陡密度梯度中的外流：行为多样性及其对潮汐瓦解事件和发光快速蓝色光学瞬变的意义
Outflows in steep density gradients: diversity of behavior and implications for tidal disruption events and luminous fast blue optical transients
AI总结：
本研究通过解析标度和流体模拟，揭示不同密度比外流在陡密度环境中的演化规律，探讨其在潮汐瓦解事件和发光快速蓝色光学瞬变中的应用。
AI中文摘要：
强大的爆炸可能会在中心引擎向周围气体发射风时经历持续的能量注入，产生由接触间断分隔的前向激波和反向激波。在绝热阶段，动力学行为强烈依赖于风与环境的密度比 $f \equiv \rho_{\rm w} / \rho_{\rm a}$。当 $f << 1$ 时，反向激波位于接触间断内部，风沉积的机械能保留在径向延伸、近似等压的受激风区域，其压力驱动被扫积的环境壳层；当 $f \gg 1$ 时，反向激波靠近接触间断，膨胀由自由膨胀的风与被扫积环境气体之间的动压相互作用主导。本研究通过解析标度关系和一维激波捕捉流体动力学模拟，确定这两种极限下的外流在环境密度剖面 $\rho_{\rm a} \propto r^{-n}$（其中 $2 \leq n \leq 3$）中的演化情况，以及它们的激波结构是加速还是以恒定速度运动。对于 $n > 2$，初始低密度外流产生加速的前向激波，其半径演化满足 $R_{\rm s} \propto t^{3/(5-n)}$；由于 $\rho_{\rm w} \propto r^{-2}$，$f$ 随半径增大，导致反向受激风区域相对于接触位置收缩，前向激波向恒定速度膨胀过渡时，该过程发生在 $t_{\rm dec} \propto f_0^{1/(2-n)}$（其中 $f_0$ 为初始风与环境密度比）且 $f \sim$ 几个值时。相比之下，初始 $f_0 \gg 1$ 的外流不会发展出延长的加速阶段，在整个绝热演化中保持近似匀速运动。本研究探讨了其在潮汐瓦解事件外流和发光快速蓝色光学瞬变中的应用，这类天体的环境通常被推断具有 $n > 2$ 的陡密度剖面。
英文摘要：
Powerful explosions may undergo sustained energy injection as a central engine launches a wind into the surrounding gas, generating a forward and a reverse shock separated by a contact discontinuity. During the adiabatic phase, the dynamics depend strongly on the wind-to-ambient density ratio $f \equiv ρ_{\rm w} / ρ_{\rm a}$. For $f << 1$, the reverse shock lies well inside the contact discontinuity, and the mechanical energy deposited by the wind is retained in a radially extended, approximately isobaric shocked-wind region whose pressure drives the swept-up ambient shell. For $f \gg 1$, the reverse shock remains close to the contact, and the expansion is governed by the ram-pressure interaction between the freely expanding wind and the swept-up ambient gas. We use analytic scalings and one-dimensional shock-capturing hydrodynamic simulations to determine how outflows in these two limits evolve in ambient density profiles $ρ_{\rm a} \propto r^{-n}$, where $2 \leq n \leq 3$, and whether their shock structures accelerate or coast at constant velocity. For $n > 2$, initially underdense outflows produce accelerating forward shocks whose radii evolve as $R_{\rm s} \propto t^{3/(5-n)}$. Because $ρ_{\rm w} \propto r^{-2}$, f increases with radius, causing the reverse-shocked wind region to contract relative to the contact position as the forward shock transitions toward constant-velocity expansion. This occurs when $f \sim$ a few at $t_{\rm dec} \propto f_0^{1/(2-n)}$, where $f_0$ is the initial wind-to-ambient density ratio. By contrast, outflows initialized with $f_0 \gg 1$ do not develop an extended accelerating phase and remain approximately coasting throughout their adiabatic evolution. We discuss applications to tidal disruption event outflows and luminous fast blue optical transients, whose environments are often inferred to have steep density profiles with $n > 2$.
