Planetary Nebula Pease 1 in M15 -

A Golden Planetary

by Leos Ondra

Anonymous and singularly placed within a globular cluster, that

small, green circle of a planetary nebula has been rousing my

curiosity since the time I became familiar with the Atlas of the

Heavens. Up to now, I had no opportunity to pick it out from a crowd

of faint cluster stars, but I know it, in spite of this, very well

from a number of papers. The planetary, most often called Pease 1

(Kustner 648 and PK 65-27 1 are the other names) plays a notable role

in our understanding of this sort of celestial object because of its

likely membership of a exclusive club of M 15’s stars. First, we can

determine its distance (and consequently a true linear diameter and

mass) more reliable than for other planetary nebulae. Further,

scrutiny of its spectrum is a very good chance to look into the

chemical composition of stars in a globular cluster. Third, if

observations of the star cluster and theory of stellar evolution are

combined, it is possible to estimate the age and mass of the nebula’s

ancestor. Finally, and perhaps most interesting, detailed radio

mapping of the shape and structure of Pease 1 is expected to suggest

where all interstellar matter produced by cluster giants and

supergiants has gone.

The first written mention of the planetary nebula is in old

photometric study of M 15, published in 1921 by Friedrich Kustner. [1]

In his extensive list, it has been included as a quite ordinary star

#648 of 13.78 photographic magnitude. Only six years later, on August

30, 1927, it put its historical signature on a plate taken by F. G.

Pease with 100-inch reflector at Mount Wilson. `Pulkovo ultra-violet’

filter was used, and Kustner 648 has appeared very bright on the

plate, compared to neighboring stars (otherwise of about equal

magnitude). The cause of this, and the true nature of the object, were

revealed by means of a spectrograph during the following summer.

Continuum belonging to a hot star, lines of hydrogen, and, first of

all, characteristic green lines of oxygen, has placed Pease 1

definitely and definitively among planetary nebulae. [2]

Naturally, F. G. Pease tried to answer a crucial question of

membership of the newly discovered nebula of the cluster. But looking

into our chances, we must admit that even today there are only

indirect, though immensely convincing, arguments against the

possibility of accidental projection onto the cluster. Measurements,

or more aptly estimates, of distances are not very promising.

Astronomers are condemned to study an architecture of the universe

from the single place in it, our Earth or, more recently, the solar

system, and parallaxes can therefore be obtained for objects only a

stone’s throw from the Sun. In the other cases, we have but to use

substitute and often very rough methods. The situation is particularly

unfavourable for planetaries and it is quite impossible to make sure

that uncertainties of Pease 1’s distance from us are smaller than the

cluster diameter.

Even if we were able to manage it by a miracle, there is still

one condition for admitting the nebula into the M 15’s club. Its

velocity, relative to the cluster’s center of gravity, cannot break

the escape limit. [3] Usually, the radial component of velocity (that

parallel with our line of sight) is considered only, because it is

much easier to measure than a proper motion. According to Pease’s

original paper, the planetary nebula approaches the Sun at 156 km/s,

while the cluster at 180 +- 50 kilometers per second. Today’s values

are 128 km/s for the nebula, and 112 km/s for M 15, both in

approaching. [4] The escape velocity from the center of the star

cluster is about 40 kilometers per second. From the place of Pease 1

it is somewhat smaller, but the nebula is nevertheless generally

accepted as a gravitationally bound member of this stellar system.

This well-reasoned assumption is, as mentioned above, ver

fruitful. Let’s begin with chemical composition. Our Sun with all its

planets, comets and the other sweepings, as well as all naked-eye

stars in the sky, have been made from a matter containing nearly all

stable elements, including carbon, so essential for the origin of

life, and iron, important in the ascent of our civilization. All

elements more complex than lithium have originated in the interior of

former stars or at fireworks of supernovae. Neglecting collisions of

cosmic rays and atoms in an interstellar medium, the only other

recognized way of producing nuclei of new elements is a evolution of a

hot universe within a few minutes after the Big Bang. Nearly all the

amount of helium, now about one quarter of the mass of the visible

universe, was syntesized then. Nonexistence of any stable nuclei with

atomic weight 5 to 8 did not permit, however, a production of carbon,

or still heavier elements, metals, in astrophysical slang.

Very old stars of first generations, with very few forerunners,

therefore contain metals in negligible amount. Such stars can be found

in SOME globular clusters, living fossils up to 16 or even 17 billion

years old. Messier 15 is such a metal-poor cluster, and Pease 1 should

share the same composition. Spectral analysis of the planetary nebula

really shows such a picture, and moreover allows us to determine

abundances also for elements which did not left any measurable traces

in spectra of cluster stars. [5] While part of helium in gas is

roughly the same as throughout the universe, there is very little

oxygen here (about forty times less than in the Sun), as well as

nitrogen (thirty times scarcer), and argon, for instance, is nearly

lacking in Pease 1 (abundances about 250 times smaller than in the

Sun). [6]

However anomalous, compared to our star, the chemical composition

is, Pease 1 seems to be otherwise a typical planetary nebula taken

from our godforsaken nook of the Galaxy. Downright run-of-the-mill

values of its parameters have been derived by R. Gathier and his

colleagues [7] nine years ago. These astronomers observed Pease 1 with

the Very Large Array (VLA) facility, at 6 cm wavelength. They were

able to draw the most detailed radio map ever prepared, showing that a

main body of Pease 1 is 1.0 +- 0.3” in diameter. At a distance of M

15, some 10 kiloparsecs (32.6 thousand light years), this corresponds

to a linear diameter of about one light month. Supposing further that

the planetary is transparent to its own radio waves at the frequency

detected, the authors estimated total mass of ionized gas in Pease 1

to be about 0.14 solar masses, with an error some 40 percent of this

value. Older, nearly classic work of C. R. O’Dell’s group [8], based

on visible light analysis, has yielded 0.21 solar masses. Theory of

stellar evolution claims, at the same time, that mass of the nebula’s

ancestor was 1.1 times that of the Sun. The rest of the matter of the

progenitor is stored chiefly in a nucleus of the planetary, destined

to become a white dwarf, and perhaps in an old and invisible stellar

wind around the nebula.

For a long time, Pease 1 was the only planetary nebula in a

globular cluster. But today His Majesty is already dethroned. A

second, equally privileged, nebula has been originally detected as a

point infrared source IRAS 18333-2357 about 1′ south of the core of

globular cluster M 22. Somewhat later F. C. Gillett [9] (of course,

together with other astronomers) identified this source as a very

peculiar planetary nebula. Lines of hydrogen and helium, two elements

widespread all over the universe, are missing in its spectrum, while

green lines of oxygen along with forbidden lines of neon are

prominent. Radial velocity and proper motion study [10] confirmed the

nebula’s membership of M 22.

Both of these planetary nebulae are unique also for a much more

fundamental reason. Until quite recently, they represented all

interstellar matter found in globular clusters. These stellar systems

are typically very rich in giants and supergiants, which lose part of

their matter by stellar wind. Each such star should contribute about

0.3 solar masses of gas to a common store of interstellar matter in a

cluster. Altogether, some thousand solar masses of gas in one bilion

years should acumulate here.

But all searches for any form of this matter, molecular, neutral

or ionized hydrogen, or dust looked for by IRAS satellite, have

failed. Over 30 globular clusters probed are utterly deserted. Only

last year, the first detection of about 200 solar masses of neutral

hydrogen in southern cluster NGC 2808 has been announced by D. J.

Faulkner et al. [11]

What cleans up globular clusters is a bit of mystery. It is true

that all gas is swept away at each passage of a cluster through a gas

disk of the Galaxy, but this happens at most twice an orbit around the

galactic center, in intervals of about ten millions years. But even in

this relatively short period, much more gas than observed (excluding

NGC 2808) should manage to build up. Possible explanation came when a

team of R. N. Manchester [12] showed that millisecond pulsars with

their powerful relativistic wind occur in globular clusters in much

larger quantity than ever expected. Before their work, astronomers

knew of 13 pulsars of this category, scattered in 12 globular

clusters. Observations of splendid 47 Tucanae with the Parkes

radiotelescope in Australia has revealed TEN new millisecond pulsars

in this cluster alone.

If other globular star clusters are equally presented with such

pulsars, and if only a small fragment of pulsars’ emitted power

interacts with cluster gas [13], the mystery of the sweeper should be

solved. [14] Scrutiny of Pease 1’s morphology could play key role in

verification of this mechanism. Already on Gathier’s radio map the

planetary nebula is elongated away from pulsars in the core of M 15 ..

Something is otherwise

Having just received a small package from the Observatoire du

Pic-du-Midi, I have to correct the article A Golden Planetary. Michel

Auriere has enclosed also interesting paper that I overlooked

previously, “K 648, the planetary nebula in the globular cluster M 15”

(Mon. Not. R. astr. Soc. 207, 471, 1984). In it, the authors started

at values of the gas density, the angular diameter and the cluster

distance nearly equal to those used by R. Gathier, but the mass of the

planetary turned out to be as small as 0.011 solar masses. Wondering

at this, I found an error in my notes on Gathier’s paper. The mass he

has really derived is 0.014, not 0.14 as given in my article. Pease 1

has thus an unusually small mass compared to that of planetaries of

the solar neighborhood.

The group of S. Adams carried out an careful spectrophotometry of

the planetary and revealed that the nebular envelope is not at all so

poor in carbon as the cluster stars are. In fact, Kustner 648 contains

slightly more carbon (compared to hydrogen, of course) than the Sun.

This is presumably caused by nuclear ash carried from the interior of

the ancestor to the outer layers which eventually became the planetary

nebula.

More interesting for observers, the V magnitude of the nebula,

14.64, with the contribution of the central star being some 75

percent, has been measured as well. Besides, the planetary has a close

faint component 0.9” to the south (the star AC 728, V 15.56 mag).

References and notes:


 [1] Veröffentlichungen  der  Universitäts-Sternwarte zu Bonn, No. 12,

     1921

 [2] PASP 40, 342, 1928

 [3] In  fact,  the concept of the escape velocity, so straightforward

     in  a case of binary star or a satellite in Earth's gravitational

     field,  has  to  used  with  caution  if  a  globular  cluster is

     considered.  Velocity  smaller  than  the  escape  one is not, in

     itself,  a  sufficient  guarantee  that a star (or a nebula) will

     remain in a cluster forever. An encounter with a close binary may

     result  in  making  the  binary still closer (harder, astronomers

     say) and in flinging the solitary star out of the cluster.

 [4] Stuart  R. Potasch: Planetary Nebulae, D. Reidel, Dordrecht, 1984

     (radial  velocity  of  Pease  1),  and  A.  Hirsfeld and Roger W.

     Sinnott: Sky Catalogue 2000.0, Vol. 2, Sky Publishing Corp., 1985

     (velocity of M 15).

 [5] There  is  a  similar  situation with helium in the Sun. It would

     seem  that  a  measurement of abundance of the element, which has

     been  discovered  just  in  our  star  (during a total eclipse in

     1868),  and  even  named  for  it, is a simple and routine thing.

     However, helium has very faint lines in the solar photosphere and

     its  abundance, entering into computers at modelling of the Sun's

     structure  and  evolution,  is  taken from observation of diffuse

     nebulae and hot stars in neighborhood of the solar system.

 [6] Data taken from Potasch's monography quoted.

 [7] Astron. Astrophys. 127, 320, 1983

 [8] Astrophys. J. 140, 119, 1964

 [9] Astrophys. J. 338, 862, 1989

[10] Astron. J. 99, 1863, 1990

[11] Astrophys. J. 374, L 45, 1991

[12] Nature 352, 219, 1991

[13] Sweeping of surrounding gas (though not in a star cluster) in the

     case  of  PSR 1957+20 is nicely illustrated by a deep photo given

     in Nature 335, 801, 1988.

[14] Nature 352, 221, 1991

Figure captions:

(BLUE STRAGGLER #1 of July 7, 1991)

Last Modification: 10 Feb 1998, 21:30 MET