Variable Star Section
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No. 22 (C97/1 ) PUBLICATIONS OF THE VARIABLE STAR SECTION ROYAL ASTRONOMICAL SOCIETY OF NEW ZEALAND Director: Frank M. Bateson P.O. Box 3093, Greerton, Tauranga, New Zealand Telephone/Fax International: (64)-07-54I-0216 email: varstara!voyager.co.nz ISSN 0111-736X PUBLICATIONS OF THE VARIABLE STAR SECTION ROYAL ASTRONOMICAL SOCIETY OF NEW ZEALAND No. 22 CONTENTS: I. V854 CENTAURI - THE FIRST 3000 DAYS Peter F. Williams 9. V2051 OPHIUCHI NEEDS CLOSER ATTENTION Frank M. Bateson 15. CW MONOCEROTIS REQUIRES CLOSER MONITORING Rod Stubbings & Ranald Mcintosh 18. VISUAL OBSERVATIONS OF THE 1995 ECLIPSE OF OW GEMINORUM Peter F. Williams 22. THE MEAN CYCLE OF IK NORMAE Frank Bateson & Ranald Mcintosh 25. THE DWARF NOVA BI ORI0NIS Frank Bateson, Ranald Mcintosh & Rod Stubbings 31. VISUAL PHASE PLOTS OF SOME BRIGHT CEPHEIDS Peter F. Williams 38. RESULTS OF MONITORING VI830 SAGITTARH OVER AN INTERVAL OF NEARLY 7000 DAYS Frank M. Bateson 41. RESULTS FROM LONG TERM MONITORING OF BB VELORUM Frank M. Bateson, Ranald Mcintosh & Rod Stubbings 44. THE DWARF NOVA MM SCORPII PROBABLY HAS A SHORT MEAN CYCLE Frank Bateson, Ranald Mc Intosh & Rod Stubbings 46. VISUAL OBSERVATIONS OF THE 1994 ECLIPSE OF BL TELESCOPH Peter F. Williams 49. REPORT OF THE VARIABLE STAR SECTION FOR 1994 & 1995 1997 February 20rt> V854 CENTAURI - THE FIRST 3000 DAYS Peter F. Williams Variable Star Section, RAstr. Soc. of N.Z. & Sutherland Astr. Soc. 3 McAuley Close, Hcathcote NSW 2233, AUSTRALIA SUMMARY: V854 Cen is the third brightest representative of the rare R Coronae Borealis class of variable stars. It was only recognised as such around 1987 which is somewhat surprising considering at maximum light this star attains a visual magnitude of 7.5. Visual observations during the 3000 day interval 1986 July through 1994 October show this a particularly active member of the RCB class. 1. INTRODUCTION: THE RCB PHENOMENON The RCB variable stars are evolved F or All of the currently recognised RCB stars G type supergiants whose atmospheres have been shown to exhibit a large are rich in carbon and deficient in infrared excess, apparently due to the hydrogen, yet with other elements at near presence of a dust shell which has solar abundances. Distribution and condensed from material expelled by the radial velocity data suggest they belong to star, or which may indicate the presence the old disk and possibly also the galactic of an old hydrogen envelope. IUE bulge population. Absolute magnitudes satellite spectra indicate dust formation appear to He within the range -3 to -5 occurs at maximum phase of the 43.2 day and may be related to the evolutionary pulsation cycle (3). Temperatures of state of the star. these shells are of the order of 500K to 900K. In the case of R Coronae Borealis, The visual light curve is typified by deep surrounding this is an extensive dust minima of amplitude up to 8 magnitudes cloud whose temperature throughout is a and which occur at essentially irregular nearly uniform 25K to 30K and which has intervals but the time of decline onset been shown by IRAS data to be some 8pc appears linked to maximum light phase of in diameter. the star's pulsation period, as has been demonstrated for RY Sgr (1) and V854 The RCB stars are comparatively rare Cen (2). The decline is generally much objects which suggests this stage of a steeper than the return to maximum star's evolution is a relatively short lived while both amplitude and duration of phenomenon. Only 26 of these stars are each event can vary considerably in presently recognised as listed in Table 1. character. Included also in this list are the three so- called "Hot RCB Stars" DY Cen, V348 At maximum the RCB stars tend to show Sgr and MV Sgr. These appear similara small amplitude variations, possibly due to the classic RCB stars but show a to radial pulsations, with periods typically considerably higher photospheric near 40 days but ranging from 20 to 230 temperature and may represent an early days. Photometric data on V854 Cen has stage in the development towards the shown a probable period of 43.2 days (2). RCB stage. 2 Also related to the RCB stars are the The rates of early decline and hydrogen deficient carbon stars (HdC photometric colour changes have been stars). These have a spectrum similar to discussed by Lawson (4) who described the RCB variables but show no evidence both "blue" and "red" events which have of circumstellar dust nor marked been interpreted as resulting from variations in brightness, though the latter different levels of obscuration of the star's may merely reflect a lack of close photosphere during the ejection process. monitoring over the long term. The presence of strong hydrogen lines in A "blue" decline occurs when the HD148839 (NSV 7820) and V854 Cen, photosphere is only partially obscured plus weak hydrogen lines in some other during the initial stages of decline, where HdC stars has given support to the (U-B) and (B-V) colours are observed to suggestion that these two groups of stars become considerably bluer prior to finally are in fact closely related and should reddening. This has also been tentatively perhaps no longer be classified as linked to a "fast" rate of initial decline independent groups. and where the cloud has been ejected from the star at a relatively low angle. The mechanism responsible for the deep minima observed in the RCB stars is still A "red" decline results from the ejected unclear and this class of variable are material totally obscuring the currently the subject of close study by a photosphere plus much of the number of professional groups. Amateur chromosphere at the time of cloud astronomers can provide valuable condensation. This has been associated assistance here by regular long term with a large angle of ejection and a "slow" monitoring of these stars and promptly rate of early decline. reporting the onset of deep declines, such information being critical in allowing 2. DISCOVERY & OBSERVATION. professional study of these events. V8S4 Cen has the dubious distinction of Multi-band photometry shows the light at having been discovered three times prior longer wavelengths of JH-K-L is less to being recognised as the important and affected during the deep minima than is interesting star it is. the visual. At L there is only minimal variation. This has been interpreted by It was first noted during a 1964 suggesting the light from U through J photographic survey for variable stars bands has as its source the central stellar conducted by the Remeis Observatory, object but at L the light originates almost Bamberg, receiving the preliminary solely from the circulstellar shell and designation BV520 and catalogued as a surrounding dust cloud. faint variable of small amplitude. This led to its inclusion in the New Catalogue During decline, the observed spectra of Suspected Variable Stars (NSV) as changes from that of absorption to one NSV6708 (5). No follow-up observations with pronounced emission lines. This is were obtained. believed to occur as the photosphere of the star becomes totally obscured by the Attention was again directed towards this expanding cloud, leaving the star in 1971 when New Zealand amateur chromosphere as the primary light Ron Welch noted it but again no follow- source. In the case of V854 Cen this has up observations came. been shown to occur some 5 or 6 magnitudes below maximum brightness. Its third discovery was on July 6, 1986 when Australian amateur Glenn Dawes 3 (6) noted it as a magnitude 7 object on In 1989 NSV6708 was assigned the patrol photographs. By this time, official variable star designation V854 however, another Australian amateur, Cen. Mati Morel, had established a service through the Variable Star Section, The first major decline observed was that RASNZ, whereby he produced and commencing in July 1987 as reported in distributed preliminary charts for newly IAUC 4419. Subsequent early declines discovered and/or transient stellar were also reported in various IAU objects. Such a service allowed for Circulars, including IAUC 4650 (1988), prompt and continuous follow-up 4879 (1989) and 5203 (1991). observation. 3. THE VISUAL LIGHT CURVE. Robert H. McNaught (6) of Siding Spring Observatory investigated survey plates The light curve presented here is based on and determined a magnitude range of 7 to observations by the author, with the 15 for this star and indicated its tendency exception of a few estimates where the to remain faint for extended periods of variable was either inaccessible from the time. He also noted this star had author's observing site or when long previously been misidentified as the 11th periods of cloud prohibited observation. magnitude CPD-39 9021 which lies only These additional observations have been 33 arcsec to south of the variable. An gleaned from the various Monthly accurate position determined by Circulars of the VSS, RASNZ. McNaught is (1950) 14h 31m 41s.54 -39 20' 13".2. All observations have been obtained using standard methods as described by Early spectra obtained at Mount Stromlo Bateson (10) for the visual observer. Observatory indicated an F type star with Binoculars have been used to record the strong hydrogen lines. variable when near maximum light while a variety of telescopes used when it faded The long periods of faintness and its close below 8.5 magnitude. The general proximity to the nearby CPD-39 9021 observing season is from March through apparently contributed to its only recent early October, though predawn 'discovery'. No doubt its location some 20 observations can commence as early as degrees from the galactic plane also aided December, thus allowing for a near its non detection for so many years, continuous record of the star's behaviour.