4,688 research outputs found
The mass-velocity and intensity-velocity relations in jet-driven molecular outflows
We use numerical simulations to examine the mass-velocity and
intensity-velocity relations in the CO J=2-1 and H S(1)1-0 lines for
jet-driven molecular outflows. Contrary to previous expectations, we find that
the mass-velocity relation for the swept-up gas is a single power-law, with a
shallow slope and no break to a steeper slope at high velocities.
An analytic bowshock model with no post-shock mixing is shown to reproduce this
behaviour very well.
We show that molecular dissociation and the temperature dependence of the
line emissivity are both critical in defining the shape of the line profiles at
velocities above 20 km s. In particular, the simulated CO J=2-1
intensity-velocity relation does show a break in slope, even though the
underlying mass distribution does not. These predicted CO profiles are found to
compare remarkably well with observations of molecular outflows, both in terms
of the slopes at low and high velocities and in terms of the range of break
velocities at which the change in slope occurs. Shallower slopes are predicted
at high velocity in higher excitation lines, such as H S(1)1-0.
This work indicates that, in jet-driven outflows, the CO J=2-1 intensity
profile reflects the slope of the underlying mass-velocity distribution only at
velocities 20 km/s, and that higher temperature tracers are required to
probe the mass distribution at higher speed.Comment: 6 pages, 8 figures. Accepted for publication in Astronomy and
Astrophysic
An explicit scheme for multifluid magnetohydrodynamics
When modeling astrophysical fluid flows, it is often appropriate to discard
the canonical magnetohydrodynamic approximation thereby freeing the magnetic
field to diffuse with respect to the bulk velocity field. As a consequence,
however, the induction equation can become problematic to solve via standard
explicit techniques. In particular, the Hall diffusion term admits fast-moving
whistler waves which can impose a vanishing timestep limit.
Within an explicit differencing framework, a multifluid scheme for weakly
ionised plasmas is presented which relies upon a new approach to integrating
the induction equation efficiently. The first component of this approach is a
relatively unknown method of accelerating the integration of parabolic systems
by enforcing stability over large compound timesteps rather than over each of
the constituent substeps. This method, Super Time Stepping, proves to be very
effective in applying a part of the Hall term up to a known critical value. The
excess of the Hall term above this critical value is then included via a new
scheme for pure Hall diffusion.Comment: 8 pages; 4 figures; accepted by MNRAS; minor corrections to
equations; addition of appendi
Multifluid magnetohydrodynamic turbulent decay
It is generally believed that turbulence has a significant impact on the
dynamics and evolution of molecular clouds and the star formation which occurs
within them. Non-ideal magnetohydrodynamic effects are known to influence the
nature of this turbulence. We present the results of a suite of 512-cubed
resolution simulations of the decay of initially super-Alfvenic and supersonic
fully multifluid MHD turbulence. We find that ambipolar diffusion increases the
rate of decay of the turbulence while the Hall effect has virtually no impact.
The decay of the kinetic energy can be fitted as a power-law in time and the
exponent is found to be -1.34 for fully multifluid MHD turbulence. The power
spectra of density, velocity and magnetic field are all steepened significantly
by the inclusion of non-ideal terms. The dominant reason for this steepening is
ambipolar diffusion with the Hall effect again playing a minimal role except at
short length scales where it creates extra structure in the magnetic field.
Interestingly we find that, at least at these resolutions, the majority of the
physics of multifluid turbulence can be captured by simply introducing fixed
(in time and space) resistive terms into the induction equation without the
need for a full multifluid MHD treatment. The velocity dispersion is also
examined and, in common with previously published results, it is found not to
be power-law in nature.Comment: 16 pages, 15 figures, Accepted for publication in Ap
High-resolution [C II] imaging of HDF850.1 reveals a merging galaxy at z=5.185
New high-resolution maps with the IRAM Interferometer of the redshifted [C
II] 158 micron line and the 0.98mm dust continuum of HDF850.1 at z = 5.185 show
the source to have a blueshifted northern component and a redshifted southern
component, with a projected separation of 0.3 arcsec, or 2 kpc. We interpret
these components as primordial galaxies that are merging to form a larger
galaxy. We think it is the resulting merger-driven starburst that makes
HDF850.1 an ultraluminous infrared galaxy, with an L(IR) of 1E13 Lsun. The
observed line and continuum brightness temperatures and the constant
line-to-continuum ratio across the source imply (1) high [C II] line optical
depth, (2) a [C II] excitation temperature of the same order as the dust
temperature, and (3) dust continuum emission that is nearly optically thick at
158 microns. These conclusions for HDF850.1 probably also apply to other
high-redshift submillimeter galaxies and quasar hosts in which the [C II] 158
micron line has been detected, as indicated by their roughly constant [C
II]-to-158 micron continuum ratios, in sharp contrast to the large dispersion
in their [C II]-to-FIR luminosity ratios. In brightness temperature units, the
[C II] line luminosity is about the same as the predicted CO(1-0) luminosity,
implying that the [C II] line can also be used to estimate the molecular gas
mass, with the same assumptions as for CO.Comment: Accepted by Astronomy and Astrophysic
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