Automorphisms and the K\"ahler cone of certain Calabi-Yau manifolds

For the Calabi-Yau threefolds $X$ constructed by C. Schoen as fiber products of generic rational elliptic surfaces, we show that the action of the automorphism group of $X$ on the K\"ahler cone of $X$ has a rationally polyhedral fundamental domain. The second author has conjectured that this statement will hold in general, the example presented here being the first non-trivial case in which the statement has been checked. The conjecture was motivated by the desire to use a construction of E. Looijenga to compactify certain moduli spaces which arise in the study of conformal field theory and ``mirror symmetry.''Comment: 8 pp., AmS-TeX v.

Model of tunneling transistors based on graphene on SiC

Recent experiments shown that graphene epitaxially grown on Silicon Carbide (SiC) can exhibit a energy gap of 0.26 eV, making it a promising material for electronics. With an accurate model, we explore the design parameter space for a fully ballistic graphene-on-SiC Tunnel Field-Effect Transistors (TFETs), and assess the DC and high frequency figures of merit. The steep subthreshold behavior can enable I_{ON}/I_{OFF} ratios exceeding 10^4 even with a low supply voltage of 0.15 V, for devices with gatelength down to 30 nm. Intrinsic transistor delays smaller than 1 ps are obtained. These factors make the device an interesting candidate for low-power nanoelectronics beyond CMOS

A chemical model for the interstellar medium in galaxies

We present and test chemical models for three-dimensional hydrodynamical simulations of galaxies. We explore the effect of changing key parameters such as metallicity, radiation and non-equilibrium versus equilibrium metal cooling approximations on the transition between the gas phases in the interstellar medium. The microphysics is modelled by employing the public chemistry package KROME and the chemical networks have been tested to work in a wide range of densities and temperatures. We describe a simple H/He network following the formation of H$_2$, and a more sophisticated network which includes metals. Photochemistry, thermal processes, and different prescriptions for the H$_2$ catalysis on dust are presented and tested within a one-zone framework. The resulting network is made publicly available on the KROME webpage. We find that employing an accurate treatment of the dust-related processes induces a faster HI--H$_2$ transition. In addition, we show when the equilibrium assumption for metal cooling holds, and how a non-equilibrium approach affects the thermal evolution of the gas and the HII--HI transition. These models can be employed in any hydrodynamical code via an interface to KROME and can be applied to different problems including isolated galaxies, cosmological simulations of galaxy formation and evolution, supernova explosions in molecular clouds, and the modelling of star-forming regions. The metal network can be used for a comparison with observational data of CII 158 $\mu$m emission both for high-redshift as well as for local galaxies.Comment: A&A accepte

H2_2 ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?

Deuterium fractionation, i.e. the enhancement of deuterated species with respect to the non-deuterated ones, is considered to be a reliable chemical clock of star-forming regions. This process is strongly affected by the ortho-to-para (o-p) H$_2$ ratio. In this letter we explore the effect of the o-p H$_2$ conversion on grains on the deuteration timescale in fully depleted dense cores, including the most relevant uncertainties that affect this complex process. We show that (i) the o-p H$_2$ conversion on grains is not strongly influenced by the uncertainties on the conversion time and the sticking coefficient and (ii) that the process is controlled by the temperature and the residence time of ortho-H$_2$ on the surface, i.e. by the binding energy. We find that for binding energies in between 330-550 K, depending on the temperature, the o-p H$_2$ conversion on grains can shorten the deuterium fractionation timescale by orders of magnitude, opening a new route to explain the large observed deuteration fraction $D_\mathrm{frac}$ in dense molecular cloud cores. Our results suggest that the star formation timescale, when estimated through the timescale to reach the observed deuteration fractions, might be shorter than previously proposed. However, more accurate measurements of the binding energy are needed to better assess the overall role of this process.Comment: Accepted for publication in ApJ Letter

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$^5$ M$_\odot$ 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 $\sim$60 K triggering fragmentation. Based on Jeans estimates the expected stellar masses are about 10 M$_\odot$. Our findings show that the merging scenario is a potential pathway for the formation of low-mass stars.Comment: Submitted to MNRA

A Non-Perturbative Superpotential With E8E_8 Symmetry

We compute the non-perturbative superpotential in $F$-theory compactification to four dimensions on a complex three-fold $\P^1\times S$, where $S$ is a rational elliptic surface. In contrast to examples considered previously, the superpotential in this case has interesting modular properties; it is essentially an $E_8$ theta function.Comment: Additional references and clarifications. Latex, 10 page

Pure-spinor superstrings in d=2,4,6

We continue the study of the d=2,4,6 pure-spinor superstring models introduced in [1]. By explicitly solving the pure-spinor constraint we show that these theories have vanishing central charge and work out the (covariant) current algebra for the Lorentz currents. We argue that these super-Poincare covariant models may be thought of as compactifications of the superstring on CY_{4,3,2}, and take some steps toward making this precise by constructing a map to the RNS superstring variables. We also discuss the relation to the so called hybrid superstrings, which describe the same type of compactifications.Comment: 27 page