A&A 372, 601-615 (2001)
DOI: 10.1051/0004-6361:20010483
M. Asplund - A. E. García Pérez
Astronomiska Observatoriet, Box 515, SE-751 20 Uppsala, Sweden
Received 18 December 2000 / Accepted 26 March 2001
Abstract
The formation of the UV OH spectral lines
has been investigated for a range of stellar parameters
in the light of 3D hydrodynamical model atmospheres.
The low atmospheric temperatures encountered at low metallicities
compared with the radiative equilibrium values enforced in classical
1D hydrostatic model atmospheres have a profound impact on the OH
line strengths. As a consequence, the derived O abundances using 3D models
are found to be systematically lower by more than 0.6dex at
[Fe/H] =-3.0 compared with previous 1D analyses,
casting doubts on the recent claims for a monotonic increase in [O/Fe] towards
lower metallicities.
In fact, taken at face value the resulting 3D LTE trend is in rough
agreement with the conventional [O/Fe] plateau.
Caution must, however, be exercised in view of the remaining assumptions
in the 3D calculations.
We have verified that the stellar parameters remain essentially
unchanged with 3D model atmospheres
provided that the infrared flux method (
K),
Hipparcos parallaxes (
)
and Fe II
lines (
dex) are utilised, leaving the
3D O abundances from OH lines largely intact
(
dex).
Greater concern stems from possible departures from LTE in
both the line formation and the molecular equilibrium, which, if present,
would increase the derived O abundances again.
Non-LTE line formation calculations with 1D model atmospheres
suggest no significant steepening of the [O/Fe] trend even if the
abundance corrections amount to about 0.2dex for all
investigated stellar parameters. We note, however, that
the 3D case may not necessarily be as metallicity-independent.
The apparent lack of laboratory or theoretical
rate coefficients at the relevant temperatures for the involved
molecular reactions unfortunately
prevents a quantitative discussion on the possible
effects of non-equilibrium chemistry.
Key words: convection - line: formation - stars: abundances -
stars: atmospheres - stars: population II -
galaxy: evolution
Following the first indication of an oxygen over-abundance relative
to iron ([O/Fe] >0) in metal-poor stars (Conti et al. 1967),
a great number of
studies have been devoted to quantify this enhancement. Although
all agree on its existence, the amount of the over-abundance is hotly
contested. Various oxygen diagnostics in different types of stars
have been applied but with disparate results. The forbidden [O I] lines
at 630.0 and 636.3nm in metal-poor giants suggest a nearly flat
plateau at [O/Fe]
for [Fe/H]
(e.g. Barbuy 1988; Sneden et al. 1991)
while the O I triplet at 777nm in metal-poor dwarfs and subgiants
tend to imply systematically higher values, often with a monotonic
increase towards lower metallicities
(e.g. Abia & Rebolo 1989;
Israelian et al. 1998, 2001; Boesgaard et al. 1999; Carretta et al. 2000).
All oxygen criteria have their pros and cons, which influence the
conclusions. The forbidden lines are immune to departures
from local thermodynamic equilibrium (LTE)
(cf. discussion in Kiselman 2001)
but the lines are very weak at low metallicities, in particular
in unevolved stars where the feature becomes undetectable for
[Fe/H]
(Nissen et al. 2001).
Furthermore, concerns regarding the primordial nature
of the oxygen in field giants have been voiced as mixing of nuclear-processed
material may pollute the surface,
as evident in many globular cluster giants (e.g. Langer et al. 1997).
The O I triplet on the other hand is more easily discerned
in metal-poor dwarfs but also more susceptible to departures from LTE
(cf. Kiselman 2001 and references therein) and inhomogeneities introduced by
stellar granulation (Kiselman & Nordlund 1995) than the [O I] lines.
The high excitation potential
of the O I lines also make them
vulnerable to errors in the effective temperature (
).
Additionally, an often overlooked source of confusion is the stellar Fe
abundances, which should be self-consistently computed in order to
obtain reliable [O/Fe] ratios.
Finally, even the question of the
absolute solar oxygen abundance is still not settled, leaving the
fundamental reference point for stellar [O/Fe] ratios insecurely anchored.
An attractive alternative to the O I and [O I] lines are
provided by the molecular OH electronic transitions in the UV
(Bessell et al. 1984, 1991; Nissen et al. 1994). In fact, at
[Fe/H]
no substitute for the OH lines as the prime oxygen diagnostic
is available. Therefore the OH spectral
line formation must be properly understood
when attempting to probe the earliest epochs of the Galactic evolution
for example to identify the nucleosynthetic fingerprints
of the elusive Population III stars (e.g. Karlsson & Gustafsson 2001).
Recently, Israelian et al. (1998, 2001) and Boesgaard et al. (1999) have
analysed the UV OH (A
- X
)
lines in stars down to
[Fe/H]
with high S/N and resolution spectra
and have found a linear trend in [O/Fe] vs. [Fe/H] with
a slope of about -0.4
, in stark
conflict with the [O I] results.
If confirmed, these results would have far-reaching consequences, as
outlined above.
The situation is complicated, however, by recent studies of the OH
vibrational-rotational lines in the infrared (IR), which suggest
a nearly flat [O/Fe] in agreement with the [O I] findings
(Balachandran et al. 2001; Melendez et al. 2001).
As all previous investigations have been based on 1D hydrostatic model atmospheres, one may worry about possible systematic errors introduced by the inherent assumptions of the analyses. Recently the first 3D hydrodynamical model atmospheres of metal-poor stars have been constructed (Asplund et al. 1999a), which have very different temperature structures compared with classical 1D model atmospheres. As a consequence, analyses of temperature sensitive spectral features can be suspected to be systematically in error if relying on 1D model atmospheres. In particular, Asplund et al. (1999a) cautioned that oxygen abundances of metal-poor stars derived from 1D studies of OH lines may be strongly overestimated due to the extreme temperature sensitivity of molecule formation. The aim of the present paper is to quantify this suspicion in terms of O abundances and investigate other possible systematic errors which may hamper the 3D analysis of OH lines. Preliminary results have been presented in Asplund (2001).
Realistic ab-initio 3D, time-dependent simulations of stellar surface
convection form the foundation
for the present study. The same compressible radiative hydrodynamical code
which previously has been applied successfully to studies of solar
(e.g. Stein & Nordlund 1998; Asplund et al. 2000a,b) and stellar granulation
(e.g. Asplund et al. 1999a; Allende Prieto et al. 1901;
Asplund et al., in preparation) has here been
used to construct sequences of 3D model atmospheres
with varying stellar parameters.
The equations of mass, momentum and energy conservation together with the
simultaneous treatment of the 3D radiative transfer equation have
been solved on a Eulerian mesh with
gridpoints.
The physical dimensions of the numerical grid were sufficiently large
to cover many (
10) granules simultaneously.
The depth scales have been optimized
to provide the best resolution where it is most needed, i.e.
in those layers with the steepest
gradients in terms of dT/dz and d2T/dz2, which for the solar-type stars
occurs around the visible surface.
Special care has been exercised to include the most appropriate input physics.
In particular, state-of-the-art equation-of-state (Mihalas et al. 1988),
which includes the effects of ionization, excitation and dissociation
of the most important atoms and molecules, and relevant continuous
(Gustafsson et al. 1975 with subsequent updates) and line (Kurucz 1993)
opacities have been employed.
During the convection simulations, the 3D radiative transfer
is solved for in total eight inclined rays under the simplifying assumptions
of LTE (
)
and grouping of the opacities into four bins (Nordlund 1982).
At regular intervals during the simulations,
the accuracy of the opacity binning technique is verified
by solving the full monochromatic radiative transfer (about 2700 wavelength
points) in the 1.5D approximation, i.e. treating each vertical column as
a separate 1D model atmosphere and ignoring all horizontal radiative
transfer effects.
Further details on the numerical procedures of the simulations
may be found in Stein & Nordlund (1998).
![]() |
logg | [Fe/H] | x,y,z-dimensions | time![]() |
[K] | [cgs] | [Mm] | [min] | |
![]() |
4.44 | +0.0 |
![]() |
50 |
![]() |
4.44 | -1.0 |
![]() |
50 |
![]() |
4.44 | -2.0 |
![]() |
40 |
![]() |
4.44 | -3.0 |
![]() |
40 |
![]() |
4.04 | +0.0 |
![]() |
60 |
![]() |
4.04 | -1.0 |
![]() |
60 |
![]() |
4.04 | -2.0 |
![]() |
60 |
![]() |
4.04 | -3.0 |
![]() |
60 |
For the present purpose, two sequences of 3D model atmospheres
have been constructed. The first series of models (here: the solar sequence)
correspond to the Sun (
K
,
logg = 4.44[cgs])
but with a range of metallicities ([Fe/H]=0.0, -1.0, -2.0 and -3.0),
while the second suite (here: the turn-off sequence)
correspond to typical turn-off stars
(
K, logg = 4.04[cgs]), again with varying
metal content ([Fe/H]=0.0, -1.0, -2.0 and -3.0).
Some details of the simulations are given in Table 1.
The individual elemental abundances have been taken from
Grevesse & Sauval (1998) scaled appropriately to the relevant [Fe/H]
with no
-element enhancements.
As long as the comparison is strictly differential between the 3D and
1D model atmospheres as in our case the omission of
-element enhancements will be negligible for the spectral line
formation.
The initial snapshots for the simulations were taken from simulations
of lower numerical resolution (
), which had been run
sufficiently long times to allow
thermal relaxation and a statistically steady state at the
wanted
to be established.
The initial snapshots for these
lower resolution runs in turn were obtained from a previous solar
simulation (Stein & Nordlund 1998) scaled appropriately to the new
stellar parameters using the experience from 1D hydrostatic stellar models
and the entropy variations in 2D hydrodynamical model atmospheres
(Ludwig et al. 1999; Freytag et al. 1999).
Classical 1D, hydrostatic MARCS model atmospheres (Asplund et al. 1997)
with identical input parameters and chemical compositions as the 3D simulations
have been constructed to allow a differential comparison in terms of
spectral line formation. Whether 3D models indeed should be assigned the same
as a corresponding 1D model atmosphere is investigated
in Sect. 5.2 when comparing the predictions for the infrared
flux method (IRFM) with the two types of models.
Although the details of the convection properties of the present and other
simulations for late-type stars
will be described elsewhere (Asplund et al., in preparation),
the most important metallicity effects on the resulting photospheric
structures are briefly discussed here in order to understand the
impact of the 3D models on the spectral line formation of the OH lines.
While the temperature remains close to the radiative
equilibrium value at solar metallicities and
mild metal-deficiencies ([Fe/H]
),
the temperature in the outer layers depart significantly from it at
lower metallicities
(Asplund et al. 1999a).
The temperature
in this optically thin region is determined from a competition between
adiabatic cooling and radiative heating. The latter mechanism arises
when continuum photons released at deeper layers are reabsorbed in
spectral lines. At solar metallicities the abundant spectral lines
succeed in providing
sufficient radiative heating to balance the adiabatic cooling and
keeping the average temperature close to the radiative equilibrium
expectation. At progressively lower metallicities, the
available spectral lines become fewer and weaker, which allows cooling
to dominate more. As a consequence, balance is not restored unless the
temperature is much below the radiative equilibrium value.
Thus, in hydrodynamical model atmospheres spectral lines have the
opposite effect compared to in hydrostatic model atmospheres which
enforce radiative equilibrium: spectral lines cause surface heating.
The effects of the lower temperatures are first visible at the
outermost layers but move towards deeper layers at progressively
lower metallicities.
When viewed as a function of optical depth instead of geometrical height,
the low surface temperatures
are less pronounced since the temperature sensitivity of the continuous
opacities partly hides the effect (Asplund et al. 1999a);
lower temperatures also imply smaller
opacities and optical depths. Nevertheless the low temperatures reach
typical line-forming regions at low metallicities. Furthermore,
the cool outer layers shift the whole line formation outwards for
temperature sensitive features such as molecular lines.
For example, at log
the average temperature difference
between 3D and 1D models can reach
1000K and thus have a profound effect on the lines
sensitive to those layers.
Differences in the gas and electron pressures could also
influence spectral line
formation. With the exception of some additional contribution from
turbulent pressure in the convective overshoot region around the
visual surface, the total pressure is well approximated by the
gas pressure in the photosphere for late-type dwarfs.
Since even in the 3D hydrodynamical
models the photosphere is typically not too far from
hydrostatic equilibrium, the pressure scale height
and thus the resulting 3D pressure structure tend to be lower
than the corresponding 1D value at a given geometric height
at low metallicities.
Naturally, the lower temperatures have an even greater
impact on the electron pressure.
The lower gas and electron pressures in metal-poor
3D model atmospheres will affect mainly
lines that are considered gravity sensitive.
The OH line calculations with the 3D hydrodynamical model
atmospheres follow the same procedure as in other recent investigations
of the influence of granulation on stellar spectroscopy
(Asplund et al. 1999a,b, 2000a,b,c; Asplund 2000a,b, 2001;
Asplund & Carlsson 2001; Allende Prieto et al. 1901;
Nissen et al. 2000, 2001; Primas et al. 2000).
The 3D convection simulations, which extend down to the essentially
adiabatic layers well below the visible photosphere,
were interpolated to a finer vertical depth-scale
although with the same number of depth-points to
improve the numerical accuracy in the spectral synthesis. The average
continuum optical depth range typically beyond log
and above log
,
in order to minimize the influence
from the artificial top and bottom boundaries.
Prior to the line transfer calculations
the horizontal resolution was decreased from
to
to ease the computational burden;
various tests ensured that the procedure did not introduce any differences in
the spatially and temporally averaged line profiles.
From the full convection simulations, which cover several convective turn-over
time-scales, representative sequences of typically one hour stellar time
with snapshots every 30s were selected for the spectral syntheses.
In terms of derived oxygen abundances, the number of snapshots were sufficient
to provide statistically significant results
(
dex),
as verified by test calculations with shorter time sequences.
The mean
's for the shorter simulation sequences are given in
Table 1.
The spectral line formation in 3D model atmospheres
was performed under the assumption of LTE.
Thus, the OH number densities were computed from instantaneous molecular
equilibrium and Saha ionization and Boltzmann excitation balances.
The line source
function was approximated with the Planck function (
).
The applicability of these strong assumptions is further investigated
in Sects. 5.5 and 5.6.
The OH molecular equilibrium was computed with scaled solar abundances
with the exception of the oxygen abundance. The removal of available
oxygen atoms due to CO formation was taken into account but
the effect was found to be negligible.
Flux profiles were computed
for two typical OH UV lines (313.9 and 316.7nm) from solving the
radiative transfer for in total 17 inclined rays (four
-angles
and four
-angles plus the vertical
).
The line transition data for the OH lines
was taken from Israelian et al. (1998); we emphasize that in a
differential 3D-1D comparison such as ours the exact choices of for
example the gf-values are not important.
In addition, we have included six of the Fe II lines
used by Nissen et al. (2001) in their study of [O/Fe] from the [O I]
line to quantify the corresponding impact on Fe abundances.
The background continuous opacities were calculated using the
Uppsala opacity package (Gustafsson et al. 1975 with subsequent updates).
Since the Doppler shifts introduced by the convective velocity field
are fully accounted for, no microturbulence or macroturbulence
parameters enter the 3D line calculations (Asplund et al. 2000b,c).
It is noteworthy that none of the various free parameters
hampering 1D analyses (e.g. mixing length parameters, micro- and
macroturbulence) are necessary with 3D hydrodynamical model atmospheres.
For the 1D calculations, a microturbulence of
1.0kms-1
has been assumed for the solar and turn-off sequences;
the choice of microturbulence, however, is only of some significance for
the solar metallicity models while unimportant at low metallicities.
The main advantage of limiting the comparison to a strictly differential
study of the 3D and 1D predictions is that the uncertainties in
e.g. the absolute transition probabilities, equivalent widths, blends,
continuum placement, missing UV opacities,
-calibration and the solar oxygen abundance can be avoided.
As a consequence we are not able to determine accurate absolute
stellar oxygen abundances.
Instead we only attempt to address the question of whether there are
systematic errors in analyses of the UV OH lines when relying
on classical 1D model atmospheres.
A star-by-star analysis of high quality OH observations
is left for a future paper.
Molecule formation is extremely temperature sensitive (as long
as the molecule in question is a trace element for its atomic constituents,
which is the case for OH in the stars studied here).
For conditions typical of the line-forming regions in the Sun,
the LTE number density of OH,
,
is proportional
to
as estimated from our adopted equation-of-state
and molecular balance routines.
This high degree of non-linearity makes molecular lines
very susceptible to errors in the adopted temperature structures
of the model atmospheres. In particular, the very different temperature
structures of typical convective up- and downflows compared with
classical 1D model atmospheres (i.e. up- and downflows can
not be represented by two theoretical hydrostatic model atmospheres with
different
,
since granules have a much steeper
temperature gradient than intergranular lanes),
can be suspected to significantly
influence the molecular number densities and line formation.
Under the assumption of LTE adopted here,
depend only
on the instantaneous local temperature.
In general the temperature contrast reverses
in the convective overshoot region some distance above the continuum
forming layers, i.e. gas above the warm, upflowing granules tend to be
cooler than average (e.g. Stein & Nordlund 1998).
In particular
in metal-poor stars the horizontal temperature contrast is very large
due to the weak coupling between the gas and the radiation field
(Asplund et al., in preparation). In general the temperatures at the high
atmospheric layers are very low but occasionally the compression from
converging gas flows or shocks can rise the temperature to the
radiative equilibrium value or even above it.
As a consequence, the UV OH LTE line strengths across the stellar granulation
are expected to be stronger in the granules
but with pronounced scatter
when viewed nearly face-on, as confirmed by the
3D line calculations (Fig. 1).
The spatially and temporally averaged OH flux profiles are
therefore strongly biased towards
upflowing regions due to their higher continuum intensities,
steeper temperature gradients and larger area coverage
.
![]() |
logg | [Fe/H] | log
![]() |
log
![]() |
log
![]() |
[K] | [cgs] | 313.9nm![]() |
316.7nm![]() |
||
5767 | 4.44 | +0.0 | 8.90 | 8.88 | 8.89 |
5822 | 4.44 | -1.0 | 8.30 | 8.03 | 8.07 |
5837 | 4.44 | -2.0 | 7.70 | 7.19 | 7.25 |
5890 | 4.44 | -3.0 | 7.10 | 6.44 | 6.52 |
6191 | 4.04 | +0.0 | 8.90 | 8.80 | 8.82 |
6180 | 4.04 | -1.0 | 8.30 | 7.97 | 8.00 |
6178 | 4.04 | -2.0 | 7.70 | 6.90 | 7.01 |
6205 | 4.04 | -3.0 | 7.10 | 6.08 | 6.22 |
HM, int.![]() |
4.44 | +0.0 | 8.90 | 8.69 | 8.72 |
HM, flux![]() |
4.44 | +0.0 | 8.90 | 8.64 | 8.69 |
Typical resulting
3D and 1D line profiles are shown in
Fig. 2, which clearly illustrates the influence of 3D models
on the derived O abundances from UV OH lines at low metallicities.
The much lower temperatures and larger concentration of OH molecules
in the 3D model atmospheres of metal-poor stars result in far
stronger OH lines than with 1D model atmospheres while the difference
is much less accentuated at solar metallicities, in accordance with
the behaviour of the temperature structures.
Table 2 lists the O abundances of the two
OH lines for the two types of model atmospheres.
The 3D O abundances are those which reproduce the equivalent widths
computed using 1D model atmospheres and an adopted
[O/Fe]
[Fe/H] trend
(Israelian et al. 1998, 2001; Boesgaard et al. 1999).
A less steep [O/Fe] trend would
have resulted in slightly smaller granulation effects due to
the shifting of the line-formation region inwards where the low
temperatures in the 3D models are less pronounced.
Similarly, the impact of the 3D models is greater for the OH 313.9nm
line than for the OH 316.7nm line in accordance with the lower
excitation potential and larger line strength of the former.
The in general larger granulation
corrections for the hotter models are in agreement with their larger
temperature differences and the high non-linearity of the OH line
formation.
For completeness we have carried out an identical 3D-1D comparison
for six Fe II lines. The resulting granulation corrections
are listed in Table 3. As expected (Asplund et al. 1999a),
the impact of 3D model atmospheres are relatively minor on the
Fe II lines since they are formed in deep atmospheric
layers and therefore not sensitive to the low temperatures
encountered in the upper layers in the 3D models. In terms
of Fe abundance, the difference between the 1D and 3D predictions
amounts to
dex.
The strong metallicity dependence of the abundance corrections
has a profound impact on the use of UV OH lines as
O diagnostic in metal-poor stars and
cast serious doubts on
recent claimed linear trends in [O/Fe] towards lower metallicities
based on 1D LTE analyses
(Israelian et al. 1998, 2001; Boesgaard et al. 1999).
In Fig. 3 the average granulation corrections of the two
investigated OH lines for the solar and turn-off sequences are shown,
while Fig. 4 presents the final mean corrections of
the different
simulations
.
The correction of the 1D LTE result due to the use of 3D model
atmospheres is here defined as relative to the solar calibration, i.e.
the abundance differences in Table 2 are subtracted with
the corresponding effect for the Sun since [O/Fe] ratios are studied.
This procedure is similar to the
common practice of determining astrophysical gf-values from the Sun,
as was also the basis for the works by Nissen et al. (1994),
Israelian et al. (1998, 2001)
and Boesgaard et al. (1999).
For the Sun, two possible calibrations are possible using either the
Holweger-Müller (1974) semi-empirical model atmosphere or the
theoretical MARCS model as the 1D representation of the solar
photosphere, yielding two possible 3D trends in Figs. 3
and 4.
The difference between the two trends therefore simply reflects the
difference in derived solar O abundances when relying on the two types
of 1D model atmospheres.
It should be noted that the granulation corrections for Fe II
lines presented in Table 3 are not included
in Figs. 3 and 4, since the adopted
[O/Fe] trend with metallicity originates from analyses of
Fe I lines (Boesgaard et al. 1999; Israelian et al. 1998, 2001,
cf. discussion in Sect. 5.4). According to Table 3,
the inclusion of the Fe II
results would bring down the 3D LTE [O/Fe] results further by
dex at the lowest metallicities.
![]() |
logg | [Fe/H] | log
![]() |
log
![]() |
![]() |
[K] | [cgs] | ||||
5767 | 4.44 | +0.0 | 7.50 | 7.48 | -0.02 |
5822 | 4.44 | -1.0 | 6.50 | 6.54 | +0.04 |
5837 | 4.44 | -2.0 | 5.50 | 5.60 | +0.10 |
5890 | 4.44 | -3.0 | 4.50 | 4.59 | +0.09 |
6191 | 4.04 | +0.0 | 7.50 | 7.49 | -0.01 |
6180 | 4.04 | -1.0 | 6.50 | 6.52 | +0.02 |
6178 | 4.04 | -2.0 | 5.50 | 5.56 | +0.06 |
6205 | 4.04 | -3.0 | 4.50 | 4.57 | +0.07 |
HM, int.![]() |
4.44 | +0.0 | 7.50 | 7.52 | +0.02 |
HM, flux![]() |
4.44 | +0.0 | 7.50 | 7.48 | -0.02 |
![]() |
Figure 3:
The impact of 3D model atmospheres on the [O/Fe] trend with
metallicity for the solar ( Upper panel) and turn-off ( Lower
panel) sequences.
The solid line represents the 1D LTE result of Boesgaard et al.
(1999) based on only the UV OH lines and the
King (1993)
![]() ![]() ![]() |
Taken at face value, with the claimed 1D [O/Fe] results and the here presented granulation corrections, the emerging trend with metallicity is in fact roughly consistent with the since long advocated [O/Fe] plateau from [O I] lines, as seen in Fig. 4. However, we caution that such a conclusion is likely premature, as there still exist inherent assumptions and approximations in the present 3D analysis, most notably the use of instantaneous molecular equilibrium and LTE radiative transfer (Sects. 5.5 and 5.6), besides of course the fact that no direct comparison with observations on a star-by-star basis has been made. Furthermore, the issues of possible missing UV opacities (Balachandran & Bell 1998; Bell et al. 2001) and stellar Fe abundances (Thevenin & Idiart 1999; King 2000, cf. Sect. 5.4) must be addressed before safe conclusions can be drawn from OH lines regarding the [O/Fe] behaviour in metal-poor stars. Therefore, we here refrain from claiming accordance between the UV OH and [O I] results and instead settle to point out a possible serious systematic error affecting recent 1D analyses of OH lines. As a result, the case for a continuous linear increase in [O/Fe] towards lower metallicities is seemingly much weakened.
![]() |
Figure 4: The impact of the 3D LTE analysis of the OH lines on the [O/Fe] trend in metal-poor stars when averaging the granulation corrections for the solar and turn-off sequences for the two OH lines 313.9 and 316.7nm. As in Fig. 3, both the cases when using the Holweger-Müller (1974) model atmosphere (dashed line) and the MARCS model atmosphere (dotted line) for the solar calibration are shown. It should be emphasized that the assumption of LTE in the molecule formation and radiative transfer may skew the results according to the discussion in Sect. 5. |
Naively, one could expect that
all OH molecular lines should be similarly affected by
the low temperatures and high molecular number densities in the
3D model atmospheres of metal-poor stars. In view of this, the low
[O/Fe] values found from OH vibrational-rotational lines in the IR
from cool (
K) metal-poor giants and dwarfs
(Balachandran et al. 2001; Melendez et al. 2001)
appear surprising.
No 3D hydrodynamical model atmospheres are yet available for
these lower
and/or surface gravities while the IR lines
are unpropitiously weak for the stellar parameters of the
current (
K) suites of models.
Hence we are unable to confirm or disprove this apparent discordance.
However, we have verified that OH lines in the IR with similar
line strengths as the UV lines suffer from as severe granulation abundance
corrections for the models listed in Table 1 using
fake OH lines with strongly enhanced transition probabilities.
Thus, the spectral locations of the transitions are not a
solution to this conundrum.
Whether the differences in stellar parameters may be a possible
resolution is discussed further in Sect. 6.
The large granulation effects on the derived O abundances for OH lines in metal-poor stars hinge crucially on the existence of the low atmospheric temperatures encountered in the OH line-forming region with 3D model atmospheres. It is therefore natural to inquire whether the resulting temperature structures are indeed accurate representations of the real stellar atmospheres. Even if the here employed 3D hydrodynamical model atmospheres no longer rely on the same simplifying assumptions as in classical 1D model atmospheres, uncertainties in the temperature structures may still remain. A consistency check on the 3D atmospheres is available from similar 2D radiative-hydrodynamical simulations (Ludwig et al. 1999; Freytag et al. 1999), which also produce the distinct sub-radiative equilibrium temperatures. This consonance is reassuring but not too surprising given the resemblance in the underlying assumptions of the two types of simulations. As explained in Sect. 4, the low atmospheric temperatures are natural consequences of no longer enforcing radiative equilibrium and instead solving explicitly the time-dependent energy equation. This phenomenon is therefore a real physical effect which must be present in the photospheres of metal-poor stars. Nevertheless, the magnitude of the effect may have been overestimated in the present simulations. Asplund et al. (1999a) indeed cautioned that the neglect of Doppler shifts in the treatment of the strong spectral lines in the construction of the 3D model atmospheres may lead to the radiative heating being underestimated. Work is currently being undertaken to construct such further improved 3D model atmospheres.
An alternative avenue to proceed is to design observational tests to confront the predictions from 3D model atmospheres with. An often-used method is a detailed comparison of spectral line asymmetries and shifts (e.g. Asplund et al. 2000a,b; Allende Prieto et al. 1901). Unfortunately, a similar study of the asymmetries of the OH lines is unlikely to give crucial clues due to the inevitable blends in the UV. Furthermore, only limited guidance is likely to be obtained from studies of line asymmetries of atomic species since they do not probe the same high atmospheric layers as the molecular transitions.
A possible venture to explore is the individual O abundances obtained
from a large sample of OH lines of different excitation potential.
Since the exact 3D abundance corrections depend
on the transition properties with low-excitation and strong lines being more
influenced by the 3D model atmospheres (Table 2), different
results are expected compared with 1D model atmospheres.
An examination of Table 3 in Israelian et al. (1998) indeed reveals that
low excitation OH lines
(e.g. 312.39, 312.81, 312.83nm,
eV)
appear to systematically give
dex higher abundance than
the high excitation lines
(e.g. 316.71, 320.39, 325.55nm,
eV)
in a 1D analysis, hinting to a possible problem in the 1D temperature
structures at low metallicities relative to the Sun.
However there is no clear trend with metallicity in the
differences in the derived
O abundances but the scatter is unfortunately very large.
A proper investigation will therefore require many more lines for a
larger stellar sample. Furthermore, a prerequisite
is an accurate determination of the stellar parameters, which, in view
of the current disagreement on the
-scale of metal-poor stars,
is unlikely to be settled for good in the near future.
In order to obtain accurate absolute abundances, not only a realistic
model of the stellar photosphere and a proper understanding of the
line formation
process are necessary but also appropriate fundamental stellar parameters.
For OH lines, the effective temperature is of special importance.
In Sect. 4 the comparison between the 3D and 1D predictions
were carried out assuming that the relevant
should be
the same for the two types of models.
One may suspect that the presence of temperature inhomogeneities in the
continuum-forming layers should make the emergent flux distribution
different in 3D model atmospheres compared with homogeneous 1D models,
in particular for metal-poor stars which often are characterized by
"naked granulation''
(Nordlund & Dravins 1990; Asplund et al., in preparation):
the region of maximum horizontal temperature contrast reaches the visible
surface whereas for example for the Sun these layes are
hidden slightly below the photosphere (Stein & Nordlund 1998).
This difference will be particularly manifested in UV colours (amounting
to
10% in continuum fluxes at 314nm, cf. Fig. 1)
while the effect is minimized at IR wavelengths.
Since IRFM is our preferred choice for
-calibrations,
we will here only investigate the impact of 3D models on this method.
The IRFM is designed to compare the observed ratio of
the total bolometric flux of a star and a
monochromatic continuum flux at IR wavelengths
with the corresponding theoretical ratio from model atmospheres
(Blackwell & Shallis 1977),
In practice, JHK photometry
normally replaces the monochromatic continuum
flux for observational convenience.
Here we follow the original idea by computing
the spatially and temporally averaged
continuum flux at 2.2m for the various 3D simulations and compare these
predictions with corresponding ones for 1D model atmospheres with
and
K to quantify typical
corrections to
-estimates based on the IRFM and
classical 1D model atmospheres.
It should be noted that our procedure thus neglects the model atmosphere
dependence in estimating the stellar fluxes outside the observed
photometric bands to obtain bolometric fluxes
(Alonso et al. 1995). However, only a small
fraction (
)
of the total flux is carried at those wavelengths for the
F-K stars of interest here and the slight inconsistency of relying on
1D models for this purpose should have a marginal effect on
the final
-calibration.
The differences in
-determinations
from 3D and 1D model atmospheres are
shown in Fig. 5. Clearly due to the small model sensitivity
of the method, the use of 1D models in available IRFM determinations
does not significantly encumber the results. For the solar sequence the
differences amount to typically 20K while the modifications
for the turn-off sequence are completely negligible.
The smaller effect for the hotter models is in accordance with the
relatively smaller fraction of the total flux emitted at IR wavelengths
and thus smaller sensitivity of the temperature structure of the
adopted model atmospheres.
In view of the
typical observational uncertainties of
100K currently
attached to IRFM, the errors in IRFM
estimates are
still very much dominated by the accuracy of the
observations rather than the adopted model atmospheres.
The small influence on
translates to only a minor impact
on the estimated O abundances.
An increase in
by +100K typically implies an
increase of the derived O by 0.2dex
for the UV OH lines (Nissen et al. 1994). Thus, a
new
-calibration based on 3D model atmospheres is only
expected to introduce a
dex alteration of the
inferred O abundances.
Furthermore, there is no significant metallicity dependence in the
-corrections, which could bias any deduced [O/Fe] trends.
We conclude that [O/Fe] determinations will not be substantially modified due to changes in the stellar parameters entailed by the adoption of 3D model atmospheres.
In addition to a specification of
(Sect. 5.2),
knowledge of the stellar surface gravity, logg, is required for
the spectral synthesis. In Sect. 4 the 3D and 1D
calculations were performed with identical logg, which
may not be appropriate.
The best method to determine stellar surface gravities
is to make use of the accurate parallaxes now available from
the Hipparcos mission. By manipulating the familiar relationships
and
,
the
trigonometric gravities are:
![]() |
+ | ![]() |
|
+ | 0.4(V + BC) - 10.51 | (1) |
Alternative methods to derive stellar surface gravities are
available from spectroscopy, in particular by enforcing ionization equilibrium
or using the pressure-damped wings of strong lines.
Unfortunately they
suffer from several drawbacks which make them less attractive
in analyses with 3D model atmospheres.
In LTE the 3D abundances
derived from Fe I lines are much lower than in classical
1D analyses (Asplund et al. 1999a), which, if correct, would lead to large
modifications of the derived logg from ionization balance.
However, the Fe I lines are almost certainly seriously
affected by departures from LTE in 3D models, and thus a 3D non-LTE
study of Fe line formation would be required, a very challenging task for
the future.
Indeed, even with 1D model atmospheres departures from LTE make the
ionization gravities discrepant from trigonometric gravities
(Allende Prieto et al. 1999b, cf. Sect. 5.4).
Similarly, the strong lines normally utilised for gravity-determinations
are from species which can be expected to be affected by departures
from LTE in 3D model atmospheres (Mg I, Ca I, Fe I),
again necessitating 3D non-LTE investigations.
Furthermore, at very low metallicities ([Fe/H]
)
also the
strongest lines become
too weak to accurately probe the photospheric pressure structure
(Fuhrmann 1998).
Stellar metallicities enter into abundance analyses both indirectly through their influence on the photospheric structure and directly through the use of Fe as a reference element for abundance ratios. While the dependence on the former is relatively weak (an error in [Fe/H] as large as 0.4dex only implies an uncertainty in the derived O abundance from OH lines of 0.05dex, Nissen et al. 1994), the latter is as important as deriving accurate O abundances when attempting to trace the evolution of [O/Fe]. This obvious fact is, however, often over-looked with values simply taken from the literature or estimated only from Fe I lines with no consideration for departures from LTE. Since the recent analyses of UV OH lines by Israelian et al. (1998) and Boesgaard et al. (1999) have utilised Fe I lines, their derived [O/Fe] trends may be systematically overestimated (King 2000; Israelian et al. 2001), which in turn could influence the 3D-1D comparison presented in Sect. 4. Recently two investigations of non-LTE effects in Fe I line formation in metal-poor stars have been published (Gratton et al. 1999; Thevenin & Idiart 1999) although with discomfortingly discordant results, which deserves further scrutiny.
We tend to view the calculations of Gratton et al. (1999) with some balanced scepticism. Their incomplete treatment of the high-excitation levels, neglect of available quantum mechanical calculations for the photo-ionization cross-sections (from e.g. the IRON Project, Bautista 1997) and their exceedingly large cross-sections for inelastic collisions with H, all combine to ensure a result close to the LTE prediction. Thevenin & Idiart (1999) on the other hand adopt more realistic atomic input data but still suffer from the incomplete handling of the line-blanketing. Since the main non-LTE effect, over-ionization, feeds on the UV radiation field it is paramount to address the UV line-blocking in the calculations of the photo-ionization rates to avoid predicting too large departures from LTE for Fe I. Furthermore, improved quantum mechanical calculations for the H-collisions are urgently needed to replace the questionable classical recipe of Drawin (1968).
Until improved non-LTE calculations are available, we urge that the determinations of stellar Fe abundances to be based on Fe II lines. Fe II lines are essentially immune to departures from LTE (e.g. Shchukina & Trujillo Bueno 1901) and as clear from Table 3 are not particularly affected by the temperature inhomogeneities and different temperature structures in 3D model atmospheres. At this stage, however, we can not exclude that the estimated [O/Fe] trend with 3D models (Fig. 4) may even need further downward adjustment due to departures from LTE for Fe I lines, on which the existing 1D [Fe/H] estimates are based (Israelian et al. 1998, 2001; Boesgaard et al. 1999). Naturally, investigations of departures from LTE for Fe in metal-poor stars should also be based on the new generation of 3D model atmospheres.
For the LTE line formation calculations presented in Sect. 4, the assumption of instantaneous molecular equilibrium has been made in the computation of the total number density of OH molecules at different times and locations in the 3D model atmospheres. According to Fig. 1, the OH line formation is strongly biased towards the upflow regions, where the gas is very rapidly cooled from about 10000K to about 4000K in a relatively thin zone around continuum optical depth unity. This transition occurs on a time-scale of merely a few minutes for the upflowing material in the Sun, which could imply that molecular equilibrium is not established. Additionally, photodissociation due to the non-local radiation field from deeper layers may cause further departures from LTE. As a result, one would expect that LTE may overestimate the OH content and therefore that the O LTE abundances may be underestimated.
In principle it is a straight-forward exercise to compute the
OH molecule formation and the resulting OH number densities
as a function of time in our 3D model atmospheres by solving
a set of coupled differential equations corresponding to
a network of chemical pathways.
However, a major obstacle is the apparent lack of rate
coefficients for the relevant reactions and temperatures, both
experimental and theoretical.
We have scoured various publically available databases such as UMIST
(Le Teuff et al. 2000)
in search of the necessary rate coefficients but
with little success in locating data for
K, which
prevents us from performing the non-LTE chemistry calculations.
Some guidance to the non-LTE behaviour may still be obtained from observations in the absence of detailed computations. Uitenbroek (2000a,b) has recently concluded from a comparison of the observed solar CO line intensities and their temporal variations with calculations based on both 1D hydrodynamical chromospheric simulations (Carlsson & Stein 1992, 1995, 1997) and 3D model atmospheres similar to those utilised here (Stein & Nordlund 1998) that the inherent assumption of instantaneous molecular equilibrium for CO may not be valid in the Sun. This may suggest that a similar phenomenon could also occur for OH. If so, it would probably be more pronounced in metal-poor stars in view of their more rapid and dramatic cooling in the photosphere.
We conclude that it can not be excluded that departures from molecular equilibrium may influence the derived O abundances when using OH lines and that its magnitude could be metallicity dependent. Naturally this should be examined further once the necessary data becomes available.
Besides the assumption of chemical equilibrium for the OH molecule formation, it should be borne in mind that LTE has also been assumed in the OH line transfer calculations presented in Sect. 4. Due to the vast number of relevant levels and transitions, detailed non-LTE calculations for the OH radiative transfer is formidable. In fact, non-LTE effects for molecules in stellar atmospheres is largely unexplored even with 1D model atmospheres.
We are not aware of any non-LTE studies for OH but CO has attracted
slightly more attention.
Uitenbroek (2000b) has recently performed detailed non-LTE calculations
for the CO vibrational-rotational lines in the Sun,
which confirm the insightful prediction by
Hinkle & Lambert (1975) that the lines are
collisionally controlled and thus that LTE is a good approximation.
Hinkle & Lambert caution on the other hand
that electronic molecular transitions, like
the UV OH lines, may be radiatively determined.
If one approximates the UV OH line formation with the two-level
approach with complete redistribution (cf. Mihalas 1978),
the line source function
will depend on
the mean intensity averaged over the absorption profile
and Planck function
as
Here
is a measure of the photon destruction probability
(
).
In the UV,
tend to be larger than
for weak lines and
therefore
.
As a consequence, one would
expect the OH lines to be weaker with scattering than in LTE, or,
equivalently, that the derived O abundance will be underestimated in LTE.
A detailed non-LTE calculation for OH including all the vibrational
and rotational levels is unfortunately
beyond the scope of the present investigation.
But we have nevertheless attempted to estimate the non-LTE corrections to
the derived O abundances from the UV OH lines using a two-level OH molecule.
Although no doubt unrealistically simplistic to allow accurate quantitative
estimates of the non-LTE effects, the approach is still expected to yield
qualitatively correct results of the non-LTE behaviour for different
stellar parameters. For the purpose, version 2.2 of
the statistical equilibrium code MULTI (Carlsson 1986)
has been used after some minor modifications to
allow treatment of OH molecular lines. The two levels correspond to the
OH 313.9nm transition with the same adopted parameters as for the
LTE calculations presented in Sect. 4. Additionally,
cross-sections for collisions with electrons and hydrogen must be specified.
For the former the classical recipe of van Regemorter (1962) was adopted
in the absence of more appropriate treatments.
For H-collisions the calculations were performed with
the formula by Drawin (1968) multiplied by a variable factor x.
It should be noted that the Drawin formula was developed for atoms
and it is not clear whether it is at all applicable for molecules.
Although often used in non-LTE calculations for late-type stars
(cf. discussion in Kiselman 2001),
evidence is now mounting that the simple-minded approach by Drawin
severely overestimates the importance of H-collisions by at least
three orders of magnitude for atomic transitions
(Fleck et al. 1991;
Belyayev et al. 1999).
The inclusions of vibrational and rotational
sub-levels and line-blanketing in the UV radiation field
are also expected to diminish the non-LTE effects due
to stronger collisional quenching and decreased
ratios in the line-forming region.
![]() |
logg | [Fe/H] | [O/Fe]![]() |
![]() |
![]() |
[K] | [cgs] | [dex] | [dex] | ||
5780 | 4.44 | +0.0 | 0.0 | 0.0 | +0.18 |
0.01 | +0.18 | ||||
0.1 | +0.18 | ||||
1.0 | +0.17 | ||||
10 | +0.12 | ||||
100 | +0.03 | ||||
5780 | 4.44 | -1.0 | +0.4 | 0.0 | +0.20 |
0.01 | +0.20 | ||||
1.0 | +0.17 | ||||
5780 | 4.44 | -2.0 | +0.8 | 0.0 | +0.25 |
0.01 | +0.25 | ||||
1.0 | +0.22 | ||||
5780 | 4.44 | -3.0 | +1.2 | 0.0 | +0.26 |
0.01 | +0.26 | ||||
1.0 | +0.24 | ||||
100 | +0.04 | ||||
5690 | 3.67 | -2.5 | +0.5 | 0.0 | +0.25 |
0.01 | +0.25 | ||||
1.0 | +0.24 | ||||
+1.0 | 0.0 | +0.27 | |||
0.01 | +0.27 | ||||
1.0 | +0.26 | ||||
6450 | 4.04 | -3.0 | +1.0 | 0.0 | +0.15 |
0.01 | +0.15 | ||||
1.0 | +0.15 |
Non-LTE calculations have been performed for four 1D model atmospheres
with the solar
and
but different metallicities
([Fe/H]=0.0, -1.0, -2.0, and -3.0), as well as for selected
metal-poor stars (e.g. HD140283 and G64-12) for different O abundances
to estimate the influence of the line strength. In all cases,
the effect of H collisions were investigated by computing the
abundance corrections for three different x: 0.0, 0.01 and 1.0.
The 1D models are not identical
to those adopted for the solar sequence in the 3D-1D LTE comparison
in order to strictly isolate
the metallicity dependence of the non-LTE effects.
A summary of the results in terms of abundance corrections
is presented in Table 4.
As expected, in all cases the assumption of LTE makes the lines stronger,
and therefore that LTE analyses underestimate the O abundances.
The difference in terms of line strengths is quite large
(Fig. 6) and therefore
relatively large non-LTE corrections are obtained, 0.2dex.
The predicted 1D non-LTE effects are almost independent on the stellar parameters.
Fortunately, the magnitude of the non-LTE corrections is only marginally dependent
on the treatment of the H collisions provided the Drawin (1968) formula
does not greatly underestimate the collisional cross-sections (x>10).
Even with x=1.0 the predicted non-LTE corrections differ only by
dex
compared to the case when neglecting the H collisions completely. In all
cases, the same results are obtained with x=0.01 and x=0.0.
We emphasize though that it is still unclear whether the Drawin
recipe can be applied also to molecules. But at this stage there are
no indications that the treatment of H collisions play any significant
role in the predicted non-LTE corrections.
![]() |
Figure 6:
The 1D OH313.9nm line
in a metal-poor (
![]() |
The departures from LTE in this two-level approach are purely due to
scattering in the line, in agreement with the prediction by
Hinkle & Lambert (1975) (Fig. 7).
Since
at the relevant
atmospheric layers for the studied model atmospheres, the line source function
exceeds the LTE value. This effect is more pronounced in
the metal-poor models but its influence on the abundance corrections
is partly balanced by the weaker lines at lower metallicities.
Since the vast majority of the OH molecules will be in the ground
electronic state and the vibrational and rotational levels within
electronic states are closely coupled through collisions,
the departure coefficient
for the lower level
will be very close to one and thus no non-LTE effects due to differences
in line opacity emerge, as evident from Fig. 7.
In spite of the significant 1D non-LTE abundance corrections, we find no evidence for a pronounced steepening of the [O/Fe] trend. However, we caution that this conclusion can not automatically be extrapolated to the 3D case, in which the departures from LTE may be more severe in metal-poor stars since the steep temperature gradients may be more prone to scattering effects than in 1D model atmospheres. Needless to say, an investigation of the non-LTE behaviour in 3D model atmospheres as a function of metallicity has very high priority.
Considering that the results presented here are among the first investigations of the impact of the new generation of 3D hydrodynamical model atmospheres on stellar spectroscopy, it is therefore in order to ask if indeed these models are more realistic than previously used classical 1D model atmospheres. Sofar the predictions from the ab-initio 3D models have been very successfully confronted with detailed observational constraints for in particular the Sun. These comparisons include such disparate tests as granulation topology and flow field (Stein & Nordlund 1998), helioseismology (Rosenthal et al. 1999), intensity brightness contrast (Stein & Nordlund 1998; Asplund et al. 2000a), flux distribution and limb-darkening (Asplund et al. 1999b) and spectral line shapes, shifts and asymmetries (Asplund et al. 2000b). No doubt the current surface convection simulations for the Sun have a very high degree of realism. In sharp contrast theoretical 1D model atmospheres fail in regards to most, if not all, of the above-mentioned tests. Recently similar 3D models have been used for studies of line asymmetries in Procyon (Allende Prieto et al. 1901) and the metal-poor halo star HD140283 (Allende Prieto et al. 1999a; Asplund et al., in preparation) with very satisfactory outcomes.
In view of the detailed discussion given in Sect. 5 on possible remaining systematic errors in the 3D analysis, the reader may get the impression that the here presented results are rather uncertain. It may therefore be in order to point out that all of these possible effects also apply equally well to any study based on 1D model atmospheres, besides the errors introduced by the assumption of hydrostatic equilibrium and by treating convection through the mixing length theory (or a close relative thereof), which is known to be a poor representation of stellar convection. However, at this stage it is probably premature to conclude that 3D model atmospheres are indeed superior to classical 1D models. It is therefore of utmost importance now to carry out the same arsenal of tests which previously has been undertaken with 1D model atmospheres (flux distribution, limb-darkening, colours, H-lines etc) as well as additional ones now possible (line asymmetries and shifts, asteroseismology), in particular for metal-poor stars. This is even more true in light of the fact that 1D model atmospheres often fail the very same tests.
Even if the present article does not deal directly with observations, in this section we will nevertheless discuss some of the recent analyses of various O diagnostics as it has bearing on our findings and may give clues to the existence or not of the large granulation corrections for OH in metal-poor stars described in Sect. 4.
A major argument for the monotonic linear trend in [O/Fe] with
metallicity derived from the UV OH lines comes from the claimed
good agreement with the O I triplet results
(Israelian et al. 1998, 2001; Boesgaard et al. 1999). Since much smaller
granulation corrections are expected for the triplet than for OH
(Asplund 2001) this would seem to contradict the findings in Sect. 4.
However, the consonance between the OH and O I results
is not as unambigous when examining some of
the published analyses in detail.
Figure 8 shows the difference between the OH-based and
O I-based abundances of Boesgaard et al. (1999) on the King (1993)
-scale; with the lower Carney (1983) scale the correlation
is slightly less pronounced but clearly present
(slope =-0.13 instead of -0.18).
The absolute value for the abundance differences is here less important than
the existence of the metallicity-trend due to the uncertainty introduced by
the choice of
-scale and the neglect of non-LTE effects for
O I.
This divergent behaviour is very close to the expected according to
the results of Sect. 4 and Asplund (2001),
which we interpret as a qualitative argument for
the aptness of the 3D calculations.
In view of this, the good general agreement found by Israelian et al. (1998)
for nine stars using the O I equivalent widths of Tomkin et al. (1992)
pose a perplexing problem.
Clearly a larger stellar sample with simultaneous analyses of the OH and
O I lines would be very helpful in this context.
One should also explore
possible differences in for example C abundances derived from C I
and CH lines
at low metallicities. Preliminary calculations reveal significant
but smaller differences between 1D and 3D analyses of CH lines
compared with for OH lines (
0.3dex instead of
0.6dex for OH at [Fe/H]=-3.0).
![]() |
Figure 8:
The difference between the OH and
O I-based 1D abundances of Boesgaard et al. (1999) on the King
(1993)
![]() |
The conventional [O/Fe] plateau indicated by the [O I] line has
recently received support from the IR OH vibrational-rotational lines
(Balachandran et al. 2001; Melendez et al. 2001).
As discussed in Sect. 4, this could be interpreted as the
here presented granulation corrections for the UV OH lines in metal-poor
stars are overestimated, since for the same star the IR and UV lines
should to first order be affected similarly (with the possible exception
of non-LTE effects in the line formation, cf. Sect. 5.6).
A more likely explanation in our opinion, however, is to be found in
the differences in stellar parameters, in particular
,
for
the sofar investigated UV and IR targets. As evident from Table 2
and Fig. 3, the magnitude of the OH granulation corrections
depend on
in the sense that larger effects are present for
higher temperatures.
Although dangerous to extrapolate, we speculate that significantly smaller
OH abundance corrections than found here
will be obtained for
K,
which is typical for the metal-poor stars with detected IR OH features.
The physical reason for the dependence on
is the strength
of the coupling between the gas and radiation field: for lower
,
more and stronger spectral lines are available which contribute additional
radiative heating, keeping the gas temperature closer to the
radiative equilibrium value (Asplund et al., in preparation).
No 3D model atmospheres are yet available for these lower
which prevent verification of our hypothesis but such convection
simulations are currently being constructed.
Due to the inconspicous nature of the [O I] line (Nissen et al. 2001),
no direct comparison with the UV OH results has sofar
been possible for metal-poor dwarfs with [Fe/H] <-2.
Recently, Fulbright & Kraft (1999) have considered the [O I] line
in the two metal-poor ([Fe/H]
)
subgiants BD
and BD
(
K and 5260K, respectively), which have been
re-analysed by Israelian et al. (2001).
The UV OH lines suggest [O/Fe] =0.60
in both of the two stars, i.e. in significantly better agreement with the traditional
[O/Fe] plateau compared with the linear trend seemingly
implied by the OH lines in the hotter stars.
As for the IR analyses, we conjecture that at
K significantly smaller granulation corrections
than those given in Sect. 4 will be deduced,
leaving the plateau-like [O/Fe] ratios largely unchallenged.
Excellent agreement between [O I] and OH is found for both
stars: the forbidden line
gives [O/Fe] =0.62 and 0.52, respectively.
The latter values come from our own analysis using 1D MARCS
model atmospheres with the stellar parameters given in
Israelian et al. (2001). The equivalent widths for [O I]
and Fe II were taken from Fulbright & Kraft except for [O I]
in BD
for
which we relied on the VLT-measurement (
pm)
by Cayrel (2001).
This [O/Fe] estimate for
BD
is for unknown reason significantly lower
than reported in Israelian et al. (2001), [O/Fe] =0.82, while
in perfect agreement with the findings of
Balachandran (2001, private communication) using Kurucz (1993) model
atmospheres but otherwise identical input.
The low atmospheric temperatures encountered in
the new generation of 3D hydrodynamical model atmospheres
(Asplund et al. 1999a, 2000a,b; Allende Prieto et al. 1901;
Asplund et al., in preparation) compared with classical
1D hydrostatic model atmospheres for
metal-poor stars have a profound impact on
the OH line formation, as described in
Sect. 4. As a consequence, a possible
severe systematic error in recent
1D LTE analyses of OH lines in metal-poor stars
(Israelian et al. 1998, 2001; Boesgaard et al. 1999) has been exposed,
making the case for a monotonic increase in [O/Fe] towards lower metallicities
less convincing.
Taken at face value, our 3D LTE OH analysis results in [O/Fe] values
in rough agreement with the conventional [O/Fe] plateau for
[Fe/H]
indicated by the [O I] lines (Fig. 4).
We emphasize though that this apparant concordance should not
be taken too literally in view of the preliminary nature of
our 3D calculations and that no star-by-star comparison has been
made with observations.
By investigating possible systematic errors for the 3D LTE results
it has been found, however, that the conclusion
of large granulation effects on the OH lines appears reasonably robust.
For example,
-calibrations using 3D model atmospheres
should not differ significantly (
K)
from previous 1D calibrations provided
they are based on IRFM; the same is not necessarily true for alternative
calibrations using Balmer lines or colours.
Similarly, estimates of logg and [Fe/H] should remain essentially
unaltered when relying on Hipparcos parallaxes and Fe II lines.
The major remaining uncertainties in the 3D analysis appear to be the
assumptions of LTE for the molecular equilibrium and in the line formation.
Although not yet investigated for 3D model atmospheres, it is possible
that such departures from LTE may indeed steepen the [O/Fe] trend once again
but unlikely as much as the original 1D LTE case.
On the other hand, departures from LTE for Fe I may diminish
the slope further.
A final verdict on this issue must therefore await detailed and improved
non-LTE calculations. But we note in the meantime that even in the
presence of possible non-LTE effects for OH and Fe I, [O/Fe]
ratios will be less affected since both the O and Fe LTE abundances
will tend to be underestimated.
The purpose of the present paper has not been to advocate a specific [O/Fe]
trend with metallicity since our investigation has been limited to a
differential 3D-1D comparison without involving observational confrontation.
Nevertheless, our findings will likely fuel the long-standing
debate on the O abundances in metal-poor stars.
As already stated several times, it would be premature to conclude from
our analysis that the monotonic linear trend in [O/Fe] claimed by
Israelian et al. (1998, 2001) and Boesgaard et al. (1999)
must now be abandoned, even if the arguments for continuously increasing
[O/Fe] ratios towards lower metallicities appear much weaker.
It should be remembered that many, albeit not all, studies of the
O I triplet at 777nm in dwarfs and subgiants
find [O/Fe] values systematically higher
than those given by the [O I] line in metal-poor giants and subgiants.
The triplet is unfortunately sensitive to the adopted
-scale
and departures from LTE, which deserve very careful treatment.
For the moment, the most reliable [O/Fe] ratios still appear to come from
the [O I] line, provided that very high S/N spectra are
utilised and the metallicities are estimated from Fe II lines
(Nissen et al. 2001; Lambert 2001).
To ignore the existing systematic errors in traditional 1D LTE analyses of the UV OH lines in the hope that departures from LTE in the molecule formation and radiative transfer will conspire to exactly compensate the effects of the low atmospheric temperatures in 3D is certainly fraught with danger and furthermore most likely misleading. As demonstrated above, in order to derive reliable oxygen abundances in very metal-poor stars, unfortunately there is no easier escape route but to perform time-dependent non-LTE calculations in 3D model atmospheres for OH. Fortunately, recent improvements in the analysis of stellar spectra ensure such studies in fact being tractable tasks for the near future, which should help remove many of the lingering uncertainties in the derived oxygen abundances and perhaps finally settle the long-standing debate on [O/Fe] in metal-poor stars.
Acknowledgements
It is a pleasure to acknowledge many stimulating discussions with S. Balachandran, B. Gustafsson, T. Karlsson, D. Kiselman, D. L. Lambert, P. E. Nissen, F. Primas and N. Ryde related to the determinations of stellar oxygen abundances and their associated intricacies. The efforts by the two referees are much appreciated. The present work has been supported by the Swedish Natural Science Foundation (grant NFR F990/1999), by the Royal Swedish Academy of Sciences and by the Nordic Optical Telescope through a Ph.D. stipend to AEGP. MA is grateful for IAU travel grants to attend the IAU Symposium 198 in Natal, Brazil, and the IAU General Assembly in Manchester, UK.