2 Observations, stellar parameters and model atmospheres

Our results are based on spectra observed mostly by Klaus Fuhrmann and in part by Andreas Korn and the late Michael Pfeiffer using the fiber optic Cassegrain échelle spectrograph FOCES fed by the 2.2 m telescope at the Calar Alto observatory during 10 observing runs in 1995-2000. The data cover an approximate spectral range of 4000-7000 Å. In total, our sample now includes 63 stars: 27 stars from our previous work (Paper I), 24 stars newly observed in January and May 2000, and 12 thin disk stars observed earlier and added to cover as best as possible the metallicity range of the thin disk. Table 1 lists all the new stars plus 19 stars from Paper I for which Sr abundances were determined. Almost all of the stars were observed at least twice. For the 1995 spectra (11 stars of our old sample) the resolving power was $\sim$$40\,000$ and the later spectra of 52 stars were observed at $\lambda/\Delta\lambda \sim 60\,000$. The signal-to-noise ratio is $\sim$200 in the spectral range where the Ba II  $\lambda4554$, $\lambda5853$ and $\lambda6496$ lines are located and $\sim$100 in the range of the Eu II $\lambda$4129 and Sr II $\lambda$4161, $\lambda$4215 lines.

We use spectra reduced according to the description given in Pfeiffer et al. (1998). Stellar element abundances are derived from line profile fitting and the instrumental profile is found from comparison of FOCES Moon spectra with the Kitt Peak Solar Flux Atlas (Kurucz et al. 1984). Observations are well fitted by a Gaussian of different values for different observing runs (i.e. from 3.2 kms^-1 to 5 kms^-1).

 

 

Table 1: Stellar parameters of the selected sample. Most of the entries are self-explanatory. $V_{\rm mic}$ is given in kms^-1.

HD/BD	$T_{\rm eff}$	$\log g$	$V_{\rm mic}$	[Fe/H]	[Ba/Fe]	[Eu/Fe]	[Sr/Fe]
3795	5370	3.82	1.0	-0.64	0.02	0.56	0.04
4614	5940	4.33	1.0	-0.30	0.03	0.11	0.00
10519	5710	4.00	1.1	-0.64	-0.05	0.38	0.06
10697	5610	3.96	1.0	0.10	-0.01	-0.08	-0.17
18757	5710	4.34	1.0	-0.28	-0.11	0.25	-0.12
19445	6060	4.44	1.4	-1.99	-0.13	-	-0.13
22879	5870	4.27	1.2	-0.86	-0.02	0.42	0.07
30649	5820	4.28	1.2	-0.47	-0.10	0.32	-0.10
30743	6300	4.03	1.6	-0.45	-0.02	0.13	-
37124	5610	4.44	0.9	-0.44	-0.12	0.30	-0.11
43042	6440	4.23	1.5	0.04	0.00	-0.02	0.10
45282	5280	3.12	1.4	-1.52	-0.07	0.60	-0.08
52711	5890	4.31	1.0	-0.16	0.01	0.05	-0.02
55575	5890	4.25	1.0	-0.36	-0.05	0.20	-0.10
58855	6310	4.16	1.4	-0.32	0.08	-	-
61421	6470	4.00	1.9	-0.01	-0.17	0.01	0.09
62301	5940	4.18	1.2	-0.69	-0.06	0.36	-0.02
64606	5320	4.54	1.0	-0.89	-0.09	0.47	-0.04
65583	5320	4.55	0.8	-0.73	-0.05	0.46	0.02
67228	5850	3.93	1.2	0.12	-0.06	-0.13	-0.06
68017	5630	4.45	0.9	-0.40	-0.12	0.28	-0.11
69611	5820	4.18	1.2	-0.60	-0.11	0.36	0.05
84937	6350	4.03	1.7	-2.07	0.00	-	-0.12
90508	5800	4.35	1.0	-0.33	-0.03	0.26	-0.12
102158	5760	4.24	1.1	-0.46	-0.13	0.34	-0.01
103095	5110	4.66	0.8	-1.35	0.00	0.55	-0.11
109358	5860	4.36	1.1	-0.21	-0.07	-	-
112758	5240	4.62	0.7	-0.43	-0.13	0.28	-0.12
114710	6000	4.30	1.1	-0.03	0.07	0.01	0.05
117176	5480	3.83	1.0	-0.11	-0.04	0.04	-0.14
121560	6140	4.27	1.2	-0.43	0.06	0.14	0.08
126053	5690	4.45	1.0	-0.35	-0.11	0.14	-0.10
130322	5390	4.55	0.8	0.04	0.03	-0.04	-
132142	5240	4.58	0.7	-0.39	-0.09	0.26	-
134987	5740	4.25	1.0	0.25	-0.12	-0.17	-0.10
142373	5840	3.84	1.2	-0.57	-0.06	0.23	-0.06
144579	5330	4.59	0.8	-0.69	-0.08	0.46	-0.04
157214	5735	4.24	1.0	-0.34	-0.13	0.34	-0.08
168009	5785	4.23	1.0	-0.03	-0.08	-0.08	-0.09
176377	5860	4.43	0.9	-0.27	0.14	-	-
179957	5740	4.38	0.9	-0.01	-0.06	0.03	-0.14
179958	5760	4.32	0.9	0.02	-0.04	0.00	-0.12
187923	5730	4.01	1.1	-0.17	-0.05	0.13	-0.10
188512	5110	3.60	0.9	-0.17	0.09	0.03	-0.10
194598	6060	4.27	1.4	-1.12	-0.03	0.58	-0.11
195019	5800	4.16	1.0	0.04	-0.03	0.00	-0.12
198149	4990	3.40	1.0	-0.14	0.04	0.01	-
201891	5940	4.24	1.2	-1.05	-0.05	0.42	-0.02
207978	6310	3.94	1.6	-0.52	0.00	-	-
209458	6080	4.33	1.1	-0.06	0.10	0.10	0.11
222794	5620	3.94	1.2	-0.69	-0.10	0.38	0.05
$2^\circ$3375	6200	4.31	1.4	-2.15	-0.18	-	-0.06

HD/BD	$T_{\rm eff}$	$\log g$	$V_{\rm mic}$	[Fe/H]	[Ba/Fe]	[Eu/Fe]	[Sr/Fe]
$0^\circ$2245	5630	3.85	1.2	-1.13	0.19	-	0.22
$34^\circ$2476	6330	4.03	1.8	-1.96	0.13	-	-0.21
$66^\circ$268	5340	4.60	0.9	-2.20	0.07	-	-0.11

As in Paper I we use stellar parameters determined mostly by Fuhrmann (1998,2001) spectroscopically: effective temperatures $T_{\rm eff}$ from Balmer line profile fitting, surface gravities $\log g$ from line wings of the Mg Ib triplet, metallicities [Fe/H] and microturbulence values $V_{\rm mic}$ from the Fe II line profile fitting. For three stars, HD19445, BD2$^\circ$3375 and BD34$^\circ$2476, we adopt the stellar parameters determined by Andreas Korn (2000) obtained with the same methods. All parameters are given in Table 1. The identification of the stellar population for all stars of our sample is from Fuhrmann (1998) and Bernkopf et al. (2001), based on the star's kinematics, $\alpha$-element enhancement and age.

For each star a line-blanketed LTE model atmosphere has been generated at given values of $T_{\rm eff}$, $\log g$, [Fe/H] and [$\alpha$/Fe], where [$\alpha$/Fe] is the relative abundance of the most abundant $\alpha$-process elements O, Mg and Si, which in cool stellar atmospheres contribute in significant amounts to the electron pressure. We assume that oxygen and silicon abundances follow magnesium and adopt [$\alpha$/Fe] = [Mg/Fe]. The [Mg/Fe] abundance ratios are taken from Fuhrmann (1998) and Bernkopf et al. (2001) analyses. Three aspects concerning model atmosphere calculations are worth mentioning

1.

The mixing-length parameter $l/H_{\rm p}$ was adopted to be 0.5;

2.

The opacity distribution functions (ODF) are interpolated from Kurucz' (1994) ODF tables for the proper stellar metallicities. In addition, they were scaled by -0.16 dex to reset the iron opacity calculated by Kurucz with $\log\varepsilon_{\rm Fe} = 7.67$ to the improved meteoritic value $\log\varepsilon_{\rm Fe} = 7.51$, which we believe to be the best representation of the solar mixture. We refer to abundances on the usual scale where $\log\varepsilon_{\rm H} = 12$;

3.

The b-f opacities were computed with solar abundances taken from Holweger (1979) and scaled according to the stellar metallicity. In addition, abundances of $\alpha$-elements O, Mg and Si were scaled by the stellar Mg/Fe ratio. We note that the additional electron pressure in $\alpha$-enhanced models results in a measurable weakening of spectral lines of dominant ionization stages such as Sr II, Ba II, Eu II. The effect increases with decreasing temperature. For example, an increase of $\alpha$-element abundances by 0.4 dex in a model atmosphere leads to Ba abundances obtained from stellar spectra increasing by about 0.1 dex.

In Paper I for stars with [Fe/H] $\leq -0.6$, model atmospheres were interpolated from a grid of $\alpha$-enhanced models with [ $\rm\alpha/Fe] = 0.4$, whereas those for thin disk stars referred to a grid of model atmospheres without any $\alpha$-enhancement. However, the observed [Mg/Fe] ratio can differ from 0.4 dex in metal-poor stars (for example, HD103095: $\rm [Mg/Fe] = 0.28$) and from 0 in thin disk stars (for example, HD117176: $\rm [Mg/Fe] = 0.08$). For this reason Ba abundances obtained in our present study may deviate by up to 0.04 dex from the corresponding values calculated in Paper I.