The observation of mira-type stars

translation Jacques VIALLE

Mira-type variables are among the amateur astronomer's most favourite targets. Although they don't offer the same kind of "suspense" as cataclysmic variables, their variations are unpredictable enough to deserve being recorded ; moreover, the amplitude of their variations is large enough to match the modest accuracy of our estimates. Many of them are bright enough, at least at maximum, to be observed with small instruments. Last of all, the slowness and relative regularity of their variations is compatible with observations necessarily scattered throughout the year because of the weather conditions or the professional duties of the observer. Out of the 26,992 observations reported for the 2nd term of 1995 in the AFOEV Bulletin n°73, 7574, i.e. 28Mira-type variables. In 1930, this proportion was well over 50variables were then much less observed than now but nowadays the number of observations of Mirae as well as the absolute number of estimates filed in the archives are far more important owing to the growing activity of variable star observers.

They had been asked to make the utmost to observe the brightest of these variables within the scope of the HIPPARCOS programme. This work, now completed, contributed to a better knowledge of their distances and of their proper motions, therefore of their physical properties.

The observations of these stars by amateurs is also appreciated by the professionals who study them in other wavelengths (particularly in the radio wavelengths) and need to know what stage of their cycle they are at. The same remark applies to astronomers who estimate their apparent diametres by interferometry.

Although these particular interventions are indeed far from being negligible, this should not make us lose from sight the importance of long-term visual monitoring which may, as a matter of fact, reveal evolving phenomena of great interest. An example is the recently discovered sudden decrease of the period of T UMi which may be related to a "helium flash".

Clearly, the chances of detecting such evolving phenomena are all the greater as the number of observed stars is higher. It would therefore be useful to update regularly the ephemerides of the greatest possible number of Mirae stars. This by no means precludes detailed observations of the brightest and easiest variables so as to detect changes not only in their ephemerides but also in the shape of their light curves. Another advantage of observing these "popular" stars is that the great number of observers and consequently of observations allows a statistical analysis that could help refine the light curve and perhaps determine each observer's personal equation.

In the AFOEV Bulletin n°36 (corrections in Bulletin n°37) and in Bulletin n°49, I had an opportunity to draw the observers' attention on some neglected Mirae. The lists that had then been published drew the attention of some observers on a few of these stars which were under-observed though AAVSO charts were available for them. In Bulletin n°49, the Chairman of our association published the estimated dates of maximum and minimum light of 370 stars (against 332 the year before), several of which had appeared in the above mentioned lists. However, there still remain many stars from these lists for which observations are not numerous enough to work out an ephemeris. For instance, in the constellation of Cygnus, ephemerides are now available for DW and GQ but there is a total uncertainty for the ephemerides of FQ, FU, KM, etc. In the case of FL and LX, there are indeed quite a lot of observations but they are not numerous and reliable enough for their maximum to be published in Bulletin n°72. (*)

It is therefore necesssary to keep on observing but is it possible to set the bar a little higher, go beyond and have a more systematic approach ? To answer this question, it is necessary to evaluate the number of potential targets and to compile a list of stars for which a chart is available. This is now possible as electronic versions of both the GCVS and the list of AAVSO charts are available.

The table below gives in column 1 the magnitude at maximum for all the Mirae of the Northern hemisphere catalogued in the GCVS as observed in the V, P (or B) bands. This magnitude is the visual magnitude given by the GCVS and it is truncated, i.e. the row corresponding to magnitude 8 includes stars between 8.0 and 8.99 ; a mean value of the colour index, i.e. 1.5, has been subtracted from the magnitude whenever the GCVS gives it as photographic. Column 2 gives the total number of stars listed in the GCVS while column 3 gives the number of stars for which an AAVSO chart is available. Column 4 gives the name of the stars.

|mv|GCVS|charts AAVSO|comments|
|3  |1  |1  |chi Cyg |
|4  |3  |3  |R Leo, R Cas, U Ori|
|5  |8  |8        ||
|6  |23 |26       ||
|7  |60 |60       ||
|8  |123   |119   ||
|9  |204   |153   ||
|10 |282   |116   ||
|11 |393   |70    ||
|12 |400   |41    ||
|13 |303   |10    ||
|14 |135   |1     ||
Of course, this table suggests that many discoveries are still to be made beyond magnitude 12, but this is not the point I wish to make here. It shows above all that a great number of stars within reach of small instruments at maximum are under-observed for lack of charts, whereas charts do exist for faint Mirae. For instance, the four "chartless" stars of magnitude 8 are :
|Star  |maximum ||
|SU Peg   |8.1 |mv   |
|WY Aqr   |10.0 |pg  |
|FW Gem   |10.4 |pg  |
|EX Peg   |10.0 |pg  | 

This of course has just a statistical value and the magnitudes given by the GCVS are not Gospel word.

It appears more rational to try to work on the brightest of the "chartless" stars than taking pains to observe very faint stars for which a chart is available. This is quite posssible now that the publication of the GSC and its availabilty through cheap easy-to-use software (see Bulletins 72 and 73) enable the amateur astronomer to produce his own home-made charts.

Let us also point to the fact that some of these "chartless" stars had been observed before World War II by A. Brun who built his own comparison sequences. Such is the case for SW Cam, RU Ser and BR Ori among other stars. Why their observation was later discontinued still remains a mystery.

The poor quality of the GSC photometry is well-known but on the whole it is not very much worse than our visual estimates. With a little care and a critical mind, usable light curves can be produced. The greatest difficulty comes from erroneous coordinates sometimes given in the GCVS which may make the identification of the star difficult. The observer has to show patience : a variable star by definition varies and in the end will reveal itself unambiguously. (**)

It is not however intended to circulate such maps. Amateur associations make sure that their members are supplied with charts that are based on a correct photometry and making them is therefore all the more painstaking. This should not prevent amateur astronomers from making their own charts from the GSC, provided they should be used with care and a critical mind and the comparison stars used be noted down. Besides, as regards the latter point, the uncertainty on the GSC photometry makes it necessary to compare the variable with several stars, selecting those whose magnitudes do not seem to be too aberrant. Lastly, eveything that looks odd should be recorded. Both the AFOEV and the AAVSO accept this kind of observations, at least as long as "official" charts are not available.

As an example, the light curve of HO Lyr in 1999, 2000 and 2001 is shown below. HO Lyr is a variable with a rather short period of 100.4 days, ranging from 11.4 mpg to 14.0 mpg according to the GCVS. The abscissae show the maxima determined from the GCVS ephemeris. It can clearly be seen that if the period is correct, the maxima are nevertheless considerably offset relative to the ephemeris. The amplitude proposed by the GCVS seems to be correct ; it is well-known that Mirae of short periods generally show fairly regular and reproducible cycles.

To conclude, let us mention two interesting, though extra-scientific, characteristics of this kind of activity : first, its inherent suspense as one never knows, at least at the beginning, what one will find. Next, as most of these stars have been discovered and catalogued photographically, one can always fancy he is the first to see them.

HO Lyrae Eo : 30584 - P : 100.4 j.

O | C |O-C |mv| |51341: |51366 |-25 |10.7| |51442 |51467 |-25 |10.7 | |51538 |51567 |-29 |10.2 | |51641 |51668 |-27 |10.9 | |51738 |51768 |-30 |10.8 | |51838 |51868 |-30 |10.5 | |51951: |51969 |-18: |10.9: | |52038 |52069 |-31 |10.7 | |

* the observational data collected in 1995 on FL and LX Cyg are published in Bulletin n°72.
** thousands of idenfication charts are available from the editor.

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