C, N and O abundances in red clump stars of the Milky Way
The Hipparcos orbiting observatory has revealed a large number of helium-core- burning ”clump” stars in the Galactic field. These low-mass stars exhibit signatures of extra-mixing processes that require modeling beyond the first dredge-up of standard models. The ratio is the most robust diagnostic of deep mixing, because it is insensitive to the adopted stellar parameters. In this work we present determinations in a sample of 34 Galactic clump stars as well as abundances of nitrogen, carbon and oxygen. Abundances of carbon were studied using the Swan (0,1) band head at 5635.5 Å. The wavelength interval 7980–8130 Å with strong CN features was analysed in order to determine nitrogen abundances and isotope ratios. The oxygen abundances were determined from the [O i] line at 6300 Å. Compared with the Sun and dwarf stars of the Galactic disk, mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs. Comparisons to evolutionary models show that the stars fall into two groups: the one is of first ascent giants with carbon isotope ratios altered according to the first dredge-up prediction, and the other one is of helium-core-burning stars with carbon isotope ratios altered by extra mixing. The stars investigated fall to these groups in approximately equal numbers.
keywords:stars: abundances – stars: evolution – stars: horizontal-branch.
During the last decades, an increasing amount of work has been done in studying the chemical composition of red clump stars of the Galaxy (e.g. McWilliam 1990; Tautvaišienė et al. 2003; Mishenina et al. 2006; Liu et al. 2007; Luck & Heiter 2007; Tautvaišienė & Puzeras 2009; Puzeras et al. 2010). From the Hipparcos catalogue (Perryman et al. 1997) containing about 600 clump stars with parallax error lower than 10% and representing a complete sample of clump stars to a distance of about 125 pc, almost a half of stars are already investigated by means of high resolution spectroscopy.
Among the fundamental questions to which investigations of clump stars should help to find an answer is a mechanism of transport of processed material to the stellar surface in low mass stars. Post-main sequence stars with masses below exhibit signatures of material mixing that require challenging modelling beyond the standard stellar theory (reviews by Charbonnel 2006; Chanamé et al. 2005 and references therein). Also it is interesting to find out how many stars in the Galactic clump belong to the first ascent giants and to He-core-burning stars.
Carbon and nitrogen abundances are among most useful quantitative indicators of mixing processes in evolved stars. Because of the first dredge-up abundances of decrease while abundances of and increase (Iben 1965). Depending on stellar mass, metallicity and evolutionary state, these alterations are growing (c.f. Boothroyd & Sackmann 1999; Charbonnel & Zahn 2007; and many other studies).
Three large studies of , N and O abundances in clump stars have been done recently. Abundances of C, N and O in 177 clump giants of the Galactic disk were determined by Mishenina et al. (2006) on a basis of spectra (=42 000) obtained on the 1.93-m telescope of the Haute-Provence Observatoire (France).
A sample of 63 red clump stars, mainly located in the southern hemisphere, was investigated by Liu et al. (2007). Abundances of oxygen were investigated on a basis of spectra (=48 000) obtained on the 1.52-m telescope of the ESO (La Silla, Chile).
A spectroscopic analysis of C, N and O (=60 000) was done for a sample of nearby giants, with red clump stars among them by Luck & Heiter (2007). We selected a subsample of 138 red clump giants based on the luminosity and effective temperature diagram in the Fig. 20 of this paper. All the stars located in the box limited by luminosities log() from 1.5 to 1.8 and effective temperatures from 4700 K to 5200 K were includes in the subsample.
A comprehensive study of abundances in Galactic clump stars was not done yet. The ratio is the most robust diagnostic of deep mixing, because it is very sensitive to mixing processes and is almost insensitive to the adopted stellar parameters.
In this paper we report C, C, N and O abundances in the 34 clump stars of the Galactic field obtained from the high-resolution spectra. The results are discussed in detail together with results of other studies of the clump stars. The preliminary results of this study were published by Tautvaišienė et al. (2003, 2007, 2010) and Tautvaišienė & Puzeras (2009).
2 Observational data
The spectra of 26 stars were obtained at the Nordic Optical Telescope (NOT, La Palma) with the SOFIN échelle spectrograph (Tuominen et al. 1999). The 2nd optical camera () was used to observe simultaneously 13 spectral orders, each of Å in length, located from 5650 Å to 8130 Å. Reduction of the CCD images, obtained with SOFIN, was done using the 4A software package (Ilyin 2000). Procedures of bias subtraction, spike elimination, flat field correction, scattered light subtraction, extraction of spectral orders were used for image processing. A Th-Ar comparison spectrum was used for the wavelength calibration. The continuum was defined from a number of narrow spectral regions, selected to be free of lines.
This sample of stars was supplemented by spectroscopic observations () of 8 red clump stars obtained on the 2.16 m telescope of the Beijing Astronomical Observatory (China) taken from the literature (Zhao et al. 2001). There are more spectra of clump stars presented in this literature source, however not all of them have regions of Swan (0,1) band head at 5630.5 Å observed, or good quality bands at 8004 Å. In Fig. 1, we show examples of spectra observed on the Nordic Optical Telescope and the telescope of Beijing Astronomical Observatory.
3 Method of analysis and physical data
The spectra were analysed using a differential model atmosphere technique. A program bsyn, developed at the Uppsala Astronomical Observatory, was used to carry out the calculations of synthetic spectra. A set of plane parallel, line-blanketed, constant-flux LTE model atmospheres was computed with an updated version of the marcs code (Gustafsson et al. 2008). Calibrations to the solar spectrum (Kurucz et al. 1984) was done for all the spectral regions investigated. For this purpose we used the solar model atmosphere from the set calculated in Uppsala with a microturbulent velocity of 0.8 , as derived from Fe i lines, and the solar abundances log, log, log , log , , , etc. (Grevesse & Sauval 2000).
For determination in stars we used 5632 – 5636 Å interval to compare with observations of Swan (0 ,1) band head at 5635.5 Å. The same molecular data of as used by Gonzalez et al. (1998) were adopted for the analysis. This feature was used in several of our previous studies of giants (Tautvaišienė et al. 2000; 2001; 2005).
The interval 7980 – 8130 Å contains strong and features, so it was used for nitrogen abundance and ratio analysis. The well known line at 8004.7 Å was analysed in order to determine ratios. The molecular data for this CN band were provided by Bertrand Plez (University of Montpellier II). All values of CN were increased by +0.03 dex to fit the model spectrum of solar atlas of Kurucz et. al. (1984).
We derived oxygen abundance from synthesis of the forbidden [O i] line at 6300 Å. The values for and isotopic line components, which blend the oxygen line, were taken from Johansson et al. (2003) and [O i] log value, as calibrated to the solar spectrum (Kurucz et al. 1984).
The atomic oscillator strengths for stronger lines of iron and other elements were taken from Gurtovenko & Kostik (1989). The Vienna Atomic Line Data Base (VALD, Piskunov et al. 1995) was extensively used in preparing the input data for the calculations. In addition to thermal and microturbulent Doppler broadening of lines, atomic line broadening by radiation damping and van der Waals damping were considered in the calculation of abundances. Radiation damping parameters of lines were taken from the VALD database. In most cases the hydrogen pressure damping of metal lines was treated using the modern quantum mechanical calculations by Anstee & O’Mara (1995), Barklem & O’Mara (1997) and Barklem et al. (1998). When using the Unsöld (1955) approximation, correction factors to the classical van der Waals damping approximation by widths were taken from Simmons & Blackwell (1982). For all other species a correction factor of 2.5 was applied to the classical ), following Mäckle et al. (1975).
Stellar rotation was taken into account when needed. The values of have been taken from Hekker & Meléndez (2007), De Medeiros et al. (2002) and Glebocki & Stawikowski (2000).
Effective temperature, gravity, [Fe/H] and microturbulent velocity values of the stars have been taken from Puzeras et al. (2010) where these values were derived using traditional spectroscopic criteria.
Determinations of stellar masses were performed using effective temperatures obtained by Puzeras et al., luminosities and Girardi et al. (2000) isochrones. The luminosities were calculated from Hipparcos parallaxes (van Leeuwen 2007), magnitudes (SIMBAD database), bolometric corrections calculated according to Alonso et al. (1999) and interstellar reddening corrections calculated using Hakkila et al. (1994) software.
3.1 Estimation of uncertainties
The sensitivity of the abundance estimates to changes in the atmospheric parameters by the assumed errors is illustrated for the star HD 141680 in Table 1.
The sensitivity of iron abundances to stellar atmospheric parameters were described in Puzeras et al. (2010).
Since abundances of C, N and O are bound together by the molecular equilibrium in the stellar atmosphere, we have also investigated how an error in one of them typically affects the abundance determination of another. causes and ; causes and . has no effect on either the carbon or the oxygen abundances.
Abundances of nitrogen were determined from 14–20 lines in the spectra obtained on the Nordic Optical Telescope and from 9–13 lines in the spectra obtained at the Beijinh Astronomical Observatory. The mean scatter of the deduced line abundances is equal to 0.07 dex. This gives an approximate estimate of uncertainties due to random errors of the analysis.
In Fig. 2 and 3, we present several examples of spectral syntheses and comparisons to the observed spectra.
4 Results and discussion
The abundances relative to hydrogen
Sp: 1 – NOT, 2 – Beijing. Evol.: g – first ascent giant, c – He-core-burning star, – may be a He-core-burning star.
4.1 Comparisons with C, N and O abundances in dwarf stars
The interpretation of the C, N and O abundances can be done by a comparison with abundances determined for dwarf stars in the Galactic disk.
As concerns carbon, we selected for the comparison the papers by Gustafsson et al. (1999) and Bensby & Feltzing (2006) since abundances of carbon were determined in these studies using the same computing programs and using the forbidden [C i] line at 8727 Å. Due to its low excitation potential, the [C i] line should not be sensitive to non-LTE effects and to uncertainties in the adopted model atmosphere parameters, contrary to what may be expected for C i and CH lines. The line, which was investigated in our work, usually gives compatible results with [C i]. A comprehensive discussion on this subject is given by Gustafsson et al. (1999) and Samland (1998).
In Fig. 4, we show results of [C/Fe] for the stars investigated in our work together with results from other studies of clump stars performed by Mishenina et al. (2006) and by Luck & Heiter (2007). Also we show the results obtained for red horizontal branch stars by Tautvaišienė et al. (2001) and by Gratton et al. (2000). Compared to [C/Fe] values in dwarf stars (Gustafsson et al. and Bensby & Feltzing), it is seen that [C/Fe] in clump stars lie by about 0.2 dex below the abundance trend of dwarfs.
Determinations of nitrogen abundances in Galactic disk stars are not numerous. For metal-abundant stars, as follows from the compilation by Samland (1989), a concentration of [N/Fe] ratios with a rather large scatter lies at the solar value in the [Fe/H] interval from dex to about dex.
For the comparison of [N/Fe] values, in Fig. 5, we show the results of clump stars together with results obtained for dwarf stars by Shi et al. (2002). By means of spectral synthesis they investigated several week N i lines. Reddy et al. (2003) also investigated nitrogen abundances in a sample of 43 F–G dwarfs in the Galactic disk by means of weak N i lines, however they used the equivalent width method, which gave, to our understanding, slightly overabundant [N/Fe] values.
As it is seen from Fig. 5, in the clump stars investigated, when compared to the Galactic field dwarf stars, the nitrogen abundances are enhanced by about 0.2 dex.
In our study, as well as in Mishenina et al. (2006), in Liu et al. (2007) and in Luck & Heiter (2007), the oxygen abundances in clump stars are similar to those observed in dwarfs (e.g. Edvardsson et al. 1993). In agreement with theoretical predictions the investigated stars do not yet show signs of evolution of the oxygen abundances after the main sequence. This allows to use oxygen abundances of clump stars for Galactic evolution studies.
4.2 Comparisons with theoretical models
In Mishenina et al. (2006), the observational results of [C/Fe] and [N/Fe] were compared with theoretical trends of the 1 dredge-up, computed using the starevol code and presented in the same paper by Mishenina et al. The modelled trends were computed using the standard mixing length theory. In the left side of Fig. 6, we plotted [C/Fe] and [N/Fe] versus effective temperatures in our sample of Galactic red clump stars compared with the theoretical tracks of abundance variations taken from Mishenina et al. The nitrogen overabundances in the clump stars are in agreement with the modeled, however carbon in the observed sample is depleted more than the theoretical model of Mishenina et al. (2006) predicts. In these models neither overshooting, nor undershooting was considered for convection. The atomic diffusion and rotational-induced mixing were also not taken into account. In order to better fit the observational results, the authors simply shifted an initial [C/Fe] by dex. In our comparison we had to make the same.
C/N and ratios we compared with the theoretical models by Boothroyd & Sackmann (1999) (the right side of Fig. 6) and found a good agreement with the observational data. These models include the deep circulation mixing below the base of the standard convective envelope, and the consequent ”cool bottom processing” (CBP) of CNO isotopes.
Recently Eggleton et al. (2006) found a mean molecular weight () inversion in their stellar evolution model, occurring after the so-called luminosity bump on the red giant branch, when the H-burning shell source enters the chemically homogeneous part of the envelope. The -inversion is produced by the reaction , as predicted by Ulrich (1972). It does not occur earlier, because the magnitude of the -inversion is small and negligible compared to a stabilizing -stratification.
The work by Eggleton et al. (2006) has inspired Charbonnel & Zahn (2007) to compute stellar models including the prescription by Ulrich (1972) and extend them to the case of a non-perfect gas for the turbulent diffusivity produced by that instability in stellar radiative zone. They found that a double diffusive instability referred to as thermohaline convection, which has been discussed long ago in the literature (Stern 1960), is important in evolution of red giants. This mixing connects the convective envelope with the external wing of hydrogen burning shell and induces surface abundance modifications in red giant stars (Lagarde & Charbonnel 2009).
In our and stellar mass plot (Fig. 6) we show the thermohaline model (TH) by Lagarde & Charbonnel (2009) as well. It fits to our observational results well at lower masses but at larger masses the theoretical ratios could be slightly lower. Nevertheless, we are sure that the thermohaline model is a promising model to be developed. Cantiello & Langer (2010) reported that thermohaline mixing is also present in red giants during core He-burning and beyond.
The comparison with theoretical models shows that according to isotope ratios, the stars fall into two groups: the one with carbon isotope ratios altered according to the 1 dredge-up prediction, and the other one with carbon isotope ratios altered by extra mixing. The stellar positions in the versus stellar mass diagram as well as comparisons to stellar evolutionary sequences in the luminosity versus effective temperature diagram by Girardi et al. (2000) show that stars fall to groups of helium-core-burning and first ascent giants in approximately equal numbers. In the last column of Table 2, we indicate the predicted evolutionary status of investigated stars. By asterisks are marked stars which have isotope ratios equal to about 20, and which could belong to helium-core-burning stars as well, especially if compared to the model of thermohaline mixing.
In the paper by Mishenina et al. (2006), according to nitrogen abundance values, the authors have suggested 21 helium-core-burning stars, about 54 candidates to helium-core-burning stars and about 100 first ascent giants.
In this work we present determinations in a sample of 34 Galactic clump stars as well as abundances of nitrogen, carbon and oxygen, obtained using high-resolution spectra observed on the Nordic Optical telescope () and at the Beijing Astronomical Observatory ().
The obtained stellar abundances together with results of other studies of Galactic clump stars (Mishenina et al. 2006; Liu et al. 2007 and Luck & Heiter 2007) and of red-horizontal-branch stars (Tautvaišienė et al. 2001 and Gratton et al. 2000) were compared with determinations of C, N and O abundances in dwarf stars of the Galactic disk. The mean abundances in the investigated clump stars suggest that carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs.
The stellar positions in the versus stellar mass diagram as well as comparisons to stellar evolutionary sequences in the luminosity versus effective temperature diagram by Girardi et al. (2000) show that the stars fall into two groups: the one is of first ascent giants with carbon isotope ratios altered according to the 1 dredge-up prediction, and the other one is of helium-core-burning stars with carbon isotope ratios altered by extra mixing. The stars investigated fall to these groups in approximately equal numbers.
And finally, we would like to point out that thermohaline convection is a fundamental physical process and is important in evolution of red giants. However, most probably thermohaline mixing is not the only physical process responsible for surface abundance anomalies in red giants (c.f. Cantiello & Langer 2010). We hope that the results presented in this work will contribute to answering fundamental questions of stellar evolution.
We wish to thank Kjell Eriksson (Uppsala Observatory) for valuable help in running the synthetic spectrum computing programs. This project has been supported by the European Commission through the Baltic Grid project as well as through “Access to Research Infrastructures Action” of the “Improving Human Potential Programme”, awarded to the Instituto de Astrofísica de Canarias to fund European Astronomers’ access to the European Nordern Observatory, in the Canary Islands. BE thanks the Swedish Research Council (VR) for support.
- pagerange: C, N and O abundances in red clump stars of the Milky Way–References
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