6 Results

6.1 Spectral displays and table

Figures 6 to 39 display the ELODIE echelle spectral orders of BD+63$^{\circ }$ 1964, BD+40$^{\circ }$ 4220 and HD 183143 over the wavelength range 3906-6812 Å. BD+63$^{\circ }$ 1964's associated standard stars, HD 205021 and HD 37128, are also shown for the identification of stellar lines. BD+63$^{\circ }$ 1964's telluric standard, HD 32630, is shown in a separate box. Also plotted is the spectrum of BD+63$^{\circ }$ 1964 corrected for stellar lines (divided by HD 37128). A synthetic spectrum of all DIBs seen in this survey of BD+63$^{\circ }$ 1964 is also included to easily identify confirmed DIBs. When necessary, a telluric correction was applied. This is indicated in the figure caption. Table 4 lists all the diffuse interstellar bands positively identified toward BD+63$^{\circ }$ 1964.

6.2 New DIBs

The 60 new DIBs discovered in BD+63$^{\circ }$ 1964 do not seem to adhere to any general pattern. They are spread throughout the spectrum, although occasionally clusters of new DIBs arise, and a slight bias can be seen towards the red end of the spectrum. The wavelength range between 6140 Å and 6200 Å is an example of a region with many new weak DIBs blended with the very broad 6173 Å DIB. The vast majority of the new DIBs are weak, narrow features, although some medium-broad DIBs are also reported. This is probably because the ELODIE spectrograph is not suited for the detection of very broad DIBs due to each order's relatively narrow wavelength coverage and the difficulty in merging adjacent orders. Few new DIBs are seen at shorter wavelengths. There are three reasons that could account for this:

• The spectrograph is not as sensitive at shorter wavelengths, where the reddened target is fainter. The noise level is much higher, especially at the beginning of each order;
• The blue range is also dominated by stellar lines, which does not ease a DIB search in that region;
• There are fewer DIBs at a given equivalent width in this wavelength region. DIBs are undoubtedly concentrated towards the red end of the spectrum (this is discussed further in Sect. 7).

6.3 Undetected DIBs mentioned in previous surveys

Some DIBs previously identified in other surveys were not included in the list of DIBs identified in BD+63$^{\circ }$ 1964's spectrum. Some appeared to be completely absent, or not rising above the noise level of the spectrum. Others were so weak that, taking into account flat-fielding deviations and other spurious absorptions, the absolute confirmation of these features as DIBs was not possible from our data. Certain very broad DIBs extending over wavelength regions which span two or more orders have been omitted due to the insufficient quality of order merging.

 

 

Table 2: Previously reported Diffuse Interstellar Bands that could not be detected or reliably measured in BD+63$^{\circ }$ 1964 and which are not included in the present survey of DIBs. "+'' and "o'' denotes the classification of certain and probable DIBs according to Jenniskens & Désert (1994)."?'' denotes Krelowski's "doubtful'' DIBs. The second column lists the DIB central wavelength quoted by the references given. The next two Col. contain data on these DIBs quoted from measurements by Jenniskens at http://www-space.arc.nasa.gov/$^\sim$leonid/DIBcatalog.html. The fourth column (EW) shows upper limits as measured in our spectrum of BD+63$^{\circ }$ 1964 to be compared with Col. 2. See Table 4 for Reference codes

	$\lambda_{\rm c}$	$\frac{W}{E_{(B-V)}}$	FW	EW	Comments	Ref.
	(Å)	(mÅ)	(Å)	(mÅ)
+	4066.0	283	15	..	Stellar?	He95
+	4880.4	22	1.35	<8	Weak	JD94
o	5845.4	4	0.7	..	Blend DIB	J96
o	5847.7	2	0.61	..	Absent	J96
+	5850.2	6	1.5	..	Blend DIB	J96
o	5902.8	7	1.2	<6	Absent	K95
+	5904.6	9	0.17	<6	Absent	K95
o	5908.3	3	0.6	<8	Weak	K95
o	5927.5	11	0.7	<6	Very Weak	K95
o	5929.6	7	0.6	..	Absent	K95
?	5941.7	-	-	<6	Absent	K95
?	5989.4	-	-	9	Probable	K95
?	5999.8	6	0.6	<8	Weak	K95
o	6032.9	5	0.6	..	Absent	K95
+	6212.7	14	1.23	..	Absent	Ch85
?	6238.8	-	-	<6	Absent	K95
o	6321.5	16	1.45	<6	Absent	JD94
+	6451.6	403	25.4	..	Absent	JD94
o	6494.9	23	1.39	<6	Absent	JD94
+	6532.1	664	17.2	..	too broad	JD94
?	6635.3	-	-	<8	Weak	K95
?	6649.8	-	-	<8	Very Weak	K95
?	6693.3	5	0.8	<6	Blend DIB	K95
?	6696.8	-	-	<6	Absent	K95
+	6741.0	13	0.97	..	too broad	He75
o	6767.6	8	0.7	<8	Weak	He88
o	6768.6	5	0.7	<6	Absent	He95
o	6779.0	3	0.54	<8	Weak	He95

Table 2 shows a list of these suggested DIBs not included in the present survey. Seven of those diffuse interstellar bands denoted "certain'' by Jenniskens & Désert (1994) cannot be accurately measured in the present spectrum of BD+63$^{\circ }$ 1964. The 6741 Å DIB lies between two orders and is therefore lost in our data. Another DIB at 6532 Å spans two orders. We cannot exclude the possiblity of imperfect merging in this case due to the nearby strong H-$\alpha$ line. The absence of a third DIB, at 6451 Å, could possibly be due to its extension over two orders. This DIB however is not visible even when an optimal merging of the orders is obtained. The rest of the "certain'' DIBs, except for one, are either too weak to be accurately measured or else absent from the spectrum. The remaining one, at 5850.2 Å, positively identified by Jenniskens et al. (1996), is absent because the resolution of the ELODIE spectrograph, at 42000, is not sufficient to distinguish this very weak DIB from the stronger and broader band at 5849.8 Å. It was detected with the AURELIE spectrograph at Observatoire d'Haute Provence, France, which has a resolution in excess of 50000 (Jenniskens et al. 1996). Alternatively this is just a red wing of the 5849.8 Å DIB. All the remaining DIBs not included in the survey have previously been classified as either probable or doubtful.

6.4 Possible additional DIBs to be confirmed

The spectrum of BD+63$^{\circ }$ 1964 provides us with a unique opportunity for finding and measuring new diffuse interstellar bands. The fact that previously known narrow DIBs are enhanced by a factor of 1.5-3 toward this target lends credence to this. An absorption band has to fulfill certain criteria as mentioned in Sect. 5 before it can be considered as a new DIB.

 

 

Table 3: Possible additional Diffuse Interstellar Bands in BD+63$^{\circ }$ 1964. These DIBs have yet to be confirmed. Central wavelength, equivalent width (EW) and full width at half maximum (FWHM) are listed

$\lambda_{\rm DIB}$	EW	FWHM	$\lambda_{\rm DIB}$	EW	FWHM
(Å)	(Å)	(Å)	(Å)	(Å)	(Å)
4968.87	65	6.54	5724.87	6	0.55
4979.28	14	1.29	5806.52	15	1.54
5098.84	5	0.48	5834.54	9	0.84
5166.33	7	0.68	5966.11	8	0.84
5167.28	8	0.69	6095.21	5	0.65
5176.67	34	3.50	6102.48	12	1.77
5428.63	12	1.11	6154.57	8	1.22
5443.22	7	0.72	6293.29	5	0.41
5525.48	8	0.74	6468.45	9	1.29
5539.73	14	1.48	6476.81	5	0.55
5559.93	6	1.06	6639.34	11	1.11
5632.81	6	0.65	6808.36	7	0.56
5634.73	7	0.89

The wealth of interstellar features in BD+63$^{\circ }$ 1964's spectrum has resulted in the discovery of 60 new "certain'' DIBs based upon these criteria. There seems to be no doubt that this number is limited by the signal to noise of the present data and the magnitude V=8.6 for this exceptional target. However this seems one of the best targets for DIB surveys when studied with cross-dispersed echelle spectrographs on 2 m class telescopes. It is likely that an analysis of more spectra of higher quality and using a larger telescope would result in the positive detection of many more diffuse bands. In searching BD+63$^{\circ }$ 1964's spectrum for new DIBs a number of weak absorption features which are probably of interstellar origin were listed. They did not fulfill the necessary criteria to be included as "certain'' DIBs in this particular survey. In some cases the uncertainty was due to blended stellar lines or atmospheric lines, or stellar lines in HD 205021 at the wavelength corresponding to that of the suspect DIB coinciding with stellar lines in the synthetic stellar spectrum. In other cases it was difficult to distinguish the feature from localised random noise. Table 3 lists these possible DIBs.

The interest of the present survey comes from the exceptional quality of the spectrum of the line-of-sight of BD+63$^{\circ }$ 1964, with additional confirmation provided by spectra of the very reddened targets HD 183143 (a B7 star with a slow rotation) and BD+40$^{\circ }$ 4220 (a hot O7 star with a fast rotation). Due to the unusual enhancement of DIBs in BD+63$^{\circ }$ 1964 we could not confirm the existence of 25 possible DIBs in the other targets. To confirm or reject these possible DIBs one needs to observe higher signal-to-noise spectra of reddened stars. Those DIBs listed as possible could then be cross-checked with other stars.