Machine-learning classification of the kinematic properties of elliptical galaxies. Using integral-field spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) galaxy survey, this study analyzes how velocity dispersion changes from the centers to the outer regions of elliptical galaxies. A combination of unsupervised and supervised machine-learning methods was used to build an automated classifier that separates the velocity-dispersion profiles into four characteristic types: Flat, Decline, Ascend, and Irregular. The trained classifier achieved an overall accuracy of 88% on the test sample. When applied to 2,624 MaNGA DR17 elliptical galaxies, approximately 68.87% were found to exhibit Flat velocity-dispersion profiles. This approach provides an efficient way to investigate the kinematic structures of large galaxy samples and offers a new statistical tool for future studies of galaxy formation, evolution, and mass distribution.
Dr. Yi Duann, Dr. Yong Tian, and Prof. Chung-Ming Ko have published a study applying machine-learning techniques to investigate the internal motions of elliptical galaxies observed by the MaNGA survey. The research analyzes velocity-dispersion profiles of elliptical galaxies, which describe how the spread of velocities changes from the centers to the outer regions of galaxies. By combining unsupervised K-means clustering with a supervised TreeBagger classifier, the team identified four characteristic profile types: Flat, Decline, Ascend, and Irregular. The supervised model achieved an overall classification accuracy of 88% on the test set.
The trained model was then applied to 2,624 elliptical galaxies in the MaNGA DR17 sample. Approximately 67.9% of these galaxies were found to exhibit Flat velocity-dispersion profiles, while about 24.5% showed declining profiles. The high fraction of Flat systems is particularly interesting because similar profiles have been reported among brightest cluster galaxies, providing a promising direction for future studies of galaxy formation, evolution, and mass distribution. This work demonstrates how machine learning can efficiently transform large astronomical surveys into statistically meaningful classifications of galaxy kinematics. The study was published in RAS Techniques and Instruments (Duann, Tian & Ko 2023, RASTAI, 2, 649–656).
https://doi.org/10.1093/rasti/rzad044
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Possible near-ultraviolet early ingress of WASP-189b and its magnetohydrodynamic interpretation. Top: CUTE near-ultraviolet observations of three WASP-189b transits. The third visit (V3) shows a tentative transit-phase offset of approximately 31.5 minutes, consistent with additional absorbing material located ahead of the planet. Bottom: Two-dimensional magnetohydrodynamic simulations illustrating how the interaction between the stellar wind and the planetary magnetosphere changes under different fast-mode Mach numbers. Distinct bow shocks form only in sufficiently fast and dense stellar-wind conditions, whereas slower winds can instead produce a compressed plasma pileup ahead of the planet. The results suggest that such cooled, dense material may provide a more favourable condition for producing detectable near-ultraviolet early-ingress absorption.
Dr. Yi Duann and her team have published a study investigating a possible near-ultraviolet early ingress of the ultra-hot Jupiter WASP-189b using observations from the 6U Colorado Ultraviolet Transit Experiment (CUTE) CubeSat. Among three observed transits, the third visit showed a tentative phase offset of about 31.5 minutes, corresponding to absorbing material extending several planetary radii ahead of the planet. Such early-ingress signatures have often been proposed as possible evidence of interactions between stellar winds and planetary magnetospheres.
To test this scenario, the team performed magnetohydrodynamic simulations across different stellar-wind conditions. The simulations show that classical bow shocks can form when both the wind speed and plasma density are sufficiently high; however, the shocked gas can then remain too hot to efficiently absorb near-ultraviolet light. In contrast, a transition toward slower stellar winds can preserve compressed plasma while allowing it to cool, creating a dense magnetic pileup that may be more readily detectable in transit observations. The study therefore highlights how time-variable stellar winds can influence the observable signatures of exoplanet magnetospheres and provides a new framework for probing star–planet interactions around ultra-hot Jupiters. The work was published in Astronomy & Astrophysics (Duann et al. 2025, A&A, 703, A24).
https://doi.org/10.1051/0004-6361/202556404
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Connecting observed exoplanet populations to planet-formation pathways with machine learning. Top: Analysis framework combining planets from the NASA Exoplanet Archive with unsupervised Gaussian mixture model (GMM) clustering and pebble-accretion synthetic populations. Observed planets are classified using their physical and dynamical properties and subsequently mapped into the same three-dimensional parameter space as simulated planets. Bottom: Distributions of three formation-related quantities predicted for the mapped populations: the gas availability at the onset of formation (G1), planetary gas mass fraction (fqas), and ice–rock mass ratio. The four populations—hot giants (HG), warm-Jupiter-dominated systems (WJD), lower-mass giants (LMG), and very-massive gas giants (VMGG)—show systematically different formation histories, with the very-massive gas giants preferentially associated with earlier formation in gas-rich disks.
Dr. Yi Duann, in collaboration with researchers at the Center for Star and Planet Formation, Globe Institute, University of Copenhagen, has published a study using machine learning to connect observed close-in exoplanets with theoretical planet-formation models. Rather than assigning planets to predefined categories, the team used a two-stage Gaussian mixture model to identify natural populations from their orbital and dynamical properties. These observational groups were then mapped onto synthetic planets produced by pebble-accretion simulations, allowing otherwise inaccessible properties, such as formation timing, gas-envelope growth, and solid composition, to be statistically inferred at the population level.
The statistical analysis shows that these populations are not simply visual groupings. Differences in formation-related parameters are highly significant, with the strongest inter-population effects found for the gas mass fraction and planet-to-star mass ratio. A complementary multinomial diagnostic separated the mapped populations with an overall accuracy of 98.2%. The very-massive gas giants show the highest median gas availability at formation and a strong correlation between early formation and gas accretion, while lower-mass giants exhibit substantially broader and more diverse formation histories. These results provide a data-driven bridge between present-day exoplanet observations and the physical processes that shaped planetary systems. The study was published in Astronomy & Astrophysics (Duann et al. 2026, A&A, 711, A238).
https://doi.org/10.1051/0004-6361/202659961
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Figure: Superorbital phase residual distributions for soft (red) and hard (black) X-rays, with the smooth evolution trend removed. Data are divided into two epochs: MJD 53500 - 57000 (top) and MJD 57000 - 60300 (bottom). Mean values and 1σ uncertainties are indicated by dashed lines and horizontal bars. A significant phase shift of 0.044 ± 0.010 cycles between soft and hard X-ray bands is evident in the bottom plot
Professor Yi Chou has published a latest study that explores the evolution of "superorbital" light variations in the high-mass X-ray binary system LMC X-4 by analyzing 33 years of data from multiple space telescopes. LMC X-4, located in the Large Magellanic Cloud, is an accreting binary system consisting of a massive star and a compact neutron star. These two stars orbit each other every 1.4 days, while the system also exhibits a "superorbital period" of approximately 30.5 days. Astronomers point out that the core mechanism of this period lies in the existence of a prominently warped accretion disk around the neutron star. This warped disk undergoes a slow "precession," periodically obscuring the X-rays emitted by the neutron star during its rotation. Professor Chou's research confirms that despite the complexity of the disk structure and superorbital phase variations, the precession period of LMC X-4 is, on average, remarkably stable. Over the past thirty-plus years, the variation in the superorbital period has been only 0.55%, making it the most stable system of its kind currently known.
Intriguingly, the study found that after late 2014 (MJD ~ 57000), observation data revealed a distinct "phase shift" between the soft and hard X-ray bands. This phenomenon indicates that the geometric structure of the warped accretion disk underwent further deformation, transitioning from a relatively symmetric warped state to an asymmetric structure. This change in disk geometry caused a lag in the timing of when rays of different energies were obscured as they passed through the warped edges. This discovery coincided with a decline in the system's hard X-ray intensity, suggesting that the warped disk is now obscuring the central object in a new and more complex manner. These findings provide astronomers with an ideal laboratory for studying accretion disk behavior in extreme physical environments. By understanding how these warped disks precess, evolve, and shift over decades, researchers can further understand how matter flows under extreme gravity and radiation pressure. This marks an important step forward in the understanding of accretion disk dynamics within high-energy astrophysics. This research paper has been published in The Astrophysical Journal ( Chou 2026, ApJ, 1000, 23).
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Dr. Chia-Lung Lin: We present a flare temperature study of the active M dwarf Wolf 359 using simultaneous multiband (u, g, r, i, z) photometry from the Lulin 1-m and 41-cm telescopes. Twelve solar-class flares (E < 1032 erg) were detected over five nights, with strong enhancements in the u, g, and r bands; only three appeared in i, and none in z. From SED fitting and g/r color ratios, we derive an average flare temperature of 5500 ± 1600 K, cooler than the canonical 10000 K. A power-law relation between flare temperature and energy, consistent with M-dwarf superflare trends, suggests a common temperature–energy scaling. Increased scatter at higher energies implies more complex physics beyond simple blackbody models. Using this relation and the flare energy-frequency distribution, we estimate the contribution to photosynthetically active radiation (PAR) on an Earth-analog, finding that even extreme superflares (≈1036 erg, ≈ 16500 K) cannot sustain an Earth-like level of photosynthetic O2 production.
These results are published in Lin et al. (2025), AJ, 170, 297 (including Prof. Wing-Huen Ip, Dr. Huang Li-Ching, Wei-Jie Hou, and Hsiang-Yao Hsiao). https://iopscience.iop.org/article/10.3847/1538-3881/ae0c0a/meta
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A study led by the NCU Transient Group investigates a mysterious class of white dwarf explosions known as “Super-Chandrasekhar explosions (or 03fg-like SNe Ia) and reveals their extraordinary ultraviolet (UV) properties. Compared to normal SNe Ia, these events exhibit significantly brighter and broader light curves, along with distinct spectral features. For the first time, the NCU team construct a sample of UV spectra obtained with the Swift satellite and find that 03fg-like SNe show a pronounced UV excess, which is challenging to explain with current explosion models for normal SNe Ia. Our analysis suggests that interaction with circumstellar material (CSM) could provide a promising explanation for their unique UV characteristics. This work has been published as Bhattacharjee & Pan et al., 2025, MNRAS, 542, 2752.
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圖：蝘蜓座 (Chamaeleon) 周圍分子暗雲以及相關的年輕恆星。圖中每個灰點代表一顆Gaia DR3的星球，用來襯托分子雲的分布。符號代表不同的年輕恆星族群：藍色與一號雲 (Cha I) 相關，橘色則屬於二號雲 (Cha II)。我們驗證三號雲 (Cha III) 沒有恆星形成活動，但驗證有三顆屬於二號雲的成員，投影在 Cha III前方，另外位於西南方的兩個分子雲 Cha East I and II （未顯示在圖中）也沒有年輕恆星。在雲氣之外，有一些之前不知道的年輕星體，以及屬於其他星團（紅色），或位於西北方向（未顯示在圖中）、鄰近的OB星協（紫色），雖然投影在相同天空區域，但分析它們的距離以及運動可以明顯區分出來。有編號的星星是我們新確認的年輕成員。這項研究是陳湘喻碩士論文的一部份，結果發表在 New Astronomy, 2025, 120, 102421
恆星從星際雲氣中成群誕生，有時受到大質量恆星影響，誘發新一代的恆星形成。碩士生陳湘喻跟指導教授陳文屏指認蝘蜓座分子暗雲區的年輕恆星，研究它們的空間分布、與雲氣以及與鄰近大質量恆星的關係，探討此區域恆星形成的過程。我們利用 Gaia 數據找出距離以及運動一致的星體，然後以紅外波段的2MASS 與 WISE資料診斷周圍是否有初生恆星特有的塵埃。這個區域有三個主要分子雲，其中已知 Cha I 恆星形成最活躍，其次是 Cha II，而 Cha III 則沒有任何年輕恆星。之前研究侷限於雲氣濃密之處，而我們使用全天數據，得以指認雲氣以外的成員星，用以解讀恆星誕生與雲氣消散的歷史，並詳細分辨出投影在在同樣天區，但屬於不同雲氣（距離、年齡稍異），以及不同星團，甚至鄰近OB星協的年輕成員（距離、年齡完全不同）。研究結果顯示大質量恆星的恆星風及輻射影響了此區域的雲氣結構（包括北方的 Musca 絲狀暗雲）以及恆星誕生的過程，但整個誘發過程止於 Cha III 之前，並未繼續往南方擴展。
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Caption: Evolution of 4U 1820-30 superorbital phases from 1987 to 2023. The dotted, solid and dashed lines represent the best fits for linear, glitch and quadratic models, respectively. The shaded area indicates the low power state between MJD 50,773 and 52,627, and the vertical dash-dot line represents the period glitch time MJD 52,380±4145, evaluated by the glitch model, with the horizontal error bar indicating the 1σ uncertainty of the glitch time.
Master student Jun-Lei Wu and Prof. Yi Chou studied the superorbital modulation of low mass X-ray binary 4U 1820-30.It was long believed by the astronomers that the superorbital modulation of the low-mass X-ray binary 4U 1820-30, with a period of 171 days, was a resonance effect induced by a third star orbiting the binary system—an explanation known as the triple model. The stability of this superorbital period predicted by this model is crucial for verifying this hypothesis. A research team led by Prof. Yi Chou analyzed X-ray light curves collected by six monitoring and scanning telescopes over a span of 36 years. They discovered that the superorbital period had significantly changed from 171 days to 167 days during this time. Further phase analysis revealed that this change either occurred abruptly between early 2001 and mid-2003 or evolved gradually with a period derivative of Ṗ=(-4.20±0.72)x10-4 day/day. Their findings indicate that the superorbital period of 4U 1820-30 is not as stable as predicted by the triple model, strongly challenging this hypothesis. Instead, they propose an irradiation-induced mass transfer instability scenario to explain the observed superorbital modulation. The research paper has been published in the Astrophysical Journal ( Yi Chou et al. 2025 ApJ 981 43 ). Its preprint, posted on arXiv in September 2024, was reported on the renowned public outreach website Phys.org under the title "Study Inspects Unusual Behavior of an X-ray Binary" .
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圖: 小行星各家族 (Hungaria, Flora, Baptistina, Phocaea, Massalia, Vesta, Nysa-Polana, Maria, Adeona, Eunomia, Juno, Agnoa, Eos, Koronis, Hygiea, Themis, Euphrosyne, and Hilda families)的相位曲線在HG1G2模型中的分佈情形
小行星的分類與大小不再局限於多色測光與光譜觀測
小行星是圍繞太陽運動的岩石或者金屬天體，它們是太陽系形成早期未能發生吸積的殘留物質，因此仍然保存著太陽系原始星雲的信息。 S型和C型是最常見的小行星類型，分別代表了小行星兩種不同的光譜特徵，與它們的物質成分相關。1984年Tholen使用色測光數據將小行星進一步分類，隨後可見光與紅外波段的光譜將小行的分類更加細分，然而小行星光譜觀測與大樣本多色測光數據耗費大量觀測資源，因此尋找替代小行星光譜的分類方法便成為科學家近年來的主要目標。近年來各研究機構陸續使用大型望遠鏡展開巡天計畫，這些巡天計畫往往為了巡天效率多使用單一濾鏡或者少數濾鏡來節省巡天時間，這縮短的時間對瞬變天體很有時效性，但對區分小行星類型是無助益的，因為無法使用多色測光數據得到小行星的色指數，進一步對 多個色指數開展主成分分析!
林忠義博士利用Zwicky Transient Facility（ZTF）(位於加利福尼亞州帕洛馬山的施密特望遠鏡擁有觀測視野達到47平方度)巡天資料將小行星進行光譜分類，論文研究方法為使用單一顏色濾鏡(雖然ZTF有三種濾鏡，但仍以r’觀測資料為最多)求得小行星光度隨相位角之間的關係，也稱之為小行星的相位函數，並將其分類。最後是用分類結果將小行星大小算出，隨後比較各小行星家族的大小分佈函數(size distribution)，了解各家族形成的時間與關聯性。這些研究成果已於今(2025)年6月20日發表於國際期刊《皇家天文學會月報》(Monthly Notices of the Royal Astronomical Society)。
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圖: SLT三色合成影像SN 2024ggi與其宿主星系NGC 3621。
由陳婷琬教授主持的Kinder計畫，透過鹿林天文台40公分SLT望遠鏡，在SN 2024ggi爆炸後僅14小時即成功取得其極早期的光度與溫度演化資料。SN 2024ggi 位於星系 NGC 3621，由大稀奇天體實驗室參與的ATLAS 計畫所發現，是近十年來距離地球最近的核心塌縮超新星之一（約 6.6 Mpc）。本研究特別結合了來自臺灣與香港的公民天文學家之力量，透過他們提供的光度觀測與鹿林SLT的早期觀測相互補強，提高時間解析度，使研究團隊得以重建SN 2024ggi爆炸初期的完整光變曲線。另外，這也是鹿林天文台首次實現「超新星夢幻連動觀測」：LOT負責光譜，SLT觀測光度！觀測顯示該超新星在早期階段光度迅速上升、顏色轉藍，並伴隨窄發射譜線，顯示其爆炸受到周圍環星物質的顯著影響。研究團隊推測，其前身星為紅超巨星，在爆炸前每年流失約10⁻³個太陽質量，並被約0.4個太陽質量的環星物質所包圍。本成果已發表於《The Astrophysical Journal》（Chen et al. 2025, ApJ, 983, 86）。
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Colloquium
- Date: 2026-09-11
Time: 14:00
Location: S4-1013
Speaker: Dr. Namitha Ramachandran
Title: Disentangling the Milky Way's Stellar Halo using DESI Milky Way Survey
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