9,401 research outputs found
The mass function and dynamical mass of young star clusters: Why their initial crossing-time matters crucially
We highlight the impact of cluster-mass-dependent evolutionary rates upon the
evolution of the cluster mass function during violent relaxation, that is,
while clusters dynamically respond to the expulsion of their residual
star-forming gas. Mass-dependent evolutionary rates arise when the mean volume
density of cluster-forming regions is mass-dependent. In that case, even if the
initial conditions are such that the cluster mass function at the end of
violent relaxation has the same shape as the embedded-cluster mass function
(i.e. infant weight-loss is mass-independent), the shape of the cluster mass
function does change transiently {\it during} violent relaxation. In contrast,
for cluster-forming regions of constant mean volume density, the cluster mass
function shape is preserved all through violent relaxation since all clusters
then evolve at the same mass-independent rate.
On the scale of individual clusters, we model the evolution of the ratio
between the dynamical mass and luminous mass of a cluster after gas expulsion.
Specifically, we map the radial dependence of the time-scale for a star cluster
to return to equilibrium. We stress that fields-of-view a few pc in size only,
typical of compact clusters with rapid evolutionary rates, are likely to reveal
cluster regions which have returned to equilibrium even if the cluster
experienced a major gas expulsion episode a few Myr earlier. We provide models
with the aperture and time expressed in units of the initial half-mass radius
and initial crossing-time, respectively, so that our results can be applied to
clusters with initial densities, sizes, and apertures different from ours.Comment: 14 pages, 10 figures, accepted for publication in MNRA
The puzzle of the cluster-forming core mass-radius relation and why it matters
We highlight how the mass-radius relation of cluster-forming cores combined
with an external tidal field can influence infant weight-loss and disruption
likelihood of clusters after gas expulsion. Specifically, we study how the
relation between the bound fraction of stars staying in clusters at the end of
violent relaxation and the cluster-forming core mass is affected by the slope
and normalization of the core mass-radius relation. Assuming mass-independent
star formation efficiency and gas-expulsion time-scale
and a given external tidal field, it is found that
constant surface density cores and constant radius cores have the potential to
lead to the preferential removal of high- and low-mass clusters, respectively.
In contrast, constant volume density cores result in mass-independent cluster
infant weight-loss, as suggested by observations. Our modelling includes
predictions about the evolution of high-mass cluster-forming cores, a regime
not yet covered by the observations. An overview of various issues directly
affected by the nature of the core mass-radius relation is presented (e.g.
cluster mass function, galaxy star formation histories, globular cluster
self-enrichment). Finally, we emphasize that observational mass-radius
data-sets of dense gas regions must be handled with caution as they may be the
imprint of the molecular tracer used to map them, rather than reflecting
cluster formation conditions. [Abridged]Comment: 14 pages, 7 figures, accepted to MNRA
Research Diary Visual Mapping : a reflective methodological tool for process and strategy-as-practice studies
Balogun, Huff and Johnson (2003) highlight the growing paradox for researchers who must focus on context and details while favouring general lines of research. These authors focus their reflection around the collection of qualitative data, particularly those of discussion groups, collaborative research and of research journal redaction techniques. We propose, in the context of collaborative research, a new utilisation of the personal diary, fuelled by our doctoral experiences in collaborative research. While the personal diary in its usual form increases the level of reflectivity on an intervening process, it is nevertheless difficult to exploit for the work of interpreting and legitimizing research. We therefore propose personal diary mapping. In addition to the advantages of personal diary mapping as a methodological tool for viewing the phenomenon, it allows a process to be described by highlighting specifics that are not obvious in reading a text. Moreover, the process of personal diary mapping provides a contribution to the epistemic work in a constructivist reference because it helps make the relationship between knowledge and empirical information explicit (Martinet 2007). After a summary bringing process studies closer to SaP and a review of the modalities of action research and their implications in terms of ethics and researcher responsibility, we present the origins, principles and benefits of visual mapping as regards the researcher's responsibility. In a second step, we illustrate the normative elements of this approach through a case study on strategic competence development based on personal diary mapping.Research Diary ; Visual Mapping ; methodological tool ; process ; strategy-as-practice
Normative Power, EU Preferences and Russia. Lessons from the Russian-Georgian War
The Russian-Georgian conflict of August 2008 proves to be a useful case in order to understand the functioning of the EU as a normative power in times of crisis. The core of the article is focused on the six EU major countries – Germany, France, Great-Britain, Italy, Poland and Spain – which embody different sets of preferences, and the way they want to deal with Russia. In the end, it tries to understand how preferences are linked with norms in that geopolitical context.Normative power, Georgian-Russian war, Mediation, Sanctions
The European Neighbourhood Policy as a Process of Democratic Norms Diffusion in Ukraine. The Case of the EU Act Beyond Conditionality?
This paper is then focused on the European Neighbourhood Policy and democracypromotion in its Eastern neighbours. Can the EU act beyond ‘hard conditionality’ experiencedduring the enlargement process in order to succeed in democratizing its neighbourhood? Whatare the obstacles to democratisation and political change within the neighbours? At the heart of the analysis lies the question of efficiency of the ENP without hardconditionality linked with adhesion. Conditionality, i.e. linking the granting of benefits to thefulfilment of some conditions, has been at the core of the EU policy when dealing withaccession candidate countries. During the enlargement process, ‘hard conditionality’(describing measures a country must meet in order to access any money or politicalretribution) was the key means for democratic norms diffusion. By contrast, neighbourhoodpolicy uses conditionality in a more flexible way, which is tantamount to ‘soft conditionality’,destined to influence policies but not to propose a one-fits-for-all policy. It is often arguedthat because of the lack of an effective or ‘hard’ conditionality, the EU cannot pretend to amajor role as a promoter of democratic norms.enlargement
The effects of the marginal tax rate in a matching model with endogenous labor supply
This paper analyzes the effects of the marginal tax rate on unemployment and economic efficiency in a matching model with homogenous agents when wages and working hours are bargained over. I show that the theoretical impact of a higher marginal tax rate on unemployment is ambiguous whatever the instantaneous utility in unemployment i.e. for an utility in unemployment that is either fixed or perfectly indexed on net wages. These results are in sharp contrast with the literature. Numerical simulations applied to France suggest that a higher marginal tax rate generally reduces the unemployment rate but at the expense of lower economic efficiency. The simulations point also out that the relation between the optimal marginal tax rate and the elasticity of labor supply is not monotonic.Matching model; Marginal Tax Rate; Labor supply; Utility in unemployment
The Origin of the Gaussian Initial Mass Function of Old Globular Cluster Systems
[Abridged] Evidence favouring a Gaussian initial globular cluster mass
function has accumulated over recent years. We show that an approximately
Gaussian mass function is naturally generated from a power-law mass
distribution of protoglobular clouds by expulsion from the protocluster of star
forming gas due to supernova activity, provided that the power-law mass
distribution shows a lower-mass limit. As a result of gas loss, the
gravitational potential of the protocluster gets weaker and only a fraction of
the newly formed stars is retained. The mass fraction of bound stars ranges
from zero to unity, depending on the local star formation efficiency
. Assuming that is independent of the protoglobular cloud
mass, we investigate how such variations affect the mapping of a protoglobular
cloud mass function to the resulting globular cluster initial mass function. A
truncated power-law cloud mass spectrum generates bell-shaped cluster initial
mass functions, with a turnover location mostly sensitive to the lower limit of
the cloud mass range. We also show that a Gaussian mass function for the
protoglobular clouds with a mean and a standard
deviation provides results very similar to those
resulting from a truncated power-law cloud mass spectrum, that is, the
distribution function of masses of protoglobular clouds influences only weakly
the shape of the resulting globular star cluster initial mass function. The gas
removal process and the protoglobular cloud mass-scale dominate the relevant
physics. Moreover, gas removal during star formation in massive clouds is
likely the prime cause of the predominance of field stars in the Galactic halo.Comment: 24 pages, accepted for publication in MNRA
A non-grey analytical model for irradiated atmospheres. I: Derivation
Context. Semi-grey atmospheric models (with one opacity for the visible and
one opacity for the infrared) are useful to understand the global structure of
irradiated atmospheres, their dynamics and the interior structure and evolution
of planets, brown dwarfs and stars. But when compared to direct numerical
radiative transfer calculations for irradiated exoplanets, these models
systematically overestimate the temperatures at low optical depth,
independently of the opacity parameters. We wish to understand why semi-grey
models fail at low optical depths, and provide a more accurate approximation to
the atmospheric structure by accounting for the variable opacity in the
infrared. Our analytical irradiated non-grey model is found to provide a range
of temperatures that is consistent with that obtained by numerical
calculations. We find that even for slightly non-grey thermal opacities the
temperature structure differs significantly from previous semi-grey models. For
small values of beta (expected when lines are dominant), we find that the
non-grey effects are confined to low-optical depths. However, for beta larger
than 0.5 (appropriate in the presence of bands with a wavelength-dependence
smaller or comparable with the width of the Planck function), we find that the
temperature structure is affected even down to infrared optical depths unity
and deeper as a result of the so-called blanketing effect. The expressions that
we derive may be used to provide a proper functional form for algorithms that
invert the atmospheric properties from spectral information. Because a full
atmospheric structure can be calculated directly, these expressions should be
useful for simulations of the dynamics of these atmospheres and of the thermal
evolution of the planets. Finally, they should be used to test full radiative
transfer models and improve their convergence.Comment: Accepted by A&A, model available at
http://www.oca.eu/parmentier/nongre
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