Leticia Carigi

The evolution of the C/O ratio in metal-poor halo stars

We report new measurements of carbon and oxygen abundances in 34 F and G dwarf and subgiant stars belonging to the halo population and spanning a range of metallicity from (Fe/H) = −0. 7t o−3.2 . The survey is based on observations of four permitted lines of C  near 9100 A and the O  λ7774 triplet, all recorded at high signal-to-noise ratios with the UVES echelle spectrograph on the ESO VLT. The line equivalent widths were analysed with the 1D, LTE, MARCS model atmosphere code to deduce C and O abundances; corrections due to non-LTE and 3D effects are discussed. When combined with similar published data for disk stars, our results confirm the metallicity dependence of the C/O ratio known from previous stellar and interstellar studies: C/O drops by a factor of ∼3-4 as O/H decreases from solar to ∼1/10 solar. Analysed within the context of standard models for the chemical evolution of the solar vicinity, this drop results from the metallicity dependence of the C yields from massive stars with mass loss, augmented by the delayed release of C from stars of low and intermediate mass. The former is, however, always the dominant factor. Our survey has also uncovered tentative evidence to suggest that, as the oxygen abundance decreases below (O/H) = −1, (C/O) may not remain constant at (C/O) = −0.5, as previously thought, but increase again, possibly approaching near-solar values at the lowest metallicities ((O/H) < ∼ − 3). With the current dataset this is no more than a 3σ effect and it may be due to metallicity-dependent non-LTE corrections to the (C/O) ratio which have not been taken into account. However, its potential importance as a window on the nucleosynthesis by Population III stars is a strong incentive for future work, both observational and theoretical, to verify its reality.

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

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

Carbon and oxygen abundances in stellar populations

Abundances of C, O, and Fe are determined for F and G main-sequence stars in the solar neighborhood in order to study trends and systematic differences in the C/Fe, O/Fe, and C/O ratios for stellar populations. Carbon abundances are determined from the CI lines at 5052 and 5380 AA and oxygen abundances from the OI triplet at 7774 AA and the [OI] line at 6300 AA. MARCS model atmospheres are applied and non-LTE corrections for the OI triplet are included. Systematic differences between high- and low-alpha halo stars and between thin- and thick-disk stars are seen in the trends of [C/Fe] and [O/Fe]. The two halo populations and thick-disk stars show the same trend of [C/O] versus [O/H], whereas thin-disk stars are shifted to higher [C/O]. Furthermore, we find some evidence of higher C/O and C/Fe ratios in stars hosting planets than in stars for which no planets have been detected. The results suggest that C and O in both high- and low-alpha halo stars and in thick-disk stars are made mainly in massive stars, whereas thin-disk stars have an additional carbon contribution from low-mass AGB and massive stars of high metallicity causing a rising trend of C/O with increasing metallicity. However, the C/O ratio does not exceed 0.8, which seems to exclude formation of carbon planets if proto-planetary disks have the same composition as their parent stars.

Chemical evolution models of local dwarf spheroidal galaxies

We calculate chemical evolution models for four dwarf spheroidal (dSph) satellites of the Milky Way (Carina, Ursa Minor, Leo I and Leo II) for which reliable non-parametric star formation histories have been derived. In this way, the independently-obtained star formation histories are used to constrain the evolution of the systems we are treating. This allows us to obtain robust inferences on the history of such crucial parameters of galactic evolution as gas infall, gas outflows and global metallicities for these systems. We can then trace the metallicity and abundance ratios of the stars formed, the gas present at any time within the systems and the details of gas ejection, of relevance to enrichment of the galaxies environment. We find that galaxies showing one single burst of star formation (Ursa Minor and Leo II) require a dark halo slightly larger that the current estimates for their tidal radii, or the presence of a metal-rich selective wind that might carry away much of the energy output of their supernovae before this might have interacted and heated the gas content, for the gas to be retained until the observed stellar populations have formed. Systems showing extended star formation histories (Carina and Leo I), however, are consistent with the idea that their tidally-limited dark haloes provide the necessary gravitational potential wells to retain their gas. The complex time structure of the star formation in these systems remains difficult to understand. Observations of detailed abundance ratios for Ursa Minor strongly suggest that the star formation history of this galaxy might in fact resemble the complex picture presented by Carina or Leo I, but localized at a very early epoch.

NGC 2579 and the carbon and oxygen abundance gradients beyond the solar circle

We present deep echelle spectrophotometry of the Galactic Hii region NGC 2579. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3550–10400 u range. This object, which has been largely neglected, shows however a rather high surface brightness, a high ionization degree and is located at a galactocentric distance of 12.4 ± 0.7 kpc. Therefore, NGC 2579 is an excellent probe for studying the behaviour of the gas phase radial abundance gradients in the outer disc of the Milky Way. We derive the physical conditions of the nebula using several emission line-intensity ratios as well as the abundances of several ionic species from the intensity of collisionally excited lines. We also determine the ionic abundances of C 2+ , O + and O 2+ – and therefore the total O abundance – from faint pure recombination lines. The results for NGC 2579 permit to extend our previous determinations of the C, O and C/O gas phase radial gradients of the inner Galactic disc (Esteban et al. 2005) to larger galactocentric distances. We find that the chemical composition of NGC 2579 is consistent with flatten gradients at its galactocentric distance. In addition, we have built a tailored chemical evolution model that reproduces the observed radial abundance gradients of O, C and N and other observational constraints. We find that a levelling out of the star formation efficiency about and beyondthe isophotal radius can explain the flattening of chemical gradients observed in the outer Galactic disc.

Carbon and oxygen abundances from recombination lines in low-metallicity star-forming galaxies. Implications for chemical evolution

We present deep echelle spectrophotometry of the brightest emission-line knots of the star-forming galaxies He 2 10, Mkn 1271, NGC 3125, NGC 5408, POX 4, SDSS J1253 0312, Tol 1457 262, Tol 1924 416 and the Hii region Hubble V in the Local Group dwarf irregular galaxy NGC 6822. The data have been taken with the Very Large Telescope Ultraviolet-Visual Echelle Spectrograph in the 3100{10420 A range. We determine electron densities and temperatures of the ionized gas from several emission-line intensity ratios for all the objects. We derive the ionic abundances of C 2+ and/or O 2+ from faint pure recombination lines (RLs) in several of the objects, permitting to derive their C/H and C/O ratios. We have explored the chemical evolution at low metallicities analysing the C/O vs. O/H, C/O vs. N/O and C/N vs. O/H relations for Galactic and extragalactic Hii regions and comparing with results for halo stars and DLAs. We

Chemical consequences of low star formation rates: stochastically sampling the initial mass function

When estimating the abundances which result from a given star formation event, it is customary to treat the initial mass function (IMF) as a series of weight factors to be applied to the stellar yields, as a function of mass, implicitly assuming one is dealing with an infinite population. However, when the stellar population is small, the standard procedure would imply the inclusion of fractional numbers of stars at certain masses. We study the effects of small number statistics on the resulting abundances by performing a statistical sampling of the IMF to form a stellar population out of discrete numbers of stars. A chemical evolution code then follows the evolution of the population, and traces the resulting abundances. The process is repeated to obtain a statistical distribution of the resulting abundances and their evolution. We explore the manner in which different elements are affected, and how different abundances converge to the infinite population limit as the total mass increases. We include a discussion of our results in the context of dwarf spheroidal galaxies and show the recently reported internal dispersions in abundance ratios for dSph galaxies might be partly explained through the stochastic effects introduced by a low star formation rate, which can account for dispersions of over 2 dex in [C/O], [N/O], [C/Fe], [N/Fe] and [O/Fe].

Carbon and oxygen abundances in low metallicity dwarf galaxies

The study of carbon and oxygen abundances yields information on the time evolution and nucleosynthetic origins of these elements, yet remains relatively unexplored. At low metallicities (12+log(O/H) < 8.0), nebular carbon measurements are limited to rest-frame UV collisionally excited emission lines. Therefore, we present UV spectrophotometry of 12 nearby, low-metallicity, high-ionization HII regions in dwarf galaxies obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We present the first analysis of the C/O ratio in local galaxies based solely on simultaneous significant detections of the UV O^+2 and C^+2 collisionally excited lines in seven of our targets and five objects from the literature, to create a final sample of 12 significant detections. Our sample is complemented by optical SDSS spectra, from which we measured the nebular physical conditions and oxygen abundances using the direct method. At low metallicity (12+log(O/H) < 8.0), no clear trend is evident in C/O vs. O/H for the present sample given the large dispersion observed. When combined with recombination line observations at higher values of O/H, a general trend of increasing C/O with increasing O/H is also viable, but with some significant outliers. Additionally, we find the C/N ratio appears to be constant (but with significant scatter) over a large range in oxygen abundance, indicating carbon is predominantly produced by similar nucleosynthetic mechanisms as nitrogen. If true, and our current understanding of nitrogen production is correct, this would indicate that primary production of carbon (a flat trend) dominates at low metallicity, but quasi-secondary production (an increasing trend) becomes prominent at higher metallicities. A larger sample will be needed to determine the true nature and dispersion of the relation.

Chemodynamical Model of the Galaxy: Abundance Gradients Predicted for H II Regions and Planetary Nebulae

We present a chemodynamical evolution model of the Galaxy to determine chemical abundance gradients of different stellar populations. From this model we have determined the abundance gradients expected for H II regions, as well as for planetary nebulae of different ages and different kinematical properties. We have compared the model predicted gradients with those derived from planetary nebulae (PNs) of types I, II, and III. From this comparison we conclude that only about half of the stars evolving toward the white dwarf stage produce PNs and that the less massive stars are less likely to produce PNs. Other arguments supporting the previous conclusions are presented.