43,231 research outputs found
Star-forming accretion flows and the low luminosity nuclei of giant elliptical galaxies
The luminosities of the centers of nearby elliptical galaxies are very low
compared to models of thin disc accretion to their black holes at the Bondi
rate, typically a few hundredths to a few tenths of a solar mass per year. This
has motivated models of inefficiently-radiated accretion that invoke weak
electron-ion thermal coupling, and/or inhibited accretion rates due to
convection or outflows. Here we point out that even if such processes are
operating, a significant fraction of the accreting gas is prevented from
reaching the central black hole because it condenses into stars in a
gravitationally unstable disc. Star formation occurs inside the Bondi radius
(typically ~100pc in giant ellipticals), but still relatively far from the
black hole in terms of Schwarzschild radii. Star formation depletes and heats
the gas disc, eventually leading to a marginally stable, but much reduced,
accretion flow to the black hole. We predict the presence of cold (~100K),
dusty gas discs, containing clustered H-alpha emission and occasional type II
supernovae, both resulting from the presence of massive stars. Star formation
accounts for several features of the M87 system: a thin disc, traced by H-alpha
emission, is observed on scales of about 100pc, with features reminiscent of
spiral arms and dust lanes; the star formation rate inferred from the intensity
of H-alpha emission is consistent with the Bondi accretion rate of the system.
Star formation may therefore help suppress accretion onto the central engines
of massive ellipticals. We also discuss some implications for the fueling of
the Galactic center and quasars.Comment: 13 pages, accepted to MNRA
A heterotic sigma model with novel target geometry
We construct a (1,2) heterotic sigma model whose target space geometry
consists of a transitive Lie algebroid with complex structure on a Kaehler
manifold. We show that, under certain geometrical and topological conditions,
there are two distinguished topological half--twists of the heterotic sigma
model leading to A and B type half--topological models. Each of these models is
characterized by the usual topological BRST operator, stemming from the
heterotic (0,2) supersymmetry, and a second BRST operator anticommuting with
the former, originating from the (1,0) supersymmetry. These BRST operators
combined in a certain way provide each half--topological model with two
inequivalent BRST structures and, correspondingly, two distinct perturbative
chiral algebras and chiral rings. The latter are studied in detail and
characterized geometrically in terms of Lie algebroid cohomology in the
quasiclassical limit.Comment: 83 pages, no figures, 2 references adde
Disk wind feedback from high-mass protostars
We perform a sequence of 3D magnetohydrodynamic (MHD) simulations of the
outflow-core interaction for a massive protostar forming via collapse of an
initial cloud core of . This allows us to characterize the
properties of disk wind driven outflows from massive protostars, which can
allow testing of different massive star formation theories. It also enables us
to assess quantitatively the impact of outflow feedback on protostellar core
morphology and overall star formation efficiency. We find that the opening
angle of the flow increases with increasing protostellar mass, in agreement
with a simple semi-analytic model. Once the protostar reaches
the outflow's opening angle is so wide that it has blown
away most of the envelope, thereby nearly ending its own accretion. We thus
find an overall star formation efficiency of , similar to that
expected from low-mass protostellar cores. Our simulation results therefore
indicate that the MHD disk wind outflow is the dominant feedback mechanism for
helping to shape the stellar initial mass function from a given prestellar core
mass function.Comment: Accepted for publication in Ap
Outflow-Confined HII regions. II. The Early Break-Out Phase
In this series of papers, we model the formation and evolution of the
photoionized region and its observational signatures during massive star
formation. Here we focus on the early break out of the photoionized region into
the outflow cavity. Using results of 3-D magnetohydrodynamic-outflow
simulations and protostellar evolution calculations, we perform post-processing
radiative-transfer. The photoionized region first appears at a protostellar
mass of 10Msun in our fiducial model, and is confined to within 10-100AU by the
dense inner outflow, similar to some observed very small hypercompact HII
regions. Since the ionizing luminosity of the massive protostar increases
dramatically as Kelvin-Helmholz (KH) contraction proceeds, the photoionized
region breaks out to the entire outflow region in <10,000yr. Accordingly, the
radio free-free emission brightens significantly in this stage. In our fiducial
model, the radio luminosity at 10 GHz changes from 0.1 mJy kpc2 at m=11Msun to
100 mJy kpc2 at 16Msun, while the infrared luminosity increases by less than a
factor of two. The radio spectral index also changes in the break-out phase
from the optically thick value of 2 to the partially optically thin value of
0.6. Additionally, we demonstrate that short-timescale variation in free-free
flux would be induced by an accretion burst. The outflow density is enhanced in
the accretion burst phase, which leads to a smaller ionized region and weaker
free-free emission. The radio luminosity may decrease by one order of magnitude
during such bursts, while the infrared luminosity is much less affected, since
internal protostellar luminosity dominates over accretion luminosity after KH
contraction starts. Such variability may be observable on timescales as short
10-100 yr, if accretion bursts are driven by disk instabilities.Comment: 9 pages, 5 figures, accepted for publication in Ap
Astrochemical confirmation of the rapid evolution of massive YSOs and explanation for the inferred ages of hot cores
Aims. To understand the roles of infall and protostellar evolution on the
envelopes of massive young stellar objects (YSOs).
Methods. The chemical evolution of gas and dust is traced, including infall
and realistic source evolution. The temperatures are determined
self-consistently. Both ad/desorption of ices using recent laboratory
temperature-programmed-desorption measurements are included.
Results. The observed water abundance jump near 100 K is reproduced by an
evaporation front which moves outward as the luminosity increases. Ion-molecule
reactions produce water below 100 K. The age of the source is constrained to t
\~ 8 +/- 4 x 10^4 yrs since YSO formation. It is shown that the chemical
age-dating of hot cores at ~ few x 10^3 - 10^4 yr and the disappearance of hot
cores on a timescale of ~ 10^5 yr is a natural consequence of infall in a
dynamic envelope and protostellar evolution. Dynamical structures of ~ 350AU
such as disks should contain most of the complex second generation species. The
assumed order of desorption kinetics does not affect these results.Comment: Accepted by A&A Letters; 4 pages, 5 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
Inside-Out Planet Formation. V. Structure of the Inner Disk as Implied by the MRI
The large population of Earth to super-Earth sized planets found very close
to their host stars has motivated consideration of formation
models. In particular, Inside-Out Planet Formation is a scenario in which
planets coalesce sequentially in the disk, at the local gas pressure maximum
near the inner boundary of the dead zone. The pressure maximum arises from a
decline in viscosity, going from the active innermost disk (where thermal
ionization of alkalis yields high viscosities via the magneto-rotational
instability (MRI)) to the adjacent dead zone (where the MRI is quenched).
Previous studies of the pressure maximum, based on -disk models, have
assumed ad hoc values for the viscosity parameter in the active zone,
ignoring the detailed physics of the MRI. Here we explicitly couple the MRI
criteria to the -disk equations, to find steady-state (constant
accretion rate) solutions for the disk structure. We consider the effects of
both Ohmic and ambipolar resistivities, and find solutions for a range of disk
accretion rates ( = - /yr),
stellar masses ( = 0.1 - 1 ), and fiducial values of
the -MRI -viscosity in the dead zone ( - ). We find that: (1) A midplane pressure maximum forms
radially the inner boundary of the dead zone; (2) Hall resistivity
dominates near the midplane in the inner disk, which may explain why close-in
planets do form in 50% of systems; (3) X-ray ionization can be
competitive with thermal ionization in the inner disk, because of the low
surface density there in steady-state; and (4) our inner disk solutions are
viscously unstable to surface density perturbations.Comment: 34 pages, 28 figures, 3 appendices. Accepted by the Astrophysical
Journa
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