863 research outputs found
On the origin of the distribution of binary-star periods
Pre-main sequence and main-sequence binary systems are observed to have
periods, P, ranging from one day to 10^(10) days and eccentricities, e, ranging
from 0 to 1. We pose the problem if stellar-dynamical interactions in very
young and compact star clusters may broaden an initially narrow period
distribution to the observed width. N-body computations of extremely compact
clusters containing 100 and 1000 stars initially in equilibrium and in cold
collapse are preformed. In all cases the assumed initial period distribution is
uniform in the narrow range 4.5 < log10(P) < 5.5 (P in days) which straddles
the maximum in the observed period distribution of late-type Galactic-field
dwarf systems. None of the models lead to the necessary broadening of the
period distribution, despite our adopted extreme conditions that favour
binary--binary interactions. Stellar-dynamical interactions in embedded
clusters thus cannot, under any circumstances, widen the period distribution
sufficiently. The wide range of orbital periods of very young and old binary
systems is therefore a result of cloud fragmentation and immediate subsequent
magneto-hydrodynamical processes operating within the multiple proto-stellar
system.Comment: 11 pages, 4 figures, ApJ, in pres
Nuclear embedded star clusters in NGC 7582
We report on the discovery of several compact regions of mid-infrared
emission in the starforming circum nuclear disk of the starburst/Seyfert2
galaxy NGC7582. The compact sources do not have counterparts in the optical and
near-infrared, suggesting that they are deeply embedded in dust. We use the
[NeII]12.8 micron line emission to estimate the emission measure of the ionized
gas, which in turn is used to assess the number of ionizing photons. Two of the
brighter sources are found to have ionizing fluxes of ~2.5x10^52, whereas the
fainter ones have ~1x10^52 photons/s. Comparing with a one Myr old starburst,
we derive stellar masses in the range (3-5)x10^5 Msun, and find that the number
of O-stars in each compact source is typically (0.6-1.6)x10^3. We conclude that
the compact mid-infrared sources are likely to be young, embedded star
clusters, of which only a few are known so far. Our observation highlights the
need for high resolution mid-infrared imaging to discover and study embedded
star clusters in the proximity of active galactic nuclei.Comment: 6 pages, 2 figures, accepted for publication in MNRAS Letter
The influence of gas expulsion and initial mass-segregation on the stellar mass-function of globular star clusters
Recently de Marchi, Paresce & Pulone (2007) studied a sample of twenty
globular clusters and found that all clusters with high concentrations have
steep stellar mass-functions while clusters with low concentration have
comparatively shallow mass-functions. No globular clusters were found with a
flat mass-function and high concentration. This seems curious since more
concentrated star clusters are believed to be dynamically more evolved and
should have lost more low-mass stars via evaporation, which would result in a
shallower mass-function in the low-mass part.
We show that this effect can be explained by residual-gas expulsion from
initially mass-segregated star clusters, and is enhanced further through
unresolved binaries. If gas expulsion is the correct mechanism to produce the
observed trend, then observation of these parameters would allow to constrain
cluster starting conditions such as star formation efficiency and the
time-scale of gas expulsion.Comment: accepted for publication in MNRAS, 10 pages, 6 figure
Star-forming accretion flows and the low luminosity nuclei of giant elliptical galaxies
The luminosities of the centers of nearby elliptical galaxies are very low
compared to models of thin disc accretion to their black holes at the Bondi
rate, typically a few hundredths to a few tenths of a solar mass per year. This
has motivated models of inefficiently-radiated accretion that invoke weak
electron-ion thermal coupling, and/or inhibited accretion rates due to
convection or outflows. Here we point out that even if such processes are
operating, a significant fraction of the accreting gas is prevented from
reaching the central black hole because it condenses into stars in a
gravitationally unstable disc. Star formation occurs inside the Bondi radius
(typically ~100pc in giant ellipticals), but still relatively far from the
black hole in terms of Schwarzschild radii. Star formation depletes and heats
the gas disc, eventually leading to a marginally stable, but much reduced,
accretion flow to the black hole. We predict the presence of cold (~100K),
dusty gas discs, containing clustered H-alpha emission and occasional type II
supernovae, both resulting from the presence of massive stars. Star formation
accounts for several features of the M87 system: a thin disc, traced by H-alpha
emission, is observed on scales of about 100pc, with features reminiscent of
spiral arms and dust lanes; the star formation rate inferred from the intensity
of H-alpha emission is consistent with the Bondi accretion rate of the system.
Star formation may therefore help suppress accretion onto the central engines
of massive ellipticals. We also discuss some implications for the fueling of
the Galactic center and quasars.Comment: 13 pages, accepted to MNRA
The Formation of Star Clusters II: 3D Simulations of Magnetohydrodynamic Turbulence in Molecular Clouds
(Abridged) We present a series of decaying turbulence simulations that
represent a cluster-forming clump within a molecular cloud, investigating the
role of magnetic fields on the formation of potential star-forming cores. We
present an exhaustive analysis of numerical data from these simulations that
includes a compilation of all of the distributions of physical properties that
characterize bound cores - including their masses, radii, mean densities,
angular momenta, spins, magnetizations, and mass-to-flux ratios. We also
present line maps of our models that can be compared with observations. Our
simulations range between 5-30 Jeans masses of gas, and are representative of
molecular cloud clumps with masses between 100-1000 solar masses. The cores
have mass-to-flux ratios that are generally less than that of the original
cloud, and so a cloud that is initially highly supercritical can produce cores
that are slightly supercritical, similar to that seen by Zeeman measurements of
molecular cloud cores. Clouds that are initially only slightly supercritical
will instead collapse along the field lines into sheets, and the cores that
form as these sheets fragment have a different mass spectrum than what is
observed. The spin rates of these cores suggests that subsequent fragmentation
into multiple systems is likely. The sizes of the bound cores that are produced
are typically 0.02-0.2 pc and have densities in the range 10^4-10^5 cm^{-3} in
agreement with observational surveys. Finally, our numerical data allow us to
test theoretical models of the mass spectrum of cores, such as the turbulent
fragmentation picture of Padoan-Nordlund. We find that while this model gets
the shape of the core mass spectrum reasonably well, it fails to predict the
peak mass in the core mass spectrum.Comment: Accepted by MNRAS. 28 pages, 16 figures. Substantial revision since
last versio
Detection Rates for Close Binaries Via Microlensing
Microlensing is one of the most promising methods of reconstructing the
stellar mass function down to masses even below the hydrogen-burning limit. The
fundamental limit to this technique is the presence of unresolved binaries,
which can in principle significantly alter the inferred mass function. Here we
quantify the fraction of binaries that can be detected using microlensing,
considering specifically the mass ratio and separation of the binary. We find
that almost all binary systems with separations greater than of
their combined Einstein ring radius are detectable assuming a detection
threshold of . For two M dwarfs, this corresponds to a limiting separation
of \gsim 1 \au. Since very few observed M dwarfs have companions at
separations \lsim 1 \au, we conclude that close binaries will probably not
corrupt the measurements of the mass function. We find that the detectability
depends only weakly on the mass ratio. For those events for which individual
masses can be determined, we find that binaries can be detected down to .Comment: 19 pages including 6 figures. Uses phyyzx format. Send requests for
higher quality figures to [email protected]
The Pleiades mass function: models versus observations
Two stellar-dynamical models of binary-rich embedded proto-Orion-Nebula-type
clusters that evolve to Pleiades-like clusters are studied with an emphasis on
comparing the stellar mass function with observational constraints. By the age
of the Pleiades (about 100 Myr) both models show a similar degree of mass
segregation which also agrees with observational constraints. This thus
indicates that the Pleiades is well relaxed and that it is suffering from
severe amnesia. It is found that the initial mass function (IMF) must have been
indistinguishable from the standard or Galactic-field IMF for stars with mass m
< 2 Mo, provided the Pleiades precursor had a central density of about 10^4.8
stars/pc^3. A denser model with 10^5.8 stars/pc^3 also leads to reasonable
agreement with observational constraints, but owing to the shorter relaxation
time of the embedded cluster it evolves through energy equipartition to a
mass-segregated condition just prior to residual-gas expulsion. This model
consequently looses preferentially low-mass stars and brown dwarfs (BDs), but
the effect is not very pronounced. The empirical data indicate that the
Pleiades IMF may have been steeper than Salpeter for stars with m > 2 Mo.Comment: 6 pages, 5 figures, accepted for publication in A&
Reverse dynamical evolution of Eta Chamaeleontis
In the scope of the star formation process, it is unclear how the environment
shapes the initial mass function (IMF). While observations of open clusters
propose a universal picture for the IMF from the substellar domain up to a few
solar masses, the young association eta Chamaeleontis presents an apparent lack
of low mass objects (m<0.1 Msun). Another unusual feature of this cluster is
the absence of wide binaries with a separation > 50 AU. We aim to test whether
dynamical evolution alone can reproduce the peculiar properties of the
association assuming a universal IMF. We use a pure N-body code to simulate the
dynamical evolution of the cluster for 10 Myr, and compare the results with
observations. A wide range of values for the initial parameters are tested in
order to identify the initial state that would most likely lead to
observations. In this context we also investigate the influence of the initial
binary population on the dynamics and the possibility of having a discontinuous
single IMF near the transition to the brown dwarf regime. We consider as an
extreme case an IMF with no low mass systems (m<0.1 Msun). The initial
configurations cover a wide range of initial density, from 10^2 to 10^8
stars/pc^3, in virialized, hot and cold dynamical state. We do not find any
initial state that would evolve from a universal single IMF to fit the
observations. Only when starting with a truncated IMF without any very low mass
systems and no wide binaries, can we reproduce the cluster core properties with
a success rate of 10% at best. Pure dynamical evolution alone cannot explain
the observed properties of eta Cha from universal initial conditions. The lack
of brown dwarfs and very low mass stars, and the peculiar binary properties
(low binary fraction and lack of wide binaries), are probably the result of the
star formation process in this association. (abridged)Comment: 13 pages, 8 figures, A&A accepte
The El Gordo galaxy cluster challenges {\Lambda}CDM for any plausible collision velocity
El Gordo (ACT-CL J0102-4915) is an extraordinarily large and bright galaxy
cluster collision. In a previous study, we found that El Gordo is in
tension with the CDM standard model when assuming the
nominal mass and infall velocity values from the hydrodynamical simulations of
Zhang et al. ( and , respectively). The recent weak lensing study of Kim
et al. showed that the mass of El Gordo is actually . Here we explore the level of tension between El
Gordo and CDM for the new mass estimate, assuming several
values. We find that in order to reduce the tension below
the level, the El Gordo subclusters should have (
when considering the combined tension with the Bullet Cluster). To the best of
our knowledge, the El Gordo hydrodynamical simulations conducted so far require
to simultaneously reproduce
its morphology and its high X-ray luminosity and temperature. We therefore
conclude that El Gordo still poses a significant challenge to CDM
cosmology. Whether the properties of El Gordo can be reconciled with a lower
should be tested with new hydrodynamical simulations that
explore different configurations of the interaction.Comment: 8 pages, 1 figure. Accepted for publication in The Astrophysical
Journal in this for
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