2 ISOGAL observations

General characteristics of the ISOGAL survey have been described extensively by others (Omont et al. 2002; Schuller et al. 2002; Omont et al. 1999a,b,c; Pérault et al. 1996) and are just briefly discussed in this section.

ISOGAL observations selected for this study are those close to the Galactic Centre, obtained by using the narrow-band filters LW5 ( ${\lambda}_{\rm ref}=6.8~\mu$m, $FWHM = 0.5~\mu$m) and LW9 ( ${\lambda} _{\rm ref}=14.9~\mu$m, $FWHM = 2~\mu$m). Reduction of the data was carried out similarly as described by Schuller et al. (2002), Glass et al. (1999) and Omont et al. (1999c). After reduction, the frames were in units of ADU/gain/sec, which in turn were converted into flux density using the following relationships:

$$F({\rm mJy}) = ({\rm ADU/gain/sec})/0.35$$ (1)

for LW5 and

$$F({\rm mJy}) = ({\rm ADU/gain/sec})/0.65$$ (2)

for LW9 (Blommaert 1998), which are conversion factors assuming a $\nu \times f(\nu ) = {\it constant}$ spectral shape. Colour-corrections for the narrow LW5 and LW9 filters amount to less than 1 percent for spectral index in the range -3.0 to +3.0 and have not been applied in the present study. The zero-magnitude fluxes for LW5 and LW9 are 81.8 and 17.7 Jansky respectively.

Table 1 summarizes the log-book of the observations. Cross-identification of OH sources observed by VLA provides an opportunity to check out the astrometry accuracies of infrared and radio observations. ISOGAL astrometry has been calibrated by cross-correlation with the DENIS survey (Simon et al. 2002; Epchtein et al. 1997), which in turn has its astrometry calculated from the USNO-A2 catalogue (Monet et al. 1998). The accuracy of the astrometry varies from field to field, and depends on several factors, mainly on the source density in the field, but in all cases is better than 2 arcsec. On the other hand, despite the fact that interferometric radio observations can provide relative positional accuracies to a fraction of an arc second, the absolute positions are not expected to be accurate at the one arc second level (Sjouwerman et al. 1998). A closer inspection of Table 2 of Sjouwerman et al. reveals systematic deviations between the three configurations arrays used, a, b (VLA), and c (ATCA). We have determined the offsets of these configurations relative to the astrometry of Lindqvist et al. (1992a), by comparing positions of the sources observed by both authors. Figure 1 shows the offsets found for each configuration. Sjouwerman's astrometry used in this paper has been corrected applying the following corrections: ${\alpha}_{\rm corr} = \alpha _{\rm Sj}-off({\alpha })$ and ${\delta}_{\rm corr} = \delta _{\rm Sj}-off({\delta })$, where $off({\alpha })$ equals to 0.5, 0.5 and 1.6 arcsec for a, b and c configurations and $off({\delta })$ are 4.2, 1.7 and -0.3 arcsec, respectively. It is also important to mention that positional discrepancies up to several arcseconds are reported in a few cases, even after after applying these corrections, as can be seen in Fig. 1.

The identification of ISOGAL counterparts of OH sources has been based essentially on the position criterion. A search for ISOGAL counterparts has been made in the [LW9] band, inside a 10$\arcsec$ radius around the radio position of double-peaked OH sources. Misidentifications are unlikely to occur, because OH/IR stars are very bright in the mid-infrared and can be easily distinguished from normal field stars. Generally, for each ISOGAL field many OH/IR stars have been identified, based on their VLA-radio position (e.g. 39 OH/IR stars identified in the TDT-82700140 observation, see Table 1).

Among the 114 OH sources searched, only 4 sources were left without a counterpart in the [LW9] and/or [LW5] band. Two of them, Li052 (OH 359.880-0.087) and Sj019 (OH 359.875-0.091), are separated by only $\sim$20$\arcsec.$ The OH spectra, double peaked in both cases, and with different central velocities, -24.4 and 19 km s^-1, suggest that those OH emissions belong to two separate OH/IR stars.

Sj019 is probably accidentally detected as H₂O maser H₂O 359.89-0.07 by Lindqvist et al. (1990), at a velocity of 29 km s^-1 which is consistent with the OH maser velocity. Thus Sj019 is expected to be a star.

Li52 (OH 359.880-0.087) was monitored by van Langevelde et al. (1993) who find the period of 759 days. Blommaert et al. (1998) find a very faint near-infrared counterpart for this OH source, showing K=18.67 and $L_{\rm short}=14.48$, which is by far the faintest object in the $L_{\rm short}$ band ( $\lambda = 3.45~\mu$m) in their sample. The inspection of the images at 6.8 and 14.9 $\mu$m reveals the presence of a dark cloud in the same direction.

Sj069 (OH 0.005+0.360) was detected at 1612 MHz with the VLA by Sjouwerman et al. (1998). Its spectrum shows a single peak at the velocity of -71.2 km s^-1. Possibly the OH emission is of interstellar origin.

Sj102 (OH 0.317-0.066) (Sjouwerman et al., op. cit.) is moderately bright at 1612 MHz and it is double-peaked as well, showing $V_{\rm exp}=13.1$ km s^-1. It was detected by ISOGAL at $\lambda = 6.8~\mu$m, but not at $14.9~\mu$m. The inspection of the images at 6.8 and 14.9 $\mu$m reveals the presence of extended emission at the position of Sj102. The faint source extracted at 6.8 $\mu$m might well be a background fluctuation and not a real point source.

Figure 1: Astrometric deviations: Sjouwerman - Lindqvist coordinates. The offsets for a, b (VLA), and c (ATCA) configurations are shown. See Sjouwerman et al. (1998) for a description of the a and b modes.

 

 

Table 1: ISOGAL Fields in which counterparts of the OH maser sources have been found. The TDT names are the usual basic reference to individual ISO observations. The names of the fields are the coordinates of its center; $\Delta l$ and $\Delta b$ are the approximate semi-widths of the fields; and N is the number of counterparts found in the field.

		LW5 ( $\lambda = 6.8~\mu$m) band				LW9 ( $\lambda = 14.9~\mu$m) band
Field	#	TDT name	$\Delta l (^{\rm o})$	$\Delta b (^{\rm o})$	Obs. Date	TDT name	$\Delta l (^{\rm o})$	$\Delta b (^{\rm o})$	Obs. Date	N
-01.12-00.33	1	-	-	-	-	31300313	0.31	0.19	24-Sep.-1996	1
-00.90-00.03	2	31300837	0.30	0.17	25-Sep.-1996	83800857	0.20	0.17	02-Mar.-1998	4
-00.81-00.15	3	-	-	-	-	84300221	0.18	0.07	07-Mar.-1998	1
-00.81-00.15	4	-	-	-	-	49101221	0.18	0.07	21-Mar.-1997	1
-00.62-00.06	5	31300236	0.16	0.17	24-Sep.-1996	83600308	0.11	0.17	28-Feb.-1998	4
-00.34+00.18	6	-	-	-	-	31300901	0.39	0.09	25-Sep.-1996	14
-00.27-00.03	7	-	-	-	-	83801051	0.03	0.08	02-Mar.-1998	3
-00.27-00.03	8	31300135	0.16	0.17	24-Sep.-1996	82700140	0.16	0.17	19-Feb.-1998	39
+00.00+01.00	9	83600419	0.15	0.06	28-Feb.-1998	83600524	0.15	0.06	28-Feb.-1998	1
+00.04+00.40	10	-	-	-	-	13600318	0.40	0.13	01-Apr.-1996	4
+00.05-00.24	11	83600855	0.16	0.13	28-Feb.-1998	83600856	0.16	0.13	28-Feb.-1998	15
+00.34-00.05	12	31300734	0.14	0.15	25-Sep.-1996	82800341	0.13	0.15	20-Feb.-1998	18
+00.37+00.17	13	84100143	0.30	0.09	05-Mar.-1998	13600503	0.30	0.09	01-Apr.-1996	9
+00.59+00.02	14	31300433	0.14	0.15	24-Sep.-1996	83800712	0.13	0.09	02-Mar.-1998	2
+00.62-00.14	15	31300433	0.14	0.15	24-Sep.-1996	84100259	0.12	0.07	05-Mar.-1998	4