Astron. Astrophys. 364, 683-688 (2000)

2. Observations and data reduction

2.1. Observations from Mt. Abu

IRAS 05361+3539 (hereafter IRS1) was observed from the 1.2 meter Infrared Telescope, Mt. Abu, India, using a 256256 HgCdTe array (NICMOS-3, made by Infrared Laboratories, Arizona, USA). The telescope is located at an elevation of 1700 meters from the mean sea level and the instrument is described in Nandakumar (2000). Both the telescope and the NIR camera are owned by the Physical Research Laboratory, Ahmedabad, India.

The source was observed in two sessions: in J, H and K' bands on 10th January 2000, and in narrow band filters centered on 2.12µm (H₂ ), 2.16µm (Br), and 2.14µm (continuum) on 25th February 2000. The FWHM of the narrow-band filters was 0.042µm. The plate scale was 1" / pixel for the broad-band images and 0.5"/ pixel for the narrow-band images. The nights were photometric during the observations. The seeing was 2" during the broad band observations and 1.5" during the narrow band observations. A large number of dithered sky frames were obtained (by shifting the telescope 3 arcmin off the source in north-south-east-west directions) in all the filters for sky subtraction and for making flat frames. Although the 2.14µm filter is supposed to exclude the nearby emission lines, we found from filter transmission curves that there is serious contamination from the 2.12µm line. We therefore will not consider the 2.14µm line image in this paper and will present the narrow band images as emission line plus continuum. We observed three standard stars (AS11, AS19, and AS21 from Hunt et al. 1998) during the observations. We used the relation given by Wainscoat & Cowie (1992) to obtain K' magnitudes from K magnitudes.

The data reduction was done using IRAF software tasks. All the NIR images went through standard pipeline procedures like sky-subtraction and flat-fielding. Individual object frames were of 30s of integration in J and H band and 3s in K' band. The images were co-added to obtain a final image in each band (J, H and K') of total integration time of 150s. Further, in each band two such images were obtained. The images were further filtered using a median filter of 33 pixels for removing noise at sky-level, thus making them suitable for photometric analysis. We used DAOPHOT (Stetson 1987) task for deriving the photometry in the unvignetted field of view of 2 arcmins. The zero point was obtained using the three observed standard stars. From the J, H and K' images containing the IRAS source, stars were identified down to 3 level (peak signal), subsequently each frame was visually inspected at different contrast levels to cross check the detection and remove false detections by DAOFIND. Aperture photometry was performed on the images using PHOT task with aperture radius of 4.5 pixels in the J, H and K' images. The sky was sampled using 5-pixel wide annuli centered on each star with inner radius at 5 pixels. The aperture size was decided using the brightest and isolated star. From the two images of 150s in each band, we found that the overall photometric error is of 0.07 mag. This estimation includes uncertainties in the determination of zero-point ( 0.03 mag) from the observed three standard stars. We found that in the K' images we could detect 12 stars up to 14.2 mag in the field of view of 2´ using the above mentioned procedure.

The narrow-band images also went through similar image processing. The total integration time was 250s in H₂ and Br filters.

2.2. 2MASS data

We extracted stars from the 2MASS point source catalogue which were within 8´ diameter of the IRAS source. The data were downloaded from the 2MASS Homepage available free for the Astronomical community. The 2MASS observations were carried out on 3rd February 1998 ¹. The 2MASS point source catalogue consists of J, H and Ks magnitudes of stars. The Ks band (bandpass =2.00µm to 2.32µm with center at 2.17µm) is very similar to K band. A histogram of the 2MASS sources in the Ks band within a field of view of 8´ around the IRAS source was plotted. The histogram showed that the completeness limit is close to 15 mag.

Since the 2MASS catalogue goes deeper than the Mt. Abu images we decided to use the catalogue magnitudes to plot color-color and color-magnitude diagram. However, the magnitudes obtained from the Mt. Abu images will be used to find prospective variable stars and to study the morphology of the IRAS source and associations.

Table 1 shows a comparison of J,H,K' and Ks magnitudes of stars detected within the unvignetted field of 2´ in the K' image with the 2MASS point source catalogue. We found that for standard stars the difference in K' and Ks magnitudes is less than 0.05 which is less than the photometric error. Hence we will treat the K' and Ks magnitude scales as similar in this paper.

Table 1. A comparison of stellar magnitudes from 2MASS data and Mt. Abu images. The 2MASS upper limit magnitude was derived from the histogram of stars within the FOV of 8´ towards the IRS1.

© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001
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