The unidentified FUV lines of hydrogen deficient dwarfs

1. David Boyce M. A. Barstow, J. K. Barstow F. James, R. Laird, P. Dobbie A. Forbes, J. Aston, B. Booles The unidentified FUV lines of hydrogen deficient dwarfs

2. The unsolved problem of EUV deficiency Introduction • DO white dwarfs are significantly EUV deficient • To reduce the predicted EUV flux, without increasing the assumed elemental abundances above the values determined from IUE and HST spectra, it is necessary to find an alternative source of photospheric opacity. • A possible source could be from additional, as yet undetected elements that are not currently included in the theoretical model calculations. The DO line problem • The FUV spectra of DO white dwarfs contain more unidentified lines than identified ones. • The result of incomplete and inaccurate atomic data?

3. Introduction Solutions for the DO line problem • Unidentified lines belong to high ionisation/ excitation species not included in the spectral line lists. • Unidentified lines belong to iron group and post iron group species not included in the spectral line lists. • Unidentified lines belong to a species wholly unexpected to be present in the DO.

4. Processing and candidate selection All hydrogen deficient dwarfs were obtained from MAST and processed in the usual way (Barstow et al 2003). Data reduction The data has been calibrated using the calfuse pipeline version 2.0.5 or later, resulting in several spectra (covering different wavelength segments) per FUSE exposure. The exposures for each segment are then co-added, weighting each according to exposure time, and finally the segments are combined, weighted by their signal-to-noise ratio, to produce a single spectrum Stars badly effected by molecular hydrogen were thrown out

5. Introduction A two component mystery • The ISM contributes a complex and poorly understood component to the DO spectra. • This component is not exactly the same as in the more familiar DA spectra • How many of the unidentified lines that were previously thought to be photospheric originate in fact from the ISM?

6. Velocity Discrimination Velocity discrimination; the basic idea Average photospheric velocity Star 1 Star 2 Line velocity Star 2 Star 1 Average ISM velocity Line position Star 2 Star 2 Star 1

7. Velocity Discrimination Velocity discrimination; real stars From well known lines we know an average velocity for the star and the ISM in that direction Using multiple stars we can differentiate between an unknown line being Photospheric or ISM without even knowing what it is. Graph of predicted lab wavelength ISM ISM ISM Photospheric Photospheric Photospheric Line=photospheric Line=ISM

8. Velocity Discrimination Velocity discrimination; the results • Velocity discrimination clearly separates ISM lines from photospheric lines • Origin of lines found without knowing line identity

9. Velocity Discrimination A side note about O VI • O VI in FUV spectra of white dwarfs considered to be tracers of hot gas in the interstellar medium • Since the distance to white dwarfs puts them just inside, on or just outside the “hot local bubble” we can test the origin of the O VI using velocity discrimination. • Our results show that all O VI detected along the lines of sights of stars within the local bubble had O VI of a stellar origin. • The hot local bubble = The cold local bubble

10. Finding the photospheric velocity for DOs Finding the photospheric velocity From a large sample of identified lines an average photospheric velocity can be calculated. Lines common to DOs and DAs can be used for this However Hot DOs and PG1159 stars are a problem due to the lack of lines common to DAs Makes velocity discrimination harder

11. Finding the photospheric velocity for DOs A new approach to finding the photospheric velocity Velocity Velocity

12. Finding the photospheric velocity for DOs Recurrent velocity analysis; DA stars Using the Morton line list we observe in the DA that there is peaks at the ISM and photospheric velocity Changing to the Kurucz line list Increases the recurrence of The photospheric velocity Kurucz more complete for higher ionisations

13. Finding the photospheric velocity for DOs Recurrent velocity analysis; DO stars Using either Morton or Kurucz We do not retrieve the photospheric velocity. The unidentified lines that make up DOs and PG1159 stars are not in the Morton or the Kurucz line list. No amount of looking will solve The DO line problem.

14. Using the photospheric velocity for DO line identification Results of velocity search Using these methods a photospheric velocity is found for a broad range of DOs and PG1159 stars

15. Using the photospheric velocity for DO line identification High ionisation/excitation species Success rate 7/8 Generation of atomic data for high ionisation species is making progress 4/4 Some lines however not so well behaved 3/8 2/5

16. Using the photospheric velocity for DO line identification Post iron group A considerable number of identifications of post iron group elements in the FUV spectra of cool DOs have been put forward by Chayer et al 2005 Do these lines appear in any of our stars? Do these lines occupy the correct velocity? Chayer et al 2005

17. Using the photospheric velocity for DO line identification Post iron group Lines tested against photospheric velocity of star Lines that fail the test

18. Conclusions • Velocity discrimination allows us to identify ISM lines from photospheric lines • Unknown lines not in linelists • Post iron group species do not occur at detectable levels. • Velocity forms a powerful tool in testing line identities • High excitation elements such as Ne VIII are the best candidates for the additional opacity required to solve the EUV deficit.