麦哲伦云中超红碳星的CO转动线发射
CO rotational line emission in very red carbon stars in the Magellanic Clouds
AI总结：
研究利用ALMA观测麦哲伦云的碳星，首次在SMC的AGB恒星周围探测到CO，发现尘埃基MLR比CO线基高1.6倍，且麦哲伦云碳星的MLR与银河系极端长周期碳星的金属丰度无相关性。
AI中文摘要：
初始质量低和中等的恒星在渐近巨星支（AGB）阶段末期会损失大部分恒星质量，确定它们的气体和尘埃质量损失率（MLRs）对于量化演化恒星对宇宙中尘埃和气体生命周期的贡献至关重要。研究使用阿塔卡马大型毫米/亚毫米阵列（ALMA）观测了大麦哲伦云（LMC）中的38颗碳星（C星）和小麦哲伦云（SMC）中的3颗碳星的CO J=2-1谱线，拟合谱线轮廓以推导恒星速度和星风膨胀速度（Vexp）。在SMC的2颗碳星和LMC的33颗碳星中检测到CO发射，这是首次在SMC的AGB恒星周围探测到一氧化碳；LMC的1颗天体显示出$^{13}$CO发射。星风膨胀速度范围约为7.5至约30 km/s。利用档案数据确定脉动周期，并使用两个尘埃辐射传输代码构建和建模光谱能量分布，通过这两个代码以及利用简单公式从CO线强度独立推导质量损失率。平均而言，基于尘埃的质量损失率比基于CO线的质量损失率高1.6倍，需要其他跃迁的额外CO数据结合适当建模来进一步研究这种可能的差异。将质量损失率、脉动周期和膨胀速度与银河系碳星样本进行比较，发现存在强烈偏差：麦哲伦云目标样本的质量损失率和光度最高，但仅占银河系样本恒星的少数；将该样本与周期超过500天的一组类似极端的银河系恒星比较，未发现金属丰度与质量损失率或Vexp之间存在相关性。
英文摘要：
Stars of low and intermediate initial mass lose most of their stellar mass at the end of their lives during the asymptotic giant branch (AGB) phase. Determining their gas and dust mass-loss rates (MLRs) is crucial for quantifying the contribution of evolved stars to the life cycle of dust and gas in the Universe. The Atacama Large Millimeter/submillimeter Array was used to observe 38 carbon stars (C stars) in the large Magellanic cloud (LMC) and three C stars in the small Magellanic cloud (SMC) in the CO J= 2-1 line. Line profiles were fitted to derive stellar velocities and wind-expansion velocities (Vexp). CO emission is detected in two C stars in the SMC and 33 C stars in the LMC. This is the first detection of carbon monoxide around an AGB star in the SMC. One object in the LMC shows emission in $^{13}$CO. The wind-expansion velocity ranges from \sim7.5 to \sim30 km/s. Archival data were used to determine the pulsation periods as well as construct and model the spectral energy distributions using two dust radiative transfer codes. Mass-loss rates were independently derived from these two codes as well as from the intensity of the CO line, using a simple formula. On average, the dust-based MLRs higher than the MLRs based on the CO line by a factor of 1.6. Additional CO data in other transitions, combined with proper modelling, is required to further investigate this possible discrepancy. Mass-loss rates, pulsation periods, and expansion velocities were compared to a sample of Galactic C stars. There is a strong bias, as the Magellanic Cloud targets sample the highest MLRs and luminosities, yet they represent only a minority of stars in a Galactic sample. Comparing this sample with a similarly extreme set of Galactic stars with periods longer than 500 days, we identify no correlation between metallicity and either the MLR or Vexp.
