A&A 399, 525-530 (2003)
DOI: 10.1051/0004-6361:20021802
S. Daflon 1 - K. Cunha 1 - V. V. Smith 2 - K. Butler3
1 - Observatório Nacional, Rua General José Cristino 77
CEP 20921-400, Rio de Janeiro, Brazil
2 -
Department of Physics, University of Texas at El Paso,
El Paso, TX 79968-0515, USA
3 -
Institut für Astronomie und Astrophysik der Universität
München, Scheinerstrasse 1, 81679 München, Germany
Received 15 October 2002 / Accepted 29 November 2002
Abstract
Non-LTE abundances of magnesium, aluminum and sulfur are derived
for a sample of 23 low-
stars belonging to six northern
OB associations of the Galactic disk within 1 kpc of the
Sun. The abundances are obtained from the fitting of synthetic
line profiles to high resolution spectra. A comparison of our
results with HII region abundances indicates good agreement
for sulfur while the cepheid abundances are higher. The derived
abundances of Mg show good overlap with the cepheid results.
The aluminum abundances for OB stars are significantly below
the cepheid values. But, the OB star results show a dependence
with effective temperature and need further investigation.
The high Al abundances in the cepheids could be the result of
mixing. A discussion of the oxygen abundance in objects near
the solar circle suggests that the current mean galactic oxygen
abundance in this region is 8.6-8.7 and in agreement with the
recently revised oxygen abundance in the solar photosphere.
Meaningful comparisons of the absolute S, Al and Mg
abundances in OB stars with the Sun must await a reinvestigation
of these elements
with 3D hydrodynamical model atmospheres for the Sun.
No abundance gradients are found within the limited range in
galactocentric distances in the present study. Such variations
would be expected only if there were large metallicity gradients
in the disk.
Key words: stars: abundances - stars: early-type
The massive OB stars, as well as other young objects such as
cepheids or H II regions, are used commonly as tracers of the
current chemical composition in the Galactic disk. Abundance
analyses of H II regions are restricted to a handful of elements,
those being most typically He, N, O, S, Ne, and Ar. In the OB
stars, besides N, O, and S, the additional elements C, Mg, Al,
Si, and Fe can be analyzed using both LTE and non-LTE techniques.
Cepheids are the more evolved, cooler descendents of a subset of
the OB stars (with masses of 5-10
)
and a rather
large number of elements (
25) can be detected in their
spectra. As all three of these types of objects are young (
few
107 yr),
it is reasonable to expect that examples
of them inhabiting the same region of the Galaxy should contain
approximately the same mixture of chemical elements. This
approximation should be as good as allowed by small scale chemical
inhomogeneities that can be produced on timescales of the lifetimes
of large, starforming regions, i.e. a few times 107 yr.
Care must be taken in comparing abundances between H II regions,
OB stars, and cepheids, however. For example, internal stellar
mixing may affect both OB stars and cepheids to varying degrees.
Some evidence of the mixing of material exposed to the CN-cycle
has been uncovered in certain OB stars, even near the main sequence
(Gies & Lambert 1992); this mixing may be driven primarily
by rotation (Heger & Langer 2000). In this case, nitrogen
abundances are measurably enhanced (by +0.3 dex) with,
perhaps, a marginal decrease in the carbon abundance
(by
-0.1 dex), while oxygen remains untouched. In the
more evolved cepheids, even deeper mixing may have occurred that
involves the full CNO cycles, such that some oxygen depletion might
be detectable (as well as larger N enhancements and C depletions).
There is also the possibility that surface abundances of sodium,
magnesium, and aluminum have been altered by the mixing of material
exposed to the Ne-Na and Mg-Al cycles. In the H II regions,
uncertainties or systematics may arise from temperature fluctuations
in the gas, unknown radiation environments, or possible depletions
of some elements out of the gas phase and onto solid particles.
By concentrating on a set of OB stars, in comparison to sample
H II regions and cepheids, all near the solar circle, abundance
trends found in these various objects can be intercompared
and ultimately be checked against their corresponding solar values.
Such checks can help uncover possible inconsistencies and provide
stronger constraints on abundance gradients derived from different
types of objects. That is the aim of this paper, which is the
fourth paper in a series whose ultimate goal is to use a large
sample of OB stars to trace abundance gradients in the Galactic
disk. Paper I (Daflon et al. 1999) presented the
first results from this survey and concentrated on 8 sharp lined
star (
60 km s-1) members of the Cep OB2
association: LTE abundances of C, N, O, Si, and Fe were derived,
as well as non-LTE abundances of C, N, O, and Si. Daflon et al.
(2001a - Paper II) added analyses of 15 members of five
additional northern OB associations (Cep OB3, Cyg OB2, Cyg OB7,
Lac OB1, and Vul OB1). The same sets of elements were analyzed in
non-LTE as in Paper I, but additional LTE results were presented
for Mg, Al, and S. In Paper III (Daflon et al. 2001b),
the analysis was expanded to include some of the more rapidly
rotating stars (
km s-1) in the northern
sample. In addition, the atomic analysis included non-LTE
calculations for Mg and Al. In this paper, the non-LTE calculations
for Mg and Al are applied to the remaining northern stars, and
sulfur is now added as an element that can be studied in non-LTE.
The observational data consist of high resolution (
),
high signal-to-noise spectra of 23 main sequence late-O/early-B
stars belonging to the OB associations of Cep OB2, Cep OB3,
Cyg OB3, Cyg OB7, Vul OB1 and Lac OB1. The spectra were obtained
at the McDonald Observatory, University of Texas, Austin, with the
2.1m telescope plus the Sandiford Echelle Spectrograph. A set of
lower resolution (R=12 000) spectra of these targets were obtained
with the 2.7 m telescope plus a Coudé Spectrograph in the H
region. More details about the observations and data reduction
are found in Papers I and II. Also in these studies are derived the
stellar parameters and microturbulences for the 23 target stars.
The effective temperatures and surface gravities have been derived
from a photometric calibration for the reddening-free index Q coupled
to the fitting of the broadened wings of H
profile. In our
earlier series of papers (Papers I and II) it has been argued that
this method results in uncertainties of
4% in
and
0.10 dex in
.
The uncertainties in gravity can be also
viewed by noting the positions of stars in a
diagram in comparison to model tracks. For stars of these effective
temperatures the
of the ZAMS is about 4.2. An average of all
surface gravities of stars here that are not clearly evolved (i.e.,
4.0) finds
;
close to what is
expected for stars near the main sequence with a scatter larger than
what we have estimated as the expected uncertainty. The microturbulent
velocities were obtained from the requirement that the non-LTE O II
abundances were independent of the line-strength. The sample stars,
their corresponding associations and spectral types, adopted effective
temperatures, surface gravities and
-value are gathered in
Table 1. We note that the surface gravity for the star
HD 214167 has been revised.
Non-LTE synthetic spectra were calculated for the few transitions
of Mg II, Al III and S III that are available in the spectra
of early-type stars. Although relatively free of blends, linelists
for each spectral region containing the lines of interest were
constructed within the interval around 3 Å. These transitions
are listed in Table 2 together with their wavelengths,
designations, excitation potentials as well as adopted gf-values.
The abundance analyses in this study are based on the fully-blanketed
and plane-parallel LTE model atmospheres calculated with the ATLAS9
code (Kurucz 1992) for a constant microturbulent velocity of
and solar composition. Departures from LTE
were considered in the line formation calculations with the newest
version of the program DETAIL (Butler 1994). The adopted model
atoms are described in Przybilla et al. (2001-Mg II),
Dufton et al. (1986-Al III) and Vrancken et al.
(1996-S III). We note that so far, all abundance
papers
including non-LTE line formation of Al published in the literature
(Vrancken et al. 1997; Vrancken et al. 2000;
Gummersbach et al. 1998) were based on the same model atom
adopted in this study, and in a sense are not completely independent.
Published non-LTE abundance analyses of magnesium (Vrancken et al.
1997; Vrancken et al. 2000; Gummersbach et al.
1998) are based on the model atom of Mihalas (1972).
All sulfur abundances in the literature are derived from LTE analysis,
except for Vrancken et al. (1996), who analyzed three B stars
as a test for their sulfur model atom. The present study is the first
systematic non-LTE analysis of sulfur abundances in OB stars.
A brief description of the adopted model atoms follows. Przybilla et al. (2001) constructed an extensive model atom for Mg I/Mg II based on recent atomic data. This model is roughly complete for levels up to n=9 for Mg I and n=10 for Mg II, whereas Mg III, that does not have a significant population in the range of effective temperatures considered here, is represented only by its ground state. The sulfur model atom (Vrancken et al. 1996) treats S II/S III simultaneously considering 81 levels of S II and 21 levels of S III. The model atom also includes the three lowest levels of S I and the two lowest levels of S IV, together with the ground state of S V that are important for the hotter stars. Vrancken et al. (1996) tested their model atom for three B2 III-V stars and concluded that S III lines yield more reliable abundances (than S II lines) for the temperature range they consider. The adopted Al III model atom of Dufton et al. (1986) is less complete than the magnesium and sulfur ones. It consists of 12 states of Al III plus the ground state of Al IV, as the populations of Al II and Al V are not significant in the temperature range between 20 000 to 35 000 K.
The synthetic line profiles were calculated with the SURFACE code
(Butler 1984), assuming Voigt profile functions. These profiles
were then broadened by means of convolution with the rotational
profile, including
and limb darkening, and instrumental
profile. The microturbulence for each target star was adopted from
O II lines, derived in previous studies, while the line abundances
and
's were allowed to vary. The best fit was chosen from
the
-minimization of the differences between theoretical and
observed profiles. The final non-LTE abundances and
's are
listed in Table 3. The abundances are represented by the
average of the individual line abundances and the respective
dispersions (and number [n] of fitted lines), whenever this is the
case. A sample of profile fitting for all the spectral regions is
shown in the panels of Fig. 1, for the star HD 197512.
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Figure 1:
Some examples of line synthetic profiles fitted
to the observed spectra of the star HD 197512 for
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In general, the published non-LTE Mg, Al and S abundances are
consistent with our results. However, aluminum abundances deserve
special attention. As a test, we recalculated the non-LTE abundances
of star 201 in NGC 2244, analyzed by Vrancken et al. (1997),
using their listed equivalent widths and, as expected, we reproduced
their derived abundance within the errors. We also compared our
non-LTE Al abundances with those interpolated in the grid of
theoretical equivalent widths of Dufton et al. (1986). The
comparison for the model atmospheres with lower
showed
that our abundances derived directly from synthesis agree within the
uncertainties with the abundances interpolated within their grid.
For the hottest models, however, Dufton's grid yields aluminum
abundances much lower than 6.0. The difference between our
synthetic profiles and Dufton's grid resides basically in the model
atmospheres, as they used non-LTE non-blanketed model atmospheres
and we use fully blanketed LTE models from Kurucz (1992).
Errors in the determination of the stellar parameters, microturbulence,
and the placement of the continuum are the main sources of
uncertainties in a chemical analysis based on the fitting of line
profiles. In Paper III we discussed the uncertainties assigned for
each of these parameters and the subsequent abundance error arising
from them. Accordingly, our total errors expected for the derived
magnesium abundances are of the order of 0.2 dex, being larger for
the coolest stars (0.3 dex for
K) and these are
dominated by the uncertainty in the microturbulent velocity (as the
abundance analysis is based on one intermediate to strong line)
and effective temperature. The uncertainties in sulfur abundances
arise chiefly from the errors in
and are estimated to be
of the order of
0.15 dex. The profile of the weak S III line
at 4364 Å was only fitted for the stars with lowest
and highest signal-to-noise spectra. For this reason, the sulfur
abundances of some stars in our sample are based only on the S III
line at 4361 Å. Formally, aluminum presents smaller errors, around
0.10 dex. However, the abundances derived from the lines
4512 and 4529 Å, that belong to the same multiplet,
show some dependence with effective temperature and introduce a larger
discrepancy in the average abundances of the hottest stars.
OB stars, as well as other young objects, such as H II regions,
track the chemical composition of the Galactic disk. With this
paper, we now have a uniform set of non-LTE abundances for C, N,
O, Mg, Al, Si, and S in 35 stars of the northern sample. All
six OB associations that are represented in this sample lie within
1 kpc of the solar circle (where we take
kpc,
McNamara et al. 2000) and, as such, will have abundances
that are, in principle, not affected significantly by possibly modest
galactic abundance gradients. The abundances of certain elements
like sulfur, magnesium and aluminum, as well as oxygen can be
compared to recent results derived from cepheids and H II regions
also lying within the same galactocentric distance interval.
The stars in this sample are all sharp-lined stars, with
km s-1. The mean magnesium abundance is
.
The magnesium abundances are quite independent
of effective temperature, as shown in the top panel of
Fig. 2. Aluminum shows significantly lower abundances
in the OB stars than the Sun, as displayed in the middle panel of
Fig. 2. Such a large difference may reside partially
in uncertainties in the non-LTE calculations for Al III. Some
evidence for this possibility may appear in the derived aluminum
abundances showing a small offset between the hotter and cooler
stars. There is a noticeable displacement between stars with
K and those with lower effective temperatures,
with the hotter sample having a mean aluminum abundance of
and the cooler having
.
For the S III
non-LTE abundances, there is no trend with
and the
scatter is small: the mean
is 0.14 dex below
the solar abundance of
(Grevesse & Sauval 1998).
Taken together, these new results for S, Mg and Al, could in principle suggest that the OB stars, on average, are slightly underabundant when compared to the Sun. However, these differences could be argued to be at the level of systematics. In fact, the recent results from 3D-hydrodynamical model atmosphere calculations for the elements C, N and O in the solar photosphere have lowered their abundances significantly. The photospheric carbon, nitrogen and oxygen abundances, for instance, have been recently revised to 8.41, 7.80, and 8.66, respectively (Asplund 2002), and are now in better agreement with the results for OB stars. In addition, the OB results rely on 1D-models while the recent solar results are from 3D hydrodynamical models. Although it is expected that the radiative OB atmospheres will not have temperature inhomogeneities that are present in the solar atmosphere (caused by convection) it is possible that some other effects like the high microturbulent velocities required for the OB stars indicates that 1D models are not a complete description of atmospheres of hot stars.
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Figure 2:
Elemental abundances of magnesium, aluminum and sulfur as
a function of effective temperature.
The abundances are represented by the average of individual lines while
errorbars represent the respective standard deviations.
The dotted lines represent the
solar abundances listed by Holweger (2001, Mg:
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Abundances for the different types of young objects, OB stars,
cepheids, and H II regions, are compared in Fig. 3
for the elements Mg, Al, and S, as well as O (with oxygen taken
from our previous papers). We restrict our discussion to
galactocentric distances that overlap those values found for the
six OB associations represented here, where
to
8.2 kpc. Oxygen is added to the discussion in Fig. 3
as it represents the most abundant element after H and He, and is
the element comprising the largest fraction of a star's abundance
of heavy elements. In Fig. 3, the mean abundances
of each OB association are plotted as filled squares, with the
errorbars representing the standard deviation found within the
abundances of the association members (in the case where only
2 members were represented, the errorbars are the average
differences from the mean). Over the limited range of distance
sampled by this particular set of stars, no significant abundance
gradient is apparent and the horizontal solid lines are the mean
abundances of the set of OB associations. The solar symbol is
plotted at
kpc. The mean oxygen abundance (non-LTE
calculations for O II from Papers I, II, and III) is
.
Again, as discussed above, in agreement with the revised solar value.
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Figure 3:
A comparison between abundances for OB stars,
cepheids, and H II regions, for the elements O, Mg, Al, and S.
The abundances are plotted as a function of the Galactocentric
distance, within 1 kpc from the Sun, represented at
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Other results plotted in Fig. 3 include the recent cepheid results from Andrievsky et al. (2002), shown as open circles, and H II regions from the optical emission lines analyzed by Deharveng et al. (2000), shown as six-pointed stars, and H II region far-IR fine-structure lines by Simpson et al. (1995) and Afflerbach et al. (1997), shown as three-pointed stars.
Beginning with oxygen in Fig. 3, with abundances shown
for cepheids and H II region optical and far-IR lines, there is
considerable overlap among all sets of results. The cepheid
O-abundances tend to be slightly higher than the OB associations,
but only by 0.1 dex: likely within possible systematic
offsets. The optical H II region results from Deharveng et al.
(2000) scatter almost perfectly within the OB association
results, while the far-IR results exhibit a larger scatter, but
with an average O-abundance that agrees with the OB associations.
Overall, the suggestion from all of the oxygen abundances is
that the current mean Galactic abundance near the solar circle
is about
,
with an intrinsic dispersion that
is not yet well-defined. The dispersion among the six OB
associations is quite small (as well as the H II region optical
emission-line results).
The panel of Mg and Al abundances in Fig. 3 contains
only OB association and cepheid results. In the case of Mg, the
overlap between OB stars and cepheids is essentially perfect.
The mean and standard deviations are
for the OB
associations and
for the cepheids. This tidy
agreement between cepheids and OB stars for Mg is not repeated
for Al, where the cepheid Al abundances are measurably above solar,
while the OB stars are well below solar. As discussed previously,
the OB star abundances are based upon non-LTE calculations using
Al III lines and there is a noticeable offset in the abundances
derived for the hotter and cooler stars. This may indicate
problems in the non-LTE calculations. On the other hand, the
rather large Al abundances for the cepheids may not be intrinsic,
but could be the result of stellar mixing involving material
exposed to the Mg-Al cycle of H-burning. An increase in aluminum
would come from a decrease in Mg, however, the observed offset
in the Al abundances seen in the cepheids would result in a
negligible decrease in Mg, due to the larger Mg abundance.
For example, if the current Galactic Al abundance is about solar,
then the cepheids have had their Al abundances enhanced by
0.2 dex. If this enhancement came from Mg atoms, it
would result in a decrease in the overall Mg abundance
by
0.02-0.03 dex, which is effectively unmeasurable.
More work will be needed to decide the best value for the
current Galactic aluminum abundance at the solar circle.
Finally, Fig. 3 also shows sulfur abundances derived from the OB stars, cepheids, and H II regions (from the far-IR lines). Here, the OB associations and H II regions again show a great degree of overlap. The cepheids exhibit systematically larger S abundances than the Sun, and these enhancements cannot be due to internal stellar mixing.
We have presented non-LTE abundances of magnesium, aluminum and
sulfur for 23 OB stars members of OB associations within 1 Kpc of
the Sun. Magnesium abundances derived for cepheids in the
literature agree well with the results for OB stars while no Mg
abundances can be derived for H II regions. On the other hand,
sulfur results point to discrepencies between the cepheids on
one side and the OB stars and H II regions on the other, that are
not easily resolved: the cepheid sulfur abundances are derived from
S I lines, while the OB stars use S III lines, and the H II region
results come from far-IR [S III] lines at 19 m. Aluminum
abundances OB stars must be interpreted with caution: the relatively
low Al abundances derived may reflect problems in the non-LTE
calculations for this species and deserves additional analysis before
further conclusions while Al in the cepheids could be enhanced due to
internal mixing.
Acknowledgements
S.D. acknowledges a CAPES fellowship and partial financial support from DAAD (Germany). KC thanks David Lambert for travel support for observing runs in 1992, 1993, and 1994. VVS acknowledges support by the National Science Foundation through grant AST99-87374.