Cesar Esteban

A reappraisal of the chemical composition of the Orion nebula based on Very Large Telescope echelle spectrophotometry

We present Very Large Telescope (VLT) UVES echelle spectrophotometry of the Orion nebula in the 3100-10 400 A range. We have measured the intensity of 555 emission lines, many of them corresponding to permitted lines of different heavy-element ions. This is the largest set of spectral emission lines ever obtained for a Galactic or extragalactic H II region. We have derived He + , C 2+ , O + , O 2+ and Ne 2+ abundances from pure recombination lines. This is the first time that O + and Ne 2+ abundances have been obtained from these kinds of lines in the nebula. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. In all cases, ionic abundances obtained from recombination lines are larger than those derived from collisionally excited lines. We have obtained remarkably consistent independent estimations of the temperature fluctuation parameter, t 2 , from different methods, which are also similar to other estimates from the literature. This result strongly suggests that moderate temperature fluctuations (t 2 between 0.02 and 0.03) are present in the Orion nebula. We have compared the chemical composition of the nebula with those of the Sun and other representative objects. The heavy-element abundances in the Orion nebula are only slightly higher than the solar ones, a difference that can be explained by the chemical evolution of the solar neighbourhood.

ON THE ABUNDANCE DISCREPANCY PROBLEM IN H II REGIONS

The origin of the abundance discrepancy, i.e., the fact that abundances derived from recombination lines are larger than those from collisionally excited lines, is one of the key problems in the physics of photoionized nebulae. In this work, we analyze and discuss data for a sample of Galactic and extragalactic H II regions where this abundance discrepancy has been determined. We find that the abundance discrepancy factor (ADF) is fairly constant and of order 2 in the available sample of H II regions. This is a rather different behavior than that observed in planetary nebulae, where the ADF shows a much wider range of values. We do not find correlations between the ADF and the O/H, O++/H+ ratios, the ionization degree, Te(High), Te(Low)/Te(High), FWHM, and the effective temperature of the main ionizing stars within the observational uncertainties. These results indicate that whatever mechanism is producing the abundance discrepancy in H II regions it does not substantially depend on those nebular parameters. On the contrary, the ADF seems to be slightly dependent on the excitation energy, a fact that is consistent with the predictions of the classical temperature fluctuations paradigm. Finally, we find that Te-values obtained from O II recombination lines in H II regions are in agreement with those obtained from collisionally excited line ratios, a behavior that is again different from that observed in planetary nebulae. These similar temperature determinations are in contradiction with the predictions of the model based on the presence of chemically inhomogeneous clumps but are consistent with the temperature fluctuations paradigm. We conclude that all the indications suggest that the physical mechanism responsible for the abundance discrepancy in H II regions and planetary nebulae are different.

Carbon and Oxygen Galactic Gradients: Observational Values from H II Region Recombination Lines*

We present results of deep echelle spectrophotometry of eight Galactic H ii regions located at Galactocentric distances between 6.3 and 10.4 kpc. The data have been taken with the Very Large Telescope Ultraviolet Echelle Spectrograph in the 3100–10360 range. We have derived C and O abundances from recombination lines for u A all the objects as well as O abundances from this kind of line for three of the nebulae. The intensity of recombination lines is almost independent of the assumed electron temperature as well as of the possible presence of spatial temperature variations or fluctuations inside the nebulae. These data allow the determination of the gas-phase C and O abundance gradients of the Galactic disk, of paramount importance for chemical evolution models. This is the first time the C gradient is derived from such a large number of H ii regions distributed in such a wide range of Galactocentric distances. Abundance gradients are found of the form dex kpc 1 , D log (O/H) p 0.044 0.010 dex kpc 1 , and dex kpc 1 . D log (C/H) p 0.103 0.018 D log (C/O) p 0.058 0.018 Subject headings: Galaxy: abundances — H ii regions — ISM: abundances

The Localized Chemical Pollution in NGC 5253 Revisited: Results from Deep Echelle Spectrophotometry*

WepresentechellespectrophotometryofthebluecompactdwarfgalaxyNGC 5253obtainedwiththeVLTUVES. We have measured the intensities of a large number of permitted and forbidden emission lines in four zones of the centralpartof thegalaxy.Wedetect faint C iiand O iirecombinationlines, thefirsttimethat these areunambiguously detected in a dwarf starburst galaxy. The physical conditions of the ionized gas have been derived using a large number of different line intensity ratios. Chemical abundances of He, N, O, Ne, S, Cl, Ar, and Fe have been determined following standardmethods.C ++ andO ++ abundanceshavebeenderivedfrompurerecombinationlinesand arelargerthanthoseobtainedfromcollisionallyexcitedlines(from0.30to0.40dexforC ++ andfrom0.19to0.28dex for O ++ ). This result is consistent with atemperaturefluctuation parameter (t 2 ) between 0.050 and 0.072. Weconfirm previous results that indicate the presence of a localized N enrichment in certain zones of NGC 5253 and detect a possible slight He overabundance in the same zones. The enrichment pattern agrees with that expected for the pollution by the ejecta of Wolf-Rayet (W-R) stars. The amount of enriched material needed to produce the observed overabundance is consistent with the mass lost by the number of W-R stars estimated in the starbursts. We discuss the possible origin of the difference between abundances derived from recombination and collisionally excited lines (the so-called abundance discrepancy problem) in H ii regions, finding that a recent hypothesis based on the delayed enrichment by SN ejecta inclusions seems not to explain the observed features. Subject headingg galaxies: abundances — galaxies: clusters: individual (NGC 5253) — galaxies: kinematics and dynamics — galaxies: starburst Online material: color figures, machine-readable table

Carbon, Nitrogen, and Oxygen Galactic Gradients: A Solution to the Carbon Enrichment Problem

ElevenmodelsofGalacticchemicalevolution,differinginthecarbon,nitrogen,andoxygenyieldsadopted,have been computed to reproduce the Galactic O/H values obtained from H ii regions. All the models fit the oxygen gradient, but only two models also fit the carbon gradient, those based on carbon yields that increase with metallicity owing to stellar winds in massive stars (MSs) and decrease with metallicity owing to stellar winds in low- and intermediate-mass stars (LIMSs). The successful models also fit the C/O versus O/H evolution history of the solar vicinity obtainedfromstellarobservations. We alsocompare thepresent-dayN/H gradient andtheN/O versusO/H and the C/Fe, N/Fe, O/Fe versus Fe/H evolution histories of the solar vicinity predicted by our two best models with those derived from H ii regions and from stellar observations. While our two best models fit the C/H and O/H gradients, as well as the C/O versus O/H history, only model 1 fits well the N/H gradient and the N/O values for metal-poor stars but fails to fit the N/H values for metal-rich stars. Therefore, we conclude that our two best models solve theC enrichmentproblembutthatfurther workneeds to be done on theN enrichmentproblem.Byadding the C and O production since the Sun was formed predicted by models 1 and 2 to the observed solar values, we find an excellent agreement with the O/H and C/H values of the solar vicinity derived from H ii region O and C recombination lines. Our results are based on an initial mass function (IMF) steeper than Salpeter’s; a Salpeter-like IMF predicts C/H, N/H, and O/H ratios higher than observed. One of the most important results of this paper is that the fraction of carbon due to MSs and LIMSs in the interstellar medium is strongly dependent on time and on the galactocentric distance; at present about half of the carbon in the interstellar medium of the solar vicinity has been produced by MSs and half by LIMSs. Subject headingg Galaxy: abundances — Galaxy: evolution — ISM: abundances — stars: mass loss

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.

Properties of the ionized gas in HH 202 – II. Results from echelle spectrophotometry with Ultraviolet Visual Echelle Spectrograph

We present results of deep echelle spectrophotometry of the brightest knot of the Herbig– Haro object HH 202 in the Orion Nebula – HH 202-S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 A. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca + and Cr + abundances in the Orion Nebula. We also calculate the He + ,C 2+ ,O + and O 2+ abundances from recombination lines. The difference between the O 2+ abundances determined from collisionally excited and recombination lines – the so-called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t 2 , of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t 2 values for both components from the analysis of the intensity ratios of He I lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202-S, finding enough evidence to conclude that the flow of HH 202-S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202-S.

Very Large Telescope Echelle Spectrophotometry of the Planetary Nebula NGC 5307 and Temperature Variations

Echelle spectrophotometry of the planetary nebula NGC 5307 is presented. The data consist of Very Large Telescope Ultraviolet Visual Echelle Spectrograph observations in the 3100-10360 A range. Electron temperatures and densities have been determined using different line intensity ratios. We determine the H, He, C, and O abundances based on recombination lines; these abundances are almost independent of the temperature structure of the nebula. We also determine the N, O, Ne, S, Cl, and Ar abundances based on collisionally excited lines; the ratios of these abundances relative to that of H depend strongly on the temperature structure of the nebula. From the O II/[O III] line intensity ratios we find a t2 = 0.056 ± 0.005. The chemical composition of NGC 5307 is compared with those of the Sun and the Orion Nebula. From the study of the relative intensities of the O II recombination lines of multiplet 1 in this and other nebulae, it is found that for electron densities smaller than about 5000 cm-3 collisional redistribution is not complete; this effect has to be taken into account to derive the O abundances for those cases in which not all the lines of the multiplet are observed. From the O II λ4649 versus Ne(Cl III) diagram we find a critical electron density of 1325 cm-3 for collisional redistribution of the O II lines of multiplet 1. Based on this diagram, we also argue that the O II and the [O III] lines originate in the same regions. We also find that the radial velocities and the FWHM of the O II and [O III] lines in NGC 5307 are similar, supporting the previous result. These two results imply that for NGC 5307 and probably for many other gaseous nebulae chemical inhomogeneities are not responsible for the large temperature fluctuations observed.

Faint emission lines in the Galactic H ii regions M16, M20 and NGC 3603★

We present deep echelle spectrophotometry of the Galactic H II regions M16, M20 and NGC 3603. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100‐10 400 A range. We have detected more than 200 emission lines in each region. Physical conditions have been derived using different continuum and line intensity ratios. We have derived He + ,C ++ and O ++ abundances from pure recombination lines as well as collisionally excited lines (CELs) for a large number of ions of different elements. We have obtained consistent estimations of the temperature fluctuation parameter, t 2 , using different methods. We also report the detection of deuterium Balmer lines up to Dδ (M16) and to Dγ (M20) in the blue wings of the hydrogen lines, which excitation mechanism seems to be continuum fluorescence. The temperature fluctuation paradigm agrees with the results obtained from optical CELs, and the more uncertain ones from far-infrared fine-structure CELs in NGC 3603, although, more observations covering the same volume of the nebula are necessary to obtain solid conclusions.

Deep echelle spectrophotometry of S 311, a Galactic H ii region located outside the solar circle

We present echelle spectrophotometry of the Galactic H II region S 311. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100‐ 10 400 A range. We have measured the intensities of 263 emission lines; 178 are permitted lines of H 0 ,D 0 (deuterium), He 0 ,C 0 ,C + ,N 0 ,N + ,O 0 ,O + ,S + ,S i 0 ,S i + ,A r 0 and Fe 0 ; some of them are produced by recombination and others mainly by fluorescence. Physical conditions have been derived using different continuum- and line-intensity ratios. We have derived He + , C ++ and O ++ ionic abundances from pure recombination lines as well as abundances from collisionally excited lines for a large number of ions of different elements. We have obtained consistent estimations of t 2 applying different methods. We have found that the temperature fluctuations paradigm is consistent with the T e(He I )v ersus T e(H I) relation for H II regions, in contrast with what has been found for planetary nebulae. We report the detection of deuterium Balmer lines up to Dδ in the blue wings of the hydrogen lines, whose excitation mechanism seems to be continuum fluorescence. Ke yw ords: ISM: abundances ‐ H II regions ‐ ISM: individual: S 311.