The Electron Temperature Gradient in the Galactic Disk

We derive the electron temperature gradient in the Galactic disk using a sample of HII regions that spans Galactocentric distances 0--17 kpc. The electron temperature was calculated using high precision radio recombination line and continuum observations for more than 100 HII regions. Nebular Galactocentric distances were calculated in a consistent manner using the radial velocities measured by our radio recombination line survey. The large number of nebulae widely distributed over the Galactic disk together with the uniformity of our data provide a secure estimate of the present electron temperature gradient in the Milky Way. Because metals are the main coolants in the photoionized gas, the electron temperature along the Galactic disk should be directly related to the distribution of heavy elements in the Milky Way. Our best estimate of the electron temperature gradient is derived from a sample of 76 sources for which we have the highest quality data. The present gradient in electron temperature has a minimum at the Galactic Center and rises at a rate of 287 +/- 46 K/kpc. There are no significant variations in the value of the gradient as a function of Galactocentric radius or azimuth. The scatter we find in the HII region electron temperatures at a given Galactocentric radius is not due to observational error, but rather to intrinsic fluctuations in these temperatures which are almost certainly due to fluctuations in the nebular heavy element abundances. Comparing the HII region gradient with the much steeper gradient found for planetary nebulae suggests that the electron temperature gradient evolves with time, becoming flatter as a consequence of the chemical evolution of the Milky Way's disk.Comment: 43 pages, 9 figures (accepted for publication in the ApJ

An open extensible tool environment for Event-B

Abstract. We consider modelling indispensable for the development of complex systems. Modelling must be carried out in a formal notation to reason and make meaningful conjectures about a model. But formal modelling of complex systems is a difficult task. Even when theorem provers improve further and get more powerful, modelling will remain difficult. The reason for this that modelling is an exploratory activity that requires ingenuity in order to arrive at a meaningful model. We are aware that automated theorem provers can discharge most of the onerous trivial proof obligations that appear when modelling systems. In this article we present a modelling tool that seamlessly integrates modelling and proving similar to what is offered today in modern integrated development environments for programming. The tool is extensible and configurable so that it can be adapted more easily to different application domains and development methods.

Stability conditions and Stokes factors

Let A be the category of modules over a complex, finite-dimensional algebra. We show that the space of stability conditions on A parametrises an isomonodromic family of irregular connections on P^1 with values in the Hall algebra of A. The residues of these connections are given by the holomorphic generating function for counting invariants in A constructed by D. Joyce.Comment: Very minor changes. Final version. To appear in Inventione

On the S-matrix renormalization in effective theories

This is the 5-th paper in the series devoted to explicit formulating of the rules needed to manage an effective field theory of strong interactions in S-matrix sector. We discuss the principles of constructing the meaningful perturbation series and formulate two basic ones: uniformity and summability. Relying on these principles one obtains the bootstrap conditions which restrict the allowed values of the physical (observable) parameters appearing in the extended perturbation scheme built for a given localizable effective theory. The renormalization prescriptions needed to fix the finite parts of counterterms in such a scheme can be divided into two subsets: minimal -- needed to fix the S-matrix, and non-minimal -- for eventual calculation of Green functions; in this paper we consider only the minimal one. In particular, it is shown that in theories with the amplitudes which asymptotic behavior is governed by known Regge intercepts, the system of independent renormalization conditions only contains those fixing the counterterm vertices with $n \leq 3$ lines, while other prescriptions are determined by self-consistency requirements. Moreover, the prescriptions for $n \leq 3$ cannot be taken arbitrary: an infinite number of bootstrap conditions should be respected. The concept of localizability, introduced and explained in this article, is closely connected with the notion of resonance in the framework of perturbative QFT. We discuss this point and, finally, compare the corner stones of our approach with the philosophy known as ``analytic S-matrix''.Comment: 28 pages, 10 Postscript figures, REVTeX4, submitted to Phys. Rev.

The Chemical Evolution Carousel of Spiral Galaxies : Azimuthal Variations of Oxygen Abundance in NGC1365

19 pages, 13 figures. Accepted to ApJThe spatial distribution of oxygen in the interstellar medium of galaxies is the key to understanding how efficiently metals that are synthesized in massive stars can be redistributed across a galaxy. We present here a case study in the nearby spiral galaxy NGC1365 using 3D optical data obtained in the TYPHOON Program. We find systematic azimuthal variations of the HII region oxygen abundance imprinted on a negative radial gradient. The 0.2 dex azimuthal variations occur over a wide radial range of 0.3 to 0.7 R25 and peak at the two spiral arms in NGC1365. We show that the azimuthal variations can be explained by two physical processes: gas undergoes localized, sub-kpc scale self-enrichment when orbiting in the inter-arm region, and experiences efficient, kpc scale mixing-induced dilution when spiral density waves pass through. We construct a simple chemical evolution model to quantitatively test this picture and find that our toy model can reproduce the observations. This result suggests that the observed abundance variations in NGC1365 are a snapshot of the dynamical local enrichment of oxygen modulated by spiral-driven, periodic mixing and dilution.Peer reviewedFinal Published versio

A Bitter Pill: The Primordial Lithium Problem Worsens

The lithium problem arises from the significant discrepancy between the primordial 7Li abundance as predicted by BBN theory and the WMAP baryon density, and the pre-Galactic lithium abundance inferred from observations of metal-poor (Population II) stars. This problem has loomed for the past decade, with a persistent discrepancy of a factor of 2--3 in 7Li/H. Recent developments have sharpened all aspects of the Li problem. Namely: (1) BBN theory predictions have sharpened due to new nuclear data, particularly the uncertainty on 3He(alpha,gamma)7Be, has reduced to 7.4%, and with a central value shift of ~ +0.04 keV barn. (2) The WMAP 5-year data now yields a cosmic baryon density with an uncertainty reduced to 2.7%. (3) Observations of metal-poor stars have tested for systematic effects, and have reaped new lithium isotopic data. With these, we now find that the BBN+WMAP predicts 7Li/H = (5.24+0.71-0.67) 10^{-10}. The Li problem remains and indeed is exacerbated; the discrepancy is now a factor 2.4--4.3 or 4.2sigma (from globular cluster stars) to 5.3sigma (from halo field stars). Possible resolutions to the lithium problem are briefly reviewed, and key nuclear, particle, and astronomical measurements highlighted.Comment: 21 pages, 4 figures. Comments welcom

The Eagle Nebula's fingers - pointers to the earliest stages of star formation?

Molecular line, millimetre/submillimetre continuum, and mid-IR observations are reported of the opaque fingers which cross the Eagle Nebula. The fingers are surprisingly warm when viewed in the CO J= 3-2 lines, with kinetic temperatures approaching 60 K, although the lines are relatively narrow. Most of the mass in the fingers is concentrated in cores which lie at the tips of the fingers, and contain from ~ 10 to 60 M⊙, representing 55-80% of the mass of the individual fingers. The integrated mass contained in the three fingers and the nearby extended material is ~ 200 M⊙. The velocity fields of the gas are complex and the material is very clumpy. The best evidence for coherent velocity structure is seen running along the central finger, which has a velocity gradient ~ 1.7 km s-1 pc-1 . The fingers contain several embedded submm continuum cores, with the most intense located at the tips of the fingers. The continuum spectra of these cores shows that they are much cooler, Tdust ~ 20 K, than Tgas ~ 60 K of their respective fingers. A simple thermal and chemical model of a finger was developed to study the physical environment, which takes into account the external UV illumination ( ~ 1700 G0), and the chemical and thermal structure of a finger. The model predictions are consistent with all of the available observations. The fingers appear to have been formed after primordial dense clumps in the original cloud were irradiated by the light of its OB stars. These clumps then shielded material lying behind from the photoevaporative dispersal of the cloud, and facilitated the formation of the finger structures. The cores in the tips of the fingers appear to be at a very early stage of pre-protostellar development: there are no embedded infrared sources or molecular outflows present. The pressure inside the cores is just less than that of the surrounding gas, allowing them to be compressed by the external pressure. The cores are probably just starting the final stages of collapse, which will lead to the formation of a condensed, warm object. It is well known that such characteristics are expected from the earliest stages of objects popularly known as `protostars'. The cores in the tips of the Eagle Nebula's fingers have characteristics similar to those expected to occur in the earliest stages of protostellar formation.</i

Influence of Gamma-Ray Emission on the Isotopic Composition of Clouds in the Interstellar Medium

We investigate one mechanism of the change in the isotopic composition of cosmologically distant clouds of interstellar gas whose matter was subjected only slightly to star formation processes. According to the standard cosmological model, the isotopic composition of the gas in such clouds was formed at the epoch of Big Bang nucleosynthesis and is determined only by the baryon density in the Universe. The dispersion in the available cloud composition observations exceeds the errors of individual measurements. This may indicate that there are mechanisms of the change in the composition of matter in the Universe after the completion of Big Bang nucleosynthesis. We have calculated the destruction and production rates of light isotopes (D, 3He, 4He) under the influence of photonuclear reactions triggered by the gamma-ray emission from active galactic nuclei (AGNs). We investigate the destruction and production of light elements depending on the spectral characteristics of the gamma-ray emission. We show that in comparison with previous works, taking into account the influence of spectral hardness on the photonuclear reaction rates can increase the characteristic radii of influence of the gamma-ray emission from AGNs by a factor of 2-8. The high gamma-ray luminosities of AGNs observed in recent years increase the previous estimates of the characteristic radii by two orders of magnitude. This may suggest that the influence of the emission from AGNs on the change in the composition of the medium in the immediate neighborhood (the host galaxy) has been underestimated.Comment: 13 pages, 13 figures, 3 table