Wednesday, 22 October 2014

The discovery of a Brown Dwarf companion to the star ζ Delphini.

Brown Dwarfs are objects intermediate to stars and planets in size; they are not large enough to fuse ordinary hydrogen in their cores, but are large enough to fuse the heavier isotope deuterium. These objects are thought to be quite variable in nature, with the largest and warmest resembling small Red Dwarf stars, but with cooler members of the group being more planet-like, and potentially having solid surfaces, planet-like atmospheres and even ice formation on their surface. Many of the Brown Dwarfs so far discovered are companions to true stars, though they are rarer as companions than both secondary stars and planets, with only a little over a hundred such companions observed since the first discovery in 1995. The larger number of companion stars discovered is easily explained, as such bodies tend to be easily visible, but as the number of planets discovered in other stellar systems has risen steadily it would have been expected that the number of Brown Dwarfs (typically larger, brighter and easier to spot than planets) would have kept pace, yet this has not happened.

In a paper published on the arXiv database at Cornell University Library on 30 September 2014, and accepted for publication in the Monthly Notices of the RoyalAstronomical Society, a team of astronomers led by Robert De Rosa of the Schoolof Earth and Space Exploration at Arizona State University and the School of Physics at the University of Exeter describe the discovery of a Brown Dwarf companion to the star ζ Delphini (Zeta Delphini) by the VAST (Volume-limted A-Star) Survey using the Near InfraRed Imager and Spectrometer (NIRI) and ALTitude conjugate Adaptive optics for the InfraRed (ALTAIR) systems on the Gemini North telescope, with additional observations made by the Canada France Hawaii Telescope and the MMT Observatory.

ζDelphini is an A3V star (a blue-white star considerably more massive than the Sun) 220 light years from Earth in the constellation of Delphinus. It is thought to be about 525 million years old with a mass of about 2.5 times that of the Sun and an effective surface temperature of 8336K (compared to 5778K for the Sun). No debris disk has been found around ζ Delphini (visible debris disks are associated with the early stages of planetary formation, though with an age of 525 million years it would be predicted that any such disk in the system would have dissipated) and the star has not been associated with any young stellar group (group of young stars with similar trajectories and ages, thought to share a common origin).

The new companion is named ζ Delphini B, making the original star ζ Delphini A. It is estimated to be between 40 and 55 times as massive as Jupiter, and to have an effective surface temperature of 1550K. ζ Delphini B is currently 912 AU from ζ Delphini A (i.e. 912 times as far from its parent star as the Earth is from the Sun), though it is thought to have an eccentric orbit with an average distance from the star of 907 AU, and an orbital period of about 10 000 years.

The Gemini/NIRI observation of the ζ Delphinisystem obtained on 2010 June 8 showing the location of the heavily saturated ζ DelphiniA, the substellar companion ζ Delphini B (zet Del B) and the seven background objects (BG 1-7) used in the astrometric analysis (indicated by the arrows). The image has been processed through a median filter to reduce the significant amount of scattered light from ζ Delphini A. The orientation and angular scale are given for reference; note that east and west are reversed in sky maps relative to ground maps as one is looking up rather than down, and that the scale is given in arcseconds (“) – the sky is divided into 360˚ (although only 180˚ is ever visible from any point on the Earth), with each degree (˚) divided into 60 arcminutes (‘) and each arcminute divided into 60 arcseconds. De Rosa et al. (2014).

There are currently three theories as to how a star could come to have a Brown Dwarf companion. The Brown Dwarf could potentially form as part of a large circumstellar disk, in a similar way to a planet, though the formation of a ~50 Jupiter-mass object at a distance of over 900 AU is improbable, both because this would require a remarkable amount of matter at the thin outer-edge of the circumstellar disk and because of the timescales that would be required for the object to coalesce (things tend to move slowly in the outer parts of stellar systems, as all motion is essentially driven by the gravity of the star, and the ζ Delphini system is only thought to be 525 million years old), however this possibility cannot be ruled out as it is possible that ζ Delphini B formed much closer to ζ Delphini A, and has subsequently migrated outwards due to gravitational interactions. Secondly it is thought possible that sometimes young pre-stellar cores can be fragmented, producing two bodies rather than one, though this is thought to be an extremely violent process, resulting in two bodies separated by distances measured in thousands of AU rather than hundreds. Finally it is possible that the two bodies formed separately and that ζ Delphini B was subsequently captured by the gravity of ζ Delphini A, though De Rosa et al. calculate that such an occurrence is unlikely, given the weak gravitational force exerted by the star at 900+ AU distances.

See also…

Brown Dwarfs are curious objects, intermediate between stars and planets. They lack the mass to fuse hydrogen in their cores like true stars, but are massive enough to fuse deuterium (a heavy isotope hydrogen, containing one proton and one neutron in its atomic nucleus), unlike planets. Brown Dwarfs therefore emit light in the infrared part of the spectrum, rather than simply reflecting light like a planet; though Brown Dwarfs within...

The KOI-13 system (Kepler Object of Interest system) comprises a pair of A-type White Dwarf stars 1630 light years from Earth, orbiting each to closely to be well differentiated. The larger of these, KOI-13α, has a mass 2.05 times that of the Sun, the smaller, KOI-13β, has...

M Class stars, or Red Dwarfs, are the most abundant stars in the Galaxy, and presumably the Universe. They are small (7.5-60% of the mass of the Sun) and cool (2300-3800 K, compared to 5778 K for our Sun), but can be very long-lived, as they burn their fuel slowly...

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1 comment:

  1. Surely you must have mistyped your thought, when you wrote that some brown dwarfs might have solid surfaces?