Symbiotic binary stars lie within our own Galaxy and generally
consist of an evolved cool giant star orbited by a
hot white dwarf companion.
Both stars are in the declining years of their lives, with the white dwarf cooling
towards obscurity, and the red giant undergoing mass loss and contributing to the
enrichment of the interstellar medium.
Occasionally the white dwarf is reheated through
thermonuclear detonation of material deposited on its surface
from the giant's atmosphere. These events typically produce nova-like increases in brightness,
typically
by factors of a hundred or so, but stronger explosions may
be the cause behind some of the most luminous supernovae, which attain
luminosities comparable with that of an entire galaxy.
Although important in terms of the overall evolution of interstellar gas,
mass loss by isolated giant stars is a relatively low-key affair, and so is
correspondingly hard to study. In symbiotic binaries, on the other hand,
the presence of a nearby hot star provides sufficient ionization to produce
a nebular spectrum that is rich in emission lines.
By studying these lines it is possible to diagnose the density, temperature
and abundance of the red giant wind, filling in details relevant to non-symbiotic
giants. For the more strongly interacting systems where nuclear detonation
occurs, it is also possible to study the time evolution in both physical
characteristics and abundances, providing data to check our theories of the
interaction in these environments.
My current research is aimed at extending my symbiotic studies by the
development and improvement of emission line diagnostics.
In addition I, together with collaborators at QUB, have been
allocated time to use NASA's Far Ultraviolet
Spectroscopic Explorer (FUSE) to study the molecular gas in those symbiotic
systems where the hot star is periodically eclipsed by the giant star. Around
the time of minimum the white dwarf provides a `pencil beam' probe of the
red giant, permitting
determination of the relative abundances of a range of molecular and atomic gas
species point-by-point through the atmosphere. These data will complement our
study of the more ionized material closer to the white dwarf.
In conjunction with graduate student Jennifer J. Birriel
and Regina E. Schulte-Ladbeck (U. Pitt.) a paper was submitted to ApJ
summarising
the far-UV and UV data for a sample of 9 symbiotics observed with the
Hopkins Ultraviolet Telscope (HUT) during the Astro-2 space
astronomy mission in March 1995 (Birriel, Espey and Schulte-Ladbeck,
2000). The main aim of this paper was to compare the OVI 1032, 1038
Angstrom doublet in the far-UV, and compare the line strength with
two optical features at 6825, 7082 Angstroms presumed to be due
to Raman scattering by neutral hydrogen of the far-UV OVI lines.
We found that the Raman scattering model is in agreement with the
observational data in that the optical lines are only present when
OVI is seen. We derived the first accurate scattering efficiencies
through the use of near-simultaneous far-UV and optical data of these
variable objects together with accurate extinction and molecular
hydrogen absorption corrections for the OVI lines. Interestingly,
the scattering efficiency for this process is relatively large in
some objects (up to 15%), attesting to the amount of cool material
in these systems. In addition, we were able to make the first
measurements of the fluorescence efficiency for OVI-pumped FeII
(< 2%) which also is produced in cool gas. These data formed the
basis of Birriel's thesis which she has now successfully defended.
Together with S. McCandliss (JHU) and other co-Is,
I continued my program of study of symbiotic binary stars using
both space-borne and ground-based data. A review of IUE and
Hopkins observations of EG And, the primary target of this work,
was presented at the Atlanta meeting of the American Astronomical
Society (Espey and McCandliess 1999).
Far-UV spectra have also been obtained as part of Espey's FUSE
Guest Investigator program. Three stars have been observed so
far, providing the best S/N high resolution observations of any
symbiotic binary to date. All objects were chosen for the presence
of soft X-ray emission which is believed to indicate the presence
of shocked gas in the region where the white dwarf and red giant
winds collide. Two observations of EG And at different phases
show distinct differences in terms of absorption line strength
and location. The other stars show interesting detail in both
emission and absorption lines, including one object where the
OVI 1032, 1039 emission is the strongest feature in the entire
far-UV to optical extinction-corrected spectrum. Analysis is
proceeding well, and a preliminary report was presented at the
January 2002 meeting of the American Astronomical Society.
As part of the Space Telescope Science Institute's Collaborative
Visitor Program, Gary Ferland (UK) worked with me on the conundrum of
the SiII emission line spectrum. No current photoionization model
for SiII can successfully explain the observed UV and optical lines.
In working on this problem we discovered an interesting
relationship between emission line properties which suggests that
the emission line gas in symbiotic binaries is more complicated
than has been assumed in photionization models to date. A short
paper summarizing the results of our findings (and also a solution
to the problem!) is in preparation.
Also in the context of diagnostic development, I continued
my collaboration with Francis Keenan (QUB) and his atomic physics
group. In a true symbiosis, observations and theory were combined
to provide strong evidence for the first observational detection of
the [AlII] 2661 Angstrom line (Keenan et al. 1999). Work is continuing
on the development of other emission line diagnostics.
Emission lines from active galactic nuclei (AGN)
A small proportion of galaxies show emission lines in their spectra,
suggesting the presence of both gas and a strong ionizing continuum.
The line and continuum radiation from the nucleus of these objects can be
exceedingly powerful, dominating the light from the entire galaxy, and providing
a means for their identification and study out to the most distant reaches of
our Universe.
In this respect these galaxies provide beacons of the physical conditions
and abundances in their cores in a similar manner to the way symbiotic binaries
illuminate otherwise dark material in their environs. This analogy can be stretched
a little further in that, to first order, the emission lines from these AGN
are similar to these seen in symbiotic systems. The relatively bright nearby
symbiotic stars therefore serve as a testbed for the development of tools and
techniques that can be extended to these more distant systems.
Recent work, in collaboration with my students, has shown that the emission
lines of AGN show a characteristic trend with luminosity which indicates that
the continuum shape alters,
presumbably reflecting a change in conditions close to the black hole at the
nucleus. Once this trend of emission accounted for, however, there remains a
discrepancy between the behaviour of the lines of ionized nitrogen
(NV 1238, 1242 Angstroms) and those of other elements, with the most luminous objects
showing a larger NV strength relative to other lines. This effect had been noticed
by previous workers, but our study has considerably extended the available database
and quantified the emission line behaviour.
The explanation is believed to be because the most luminous AGN are also
the most distant, and the light we see now left these objects when they
were at an earlier stage in evolution. The emission lines (and abundances) we
observe thus represent an earlier stage of processing of their interstellar gas
and reflect the ejecta of their most recent stage of star formation at that time.
For nitrogen to be enhanced relative to other elements when the Universe was
relatively young, a combination of early and vigorous star formation must have
occurred. At later
epochs, this overabundance is dilluted as further elements are processed in stars
and added to the interstellar gas.
Having confirmed the `anomalous' behaviour of NV, I hope
to extend these studies to fainter AGN in order to isolate the dependencies of line
strength on luminosity and distance.
Publications
Recent references can be found
here.
