How do a white dwarf and a brown dwarf live together? – Observations of an irradiated brown dwarf

Sarah Casewell (Uni. of Leicester) in collaboration, amongst others, with Paul Rimmer and Christiane Helling from the LEAP Group, recently published a paper in Monthly Notices of the Royal Astronomical Society (MNRAS) on the effects of irradiation in a close white dwarf-brown dwarf binary system.

Brown Dwarfs are sometimes called “failed” stars. Their interior temperature is too low to fuse hydrogen, which is the energy source of stars like our Sun. They don’t hold the conditions for hydrogen fusion. They have masses from Jupiter- mass to a couple of times of Jupiter-mass. Regarding their sizes and atmosphere they are the missing link between stars and planets, as they host surface temperatures of a couple of thousand K and observations indicate cloud formation on them. Stars with birth masses higher than 8 times the mass of our Sun (MSun) live a shorter, quicker life than their lower mass siblings. Stars with masses > 8 MSun end their lives as neutron stars or black holes. The final evolutionary stage of stars with masses less than 8 MSun, on the other hand, is to become a white dwarf. White dwarfs are Earth-size objects with the mass of the Sun or higher. They don’t fuse elements any more for energy production, instead, their interior is composed of degenerated matter that keeps them from collapsing inward under their own weight. They slowly lose all their energy and stop radiating.

Brown dwarfs' place in the "sequence" of stars and planets. (Credit: NASA)

Brown dwarfs’ place in the “sequence” of stars and planets. (Credit: NASA)

White dwarfs are the about the size of the Earth with masses of the Sun, which makes them incredibly dense objects. (Credit: NASA, S. Charbinet)

White dwarfs are the about the size of the Earth with masses of the Sun, which makes them incredibly dense objects. (Credit: NASA, S. Charbinet)

How can a white dwarf – brown dwarf binary system form? If the star formation is efficient enough, hence, if there is enough material in the contracting molecule cloud, two or more stars may form at the same time. If they are close enough that their gravitational forces hinder them to fly apart, they start to orbit each other and a binary star system has formed. Binary systems, where a star has a companion are very common. Our Sun is very unusual in that it doesn’t have a stellar companion. There is a huge variety in these binary systems, which excites many scientists. However, white dwarf-brown dwarf systems are extremely rare – only 5 have been confirmed to date! This is partly due to the fact that forming a brown dwarf in a close binary with a low-mass main sequence star (the white dwarf progenitor with M ~ MSun) is very difficult. The other possible explanation is, as the main sequence star evolves into the white dwarf, it expands during its red giant phase and engulfs the brown dwarf, causing the brown dwarf to spiral inwards. This process will only stop when the outer envelope of the star is ejected to leave the hot core, which is the white dwarf. The result is a beautiful planetary nebula with the dim, mostly invisible white dwarf in the center of it. At this point, the binary has ended up with a very short period, hence the brown dwarfs orbits the white dwarfs in a few hours.

An artists impression of the brown dwarf orbiting the white dwarf WD0137-349

An artists impression of the brown dwarf orbiting the white dwarf WD0137-349. (Credit: ESO)

These systems are tidally locked – where the same side of the brown dwarf always faces the white dwarf. As the white dwarf has a temperature of 16500 K, and the brown dwarf is only 1000 K, this irradiation heats up the day side. This continual heating of one side means we see photometric variability on the period of the binary in all wavelengths from the optical through to 8 microns in the mid- infrared. In the case of WD0137-349, the system we studied, the day side is 500 K hotter than the night side.

Photometric variability caused by the uneven irradiation of the 2 sides of the brown dwarf. [3.6] (black plus signs), [4.5] (blue circles), [5.8] (green diamonds) and [8.0] (red squares) μm light curves of WD0137-349 folded on the ephemeris of the system.

Photometric variability caused by the uneven irradiation of the 2 sides of the brown dwarf. [3.6] (black plus signs), [4.5] (blue circles), [5.8] (green diamonds) and [8.0] (red squares) μm light curves of WD0137-349 folded on the ephemeris of the system. (Casewell et al. 2015)

As the white dwarf is primarily emitting in the ultraviolet wavelengths, the irradiation on the brown dwarf’s atmosphere has the capability to cause unusual photochemistry. By using the hydrogen molecules present, the ultraviolet light can cause it to fluoresce via a pumping mechanism, or even form more exotic molecules such as H3+, a molecule which is commonly seen in the atmospheres of Jupiter, Saturn and Uranus and is associated with the heating caused by the aurorae in these objects.

We can study the effects of the irradiation in these systems to determine what the effects of irradiation on exoplanet atmospheres are likely to be. Brown dwarfs are extremely similar to hot jupiters in their atmospheric composition, and we imagine that in a planet bombarded by ultraviolet irradiation, similar effects due to the heating will occur.

 

For more details check out the original paper on ADS:

Casewell, S. at all. 2015, MNRAS, 447, 3218

The LEAP Group can be found here:

http://leap2010.wp.st-andrews.ac.uk/

leap-2010.eu

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