Project Leader: Stan Walker

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 distinct maxima 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, now V415 Velorum, the existence of which was drawn to our attention by Peter Williams. The periods of these stars are all in excess of 400 days and usually 500 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 These have been replaced by a prominent ‘bump’ on the light curve.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 from 560 to ~500 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. (See DMM Light Curves for ASAS light curves of these stars.)

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 or SRs which could possibly be double peaked. There is no doubt that BX Carinae is one of these, although its low amplitude puts it in the SR region. TT Centauri and UZ Circini show some of the characteristics of these stars.

None of these stars are very far from the galactic plane, Bo, but all lie along one spiral arm, or even in a direction away from the galactic centre. John Greaves noted another interesting feature, namely that these stars are all in a quite small region of the southern Milky Way.

CK Carinae, CL Carinae and FK Puppis all SR stars and quite bright, with low amplitudes. The periods are unusually long for SR stars of such regularity, even if we take the half-period as the true period. FK Puppis may be merely an SR star with alternating deep and shallow minima Coordinates and present ranges are:

The project is intended to find out more about these stars by examining the overall light curves and obtaining colours. The spectral classes are a mixture, ranging from normal Miras with mid-M spectra to C and CS types.

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 V415 Velorum, 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. 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 may be a bit artificial. For charts of these stars refer either to DMM Visual Chart Notes or the list of charts.

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.

Amongst other things the more of pulsation is important. Miras are believed to pulsate in the fundamental mode, m0, although that is still debated. But the recent period changes in BH Crucis and a northern object, LX Cygni, look rather like changes from m1 to m2, first to second overtones. LX Cygni at +48o is not a good SH target but if anyone from the NH would like to participate.

Another area of discussion centres around the reasons and differences in the maxima. One group argued that the true period was the half period. This seems to conflict with the meagre data that is available now. Information on colour changes (and pulsation radial velocities – which are probably beyond amateur spectroscopy at present) will answer this question.

Project Description

Objective

Determining the colour changes and light curves for a complete cycle of selected Mira and Semi-Regular stars which show two discrete maxima during each cycle. Other similar stars with distinct and regular bumps on the light curves may be included.

Equipment and Filters

The skeleton against which the colour measured will be fitted will comprise standard visual observations. Colour systems will include UBV, VRI and JH. Single filter V measures will be accepted if offered.

Observing Requirements

We wish to obtain the following measures each cycle:

• Visual 100 per star, or about one each 5 days.
• UBV VRI, JH 50 per star, about each 10 days

A slight oversupply of measures around each maximum would probably be useful on the stars reaching maximum early so that we can evaluate the suitability of this scale. For the stars with low amplitudes, visual observers should preferably be quite experienced.

Time Frame of Project

As most of the target stars have periods of between 400 and 600 days this project will extend to 31 December 2010. Progressive results will be published in the Quarterly Newsletters and the website. Two of the stars, BH Crucis and R Centauri, seem in an active evolutionary state and might well be followed after that. They certainly should be reobserved at a further ten-year interval.

Publication of Results

This will be done in early 2011 in an approved Journal.

Project Targets

There are eleven stars on the preliminary target list. Please see page 21 of the VSS Newsletter – Feb 2009 for the details. These may be changed if they appear after several months’ measures to be unsuitable. They all lie close to the galactic plane, as evidenced by the Bo column. It has been suggested that these stars form a single small population, being found in such a small part of the sky. But if you know of others, let us know.

Project Information Files

Charts

Some of these are for visual observations, others for CCD observations.