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Astron. Astrophys. 349, 605-618 (1999)

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2. Observations

The near infrared observations reported in this paper were carried out between 3 and 13 July 1998 using MAGIC, the near-infrared camera and spectrograph of the German-Spanish Astronomical Center on Calar Alto, Spain, mounted at the 1.23 m telescope. Additional observations in the visible could be obtained in an earlier run with the same telescope in Calar Alto, and spectroscopy of the central star in the visible was performed using the ESO New Technology Telescope in La Silla, Chile, in April 1999.

2.1. Near-infrared imaging and photometry

The images presented here are centered on the position of the star ioinizing the DR 18 nebula (hereafter referred to as "the central star"), [FORMULA], [FORMULA]. The pixel scale of MAGIC at the 1.23 m telescope is [FORMULA] pixel-1, with each individual [FORMULA] frame covering 23.9 arcmin2 on the sky. Individual sequences of images were obtained in each filter, with each sequence consisting of a raster of [FORMULA] points defining a [FORMULA] square grid. Sixty individual exposures of 1 sec were stacked together at each raster position. Upon completion of each raster, an area of the sky near DR 18 field but farther from the Milky Way was observed to obtain a sky frame. The sky frames were constructed by median averaging and deviant pixel clipping of images obtained on a five-point raster. After sky subtraction and flat fielding, the target frames were shifted to compensate for the telescope offset between raster positions and combined, with rejection of the highest and lowest pixel value at each point of the combined image to remove bad pixels and cosmic rays in the original images. Image reduction was performed using standard IRAF tasks and dedicated scripts.

In addition to the broad band observations in the J (1.25 µm), H (1.65 µm), and K (2.2 µm) filters, images were obtained as well in narrow band filters ([FORMULA]) centered on the wavelenght of the (1,0) H2 [FORMULA] line at 2.122 µm, the Br[FORMULA] line at 2.166 µm, and the (2,1) H2 [FORMULA] line at 2.248 µm. Moreover, observations were also obtained through a "CO-continuum" filter centered at 2.260 µm, used for another observing program executed during another part of the night. Having ([FORMULA]), the CO continuum filter actually includes the entire passband of the (2,1) H2 [FORMULA] filter. The total exposure times were 18 min for the J, H, and K filters; 36 min for the 2.122 µm and 2.248 µm filters; and 54 min for the 2.166 µm and 2.260 µm filters.

Digital photometry was carried out on the J, H, and K images in order to construct color-magnitude and color-color diagrams of the stars in the area, which may make it possible to identify very young stars possibly associated with the DR 18 HII region. The tasks in the DAOPHOT package layered on IRAF were used to identify the stars, adjust the point spread function, and perform digital photometry. The instrumental photometry was calibrated by observing the nearby infrared standard HD 203856 (Elias et al. 1982).

2.2. Near-infrared spectroscopy

We obtained a spectrum of the central star of DR 18 in the K-band window using a 2 pixel slit and the JK grism. The resulting spectrum has a resolution of [FORMULA]. To obtain it, we took several sequences of five individual spectra moving the star along the slit between each pair of exposures, in order to enable the removal of detector defects. The reduction of the spectra was carried out by means of IRAF scripts which used tasks in the NOAO/ONEDSPEC and TWODSPEC packages. One-dimensional spectra were extracted, combined, and wavelength-calibrated using the OH airglow lines as a wavelength reference (Oliva & Origlia 1992). Telluric absorption features were removed by ratioing the spectra by those of the nearby star HD 196850, whose G0 spectral type indicates the practical absence of any noticeable features in this wavelength domain, with the exception of a faint Br [FORMULA] line (Kleinmann & Hall 1986). This star was observed inmediately after each observation of the central star of DR 18, at nearly the same airmass. Given the availability of time, a spectrum was also obtained of the brightest star of the field in the K band.

2.3. Imaging in the visible

Images of DR 18 in the V band and H[FORMULA] were obtained by combining exposures obtained with the CCD camera at the 1.23 m telescope in September 1996. These observations were carried out under less than ideal conditions, but they are useful to compare the near-infrared and visible morphologies of the object, as well as to measure the visible magnitude of the central star. The latter was calibrated by observing the standard star HD 197037.

2.4. Spectroscopy in the visible

Spectra of the central star of DR 18 were obtained on 20 and 21 April 1999, using EMMI in low-resolution spectroscopy mode at the the 3.5 m ESO New Technology Telescope (NTT). Grism #3 was used in combination with a [FORMULA] slit, yielding a resolution of [FORMULA]. The rather wide slit, oriented following the parallactic angle, was chosen due to the degraded image quality resulting from the low altitude at which DR 18, a mid-northern object, has to be observed from La Silla. Three spectra obtained with the described setup were added together, totalling 60 min. of exposure time. Wavelength calibration was performed using a Th-Ar lamp. Relative flux calibration was achieved by ratioing the extracted spectra, corrected for atmospheric extinction, by the throughput curve of the system as previously determined by the team operating the NTT.

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999