Planetary nebulae are gaseous envelopes produced by stars at the end of their evolution. When a Sun-like star has finished burning hydrogen and helium in its interior, only a very dense core is left surrounded by a tenuous atmosphere, all the rest having escaped from the star through stellar winds. Collisions of stellar winds produced at various epochs induce local density enhancements, which can be observed as shells, more or less complex in shape, when they are ionized and excited by the photons emitted by the central star now very hot. Until recently, the number of planetary nebulae known in the halo of our galaxy amounted to roughly ten. This number is about to increase rapidly, thanks to spectroscopic surveys aimed at finding quasars and emission line galaxies and which also allow to discover emission line stars and planetary nebulae. A new planetary nebula in the Galactic halo The famous Second Byurakan Survey,carried out in Armenia in the 80-90ties, is at the origin of a particularly interesting discovery. Gaghik Tovmassian, presently working in Mexico, became intreagued by an object originally classified as a cataclysmic variable : SBS 1150+599A. The very unusual spectrum of this object shows a blue continuum, a few narrow and blue-shifted emission lines, and the forbidden line [OIII] at 5007 Angstrom, of very weak intensity. During an Astronomical meeting held in Mexico, Gaghik Tovmassian showed this spectrum to Grazyna Stasinska, from Meudon Observatory, who at once emitted the hypothesis of a planetary nebula.
However, the spectrum is not really typical of a planetary nebula either. It has only one forbidden line in the 4000 - 7000 Angstrom range, and this line is extremely weak, as seen in the inset of the figure which represents the spectrum of the object obtained with a total 8400 sec integration time at the 2.1m telescope of San Pedro Martir (Mexico). In order to produce the observed spectrum, a planetary nebula must be excited by a very hot star (of effective temperature at least 70000K). Moreover, its gaseous envelope must be partly transparent to the ionizing photons emitted by the star in order to explain the absence of a zone producing the so-called low-excitation lines. Actually, these two conditions are not exceptional for planetary nebulae. What is exceptional is the extreme weakness of the [OIII]5007 line, which implies a very small oxygen abundance. A more detailed study allowed to eliminate with good confidence the alternative interpretations concerning the nature of this object (cataclysmic variable, symbiotic star, galaxy from the local group etc...) and to retain the planetary nebula hypothesis as being by far the most probable. Photonionization models constructed by Grazyna Stasinska showed that the oxygen abundance of this planetary nebula (now called PN G135.9+55.9 following the nomenclature of the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae)is by far inferior to the one of the planetary nebulae known so far. Depending on the hypothesis adopted for the characteristics of the nebula and its central star, the O/H ratio in PN G135.9+55.9 would lie between one hundredth and one thousandth of the value measured in the Sun. This would be the most oxygen-poor planetary nebula known so far ! The abundance of oxygen in the nebular envelope probably reflects the chemical composition of the gas out of which the parent star was formed. This could mean that the star is very old, may be older than the oldest globular clusters. Or may be the star formed out of almost pristine meterial accreted by our Galaxy. Whatever the answer will be, it will bring further questions regarding the formation of the halo of our Galaxy and the evolution of extremely metal-poor stars. To be continued ...
Reference : Tovmassian, G.H., Stasinska, G., Chavushyan, V.H., Zharikov, S.V., Gutierrez, C., Prada, F., "SBS 1150+599A : an extremely oxygen-poor planetary nebula in the Galactic halo ?", 2001, Astronomy & Astrophysics, in press, astro-ph/0104222. Contact : Grazyna Stasinska (Département DAEC, Observatoire de Paris)
Dernière modification le 4 mars 2013