Irradiated brown dwarfs - hot Jupiter analogues? Sarah L Casewell1* STFC Ernest Rutherford Research Fellow 1. University of Leicester, Department of Physics and Astronomy, Leicester, UK *email: [email protected]

As brown dwarfs have atmospheres similar to those we see in Jupiter and in hot Jupiter exoplanets, irradiated brown dwarfs have been described as “filling the fourth corner of parameter space between the solar system planets, hot Jupiter exoplanets and isolated brown dwarfs”. Irradiated brown dwarfs are however, rare. There are only about 22 known around main sequence stars, and even fewer that have survived the evolution of their host star to form close, post-common envelope systems where the brown dwarf orbits a white dwarf. These systems are however, extremely useful. The white dwarf emits more at shorter wavelengths, where the brown dwarf is brightest in the infrared, meaning it is possible to directly observe the brown dwarf – something that is extremely challenging for the main sequence star-brown dwarf systems.

BROWN DWARF DESERT ATMOSPHERES AND IRRADIATION

Despite there being a plethora of Hot Jupiters known, there are very few The brown dwarfs in these systems are highly irradiated leading to emission brown dwarfs discovered in similar orbits, a phenomenon known as the lines being seen from hydrogen, Ca I, Na I, K I, Ti I and Fe in the systems with brown dwarf desert. This is thought to be caused by the difference in white dwarfs hotter than 13000 K. The hottest primaries show fewer emission formation mechanisms, planets forming via core accretion around stars, lines due to dissociation, but large day- night temperature differences of ~500 but brown dwarfs forming via gravitational instabilty . . K (Casewell et al., 2015; 2018)

planets brown stars dwarfs 50

40 ~25

30 deuterium burning hydrogen burning

number of objects 20

10 WASP planets EBLM IV

0 0.1 1.0 10.0 100.0 WD0137-349 SDSSJ1205 Ca I 866.2 nm Na I 588.9 / 589.5 nm . companion mass [Mjup] . Figure 1: Brown dwarf desert (Triaud et al., 2017). Despite thousands of Figure 3: Halpha emission for WD0137 and SDSS1205 as well as Ca I and Na I exoplanets discovered, and ~50% of sun like stars being part of a multiple emission from WD0137 (Longstaff et al., 2017; Parsons et al., 2017). system, there are only ~25 brown dwarfs orbiting main sequence stars within 3 AU (Carmichael et al., 2020).

EVOLVED SYSTEMS BROWN DWARF INFLATION The evolved form of these systems, detached, post-common envelope white dwarf brown dwarf binaries however show huge potential for Figure 4 shows that low mass brown dwarfs (<35 MJup) are generally inflated comparison to hot Jupiters. The periods are ~hrs, but the insolation is irrelevant of the insolation from their host star. Higher mass brown dwarfs are often similar to systems with a main sequence primary and a brown dwarf harder to inflate, and the majority are not. The brown dwarf WD1032+011B is in an orbit of ~days. eclipsing and nearly 70 MJup, and at least 5 Gyr old. Despite this, it is considerably inflated (Casewell et al., 2020a). Such inflation is not due to tidal 22000 WD1032+011 deformation or rotation, as the white dwarf primary only has an effective 20000 temperature of 10000 K it is also unlikely to be due to the UV irradiation from EPIC212235321 the white dwarf. 18000 SDSS1411+2009 ) K 2 3000 K ( 100 Myr 600 Myr 2 Gyr 6 Gyr

f

f 10 Gyr [Fe/H]=-0.5, 2 Gyr [Fe/H]=-0.5, 6 Gyr [Fe/H]=-0.5, 10 Gyr e T SDSS1205 16000 y

r 1.8 4000 K a n i r

SDSS1557 P 14000 5000 K 1.6 NLTT5306 12000 1.4 7000 K ) WD0837+185 p u J R (

10000 s 9000 K u GD1400 i 1.2 d a R 8000 WD0137-349 1 13000 K 6000 0 0.1 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 Separation (AU) 0.8

Figure 2:The known white dwarf-brown dwarf binaries. Periods range from 68 0.6 24000 K min (EPIC212235321) to 10 hrs (GD1400). The white dwarf effective temperature 10 20 30 40 50 60 70 80

is represented by the colour. The pink circles are the brown dwarf secondaries. Mass (MJup ) The size of the circles are proportional to the masses of the binary components. Figure 4: Mass-Radius relationship for all the known irradiated brown dwarfs. Colour is the effective temperature of the primary star, the circle size is proportional to the bolometric insolation. The 3 eclipsing systems with white dwarf primaries are plotted CONCLUSIONS as squares. Adapted from Casewell et al., (2020b). REFERENCES

While there are only ~25 Main sequence-Brown dwarf binaries Carmichael et al., 2020 AJ, submitted ; known, there are 9 known evolved forms of the binaries. With periods Casewell et al., 2020a, MNRAS, 497, 3571; of only a few hours the brown dwarfs are tidally locked and often Casewell et al., 2020b, MNRAS in press; highly irradiated resulting in day-nightside temperatures of up to Casewell et al., 2018, MNRAS, 481, 5216; Casewell et al., 2015, MNRAS, 447, 3218; 500~K, and emission lines from the brown dwarf chromosphere. For a Lee et al., 2020, MNRAS, 496, 4674; discussion of the modelling of these systems see Lee et al., (2020) Longstaff et al., 2017, MNRAS, 471, 1728 ; and Lothringer & Casewell (2020). Lothringer & Casewell, 2020, ApJ, accepted; Parsons et al., 2017, MNRAS, 471, 976.