O/Fe in metal-poor main sequence and subgiant stars ?

By P. Nissen, F. Primas, Martin Asplund, D. Lambert. 2002

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A study of the O/Fe ratio in metal-poor main sequence and subgiant stars is presented using the (Oi) 6300 A line, the Oi 7774 A triplet, and a selection of weak Feii lines observed on high-resolution spectra acquired with the VLT UVES spectrograph. The (Oi) line is detected in the spectra of 18 stars with 2:4< (Fe/H)< 0:5, and the triplet is observed for 15 stars with (Fe/H) ranging from 1: 0t o 2:7. The abundance analysis was made first using standard model atmospheres taking into account non-LTE eects on the triplet: the (Oi) line and the triplet give consistent results with (O/Fe) increasing quasi-linearly with decreasing (Fe/H) reaching (O/Fe)'+0: 7a t (Fe/H)= 2:5. This trend is in reasonable agreement with other results for (O/Fe) in metal-poor dwarfs obtained using standard atmospheres and both ultraviolet and infrared OH lines. There is also broad agreement with published results for (O/Fe) for giants obtained using standard model atmospheres and the (Oi) line, and the OH infrared lines, but the Oi lines give higher (O/Fe) values which may, however, fall into place when non-LTE eects are considered. When hydrodynamical model atmospheres representing stellar granulation in dwarf and subgiant stars replace standard models, the (O/Fe) from the (Oi )a nd Feii lines is decreased by an amount which increases with decreasing (Fe/H). These 3D eects on (O/Fe) is compounded by the opposite behaviour of the (Oi) (continuous opacity eect) and Feii lines (excitation eect). The (O/Fe) vs. (Fe/H) relation remains quasi-linear extending to (O/Fe)'+0.5 at (Fe/H)= 2:5, but with a tendency of a plateau with (O/Fe)'+0.3 for 2:0< (Fe/H)< 1:0, and a hint of cosmic scatter in (O/Fe) at (Fe/H)' 1:0. Use of the hydrodynamical models disturbs the broad agreement between the oxygen abundances from the (Oi), Oi and OH lines, but 3D non-LTE eects may serve to erase these dierences. The (O/Fe) values from the (Oi) line and the hydrodynamical model atmospheres for dwarfs and subgiant stars are lower than the values for giants using standard model atmospheres and the (Oi )a nd Oi lines.

Subfields: Astronomy and Astrophysics, Instrumentation

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Abundances for metal-poor stars with accurate parallaxes , I. Basic data (2003, methodology): “This agreement is comfortable, and supports the result obtained by Nissen et al. (2002) on a somewhat smaller sample.”
The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3 (2003, background): “In addition, Fe ii abundances have been shown to remain virtually unaffected if three-dimensional calculations replace one-dimensional calculations (Asplund & Garcia Perez 2001); this may not be the case for Fe i (Nissen et al. 2002).”
Oxygen trends in the Galactic thin and thick disks (2003, methodology): “Apart from the 4 halo stars this left us with 10 thick disk and 17 thin disk stars from Nissen et al. (2002).”
Galactic Chemical Evolution: Carbon through Zinc (2006, unknown): “A plateau of [O/Fe] ∼ 0.3− 0.45 is seen at −2 < ∼ [Fe/H] < ∼ − 1: [O/Fe] ∼ 0.45 (Carretta et al. 2000), 0.35 (Melendez & Barbuy 2002), and 0.3 with 3D correction (Nissen et al. 2002).”
The evolution of the Milky Way from its earliest phases: Constraints on stellar nucleosynthesis (2004, unknown): “…(1999), Carney et al. (1997), Nissen and Schuster (1997), Fulbright (2000), Gilroy et al. (1998), Gratton & Sneden (1988; 1994), Ryan et al. (1991), Carretta et al. (2002), Edvardsson et al. (1993), McWilliam et al. (1995), Nissen et al. (2002), Matteucci et al. (1993 and references therein).”
Quantifying the uncertainties of chemical evolution studies II. Stellar yields (2010, methodology): “Furthermore, switching from 1D to 3D models changes the [O I]-based abundances by as much as−0.2 dex in turn-off stars with [Fe/H] = −2; a similar behaviour can be anticipated for metal-poor giants (Nissen et al. 2002).”
First stars V - Abundance patterns from C to Zn and supernova yields in the early Galaxy (2003, methodology): “Thus, we assumed as a first approximation that the correction computed by Nissen et al. (2002) is valid also for metal-deficient giants.”
A Globular Cluster Metallicity Scale Based on the Abundance of Feii (2002, methodology): “…unaffected by departures from LTE, metallicities for globular clusters could safely be based on LTE analysis of Fe ii, a conclusion supported by Asplund & Garcia Perez (2001) and Nissen et al. (2002) in their discussions of three-dimensional (“3D”) model atmospheres for metal-poor stars.”
The Binary Frequency Among Carbon-enhanced, s-Process-rich, Metal-poor Stars (2004, methodology): “A crosscorrelation technique was adopted for these objects as well; this uses the metal-poor (−2.60 ≤ [Fe/H ] ≤ −2.40; Lambert & Allende-Prieto 2002, Nissen et al. 2002, Aoki et al. 2002), carbon-mildly enhanced (0.22 ≤ [C/Fe] ≤ 0.6; Tomkin et al. 1992; Norris, Ryan & Beers 1997) subgiant HD 140283 ”
The Evolution of the C/O Ratio in Metal-poor Halo Stars (2003, methodology): “30 corrected for the Ni i blend as discussed by Nissen et al. (2002).”
Three-dimensional hydrodynamical simulations of surface convection in red giant stars: impact on spectral line formation and abundance analysis (2007, methodology): “…same procedure adopted in other recent investigations of the effects of surface convection and granulation on solar and stellar spectroscopy (Asplund et al. 1999, 2000a,b, 2005; Asplund 2000; Asplund & Garcı́a Pérez 2001; Allende Prieto et al. 2001, 2002b; Nissen et al. 2002; Collet et al. 2006).”
The chemical compositions of Galactic disc F and G dwarfs (2002, background): “The abundance ratio [O/Fe] is found to increase with decreasing [Fe/H], in general agreement with other studies where the blend was recognized (Nissen et al. 2002).”
The Hamburg/ESO R-process enhanced star survey (HERES) - III. HE 0338-3945 and the formation of the r + s stars (2006, background): “A value of∆NLTE ∼ −0.1 dex has been adopted for stars with similar fundamental parameters by Nissen et al. (2002).”
Line formation in solar granulation IV. (O I), O I and OH lines and the photospheric O abundance (2003, background): “Recent disputes re-volve around the overabundance of oxygen in metal-poor halo stars (see Asplund & Garc´ıa P´erez 2001; Nissen et al. 2002 and references therein), the Galactic radial abundance gradient (Rolleston et al. 2000; Cunha & Daﬂon 2003), and, astonish-ingly, the solar oxygen abundance.”
First stars VI - Abundances of C, N, O, Li, and mixing in extremely metal-poor giants. Galactic evolution of the light elements (2004, background): “So far, explicit 3D corrections have only been computed for dwarfs (Nissen et al. 2002).”