# Presentation: 799742

## FUSE Observations of the Loop I / Local Bubble Interaction Region

- S.M. Sallmen (Physics, UW – La Crosse)
- E.J. Korpela (SSL, UC Berkeley)
- H. Yamashita (Former UW-L Undergraduate)

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Schematic Orientation

**Schematic Orientation**

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Interaction Zone

**Interaction Zone**

- Neutral H map of IZ
- Dickey & Lockman (1990)
- IZ annulus
- Egger & Aschenbach (1995)

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Shadowing Strategy

**Shadowing Strategy**

- Compare OVI (O5+) emission along two adjacent sightlines along edge of IZ
- “Shadowed” goes through IZ
- “Unshadowed” goes outside IZ
- Compared with “Unshadowed” sightline, “Shadowed” sightline
- has low RASS 1⁄4 keV emission
- has higher N(HI)
- distant OVI should be blocked

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## X-ray Shadowing

- RASS 1⁄4 keV map near our pointings
- Snowden et al. (1997)

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## FUSE data

- “Unshadowed”: (*l, b*) = (276.26°, +10.692°)
- 2004 April 3 / 4,
- 43 ksec total, 30 ksec orbital night
- “Shadowed”: (*l, b*) = (278.23°, +8.02°)
- 2002 June 27-29, 2003 Feb 18 / 19
- 60 ksec total, 40.6 ksec orbital night
- Only consider data from orbital night
- Emission lines:
- Modeled as 106 km/s tophat convolved with Gaussian
- Gaussian contains instrumental contribution of ~ 25 km/s (Dixon et al. 2006)

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## OVI emission spectra

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Emission Intensities

| Species | Wavelength
(Å) | Shadowed Intensity (LU) | Unshadowed Intensity (LU) |
| --- | --- | --- | --- |
| O VI | 1031.93 | 2750 ± 550 | 10800 ± 1200 |
| C III | 977.02 | 13300 ± 2200 | 15000 ± 5000 |

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Comparison with other Superbubble measurements

*FUSE: *Dixon et al. 2006 survey
- IOVI* > *8000 LU for 2 sightlines (Monogem Ring, Outskirts of Vela / Gum Nebula)
*SPEAR: *
- IOIV ~ 180 000 LU towards much younger Vela (Nishikida et al. 2006)
- IOVI ~ 7000 LU along edge of* *Orion-Eridanus Superbubble (Kregenow et al. 2006)
- Confined to region ~ 5-10 pc across

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Modeling Physical Conditions

**Modeling Physical Conditions**

- Expect IZ to contain most absorbing material
*τ**us* may be non-zero (boundaries of LB & Loop I)
- For now, set *τ**us** = *0   &  *τ**s** = **τ**IZ** *

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## OVI Emission Within Loop I

- Assumptions:
*I**LB** *& *I**beyond *are the same along both sightlines
- Difference in optical depth between sightlines (*τ**IZ*) is entirely due to material in the IZ

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## CASE I: IZ Completely Opaque

*τ**s**= **τ**IZ** = ∞**	*
- 		 	*I**LB** =** *2750 ± 550 LU
- 			*I**beyond** =** *8050 ± 1320 LU
- Unlikely to be correct!
- Gives lower limit on OVI interior to Loop I
***Substantial OVI emission arises beyond the IZ***

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## CASE II: Shadowed sightline has average IZ properties

*N*(*H I*) = 7 × 1020 cm−2 (Egger & Aschenbach 1995)
*	**τ**IZ** =** *2.3 			(Sasseen et al. 2002)
- 		 	 *I**LB** **=** *1890 ± 620 LU
*	**	**	**I**beyond** **=** *8910 ± 1460 LU
-  *I**LB* inconsistent with Shelton (2003) upper limit
***More OVI emission arising in interfaces on near side of IZ than elsewhere in LB***
***	******	******	******	******	******OR***
***Neutral H in this direction is less than average *****(*****Case III*****)**

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## CASE III: No Local Emission

*I**LB** = *0 LU
- 		 	*I**beyond** =** *10800 ± 1200 LU
*	**	**	**τ**IZ** =** *1.4 ± 0.2
- 			*N*(*H I*) = (4.1 ± 0.7) × 1020 cm−2
- 						(Sasseen et al. 2002)
- Gives ***lower limit*** on neutral H column density of IZ
*N*(*H I*) less than this gives unphysical (−ve) Local Bubble Emission
- No significant change if use Shelton (2003) upper limit *I**LB** = *600 LU
***This is our preferred scenario***

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## IZ Column Density

- Egger & Aschenbach (1995):
- Avg. N(HI) ~ 7 × 1020 cm−2
- Too much local OVI emission
- Reis & Corradi (2007):
- N(HI) near our sightlines ~ 2 × 1020 cm−2
- 2 × smaller than lower limit consistent with our assumptions
- Our “Shadowed” sightline chosen due to notable X-ray shadow, so could easily be higher

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Physical Conditions Inside Loop I

- Model parameters:
- Input: *ROSAT *count rates (R12 and R34) along both sightlines
*CHIANTI *equilibrium plasma model
- Assume Tplasma & X-ray emission measure same on both sightlines
- Output spectrum absorbed along:
- “Unshadowed” sightline by a non-IZ neutral H column (*τ**us*)
- “Shadowed” sightline by that + additional IZ contribution (*τ**s** = **τ**IZ** **+** **τ**us*)
- Results:
- T = 1.26 ± 0.06 million K
- Non-IZ column density = (1.5 ± 0.2) × 1020 cm−2
- IZ column density = (4.0 ± 0.2) × 1020 cm−2
- 					(comparable to Case III above)

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## OVI Emission

- Use these neutral H column densities to determine *τ**us *& *τ**s*
*	**	**τ**us**= *0.50 ± 0.07	;	*τ**IZ** = *1.33 ± 0.07* **	*
*	** **	**	**	**τ**s**= **τ**us** + **τ**IZ** = *1.83 ± 0.09
- 		 	*I**LB** =**  −*130 ± 900 LU
- 			*I**beyond** =** *18000 ± 3300 LU
***Consistent with no local emission***

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## CIII Emission

- Optical Depth to CIII emission 13% higher
*	**	* 	*I**LB** =** *12800 ± 3400 LU
- 			*I**beyond** =** *3800          LU
- Large errors on distant emission due to opacity of intervening material
***Much of the emission arises on the near side of IZ***
- Substantially more than Shelton (2003) observation within LB
- Suggests CIII production at inhomogeneous interfaces

| +12000 |
| --- |
| −3800 |

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Conclusions

- Observed OVI (1032Å) emission:
- “Shadowed”: ~ 2750 ± 550 LU
- “Unshadowed”: 10800 ± 1200 LU
- Implications:
- ≤ ~ 800 LU arises from near side of IZ
- OVI emission arising within Loop I: ~104 LU
- If ne ~ 0.1 cm−3 , then *l **~ *2.5 pc, *P ~ *30,000 cm−3 K
- Observed CIII (977Å) similar along both sightlines
- Much of emission likely arises on near side of IZ

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## OVI Emission Intensities

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Pathlength

- IOVI  EMOVI ~ 0.012 ± 0.002 cm−6 pc
- Assume
- T ~ 3 × 105 K for OVI-emitting gas
- pressure equilibrium (X-ray / OVI)
- ne either 0.01 cm−3 or 0.1 cm−3 	(Dixon et al. 2006)
-  OVI-emitting pathlength 0.7 to 1.2 times X-ray emitting pathlength
- OVI pathlength ~ 120 pc OR ~ 1.2 pc
- Total OVI + X-ray ~ 250 pc or ~ 2.5 pc
- Path through edge of Loop I << 250 pc
- ne ~ 0.1 cm−3
*l **~ *2.5 pc
-  P ~ 30 000 cm−3 K

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Emission Line Velocities

- “Shadowed”:
- VLSR(OVI) = −29 ± 6 km/s
- VLSR(CIII) = −10 ± 10 km/s
- “Unshadowed”:
- VLSR(OVI) = 15 ± 8 km/s
- VLSR(CIII) = 45 ± 35 km/s
- Velocity shift  inhomogeneous properties for gas?
- Non-uniform surface brightness in aperture could mimic velocity shift (e.g. Shadowed sightline)

- Local Bubble & Beyond: Philadelphia

- April 24, 2008

## Emission Line Widths

- Remove instrumental contribution from convolving Gaussian
- FWHM < 25km/s  unfilled aperture
- True for best-fit on Shadowed sightline
- “Shadowed”:  upper edge of 1-σ range on convolving Gaussian  T ~ 2 × 105 K
- “Unshadowed”:  T ~ 4 million K
- Turbulent broadening?
- More than one emitting region?

- Local Bubble & Beyond: Philadelphia

- April 24, 2008