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Astron. Astrophys. 333, 613-618 (1998)

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2. Sample and observation

All S stars observed in JHK bands either with or without Tc are selected from Groenewegen (1993). Most of Tc-deficient stars (20, out of 24 from his Table 2) and a majority of Tc-rich stars (24, out of 36 from his Table 1) were observed in the JHK bands according to the condition and location of the telescope used.

Near infrared observations were made during 1993-1994 using the InSb photometer attached to the 1.26 m infrared telescope at the Xinglong Station, Beijing Astronomical Observatory, China. The InSb detector and filters in JHK bands were cooled at 77 K by liquid nitrogen. The JHK photometric system which is very close to the standard one is listed in Table 1. In order to reduce the influence of local changes in atmospheric conditions, all observations were made near the zenith. During observations, atmospheric extinction coefficients at JHK bands were obtained each night by measuring a standard star at different zenith distances. Following Chen et al. (1987), the correction for interstellar reddening was estimated for every star, although it is quite small for most of them. The accuracy of the photometry is, on the average, 0.05, 0.04 and 0.05 for J, H and K, respectively. The standard stars used are listed in Table 2.


[TABLE]

Table 1. The JHK photometric system used



[TABLE]

Table 2. Standard stars for near infrared observation


The final sample consists of 20 stars without Tc and 24 ones with Tc, and their magnitudes in JHK bands with observing date are listed in Tables 3 and 4 respectively. The star number from Stephenson (1984, hereafter GCGSS) and the GCVS name or HR/HD/BD number are also given. The IRAS associations for all stars observed have been taken from Chen et al. (1995), and only good quality fluxes ([FORMULA]) in the IRAS bands are taken from the PSC. The IRAS LRS classifications are taken from Chen & Kwok (1993). In the Tables, B or V magnitudes from the GSC Catalog are also presented. In the last column of Tables 3 and 4 the symbol "[FORMULA] " and "[FORMULA] " denote that there were no K and JHK observations appeared in the Catalog of Infrared Observations (Gezari et al. 1993), respectively, and symbol "V" indicates some difference of more than 0.2 compared with previous data from Gezari et al. (1993).


[TABLE]

Table 3. Observed S stars with Tc



[TABLE]

Table 4. Observed S stars without Tc


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© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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