437 research outputs found
Polarization measurements analysis II. Best estimators of polarization fraction and angle
With the forthcoming release of high precision polarization measurements,
such as from the Planck satellite, it becomes critical to evaluate the
performance of estimators for the polarization fraction and angle. These two
physical quantities suffer from a well-known bias in the presence of
measurement noise, as has been described in part I of this series. In this
paper, part II of the series, we explore the extent to which various estimators
may correct the bias. Traditional frequentist estimators of the polarization
fraction are compared with two recent estimators: one inspired by a Bayesian
analysis and a second following an asymptotic method. We investigate the
sensitivity of these estimators to the asymmetry of the covariance matrix which
may vary over large datasets. We present for the first time a comparison among
polarization angle estimators, and evaluate the statistical bias on the angle
that appears when the covariance matrix exhibits effective ellipticity. We also
address the question of the accuracy of the polarization fraction and angle
uncertainty estimators. The methods linked to the credible intervals and to the
variance estimates are tested against the robust confidence interval method.
From this pool of estimators, we build recipes adapted to different use-cases:
build a mask, compute large maps, and deal with low S/N data. More generally,
we show that the traditional estimators suffer from discontinuous distributions
at low S/N, while the asymptotic and Bayesian methods do not. Attention is
given to the shape of the output distribution of the estimators, and is
compared with a Gaussian. In this regard, the new asymptotic method presents
the best performance, while the Bayesian output distribution is shown to be
strongly asymmetric with a sharp cut at low S/N.Finally, we present an
optimization of the estimator derived from the Bayesian analysis using adapted
priors
The Galactic dust as a foreground to Cosmic Microwave Background maps
We present results obtained with the PRONAOS balloon-borne experiment on
interstellar dust. In particular, the submillimeter / millimeter spectral index
is found to vary between roughly 1 and 2.5 on small scales (3.5' resolution).
This could have implications for component separation in Cosmic Microwave
Background maps.Comment: 4 pages, 1 figure, proceeding of the Multi-Wavelength Cosmology
conference held in Mykonos, Greece, June 2003, ed. Kluwe
Toward a better understanding of the mid-infrared emission in the LMC
In this paper we aim to constrain for the first time the dust emission in the
mid-to-far infrared domain, in the LMC, with the use of the Spitzer IRS and
MIPS SED data, combined with Herschel data. We also consider UV extinction
predictions derived from modeling. We selected 10 regions observed as part of
the SAGE-Spec program, to probe dust properties in various environments
(diffuse, molecular and ionized regions). All data were smoothed to the
40arcsec angular resolution. The SEDs were modeled with DustEM models, using
the standard Mathis RF, as well as three additional RFs, with stellar clusters
ages ranging from 4 Myr to 600 Myr. Standard dust models used to reproduce the
Galactic diffuse medium are clearly not able to reproduce the dust emission in
the MIR wavelength domain. This analysis evidences the need of adjusting
parameters describing the dust size distribution and shows a clear distinct
behavior according to the type of environments. In addition, whereas the small
grain emission always seems to be negligible at long wavelengths in our Galaxy,
the contribution of this small dust component could be more important than
expected, in the submm-mm range, in the LMC averaged SED. Properties of the
small dust component of the LMC are clearly different from those of our Galaxy.
Its abundance, significantly enhanced, could be the result of large grains
shattering due to strong shocks or turbulence. In addition, this grain
component in the LMC systematically shows smaller grain size in the ionized
regions compared to the diffuse medium. Predictions of extinction curves show
significantly distinct behaviors depending on the dust models but also from one
region to another. Comparison of model predictions with the LMC mean extinction
curve shows that no model gives satisfactory agreement using the Mathis
radiation field while using a harder radiation field tends to improve the
agreementComment: Accepted for publication in A&
A CO survey on a sample of Herschel cold clumps
Context. The physical state of cold cloud clumps has a great impact on the process and efficiency of star formation and the masses of the forming stars inside these objects. The sub-millimetre survey of the Planck space observatory and the far-infrared follow-up mapping of the Herschel space telescope provide an unbiased, large sample of these cold objects. Aims. We have observed (CO)-C-12(1-0) and (CO)-C-13(1-0) emission in 35 high-density clumps in 26 Herschel fields sampling different environments in the Galaxy. Here, we aim to derive the physical properties of the objects and estimate their gravitational stability. Methods. The densities and temperatures of the clumps were calculated from both the dust continuum and the molecular line data. Kinematic distances were derived using (CO)-C-13(1-0) line velocities to verify previous distance estimates and the sizes and masses of the objects were calculated by fitting 2D Gaussian functions to their optical depth distribution maps on 250 mu m. The masses and virial masses were estimated assuming an upper and lower limit on the kinetic temperatures and considering uncertainties due to distance limitations. Results. The derived excitation temperatures are between 8.5-19.5 K, and for most clumps between 10 15 K, while the Herschel-derived dust colour temperatures are more uniform, between 12 16 K. The sizes (0.1-3 pc), (CO)-C-13 column densities (0.5-44 x 10(15) cm(-2)) and masses (from less than 0.1 M-circle dot to more than 1500 M-circle dot) of the objects all span broad ranges. We provide new kinematic distance estimates, identify gravitationally bound or unbound structures and discuss their nature. Conclusions. The sample contains objects on a wide scale of temperatures, densities and sizes. Eleven gravitationally unbound clumps were found, many of them smaller than 0.3 pc, but large, parsec-scale clouds with a few hundred solar masses appear as well. Colder clumps have generally high column densities but warmer objects appear at both low and higher column densities. The clump column densities derived from the line and dust observations correlate well, but are heavily affected by uncertainties of the dust properties, varying molecular abundances and optical depth effects.Peer reviewe
Revisiting the dust properties in the molecular clouds of the Large Magellanic Cloud
Context. Some Galactic molecular clouds show signs of dust evolution as compared to the diffuse interstellar medium, most of the time through indirect evidence such as color ratios, increased dust emissivity, or scattering (coreshine). These signs are not a feature of all Galactic clouds. Moreover, molecular clouds in the Large Magellanic Cloud (LMC) have been analyzed in a previous study based on Spitzer and IRIS data, at 4' angular resolution, with the use of one single dust model, and did not show any signs of dust evolution. Aims. In this present analysis we investigate the dust properties associated with the different gas phases (including the ionized phase this time) of the LMC molecular clouds at 1' angular resolution (four times greater than the previous analysis) and with a larger spectral coverage range thanks to Herschel data. We also ensure the robustness of our results in the framework of various dust models. Methods. We performed a decomposition of the dust emission in the infrared (from 3.6 to 500 mu m) associated with the atomic, molecular, and ionized gas phases in the molecular clouds of the LMC. The resulting spectral energy distributions were fitted with four distinct dust models. We then analyzed the model parameters such as the intensity of the radiation field and the relative dust abundances, as well as the slope of the emission spectra at long wavelengths. Results. This work allows dust models to be compared with infrared data in various environments for the first time, which reveals important differences between the models at short wavelengths in terms of data fitting (mainly in the polycyclic aromatic hydrocarbon bands). In addition, this analysis points out distinct results according to the gas phases, such as dust composition directly affecting the dust temperature and the dust emissivity in the submillimeter and different dust emission in the near-infrared (NIR). Conclusions. We observe direct evidence of dust property evolution from the diffuse to the dense medium in a large sample of molecular clouds in the LMC. In addition, the differences in the dust component abundances between the gas phases could indicate different origins of grain formation. We also point out the presence of a NIR-continuum in all gas phases, with an enhancement in the ionized gas. We favor the hypothesis of an additional dust component as the carrier of this continuum.Peer reviewe
Multiwavelength study of the high-latitude cloud L1642: chain of star formation
L1642 is one of the two high galactic latitude (|b| > 30deg) clouds confirmed
to have active star formation. We examine the properties of this cloud,
especially the large-scale structure, dust properties, and compact sources in
different stages of star formation. We present high-resolution far-infrared and
submm observations with the Herschel and AKARI satellites and mm observations
with the AzTEC/ASTE telescope, which we combined with archive data from near-
and mid-infrared (2MASS, WISE) to mm observations (Planck). The Herschel
observations, combined with other data, show a sequence of objects from a cold
clump to young stellar objects at different evolutionary stages. Source B-3
(2MASS J04351455-1414468) appears to be a YSO forming inside the L1642 cloud,
instead of a foreground brown dwarf, as previously classified. Herschel data
reveal striation in the diffuse dust emission around L1642. The western region
shows striation towards NE and has a steeper column density gradient on its
southern side. The densest central region has a bow-shock like structure
showing compression from the west and a filamentary tail extending towards
east. The differences suggest that these may be spatially distinct structures,
aligned only in projection. We derive values of the dust emission cross-section
per H nucleon for different regions of the cloud. Modified black-body fits to
the spectral energy distribution of Herschel and Planck data give emissivity
spectral index beta values 1.8-2.0 for the different regions. The compact
sources have lower beta values and show an anticorrelation between T and beta.
Markov chain Monte Carlo calculations demonstrate the strong anticorrelation
between beta and T errors and the importance of mm Planck data in constraining
the estimates. L1642 reveals a more complex structure and sequence of star
formation than previously known.Comment: 22 pages, 18 figures, accepted to Astronomy & Astrophysics; abstract
shortened and figures reduced for astrop
Submillimeter mapping and analysis of cold dust condensations in the Orion M42 star forming complex
We present here the continuum submillimeter maps of the molecular cloud
around the M42 Nebula in the Orion region. These have been obtained in four
wavelength bands (200, 260, 360 and 580 microns) with the ProNaOS two meter
balloon-borne telescope. The area covered is 7 parsecs wide (50 arcmin at a
distance of 470 pc) with a spatial resolution of about 0.4 parsec. Thanks to
the high sensitivity to faint surface brightness gradients, we have found
several cold condensations with temperatures ranging from 12 to 17 K, within 3
parsecs of the dense ridge. The statistical analysis of the temperature and
spectral index spatial distribution shows an evidence of an inverse correlation
between these two parameters. Being invisible in the IRAS 100 micron survey,
some cold clouds are likely to be the seeds for future star formation activity
going on in the complex. We estimate their masses and we show that two of them
have masses higher than their Jeans masses, and may be gravitationally
unstable.Comment: 4 figures, The Astrophysical Journal, Main Journal, in pres
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