在遥远的星系外围富钙间隙瞬变
Calcium-rich gap transients in the remote outskirts of galaxies
摘要：
从帕洛玛瞬态工厂的前两个赛季，我们确定了三个奇特的瞬态（PTF 09 dav、PTF 10 iuv、PTF 11bij），具有五个显著特征：新星与超新星间隙的峰值光度（M_R = 15.5 ~-16.5），快速光度演化（上升时间~12- 15天），大光球速度（~6000 ~ 11000 km/s），早期光谱演化到星云相（~1 ~ 3个月），以及以钙为主的奇特星云光谱。我们还挑选了Lick天文台长达十年的超新星搜索数据库，并确定了这个群体的另一个成员SN 2007 ke。我们对光度和光谱特性的选择是基于SN 2005 E（Perets et al. 2010）。令我们惊讶的是，就像SN 2005 E的情况一样，这个星系群的所有四个成员都明显地偏离了它们的宿主星系。给出了采样良好的早期和晚期光变曲线，我们得到的喷出物质量在0.4- 0.7 Msun的范围内。在光谱学上，我们发现光球相可能存在差异，但共同之处在于不寻常的星云光谱。我们广泛的后续观测排除了这类“富钙间隙”瞬变的标准热核和标准核心坍塌爆炸。如果它的前身是一颗白矮星，我们很可能会看到白矮星核心的爆炸，甚至可能会看到激波与先前喷出的新星外壳的相互作用。在大质量恒星祖先的可能性较小的情况下，需要调用一个非常非标准的低金属丰度环境通道（例如，导致黑洞形成的喷出物回落）。探测到（或缺乏）一个微弱的潜在宿主（矮星系，星团）将提供一个关键和决定性的诊断，在这些选择之间做出选择。
英文摘要：
From the first two seasons of the Palomar Transient Factory, we identify three peculiar transients (PTF09dav, PTF10iuv, PTF11bij) with five distinguishing characteristics: peak luminosity in the gap between novae and supernovae (M_R = 15.5 to -16.5), rapid photometric evolution (rise-time ~12--15 days), large photospheric velocities (~6000 to 11000 km/s), early spectroscopic evolution into nebular phase (~1 to 3 months) and peculiar nebular spectra dominated by Calcium. We also culled the extensive decade-long Lick Observatory Supernova Search database and identified an additional member of this group, SN 2007ke. Our choice of photometric and spectroscopic properties was motivated by SN 2005E (Perets et al. 2010). To our surprise, as in the case of SN 2005E, all four members of this group are also clearly offset from the bulk of their host galaxy. Given the well-sampled early and late-time light curves, we derive ejecta masses in the range of 0.4--0.7 Msun. Spectroscopically, we find that there may be a diversity in the photospheric phase, but the commonality is in the unusual nebular spectra. Our extensive follow-up observations rule out standard thermonuclear and standard core-collapse explosions for this class of "Calcium-rich gap" transients. If the progenitor is a white dwarf, we are likely seeing a detonation of the white dwarf core and perhaps, even shockfront interaction with a previously ejected nova shell. In the less likely scenario of a massive star progenitor, a very non-standard channel specific to a low-metallicity environment needs to be invoked (e.g., ejecta fallback leading to black hole formation). Detection (or lack thereof) of a faint underlying host (dwarf galaxy, cluster) will provide a crucial and decisive diagnostic to choose between these alternatives.
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