A&A 381, 982-992 (2002)
DOI: 10.1051/0004-6361:20011530
J. R. Shi 1,2 - G. Zhao 1 - Y. Q. Chen 1
1 - National Astronomical Observatories, Chinese Academy of Sciences, Beijing
100012, PR China
2 -
Chinese Academy of Sciences - Peking University Joint Beijing Astrophysical
Center, Beijing 100871, PR China
Received 31 May 2001 / Accepted 29 October 2001
Abstract
Abundance analysis of carbon and nitrogen has been performed for a sample of 90 F and G
type main-sequence disk stars with a metallicity range of -1.0 < [Fe/H] <+0.2 using the C I
and N I lines. We confirm a moderate carbon excess in the most metal-poor disk dwarfs found in
previous investigations. Our results suggest that carbon is enriched by superwinds of metal-rich
massive stars at the beginning of the disk evolution, while a significant amount of carbon
is contributed by low-mass stars in the late stage.
The observed behavior of [N/Fe] is about solar in the disk stars, irrespective of the metallicity.
This result suggests that nitrogen is produced mostly by intermediate-mass stars.
Key words: stars: abundances - stars: late-type - Galaxy: evolution - Galaxy: solar neighbourhood
The literature on the origin of carbon has been reviewed by Timmes
et al. (1995) and Gustafsson et al. (1999). The former paper concluded that massive star
synthesis of primary carbon is sufficient to explain the metal-poor halo dwarf observations, while at
[Fe/H]
-0.8 dex,
intermediate- and low-mass stars begin to deposit large amounts of carbon. However, the latter paper
states that "the behavior of C/O ratios as a function of O/H is best explained by the effects of
stellar mass loss on massive star yields.''
Nitrogen is not directly produced in a large
amount by hydrostatic He-burning. Nitrogen synthesis requires recycling of material from a region
where H-burning occurs through the CNO cycle. Timmes et al. (1995) suggested
that no primary nitrogen is produced in the standard massive star models.
However, recently, Maeder & Meynet (2000)
have shown that a primary nitrogen is produced in rapidly rotating stars within the mass
range 10-15 .
The nitrogen production for intermediate- and low-mass stars is discussed by Renzini & Voli (1981),
van der Hoek & Groenewegen (1997), Marigo et al. (1996, 1998), Busso et al.
(1999) and Marigo (2001).
Observational data
indicate that large amounts of nitrogen are present in the outer layers of evolved stars over a wide mass
range. From there, nitrogen can be lost to the ISM through quiescent or violent stellar winds, e.g.
in the case of planetary nebulae. Lack of adequate knowledge of the
relevant mechanisms prevents accurate predictions of the run of [N/Fe] ratio with metal abundance.
The literature on the origin of nitrogen was reviewed by
Henry et al. (2000). Nitrogen has a primary component
and a secondary component. At higher
abundances the secondary component dominates. Henry et al. (2000) suggested that
"nitrogen is produced mostly (>90%) by intermediate-mass stars, and the relevant stellar
masses for production are 4-8 ''.
As far as the disk stars concerned, many general trends have been discovered during the past decades. The most notable results are correlations of metallicity with age, Galactocentric distance, and vertical distance from the Galactic plane based on photometric, low- and high-resolution observations (e.g. Eggen et al. 1962; Twarog 1980; Edvardsson et al. 1993a; Chen et al. 2000, hereafter Chen00). For the case of carbon, the particularly important paper is the detailed abundance analysis of 80 F and G dwarfs with -1.1 < [Fe/H] < +0.25 by Gustafsson et al. (1999). The main result from their work was the moderate carbon excess in the most metal-poor disk stars. They also suggested that carbon enrichment is by superwinds of metal-rich massive stars.
The present work, based on a differently selected sample of disk stars, aims at exploring and extending the results of Gustafsson et al. (1999). In Sect. 2 we present the observational techniques. The methods of analysis are discussed in Sect. 3. While in Sect. 4 we discuss the possible errors in our results. The relations between abundances, kinematics and ages are given in Sect. 5. And the discussion and conclusions are presented in the last section.
The spectra were reduced with standard MIDAS routines for order identification, background subtraction, flat-fielding correction, extraction of echelle orders, wavelength calibration, and continuum fitting (see Chen00 for details). Figure 1 shows a portion of the spectrum of HD34411A around 7115Å.
![]() |
Figure 1:
The C I lines around ![]() |
Open with DEXTER |
Surface gravities in Chen00 were based on Hipparcos parallaxes (ESA 1997). Combining the uncertainties of mass, bolometric correction and temperature, the total error is less than 0.10 dex.
Microturbulence velocities were estimated by
requesting that the iron abundance derived from Fe I lines with
eV should not
depend on equivalent width. The typical error of the microturbulence parameter is about
kms-1.
We employ the macroturbulence parameters in the
radial-tangential form and adopt values as described in Gray (1984).
The projected rotational velocity is
obtained as the residual to the observed line profiles,
after excluding the known instrumental profiles obtained from the Moon spectra.
Species | ![]() |
![]() |
![]() |
log gf | log C6 |
[Å] | [eV] | [eV] | |||
C I | 6587.61 | 8.54 | 10.42 | -1.15 | -29.67 |
C I | 7111.48 | 8.64 | 10.38 | -1.23 | -29.68 |
C I | 7113.48 | 8.65 | 10.39 | -0.85 | -29.67 |
C I | 7115.19 | 8.64 | 10.38 | -1.10 | -29.12 |
C I | 7116.99 | 8.65 | 10.39 | -0.93 | -29.12 |
C I | 7119.67 | 8.64 | 10.38 | -1.15 | -29.12 |
C I | 8335.15 | 7.68 | 9.17 | -0.38 | -30.73 |
[C I] | 8727.17 | 1.26 | 2.68 | -8.17 | -32.71 |
N I | 7468.31 | 10.34 | 12.00 | -0.11 | -31.02 |
N I | 8216.34 | 10.34 | 11.84 | 0.15 | -31.09 |
N I | 8683.40 | 10.33 | 11.76 | 0.12 | -31.13 |
Stürenburg & Holweger (1990) and Tomkin et al. (1995) have discussed the gfvalues of C I lines. The scatter of the individual abundance, caused by oscillator strength
error, is the major
contributor. In this paper, the oscillator strengths of selected C I and
N I lines have been improved from the analysis of
corresponding profiles in the Kitt Peak Solar Flux Atlas (Kurucz et al. 1984),
with the solar abundance of log
(C) = 8.56 and log
(Anders & Grevesse 1989) respectively. The stellar parameters we adopted in the
solar model are:
K,
log g=4.44 and
.
We note that those C I line gf values
are similar to the theoretical gf values of Hibbert et al. (1993),
for the common C I
lines, the average difference log gf (solar) - log gf (Hibbert) is -0.07
0.09 dex;
the number after the "
'' here and throughout is standard deviation.
Also those gf values are in good agreement with the gf values adopted by Stürenburg & Holweger
(1990). Our gf values of three N I lines are nearly the same as the theoretical values of
Hibbert et al. (1991), and the difference is very small, less than 0.02 dex.
Thus the adoption of theoretical instead of
solar oscillator strengths would not significantly affect the absolute or differential
carbon or nitrogen abundances of the program stars (see Table 1).
The derived abundances are presented in Table 2. We note that the difference in carbon abundances between the high excitation lines and forbidden line are small, this is in agreement with the results of Carretta et al. (2000). In the spectrum of HD212029, carbon and nitrogen lines were too weak to allow an accurate measurement of the abundances, thus this star is excluded from further analysis.
Star |
![]() |
logg | [Fe/H] | ![]() |
[C/Fe] | [N/Fe] | |||||||||||
HD | 6587 | 7111 | 7113 | 7115 | 7116 | 7119 | 8335 | 8727 |
![]() |
7468 | 8216 | 8683 |
![]() |
||||
400 | 6122. | 4.13 | -0.23 | 1.83 | -0.08 | -0.09 | -0.02 | -0.02 | -0.05 | -0.15 | -0.17 | -0.16 | |||||
3454 | 6056. | 4.29 | -0.59 | 1.57 | -0.08 | -0.08 | -0.02 | 0.00 | -0.05 | 0.00 | 0.00 | ||||||
5750 | 6223. | 4.21 | -0.44 | 1.93 | 0.01 | 0.05 | 0.06 | 0.01 | 0.06 | 0.04 | -0.05 | -0.06 | -0.05 | ||||
6834 | 6295. | 4.12 | -0.73 | 1.98 | 0.28 | 0.30 | 0.30 | 0.32 | 0.28 | 0.30 | |||||||
6840 | 5860. | 4.03 | -0.45 | 2.23 | 0.07 | 0.06 | 0.04 | 0.04 | 0.05 | 0.05 | 0.05 | ||||||
10307 | 5776. | 4.13 | -0.05 | 1.80 | 0.04 | 0.00 | 0.03 | 0.02 | 0.02 | 0.00 | 0.00 | 0.02 | 0.05 | 0.00 | 0.10 | 0.05 | |
11007 | 6027. | 4.20 | -0.16 | 1.73 | 0.05 | 0.03 | 0.06 | 0.05 | 0.06 | 0.06 | 0.02 | 0.05 | 0.22 | 0.12 | 0.22 | 0.19 | |
11592 | 6232. | 4.18 | -0.41 | 2.16 | 0.00 | 0.04 | 0.05 | 0.09 | 0.02 | 0.05 | 0.06 | 0.04 | 0.09 | 0.10 | 0.10 | ||
19373 A | 5867. | 4.01 | 0.03 | 1.84 | 0.07 | 0.03 | 0.00 | 0.00 | -0.03 | 0.07 | 0.02 | 0.02 | 0.05 | 0.05 | 0.05 | ||
22484 | 5915. | 4.03 | -0.13 | 1.97 | 0.08 | 0.10 | 0.06 | 0.16 | 0.06 | 0.08 | 0.07 | 0.09 | 0.00 | 0.00 | |||
24421 | 5986. | 4.10 | -0.37 | 1.81 | 0.06 | 0.10 | 0.13 | 0.13 | 0.10 | 0.00 | 0.00 | 0.00 | |||||
25173 | 5867. | 4.07 | -0.62 | 1.79 | 0.25 | 0.29 | 0.26 | 0.24 | 0.26 | 0.00 | 0.00 | 0.00 | |||||
25457 | 6162. | 4.28 | -0.11 | 2.77 | -0.02 | -0.04 | 0.04 | 0.04 | 0.06 | 0.01 | |||||||
25998 | 6147. | 4.35 | -0.11 | 3.24 | 0.20 | 0.14 | 0.16 | 0.14 | 0.17 | 0.16 | |||||||
33632 A | 5962. | 4.30 | -0.23 | 1.56 | 0.13 | 0.05 | 0.14 | 0.14 | 0.13 | 0.14 | 0.12 | 0.00 | 0.00 | 0.00 | |||
34411 A | 5773. | 4.02 | 0.01 | 1.69 | 0.04 | 0.01 | 0.01 | -0.04 | -0.04 | -0.04 | 0.04 | 0.04 | 0.00 | 0.08 | 0.00 | 0.04 | |
35296 A | 6015. | 4.24 | -0.14 | 2.10 | 0.02 | 0.00 | -0.02 | 0.02 | 0.03 | 0.00 | 0.01 | 0.00 | 0.00 | ||||
39587 | 5805. | 4.29 | -0.18 | 2.16 | -0.05 | 0.00 | 0.03 | 0.08 | 0.02 | 0.08 | 0.00 | 0.04 | |||||
39833 | 5767. | 4.06 | 0.04 | 1.88 | -0.17 | -0.10 | -0.18 | -0.17 | -0.24 | -0.17 | 0.00 | 0.00 | |||||
41640 | 6004. | 4.37 | -0.62 | 1.98 | 0.08 | 0.08 | 0.10 | 0.09 | |||||||||
43947 | 5859. | 4.23 | -0.33 | 1.73 | 0.00 | 0.00 | 0.03 | 0.00 | 0.01 | ||||||||
46317 | 6216. | 4.29 | -0.24 | 1.79 | 0.03 | 0.05 | 0.02 | 0.04 | 0.02 | 0.03 | 0.03 | -0.05 | -0.05 | ||||
49732 | 6260. | 4.15 | -0.70 | 1.91 | 0.05 | 0.05 | 0.06 | 0.10 | 0.07 | 0.00 | 0.00 | ||||||
54717 | 6350. | 4.26 | -0.44 | 2.00 | 0.04 | 0.05 | 0.05 | 0.08 | 0.05 | 0.03 | 0.05 | 0.05 | 0.03 | 0.04 | |||
55575 | 5802. | 4.36 | -0.36 | 1.62 | 0.08 | 0.09 | 0.08 | 0.09 | 0.09 | 0.02 | 0.00 | 0.01 | |||||
58551 | 6149. | 4.22 | -0.54 | 2.15 | 0.00 | -0.09 | 0.02 | 0.01 | -0.02 | -0.02 | |||||||
58855 | 6286. | 4.31 | -0.31 | 2.06 | -0.05 | -0.01 | -0.04 | -0.02 | -0.03 | -0.03 | -0.03 | -0.20 | -0.22 | -0.21 | |||
59380 | 6280. | 4.27 | -0.17 | 2.09 | 0.07 | 0.06 | 0.05 | 0.05 | 0.04 | 0.07 | 0.06 | -0.04 | -0.04 | ||||
59984 A | 5900. | 4.18 | -0.71 | 1.78 | 0.22 | 0.22 | 0.02 | 0.02 | |||||||||
60319 | 5867. | 4.24 | -0.85 | 1.56 | 0.29 | 0.32 | 0.32 | 0.35 | 0.30 | 0.32 | 0.02 | 0.05 | 0.04 | ||||
62301 | 5837. | 4.23 | -0.67 | 1.72 | 0.33 | 0.34 | 0.30 | 0.32 | 0.35 | 0.33 | 0.05 | 0.05 | |||||
63333 | 6057. | 4.23 | -0.39 | 1.92 | 0.08 | 0.02 | 0.16 | 0.09 | 0.00 | 0.00 | |||||||
68146 A | 6227. | 4.16 | -0.09 | 2.12 | 0.02 | 0.00 | -0.02 | 0.01 | 0.00 | 0.02 | 0.02 | ||||||
69897 | 6243. | 4.28 | -0.28 | 2.02 | -0.04 | -0.04 | 0.00 | 0.02 | -0.06 | -0.02 | -0.09 | -0.09 | |||||
72945 A | 6202. | 4.18 | 0.00 | 2.28 | -0.03 | -0.09 | -0.06 | -0.04 | -0.05 | -0.06 | -0.03 | 0.02 | 0.00 | ||||
75332 | 6130. | 4.32 | 0.00 | 2.34 | 0.05 | -0.03 | -0.04 | -0.03 | -0.02 | -0.02 | -0.02 | 0.00 | 0.00 | ||||
76349 A | 6004. | 4.21 | -0.49 | 2.09 | 0.03 | 0.05 | 0.04 | ||||||||||
78418 A | 5625. | 3.98 | -0.26 | 1.55 | 0.20 | 0.23 | 0.20 | 0.18 | 0.30 | 0.22 | 0.07 | 0.07 | |||||
79028 | 5874. | 4.06 | -0.05 | 1.96 | 0.07 | 0.09 | 0.02 | 0.03 | 0.02 | 0.01 | 0.04 | 0.05 | 0.05 | ||||
80218 | 6092. | 4.14 | -0.28 | 1.87 | 0.18 | 0.07 | 0.03 | 0.07 | 0.12 | 0.19 | 0.11 | 0.02 | 0.05 | 0.04 | |||
89125 A | 6038. | 4.25 | -0.36 | 1.66 | -0.03 | -0.10 | -0.11 | -0.03 | -0.07 | -0.09 | -0.09 | ||||||
90839 A | 6051. | 4.36 | -0.18 | 2.14 | 0.06 | 0.03 | 0.04 | -0.01 | -0.02 | 0.03 | 0.01 | 0.07 | 0.05 | 0.06 | |||
91889 A | 6020. | 4.15 | -0.24 | 1.66 | -0.08 | -0.14 | -0.07 | -0.06 | -0.07 | -0.08 | -0.12 | -0.12 | |||||
94280 | 6063. | 4.10 | 0.06 | 2.00 | -0.03 | -0.02 | -0.02 | -0.04 | -0.06 | -0.03 | -0.15 | -0.15 | |||||
95128 | 5731. | 4.16 | -0.12 | 1.87 | 0.17 | 0.16 | 0.16 | 0.12 | 0.19 | 0.16 | -0.08 | -0.05 | -0.06 | ||||
97916 | 6445. | 4.16 | -0.94 | 2.11 | -0.08 | 0.00 | -0.04 |
Star |
![]() |
logg | [Fe/H] | ![]() |
[C/Fe] | [N/Fe] | |||||||||||
HD | 6587 | 7111 | 7113 | 7115 | 7116 | 7119 | 8335 | 8727 |
![]() |
7468 | 8216 | 8683 |
![]() |
||||
100180 A |
5866. | 4.12 | -0.11 | 1.87 | 0.05 | 0.06 | 0.03 | 0.00 | 0.00 | 0.03 | 0.06 | 0.03 | 0.15 | 0.15 | |||
100446 | 5967. | 4.29 | -0.48 | 1.86 | 0.12 | 0.12 | 0.13 | 0.16 | 0.10 | 0.12 | 0.15 | 0.15 | |||||
100563 | 6423. | 4.31 | -0.02 | 2.63 | 0.20 | 0.20 | 0.12 | 0.12 | 0.16 | 0.17 | 0.16 | -0.02 | 0.02 | 0.00 | |||
101676 | 6102. | 4.09 | -0.47 | 1.96 | 0.10 | 0.07 | 0.07 | 0.06 | 0.10 | 0.08 | 0.15 | 0.15 | 0.15 | ||||
106516 A | 6135. | 4.34 | -0.71 | 1.48 | 0.43 | 0.38 | 0.41 | 0.41 | 0.39 | 0.45 | 0.41 | 0.48 | 0.38 | 0.43 | |||
108510 | 5929. | 4.31 | -0.06 | 2.00 | 0.03 | 0.02 | 0.01 | 0.01 | -0.04 | 0.11 | 0.11 | 0.04 | -0.42 | -0.40 | -0.41 | ||
109303 | 5905. | 4.10 | -0.61 | 1.69 | 0.29 | 0.24 | 0.26 | 0.20 | 0.16 | 0.18 | 0.20 | 0.22 | |||||
114710 | 5877. | 4.24 | -0.05 | 1.77 | 0.03 | -0.02 | -0.02 | -0.03 | -0.02 | 0.03 | 0.01 | -0.15 | -0.18 | -0.17 | |||
115383 A | 5866. | 4.03 | 0.00 | 2.09 | 0.05 | 0.01 | 0.02 | 0.00 | 0.02 | 0.07 | 0.07 | ||||||
118244 | 6234. | 4.13 | -0.55 | 2.29 | 0.05 | 0.09 | 0.03 | 0.06 | 0.04 | 0.05 | 0.00 | 0.00 | |||||
121560 | 6059. | 4.35 | -0.38 | 1.69 | -0.10 | -0.09 | -0.10 | -0.07 | -0.09 | 0.00 | 0.00 | ||||||
124244 | 5853. | 4.11 | 0.05 | 1.92 | -0.02 | -0.02 | 0.05 | -0.07 | -0.05 | -0.04 | -0.15 | -0.15 | |||||
128385 | 6041. | 4.12 | -0.33 | 2.07 | 0.00 | -0.05 | 0.02 | 0.04 | 0.0 | -0.20 | -0.10 | -0.15 | |||||
130948 | 5780. | 4.18 | -0.20 | 2.15 | -0.01 | 0.02 | 0.02 | 0.05 | 0.03 | 0.02 | 0.00 | 0.04 | 0.02 | ||||
132254 | 6231. | 4.22 | 0.07 | 2.05 | -0.16 | -0.12 | -0.14 | -0.10 | -0.15 | -0.13 | -0.09 | -0.15 | -0.12 | ||||
139457 | 5941. | 4.06 | -0.52 | 1.95 | 0.24 | 0.22 | 0.18 | 0.20 | 0.16 | 0.20 | 0.00 | 0.00 | |||||
142373 | 5920. | 4.27 | -0.39 | 1.48 | -0.02 | 0.00 | 0.02 | -0.01 | 0.00 | 0.00 | |||||||
142860 A | 6227. | 4.18 | -0.22 | 2.15 | -0.05 | -0.03 | -0.03 | -0.03 | -0.05 | -0.05 | -0.07 | -0.04 | 0.00 | -0.02 | -0.01 | ||
146099 A | 5941. | 4.10 | -0.61 | 1.79 | 0.08 | 0.14 | 0.11 | 0.02 | 0.02 | ||||||||
149750 | 5792. | 4.17 | 0.08 | 1.96 | 0.08 | 0.09 | 0.06 | 0.07 | 0.08 | ||||||||
154417 | 5925. | 4.30 | -0.04 | 1.78 | 0.09 | 0.12 | 0.08 | 0.15 | 0.11 | 0.08 | 0.06 | 0.07 | |||||
157347 | 5654. | 4.36 | -0.02 | 1.89 | -0.12 | -0.10 | -0.09 | -0.12 | -0.10 | -0.08 | -0.05 | -0.09 | -0.29 | -0.29 | -0.29 | ||
157466 | 5935. | 4.32 | -0.44 | 1.89 | -0.10 | -0.08 | -0.04 | -0.08 | -0.12 | -0.08 | 0.08 | 0.00 | 0.04 | ||||
162004 B | 6059. | 4.12 | -0.08 | 2.34 | 0.00 | -0.04 | -0.03 | 0.00 | -0.02 | 0.03 | -0.01 | -0.06 | -0.06 | -0.06 | |||
167588 | 5894. | 4.13 | -0.33 | 1.72 | 0.15 | 0.07 | 0.09 | 0.07 | 0.09 | -0.04 | -0.06 | -0.05 | |||||
168009 | 5719. | 4.08 | -0.07 | 1.82 | 0.25 | 0.19 | 0.25 | 0.24 | 0.13 | 0.21 | 0.12 | 0.06 | 0.09 | ||||
170153 A | 6034. | 4.28 | -0.65 | 2.36 | 0.38 | 0.43 | 0.37 | 0.35 | 0.40 | 0.39 | 0.00 | -0.03 | -0.01 | ||||
184601 | 5830. | 4.20 | -0.81 | 1.50 | 0.33 | 0.37 | 0.35 | 0.39 | 0.40 | 0.35 | 0.37 | 0.15 | 0.21 | 0.18 | |||
189340 | 5888. | 4.26 | -0.19 | 2.28 | 0.05 | -0.05 | -0.02 | -0.01 | -0.05 | -0.05 | |||||||
191862 A | 6328. | 4.19 | -0.27 | 2.88 | -0.02 | -0.05 | -0.10 | -0.04 | -0.05 | -0.40 | -0.49 | -0.45 | |||||
198390 | 6339. | 4.20 | -0.31 | 1.92 | 0.19 | 0.20 | 0.20 | ||||||||||
200580 | 5829. | 4.39 | -0.58 | 1.66 | 0.32 | 0.32 | |||||||||||
201891 | 5827. | 4.43 | -1.04 | 1.55 | 0.38 | 0.41 | 0.32 | 0.37 | |||||||||
204306 | 5896. | 4.09 | -0.65 | 1.70 | 0.20 | 0.22 | 0.26 | 0.15 | 0.21 | 0.07 | 0.06 | 0.07 | |||||
204363 | 6141. | 4.18 | -0.49 | 2.06 | 0.20 | 0.13 | 0.10 | 0.15 | 0.15 | 0.15 | 0.15 | ||||||
206301 | 5682. | 3.98 | -0.04 | 2.12 | -0.09 | -0.09 | -0.11 | -0.07 | -0.10 | 0.09 | -0.04 | 0.00 | -0.02 | -0.02 | |||
206860 | 5798. | 4.25 | -0.20 | 2.33 | -0.08 | -0.03 | -0.01 | 0.00 | 0.00 | -0.02 | 0.12 | 0.10 | 0.11 | ||||
208906 A | 5929. | 4.39 | -0.73 | 1.47 | 0.26 | 0.24 | 0.23 | 0.28 | 0.24 | 0.25 | 0.05 | 0.05 | |||||
209942 A | 6022. | 4.25 | -0.29 | 2.26 | 0.11 | 0.08 | 0.15 | 0.08 | 0.12 | 0.11 | 0.06 | 0.05 | 0.08 | 0.06 | |||
210027 A | 6496. | 4.25 | -0.17 | 1.97 | 0.00 | -0.01 | 0.03 | -0.03 | 0.00 | -0.03 | -0.01 | -0.06 | -0.08 | -0.07 | |||
210752 | 5847. | 4.33 | -0.68 | 2.39 | 0.16 | 0.15 | 0.16 | 0.16 | 0.00 | 0.00 | |||||||
212029 A | 5875. | 4.36 | -1.01 | 2.03 | |||||||||||||
215257 | 5976. | 4.36 | -0.65 | 1.74 | 0.06 | 0.06 | |||||||||||
219623 A | 6039. | 4.07 | 0.02 | 1.84 | 0.12 | 0.04 | 0.08 | 0.04 | 0.07 | 0.00 | 0.02 | -0.01 | 0.00 |
The propagation of these parameter errors to the carbon/nitrogen abundance estimation has been investigated by changing the fundamental parameters by a representative amount and then running the synthetic spectrum program to obtain the change in carbon/nitrogen abundance. This is shown in Table 3 for HD34411A. The statistical errors in the carbon/nitrogen to iron ratio due to uncertainties in the fundamental parameters are around 0.02 and 0.05 dex respectively. The systematic error is about 0.07 dex for carbon, 0.09 dex for nitrogen, which mainly reflects the errors in temperature and surface gravity. The statistical errors in the model metallicity and in the microturbulence are unimportant for the derived carbon/nitrogen abundance (Andersson & Edvardsson 1994; Gustafsson et al. 1999, and references therein).
We have also searched for correlations between carbon/nitrogen abundances and atmospheric parameters.
There is no correlation of [C/Fe] and [N/Fe] with either
or log g. The
weak correlation between [C/Fe] and microturbulence parameter found by
Andersson & Edvardsson (1994) is not confirmed in this work.
We estimate that the total random error is 0.1 dex in the carbon abundance, 0.2 dex in the
nitrogen abundance.
![]() |
Our program stars are not greatly different from the Sun and Procyon, so the non-LTE correction
of carbon/nitrogen abundances is not affected significantly.
![]() |
Figure 2:
Comparison of derived [C/Fe] for stars in common with other studies.
Plusses (+) are from Clegg et al. (1981); open circles (![]() ![]() ![]() |
Open with DEXTER |
![]() |
Figure 3:
Comparison of derived [N/Fe] for the stars in common with other studies.
Plusses (+) are from Clegg et al. (1981); open circles (![]() ![]() |
Open with DEXTER |
We have 4 stars in common with Clegg et al. (1981) and 8 stars with
Larid et al. (1985). Although the systematic deviation of [N/Fe] is small,
and
respectively, the rms is very large, reflecting the difficulties in the
determination of nitrogen abundances.
The comparison of this work with others is plotted in Fig. 3.
The plot of [C/Fe] against [Fe/H] shown in Fig. 4a confirms the enrichment of carbon relative
to iron
for the most metal-poor disk stars found in previous investigations
(Friel & Boesgaard 1992; Andersson & Edvardsson 1994;
Tomkin et al. 1995; Gustafsson et al. 1999). For our group C stars,
the [C/Fe] is around 0.3 dex, while [C/Fe] is flat for [Fe/H] > -0.4.
A striking feature of the plot is that the transition from [C/Fe]
0.0 to [C/Fe]
+0.4 for
group A stars is made over a small range in [Fe/H] or rather
abruptly at [Fe/H]
-0.7. There are also some group C and A stars in Tomkin et al.
(1995, in Fig. 8) and the same trend can be found in their sample, as well as the same decreasing
trend for the metal-poor part of group B stars. We note, however, that Gustafsson et al. (1999)
did not find this trend, which may be due to the small number of group A stars in their sample or different
lines used to determine the carbon abundances. In short, there seems to be
an abrupt drop of [C/Fe] at [Fe/H]
-0.7. Two scenarios may explain this result: one is that
it may indicate the large contribution of iron from type Ia SNe at that time; another is that when the
metallicity is relatively higher, the mass loss
rate is more important, the star enters the WC phase at an earlier stage, thus less helium has been
changed into carbon (Maeder & Meynet 1993).
Portinari et al. (1998) also pointed out that, for massive stars
(
),
carbon yields decrease again moving from the Z = 0.008 to the Z = 0.05,
both because a higher mass loss rate is able to take more helium away before it turned to carbon,
and because with an increasing metallicity an increasing fraction of the original carbon is
turned to 14N in the CNO cycle, while Prantzos et al. (1994),
Gustafsson et al. (1999) and Henry et al. (2000) suggested that carbon production could be
favoured at higher metallicities, in order to explain the observed correlation between C/O and O/H.
![]() |
Figure 4:
Abundance patterns for [C/Fe] and [N/Fe] with three groups of stars: open triangles
(![]() ![]() ![]() ![]() |
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The decrease of [C/Fe] with increasing [Fe/H] in the galactic disk is very different from
the variation of the s-element abundances with [Fe/H].
Using the standard infall Galactic chemical evolution (hereafter GCE) models and yields of Maeder
(1992) or Portinari et al. (1998), Liang et al. (2001) found that [C/Fe]
increases from
to
,
which was in contradiction with the
observations (see also Portinari et al. 1998). In addition, [C/Fe] was above the observations in
the metal-rich region. Prantzos et al. (1994) also studied the
formation of oxygen and carbon in the Galaxy and found that, if the duration of the halo phase was
on the order 1-2 Gyrs as is currently believed, intermediate- or low-mass stars should not
have been the main carbon sources. As noted already by Gustafsson & Ryde (2000),
however, the GCE model calculations of Prantzos et al.
(1994), with contributions added also from the intermediate- and low-mas stars according to
Renzini & Voli (1981), fit the recent observation results (Tomkin et al. 1995;
Gustafsson et al. 1999 and this work) rather well, although
the C/O ratios of comparatively metal-rich halo stars become too high. In that model, a significant amount
of the carbon in the disk stars was contributed by intermediate- and low-mass stars.
If low-mass stars produce significant amounts of carbon as reviewed by Busso et al. (1999),
the discrepancy for the metal-rich halo stars would diminish, but they may produce too much carbon at late stages. This conflict could possibly be resolved by advocating more efficient carbon production by
low-mass metal-poor stars, as Marigo (2001) suggested that, for intermediate-
and low-mass stars, the carbon yields are larger at low metallicities, both because the third
dredge-up and hot-bottom burning are more efficient, and because TP-AGB lifetimes are longer.
The spread of [C/Fe] at [Fe/H]
is rather large. Does it reflect the intrinsic
scatter of the stellar [C/Fe]
ratios or can it, alternatively, be attributed to observational error? Additional observations
will be needed to examine the intriguing patterns.
In addition, the [C/Fe] ratio of the halo star HD97916 is
nearly solar, this result is in agreement with other halo stars investigated by Tomkin et al.
(1992).
Relative abundances of [N/Fe] as a function of [Fe/H] are shown in Fig. 4b. The result generally
confirms the published trend based on lower quality spectroscopic analysis (Larid 1985;
Carbon et al. 1987). However,
there is a N-rich metal-poor star HD106516A, which was found by Larid (1985) and this type
of star was discussed in detail by Beveridge & Sneden (1994).
The three metal-rich stars, namely HD108510, HD157347 and HD191862A, show unexpectedly low nitrogen
abundances
.
They are interesting because none of the present scenarios can explain
their low nitrogen abundances, and the previous discovery of these low nitrogen abundance focus on the
metal-poor stars as shown in Clegg (1977). The behavior of other elements and
kinematics for the three stars is not special.
![]() |
Figure 5: Relations of [C/O] and [N/O] vs. [Fe/H]. Symbols are same as Fig. 4. |
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![]() |
Figure 6: Relations of [C/Ba] and [N/Ba] vs. [Fe/H]. Symbols are same as Fig. 4. |
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The ratios of heavy elements are expected to reveal the
characteristics of stellar yields. Massive stars (
),
which become type II SNe, are responsible for the nucleosynthesis of oxygen and part of iron in disk stars
(Timmes et al. 1995),
while the less massive AGB stars are thought to be the primary source of barium.
The masses of AGB stars run up to about 8
,
but it is probably the lower mass
(about 1-3
)
and longer-lived stars that dominate the synthesis of elements like barium (Busso
et al. 1999).
There are two features that can be found in Figs. 5a and 6a. Firstly, the large spread
of [C/O] or [C/Ba] around [Fe/H]
-0.7 is evident. As oxygen is produced mainly by massive stars,
the steep slope of [C/O] may reflect the fact that less carbon is ejected from high metallicity massive
stars (Maeder & Meynet 1993; Portinari et al. 1998), while the decrease of [C/Ba] could
be due to large barium contribution from low-mass stars. Secondly, there is a slight increase in [C/O]
or [C/Ba] with increasing metallicity for -0.4 < [Fe/H] < +0.2, this behavior could be explained
by the large contribution of carbon, but no (or a slight) contribution of oxygen from low-mass stars,
which is in agreement with the model calculations by Marigo et al. (1996, 1998),
Forestini & Charbonnel (1997), Busso et al. (1999) and Marigo (2001);
or could instead be explained by the increase of carbon yields from massive stars with metallicity, as
suggested by Gustafsson et al. (1999).
We note, however, that, although the model calculations of Gustafsson et al. (1999) were in general
agreement with the observed relations for [C/O] vs. [Fe/H], the fit is not perfect. For example, for
[C/O] a value is predicted of about -0.4 at [Fe/H]
-0.6, this is not seen in their observations.
Alternatively, the increase in [C/Ba] may be due
to the decrease of barium abundances for more metal-rich stars (Chen00). In addition, there seems
to be a tendency for the halo stars to have low [C/O] ratios as compared with disk stars
(McWilliam 1997;
Gustafsson et al. 1999); this is the case for our halo star HD 97916.
Excluding the low nitrogen abundance stars, a gradual increase of [N/O] ratio with [Fe/H] can be found in Fig. 5b, reflecting the difference in time scale between oxygen and nitrogen producing stars. Oxygen is expected to be produced predominantly in the SN II explosion of short lived massive stars, while nitrogen is probably produced by intermediate-mass stars, as suggested by Timmes et al. (1995), Portinari et al. (1998) and Liang et al. (2001). Recently, Marigo (2001) have shown that the nitrogen produced by intermediate-mass stars with hot-bottom burning is of primary origin and that nitrogen production is favoured at lower metallicities. Liang et al. (2001) found that the primary nitrogen from intermediate-mass stars plays an important (possibly dominant) role in the nitrogen production (their Fig. 8b).
Although there is a large scatter, the [N/Ba] is about solar through the whole metallicity region explored in this paper (Fig. 6b), which may be explained by the common sources of nitrogen and barium. This suggestion is further supported by the N-rich stars, which are also rich in s-process elements (Beveridge & Sneden 1994). However, this argument is weakened by the possibility that the barium is favorably produced by intermediate metallicity low-mass stars (Busso et al. 1999).
Generally, no correlation between nitrogen abundances
and kinematics can be found in our samples (Fig. 7b).
![]() |
Figure 7: Rotation velocity vs. [C/Fe] and [N/Fe]. Symbols are same as Fig. 4. |
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Unexpectedly, carbon abundances of group A stars with larger
galactocentric distances slightly decrease with decreasing age. Tomkin et al. (1995)
found the same result, and suggested "it is a surprising possibility.''
A large sample of these type stars is needed to confirm this trend.
![]() |
Figure 8: Relation of [C/H] and [N/H] vs. age. Symbols are same as Fig. 4. |
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Wheeler et al. (1989) reviewed carbon abundances and concluded that [C/Fe]
0.0 independent of
[Fe/H] but with a possible increase in [C/Fe] below [Fe/H]
-1.5. Recent studies of carbon abundance
indicate that [C/Fe] is enhanced with declining [Fe/H] in the Galactic disk (e.g. Fried & Boesgaard
1992; Andersson & Edvardsson 1994; Tomkin et al. 1995; Gustafsson et al. 1999;
and this work). Thus, in the disk [C/Fe] and [
/Fe] show morphologically
similar trends with [Fe/H].
The threshold temperature of He burning and production of 12C via the triple alpha process is
K, a temperature that is reached in both massive (
)
and intermediate-mass
(
)
stars.
The main nucleosynthesis site of carbon has been argued for many years. Some authors suggested that the
main sources of carbon are intermediate- and low-mass stars (Wheeler et al. 1989; Timmes et al.
1995; Chiappini et al. 1997; Kobulnicky & Skillman 1998; Oberhummer et al.
2000). Others suggested that the massive stars with wind driven mass loss could be the main carbon
source during the whole Galactic evolution (Prantzos et al. 1994; Gustafsson et al. 1999;
Karlsson et al. 1998; Henry et al. 2000; Carigi 2000), while, some authors
argued that the carbon sources are still not clear (Garnett et al. 1999;
Gustafsson & Ryde 2000; Hou et al. 2000; Liang et al. 2001). Our observational results
show that high carbon abundances are found for group C stars. Considering their old ages,
intermediate- and low-mass stars have no time to contribute significant amounts of carbon. More probably
it comes from metal-rich Wolf-Rayet stars, which eject a significant amount of carbon into
the ISM by radiative-driven stellar wind at the early time of the disk evolution (Maeder 1992).
Moreover, the steep decrease of
[C/O] at [Fe/H]
-0.7 could indicate that less
carbon is ejected by massive stars at this metallicity (Maeder & Meynet 1993;
Portinari et al. 1998). The
increase of [C/O] or [C/Ba] with increasing [Fe/H] for -0.4 < [Fe/H] < +0.2 could be due to
the increase of carbon yields from massive stars with metallicity, as already suggested by Gustafsson et al.
(1999); or could instead be due to a large carbon contribution from low mass stars.
Although carbon abundances of the most metal-poor disk stars may resemble the
element
pattern, the abundance trends are quite different and uncertain. It is clear that
further study is required to understand [C/Fe] as a function of [Fe/H].
Nitrogen abundances have been obtained by Clegg et al. (1981), Tomkin & Lambert (1984), Laird (1985) and Carbon et al. (1987) with the result that [N/Fe] is essentially solar, irrespective of the metal content of the star, but with considerable scatter. Our preliminary analysis confirms previous results that [N/Fe] is nearly constant in a wide range of [Fe/H].
Like carbon, nitrogen production via the CNO cycles may occur in either massive or intermediate mass
stars. However, discovering the origin of nitrogen is further complicated by the fact that the seed carbon
needed for its production may either have been present when the star was born or is synthesized within the
star during its lifetime.
The main nucleosynthesis site of nitrogen was discussed by Vila-Costas & Edmunds (1993),
Timmes et al. (1995), Henry et al. (2000), and Liang et al. (2001), they
suggest that nitrogen is produced principally in intermediate-mass stars of
.
A gradual increase of [N/O]
with [Fe/H] for the disk stars found in this work may confirm this suggestion.
The primary nitrogen from intermediate-mass stars may also play an important role in
the nitrogen production.
Combined with models of galactic chemical evolution, our results allow us to throw some light on the
Galaxy evolution. As already discussed by Chen00, group C stars are most like thick disk stars
(very old, metal-poor and low
), for which Fuhrmann (1998, 2000) found a similar
[Mg/Fe] to the halo stars, however, the behavior of [C/Fe] is
quite different between the thick disk and the halo stars at the same [Fe/H]. This result favors the
suggestion that the halo and thick disk have their own distinct evolution paths
(Gilmore 1995; Fuhrmann 2000).
In order to confirm the present results for carbon and nitrogen, it is necessary to investigate a large sample of stars with various metallicities, especially for the metal-poor stars of the thick disk and the halo, which will be an important step in improving our present understanding of stellar nucleosynthesis and the chemical evolution of the Galaxy.
Acknowledgements
We are very grateful to Prof. Thomas Gehren for his valuable comments and many stimulating discussions. The authors are grateful to Dr. Johannes Reetz for providing the code SIU for synthetical spectrum computations. We are thankful for the suggestions of an anonymous referee that much improved the final version of this paper. This research was supported by the National Natural Science Foundation of China under the grant No. 19725312 and NKBRSF 1999075406.