8,506 research outputs found
An Introduction to the Covariant Quantization of Superstrings
We give an introduction to a new approach to the covariant quantization of
superstrings. After a brief review of the classical Green--Schwarz superstring
and Berkovits' approach to its quantization based on pure spinors, we discuss
our covariant formulation without pure spinor constraints. We discuss the
relation between the concept of grading, which we introduced to define vertex
operators, and homological perturbation theory, and we compare our work with
recent work by others. In the appendices, we include some background material
for the Green-Schwarz and Berkovits formulations, in order that this
presentation be self contained.Comment: LaTex, 23 pp. Contribution to the Proceedings of the Workshop in
String Theory, Leuven 2002, some references added and a comment on ref. [16
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
The synergistic effect between positivity, socio-demographic factors and smoking cessation: results of a cohort study
OBJECTIVE: To examine the extent to which a effect does exist between Positivity (POS), smoking and socio-demographic factors in determining quitting smoking in subjects participating in a Group Counselling Program (GCP) for smoking cessation.METHODS: 481 subjects were contacted through a telephone call. A logistic regression analysis was carried out. Possible interaction between sociodemographic variables and POS level was tested using the Synergism Index (SI).RESULTS: For individuals with a POS level over or equal to 3.4 the odds of being smoker was significantly higher among females (OR = 1.55), who smoked at home (OR = 2.16) and lower if there had children at home (OR = 0.53). For individuals with a POS level under 3.4, the only significant variable associated with smoking was beinga female (OR = 2.58). As far concerns the synergistic effect between the variables considered does exist between POS levels and having children at home (SI=1.13) and female gender (SI = 2.8).CONCLUSIONS: The synergistic effect between POS and sociodemographic factors adds evidence on the use of POS as possible determinants of individual happiness
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
Investigation on the thermal gelation of Chitosan/\u3b2-Glycerophosphate solutions
This work deals with the effect of temperature on the thermal-gelation process of water solutions containing
chitosan \u3b2-glycerolphosphate disodium salt hydrate. In particular, the attention is focused on the role played by
temperature on the gel final properties, a very important aspect in the frame of drug delivery systems. The study
was performed by combining rheology and low field nuclear magnetic resonance, two approaches that revealed
to be highly synergic as they can detect different aspects of the developing polymeric network. This study
indicates that 30 \ub0C represent a sort of threshold for both the gelation kinetics and the gel final properties.
Indeed, above this temperature, gelation kinetics was rapid and yielded to a strong gel. On the contrary, a slow
kinetics and a final weak gel occurred below 30 \ub0C. Finally, rheology and low field NMR allowed, independently,
evaluating the time evolution of the network mesh size upon gelation
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
Equilibrium molecular energies used to obtain molecular dissociation energies and heats of formation within the bond-order correlation approach
Ab initio calculations including electron correlation are still extremely
costly except for the smallest atoms and molecules. Therefore, our purpose in
the present study is to employ a bond-order correlation approach to obtain, via
equilibrium molecular energies, molecular dissociation energies and heats of
formation for some 20 molecules containing C, H, and O atoms, with a maximum
number of electrons around 40. Finally, basis set choice is shown to be
important in the proposed procedure to include electron correlation effects in
determining thermodynamic properties. With the optimum choice of basis set, the
average percentage error for some 20 molecules is approximately 20% for heats
of formation. For molecular dissociation energies the average error is much
smaller: ~0.4.Comment: Mol. Phys., to be publishe
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