[论文解读] High Metallicity LGRB Hosts

J. F. Graham, A. S. Fruchter+10 more|arXiv (Cornell University)|Nov 2, 2015

Astronomy and Astrophysical Research被引用 7

一句话总结

本文研究了高金属度宿主星系中罕见的长伽马射线暴（LGRBs），挑战了长期以来认为LGRBs仅在低金属度环境中形成的理论。通过使用Gemini/GMOS、Keck/NIRSPEC和哈勃空间望远镜对LGRB 051022和LGRB 020819B的深成像与光谱观测，作者发现这些暴发发生在金属度接近太阳值的宿主星系中（12+log(O/H) ≈ 8.6–8.7），表明高金属度LGRB形成虽罕见但可能，提示可能存在不依赖金属度的形成路径，可能涉及双星相互作用。

ABSTRACT

We present our imaging and spectroscopic observations of the host galaxies of two dark long bursts with anomalously high metallicities, LGRB 051022 and LGRB 020819B, which in conjunction with another LGRB event with an optical afterglow comprise the three LGRBs with high metallicity host galaxies in the Graham & Fruchter (2013) sample. In Graham & Fruchter (2013), we showed that LGRBs exhibit a strong and apparently intrinsic preference for low metallicity environments (12+log(O/H) < 8.4 in the KK04 scale) in spite of these three cases with abundances of about solar and above. These exceptions however are consistent with the general star-forming galaxy population of comparable brightness & redshift. This is surprising: even among a preselected sample of high metallicity LGRBs, were the metal aversion to remain in effect for these objects, we would expect their metallicity to still be lower than the typical metallicity for the galaxies at that luminosity and redshift. Therefore we deduce that it is possible to form an LGRB in a high metallicity environment although with greater rarity. From this we conclude that there are three possible explanations for the presence of the LGRBs observed in high metallicity hosts as seen to date: (1) LGRBs do not occur in high metallicity environments and those seen in high metallicity hosts are in fact occurring in low metallicity environments that have become associated with otherwise high metallicity hosts but remain unenriched. (2) The LGRB formation mechanism while preferring low metallicity environments does not strictly require it resulting in a gradual decline in burst formation with increasing metallicity. (3) The typical low metallicity LGRBs and the few high metallicity cases are the result of physically different burst formation pathways with only the former affected by the metallicity and the later occurring much more infrequently.

研究动机与目标

• 调查长伽马射线暴（LGRBs）在高金属度宿主星系中的起源，这些宿主与既定的低金属度环境偏好相矛盾。
• 确定观测到的高金属度LGRB宿主是否为真实宿主，或是否由于宿主内局部低金属度区域的污染所致。
• 评估LGRB形成是否可在高金属度环境中通过与标准低金属度路径不同的机制发生。
• 评估双星伴星在高金属度恒星中通过增强质量损失仍能促进喷流形成的作用。

提出的方法

• 利用Gemini/GMOS、Keck/NIRSPEC和哈勃空间望远镜仪器，对LGRB 051022和LGRB 020819B的宿主星系进行深成像与光谱观测。
• 使用KK04标度测量氧丰度，以确定宿主金属度，结果为12+log(O/H) ≈ 8.6–8.7，与太阳或近太阳值一致。
• 比较LGRB事件在其宿主星系内的空间分布与恒星形成区及金属度梯度的关系，检验暴发是否发生在低金属度区域。
• 建议使用激光导星自适应光学与红外积分场单元光谱技术，在红移z > 0.8时实现空间分辨的金属度测量。
• 建议结合可调窄带ACS扫掠滤波器与IFU观测，以克服高红移下大气视宁度的限制。
• 评估双星演化路径作为潜在机制的可能性，以提升高金属度前身星的角动量，从而在增强质量损失下仍能实现喷流形成。

实验结果

研究问题

• RQ1 长伽马射线暴是否可在高金属度环境中形成，还是观测到的宿主因局部低金属度区域而被误标？
• RQ2 LGRB形成机制是否严格依赖于低金属度，还是暴发率随金属度增加而逐渐下降？
• RQ3 高金属度LGRBs是否通过与典型低金属度LGRBs不同的物理机制形成？
• RQ4 在高金属度系统中，双星相互作用是否能提供足够的角动量以实现喷流准直，从而绕过金属度限制？

主要发现

• LGRB 051022和LGRB 020819B被发现在12+log(O/H) ≈ 8.6–8.7的宿主星系中，表明其金属度为太阳或近太阳值，与LGRB宿主通常偏好低金属度的特征相矛盾。
• 这两个高金属度LGRB宿主与同光度、同红移的典型星暴星系群体一致，表明不存在对低金属度的内在偏好。
• 这些高金属度LGRBs的存在表明，尽管低金属度强烈偏好，但并非LGRB形成的严格必要条件。
• 作者得出结论：可能存在一种不依赖金属度的形成路径，可能涉及双星伴星，从而解释高金属度环境中LGRB的罕见现象。
• 建议开展空间分辨光谱观测，以确定暴发是否发生在宿主星系内的低金属度区域，或如典型LGRB宿主一样发生在高金属度恒星形成区。
• 作者假设双星相互作用可能提升高金属度前身星的角速度，使其在增强质量损失下仍能实现喷流形成，为标准单星模型提供一种可行的替代机制。