1. Irradiated brown dwarf companions to white dwarf stars
Emma Longstaff
Working with Sarah Casewell, Graham Wynn, Christiane Helling, Pierre Maxted
Planetary systems beyond the main sequence II

2. Brown dwarfs Sub-stellar objects. 13 – 72 MJupiterY
Temperature (K)
TiO  VO
FeH
CaH
H2O CO K Na
CH4
NH3
Sub-stellar objects.
13 – 72 MJupiter
Form through gravitational collapse.
Have not burned hydrogen in their lifetimes.
Their atmospheres are more akin to planets than stars.
Characterised by strong absorption features.
Helling & Casewell (2014)

3. Brown dwarf desert
There is a distinct lack of brown dwarf companions to main sequence stars in short period orbits.
Likely due to difficulty in the formation of these binaries.
Observing their atmospheres in detail proves difficult due to the active nature of their host.
Solution: Observe these systems in their more evolved form.
Only <0.5% of white dwarfs have a brown dwarf companion.
Grether & Lineweaver (2005)

4. Close white dwarf – brown dwarf systems
They arise through common envelope evolution (CEE).
The white dwarf progenitor evolves off the main sequence.
The star expands along the red giant branch.
The brown dwarf companion is unable to accept all the donated material and is engulfed: a common envelop is formed.
Drag forces cause the companion to lose orbital angular momentum and spiral in toward the core.
The envelope is then ejected leaving a close WD + BD binary.

5. Detached survivors of CEE
Survival is not easy! Only a few confirmed WD + BD detached post common envelope binaries.
The systems that have survived are very useful, especially for characterising planetary atmospheres.
The contrast between the white dwarf and brown dwarf makes separating their spectra easier.
Brown dwarfs can be directly detected at wavelengths long-wards of 1.2 microns.
Brown dwarf models are more advanced than exoplanet atmosphere models.
Name
Spectral type
Period WD
WD + L6-L8
114 mins
NLTT 5306
WD + L4-L7
101 mins

6. WD Hα
Performed a multiple Gaussian fit of the Hα line to measure the radial velocities.
Fitted the radial velocities with,
to refine the ephemeris.
Properties
Period
114 mins T0
WD mass1
0.39 ± MSun
BD mass1
53 ± 6 MJup
SpT
L6 – L8
WD temperature
16,500K
1Maxted et al. (2006)

7. Emission lines Discovered metal emission lines:
Na I, Mg I, Si I, K I, Ca I & II, Ti I, and Fe I & II
All emission is coming from the brown dwarf.
No indication of any mass transfer.
This emission can be completely attributed to the effects of irradiation.

8. Equivalent Widths
Significant difference between the day and night side line strengths.
Line
Equivalent width Hα
-0.79 ± 0.021
-0.03 ± 0.021
Ca II (8662 A)
-0.73 ± 0.021
-0.05 ± 0.021
Hα line strength displays a significant phase dependence.
Supports 500 K day/night temperature difference found by Casewell et al. (2015).
This can be found in Longstaff et al. (submitted)

9. Comparison V471 Tau is a white dwarf + M dwarf system.
2MASS J0418 and SDSS J0423 are “hyperactive” brown dwarfs.
WD more closely resembles irradiated stellar companions.
This could aid future models of atmospheres and the variation in line width we observe should be replicated.
We expect to see this in other irradiated atmospheres such as hot Jupiters.

10. NLTT 5306 Shortest period detached WD+BD binary and has undergone CEE.
Followed the same procedure as before.
Hα emission moves in phase with absorption feature.
We’ve added our new XSHOOTER data to previously published radial velocities (Steele et al 2013)
We detect no lines from the brown dwarf.

11. NLTT 5306 We confirmed the presence of Hα emission.
Hα emission is coming from the white dwarf.
We also detected Na I doublet in absorption (5889 / 5895Å) also originating from white dwarf.
Steele et al. (2013) attributed Hα emission to low level accretion via a wind from the brown dwarf.
Much cooler than WD
Properties
NLTT
WD0137
Period (mins)
101
114
WD mass (MSun)
0.44 ± 0.04
0.39 ± 0.035
BD mass (MJup)
56 ± 3
53 ± 6
SpT
L4 – L7
L6 – L8
WD temperature
7756 ± 35 K
16,500 K

12. Swift We used the Ultraviolet/Optical Telescope: UVOT.
We detected NLTT 5306 in all three wavebands.
We predicted magnitudes by convolving a white dwarf model spectrum with each of the filter profiles.
Observed at Φ = 0.2
No X-ray detection.
Filter
Central λ
Predicted
Observed W1
2600 Å
18.36
18.32 ± 0.06 M2
2246 Å
19.24
18.76 ± 0.07 W2
1928 Å
19.32
19.29 ± 0.07 W1 M2 W2 W1 M2 W2

13. Future analysis We now know we can detect NLTT 5306 with UVOT.
We have had a Swift proposal accepted to get full orbital coverage in the 3 wavebands.
Accretion is likely to be occurring here however the mechanism is as yet unknown.
Roche lobe overflow?
Calculations suggest it is under-filling its Roche lobe.
Inflation due to incident irradiation?
This white dwarf is cooler than WD and we see nothing like this in the hotter system.
Magnetic fields?
We do not see any evidence of Zeeman splitting however, this object is cool
NLTT 5306B does not appear to be a “hyper-active” brown dwarf.

14. Summary & Conclusions
We have analysed new data on the two shortest period detached white dwarf + brown dwarf binaries.
They have both survived common envelope evolution unscathed.
WD B displays metal emission lines as a direct result of the intense irradiation from it’s host star.
Despite their similar masses and orbital properties NLTT 5306 displays evidence that accretion is occurring.
Initial Swift observations indicate a UV excess and further observations should give insights into the accretion mechanism.
These systems are rare but invaluable for understanding CEE and providing templates for future models of irradiated atmospheres such as hot Jupiters.