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One of the most common methods to measure chemical abundances in the
Universe is the study of the emission of the ionized gas that composes
a number of nebulae related to both star formation (HII regions)
and death (planetary nebulae, supernova remnants, stellar outflows).
However, it is well known that in all these nebulae the computed
abundance values depend on the kind of emission lines
considered. Specifically, optical recombination lines (ORLs) provide
chemical abundance values that are systematically larger than those
obtained using collisionally excited lines (CELs). The abundance
discrepancy factor between ORLs and CELs is usually between 1.5 and 3,
but in planetary nebulae it has a significant tail extending to much
larger values. This is generally known as the "abundance discrepancy
problem". It has been around for more than seventy years, and is one
of the major unresolved problems in nebular astrophysics.
Spectroscopic observations with the William Herschel Telescope of
three planetary nebulae have shed new light on the
problem. Astronomers from the Instituto de Astrofísica de Canarias
have shown that the largest abundance discrepancies (as high as 300 in
certain positions in the nebula, see Figure 1) are reached in planetary
nebulae that have a close binary central star. The spectroscopic
analysis supports the interpretation that two different gas phases
coexist in these nebulae: hot gas at 10,000 K with standard chemical
abundances metallicity where the CELs can be efficiently excited, and
a much cooler (~1000 K) plasma with a highly enhanced content of heavy
elements (which is the cause of the cooling) where only ORLs form.
This dual nature of the stellar ejecta is not predicted by mass loss
theories. How much each gas component contributes to the total mass,
and how they are distributed and mixed, is poorly known.
Figure 1: INT Hα image of Abell 46, one of the planetary nebulae
studied in this work, and the one with the highest abundance
discrepancy found so far
[TIFF |JPEG|JPEG(inset) ].
These observations add a new unexpected ingredient to understanding the
abundance discrepancy problem: high abundance discrepancies should be
explained in a framework of binary evolution. Several explanations
have been proposed, some of which are naturally linked to binarity, like
the hypothesis that the ORL emitting gas is high-metallicity
ejecta from nova explosions. Other explanations involve the presence
of planetary debris that survived the whole evolution of the central
stars, or -as proposed in this study- the effects of tidal
destruction, accretion and ejection of Jupiter-like planets. At this
stage it is difficult to favour any scenario because observational
constraints are still limited. Very deep spectroscopy at the
highest spatial resolution is perhaps the key to gain further insight
into this relevant astrophysical problem.
Figure 2: Portion of the WHT spectrum of Ou5. Some of the strongest OII
and N III ORLs are indicated. These lines are extremely faint in
most ionized nebulae, but in objects with large abundance discrepancy
factors can reach an intensity of few percents the flux of the
Romano L.M. Corradi, Jorge García-Rojas, David Jones and Pablo Rodríguez-Gil
"Binarity and the abundance discrepancy problem in planetary nebulae", 2015,
ApJ, 803, 99.Paper.