Astron. Astrophys. 332, 877-903 (1998)

Appendix A: setting up a photometric index probing the chemical peculiarities in barium stars

To be able to compare in a homogeneous way the level of chemical peculiarities in different barium stars is clearly of paramount importance when trying to identify the mass transfer scenario that operated in these stars. A key constraint may indeed be obtained by assessing whether or not there is a correlation between the level of chemical peculiarities and some dynamical parameter like the orbital period. As detailed abundance analyses are only available for a small subset of barium stars, the aim of this Appendix is to design a photometric index available for (nearly) all barium stars and probing their chemical peculiarities.

Lü et al. (1983) and Lü (1991) pointed out that barium stars segregate according to their Ba index (as defined in the scale of Warner 1965 from visual inspection of the strength of the Ba line) in the color-color diagram constructed from DDO photometric indices (see McClure & van den Bergh 1968). This segregation is caused by the so-called Bond-Neff depression, a broad absorption feature present in the spectrum of barium stars and extending from about 350 to 450 nm (Bond & Neff 1969; Lü & Sawyer 1979). A veil of many heavy-element lines has been proposed as the origin of this broad feature (McWilliam & Smith 1984; Wing 1985, and references therein), though that issue is still controversial as CN bands are also important contributors in this spectral region (Tripicco & Bell 1991).

According to these findings, it may be expected that a photometric index of the form be related to the level of chemical peculiarities in barium stars (see Fig. 4 of Lü 1991). The m and n parameters in the above relation may actually be chosen so as to yield the maximum correlation between and some given abundance indicator. McWilliam (1988, 1990) showed that the abundance ratio YII/TiII, derived from the YII and TiII lines, is a powerful indicator for detecting heavy-element abundance peculiarities in red giants, since (i) that abundance ratio is relatively insensitive to the atmospheric parameters, and (ii) the distribution of YII/TiII ratios in a sample of about 600 G - K giants is very narrow, with a FWHM of only 0.3 dex centered at log(YII/TiII) (adopted as normalization value for [YII/TiII] in the following).

The index has therefore been calibrated in terms of [YII/TiII] abundances, using 131 G and K giants for which both DDO colors (McClure & Forrester 1981) and abundances (McWilliam 1990) are available.

The value is obtained by requiring maximum linear correlation between [Y/Ti] and for that sample, and ensures that barium stars have , whereas nearly all normal giants have (Fig. A.1).

Fig. A.1. The color index vs. [YII/TiII] for 131 G - K giants with DDO colors from McClure & Forrester (1981) and abundances from McWilliam (1990). Typical error bars have been indicated. Double-circled triangles identify the barium stars HD 116713, HD 139195, HD 204075, HD 202109 and HD 212320, whereas squares correspond to heavy element-rich stars according to McWilliam (1990), not previously reported as barium stars (see text). Strong CN stars from the list of Keenan, Yorka & Wilson (1987) have been represented by single-circled triangles

Although the expected trend is clearly present (taking into account the  dex uncertainty on [Y/Ti]), there are a few stars in `forbidden' regions [namely (i) non-barium stars [Y/Ti] with , or (ii) possible barium stars [Y/Ti] with ], degrading the ability of the index to identify barium stars. The two stars (HR 7754 and HR 8590) in region (ii) were not previously identified as barium stars. HR 7754 is in fact present in the list of MK standards provided by Keenan & McNeil (1989), and there is no mention whatsoever of the barium nature of that star, classified as G9III. Apart from the fact that HR 7754 is member of a complex multiple visual system, there is currently no clue as to the origin of this discrepancy.

Stars in region (i) ([Y/Ti] , ) may actually be bright giants or CN-strong stars, spilling somewhat over into the region of barium stars. It is indeed clear from the [ ] diagram (Fig. A.2), where the fiducial loci of dwarfs, giants and Ib supergiants from McClure & Forrester (1981) have been indicated, that Ib supergiants with may be found among barium stars. As CN bands are strong contributors to the the index (Tripicco & Bell 1991), CN-strong stars tend to have indices smaller than average as well, and some (like HR 3905) may also contaminate region (i) (Fig. A.2).

Fig. A.2. The color index vs. for barium stars (open squares: Ba0 - Ba1, open triangles: Ba2 - Ba3, filled circles: Ba4 - Ba5). The loci of main sequence stars (class V), subgiant stars (class IV), giant stars (class III) and supergiant stars (class Ib), as provided by McClure & Forrester (1981), are indicated by the various lines. Small dots correspond to 131 G - K giants common to the samples of McClure & Forrester (1981) and McWilliam (1990), as plotted in Fig. A.1. Double-circled and single-circled symbols identify the same stars as in Fig A.1. The arrow corresponds to a reddening by

No reddening correction has been applied to the stars plotted in Fig. A.2. According to the reddening correction factors provided by McClure (1979), reddening is expected to have a very limited impact on the index, as the de-reddened index writes .

Despite ambiguities in identifying barium stars when is close to 0, smaller values of that index correlate fairly well with heavy-element overabundances, as indicated by the barium stars present in Fig. A.1. In fact, a similar correlation was already obtained for the closely-related index defined by Jorissen et al. (1992b) from Strömgren photometry.

With the adopted normalization, mild barium stars typically have while strong barium stars (with Ba4 and Ba5 indexes) have .

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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