Fig.1) brightness changes of V832 Cyg from May 2008 to April 2023
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Béla Hassforther
Can a star that shows just three maxima and three minima in fifteen years be of interest to an observer? It can! That's what I want to show here by the example of V832 Cyg = 59 Cyg.
I first noticed V832 Cyg while browsing the "Hipparcos Variability Annex: Light Curves" of ESA [1]: V832 Cyg shows there an almost linear brightness decrease of about 0.03mag for two years in the covered observation period, which seems to be followed by a not so clear slight increase in the next 300 days. This whetted my appetite to learn more about the brightness changes before and after. After some initial research, the star seemed interesting enough to measure it on my own images.
My observation material consists of two long time-series:
The time-series overlap, so I could determine which corrections had to be applied to the rows. At first I measured the star only on the EOS shots, but then I wanted to know if my old IXUS shots could also be used, made some experiments and finally measured all shots. So the following figure 1) represents the brightness changes by exactly 800 values.
Three maxima can be seen and at the end of the observation period a steep rise to a fourth maximum. The observations appear to start at a minimum and overlap three other minima. While the minima differ little in depth, the maxima are of different heights: the second observed maximum is much weaker than the other two maxima and also already noticeably weaker than the current approaching maximum. The two high maxima show a slow increase and a faster decrease in brightness. The total amplitude is just 0.3mag in the visual.
It is easy to see that during a minimum the star hardly changes in its brightness for more than a year, so each new observation increases the suspense as to when the rise to the next maximum will begin. It is similar, but not quite so pronounced, during maxima. My assertion that the brightness changes is exciting to observe finds its explanation here.
The solid curve of Fig.1 was calculated with the "smoothing of observational data" procedure described by Jan Vondrak in [2] and [3], using the software HEC13, version 7 [4] provided by Petr Harmanec. Getting this old DOS software to work on today's computers can be quite a challenge. It is nice that the software also outputs the residuals to the "smoothed" value for each observation.
Fig.1) brightness changes of V832 Cyg from May 2008 to April 2023
Authors P. Harmanec et al. published an excellent paper on V832 Cyg in 2002 [5] and collected historical photometric and spectroscopic observations for it. With their work, the older literature on V832 Cyg became almost obsolete. They make available on VizieR [6] their collected measurements. If one additionally downloads the V measurements from AAVSO [7], the following exciting light curve is obtained (Fig. 2). As usual, the AAVSO data contain many outliers, which have been removed here. Furthermore the AAVSO data contain values which are also found in P.Harmanec, but too bright by 0.02mag. Therefore I added 0.02mag to all AAVSO values for this light curve.
Fig.2) brightness changes of V832 Cyg from 1958 to 2023 (without IXUS values)
The following observations can be made:
And with the trend to higher and higher maxima the star is of course very exciting for observers. Perseverance and patience are required, however.
How can the brightness changes be explained? The paper by Harmanec et al. cited above [5] provides a succinct summary:
"We have no clear explanation for either the cyclic changes of the (U - B) index or similar (but somewhat shorter) cyclic changes of the brightness in V observed since about JD 2 447 000. Their origin must probably be sought in physical variations in the Be envelope during the process of its gradual evolution." [8]
It would be presumptuous at this point to try to explain more than experts can do at present, but some of the system properties are quite certainly known and can be summarized.
The components Aa1 and Aa2: The main star Aa1 in the system V832 Cyg is a Be-star (spectral type B1V) and like most - or all? - representatives of these stars a binary. The companion Aa2 is a so-called sdO star, i.e. a hot but low-mass star. While the binary nature was known for a long time by RG measurements of the orbital motion of the main star, the companion could be confirmed spectroscopically only in 2005 [9]. The orbital period of this double star is 28.2 days according to several independent studies. Also photometrically Harmanec et al. could clearly detect a brightness changes with this period, but with a total amplitude of just 0.015mag. Here the variable disk around the Be star is likely to play a role.
The component Ab: For more than 40 years a 0".2 distant component of spectral type A3V and magnitude 7.64mag has been tracked with speckle measurements. The observed orbital arc in 34 years spans 84, so the orbital period should be around 200 years. This companion is, of course, too far from the close binary star system to play any role in its evolution.
Component B: This distant component of magnitude 9.43mag and spectral type A8III was previously assumed to have a common direction of motion, however this is clearly refuted by Gaia-DR3 data [10]: Both the parallaxes and proper motions are too different.
The components C, D and E mentioned in the Washington Double Star Catalog (WDS) differ in their Gaia-DR3 data partly considerably from the data of V832 Cyg and therefore need not be considered.
[1] ESA, 1997, The Hipparcos and Tycho Catalogues, ESA SP-1200. volume 12. Hipparcos Variability Annex: Light Curves.
[2] Vondrak, Jan - A contribution to the problem of smoothing observational data, in: Bulletin of the Astronomical Institute of Czechoslovakia, vol. 20, p.349, 1969.
[3] Vondrak, Jan - Problem of Smoothing Observational Data II, in: Bulletin of the Astronomical Institute of Czechoslovakia, vol. 28, p.84, 1977
[4] https://astro.troja.mff.cuni.cz/ftp/hec/HEC13/
[5] P. Harmanec et al, Properties and nature of Be stars, XXI. The long-term and the orbital variations of V832 Cyg = 59 Cyg, A&A 387, 580-594 (2002).
[6] https://cdsarc.cds.unistra.fr/viz-bin/qcat?J/A+A/387/580#/browse
[7] https://www.aavso.org/
[8] P. Harmanec et al, op. cit., p.593.
[9] M. Maintz et al, Publ. Astron. Inst. ASCR 93, 21 - 28 (2005).
[10] This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium).
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Autor: Béla Hassforther.
Letzte Änderung: 09.05.2023
Adresse dieser Seite: http://www.bela1996.de/astronomy/cyg-v832-en.html