AAQ Variable Star Section
V854 Centauri - A Remarkable RCB Type Variable Starby Peter Williams IntroductionV854 Cen has the rather dubious distinction of having been discovered on 3 separate occasions while its true character remained unrecognised for almost 2 decades. This is despite it being third brightest of the RCB stars with a maximum visual brightness near magnitude 7.5. The initial discovery was made in 1964 during a photographic survey for previously unknown southern variable stars conducted by the Remeis Observatory of Bamberg. It was recorded as a faint star of magnitude 9.7 and showing small amplitude variations of just 0.8 magnitudes. The star received the preliminary designation BV520 and was subsequently included in the New Catalogue of Suspected Variable Stars (NSV) with the designation NSV6708. Being one of many hundreds of seemingly unremarkable faint suspected variable stars no follow up investigation was made. Attention was again directed to it in 1971 when New Zealand amateur Ron Welch noted it during a photographic nova search. Again, there was no follow up investigation and so its true nature again remained unresolved. Another decade passed before its third discovery when in July 1986 Australian amateur Glenn Dawes noted NSV6708 as a magnitude 7 object on his nova patrol photos. By this time, however, renowned Australian chart maker Mati Morel had established a service in conjunction with the Variable Star Section, Royal Astronomical Society of New Zealand whereby he produced and distributed preliminary charts for newly discovered and transient stellar objects. This service allowed for prompt and continuous follow up observation. Prompted by Dawes’ discovery, Robert McNaught of Siding Spring Observatory investigated survey plates and determined a magnitude range of 7 to 15 for this and noted a tendency for it to remain faint for extended periods of time. He also noted NSV6708 had previously been misidentified with the magnitude 11 star CPD-39 9021 that lies just 33 arcsec south of the variable. NSV6708 was then allocated the official variable star designation V854 Centauri. The RCB PhenomenonThe RCB variable stars are evolved F and G supergiants whose atmosphere is rich in carbon and deficient in hydrogen, but with other elements at near solar abundances. Distance and radial velocity data suggests they belong to the old disk and possibly also the galactic bulge population. Absolute magnitudes lie within the range of –3 to –5 and may be related to the evolutionary state of the star. The visual light curve is typified by deep minima of amplitude up to 8 magnitudes. These minima occur at essentially irregular intervals but the time of decline onset appears to be linked to maximum phase of the star’s semi regular pulsations. The decline is generally much steeper than the return to maximum while both amplitude and duration of each event can vary considerably. The RCB stars often show small amplitude variations when at maximum with periods typically near 40 days but ranging from 20 to 230 days. In the case of V854 Cen a period near 43.2 days is evident. All of the currently known RCB stars have been shown to exhibit a large infrared excess. This is apparently due to the presence of a dust shell that has condensed from material expelled by the star and may also indicate the presence of an old hydrogen envelope. Spaced based observations of V854 Cen at ultra violet wavelengths show the formation of dust coincides with maximum phase of the 43.2 day pulsation cycle. Temperature of these circumstellar shells is of the order of 500°K to 900°K. In the case of the star R CrB, the presence of an extensive dust cloud of some 8 parsecs diameter has been inferred from infrared observations. Multi-band photometry shows the light at longer wavelengths is less affected during the deep declines than is light in the visual wavelengths. At L band (Johnson L Band filter - infrared light at a wavelength of 3500 nanometres [nm]) there is only minimal variation and this has been interpreted as the light of U (Johnson U Band filter - ultraviolet light at a wavelength of 365 nm) through to J (Johnson J Band filter - infrared light at a wavelength of 1250 nm) bands originate from the central stellar object whereas the L band light originates almost solely from the circumstellar shell and surrounding dust cloud. During decline the observed spectrum changes from absorption to one with pronounced emissions. This is believed to occur as the star’s photosphere becomes totally obscured by the expanding cloud of obscuring material, leaving the chromosphere as the primary light source. In the case of V854 Cen this has been shown to occur some 5 or 6 magnitudes below maximum brightness. The RCB stars are comparatively rare objects. This suggests the RCB stage of the star’s evolution is a relatively short-lived phenomenon. Currently, as at 2005, there are just 37 stars known to be of the RCB or related types. This total includes 3 so-called “Hot RCB” stars, 6 hydrogen deficient carbon stars (HdC stars) and 3 LMC stars of the RCB type. How To Find V854 Centauri
The photograph below provides an ~ 15° field of view in the northeast portion of Centaurus. The photograph was taken on 4 June 2005 (JD 2453525.87). North is to the left and east is to the bottom. Hold your mouse over the photograph to identify Theta, Phi, Eta and V854 Centauri. The magnitudes of some of the comparison stars are shown in orange (the decimal point has been omitted).
Variable Star Charts
ObservationsV854 Cen can be observed with binoculars when near maximum from relatively dark suburban locations but requires a moderate to large telescope and dark skies to follow it when near its minimum. The general observing season for V854 Cen is from March through to early October, though pre-dawn observations can commence as early as December thus allowing for a near continuous record of this star’s behaviour. Conjunction with the Sun occurs in November. AAQ members are encouraged to observe V854 Cen and to submit their observations to variable star databases. Observations can be submitted to the Variable Star Section of the Royal Astronomical Society of New Zealand. Contact for the data format required to submit observations to the RASNZ. Observations can also be submitted to the American Association Of Variable Star Observers (AAVSO). Instructions on how to submit your observations to the AAVSO can be found here. The preferred method is to submit your data online using WebObs. Please also provide a copy of your observations to the AAQ Variable Star Section Director. The AAQ member’s observations will be collated and used to regularly update our light curve for this star . The Visual Light Curve Of V854 CentauriA light curve for V854 Cen based on 9 years of regular visual monitoring by Peter Williams is shown below (click on the image to obtain full screen width resolution light curve). When at maximum light, V854 Cen shows small variations between magnitudes 7.3 and 7.6 with occasional well-defined dips to near magnitude 8. The six week semi-regular cycle is often evident in the light curve. The deep obscuration declines show a wide range in both amplitude and duration, as is typical for this class of variable star. Minimum brightness is near magnitude 14.5. V854 Cen shows a tendency to produce declines with an initial slow decrease in brightness, followed by a much more rapid fall to minimum. The subsequent recovery is often steep at first followed by a more gradual return to maximum. Perhaps the most striking feature of the light curve is the number of deep minima observed. V854 Cen is clearly a very active representative of the RCB type stars with no fewer than 17 deep declines observed in the 2 decades 1986 through to mid 2005. This is nearly twice as often as other active RCB stars and up to 10 times more frequent than many of the other well observed RCB stars.
The unpredictable but frequent nature of its variations makes it a fascinating and rewarding object for the amateur variable star observer. V854 Cen is a star that should be included in each observer’s working list.
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