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95 Nearby Cool Brown Dwarfs Identified

The boundary between planet and star is hard enough to pin down without thinking of some recently discovered brown dwarfs that are cool enough to approach Earth temperatures. Yet worlds/stars like these are among the haul assembled by volunteers working data for Backyard Worlds: Planet 9, a citizen science project whose latest findings include 95 cool brown dwarfs in the Sun’s neighborhood, as reported in the Astrophysical Journal.

Despite a determined search, we’ve yet to find such an object closer than the nearest stars at Alpha Centauri. But 23 light years out — the distance of the closest of these brown dwarfs — is definitely close in galactic terms, and most of the brown dwarfs tracked in the new work are between 30 and 60 light years from Earth. That makes sense, for objects like these are faint enough that identifying them at greater range is all but impossible.

The data used in the brown dwarf collection come from a range of observatories including W. M. Keck, Mont Mégantic, Las Campanas, Kitt Peak and Cerro Tololo. Space-based data from WISE (Wide-field Infrared Survey Explorer) were also valuable, as were follow-up observations from the Spitzer Space Telescope providing photometric confirmation. Low temperature brown dwarfs like these build our catalog while also clarifying gaps in the low-temperature population.

Jackie Faherty (American Museum of Natural History) is a co-author of the paper, whose lead author is Aaron Meisner (NSF NOIRLab). Faherty places the work in context, while giving a nod to the Backyard Worlds participants:

“This paper is evidence that the solar neighborhood is still uncharted territory and citizen scientists are excellent astronomical cartographers. Mapping the coldest brown dwarfs down to the lowest masses gives us key insights into the low-mass star formation process while providing a target list for detailed studies of the atmospheres of Jupiter analogs.”

Image: Artist’s impression of one of this study’s superlative discoveries, the oldest known wide-separation white dwarf plus cold brown dwarf pair. The small white orb represents the white dwarf (the remnant of a long-dead Sun-like star), while the brown/orange foreground object is the newly discovered brown dwarf companion. This faint brown dwarf was previously overlooked until it was spotted by citizen scientists, because it lies right within the plane of the Milky Way. Credit: NOIRLab/NSF/AURA/P. Marenfeld. Acknowledgement: William Pendrill.

We can also thank NOIRlab’s Community Science and Data Center (CSDC), which made the large survey sets available through its Astro Data Lab science platform. The web portal makes matching data sets from a variety of observatories available to citizen scientists. Meisner adds:

“The technical burden of downloading billion-object astronomical catalogs is typically insurmountable for individual investigators—including most professional astronomers. Thankfully, the Astro Data Lab’s open and accessible web portal allowed Backyard Worlds citizen scientists to easily query massive catalogs for brown dwarf candidates.”

The authors make the point that some of the objects in this study will be potential targets for follow-up spectroscopy by the James Webb Space Telescope:

Among our most exciting discoveries are new candidate members of the 10 pc sample, two objects moving faster than 200”/yr, three T type subdwarf candidates, five Y dwarf candidates, and a new T8 plus white dwarf co-moving system. Our Y dwarf candidates begin bridging the gap between the bulk of the Y dwarf population and the coldest known brown dwarf, making them potential targets for JWST spectroscopy. Backyard Worlds is actively pursuing additional ground-based follow-up of the discoveries presented in this study, especially spectroscopy where feasible. While this work’s new brown dwarf candidates already demonstrate the power of citizen science for mapping the solar neighborhood, these objects make up only a small fraction of Backyard Worlds moving object discoveries to date. As NEOWISE continues scanning the sky, Backyard Worlds will endeavor to search all of its newly delivered data for yet more cold and close neighbors to the Sun.

Backyard Worlds volunteers have discovered more than 1,500 stars and brown dwarfs near the Sun. The current batch represents some of the coldest brown dwarfs in that collection. More than 100,000 citizen scientists are involved in the project, a network that examines telescope images to identify the movements of nearby dwarfs and planets. Putting human eyes on large data sets is integral to the search for rare objects, an approach astronomers will exploit with data gathered by the Vera C. Rubin Observatory (VRO). Now under construction in the Chilean Andes, the instrument will survey the southern hemisphere sky every three nights over a ten year period.

The paper is Meisner et al., “Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project,” accepted at the Astrophysical Journal (preprint).

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Comments on this entry are closed.

  • Melina Thévenot August 19, 2020, 14:48

    Thanks for sharing our work :)

    • Paul Gilster August 19, 2020, 20:06

      Absolutely. And thank you for another great job.

  • Henry Cordova August 19, 2020, 16:26

    Typo alert?

    The phrase “two objects moving faster than 200”/yr”, is probably a typographical error. As far as I know, the largest known stellar proper motion is Barnard’s Star, 10.3”/yr. Not only is Barnard’s Star very close to us, (distance = 1.8 pc), it is a halo star zipping through the galactic disc at a high relative velocity to the Local Standard of Rest.

    The ApJ monograph abstract linked to in the article,

    https://arxiv.org/abs/2008.06396

    does contain the following, (much more reasonable), phrase and mu value,

    “our fastest-moving discovery is WISEA J155349.96+693355.2 (total motion ~2.15″/yr)”.

    Any stellar proper motion of more than an order of magnitude greater than Barnard’s Star would certainly be extraordinary!

    • Paul Gilster August 19, 2020, 20:06

      Good catch, Henry!

  • Michael Fidler August 19, 2020, 23:40

    So are these planets or stars? The 4th star system from our sun is a Y2 Sub brown dwarf or should we call them Infrared Dwarf? WISE 0855−0714 is in the same group as the 95 Infrared Dwarfs with temperature between 225-260 Kelvin (−55 to 8 °F) at only 7.27 light-years. Its mass thou is estimated to be between 3 to 10 MJup which is lower then what the International Astronomical Union considers a brown dwarf at a mass above 13 MJup.
    Now any planets around these objects could have a pleasant temperature do to internal heating from Io type interactions with its primary or tidal heating when reaching near to the roche limits. Super earths could develop a habitable zone temperatures on there surface from volcanic and interior heat with mild atmospheric temperatures.
    WISE 0855−0714 would be a good place to look at radio and aurora activity over long periods and any other signatures that may indicate active super earth or earth size planets in orbit around it. Could life develop on self warming ocean planets over billions of years? What of WISE 0855−0714 itself, its temperature beneath the H2O clouds could be very mild, the James Webb Space Telescope should tell us a lot more in the coming years.

    • Michael Fidler August 20, 2020, 9:52

      Looks like WISE 0855−0714 is also a halo star?

      WISE 0855−0714 has the third-highest proper motion (8118±8 mas/yr) after Barnard’s Star (10300 mas/yr) and Kapteyn’s Star (8600 mas/yr).

      https://en.wikipedia.org/wiki/WISE_0855%E2%88%920714

      • Sam Goodman August 20, 2020, 11:51

        A lot of the high proper motion is simply due to being very close-by, the approximate tangential velocity based on the proper motion and distance I work out to be ~86 km/s, that’s nowhere near Halo velocities. Barnard’s Star is also not clearly Halo.

    • Bruce D Mayfield August 20, 2020, 12:33

      Good question Michael, since classification of things is a fundamental function of scientific endeavors. Often what an object is called depends on what the research team’s target of study or aims are. Thus if the group is looking for exoplanets, a 15 J body will be called a planet, while another group looking for brown dwarfs might call an 11 J body a BD.

      13 Jupiters should be the cut line here, since that is the minimum mass needed for fusion of deuterium to begin. The cut line between BD and proton fusing, main sequence Red Dwarf stars is less clear cut. Here’s what wikipedia says about the definition of BDs:
      “A brown dwarf is a type of substellar object that has a mass between those of the heaviest gas giant planets and the least massive stars, i.e. about 13 to 75–80 times that of Jupiter, or about 2.5×10²⁸ kg to 1.5×10²⁹ kg. Below this range are the sub-brown dwarfs, and above it are the red dwarfs. Wikipedia”

  • Sam Goodman August 20, 2020, 11:28

    This is one area where the IAU confuses more than it helps. For a start having had the opportunity to talk with a lot of brown dwarf researchers precisely zero have used the term “sub-brown dwarf”; if distinguished at all from other brown dwarfs the generic “planetary mass object” seems preferred for anything in this ambiguous range. Secondly a lot of the researchers seem disinclined to the somewhat arbitrary mass definition the IAU has set, preferring instead to classify based on formation.

    There’s growing evidence that brown dwarfs are formed in a “star-like” process while giant planets form via accretion, e.g.;
    https://arxiv.org/abs/1904.05358

    While other research shows that based on the field sample the “brown dwarf” minimum mass is somewhere below around 5 Mjup.

    https://arxiv.org/abs/1812.01208

  • Henry Cordova August 20, 2020, 13:51

    I feel compelled to make a few remarks about proper motion for the benefit of those readers who may not be familiar with astronomical jargon and general galactic dynamics.

    Proper motion (PM) is a star’s amount of angular motion relative to the Earth-Sun system ACROSS the line of sight, measured in units of seconds of arc per year. It is determined by astrometric methods. Motion IN the line of sight is called “radial velocity” (RV) and is usually listed in km/sec, positive if toward Earth, negative away. It is determined with spectroscopic (Doppler) measurements.

    PM is a function of the star’s true velocity (relative to us) and its distance from us. Its value is also influenced by the geometry of its orbit relative to us. Nearby stars often have larger proper motions than more distant ones, so that the PM is considered a (very) rough indicator of a star’s distance; indeed, when measuring stellar parallaxes first became possible, the most likely candidates for investigation were chosen from stars with known large proper motions.

    Most stars in the Solar neighborhood are galactic disc stars, (Population I) and share in the same general drift as the Sun’s orbit around the galactic nucleus. Their proper motions are small random variations in their orbital motion superimposed on the general galactic drift in the Solar neighborhood, what is termed the Local Standard of Rest or LSR. Halo stars (Pop II) orbit the Milky Way in random elliptical orbits, and intersect the galactic disc at acute angles at very high apparent velocities. Very often, these high proper motions identify them as a separate stellar population altogether, not just from their kinematic behavior but from their own intrinsic properties. The observation that stars of different kinematics also have different intrinsic properties is a clue as to how the galaxy is structured and how it evolved.

    So for example, Alpha Centauri (dist = 4.4 ly), another disc star, has a PM of 3.7″/yr and an RV of about -20 km/sec. Barnard’s Star (dist = 6.0 ly), a visitor from the Halo, is passing through our neighborhood at a PM of 103″/yr and an RV of -111 km/sec. Its pretty safe to speculate these stars are fundamentally different and probably have very different histories. This concept of stellar populations is key to understanding galactic evolution.

    • Henry Cordova August 20, 2020, 15:45

      In response to Mr Goodman’s comments that Barnard’s star is NOT a Halo star, I did some further research that indicates he may indeed have a point. According to the Wikipedia entry on this object;

      “Barnard’s Star seems to be typical of the old, red dwarf population II stars, yet these are also generally metal-poor halo stars. While sub-solar, Barnard’s Star’s metallicity is higher than that of a halo star and is in keeping with the low end of the metal-rich disk star range; this, plus its high space motion, have led to the designation “intermediate population II star”, between a halo and disk star. Although some recently published scientific papers have given much higher estimates for the metallicity of the star, very close to the Sun’s level, between 75–125% of the solar metallicity.”

      My speculations were based solely on the observation that Barnard’s Star has both the highest proper motion and radial velocity by far of all 53 systems within 15 pc of Sol. (RASC Observer’s Handbook, 2020).

      This may be persuasive, but not conclusive. Perhaps we need to get that Daedalus probe up and running.