Infrared OH Lines Research Articles

Three-dimensional hydrodynamical CO5BOLD model atmospheres of red giant stars

Context. Although oxygen is an important tracer of Galactic chemical evolution, measurements of its abundance in the atmospheres of the oldest Galactic stars are still scarce and rather imprecise. At the lowest end of the metallicity scale, oxygen can only be measured in giant stars and in most of cases such measurements rely on a single forbidden [O I] 630 nm line that is very weak and frequently blended with telluric lines. Although molecular OH lines located in the ultraviolet and infrared could also be used for the diagnostics, oxygen abundances obtained from the OH lines and the [O I] 630 nm line are usually discrepant to a level of ~0.3-0.4 dex. Aims. We study the influence of convection on the formation of the infrared (IR) OH lines and the forbidden [O I] 630 nm line in the atmospheres of extremely metal-poor (EMP) red giant stars. Methods. We used high-resolution and high signal-to-noise ratio spectra of four EMP red giant stars obtained with the VLT CRIRES spectrograph. For each EMP star, 4-14 IR OH vibrational-rotational lines located in the spectral range of 1514-1548 and 1595-1632 nm were used to determine oxygen abundances by employing standard 1D LTE abundance analysis methodology. We then corrected the 1D LTE abundances obtained from each individual OH line for the 3D hydrodynamical effects. Results. We find that the influence of convection on the formation of [O I] 630 nm line in the atmospheres of EMP giants studied here is minor, which leads to very small 3D-1D abundance corrections (< -0.01 dex). On the contrary, IR OH lines are strongly affected by convection and thus the abundance corrections for these lines are significant, 3D-1D ~ -0.2 ... -0.3 dex. These abundance corrections do indeed bring the 1D LTE oxygen abundances of EMP red giants obtained using IR OH lines into better agreement with those determined from the [O I] 630 nm line.

A Low Solar Oxygen Abundance from the First‐Overtone OH Lines

An extremely high resolution (>105) high signal-to-noise ratio (>103) solar spectrum has been used to measure 15 very weak first-overtone (Δv = 2) infrared OH lines, resulting in a low solar abundance of AO ≈ 8.6 when MARCS, three-dimensional, and spatially and temporally averaged three-dimensional model atmospheres are used. A higher abundance is obtained with Kurucz (AO ≈ 8.7) and Holweger & Müller (AO ≈ 8.8) model atmospheres. The low solar oxygen abundance obtained in this work is in good agreement with a recent three-dimensional analysis of [O I], O I, OH fundamental (Δv = 1) vibration-rotation, and OH pure rotation lines. The present result gives further support for a low solar metallicity, and although using a low solar abundance with OPAL opacities ruins the agreement between the calculated and the helioseismic measurement of the depth of the solar convection zone, recent results from the Opacity Project show that the opacities near the base of the solar convection zone are larger than previously thought, lending further confidence to a low solar oxygen abundance.

Oxygen Abundance in the Template Halo Giant HD 122563

HD 122563 is a well-known bright (V ¼ 6:2) halo giant of low metallicity ((Fe/H ��� 2:7). We have observed HD 122563 for infrared OH lines at 1.5-1.7 lm in the H band with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope. Optical spectra were obtained with the UVES spectrograph at the 8 m VLT UT2 telescope at ESO (Paranal) and the FEROS spectrograph at ESO (La Silla). Based on the opti- cal high-resolution data, a detailed analysis has been carried out, and data on the forbidden (O i) 6300 Aline, unblended by telluric or sky lines, was obtained with the FEROS spectrograph. Signal-to-noise ratios of 200- 400 were obtained at resolutions of 37,000 in the H band and 45,000 in the optical. For the analysis we have adopted a photometric effective temperature Teff ¼ 4600 K. Two values for the gravity were adopted: a value deduced from ionization equilibrium, log g ¼ 1:1, with corresponding metallicity (Fe/H �¼� 2:8 and micro- turbulence velocity vt ¼ 2: 0k m s � 1 ; and log g ¼ 1:5, derived from the Hipparcos parallax, implying (Fe/H �¼� 2:71 and vt ¼ 2: 0k m s � 1 . The forbidden (O i) 6300 Aand the permitted O i 7771 Alines give O/Fe ratios essentially insensitive to model parameter variations, whereas the oxygen abundances from OH lines are sensitive to gravity, giving (O/Fe �¼þ 0:9 and +0.7, respectively, for log g ¼ 1:1 and 1.5. We derive the following oxygen abundances: for model 1, (O/Fe �¼þ 0:6, +1.1, and +0.9; and for model 2, (O/Fe �¼þ 0:6, +1.1, and +0.7, based on the (O i) 6300 A ˚ ,O i 7771 A ˚ , and IR OH 1.6 lm lines, respectively. The different oxygen abundance indicators give different oxygen abundances, illustrating the problem of oxy- gen abundance derivation in metal-poor giants. This is important because the age of globular clusters and the production of Li, Be, and B from spallation of C, N, and O atoms in the early Galaxy depend on the oxygen abundance adopted for the metal-poor stars. Subject headings: stars: abundances — stars: individual (HD 122563) — stars: Population II On-line material: machine-readable table

Keck NIRSPEC Infrared OH Lines: Oxygen Abundances in Metal‐poor Stars down to [Fe/H] = −2.9

Infrared OH lines at 1.5-1.7 μm in the H band were obtained with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope for a sample of seven metal-poor stars. Detailed analyses have been carried out, based on optical high-resolution data obtained with the Fiber-fed Extended Range Optical Spectrograph at ESO. Stellar parameters were derived by adopting infrared flux method effective temperatures, trigonometric and/or evolutionary gravities, and metallicities from Fe II lines. We obtain that the sample stars with metallicities [Fe/H] < -2.2 show a mean oxygen abundance [O/Fe] ≈ 0.54 for a solar oxygen abundance of (O) = 8.87, or [O/Fe] ≈ 0.64 if (O) = 8.77 is assumed.

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

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.

Oxygen Abundances from Infrared OH Lines

AbstractOxygen abundances are derived from IR-OH lines and compared to published results from UV-OH lines.

Oxygen Abundances in Metal‐poor Stars (−2.2 &lt; [Fe/H] &lt; −1.2) from Infrared OH Lines

Infrared OH lines at 1.55 - 1.56 um in the H-band were obtained with the Phoenix high-resolution spectrograph at the 2.1m telescope of the Kitt Peak National Observatory for a sample of 14 metal-poor stars. Detailed analyses of the sample stars have been carried out, deriving stellar parameters based on two methods: (a) spectroscopic parameters; (b) IRFM effective temperatures, trigonometric gravities and metallicities from Fe II lines. The Fe I lines present in the H-band region observed showed to be well fitted by the stellar parameters within $\Delta$[Fe/H] < 0.15 dex. The oxygen abundances were derived from fits of spectrum synthesis calculations to the infrared OH lines. CO lines in the H- and K-bands were obtained for a subsample in order to determine their carbon abundances. Adopting the spectroscopic parameters a mean oxygen-to-iron ratio of [O/Fe] ~ +0.52 is obtained, whereas using the IRFM temperatures, Hipparcos gravities and [FeII/H], [O/Fe] ~ +0.25 is found. A mean of the two methods gives a final value of [O/Fe] ~ +0.4 for the metallicity range -2.2 < [Fe/H] < -1.2 of the sample metal-poor stars.