VARIABLE STARS in NGC 4755

NGC 4755 CATALOGUED VARIABLES

All variable stars within any star cluster are very important, as they can give us some additional indictions of true distance and of its evolution. For most of the younger clusters, the brighter stars are usually hot and distinctly blue in colour. Sometimes several of these are variable stars, often being ones that have started to evolve away from the main-sequence. Such stars have entered one of the so-called instability strips that frequent several regions across all H-R diagrams. For lower massed stars, none are usually found to as variable, as they continue to remain in a long peaceful phase of their hydrogen burning lives.

Young clusters usually offer one of the principle types of variables known as Beta Cepheids, often simply abbreviated as the BCEP or β Cep type. In 1979, some twenty-four Beta Cepheid variables were suspected in all the listed open clusters, but numbers of known examples has now grown exponentially and are now commomplace stars in clusters.

All Beta Cepheids vary in sinusoidal periods between 0.1 to 0.7 days, displaying very small changes in luminosity by between 0.1 and 0.3 magnitudes. Spectral classes are between B0 to B3, and this also applies across all the five main categories of the luminosity classes. Common bright examples of field Beta Cepheids included nearby to the Jewel Box — β Crucis (Mimosa), δ Crucis λ Crucis and β Centauri (In NSP 16), and β Canis Majoris. Most of these stars are members of the lower Crux-Centauri Association, showing similar proper motions and velocities through space.

Astrophysicists think that these brightness variations in Beta Cepheids are more likely caused by radial pulsation from various instabilities that changes the hydrogen burning occuring within their cores. They are general found by finding those stars that fit within the spectral class range and then by observing them photometrically over several successive nights to see if they are variable in brightness. The proportion of uncertain cluster Beta Cepheids is currently large. Like those in the Jewel Box, several stars will appear to mimic these Beta Cepheids, having characteristic of similar apparent magnitude changes or periods. Many of these usually prove to be of differing spectral classes than the B-types — immediately showing they are different types of stars. Another reason for uncertainty is simply the lack of long-periods of observations. A few long-term results suggests there are several different lengths for the harmonic periods, derived by the frequency-period analysis. Repeating observations at another time general helps refine these periods. Interestingly, further highly-detailed investigation has unveiled the existance of multiple periods like those seen in the nearby β Crucis. This particular star shows some seven different harmonic periods.

A sub-type of the BCEP (BCEPS), which have even smaller variations in brightness over periods in the order of >0.1 days. None of these types have been found in NGC 4755.

There are twenty-five known catalogued variables and several other suspected variables within “The Jewel Box” (2013). Until about 1990, only three were certain Beta Cepheid variables, being (“Individual Stars”; 8 (BV Cru), 12 (BU Cru), 13 (BT Cru) — plus one suspected BCEP-type (BW Cru) discovered by S.M. Jakate in 1977. (“Individual Stars”; 14 (BS Cru) (“A Search For Beta Cephei Stars II: NGC 4755.”; Astron.J., 83, 10, p.1179 (1978)) In 2002 this has increased to eight, with only one remaining suspect — BW Cru. All the positions of these stars appear in the attached Figure above, and also the individual stars table presented in the text below. Remaining notable variables are especially CV Cru (NGC 4755 417) and CT Cru (NGC 4755 418), are 9.86 and 9.83 magnitude, may also be Beta Cepheids, with B−V’s of +0.12 and +0.16, respectively. However, the all data on these stars is presently still scant, but some extensive studies have been made in recent years.

Among the eclipsing binaries, are the E-II eclipsing binary BU Cru (“Individual Stars”; 4 (BU Cru)), and the eclipsing binary CN Cru (“Individual Stars”; 17), followed by the elliptical binary variable CC Cru, which is Star 6 and is seen opposite the “crossbar”. Since then the numbers have increased. (2002)

As of 2003, recent additions to the family of true Jewel Box variables are;
CU Cru, CV Cru, CW Cru, CX Cru, CY Cru, CZ Cru, DS Cru and DU Cru

In 2013, newrecent additions to the catalogued Jewel Box variables are;
EE Cru, EF Cru, EG Cru, EH Cru, EI Cru and ET Cru

Among the other variables is the E-II eclipsing binary of BU Cru. This is listed as “Star #4” (See“Individual Stars”). Another eclipsing binary is CN Cru which is listed as “Star #17” See “Individual Stars”). The last elliptical binary variable is CC Cru.

In the GVSC4 Notes (2001) it is suggested that the variable V496 might also be a “Possible member of the open cluster” NGC 4755. V496 Cen; B-V=+1.02, U-B=+1.33.

TABLE ACCEPTED VARIABLE STARS in the JEWEL BOX

COMMENTS

Star 03: JDE 2443228.61
Star 18: HIP 62732
Star 19: HIP 62918 (Brightest Red Star "No.5")

NOTES

Minimum Magnitudes marked asn.nn or n.nn B are estimated from the information available from other sources.

Data adapted from the Electronic form of the “General Variable Star Catalogue” (GVSC) 4.0 (Vol. I-III) by N.N. Samus (Moscow Inst. Astron.), O.V. Durlevich (Sternberg Astron. Inst., Moscow) Kholopov (1988) 10 Dec 2001 [See http://lnfm1.sai.msu.ru/GCVS/gcvs/]

Includes the update page “Namelists of Variable Stars” Nos.67-76 (Kholopov+, 1985-2001) 10 Dec 2001

Last Update : 9th March 2013

Southern Astronomical Delights © (2012)

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