331 research outputs found
Self-Consistent Analysis of OH-Zeeman Observations: Too Much Noise about Noise
We had recently re-analyzed in a self-consistent way OH-Zeeman observations
in four molecular-cloud envelopes and we had shown that, contrary to claims by
Crutcher et al., there is no evidence that the mass-to-flux ratio decreases
from the envelopes to the cores of these clouds. The key difference between our
data analysis and the earlier one by Crutcher et al. is the relaxation of the
overly restrictive assumption made by Crutcher et al, that the magnetic field
strength is independent of position in each of the four envelopes. In a more
recent paper, Crutcher et al. (1) claim that our analysis is not
self-consistent, in that it misses a cosine factor, and (2) present new
arguments to support their contention that the magnetic-field strength is
indeed independent of position in each of the four envelopes. We show that the
claim of the missing cosine factor is false, that the new arguments contain
even more serious problems than the Crutcher et al. original data analysis, and
we present new observational evidence, independent of the OH-Zeeman data, that
suggests significant variations in the magnetic-field strength in the four
cloud envelopes.Comment: 8 pages, 3 figures, MNRAS in pres
The Initial Core Mass Function due to Ambipolar Diffusion in Molecular Clouds
We show that the ambipolar-diffusion--initiated fragmentation of molecular
clouds leads simply and naturally to an initial core mass function (CMF) which
is very similar to the initial stellar mass function (IMF) and in excellent
agreement with existing observations. This agreement is robust provided that
the three (input) free parameters remain within their range of values suggested
by observations. Other, observationally testable, predictions are made.Comment: 5 pages, 4 figures, accepted by MNRAS-
Long-lived Magnetic-Tension-Driven Modes in a Molecular Cloud
We calculate and analyze the longevity of magnetohydrodynamic (MHD) wave
modes that occur in the plane of a magnetic thin sheet. Initial turbulent
conditions applied to a magnetically subcritical cloud are shown to lead to
relatively rapid energy decay if ambipolar diffusion is introduced at a level
corresponding to partial ionization primarily by cosmic rays. However, in the
flux-freezing limit, as may be applicable to photoionized molecular cloud
envelopes, the turbulence persists at "nonlinear" levels in comparison with the
isothermal sound speed \cs, with one-dimensional rms material motions in the
range of \approx 2\,\cs -5\,\cs for cloud sizes in the range of \approx
2\,\pc - 16\,\pc. These fluctuations persist indefinitely, maintaining a
significant portion of the initial turbulent kinetic energy. We find the
analytic explanation for these persistent fluctuations. They are
magnetic-tension-driven modes associated with the interaction of the sheet with
the external magnetic field. The phase speed of such modes is quite large,
allowing residual motions to persist without dissipation in the flux-freezing
limit, even as they are nonlinear with respect to the sound speed. We speculate
that long-lived large-scale MHD modes such as these may provide the key to
understanding observed supersonic motions in molecular clouds.Comment: Accepted by The Astrophysical Journal, 6 pages, 5 figures. Animations
and a 3D pdf file are available at http://www.astro.uwo.ca/~basu/pb.ht
DR21 Main: A Collapsing Cloud
The molecular cloud, DR21 Main, is an example of a large-scale gravitational
collapse about an axis near the plane of the sky where the collapse is free of
major disturbances due to rotation or other effects. Using flux maps,
polarimetric maps, and measurements of the field inclination by comparing the
line widths of ion and neutral species, we estimate the temperature, mass,
magnetic field, and the turbulent kinetic, mean magnetic, and gravitational
potential energies, and present a 3D model of the cloud and magnetic field.Comment: 27 Pages, submitted to ApJ, corrected typos, referee comments adde
Self-Consistent Analysis of OH Zeeman Observations
Crutcher, Hakobian, and Troland (2009) used OH Zeeman observations of four
nearby molecular dark clouds to show that the ratio of mass to magnetic flux
was smaller in the ~0.1 pc cores than in the ~1 pc envelopes, in contradiction
to the prediction of ambipolar diffusion driven core formation. A crucial
assumption was that the magnetic field direction is nearly the same in the
envelope and core regions of each cloud. Mouschovias and Tassis (2009) have
argued that the data are not consistent with this assumption, and presented a
new analysis that changes the conclusions of the study. Here we show that the
data are in fact consistent with the nearly uniform field direction assumption;
hence, the original study is internally self-consistent and the conclusions are
valid under the assumptions that were made. We also show that the Mouschovias
and Tassis model of magnetic fields in cloud envelopes is inconsistent with
their own analysis of the data. However, the data do not rule out a more
complex field configuration that future observations may discern.Comment: 3 pages, 1 figure, accepted for publication by MNRAS Letter
Static equilibria of the interstellar gas in the presence of magnetic and gravitational fields
Re-examining Larson's Scaling Relationships in Galactic Molecular Clouds
The properties of Galactic molecular clouds tabulated by Solomon etal (1987)
(SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey
of 13CO J=1-0 emission. These new data provide a lower opacity tracer of
molecular clouds and improved angular and spectral resolution than previous
surveys of molecular line emission along the Galactic Plane. We calculate GMC
masses within the SRBY cloud boundaries assuming LTE conditions throughout the
cloud and a constant H2 to 13CO abundance, while accounting for the variation
of the 12C/13C with Galacto-centric radius. The LTE derived masses are
typically five times smaller than the SRBY virial masses. The corresponding
median mass surface density of molecular hydrogen for this sample is 42
Msun/pc^2, which is significantly lower than the value derived by SRBY (median
206 Msun/pc^2) that has been widely adopted by most models of cloud evolution
and star formation. This discrepancy arises from both the extrapolation by SRBY
of velocity dispersion, size, and CO luminosity to the 1K antenna temperature
isophote that likely overestimates the GMC masses and our assumption of
constant 13CO abundance over the projected area of each cloud. Owing to the
uncertainty of molecular abundances in the envelopes of clouds, the mass
surface density of giant molecular clouds could be larger than the values
derived from our 13CO measurements. From velocity dispersions derived from the
13CO data, we find that the coefficient of the cloud structure functions,
vo=sigma_v/R^{1/2}, is not constant, as required to satisfy Larson's scaling
relationships, but rather systematically varies with the surface density of the
cloud as Sigma^{0.5} as expected for clouds in self-gravitational equlibrium.Comment: Accepted by ApJ. Newest version includes modifications from the
refere
Protostellar disk formation and transport of angular momentum during magnetized core collapse
Theoretical studies of collapsing clouds have found that even a relatively
weak magnetic field (B) may prevent the formation of disks and their
fragmentation. However, most previous studies have been limited to cases where
B and the rotation axis of the cloud are aligned. We study the transport of
angular momentum, and its effects on disk formation, for non-aligned initial
configurations and a range magnetic intensities. We perform 3D AMR MHD
simulations of magnetically supercritical collapsing dense cores using the code
Ramses. We compute the contributions of the processes transporting angular
momentum (J), in the envelope and the region of the disk. We clearly define
what could be defined as centrifugally supported disks and study their
properties. At variance with earlier analyses, we show that the transport of J
acts less efficiently in collapsing cores with non-aligned rotation axis and B.
Analytically, this result can be understood by taking into account the bending
of field lines occurring during the gravitational collapse. For the transport
of J, we conclude that magnetic braking in the mean direction of B tends to
dominate over both the gravitational and outflow transport of J. We find that
massive disks, containing at least 10% of the initial core mass, can form
during the earliest stages of star formation even for mass-to-flux ratios as
small as 3 to 5 times the critical value. At higher field intensities, the
early formation of massive disks is prevented. Given the ubiquity of Class I
disks, and because the early formation of massive disks can take place at
moderate magnetic intensities, we speculate that for stronger fields, disks
will form later, when most of the envelope will have been accreted. In
addition, we speculate that some observed early massive disks may actually be
outflow cavities, mistaken for disks by projection effects. (Abridged version
of the abstract.)Comment: 23 pages, 23 figures, to be published in A&
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