for the fragmentation of the baryonic component during merging
of dark matter halos in the early Universe are studied. We assume that the
baryonic component undergoes a shock compression. The characteristic masses of
protostellar molecular clouds and the minimum masses of protostars formed in
these clouds decrease with increasing halo mass. This may indicate that the
initial stellar mass function in more massive galaxies was shifted towards
lower masses during the initial stages of their formation. This would result in
an increase of the number of stars per unit halo mass, i.e., the efficiency of
star formation.Comment: 18 pages, 7 figure
Pion decay constant in quenched QCD with Kogut-Susskind quarks
We present a non-perturbative calculation for the pion decay constant with
quenched Kogut-Susskind quarks. Numerical simulations are carried out at and 6.2 with various operators extending over all flavors. The
renormalization correction is applied for each flavor by computing
non-perturbative renormalization constants, and it is compared with a
perturbative calculation. We also study the behavior of in the
continuum limits for both non-perturbative and perturbative calculations. The
results in the continuum limit is also discussed.Comment: LATTICE99(matrix elements) 3 pages, 4 eps figure
The effect of near-surface plastic deformation on the hot corrosion and high temperature corrosion-fatigue response of a nickel-based superalloy
Surface treatments such as shot peening to inhibit fatigue crack initiation are essential processes when designing gas turbine components for aerospace applications. It is therefore crucial to understand the effects of shot peening in representative service environments. Here, the influence of surface treatment on the high temperature corrosion fatigue response of a polycrystalline nickel-based superalloy is considered, an area that has not previously been explored. Two shot peening conditions; 110H 7A 200% and 330H 7A 200%, along with a polished surface were chosen. Specimens were salted and exposed to SO2 gas during fatigue testing at 700âŻÂ°C. A range of novel techniques including SEM, EBSD and axial chromatism profilometry were used to analyse the near surface cold work and surface condition before and after testing. EBSD local misorientation maps, paired with an increase in corrosion-fatigue life, suggest that a greater depth of cold work produced by the smaller shot size (110H), is providing a significant benefit in terms of hot corrosion and corrosion-fatigue performance. This paper concludes that the presence of a substantial layer of cold work is required to account for any metal loss due to the effects of hot corrosion. It is also evident that cold work hinders fatigue crack initiation and delays the onset of pit to crack transition
Effect of Dust Extinction on Estimating Star Formation Rate of Galaxies: Lyman Continuum Extinction
We re-examine the effect of Lyman continuum ( \AA)
extinction (LCE) by dust in H {\sc ii} regions in detail and discuss how it
affects the estimation of the global star formation rate (SFR) of galaxies. To
clarify the first issue, we establish two independent methods for estimating a
parameter of LCE (), which is defined as the fraction of Lyman continuum
photons contributing to hydrogen ionization in an H {\sc ii} region. One of
those methods determines from the set of Lyman continuum flux, electron
density and metallicity. In the framework of this method, as the metallicity
and/or the Lyman photon flux increase, is found to decrease. The other
method determines from the ratio of infrared flux to Lyman continuum flux.
Importantly, we show that f \la 0.5 via both methods in many H {\sc ii}
regions of the Galaxy. Thus, it establishes that dust in such H {\sc ii}
regions absorbs significant amount of Lyman continuum photons directly. To
examine the second issue, we approximate to a function of only the
dust-to-gas mass ratio (i.e., metallicity), assuming a parameter fit for the
Galactic H {\sc ii} regions. We find that a characteristic , which is
defined as averaged over a galaxy-wide scale, is 0.3 for the nearby spiral
galaxies. This relatively small indicates that a typical increment
factor due to LCE for estimating the global SFR () is large () for the nearby spiral galaxies. Therefore, we conclude that the effect of
LCE is not negligible relative to other uncertainties of estimating the SFR of
galaxies.Comment: 18 papges, 11 figures, accepted by Ap
I=2 Pion Scattering Length with Wilson Fermions
We present results for I=2 pion scattering length with the Wilson fermions in
the quenched approximation. The finite size method presented by L\"uscher is
employed, and calculations are carried out at , 6.1, and 6.3. In the
continuum limit, we obtain a result in reasonable agreement with the
experimental value.Comment: LATTICE99(matrixelement), 3 pages, 4 eps figure
Velocity Dispersion of Dissolving OB Associations Affected by External Pressure of Formation Environment
This paper presents a possible way to understand dissolution of OB
associations (or groups). Assuming rapid escape of parental cloud gas from
associations, we show that the shadow of the formation environment for
associations can be partially imprinted on the velocity dispersion at their
dissolution. This conclusion is not surprising as long as associations are
formed in a multiphase interstellar medium, because the external pressure
should suppress expansion caused by the internal motion of the parental clouds.
Our model predicts a few km s as the internal velocity dispersion.
Observationally, the internal velocity dispersion is km s which
is smaller than our prediction. This suggests that the dissipation of internal
energy happens before the formation of OB associations.Comment: 6 pages. AJ accepte
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