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Astron. Astrophys. 325, 1025-1030 (1997)

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3. [FORMULA] and other Balmer lines

Fig. 3 illustrates the behaviour of EW([FORMULA]) as a function of the stellar parameters; values are taken from Table 8 by Kurucz (1979). The [FORMULA] line is strong in a range of temperatures (Fig. 3a), corresponding approximately to a lifetime interval of 0.2-2 Gyr on the main sequence. Moreover Fig. 3 shows that high EWs are reached in that range of temperatures only for high surface gravities (Fig. 3b). The maximum [FORMULA] is therefore reached for an A0V-type star.

[FIGURE] Fig. 3a-c. [FORMULA] of single stars: a) as a function of [FORMULA] for solar metallicity and two different gravities (filled circles and solid line Log g=4; empty circles and dotted line Log g=1.5); b) as a function of the gravity for solar metallicity and 4 different effective temperatures; c) as a function of the metallicity for 2 different gravities and 2 different effective temperatures

The [FORMULA] lines show the same behaviour of [FORMULA], the only difference being a decrease of the maximum EW at lower order lines; all the following conclusions are therefore valid also for the other Balmer lines.

[FIGURE] Fig. 4a-c. 14 Gyr old isochrones of Z=0.02 (top), Z=0.001 (center) and Z=0.0001 (bottom). Rectangles show the region of highest EW([FORMULA]); the highest values are reached in the inner rectangle. The zero age [FORMULA] main sequence is also shown (empty circles)

In order to verify if only main sequence stars have the range of temperatures and the high gravities necessary to originate strong Balmer lines, old isochrones (14 Gyr) of different metallicities are shown in Fig. 4 on a modified HR diagram (log g versus [FORMULA]. These diagrams reproduce only the isochrones and do not give the density distribution along them according to the IMF. The main evolutionary phases are easily recognized (main sequence, giant branch, HB, AGB and Post-AGB). The hottest Post-AGB stars lie out of the diagram on the left-bottom side (high [FORMULA] and g) and therefore cannot contribute significantly to the [FORMULA] line. The only stars able to reach the [FORMULA] -strong region of Fig. 4 are the bluest horizontal branch stars with low Z. Stars with solar metallicity do not reach high enough temperatures in this phase. On the contrary, for an age 800 Myr the stellar objects in the rectangles are those around the main sequence turn-off ([FORMULA]), regardless of Z.

In extremely metal-rich populations, other types of stars are expected to have the required temperatures and gravities: Hot and Very Hot HB objects and AGB-manquè stars (Greggio & Renzini 1990, Liebert et al. 1994, Whitney et al. 1994, Bressan et al. 1994). Their contribution to the integrated spectra of stellar systems needs to be verified. On the other hand, if metal-rich HB stars would be responsible for the observed strong Balmer lines in some galaxies, these should belong to a restricted class of objects (the most metal-rich and most luminous) and this luminosity selection is not observed.

The [FORMULA] of [FORMULA] SSPs of different ages are shown in Fig. 5; they are computed using the Jacoby et al. stellar library (1984), whose higher spectral resolution (4 Å) compared to Kurucz's models (20 Å) allows one to study spectral features such as the Balmer lines. The typical uncertainty on the measure of EW is 0.2 Å. If the gaseous emission is not considered, [FORMULA] reaches values [FORMULA] Å already at 3 Myr and remains strong for about 1.5 Gyr; the maximum is reached at about 300 Myr. Comparing our results with the Balmer lines observed in globular clusters (Bica & Alloin 1986, Cohen et al. 1984), a very good agreement is found. In older SSPs (5-15 Gyr), [FORMULA] Å. If the gaseous contribution is included, the line is in emission during the first 10 Myr (having a value of about 40 Å during the first 3 Myr), while the values at the following times remain unchanged.

[FIGURE] Fig. 5. [FORMULA] of single stellar populations of solar metallicity as a function of the age of the SSP; in this case the gaseous emission is not taken into account

The stars of the spectral library employed have [FORMULA], therefore in principle the metallicity dependence of the results could not be investigated. We have seen however (Fig. 3c) that, given the effective temperature and surface gravity, the [FORMULA] does not depend on the metallicity of the atmosphere. The internal structure, and consequently the position of the stars of an SSP on the HR diagram, are instead strongly dependent on the metal content. Therefore the strong Balmer lines observed in the integrated spectra of globular clusters can be studied also using the stellar library of Jacoby et al., by considering isochrones of different metallicities and adopting the Jacoby et al. spectra for any isochrone metal content. The main difference with respect to the solar metallicity case is that the [FORMULA] at low Z (0.0001) is not negligible also for old populations, due to the presence of hot HB stars previously discussed. The dependence of the EW of two Balmer lines on the metallicity for an old SSP (14 Gyr) is shown in Fig. 6. For the [FORMULA] line two distinct regimes are clearly visible and the relations are linear with a correlation coefficient greater than 98%:

[EQUATION]

[EQUATION]

[FIGURE] Fig. 6a-d. EW(H [FORMULA]) and EW(H [FORMULA]) of 14 Gyr old SSPs as a function of the metallicity. [Fe/H] values are computed from Z using solar abundances ratios.

The strong gradient at low metallicities and the shallower at [FORMULA] are the direct consequence of the non-monotonic behaviour of HB stars with metallicity illustrated in Fig. 4. This can account for the large spread in [FORMULA] observed by Bica & Alloin (1986) for old globular clusters.

In the case of the [FORMULA] line (Fig. 6c, d), after the steep gradient at low Z, the line is independent of metallicity for [FORMULA]. This could account for the large scatter and the lack of any correlation of the Balmer lines with metallicity above [Fe/H]=-1 reported by Gregg (1993), Faber & Worthey (1993) and Gregg (1994), for which alternative hypotheses have been proposed but seem unsatisfactory (interstellar [FORMULA] emission filling, blue stragglers in post-core-collapse clusters).

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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