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Astron. Astrophys. 363, 1019-1025 (2000)

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2. Observational data and reduction

2.1. Photometric data

Observations in standard UBVR bands were carried out in two sets: November 26-December 3, 1997 and November 7-12, 1999, using the 50 cm AZT-14 telescope of the Byurakan Astrophysical Observatory (Armenia) equipped with a direct current intensification photopolarimeter that works as a conventional photometer when the polarizer is removed.

Polarimetry was performed in UBVR bands but photometry in BV bands only. The standard deviation for polarimetric and photometric mesurements was about 0.1-0.2% and [FORMULA]-[FORMULA], respectively. The star SAO 74365 was used as a standard. More details on the applied method and equipment, as well as reduction to the standard UBV system, can be found in Eritsian & Nersisian (1984).

A description of photometric observations and their results are presented in Table 1. Both conventional and Julian date, UTC time, apparent standard B and V magnitudes and B-V color index are given.


Table 1. Photometric data for WDS 00550+2338

As mentioned above, two photometric runs were carried out - first in November 1997 when a brightness increase was detected for the first time, and second in November 1999 aimed to further monitor the star. The total observation time was about 27 hours. Photometric data obtained in 1997 were reported briefly in IBVS (Tamazian et al. 1998). To present a complete set of photometric data, observations in the 1997 run are included in Table 1 along with the results obtained in 1999. Photometric behavior of WDS 00550+2338 during these two sets of observations is shown in Fig. 1. No appreciable polarimetric signal was detected during the entire observational period.

[FIGURE] Fig. 1. Light curves and B-V index behavior for WDS 00550+2338. The dashed line separates two runs conducted in 1997 and 1999. Dates when observations were not carried out are indicated "n.o." (no observations, see data in Table 1).

2.2. Speckle interferometric measurements

Speckle interferometric measurements were carried out in September 21, 1999 using an intesified CCD (ICCD) camera of the Astronomical Observatory Ramon Maria Aller of the University of Santiago de Compostela, developed in cooperation with Special Astrophysical Observatory (Russia) and installed at the Cassegrain focus of the Spanish 1.52 m telescope at the Spanish-German Astronomical Center in Calar Alto (Spain).

The ICCD is based on the SensiCam CCD camera (manufactured by PCO Computer Optics GmbH, Germany) coupled to a 20 mm multialkaline photocathode 3-stage electrostatic image tube.

The observations consist of between 1000 and 3000 short exposures (electromechanical shutter synchronized with the detector allows selection of exposure times between 5 and 40 ms; up to 5 images per second can be stored on hard disk) depending on the seeing conditions and the object brightness. Each short exposure image is fully digitized at 12 bits per pixel and sampled by 512[FORMULA]512 pixels.

In the course of WDS 00550+2338 observations a [FORMULA] magnification microobjective producing a scale [FORMULA].0120 per pixel at the detector plane was used. For precise scale determination we used a set of calibration stars with well known ephemerides.

After image correction for flat field and geometrical distortion, a standard speckle interferometric reduction technique was used. The last step is the power spectrum correction for photon noise as well as the circular cuts extraction from the set of spatial frequencies up to the cut-off. We assumed that the optical transfer function for each cut is constant, then fit a function based on the binary model, finally determining the precise separation and position angle.

The typical error of the measurement which depends on signal-to-noise ratio in the power spectrum was about [FORMULA].01 in separation and 1o in position angle. Geometrical aberration, mainly due to the image tube, was about 2%.

2.3. Speckle spectroscopy and spectrum

Speckle spectroscopy was employed to obtain separately the spectra of the primary and secondary stars. In our scheme a speckle image and its dispersed specklegram are simultaneously detected (Baba et al. 1994a). The cross-correlation of speckle images and dispersed specklegrams results in objective spectra with high spatial resolution (Baba et al. 1994b).

Speckle spectroscopic observations were conducted on September 10, 1998 at the San Pedro Martir Observatory in Mexico. The speckle spectroscopic camera was attached to the Cassegrain focus of the 1.5 m reflector. An interference filter of [FORMULA].5 nm ([FORMULA].5 nm) was used to detect speckle images, and dispersed specklegrams were formed with the use of a holographic blazed reflection grating (600 grooves/mm).

Two synchronized ICCDs, which were equipped with a micro channel plate (MCP), were emplyoed to detect speckle images and dispersed specklegrams, respectively. These ICCDs were operated at a video rate of 30 Hz. The effective exposure time of one data frame was reduced to 16ms after separating the odd and even fields of one video frame computationally.

Fig. 2 shows a reconstructed image (top) and its objective spectra (bottom) from 3920 data frames. The image was reconstructed by the shift-and-add method (Kuwamura et al. 1993) while objective spectra were reconstructed by the cross-correlation method under the assumption that the point spread function is the same as the imaging channel in the spectral reconstruction region. The secondary star spectrum yields displaced horizontaly relative to the primary exactly by the same distance as its direct image shown at the top. Each spectrum of both components was calculated by averaging 7 lines, which is the effective width of the reconstructed spectrum.

[FIGURE] Fig. 2. Shift-and-add reconstructed image (top) and its spectra derived from the cross-correlation method (bottom). The spectral range is 639-695 nm for the primary star and 635-691 nm for the secondary.

Fig. 3 show the spectra (intensity is given in arbitrary units) of the primary and secondary components, respectively. The wavelength range was limited to [FORMULA] nm since the speckle images which served as a reference to the dispersed specklegrams were detected through the same filter, mentioned above.

[FIGURE] Fig. 3. Spectra of the primary (A) and secondary (B) components. Intensity is given in arbitrary units.

The composite spectrum of WDS 00550+2338 was obtained on December 25, 1998 using the Boller & Chivens spectrograph attached to the 2.1 m telescope of the "Guillermo Haro" Observatory in Cananea, Mexico. A TK1024 (1024[FORMULA]1024 pix) CCD was used whose pixel size is 24 microns and readout noise [FORMULA].

A 600 lines/mm grating was used, giving a dispersion of 30 A/mm with an effective instrumental spectral resolution of about 4 A.

All frames were reduced using the IRAF environment following standard bias substraction, flatfielding, cosmic rays bias removal and other usual proceedings. For flatfielding and wavelength calibration, halogen and He-Ar built-in lamps were used respectively, and standard stars were observed along with the target for calibration purposes. The spectrum is shown in Fig. 4 where several representative lines mentioned below in the text are marked.

[FIGURE] Fig. 4. Composite spectrum of WDS 00550+2338. Several representative lines are marked.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000