A Brown Dwarf Benchmark

by Paul Gilster on January 21, 2014

Couple the Keck I 10-meter telescope on Mauna Kea with HIRES (the High-Resolution Echelle Spectrometer) and you get extremely high spectral resolution, making the combination a proven champion at finding planets around other stars. But it was when Justin Crepp (University of Notre Dame) and team followed up seventeen years of HIRES measurements with new observations using NIRC2 (the Near-Infrared Camera, second generation), mounted on the Keck II telescope with adaptive optics, that a nearby brown dwarf could be directly imaged.

HD 19467 B is a T-dwarf more than 100,000 times fainter than its host, a nearby star whose distance (roughly 101 light years) is well established. The team believes the discovery will allow scientists to establish benchmarks that will help define objects with masses between stars and planets. Says Crepp:

“This object is old and cold and will ultimately garner much attention as one of the most well-studied and scrutinized brown dwarfs detected to date. With continued follow-up observations, we can use it as a laboratory to test theoretical atmospheric models. Eventually we want to directly image and acquire the spectrum of Earth-like planets. Then, from the spectrum, we should be able to tell what the planet is made out of, what its mass is, radius, age, etc., basically all relevant physical properties.”

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Image: Direct image detection of a rare brown dwarf companion taken at Keck Observatory. Stellar speckles have been removed using PSF subtraction [used to study faint features around bright objects]. The companion is 100,000 times fainter than its host star in the K-band. Credit: Crepp et al./ 2014 APJ.

The work grows out of TRENDS (TaRgetting bENchmark-objects with Doppler Spectroscopy), a high-contrast imaging survey using adaptive optics to target older objects orbiting nearby stars. Imaging surveys like these have in general targeted young stars, but TRENDS focuses on older targets for which the existence of an unseen companion has been suggested by earlier radial velocity data. The paper on this work notes that our ability to see these older, fainter objects is improving with recent advances in high-contrast imaging techniques and hardware.

TRENDS looks for faint companions and thus far has uncovered a number of high mass ratio binary stars, a triple star system (HD 8375) and a white dwarf companion orbiting HD 114174. But here’s why Crepp speaks of using this brown dwarf discovery as a model. From the paper:

By connecting the properties of directly imaged companions to that of their primary star (such as metallicity and age), these objects serve as useful test subjects for theoretical models of cool dwarf atmospheres… Further, the combination of Doppler observations and high-contrast imaging constrains the companion mass and orbit, essential information that brown dwarfs discovered in the field or at wide separations by seeing-limited instruments do not provide.

I learned from the paper that while many nearby brown dwarfs have been discovered by surveys scanning large areas of the sky at optical, near-infrared and mid-infrared wavelengths, only a few are members of multiple systems. Even these are at large separations from the host star given that the glare of the primary makes it so difficult to see ultra-cold dwarfs in closer orbits. The significance of HD 19467 B, then, is that this is the first directly imaged T-dwarf orbiting a Sun-like star with a measured Doppler acceleration, meaning it will be among the first to have a dynamically measured mass. As studies continue, what Crepp and team have found should turn out to be an important benchmark in the investigation of how brown dwarfs evolve.

The paper is Crepp at al., “The TRENDS High-Contrast Imaging Survey. V. Discovery of an Old and Cold Benchmark T-dwarf Orbiting the Nearby G-star HD 19467,” The Astrophysical Journal Vol. 781, No. 1 (2014), p. 29 (abstract / preprint).

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{ 5 comments… read them below or add one }

Wojciech J January 21, 2014 at 15:08

Out of curiosity how would the sky look from a planet orbiting a brown dwarf? Does it emit enough light at all to provide any illumination?

torque_xtr January 21, 2014 at 17:21

“Does it emit enough light at all to provide any illumination?”

It depends very much on the spectral class. For the same liquid water-supporting total flux, L-type dwarfs would provide an incadescent bulb-like illumination in visible light, T-type would illuminate like a fire (earlier) or glowing embers (later), but possibly with some purple or deep blue tinge, especially for T-type. Y-type would be a huge, but barely visible deep red disk in the sky for human eye, or not visible at all, just the ink-black patch in the sky, blotting the stars out (but producing substantial heat, it would feel like standing next to an oven, and glowing fiercely in the night-vision goggles)

Gerry January 22, 2014 at 0:36

In follow-up to the previous two comments, in the case of the liquid-water supporting flux as described by torque_xtr, what would be the associated orbital radius of the planet receiving amount of heat from the Brown Dwarf “sun”?

The comparison of a T-Type dwarf’s illumination to a fire is interesting. One usually associates camp-fires or embers with dim, orange/yellow illumination, but I find myself trying to imagine how bright it would seem when those dim flames or embers were filling half the sky…

torque_xtr January 22, 2014 at 16:04

Quick calculations: the orbital distance/primary radius ratio is roughly proportional to the half of the square of the ratio of the effective temperatures of the primary and the planet, so for 2000 K L-dwarf it would be about 25 R(primary) (8 times the visible diameter of the full Moon), for 1000 K T-dwarf – only about 6R(primary) (nearly sixth of the sky), and then it rapigly goes below Roche’s limit for T(primary) around 700 K. This doesn’t include the absorption or radiative coefficients and the effects of the tidal lock, but it means the water HZ around the completely black for human eyes primary would be well within the zone of tidal disruption – except maybe only for the farthest reaches. Even 700K-body would glow slightly because it’s atmosphere would possibly be transparent in visible light down to the salt cloud layer, and this would be considerably hotter than T(eff)…

kzb January 24, 2014 at 8:55

Just to point out, there was a paper a couple of weeks ago on lightning in brown dwarf atmospheres. Apparently there is a lot of it going on, so maybe with a Y-dwarf you would see a disk lit up with constant electrical discharges….

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