64 research outputs found
The formation of massive primordial stars in the presence of moderate UV backgrounds
Radiative feedback from populations II stars played a vital role in early
structure formation. Particularly, photons below the Lyman limit can escape the
star forming regions and produce a background ultraviolet (UV) flux which
consequently may influence the pristine halos far away from the radiation
sources. These photons can quench the formation of molecular hydrogen by
photo-detachment of . In this study, we explore the impact of such
UV radiation on fragmentation in massive primordial halos of a few times ~M. To accomplish this goal, we perform high resolution
cosmological simulations for two distinct halos and vary the strength of the
impinging background UV field in units of . We further make use of
sink particles to follow the evolution for 10,000 years after reaching the
maximum refinement level. No vigorous fragmentation is observed in UV
illuminated halos while the accretion rate changes according to the thermal
properties. Our findings show that a few 100-10, 000 solar mass protostars are
formed when halos are irradiated by at and
suggest a strong relation between the strength of UV flux and mass of a
protostar. This mode of star formation is quite different from minihalos, as
higher accretion rates of about M/yr are observed by
the end of our simulations. The resulting massive stars are the potential
cradles for the formation of intermediate mass black holes at earlier cosmic
times and contribute to the formation of a global X-ray background.Comment: Submitted to APJ, comments are welcome. High resolution copy is
available at http://www.astro.physik.uni-goettingen.de/~mlatif/IMBHs_apj.pd
Formation of carbon-enhanced metal-poor stars in the presence of far ultraviolet radiation
Recent discoveries of carbon-enhanced metal-poor stars like SMSS
J031300.36-670839.3 provide increasing observational insights into the
formation conditions of the first second-generation stars in the Universe,
reflecting the chemical conditions after the first supernova explosion. Here,
we present the first cosmological simulations with a detailed chemical network
including primordial species as well as C, C, O, O, Si, Si, and
Si following the formation of carbon-enhanced metal poor stars. The
presence of background UV flux delays the collapse from to and
cool the gas down to the CMB temperature for a metallicity of
Z/Z=10. This can potentially lead to the formation of lower mass
stars. Overall, we find that the metals have a stronger effect on the collapse
than the radiation, yielding a comparable thermal structure for large
variations in the radiative background. We further find that radiative
backgrounds are not able to delay the collapse for Z/Z=10 or a
carbon abundance as in SMSS J031300.36-670839.3.Comment: submitted to ApJ
Dark-matter halo mergers as a fertile environment for low-mass Population III star formation
While Population III stars are typically thought to be massive, pathways
towards lower-mass Pop III stars may exist when the cooling of the gas is
particularly enhanced. A possible route is enhanced HD cooling during the
merging of dark-matter halos. The mergers can lead to a high ionization degree
catalysing the formation of HD molecules and may cool the gas down to the
cosmic microwave background (CMB) temperature. In this paper, we investigate
the merging of mini-halos with masses of a few 10 M and explore the
feasibility of this scenario. We have performed three-dimensional cosmological
hydrodynamics calculations with the ENZO code, solving the thermal and chemical
evolution of the gas by employing the astrochemistry package KROME. Our results
show that the HD abundance is increased by two orders of magnitude compared to
the no-merging case and the halo cools down to 60 K triggering
fragmentation. Based on Jeans estimates the expected stellar masses are about
10 M. Our findings show that the merging scenario is a potential
pathway for the formation of low-mass stars.Comment: Submitted to MNRA
How realistic UV spectra and X-rays suppress the abundance of direct collapse black holes
Observations of high redshift quasars at indicate that they harbor
supermassive black holes (SMBHs) of a billion solar masses. The direct collapse
scenario has emerged as the most plausible way to assemble SMBHs. The nurseries
for the direct collapse black holes are massive primordial halos illuminated
with an intense UV flux emitted by population II (Pop II) stars. In this study,
we compute the critical value of such a flux () for
realistic spectra of Pop II stars through three-dimensional cosmological
simulations. We derive the dependence of on the radiation
spectra, on variations from halo to halo, and on the impact of X-ray
ionization. Our findings show that the value of is a few
times and only weakly depends on the adopted radiation spectra in
the range between K. For three simulated halos
of a few times ~M, varies from . The impact of X-ray ionization is almost
negligible and within the expected scatter of for
background fluxes of . The computed estimates of
have profound implications for the quasar abundance at
as it lowers the number density of black holes forming through an
isothermal direct collapse by a few orders of magnitude below the observed
black holes density. However, the sites with moderate amounts of
cooling may still form massive objects sufficient to be compatible with
observations.Comment: Accepted for publication in MNRAS, comments are welcom
Effects of turbulence and rotation on protostar formation as a precursor to seed black holes
Context. The seeds of the first supermassive black holes may have resulted
from the direct collapse of hot primordial gas in K haloes,
forming a supermassive or quasistar as an intermediate stage.
Aims. We explore the formation of a protostar resulting from the collapse of
primordial gas in the presence of a strong Lyman-Werner radiation background.
Particularly, we investigate the impact of turbulence and rotation on the
fragmentation behaviour of the gas cloud. We accomplish this goal by varying
the initial turbulent and rotational velocities.
Methods. We performed 3D adaptive mesh refinement simulations with a
resolution of 64 cells per Jeans length using the ENZO code, simulating the
formation of a protostar up to unprecedentedly high central densities of
cm, and spatial scales of a few solar radii. To achieve this
goal, we employed the KROME package to improve modelling of the chemical and
thermal processes.
Results. We find that the physical properties of the simulated gas clouds
become similar on small scales, irrespective of the initial amount of
turbulence and rotation. After the highest level of refinement was reached, the
simulations have been evolved for an additional ~5 freefall times. A single
bound clump with a radius of AU and a mass of ~ M is formed at the end of each simulation, marking the onset
of protostar formation. No strong fragmentation is observed by the end of the
simulations, regardless of the initial amount of turbulence or rotation, and
high accretion rates of a few solar masses per year are found.
Conclusions. Given such high accretion rates, a quasistar of
M is expected to form within years.Comment: 18 pages, 7 figures, fixed typos, added references and clarified some
details; accepted for publication in A&
A UV flux constraint on the formation of direct collapse black holes
The ability of metal free gas to cool by molecular hydrogen in primordial
halos is strongly associated with the strength of ultraviolet (UV) flux
produced by the stellar populations in the first galaxies. Depending on the
stellar spectrum, these UV photons can either dissociate molecules
directly or indirectly by photo-detachment of as the latter
provides the main pathway for formation in the early universe. In
this study, we aim to determine the critical strength of the UV flux above
which the formation of molecular hydrogen remains suppressed for a sample of
five distinct halos at by employing a higher order chemical solver and a
Jeans resolution of 32 cells. We presume that such flux is emitted by PopII
stars implying atmospheric temperatures of ~K. We performed
three-dimensional cosmological simulations and varied the strength of the UV
flux below the Lyman limit in units of . Our findings show that the
value of varies from halo to halo and is sensitive to the
local thermal conditions of the gas. For the simulated halos it varies from
400-700 with the exception of one halo where .
This has important implications for the formation of direct collapse black
holes and their estimated population at z > 6. It reduces the number density of
direct collapse black holes by almost three orders of magnitude compared to the
previous estimates.Comment: 10 pages, 6 figures, matches the accepted version to ber published in
MNRAS, higher resolution version is available at
http://www.astro.physik.uni-goettingen.de/~mlatif/Jcrit.pd
Control of Au nanoantenna emission enhancement of magnetic dipolar emitters by means of VO2 phase change layers
Active, ultra-fast external control of the emission properties at the nanoscale is of great interest for chip-scale, tunable and efficient nanophotonics. Here we investigated the emission control of dipolar emitters coupled to a nanostructure made of an Au nanoantenna, and a thin vanadium dioxide (VO2) layer that changes from semiconductor to metallic state. If the emitters are sandwiched between the nanoantenna and the VO2 layer, the enhancement and/or suppression of the nanostructure’s magnetic dipole resonance enabled by the phase change behavior of the VO2 layer can provide a high contrast ratio of the emission efficiency. We show that a single nanoantenna can provide high magnetic field in the emission layer when VO2 is metallic, leading to high emission of the magnetic dipoles; this emission is then lowered when VO2 switches back to semiconductor. We finally optimized the contrast ratio by considering different orientation, distribution and nature of the dipoles, as well as the influence of a periodic Au nanoantenna pattern. As an example of a possible application, the design is optimized for the active control of an Er3+ doped SiO2 emission layer. The combination of the emission efficiency increase due to the plasmonic nanoantenna resonances and the ultra-fast contrast control due to the phase-changing medium can have important applications in tunable efficient light sources and their nanoscale integration
Establishing the evolutionary timescales of the massive star formation process through chemistry
(Abridged) Understanding the details of the formation process of massive
(i.e. M<8-10M) stars is a long-standing problem in astrophysics. [...]
We present a method to derive accurate timescales of the different evolutionary
phases of the high-mass star formation process. We model a representative
number of massive clumps of the ATLASGAL-TOP100 sample which cover all the
evolutionary stages. The models describe an isothermal collapse and the
subsequent warm-up phase, for which we follow their chemical evolution. The
timescale of each phase is derived by comparing the results of the models with
the properties of the sources of the ATLASGAL-TOP100 sample, taking into
account the mass and luminosity of the clumps, and the column densities of
methyl acetylene (CHCCH), acetonitrile (CHCN), formaldehyde (HCO)
and methanol (CHOH). We find that the chosen molecular tracers are affected
by the thermal evolution of the clumps, showing steep ice evaporation gradients
from 10 to 10 AU during the warm-up phase. We succeed in reproducing
the observed column densities of CHCCH and CHCN, while HCO and
CHOH show a poorer agreement with the observed values. The total (massive)
star formation time is found to be yr, which is defined by
the timescales of the individual evolutionary phases of the ATLASGAL-TOP100
sample: yr for 70-m weak, yr for
mid-IR weak, yr for mid-IR bright and
yr for HII-regions phases. Our models, with an appropriate selection of
molecular tracers that can act as chemical clocks, allow to get robust
estimates of the duration of the individual phases of the high-mass star
formation process, with the advantage of being capable to include additional
tracers aimed at increasing the accuracy of the estimated timescales.Comment: Published on A&A (19 pages, 9 figures, 7 tables
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