Tuesday, 1/29/13

Three papers today. The first is with a PAH theme for Carl on "Evolution of PAHs in photodissociation regions: Hydrogenation and charge states", it’s mostly modeling with a PDR code, but also applied to the PDR of local nebula NGC 7023 as well as the diffuse medium. The next is on H2O, and really how plentiful it is, in high-redshift ULIRGS (detected with the help of lensing). Lastly, there’s a paper that Eric has let me know is actually on a PNe and not an HII region, but is still doing state-of-the-art stuff modeling the physical conditions of the NGC 6720 nebula using HST WFC3 emission-line filter images (He II 4686 , Hbeta, [OIII] 5007, Halpha, [NII] 6583, [SII] 6716, [OIII] 4363, [NII] 5755, [S II] 6731, holy moly that’s a lot of lines!). Enjoy! – Alison

Title: Evolution of PAHs in photodissociation regions: Hydrogenation and charge states
Authors: Montillaud, J.; Joblin, C.; Toublanc, D.
Bibliographic Code: 2013arXiv1301.6507M


Various studies have emphasized variations of the charge state and composition of the interstellar polycyclic aromatic hydrocarbon (PAH) population in photodissociation regions (PDRs). We aim to model the spatial evolution of the charge and hydrogenation states of PAHs in PDRs. We focus on the specific case of the north-west (NW) PDR of NGC 7023 and also discuss the case of the diffuse interstellar medium (ISM). The physical conditions in NGC 7023 NW are modelled using a state-of-the-art PDR code. We then use a new PAH chemical evolution model that includes recent experimental data on PAHs and describes multiphoton events. We consider a family of compact PAHs bearing up to 96 carbon atoms. The calculated ionization ratio is in good agreement with observations in NGC 7023 NW. Within the PDR, PAHs evolve into three major populations: medium-sized PAHs (50<Nc<90) are normally hydrogenated, larger PAHs (Nc>90) can be superhydrogenated, and smaller species (Nc<50) are fully dehydrogenated. In the cavity, where the fullerene C60 was recently detected, all the studied PAHs are found to be quickly fully dehydrogenated. PAH chemical evolution exhibits a complex non-linear behaviour as a function of the UV radiation field because of multiphoton events. Steady state for hydrogenation is reached on timescales ranging from less than a year for small PAHs, up to 10000 years for large PAHs at Av=1. We identified critical reactions that need more studies. Our new model allows us to rationalize the observational constraints without any fitting parameter. PAHs smaller than 50 carbon atoms are not expected to survive in the NGC 7023 NW PDR. A similar conclusion is obtained for the diffuse ISM. Carbon clusters turn out to be end products of PAH photodissociation, and the evolution of these clusters needs to be investigated further to evaluate their impact on the chemical and physical evolution of PDRs.

Title: H2O emission in high-z ultra-luminous infrared galaxies
Authors: Omont, A.; Yang, Chentao; Cox, P.; Neri, R.; Beelen, A.; Bussmann, S.; Gavazzi, R.; van der Werf, P.; Riechers, D.; Downes, D.; Krips, M.;Dye, S.; Ivison, R.; Vieira, J. D.; Weiss, A.; Aguirre, J. E.; Baes, M.; Baker, A. J.; Bertoldi, F.; Cooray, A.; Dannerbauer, H.; De Zotti, G.;Eales, S. A.; Fu, H.; Gao, Y.; Guelin, M.; Harris, A. I.; Jarvis, M.; Lehnert, M.; Leeuw, L.; Lupu, R.; Menten, K.; Michalowski, M. J.;Negrello, M.; Serjeant, S.; Temi, P.; Auld, R.; Dariush, A.; Dunne, L.; Fritz, J.; Hopwood, R.; Hoyos, C.; Ibar, E.; Maddox, S.; Smith, M. W. L.;Valiante, E.; Bock, J.; Bradford, C. M.; Glenn, J.; Scott, K. S.
Bibliographic Code: 2013arXiv1301.6618O


Using IRAM PdBI we report the detection of H2O in six new lensed ultra-luminous starburst galaxies at high redshift, discovered in the Herschel H-ATLAS survey. The sources are detected either in the 2_{02}-1_{11} or 2_{11}-2_{02} H_2O emission lines with integrated line fluxes ranging from 1.8 to 14 Jy.km/s. The corresponding apparent luminosities are mu x L_H2O ~ 3-12 x 10^8 Lo, where mu is the lensing magnification factor (3 < mu < 12). These results confirm that H2O lines are among the strongest molecular lines in such galaxies, with intensities almost comparable to those of the high-J CO lines, and same profiles and line widths (200-900 km/s) as the latter. With the current sensitivity of PdBI, H2O can therefore easily be detected in high-z lensed galaxies (with F(500um) > 100 mJy) discovered in the Herschel surveys. Correcting the luminosities for lensing amplification, L_H2O is found to have a strong dependence on the IR luminosity, varying as ~L_IR^{1.2}. This relation which needs to be confirmed with better statistics, may indicate a role of radiative (IR) excitation of the H2O lines, and implies that high-z galaxies with L_IR >~ 10^13 Lo tend to be very strong emitters in H2O, that have no equivalent in the local universe.

Title: Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images–II:Physical Conditions
Authors: O’Dell, C. R.; Ferland, G. J.; Henney, W. J.; Peimbert, Manuel
Bibliographic Code: 2013arXiv1301.6642O


We have performed a detailed analysis of the electron temperature and density in the the Ring Nebula using the calibrated HST WFC3 images described in the preceding paper. The electron temperature (Te) determined from [N II] and [O III] rises slightly and monotonically towards the central star. The observed equivalent width (EW) in the central region indicates that Te rises as high as 13000 K. In contrast, the low EW’s in the outer regions are largely due to scattered di?ffuse Galactic radiation by dust. The images allowed determination of unprecedented small scale variations in Te. These variations indicate that the mean square area temperature fluctuations are signi?ficantly higher than expected from simple photoionization. The power producing these fluctuations occurs at scales of less than 3.5E15 cm. This scale length provides a strong restriction on the mechanism causing the large t^2 values observed.

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