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,
 [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:

  1. The planetary nebula Pease 1 in M 15 sometimes caues trouble even to happy owners of large instruments because of lack of good finder charts. Our problem seems to be quite the opposite. Having no sufficient scope, we keep several charts in the Amateur Sky Survey archives. This one is reprinted (and slightly adapted) from Perek & Kohoutek's Catalogue of Galactic Planetary Nebulae, NCSAV, Prague, 1967, Plate 103.
  2. To make this, second finder chart, stars were plotted after original Kustner's list. The nebula (star #648 of about 14 mag) is marked with bars. The empty area in the right lower corner is due to Kustner's omission of stars in the crowded center of M 15.
  3. The last chart is a pair of photos taken by M. Auriere and his colleagues (Astron. Astrophys. 63, 341, 1978). The picture to the left is unfiltered, while the right one was exposed through nebular [OIII] filter. Note that the nebula is much brighter on the latter. Each frame is 35'' by 50'' with north up and west to the right.
  4. The most detailed radio portrait of Pease 1 ever prepared, by R. Gathier at 6 cm wavelength. Further scrutiny of the nebula's morphology is expected to suggest why gas is completely lacking in most of globular clusters.
  5. The position of the planetary nebula (arrowed) between the brightest stars of M 15 and pulsars PSR 2127+11A to E. All but one pulsars lie in what is a classic example of so-called post-collapse-core, an inevitable result of dynamical evolution of globular star clusters. Core collapse (remember strong concentration of M 15 in your telescope) is stopped only when a sufficient number of close binaries is formed by tidal captures. Encounters of core stars with such binaries may result in increasing of their velocity what works against collapsing. The 8-hour binary pulsar PSR 2127+11C is believed to have been expelled to outskirts of M 15 just by such close encounter. It seems, nevertheless, that this was not fatal, the pulsar is most likely still bound to the cluster. Redrawn from a chart given in paper by T. A. Prince et al. (Astrophys. J. 374, L 41, 1991), Pease 1 added according to its coordinates determined by R. Gathier.
(BLUE STRAGGLER #1 of July 7, 1991)

Leos Ondra (leo at sky.cz)

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