1,833 research outputs found
N-body modeling of globular clusters: Masses, mass-to-light ratios and intermediate-mass black holes
We have determined the masses and mass-to-light ratios of 50 Galactic
globular clusters by comparing their velocity dispersion and surface brightness
profiles against a large grid of 900 N-body simulations of star clusters of
varying initial concentration, size and central black hole mass fraction. Our
models follow the evolution of the clusters under the combined effects of
stellar evolution and two-body relaxation allowing us to take the effects of
mass segregation and energy equipartition between stars self-consistently into
account. For a subset of 16 well observed clusters we also derive their
kinematic distances. We find an average mass-to-light ratio of Galactic
globular clusters of , which agrees very well with the
expected M/L ratio if the initial mass function of the clusters was a standard
Kroupa or Chabrier mass function. We do not find evidence for a decrease of the
average mass-to-light ratio with metallicity. The surface brightness and
velocity dispersion profiles of most globular clusters are incompatible with
the presence of intermediate-mass black holes (IMBHs) with more than a few
thousand in them. The only clear exception is Cen, where the
velocity dispersion profile provides strong evidence for the presence of a
40,000 IMBH in the centre of the cluster.Comment: 31 pages, 21 figures, accepted for publication in MNRA
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The Encyclopaedia of Literature in African Languages
.wav and .mp3 versions of audio recording of part of the lectureThe Encyclopaedia of Literature in African Languages (ELLAF) project focuses on oral and written literature in African languages. The ELLAF project proposes the creation of a website presenting and analysing literary texts in African languages, in order to make a wide range of these written or oral texts, in Sub-Saharan African and Malagasy languages, available to enthusiasts, students and specialists from around the world. The project aims to build up a research database based on literary works produced in their original languages, translated into French and/or English and presented in their linguistic, social and cultural contexts.The website signifies the creation of new tools for presenting and analysing textual data, centred on written/oral African literature in African languages regardless of their sociolinguistic status. This presentation favoured research through various fields (language, author, literary genre, predefined keyword and/or full text keyword) using a cross-disciplinary and comparative method. The method provides access to, for example: every text from a particular literature, the same genre found in several literatures, one theme in several literatures, a figurative element in several genres, notional degrees not necessarily made explicit by lexical occurrences. The material available here is ready for teaching straight away in regards to the transmission of factual knowledge: presentation of a language, overview of literature, contextualisation of literary production, bibliographic data. Furthermore, the bringing together of oral and written literary texts is not only of interest in a practical and documentary sense, but equally from a theoretical point of view.World Oral Literature Projec
The Global Mass Functions of 35 Galactic globular clusters: II. Clues on the Initial Mass Function and Black Hole Retention Fraction
In this paper we compare the mass function slopes of Galactic globular
clusters recently determined by Sollima & Baumgardt (2017) with a set of
dedicated N-body simulations of star clusters containing between 65,000 to
200,000 stars. We study clusters starting with a range of initial mass
functions (IMFs), black hole retention fractions and orbital parameters in the
parent galaxy. We find that the present-day mass functions of globular clusters
agree well with those expected for star clusters starting with Kroupa or
Chabrier IMFs, and are incompatible with clusters starting with single
power-law mass functions for the low-mass stars. The amount of mass segregation
seen in the globular clusters studied by Sollima & Baumgardt (2017) can be
fully explained by two-body relaxation driven mass segregation from initially
unsegregated star clusters. Based on the present-day global mass functions, we
expect that a typical globular cluster in our sample has lost about 75% of its
mass since formation, while the most evolved clusters have already lost more
than 90% of their initial mass and should dissolve within the next 1 to 2 Gyr.
Most clusters studied by Sollima & Baumgardt also show a large difference
between their central and global MF slopes, implying that the majority of
Galactic globular clusters is either near or already past core collapse. The
strong mass segregation seen in most clusters also implies that only a small
fraction of all black holes formed in globular clusters still reside in them.Comment: 8 pages, 6 figures, MNRAS, 472, 74
Testing Photometric Diagnostics for the Dynamical State and Possible IMBH presence in Globular Clusters
Surface photometry is a necessary tool to establish the dynamical state of
stars clusters. We produce realistic HST-like images from N-body models of star
clusters with and without central intermediate-mass black holes (IMBHs) in
order to measure their surface brightness profiles. The models contain ~600,000
individual stars, black holes of various masses between 0% to 2% of the total
mass, and are evolved for a Hubble time. We measure surface brightness and star
count profiles for every constructed image in order to test the effect of
intermediate mass black holes on the central logarithmic slope, the core
radius, and the half-light radius. We use these quantities to test diagnostic
tools for the presence of central black holes using photometry. We find that
the the only models that show central shallow cusps with logarithmic slopes
between -0.1 and -0.4 are those containing central black holes. Thus, the
central logarithmic slope seems to be a good way to choose clusters suspect of
containing intermediate-mass black holes. Clusters with steep central cusps can
definitely be ruled out to host an IMBH. The measured r_c/r_h ratio has similar
values for clusters that have not undergone core-collapse, and those containing
a central black hole. We notice that observed Galactic globular clusters have a
larger span of values for central slope and r_c/r_h than our modeled clusters,
and suggest possible reasons that could account for this and contribute to
improve future models.Comment: Accepted for publication in Ap
Dynamical Constraints on the Origin of Multiple Stellar Populations in Globular Clusters
We have carried out a large grid of N-body simulations in order to
investigate if mass-loss as a result of primordial gas expulsion can be
responsible for the large fraction of second generation stars in globular
clusters (GCs) with multiple stellar populations (MSPs). Our clusters start
with two stellar populations in which of all stars are second generation
stars. We simulate clusters with different initial masses, different ratios of
the half-mass radius of first to second generation stars, different primordial
gas fractions and Galactic tidal fields with varying strength. We then let our
clusters undergo primordial gas-loss and obtain their final properties such as
mass, half-mass radius and the fraction of second generation stars. Using our
N-body grid we then perform a Monte Carlo analysis to constrain the initial
masses, radii and required gas expulsion time-scales of GCs with MSPs. Our
results can explain the present-day properties of GCs only if (1) a substantial
amount of gas was present in the clusters after the formation of second
generation stars and (2) gas expulsion time-scales were extremely short
( yr). Such short gas expulsion time-scales are in agreement
with recent predictions that dark remnants have ejected the primordial gas from
globular clusters, and pose a potential problem for the AGB scenario. In
addition, our results predict a strong anti-correlation between the number
ratio of second-generation stars in GCs and the present-day mass of GCs. So
far, the observational data show only a significantly weaker anti-correlation,
if any at all.Comment: 14 pages, 6 figures, 3 tables, typos corrected. Accepted for
publication in MNRA
Proper Motions and Internal Dynamics in the Core of the Globular Cluster M71
We have used Gemini North together with the NIRI-ALTAIR adaptive optics
imager in the H and K bands to explore the core of the Galactic globular
cluster M71 (NGC 6838). We obtained proper motions for 217 stars and have
resolved its internal proper motion dispersion. Using a 3.8 year baseline, the
proper motion dispersion in the core is found to be 179 +/- 17 microarcsec/yr.
We find no evidence of anisotropy in the motions and no radial variation in the
proper motions with respect to distance from the cluster center. We also set an
upper limit on any central black hole to be ~150 Msun at 90% confidence level.Comment: 5 pages, 4 figures, accepted for publication in Astrophysical Journal
Letter
The evolution of two stellar populations in globular clusters I. The dynamical mixing timescale
We investigate the long-term dynamical evolution of two distinct stellar
populations of low-mass stars in globular clusters in order to study whether
the energy equipartition process can explain the high number of stars
harbouring abundance anomalies seen in globular clusters. We analyse N-body
models by artificially dividing the low-mass stars (m<0.9 Msun) into two
populations: a small number of stars (second generation) consistent with an
invariant IMF and with low specific energies initially concentrated towards the
cluster-centre mimic stars with abundance anomalies. These stars form from the
slow winds of fast-rotating massive stars. The main part of low-mass (first
generation) stars has the pristine composition of the cluster. We study in
detail how the two populations evolve under the influence of two-body elaxation
and the tidal forces due to the host galaxy.Stars with low specific energy
initially concentrated toward the cluster centre need about two relaxation
times to achieve a complete homogenisation throughout the cluster. For
realistic globular clusters, the number ratio between the two populations
increases only by a factor 2.5 due to the preferential evaporation of the
population of outlying first generation stars. We also find that the loss of
information on the stellar orbital angular momentum occurs on the same
timescale as spatial homogenisation.Comment: 9 pages, 9 figures, accepted for publication in A&A, references adde
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