Hydrogen Two-Photon Continuum Emission from the Horseshoe Filament in NGC 1275

Far ultraviolet emission has been detected from a knot of Halpha emission in the Horseshoe filament, far out in the NGC 1275 nebula. The flux detected relative to the brightness of the Halpha line in the same spatial region is very close to that expected from Hydrogen two-photon continuum emission in the particle heating model of Ferland et al. (2009) if reddening internal to the filaments is taken into account. We find no need to invoke other sources of far ultraviolet emission such as hot stars or emission lines from CIV in intermediate temperature gas to explain these data.Comment: 9 pages, 8 figures. Accepted for publication in MNRA

Atomic Data for Permitted Resonance Lines of Atoms and Ions from H to Si, and S, Ar, Ca and Fe

We list vacuum wavelengths, energy levels, statistical weights, transition probabilities and oscillator strengths for permitted resonance spectral lines of all ions of 18 astrophysically important elements (H through Si, S, Ar, Ca, Fe). Using a compilation of experimental energy levels, we derived accurate wavelengths for 5599 lines of 1828 ground-term multiplets which have gf-values calculated in the Opacity Project. We recalculated the Opacity Project multiplet gf-values to oscillator strengths and transition probabilities of individual lines. For completeness, we added 372 resonance lines of NeI, ArI, FeI and FeII ions which are not covered by the Opacity Project. Intercombination and forbidden lines are not included in the present compilation.Comment: 6 pages of text, latex, 1 figure, 4 tables; tables in ASCII format available at ftp://asta.pa.uky.edu/dima/lines/ or at http://www.pa.uky.edu/~verner/atom.html Accepted by Atomic Data Nucl. Data Table

Studies of NGC 6720 with Calibrated HST WFC3 Emission-Line Filter Images--III:Tangential Motions using AstroDrizzle Images

We have been able to compare with astrometric precision AstroDrizzle processed images of NGC 6720 (the Ring Nebula) made using two cameras on the Hubble Space Telescope. The time difference of the observations was 12.925 yrs. This large time-base allowed determination of tangential velocities of features within this classic planetary nebula. Individual features were measured in [N II] images as were the dark knots seen in silhouette against background nebular [O III] emission. An image magnification and matching technique was also used to test the accuracy of the usual assumption of homologous expansion. We found that homologous expansion does apply, but the rate of expansion is greater along the major axis of the nebula, which is intrinsically larger than the minor axis. We find that the dark knots expand more slowly that the nebular gas, that the distance to the nebula is 720 pc +/-30%, and the dynamic age of the Ring Nebula is about 4000 yrs. The dynamic age is in agreement with the position of the central star on theoretical curves for stars collapsing from the peak of the Asymptotic Giant Branch to being white dwarfs

Improved He I Emissivities in the Case B Approximation

We update our prior work on the case B collisional-recombination spectrum of He I to incorporate \textit{ab initio} photoionisation cross-sections. This large set of accurate, self-consistent cross-sections represents a significant improvement in He I emissivity calculations because it largely obviates the piecemeal nature that has marked all modern works. A second, more recent set of \textit{ab initio} cross-sections is also available, but we show that those are less consistent with bound-bound transition probabilities than our adopted set. We compare our new effective recombination coefficients with our prior work and our new emissivities with those by other researchers, and we conclude with brief remarks on the effects of the present work on the He I error budget. Our calculations cover temperatures $5000 \le T_e \le 25000$ K and densities $10^1 \le n_e \le 10^{14}$ cm$^{-3}$. Full results are available online.Comment: Accepted to MNRAS Letters; 4 pages, 4 figures, 2 tables, 1 supplemental fil

The H II Region/PDR Connection: Self-Consistent Calculations of Physical Conditions in Star-Forming Regions

We have performed a series of calculations designed to reproduce infrared diagnostics used to determine physical conditions in star forming regions. We self-consistently calculate the thermal and chemical structure of an H II region and photodissociation region (PDR) that are in pressure equilibrium. This differs from previous work, which used separate calculations for each gas phase. Our calculations span a wide range of stellar temperatures, gas densities, and ionization parameters. We describe improvements made to the spectral synthesis code Cloudy that made these calculations possible. These include the addition of a molecular network with ~1000 reactions involving 68 molecular species and improved treatment of the grain physics. Data from the Spitzer First Look Survey, along with other archives, are used to derive important physical characteristics of the H II region and PDR. These include stellar temperatures, electron densities, ionization parameters, UV radiation flux, and PDR density. Finally, we calculate the contribution of the H II region to PDR emission line diagnostics, which allows for a more accurate determination of physical conditions in the PDR.Comment: 60 pages, 35 figures, to be published in the Astrophysical Journal. Version with full resolution is available at http://www.pa.uky.edu/~nicholas/hii_pdr_high_res.pd

Ultraviolet emission lines of Si II in quasars --- investigating the "Si II disaster"

The observed line intensity ratios of the Si II 1263 and 1307 \AA\ multiplets to that of Si II 1814\,\AA\ in the broad line region of quasars are both an order of magnitude larger than the theoretical values. This was first pointed out by Baldwin et al. (1996), who termed it the "Si II disaster", and it has remained unresolved. We investigate the problem in the light of newly-published atomic data for Si II. Specifically, we perform broad line region calculations using several different atomic datasets within the CLOUDY modeling code under optically thick quasar cloud conditions. In addition, we test for selective pumping by the source photons or intrinsic galactic reddening as possible causes for the discrepancy, and also consider blending with other species. However, we find that none of the options investigated resolves the Si II disaster, with the potential exception of microturbulent velocity broadening and line blending. We find that a larger microturbulent velocity ($\sim 500 \rm \, kms^{-1}$) may solve the Si II disaster through continuum pumping and other effects. The CLOUDY models indicate strong blending of the Si II 1307 \AA\ multiplet with emission lines of O I, although the predicted degree of blending is incompatible with the observed 1263/1307 intensity ratios. Clearly, more work is required on the quasar modelling of not just the Si II lines but also nearby transitions (in particular those of O I) to fully investigate if blending may be responsible for the Si II disaster.Comment: Accepted for publication in Ap