Jorge García-Rojas

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

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.

Star formation and stellar populations in the Wolf-Rayet(?) luminous compact blue galaxy IRAS 08339+6517 ,

Aims. IRAS 08339+6517 is a luminous infrared and Lyα-emitting starburst galaxy that possesses a dwarf companion object at a projected distance of 56 kpc. An H i tidal tail has recently been detected between both galaxies, suggesting that about 70% of the neutral gas has been ejected from them. Methods. We present deep broad-band optical images, together with narrow band Hα CCD images, and optical intermediate-resolution spectroscopy of both galaxies. Results. The images reveal interaction features between both systems and strong Hα emission in the inner part of IRAS 08339+6517. The chemical composition of the ionized gas of the galaxies is rather similar. The analysis of their kinematics also indicates interaction features and reveals an object that could be a candidate tidal dwarf galaxy or a remnant of an earlier merger. Our data suggest that the H i tail has been mainly formed from material stripped from the main galaxy. We find weak spectral features that could be attributed to the presence of Wolf-Rayet stars in this starburst galaxy and estimate an age of the most recent burst of around 4–6 Myr. A more evolved underlying stellar population, with a minimal age between 100–200 Myr, is also detected and fits an exponential intensity profile. A model which combines 85% young and 15% old populations can explain both the spectral energy distribution and the H i Balmer and He i absorption lines presented in our spectrum. The

Chemical Abundances of the Galactic H II Region NGC 3576 Derived from Very Large Telescope Echelle Spectrophotometry

We present echelle spectrophotometry of the Galactic H II region NGC 3576. The data have been taken with the VLT UVES echelle spectrograph in the 3100 to 10400 angstroms range. We have measured the intensities of 458 emission lines, 344 are permitted lines of H0, He0, C+, N0, N+, N++, O0, O+, Ne+, S++, Si0, Si+, Ar0 and Ar+; some of them are produced by recombination and others mainly by fluorescence. Electron temperatures and densities have been determined using different continuum and line intensity ratios. We have derived He+, C++, O+, O++ and Ne++ ionic abundances from pure recombination lines. We have also derived abundances from collisionally excited lines for a large number of ions of different elements. Remarkably consistent estimations of t2 have been obtained by comparing Balmer and Paschen to [O III] temperatures, and O++ and Ne++ ionic abundances obtained from collisionally excited and recombination lines. The chemical composition of NGC 3576 is compared with those of other Galactic H II regions and with the one from the Sun. A first approach to the gas-phase Galactic radial abundance gradient of C as well as of the C/O ratio has been made.

The planetary nebula IC 4776 and its post-common-envelope binary central star

We present a detailed analysis of IC 4776, a planetary nebula displaying a morphology believed to be typical of central star binarity. The nebula is shown to comprise a compact hourglass-shaped central region and a pair of precessing jet-like structures. Time-resolved spectroscopy of its central star reveals a periodic radial velocity variability consistent with a binary system. Whilst the data are insufficient to accurately determine the parameters of the binary, the most likely solutions indicate that the secondary is probably a low-mass main-sequence star. An empirical analysis of the chemical abundances in IC 4776 indicates that the common-envelope phase may have cut short the asymptotic giant branch evolution of the progenitor. Abundances calculated from recombination lines are found to be discrepant by a factor of approximately 2 relative to those calculated using collisionally excited lines, suggesting a possible correlation between low-abundance discrepancy factors and intermediate-period post-common-envelope central stars and/or Wolf-Rayet central stars. The detection of a radial velocity variability associated with the binarity of the central star of IC 4776 may be indicative of a significant population of (intermediate-period) post-common-envelope binary central stars that would be undetected by classic photometric monitoring techniques.