高能暂现源 · 每日文献综述

We report on the evolution of extinction, mass accretion rate, and spectral index from analysis of SpeX observations and AllWISE spectral energy distributions, compared to stellar parameters (mass, age, and veiling) derived from analysis of previously reported and newly analyzed iSHELL observations of the same young stellar objects (YSOs). By using stellar gravity as an age indicator, we are able to infer age without any assumptions about the circumstellar environment. Our sample consists of 109 Class I, II, and III YSOs. The spectral index broadly decreases with time but with sufficient scatter that Class I-III YSOs can all be found at the same age. There is a wide range of extinction (A_V) from nearly 0 to greater than 30 mag for ages up to 5 Myr, showing no clear trend with age. The mass accretion rate declines from a median of 10^-8 solar masses per year at ~1 Myr to less than 10^-10 solar masses per year at ages greater than 10 Myr, with a scatter of the mass accretion rate of +/- 1 dex. Accretion seems to effectively end by ~ 5 Myr, by which time the median mass gained is 0.05 solar masses. We find a wide range of veiling (rK) from zero to three (limited by our need to detect photospheric lines) for Class I and II YSOs, with the median veiling declining after ~2 Myr. Class I YSOs appear to be physically very similar to Class II YSOs, with similar accretion rates and overlapping age and mass distributions. There are no objects with a high spectral index and low extinction, suggesting that once starved of circumstellar material, the spectral index rapidly declines.

The transmission of three-dimensional anelastic internal gravity wave packets incident upon a retrograde background jet flow are examined through fully nonlinear numerical simulations. The background winds are chosen to be representative of the eastward and westward flows of the Quasi-Biennial Oscillation (QBO) in the equatorial stratosphere. Transmission, which is quantified by the relative pseudomomentum of waves passing above the jet, is found to be smaller for incident waves with larger initial frequency. Due to anelastic effects, the amplitude of the transmitted wave packets tends to grow exponentially, ultimately inducing a local mean flow that acts to drive the waves to overturn and break turbulently. The results are compared with previous studies that predicted transmission of small amplitude wave packets, and implications are discussed for the influence of wave transmission across the QBO.