My current research focuses on studies of gas in both galactic and extragalactic systems using data from ground-based, and space-based telescopes (e.g., Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer). I previously worked for the European Space Agency at the Space Telescope Science Institute.
I have deliberately built up expertise in a number of areas of emission and absorption line spectroscopy, extending my skills over a wide range of wavelengths. First amongst these are the development of suitable diagnostics which can be applied over a broad range of conditions, and which make use of the best available atomic data. This work has proceeded in conjunction with Prof. Keenan and others at the Queen's University of Belfast (Northern Ireland), and with Prof. Ferland (University of Kentucky).
In the following sections I provide a brief overview of the applications of this work.
Symbiotic binary stars
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 Telescope (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 photoionization 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, presumably 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 diluted 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.