Abstract
Models aiming to explain the formation of massive black hole seeds, and in particular the direct collapse scenario, face substantial difficulties. These are rooted in rather ad hoc and ...fine-tuned initial conditions, such as the simultaneous requirements of extremely low metallicities and strong radiation backgrounds. Here, we explore a modification of such scenarios where a massive primordial star cluster is initially produced. Subsequent stellar collisions give rise to the formation of massive (104−105 M⊙) objects. Our calculations demonstrate that the interplay among stellar dynamics, gas accretion, and protostellar evolution is particularly relevant. Gas accretion on to the protostars enhances their radii, resulting in an enhanced collisional cross-section. We show that the fraction of collisions can increase from 0.1 to 1 per cent of the initial population to about 10 per cent when compared to gas-free models or models of protostellar clusters in the local Universe. We conclude that very massive objects can form in spite of initial fragmentation, making the first massive protostellar clusters viable candidate birth places for observed supermassive black holes.
Fragmentation often occurs in disk-like structures, both in the early Universe and in the context of present-day star formation. Supermassive black holes (SMBHs) are astrophysical objects whose ...origin is not well understood; they weigh millions of solar masses and reside in the centers of galaxies. An important formation scenario for SMBHs is based on collisions and mergers of stars in a massive cluster with a high stellar density, in which the most massive star moves to the center of the cluster due to dynamical friction. This increases the rate of collisions and mergers since massive stars have larger collisional cross sections. This can lead to a runaway growth of a very massive star which may collapse to become an intermediate-mass black hole. Here we investigate the dynamical evolution of Miyamoto-Nagai models that allow us to describe dense stellar clusters, including flattening and different degrees of rotation. We find that the collisions in these clusters depend mostly on the number of stars and the initial stellar radii for a given radial size of the cluster. By comparison, rotation seems to affect the collision rate by at most 20%. For flatness, we compared spherical models with systems that have a scale height of about 10% of their radial extent, in this case finding a change in the collision rate of less than 25%. Overall, we conclude that the parameters only have a minor effect on the number of collisions. Our results also suggest that rotation helps to retain more stars in the system, reducing the number of escapers by a factor of 2−3 depending on the model and the specific realization. After two million years, a typical lifetime of a very massive star, we find that about 630 collisions occur in a typical models with
N
= 10
4
,
R
= 100
R
⊙
and a half-mass radius of 0.1 pc, leading to a mass of about 6.3 × 10
3
M
⊙
for the most massive object. We note that our simulations do not include mass loss during mergers or due to stellar winds. On the other hand, the growth of the most massive object may subsequently continue, depending on the lifetime of the most massive object.
ABSTRACT
While large numbers of supermassive black holes have been detected at z > 6, their origin is still essentially unclear. Numerical simulations have shown that the conditions for the classical ...direct collapse scenario are very restrictive and fragmentation is very difficult to be avoided. We thus consider here a more general case of a dense massive protostar cluster at low metallicity (≲10−3 Z⊙) embedded in gas. We estimate the mass of the central massive object, formed via collisions and gas accretion, considering the extreme cases of a logarithmically flat and a Salpeter-type initial mass function. Objects with masses of at least 104 M⊙ could be formed for inefficient radiative feedback, whereas ∼103 M⊙ objects could be formed when the accretion time is limited via feedback. These masses will vary depending on the environment and could be considerably larger, particularly due to the continuous infall of gas into the cloud. As a result, one may form intermediate mass black holes of ∼104 M⊙ or more. Upcoming observations with the James Webb Space Telescope and other observatories may help us to detect such massive black holes and their environment, thereby shedding additional light on such a formation channel.
ABSTRACT
The centres of galaxies host nuclear stellar clusters, supermassive black holes, or both. The origin of this dichotomy is still a mystery. Nuclear stellar clusters are the densest stellar ...system in the Universe, so they are ideal places for runaway collisions to occur. Previous studies have proposed the possible existence of a critical mass scale in such clusters, for which the occurrence of collisions becomes very frequent and leads to the formation of a very massive object. While it is difficult to directly probe this scenario with simulations, we here aim for a proof of concept using toy models where the occurrence of such a transition is shown based on simplified compact systems, where the typical evolution time-scales will be faster compared to the real Universe. Indeed our simulations confirm that such a transition takes place and that up to 50 per cent of the cluster mass can go into the formation of a central massive object for clusters that are above the critical mass scale. Our results thus support the proposed new scenario on the basis of idealized simulations. A preliminary analysis of observed nuclear star clusters shows similar trends related to the critical mass as in our simulations. We further discuss the caveats for the application of the proposed scenario in real nuclear star clusters.
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•Invasive Acacia dealbata tree causes environmental problems in their non native area.•Bark, wood, leaves and flowers of this renewable source offers bioactive compounds.•Conventional ...and novel extraction methods are useful to develop biorefinery based processes.•Bioethanol and adsorbents can be produced from this wild biomass.•dealbata’ biocompounds can be added to food, cosmetic or pharmaceutical matrices.
The forest wastes removed as a strategy to control the invasive shrub Acacia dealbata Link (silver wattle) can be regarded as a low cost renewable source of biomass for the production of biofuels, bioactives and chemicals. A number of studies have reported both conventional and novel processes for the utilization of different parts of the plant. This review presents an overview of the environmental problems associated to this shrub in invaded areas and a compilation of the technologies proposed for the extraction and production of commercially interesting compounds from silver wattle. Combination of such tools for the fractionated valorization of this resource following a zero waste biorefinery approach is discussed, with emphasis on the use of greener flexible technologies.
Abstract
Recently a theory about the formation of overdensities of stars along tidal tails of globular clusters has been presented. This theory predicts the position and the time of the formation of ...such overdensities and was successfully tested with N-body simulations of globular clusters in a point-mass galactic potential. In this work,
we present a comparison between this theory and our simulations using a dwarf galaxy orbiting two differently shaped dark matter haloes to study the effects of a cored and a cuspy halo on the formation and the evolution of tidal tails. We find no difference using a cuspy or a cored halo, however, we find an intriguing asymmetry between the leading arm and the trailing arm of the tidal tails. The trailing arm grows faster than the leading arm. This asymmetry is seen in the distance to the
first overdensity and its size as well. We establish a relation between the distance to the first overdensity and the size of this overdensity.
•Sequential extraction was proposed to obtain antioxidant extracts from paulownia petioles.•Supercritical CO2 extraction and microwave heated extraction were the studied technologies.•Optimal ...conditions for supercritical CO2 (20 MPa, 10% ethanol) & microwave heating (120 °C) were found.•Exhausted solid were proposed to develop gelled matrices with strong mechanical features.
Attention must be paid to the valorization of forest residues to advance towards a circular economy maximizing the use of resources. Petioles of paulownia were subjected to sequential extraction using green technologies to obtain extracts with antioxidant properties. Firstly, supercritical CO2 extraction was performed for 90 min at 45 °C at different extraction pressures (20 and 30 MPa) and ethanol concentrations (0. 5 and 10%). Subsequently, the solid residue remaining after supercritical extraction was subjected to microwave heated autohydrolysis for 3 min at different temperatures in the range 120−220 °C. The combination of both extraction technologies led to an extraction yield of 39%, and generated extracts with 10 g gallic acid equivalents (GAE)/100 g extract, and antiradical capacity against the ABTS radical ranging between 30−50 g/100 g extract. The waste solids remaininig after conventional extraction and microwave heated autohydrolysis of paulownia petioles were used in different mass proportions (1–9%) for the preparation of hydrogels with potential application in personal care products. Starch based gelling matrices enriched with the selected residual solids exhibited strong viscoelastic properties.
The common bean is a legume crop distributed worldwide. Dry bean production has gone through increasing difficulties due to relatively low yields in the last few years.
Rhizoctonia solani
is one of ...the root and hypocotyl pathogens that causes most of the economic losses in this crop. One promising strategy to control plant diseases is the use of biological control agents, able to reduce the negative effects of pathogens and to promote positive responses in the plant.
Trichoderma
spp. is a fungal genus ubiquitous in soil that can grow in soil or in any of the above ground parts of plants. The aims of this work were to study the effect of
Trichoderma
on bean plant growth, in the presence of the phytopathogen
R. solani
, according to the
Trichoderma
isolation source (seed or soil). Fifty-five
Trichoderma
isolates collected from bean seeds and from bean field soils were analyzed. Among them, those isolated from soil samples showed a higher plant growth promotion activity than those strains isolated from seeds
,
in the presence of
R. solani.
Furthermore, bean plants inoculated with
Trichoderma
-soil isolates showed a higher percentage of germination, hypocotyl diameter, length of the root system, and dry weight of aerial parts and root system than plants inoculated with
Trichoderma-
seed isolates.
Collisions in primordial star clusters Reinoso, B.; Schleicher, D. R. G.; Fellhauer, M. ...
Astronomy & astrophysics,
06/2018, Letnik:
614
Journal Article
Recenzirano
Odprti dostop
Collisions were suggested to potentially play a role in the formation of massive stars in present day clusters, and have likely been relevant during the formation of massive stars and intermediate ...mass black holes within the first star clusters. In the early Universe, the first stellar clusters were particularly dense, as fragmentation typically only occurred at densities above 109 cm−3, and the radii of the protostars were enhanced as a result of larger accretion rates, suggesting a potentially more relevant role of stellar collisions. We present here a detailed parameter study to assess how the number of collisions and the mass growth of the most massive object depend on the properties of the cluster. We also characterize the time evolution with three effective parameters: the time when most collisions occur, the duration of the collisions period, and the normalization required to obtain the total number of collisions. We apply our results to typical Population III (Pop. III) clusters of about 1000 M⊙, finding that a moderate enhancement of the mass of the most massive star by a factor of a few can be expected. For more massive Pop. III clusters as expected in the first atomic cooling halos, we expect a more significant enhancement by a factor of 15–32. We therefore conclude that collisions in massive Pop. III clusters were likely relevant to form the first intermediate mass black holes.
Runaway stellar collisions in dense star clusters are invoked to explain the presence of very massive stars or blue stragglers in the center of those systems. This process has also been explored for ...the first star clusters in the Universe and shown to yield stars that may collapse at some points into an intermediate mass black hole. Although the early evolution of star clusters requires the explicit modeling of the gas out of which the stars form, these calculations would be extremely time-consuming and often the effects of the gas can be accurately treated by including a background potential to account for the extra gravitational force. We apply this approximation to model the early evolution of the first dense star clusters formed in the Universe by performing N-body simulations, our goal is to understand how the additional gravitational force affects the growth of a very massive star through stellar mergers in the central parts of the star cluster. Our results show that the background potential increases the velocities of the stars, causing an overall delay in the evolution of the clusters and in the runaway growth of a massive star at the center. The population of binary stars is lower due to the increased kinetic energy of the stars, initially reducing the number of stellar collisions, and we show that relaxation processes are also affected. Despite these effects, the external potential enhances the mass of the merger product by a factor ∼2 if the collisions are maintained for long times.
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