144 research outputs found
A method for reconstructing the PDF of a 3D turbulent density field from 2D observations
We introduce a method for calculating the probability density function (PDF)
of a turbulent density field in three dimensions using only information
contained in the projected two-dimensional column density field. We test the
method by applying it to numerical simulations of hydrodynamic and
magnetohydrodynamic turbulence in molecular clouds. To a good approximation,
the PDF of log(normalised column density) is a compressed, shifted version of
the PDF of log(normalised density). The degree of compression can be determined
observationally from the column density power spectrum, under the assumption of
statistical isotropy of the turbulence.Comment: 5 pages, 2 figures, accepted for publication in MNRAS Letter
Assessment of Altimetry Using Ground-Based GPS Data from the 88S Traverse, Antarctica, in Support of ICESat-2
We conducted a 750 km kinematic GPS survey, referred to as the 88S Traverse, based out of South Pole Station, Antarctica, between December 2017 and January 2018. This ground-based survey was designed to validate space-borne altimetry and airborne altimetry developed at NASA. The 88S Traverse intersects 20% of the ICESat-2 satellite orbits on a route that has been flown by two different Operation IceBridge airborne laser altimeters: the Airborne Topographic Mapper (ATM; 26 October 2014) and the University of Alaska Fairbanks (UAF) Lidar (30 November and 3 December 2017). Here we present an overview of the ground-based GPS data quality and a quantitative assessment of the airborne laser altimetry over a flat section of the ice sheet interior. Results indicate that the GPS data are internally consistent (1:1+- 4:1 cm). Relative to the ground-based 88S Traverse data, the elevation biases for ATM and the UAF lidar range from -9:5 to 3.6 cm, while surface measurement precisions are equal to or better than 14.1 cm. These results suggest that the ground-based GPS data and airborne altimetry data are appropriate for the validation of ICESat-2 surface elevation data
Modification of Projected Velocity Power Spectra by Density Inhomogeneities in Compressible Supersonic Turbulence
(Modified) The scaling of velocity fluctuation, dv, as a function of spatial
scale L in molecular clouds can be measured from size-linewidth relations,
principal component analysis, or line centroid variation. Differing values of
the power law index of the scaling relation dv = L^(g3D) in 3D are given by
these different methods: the first two give g3D=0.5, while line centroid
analysis gives g3D=0. This discrepancy has previously not been fully
appreciated, as the variation of projected velocity line centroid fluctuations
(dv_{lc} = L^(g2D)) is indeed described, in 2D, by g2D=0.5. However, if
projection smoothing is accounted for, this implies that g3D=0. We suggest that
a resolution of this discrepancy can be achieved by accounting for the effect
of density inhomogeneity on the observed g2D obtained from velocity line
centroid analysis. Numerical simulations of compressible turbulence are used to
show that the effect of density inhomogeneity statistically reverses the effect
of projection smoothing in the case of driven turbulence so that velocity line
centroid analysis does indeed predict that g2D=g3D=0.5. Using our numerical
results we can restore consistency between line centroid analysis, principal
component analysis and size-linewidth relations, and we derive g3D=0.5,
corresponding to shock-dominated (Burgers) turbulence. We find that this
consistency requires that molecular clouds are continually driven on large
scales or are only recently formed.Comment: 28 pages total, 20 figures, accepted for publication in Ap
The Molecular Environment of the Gamma-ray Source TeV J2032+4130
The mysterious very high energy gamma-ray source, TeV J2032+4130, is
coincident with the powerful Cygnus OB2 stellar association, though a physical
association between the two remains uncertain. It is possible that the detected
very high energy photons are produced via an overdensity of locally accelerated
cosmic rays impinging on molecular clouds in the source region. In order to
test this hypothesis, we used the Kitt Peak 12m, the Heinrich-Hertz
Submillimeter Telescope (HH-SMT), and the Five College Radio Astronomy
Observatory (FCRAO), to obtain observations in the J=1-->0 and J=2-->1 lines of
both 12CO and 13CO. We report here on the detection of significant molecular
material toward the TeV source region which could be acting as the target of
locally accelerated CRs. We also find evidence of compact molecular clumps,
showing large line widths in the CO spectra, possibly indicative of energetic
processes in this region of Cygnus OB2.Comment: Accepted for publication in ApJ
The Supernova Remnant CTB104A : Magnetic Field Structure and Interaction with the Environment
We present new, high resolution 1420 and 408 MHz continuum images and HI and
12CO (J=1-0) spectral line maps of the diffuse supernova remnant CTB104A
(G93.7-0.3). Analysis of the complex continuum emission reveals no significant
spectral index variations across the remnant. Three prominences around CTB104A
are found to be related to the SNR, while one extension to the east is
identified as an HII region associated with a background molecular shell. Small
scale polarization and rotation measure (RM) structures are turbulent in
nature, but we find a well-ordered RM gradient across the remnant, extending
from southeast to northwest. This gradient does not agree with the direction of
the global Galactic magnetic field, but does agree with a large-scale RM
anomaly inferred from rotation measure data by Cleg et al. (1992). We show that
the observed morphology of CTB104A is consistent with expansion in a uniform
magnetic field, and this is supported by the observed RM distribution. By
modeling the RM gradient with a simple compression model we have determined the
magnetic field strength within the remnant as Bo ~ 2.3 micro G. We have
identified signatures of the interaction of CTB104A with the surrounding
neutral material, and determined its distance, from the kinematics of the HI
structure encompassing the radio emission, as 1.5 kpc. We also observed clear
breaks in the HI shell that correspond well to the positions of two of the
prominences, indicating regions where hot gas is escaping from the interior of
the SNR.Comment: 7 pages, Latex with aastex and emulateapj5, 12 figures, ApJ accepte
Interstellar Turbulence: II. Energy Spectra of Molecular Regions in the Outer Galaxy
The multivariate tool of Principal Component Analysis (PCA) is applied to 23
fields in the FCRAO CO Survey of the Outer Galaxy. PCA enables the
identification of line profile differences which are assumed to be generated
from fluctuations within a turbulent velocity field. The variation of these
velocity differences with spatial scale within a molecular region is described
by a singular power law, delta v= c L^alpha which can be used as a powerful
diagnostic to turbulent motions. For the ensemble of 23 fields, we find a mean
value alpha = 0.62 +- 0.11. From a recent calibration of this method using
fractal Brownian motion simulations (Brunt & Heyer 2001), the measured velocity
difference-size relationship corresponds to an energy spectrum, E(k), which
varies as k^-beta, where beta = 2.17 +- 0.31. We compare our results to both
decaying and forced hydrodynamic simulations of turbulence. We conclude that
energy must be continually injected into the regions to replenish that lost by
dissipative processes such as shocks. The absence of large, widely distributed
shocks within the targeted fields suggests that the energy is injected at
spatial scales less than several pc.Comment: 24 pages, 10 figures, accepted by Ap
Spitzer observations of the Massive star forming complex S254-S258: structure and evolution
We present Spitzer-IRAC, NOAO 2.1meter-Flamingos, Keck-NIRC, and
FCRAO-SEQUOIA observations of the massive star forming complex S254-S258,
covering an area of 25x20 arc-minutes. Using a combination of the IRAC and NIR
data, we identify and classify the young stellar objects (YSO) in the complex.
We detect 510 sources with near or mid IR-excess, and we classify 87 Class I,
and 165 Class II sources. The YSO are found in clusters surrounded by isolated
YSO in a low-density distributed population. The ratio of clustered to total
YSO is 0.8. We identify six new clusters in the complex. One of them,
G192.63-00, is located around the ionizing star of the HII region S255. We
hypothesize that the ionizing star of S255 was formed in this cluster. We also
detect a southern component of the cluster in HII region S256. The cluster
G192.54-0.15, located inside HII region S254 has a VLSR of 17 km/s with respect
to the main cloud, and we conclude that it is located in the background of the
complex. The structure of the molecular cloud is examined using 12CO and 13CO,
as well as a near-IR extinction map. The main body of the molecular cloud has
VLSR between 5 and 9 km/s. The arc-shaped structure of the molecular cloud,
following the border of the HII regions, and the high column density in the
border of the HII regions support the idea that the material has been swept up
by the expansion of the HII regions.Comment: Accepted for publication in The Astrophysical Journa
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