CITA Research September 2002 - August 2003
Interstellar Medium
Studies of the interstellar medium (ISM) address the diffuse gas and dust
between stars, and their role in the life cycles of stars and of galaxies.
Derived from the infall of gas from intergalactic space, the ISM is the
substrate from which stars condense and into which stellar winds and
supernovae expand. All aspects of galactic evolution -- the way galaxies
appear from afar, the rate at which they form stars of various types, and
the manner in which they intensify their internal magnetic fields -- are
manifestations of how the ISM reacts to internal forces (e.g., supernovae)
and to external forces (e.g., collisions between galaxies). Despite fifty
years of progress, many open questions remain: for instance, how do
newborn stars affect the gas clouds from which they formed? What is the
mechanism by which dust grains align with the magnetic field? How can
this effect be used to interpret observations of dense, cold molecular
clouds? What are the basic dynamics and structure of these objects, from
which stars form? In 2002, CITAzens have been addressing these questions
with techniques ranging from fundamental physics, to analytical and
numerical modeling of important phenomena, to the detailed investigation
of individual regions, as outlined below.
Study of interstellar turbulence using spectro-imagery observations of HI clouds
In collaboration with F. Levrier and E. Falgarone (ENS, France),
Marc-Antoine Miville-Deschenes used fractional Brownian motion (fBm)
simulations to study how the statistical properties of 3D density and
velocity fields can be retrieved from spectro-imagery observations of
an optically thin tracer. Miville-Deschenes showed that for typical
turbulent power spectra, the centroid velocity field of
spectro-imagery data allows one to retrieve the power spectrum of the
3D velocity field and therefore determine directly the energy spectrum
of interstellar turbulence.
With G. Joncas (Laval), F. Boulanger (IAS, France) and E. Falgarone
(ENS, France), Marc-Antoine Miville-Deschenes used 21 cm
interferometric observations to determine the power spectrum of the 3D
density and velocity field in a local HI cloud (the Ursa Major
cirrus), over 3 orders of magnitude in scale. This work shows that
interstellar turbulence has a similar power spectrum than
incompressible turbulence (Kolmogorov). This result puts important
constraints on the impact of compressibility and magnetic field in MHD
turbulent fluids.
In collaboration with A. Daigle and G. Joncas (Laval), Marc-Antoine
Miville-Deschenes works on the use of neural network to detect HI
expanding shells in 21 cm spectro-imagery observations. Using fBm
simulations they showed that turbulence does not produce significant
spurious detections.
Dust evolution in the ISM
Marc-Antoine Miville-Deschenes conducted several projects on dust
evolution in the ISM using Infrared Space Observatory (ISO) data,
including work with the ISOGAL team that published catalogs of points
sources and diffuse images of Galactic plane regions observed with
ISO, but also a study of the PAH properties in a star-forming core in
the Trifid Nebula, in collaboration with B. Lefloch (Grenoble) and
J. Cernicharo (Madrid). Miville-Deschenes also participated in the
discovery in L134N of one of the coldest core found in molecular
clouds (dust temperature ~ 7.6K). This work, done in collaboration
with L. Pagani (Paris), brings very interesting insights on dust
evolution scenari (formation of fractal aggregates) and really
challenges dust theory.
Miville-Deschenes also used dust models and radiative transfer code
to characterize dust properties in diffuse interstellar clouds using
infrared observations. With M. Sc. student F. Fressin he showed that
the smallest dust grains (PAH-like) coagulate on bigger grains very
rapidly in the gas condensation process. This has an impact on our
understanding of the thermodynamical and chemical scenario of the
formation of long-lived molecular structures.
Component separation
Because of his interest in diffuse interstellar clouds, Marc-Antoine
Miville-Deschenes also worked on component separation which is
essential for the detection and characterization of extra-galactic and
cosmological signals. In particular he made the first detection of the
cosmic infrared background fluctuations at 60 and 100 micron using
IRAS data, in collaboration with G. Lagache and J.-L. Puget (IAS,
France).
Miville-Deschenes is involved in several projects devoted to the
observation of very low column density HI clouds, to characterize
their gas and dust properties and to evaluate the possibility of using
21 cm observations to remove foreground emissions. These observations
are done with the DRAO (Penticton, Canada) and GBT (Virginia, USA)
radio-telescopes, and with SIRTF. This work is done in collaboration
with P. Martin (CITA), J. Lockman (NRAO) and the IAS team (France).
Dynamics of grains exposed to radiative torques
Weingartner and Draine (Princeton) have developed a method for
studying the dynamics of grains exposed to radiative torques (due to
the absorption and scattering of starlight by an irregular grain) that
includes the effects of thermal fluctuations coupling the grain
magnetism and rotation. They find that inclusion of thermal
fluctuations can lead to qualitative changes in the dynamics from
previous treatments that did not take this effect into account.
Radiative torques are thought to play a dominant role in the alignment
of grains with the interstellar magnetic field, and future work
employing the method developed here will explore this possibility in
detail.
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