arXiv subjects

John R. Percy

Publications and source records attributed to John R. Percy.

16 recordsLinked to original sources

arXiv2023

The Long Secondary Period (LSP) Variables: Overview and Some Analysis

We briefly review the phenomenon of long secondary periods (LSPs) in red giants, and the LSP variable stars classification introduced in the All-Sky Automated Survey for Supernovae (ASAS-SN) variable star catalog; they are red giant Long Period Variables (LPVs) in which their LSP variability is significantly greater than their pulsational variability. We then describe and discuss the results of a period and amplitude analysis of a random sample of 35 LSP variables in the ASAS-SN catalog, using ASAS-SN data and the AAVSO VStar time-series analysis software. The pulsation period and amplitude, and LSP, all increase with increasing luminosity or size of the star, as expected. The behavior of the LSP amplitude is more complicated; it appears to be larger in moderate-luminosity stars, and smaller in low- and high-luminosity stars. In particular, it is relatively small in a sample of 27 Mira stars, analyzed separately using AAVSO visual data. These results are discussed in the context of the current model for the LSP phenomenon, namely that it is caused by eclipses of the red giant star by a dust-enshrouded companion.

astro-ph.SR↗

arXiv2022

Long Secondary Periods in Pulsating Red Giants: A Century of Investigation

Red giants are unstable to radial pulsation. About a third of them also show a long secondary period, 5 to 10 times the pulsation period. The long secondary periods were recently ascribed to eclipses of the red giant by a low-mass dust-enshrouded companion. Long secondary periods have been known for over a century. In this paper, I use primarily American Association of Variable Star Observers visual and photoelectric observations to look for evidence of long secondary periods in 103 red giant stars listed by Nancy Houk in 1963 as having long secondary periods, based mostly on photographic photometry. I have determined long secondary periods in 37 stars, and upper limits (some of them not very stringent) in 25. In the former, the ratio of long secondary period to pulsation period peaks strongly at 10, which suggests that most of the stars are pulsating in the first overtone. The loong secondary periods are consistent with those o0f Houk in 33 of the 37 stars. I have identified 16 stars as bimodal pulsaters;their period ratios are consistent with previous observational and theoretical results. For 14 stars,the periods in the General Catalogue of Variable Stars are incorrect or absent.

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arXiv2019

Period Analysis of All-Sky Automated Survey for Supernovae (ASAS-SN) Data on Pulsating Red Giants

The All-Sky Automated Survey for Supernovae (ASAS-SN) has recently used over 2000 days of data to identify more than 50,000 variable stars, automatically classify these, determine periods and amplitudes for those that are periodic -- part of a remarkable project to classify 412,000 known variable stars, and determine their basic properties. This information about the newly-discovered variables, along with the photometric data is freely available on-line. In this paper, we analyze ASAS-SN data on two small random samples of pulsating red giants (PRGs) in detail, and compare our results with those found by ASAS-SN. For the majority of a sample of 29 mostly semi-regular (SR) PRGs, the ASAS-SN results are incorrect or incomplete: either the ASAS-SN periods are 2, 3, or 4 times the actual period, or the ASAS-SN period is a "long secondary period" with a shorter pulsation present, or the star is multi-periodic or otherwise complex, or the star's data are contaminated by instrumental effects. For almost all of a sample of 20 of the longest-period Mira stars (period 640 days or more), the ASAS-SN period is actually 2 or more times the actual period. Our results are not surprising, given the very complex behavior of PRGs.

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arXiv2019

ASAS-SN Observations of the Pulsation of some R Coronae Borealis (RCB) Stars

Photometry from the All-Sky Automated Survey for Supernovae (ASAS-SN), along with the AAVSO VSTAR time-series analysis package has been used to study the pulsational properties of 31 R Coronae Borealis (RCB) stars. Periods have been derived for many of the stars, but the variability tends to be small and complex, occasionally multiperiodic, with noticeable variation in amplitude. As with other RCB stars, the periods are a few weeks.

astro-ph.SR↗

arXiv2018

A Study of Pulsation and Fadings in some R Coronae Borealis Stars

We have measured the times of onset of recent fadings in four R CrB stars -- V854 Cen, RY Sgr, R CrB, and S Aps. These times continue to be locked to the stars' pulsation periods, though with some scatter. In RY Sgr, the onsets of fading tend to occur at or a few days after pulsation maximum. We have studied the pulsation properties of RY Sgr through its recent long maximum using (O-C) analysis and wavelet analysis. The period "wanders" by a few percent. This wandering can be modelled by random cycle-to-cycle period fluctuations, as in some other types of pulsating stars. The pulsation amplitude varies between 0.05 and 0.25 in visual light, non-periodically but on a time scale of about 20 pulsation periods.

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arXiv2018

Long-Term Changes in the Variability of Pulsating Red Giants (and One R CrB Star)

We have used many decades of visual observations from the AAVSO International Database, and the AAVSO time-series analysis package VSTAR, to study the long-term changes in period, amplitude, and mean magnitude in about 30 normal pulsating red giants (PRGs) i.e. those without large secular changes in period, as well as a few of the rare PRGs which do have such secular period changes. The periods of the typical PRGs "wander" on time scales of about 40 pulsation periods -- significantly longer than the time scales of amplitude variation which are 20-35 with a mean of 27 pulsation periods. We have also studied the range and time scale of the long-term changes in pulsation amplitude and mean magnitude, as well as period, and looked for correlations between these. Changes in mean magnitude are larger in stars in stars with larger mean amplitude, but correlate negatively with changes in amplitude. There is a weak positive correlation between the long-term period changes and amplitude changes. The causes of these three kinds of long-term changes are still not clear. We note, from the presence of harmonics in the Fourier spectra, that the longest-period PRGs have distinctly non-sinusoidal phase curves. For studying PRGs, we demonstrate the advantage of studying stars with minimal seasonal gaps in the observations, such as those near the celestial poles. We studied Z UMi, misclassified as a possible Mira star, but actually an R CrB star. We determined times of onset of 16 fadings. We were not able to determine a coherent pulsation period for this star at maximum, with an amplitude greater than 0.05. We did, however, find that the times of onset of fadings were "locked" to a 41.98-day period -- a typical pulsation period for an R CrB star.

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arXiv2018

Sparsely-Observed Pulsating Red Giants in the AAVSO Observing Program

This paper reports on time-series analysis of 156 pulsating red giants (21 SRa, 52 SRb, 33 SR, 50 Lb) in the AAVSO (American Association of Variable Star Observers) observing program for which there are no more than 150-250 observations in total. Some results were obtained for 68 of these stars: 17 SRa, 14 SRb, 20 SR, and 17 Lb. These results generally include only an average period and amplitude. Many, if not most of the stars are undoubtedly more complex; pulsating red giants are known to have wandering periods, variable amplitudes, and often multiple periods including "long secondary periods" of unknown origin. These results (or lack thereof) raise the question of how the AAVSO should best manage the observation of these and other sparsely-observed pulsating red giants.

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arXiv2017

Amplitude Variations in Pulsating Red Giants. II. Some Systematics

In order to extend our previous studies of the unexplained phenomenon of cyclic amplitude variations in pulsating red giants, we have used the AAVSO time-series analysis package VSTAR to analyze long-term AAVSO visual observations of 50 such stars, mostly Mira stars. The relative amount of the variation, typically a factor of 1.5, and the time scale of the variation, typically 20-35 pulsation periods, are not significantly different in longer-period, shorter-period, and carbon stars in our sample, and they also occur in stars whose period is changing secularly, perhaps due to a thermal pulse. The time scale of the variations is similar to that in smaller-amplitude SR variables, but the relative amount of the variation appears to be larger in smaller-amplitude stars, and is therefore more conspicuous. The cause of the amplitude variations remains unknown.

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arXiv2017

Forty Years of Linking Variable Star Research with Education

In this review, I reflect on four decades of my experience in linking astronomy research and education by supervising variable-star research projects by undergraduates, and by outstanding senior high school students. I describe the evolution of my experience, the students I have supervised, the nature of their projects, the educational contexts of the projects, the need for "best practices", the journals in which we publish, and the special role of the American Association of Variable Star Observers (AAVSO). I then describe our recent research on pulsating red giants and related objects, including three astrophysical mysteries that we have uncovered. Finally, I suggest how my projects might be scaled up or extended by others who supervise student research.

physics.ed-ph↗

arXiv2016

Studies of the Long Secondary Periods in Pulsating Red Giants. II. Lower-Luminosity Stars

We have used AAVSO visual and photoelectric V data, and the AAVSO time-series package VSTAR and the Lomb-Scargle time-series algorithm to determine improved pulsation periods, "long secondary periods" (LSPs). and their amplitudes in 51 shorter-period pulsating red giants in the AAVSO photoelectric photometry program, and the AAVSO LPV (long period variable) binocular program. As is well known, radial pulsation becomes detectable in red giants at about spectral type M0, with periods of about 20 days. We find that the LSP phenomenon is also first detectable at about M0. Pulsation and LSP amplitudes increase from near zero to about 0.1 at pulsation periods of about 100 days. At longer periods, the pulsation amplitudes continue to increase, but the LSP amplitudes are generally between 0.1 and 0.2 on average. The ratios of LSP to pulsation period cluster around 5 and 10, presumably depending on whether the pulsation period is the fundamental or first overtone. The pulsation and LSP phase curves are generally close to sinusoidal, except when the amplitude is small, in which case they may be distorted by observational scatter or, in the case of the LSP amplitude, by the pulsational variability. As with longer-period stars, the LSP amplitude increases and decreases by a factor of two or more, for unknown reasons, on a median time scale of about 20 LSPs. The LSP phenomenon is thus present and simiklar in radially pulsating red giants of all periods. Its cause remains unknown.

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arXiv2016

Period Changes and Evolution in Pulsating Variable Stars

We review ways in which observations of the changing periods of pulsating variable stars can be used to detect and directly measure their evolution. We briefly describe the two main techniques of analysis -- (O-C) analysis and wavelet analysis -- and results for pulsating variable star types which are reasonably periodic: type I and II Cepheids, RR Lyrae stars, beta Cephei stars, and Mira stars. We comment briefly on delta Scuti stars and pulsating white dwarfs. For some of these variable star types, observations agree approximately with the predictions of evolutionary models, but there still exist significant areas of disagreement that challenge future models of stellar evolution. There may be a need, for instance, to include processes such as rotation, mass loss, and magnetic fields. There may also be non-evolutionary processes which are contributing to the period changes.

astro-ph.SR↗

arXiv2016

Studies of the Long Secondary Periods in Pulsating Red Giants

We have used systematic, sustained visual observations from the AAVSO International Database, and the AAVSO time-series analysis package VSTAR to study the unexplained "long secondary periods" (LSPs) in 27 pulsating red giants. In our sample, the LSPs range from 479 to 2967 days, and are on average 8.1 +/- 1.3 times the pulsation period. There is no evidence for more than one LSP in each star. In stars with both the fundamental and first overtone radial period present, the LSP is more often about 10 times the latter. The visual amplitudes of the LSPs are typically 0.1 magnitude and do not correlate with the LSP. The phase curves tend to be sinusoidal, but at least two are sawtooth. The LSPs are stable, within their errors, over the timespan of our data, which is typically 25,000 days. The amplitudes, however, vary by up to a factor of two or more on a timescale of roughly 20-30 LSPs. There is no obvious difference between the behavior of the carbon (C) stars and the normal oxygen (M) stars. Previous multicolor photoelectric observations showed that the LSP color variations are similar to those of the pulsation period, and of the LSPs in the Magellanic Clouds, and not like those of eclipsing stars. We note that the LSPs are similar to the estimated rotation periods of the stars, though the latter have large uncertainties. This suggests that the LSP phenomenon may be a form of modulated rotational variability.

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arXiv2014

A Study of Multiple-Mode Pulsating Red Giants

This project began as a search for triple-mode pulsating red giants (PRGs) among known double-mode PRGs,, using AAVSO visual data. It then expanded to include a check on the reality and significance of some double-mode PRGs. We identified several "double-mode" PRGs in which one of the periods might simply be an alias of the other. We also comment on the possibility that the additional periods might be harmonics, rather than overtones. As for the triple-mode pulsators: in our sample, we find no convincing cases in which there is a significant, non-alias third period which might be an additional radial period. We note, however, that a few triple-mode pulsators have been found in a very large sample of PRGs in the LMC/ We discuss the results in the context of recent theoretical models of pulsating red giants.

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arXiv2014

Amplitude Variations in Pulsating Yellow Supergiants

It was recently discovered that the amplitudes of pulsating red giants and supergiants vary significantly on time scales of 20-30 pulsation periods. Here, we analyze the amplitude variability in 29 pulsating yellow supergiants (5 RVa, 4 RVb, 9 SRd, 7 long-period Cepheid, and 4 yellow hypergiant stars), using visual observations from the AAVSO International Database, and Fourier and wavelet analysis using the AAVSO's VSTAR package. We find that these stars vary in amplitude by factors of up to 10 or more (but more typically 3-5), on a mean time scale (L) of 33 +/- 4 pulsation periods (P). Each of the five sub-types shows this same behavior, which is very similar to that of the pulsating red giants, for which the median L/P was 31. For the RVb stars, the lengths of the cycles of amplitude variability are the same as the long secondary periods, to within the uncertainty of each.

astro-ph.SR↗

arXiv2014

Does the Period of a Pulsating Star Depend on its Amplitude?

Several classes of pulsating stars are now known to undergo slow changes in amplitude; these include pulsating red giants and supergiants, and yellow supergiants. We have used visual observations from the AAVSO International Database, and wavelet analysis of 39 red giants, 7 red supergiants, and 3 yellow supergiants, to test the hypothesis that an increase in amplitude would result in an increase in period, because of non-linear effects in the pulsation. For most of the stars, the results are complex and/or indeterminate, due to the limitations of the data, the small amplitude or amplitude variation, or other processes such as random cycle-to-cycle period fluctuations. For the dozen stars which have substantial amplitude variation, and reasonably simple behavior, there is a 75-80% tendency to show a positive correlation between amplitude and period.

astro-ph.SR↗

arXiv2013

Amplitude Variations in Pulsating Red Supergiants

We have used long-term AAVSO visual observations and Fourier and wavelet analysis to identify periods and study long-term amplitude variations in 44 red supergiants. Of these, 12 stars had data which were too sparse and/or had low amplitude and/or were without conspicuous peaks in the Fourier spectrum; 6 stars had only long (2500-4000 days) periods without significant amplitude variation. The other 26 stars had one or two periods, either "short" (hundreds of days) or "long" (thousands of days), whose amplitudes varied by up to a factor of 8, but more typically 2-4. The median timescale of the amplitude variation was 18 periods. We interpret the shorter periods as due to pulsation, and the longer periods as analogous to the "long secondary periods" found in pulsating red giants.

astro-ph.SR↗