DOUBLES and TRIPLES in the JEWEL BOX
Although pairs are known to exist in open star clusters, more often than not, none are usually listed. This mainly because all the cluster members are already gravitionally bound in the cluster system. It is also thought that many pairs form is clusters, but these are commonly only temporary binaries, which can be torn apart by the internal cluster’ dynamics.
For NGC 4755 only four (4) pairs can be found in the Washington Double Star Catalogue (WDS), and these are B 805, JSP 561, JSP 563 and JSP 562 / HDS 1808.
RECOGNISED WDS PAIRS
B 805 (12540-6020) is listed as 4.3 arcsec separation and
position angle 265°, whose description appears as Star 3
in ‘Some Bright Individual Cluster
Stars’ (See Page014.htm). However, no new measures have
been obtained since W.H. van den Bos discovery 1927. Re-measuring
the pair using the CDS ‘Aladin’ was impossible to do even with significant
image manipulation. I originally suspected that star B805 C
was the component which lies 15.1 arcsec along a similar PA of
274°, but this seems a little too large a displacement in the
years that have passed since 1927. Examining the proper motion of
the other bright Jewel Box stars finds no unusual motions either in
velocity or direction. We can only conclude that unless the
companion star B 805 C has significant motion in the opposing
directing to the primary, this cannot be the star that van den Bos
HDS 1808 (12536-6021) are the stars No.4 and No.8 that appears in the ‘Bar’. It is the Harvard Double Star (HDS) discovered from Cordoba, Argentina in 1892, whose separation was measured as 17.3 arcsec along the position angle of 43°. According to the latest measure in the WDSDec04, the position angle has remained as 43°, and whose movement in minor even when taking precession into account. Separation is 17.1 arcsec (2000), and it is perhaps decreasing, judging by the changed by -0.2 arcsec. The pair is easily and cleanly divided in 5cm — being the brightest of the recognised and unrecognised pairs in the Jewel Box Cluster. A spectral class of B3Ib: is given while the companion is tentatively given as B0. Needless to say both appear either white or bluish-white in the telescope.
JSP 562 (12536-6021) is a faint pair of 10.9v and 11.0v magnitude whose separation in the WDSDec04 is given as 3.3 arcsec along PA 205° that was last made in 1965. From this, the pair may be decreasing both in separation and position angle. Megastar 4.0 and 5.0 place the position of the pair in the wrong place, being the star in the Figure below, immediately eastward of the pair P17. It is in fact 1′ west of the‘Bar’ or 36 arcsec NW of HDS 1808, and this position agrees with that given in the WDS04. Using ALADIN I measured the pair as 5.76 arcsec along PA 338°, suggesting significant change in the last forty years. As first I questioned if I had the correct star but no others match the brightness or placement.
JSP 561 (12536-6025) this 9.9v and 10.9v magnitude pair
lies about 2.8′SE (PA 224°) from Kappa Crucis itself. JSP
561 was discovered by M.K. Jessup in1929 who measured 2.1° with
an easterly position angle of 93°, but unfortunately the latest
version of the WDS04 still gives only this solitary 1929 position. I
remeasured this using ALADIN 2.5v, finding a mean of 1.53 arcsec
along PA 97.0° (n=5) On observation of the pair I found it
could not be resolved cleanly in either 20cm or 30cm, and this
closer measure may explain why. A future observation or two would be
quite useful to confirm the pair is seemingly closing. Spectral
class of the primary is B0.
JSP 563 (12539-6023) is the third recognised system and is
one of the easiest of the pairs to resolve that lies some 41.0
arcsec SE (PA 134°) from the bright star Kappa Crucis.
Figure 9. shows the positions of various doubles and triples that can possible be seen optically within the cluster. I have selected only those with the most interesting or obvious ones that are possibly visible in apertures anywhere between 10cm and 50cm. Eighteen pairs (P) and seven triples (T) are given in the 5′ area of NGC 4755. Most within the selection range between 11.0 and 16.5 magnitude.
UNRECOGNISED PAIRS, TRIPLES and MULTIPLES
Most of the following wide pairs and triples have been measured using ALADIN v2.5. Although they may or may not be physically connected as pairs or doubles they are likely visible in amateur apertures above 20cm or so. Some may provide interesting challenges for larger apertures, even though I have not seen some of the fainter ones from my own lack of aperture.
P4 — Both stars appear yellow.
P9 — The secondary appear also as a double whose separation is 2.4⁑ along PA 316°. The companion is estimated at about 15th magnitude.
P10c (Not plotted) — Another faint pair lies due south by 55 arcsec of 5.7 magnitude Star A / HIP 62894. P10c can be easily spotted a little south of half distance between T5 and P10, and on the Figure lies next to thetelescopic ‘spike’ from the bright Star A. I saw this pair as elongated in 20cm but did not inspect it in 30cm. The occultation bar certainly made the stars more pronounced and an deep-sky observer would be advised to do the same.
P13b — This pair lies between P13 and T7
P17a AB — Is likely a real pair that should appear in the WDS, mainly because of the smallish separation. I have seen this in 30cm on a steady seeing night. (See attached figure for its position west of the ‘Bar’.)
T4 — lies on the edge of the given chart, and although not marked T4 on the Figure, the triple can be found half-way between the labels P9 and P8. I could not see this in either 20cm or 30cm — suspecting the stars might be too faint for the aperture. T4 C seems elongated along the east-west axis. Both seem of similar magnitude with an estimated separation of 1.9 arcsec along position angle 270°
T5 AC — This star was originally seen as elongated. With image manipulation using ALADIN v2.5 the separation of 3.49 arcsec and the PA 100.1 (n=4) was obtained. ‘T5A’ is only 41 arcsec NE (PA 39°) from 5.7 magnitude Star A / HIP 62894. To see the AC pair I used an occultation bar to remove the brightness of Star A in 20cm. I could see all three stars and just resolve T5AC in the 30cm. I’d also recommend viewing this, like T6a below, using as high as power as the seeing conditions allow. Definitely not for the faint hearted!
T6 AC — This star was originally seen as elongated but is clearly double. With image manipulation the separation was obtained with ALADIN v2.5 as 2.83 arcsec and the PA 128.6 (n=3). I considered removing this pair as the A and B components are fairly wide, but this additional possible ‘new ’ pair I decided to add it. ‘T6A’ is placed only 47 arcsec (PA 146°) from 5.7 magnitude Star A / HIP 62894. To see the triple I used an occultation bar to remove the brightness of Star A in 20cm. I could clearly see all three stars but did not resolve T5CD in either 20cm or 30cm.
T7 — This is actually a polygon of stars, only the main pair is likely visible. This is a good test for large to very large amateur apertures. ‘T7 E’ is certainly very challenging. In 20cm I could see only T7 AB, while 30cm revealed AB, AC and elongated AD.
Last Update : 25th April 2016
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