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2. Near infrared observations

The near infrared observations have been carried out with an InSb photometer built by Observatoire de Paris and modified by the Institut d'Astrophysique de Paris and the Bureau des Longitudes to include a CCD Thomson camera in the focal plane that allows field recognition and provides approximate photometric measurements in the R and I bands. Standard JHKL filters were used. The field aperture of the IR photometer represents 30'' on the sky and the beamswitching is made in east-west direction at a frequency of 10 Hz, with a beamthrow of 45''. This instrument was attached at the f/15 cassegrain focus of the 80 cm telescope of Observatoire de Haute Provence, France (OHP) during 4 periods in 1992 and 1993 (Table 1). The CCD camera was only available in periods 2 and 3 and was only used for field recognition.

 table216
Table 1: Journal of observations

To pick up the IRAS source, the telescope was pointed by offseting from nearby stars of the Guide Star Catalogue (Space Telescope Science Institute, 1992, hereafter GSC) and the objects were searched for in the K band. Owing to the large field aperture, in most cases, the infrared sources were immediately detected. In a few peculiar cases, however, the CCD image obtained in real time was used to identify the location of the IRAS sources with respect to the field stars. Their optical counterparts, if visible, were identified by simple comparison between GSC charts and the observed field images (Fig. 1). In addition, these CCD images (typically, the tex2html_wrap_inline947 limit of detection is about 16-17 mag) allow to check a possible confusion with closeby field stars that might influence the IR photometric measurements.

 figure225
Figure 1: Identification charts (tex2html_wrap_inline949) of IRAS 20072+3116 and IRAS 21076+2820. Dots represent the GSC stars. Zooms of tex2html_wrap_inline951 around the IRAS positions are plotted at the bottom right of these diagrams. The GSC positions are represented by crosses, the IRAS positions by their ellipses of uncertainty, and the near-IR sources by dot-circles. In both cases, the near-IR positions and the IRAS ellipses coincide. One notice that 21076+2820 is associated with a GSC star although 20072+3116 is not

The JHKL photometry was derived with the help of frequent measurements of standard stars (Blackwell et al. 1979; Engels et al. 1981; Koornneef 1983; Campins et al. 1985) at similar air-masses and as close as possible to the observed source. An average accuracy of +/- 0.05 magnitude in the JHK bands and +/- 0.1 magnitude in L was achieved most of the time for objects brighter than JHK = 7 and L = 3.5.

The new infrared photometric data, are summarized in Table 2, where Col. 1 gives the IRAS name, Col. 2 refers to the epoch of the observations, Cols. 3 to 6 list the photometric data, Cols. 7 to 9 the IRAS coloursgif and LRS types, Cols. 10 and 11 the classification in our system (Epchtein et al. 1988) and in the VH system, respectively. Column 12 contains flags that indicate that the objects have an entry in one or several of the following catalogs: the GCCCS (flag C), "a general catalog of S stars" (Stephenson 1976) (S), the GSC (G), another stellar catalog (I) quoted in the IRAS PSC, or have been detected in one or several radio lines: CO/HCN (Loup et al. 1993), Htex2html_wrap_inline967O maser (Comoretto et al. 1990), OH maser (te Lintel-Hekkert et al. 1989) (M). More detailed information is given in Table 3.

Despite our large aperture, none of the measurement seems to be significantly contaminated by the emission of a nearby star. Out of the 84 stars observed when the CCD camera was available, only 9 of them, namely, 03118+6144, 03174+5247, 06234+0612, 06360+3335, 19122+2318, 20072+3116, 21131+3215, 21254+5709, 21309+6507 have a field star located at less than 15'', but, in all cases they are much fainter than the main source in the near-IR. This has been systematicallly checked by centering the photometer successively on the 2 objects and comparing the 2 fluxes. In two cases the fluxes may, however, have been contaminated by a nearby star, i) a GSC star of tex2html_wrap_inline971 is located at 19'' of the IRAS position of 05581+2232, ii) 06523+2437 is close to 2 GSC stars.


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