原恒星幕式吸积的年代学--CO和H $_2$ O雪线的PDKEEPAAAAA巡天
Chronology of Episodic Accretion in Protostars -- an ALMA survey of the CO and H$_2$O snowlines
摘要：
幕式吸积被用来解释原恒星光度的广泛变化，但它的起源和对恒星形成过程的影响尚未完全了解。我们提出了一个ALMA调查的N $_2$ H $^+$（$1-0$）和HCO $^+$（$3-2$）对39类0和I类来源的英仙座分子云。N $_2$ H $^+$和HCO $^+$分别通过与CO和H $_2$ O的气相反应被破坏，从而追踪CO和H $_2$ O雪线位置。雪线的位置比目前的光度预期的半径大得多，这表明过去发生了一次吸积爆发，使雪线向外移动。在$_2$ H $^+$中，我们发现了18/18的0类和9/10的I类爆发后源，在HCO $^+$中，我们发现了7/17的0类和1/8的I类爆发后源，在过去的爆发中，我们发现吸积光度是$\sim10-100~L_\odot$。这一结果可以解释为爆发频率的演变或盘的演变。在前一种情况下，假设\ce{H2O}的再冻结时间尺度为1000年，CO为10，000年，我们发现爆发之间的间隔从0级的2400年增加到I级的8000年。这种爆发频率的降低可能反映了碎片更有可能发生在早期的演化阶段，当年轻的恒星物体更容易不稳定时。
英文摘要：
Episodic accretion has been used to explain the wide range of protostellar luminosities, but its origin and influence on the star forming process are not yet fully understood. We present an ALMA survey of N$_2$H$^+$ ($1-0$) and HCO$^+$ ($3-2$) toward 39 Class 0 and Class I sources in the Perseus molecular cloud. N$_2$H$^+$ and HCO$^+$ are destroyed via gas-phase reactions with CO and H$_2$O, respectively, thus tracing the CO and H$_2$O snowline locations. A snowline location at a much larger radius than that expected from the current luminosity suggests that an accretion burst has occurred in the past which has shifted the snowline outward. We identified 18/18 Class 0 and 9/10 Class I post-burst sources from N$_2$H$^+$, and 7/17 Class 0 and 1/8 Class I post-burst sources from HCO$^+$.The accretion luminosities during the past bursts are found to be $\sim10-100~L_\odot$. This result can be interpreted as either evolution of burst frequency or disk evolution. In the former case, assuming that refreeze-out timescales are 1000 yr for \ce{H2O} and 10,000 yr for CO, we found that the intervals between bursts increases from 2400 yr in the Class 0 to 8000 yr in the Class I stage. This decrease in the burst frequency may reflect that fragmentation is more likely to occur at an earlier evolutionary stage when the young stellar object is more prone to instability.
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