A model for atomic and molecular interstellar gas: The Meudon PDR code

We present the revised ``Meudon'' model of Photon Dominated Region (PDR code), presently available on the web under the Gnu Public Licence at: http://aristote.obspm.fr/MIS. General organisation of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelisation highlighted.Comment: accepted in ApJ sup

UB Knightlines Spring 2013

The UB Knightlines newsletter for Spring 2013. This issue contains articles discussing alumni and their continued relationship with the University, a new cross disciplinary graduate program between design and business, students report on the University's SNAP Challenge, UB Career Day, a SGA hosted event about gun violence with a survivor of the Virginia Tech shooting, a memorial scholarship for Lauren Gabrielle Rousseau who died during the Sandy Hook shooting, the School of Engineering partnering the NASA on a satellite launch, Fones recieving a Health Resources and Services Administration grant, the renaming of the International College to College of Public and International Affairs, the Innovators series at UB, the renaming of Arnold Bernhard Center gallery to the Schelfhaudt Gallery, a 1941 alum of Junior College of Connecticut Fanny Gabriel, works of faculty members, alumni news, an alumni reunion in Malaysia, the annual Alumni Soccer Game, the Volleyball team winning the East Coast Conference, the death of UB Soccer coach Fran Bacon, UB Soccer goalkeeper Julia Hansson being named to the 2012 Capital One National Academic All-America Second Team by the College Sports Information Directors of America (CoSIDA), UB alums playing professional sports world-wide, and other campus and sports news

On the missing 2175 Angstroem-bump in the Calzetti extinction curve

The aim of the paper is to give a physical explanation of the absence of the feature in the Calzetti extinction curve. We analyze the dust attenuation of a homogeneous source seen through a distant inhomogeneous distant screen. The inhomogeneities are described through an idealized isothermal turbulent medium where the probability distribution function (PDF) of the column density is log-normal. In addition it is assumed that below a certain critical column density the carriers of the extinction bump at 2175 Angstroem are being destroyed by the ambient UV radiation field. Turbulence is found to be a natural explanation not only of the flatter curvature of the Calzetti extinction curve but also of the missing bump provided the critical column density is N_H >= 10^21 cm^-2. The density contrast needed to explain both characteristics is well consistent with the Mach number of the cold neutral medium of our own Galaxy which suggests a density contrast sigma_(rho/) 6.Comment: 6 pages, 6 figures accepted for publication in A&A, section

Photoelectric effect on dust grains across the L1721 cloud in the rho Ophiuchi molecular complex

We present ISO-LWS measurements of the main gas cooling lines, C+ 158 mum and O 63 mum towards a moderate opacity molecular cloud (Av=3), L1721, illuminated by the B2 star nu Sco (X = 5-10). These data are combined with an extinction map and IRAS dust emission images to test our understanding of gas heating and cooling in photo-dissociation regions (PDRs). This nearby PDR is spatially resolved in the IRAS images; variations in the IRAS colors across the cloud indicate an enhanced abundance of small dust grains within the PDR. A spatial correlation between the gas cooling lines and the infrared emission from small dust grains illustrates the dominant role of small dust grains in the gas heating through the photoelectric effect. The photoelectric efficiency, determined from the observations by ratioing the power radiated by gas and small dust grains, is in the range 2 to 3% in close agreement with recent theoretical estimates. The brightness profiles across the PDR in the C+ 158 mum and O 63 mum lines are compared with model calculations where the density profile is constrained by the extinction data and where the gas chemical and thermal balances are solved at each position. We show that abundance variations of small dust grains across the PDR must be considered to account for the LWS observations.Comment: 10 pages, 15 figure

A forming disk at z~0.6: Collapse of a gaseous disk or major merger remnant?

[Abridged] We present and analyze observations of J033241.88-274853.9 at z=0.6679, using multi-wavelength photometry and imaging with FLAMES/GIRAFFE 3D spectroscopy. J033241.88-274853.9 is found to be a blue, young (~320Myr) stellar disk embedded in a very gas-rich (fgas=73-82% with log(Mstellar/Mo)=9.45) and turbulent phase that is found to be rotating on large spatial scales. We identified two unusual properties of J033241.88-274853.9. (1) The spatial distributions of the ionized gaseous and young stars show a strong decoupling; while almost no stars can be detected in the southern part down to the very deep detection limit of ACS/UDF images, significant emission from the [OII] ionized gas is detected. (2) We detect an excess of velocity dispersion in the southern part of J033241.88-274853.9 in comparison to expectations from a rotating disk model. We considered two disk formation scenarios, depending on the gaseous phase geometry. In the first one, we examined whether J033241.88-274853.9 could be a young rotating disk that has been recently collapsed from a pre-existing, very gas-rich rotating disk. This scenario requires two (unknown) additional assumptions to explain the decoupling between the distribution of stars and gas and the excess of velocity dispersion in the same region. In a second scenario, we examine whether J033241.88-274853.9 could be a merger remnant of two gas-rich disks. In this case, the asymmetry observed between the gas and star distributions, as well as the excess of velocity dispersion, find a common explanation. Shocks produced during the merger in this region can be ionized easily and heat the gas while preventing star formation. This makes this scenario more satisfactory than the collapse of a pre-existing, gas-rich rotating disk.Comment: Accepted for publication in A&A. 8 pages & 5 figure

Bridging Model and Observed Stellar Spectra

Accurate model stellar fluxes are key for the analysis of observations of individual stars or stellar populations. Model spectra differ from real stellar spectra due to limitations of the input physical data and adopted simplifications, but can be empirically calibrated to maximise their resemblance to actual stellar spectra. I describe a least-squares procedure of general use and test it on the MILES library.Comment: 7 pages, 6 figures, accepted for publication in MNRA

The penetration of FUV radiation into molecular clouds

The solution of the FUV radiative transfer equation can be complicated if the most relevant radiative processes such as dust scattering and gas line absorption are included, and have realistic (non-uniform) properties. We have extended the spherical harmonics method to solve for the FUV radiation field in illuminated clouds taking into account gas absorption and coherent, nonconservative and anisotropic scattering by dust grains. Our formalism allows us to consistently include: (i) varying dust populations and (ii) gas lines in the FUV radiative transfer. The FUV penetration depth rises for increasing dust albedo and anisotropy of the scattered radiation (e.g. when grains grow towards cloud interiors). Illustrative models of illuminated clouds where only the dust populations are varied confirm earlier predictions for the FUV penetration in diffuse clouds (A_V1) we show that the FUV radiation field inside the cloud can differ by orders of magnitude depending on the grain properties. We show that the photochemical and thermal gradients can be very different depending on grain growth. Therefore, the assumption of uniform dust properties and averaged extinction curves can be a crude approximation to determine the resulting scattering properties, prevailing chemistry and atomic/molecular abundances in ISM clouds or protoplanetary disks.Comment: Accepted for publication in Astronomy & Astrophysics. Section 2. Astrophysical processes. Version 2: minor language corrections added. Figs. 2, 4 and 8 bitmapped to lower resolutio

Dehydrogenated polycyclic aromatic hydrocarbons and UV bump

Recent calculations have shown that the UV bump at about 217.5 nm in the extinction curve can be explained by a complex mixture of PAHs in several charge states. Other studies proposed that the carriers are a restricted population made of neutral and singly-ionised dehydrogenated coronene molecules (C24Hn, n less than 3), in line with models of the hydrogenation state of interstellar PAHs predicting that medium-sized species are highly dehydrogenated. To assess the observational consequences of the latter hypothesis we have undertaken a systematic study of the electronic spectra of dehydrogenated PAHs. We use our first results to see whether such spectra show strong general trends upon dehydrogenation. We used state-of-the-art techniques in the framework of the density functional theory (DFT) to obtain the electronic ground-state geometries, and of the time- dependent DFT to evaluate the electronic excited-state properties. We computed the absorption cross-section of the species C24Hn (n=12,10,8,6,4,2,0) in their neutral and cationic charge-states. Similar calculations were performed for other PAHs and their fullydehydrogenated counterparts. pi electron energies are always found to be strongly affected by dehydrogenation. In all cases we examined, progressive dehydrogenation translates into a correspondingly progressive blue shift of the main electronic transitions. In particular, the pi-pi* collective resonance becomes broader and bluer with dehydrogenation. Its calculated energy position is therefore predicted to fall in the gap between the UV bump and the far-UV rise of the extinction curve. Since this effect appears to be systematic, it poses a tight observational limit on the column density of strongly dehydrogenated medium-sized PAHs.Comment: 5 pages, 7 figures, Astronomy & Astrophysics, in pres

Constraining the low-mass end of the Initial Mass Function with Gravitational Lensing

The low-mass end of the stellar Initial Mass Function (IMF) is constrained by focusing on the baryon-dominated central regions of strong lensing galaxies. We study in this letter the Einstein Cross (Q2237+0305), a z=0.04 barred galaxy whose bulge acts as lens on a background quasar. The positions of the four quasar images constrain the surface mass density on the lens plane, whereas the surface brightness (H-band NICMOS/HST imaging) along with deep spectroscopy of the lens (VLT/FORS1) allow us to constrain the stellar mass content, for a range of IMFs. We find that a classical single power law (Salpeter IMF) predicts more stellar mass than the observed lensing estimates. This result is confirmed at the 99% confidence level, and is robust to systematic effects due to the choice of population synthesis models, the presence of dust, or the complex disk/bulge population mix. Our non-parametric methodology is more robust than kinematic estimates, as we do not need to make any assumptions about the dynamical state of the galaxy or its decomposition into bulge and disk. Over a range of low-mass power law slopes (with Salpeter being Gamma=+1.35) we find that at a 90% confidence level, slopes with Gamma>0 are ruled out.Comment: 5 pages, 6 figures. Accepted for publication in MNRAS Letter

Herschel observations of interstellar chloronium

Using the Herschel Space Observatory's Heterodyne Instrument for the Far-Infrared (HIFI), we have observed para-chloronium (H2Cl+) toward six sources in the Galaxy. We detected interstellar chloronium absorption in foreground molecular clouds along the sight-lines to the bright submillimeter continuum sources Sgr A (+50 km/s cloud) and W31C. Both the para-H2-35Cl+ and para-H2-37Cl+ isotopologues were detected, through observations of their 1(11)-0(00) transitions at rest frequencies of 485.42 and 484.23 GHz, respectively. For an assumed ortho-to-para ratio of 3, the observed optical depths imply that chloronium accounts for ~ 4 - 12% of chlorine nuclei in the gas phase. We detected interstellar chloronium emission from two sources in the Orion Molecular Cloud 1: the Orion Bar photodissociation region and the Orion South condensation. For an assumed ortho-to-para ratio of 3 for chloronium, the observed emission line fluxes imply total beam-averaged column densities of ~ 2.0E+13 cm-2 and ~ 1.2E+13 cm-2, respectively, for chloronium in these two sources. We obtained upper limits on the para-H2-35Cl+ line strengths toward H2 Peak 1 in the Orion Molecular cloud and toward the massive young star AFGL 2591. The chloronium abundances inferred in this study are typically at least a factor ~10 larger than the predictions of steady-state theoretical models for the chemistry of interstellar molecules containing chlorine. Several explanations for this discrepancy were investigated, but none has proven satisfactory, and thus the large observed abundances of chloronium remain puzzling.Comment: Accepted for publication in the Astrophysical Journa