After producing the comprehensive elemental catalog, the secondary goal of this project was to look for evidence of alteration in stellar composition due to fusion processes during the main-sequence lifetime of the stars. Of the known processes, I found strong observational evidence of two, and possible evidence for a third.
7.3.1 Carbon, Nitrogen and Oxygen
With the assumed age range between 700 Myr and 8 Gyr, and Main Sequence Turn Off (MSTO) masses between 2.4M and 1.2M , I expected the evolved members of my cluster stars to show evidence of CNO-cycle processing. Even though the CNO processing is ongoing throughout a star’s lifetime, conditions at the core-radiative zone boundary isolate core materials until core hydrogen fusion ends. Convective processes during the H shell fusion, and core He fusion phases allow the altered core materials to mix into the stars’ atmosphere, becoming detectable in giant star spectra. Iben(1964) and
Iben(1991) summarize the atmospheric differences I expected to see as a result of this “first dredge- up”. Specifically, there should be increases in13C (relative to12C) and14N.
Böcek Topcu et al. explained the relative N enhancement and C depletion to main-sequence evo- lutionary processes, namely He production in the core through the CNO cycle. The relevant portion of the CNO cycle is that which occurs at the lowest temperature:
12C(p, γ)13N(, e+γ
e)13C(p, γ)14N(p, γ)15O(, e+γe)15N(p, α)12C
The slow step, or “bottleneck”, is the14N proton capture. Over the course of the main-sequence lifetime of a star, this results in an increase of N abundance at the expense of the 12C abundance. With the IC 4756, Hyades, and Praesepe MSTO masses above 2.0M , evidence of processing from the higher temperature branch of the CNO-I-cycle (CNO-II) might also be present. Specifically, the high temperature branch has two steps which require proton capture by O. Like the N increase in the CNO-I cycle, increased O abundance might be explained through the capture steps in the higher temperature CNO-II cycle. I did not measure a significantly increased O abundance over any of the clusters, including the three with the highest turnoff masses. Therefore, I conclude that these higher temperature branches are not significant contributors to the elemental alteration of the sample clusters. Admittedly, measuring the CNO abundances in giant stars alone only shows half the picture. The critical comparison should occur between main-sequence and evolved members. To that end, I mea- sured C and O abundances using the absorption features at 7115 Å and 7775 Å, respectively. These abundance measurements do hint at the expected trend for CNO processing, but measurement un- certainty, particularly due to the lack of strong CN features in our dwarf sample, prevents me from making a definitive statement to that effect, based on C and O measures, alone.
The third “piece” - N abundance - shows stronger evidence of the expected trend for CNO pro- cessing. N abundance measurements are mostly the result of molecular CN feature synthesis, so I was largely unable to determine N abundances for a large fraction of the dwarf population. The few dwarf N abundance measurements usually are a single EQW measurement of the 7442.29 Å N I feature. Fre- quently, the software was able to detect the 7442.29 Å line, but disqualified it due to the EQW measure- ments falling below the acceptable minimum width threshold for the spectra’s S/N ratio. However, in the few cases where I have dwarf and giant N abundance measures, they show a marked increase from dwarfs to giants.
7.3. Elemental Evolution 79
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[Fe/H] = 0.00
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C/N
FIGURE 7.1: C/N ratios in the giants of my selected clusters (plotted by age). Included in the figure are the analytical fits for metallicity ([Fe/H]) values of
-0.10, 0.00, and +0.30 fromSalaris et al. (2015).
In paper I we noted that the [C/N] ratio of -0.49, as measured in our NGC 752 giant sample, agreed with the expected [C/N] of -0.54 for a 1.5Gyr giant just after its first dredge-up ([C/N]FDU) at solar metallicity given inSalaris et al. (2015).Salaris et al. provide an analytical solution for the Age- (C/N, [Fe/H]) relation, which I have plotted in Figure7.1along with all of my target cluster [C/N] ratios. Although my error bars are large, and the Salaris relation is not well defined below 900Myr, my measurements do appear to follow the expected trend.
7.3.2 Na, Mg, Al, and Si - Evidence for nucleosynthesis
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[N
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] (
de
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FIGURE 7.2: The increase of Na abundance when comparing giant to dwarf atmospheres, defined as∆[Na/Fe], plotted against cluster age. The points show definite Na engancement in the giants, decreasing with cluster age, possibly due
to lower core temperatures in less massive stars.
When comparing Na abundances between the dwarf and giant populations, I found that Na abun- dance shows a clear increase between the dwarf and giant populations, as shown in Figure7.2, above. For simplicity,∆Na = A(Nagiant) −A(Nadwarf). While there does appear to be a trend in the mean ∆Na, when uncertainties are taken into account, it is hard to make a claim on a temporal trend. Even though my choice of Na lines did not include the strong 5890/5896 Å or 8183/8195 Å doublets, I did correct for NLTE effects, as suggested inLind et al. (2011). The measured Na increase in the giants is possibly due to the same mechanism asBoesgaard, Roper & Lum(2013) noted in comparing their Praesepe dwarf population with the giants studied byCarrera & Pancino(2011). Specifically, that the enrichment is a result of the NeNa cycle as detailed inArnould & Mowlavi(1995). As with the O and N enhancement during CNO processing, the23Na proton capture has the smallest cross-section, which provides an explanation for my measured enhancement. The relevant cycle is:
20Ne(p, γ)21Na(e+γ