The explosive alphaelements silicon calcium and titanium

The elements silicon, calcium, and titanium are all believed to be created in the explosive nucleosynthesis phase of Type-II supernovae (Woosley & Weaver 1995). Owing to our belief in their common origin, we have averaged the three [X/Fe] ratios of these elements to reduce the effects of random observational scatter. We define [aEx/Fe] = [(Si + Ca + Ti)/Fe] = ([Si/Fe] + [Ca/Fe] + [Ti/Fe])/3.

We plot the results for this combination in Figure 12.3. As we have seen in Figures 12.1 and 12.2, the bulge field stars and cluster stars have [aEx/Fe] values as high as or higher than those of the other populations of the galaxy (again with the exception of HP-1 and the Zoccali et al. result for NGC 6528). As before, we conclude that this indicates a higher ratio of Type-II to Type-Ia supernovae contributions than elsewhere in the Galaxy at a given metallicity. Note that MR94 found high [Ti/Fe] values for their metal-rich stars. Owing to the superior spectra of the Fulbright et al. (2006b) analysis, we favor the more recent result.

As in the case of the decrease in bulge [O/Fe] ratios at high metallicity, the drop in [aEx/Fe] values cannot arise purely from Type-Ia contributions. Type-Ia supernovae make large amounts of Fe-group elements, but they do contribute some lighter elements as well. The yields decrease with decreasing atomic number - a given Type Ia makes more Ti than Ca, but more Ca than Si, and so on (Thielemann et al. 1996). These heavy alpha-element contributions from Type-Ia SNe can

1

| 1 1 1

1 1 1 1 1 1 1 1 1 Fulbright 2000

Bensby et al. 2005 m

A

Reddy et al. 2003 *

"x

* if x m x X * • • x x x £ x x x x , x xx x x

\ * m

• .

X >S< X X

± A S

• _

A

X XX *

Fulbright 2000 x Bensby et al. 2005 aa Reddy et al. 2003 *

• Baade's Window H HP-1 ADO NGC 6553 ▲ ■ # NGC 6528 B

Figure 12.3. Top panel: [(Si + Ca + Ti)/Fe] (i.e. [aEx/Fe]) versus [Fe/H] ratios for bulge, disk, and halo stars from several surveys. The bulge field-star data are from Fulbright et al. 2006, while the thin-disk data are from Reddy et al. and Bensby et al. (small triangles), and the three-point crosses indicate the region's thick-disk stars. The Baade's Window data lie at or above those for all of the other populations in the Galaxy at all metallicities. Bottom panel: the same as above, but with the bulge globular-cluster data added (using the same symbols as in Figures 12.1 and 12.2). As with the oxygen and magnesium data, data for the bulge clusters lie along the range of the bulge field-star data. The cluster HP-1 and the Zoccali et al. data point for NGC 6528 lie well below all other points. The HP-1 point here agrees with what was seen with its [Mg/Fe] ratio. The NGC 6528 point is well below that of the other two studies of that cluster (the Zoccali et al. [Mg/Fe] ratio for NGC 6528 in Figure 12.2 also lies below that of the other two studies, whereas the [O/Fe] ratios are in good agreement).

be seen as a lower decline in [X/Fe] ratios from the "plateau" values in the metal-poor halo to Solar values in the disk. For example, the "plateau" value for [Mg/Fe] in the halo is about +0.5 dex, compared with +0.3 dex for [Ti/Fe] (Fulbright 2000). The same amount of Fe is added to the system in both cases, but more Ti was added at the same time (presumably by Type-Ia SNe) to lessen the extent of the "dilution."

This means that if Type-Ia SNe were the reason for the drop in the [aEx/Fe] ratios at high metallicities then the [Mg/Fe] ratio should have dropped more. Yet the opposite is seen in the bulge. We cannot exclude all Type-Ia contributions, of course - if the previous estimates of a 500-Myr timescale for star formation in the bulge still hold then we expect a reasonable amount of Type-Ia ejecta to be included in the stars.

In the top panel of Figure 12.3 we have included the data for individual field halo and disk stars from various surveys to show the internal scatter within the various populations. The rms scatter between bulge points (about 0.04 dex) from Fulbright et al. (2006b) is similar to that of the thin disk (Reddy et al. and Bensby et al. points) and smaller than that of the halo (the metal-poor Fulbright (2000) points).

The lesser scatter seen for the bulge stars indicates that the bulge composition evolved much more homogeneously than that of the halo. The high [aEx/Fe] values in the bulge suggest that either Type-II SNe in the bulge had more massive progenitor masses than did that in the halo (equal to the highest mass function in the halo), or the halo experienced more nucleosynthesis contributions from Type-Ia SNe than did the bulge. The range of [a/Fe] ratios seen in the halo certainly indicates that it experienced a very inhomogeneous enrichment history. More extreme evidence of alpha/Fe dispersion in the halo is already well established (Nissen & Schuster 1997, Brown et al. 1997, Fulbright 2002).

Wyse & Gilmore (1992) proposed that the bulge formed from Galactic-spheroid (halo) gas, because of the similarity of the specific angular momenta of these two systems, and because the low mean metallicity indicates that 90% of the spheroid gas was lost. These abundances provide an interesting test of this bulge-formation idea: since the [aEx/Fe] ratios in the most metal-poor bulge stars are higher than those of stars in the halo, and because the most metal-poor bulge stars, at

[Fe/H] ---1.3 dex, are of metallicity similar to the mean metallicity for the halo, the metal-poor bulge stars could not have been made from halo gas with the average metallicity and composition seen today. However, the metal-poor bulge stars could have been produced from halo gas, provided that the halo composition at that time was similar to the top ~20% of the halo [aEx/Fe] ratios seen today, i.e. provided that the mean halo composition changed with time. Thus, this result suggests that, if the bulge formed out of halo gas, then the onset of bulge formation occurred before ~80% of the chemical enrichment of today's stellar halo had occurred.

Was this article helpful?

0 0

Post a comment