Issue |
A&A
Volume 509, January 2010
|
|
---|---|---|
Article Number | A20 | |
Number of page(s) | 15 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361/200912687 | |
Published online | 12 January 2010 |
Chemical abundances of 11 bulge stars
from high-resolution, near-IR spectra![[*]](/icons/foot_motif.png)
N. Ryde1,2 - B. Gustafsson2 - B. Edvardsson2 - J. Meléndez3 - A. Alves-Brito4 - M. Asplund5 - B. Barbuy4 - V. Hill6 - H. U. Käufl7 - D. Minniti8,9 - S. Ortolani10 - A. Renzini11 - M. Zoccali8
1 - Lund Observatory, Box 43, 221 00 Lund, Sweden
2 - Department of Physics and Astronomy, Uppsala University, Box 515,
751 20 Uppsala, Sweden
3 - Centro de Astrofisica da Universidade do Porto, Rua das Estrelas,
4150-762 Porto, Portugal
4 - Department of Astronomy, University of São Paulo, IAG, Rua do Matão
1226, São Paulo 05508-900, Brazil
5 - Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1,
85748 Garching, Germany
6 - OCA, Boulevard de l'Observatoire, BP 4229, 06304 Nice Cedex 4,
France
7 - ESO, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
8 - Department of Astronomy and Astrophysics, Universidad Catolica de
Chile, Casilla 306, Santiago 22,
Chile
9 - Vatican Observatory, V00120 Vatican City State, Italy
10 - Department of Astronomy, Padova University, Vicolo
dell'Osservatorio 2, 35122 Padova, Italy
11 - Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5,
35122 Padova, Italy
Received 12 June 2009 / Accepted 22 September 2009
Abstract
Context. It is debated whether the Milky Way bulge
has characteristics more similar to those of a classical bulge than
those of a pseudobulge. Detailed abundance studies of bulge stars are
important when investigating the origin, history, and classification of
the bulge. These studies provide constraints on the star-formation
history, initial mass function, and differences between stellar
populations. Not many similar studies have been completed because of
the large distance and high variable visual extinction along the
line-of-sight towards the bulge. Therefore, near-IR investigations can
provide superior results.
Aims. To investigate the origin of the bulge and
study its chemical abundances determined from near-IR spectra for bulge
giants that have already been investigated with optical spectra. The
optical spectra also provide the stellar parameters that are very
important to the present study. In particular, the important
CNO elements are determined more accurately in the near-IR.
Oxygen and other elements
are important for investigating the star-formation history.
The C and N abundances are important for determining
the evolutionary stage of the giants and the origin of C in
the bulge.
Methods. High-resolution, near-infrared spectra in
the H band were recorded using the CRIRES
spectrometer mounted on the Very Large Telescope.
The CNO abundances are determined from the numerous molecular
lines in the wavelength range observed. Abundances of the elements
Si, S, and Ti are also determined from the near-IR spectra.
Results. The abundance ratios [O/Fe], [Si/Fe], and
[S/Fe] are enhanced to metallicities of at least [Fe/H] =
-0.3, after which they decline. This suggests that the
Milky Way bulge experienced a rapid and early burst of star
formation similar to that of a classical bulge. However, a similarity
between the bulge trend and the trend of the local thick disk seems to
be present. This similarity suggests that the bulge could have had a
pseudobulge origin. The C and N abundances suggest
that our giants are first-ascent red-giants or clump stars, and that
the measured oxygen abundances are those with which the stars were
born. Our [C/Fe] trend does not show any increase with [Fe/H],
which is expected if W-R stars contributed substantially to
the C abundances. No ``cosmic scatter'' can be traced
around our observed abundance trends: the measured scatter is expected,
given the observational uncertainties.
Key words: stars: abundances - stars: carbon - Galaxy: bulge - infrared: stars - stars: late-type
1 Introduction
An unsolved problem in cosmology is how galaxies formed and obtained their stellar populations (see for instance Renzini 2006). Two scenarios have traditionally been presented for the formation of bulges, one leading to ``classical'' bulges, whereby they originate from merger-driven starbursts, and another leading to ``pseudobulges'', related to the ``secular'', dynamical evolution of disks (Kormendy & Kennicutt 2004). In the classical bulge scenario, most stars originate in a short phase of star formation when the universe was only a few Gyr old, and bulge formation may precede that of the disk, which possibly formed later. In the so-called ``pseudobulges'', stars form in the disk, over an extended period of time, and the bulge is produced by the secular evolution of the disk driven by the development of a bar. Thus, one expects classical bulges consist almost entirely of old stars, and stars in pseudobulges to have an age spread comparable to the Hubble time.
The bulges of early-type spirals (Sa and Sb) are generally
considered to be ``classical'', while pseudobulges are thought to
inhabit preferentially late-type
spirals (Sc and Sd). The Sbc
Milky Way (MW) galaxy sits morphologically on the borderline,
hence it is no surprise that the origin of its bulge is currently a
matter of debate. What is especially puzzling about the
MW bulge is that it is
dynamically ``pseudo'' (due to its boxy, peanut-shaped bar, Kormendy & Kennicutt 2004),
while its stellar content is that expected of a ``classical''
bulge. Indeed, deep color-magnitude diagrams (CMD) of the
MW bulge show no detectable trace of stellar populations
younger than Gyr
(Zoccali
et al. 2003; Ortolani et al. 1995),
a conclusion for which compelling evidence was provided by
the deep CMD obtained with HST for a proper-motion selected sample of
bulge stars (Clarkson et al.
2008).
This sharp distinction between these two scenarios of bulge
formation has been called into question: observations of disk galaxies
at redshift
(a lookback time
Gyr) indicate that
early disks have radically different physical properties from local
disks of similar mass. The early disks are characterized by far higher
velocity dispersions and gas fractions, and
harbor massive, highly star-forming clumps (Förster Schreiber et al. 2009;
Genzel
et al. 2006). Thus, dynamical instabilities in these
early disks appear to occur on much shorter timescales (few 108 years)
than for local disks, with a secular, but rapid evolution of the early
disks resulting in the early formation of a bulge (Genzel et al. 2008). In
parallel to observations, theories have also envisaged the early
production of bulges by the migration and central coalescence of
gas-rich clumps in high-redshift disks (e.g., Immeli et al. 2004; Bournaud &
Elmegreen 2009; Carollo et al. 2007; Elmegreen
& Elmegreen 2005).
Determinations of detailed chemical compositions are key data
for studies of the origin and
evolution of stellar populations, since they provide characteristic
signatures of the objects that enrich the interstellar gas. Abundance
ratios are sensitive to both the time scales of star formation and the
initial mass function (IMF), and may illustrate relations between
different stellar groups, since different elements are synthesized by
different processes and stars. For the bulge, a high ratio of -element
abundances relative to Fe is observed, suggesting that the
star-formation period was early and very short (Fulbright et al. 2007;
Lecureur
et al. 2007) and that the bulge formed more rapidly
than the thin, and perhaps even the thick, Galactic disk (McWilliam et al. 2008).
Since stellar abundance ratios carry this genetic information on the origin of different stellar populations, Zoccali et al. (2006) compared the [O/Fe] ratios of bulge stars with those of thin disk and thick disk stars from Bensby et al. (2004). Finding bulge ratios that are systematically higher than disk ones, Zoccali et al. argued against the bulge having been appreciably contaminated by the migration of thick disk stars analogous to those in the solar neighborhood. A similar consideration was put forward by Lecureur et al. (2007), based on the much higher values of [Na/Fe], [Mg/Fe], and [Al/Fe] ratios in bulge stars compared to those in thin and thick disk stars measured by Bensby et al. (2005), Bensby & Feltzing (2006), and Reddy et al. (2006). However, differences in abundance ratios may also be caused by differences in systematic errors when the abundance analysis is completed by different groups, using different data and methods. For example, cool giants with crowded spectra are affected by many systematic uncertainties (see, for instance, Santos et al. 2009) and comparing abundances derived from metal-rich, cool giants with those of solar-type dwarfs could be uncertain because of relative systematic errors.
In this context, Meléndez
et al. (2008) carried out a homogeneous abundance
analysis of bulge, thick disk, and thin disk giant stars, confirming
the [O/Fe] trend found by Zoccali
et al. (2006) for bulge
stars, but finding that [O/Fe] ratios for thick disk stars are
much higher than those derived by Bensby
et al. (2004). Thus, in the study by Meléndez
et al., the high thick-disk [O/Fe]-ratios appear to be
indistinguishable from those of the bulge. This similarity between the
[O/Fe] ratios of bulge and thick disk stars weakens the
conclusion of Zoccali et al.
(2006) about the genetic difference between the bulge and the
thick disk, although it remains to be seen whether this similarity also
holds for other [/Fe] ratios,
that appear to be so different in Lecureur
et al. (2007). If thick disk and bulge giants follow
the same [O/Fe] versus [Fe/H] relation, the inference
would be that both have formed ``rapidly''. The properties of
disks,
with their high velocity dispersion and high star formation rates,
implies that we are witnessing thick-disk formation.
Determination of abundances for a large sample of red giant
stars and planetary nebulae (cf. Chiappini
et al. 2009) in various bulge fields as well as in
the inner Galactic disk will obviously provide a most powerful method
for constraining the chemical evolution and models of the bulge (Silk & Wyse
1993; Matteucci
& Romano 1999). A way to achieve this is by
high-resolution, near-IR spectroscopy of red-giant stars. In the IR,
the obscuration in the direction of the bulge is considerably lower,
offering us the opportunity to target heavily reddened regions.
Furthermore, near-IR spectra are far less affected by line blending
than spectra at optical wavelengths, which means that it is possible to
accurately fit the continuum and avoid abundance criteria marred with
blending lines, so important in abundance analysis. Moreover,
only the IR offers, even within a small wavelength range, all
the indicators necessary to accurately determine the
CNO abundances by the simultaneous observations of many
uncontaminated CO, CN, and OH lines. Here, we present the
first data from our VLT/CRIRES program in which we study
systematically the stellar abundance ratios in different parts of the
Galactic bulge. In particular, we study the key elements C, N,
and O, but also include some elements.
2 Observations
In the VLT program ``Unveiling the secrets of the Galactic bulge: an infrared spectroscopic study of bulge giants'' we have as yet observed 8 bulge stars in the H band with the CRIRES spectrometer (Käufl et al. 2004; Moorwood 2005; Käufl et al. 2006). CRIRES is a cryogenic echelle spectrograph designed for high spectral resolution, near-infrared observations. Adaptive optics (MACAO - multi-applications curvature adaptive optics) was used, enhancing both the spatial resolution and the signal-to-noise ratio. The adaptive optics, which is only feasible and available in the near-IR, also has the advantage of rejecting diffuse starlight that may affect observations in regions of high star density.
The giants that we observed were chosen from the optical
investigation of Lecureur
et al. (2007), to be in three fields of the Galactic
bulge at (l, b) =
(,
), (
,
)
[Baade's Window], and (
,
)
[Globular Cluster NGC 6553]. Lecureur
et al. (2007) analyzed UVES/FLAMES spectra and
derived chemical abundances, but their determinations of the important
C, N, and O elements can be improved. The K giants
are chosen to be half way up along the red-giant branch (RGB), with
4000 K
4500 K, a range where molecular diagnostics can be
optimally observed and where adequate S/N ratios
can be achieved. Spectra of
these stars may be modelled more accurately than those of stars higher
in the RGB, making their
abundance analysis much more reliable. The surface compositions are
characteristic of the gas from which the stars once formed, with the
exception of changes in C and N, the sums of which are, however,
expected to be left unaltered by the dredge-up of CN-processed material
from the stellar interiors. The H magnitudes
and the total integration times (ranging from 32 to
80 min) for each of the program stars are given in
Table 1.
Our observations were performed between May 2007 and
October 2008.
Zoccali et al.
(2008) determined the iron abundance distribution of the
Galactic bulge from approximately 800 K giants (including
those we observed) in four fields toward the bulge with the VLT/FLAMES
in the GIRAFFE mode at R=20 000. They
detected a clear gradient toward lower latitudes. The iron distribution
functions of the stars in the different fields are mainly in the range ,
but the peaks of the distributions occur at lower [Fe/H] for
lower latitudes. All of our stars in three fields in the bulge have
metallicity less than solar.
The projected slit width on the sky was 0.30
yielding a spectral resolution of
with 3.0 pixels per spectral resolution element
. This allows us to resolve
blends, define the continuum, and adequately take care of telluric
lines. In principle, with ideal adaptive optics and perfect image
quality the
true effective entrance slit would be the diffraction-limited image of
the star itself transmitted by the instrument
.
However, CRIRES does not reach this limit because of the finite pixel
size (
ms-1
equivalent) and the limited optical quality of the internal optics,
mostly due to the relatively large ZnSe pre-dispersion prism. Indeed,
the optical quality of the complete pre-disperser is just marginally
sufficient for the nominal resolution of R=100 000.
In addition, at the wavelength of interest here, the adaptive optics
does not produce diffraction-limited images, but only a core with
a halo. This implies that the finite slit width controls the
effective spectral resolution. During the commissioning of the
CRIRES-MACAO system, the effective point-spread function was analyzed
in great detail, and even in the K-band in perfect
conditions the energy fraction within the nominal 0.2'' slit
never exceeded 60%, while for fainter stars and normal seeing
conditions this fraction was similar to 40-50% (for more details, see
Table 1
in Paufique et al. 2006).
The wavelength range expected from the ``36/-1/i''-setting
of CRIRES (i.e., in Echelle order 36) was
1539.3-1565.4 nm over the detector arrays, consisting of a
mosaic of four Aladdin III InSb arrays in the focal plane.
At order 36, the blaze function of the grating limits
the throughput of the detector arrays #1 and #4
markedly. We, therefore, concentrated our analysis on detector
arrays #2 and #3. Data for the first and fourth
detector arrays were used to check the abundances when possible. This
is, nevertheless, an improvement in wavelength coverage compared with
the Phoenix spectrometer (Hinkle
et al. 1998) at the Gemini telescope, an instrument
that inspired the design of the CRIRES spectrometer, with a total
wavelength range that corresponds to approximately one of CRIRES's
detector arrays (
).
It should be noted that there are small gaps of approximately
2 nm between the spectra on the detector arrays.
Table 1: Account of our observations.
Table 2:
Stellar parameters for the model atmospheres of our program stars given
as .
The approximate signal-to-noise ratios (S/N) per pixel of the observed spectra at 1554.8 nm (a carefully selected continuum region in the third detector array) are also given in Table 1. The S/N per resolution element is close to being a factor of 2 higher. The S/N is difficult to measure because the many spectral lines make it difficult to find a large enough region of continuum in which it can be estimated. The numbers given here are therefore indicative. The S/N varies by a factor of two between the detector arrays, mainly because the blaze function. The S/N of the third detector array increases with wavelength from 90 to 110% of the S/N at the reference wavelength of the standard setting, here chosen to be at 1557.3 nm. For the second detector, the S/N varies linearly with wavelength from approximately 70% to 100% of the S/N at the reference wavelength.
The observed data were reduced with the ESO standard pipeline
reduction package. The CRIRES pipeline is based on the general and
coherent approach by ESO using common routines, also employed in other
instruments (Ballester et al.
2006). The wavelength solution is based on a physical model
approach for CRIRES (Kerber
et al. 2008a) using telluric emission lines and the
new infrared line catalog for ThAr hollow arc lamps (Kerber
et al. 2008b). For the extraction of one-dimensional
spectra, the data taken for
different positions of the star along the slit (resulting from nodding
and dithering) are corrected in the usual way for glitches and bad
pixels and then rebinned in wavelength space before co-addition. The
final extraction is based on the ``optimal extraction'' method, which
preserves the flux without sacrificing S/N-ratio.
In addition to these eight giants, we reanalyzed the data of
three bulge giants studied in Ryde
et al. (2009). These were also observed with CRIRES,
during its science verification,
on 12 August 2006. The H magnitudes
and the total integration times are given in Table 1. The observations
and data are similar to those of our eight stars, although they were
observed at R=50 000 and processed with
routines in the reduction package
IRAF (Tody 1993) to obtain one-dimensional,
continuum-normalized, and wavelength-calibrated stellar spectra.
3 Analysis
We analyze our spectra by modeling the stellar atmosphere and creating synthetic spectra for the observed spectral region. These are thereafter convolved to fit the shapes and widths of the lines, including the stellar macroturbulence and instrumental broadening. We then derive elemental abundances by fitting the synthetic to the observed spectra. In this section, we discuss the model atmospheres, the stellar parameters and their uncertainties, and the spectrum synthesis.
3.1 Model atmospheres
We derived model atmospheres with the MARCS
code (Gustafsson et al. 2008)
after adopting the stellar effective temperature, logarithmic surface
gravity, metallicity, microturbulence, and [/Fe] enhancement for
the model of each of our stars (see Table 2, Col. 4).
The MARCS standard models are hydrostatic
and computed based on the assumptions of local
thermodynamic equilibrium (LTE), chemical equilibrium, homogeneous
spherically symmetric stratification (in our case with
),
and the conservation of the total flux (radiative plus convective, the
convective flux being computed using the local mixing length recipe).
The radiation field used in the model generation is calculated by
assuming absorption from atoms and molecules by opacity sampling at
approximately 100 000 wavelength points over the
wavelength range
m.
The models are calculated with 56 depth points from
a Rosseland optical depth of
to
.
Data on absorption by atomic species are collected from the VALD
database (Piskunov et al. 1995)
and Kurucz and other authors (for details, see Gustafsson
et al. 2008). The molecular-line opacity of CO, CN,
CH, OH, NH, TiO, VO, ZrO, H2O, FeH, CaH, C2,
MgH, SiH, and SiO is included and up-to-date dissociation energies and
partition functions are used. It should be noted that, for our
targeted elements, we iterate and specify these abundances for each new
iteration of the model atmosphere calculation, in order to be
self-consistent.
The atomic line absorption files used in a MARCS
model calculation are precalculated within a grid. Those used for our
models are thus files with metallicities closest to the metallicity
of the stars. The grid is given in steps of
within the relevant metallicity range. For stars with
[Fe/H] < 0.0, [
/Fe] can be solar or
-enriched by
up to
.
For more metal-rich stars, the abundance ratios are solar.
Microturbulence parameters of 1 or
were used in the model calculations.
3.2 Fundamental stellar parameters and their uncertainties
3.2.1 Deriving the fundamental parameters of our stars
The fundamental stellar parameters, i.e., ,
,
[Fe/H], and
,
are needed as input to our model photosphere and spectrum synthesis.
Optical Fe I and Fe II
equivalent widths were used to obtain spectroscopic surface gravities
and effective temperatures by requiring a relative ionization and
excitation equilibrium with respect to a set of standard bright giants
(see Meléndez et al.
2008). The iron linelist was carefully chosen to avoid
significant blends in K giants (Alves-Brito et al.
2009, in preparation) and is an extension of the linelist
presented in Hekker & Meléndez
(2007). It has an identical scale to that used by Meléndez et al. (2008),
which means that the stellar parameters for our stars are on the same
scale as those of well-studied nearby giants, which ultimately have
effective temperatures determined from the infrared flux method
temperature scale of Ramírez
& Meléndez (2005a,b) and surface gravities based on
absolute magnitudes inferred from Hipparcos parallaxes by adopting
stellar masses from isochrones.
The equivalent widths ()
of the iron lines for our new CRIRES stars were taken from Lecureur et al. (2007),
who obtained automatic
measurements
from spectra observed with VLT/FLAMES in the UVES mode, providing a
spectral resolution of R=45 000. They used
the DAOSPEC code, which is described in Stetson
& Pancino (2008). For one star, B6-b8 (which can be
considered as one of the most difficult cases, because the star is cool
and metal-rich), we also measured the equivalent widths manually.
The microturbulence, ,
was obtained by assuming that the derived Fe I abundances
were independent of line strengths. This procedure also yields the iron
abundances. The [
/Fe]
used in our models are based on our preliminary optical analysis of
different
-elements
in bulge stars (Alves-Brito et al. 2009,
in preparation) and so is based on a preliminary mean bulge
relationship between [
/Fe]
and [Fe/H].
Our new stellar parameters for our bulge stars, including the
three stars from Ryde
et al. (2009) and our reference star Arcturus, are
given in Table 2.
In this table, we also give the [/Fe] values that we
use to determine the parameters and later adopt in the model
calculations. The earlier determinations of the stellar parameters for
these stars from the literature are also given in the table, namely
those of Lecureur et al.
(2007), the updated parameters based on UVES data as
described in Zoccali
et al. (2008), and finally Fulbright et al. (2006),
the last set also being used by Ryde
et al. (2009). The differences in the stellar
parameters, which are sometimes quite large, are mainly caused by the
different methods applied to the different
stellar parameter determinations. For instance, the different line sets
used and our relative ionization-equilibrium constraint to obtain the
surface gravities, can cause differences. The different values
illustrate the general uncertainty, in particular the difficulties in
determining the fundamental parameters from the equivalent widths
observed with UVES/FLAMES (Lecureur
et al. 2007). Our new effective temperatures are
systematically cooler (on average by 159 K)
than the updated parameters based on the UVES data, as described in Zoccali et al. (2008).
For the three stars from Fulbright
et al. (2006), the differences are in both
directions with a maximum of 144 K.
We redetermined the fundamental parameters of the stars from Ryde et al. (2009),
which we also analyse here, based on the same scale used for other
stars presented here. We also redetermined the fundamental parameters
for our reference star Arcturus ( Boo) in a similar
way. The new parameters for these stars are given in the lower part of
Table 2.
The changes in the derived C, N, and O abundances for these
stars compared with those derived by Ryde
et al. (2009) agree with that expected from the
sensitivities of these abundances to the stellar parameters, and
keeping in mind that different solar C, N, and O values are
used in our paper compared to Ryde
et al. (2009).
It would be preferable to obtain accurate effective
temperatures directly from our IR spectra because of the
substantial extinction towards the bulge in the visual wavelength
region.
This is, however, a difficult task. Although we are exploring the use
of other infrared wavelength regions to improve the determination of
stellar parameters, the current infrared spectrographs available to us
on 8 m-class telescopes cover only narrow regions, therefore
requiring considerable amounts of telescope time to improve the stellar
parameters based solely on infrared data. Furthermore, the limited
ranges in excitation energy for lines of various molecular species
limit their use for determination.
Effective-temperature sensitive features are in particular the
OH molecular lines used in oxygen abundance determinations.
Other lines are only weakly sensitive. Carbon, which appears in
numerous lines of four different species, should
in principle be useful for the
determination.
Our tests show, however, that for a 100 K increase
in
and for fundamental parameters typical of our targets the
high-excitation C I lines
become only slightly stronger, the CN lines remain unchanged,
the CO lines become slightly weaker, and the C2 lines
remain almost unchanged. High S/N
spectra or broad wavelength regions would therefore be necessary. In
the near future, we, therefore, intend
to rely on spectroscopic equilibrium based on optical FeI and
FeII lines. Besides the current work of Alves-Brito
et al. to define the zeropoints of the spectroscopic stellar
parameters, we are currently acquiring more high resolution optical
data of bright K giants, which will allow us to further
improve our stellar parameter scale. We are also developing simulations
to estimate realistic uncertainties in the atmospheric parameters of
cool giants (Meléndez et al., in preparation).
3.2.2 Uncertainties in the fundamental parameters
Our
values are uncertain, limited by the uncertainties in the
measurements.
However, the small dispersion in the [O/Fe] versus [Fe/H] plot
(Fig. 2)
suggests that the random errors are of the order
of 75 K. This is the value that we use to derive the
impact of this uncertainty on the derived abundances
(see Table 4).
However, the uncertainty could be underestimated.
Our spectroscopic surface gravity determinations are also very
uncertain with uncertainties estimated to be 0.3 dex
(in some cases by as much as 0.5 dex), again mainly
because of the quality of the measurements of the
few weak Fe II lines but also
because we assume LTE in the ionization equilibrium, which may not be
valid. However, our
values
more or less follow our expectations for giants with
K
(
)
and 4300 K (
). On the other hand, these
gravities locate a few stars at distances incompatible with the bulge
(all distances being between 4.5 and
12 kpc). Our stars are chosen from the bulge giants of Lecureur et al. (2007),
which were selected to have a high probability of being bulge members.
We therefore also calculated the photometric surface gravities, using
the PARAM tool (see da Silva
et al. 2006), by assuming the stars to be at a
distance of 8 kpc (Reid 1993)
and calculating the extinction in the same way as Lecureur et al. (2007).
Our resulting gravities are given in Table 3 together with our
spectroscopic ones. The
values
obtained by Zoccali et al. (and Lecureur et al.) are
also determined photometrically and are given by 1.7
0.1 dex, in good agreement with our photometric values. The
main sources of uncertainty in the photometric gravities are that the
true distances to the stars are not well known and the bulge has an
extension of a few kpc. For a conservative uncertainty of
kpc
in the distance of 8 kpc, we find an uncertainty in the
gravities of 0.25 dex, only because of the distance
uncertainty. The sensitivity of the determination of
to the extinction and the differential reddening, of which the latter
is not taken into account, is small (see Lecureur
et al. 2007). The gravities based on the two
methods, given in Table 3,
are compatible with each other within the uncertainties, the
differences being mostly <0.3 dex, except for one case,
B6-b8, which is a cool, metal-rich giant. Fortunately, in our case,
whichever
determination
we assume does not make a large difference, especially for the oxygen
abundance from the OH lines, as can be seen from
Table 4
and Fig. 2.
The carbon abundance determined from the CO lines is, however,
more affected (see also Fig. 4).
In the following discussion, we have chosen to use the surface
gravities determined spectroscopically.
Table 3: Spectroscopically and photometrically determined surface gravities.
Finally, we estimate the uncertainty in the metallicity to be, in general, of the order of 0.05 dex and that in the microturbulence to be of the order of 0.25 km s-1.
The changes in the fundamental parameters caused only by the
two different measurements of equivalent widths (automatic DAOSPEC or
manually) of the cool, metal-rich giant, B6-b8,
are 20 K in the temperature, 0.16 dex in ,
-0.17 dex in [Fe/H], and 0.35 km s-1
in the microturbulence. The differences are within our estimated total
uncertainties, except for
and the metallicity. This star is, however, particularly cool and
metal-rich, which means that the iron line measurements are especially
difficult for this star.
3.3 Synthetic spectra
For the analysis of the observed spectra, we generated synthetic
spectra, calculated for spherical symmetry in our model photospheres.
We sample the spectra with a resolution of R=600 000.
With a microturbulence velocity of ,
this ensures an adequate sampling. To fit the observed
spectra, we finally convolve our synthetic spectra with a
macroturbulent broadening, represented by a radial-tangential function (Gray 1992), and fitted to
include the effects of both macroturbulence and the instrumental
profile. The final macroturbulence parameters used are given in
Table 2.
The code used for calculating the synthetic spectra is
BSYN v. 7.06, which is based on routines from the MARCS
code. Full consistency with the model atmosphere is achieved by
choosing the same fundamental parameters, [
/Fe], individual abundances
and in both calculations, including the molecular equilibria.
A
ratio
of 24 (96%
)
is used for the bulge stars, and 9 for Arcturus.
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Figure 1: Sections of the observed CRIRES spectra of two of our bulge giants are shown with full, black lines. The observations are wavelength-shifted to laboratory wavelengths to enable a more direct comparison between the different stellar spectra. Therefore, our observed spectra cover slightly different wavelength ranges. For comparison, the Arcturus atlas spectrum (Hinkle et al. 1995b) is also shown. The parts of the spectra with the highest signal-to-noise ratios are shown. From these parts the C, N, and O elements can be determined. Our best-fit synthetic spectra is shown in red. All synthetic lines deeper than a factor of 0.97 of the continuum are identified. A few features cannot be identified in the Arcturus spectrum and are labeled with question marks. These features also show up in the bulge-star spectra. A few lines in BW-f6 are conspicuously stronger than expected. They are probably affected by cosmic rays in the detector array. |
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The atomic linelist used in our calculations is compiled from the VALD
database (Piskunov et al. 1995)
and from Meléndez & Barbuy
(1999). The stellar parameters of our bulge stars resemble
those of Boo,
which is therefore a good choice of reference star. However, most of
the lines used in our analysis can also be analyzed in the solar
spectrum. We, therefore, primarily checked the linelist against the
solar spectrum and corrected the line-strengths, if needed, by
determining ``astrophysical
-values'', and fitting the
atomic lines in synthetic solar spectra to the observed ones (Livingston & Wallace 1991).
Hence, in our linelist we adjusted the wavelengths of 96 lines
based on the solar spectrum (see Table 7). The lines
fitted were, among others, some Fe, Ni, Si, S and Ti lines. In
addition, 4 Ti lines and
4 Si lines, which were too weak in the solar
spectrum, were fitted to the
Boo spectrum from the Hinkle
et al. (1995a) atlas. These 8 lines are also given
in Table 7.
To determine the astrophysical
-values of these lines, we
need to know the abundances of these elements in
Boo. Furthermore,
apart from iron, Mg and Si are the most important electron donors in
the continuum-forming regions, and therefore affect the line strengths
by means of the continuous opacity (H
). It is therefore
also important to estimate the abundance of Fe and Mg, as reliably as
possible.
Hence, the Fe, Mg, Si, and Ti abundances of our Arcturus
modeling are based on the optically determined abundances derived by Fulbright et al. (2007),
but taking into consideration our slightly different fundamental
parameters, which, however, increased these abundances by only 0.00,
0.01, 0.03, and 0.01 dex, respectively. The other Fe, Si, and
Ti lines in our near-IR list, yield the same abundance for
Arcturus, within a few tenth of a dex. To conclude,
the linelist used here is similar to the one described in Ryde et al. (2009),
except that some new lines are added, a few omitted, the strengths of
nine lines were adjusted slightly (less than 0.1 dex), and the
strengths of 2 Ti lines (15 334.84 and
15 543.78 Å) and one Si line
(15 506.98 Å) strengths were adjusted by a larger
amount. The latter three lines are all visible in the sun.
The molecular lists included are for CO (Goorvitch
1994), SiO (Langhoff &
Bauschlicher 1993), CH (Jørgensen
et al. 1996), CN (Jørgensen
& Larsson 1990; and Plez 1998, private
communications), OH (Goldman
et al. 1998), and C2 (Querci et al. 1971; and
Jørgensen 2001, private communications). For the molecules, the
linelists were adopted as they were, leading to the following C, N, and
O abundances for Arcturus from the spectra in the Arcturus
atlas (Hinkle et al.
1995a):
(from CO lines),
(from CN lines), and
(from OH lines), which are in excellent agreement with the
values derived by Ryde
et al. (2009) and by the optical work by Lecureur (2007), who derived
,
,
and
.
From our spectra, we determined elemental abundances from the
CO [v=3-0] band, around 20 suitable
CN lines, some 20 suitable OH [
v=4-2,
3-1, 2-0] lines, and numerous Fe I lines.
Silicon could be measured from 2 or 3 lines, sulfur
from 2 lines, and titanium from only 1 line, which made the
derived Ti abundance the most uncertain of the elements.
We derive the best fits, line by line, by synthesizing a grid of model
spectra with incremental differences of 0.05 dex in the
abundance sought for and finding the best fit by visual inspection.
While for the OH and CN lines, and the atomic lines, every
suitable line was inspected, for the CO band the entire band
was fitted. In Fig. 1,
we present two examples of spectra of our 8 bulge stars,
namely those of B3-b8 and BW-f6. These are spectra for which we
obtained the highest S/N.
Only the third detector array, providing the highest S/N,
is shown, but spectra from the other detector arrays were also
used in the analysis. The reference spectrum of Arcturus is also shown
in the figure.
Table 4: Uncertainties in the derived abundances due to uncertainties in the stellar parameters.
Table 5: [C/Fe]a,b, [N/Fe], [(C+N)/Fe], [O/Fe], [Si/Fe], [S/Fe], [Ti/Fe], and [Fe/H] for our 11 bulge giants and Arcturus.
3.4 Uncertainties in the derived abundances
The propagation of uncertainties in the stellar parameters to
uncertainties in the C, N, and O abundances is presented in
Table 4,
based on the discussion in Ryde
et al. (2009). These uncertainties are derived for a
typical star of our sample, namely BW-f6. The uncertainty in the [/Fe] ratio
is included but ambiguous since the different
-elements show different
trends. We adopted a general enhancement of [
/Fe] = +0.2 in both
the model calculations and the calculation of the synthetic spectra. We
estimate the total internal uncertainties in the derived C, N, and
O abundances to be approximately
,
,
and
dex.
As a comparison, the standard deviations in the determinations
of the C, N, and O abundances from the many observed CO, CN,
and OH lines for a given model are small, less than
0.05 dex. For example, for a given star and model atmosphere,
the determination of the oxygen abundances from each of the
approximately 20 suitable OH lines provides a mean
oxygen abundance with a standard deviation of 0.04
(line-to-line scatter) and a standard deviation of the mean of 0.01
dex. We note, however, that the
error in the CNO abundances generated by errors in the
fundamental parameters will correlate, e.g. according to
Table 4
an underestimated effective temperature will lead to underestimated
abundances of C, N, as well as O. For the
elements,
represented here by sulfur, we estimate that
dex,
i.e., of the same order as the molecular lines. We see, however, that
the oxygen abundance is affected by the largest uncertainty, which is
mainly due to the uncertainties in the effective temperature.
The relatively low star-to-star scatter in the
[O/Fe] versus [Fe/H] plot (see Fig. 2) confirms that our
errorbar in the stellar parameters, especially
,
are reasonable. Although our [O/Fe] values are sensitive
to
,
they are not affected much by the uncertainties in
(compare Table 4
and the two panels in Fig. 2).
On the other hand, the oxygen abundance of seven of our stars as
derived by Zoccali et al.
(2006) from the [O I] line
at 6300 Å is not affected much by the effective temperature
but far more by the surface gravity. For instance, a change in
of +0.3 dex produces a change in the oxygen abundances of
+0.13 dex from the [O I] line
(cf.
dex from the
OH lines), whereas a change in the temperature of
100 K produces a change of +0.02 dex
(cf. +0.16 dex from the OH lines).
Other systematic uncertainties that could affect the abundance
results include those related to, for instance, the continuum
placement, the model atmosphere assumptions (such as the
treatment of convection and the assumption of spherical symmetry), the
uncertainties in the line strengths ( values), and the
dissociation energies of the molecules. Furthermore, possible
non-LTE effects in the line formation of both atomic and
molecular lines could affect the results in a systematic way. In the
future, only a full non-LTE analysis of all relevant atoms and
molecules would be able to disclose the magnitude of these systematic
uncertainties that our LTE analysis might be
plagued by. For abundance ratios, several uncertainties partly
cancel leading to smaller uncertainties. The uncertainty due to the
placement of the continuum is estimated to be relatively small, less
than 0.03 dex.
![]() |
Figure 2:
Logarithmic ratios of oxygen to iron normalized by the solar value for
bulge stars. Filled circles show all of our data from this paper,
except for the special giant Arp 4203, which is denoted by an
open circle. Crosses show the results of Meléndez
et al. (2008). Typical uncertainties are indicated
in the upper right corner. The upper panel shows
the oxygen abundances we derive when we use our spectroscopically
derived |
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4 Results
In Table 5,
we present our derived [C/Fe], [N/Fe], [O/Fe], [Si/Fe], [S/Fe],
[Ti/Fe], and [Fe/H] for our eleven bulge giants. We also provide the
derived C, N, C+N, O, and S abundances for Arcturus in
addition to the Si, Ti, and Fe abundances used. Our iron
abundances of the bulge stars, determined from 8 to
20 Fe-lines depending on the different S/N
of the IR spectra, are systematically between 0.00 to
0.10 dex larger than the optically determined metallicity as
given in Table 2,
which is satisfactory. In Fig. 2,
we plot the [O/Fe] versus metallicity for our
11 bulge giants, together with the bulge giants from Meléndez et al. (2008).
These abundance ratios were also determined from near-IR spectra and
the adopted stellar parameters were on the same scales. We find
enhanced values of [O/Fe]
+0.4 up to approximately [Fe/H]
-0.3, after which they decrease. In the figure, we highlight the giant
star Arp 4203 which shows a large depletion of carbon, a large
enhancement of nitrogen, and a [C+N/Fe] that is far from
solar, making this giant special. This was also noted by Fulbright et al. (2007),
Meléndez et al. (2008),
and Ryde et al. (2009),
who concluded that the oxygen abundance in this star should probably
not be used to represent the unprocessed [O/H] value of this
bulge giant.
![]() |
Figure 3: Logarithmic ratios of Si, S, and Ti to iron normalized by the solar value for our bulge stars are shown with filled circles. The [O/Fe] versus [Fe/H] trend is indicated by a full line for reference in all panels. The measured values for the special giant Arp 4203 are shown with an open circle. Typical uncertainties are indicated in the upper right corners. |
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In Fig. 3,
we plot our derived [Si/Fe], [S/Fe], and [Ti/Fe] versus our
derived metallicity. For reference, we also plot the trend we estimate
from our [O/Fe] versus [Fe/H] values. The abundances
of the elements
are more uncertain than our derived C, N, O, and Fe abundances
since there are much fewer lines to measure. The uncertainties are
therefore larger for these elements than for the C, N, O, and
Fe abundances. In spite of this, we find that the [
/Fe] values
are enhanced for metallicities up to at least [Fe/H]
-0.3 after which they appear to decline. Below this
metallicity [Si/Fe] and [S/Fe] are enhanced at a
level of [
/Fe]
+0.3. For higher metallicities, they appear to decline and follow
[O/Fe] for a given metallicity. The ratio [Ti/Fe] is generally
lower for all metallicities. Our
element trends
overall corroborate the [O/Fe] enhancement trend, although the
[Ti/Fe] ratios are lower. In this figure, we also
highlight Arp 4203.
All our stars except B3-b1 show significantly enhanced
[N/Fe] values. If a star has experienced the first
dredge-up, CN-cycled material is exposed at its surface. The abundances
of C and N are then expected to change but their sum is left unaltered.
In Table 5,
we also provide the calculated [(C+N)/Fe]. This is plotted in
Fig. 4
together with the [(C+N)/Fe] for the bulge giants from Meléndez et al. (2008).
These two data sets show approximately the
same pattern when it comes to the mean, standard deviation, and slope.
We find a slope from a linear regression analysis for our data of k=+0.07
0.09 and for both sets of k=+0.04
0.04, i.e., both data sets are consistent with being sampled from a
flat distribution. Furthermore, we find a mean for both data sets of
[(C+N)/Fe]
=
0.08
0.09 (s.d.) and an error in the mean of 0.02 dex.
Thus, both data sets show a systematic enhancement in the
[(C+N)/Fe] ratios and they are therefore not consistent with
being at solar values for all metallicities. Given our estimated
uncertainties, our stars show no cosmic scatter. More stars and higher
accuracy would be needed to judge whether an offset from solar values,
the slight increase with metallicity, or the curved tendency in
Fig. 4
are real. We note that the highest value, apart from that of
Arp 4203, is that of Arcturus, with
[(C+N)/Fe] = 0.23.
![]() |
Figure 4:
Logarithmic ratios of carbon+nitrogen to iron normalized by the solar
value.
Our data are shown by filled circles, except Arp 4203 which is
shown by an open circle. Crosses show the results of Meléndez et al. (2008).
The [(C+N)/Fe] value we derive for Arcturus is shown with a
star. Typical uncertainties are indicated in the upper right corner.
The upper panel shows the values we retrieve when assuming
spectroscopically derived |
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In Fig. 5, we plot C/N for our stars in the theoretical HR diagram and show how C/N varies with position in it. We see that the stars are aligned along the giant branch and that the C-N ratios decrease along it, as expected. Arp 4203 has evolved the furthest.
![]() |
Figure 5: C/N ratios plotted in the theoretical HR diagram. The dot diameters are proportional to C/N with a largest value of 4.6 for B3-b1, 2.8 for Arcturus, and 0.08 for Arp 4203. Stars with [Fe/H] < -0.4 are plotted as filled dots and the more metal-rich ones as open circles. |
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5 Discussion
Here, we will discuss the trends of the
elements versus metallicity, and end with a discussion on our
carbon and nitrogen abundances.
5.1 The [
/Fe] trends
The trends of the
elements (O, Mg, Si, S, Ca, and Ti) are of particular interest, since
accurate [
/Fe] ratios
in pre-AGB bulge stars place strong constraints on the star-formation
history.
5.1.1 Oxygen
Our oxygen abundances show good
agreement in the downward [O/Fe] versus [Fe/H] trends with the
results found by other authors. However, differences between these
exist. In Fig. 6,
we plot the [O/Fe] trends for the bulge stars based on analyses of
near-IR spectra. These are, in addition to our new results, values from
Cunha & Smith (2006),
Rich & Origlia (2005),
Meléndez et al. (2008),
and Ryde et al. (2009).
The [O/Fe] versus [Fe/H] trend of Rich
& Origlia (2005) rises in value by 0.11 dex
to adjust to the authors' assumed solar oxygen abundance
of 8.83 compared to our value of 8.72.
It would be speculative to quantify the similarities between
the different trends because of the small number of stars, but assuming
a constant [O/Fe] versus [Fe/H] up to a metallicity of
[Fe/H] = -0.3, and assuming a constant slope thereafter up to
[Fe/H]
+0.4, we find agreement between the slope of our data (k=-1.0
0.3) and that of Meléndez
et al. (2008) (k=-0.6
0.15). Our data are also marginally consistent with the data of Rich & Origlia (2005),
which are confined to a narrow range in metallicity. The Cunha & Smith (2006)
data set seems, however, to infer a more shallow slope (k=-0.1
0.2).
In Fig. 7, we
plot the data points from the optical work of Fulbright et al. (2007),
which also show an agreement in the slope (k=-0.8
0.15). In Fig. 8
we plot the optical results from Zoccali
et al. (2006), also presented in Lecureur et al. (2007)
with a slope of k=-0.6
0.15, together with our determinations. Our results are similar to
these in scatter and slope.
![]() |
Figure 6: Logarithmic ratios of oxygen to iron normalized by the solar value. The oxygen abundances are here all determined from near-IR spectra. Filled circles show our data from this paper, except that of Arp 4203. Arcturus at [Fe/H] = -0.53 is indicated with a black star at [O/Fe] = 0.51, in agreement with that of Lecureur (2007). Crosses show the results of Meléndez et al. (2008), green squares represent the abundances determined by Cunha & Smith (2006), and red squares the data from Rich & Origlia (2005). The blue star and plus signs represent thick disk and thin disk giants, respectively, from Meléndez et al. (2008). |
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![]() |
Figure 7: Logarithmic ratios of oxygen to iron normalized by the solar value. Filled circles show our data from this paper, except that of Arp 4203, and a black star indicates Arcturus at [Fe/H] = -0.53. Triangles show the optically determined values by Fulbright et al. (2007). The two small dots represent the stars from Fulbright et al. (2007), which were analyzed by Ryde et al. (2009) from near-IR lines and reanalyzed by us. The determinations by Fulbright et al. and our determinations are connected by full lines. The blue stars and plus signs represent thick disk and thin disk giants, respectively, from Meléndez et al. (2008). |
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![]() |
Figure 8: Logarithmic ratios of oxygen to iron normalized by the solar value. Filled black circles show our data from this paper, except that of Arp 4203, and a black star indicates Arcturus at [Fe/H] = -0.53. Filled green circles show the optically determined values by Lecureur et al. (2007) and Zoccali et al. (2006). The small black dots mark the stars from Lecureur et al. (2007) and Zoccali et al. (2006) for which we have determined the oxygen abundances from near-IR lines. These two determinations are connected. For the star B6-f7, there is only a near-IR determination of the oxygen abundance. When comparing these two determinations for the same stars, it should be noted that we have used new stellar parameters when determining the abundances from the near-IR spectra. The blue star and plus-sign symbols represent thick disk and thin disk giants, respectively, from Meléndez et al. (2008). |
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Table 6: [Fe/H] and [O/Fe] determined from near-IR lines in this paper and determinations based on optical lines.
All our stars are in common with the optical analyses of Zoccali et al. (2006) but
for one of them it was impossible to determine an oxygen abundance from
the [O I] lines at 6300 Å. The metallicities
and [O/Fe] for these stars from our and their analyses are given in
Table 6.
We note that the stellar parameters differ between the two
determinations. In Fig. 8, we mark
and connect the two determinations for the same stars. When comparing
the two analyses, we see that the metallicities are within the
uncertainties, marginally also for B6-b8 (
dex).
The oxygen abundances generally agree within uncertainties, the largest
difference being
dex.
However, our abundances tend to be systematically lower. Given that our
OH lines are very temperature sensitive, one reason for the
differences could be that the effective temperatures are still not
determined accurately enough. Another reason could be an overestimation
of the line strengths in the optical spectra. Although the line
strengths of the [O I] and the near-IR OH
lines are comparable
,
the optical spectra used in Zoccali
et al. (2006) reaches S/N
50 per resolution element, whereas our spectra have a S/N
a factor of 2-4 higher and many more lines to use as oxygen
criteria. Unknown blends might also affect the [O I] line.
Our [O/Fe] data suggest a high value of +0.4 up to
[Fe/H]
-0.3, after which the values decline rapidly with a slope of k=-1.0
0.3. This is the mean trend plotted in Fig. 3.
A combination of our [O/Fe] data and those of Meléndez et al. (2008)
(see Fig. 2)
corroborates our finding. This trend also fits well with the values
found by Rich & Origlia
(2005) (which are similar to those in Rich
et al. 2007) and is consistent with the trends found
by Fulbright et al.
(2007).
In all the three comparison figures, we also plotted [O/Fe] versus [Fe/H] for the thin and thick disks from Meléndez et al. (2008). When studying all [O/Fe] determinations in Figs. 6 to 8, one obtains the impression that these together may suggest similar, or possibly even higher values than those of the local thick disk of Meléndez et al. (2008). We note that there may be important and different systematic errors in all of these comparisons.
All trends seem to show a scatter that is similar or larger than the trends found for the thick and thin disks by Meléndez et al. (2008). This might, however, be expected for analyses of bulge stars since these are more difficult to analyse; our scatter reflects the expected uncertainties, and a cosmic scatter, if any, must be smaller than that.
Table 7: Linelist of metal lines with astrophysical oscillator strengths.
5.1.2 Mg, Si, S, Ca, and Ti
We find that [Si, S/Fe]
+0.3 for metallicities up to at least [Fe/H]
-0.3 above which they seem to decline. Although Si and S may exhibit
slightly different trends, published abundance trends based on detailed
abundance analyses for bulge stars all suggest that the ratio
of
-element
abundances relative to Fe have more or less enhanced values for all
metallicities [Fe/H] < 0.0 (e.g., Origlia
et al. 2005b; McWilliam et al. 2008;
Origlia
et al. 2008; Cunha & Smith 2006; Origlia
& Rich 2004; Fulbright et al. 2007;
Origlia
et al. 2002; McWilliam & Rich 2004; Rich
et al. 2007; Origlia et al. 2005a;
Lecureur
et al. 2007; Rich & Origlia 2005; Carretta
et al. 2001). For instance, Lecureur
et al. (2007) found high [
/Fe] ratios in the
bulge compared to the disks, which suggests an enrichment by mostly
massive stars at all metallicities. Ryde
et al. (2009) measured sulfur (a product of
explosive nucleosynthesis) from near-IR spectra and found enhanced
values. McWilliam & Rich
(2004) demonstrated that Mg and Si are enhanced by
0.3-0.5 dex to super-solar metallicities. It should
be noted here that our data do not support an enhanced [Si/Fe] and
[S/Fe] values for [Fe/H] > -0.3. The McWilliam & Rich (2004)
[Ti/Fe] and [Ca/Fe] trends show a steeper decline than for Mg and Si
with metallicity but not as much as for oxygen. To reproduce
the different trends, they suggest that the initial mass function (IMF)
is skewed with more massive stars in the bulge. Fulbright et al. (2007)
measured the abundances of O, Mg, Si, Ca, and Ti and found that these [
/Fe] decline
as a function of metallicity but that they also retain higher values
than those of the disks for all metallicities. Mg was found to
be enhanced the most, while Si, Ca, and Ti (which are understood to be
the products of the explosive nucleosynthesis phase of type II
supernovae) follow each other well at lower enhancement levels. The
sharp decline in Si, Ca, and Ti compared to that of Mg was assumed to
be caused by a metallicity-dependent decline of the former yields.
Our results for the -elements do not support the
existence of any significant ``cosmic scatter'' in the
-element
abundances relative to iron in the bulge, but this is hardly conclusive
since the observational scatter is considerable. Alves-Brito
et al. (2009, in prep.) show, however, that their
measurement of the [
/Fe] ratio
in both bulge and thick disk giants have a scatter of only
0.03 dex. Fulbright
et al. (2007) also find a small scatter in their
mean [
SiCaTi
/Fe], much
smaller than that of halo stars, and they interpret this as an
indication that the bulge composition developed homogeneously, for
example, due to efficient mixing.
5.1.3 The thick disk-bulge similarity
Fulbright et al. (2007),
Lecureur et al. (2007),
and Zoccali et al. (2006)
find abundance trends, including those for oxygen, that differ from
those of the thick disk dwarfs and turn-off stars measured by Bensby et al. (2004)
and Reddy et al. (2005), and bulge giants measured from their
own optical spectra. In contrast, Meléndez
et al. (2008), by means of a homogeneous analysis of
near-IR spectra of bulge and disk giants, find no chemical distinction
between the local thick disk (up to [Fe/H]
-0.2) and the bulge, suggesting that the two populations show a similar
chemical evolution and that the star-formation rates should not differ
significantly. As shown in Figs. 6 to 8, the
abundances measured here for bulge giants are consistent, within the
uncertainties, with previous ones, measured using data acquired in both
optical and near-IR. In the comparison with the thick disk, we follow
the approach of Meléndez et al., by restricting the comparison
to giants in both components, measured in a fully consistent way. Our
bulge stars have abundances similar to those of thick disk giants.
5.2 The carbon and nitrogen abundances
Estimates derived from the optical wavelength region of the carbon and nitrogen abundances are highly uncertain (in many cases only upper limits are known), whereas the CO, CN, and OH lines in the near-IR together easily infer these abundances. The C and N abundances might provide clues to, for instance, the importance of W-R winds and the evolutionary state of the giants, thereby indicating whether the measured oxygen abundances are the original, unprocessed ones.
5.2.1 First dredge-up and the measured oxygen abundance
Low-mass giants that have ascended the giant branch for the first time
have only experienced the first dredge-up of CN-processed material from
the interior. Thus, the CN cycle's products, which are
mainly
and some
converted from
,
are dredged-up to their surfaces. This is not expected to alter the sum
of the number of carbon and nitrogen nuclei, while the measured oxygen
abundances should reflect the original abundances in the giants.
From Table 5,
we see that all stars (with the exception of Arp 4203) only
show signs of the first dredge-up and thus no further processing of
oxygen by means of the NO cycle, nor any increase in the
C abundance characteristic of the third dredge-up on the
asymptotic giant branch.
The galactic chemical evolution of carbon and nitrogen remains
uncertain. Bensby &
Feltzing (2006) find a constant [C/Fe] close to +0.1
for -0.9 < [Fe/H] < 0.0 for disk stars
and the summarized observational trend of [N/Fe] versus [Fe/H]
presented in Goswami &
Prantzos (2000) is constant at a
solar value. Thus, if the (C+N) abundances are expected to
follow that of iron, one would expect the [(C+N)/Fe] to be slightly
below +0.1 for all metallicities in the Galactic disk, which
is consistent with what we find. If the atmospheres of our
stars had also been exposed to ON-cycled
material (in which
is converted to
), their
nitrogen abundances
would have been higher (resulting in higher [(C+N)/Fe] ratios
and a larger scatter) with an accompanying lower oxygen abundance.
It therefore seems likely that our measured oxygen
abundances can be taken as the stars original unprocessed abundance.
This is also what is to be expected if the stars have relatively low
masses, and are still in the H-shell burning or He-core burning phase,
i.e., are on their first ascent along the giant branch or are clump
stars.
5.2.2 The role of W-R stars
The oxygen abundance trends found from optical spectra of
K giants in the bulge by McWilliam
& Rich (2004), indicate a surprising interruption of
oxygen production in the bulge for high metallicities. The decrease in
oxygen abundance is consistent with the strange scenario of no oxygen
production for .
McWilliam & Rich (2004)
suggest that this could be connected to the onset of the Wolf-Rayet
(W-R) phenomenon, which would be vital for the production of
the CNO elements. Carbon is lost in metallicity-sensitive,
radiation-driven stellar winds of metal-rich W-Rs preventing carbon
being converted into oxygen, thereby reducing the oxygen production.
Hence, the steep oxygen decline would not be specific for the stellar
population(s) in the bulge, but a metal-dependent phenomenon, with
metallicity-sensitive stellar yields from massive stars, playing an
important role. Indeed, McWilliam
et al. (2008) show that the Galactic bulge and the
thin disk experience the same decline in the [O/Mg] versus
[Mg/H] diagram, supporting this hypothesis (or the
alternative hypothesis that the IMF is considerably metallicity
dependent), since both oxygen and magnesium are synthesized in the
hydrostatic cores of massive stars in a similar fashion. In this
diagram, the effects of the Fe-producing type Ia supernovae
are eliminated. The decline in the [O/Fe] versus [Fe/H] plot
in the bulge would also reflect the decrease in oxygen yields due to
W-R stars, and the onset of Fe production from
type Ia supernovae which provide clues to the timescales of
the rates and duration of the star formation in the early bulge.
Similarly, Fulbright
et al. (2007) also relate their low oxygen
overabundances to lower oxygen yields at higher metallicities, because
of metallicity-dependent W-R winds. This finds support in the
calculations for rotating massive-star models by Meynet & Maeder (2005),
who conclude that higher C/O abundances are expected from
high-metallicity WR stars. This idea is strengthened by Cunha et al. (2008), who
derive fluorine (
) abundances for a
sample of bulge stars. Their results suggest that winds from metal-rich
W-R stars contribute more to the production of this element
than the AGB stars in the bulge do compared to the situation
in the disk.
If the W-R hypothesis, invoked to explain the steeper decline
of the [O/Fe] ratio compared to the other elements
is correct, it would mean a dramatic increase in the carbon
yields and thereby of the carbon abundances versus metallicity, since
the carbon lost from the star is material that would otherwise be
expected to be transformed into oxygen at later stages and then
expelled by supernovae explosions. However, it is fully
conceivable that much of the matter, lost during the
W-R stage, which would otherwise be transformed into oxygen,
is as yet primarily helium. More
detailed model calculations are needed to explore which type of
C enrichment is expected. The mass loss in massive stars
would, in any case, have a large impact on the formation of carbon and
oxygen, especially in metal-rich populations. The carbon versus
metallicity trend is therefore a crucial test of the
W-R scenario. However, assuming that nitrogen is not affected,
we are unable to detect a dramatic increase in carbon production from
our data. The [(C+N)/Fe] ratio that we find has a mean of
+0.08
0.09 dex, and the [(C+N)/Fe] versus [Fe/H] trend
shows a modest, if any, slope of
0.09. Thus, our data do not provide strong support for
this hypothesis.
6 Conclusions
The chemical abundance determinations of stars are known to be plagued
by systematic errors that may be difficult to estimate.
To discuss the properties of different stellar populations,
homogeneous differential spectroscopic studies and detailed comparisons
of results from different studies are significant. In the present
study, we have tried to follow this route, and find satisfactory
agreement with results obtained in the optical, as well as IR,
when a common temperature scale is used for the stars. With our
high-resolution IR spectroscopy, we have explored the
CNO abundances, as well as the abundances of Si, S, Ti, and Fe
for 11 bulge giants. We have found enhanced [O/Fe],
[Si/Fe], and [S/Fe] values with increasing [Fe/H] up to
approximately [Fe/H]
-0.3, after which these abundance ratios relative to Fe decrease. This
suggests an early and rapid star formation in the bulge. Our
investigation is not designed to allow a detailed comparison with thick
disk stars and to determine the relationship between these two
populations; this type of study should be made differentially to
minimise systematic uncertainties. Our abundance trends are, however,
consistent with there being a similarity between these populations as
found in the differential study by Meléndez
et al. (2008). This type of similarity suggests that
the picture of an isolated classical bulge may be oversimplified. Inner
disk stars, at smaller galactocentric distances, should be explored to
deepen the understanding of a possible physical connection between the
bulge and the thick disk.
From our C and N abundances, we conclude that our stars are first-ascent red-giants or clump stars, suggesting that their oxygen abundances are unaffected by CNO cycling. Furthermore, we find that there is no significant increase in the carbon abundances at high metallicities, which is expected if W-R stars are responsible for the sharp decline in [O/Fe] versus metallicity.
We have demonstrated that for the same stars several different determinations of the stellar parameters from optical spectra produce significantly different results, implying that important systematic uncertainties exist. Attempts to reduce these should be made. We note also that Chiappini et al. (2009) compare, among others, the oxygen abundances derived from planetary nebulae (PNe) and giants in the bulge and find that the abundances determined from giant star spectra are systematically higher by 0.3 dex. They conclude that this discrepancy may be caused by systematic uncertainties in either the PNe or giant star abundance determinations, or both.
To fully clarify the situation of the origin and evolution of
the galactic bulge, additional near-IR abundance surveys of elements
(especially more elements)
are needed. Most previous investigations have been restricted to
Baade's window. Different regions of the bulge are now being explored,
and further systematic work is needed.
N.R. is a Royal Swedish Academy of Sciences Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation. Funds from Kungl. Fysiografiska Sällskapet i Lund are acknowledged. N.R., B.E., and B.G. acknowledge support from the Swedish Research Council, VR. J.M. acknowledge support from the Portuguese FCT (PTDC/CTE-AST/65971/2006, Ciencia 2007). M.Z. and D.M. are supported by FONDAP Center for Astrophysics 15010003, by BASAL CATA PFB 0609, and by FONDECYT. Kjell Eriksson is thanked for valuable help and discussions concerning the running of the MARCS program. The referee is thanked for valuable suggestions.
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Footnotes
- ... spectra
- Based on observations collected at the European Southern Observatory, Chile (ESO program 079.B-0338(A)).
- ... element
- The scale in dispersion direction is 0.10
/pixel at the center of the order and
/pixel over the focal plane from the long wavelength-side to the short. In the spatial direction, the scale is 0.087
/pixel. This change in scale is due to the change in the beam-diameter induced by the off-axis reflection at the Echelle grating, which produces an anamorphism.
- ... instrument
- In such a case, the spectrograph entrance slit would only be a technicality to reduce the background and to establish geometric alignment, here mostly compensating pointing errors.
- ...-ratio
- Full details of the pipeline can be found under http://www.eso.org/observing/dfo/quality/CRIRES/pipeline/pipe_reduc.html
- ...(Tody 1993)
- IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.
- ... comparable
- For example, for the giant B3-b8 the [O I] line has a strength of approximately
and the OH lines have line-strengths in the approximate range of
.
All Tables
Table 1: Account of our observations.
Table 2:
Stellar parameters for the model atmospheres of our program stars given
as .
Table 3: Spectroscopically and photometrically determined surface gravities.
Table 4: Uncertainties in the derived abundances due to uncertainties in the stellar parameters.
Table 5: [C/Fe]a,b, [N/Fe], [(C+N)/Fe], [O/Fe], [Si/Fe], [S/Fe], [Ti/Fe], and [Fe/H] for our 11 bulge giants and Arcturus.
Table 6: [Fe/H] and [O/Fe] determined from near-IR lines in this paper and determinations based on optical lines.
Table 7: Linelist of metal lines with astrophysical oscillator strengths.
All Figures
![]() |
Figure 1: Sections of the observed CRIRES spectra of two of our bulge giants are shown with full, black lines. The observations are wavelength-shifted to laboratory wavelengths to enable a more direct comparison between the different stellar spectra. Therefore, our observed spectra cover slightly different wavelength ranges. For comparison, the Arcturus atlas spectrum (Hinkle et al. 1995b) is also shown. The parts of the spectra with the highest signal-to-noise ratios are shown. From these parts the C, N, and O elements can be determined. Our best-fit synthetic spectra is shown in red. All synthetic lines deeper than a factor of 0.97 of the continuum are identified. A few features cannot be identified in the Arcturus spectrum and are labeled with question marks. These features also show up in the bulge-star spectra. A few lines in BW-f6 are conspicuously stronger than expected. They are probably affected by cosmic rays in the detector array. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
Logarithmic ratios of oxygen to iron normalized by the solar value for
bulge stars. Filled circles show all of our data from this paper,
except for the special giant Arp 4203, which is denoted by an
open circle. Crosses show the results of Meléndez
et al. (2008). Typical uncertainties are indicated
in the upper right corner. The upper panel shows
the oxygen abundances we derive when we use our spectroscopically
derived |
Open with DEXTER | |
In the text |
![]() |
Figure 3: Logarithmic ratios of Si, S, and Ti to iron normalized by the solar value for our bulge stars are shown with filled circles. The [O/Fe] versus [Fe/H] trend is indicated by a full line for reference in all panels. The measured values for the special giant Arp 4203 are shown with an open circle. Typical uncertainties are indicated in the upper right corners. |
Open with DEXTER | |
In the text |
![]() |
Figure 4:
Logarithmic ratios of carbon+nitrogen to iron normalized by the solar
value.
Our data are shown by filled circles, except Arp 4203 which is
shown by an open circle. Crosses show the results of Meléndez et al. (2008).
The [(C+N)/Fe] value we derive for Arcturus is shown with a
star. Typical uncertainties are indicated in the upper right corner.
The upper panel shows the values we retrieve when assuming
spectroscopically derived |
Open with DEXTER | |
In the text |
![]() |
Figure 5: C/N ratios plotted in the theoretical HR diagram. The dot diameters are proportional to C/N with a largest value of 4.6 for B3-b1, 2.8 for Arcturus, and 0.08 for Arp 4203. Stars with [Fe/H] < -0.4 are plotted as filled dots and the more metal-rich ones as open circles. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Logarithmic ratios of oxygen to iron normalized by the solar value. The oxygen abundances are here all determined from near-IR spectra. Filled circles show our data from this paper, except that of Arp 4203. Arcturus at [Fe/H] = -0.53 is indicated with a black star at [O/Fe] = 0.51, in agreement with that of Lecureur (2007). Crosses show the results of Meléndez et al. (2008), green squares represent the abundances determined by Cunha & Smith (2006), and red squares the data from Rich & Origlia (2005). The blue star and plus signs represent thick disk and thin disk giants, respectively, from Meléndez et al. (2008). |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Logarithmic ratios of oxygen to iron normalized by the solar value. Filled circles show our data from this paper, except that of Arp 4203, and a black star indicates Arcturus at [Fe/H] = -0.53. Triangles show the optically determined values by Fulbright et al. (2007). The two small dots represent the stars from Fulbright et al. (2007), which were analyzed by Ryde et al. (2009) from near-IR lines and reanalyzed by us. The determinations by Fulbright et al. and our determinations are connected by full lines. The blue stars and plus signs represent thick disk and thin disk giants, respectively, from Meléndez et al. (2008). |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Logarithmic ratios of oxygen to iron normalized by the solar value. Filled black circles show our data from this paper, except that of Arp 4203, and a black star indicates Arcturus at [Fe/H] = -0.53. Filled green circles show the optically determined values by Lecureur et al. (2007) and Zoccali et al. (2006). The small black dots mark the stars from Lecureur et al. (2007) and Zoccali et al. (2006) for which we have determined the oxygen abundances from near-IR lines. These two determinations are connected. For the star B6-f7, there is only a near-IR determination of the oxygen abundance. When comparing these two determinations for the same stars, it should be noted that we have used new stellar parameters when determining the abundances from the near-IR spectra. The blue star and plus-sign symbols represent thick disk and thin disk giants, respectively, from Meléndez et al. (2008). |
Open with DEXTER | |
In the text |
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