April 2019 - Double Star of the Month

This month's pairs consist of a very easy pair in the southern hemisphere, and a much more difficult one in the north.

When I used the 28-inch refractor at Herstmonceux in 1970, I observed STF 1606, an orbital pair in Canes Venatici (12 10 47.34 +39 53 29.5), which was found at a separation of 0".41. The pair then closed to 0".29 in 1990 and is now opening.

For 2019.0 the position angle and separation will be 141 degrees and 0".6, so it should be just resolvable in 20-cm and I will look forward to seeing this pair as double for the first time in 49 years.

It sits in a little group of three Struve pairs which also includes STF 1622 (see the column for April 2012) and STF 1624. The group is four degrees preceding and slightly south of beta CVn. STF 1606 is also practically coincident with NGC 4145, abarred spiral galaxy of V=11.3.

In 2011, Shaya and Olling published a paper in Astronomical Journal in which they identified over 800 very wide pairs which they concluded were physically connected. From that list, number 588 (SHY 588) (12 02 39.44 -10 42 48.9) is a pair of stars with V magnitudes of 7.5 and 8.6. The current separation is 331" at PA 115 degrees.

Gaia DR2 indicates that the brighter star has a distance of 177.6 light-years whilst the fainter is 186.8 light-years distant. The proper motions are similar but the difference in distance is supiciously large for them to constitute a binary, although Shaya and Olling used the Hipparcos data which actually suggests that the two stars are further apart in distance than does DR2. The telling factor may be the quoted error on the parallax of A which is seven times that on the parallax of B and suggests higher multiplicity.

For the binocular user this is an easy pair and the field is enlivened by a V=8.5 star some 19 degrees and 262" distant from A. DR2 indicates that this is more than three times more distant than the SHY pair and therefore unrelated.

Bob Argyle - Double Star Section Director