标题： 年轻恒星天体的支点波长——LRLL 31案例 显示英文标题

标题： The fulcrum wavelength of young stellar objects -- the case of LRLL 31

摘要： 一小部分年轻恒星天体（YSOs）在其中红外波段的SED表现出“跷跷板”式的时变特性；当短于某个支点波长（$\lambda_{f}$）的流量增加时，该波长长波一侧的流量会减少（反之亦然），变化周期为数周到数年。 虽然先前的研究表明，一个不透明且轴对称的可变高度遮挡物可以导致这些天体SED出现这种行为，但单个$\lambda_{f}$出现的条件之前尚未确定，也未明确决定其值的因素。 通过辐射传递建模，我们对该类天体的典型代表LRLL 31进行了参数研究，以探索这一现象，并确认这种流量变化的原因很可能是由于吸积盘内缘在尘埃亚升华半径处光学厚度的变化，或者一些产生类似表象的其他现象。 我们还发现，只有在视线与吸积盘相交的高度倾斜情况下才会出现支点波长。接受LRLL 31的盘面高度倾斜这一前提后，分别独立改变内缘半径、径向和垂直密度剖面，以评估这对$\lambda_{f}$及其相对于约$10 \mu$米附近的硅酸盐特征的位置的影响。尽管$\lambda_{f}$是这些参数的函数，但它被发现对垂直密度指数$\beta$最为敏感。 在所有其他因素保持不变的情况下，仅对于更扁平的圆盘 ($\beta < 1.2$)，我们在硅酸盐特征之外发现了 $\lambda_{f}$。 显示英文摘要

摘要： A small subset of young stellar objects (YSOs) exhibit "see-saw" temporal variations in their mid-infrared SED; as the flux short-ward of a fulcrum wavelength ($\lambda_{f}$) increases the flux long-wards of this wavelength decreases (and vice-versa) over timescales of weeks to years. While previous studies have shown that an opaque, axisymmetric occulter of variable height can cause this behaviour in the SED of these objects, the conditions under which a single $\lambda_{f}$ occurs have not previously been determined, nor the factors determining its value. Using radiative transfer modelling, we conduct a parametric study of the exemplar of this class, LRLL 31 to explore this phenomenon, and confirm that the cause of this flux variation is likely due to the change in height of the optically thick inner rim of the accretion disc at the dust sublimation radius, or some other phenomenon which results in a similar appearance. We also determine that a fulcrum wavelength only occurs for high inclinations, where the line of sight intersects the accretion disc. Accepting that the disc of LRLL 31 is highly inclined, the inner rim radius, radial and vertical density profiles are independently varied to gauge what effect this had on $\lambda_{f}$ and its position relative to the silicate feature near $10 \mu$m. While $\lambda_{f}$ is a function of each of these parameters, it is found to be most strongly dependent on the vertical density exponent $\beta$. All other factors being held constant, only for flatter discs ($\beta < 1.2$) did we find a $\lambda_{f}$ beyond the silicate feature.