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Astron. Astrophys. 360, 509-519 (2000)

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2. Observational material

Table 1 summarizes some parameters characterizing the two open clusters NGC 2360 and NGC 2447 studied in this paper. The distance modulus has been re-determined using Mermilliod's database (1995 , 1999) and the age is determined with the theoretical isochrones of Schaller et al. (1992). They lead to turn-off masses of 1.98 and [FORMULA] for NGC 2360 and NGC 2447, respectively.


Table 1. Adopted parameters for NGC 2360 and 2447, including the iron abundance [Fe/H] and metallicity [M/H] obtained in this paper (Sect. 4). The mass [FORMULA] at the turn-off (defined as point T in Fig. 1 of Maeder & Meynet 1991) is given too.

The seven red giants studied in this paper have been selected on the basis of their constant radial velocity, as determined by CORAVEL observations, in order to ensure that they are single stars. This point is important since the light of a secondary component could bias the equivalent widths used to estimate the abundances of the primary.

For each star, one echelle spectrum has been obtained by DR in November 1995 using the EMMI spectrograph attached to the NTT 3.5m telescope at the European Southern Observatory in La Silla, Chile. The grism #5 and the echelle grating #10 were used, yielding spectra in the wavelength range from 4050Å to 6650Å with a resolving power of [FORMULA]. The detector was the ESO CCD #36, with [FORMULA] pixels. The signal-to-noise ratio per pixel of the extracted spectra varies from 54 to 226, depending on the wavelength range and exposure time.

The journal of the observations is given in Table 2, together with the main characteristics of each stars (identification, V magnitude, Geneva [FORMULA] color index, S/N ratio, Julian date, exposure time, radial velocity derived from our spectra and average RV from CORAVEL observations). The radial velocities are determined from our spectra by cross-correlation with a CORAVEL-type binary mask optimized for the K0 spectral type. They are compatible with those determined earlier from CORAVEL observations by Mermilliod & Mayor (1989 , 1990), confirming that the red giants of our sample are very probably single.


Table 2. Observational data of the 7 red giants and signal-to-noise ratio per pixel of their extracted spectra. The V magnitude and [FORMULA] index are those of Geneva photometry. The adopted numbering is that of Becker et al. (1976), but we also give the numbers in the system of Eggen (1968) for NGC 2360. There is no other numbering system for NGC 2447, but the map of this cluster is available on the web (Mermilliod 1999).The radial velocities of the 7 red giants and of two reference stars are also given, as determined in this work ("EMMI") at the indicated Julian dates. The exposure times are also listed. The CORAVEL RV values ("COR.") of the cluster stars are average ones from Mermilliod & Mayor (1989 , 1990), while those of the reference stars (IAU standards) are taken from Udry et al. (1999). The errors quoted for the CORAVEL observations are related to the average RV values; the last column ("N") gives the number of CORAVEL measurements.

Observations of two IAU RV standards, HD 66141 and HD 80170 (of which Udry et al. 1999give the precise RV value from CORAVEL observations), show that our results can be trusted to within [FORMULA] km s-1. To reach this accuracy, we had to monitor the instrumental drift by taking a few short Th-Ar calibration exposures during the night and correlate them with a longer calibration exposure taken in the preceding afternoon; then we linearly interpolated the instrumental velocity shifts for the epochs of the science exposures.

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Online publication: August 17, 2000