55 research outputs found
Three-dimensional structure of the Upper Scorpius association with the Gaia first data release
Using new proper motion data from recently published catalogs, we revisit the
membership of previously identified members of the Upper Scorpius association.
We confirmed 750 of them as cluster members based on the convergent point
method, compute their kinematic parallaxes and combined them with Gaia
parallaxes to investigate the 3D structure and geometry of the association
using a robust covariance method. We find a mean distance of ~pc
and show that the morphology of the association defined by the brightest (and
most massive) stars yields a prolate ellipsoid with dimensions of
~pc, while the faintest cluster members define a more
elongated structure with dimensions of ~pc. We
suggest that the different properties of both populations is an imprint of the
star formation history in this region.Comment: 5 pages, 1 figure, MNRAS letters (in press
Hubble-Lema\^itre fragmentation and the path to equilibrium of merger-driven cluster formation
This paper discusses a new method to generate self-coherent initial
conditions for young substructured stellar cluster. The expansion of a uniform
system allows stellar sub-structures (clumps) to grow from fragmentation modes
by adiabatic cooling. We treat the system mass elements as stars, chosen
according to a Salpeter mass function, and the time-evolution is performed with
a collisional N-body integrator. This procedure allows to create a
fully-coherent relation between the clumps' spatial distribution and the
underlying velocity field. The cooling is driven by the gravitational field, as
in a cosmological Hubble-Lema\^itre flow. The fragmented configuration has a
`fractal'-like geometry but with a self-grown velocity field and mass profile.
We compare the characteristics of the stellar population in clumps with that
obtained from hydrodynamical simulations and find a remarkable correspondence
between the two in terms of the stellar content and the degree of spatial
mass-segregation. In the fragmented configuration, the IMF power index is ~0.3
lower in clumps in comparison to the field stellar population, in agreement
with observations in the Milky Way. We follow in time the dynamical evolution
of fully fragmented and sub-virial configurations, and find a soft collapse,
leading rapidly to equilibrium (timescale of 1 Myr for a ~ 10^4 Msun system).
The low-concentration equilibrium implies that the dynamical evolution
including massive stars is less likely to induce direct collisions and the
formation of exotic objects. Low-mass stars already ejected from merging clumps
are depleted in the end-result stellar clusters, which harbour a top-heavy
stellar mass function.Comment: 22 pages, accepted for publication in MNRA
Reflection Positivity and Monotonicity
We prove general reflection positivity results for both scalar fields and
Dirac fields on a Riemannian manifold, and comment on applications to quantum
field theory. As another application, we prove the inequality
between Dirichlet and Neumann covariance operators on a manifold with a
reflection.Comment: 11 page
The lower mass function of the young open cluster Blanco 1: from 30 Mjup to 3 Mo
18 pages, 15 figures and 5 tables accepted in A&AWe performed a deep wide field optical survey of the young (~100-150 Myr) open cluster Blanco1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.The survey covers 2.3 square degrees in the I and z-bands down to I~z~24 with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the (I,I-z) CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy. We find that our survey should contain about 57% of the cluster members in the 0.03-0.6 Mo mass range, including 30-40 brown dwarfs. The candidate's radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between 0.03Mo and 0.6Mo, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index alpha=0.69 +/- 0.15. Over the whole mass domain, from 0.03Mo to 3Mo, the mass function is better fitted by a log-normal function with m0=0.36 +/- 0.07Mo and sigma=0.58 +/- 0.06. Comparison between the Blanco1 mass function, other young open clusters' MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution
The Origin and Universality of the Stellar Initial Mass Function
We review current theories for the origin of the Stellar Initial Mass
Function (IMF) with particular focus on the extent to which the IMF can be
considered universal across various environments. To place the issue in an
observational context, we summarize the techniques used to determine the IMF
for different stellar populations, the uncertainties affecting the results, and
the evidence for systematic departures from universality under extreme
circumstances. We next consider theories for the formation of prestellar cores
by turbulent fragmentation and the possible impact of various thermal,
hydrodynamic and magneto-hydrodynamic instabilities. We address the conversion
of prestellar cores into stars and evaluate the roles played by different
processes: competitive accretion, dynamical fragmentation, ejection and
starvation, filament fragmentation and filamentary accretion flows, disk
formation and fragmentation, critical scales imposed by thermodynamics, and
magnetic braking. We present explanations for the characteristic shapes of the
Present-Day Prestellar Core Mass Function and the IMF and consider what
significance can be attached to their apparent similarity. Substantial
computational advances have occurred in recent years, and we review the
numerical simulations that have been performed to predict the IMF directly and
discuss the influence of dynamics, time-dependent phenomena, and initial
conditions.Comment: 24 pages, 6 figures. Accepted for publication as a chapter in
Protostars and Planets VI, University of Arizona Press (2014), eds. H.
Beuther, R. S. Klessen, C. P. Dullemond, Th. Hennin
The Monitor project: Rotation of low-mass stars in the open cluster M34
We report on the results of a V and i-band time-series photometric survey of
M34 (NGC 1039) using the Wide Field Camera (WFC) on the Isaac Newton Telescope
(INT), achieving better than 1% precision per data point for 13 <~ i <~ 17.
Candidate cluster members were selected from a V vs V-I colour-magnitude
diagram over 14 < V < 24 (0.12 <~ M/Msun <~ 1.0), finding 714 candidates, of
which we expect ~ 400 to be real cluster members (taking into account
contamination from the field). The mass function was computed, and found to be
consistent with a log-normal distribution in dN/dlogM. Searching for periodic
variable objects in the candidate members gave 105 detections over the mass
range 0.25 < M/Msun < 1.0. The distribution of rotation periods for 0.4 <
M/Msun < 1.0 was found to peak at ~ 7 days, with a tail of fast rotators down
to periods of ~ 0.8 days. For 0.25 < M/Msun < 0.4 we found a peak at short
periods, with a lack of slow rotators (eg. P >~ 5 days, consistent with the
work of other authors (eg. Scholz & Eisloffel 2004) at very low masses. Our
results are interpreted in the context of previous work, finding that we
reproduce the same general features in the rotational period distributions. A
number of rapid rotators were found with velocities ~ a factor of two lower
than in the Pleiades, consistent with models of angular momentum evolution
assuming solid body rotation without needing to invoke core-envelope
decoupling.Comment: 22 pages, 19 figures, 5 tables, MNRAS accepte
On the mass segregation of stars and brown dwarfs in Taurus
We use the new minimum spanning tree (MST) method to look for mass segregation in the Taurus association. The method computes the ratio of MST lengths of any chosen subset of objects, including the most massive stars and brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in the cluster. This mass segregation ratio (ΛMSR) enables a quantitative measure of the spatial distribution of high- and low-mass stars, and brown dwarfs to be made in Taurus. We find that the most massive stars in Taurus are inversely mass segregated with ΛMSR= 0.70 ± 0.10 (ΛMSR= 1 corresponds to no mass segregation), which differs from the strong mass segregation signatures found in more dense and massive clusters such as Orion. The brown dwarfs in Taurus are not mass segregated, although we find evidence that some low-mass stars are, with an ΛMSR= 1.25 ± 0.15. Finally, we compare our results to previous measures of the spatial distribution of stars and brown dwarfs in Taurus, and briefly discuss their implication
On the universal outcome of star-formation: Is there a link between stars and brown-dwarfs?
(abridged) The recent evidence obtained by Briceno et al. that star-formation
in Taurus-Auriga (TA) may be producing significantly fewer brown dwarfs (BDs)
per star than the ONC is investigated by setting up a realistic model stellar
plus BD population and explicitly taking into account a high binary proportion
and dynamical evolution in the TA groups and the ONC. The Briceno result is
reproduced almost exactly despite an identical IMF in both systems because many
BD-BD and star-BD binaries are disrupted in the ONC thus freeing BDs, while the
TA groups remain unevolved dynamically. However, the resulting populations do
not have the correct star-star, star-BD and expecially BD-BD binary properties,
even if a variable BD IMF is allowed for. The conclusion is therefore that BDs
need to be added as a separate population which has its own binary properties.
Such an extra population can have various origins which are briefly discussed
in this contribution but more fully in an associated paper.Comment: MNRAS, accepted, 23 pages, 14 figures, LaTeX, two references adde
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