DISTANCE of NGC 4755
The first realistic distance of 1.09kpc was found by Trumpler in 1930, during his extensive cluster surveys in both hemispheres (Lick Obs.Bull., 14, 173). Yet, it was only when good photometric observations were obtained between about 1959 and 1965, that the estimates tended to ‘settle’ around 2.3 to 2.4kpc, though Sanners et al. (2010) latest value of c.3.4kpc. seems an odd aberration. Important examples of estimations at this time and of the modern determinations includes:
AOST2 says the distance is 2.4kpc, while Sky Catalogue 2000.0 Vol.2 says 2.34kpc. Both of these values are the rounded results of measures made in the years 1966 and 1967, and these value has remained fairly constant between 1965 until the beginning of the 1990s.
One of the best distance estimates, with the lowest error, is by Sagar. R. & Cannon, R.D. (1995) of 2.1±0.2kpc (6,850±650ly.) and Sanner et al. (2001) of 1.9±0.3kpc. Another of the latest distance determinations was found using the brighter component stars by Koenig, et. al. (AGM), 14, 35 (1998) in distance modulus as V–MV of 11.85±0.30 magnitudes, and this was also confirmed by the red M2Iab supergiant as 11.84±0.50. This translates as 2.34±0.35kpc. (7,630±114ly) and 2.36±0.57kpc. (8,000±186ly), respectively. and such a result suggests to reconfirms the quoted distances of the 1960s.
There may be some significance to these results, as the brighter stars are giving a slightly different distance of 0.2 to 0.3pc. to those of the much deeper magnitude surveys. When combined with the ‘double’ main sequences suggest that the Jewel Box might be superimposed cluster — as hinted at in Sagar and Cannon’s paper (1995). The only other likelihood is that observations between the bright and fainter stars are somehow flawed — either by the measurements or in the stellar evolution theory. Problems may also still exist in the determination of precise distance, mainly because of the uncertainty in the quantities of interstellar gas obscuring in this part of the Milky Way. Either way, cluster observations of NGC 4755 will still be important for many decades that are to come.
Last Update : 24th April 2016
Southern Astronomical Delights © (2016)
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