639 research outputs found
The Shapes of Molecular Cloud Cores in Orion
We investigate the intrinsic shapes of starless cores in the Orion GMC, using
the prestellar core sample of Nutter and Ward-Thompson (2007), which is based
on submillimeter SCUBA data. We employ a maximum-likelihood method to
reconstruct the intrinsic distribution of ellipsoid axial ratios from the axial
ratios of projected plane-of-the-sky core ellipses. We find that, independently
of the details of the assumed functional form of the distribution, there is a
strong preference for oblate cores of finite thickness. Cores with varying
finite degrees of triaxiality are a better fit than purely axisymmetric cores
although cores close to axisymmetry are not excluded by the data. The incidence
of prolate starless cores in Orion is found to be very infrequent. We also test
the consistency of the observed data with a uniform distribution of intrinsic
shapes, which is similar to those found in gravoturbulent fragmentation
simulations. This distribution is excluded at the 0.1% level. These findings
have important implications for theories of core formation within molecular
clouds.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
A new method for probing magnetic field strengths from striations in the interstellar medium
Recent studies of the diffuse parts of molecular clouds have revealed the
presence of parallel, ordered low-density filaments termed striations. Flows
along magnetic field lines, Kelvin-Helmholtz instabilities and hydromagnetic
waves are amongst the various formation mechanisms proposed. Through a synergy
of observational, numerical and theoretical analysis, previous studies singled
out the hydromagnetic waves model as the only one that can account for the
observed properties of striations. Based on the predictions of that model, we
develop here a method for measuring the temporal evolution of striations
through a combination of molecular and dust continuum observations. Our method
allows us to not only probe temporal variations in molecular clouds but also
estimate the strength of both the ordered and fluctuating components of the
magnetic field projected on the plane-of-the-sky. We benchmark our new method
against chemical and radiative transfer effects through two-dimensional
magnetohydrodynamic simulations coupled with non-equilibrium chemical modelling
and non-local thermodynamic equilibrium line radiative transfer. We find good
agreement between theoretical predictions, simulations and observations of
striations in the Taurus molecular cloud. We find a value of for the plane-of-sky magnetic field, in agreement with previous
estimates via the Davis-Chandrasekhar-Fermi method, and a ratio of fluctuating
to ordered component of the magnetic field of 10\%.Comment: 12 pages, 14 figures, Accepted for publication in MNRA
The Magnetic Field of L1544: I. Near-Infrared Polarimetry and the Non-Uniform Envelope
The magnetic field (B-field) of the starless dark cloud L1544 has been
studied using near-infrared (NIR) background starlight polarimetry (BSP) and
archival data in order to characterize the properties of the plane-of-sky
B-field. NIR linear polarization measurements of over 1,700 stars were obtained
in the H-band and 201 of these were also measured in the K-band. The NIR BSP
properties are correlated with reddening, as traced using the RJCE (H-M)
method, and with thermal dust emission from the L1544 cloud and envelope seen
in Herschel maps. The NIR polarization position angles change at the location
of the cloud and exhibit their lowest dispersion of position angles there,
offering strong evidence that NIR polarization traces the plane-of-sky B-field
of L1544. In this paper, the uniformity of the plane-of-sky B-field in the
envelope region of L1544 is quantitatively assessed. This allowed evaluating
the approach of assuming uniform field geometry when measuring relative
mass-to-flux ratios in the cloud envelope and core based on averaging of the
envelope radio Zeeman observations, as in Crutcher et al. (2009). In L1544, the
NIR BSP shows the envelope B-field to be significantly non-uniform and likely
not suitable for averaging Zeeman properties without treating intrinsic
variations. Deeper analyses of the NIR BSP and related data sets, including
estimates of the B-field strength and testing how it varies with position and
gas density, are the subjects of later papers in this series.Comment: 16 pages, 9 figures; accepted for publication in The Astrophysical
Journa
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