Stephen C. Russell

Bio: Stephen C. Russell is an academic researcher. The author has contributed to research in topics: Supergiant & Galaxy. The author has an hindex of 6, co-authored 7 publications receiving 1059 citations.

Abundances of the heavy elements in the Magellanic Clouds. III - Interpretation of results

Abstract: Chemical and structural evolutionary models for the Magellanic Clouds assuming bimodal star formation and gas infall are presented. The models are discussed in relation to the observed chemical abundances of the Clouds and are compared with our own Galaxy. The detailed abundances derived from previous work are investigated for any obvious trends with metallicity or differences compared with the Galaxy. Considering all the data, conclusions are drawn on the possible star formation histories of the Magellanic Clouds and the implications for our own Galaxy

Abundances of the Heavy Elements in the Magellanic Clouds. II. H II Regions and Supernova Remnants

Abstract: The first consistent abundance analysis of both H II regions and SNRs in the Magellanic Clouds is presented. Detailed modeling of the SNRs results in abundance measures in good agreement with those from H II regions, confirming the overall accuracy of the generalized modeling code MAPPINGS for both photoionized and shock-excited nebulae. A differential analysis performed between SNRs and the Galactic Herbig-Haro (H-H) objects for elements heavier than Ar reveals that there is no difference for grain depletion in either Cloud SNR or Galactic H-H objects. It shows that there is no measurable zero point error between the Fe abundances determined from supergiants and those determined from SNRs. This permits the two traditionally separated abundance scales of the H II regions and supergiants to be combined for the first time into one comprehensive system for the first time. The importance of this overall abundance pattern for the determination of the star formation history in the Clouds is addressed. 118 refs.

Abundances of the heavy elements in the Magellanic Clouds. I. Metal abundances of F-type supergiants

Abstract: Metal abundances of eight F-type supergiants in each of the Magellanic Clouds were determined using the results of high-resolution spectroscopy analysis of these stars, together with new Stromgren uvby and Cousins (1980) BVRI photometry. It was found that the mean Fe abundance (Fe/H) for the SMC is -0.65 + or - 0.2 dex, and the mean Fe abundance for the LMC is -0.30 + or - 0.2 dex. The abundances of stars in both the SMC and LMC appear relatively uniform, and the abundances of the elements relative to Fe are very similar in both Magellanic Clouds and in Canopus (the carbon-to-iron abundances are the same for all three). It was also found that Nd and Sm are overabundant in both clouds, supporting the trends found by Spite et al. (1988) for the three SMC stars they studied. 140 refs.

Theoretical Modeling of Starburst Galaxies

Abstract: We have modeled a large sample of infrared starburst galaxies using both the PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution (SED) of the young star clusters. PEGASE utilizes the Padova group tracks, while STARBURST99 uses the Geneva group tracks, allowing comparison between the two. We used our MAPPINGS III code to compute photoionization models that include a self-consistent treatment of dust physics and chemical depletion. We use the standard optical diagnostic diagrams as indicators of the hardness of the EUV radiation field in these galaxies. These diagnostic diagrams are most sensitive to the spectral index of the ionizing radiation field in the 1-4 ryd region. We find that warm infrared starburst galaxies contain a relatively hard EUV field in this region. The PEGASE ionizing stellar continuum is harder in the 1-4 ryd range than that of STARBURST99. As the spectrum in this regime is dominated by emission from Wolf-Rayet (W-R) stars, this discrepancy is most likely due to the differences in stellar atmosphere models used for the W-R stars. The PEGASE models use the Clegg & Middlemass planetary nebula nuclei (PNN) atmosphere models for the W-R stars, whereas the STARBURST99 models use the Schmutz, Leitherer, & Gruenwald W-R atmosphere models. We believe that the Schmutz et al. atmospheres are more applicable to the starburst galaxies in our sample; however, they do not produce the hard EUV field in the 1-4 ryd region required by our observations. The inclusion of continuum metal blanketing in the models may be one solution. Supernova remnant (SNR) shock modeling shows that the contribution by mechanical energy from SNRs to the photoionization models is 20%. The models presented here are used to derive a new theoretical classification scheme for starbursts and active galactic nucleus (AGN) galaxies based on the optical diagnostic diagrams.

Using Strong Lines to Estimate Abundances in Extragalactic H II Regions and Starburst Galaxies

Abstract: We have used a combination of stellar population synthesis and photoionization models to develop a set of ionization parameter and abundance diagnostics based only on the use of the strong optical emission lines. These models are applicable to both extragalactic H II regions and star-forming galaxies. We show that, because our techniques solve explicitly for both the ionization parameter and the chemical abundance, the diagnostics presented here are an improvement on earlier techniques based on strong emission-line ratios. Our techniques are applicable at all metallicities. In particular, for metallicities above half solar, the ratio [N II]/[O II] provides a very reliable diagnostic since it is ionization parameter independent and does not have a local maximum. This ratio has not previously been used historically because of worries about reliable calibration over such a long baseline, and reddening correction concerns. However, we show that the use of classical reddening curves and standard calibration are quite sufficient to allow this [N II]/[O II] diagnostic to be used with confidence as a reliable abundance indicator. As we had shown, the commonly used abundance diagnostic R23 depends strongly on the ionization parameter, while the commonly used ionization parameter diagnostic [O III]/[O II] depends strongly on abundance. The iterative method of solution presented here allows both of these parameters to be obtained without recourse to the use of temperature-sensitive line ratios involving faint emission lines. We compare three commonly used abundance diagnostic techniques and show that individually, they contain systematic and random errors. This is a problem affecting many abundance diagnostics, and the errors generally have not been properly studied or understood due to the lack of a reliable comparison abundance, except for very low metallicities, where the [O III] λ4363 auroral line is used. Here we show that the average of these techniques provides a fairly reliable comparison abundance indicator against which to test new diagnostic methods. The cause of the systematic effects are discussed, and we present a new optimal abundance diagnostic method based on the use of line ratios involving [N II], [O II], [O III], [S II], and the Balmer lines. This combined diagnostic appears to suffer no apparent systematic errors, can be used over the entire abundance range and significantly reduces the random error inherent in previous techniques. Finally, we give a recommended procedure for the derivation of abundances in the case that only spectra of limited wavelength coverage are available so that the optimal method can no longer be used.

Physical Properties of Wolf-Rayet Stars

Abstract: The striking broad emission line spectroscopic appearance of Wolf-Rayet (WR) stars has long defied analysis, owing to the extreme physical conditions within their line- and continuum-formin...

The MAPPINGS III Library of Fast Radiative Shock Models

Abstract: We present a new library of fully-radiative shock models calculated with the MAPPINGS iii shock and photoionization code. The library consists of grids of models with shock velocities in the range vs=100-1000 km s −1 and magnetic parameters B/ p n of 10 −4 -10 µG cm 3/2 for five different atomic abundance sets, and for a pre-shock density of 1.0 cm −3 . Additionally, Solar abundance model grids have been calculated for densities of 0.01, 0.1, 10, 100, and 1000 cm −3 with the same range in vs and B/ p n. Each model includes components of both the radiative shock and its photoionized precursor, ionized by the EUV and soft X-ray radiation generated in the radiative gas. We present the details of the ionization structure, the column densities, and the luminosities of the shock and its precursor. Emission line ratio predictions are separately given for the shock and its precursor as well as for the composite shock+precursor structure to facilitate comparison with observations in cases where the shock and its precursor are not resolved. Emission line ratio grids for shock and shock+precursor are presented on standard line ratio diagnostic diagrams, and we compare these grids to observations of radio galaxies and a sample of AGN and star forming galaxies from the Sloan Digital Sky Survey. This library is available online, along with a suite of tools to enable the analysis of the shocks and the easy creation of emission line ratio diagnostic diagrams. These models represent a significant increase in parameter space coverage over previously available models, and therefore provide a unique tool in the diagnosis of emission by shocks. Subject headings: hydrodynamics - shock waves - ISM: abundances,- Galaxies: Nuclei, Galaxies: Seyfert - infrared: ISM, Ultraviolet: ISM, X-rays: ISM

The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 M⊙ stellar mass limit

Abstract: Spectroscopic analyses of hydrogen-rich WN5‐6 stars within the young star clusters NGC 3603 and R136 are presented, using archival Hubble Space Telescope and Very Large Telescope spectroscopy, and high spatial resolution near-IR photometry, including MultiConjugate Adaptive Optics Demonstrator (MAD) imaging of R136. We derive high stellar temperatures for the WN stars in NGC 3603 (T∗ ∼ 42±2 kK) and R136 (T∗ ∼ 53± 3 kK) plus clumping-corrected mass-loss rates of 2 ‐ 5 ×10 −5 M⊙ yr −1 which closely agree with theoretical predictions from Vink et al. These stars make a disproportionate contribution to the global ionizing and mechanical wind power budget of their host clusters. Indeed, R136a1 alone supplies ∼7% of the ionizing flux of the entire 30 Doradus region. Compar isons with stellar models calculated for the main-sequence evolution of 85 ‐ 500 M⊙ accounting for rotation suggest ages of ∼1.5 Myr and initial masses in the range 105 ‐ 170 M⊙ for three systems in NGC 3603, plus 165 ‐ 320 M⊙ for four stars in R136. Our high stellar masses are supported by consistent spectroscopic and dynamical mass determinations for the components of NGC 3603 A1. We consider the predicted X-ray luminosity of the R136 stars if they were close, colliding wind binaries. R136c is consistent with a colliding wind binary system. However, short period, colliding wind systems are excluded for R136a WN stars if mass ratios are of order unity. Widely separated systems would have been expected to harden owing to early dynamical encounters with other massive stars within such a high density environment. From simulated star clusters, whose constituents are randomly sampled from the Kroupa initial mass function, both NGC 3603 and R136 are consistent with an tentative upper mass limit of ∼300 M⊙. The Arches cluster is either too old to be used to diagnose the upper mass limit, exhibits a deficiency of very massive stars, or mo re likely stellar masses have been underestimated ‐ initial masses for the most luminous stars in the Arches cluster approach 200 M⊙ according to contemporary stellar and photometric results. The potential for stars greatly exceeding 150 M⊙ within metal-poor galaxies suggests that such pair-instab ility supernovae could occur within the local universe, as has been claimed for SN 2007bi.