825 research outputs found
Proper motion Pleiades candidate L-type brown dwarfs
We present results of an optical and near-infrared (IR) 1.8 deg^2 survey in
the Pleiades open cluster to search for substellar objects. From optical I-band
images from the CFHT and J-band images from the 3.5 m CAHA Telescope, we
identify 18 faint and very red L brown dwarf candidates, with I> 20.9 and I-J>
3.2. The follow-up observations of nine objects in the H- and Ks-bands confirm
that eight belong to the IR sequence of the cluster and the proper motion
measurements of seven candidates confirm that they are Pleiades members. A
preliminary estimation of the substellar mass spectrum dN/dM in the form of a
power law M^-alpha provides alpha=0.57+-0.14. We extrapolate this function to
estimate the number of very low-mass brown dwarfs and planetary mass objects
that could be present in the cluster down to 1 M_Jup. Sensitive searches
combining far red and near infrared observations may unveal these objects in a
near future.Comment: 2 pages, 2 figures, ULMSF Workshop, accepted for publication in
Astron. Nachrichte
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
Optimal Payoffs under State-dependent Preferences
Most decision theories, including expected utility theory, rank dependent
utility theory and cumulative prospect theory, assume that investors are only
interested in the distribution of returns and not in the states of the economy
in which income is received. Optimal payoffs have their lowest outcomes when
the economy is in a downturn, and this feature is often at odds with the needs
of many investors. We introduce a framework for portfolio selection within
which state-dependent preferences can be accommodated. Specifically, we assume
that investors care about the distribution of final wealth and its interaction
with some benchmark. In this context, we are able to characterize optimal
payoffs in explicit form. Furthermore, we extend the classical expected utility
optimization problem of Merton to the state-dependent situation. Some
applications in security design are discussed in detail and we also solve some
stochastic extensions of the target probability optimization problem
Young and embedded clusters in Cygnus-X: evidence for building up the IMF?
We provide a new view on the Cygnus-X north complex by accessing for the
first time the low mass content of young stellar populations in the region.
CFHT/WIRCam camera was used to perform a deep near-IR survey of this complex,
sampling stellar masses down to ~0.1 M. Several analysis tools,
including a extinction treatment developed in this work, were employed to
identify and uniformly characterise a dozen unstudied young star clusters in
the area. Investigation of their mass distributions in low-mass domain revealed
a relatively uniform log-normal IMF with a characteristic mass of 0.320.08
M and mass dispersion of 0.400.06. In the high mass regime, their
derived slopes showed that while the youngest clusters (age < 4 Myr) presented
slightly shallower values with respect to the Salpeter's, our older clusters (4
Myr < age < 18 Myr) showed IMF compliant values and a slightly denser stellar
population. Although possibly evidencing a deviation from an 'universal' IMF,
these results also supports a scenario where these gas dominated young clusters
gradually 'build up' their IMF by accreting low-mass stars formed in their
vicinity during their first ~3 Myr, before the gas expulsion phase, emerging at
the age of ~4 Myr with a fully fledged IMF. Finally, the derived distances to
these clusters confirmed the existence of at least 3 different star forming
regions throughout Cygnus-X north complex, at distances of 500-900 pc, 1.4-1.7
kpc and 3.0 kpc, and revealed evidence of a possible interaction between some
of these stellar populations and the Cygnus-OB2 association.Comment: 20 pages, 19 figures. Contains an appendix with 10 extra figure
The 0.03-10Mo mass function of young open clusters
We report the present day mass functions (PDMFs) of 3 young open clusters
over a mass range from 30 Jupiter masses to 10~\msunn. The PDMFs of the 3
clusters are remarkably similar, suggesting little impact of specific
conditions (stellar density, metallicity, early dynamical evolution) on the
mass distribution. Functional forms are provided to allow quantitative
comparison with MFs derived in other environments.Comment: 6 pages, to appear in "IMF@50", Corbelli, Palla, Zinnecker ed
The relation between accretion rates and the initial mass function in hydrodynamical simulations of star formation
We analyse a hydrodynamical simulation of star formation. Sink particles in
the simulations which represent stars show episodic growth, which is presumably
accretion from a core that can be regularly replenished in response to the
fluctuating conditions in the local environment. The accretion rates follow
, as expected from accretion in a gas-dominated
potential, but with substantial variations over-laid on this. The growth times
follow an exponential distribution which is tapered at long times due to the
finite length of the simulation. The initial collapse masses have an
approximately lognormal distribution with already an onset of a power-law at
large masses. The sink particle mass function can be reproduced with a
non-linear stochastic process, with fluctuating accretion rates , a distribution of seed masses and a distribution of growth times. All
three factors contribute equally to the form of the final sink mass function.
We find that the upper power law tail of the IMF is unrelated to Bondi-Hoyle
accretion.Comment: 13 pages, 13 figures, MNRAS accepte
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
Trade credit contracts : design and regulation
This paper provides a theoretical analysis of trade credit within a real options framework. We show that under trade credit the buyer delays the decision to stop production, getting closer to the supply chain optimal stopping decision. Therefore, trade credit may serve as a coordination device. The supplier can optimally choose to offer trade credit for free, since this will guarantee her business for a longer period of time. Optimal trade credit design is analyzed for an integrated supply chain (cooperative solution) and for external procurement (Nash bargaining and Stackelberg solutions). When regulation imposes a limit on trade credit maturity, the wholesale price is reduced, trade credit decreases and internal procurement increases. The model's predictions are in line with recent empirical evidence on the effects of regulation in the retail industry
A photometric and astrometric investigation of the brown dwarfs in Blanco 1
We present the results of a photometric and astrometric study of the low mass
stellar and substellar population of the young open cluster Blanco 1. We have
exploited J band data, obtained recently with the Wide Field Camera (WFCAM) on
the United Kingdom InfraRed Telescope (UKIRT), and 10 year old I and z band
optical imaging from CFH12k and Canada France Hawaii Telescope (CFHT), to
identify 44 candidate low mass stellar and substellar members, in an area of 2
sq. degrees, on the basis of their colours and proper motions. This sample
includes five sources which are newly discovered. We also confirm the lowest
mass candidate member of Blanco 1 unearthed so far (29MJup). We determine the
cluster mass function to have a slope of alpha=+0.93, assuming it to have a
power law form. This is high, but nearly consistent with previous studies of
the cluster (to within the errors), and also that of its much better studied
northern hemisphere analogue, the Pleiades.Comment: 8 Pages, 5 Figures, 2 Tables and 1 Appendix. Accepted for publication
in MNRA
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