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Astron. Astrophys. 345, 419-429 (1999)

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2. The line-strength indices

2.1. H+K(CaII)

According to its definition (see Longhetti et al. 1998a), the index H+K(CaII) is actually a measure of the Balmer H[FORMULA] line. It is in fact only the blend of H[FORMULA] with the H(CaII) line that is able to affect the apparent ratio between H and K(CaII) lines.

Rose (1985) has pointed out that stars of type later than F5 show a constant ratio between H and K(CaII) lines, while hotter stars, with a deeper Balmer series, are characterized by a stronger blend H(CaII)+H[FORMULA] relative to K(CaII) and consequently by a smaller value of the H+K(CaII)index, which reaches the minimum value in A-type stars characterized by the maximum Balmer absorption.

2.2. H[FORMULA]/FeI

This index, introduced by Rose (1985), is a measure of the Balmer H[FORMULA] line, whose central intensity is related to the average intensity of two FeI lines. At increasing surface temperature of a star, this index decreases because the H[FORMULA] and FeI lines increase and decrease, respectively. The H[FORMULA]/FeI index reaches its minimum value in A0-type stars (whereby the Balmer absorption lines have their maximum intensity). It turns out to be a good age indicator (just like all the Balmer lines) even though it is also sensitive to metallicity via the dependence on the FeI lines. In addition to this, like the other Balmer lines indices, it might be contaminated by emission lines.

2.3. [FORMULA]4000

The spectral break at 4000 Å is produced by two factors: the concentration of Balmer lines toward the asymptotic limit of 3650 Å and the increase in stellar opacity at shortward of 4000 Å caused by ionized metals. The [FORMULA]4000 index decreases with increasing surface temperature, it depends upon the stellar gravity only for stars cooler than F5 (Hamilton 1985) and is also sensitive to the chemical composition (Van den Bergh 1963, van den Bergh & Sackmann 1965, Hamilton 1985). This index provides information about the stellar population at the turn off , blending metallicity and age effects (Worthey 1992). Furthermore, the [FORMULA]4000 index is useful for studies of distant galaxies, because thanks to the large band-passes adopted for its definition it can be measured with small errors also on spectra at low signal to noise. Finally, just like all other "blue" indices, cosmological recession shifts this index toward redder spectral ranges whereby modern CCDs are more sensitive.

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© European Southern Observatory (ESO) 1999

Online publication: April 19, 1999
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