Star formation and molecular hydrogen in dwarf galaxies: a non-equilibrium view

We study the connection of star formation to atomic (HI) and molecular hydrogen (H$_2$) in isolated, low metallicity dwarf galaxies with high-resolution ($m_{\rm gas}$ = 4 M$_\odot$, $N_{\rm ngb}$ = 100) SPH simulations. The model includes self-gravity, non-equilibrium cooling, shielding from an interstellar radiation field, the chemistry of H$_2$ formation, H$_2$-independent star formation, supernova feedback and metal enrichment. We find that the H$_2$ mass fraction is sensitive to the adopted dust-to-gas ratio and the strength of the interstellar radiation field, while the star formation rate is not. Star formation is regulated by stellar feedback, keeping the gas out of thermal equilibrium for densities $n <$ 1 cm$^{-3}$. Because of the long chemical timescales, the H$_2$ mass remains out of chemical equilibrium throughout the simulation. Star formation is well-correlated with cold ( T $\leqslant$ 100 K ) gas, but this dense and cold gas - the reservoir for star formation - is dominated by HI, not H$_2$. In addition, a significant fraction of H$_2$ resides in a diffuse, warm phase, which is not star-forming. The ISM is dominated by warm gas (100 K $<$ T $\leqslant 3\times 10^4$ K) both in mass and in volume. The scale height of the gaseous disc increases with radius while the cold gas is always confined to a thin layer in the mid-plane. The cold gas fraction is regulated by feedback at small radii and by the assumed radiation field at large radii. The decreasing cold gas fractions result in a rapid increase in depletion time (up to 100 Gyrs) for total gas surface densities $\Sigma_{\rm HI+H_2} \lesssim$ 10 M$_\odot$pc$^{-2}$, in agreement with observations of dwarf galaxies in the Kennicutt-Schmidt plane.Comment: Accepted for publication in MNRAS. Changes (including a pamameter study in Appendix C) highlighte

ALMA Maps of Dust and Warm Dense Gas Emission in the Starburst Galaxy IC 5179⋆^\star

We present our high-resolution ($0^{\prime\prime}.15\times0^{\prime\prime}.13$, $\sim$34 pc) observations of the CO(6-5) line emission, which probes the warm and dense molecular gas, and the 434 $\mu$m dust continuum emission in the nuclear region of the starburst galaxy IC 5179, conducted with the Atacama Large Millimeter Array (ALMA). The CO(6-5) emission is spatially distributed in filamentary structures with many dense cores and shows a velocity field that is characteristic of a circum-nuclear rotating gas disk, with 90% of the rotation speed arising within a radius of $\lesssim150$ pc. At the scale of our spatial resolution, the CO(6-5) and dust emission peaks do not always coincide, with their surface brightness ratio varying by a factor of $\sim$10. This result suggests that their excitation mechanisms are likely different, as further evidenced by the Southwest to Northeast spatial gradient of both CO-to-dust continuum ratio and Pa-$\alpha$ equivalent width. Within the nuclear region (radius$\sim$300 pc) and with a resolution of $\sim$34 pc, the CO line flux (dust flux density) detected in our ALMA observations is $180\pm18$ Jy km/s ($71\pm7$ mJy), which account for 22% (2.4%) of the total value measured by Herschel.Comment: Accepted for publication in Ap

Flexible delivery of Er:YAG radiation at 2.94 µm with negative curvature silica glass fibers:a new solution for minimally invasive surgical procedures

We present the delivery of high energy microsecond pulses through a hollow-core negative-curvature fiber at 2.94 µm. The energy densities delivered far exceed those required for biological tissue manipulation and are of the order of 2300 J/cm(2). Tissue ablation was demonstrated on hard and soft tissue in dry and aqueous conditions with no detrimental effects to the fiber or catastrophic damage to the end facets. The energy is guided in a well confined single mode allowing for a small and controllable focused spot delivered flexibly to the point of operation. Hence, a mechanically and chemically robust alternative to the existing Er:YAG delivery systems is proposed which paves the way for new routes for minimally invasive surgical laser procedures

A purely reflective large wide-field telescope

Two versions of a fast, purely reflective Paul-Baker type telescope are discussed, each with an 8.4-m aperture, 3 deg diameter flat field and f/1.25 focal ratio. The first version is based on a common, even asphere type of surface with zero conic constant. The primary and tertiary mirrors are 6th order aspheres, while the secondary mirror is an 8th order asphere (referred to here for brevity, as the 6/8/6 configuration). The D_80 diameter of a star image varies from 0''.18 on the optical axis up to 0''.27 at the edge of the field (9.3-13.5 mcm). The second version of the telescope is based on a polysag surface type which uses a polynomial expansion in the sag z, r^2 = 2R_0z - (1+b)z^2 + a_3 z^3 + a_4 z^4 + ... + a_N z^N, instead of the common form of an aspheric surface. This approach results in somewhat better images, with D_80 ranging from 0''.16 to 0''.23, using a lower-order 3/4/3 combination of powers for the mirror surfaces. An additional example with 3.5-m aperture, 3.5 deg diameter flat field, and f/1.25 focal ratio featuring near-diffraction-limited image quality is also presented.Comment: 14 pages, 6 figures; new examples adde

Global Calculations of Density Waves and Gap Formation in Protoplanetary Disks using a Moving Mesh

We calculate the global quasi-steady state of a thin disk perturbed by a low-mass protoplanet orbiting at a fixed radius using extremely high-resolution numerical integrations of Euler's equations in two dimensions. The calculations are carried out using a moving computational domain, which greatly reduces advection errors and allows for much longer time-steps than a fixed grid. We calculate the angular momentum flux and the torque density as a function of radius and compare them with analytical predictions. We discuss the quasi-steady state after 100 orbits and the prospects for gap formation by low mass planets.Comment: Accepted to Ap