2 Observations and data analysis

 

 

Table 1: GSC2 plate material used for the astrometry and the photographic photometry of the new DQ white dwarf.

Field	Survey	Center (J2000)	Epoch	Pixel	Color	Emulsion + Filter
XJ443	POSS-II	13:04:14.7 +29:48:37	1995.234	15 $\mu$m	$B_{\rm J}$	IIIaJ + GG385
XP443	POSS-II	13:04:15.2 +29:48:42	1993.288	15 $\mu$m	$R_{\rm F}$	IIIaF + RG610
XI443	POSS-II	13:04:20.7 +29:44:17	1991.299	15 $\mu$m	$I_{\rm N}$	IV-N + RG9
N322	Quick V	13:06:56.6 +29:13:25	1983.294	25 $\mu$m	V12	IIaD+Wratten 12
XE322	POSS-I	13:06:55.5 +29:13:25	1955.288	25 $\mu$m	E	103a-E + red plexiglass
XO322	POSS-I	13:06:56.1 +29:13:24	1955.288	25 $\mu$m	O	103a-O unfiltered

2.1 Astrometry and photometry

Our material consists of Schmidt plates from the Northern photographic surveys (POSS-I, Quick V and POSS-II) carried out at the Palomar Observatory (see Table 1). All plates were digitized at STScI utilizing modified PDS-type scanning machines with 25 $\mu$m square pixels (1.7''/pixel) for the first epoch plates, and 15 $\mu$m pixels (1''/pixel) for the second epoch plates (Laidler et al. 1996). These digital copies of the plates were initially analyzed by means of the standard software pipeline used for the construction of the GSC-II. The pipeline performs object detection and computes parameters and features for each identified object. Further, the software provides classification, position, and magnitude for each object by means of astrometric and photometric calibrations which utilized the Tycho2 (Høg et al. 2000) and the GSPC-2 (Bucciarelli et al. 2001) as reference catalogs. Accuracies better than 0.1-0.2 arcsec in position and 0.15-0.2 mag in magnitude are generally attained.

Figure 1: First epoch (POSS-I, XE322) and second epoch (POSS-II, XP443) plates in the direction of the newly discovered WD, the encircled star near the field center. The large relative motion of the object is evident.

 

 

Table 2: Astrometry and photometry of GSC2U J131147.2+292348. The position was determined from plate XJ443 (epoch 1995.234, equinox J2000), while all of the available plates were used for the proper motions. The error of the photographic photometry is better than 0.2 mag (1$\sigma$).

$\alpha$ (h m s)	$\delta$ (d m s)	$\mu_{\alpha}\cos\delta$	$\mu_{\delta}$
(J2000)	(J2000)	(arcsec/yr)	(arcsec/yr)
13 11 47.21	+29 23 48.0	$-0.382 \pm 0.002$	$0.286 \pm 0.005$

$B_{\rm J}$	V12	$R_{\rm F}$	$I_{\rm N}$
19.6	18.7	18.1	17.5

J	H	$K_{\rm s}$
$17.48\pm 0.05$	$17.13\pm 0.10$	$17.08\pm 0.12$

Star GSC2U J131147.2+292348 was part of the sample of WD candidates discovered after screening the high proper motion stars found in survey field 443 (Table 1). These were selected on the basis of their relative proper motions as derived by applying the procedure described in Spagna et al. (1996) to just the POSS-II plates, spanning $\sim$4 years. The finding charts in Fig. 1 show the high proper motion of this object.

The astrometry and photometry of GSC2U J131147.2+292348 are given in Table 2. The position refers to the epoch of the most recent plate (XJ443), while the accurate proper motion was computed by combining the image locations of the star as measured on the 6 different plates of Table 1, which span $\sim$40 years. The photographic magnitudes are given in the natural photometric system of the POSS-II and Quick-V plates as defined by the emulsion-filter combinations in Table 1.

In particular, the transformation between the photographic and Johnson V is V₁₂=V - 0.15 (B-V) according to Russell et al. (1990). Also, recently acquired NIR images provided the J, H, $K_{\rm s}$ magnitudes in Table 2. Finally, Moreau & Reboul (1995) published the values $U\simeq 19.15$ and $V\simeq 19.10$. Note that their visual magnitude is fairly consistent with our V₁₂, considering the above color transformation and the errors of the photographic photometry.

2.2 Spectroscopy

Spectroscopy of GSC2U J131147.2+292348 was obtained on the night of 2001 January 29 using the intermediate dispersion spectrographic and imaging system (ISIS) on the 4.2-m William Herschel Telescope on the island of La Palma. The 5700 Å dichroic was used to split the light and feed to the blue and red arms of the spectrograph.

We used the R158B grating on the blue arm, which gave a nominal dispersion of 1.62 Å/px and useful wavelength coverage from 3200 to 5700 Å. (The dichroic cuts in at wavelengths >5700 Å, and at short wavelengths, the sensitivity falls off with the quantum efficiency of the detector.) On the red arm, we used the R158R grating to give a nominal dispersion of 2.9 Å/px covering from 5500 to 8000 Å. A blocking filter (GG495) was also used on the red arm to cut out second order blue light. A 30-min exposure was made using a 1-arcsec slit. Subsequent exposures were taken of the spectrophotometric standards Feige 67 and Feige 34 to enable flux calibration of the primary target. We took arc lamp exposures to enable wavelength calibration and tungsten lamp exposures for the pixel-to-pixel sensitivity variation and enable flat fielding.

The data were reduced within the IRAF environment, following standard procedures. No attempt was made to correct for extinction, both standards and targets were measured with an airmass $\le$1.1. Observations were made with a slit width of 1.02 arcsec, which corresponds to 4 detector pixels in the blue, i.e. a dispersion of 6.5 Å per resolution element. For the red arm, the pixel scale is 0.36 arcsec per pixel, leading to a resolution element of size 3 pixels, i.e. a resolution of 8.2 Å. The blue and red arm spectra have been Gaussian smoothed at these resolutions.

Good agreement between the red and blue arm spectra was found in the overlap region, with fluxes agreeing to better than 10% in the range 5600-5700 Å.

Figure 2: The WHT optical spectrum of GSC2U J131147.2+292348. Vertical marks indicate the locations of the strong C2 Deslandres-d'Azambuja and Swan bands, and of the telluric O2. The crosses refer to the fluxes (with $\pm 20\%$ error bars) derived from the $B_{\rm J}$, V12, $R_{\rm F}$, $I_{\rm N}$ photographic photometry of Table 2 and the U mag from Moreau & Reboul (1995).