Variables Stars in NGC 4755

   

NGC 4755 CATALOGUED VARIABLES

| BS Cru | BT Cru | BU Cru | BV Cru | BW Cru | CC Cru | CN Cru |
| CQ Cru | CR Cru | CS Cru | CT Cru | CU Cru | CV Cru | CW Cru |
| CX Cru | CY Cru | CZ Cru | DS Cru | DU Cru |

All variable stars within any star cluster are very important, as they can give us some indiction of distance and the cluster's evolution. For most of the young clusters, many of the brighter stars that are variables, and more often than not, hot blue stars. These stars have started to evolve away from the main sequence, and have entered one of the so-called instability strips that frequent several places across all C-M diagrams. For the lower mass stars, none are usually found to be variable, as they remain in a long peaceful phase of hydrogen burning.

For any of the young clusters, one of the principle variable types are the Beta Cepheid stars, often abbreviated as the BCEP or β Cep type. In 1979, some twenty-four Beta Cepheid variables were suspected in all open clusters, but the number of known examples has now grown to many dozens. Observation of Beta Cepheid’s are often found to vary in sinusoidal periods over 0.1 to 0.7 days, displaying small changes in luminosity by between 0.1 and 0.3 magnitudes. Spectral classes are often found between B0 to B3, and this applies across all the seven main categories of luminosity classes. Common bright examples of field Beta Cepheids included nearby to the Jewel Box - β Crucis (Mimosa), δ Cruciss λ Crucis and β Centauri (See NSP 16 at http://www.blackskies.com/nsp16.htm), and β Canis Majoris.

Astrophysicists think that these variations seen in Beta Cepheids are more likely caused by radial pulsation from various instabilities with hydrogen burning in the core. They are general found by finding those fitting within the spectral class range then observing them photometrically over a period of some nights. The number of uncertain Beta Cepheids are large, like in the Jewel Box, mainly because there are a number of variable mimics that may characterise the same variability or periods. Another reason is simply lack of observations. Also few obtained results will suggest there are several different possible lengths for the harmonic periods produced by the frequency-period analysis. Repeating the observations at another time general help refine these periods. Interestingly, further highly-detailed observations have unveil the existance of multiple periods like those seen in the nearby β Crucis. This star shows some seven different harmonic periods.

A sub-type of the BCEP’s are the BCEPS’s, 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 nineteen known variables or suspected variables within “The Jewel Box” (2002). Until about 1990 only three were certain Beta Cepheid varibles (“Individual Stars”; 8 (BV Cru), 12 (BW Cru), 13 (BT Cru)) with 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.1179-1182 (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 xx, and also the individual stars table presented in the text below. The remaining variables. 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 data on these stars is presently remains scant.

Among the eclipsing binaries, is 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)

More 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

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 GVSC 4 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 the JEWEL BOX VARIABLE STARS

COMMENTS : Star 3: JDE 2443228.61 ; Star 19: HIP 62732 ; Star 19: HIP 62918

N O T E S

• Minimum Magnitudes marked as n.nn or n.nn B are estimated from the information available.

• 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 http://lnfm1.sai.msu.ru/GCVS/gcvs/

• Includes the latest update page “NAMELISTS OF VARIABLE STARS” NOS.67-76 (KHOLOPOV+, 1985-2001) 10 Dec 2001

“Southern Astronomical Delights”   © Andrew James (2002)          Sydney, Australia