# Presentation: 553016

## The OVI Issue, Part II 

**M.A. Barstow****1****, D.D. Boyce****1****, B.Y.Welsh****2****, R. Lallement****3****, A.E. Forbes****1**** & J.K. Barstow****1**** **
**1**** Department of Physics & Astronomy, University of Leicester, ****2**** Space Sciences Laboratory, University of California, Berkeley, ****3**** Service d’Aeronomie du CNRS, Verrieres-le-Buisson, France**

## Introduction

- White Dwarf spectra in the far-UV
- Photospheric, interstellar and circumstellar material
- Our new analysis of the FUSE sample
- Location of OVI in the Local Bubble/Cavity
- Conclusions

## White Dwarf Composition

**White Dwarf Composition**

- Nitrogen

- Nickel

## Nitrogen (NV)

**Nitrogen (N****V****)**

- GD659

- REJ1614-085

- REJ1032+532

## Stratification of nitrogen

## Stratification of nitrogen

- REJ1032+532

## Summary

- Conventional view
- White dwarf composition has a strong dependency on Teff
- Balance of radiative levitation vs. gravity
- High ionization in cooler stars not photospheric
- But stratified high ionization material CAN be present in cooler stars!
- Almost all WDs have photospheric material (e.g. PV  in GD71 @ 33,000K; Dobbie et al. 2005)
- OVI seen <45,000K could be photospheric
- Best (only?) discriminant between photosphere and ISM is radial velocity

## FUSE Observations

- Previous work on OVI with FUSE
- Oegerle et al., 2005, ApJ, 622, 389
- Savage & Lehner, 2006, ApJS, 162, 134
- Our work
- Uniform data processing (latest pipeline)
- Larger stellar sample (83 vs. 46)
- New photospheric velocity analyses
- Compare results with LISM density maps

## Slide 9

## Slide 10

## *

## Example Results

| Star | vphot | vism |  δ v | vOVI | Comment |
| --- | --- | --- | --- | --- | --- |
| WD2350-706  | 35.7 | -6.5 | 42.3 | 10.6 | IR |
| WD2331-475 | 22.5 | -0.7 | 23.2 | 3.0 | I |
| WD1029+537 | 13.8 | -22.8 | 36.6 | 25.7 | P |
| WD0050-332 | 11.1 | -14.9 | 26.0 | 13.7 | P |
| WD0501+524 | 21.5 | 14.3 | 7.2 | 20.4 | ? |
| WD2321-549 | 12.5 | -16.6 | 29.2 | -1.6 | I & P |
| WD2257-073 | 15.3 | -10.2 | 25.5 | 30.8 | IR |
| WD2152-548 | -3.8 | -1.6 | -2.3 | -2.6 | ? |

- Key: I = interstellar at CII/OI velocity; P = photospheric
-         IR = interstellar redshifted wrt CII/OI
-         ? = cannot distinguish

## Summary of Results

- We detect ISM OVI in 14 stars
- 3 I, 7 IR, 4 blend of I and P
- 15 clearly photospheric only
- 17 OVI detected but not resolved
- 37 non-detections > upper limits
- How do these compare with e.g. Savage & Lehner
- 6 ISM detections in agreement
- 14 ISM detections we believe are P/?/ND
- 1 ND and 1 P that we have as ISM detections
- 14 non-detections in agreement
- Main differences due to new photospheric velocity measurements

## l=0

## l=0

## l=0

## l=0

## l=0

## Slide 19

## Summary of Results

- Range of column densities detected
- 4.5 x 1012 - 4.2 x 1013 cm-2
- 2.1 x 1012 – 7.6 x 1012 cm-2 (upper limits)
- Range of volume densities
- 8.8 x10-9 – 9.8 x 10-8 cm-3
- Ave = 4.2 x 10-8 (S&L = 4.6 x 10-8)
- Are we seeing a threshold effect?
- No detections within ~50pc
- Densities do not seem to be uniformly distributed
- Some long lines of sight with no detection, but which are close to detected material!

## Conclusion

**Conclusion**

- In common with Oegerle et al. + Savage & Lehner
- distn of OVI is patchy
- Mainly at high galactic latitude
- In majority of cases OVI is detected in sightline crossing or near a cold cloud

## Density maps of the Local ISM

**Density maps of the Local ISM**