When last we left Delphinus hanging silently in a dark sky, we were exploring an area around Epsilon (ε) Delphini, located near the base of the Dolphin’s tail (here’s a link to that excursion if you missed it).
We’ll use Epsilon (ε) as a starting point once again and work our way a short distance to the east in search of three more double or multiple stars, two of which have some faint and shadowy company.
If ENG 75 rings a familiar bell, it’s probably because we looked at its numerical sibling, ENG 74, on our last tour. That faint triple star was a real chore to find, but sixth magnitude ENG 75 is impossible to miss. From 4.04 magnitude Epsilon (ε ) Delphini, a short jaunt of one and a half degrees due east will get you there. 5.43 magnitude Iota (ι) Delphini, located one degree east of Epsilon, is perfectly situated for use as a reference point.
ENG 75 HIP: 101966 SAO: 106360
RA: 20h 39.9m Dec: +11° 15’
|CVN 17 Aa, Ab:||5.50, 9.10||0.70″||66°||2006|
|ENG 75 AB:||6.45, 10.60||182.20″||6°||2010|
|Distance: 108 Light Years|
|Spectral Classification: “A” is F8, “B” is MI|
|Note: Simbad shows Aa, Ab with a visual magnitude of 6.385|
Even though the data above shows ENG 75 to be a triple star, we’ll have to settle for a telescopic double star view since the sub-arc second secondary of the Aa, Ab pair is buried securely in the primary’s glare.
With “B” being just over three arc minutes away from the primary, it would seem unlikely there would be any kind of gravitational interaction between the two stars, which is what I found when I looked up the proper motion numbers (Simbad link):
The Aa, Ab secondary, on the other hand, definitely appears to be gravitationally related to the primary. G. Chauvin (which is where the CVN comes from for the WDS designation) has led a group (see the next two links) who have determined the secondary of the Aa, Ab pair is 20 to 23 astronomical units (AU) from the primary. To put that into perspective, 20 AU’s is the average distance of Uranus from the sun.
I was able to find two images of the pair, the one on the left taken from this source (photo on fourth page) and the one on the right from this source (photo on second page). The HD 196885 designation shown on the photos is one of the numerous identifications assigned to ENG 75 A (scroll down to the middle of this page to see the entire list of thirty-four).
And you may have noticed there’s also a reference to a planet on the Simbad proper motion chart above, which is mentioned by Chauvin in both of the papers in the two previous links. The data on that planet, known as HD 196885 A b (not to be confused with the binary Aa, Ab pair) can be found here and here.
This was one of the first discoveries of an exoplanet in a close binary pair, which until that point had been considered highly unlikely because of the gravitational influences of the two closely orbiting stars. Keep in mind, the secondary referred to in this case is about as far away from the primary as Uranus is from the sun – and yet there’s a planet with three times the mass of Jupiter orbiting the primary at 2.6 AU’s from the primary (Mars is at about 1.7 AU’s when at its farthest point from the sun). Amazing, to say the least.
Preceding ENG 75 in the western shadows is J 1242, with magnitudes of 9.6 and 11.0, a separation of 5.1”, and a position angle of 204° (WDS 2011). And to the south of ENG 75 at a distance of 477 light years is SCJ 27 with magnitudes of 8.67 and 10.03, separated by 6.00” at a position angle of 264° (WDS 2011). SCJ 27 also has a 13.90 magnitude third component at a separation of 14.20” and a PA of 79° (WDS 2006).
Although the secondaries were faint and very close to the primaries, I was able to pry both of those pairs apart with my six inch refractor at 89x. On the other hand, the 13.90 magnitude “C” component of SCJ 27 eluded me, probably because it was too faint for the six inch lens.
On to Kappa Delphini now, also known as OΣ 533 (or STT 533). For navigational purposes, it lies a bit more than a degree (1° 11’) south and very slightly west (188°) of ENG 75. (Here’s our second chart once again).
OΣ 533 (Kappa Delphini) HIP: 101916 SAO: 126059
RA: 20h 39.1m Dec: +10° 05’
Magnitudes AB: 5.15, 11.63 AC: 5.15, 8.62
Separations AB: 44.80” AC: 212.30”
Position Angles AB: 278° (WDS 2001) AC: 100° (WDS 2011)
Distances: “A” is 98 Light Years (WDS); “C” is 9.34 Light Years (Simbad)
Spectral Classifications: “A” is G1+K2, “C” is K2
Shown to the southwest of the primary (left in the sketch) about halfway to the edge of the field is a dim and difficult pair, BRT 2187 (11.1, 11.7, 5.1”, 161°), which is also shown in the inset at the left at a higher magnification. And just beyond the field of view in the south corner is another pair, OL 218 (9.5, 10.0, 5.5”, 274°), which was slightly easier to separate thanks to it being over a magnitude brighter.
You’ll notice several stars clustered around the primary which haven’t been designated as components of OΣ 533. The one between the primary and “C” is shown at a magnitude of 12.46 in Sky Tools 3, the one southeast of the primary (below it in the sketch) at 11.92, and the brighter of the pair southwest of the primary (below and to the left in the sketch) at 11.56. Probably the main reason the more distant “C” component has been included in the system, as opposed to those fainter but closer stars, is because “C” is moving in tandem with the primary, which S.W. Burnham took note of over a hundred years ago (source):
When the proper motion for “A” and “C” is plotted, this is what results (Simbad link):
From that plot it’s clear there’s a physical link between “A” and “C”, while “B” appears to have no physical connection to either of the two stars.
Which takes us to the final star of this tour, Σ 2713 – and it’s not an easy one to track down. The problem is Σ 2713 is dim, coming in at just below tenth magnitude, which makes it rather difficult to see in an 8×50 finder.
The short version instructions are: 43” northeast of OΣ 533 at a PA of 159°, or 39” southeast of ENG 75 at a PA of 42°. The longer version instructions require looking at the chart at the right, which shows Σ 2713 forming a triangle with ENG 75 and OΣ 533, with Σ 2713 located about halfway between and slightly east of those stars. Fortunately Σ 2713 also forms a smaller triangle with 8.64 magnitude SAO 106371 and 7.94 magnitude HIP 101993, which provides another way to pin down its location. In fact, if you center SAO 106371 in your finder, you’ll also snag Σ 2713 in the same field.
Σ 2713 No HIP SAO: 160375
RA: 20h 40.9m Dec: +10° 35’
Magnitudes: 9.8, 9.8
Position Angle: 63° (WDS 2008)
Distance: 1019 Light Years for “A”; 140.5 LY for “B” (Tycho)
Spectral Classification: B9
You’ll notice two widely differing distances in the data above for “A” and “B”. That’s because the Tycho catalog shows two different parallaxes for the two stars, 3.20 milli-arc seconds (mas) for “A” and 23.20 mas for “B”. I suspect there’s an error in one of those numbers, either a dropped initial digit or one that shouldn’t be there. Unfortunately, Hipparcos doesn’t have a distance for Σ 2713, so the Tycho data is all there is until the Gaia results begin to come in.
As to whether the two stars are gravitationally linked in some way, the data there is also conflicting. Burnham described the Σ 2713 pair as fixed in his 1906 catalog entry, but the history of measures between 1830 and 1905 are a bit inconclusive (sources for info below: Lewis on left, Burnham on right):
The WDS shows a proper motion of +006 -002 (.006”/year east, .002”/year south) for the primary and +003 +000) for the secondary, which would indicate a very minimal change in relative positions of the two stars. However Simbad shows a significantly higher rate of proper motion in the opposite direction for the primary -040 -005 (.040”/year west, .005”/year south). Its numbers for the secondary (+0001.5 -000.6) are just slightly different from the WDS. When the Simbad data is superimposed on an Aladin image of Σ 2713, this is what results:
Based on Simbad’s data (in that link, HD 197078 is “A” and BD+10 4357B is “B”), it would appear the two stars are unrelated. On the other hand, in ten or twenty or thirty years, it may turn out the WDS data is more accurate — you just never know in some of these cases.
Our next trip will take us to southeastern Delphinus for a look at three more enigmatic and mysterious pairs. Until then, hope your skies are more cooperative than mine are lately! 😎