December 2016 - Double Star of the Month

pi Arietis (02 49 17.55 +17 27 51.5), also known as STF 311, is a very unequal triple and as such probably needs 10-cm to see all three well. The stars are magnitudes 5.3, 8.0 and 10.7, and the separations of AB and AC are 3".2 and 24".1.

AB first came to the attention of the elder Herschel in 178x and is listed as H I 64. Sissy Haas notes that this pair was not seen in 12.5-cm, but Admiral Smyth is enthusiastic about this superb trio. Pale yellow, flushed and dusky is his conclusion on the star hues.

The proximity of pi Ari to the ecliptic has resulted in lunar occultations occurring and it was during one of these events that a close companion to A was found. A also appears to be an SB1 but it seems unlikely that this is the occultation pair so A would appear to be a group of 4 stars.

Looking much further out David Arnold finds a mag 10.5 star at 220" whilst Tofol Tobal has imaged stars of mags 14 and 15, at respectively 14".2 and 17".5 from A.

DUN 16 = f Eri (03 48 35.88 -37 37 12.6) is a member of the Tucana/Horologium + Columba moving group - a cadre of bright stars with similar space motions which are between 37 and 65 light years away from the Sun.

The pair consists of a magnitude 4.7 B8 dwarf paired with a 5.3 magnitude A1 dwarf. The separation has increased from 7" in 1826 to 8".4 in 2009 whilst the position angle has changed from 202° to 216° in the same interval.

The significant proper motion of star A would have moved it further away from B had B been a field star, rather than a binary companion, as seems to be the case.

One star is possibly a beta Lyrae-type eclipsing binary. The revised Hipparcos parallax gives a distance of 50.8 parcsecs. Ernst Hartung says that This beautiful pale yellow pair dominates a field of scattered stars and is a fine sight with 7.5 cm. Sissy Haas calls it a Showcase pair.

Bob Argyle - Double Star Section Director