34 research outputs found
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
Herschel dust emission as a probe of starless cores mass: MCLD 123.5+24.9 of the Polaris Flare
We present newly processed archival Herschel images of molecular cloud MCLD
123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using
the spectral synthesis code Cloudy, we explore uncertainties in the derivation
of column densities, hence, masses of molecular cores from Herschel data. We
first consider several detailed grain models that predict far-IR grain
opacities. Opacities predicted by the models differ by more than a factor of
two, leading to uncertainties in derived column densities by the same factor.
Then we consider uncertainties associated with the modified blackbody fitting
process used by observers to estimate column densities. For high column density
clouds (N(H) 10 cm), this fitting technique can
underestimate column densities by about a factor of three. Finally, we consider
the virial stability of the five starless cores in MCLD 123.5+24.9. All of
these cores appear to have strongly sub-virial masses, assuming, as we argue,
that CO line data provide reliable estimates of velocity dispersions.
Evidently, they are not self-gravitating, so it is no surprise that they are
starless.Comment: ApJ, Accepted. Minor typographical errors corrected and figures 6 & 7
updated in v
Magnetic Fields in Dark Cloud Cores: Arecibo OH Zeeman Observations
We have carried out an extensive survey of magnetic field strengths toward
dark cloud cores in order to test models of star formation: ambipolar-diffusion
driven or turbulence driven. The survey involved hours of observing
with the Arecibo telescope in order to make sensitive OH Zeeman observations
toward 34 dark cloud cores. Nine new probable detections were achieved at the
2.5-sigma level; the certainty of the detections varies from solid to marginal,
so we discuss each probable detection separately. However, our analysis
includes all the measurements and does not depend on whether each position has
a detection or just a sensitive measurement. Rather, the analysis establishes
mean (or median) values over the set of observed cores for relevant
astrophysical quantities. The results are that the mass-to-flux ratio is
supercritical by , and that the ratio of turbulent to magnetic energies
is also . These results are compatible with both models of star
formation. However, these OH Zeeman observations do establish for the first
time on a statistically sound basis the energetic importance of magnetic fields
in dark cloud cores at densities of order cm, and they lay
the foundation for further observations that could provide a more definitive
test.Comment: 22 pages, 2 figures, 2 table
Testing Magnetic Star Formation Theory
We report here observations of the Zeeman effect in the 18-cm lines of OH in
the envelope regions surrounding four molecular cloud cores toward which
detections of B(LOS) have been achieved in the same lines, and evaluate the
ratio of mass to magnetic flux, M/Phi, between the cloud core and envelope.
This relative M/Phi measurement reduces uncertainties in previous studies, such
as the angle between B and the line of sight and the value of [OH/H]. Our
result is that for all four clouds, the ratios R of the core to the envelope
values of M/Phi are less than 1. Stated another way, the ratios R' of the core
to the total cloud M/Phi are less than 1. The extreme case or idealized (no
turbulence) ambipolar diffusion theory of core formation requires the ratio of
the central to total M/Phi to be approximately equal to the inverse of the
original subcritical M/Phi, or R' > 1. The probability that all four of our
clouds have R' > 1 is 3 x 10^{-7}; our results are therefore significantly in
contradiction with the hypothesis that these four cores were formed by
ambipolar diffuson. Highly super-Alfvenic turbulent simulations yield a wide
range of relative M/Phi, but favor a ratio R < 1, as we observe. Our experiment
is limited to four clouds, and we can only directly test the predictions of the
extreme-case "idealized" models of ambipolar-diffusion driven star formation
that have a regular magnetic field morphology. Nonetheless, our experimental
results are not consistent with the "idealized" strong field, ambipolar
diffusion theory of star formation.Comment: 30 pages, 6 figures; paper revised after journal review, now accepted
by Ap
\u3cem\u3eHerschel\u3c/em\u3e Dust Emission as a Probe of Starless Cores Mass: MCLD 123.5+24.9 of the Polaris Flare
We present newly processed archival Herschel images of molecular cloud MCLD 123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using the spectral synthesis code Cloudy, we explore uncertainties in the derivation of column densities, and hence masses of molecular cores from Herschel data. We first consider several detailed grain models that predict far-infrared grain opacities. Opacities predicted by the models differ by more than a factor of two, leading to uncertainties in derived column densities by the same factor. Then we consider uncertainties associated with the modified blackbody fitting process used by observers to estimate column densities. For high column density clouds (N(H) ≫ 1 x 1022 cm−2), this fitting technique can underestimate column densities by about a factor of three. Finally, we consider the virial stability of the five starless cores in MCLD 123.5+24.9. All of these cores appear to have strongly sub-virial masses, assuming, as we argue, that 13CO line data provide reliable estimates of velocity dispersions. Evidently, they are not self-gravitating, so it is no surprise that they are starless
Optically Thick [O I] and [C II] Emission toward NGC 6334A
This work focuses on [O I] and [C II] emission toward NGC 6334A, an embedded H+ region/PDR only observable at infrared or longer wavelengths. A geometry in which nearly all the emission escapes out the side of the cloud facing the stars, such as Orion, is not applicable to this region. Instead, we find the geometry to be one in which the H+ region and associated PDR is embedded in the molecular cloud. Constant-density PDR calculations are presented which predict line intensities as a function of AV [or N(H)], hydrogen density (nH), and incident UV radiation field (G0). We find that a single-component model with AV~650 mag, nH=5×105 cm-3, and G0=7×104 reproduces the observed [O I] and [C II] intensities, and that the low [O I] 63 to 146 μm ratio is due to line optical depth effects in the [O I] lines, produced by a large column density of atomic/molecular gas. We find that the effects of a density law would increase our derived AV, while the effects of an asymmetric geometry would decrease AV, with the two effects largely canceling. We conclude that optically selected H+ regions adjacent to PDRs, such as Orion, likely have a different viewing angle or geometry than similar regions detected through IR observations. Overall, the theoretical calculations presented in this work have utility for any PDR embedded in a molecular cloud
Orion\u27s Veil. IV. H\u3csub\u3e2\u3c/sub\u3e Excitation and Geometry
The foreground Veil of material that lies in front of the Orion Nebula is the best studied sample of the interstellar medium because we know where it is located, how it is illuminated, and the balance of thermal and magnetic energy. In this work, we present high-resolution STIS observations toward the Trapezium, with the goal of better understanding the chemistry and geometry of the two primary Veil layers, along with ionized gas along the line of sight. The most complete characterization of the rotational/vibrational column densities of H2 in the almost purely atomic components of the Veil are presented, including updates to the Cloudy model for H2 formation on grain surfaces. The observed H2 is found to correlate almost exclusively with Component B. The observed H2, observations of CI, CI*, and CI**, and theoretical calculations using Cloudy allow us to place the tightest constraints yet on the distance, density, temperature, and other physical characteristics for each cloud component. We find the H2 excitation spectrum observed in the Veil is incompatible with a recent study that argued that the Veil was quite close to the Trapezium. The nature of a layer of ionized gas lying between the Veil and the Trapezium is characterized through the emission and absorption lines it produces, which we find to be the blueshifted component observed in S iii and P iii absorption. We deduce that, within the next 30–60 thousand years, the blueshifted ionized layer and Component B will merge, which will subsequently merge with Component A in the next one million years
Orion\u27s Veil: Magnetic Field Strengths and Other Properties of a PDR in Front of the Trapezium Cluster
We present an analysis of physical conditions in the Orion Veil, an atomic photon-dominated region (PDR) that lies just in front (≈2 pc) of the Trapezium stars of Orion. This region offers an unusual opportunity to study the properties of PDRs, including the magnetic field. We have obtained 21 cm H i and 18 cm (1665 and 1667 MHz) OH Zeeman effect data that yield images of the line-of-sight magnetic field strength B los in atomic and molecular regions of the Veil. We find B los ≈ −50 to −75 μG in the atomic gas across much of the Veil (25\u27\u27 resolution) and B los ≈ −350 μG at one position in the molecular gas (40\u27\u27 resolution). The Veil has two principal H i velocity components. Magnetic and kinematical data suggest a close connection between these components. They may represent gas on either side of a shock wave preceding a weak-D ionization front. Magnetic fields in the Veil H i components are 3–5 times stronger than they are elsewhere in the interstellar medium where N(H) and n(H) are comparable. The H i components are magnetically subcritical (magnetically dominated), like the cold neutral medium, although they are about 1 dex denser. Comparatively strong fields in the Veil H i components may have resulted from low-turbulence conditions in the diffuse gas that gave rise to OMC-1. Strong fields may also be related to magnetostatic equilibrium that has developed in the Veil since star formation. We also consider the location of the Orion-S molecular core, proposing a location behind the main Orion H+ region
Physical Conditions in Orion\u27s Veil. II. A Multicomponent Study of the Line of Sight toward the Trapezium
Orion\u27s Veil is an absorbing screen that lies along the line of sight to the Orion H II region. It consists of two or more layers of gas that must lie within a few parsecs of the Trapezium cluster. Our previous work considered the Veil as a whole and found that the magnetic field dominates the energetics of the gas in at least one component. Here we use high-resolution STIS UV spectra that resolve the two velocity components in absorption and determine the conditions in each. We derive a volume hydrogen density, 21 cm spin temperature, turbulent velocity, and kinetic temperature for each. We combine these estimates with magnetic field measurements to find that magnetic energy significantly dominates turbulent and thermal energies in one component, while the other component is close to equipartition between turbulent and magnetic energies. We observe H2 absorption for highly excited v, J levels that are photoexcited by the stellar continuum, and detect blueshifted S+2 and P+2 ions. These ions must arise from ionized gas between the mostly neutral portions of the Veil and the Trapezium and shields the Veil from ionizing radiation. We find that this layer of ionized gas is also responsible for He I λ3889 absorption toward the Veil, which resolves a 40 year old debate on the origin of He I absorption toward the Trapezium. Finally, we determine that the ionized and mostly atomic layers of the Veil will collide in less than 85,000 yr