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Thread: Object of the Week, February 5 2017 Sanduleak 2-21, a surprising 1975 discovery

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    Member Steve Gottlieb's Avatar
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    Object of the Week, February 5 2017 Sanduleak 2-21, a surprising 1975 discovery

    Sa 2-21 = ESO 561-016 = PK 238+7.2 = PN G238.9+07.3
    RA: 08h 08m 44.2s
    DEC: -19 14' 02"
    Type: Planetary Nebula (elliptical)
    Size: 42"x36"
    P.A.: 138
    Mag: V ≈ 13.7

    Sanduleak 2-21 (Sa 2-21) is a moderately bright planetary that escaped detection (as a planetary) until 1975. It was discovered by Warner & Swasey Observatory astronomer Nicholas Sanduleak on an objective-prism plate taken with the Curtis Schmidt telescope at Cerro Tololo in Chile. It is clearly visible on the POSS, but was overlooked by the time George Abell compiled his famous list of 86 Abell planetaries in 1966 based on the POSS (many were actually found in the 1950's). It was picked up, though, by Russian astronomer Boris Vorontsov-Velyaminov in the 1960's when he compiled the Morphological Catalog of Galaxies (MCG -03-21-004) and as a result also carries the galaxy designation PGC 22854. Even in 1998, the planetary was included in the paper "Accurate positions for MCG galaxies" (1998PASP..110..779C), so its not surprising that some amateur software may plot it as both a galaxy and a planetary.


    How could this planetary have been missed until 1975? Sanduleak stated "It is easy to see how this otherwise conspicuous object might be overlooked on the blue-sensitive print since it lies only about 4 arc-minutes from the 4th magnitude star 16 Puppis and falls within the halation ring surrounding that bright star." Still, Abell noticed Abell 12, which lies only 1.2 arc-minutes from the 4th magnitude star Mu Orionis, so Sa 2-21 was somewhat of an oversight by Abell and others. Here's an image from ESO's La Silla Observatory in Chile taken in 1984.

    Sa 2-21.jpg

    Sa 2-21 is a snap to locate, as it's just 4' west of naked-eye 16 Puppis (V = 4.4). I'd suggest using an OIII or narrowband filter as it will dim the bright star and increase contrast with the planetary. In an 8- or 10-inch scope, you'll probably need a filter to observe it, but in larger scopes its faintly visible unfiltered. My first observation was using a 13-inch f/4.5 in February 1985 using a UHC filter. My notes read "fairly faint, moderately large, slightly elongated ~E-W."

    With my 18" it's faint but easily visible unfiltered at 150x and quite striking when I add an OIII or NPB filter. I prefer the narrowband NPB filter as it gives an aesthetically more pleasing view with the stars less suppressed. Sa 2-21 appears as a crisply defined, moderately bright oval, 4:3 NW-SE, with a size of ~45"x35". It displays a weak annular appearance with a slightly brighter rim. The planetary forms the northwest vertex of an equilateral triangle with a mag 11 star 1.5' S and a mag 12/12.8 double star at 5" (BRT 1449) 1.5' SE.

    Images reveal a barrel-shaped planetary with slightly brighter knots at the west and east ends and [N II] images add ring-like features. The 2003 paper by Harman et al: "Morphology, kinematics and modelling of the elliptical planetary nebula Sa 2-21", includes images and [O III] and [N II] models for its morphology. Here's a Pan-STARRS image from Haleakala (Maui).

    Sa 2-21.jpg

    I usually take a look at Sa 2-21 every couple of years as I enjoy this "late" discovery. If you've never seen it,

    GIVE IT A GO AND LET US KNOW!
    Last edited by Steve Gottlieb; February 6th, 2017 at 06:50 PM.
    Steve
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    Indeed surprisingly late discovery. I wasn't aware about this fact. But of course you are right, other Sanduleak PN are much smaller or fainter.

    I observed the PN from the dessert of Southern Morocco. One of the few observation through an 16" SCT full of sand. But the PN was really visible, I promise
    16", 406x, NELM 7m0+, Seeing III
    Sa2-21.jpg
    Clear Skies, uwe
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    Member Howard B's Avatar
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    "an 16" SCT full of sand" - that's funny!

    I haven't seen this PN yet but it's on my list now, thanks Steve.
    Howard
    28 inch f/4 alt-az Newtonian

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    Hi Steve & All,

    My only recorded observations of this PNe are 20 years ago with my 25cm GEM Newtonian. The first was from outer suburban Engadine and a week later at our society's "nearly dark" site near Bargo NSW. On the latter occasion was at x181 with a Lumicon UHC filer:

    "This PNe is much easier in a DK sky. Can almost hold with D.V. Makes a RA Tri with 2 mag 12 *s. About 6' W of mag 4 16 Puppis. Maybe annular, centre seems slightly darker than the rim.

    Best,

    L.

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    Hi Steve & All,

    After discovering that my last observation of this neat PNe was two decades ago, I thought it deserved a re-look with larger aperture and vastly better conditions.

    Tonight, I viewed it at 9.40pm under a sky with an SQM-L reading of 22.08 at zenith, the seeing was 7/10. 9mm T1 Nagler, x247 with a Lumicon UHC filter. FWIW, I agree with Steve's comments above regarding the choice of filter -- and prefer a NPB to a line filter for the exact same reason.

    x247 20' TF

    Sanduleak 2-21 PK 238+7.2 PNG 238.9+7.3 Planetary Nebula *
    RA: 08h 08m 44.1s Dec: -19 14' 01"
    Mag: 13.6 Size: 40.0" Class: Mag C. Star: -

    This is a moderately faint PNe that is easy to find as it is about 4' WNW of mag 4.4 16 Puppis -- this star makes the observation more difficult than it would otherwise be. Visible without the filter with D.V , it forms an Eq Tri with two stars each about 1.5' to its ESE and S. The former is a close pair of mag 12.5/ 13.0 and the later about mag 11.5.

    A slightly diffuse edged disc approx 30" diameter with no visible central *. Lowish SB disc form with overall, a weak indefinite dimming to centre indicating it may be annular. With concentration, it seems weakly elong in PA 135.

    Best,

    L.

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    My observations of Saturday night (25 Feb) from SQM 21.4 skies with excellent seeing:

    At 256x, 320x and 427x an oval patch of light inside a triangle of stars of which two legs are very long and one tip at the short side a double star. UHC and OIII don't help much though UHC gives the best view. Lots of humidity in the air which may have affected the observation.


    Clear skies, Wouter

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    Steve,

    I observed this last weekend after reading your OOTW. This was with a 16" under Bortle 3 skies with moderate-good seeing, at 260x. My log reads "Smallish, almost round object. Relatively faint, but visible well with AV. The outer edge is brighter and crisp."

    Regards,
    Adarsha

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