2 Carbon star selection

The full HES database consists of $\sim$10 million extracted, wavelength calibrated spectra. The input catalog for extraction of objective-prism spectra is generated by using the Digitized Sky Survey I (DSS I). An astrometric transformation between DSS I plates and HES plates yields, for each object in the input catalog, the location of its spectrum on the relevant HES plate, and provides a wavelength calibration zero point (Wisotzki et al. 2000).

Carbon stars can be identified in the HES database by their strong C₂ and CN bands. We select carbon star candidates when the mean signal-to-noise ratio (S/N) in the relevant wavelength range is >5 per pixel and both of the C₂ bands $\lambda\lambda\,5165$, 4737, or both of the CN bands $\lambda\lambda\,4216$, 3883 are stronger than a selection threshold. Band strengths are measured by means of line indices - ratios of the mean photographic densities in the carbon molecular absorption features and the continuum bandpasses shown in Fig. 1, and listed in Table 1. The use of pairs of indices prevents confusion with plate artifacts, e.g., scratches. It is very unlikely that two such artifacts are present at the positions of two molecular bands. Selection boxes in the $I\,(\mbox{C}_2\;\lambda\,5165)$ versus $I\,(\mbox{C}_2\;\lambda\,4737)$ and $I\,(\mbox{CN}\;\lambda\,4216)$ versus $I\,(\mbox{CN}\;\lambda\,3883)$ planes were chosen well-separated from the dense locus of "normal'' stars (see Fig. 2). The selection criteria are listed in Table 2.

  

Table 1: Wavelengths of passbands used for computation of C band indices in the HES. "cont'' = continuum; "flux'' = feature passband.

$$\begin{tabular}{lcccc}\hline\hline \rule{0.0ex}{2.3ex} & \mu... ... 3610--3740\,{\AA} & & & & cont\\ \hline\hline \end{tabular}$$

Figure 2: Selection of carbon stars in the $I\,(\mbox{C}_2\;\lambda\,5165)$ versus $I\,(\mbox{C}_2\;\lambda\,4737)$ plane. Band strengths are measured by line indices. Dots: all spectra on a randomly chosen HES plate; boxes: test sample of known FHLC stars present on HES plates (see Table 4); dashed box - selection region. Spectra in which only one high C2 band index value was measured suffer either from an overlapping spectrum, or from a plate artifact. The selection in the $I\,(\mbox{CN}\;\lambda\,4216)$ versus $I\,(\mbox{CN}\;\lambda\,3883)$ plane is done analogously. The two test sample objects outside the selection box are CGCS 525 and CGCS 3180. They are selected by CN band indices (see Table 4).

 

 

Table 2: Carbon star selection criteria. The maximum allowed band index values correspond to an integrated density of zero in the feature passbands. That is, larger band indices can only be due to artifacts, e.g. scratches, causing photographic densities (above skybackground) $<\!0$. Stars are selected if both of their C₂ indices or both of their CN indices fall into the indicated ranges.

Feature	Index range [Å]
$\mbox{C}_2\;\lambda\,5165$	[10,91]
$\mbox{C}_2\;\lambda\,4737$	[15,114]
$\mbox{CN}\;\lambda\,4216$	[2,56]
$\mbox{CN}\;\lambda\,3883$	[13,55]

Carbon stars can be distinguished reliably from other late type stars, e.g. M or S stars, even if only weak C bands are present in their spectra (cf. Fig. 3).

Figure 3: Comparison of HES spectra of the C star HE 1524-0210, exhibiting a weak C band only, with two M stars. The abscissa is the same as in Fig. 1.

Other potential sample contaminators are white dwarfs of type DQ, which show carbon molecular bands. However, since the latter usually have a much bluer continuum (see Fig. B.1), they can easily be recognized by visual inspection of the spectra, and by their U-B color. McCook & Sion (1999) list 49 DQs, of which 30 have an available U-Bmeasurement. The average U-B of those is -0.58, i.e., $\sim$1.5mag away from the average U-B of the HES C star sample. Our U-B colors are measured directly from the HES spectra with a mean accuracy of $\sigma_{U-B}=0.09$mag (Christlieb et al. 2001 hereafter Paper I). The average U-B of HES C stars is $\sim$0.9, more than 90% have U-B>0.5, and there is no C star of U-B<0 in the HES sample. While four (i.e., 13%) of the 30 DQs with U-B in McCook & Sion (1999) have U-B>0.0, the pressure-broadened features of DQs are easily distinguished by visual inspection of the carbon bands (see Fig. B.1).

Figure 4: V magnitude and B-V distribution of the HES FHLC sample. B-V was derived from HES spectra with the procedures described in Paper I.

With a rough estimate of their surface density, we can quantify an upper limit for the contamination of the HES C star sample by "red'' (U-B>0.0) DQs. First of all, we have to take into account that the ratio of northern hemisphere to southern hemisphere DQs is unbalanced in McCook & Sion (1999), as much as is the total catalog. This is because the southern hemisphere so far has been surveyed less extensively for white dwarfs. Assuming that the northern hemisphere sample of DQs is complete, we derive a surface density of 9 DQs brighter than V=16.5 in $20\,000$deg², i.e. $4.5\times 10^{-4}$deg^-2. Hence, the surface density of U-B>0.0 DQs is $5.9\times 10^{-5}$deg^-2, and we expect 0.44 DQs to be present on all 329 plates currently used for the exploitation of the stellar content of the HES. Therefore, even if we assume that the sample of DQs known so far is incomplete by a factor of 2, we statistically expect less than 1 DQ to be present in the HES C star sample.

On the 329 HES plates (effective area $6\,400$deg²) we found 403 FHLCs. 90 of them were selected by C₂ band indices only, 171 by CN band indices only, and 144 by C₂ and CN indices. The V and B-V distributions are displayed in Fig. 4. The faintest objects have $V\sim16.5$, and the most distant objects reach $\sim$35kpc (cf. Fig. 5), assuming they are all giants with M_V=-1mag.

Figure 5: Distance distribution of the 393 HES FHLCs with available V magnitudes, assuming that they are all giants with MV=-1mag.