Research column

Caption: Temporal mass accretion rates during unstable accretion bursts. The upper panel shows DL Tau, and the lower panel shows Haro 6-13. The left side of the figure displays the simultaneous accretion luminosity bursts observed by TESS, ZTF, or ASAS-SN. The horizontal axis represents time (in days), and the vertical axis represents normalized flux. The right side of the figure presents the derived temporal mass accretion rates. The horizontal axis represents time (in days), and the left vertical axis (black) represents accretion rates (in units of M_☉/yr), while the right vertical axis (blue) represents accretion luminosity (in units of erg/s).

PhD student Chia-Lung Lin: We have combined photometric and spectroscopic surveys from both space telescopes (TESS) and ground-based telescope (LAMOST, ZTF, and ASAS-SN) to study the mass accretion rates, flare activities, and variability characteristics of 16 Classical T Tauri Stars (CTTS) in Taurus. By calculating the flux of various spectral lines such as H-alpha, we derived an average mass accretion rate of 1.76×10^-9 M_☉/yr for these 16 CTTSs. Among them, two stars, DL Tau and Haro 6-13, exhibited temporary brightness increases in their TESS light curves due to bursts of unstable mass accretion rates. By analyzing simultaneous monitoring data from ZTF, ASAS-SN, and TESS, we estimated the time series variations in mass accretion rates for these two stars over a span of 50 days. If the results from spectral data reflect a stable accretion pattern, we concluded that the mass accretion behavior of these two stars is primarily driven by stable accretion. Additionally, we detected a total of 13 large flares across these 16 stars, with energy ranging from 2×10³⁴ to 6×10³⁵ erg, making their flare activity more than a hundred times more active than that of solar-like stars. We also found that the variability classes of these 16 stars change over time, with the timescales of these changes falling roughly between 1.6 to 4 years compared to results in the literature.

This study has been published in Astronomical Journal in 2023, August: Lin, C.-L., Ip, W.-H., Hsiao, Y., et al. 2023, AJ, 166, 82 (Including Prof. Ip, Wing-Huen, master graduate Hsiao, Yao and Cheng, Tzu-Hueng in the IANCU)
DOI: 10.3847/1538-3881/ace322

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Master student Yao-Wen Jhang and Prof. Yi Chou report the analysis results of X-ray light curves collected by Neutron star Interior Composition ExploreR (NICER) from June 2017 to July 2022 of the X-ray binary 4U 1820-30, located at the globular cluster NGC 6624. From the yearly orbital phases obtained by the orbital modulation light curves and historical records, we measured that the observed orbital period derivative is 1/P dP/dt = (-5.21±0.13)×10^-8 yr^-1 from a time span of 46.3 years database. No significant second order orbital period derivative can be found with 2σ upper limit of | d²P / dt² | < 5.48×10^-22s s^-1. To explain the complete different orbital period evolution trends from theoretical prediction and observation result, we agreed that it is caused by the acceleration of the binary system in the gravitational field of globular cluster NGC 6624, as suggested by previous studies. However, we pointed out that it is improper to estimate the acceleration of 4U 1820-30 by observed orbital period derivative because there are too much uncertainties in the intrinsic orbital period derivative. Furthermore, we detected a modulation with a period of 691.6±0.7 s in the NICER X-ray light curves, which is consistent with the superhump period discovered in the far-ultraviolet band of Hubble Space Telescope. In addition to explaining how the superhump modulation can be detected in X-ray band, we also suggested that this periodic modulation may be induced by a hierarchical third star that orbits around the binary system. These results have been published in Chou & Jhang 2023 ApJ, 951, 42.

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Based on light curves data taken from ZTF and published distances, the team led by Prof. Chow-Choong Ngeow derived the gri-band period-luminosity (PL) relations for various types of pulsating stars located in the globular clusters. These include RR Lyrae, Type II Cepheids, and anomalous Cepheids. Many of these PL relations were derived for the first-time in gri filters, with potential applications in the era of Rubin Observatory’s LSST. These works are published in Ngeow et al (2022, AJ 163:239, AJ 164:154, and AJ 164:191).

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