Discovery of a Nearby Star-Brown Dwarf Binary

The nearest star systems to the Sun are some of the most heavily studied, as their proximity makes them brighter and easier to observe. Moreover, nearby systems can be studied at finer resolution than distant ones, making it easier to detect astrometric motion (parallax, proper motion, orbital motion), close companions, and even circumstellar structures such as disks and jets.  As astronomers probe ever cooler stars and brown dwarfs, we are constantly finding new neighbors, such as the recently discovered L dwarf + T dwarf binary Luhman 16AB (3rd closest to the Sun) and the frigid Y dwarf WISE J0855-0714, both around 2 pc (6 lightyears) away.

One of the recent nearby star discoveries is WISE J0720-0846 (Figure 1), uncovered by Ralf-Dieter Scholz in a cross-match of the WISE and 2MASS surveys.  At ~7 pc, this apparently cool late M or L dwarf is an exciting new addition to the Solar Neighborhood. So of course we had to get a peek at it.

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Figure 1: WISE J0720-0846 is the red source at the center of this image, a combination of Digital Sky Survey blue, red and infrared plates. The source appears rainbow colored because it moved between imaging epochs. (Image generated with IRSA Finderchart)

Spectral Evidence of a Brown Dwarf Companion

In December 2013, we obtained a low-resolution near-infrared spectrum of WISE J0720-0846 with the SpeX spectrograph, shown in Figure 2. While this source looks like a normal late M dwarf, we noticed some intriguing peculiarities: a little excess flux at 1.25 µm and a “dip” at 1.62 µm that isn’t commonly seen in M dwarf spectra. Based on our previous experience, we suspected that this nearby star was probably harboring a brown dwarf companion.  Indeed, a spectral template combining an M9 dwarf with a T5 dwarf provides an excellent fit to the observed data.

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Figure 2: (Top) Comparison of the SpeX spectrum of WISE J0720-0846 (black line) to the M9 LHS 2924 (red line). There are slight differences that hint at a brown dwarf companion. (Bottom) Best binary template fit (purple line), a combination of LHS 2924 (red line) and the T5 2MASS J0407+1514 (blue line). Note how the 1.6 µm region is reproduced almost exactly.

Excited at the prospect of finding a very nearby star-brown dwarf binary, we set all of our resources in action:

  • Low resolution optical spectroscopy to determine the spectral type of the primary, which turned out to be an M9.5 dwarf;
  • High resolution optical and near-infrared spectroscopy to search for radial velocity tugs by the brown dwarf companion;
  • High resolution imaging with the Keck Laser Guide Star Adaptive Optics system to try to directly image the companion; and
  • Photometric monitoring with the TRAPPIST telescope over several weeks to see if we might pick up a transit of the companion or other variations.

Companion Confirmed?

A first look at our Keck images, taken in marginal conditions, revealed no obvious companion.  However, we dug a little deeper, subtracting the primary star from itself, and noticed a faint little speck just 0.14 arcseconds to the west, corresponding to a separation of 0.8 AU (Figure 3).

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Figure 3: (Left) High resolution Image of WISE J0720-0846; the image is 2 arcseconds across. (Right) After rotating and subtracting the image on left, a faint candidate companion is revealed.

Is this the companion? It’s hard to tell given how faint the speck is and how close it is to the primary target, but it has the same relative brightness we expect for a T dwarf companion based on our spectral analysis. Our high resolution spectroscopic observations failed to detect any shifts in motion caused by the gravitational pull of a companion, but this only rules out companions closer than 0.3 AU, 3 times closer in that the speck we detect.  We plan to image this source again to validate this companion and get better constraints on the relative brightnesses, and we will continue spectroscopic monitoring for longer-term radial velocity variations.

Cool Facts about WISE J0720-0846

Our observations revealed some interesting features about the WISE J0720-0846 system:

A fast-moving, old system: Scholz found that WISE J0720-0846 was a relatively slow-moving source across the sky, so we were surprised to find that our high resolution spectroscopy shows this system to be moving rapidly away from us at a speed of 83 km/s (186,000 miles per hour!).  When we transform its 3D motion into a Galactic orbit (Figure 4), we find that the motion of WISE J0720-0846 is pretty eccentric, moving between to outer and inner parts of the Milky Way, right down to the outer edge of the bulge. Such orbits are common for older stars whose orbits start off circular, but over time are perturbed by the spiral arms or encounters with giant molecular clouds. This motion tells us that WISE J0720-0846 is a mature system, probably a few billion years or older.

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Figure 4: The billion-year Galactic orbit of WISE J0720-0846 based on its measured 3D motion. A schematic of the Galaxy is indicated by shaded colors, including spiral arms (lightest grey), bulge (medium grey) and bar (dark grey). The Sun is indicate by the large solid point at (-8.5,0) kiloparsecs. 

 

An active, fast-rotating primary: In our optical data, we see strong hydrogen emission at 656 nm, a sure sign of magnetic activity.  The emission actually varied by over a factor of 10 during the few months we observed WISE J0720-0846. Our TRAPPIST light curve also shows mini-flares, with 2-7% bursts in brightness (Figure 5). Magnetic activity is a common feature of young stars, but tends to decline over time. So if WISE J0720-0846 is old, why is it so active?

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Figure 5: Four mini-flares captured in our TRAPPIST monitoring observations.

One of the main drivers of magnetic activity is rotation, which powers the dynamo that generates magnetic fields in stars. At it turns out, WISE J0720-0846A is a pretty fast rotator. Our high resolution spectroscopy sets a rotation period limit of less than 15 hours; for comparison, the Sun rotates once every 25-35 days (depending on latitude). Apparently, this source hasn’t spun down like most stars do, a common feature of very late M dwarfs. Unfortunately, we didn’t see any significant periodic variability in our TRAPPIST data to pin down the rotation rate.

 

Even closer than we thought: The TRAPPIST observations also provided positional measurements that we could use to improve the parallax and proper motion of this source (Figure 6).  We found that WISE J0720-0846 is only 6±1 pc (20 light-years) away from the Sun, closer than previously surmised. This makes it the 4th closest late M dwarf known.

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Figure 6: (Left) Motion of WISE J0720-0846 across the sky over the past 55 years. The black points indicate position measurements (with lines indicating uncertainties; the grey curlycue is the best fit model for the motion that takes into account the source’s parallax and proper motion. (Right): Zoom in on the position measurements made in this study.

 

WISE J0720-0846AB in Context

It is exciting to find such a nearby and tightly-bound star/brown dwarf binary system to the Sun, since monitoring the position of both components over time will provide an accurate measure of the system’s mass in just a few years. It turns out, however, that such systems may be relatively common.  Of the 13 late-M dwarf primaries within 10 pc of the Sun (including WISE J0720-0846), two have T dwarf companions, the other being SCR 1845-6357AB at 3.8 pc.  Surprisingly, these are the only two binaries in this select sample! Is this a fluke of small number statistics? Perhaps, but our calculations suggest that at least 25% of late M dwarf binaries have a brown dwarf as a companion, not a star. And since cool M dwarfs are the most common type of star in the Galaxy, these systems may be productive hunting grounds for studying the nearest brown dwarfs to the Sun.

 

This research has been accepted for publication in Burgasser et al., 2014, Astronomical Journal, in press (arxiv/1410.4288)

 

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