3,324 research outputs found
Unusual Carbonaceous Dust Distribution in PN G095.2+00.7
We investigate the polycyclic aromatic hydrocarbon features in the young
Galactic planetary nebula PN G095.2+00.7 based on mid-infrared observations.
The near- to mid-infrared spectra obtained with the AKARI/IRC and the
Spitzer/IRS show the PAH features as well as the broad emission feature at 12
{\mu}m usually seen in proto-planetary nebulae (pPNe). The spatially resolved
spectra obtained with Subaru/COMICS suggest that the broad emission around 12
{\mu}m is distributed in a shell-like structure, but the unidentified infrared
band at 11.3 {\mu}m is selectively enhanced at the southern part of the nebula.
The variation can be explained by a difference in the amount of the UV
radiation to excite PAHs, and does not necessarily require the chemical
processing of dust grains and PAHs. It suggests that the UV self-extinction is
important to understand the mid-infrared spectral features. We propose a
mechanism which accounts for the evolutionary sequence of the mid-infrared dust
features seen in a transition from pPNe to PNe.Comment: 6 pages, 4 figure
The Impact of Feedback in Massive Star Formation. II. Lower Star Formation Efficiency at Lower Metallicity
We conduct a theoretical study of the formation of massive stars over a wide
range of metallicities from 1e-5 to 1Zsun and evaluate the star formation
efficiencies (SFEs) from prestellar cloud cores taking into account multiple
feedback processes. Unlike for simple spherical accretion, in the case of disk
accretion feedback processes do not set upper limits on stellar masses. At
solar metallicity, launching of magneto-centrifugally-driven outflows is the
dominant feedback process to set SFEs, while radiation pressure, which has been
regarded to be pivotal, has only minor contribution even in the formation of
over-100Msun stars. Photoevaporation becomes significant in over-20Msun star
formation at low metallicities of <1e-2Zsun, where dust absorption of ionizing
photons is inefficient. We conclude that if initial prestellar core properties
are similar, then massive stars are rarer in extremely metal-poor environments
of 1e-5 - 1e-3Zsun. Our results give new insight into the high-mass end of the
initial mass function and its potential variation with galactic and
cosmological environments.Comment: 13 pages, 9 figures, accepted for publication in The Astrophysical
Journa
Electron-capture supernovae exploding within their progenitor wind
The most massive stars on the asymptotic giant branch (AGB), so called
super-AGB stars, are thought to produce supernovae (SNe) triggered by electron
captures in their degenerate O+Ne+Mg cores. Super-AGB stars are expected to
have slow winds with high mass-loss rates, so their wind density is high. The
explosions of super-AGB stars are therefore presumed to occur in this dense
wind. We provide the first synthetic light curves (LCs) for such events by
exploding realistic electron-capture supernova (ecSN) progenitors within their
super-AGB winds. We find that the early LC, i.e. before the recombination wave
reaches the bottom of the H-rich envelope of SN ejecta (the plateau phase), is
not affected by the dense wind. However, after the plateau phase, the
luminosity remains higher when the super-AGB wind is taken into account. We
compare our results to the historical LC of SN 1054, the progenitor of the Crab
Nebula, and show that the explosion of an ecSN within an ordinary super-AGB
wind can explain the LC features. We conclude that SN 1054 could have been a
Type IIn SN without any extra extreme mass loss which was previously suggested
to be necessary to account for its early high luminosity. We also show that our
LCs match Type IIn SNe with an early plateau phase (`Type IIn-P') and suggest
that they are ecSNe within super-AGB winds. Although some ecSNe can be bright
in the optical spectral range due to the large progenitor radius, their X-ray
luminosity from the interaction does not necessarily get as bright as other
Type IIn SNe whose optical luminosities are also powered by the interaction.
Thus, we suggest that optically-bright X-ray-faint Type IIn SNe can emerge from
ecSNe. Optically-faint Type IIn SNe, such as SN 2008S, can also originate from
ecSNe if their H-rich envelope masses are small. Some of them can be observed
as `Type IIn-b' SNe due to the small H-rich envelope mass.Comment: 8 pages, 6 figures, accepted by Astronomy & Astrophysics, abstract
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Photoabsorption spectra in the continuum of molecules and atomic clusters
We present linear response theories in the continuum capable of describing
photoionization spectra and dynamic polarizabilities of finite systems with no
spatial symmetry. Our formulations are based on the time-dependent local
density approximation with uniform grid representation in the three-dimensional
Cartesian coordinate. Effects of the continuum are taken into account either
with a Green's function method or with a complex absorbing potential in a
real-time method. The two methods are applied to a negatively charged cluster
in the spherical jellium model and to some small molecules (silane, acetylene
and ethylene).Comment: 13 pages, 9 figure
Synthetic Light Curves of Shocked Dense Circumstellar Shells
We numerically investigate light curves (LCs) of shocked circumstellar shells
which are suggested to reproduce the observed LC of superluminous SN 2006gy
analytically. In the previous analytical model, the effects of the
recombination and the bolometric correction on LCs are not taken into account.
To see the effects, we perform numerical radiation hydrodynamic calculations of
shocked shells by using STELLA, which can numerically treat multigroup
radiation transfer with realistic opacities. We show that the effects of the
recombination and the bolometric correction are significant and the analytical
model should be compare to the bolometric LC instead of a single band LC. We
find that shocked circumstellar shells have a rapid LC decline initially
because of the adiabatic expansion rather than the luminosity increase and the
shocked shells fail to explain the LC properties of SN 2006gy. However, our
synthetic LCs are qualitatively similar to those of superluminous SN 2003ma and
SN 1988Z and they may be related to shocked circumstellar shells.Comment: 7 pages, 7 figures, 1 table, accepted by Monthly Notices of the Royal
Astronomical Societ
The Formation and Destruction of Molecular Clouds and Galactic Star Formation
We describe an overall picture of galactic-scale star formation. Recent
high-resolution magneto-hydrodynamical simulations of two-fluid dynamics with
cooling/heating and thermal conduction have shown that the formation of
molecular clouds requires multiple episodes of supersonic compression. This
finding enables us to create a scenario in which molecular clouds form in
interacting shells or bubbles on a galactic scale. First we estimate the
ensemble-averaged growth rate of molecular clouds over a timescale larger than
a million years. Next we perform radiation hydrodynamics simulations to
evaluate the destruction rate of magnetized molecular clouds by the stellar FUV
radiation. We also investigate the resultant star formation efficiency within a
cloud which amounts to a low value (a few percent) if we adopt the power-law
exponent -2.5 for the mass distribution of stars in the cloud. We finally
describe the time evolution of the mass function of molecular clouds over a
long timescale (>1Myr) and discuss the steady state exponent of the power-law
slope in various environments.Comment: 7 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
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