79 research outputs found
Shaping Disk Galaxy Stellar Populations via Internal and External Processes
In recent years, effects such as the radial migration of stars in disks have been recognized as important drivers of the properties of stellar populations. Radial migration arises due to perturbative effects of disk structures such as bars and spiral arms, and can deposit stars formed in disks to regions far from their birthplaces. Migrant stars can significantly affect the demographics of their new locales, especially in low-density regions such as in the outer disks. However, in the cosmological environment, other effects such as mergers and filamentary gas accretion also influence the disk formation process. Understanding the relative importance of these processes on the detailed evolution of stellar population signatures is crucial for reconstructing the history of the Milky Way and other nearby galaxies. In the Milky Way disk in particular, the formation of the thickened component has recently attracted much attention due to its potential to serve as a diagnostic of the galaxy's early history. Some recent work suggests, however, that the vertical structure of Milky Way stellar populations is consistent with models that build up the thickened component through migration. I discuss these developments in the context of cosmological galaxy formatio
Circumstellar disks in high-mass star environments: the early solar system
The early solar system represents the only case we have of a circumstellar disk that can be investigated "in situ” -albeit 4.6 Gyr after its formation. Meteorites studies give mounting evidence for an intense irradiation phase of the young circumsolar disk by energetic particles, and also for contamination by products of high-mass stellar and/or explosive nucleosynthesis. We thus discuss the conditions of the birth of the solar system in a high-mass star environmen
Cosmic ray induced ionisation of a molecular cloud shocked by the W28 supernova remnant
Cosmic rays are an essential ingredient in the evolution of the interstellar
medium, as they dominate the ionisation of the dense molecular gas, where stars
and planets form. However, since they are efficiently scattered by the galactic
magnetic fields, many questions remain open, such as where exactly they are
accelerated, what is their original energy spectrum, and how they propagate
into molecular clouds. In this work we present new observations and discuss in
detail a method that allows us to measure the cosmic ray ionisation rate
towards the molecular clouds close to the W28 supernova remnant. To perform
these measurements, we use CO, HCO, and DCO millimetre line
observations and compare them with the predictions of radiative transfer and
chemical models away from thermodynamical equilibrium. The CO observations
allow us to constrain the density, temperature, and column density towards each
observed position, while the DCO/HCO abundance ratios provide us with
constraints on the electron fraction and, consequently, on the cosmic ray
ionisation rate. Towards positions located close to the supernova remnant, we
find cosmic ray ionisation rates much larger () than those in
standard galactic clouds. Conversely, towards one position situated at a larger
distance, we derive a standard cosmic ray ionisation rate. Overall, these
observations support the hypothesis that the rays observed in the
region have a hadronic origin. In addition, based on CR diffusion estimates, we
find that the ionisation of the gas is likely due to GeV cosmic rays.
Finally, these observations are also in agreement with the global picture of
cosmic ray diffusion, in which the low-energy tail of the cosmic ray population
diffuses at smaller distances than the high-energy counterpart.Comment: Accepted to A\&
Overview of dynamical mechanisms of secular evolution
Gravity-bound isolated systems, from stars, planetary systems, star clusters to galaxies, share common properties where evolution is the rule. Typically if they start forming at a well defined epoch they tend to change significantly over a timescale comparable to their present age. So evolution is never truly stopped, it just proceeds slower and slower: after a rapid, violent phase a slower, secular phase follows. In galactic astronomy for many decades the paradigm was rather that after a short violent time galaxies would settle in a stable steady state just consuming gas into stars. Actually today it appears that the progressive appearance of galaxy systematic morphologies and the slowing pace of mergers indicate that common intrinsic dynamical factors continue to shape galaxies towards similar properties irrespective of their largely different formation histories and initial conditions. Newtonian physics supplemented by a weakly dissipative component provides an amazing amount of explanations for the galaxy properties, like exponential stellar disks, spirals, bars, and peanut-shaped bulges. The purpose of this talk is to review these mechanisms of dynamical secular evolutio
Exploring the origin of neutron star magnetic field: magnetic properties of the progenitor OB stars
Ferrario & Wickramasinghe (2006) explored the hypothesis that the magnetic
fields of neutron stars are of fossil origin. In this context, they predicted
the field distribution of the progenitor OB stars, finding that 5 per cent of
main sequence massive stars should have fields in excess of 1kG. We have
carried out sensitive ESPaDOnS spectropolarimetric observations to search for
direct evidence of such fields in all massive B- and O-type stars in the Orion
Nebula Cluster star-forming region. We have detected unambiguous Stokes V
Zeeman signatures in spectra of three out of the eight stars observed (38%).
Using a new state-of-the-art Bayesian analysis, we infer the presence of strong
(kG), organised magnetic fields in their photospheres. For the remaining five
stars, we constrain any dipolar fields in the photosphere to be weaker than
about 200G. Statistically, the chance of finding three ~kG fields in a sample
of eight OB stars is quite low (less than 1%) if the predictions of Ferrario &
Wickramasinghe are correct. This implies that either the magnetic fields of
neutron stars are not of fossil origin, that the flux-evolution model of
Ferrario & Wickramasinghe is incomplete, or that the ONC has unusual magnetic
properties. We are undertaking a study of other young star clusters, in order
to better explore these possibilities.Comment: 40 Years of Pulsars conference: Millisecond Pulsars, Magnetars and
More. McGill University, Montreal, Canada, August 12-17, 2007. 5 pages, 4
figure
Million-Degree Plasma Pervading the Extended Orion Nebula
Most stars form as members of large associations within dense, very cold (10 to 100 kelvin)
molecular clouds. The nearby giant molecular cloud in Orion hosts several thousand stars of ages
less than a few million years, many of which are located in or around the famous Orion Nebula,
a prominent gas structure illuminated and ionized by a small group of massive stars (the Trapezium).
We present x-ray observations obtained with the X-ray Multi-Mirror satellite XMM-Newton, revealing
that a hot plasma with a temperature of 1.7 to 2.1 million kelvin pervades the southwest extension of
the nebula. The plasma flows into the adjacent interstellar medium. This x-ray outflow phenomenon
must be widespread throughout our Galaxy
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