R. J. Dufour

Interstellar Abundance Gradients in NGC 2403: Comparison to M33*

We present new long-slit spectra of 12 H II regions in the Scd spiral galaxy NGC 2403. Gas-phase element abundances for O, N, S, Ne, and Ar were estimated using direct measurements of electron temperature based on detections of [O III] λ4363, [S III] λ6312, and [O II] λ7320-7330, and from theoretical photoionization analysis. We find abundance gradients for O/H and N/O of -0.102 ± 0.009 dex kpc-1 and -0.032 ± 0.005 dex kpc-1, respectively. The relatively flat N/O gradient suggest a significant source of primary nitrogen. An upper limit for the C/O ratio for one H II region was determined from a Hubble Space Telescope FOS spectrum. A mild outward increasing gradient in S/O is seen at marginal significance (0.03 ± 0.02 dex kpc-1). We compare the abundance gradient and effective oxygen yield deduced for NGC 2403 with those determined for M33, another Scd galaxy with very similar structural parameters. We find close agreement in most of the chemical properties between NGC 2403 and M33. However, the effective yield for oxygen determined from closed box chemical evolution calculations is higher in M33 than in NGC 2403. The higher yield derived for M33 is similar to the case of H I-deficient Virgo spiral galaxies. We also compare NGC 2403 and M33 with a larger sample of unbarred spirals having abundance measurements. The global metallicity of the spirals correlates well with galaxy luminosity, as already noted from earlier investigations. The O/H gradient per unit disk scale length does not correlate with galaxy luminosity with possibly small intrinsic scatter, suggesting that spiral galaxies are homologous with regard to chemical evolution. The correlation between gas abundances and local surface brightness (mass density) in late-type spirals appears to be described well by chemical evolution models incorporating self-regulating star formation. However, the characteristic abundance at a given value of surface brightness correlates with galaxy luminosity. This suggests that an additional parameter which scales with galaxy mass influences the chemical properties of disks.

The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations

We present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III 1666 A and C III 1909 A in our spectra, enabling us to determine C(+2)/O(+2) with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of carbon from intermediate-mass stars. Our results also suggest that it may not be appropiate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver & Woosleys best estimate for the 12C(alpha, gamma) 16O nuclear reaction rate, assuming negligible contanmination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III 1750 A multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma uppr limits on the 1750 A feature indicate that the N(+2)/O(+2) ratios in these objects are not significantly larger than the N(+)/O(+) ratios measured from optical spectra. This behavior is consistent with predictions of photionization models, although better detections of N III are needed to confirm the results.

Carbon in Spiral Galaxies from HUBBLE SPACE TELESCOPE Spectroscopy

We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space Telescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O≈-0.8 at log O/H=-4.0 to log C/O≈-0.1 at log O/H=-3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, confirming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemical-evolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much flatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important effect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

The carbon abundance in the Magellanic Clouds from IUE observations of H II regions

Observations of C II), C III), and C IV lines in the ultraviolet spectra of three H II regions in the Small Magellanic Cloud (SMC) and of four H II regions in the Large Magellanic Cloud (LMC) were obtained with the International Ultraviolet Explorer satellite and used to derive the carbon abundance in the Megellanic Clouds by nebular model analyses. Based on absolute emission line fluxes between the IUE and ground-based observations for the individual H II regions, it is found that 12+log C/H = 7.16 +- 0.04 for the SMC and 7.90 +- 0.15 for the LMC (where the errors quoted are statistical; systematic errors due to aperture geometry and positioning differences between the IUE and ground-based observations may be several times larger). The corresponding values for the average C/O ratios are log C/O = -0.89 +- 0.02 for the SMC and -0.48 +- 0.04 for the LMC (statistical errors). For one SMC H II region, N81, the O III) lambda1663 lines were observed in the IUE spectra and permitted direct calculation of the C/O ratio independent of absolute fluxes. The result (log C/O = -0.94) is in excellent agreement with ratio based on absolute fluxes.

The Recent Star Formation History of GR 8 from Hubble Space Telescope Photometry of the Resolved Stars

?????We have used the Hubble Space Telescope to observe the resolved stars in the dwarf irregular galaxy GR 8 (DDO 155, UGC 8091). The data consisted of dithered Wide Field Planetary Camera 2 images in three bands: F439W (1 hr), F555W (30 minutes), and F814W (30 minutes). The stellar photometry was extracted with a modified version of DoPHOT. Artificial star tests showed the data to be 50% complete to V = 26.3, B = 25.4, and I = 25.2. The color-magnitude diagrams contain well-defined populations, including a very young main sequence (MS) (<10 Myr), and a red giant branch as old as several Gyr. These features align well with stellar evolution models of the appropriate metallicity. The distance based on the tip of the red giant branch is in excellent agreement with the Cepheid determination of ? = 26.75 ? 0.35 (2.2 Mpc), which we adopted. An extended stellar halo was discovered well beyond the H I. Based on the MS and blue HeB luminosity function, we calculated the star formation rate (SFR) over the past 500 Myr. The SFR has been fairly constant, at 400 M? Myr-1 kpc-2 with up to 60% variations. The blue HeB stars were used as a tracer for the location of star formation over this time period. The star formation occurred in superassociation size regions (100?200 pc), which lasted ~100 Myr. These regions come and go with no obvious pattern, except that they seem to concentrate in the current locations of H I clumps. This suggested that the H I clumps are long-lived features that support several star-forming events over time. The most likely explanation is that the star-forming regions are gravitationally bound. We estimated the gas-to-star conversion efficiency to be 6%. We compared our results with those of three other dI galaxies: Sextans A, Pegasus DIG, and Leo A. There is a trend of higher SFR per area with larger MH I/LB. Also, the star formation pattern is similar in all four galaxies. Finally, none of the four dI galaxies contained a large starburst, comparable to those in BCD galaxies. Combining the histories of all four galaxies, this implies that less than 5% of dI galaxies are hosting a strong burst of star formation at any given time. Observations of more galaxies are needed to improve this statistic.

High carbon in I Zwicky 18: New results from hubble space telescope spectroscopy

We present new measurements of the gas-phase C/O abundance ratio in both the northwest (NW) and southeast (SE) components of the extremely metal-poor dwarf irregular galaxy I Zw 18, based on ultraviolet spectroscopy of the two H II regions using the Faint Object Spectrograph on the Hubble Space Telescope. We determine values of log C/O = -0.63 ± 0.10 for the NW component and log C/O = -0.56 ± 0.09 for the SE component. In comparison, log C/O = -0.37 in the Sun, while log C/O = -0.85 ± 0.07 in the three most metal-poor irregular galaxies measured by Garnett et al. Our measurements show that C/O in I Zw 18 is significantly higher than in other comparably metal-poor irregular galaxies and above predictions for the expected C/O from massive star nucleosynthesis. These results suggest that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass carbon-producing stars; this idea is supported by stellar photometry of I Zw 18 and its companion, which demonstrate that the current bursts of massive stars were not the first. Despite its very low metallicity, it is likely that I Zw 18 is not a primeval galaxy.

Exploring Broadband GRB Behavior during γ-Ray Emission

S. A. Yost, H. F. Swan, E. S. Rykoff, F. Aharonian, C. W. Akerlof, A. Alday, M. C. B. Ashley, S. Barthelmy, D. Burrows, D. L. Depoy, R. J. Dufour, J. D. Eastman, R. D. Forgey, N. Gehrels, E. Gogus, T. Guver, J. P. Halpern, L. C. Hardin, D. Horns, U. Kizilolu, H. A. Krimm, S. Lepine, E. P. Liang, J. L. Marshall, T. A. McKay, T. Mineo, N. Mirabal, M. Ozel, A. Phillips, J. L. Prieto, R. M. Quimby, P. Romano, G. Rowell, W. Rujopakarn, B. E. Schaefer, J. M. Silverman, R. Siverd, M. Skinner, D. A. Smith, I. A. Smith, S. Tonnesen, E. Troja, W. T. Vestrand, J. C. Wheeler, J. Wren, F. Yuan, and B. Zhang

Morphology and composition of the helix nebula

We present new narrowband filter imagery in Hα and [N II] λ6584, along with UV and optical spectrophotometry measurements from 1200 to 9600 A of NGC 7293, the Helix Nebula, a nearby, photogenic planetary nebula of large diameter and low surface brightness. Detailed models of the observable ionized nebula support the recent claim that the Helix is actually a flattened disk whose thickness is roughly one-third its diameter, with an inner region containing hot, highly ionized gas that is generally invisible in narrowband images. The outer visible ring structure is of lower ionization and temperature and is brighter because of a thickening in the disk. We also confirm a central star effective temperature and luminosity of 120,000 K and 100 L☉, and we estimate a lower limit to the nebular mass to be 0.30 M☉. Abundance measurements indicate the following values: He/H=0.12 (± 0.017), O/H=4.60×10-4 (± 0.18), C/O=0.87 (± 0.12), N/O=0.54 (± 0.14), Ne/O=0.33 (± 0.04), S/O=3.22 × 10-3 (± 0.26), and Ar/O=6.74 × 10-3 (± 0.76). Our carbon abundance measurements represent the first of their kind for the Helix Nebula. The S/O ratio that we derive is anomalously low; such values are found in only a few other planetary nebulae. The central star properties, the supersolar values of He/H and N/O, and a solar level of C/O are consistent with a 6.5 M☉ progenitor that underwent three phases of dredge-up and hot bottom burning before forming the planetary nebula.

The H II regions of the galaxy M101

The physical properties and composition of the H II region population of the spiral galaxy M101 (NGC 5457) were examined using spectrophotometrically calibrated CCD imagery of two overlapping 16-arcmin fields in the M101, in which individual H II regions were identified and mapped using an automated algorithm. The characteristics obtained for the H II region population of M101 include the H II region luminosity function and radial variations in extinction; the forbidden O III line/H-beta, O/H, and forbidden S II line/H-alpha ratios; the ionization parameter U; and the numbers of ionizing photons. Global variations of physical properties of the H II regions in M101 are compared to model expectations derived for other galaxies. The evident star formation in the M101 galaxy is discussed.

Temperature variations from Hubble Space Telescope spectroscopy of the Orion Nebula

We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the fractional mean-square Te variation (t 2 ) across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in order to estimate Te and t 2 in the N + region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [N II] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5 ′′ square (matching the slit width). The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t 2 varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 K to 10232 K; t 2 varies from 0.00584 to 0.0175. The measurements of Te reported here are an average along each line of sight. Therefore, despite finding remarkably low t 2 , we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II] Te exceeds the average [O III] Te confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by θ 1 Ori C; and indirectly, evidence of high electron density.