Browsing by Author "Guo, Zhen"

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    Measurement of Rayleigh wave ellipticity and its application to the joint inversion of high-resolutionﾠSwave velocity structure beneath northeast China
    (Wiley, 2016) Li, Guoliang; Chen, Haichao; Niu, Fenglin; Guo, Zhen; Yang, Yingjie; Xie, Jun
    We present a new 3-D S wave velocity model of the northeast (NE) China from the joint inversion of the Rayleigh wave ellipticity and phase velocity at 8–40 s periods. Rayleigh wave ellipticity, or Rayleigh wave Z/H (vertical to horizontal) amplitude ratio, is extracted from both earthquake (10–40 s) and ambient noise data (8–25 s) recorded by the NorthEast China Extended SeiSmic Array with 127 stations. The estimated Z/H ratios from earthquake and ambient noise data show good consistency within the overlapped periods. The observed Z/H ratio shows a good spatial correlation with surface geology and is systematically low within the basins. We jointly invert the measured Z/H ratio and phase velocity dispersion data to obtain a refined 3-D S wave velocity model beneath the NE China. At shallow depth, the 3-D model is featured by strong low-velocity anomalies that are spatially well correlated with the Songliao, Sanjiang, and Erlian basins. The low-velocity anomaly beneath the Songliao basin extends to ~ 2–3 km deep in the south and ~5–6 km in the north. At lower crustal depths, we find a significant low-velocity anomaly beneath the Great Xing'an range that extends to the upper mantle in the south. Overall, the deep structures of the 3-D model are consistent with previous models, but the shallow structures show a much better spatial correlation with tectonic terranes. The difference in sedimentary structure between the southern and northern Songliao basin is likely caused by a mantle upwelling associated with the Pacific subduction.
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    Twenty-five Years of Accretion onto the Classical T Tauri Star TW Hya
    (IOP Publishing Ltd, 2023) Herczeg, Gregory J.; Chen, Yuguang; Donati, Jean-Francois; Dupree, Andrea K.; Walter, Frederick M.; Hillenbrand, Lynne A.; Johns-Krull, Christopher M.; Manara, Carlo F.; Günther, Hans Moritz; Fang, Min; Schneider, P. Christian; Valenti, Jeff A.; Alencar, Silvia H. P.; Venuti, Laura; Alcalá, Juan Manuel; Frasca, Antonio; Arulanantham, Nicole; Linsky, Jeffrey L.; Bouvier, Jerome; Brickhouse, Nancy S.; Calvet, Nuria; Espaillat, Catherine C.; Campbell-White, Justyn; Carpenter, John M.; Chang, Seok-Jun; Cruz, Kelle L.; Dahm, S. E.; Eislöffel, Jochen; Edwards, Suzan; Fischer, William J.; Guo, Zhen; Henning, Thomas; Ji, Tao; Jose, Jessy; Kastner, Joel H.; Launhardt, Ralf; Principe, David A.; Robinson, Connor E.; Serna, Javier; Siwak, Michal; Sterzik, Michael F.; Takasao, Shinsuke
    Accretion plays a central role in the physics that governs the evolution and dispersal of protoplanetary disks. The primary goal of this paper is to analyze the stability over time of the mass accretion rate onto TW Hya, the nearest accreting solar-mass young star. We measure veiling across the optical spectrum in 1169 archival high-resolution spectra of TW Hya, obtained from 1998–2022. The veiling is then converted to accretion rate using 26 flux-calibrated spectra that cover the Balmer jump. The accretion rate measured from the excess continuum has an average of 2.51 × 10−9 M ⊙ yr−1 and a Gaussian distribution with an FWHM of 0.22 dex. This accretion rate may be underestimated by a factor of up to 1.5 because of uncertainty in the bolometric correction and another factor of 1.7 because of excluding the fraction of accretion energy that escapes in lines, especially Lyα. The accretion luminosities are well correlated with He line luminosities but poorly correlated with Hα and Hβ luminosity. The accretion rate is always flickering over hours but on longer timescales has been stable over 25 years. This level of variability is consistent with previous measurements for most, but not all, accreting young stars.