Khagol Bulletin # 136 (Apr 2025) - ENG

| 11 | KHAG L | No. 136 - APRIL 2025 M2Ia-Ibe, V = 0.2 to 1.3] is an especially bright star, a red supergiant, whose intensity variability makes it a semiregular variable star, off the main sequence. The interest in this star suddenly increased at the end of 2019 because of a dimming and subsequent rapid brightening event that took place during 2019-2020. This historic event, called the Great Dimming, caught the attention of astronomers and the general public worldwide. Multiple hypotheses were suggested to explain this dimming, including the formation of a dust cloud, changes in the photosphere, the presence of spots on the stellar surface and cooling in convective cells, but these could not be validated by later studies. We examined whether the dimming and brightening event of 2019–20 could be due to a critical transition in the pulsation dynamics of Betelgeuse by studying the characteristics of the light curve prior to the transition fromthe AAVSO International Database. For this, the light curve of Betelgeuse was binned into ten-day bins to examine the long trends in the data. The quantifiers, DET and LAM, were calculated over moving windows with a size of 300 points by one data point at a time. The corresponding RPs are constructed for each window and plotted against time to check for any significant variations over time. (Fig1) We find DET and LAM both increase systematically as the dimming episode approaches, which is an indication of a dynamical transition (Fig 2). We extended the analysis to multiwavelength data from different bands in the photometry data collected from the Wing photometry (IR/near-IR) and Wasatonic observatory (V-band)[5]. We found the changes were simultaneous in all wavelengths, which means the dimming was not a surface phenomenon. It should be related to a dynamical process that affected the whole star, with a change in its pulsation dynamics. Some of the later studies found increased prominence of the ~180-day period post dimming, supporting our conclusions. Thus, signatures of an approaching change in the underlying dynamics could be observed from the changes in the recurrence measures that capture changes in the dynamical structure of the light curve of the star. In addition to gaining insight into the nature of dynamics, the method of reconstruction of dynamics from data, followed by analysis of the measures from its recurrence patterns, can help to understand transitions in dynamics. We see several applications in astrophysical data analysis, in classifying sources based on the nature of their dynamics, understanding the nature of transitions and more importantly, identifying measures that can predict an approaching transition. References: 1. G. Ambika, M. Takeuti &A K Kembhavi- Chaotic pulsations in irregular variables-2003, “Mass losing Pulsating stars and their Circumstellar Matter”- eds. Y Nakada, M Honma & M Seki- Kluwer Aca. Pub- p-95. 2. Buchler, J.R., Kollath, Zoltan and Cadmus, R. 2004, "Evidence for Low- Dimensional Chaos in Semiregular Variable Stars", Astrophysical Journal, 613 532DOI 10.1086/422903. 3. Ambika, G. & Harikrishnan, K. P. Methods of nonlinear time series analysis and applications: A review. 2020, A. Mukhopadhyay, S. Sen, D. N. Basu & S. Mondal (Eds.), Dynamics and control of energy systems (pp. 9-27). Springer. doi:10.1007/978-981-15-0536-2_2. 4. Sandip V George, Sneha Kachhara, Ranjeev Misra, and G. Ambika. Early warning signals indicate a critical transition in Betelgeuse, 2020, Astronomy&Astrophysics, 640, L21; h t t ps : / / do i . o r g / 10 . 1051 /0004- 6361/202038785. 5. Kachhara, S., George, S. V., Misra, R., & Ambika, G. Evidence for dynamical changes in Betelgeuse using multi- wavelength data. 2023, The Sixteenth Marcel Grossmann Meeting, pp. 3485- 3493. DOI: 10.1142/9789811269776_0288. Fig 1. Recurrence plots calculated from the light curve of Betelgeuse before (left) and during (right) dimming. Fig 2. Recurrence measures (a) Determinism (DET) and (b) Laminarity (LAM) before dimming (red), during (blue), and post dimming (green).