A brief history of supermetallicity

In the 1950s, spectroscopists were aware that the nucleus of M31 has strong CN lines, with strengths similar to the line strengths found for stars in the Solar vicinity (Morgan 1958). Baade (1963) stated that "After the first generation of stars has been formed, we can hardly speak of a generation, because the enrichment takes place so soon, and there is probably very little time difference. So the CN giants that contribute most of the light in the nuclear region of the Nebula must also be called old stars; they are not young." This is a remarkably prescient insight that largely describes the present-day picture of the chemical evolution of spheroids.

The work of Spinrad & Taylor (1969) is considered the seminal work on superme-tallicity, but it was predated by an interest in the spectroscopy of galaxies (Spinrad 1961) that resulted in the identification of strong Na lines in the nucleus of M31, a finding that was to be a subject of debate for over two decades. Hindsight finds the most extreme metallicities ([Fe/H] = +0.75 for NGC 6791) to be skirting the bounds of credibility, and this no doubt has contributed to the general atmosphere of skepticism; but one must consider the state of abundance analysis at that time. No doubt NGC 6791 remains today the most metal-rich open cluster; Spinrad and Taylor at least succeeded in getting the correct ranking. Was the supermetallicity in fact a real phenomenon? On the one hand, Gustafsson et al. (1974) argued for its reality, using narrow-band photometry (with a pulse-counting photometer) of spectral regions selected to have clumps of weak iron lines adjacent to clean continuum; this was an innovative method for its time and turns out to have given the correct answer. On the other hand, Peterson (1976) argued that supermetallicity was spurious and arises from the temperature profile of the stellar atmosphere (boundary cooling). The argument stated that excess CN causes a steepening of the boundary temperature gradient; the anomalous cooling strengthens the lines of neutral metals. This in turn masquerades as supermetallicity. The dispute was largely settled by two papers. The first was that of Branch et al. (1978), which was the first modern analysis of the prototype super-metal-rich (SMR) star |^-Leonis, using a reticon detector that yielded spectra with very high SNR (Figure 1.1). The application of a modern detector with a linear response and the capability to produce high-SNR

a-Ari a-Ari

7,732

7,734

7,772

7,774

7,800

7,802

WAVELENGTH (A)

Figure 1.1. One of the first high-resolution digital reticon spectra of ^-Leo, the comparison with Solar-metallicity a-Ari shows the clear enhancement of the weak iron lines relative to the continuum (Branch et al. 1978). The enhancement of metals in the super-metal-rich star ^-Leo ([Fe/H] = +0.3) is obvious.

data represented a breakthrough in the subject. While the abundance of |x-Leonis is debated at the 0.1-dex level, that study finds what is essentially the modern value of [Fe/H] for |x-Leonis. The second was that of Deming & Butler (1979), who considered the abundances of binary-star companions of SMR stars; these authors also find a genuinely elevated iron abundance in SMR stars. Deming (1980) argued that Peterson's work had been affected by a subtle misplacement of the continuum in her photographic spectra (a continuum misplacement of 1% leading to a 0.1-dex effect on metallicity).

A sidebar to this debate was Spinrad's (1961) claim that enhanced Na D lines in the M31 nucleus argued for dwarf enhancement in that population. The resulting controversy continued through studies by Whitford (1977), on the FeH Wing-Ford bands, and Faber & French (1980), on the Na 8190 lines in the nucleus of M31, while ultimately settling on giant-dominated light on the basis of infrared studies (e.g. Frogel & Whitford 1987). Studies of the Galactic-bulge luminosity function (Zoccali et al. 2000) do not find an abnormally bottom-heavy mass function; the issue is resolved in favor of giant-dominated light.

The effect of the debate over supermetallicity has been, nonetheless, to cast aspersions on the subject. That is in part why we had to wait until 2006 for a meeting on the metal-rich Universe.

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