122 research outputs found
Instabilities in multi-asset and multi-agent market impact games
We consider the general problem of a set of agents trading a portfolio of assets in the presence of transient price impact and additional quadratic transaction costs and we study, with analytical and numerical methods, the resulting Nash equilibria. Extending significantly the framework of Schied and Zhang (2019) and Luo and Schied (2020), who considered the single asset case, we prove the existence and uniqueness of the corresponding Nash equilibria for the related mean-variance optimization problem. We then focus our attention on the conditions on the model parameters making the trading profile of the agents at equilibrium, and as a consequence the price trajectory, wildly oscillating and the market unstable. While Schied and Zhang (2019) and Luo and Schied (2020) highlighted the importance of the value of transaction cost in determining the transition between a stable and an unstable phase, we show that also the scaling of market impact with the number of agents J and the number of assets M determines the asymptotic stability (in J and M) of markets
Extended main sequence turnoff as a common feature of Milky Way open clusters
We present photometric analysis of twelve Galactic open clusters and show
that the same multiple-population phenomenon observed in Magellanic Clouds
(MCs) is present in nearby open clusters. Nearly all the clusters younger than
2.5 Gyr of both MCs exhibit extended main-sequence turnoffs (eMSTOs) and
all the cluster younger than 700 Myr show broadened/split main sequences
(MSs). High-resolution spectroscopy has revealed that these clusters host stars
with a large spread in the observed projected rotations. In addition to
rotation, internal age variation is indicated as a possible responsible for the
eMSTOs, making these systems the possible young counterparts of globular
clusters with multiple populations. Recent work has shown that the
eMSTO+broadened MSs are not a peculiarity of MCs clusters. Similar photometric
features have been discovered in a few Galactic open clusters, challenging the
idea that the color-magnitude diagrams (CMDs) of these systems are similar to
single isochrones and opening new windows to explore the eMSTO phenomenon. We
exploit photometry+proper motions from Gaia DR2 to investigate the CMDs of open
clusters younger than 1.5 Gyr. Our analysis suggests that: (i) twelve
open clusters show eMSTOs and/or broadened MSs, that cannot be due neither to
field contamination, nor binaries; (ii) split/broadened MSs are observed in
clusters younger than 700 Myr, while older objects display only an eMSTO,
similarly to MCs clusters; (iii) the eMSTO, if interpreted as a pure age
spread, increases with age, following the relation observed in MCs clusters and
demonstrating that rotation is the responsible for this phenomenon.Comment: 17 pages, 42 figures, 1 table, accepted for publication in ApJ
(31/10/2018
Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419
Recent work revealed that both the helium variation within globular clusters
(GCs) and the relative numbers of first and second-generation stars (1G, 2G)
depend on the mass of the host cluster. Precise determination of the internal
helium variations and of the fraction of 1G stars are crucial constraints to
the formation scenarios of multiple populations (MPs). We exploit multi-band
Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one
of the most-massive and distant GCs of the Galaxy, almost isolated from its
tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and
identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which
comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the
observed colors of these four populations with the colors derived from
appropriate synthetic spectra to infer the relative helium abundances. We find
that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY
~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium
(Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most
of the current scenarios for MPs. Furthermore, the relatively larger fraction
of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting
literature results, we find that the fractions of 1G stars of GCs with large
perigalactic distance are typically higher than in the other GCs with similar
masses. This suggests that NGC 2419, similarly to other distant GCs, lost a
lower fraction of 1G stars.Comment: 10 pages, 8 figures, submitted to MNRAS January 22n
The Hubble Space Telescope UV Legacy Survey of Galactic Globular Clusters. XIX. A Chemical Tagging of the Multiple Stellar Populations Over the Chromosome Maps
The HST UV Survey of Globular Clusters (GCs) has investigated GCs and their
stellar populations. In previous papers of this series we have introduced a
pseudo two-color diagram, "chromosome map" (ChM), that maximises the separation
between the multiple populations. We have identified two main classes of GCs:
Type I (~83% of the objects) and Type II, both hosting two main groups of
stars, referred to in this series as first (1G) and second generation (2G).
Type II clusters exhibit two or more parallel sequences of 1G and 2G stars in
their ChMs. We exploit elemental abundances from literature to assign the
chemical composition to the distinct populations as identified on the ChMs of
29 GCs. We find that stars in different regions of the ChM have different
composition: 1G stars share the same light-element content as field stars,
while 2G stars are enhanced in N, Na and depleted in O. Stars enhanced in Al
and depleted in Mg populate the extreme regions of the ChM. We investigate the
color spread among 1G stars observed in many GCs, and find no evidence for
variations in light elements, whereas either a 0.1 dex Fe spread or a variation
in He remain to be verified. In the attempt of analysing the global properties
of the multiple populations, we have constructed a universal ChM, which
highlights that, though variegate, the phenomenon has some common pattern. The
universal ChM reveals a tight connection with Na, for which we have provided an
empirical relation. The additional ChM sequences typical of Type II GCs are
enhanced in metallicity and, often, in s elements. Omega Cen can be classified
as an extreme Type II GC, with a ChM displaying three main streams, each with
its own variations in chemical abundances. One of the most noticeable
differences is between the lower and upper streams, with the latter (associated
with higher He) having higher Fe and lower Li. We publicly release ChMs.Comment: 35 pages, 28 figures, 3 tables. Submitted to MNRA
Mass loss of different stellar populations in Globular Clusters: the case of M4
In a Globular Cluster (GC), the mass loss during the red-giant branch (RGB)
phase and the helium content are fundamental ingredients to constrain the
horizontal branch (HB) morphology. While many papers have been dedicated to the
helium abundance in the different stellar populations, small efforts have been
done to disentangle the effects of mass loss and helium content. We exploit the
nearby GC NGC6121 (M4), which hosts two well-studied main stellar populations,
to infer both helium and RGB mass loss. We combine multi-band Hubble Space
Telescope photometry of RGB and main sequence (MS) stars of M4 with synthetic
spectra to constrain the relative helium content of its stellar populations. We
find that the second generation stars in M4 is enhanced in helium mass fraction
by with respect to the remaining stars that
have pristine helium content. We then infer the mass of the HB stars by
searching for the best match between the observations and HB populations
modelled assuming the helium abundance of each population estimated from the
MS. By comparing the masses of stars along the HB, we constrain the mass loss
of first- and second-generation stars in M4. We find that the mass lost by the
helium enriched population is % larger than the mass lost by the first
generation stars (). We discuss the
possibility that this mass loss difference depends on helium abundance, the
different formation environment of the two generations, or a combination of
both.Comment: 10 pages, 4 figures, 1 table. Accepted for publication in Ap
The Generalized Stochastic Microdosimetric Model: the main formulation
The present work introduces a rigorous stochastic model, named Generalized
Stochastic Microdosimetric Model (GSM2), to describe biological damage induced
by ionizing radiation. Starting from microdosimetric spectra of energy
deposition in tissue, we derive a master equation describing the time evolution
of the probability density function of lethal and potentially lethal DNA damage
induced by radiation in a cell nucleus. The resulting probability distribution
is not required to satisfy any a priori assumption. Furthermore, we generalized
the master equation to consider damage induced by a continuous dose delivery.
In addition, spatial features and damage movement inside the nucleus have been
taken into account. In doing so, we provide a general mathematical setting to
fully describe the spatiotemporal damage formation and evolution in a cell
nucleus. Finally, we provide numerical solutions of the master equation
exploiting Monte Carlo simulations to validate the accuracy of GSM2.
Development of GSM2 can lead to improved modeling of radiation damage to both
tumor and normal tissues, and thereby impact treatment regimens for better
tumor control and reduced normal tissue toxicities
On the role of dust and mass loss in the extended main sequence turnoff of star clusters: the case of NGC 1783
The Color Magnitude Diagram (CMD) morphology of the "extended" main sequence
turnoff (eMSTO) and upper main sequence (MS) of the intermediate age ( Gyr) Large Magellanic Cloud Cluster NGC 1783 shows the presence of a small
group of UV-dim stars, that, in the ultraviolet Hubble Space Telescope filters,
are located at colors on the red side of the typical "fan" shape displayed by
the eMSTO. We model the UV-dim stars by assuming that some of the stars which
would intrinsically be located on the left side of the eMSTO are obscured by a
ring of dust due to grain condensation at the periphery of the excretion disc
expelled when they spin at the high rotation rates typical of stars in the Be
stage. A reasonably low optical depth at 10 is necessary to model the
UV-dim group. Introduction of dust in the interpretation of the eMSTO may
require a substantial re-evaluation of previous conclusions concerning the role
of age and/or rotation spreads in the MC clusters: the entire eMSTO can be
populated by dusty stars, and the reddest UV-dim stars simply represents the
tail of the distribution with both maximum obscuration and the dust ring seen
along the line of sight. The model stars having higher rotational projected
velocity ( i) are predicted to be preferentially redder than the
slowly-rotating stars. The mass loss responsible for the dust may also cause
the non-monotonic distribution of stars in the upper main sequence, with two
peaks and gaps showing up in the UV CMD.Comment: 11 pages, 13 figures. This is a pre-copyedited, author-produced PDF
of an article (stad851) accepted for publication in MNRAS following peer
revie
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