Student research - Department of Astronomy and Astrophysics
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Item A Note on the Long-Term Light Curves of SRd Variable Stars(2024) Percy, John R.; Taiger, Chelsea WillowWe briefly review some recent results on SRd variables – low-mass yellow supergiants with semi-regular brightness variability. We then describe some interesting features of the long term light curves of 19 SRd variables for which there are many decades of sustained visual and photoelectric data in the AAVSO International Database. DE Her, RS Lac, UY CMa and possibly RX Ret showed a large-amplitude, long-term cyclic variation; TV Per brightened by 2.5 magnitude in the last 10,000 days; Z Aur switched between periods of 110 and 135 days; UU Her switched between periods of 44 and 72 days, as it is well-known to do; TX Aql showed an unusually short period of 35 days. We have used the AAVSO VSTAR time-series analysis package to determine the average periods and amplitudes in each of the stars. The amplitudes are strongly variable. There is no strong evidence for detectable long secondary periods of the kind found in RV Tauri stars or red giants in any of these stars. Z Aur and UU Her each show two possible pulsation periods but, especially in the former, the period ratio would be difficult to explain.Item Semi-Regular Yellow (SRd) Variables: New Results(2022) Percy, John R.; Wang, YijieSRd stars are semi-regular yellow supergiant pulsating variables, mostly low-mass stars (like the sun) in an advanced stage of evolution. They are closely related to the better-known RV Tauri (RV) variables, which are characterized by alternating deep and shallow minima. Despite their interest and importance, SRd stars have been rather neglected by astronomers. AAVSO observers, however, have been productively observing both SRd and RV stars for many decades. In a recent paper (JAAVSO 50, 96 (2022)), we investigated the cause of the semi-regularity of SRd stars: notably variable pulsation amplitude (of unknown cause), but also period shifts and “wandering”, bimodal pulsation, and “long secondary periods” (LSPs), an order of magnitude longer than the pulsation period P, In the present paper, we build on these results, and previous results for SRd variables based on AAVSO observations. We are especially interested in the relation between the pulsation period and the luminosity, and the relation between LSP and the luminosity. Period-luminosity relations are well-known in Cepheids and, more recently, in RV variables. We are also interested in the behavior and cause of the LSPs, which may be due to binarity. We use V observations from the All-Sky Automated Search for Supernovae (ASAS-SN) on-line database, and the AAVSO's time-series analysis package VSTAR to investigate light curves, pulsation periods and LSPs in several dozen stars. We use GAIA distances from the ASAS-SN variable star catalog to determine absolute magnitudes Mv. We then correct these for interstellar absorption. From these absolute magnitudes, period-luminosity relations are derived. Contrary to previous results, we find LSPs to be quite common in SRd stars. There is considerable scatter in the P-Mv and LSP-Mv relations, but the LSP/P ratio peaks strongly at 8-10, as in pulsating red giants with LSPs. Finally: we compare the behavior of the SRd stars with that of semi-regular red giant (SRa/SRb) variables, and also point out some unsolved problems with SRd stars – such as the cause of the variability of the pulsation amplitudes. AAVSO observations may help to solve these.Item Studies of Low-Mass Pulsating Yellow Supergiants(2021) Percy, John; Haroon, SaraWe have carried out Fourier and wavelet analysis of long-term AAVSO visual observations of 14 RV, SRd, CW and related variable stars in some detail, and of 27 other such variables in less detail. The purpose was to investigate aspects of their complex variability, especially long-term changes in pulsation period, amplitude, and mean magnitude, and also the possible relationships between these types of variables. The mean amplitude variability does not seem to be a function of period, and is similar in RV and SRd stars. We have studied the periods of bimodal variables; there are very few with period ratios different from 2:1. We note with interest the apparent scarcity of "long secondary periods" in SRd stars, as compared with RV variables (the RVb phenomenon) which is believed to be due to binarity. We use wavelet analysis to study possible evolutionary period changes, as an alternative to the usual (O-C) method. We include miscellaneous comments on 17 stars in our total sample. Finally: we note that mean pulsational amplitudes of these stars, determined by Fourier analysis, are smaller than values determined by wavelet analysis, presumably because of the ":wandering" periods in these stars.Item Analysis of ASAS-SN Observations of Short-Period Mira Stars(2020-06-12) Percy, John; Golaszewska, PatriciaWe have analyzed observations of a uniform sample of 36 stars in the ASAS-SN variable star catalog, with mean magnitudes between 10 and 12, classified as Mira by the catalog, and with periods of 150 days or less. They presumably represent a transition from Mira type to semi-regular type. The stars show a wide variety of light-curve shapes, and of deviation from periodicity. The amplitude increases with increasing period, as is well known, but no other properties, including the degree of periodicity seem to depend on period.Item Flare Stars: A Short Review(2019-10-31) Dzombeta, Krstinja; Percy, JohnFlare stars, or UV Ceti stars, are a type of eruptive variable star, defined by their flaring behaviour -- a rapid (minutes) increase in brightness, followed by a slower (hours) decrease. This short review outlines current knowledge about flare stars, their importance, recent research developments, future research directions, and some practical activities for skilled amateur astronomers and students -- the primary audience for this review. Over the past decade, flare stars have been the subject of intensive research, as a result of an abundance of new data, especially from the Kepler and TESS space telescopes. The large statistical samples of data have clarified the relation between flaring and stellar spectral type, luminosity, and rotation. They have allowed for the expansion of the range of spectral types of flare stars, from K and M type dwarfs, to F and G, and possibly even A. They have confirmed the greater frequency of flares on M dwarfs, compared to K, and that flare stars' energies follow a decreasing power law for the number of high-energy flares, although a break in the relationship has also been demonstrated. Current problems in flare-star research include improved modelling of the new observational results, using the dynamo theory which explains the stars' magnetic field. What is the difference, if any, between the dynamo in completely-convective stars such as M dwarfs, and in stars such as the sun with only partial convection zones?