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Why observe LPV's

Why Observe LPV's?

What is the value of observing long-period variables (LPVs), especially visually, and continuing to do so?  Let me illustrate by listing the various work that my students and I have done with your observations.  Most of our results are published in JAAVSO, and some in PASP (Publications of the Astronomical Society of the Pacific).  Also be sure to read the review articles by Kiss and Percy (2012 JAAVSO 40, 528) and by Willson and Marengo (2012 JAAVSO 40, 516) in the JAAVSO Centennial Issue, and check out the “LPVs in the News” web page.  In my case, your observations have had educational as well as scientific value, by enabling my students to develop and integrate their science, math, and computing skills, motivated by doing real science, with real data – your data.

  • An astrophysical mystery: why do a third of LPVs have a long secondary period (LSP)?  Our work has discovered or refined LSPs in over a hundred LPVs (arxiv.org/abs/1607.06482, 2013 JAAVSO 41, 1; 41, 15) and provided new information which may help to solve the mystery.
  • Another astrophysical mystery: why do the pulsation and LSP amplitudes in almost all LPV giants (2013 JAAVSO 41, 193)  and supergiants (2014 JAAVSO 42, 1) increase and decrease by factors of 2 to 10 on time scales of about 20 pulsation periods or LSPs?
  • Watching LPVs evolve: a century of your observations enables us to measure the slow period changes caused by the LPV's evolution (1999 PASP 111, 98; see also Tomas Karlsson's excellent recent work: 2014 JAAVSO 42, 280).  The accuracy of this work increases as the square of the length of time of observation, so keep up the good work!
  • A few LPVs vary more rapidly in pulsation period, and are probably undergoing a nuclear event called a “thermal pulse”.  Your observations can help us to observe and understand the thermal pulse phenomenon (Templeton et al. 2005 Astron J 130, 776).
  • Another astrophysical mystery: why are there random cycle-to-cycle period fluctuations in almost all LPVs (1999 PASP 111, 94)?  Are they due to the effect of giant convection cells?
  • What about the “semi-regular” (SR) LPVs?  Many of them turn out to have two pulsation periods, and two periods are even better than one (2013 JAAVSO 41, 1; 41, 15)!  They provide a tool for “precision astrophysics” using the Petersen diagram, a plot of period ratio versus period  (2015 JAAVSO 43, 118).
  • And what about all the “irregular” (L-type) LPVs in the AAVSO database?  Most of them turn out to be non-variable, or micro-variable at best.  In my humble opinion, they should be dropped from the program, though not everyone agrees with me  (2009 JAAVSO 37, 71; 2010 JAAVSO 38, 161; 2011 JAAVSO 39, 1).
  • Your observations also help us to identify LPVs with unique or peculiar behavior, which makes them worthy of more intensive study (1990 ASP CS 11, 446).
  • And your observations enable us to determine reliable periods and amplitudes for hundreds of LPVs, providing raw material for many areas of astronomy and astrophysics: describing and classifying, comparing with observations at other wavelengths, scheduling observing runs, theoretical modelling of evolution, pulsation, and mass loss, and much more

This summer, my student Henry Leung analyzed shorter-period LPVs from the binocular program and the PEP program, and found that the mysterious LSP phenomenon persisted to the shortest periods.  This coming year, Alex Gomes will be working with me on various aspects of the evolution of LPVs – all using your observations!

John Percy

University of Toronto

JAAVSO Editor

September 2016

 

Pulsating Red Giants  Showing Interesting and/or Unusual Behavior

Which pulsating red giants (PRGs), among the LPVs, need more observations?  One answer would be: those which are being sparsely observed.  If their behavior is normal, however, sparse observations are probably sufficient to monitor their variability.  But what is “normal”?  All PRGs show meandering variations in period, of a few percent, probably due to random, cycle-to-cycle fluctuations in period (Percy and Colivas 1999).  And all PRGs show significant variations in amplitude, on time scales of 20-30 pulsation periods (Percy and Abachi 2013).  About a third of PRGs show “long secondary periods” (LSPs), a few times longer than the pulsation periods, with amplitudes of 0.1-0.2 magnitude (e.g. Wood 2000).  But there are a few PRGs with rapid period changes (Templeton et al. 2005), probably because they are undergoing helium shell flashes.  There are some which undergo large changes in amplitude, for unknown reasons.  And there are a few which undergo large, slow variations in mean magnitude – much slower than LSPs.  The table below lists the most conspicuous of these.   The columns give: the star name; the average period (P) and its average amplitude (A) as determined from visual observations in the AAVSO International Database, using VSTAR; the period (PK) determined by Karlsson (2013) using (O-C) analysis; and notes on each star..  Because of the peculiarities of these stars, the periods are approximate only.  I have restricted the table to stars having significant amplitude -- 0.2 or greater – and therefore suitable for visual observation. In a sense, this list is an update of the list in Percy et al. (1990).  The choice of stars is arbitrary in the sense that the boundary between normal and abnormal is fuzzy.  Where to draw the line?  What is a “large” change in amplitude?  Or “large” changes in mean magnitude?  Nevertheless, I can guarantee you that all of the stars in the table are interesting, and are definitely worthy of observation.

The list has been designated "The Percy List" and can be found in our file section (the file is titled "The Percy List.pdf").

References

Karlsson, T. 2013, JAAVSO, 41, 348.

Percy, J.R., Colivas, T., Sloan, W.B., and Mattei, J.A. 1990, ASP Conference. Series 11, 446.

Percy, J.R. and Colivas, T. 1999, Publ. Astron. Soc. Pacific, 111, 94.

Percy, J.R. and Abachi, R. 2013, JAAVSO, 41, 193.

Templeton, M.R., Mattei, J.A. and Willson, L.A. 2005, Astron. J., 130, 776.

Wood, P.R. 2000, Publ. Astron. Soc. Australia, 17, 18.

John Percy

University of Toronto

JAAVSO Editor

March 2017

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