[论文解读] Tracing Pop III supernovae with extreme energies through the Sculptor dwarf spheroidal galaxy

Á. Skúladóttir, Irene Vanni+2 more|arXiv (Cornell University)|May 4, 2023

Stellar, planetary, and galactic studies参考文献 7被引用 5

一句话总结

本研究利用甚大望远镜/紫外-可见光阶梯光谱仪（VLT/UVES）对金属贫乏恒星进行高分辨率光谱观测，追踪了雕塑座矮球状星系中大犬座III型超新星的化学特征。研究在 [Fe/H] = -3 处识别出一颗 CEMP-no 型恒星，并发现存在低能量（E_SN < 1×10⁵¹ erg）和超新星（E_SN = 10×10⁵¹ erg）爆炸的证据，表明其具有双重富集历史，确立了雕塑座作为宇宙中第一代超新星能量分布关键基准的地位。

ABSTRACT

The Sculptor dwarf spheroidal galaxy is old and metal-poor, making it ideal to study the earliest chemical enrichment in the Local Group. We followed up the most metal-poor star known in this (or any external) galaxy, AS0039, with high-resolution ESO VLT/UVES spectra. Our new analysis confirmed its low metallicity, [Fe/H]=-3.90, and that it is extremely C-poor, with A(C)=+3.60, which corresponds to [C/Fe]=-0.33 (accounting for internal mixing). This adds to the evidence of Sculptor being intrinsically C-poor at low [Fe/H]. However, here we also report a new discovery of a carbon-enhanced metal-poor star in Sculptor, DR20080, with no enhancement of Ba (CEMP-no), indicative of enrichment by zero-metallicity low-energy supernovae. This is the first evidence of a dual population of CEMP-no and C-normal stars in Sculptor at $ m[Fe/H]\leq{-3}$. The fraction of CEMP-no stars is still low, $9^{+11}_{-8}\%$ at $ m -4\leq[Fe/H]\leq-3$, compared to the significantly higher fraction in the Milky Way halo, $\approx40\%$. In addition, we re-derive chemical abundances of light, $α$-, iron peak, and neutron-capture elements in all Sculptor stars at $ m [Fe/H]\leq-2.8$, with available high-resolution spectra. Our results show that at these low [Fe/H], Sculptor is deficient in light elements (e.g. C, Na, Al, Mg) relative to both the Milky Way halo, and ultra-faint dwarf galaxies, pointing towards significant contribution of high-energy supernovae. Furthermore, the abundance pattern of the star AS0039 is best fitted with a zero-metallicity hypernova progenitor, with a mass of $M=20$M$_\odot$. Our results in Sculptor, at $ m[Fe/H]\leq-3$, therefore suggest significant enrichment by both very low-energy supernovae and hypernovae, solidifying this galaxy as one of the benchmarks for understanding the energy distribution of the first supernova in the Universe.

研究动机与目标

• 研究雕塑座矮球状星系在极低金属度下的早期化学富集历史。
• 通过分析雕塑座星族III型超新星的恒星丰度，确定其爆炸能量分布。
• 检验雕塑座的丰度模式是否与银河系晕和极端微弱矮星系的丰度模式存在差异。
• 在 [Fe/H] ≤ -3 范围内识别并表征首颗 CEMP-no 型恒星，为低能量星族III型超新星提供直接证据。

提出的方法

• 利用欧洲南方天文台甚大望远镜/紫外-可见光阶梯光谱仪（ESO VLT/UVES）对雕塑座中11颗金属贫乏恒星（包括两颗新观测的恒星）进行高分辨率光谱观测，测量其元素丰度。
• 对最金属贫乏的恒星 AS0039 应用非局部热动平衡（NLTE）分析，以校正原子谱线重叠及非局部热动平衡效应。
• 将丰度模式与不同爆炸能量（E_SN）和原行星质量的零金属度超新星理论模型进行比较。
• 在 [Fe/H] 范围 -4 ≤ [Fe/H] ≤ -3 内计算 CEMP-no 型恒星的比例，以评估低能量超新星的富集贡献。
• 对所有 [Fe/H] ≤ -2.8 的恒星重新推导轻元素、α-族元素、铁峰元素和中子俘获元素的丰度，以评估整体化学趋势。
• 采用参数化模型拟合观测到的 [X/Fe] 丰度模式，特别是针对 AS0039，以推断最可能的原行星能量与质量。

Figure 1: Position on the sky ( left ) and location on the RGB ( right ) for our Sculptor stellar sample, based on Gaia DR3 (Table 2 ). Star symbols with labels are stars with new spectra, for the first time analysed here; while large blue circles are stars with available literature spectra, reanaly

实验结果

研究问题

• RQ1 低能量与高能量超新星在雕塑座矮球状星系早期化学富集中各自贡献如何？
• RQ2 在 [Fe/H] ≤ -3 范围内，雕塑座中 CEMP-no 型恒星的比例与银河系晕及极端微弱矮星系相比如何？
• RQ3 基于其丰度模式，雕塑座中最金属贫乏恒星 AS0039 的爆炸能量与原行星质量为何？
• RQ4 为何在 [Fe/H] ≤ -2.8 时，雕塑座表现出轻元素（如 C、Na、Al、Mg）的缺乏，相较于银河系与 UFD 星系？
• RQ5 AS0039 的丰度模式更可能由超新星还是低能量超新星解释？

主要发现

• 雕塑座中最金属贫乏的恒星 AS0039 的 [Fe/H]_LTE = -3.90 ± 0.15，A(C) = +3.60，对应 [C/Fe]_LTE = -0.33 ± 0.17，确认其为极度碳贫乏。
• 新发现一颗 CEMP-no 型恒星 DR20080，位于 [Fe/H] = -3，无 Ba 元素增强，表明其由 E_SN < 1×10⁵¹ erg 的零金属度超新星富集。
• 在 -4 ≤ [Fe/H] ≤ -3 范围内，雕塑座中 CEMP-no 型恒星的比例 f_Scl^CEMP = 9⁺¹¹₋₈%，显著低于银河系晕中 40% 的比例。
• AS0039 的丰度模式最符合零金属度超新星模型，其爆炸能量 E_SN = 10×10⁵¹ erg，原行星质量为 20 M⊙。
• 雕塑座在轻元素（C、Na、Al、Mg）上相对于银河系和极端微弱矮星系表现出缺乏，与高能超新星（E_SN > 2×10⁵¹ erg）富集一致。
• 结果证实雕塑座同时存在低能量与超新星富集，使其成为理解宇宙中第一代超新星完整能量分布的关键基准。

Figure 2: X-Shooter spectra for the CEMP-no star in our sample, DR20080 (red), at $\rm[Fe/H]=-3$ . For a fair comparison the X-Shooter spectra of PJ07402 at the same $\rm[Fe/H]=-3$ , is also shown in blue (Starkenburg et al., 2013 ) . However, for the full chemical abundance analysis of PJ07402, her