Towards the resolution of the problem of
oxygen and iron abundances in metal-poor stars
1
2
Nataliya Shchukina & Javier Trujillo Bueno
1Main Astronomical Observatory, National Academy of Sciences, 03680, Kyiv, Ukraine, shchukin@mao.kiev.ua
2Instituto de Astrofisica de Canarias, 38205, La laguna, Tenerife, Spain, jtb@.iac.es
Sun,
Sun, disk center,
center, 3D
Height (Km)
Vertical velocity
Velocity (Km/s)
-400
We present the results of a detailed theoretical investigation on the
impact of NLTE effects and of granulation inhomogeneities on the iron
& oxygen abundances in the metal-poor star HD140283. This classical
halo subgiant is widely used for studies of the chemical evolution of the
Galaxy and the origin of the chemical elements.
100
Height (Km)
Temperature
Temperature (K)
Surface distance ( Mm)
-400
100
The O I IR triplet
7772-5 A lines
are formed in
the deep photosphere
( 100 --- 200 km) ).
Surface distance ( Mm)
Fig. 1. The 3D model of the solar granulation.
Vertical velocity (top) and temperature (bottom)
stratification with height and along one of the solar
snapshot slices
NLTE
HD140283, disk center
3D
LTE
Introduction
Fe I
l5429 A
Synthetic
spatially resolved
intensity profiles
Fig. 2. 3D modelling of the stellar Fe I lines.
Top panels: The disk-center synthetic «wiggly-line» spectrograms of the
Fe I line l 5429.699 A calculated in the 3D hydrodynamical model of the
metal-poor subgiant HD140283. The hypothetical slit is located along the
position Y=24 in the 3D model. Left panels: NLTE. Right panels: LTE. The
«wiggles» are caused by the Doppler shifts associated with the granulation
velocities. Bright strips cprrespond to granules, while dark ones to
intergranules. Black lines show variations of the line equivalent widths W
along the slit. Bottom panels:The spatially resolved emergent intensity
profiles at each of the 50 surface gridpoints along Y=24. The thick orange
lines show the resulting spatially averaged intensity. Intensities are given
in absolute energy units (erg cm-2 s-1 ster-1 Hz-1).
Our analysis is based on both the classical one-dimensional (1D) stellar
atmosphere models and on the new generation of three-dimensional
(3D) hydrodynamical models (Fig. 1) .
3D, NLTE
FeI: log e=5.77
FeII: log e= 5.60
1D, NLTE
FeI: log e=5.77
FeII: log e= 5.34
3D, LTE
FeI: log e=4.85
FeII: log e= 5.18
1D, LTE
FeI: log e=5.20
FeII: log e= 5.18
We consider the Sun as a reference star. We investigate also the solar
Fe & O abundances.
Results
The Fe I lines in the 3D model of HD140283 are much weaker in NLTE
than in LTE (Fig. 2) because the UV overionization mechanism produces
a strong underpopulation of the Fe I levels in the granular regions
(Fig.3) . As a result, the NLTE effects on the derived iron abundance are
very important, amounting to ~0.9 dex and ~0.6 dex in the 3D and 1D
cases, respectively (Fig. 4) .
We find that when NLTE effects are taken into account the Fe I spectrum
of the star HD140283 is insensitive to the 3D effects (Fig. 4).
Fig. 4. The iron abundance determinations for HD140283.
The iron abundances derived from Fe I lines (filled circles) and from Fe II
lines (stars) vs. the lower level excitation potential of the chosen lines.
Left panels show the results for the 3D model while the right panels for the
1D model. Top panels: NLTE, Bottom panels: LTE. The four weak Fe II lines
(ll 6149.2, 6247.5, 6432.6, 6456.3)
used by Nissen et al. (2002) are marked with squares. The dashed & dotted
horizontal lines indicate the mean abundances found from the set of Fe I &
Fe II lines, respectively. The mean abundances are given in panels. The
stellar parameters used : Teff=5700 K, log g=3.7, [Fe/H]=-2.5.
NLTE and 3D effects have to be taken into account for a reliable
determination of the iron abundances in HD140283 from weak Fe II lines
(Fig. 4) because the significant overexcitation of their upper levels in
the granules tend to produce emission features (Fig. 5). Such Fe II lines
are weaker than in LTE: the abundace correction is ~0.4 dex for the 3D
case.
We derive the oxygen abundance in the HD140283 star by using the O I
triplet at l 7772-5 A and the forbidden [O I] line at l 6300 A. While the
oxygen abundance derived from the O I triplet is insensitive to the
presence of granulation inhomogeneities, such 3D effects amount to
~ -0.2 dex for the [O I] line. The NLTE abundance correction for the O I
triplet is ~ -0.2 dex.
The solar iron abundance is similar to the meteoritic value, i.e.
log e =7.50±0.10 (Shchukina & Trujillo Bueno 2001).
We derived the solar oxygen abundance by fitting the observed disk
center profiles (Fig. 6) and equivalent widths of the O I triplet via
synthesis in the 3D model. Taking into account NLTE effects we find
log e =8.70±0.06 while the LTE approximation gives log e =8.93±0.06.
Conclusions
HD140283
The iron abundance in HD140283 (Teff=5700, log g=3.7, [Fe/H]=-2.5)
• The oxygen and iron discrepancies in the HD140283 cannot be
removed by taking into account NLTE and/or 3D effects.
l 6247
Teff= 5600K
•[Fe/H] = -2.5
[Fe/H] = -2.0
• We find [O/Fe]=0.5 at [Fe/H]= -2.
Fig. 3. Departure coefficients (b) vs. height in the 3D
model of the metal-poor subgiant HD140283.
Left panel: Representantive «granular» point. Right panel: «Intergranular»
point. Solid lines: b-coefficients of the FeI levels with c < 5 eV. Dashed
and dashed-dotted lines: b-coefficients of the lower and upper FeII levels,
respectively. The thick lines correspond to the Fe I ground level a5D, to the
Fe II ground level a6D, and to the Fe II excited level of odd parity z6Do,
respectively. Arrows: heights below which one of the Fe I lines (l5429.699)
and the Fe II lines considered originate in the NLTE and LTE cases.
• At first sight, one might be tempted to conclude that such a
value falls on the linear rise trend in the [O/Fe]-[Fe/H] diagram.
• However, one may also argue that such a value supports the
possibility of a plateau at 0.5 dex in the range -3 < [Fe/H] < -1.
• The analysis of a single metal-poor star is not enough to opt
for one of these two possibilities. Other stars will be analyzed.
12-18 September JENAM-2004 Granada
We thank Martin Asplund for providing the 3D atmospheric models.
This research has been supported by the INTAS program of the European
Commision and by the Spanish Plan Nacional de Astronomia y Astrofisica.
Fe II lines
l 6247 & 5316 A
l 5316
Synthetic
flux profiles
NLTE
LTE
Fig. 5. 3D modelling of the stellar Fe II lines.
The flux profiles of the weak Fe II line l 6247.56 A ( top panels ) and of moderately
strong Fe II line l 5316.62 A ( bottom panels ) calculated in the 3D hydrodynamical
model of the metal-poor subgiant HD140283. The profiles were synthesized
using the iron abundance log e =5.0. Left panels : NLTE. Right panels : LTE.
Individual thin curves show the computed emergent fluxes for the «granular » and
«intergranular » models corresponding to the 50 spatial gridpoints located along
the slice Y=24. The thick solid lines with filled circles on each of the panels : the
resulting spatially averaged flux . The intergranular profiles have a redshift and a
lower continuum intensity . Bright granules are characterized by a higher
continuum flux and a blue line shift . Note that the flux profile of the weak Fe II line
l 6247.56 A is observed in absorption while the synthesized NLTE flux profile is
in emission . Fluxes are given in absolute energy units (erg cm -2 s-1 Hz -1 ).
• The effective temperature Teff & the metallicity [Fe/H], hence,
the 3D-model, need to be revised:
•Teff =5700 K
HD140283, disk center,
center,
3D
Sun,
Sun, disk center
NLTE+3D
O I IR triplet
l7772 A
Obs.
l 7772
m=1
log e =8.70
Spatially averaged
residual intensity
profile
RDl vs
vs.. Dl (mA)
Fig. 6. Spatially averaged O I IR line profile for the solar
disk center.
Solid line: The disk-center spatially averaged residual intensity profile of
the O I line l 7772 A calculated in the 3D hydrodynamical model of the
Sun. Filled circles: observations (Jungfraujoch atlas , Delbouille et al.
1973). The solar oxygen abundance log e =8.70.