SELECTED SOUTHERN DOUBLES and VARIABLES
R.A. 18 Hours

Δ221 (18243-4407) is a very wide double that lies in the small Milky Way constellation of Corona Australis, and is located near the southern border of the constellation - being some 2.4°SW of 4.6 magnitude Theta CrA. θ CrA is easily identified as it is the last bright star of the arc that makes the principal stars of Corona Australis and follows about 3° SE of the 5.4 and 5.6 magnitude optical duo of η¹ and η² CrA. These latter stars are separated by 16′ along PA 27°, making this a fine optical duo for the naked-eye.

The field itself contains many faint stars but does contain one faint yellow 8.4 magnitude star HIP 90323 / SAO 229009 / PPM 324076 (18258-4407) some 16′ due east of the pair. Δ221 itself lies 3.0°E (PA 99°) from the globular star cluster, NGC 6541 - the only recognised globular within Corona Australis.

Dunlop lists the star as ρ Telescopii, whose Bayer designation for some reason has not been recognised today, also stating this is one of the original Lacaillé doubles. Magnitudes are given as 5.2v and 10.0v (5.03V & 10.13V) but Dunlop oddly lists these magnitudes as “6,11” - one magnitude different than what we observe today.

Dunlop lists Δ221 as being separated by 40 arcsec - but does not give any corresponding position angle. For some reason the current Washington Double Star Catalogue (WDS04) wrongly gives the impression the PA of 180° was created by Dunlop. This is really an improper assumption because he never says this. The duo was measured ten years later by John Herschel was found the as 75.3 arcsec along PA 164° which was agreed to be the same by Innes measures in 1914. This pair remained unmeasured until 1962 (IDS 1963) which found the separation as 74.8 arcsec along PA 162°. Such latter measures implies some change and this is confirmed after the WDS01, which gives the Hipparcos positions of 74.3 arcsec along PA 162°.

Primary Δ225 A is listed as HIP 90200 / SAO 228982 / PPM 24039 whose parallax of 6.22±0.85 mas. This makes the distance 161±22pc. or 524±73ly. As for the companion Δ225 B, we cannot directly compare it with the primary as the secondary only appears in the Tycho catalogue as T 7913:735:1 / PPM 763523 - with the i naccurate parallax of 21.90±28.40mas. If we also use the distance of only the primary, then the real separation of the two stars would be more than 12 000 AU or 1.8×10¹² km. This is a little too far for true double star.

However, the inaccurate proper motions of ‘B’ is telling - whose common proper motions are travelling in the directly opposite direction. I.e. Aiming direct towards each other. We can likely conclude that Δ221 is an optical pair that is slowly decreasing in separation by around 0.5 arcsec per century while the PA is fixed at about 162°.

Although the proper motion of the secondary is too inaccurate - clearly explaining the apparent motions seen in the measures made to date. (It also discounts Dunlop’s presumed 180° PA.)

The primary is a suspected variable that is listed as NSV 10724 / BV 481. Little is known about the variability or the type of variable, except for the 5.05B magnitude. Spectral class for the main star is given as B2.5Vn.

There is little doubt this is an optical pair.

HJ 5014 (18068-4326) is a bright double in the constellation of Corona Australis with in the surrounding fields of Δ221. Discovered in 1836 by John Herschel, this well-known binary star, this visual pair lies some 22′ NW (PA 322°) from the globular star NGC 6541 (18080-4342) so it appears in same field of a low magnification eyepiece. This 4.9v and 5.0v (4.92V and 5.00V) magnitude duo is a magnificent spectacle in aperture above 10.5cm and presently visible in 7.5cm was care and reasonable seeing. At present the stars are separated by 1.7 arcsec, with the WDS04 the latest observed position being 8° and 1.7 arcsec (2002).

HJ 5014 is visible to the naked-eye as a 4.8 magnitude star and is catalogued as HIP 88726 / SAO 228708 / PPM 323736 / HD 165189, having a B-V of +0.255. This equally white coloured pair, or as some have stated, yellowish-white. These star colours match fair well with the given spectral classes of A5V and A5V.

Based on the Hipparcos parallax of 22.79±1.12, HJ 5014 lies 43.88±2.16pc. or 143.1±7.1 ly. (Error of 4.9%) from the Sun. It measured the mean separation as 1.674±0.002 and the PA of 12° (1991), obtaining the system’s individual proper motions of pmRA; +9.46±1.31 and pmDec; -104.72±0.66 mas.yr^-1. Common proper motion (cpm) is 105.15 mas.yr^-1 or 105 arcsec per century. Calculations show that HJ 5014’s skyward motion will be towards the southeastern quadrant (Quadrant 2) along PA 161°, which is towards the south-western edge of the globular NGC 6541. During the year 3 200 A.D. the binary will pass within 7.4′ of NGC 6541’s centre and placed with the outliers of cluster. Future observers no doubt will find this wonderful and thrilling celestial match-up!

Stellar absolute magnitude of the stars are +1.7 and +1.8, respectively, whose masses from the available data are equally about 2.1M⊙ (ΣM⊙ of 4.2). Surface temperatures are 8 170K with both having the luminosity (L∗/L⊙) eighteen times that of the Sun and diameters around three million kilometers. A similar bright star would be 1st magnitude Altair in the constellation of Aquila. Adopting the mean separation (semi-major axis ‘a’) of 1.86 arcsec the period of the two stars in about 414 years, whose true separation becomes 90 AU or 1.3×10¹⁰ km. This period happens to match quite well with recent upgraded period of 450 years by Argyle and Alzner in 2001. (Arv2001a)

I 250 (18412-4210) in CrA is 18.1′E of Dunlop’s sky position for Δ223. This is not an easy pair and is a challenge for the moderate to larger apertures. I once glimpsed the stars in 20cm, but the stars appeared elongated and subject to the seeing on occasions. Listed as 7.1v and 8.3v magnitude, this fixed pair was discovered by Innes in 1897. Hipparcos has measured the latest position as 1.116±0.006 arcsec PA 114° (1991). There has been twenty-two measures so far recorded to date.

It is clear that this is very likely a binary star whose orbital is face-on than severely inclined to our line-of-sight. Based on the change in PA by 1.32° per decade, the period can be interpolated to be 2,700 years. To confirm this, we can use the combined parallax of 5.86±1.46 mas, finding the distance is 171pc. or 557 ly. If these two magnitudes are 7.1 and 8.3, then the absolute magnitudes are approximately +0.9 are +2.1. Using the calculator, the observed true separation is 204 AU or 3×10¹⁰ km., estimating the minimum period of 1 390 years and agreeing well with the earlier estimate. This pair will be interesting to watch in the ensuing centuries to see if the position angle continues to reduce at the current rate. A combined spectral class of A2IV appears in the WDS04.