8 Attachment(s)
Object of the Week October 22nd, 2023 – Pease 1 in M15
K648, Ps 1, PK 065-27.1, PN G 065.0-27.3
Pegasus
Planetary Nebula
RA 21:29:59.4
DEC +12:10:26
Mag +14.7 (central star), +15.1 (ionized nebula)
Size 3”
Dist 33,900 l-y
Part I – Discovery, Confirmation, and Study
In 1921, the then 65-year-old Dr. Karl Friedrich Küstner published his paper titled “Der Kugelförmige Sternhaufen Messier 15” (The Spherical Starcluster Messier 15) in the Veröffentlichungen der Universitäts-Sternwarte zu Bonn (Publications of the University Observatory in Bonn) [1]. He had taken several photographic plate images of the globular cluster M15 and used them to catalog over 1,000 stars in an area just 20’ x 20’. One thing he included for each numbered star was a photographic magnitude he had independently deduced.
Later in the same decade, the American astronomer Francis G. Pease (age 47) studied plates taken of M15 using the 100-inch Hooker reflector atop Mount Wilson [2]. He was a staff member there and noticed that one star about 30” from the center “appeared very bright” as compared to all other nearby stars on the plate taken through the “Pulkowa ultraviolet” color filter. It probably took some time, but he was able to find that in Dr. Küstner’s catalog the star carried the number 648 along with a magnitude of +13.78.
Attachment 5261
Pease 1 (ESA/NASA)
To follow up on his odd finding, Pease was able to obtain two spectrograms of Küstner 648 (K648) using, again, the 100-inch. These showed the star to instead be a compact planetary nebula while the ever-helpful Humason was able to use them to measure radial velocity that was similar to the stars in the cluster. However, while the former was concrete, the latter was not, and would need to be re-checked since the idea of such a young object existing in such an old cluster was extraordinary!
It seems Alfred Joy was the first to do around 1948 when he used the 100-inch to obtain spectra of variable stars in several globular clusters [3]. He found that the radial velocity was -122 km/sec, which was very similar to what others were getting for the cluster's stars. He even noted of the nebula saying that “the close agreement of the radial velocity with that of the cluster indicates that the nebula is, without a doubt, a member of the cluster.”
Attachment 5262
Pease 1 (ESA/NASA)
In 1951, while trying to deduce the member stars of the cluster through proper motion, Archibald Brown found that Pease 1 had a photographic and photovisual magnitude of +13.39 and +14.06, respectively [4]. But the first paper to solely study the planetary nebula came in 1964 when O’Dell, Peimbert, and Kinman used the 120-inch reflector at Lick Observatory [5]. The radial velocity they obtained confirmed Joy’s measurements and “thereby strengthening the assumption of a common motion.” They also were able to judge the nebula’s size as 1.1” in diameter.
Attachment 5260
Pease 1 (ESA/NASA)
At the beginning of the 1980s, Aurière and Cordoni (1981) performed a photometric study of M15 and found the planetary to be magnitude +14.64 [6]. At the time, Küstner 648 was an “only child”, but by the end of the decade a second planetary had been discovered in M22 [7]! However, it continued to be of particular interest because not only was it rare halo planetary nebula, but also one in a globular cluster. How did it form? At the present time, the most massive main-sequence stars in M15 have about 80% of the mass of our Sun [8]. And it’s almost impossible for such low-mass stars to ascend to the AGB and eject a PN!
The Hubble Space Telescope obtained its first images of it using the Planetary Camera in May of 1991 [9]. And despite its flawed primary mirror, it was able to resolve not only an inner bipolar structure spanning 2” in roughly a north to south orientation, but also a tenuous, more spherical halo extending out to 5”. Later observations by Alves, Bond, and Livio (2000) using the HST showed that the central star of Pease 1 doesn’t have a companion to have borrowed mass from, so they suggested that two stars might have merged to become the progenitor of Pease 1 [10].
Attachment 5259
(CloudyNights member james7ca)
Attachment 5257
Phil Harrington's Chart
Part II – Observations
Before writing this OotW, I tried to answer three questions I had. The first was how bright the planetary nebula is compared to its central star, the second was what does it take to claim seeing the planetary, and the third was what is the minimum aperture to procure such a sighting?
The answer to the first question is the most important one and, as one might expect, proven to be the hardest. But for all intents and purposes, I believe I can firmly state that the light we see in our telescope is over 50% from the central star. In Alves et al (2000), they found it to be +14.73 after trying to isolate the central star by using a certain filter onboard the HST and then converting that back to a V mag. Brian Skiff has dug out Peter Stetsen’s photometry for the planetary and believes that his magnitude (+14.16) is probably the combined light of nebula and central star. This would mean that the brightest part of the nebula, which is nearly 1” in diameter, is around +15.1.
Attachment 5256
Enhanced Pease 1 (ESA/NASA/Judy Schmidt)
So, to claim seeing the nebula and be 100% in doing so, you need to use an O-III filter. On a recent night with exceptional seeing, I was able to push my 10-inch SCT as high as 820x on M15. I found that at 400x I could still see Pease 1 as it didn’t dim as much as the cluster’s stars did with an O-III filter. From this, I would say that it should be possible with a telescope as small as 8-inches if the location and viewing conditions are exceptional.
Just this month, I’ve taken the time to scrutinize the star-field at 820x in my 10-inch, 1,000x in my 16-inch, and 664x in a 36-inch. And it’s my opinion that seeing the disk of Pease 1 is going to be extremely challenging when you consider that the brighest part is under 1". Also, instead of lying around somewhat separate, it lies at the northern tip of a "shark fin" shaped glow formed by mostly fainter stars. In Alves et al (2000), they write that " K648 shows three main structural features, which are revealed clearly in our new images and can be described approximately as follows : (1) a very bright, hollow, inner elliptical shell, with outer major and minor axes of 0.8" and 0.6"; (2) a larger, somewhat fainter, and fairly sharp-edged elliptical shell of dimensions and (3) a much 3.1" x 2.7"; fainter, diffuse elliptical halo that fades away gradually with radius but can be detected out to dimensions of at least 6.5 x 5.5". The corresponding dimensions in parsecs are inner shell, 0.05 x 0.04 pc ; outer shell, 0.19 x 0.16 pc ; and faint halo, 0.38 x 0.33 pc."
Attachment 5263
Attachment 5258
(Mathieu Senegas/Robert Cazilhac)
So, now it's your turn to “Give it a go and let us know!”
The author would like to express his gratitude to Lowell Observatory's Brian Skiff, who provided him with scans from Küstner's obscure paper and other similar support. He would also like to apologize for posting a day late.
[1] Küstner 1921 (https://ui.adsabs.harvard.edu/abs/19......1K/abstract)
[2] Pease 1928 (https://ui.adsabs.harvard.edu/abs/19....342P/abstract)
[3] Joy 1949 (https://ui.adsabs.harvard.edu/abs/19....105J/abstract)
[4] Brown 1951 (https://ui.adsabs.harvard.edu/abs/19....344B/abstract)
[5] O’Dell et al 1964 (https://ui.adsabs.harvard.edu/abs/19....119O/abstract)
[6] Aurière & Cordoni 1981 (https://ui.adsabs.harvard.edu/abs/19....347A/abstract)
[7] Cohen & Gillett 1989 (https://ui.adsabs.harvard.edu/abs/19....803C/abstract)
[8] Rauch 2002 (https://ui.adsabs.harvard.edu/abs/20...1007R/abstract)
[9] Bianchi et al 1995 (https://ui.adsabs.harvard.edu/abs/19....537B/abstract)
[10] Alves et al 2000 (https://ui.adsabs.harvard.edu/abs/20...2044A/abstract)
