Saturday, September 7, 2013

A Very Dim and Cool Brown Dwarf

In a paper submitted to the astrophysical Journal on 25 August 2013, Kirkpatrick et al. (2013) present the discovery of an interesting nearby brown dwarf using data from NASA’s Wide-field Infrared Survey Explorer (WISE) satellite. This particular brown dwarf is very cold and very low mass; and it is designated as WISE 0647-6232. Observations by the Hubble Space Telescope confirm WISE 0647-6232 to be a Y dwarf. A brown dwarf is an object too small to be a star, but formed as stars are. Brown dwarfs are categorized according to the spectral types - M, L, T and Y, with Y dwarfs being the coldest class. In fact, Y dwarfs can be cold enough for water clouds to condense in their upper atmosphere. For Y dwarfs colder than ~350 K, water clouds may be a dominant atmospheric feature.

An artist’s rendition shows the relative sizes and likely appearance from left to right of the Sun, a very cool star, a warm brown dwarf, a cooler brown dwarf, and finally Jupiter; as how they might appear through an infrared camera. Credit: Robert Hurt.

Combining imaging data from the Spitzer Space Telescope, HST and WISE, the distance of WISE 0647-6232 is estimated to be 8.7±0.9 pc (28.4±2.9 ly). The best fit model for WISE 0647-6232 based on collected observational data indicates an effective temperature of 350 to 400 K and a mass of ~5 to 30 Jupiter-masses. A brown dwarf cools as it ages and the more massive a brown dwarf, the longer it will take to cool down to a given temperature. As a result, for WISE 0647-6232, a mass of ~5 Jupiter-masses suggests an age of ~100 million years, while a mass of 30 Jupiter-masses suggests an age of over 10 billion years.

By observing the motion of WISE 0647-6232 through space, it seems that this brown dwarf may be a member of a group of stars that are moving in the same direction. This group is known as the Columba group and its member stars form in the same star cluster ~30 million years ago. If WISE 0647-6232 is part of the Columba group, then an age of ~30 million years would imply an even lower mass of less than 2 Jupiter-masses.

The discovery of a single Y dwarf such as WISE 0647-6232 is important because only a handful of Y dwarfs are currently known, partly due to the faintness of such cold objects. Ground-based observations of Y dwarfs are very difficult because the Earth’s atmosphere is opaque to the thermal and mid-infrared wavelengths where these objects emit most of their light. As a result, space telescopes such as the Spitzer Space Telescope and HST serve as vital platforms in the study of Y dwarfs because they are not affected by the limitations imposed by the Earth’s atmosphere. Using space-based telescopes to study Y dwarfs can, for instance, search for transiting exoplanets around these brown dwarfs and observe the evolution of cloud features via photometric monitoring.

The discovery of WISE 0647-6232, a Y dwarf at a distance of less than 10 pc, is significant because it helps improve our understanding of the Y dwarf space density (i.e. number of Y dwarfs per volume of space). An accurate Y dwarf space density can reveal the cut-off for the lowest mass objects that can still form in the same way stars do. In addition to WISE 0647-6232, other notable Y dwarfs include WISE 1828+2650 - one of the coldest known brown dwarf or free floating planet with an effective temperature in the range 250 to 400 K (Beichman at al., 2013); and WISE 1541-2250 - a nearby brown dwarf located ~2.8 pc away and has an estimated temperature of ~350 K (Kirkpatrick et al., 2011).

- Kirkpatrick et al. (2013), “Discovery of the Y1 Dwarf WISE J064723.23-623235.5”, arXiv:1308.5372 [astro-ph.SR]
- Beichman at al. (2013), “The Coldest Brown Dwarf (Or Free Floating Planet)?: The Y Dwarf WISE 1828+2650”, arXiv:1301.1669 [astro-ph.SR]
- Kirkpatrick et al. (2011), “The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE)”, arXiv:1108.4677 [astro-ph.SR]