SDSS 1557 (SDSS J155720.77+091624.6, WD 1554+094) is a

binary system A binary system is a system of two astronomical bodies of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples.) The most common ki ...

composed of a

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

and a

brown dwarf Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...

. The system is surrounded by a circumbinary debris disk. The debris disk was formed when a minor planet was tidally disrupted around the white dwarf in the past.

The brown dwarf companion

In 2011 it was found that the system did show Y- and J-band excess, which hinted at a companion. Follow-up observations with instruments on the

Gemini Observatory The Gemini Observatory comprises two 8.1-metre (26.6 ft) telescopes, Gemini North and Gemini South, situated in Hawaii and Chile, respectively. These twin telescopes offer extensive coverage of the northern and southern skies and rank among ...

and the

Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...

revealed the secondary, the brown dwarf SDSS 1557B, and a circumbinary disk around the binary. The researchers measured the

radial velocity The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity ...

changes with the help of the

Magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

absorption line at 4482 Å and found that a 66 brown dwarf orbits the white dwarf at around 0.7 , with the orbital period being around 2.27 hours. The irradiated brown dwarf also shows a

hydrogen-alpha Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is emit ...

emission line. Additional follow-up came with Hubble WFC3, using time-resolved spectrophotometry. SDSS 1557B is similar to

ultra-short period planets An ultra-short period (USP) planet is a type of exoplanet with an orbital period of less than one Earth day. At this short distance, tidal interactions lead to relatively rapid orbital and spin evolution. Therefore when there is a USP planet arou ...

and is likely

tidally locked Tidal locking between a pair of co-orbiting astronomical bodies occurs when one of the objects reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit. In the case where a tidally locked ...

. White dwarfs give off more radiation in the

ultra-violet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of the ...

than it is the case for

main-sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

stars. This leads to a higher UV-exposure for SDSS 1557B when compared to a regular

hot Jupiter Hot Jupiters (sometimes called hot Saturns) are a class of gas giant exoplanets that are inferred to be physically similar to Jupiter (i.e. Jupiter analogue, Jupiter analogues) but that have very short orbital periods (). The close proximity to t ...

. The fact that SDSS 1557B is tidally locked creates vast temperature changes in the dayside and nightside of the brown dwarf. The researchers found that the brown dwarf is inefficient at redistributing the heat from the dayside to the nightside. They also find that the nightside is likely dominated by

clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may c ...

and the dayside is likely dominated by opaque H⁻ and likely has a temperature inversion.

The circumbinary disk

The system was first suspected to be a white dwarf with a circumstellar disk in 2011 from K-band excess. The system also displayed high

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

abundances ( Ca, Mg, Si), showing that the white dwarf was polluted with planetary debris. The disk ring lies at around 3.3 , exterior to the

Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, ...

. The dust grains of the disk have a temperature of 1,100

Kelvin The kelvin (symbol: K) is the base unit for temperature in the International System of Units (SI). The Kelvin scale is an absolute temperature scale that starts at the lowest possible temperature (absolute zero), taken to be 0 K. By de ...

(K). The material from the disk crosses the gap between disk and white dwarf in streams. A process that is well known for binaries and seen in other binaries, such as CoRoT 223992193.

Past evolution of the system

The system formed at least 1.5 Gyr ago as a low-mass-ratio binary of a star (1.06–1.85 ) and a companion with a semi-major axis of less than one

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

(AU) in the past. The minor planet on the other hand had an orbit that was larger than a few AU. The brown dwarf was engulfed when the star became a

giant In folklore, giants (from Ancient Greek: ''wiktionary:gigas, gigas'', cognate wiktionary:giga-, giga-) are beings of humanoid appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''gia ...

, an evolutionary stage known as

common envelope In astronomy, a common envelope (CE) is gas that contains a binary star system. The gas does not rotate at the same rate as the embedded binary system. A system with such a configuration is said to be in a common envelope phase or undergoing comm ...

. Around 33 Myrs ago the common envelope was ejected, forming a low-mass

Helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

core white dwarf. This formed the present binary, called SDSS 1557. A minor planet, likely an asteroid larger than 4 km, with a mass of at least 10¹⁴ kg survived the giant phase of the star. It was scattered towards the binary and tidally disrupted by the white dwarf when it crossed the Roche radius. The resulting debris cloud became the disk we see today.

References

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The brown dwarf companion

The circumbinary disk

Past evolution of the system

See also

References