3 Observations and data reduction

3.1 ELODIE spectrograph performances

Observations were obtained on July 24th and November 8th, 1995 at the Observatoire de Haute Provence (OHP). We used the 1.93 m telescope, equipped with the spectrograph ELODIE, which is a fiber-fed echelle spectrograph, covering the wavelength range from 3906 to 6811 Å with a resolution of $\sim$ 42000 (Baranne et al. 1996). The fibres are POLYMICRO fibres with a diameter of 100 $\mu$m. The grating used is a 408$\times$102 mm echelle grating with 31 grooves/mm and a $\theta=76^{\circ}$ blaze angle. The dispersion crossing is done with two optical components, a 40$^{\circ }$ flint prism and a 8.63$^{\circ }$ crown grism with 150 grooves/mm. The combination results in a constant interorder spacing over the 67 spectral orders. The detector is a back-illuminated Tk1024 CCD with 24 $\mu$m pixels, cooled to 183 K. At a readout of 100 $\mu$s/pixel the typical readout noise is 8.5 e^-. The CCD has a linear response up to 100000 e^-.

The preliminary reduction of the ELODIE data was performed on-line at the observatory. The normal CCD reduction procedure was carried out, incorporating the correction of bad-pixels and cosmic rays, and the subtraction of the bias and dark current. No systematic post-reduction deglitching was applied except for very obvious spikes.

   
3.2 The program stars and optimal reduction

In addition to BD+63$^{\circ }$ 1964, observations of three different standard stars were performed during the same night. Table 1 summarises the physical and observational characteristics of all the stars used in this survey. HD 37128, with its comparable rotation and spectral type to BD+63$^{\circ }$ 1964 was used as the main stellar standard. HD 205021, a slow rotator, further enabled us to control the identification of stellar lines in BD+63$^{\circ }$ 1964. HD 32630 was used as a telluric standard.

 

 

Table 1: Physical and observational parameters for the program stars used in the DIB survey. The radial and rotational velocities were determined from the spectra

Target	BD+63$^{\circ }$	HD	BD+40$^{\circ }$	HD	HD	HD	HD	HD
	1964	183143	4220	32630	37128	205021	164353	188209
V magnitude	8.6	6.86	9.05	3.20	1.70	3.19	3.97	5.62
Spectral Type	B0 II	B7Iae	O7e	B3 V	B0 I	B1 IV	B5Ib	O9.5Ib
E (B-V)	1.01	1.28	2	0.02	0.08	0	0.12	0.15
$v\sin i$ (km s-1)	84	60	>300	132	87	28	5	77
$v_{\rm rad}$ (km s-1)	-91	+34	$\sim$-140	+7	+26	-8	+20	+14
Observed at:
S/N	205	200	88	274	387	390	228	214
Airmass (Sec z)	1.07	1.11	1.03	1.46	1.70	1.12	1.36	1.03

BD+63$^{\circ }$ 1964, is a B0 II star and has therefore relatively few stellar lines. Nevertheless the spectrum has, especially towards the blue, some prominent stellar lines which hamper the detection and measurement of possible new DIBs. Few stellar lines appear above 4800 Å where most of the diffuse interstellar bands are concentrated. To distinguish DIBs from stellar lines the spectrum of BD+63$^{\circ }$ 1964 was divided by the stellar standard HD 37128, which is a very good match in spectral type and rotational velocity. The resulting spectrum is characterised mostly by interstellar features, with a few residual stellar lines.

Telluric contamination limits the detection and measurement of DIBs and affects a significant fraction of our total wavelength range. The contamination is due to oxygen lines, mostly around the diffuse band at 6284 Å and water lines above 5800 Å. Although the oxygen column density remains stable throughout the observations, the H₂O column density can vary slightly between exposures. For telluric correction the bright star HD 32630 was chosen. To remove the atmospheric contamination the relation below is used:

$$I_{\rm TC} = \frac{I}{I_{\rm t}^\frac{z}{z_{t}}}$$

I and $I_{\rm t}$ are the object spectrum and the telluric standard spectrum respectively. $\frac{z}{z_{t}}$ is the ratio of the respective airmasses. The inaccuracy in airmass estimations usually requires some iterations to obtain the true ratio. In removing stellar lines from regions contaminated by telluric absorption, both BD+63$^{\circ }$ 1964 and HD 37128 were first divided by HD 32630.

The heavily reddened star BD+40$^{\circ }$ 4220 (E_(B-V)=2) and HD 183143 (E_(B-V)=1.28), up to now the reference star for studies of DIB behaviour, were used to add confirmation to the newly detected DIBs in this present survey. HD 183143, a B7 star, has been used in previous DIB surveys (Herbig 1975; Jenniskens & Désert 1994; Herbig 1995). BD+40$^{\circ }$ 4220 is a hot star (spectral type O7) with a fast rotation which is ideal for detecting weak DIBs, especially in the visible and infrared. HD 164353 was used to identify stellar lines and remove telluric contamination in HD 183143, and HD 188209 was used as BD+40$^{\circ }$ 4220's telluric standard. Due to slight reddening in the standards HD 164535 and HD 188209, the large DIBs at 6284 Å and 6283 Å in these stars incurred an additional error of $\sim$10$\%$. See Fig. 40 for samples of corrected and uncorrected telluric contamination in the target stars.

Figure 1: A synthetic spectrum of all 226 DIBs confirmed towards BD+63$^{\circ }$ 1964 between 3906 Å and 6812 Å

Figure 2: A plot of Log 10(FWHM) against wavelength for the DIBs measured toward BD+63$^{\circ }$ 1964. A dominant population of DIB widths lies between 0.6 and 1.4 Å, more abundant towards the red and clustered around some ranges. A second population of medium-broad DIBs is apparent with FWHM between 1.4 and 3.2 Å. It also illustrates the clustering of some DIBs around "magic'' wavelengths, at 550, 580 and 630 nm

Figure 3: Histogram of Full Widths of DIBs in BD+63$^{\circ }$ 1964. This plot illustrates the three populations of DIBs discussed in the text: narrow DIBs, medium broad DIBs and broad DIBs

Figure 4: A plot of Log10(EW) against wavelength for the DIBs measured toward BD+63$^{\circ }$ 1964. This plot illustrates the increased number of DIBs detected for a given equivalent width towards the red, and the clustering of DIBs around 550, 580, and 630 nm

Figure 5: Histogram of Equivalent Widths of DIBs in BD+63$^{\circ }$ 1964. The cut off at $\sim$ 5 mÅ corresponds to the sensitivity limit for confident detection