Silvia Torres-Peimbert

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.

Optical Recombination Lines of Heavy Elements in Giant Extragalactic H II Regions

We present high-resolution observations of the giant extragalactic H II regions NGC 604, NGC 2363, NGC 5461, and NGC 5471, based on observations taken with the ISIS spectrograph on the William Herschel Telescope. We have detected, for the first time, C II and O II recombination lines in these objects. We find that recombination lines give larger C++ and O++ abundances than collisionally excited lines, suggesting that temperature variations may be present in the objects. We detect [Fe IV] lines in NGC 2363 and NGC 5471, the most confident detection of optical lines of this kind in H II regions. Considering the temperature structure, we derive their H, He, C, N, O, Ne, S, Ar, and Fe abundances. From the recombination lines of NGC 5461 and NGC 5471, we determine the presence of C/H and O/H gradients in M101. We calculate the ΔY/ΔO and ΔY/ΔZ values considering the presence of temperature variations and under the assumption of constant temperature. We obtain a better agreement with models of galactic chemical evolution by considering the presence of temperature variations than by assuming that the temperature is constant in these nebulae.

Physical conditions of H II regions in M101 and the pregalactic helium abundance

Spectrophotometry in the 3400-7400 A range is presented for eight H II regions and the nucleus of M101. The He, N, O, Ne, S, and Ar abundances relative to H are derived. The O/H ratios are smaller than previously found by about 0.2 dex. Negative gradients with galactocentric distance of the O/H, N/O, and He/H ratios are found. No gradients in the S/O, Ne/O, and Ar/O ratios are found. The pregalactic helium abundance by mass has been determined from NGC 2363, NGC 5471, and the SMC H II regions and amounts to 0.230 + or - 0.006. 67 refs.

The O++/H+ abundance ratio in gaseous nebulae derived from recombination lines

We present O +2 /H + values for the Orion Nebula, M17, and NGC 6572 based on O II recombination lines, which are independent of the temperature structure of the observed nebulae. In the H II regions sampled (Orion and M17) the O +2 /H + values derived from O II recombination lines are about a factor of 2 higher than those derived from O III forbidden lines. These differences can be accounted for by the presence of spatial temperature variations over the observed volumes. The abundances derived from the recombination lines eliminate the O/H discrepancy between the stellar values and the H II region values of the solar neighborhood

Gradients in the physical conditions of M101 and the pregalactic helium abundance

Photoelectric spectrophotometry in the 3400-7400 A range is presented for five H II regions and the nucleus of M101. The abundances of He, N, O, Ne, S, and Ar relative to H are derived for three of the H II regions. Gradients were found in: electron temperature, He/H, N/H, O/H, S/H, and Ar/H. It was found that for a given O/H the N/O values are smaller in M101 than in the Galaxy, probably indicating that on the average the stellar population of M101 is younger than that of the Galaxy. For the first time the effects due to He and H stellar underlying emission and absorption in the N(He)/N(H) ratio were estimated quantitatively and they were found to be almost negligible. A pregalactic helium abundance of Y/sub p/ = 0.216 +- 0.0010 (3sigma) was derived.

Si/O Abundance Ratios in Extragalactic H II Regions from Hubble Space Telescope UV Spectroscopy

We have measured the Si III] intercombination doublet at 1883 ? and 1892 ? in seven extragalactic H II regions using the Faint Object Spectrograph on the Hubble Space Telescope. These measurements were used to derive the Si/C and Si/O gas phase abundance ratios in the ionized gas. We find that Si/O shows no systematic variation with O/H over the range -4.8 < log O/H < -3.4. The weighted mean value for Si/O over this range is log Si/O = -1.59 ? 0.07. For comparison, the solar value is log Si/O = -1.37, while for Galactic B stars and supergiants -1.6 < log Si/O < -1.2. Uncertainty in the stellar reference value for Si/O prevents a straightforward interpretation of our Si/O ratios. However, if the solar Si/O ratio represents the intrinsic cosmic ratio, our results imply that, on average, only about 50% of the total abundance of silicon in these H II regions is incorporated into dust grains, a value significantly smaller than is typically measured in dense interstellar clouds. Our results suggest that some grain modification is occurring within the H II region environment.

Hubble Space Telescope Observations of the Dusty Small Magellanic Cloud H II Region N88A

We present results from the analysis of Hubble Space Telescope Faint Object Spectrograph (FOS), WFPC1, IUE, and Cerro Tololo Inter-American Observatory 4 m observations of the morphology, physical conditions, and chemical abundances in the anomalous H II region N88A in the Small Magellanic Cloud. Not only is N88A unusual among SMC H II regions because it contains much dust, it also is found to have a high electron density and complex ionization structure. The derived reddening curve for the nebula is flatter in the UV than the general SMC extinction curve, suggesting the absence of small grains. A detailed abundance analysis, using both empirical emission-line diagnostics and photoionization model comparisons, indicates that carbon and silicon are enriched in the nebula, while He, O, N, Ne, and Ar are similar in abundance to other SMC H II regions. There is no evidence for the large fluctuations in temperature or density that are seen to occur in some comparably dense planetary nebulae, so we believe that our rather traditional analyses are well founded. We conclude that SMC N88A is a very young H II region forming out of a small dusty molecular cloud that is being disrupted by star formation in a larger OB association in the area. The high carbon and silicon abundances in N88A are attributed to photoevaporation of dust grains existing in the molecular cloud material—rather than being ejected from recent stellar mass loss from stars in N88A itself.

Temperature and Density Fluctuations in Planetary Nebulae

The determination of the chemical composition of gaseous nebulae depends on whether they contain fluctuations in density and/or temperature beyond those predicted by photoionization models or pressure balance. These fluctuations are strongly suggested by the unexpectedly large observed strengths of O II and C II recombination lines. If there are dense clumps, the derived abundances can be wrong by a factor of 2 or more. This paper does not address the physics of producing fluctuations but rather examines the spectroscopic consequences if the fluctuations exist. There are 10 planetary nebulae (PNs) or portions of spatially resolved PNs in which the recombination line O II λ4649 has been measured along with collisionally excited O III] λ1664, [O III] λ4363, λ5007, C III] λ1908, and the recombination line C II λ4267. The C+2/O+2 obtained from λ1908/λ1664 is the same as that from the recombination lines, λ4267/λ4649. This result, which is little affected by reddening, positioning of the ultraviolet observations relative to the optical, or any temperature or density fluctuations, strongly suggests that the standard physics of the recombination lines is correct. We define T4363 as the temperature that would produce the observed dereddened [O III] λ4363/λ5007 ratio at low densities (ne < 104 cm-3, so that the collisional de-excitation of the 1D level of O+2 is negligible.) Similarly, T1664 describes (O III] λ1664)/λ5007, T4649 describes (O II λ4649)/λ5007, and T1908 describes (C II λ1908)/(C III] λ4267). We have extensively investigated the case where two separate zones along the line of sight have arbitrary densities and temperatures. We show by models and physical reasoning that the inequalities T1664 ≥ T4363 ≥ T1908 ≥ T4649 must hold with a general distribution of temperatures within the nebulae for low densities. For high densities, T4363 ≥ T1664 is possible but the other inequalities must hold. Plots of various temperature ratios are given showing permitted and forbidden regions with and without density fluctuations. We analyze the line strengths in the PNs for which there are measurements of C II λ4267 as well as ultraviolet and optical lines of [O III] and [O II]. Only 12 of the 47 lines of sight have understandable line ratios if no fluctuations are present, even if errors in line ratios of ~30% in the line-strength ratios are allowed. The most discrepant objects require an increase in the observed carbon line ratio λ1908/λ4267 by an order of magnitude to bring the C+2 abundances into agreement unless there are severe fluctuations. Spatially resolved photometry is needed to determine whether it is very local variations of temperature and/or density that produce anomalous line strengths with present-day spatial resolution.

Broad Hα Wings in Nebulae around Evolved Stars and in Young Planetary Nebulae

Eleven objects that have been reported as proto-planetary nebula or as young planetary nebulae that show very extended Hα wings are presented. The extension of these wings is larger than 800 km s-1. Data for two symbiotic stars that show this same characteristic are also presented. Raman scattering is the mechanism that best explains the wings in 10 of the PNe and in the two symbiotic stars. In the PN IRAS 20462+3416 the wing profile can be explained by very intense stellar wind.