Level structure of the $T_z=-1$ nucleus ${}^{34}\mathrm{Ar}$ and its relevance for nucleosynthesis in ONe novae

The ${}^{24}\mathrm{Mg}+{}^{12}\mathrm{C}$ fusion reaction was used to perform a detailed $\gamma$-ray spectroscopy study of the astrophysically important nucleus ${}^{34}\mathrm{Ar}$. In particular, an experimental setup, coupling the advanced $\gamma$-ray tracking array GRETINA with the well-established Argonne fragment mass analyzer (FMA), was employed to obtain excitation energies and spin-parity assignments for excited states in ${}^{34}\mathrm{Ar}$, both above and below the proton separation energy. For the first time, an angular distribution analysis of in-beam $\gamma$ rays from fusion-evaporation reactions, using a tracking array, has been performed and Coulomb energy differences of analog states in the $T=1,$ $A=34$ mirror system, explored from 0 to 6 MeV. Furthermore, we present a comprehensive discussion of the astrophysical ${}^{33}\mathrm{Cl}(p,\gamma )$ stellar reaction rate, together with implications for the identification of nova presolar grains from sulfur isotopic abundances.

Figure 1

Portions of the $\gamma$-ray singles spectra observed in coincidence with (a) all recoils, (b) S, (c) Cl, and (d) Ar recoils, following the application of suitable gating conditions in the ionization chamber (IC), as discussed in the text.

Figure 2

A $\gamma$-ray spectrum gated on the 2091-keV, $2_1^{+}\to 0_1^{+}$ transition in ${}^{34}\mathrm{Ar}$. The peaks are labeled with transition energies given in keV.

Figure 3

Example $\gamma$-ray angular distributions obtained in the present work for both ${}^{34}\mathrm{Ar}$ and ${}^{34}\mathrm{S}$. $\mathrm{\Delta }J=1$ transitions are shown on the top row while $\mathrm{\Delta }J=2$ transitions are given at the bottom. The error bars on the data points reflect the statistical uncertainty on the extracted $\gamma$-ray intensities.

Figure 4

Proposed assignments of analog states in the $T=1$, ${}^{34}\mathrm{Ar}\text{−}{}^{34}\mathrm{S}$ mirror system for excitation energies up to $\approx 6$ MeV, together with a comparison with shell-model calculations. Excitation energies for levels in ${}^{34}\mathrm{Ar}$ above 5061 keV are taken from the ${}^{36}\mathrm{Ar}(p,t$) reaction study of Long et al. [25].

Figure 5

Uncertainties in the ${}^{33}\mathrm{Cl}(p,\gamma$) stellar reaction rate based on the present data, in comparison with previous estimates. See text for details [32].

Figure 6

Expected $\delta {}^{33}\mathrm{S}$/${}^{32}\mathrm{S}$ and $\delta {}^{34}\mathrm{S}$/${}^{32}\mathrm{S}$ ratios from the present data in comparison with extracted values from isolated presolar grains. The most promising nova grain candidate, $\mathrm{Ag}2\_6$ [39], is highlighted by a purple diamond, while blue circles represent data obtained on the grains: M7-C [40], M7-D [40], KJE-al-5-7 [18], $\mathrm{G}270\_2$ [39], GAB [39], Ag2 [39], M1-A8-G145 [41], KJD-1-11-5 [42], and KJD-3-23-5 [42]. Finally, the shaded region represents the $\delta {}^{33}\mathrm{S}$/${}^{32}\mathrm{S}$ and $\delta {}^{34}\mathrm{S}$/${}^{32}\mathrm{S}$ ratio ranges of type-II supernovae [37].