2. Observations

The results presented here include the near infrared photometry carried out in the standard J (1.25 m), H (1.65 m), K (2.2 m), and in some cases, L' (3.8 m) and M (4.8 m) bands during different observing runs both with the 1.55 m CST telescope operated by the Instituto de Astrofísica de Canarias at the Spanish Observatorio del Teide (Tenerife, Spain), and with the 1 m ESO telescope at the Observatorio de La Silla (Chile) since May 1989 until December 1993. The log-in of the observations is shown in Table 1 (click here), where we also quote the number of objects observed during each run, together with the number of objects detected (in brackets) in each case.

 

Run Period Telescope IRAS sources

(1) May 23, 1989 - June 5, 1989 1.5 m CST 28 (26)

(2) November 26, 1989 - December 4, 1989 1.5 m CST 6 (6)

(3) April 18, 1990 - April 24, 1990 1.5 m CST 19 (17)

(4) May 6, 1990 - May 11, 1990 1 m ESO 64 (56)

(5) June 19, 1990 - June 25, 1990 1.5 m CST 65 (48)

(6) March 19, 1992 - March 25, 1992 1 m ESO 31 (29)

(7) May 16, 1992 - May 22, 1992 1 m ESO 23 (21)

(8) October 15, 1992 - October 21, 1992 1.5 m CST 10 (9)

(9) December 1, 1993 - December 7, 1993 1.5 m CST 52 (42)

 

At both telescopes we used infrared photometers equipped with InSb photovoltaic detectors, operating at the temperature of liquid nitrogen, with a photometric aperture of 15'' and a chopper throw of 20'' in R.A. direction. The Teide photometric system is described in Arribas & Martínez-Roger (1987), as well as its relations with other standard photometric systems. The ESO photometric system is described in Bouchet et al. (1991). For flux calibration we used the list of standard stars given by Koornneef (1983) in the case of those stars observed at the CST while, while for those observed at the 1 m ESO telescope we used the standard stars included in Bouchet et al. (1991). Several standard stars were observed at least twice each night at different air masses to determine the atmospheric extinction in each filter.

Prior to our observations, we searched the ESO or Palomar blue and red prints looking for the presence of possible optical counterparts. Usually only one candidate was found inside the IRAS error ellipse but sometimes, specially towards the Galactic Center, several objects were observed and, in other cases, nothing was seen around the IRAS position. When several possible optical counterparts are found in a goven field, a good method to determine the which one is the best candidate is to compare the blue and red prints, since one should expect these stars to be strongly reddened as a consequence of the dust present in their circumstellar envelopes. Unfortunately, as we have already mentioned, this is not always valid. If no optical counterpart is observed, we chose a reference star nearby, bright enough to be detected on the TV screen at the telescope, and calculate the blind offset necessary to move the telescope to the IRAS position.

Once at the telescope we made raster scans wide centered on the IRAS position in the K band resulting in an 80% of positive detections. The limiting magnitude is estimated to be between 11th and 13th with this method at both telescopes depending on the atmospheric conditions. Usually a single bright near infrared counterpart was found in each field, in most cases coincident with the best candidate previously determined through the visual inspection of the ESO or Palomar prints. If more than one near infrared source was detected, we always measured that closest to the original IRAS coordinates. Mean discrepancies between the IRAS coordinates and the position of the near infrared counterparts found are around 16'' in right ascension and 8'' in declination. In a few cases, however, our identification is more doubtful, since the near infrared counterpart was found at distances of around 1' from the IRAS position. In these cases, one must take into consideration other circumstances such as, for instance, the characteristics of the optical spectrum, if available, or whether the near infrared and optical brightness are consistent with the properties observed in other spectral ranges, as we will discuss later. It is important to remark that, although we cannot rule out the possibility of having observed spurious sources in a few cases, the probability of this is very small and we are confident that this does not affect the statistical conclusions derived in this Paper.

In the absence of problems, and once the near infrared counterpart was determined, we performed the photometry in the standard J, H and K filters. L' and M were used only in the case of very good atmospheric conditions (humidity below 40%) and when the object was bright enough (or red enough) to expect a positive detection in these two filters.