Binary stars in the heart of planetary nebulae give clues to understand the chemistry of the Universe
27/04/2015
A detailed study by researchers from the IAC, shows that these nebulae have the greatest differences in their chemical compositions obtained with different methods, which sheds new light on an unsolved problem in Astrophysics
One of the methods most commonly used to measure the chemical composition of the universe is the study of the nebulae where the stars are born (“HII regions”) or die (“planetary nebulae”, supernova remnants, etc.). Each chemical element in the hot gas which makes up these nebulae emits its own characteristic radiation, analogous to a fingerprint, formed by spectral lines at specific wavelengths, whose physical basis is well understood. So studying the spectra which these nebulae emit we can, in principle, determine accurately their chemical composition. However there is a basic problem with the method: the composition inferred differs depending on the physical mechanism which is assumed to produce the spectral emission lines, which poses a serious problem for astrophysicists.
A group of researchers at the Institute of Astrophysics of the Canaries (IAC) and the University of La Laguna (ULL), among them the Severo Ochoa Postdoc Jorge García-Rojas, has carried out a detailed spectroscopic study of three planetary nebulae, revealing that one of the factors which lies behind this problem is the existence of two (or more) gas phases in the nebulae, with different temperatures, densities, and chemical compositions. This result has just been published in the specialized journal “The Astrophysical Journal”.
The problem of the abundance discrepancy
Emission nebulae form near stars which emit highly energetic radiation (such as massive young stars and white dwarfs) capable of ionizing their constituent gas. The lines which are observed in the spectra of these nebulae are basically of two types: those produced by collisions between atoms or ions (atoms which have lost one or more electrons because they have been affected by the energetic radiation from a nearby star) and free electrons (which have been stripped from the atoms during the ionization process), which are termed “collisionally excited” lines, and those which are produced when the ions recapture free electrons, which are termed “recombination” lines.
The problem of the discrepancy in the chemical abundances which is found in observations of nebulae goes back as far as the 1940’s. The composition found using collisionally excited lines of elements such as oxygen, carbon, nitrogen or neon, which are the brightest lines, is different (usually between 2 and 3 times less) than the composition obtained using the recombination lines of the same elements, which are themselves fainter. This effect is found both in HII regions and planetary nebulae, although the discrepancy in the latter tends to be much greater.
Extreme cases
The team, using the William Herschel Telescope at the Roque de los Muchachos Observatory on the island of La Palma, obtained the spectra of three planetary nebulae, which they thought would show strong differences in their chemical abundances when observed using the two methods. “And indeed”, comments Jorge Garcia-Rojas “we found a discrepancy of more than a factor 50 in two of the objects, which rises to as high as 300 in the internal zones of the nebula Abell 46, the most extreme value found in any planetary nebula so far”. The observations showed the presence, as well as of hot gas which is typical of this kind of nebulae, of a second gaseous component, cooler and denser, which a higher oxygen abundance. It is this second component that produces the bulk of the emission from the recombination lines. However, the origin of this component is not known.
The three nebulae have another characteristic in common, which should not be related, a priori, with the problem of the abundance discrepancy. Romano Corradi, the researcher who is leading the investigation, explains that “the central star in these nebulae, which is what is left of the progenitor star when it has lost its gaseous envelope which now forms the nebula is, in these objects, in fact a pair of stars which are orbiting at high speed around one another, making complete orbits in less than ten hours”. The fact that they are so close implies that these pairs of stars had a dramatic past, in which there was a brief period when each was orbiting within the outer envelope of the other, and they have escaped “by the skin of their teeth” a possible collision and merger. “The evolution of these binary systems is not at all clear” David Jones, another of the researchers, told us “and now we have added an extra confusing factor: the associated nebulae show the highest abundance discrepancies ever measured”.
“These new observations add valuable information which can help with the abundance discrepancy problem” adds Pablo Rodríguez-Gil, another of the co-authors of the article, “showing that there is a clear relation between the high discrepancies found in some planetary nebulae and the binary nature of their stars”. One possible explanation could be the ejection of material rich in oxygen, nitrogen and other elements in a nova explosion, a phenomenon which occurs naturally in short period binary systems. Another possible alternative which is put forward in this study is that the component of the nebula which is rich in these elements was formed by the capture and disintegration of Jupiter-like planets by one of the stars. This could explain the presence of the two components of the nebulae, one due to the outer envelope of the planet, which is very rich in hydrogen and with “normal” abundances, and the other due to the nucleus of the planet, which is very rich in heavier elements. If this was really the case, the name “planetary nebula” which was given by Sir William Herschel to this type of objects because they looked a lot like planets through the telescopes of his epoch, would not be as far from the truth as we have thought up to now.
MORE INFORMATION:
Scientific article: “BINARITY AND THE ABUNDANCE DISCREPANCY PROBLEM IN PLANETARY NEBULAE”. The Astrophysical Journal, 2015, 803, 99. doi:10.1088/0004-637X/803/2/99
Authors: Romano L. M. Corradi, Jorge García-Rojas, David Jones and Pablo Rodríguez-Gil
