Barnard 5中中性与离子线宽：MHD波穿透的证据
Neutral vs Ion Linewidths in Barnard 5: Evidence for Penetration by MHD Waves
摘要：
稠密的核心是湍流消散的最终场所。从理论上已经提出，对于分子离子（与中性粒子相比）和具有更高临界密度的跃迁，非热速度色散应该更窄。为了检验这些假设，我们比较了巴纳德5号致密核中N $_2$ H $^+$（1- 0）（n $_{\rm crit}$ = $6\times10^4$ cm $^{-3}) and NH$ _3 $ (n$ _{ \rm crit}=2 \times 10^3 $ cm$ ^{-3}）的速度弥散。我们分析了结合GBT和VLA数据获得的NH $_3$（1，1）和（2，2）以及GBT Argus获得的N $_2$ H $^+$（1- 0）的高分辨率和高信噪比观测，它们呈现出相似的形态。令人惊讶的是，离子的非热速度分散系统地比中性的高20%。导出的音速马赫数$\mathcal{M}_s = σ_{\rm NT}/c_s$，对于N $_2$ H $^+$和NH $_3$分别有峰值$\mathcal{M}_{s, {\rm N_2H^+}} = 0.59$和$\mathcal{M}_{s, {\rm NH}_3} = 0.48$。这种观察到的差异可能表明，即使在致密核心内部深处的磁场仍然在振荡，因为它在核心外部的湍流区域中。离子应该比中性粒子更强地动态耦合到这个振荡场，从而解释它们更宽的线宽。如果进一步的观察证实，这一发现将为致密核心向静止的转变提供更多的线索。
英文摘要：
Dense cores are the final place where turbulence is dissipated. It has been proposed from theoretical arguments that the non-thermal velocity dispersion should be narrower both for molecular ions (compared to neutrals) and for transitions with higher critical densities. To test these hypotheses, we compare the velocity dispersion of N$_2$H$^+$ (1--0) (n$_{\rm crit}$ = $6\times10^4$ cm$^{-3}) and NH$_3$ (n$_{\rm crit}=2\times10^3$ cm$^{-3}), in the dense core Barnard 5. We analyse well resolved and high signal-to-noise observations of NH$_3$ (1,1) and (2,2) obtained with combining GBT and VLA data, and N$_2$H$^+$ (1--0) obtained with GBT Argus, which present a similar morphology. % Surprisingly, the non-thermal velocity dispersion of the ion is systematically higher than that of the neutral by 20\%. The derived sonic Mach number, $\mathcal{M}_s = σ_{\rm NT}/c_s$, has peak values $\mathcal{M}_{s, {\rm N_2H^+}} = 0.59$ and $\mathcal{M}_{s, {\rm NH}_3} = 0.48$ for N$_2$H$^+$ and NH$_3$, respectively. % This observed difference may indicate that the magnetic field even deep within the dense core is still oscillating, as it is in the turbulent region outside the core. The ions should be more strongly dynamically coupled to this oscillating field than the neutrals, thus accounting for their broader linewidth. If corroborated by further observations, this finding would shed additional light on the transition to quiescence in dense cores.
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