The Dual Maxima Mira Stars

Article supplied by Stan Walker 11 August 2008

The Mira stars are an interesting group of variable stars, well suited to visual observing. (And little observed in colours such as UBV) In most cases there is a quick rise to maximum brightness, followed by a slower decline to a rather faint level. The amplitudes are usually quite large, and the periods of 200-600 days make them easy to observe for the casual observer.

Amongst these stars there are a few unusual objects. These are Miras which, at times, show two bright maxima and two minima. The most well known of these stars in the 1960s were R Centauri and R Normae. Since then, two other southern objects have been observed - BH Crucis, discovered by Ron Welch in Auckland, and NSV 4721, the existence of which was drawn to our attention by Peter Williams.. The periods of these stars are all in excess of 400 days. Colour photometry reveals another interesting feature, namely that the first maximum in R Centauri is bluer, hence hotter, than the second maximum, whereas the reverse is the case with BH Crucis.

More recently, both BH Crucis and R Centauri have shown major changes in period and have largely lost their dual maxima features. These have been replaced by a prominent 'bump' on the light curve. The period of BH Crucis has increased from 421 to ~530 days, whereas that of R Centauri has decreased form 560 to ~510 days. The light curves now resemble stars such as R Hydrae, which frequently has a pronounced bump on the rise. R Hydrae also resembles some of these stars in that its period was ~560 days in 1660, but is now ~380 days. Another difference in R Centauri and BH Crucis is that the former now has a pronounced bump on the falling light curve, the latter on the rising curve.

John Greaves noted another interesting feature, namely that these stars are all in a quite small region of the southern Milky Way. Coordinates and present ranges are:

NSV 4721    10 03 30   -46 49.2   9.6-11.8
BH Crucis   12 16 17   -56 17.2   6.5-9.8
R Centauri  14 16 34   -59 54.8   5.8-9.0
R Normae    15 35 57   -49 30.5   6.4-12.0

At times other stars have been noted as having double maxima but an examination of the light curves doesn't support these claims. However, there might be many that I have not come across. A search of ASAS data on LPVs with periods of 400+ days showed several other Miras which could possibly be double peaked. There is no doubt that BX Carinae is one of these, and CK Carinae may well be, although there are not enough observations to be certain and the light curve is more sinusoidal than the other stars. Two other stars, V865 Aquila and TT Centauri, have flattish maxima with dips. TT Cen is the more likely one of these. The light curve of BX Carinae from ASAS is shown.

ASAS 105206-6229.0 Light Curve (asas3)

The pulsation mechanism which causes the changes in brightness is probably similar to that of the Cepheid stars where variations in opacity of the envelope set up self-sustaining oscillations or shock waves which the observer sees as a very regular bright/faint periodicity. Because of the much more extensive outer envelope in Mira stars, however, the visual effects are not quite as regular.

With Cepheid stars, those with periods around 10 days show dual maxima. R Muscae is a well known example of this. Stars with periods around this show humps on the rise or decline but these are closely related to the periods. These are stated to be caused by reflected shock waves from one of the convection/radiation interfaces in the star. It's interesting that many of the Miras with periods of over a year show similar bumps, but with no particularly regular distribution.

All of these stars are reasonably well covered by ASAS, although the maxima of three of them are in the saturated area of that equipment and careful visual measures are valuable. Colour measures of the two objects poorly observed in UBV, R Normae and NSV 4721, would be interesting in view of the contrast between R Centauri and BH Crucis. UBV is preferred, but BVR would give us a reasonable handle on colour changes.

Coincidentally, three of these stars have passed the first maximum and are approaching the second or - in R Carinae's case - a pronounced hump on the decline. BH Crucis is the odd one out, approaching minimum at this time. Of these stars, only R Normae has a 'normal' mira amplitude of ~5.6 magnitudes. The others are near the minimum level of 2.5 magnitudes, which only seems to show that the mira definition is a bit artificial. Charts of all these stars, and comparisons, have been published by the Section. BX Carinae is presently between the two maxima, but I'm uncertain about the existence of a chart for this star. BH Crucis is rather faint, but will begin the rise to maximum shortly.

For the CCD and pep exponents with filters it would be rewarding to follow a full cycle of these stars. All are more or less circumpolar and in reasonably dense parts of the sky so that any comparison star of similar brightness may well have known colours.

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