31,153 research outputs found
Nonlinear Single-Armed Spiral Density Waves in Nearly Keplerian Disks
Single-armed, stationary density waves can propagate in very weakly
self-gravitating gas disks dominated by a central mass. Examples include
circumstellar disks of protostars and molecular disks in galactic nuclei. We
explore the linear and nonlinear dynamics of such waves. Variational methods
yield nonlinear versions of the dispersion relation, angular momentum flux, and
propagation velocity in the tight-winding limit. The pitch angle increases with
amplitude until the tight-winding approximation breaks down. We also find a
series of nonlinear logarithmic spirals which is exact in the limit of small
disk mass and which extends to large pitch angle.Comment: 16 pages, 3 figures. Uses mn.sty and mncite.sty. Accepted by MNRA
Maximally Star-Forming Galactic Disks I. Starburst Regulation Via Feedback-Driven Turbulence
Star formation rates in the centers of disk galaxies often vastly exceed
those at larger radii. We investigate the idea that these central starbursts
are self-regulated, with the momentum flux injected to the ISM by star
formation balancing the gravitational force confining the gas. For most
starbursts, supernovae are the largest contributor to the momentum flux, and
turbulence provides the main pressure support for the predominantly-molecular
ISM. If the momentum feedback per stellar mass formed is p_*/m_* ~ 3000 km/s,
the predicted star formation rate is Sigma_SFR=2 pi G Sigma^2 m_*/p_*
~0.1(Sigma/100Msun/pc^2)^2 Msun/kpc^2/yr in regions where gas dominates the
vertical gravity. We compare this prediction with numerical simulations of
vertically-resolved disks that model star formation including feedback, finding
good agreement for gas surface densities Sigma ~ 10^2-10^3 Msun/pc^2. We also
compare to a compilation of star formation rates and gas contents from local
and high-redshift galaxies (both mergers and normal galaxies), finding good
agreement provided that X_CO decreases weakly as Sigma and Sigma_SFR increase.
Star formation rates in dense, turbulent gas are also expected to depend on the
gravitational free-fall time; if the efficiency per free-fall time is
epsilon_ff ~ 0.01, the turbulent velocity dispersion driven by feedback is
expected to be v_z = 0.4 epsilon_ff p_*/m_* ~ 10 km/s, relatively independent
of Sigma or Sigma_SFR. Turbulence-regulated starbursts (controlled by kinetic
momentum feedback) are part of the larger scheme of self-regulation;
primarily-atomic low-Sigma outer disks may have star formation regulated by UV
heating feedback, whereas regions at extremely high Sigma may be regulated by
feedback of radiation that is reprocessed into trapped IR.Comment: 35 pages, 5 figures; accepted by the Ap
A law for star formation in galaxies
We study the galactic-scale triggering of star formation. We find that the
largest mass-scale not stabilized by rotation, a well defined quantity in a
rotating system and with clear dynamical meaning, strongly correlates with the
star formation rate in a wide range of galaxies. We find that this relation can
be understood in terms of self-regulation towards marginal Toomre stability and
the amount of turbulence allowed to sustain the system in this self-regulated
quasi-stationary state. We test such an interpretation by computing the
predicted star formation rates for a galactic interstellar medium characterized
by lognormal probability distribution function and find good agreement with the
observed relation.Comment: 13 pages, 2 figures, Accepted in Ap
Planetary embryos and planetesimals residing in thin debris disks
We consider constraints on the planetesimal population residing in the disks
of AU Microscopii, Beta Pictoris and Fomalhaut taking into account their
observed thicknesses and normal disk opacities. We estimate that bodies of
radius 5, 180 and 70 km are responsible for initiating the collisional cascade
accounting for the dust production for AU-Mic, Beta-Pic and Fomalhaut's disks,
respectively, at break radii from the star where their surface brightness
profiles change slope. Larger bodies, of radius 1000km and with surface density
of order 0.01 g/cm^2, are required to explain the thickness of these disks
assuming that they are heated by gravitational stirring. A comparison between
the densities of the two sizes suggests the size distribution in the largest
bodies is flatter than that observed in the Kuiper belt. AU Mic's disk requires
the shallowest size distribution for bodies with radius greater than 10km
suggesting that the disk contains planetary embryos experiencing a stage of
runaway growth.Comment: submitted to MNRA
Discovery of Recent Star Formation in the Extreme Outer Regions of Disk Galaxies
We present deep Halpha images of three nearby late-type spiral galaxies
(NGC628, NGC1058 and NGC6946), which reveal the presence of HII regions out to,
and beyond, two optical radii (defined by the 25th B-band isophote). The
outermost HII regions appear small, faint and isolated, compared to their inner
disk counterparts, and are distributed in organized spiral arm structures,
likely associated with underlying HI arms and faint stellar arms. The
relationship between the azimuthally--averaged Halpha surface brightness
(proportional to star formation rate per unit area) and the total gas surface
density is observed to steepen considerably at low gas surface densities. We
find that this effect is largely driven by a sharp decrease in the covering
factor of star formation at large radii, and not by changes in the rate at
which stars form locally. An azimuthally--averaged analysis of the
gravitational stability of the disk of NGC6946 reveals that while the existence
of star formation in the extreme outer disk is consistent with the Toomre-Q
instability model, the low rates observed are only compatible with the model
when a constant gaseous velocity dispersion is assumed. We suggest that
observed behaviour could also be explained by a model in which the star
formation rate has an intrinsic dependence on the azimuthally-averaged gas
volume density, which decreases rapidly in the outer disk due to the vertical
flaring of the gas layer.Comment: 10 pages, 2 embedded postscript files, 3 jpeg images; accepted for
publication in ApJ Letter
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