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Astron. Astrophys. 357, 931-937 (2000)

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2. Observational data and evolutionary status

2.1. Working sample

Our analysis is based on the observational data from Paper I. This sample is composed of 120 Pop I subgiant stars with F, G and K spectral types which belong to the "Catalogue of rotational and radial velocities for evolved stars" (De Medeiros & Mayor 1999), as well as to the Bright Star Catalogue (Hoffleit & Jaschek 1982). We thus use the rotational velocities given in Paper I, as well as the values derived for log g , ALi and Teff with their respective errors.

2.2. Evolutionary status

In order to interpret the observations accurately, we need to know the mass and the evolutionary stage of the sample stars. We use the HIPPARCOS (ESA 1997) trigonometric parallax measurements to locate precisely our objects in the HR diagram. Among our 120 stars, only one object (HD 144071) has no available Hipparcos parallax and is thus rejected from further analysis. Intrinsic absolute magnitudes [FORMULA] are derived from the parallaxes and the [FORMULA] magnitudes given by Hipparcos. We determine the bolometric corrections BC by using the Buser & Kurucz's relation (1992) between BC and V-I (again taken from the Hipparcos Catalogue). Finally, we compute the stellar luminosity and the associated error from the sigma error on the parallax. The uncertainties in luminosity lower than [FORMULA] have an influence of [FORMULA] in the determination of the masses. We show the results of these determinations in Fig. 1 and in Table 1. This table displays the stellar masses and luminosities derived for all objects of our sample. Moreover complementary data used for the present analysis (Vsini , Teff , log g , [Fe/H] and ALi ) can be found in Papier I (see Tables 1 to 3). The error adopted on [FORMULA] ([FORMULA]K) is typical for this class of stars, as already discussed in Paper I.

[FIGURE] Fig. 1. Distribution of the sample stars in the HR diagram. Luminosities and related errors have been derived from the Hipparcos parallaxes. The typical error on Teff is [FORMULA] 200 K (Paper I). Evolutionary tracks at [Fe/H]=0 are shown for stellar masses between 1 and [FORMULA]. The turnoff and the beginning of the ascent on the red giant branch are indicated by the dashed and dotted lines respectively in order to discriminate dwarfs, subgiants and giants


[TABLE]

Table 1. Derived masses and luminosities for our program stars


We have computed evolutionary tracks with the Toulouse-Geneva code for a range of stellar masses between 1 and 4 [FORMULA] and for different metallicities consistent with the range of our sample stars (see Paper I). However, solar composition being relevant to most of the objects of our sample (about 65%), only tracks computed with [Fe/H]=0 will be displayed in the figures. The evolution was followed from the Hayashi fully convective configuration. We used the radiative opacities by Iglesias & Rogers (1996), completed with the atomic and molecular opacities by Alexander & Ferguson (1994). The nuclear reactions are from Caughlan & Fowler (1988) and the screening factors are included according to the prescription by Graboske et al. (1973). No transport processes except for the classical convective mixing (with a value of 1.6 for the mixing length parameter) are taken into account.

2.3. Discrimination between dwarfs, subgiants and giants among the sample

In Fig. 1 we compare the observational HR diagram with the evolutionary tracks computed with [Fe/H] = 0. The dashed line indicates the evolutionary point where the subgiant branch starts and which corresponds to the hydrogen exhaustion in the stellar central regions (i.e., turnoff point). About 30 stars are located below the turnoff line and therefore appear to be genuine dwarfs, although classified as subgiants in the Bright Star Catalogue. On the other hand, about 15 stars located on the right side of the dotted line have started the ascent of the RGB and are thus considered as giants.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000
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