Spectroscopic Observations by Cool Star Lab Researchers Contribute to Expanding the Census of the Solar Neighborhood

2020 is the year of the US Census. Just as an accurate count of people is essential to understanding the composition and needs of the nation, an accurate count of stars is essential to understanding the composition and properties of the Milky Way Galaxy. And just like our Census, we have to put in additional effort to make sure everyone – and every star – is counted.

One project that is working hard to assure that every star is counted is the Backyard Worlds: Planet 9 project. Backyard Worlds: Planet 9 uses data from NASA’s Wide-Field Infrared Survey Explorer (WISE) and Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) satellites to uncover faint, cool, nearby objects. And just like the Census, the success of this count hinges on the efforts of citizens. In this case, it is the over 100,000 citizen scientists from the around the world who have examined trillions of pixels of telescope images to identify the subtle movements of nearby star and brown dwarf candidates that distinguish them from an overwhelming number of background stars.

Sky map of the 96 nearby, cool brown dwarf discoveries made by Backyard Worlds, in Galactic coordinates (the Galactic plane runs across the middle). Potential star-brown dwarf companion systems are indicated in green, while rejected candidates are indicated in red. candidates (Table 1) for which no Spitzer counterpart was found (from Meisner et al. 2020).

This effort has led to the discovery of nearly 100 nearby, cool brown dwarfs, never before observed, as reported in the recently published paper “Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project” by Meisner et al.

Brown dwarfs are small “stars” incapable of fusing hydrogen, and are particularly hard to find because of their small sizes and low temperatures. Nevertheless, these nearly invisible astronomical neighbors are fundamental to understanding the properties of brown dwarfs and giant exoplanets, the process of star formation, and the history of our Milky Way Galaxy.

NIRES & FIRE spectra reported in the Meisner et al. (2020) study, with the T8 spectral standard 2MASSI J0415195−093506 overplotted in red. All of these sources are among the coldest brown dwarfs currently known, and new members of the immediate Solar Neighborhood (from Meisner et al. 2020).

The Cool Star Lab is contributing to this effort through the acquisition of near-infrared spectra of candidates using the Near-Infrared Echellette Spectrometer instrument, or NIRES, on the 10-meter W. M. Keck Observatory on Maunakea, Hawaii. Keck/NIRES is essential to this work, as low-temperature brown dwarfs emit most of their light at near-infrared wavelengths, while Keck provides the aperture to detect this light. The spectra, obtained in this case by Cool Star Lab members Christian Aganze, Roman Gerasimov, and Christopher Theissen, allow the team to confirm the Backyard World candidates as cool, nearby brown dwarfs, as well as classify and characterize their physical properties. Spectral data were also acquired with the Magellan/FIRE spectrograph, an instrument Cool Star Lab PI Adam Burgasser helped construct at MIT. In addition, archival and catalog data were made available through NSF’s NOIRLab Astro Data Lab science platform, which proved critical to allowing Backyard World citizen and professional scientists to search through billion-object catalogs to characterize the new discoveries.

Three epochs of infrared images of the common proper motion white dwarf + brown dwarf binary system LSPM J0055+5948 (top right) and WISEU 0055+5947 (center, in red). The brown dwarf stands out with its strikingly orange color in this infrared color map, reflective of its very low temperature (from Meisner et al. 2020).

One of the key discoveries reported in this paper is a new wide cool brown dwarf companion to the white dwarf LSPM J0055+5948, a 0.46 solar-mass remnant of a long-dead star about 23 parsecs from the Sun. The newly-discovered companion, WISE J0055+5947 is separated by 400 Astronomical Units from the white dwarf. The Keck/NIRES data allows us to classify this companion as a T8 dwarf, making it among the coolest of brown dwarfs known. Brown dwarf companions to stars are particularly important for testing brown dwarf models, since the stellar primary can provide the age and composition of the entire system. In this case, optical spectroscopy of the white dwarf allowed us to determine its temperature and surface gravity, a cooling time (time since stellar death) of 5 billion years, and the identify of its progenitor as a roughly solar-type star. Adding in the lifetime of the progenitor implies a total age of about 10 billion years. This makes WISE J0055+5947 one of the few brown dwarfs with a well-determined age – and also one of the oldest – an ideal source for testing brown dwarf models.

Artist rendering depicting the white dwarf + brown dwarf binary LSPM J0055+5948 (small white orb at left) and WISEU 0055+5947 (purple foreground sphere at right). The companion was previously unknown until it was spotted by citizen scientists, because it lies in direction of the Milky Way, shown as a dense band of background stars. This image was created by William Pendrill, a Backyard Worlds citizen scientist and co-author on the Meisner et al. (2020) study.

There are many other exciting sources in this discovery pool, including brown dwarfs closer than 10 pc from the Sun; three brown dwarfs moving at over 200 km/s relative to the Sun, that may be halo brown subdwarfs; additional brown dwarf candidate companions; and five candidate Y dwarfs which would have temperatures that are similar to Earth and potentially host water clouds. These temperatures were inferred from astrometric and photometric measurements made by the recently decommissioned Spitzer Space Telescope.

In addition to the 20 citizen scientists named as coauthors on the paper, the full collaboration included research teams from: NSF’s NOIRLab the American Museum of Natural History, Caltech/IPAC, Arizona State University, Université de Montréal, NASA Goddard Space Flight Center, University of California San Diego, University of Leicester, European Space Agency, Space Telescope Science Institute, City University of New York, Bucknell University, University of Oklahoma, Universidad Nacional de Córdoba-CONICET and the University of Central Florida. The work was supported by funding from NASA through the Astrophysics Data Analysis Program and Hubble Fellowship Program.

The paper “Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project” by Meisner et al. (2020) is scheduled for publication in the Astrophysical Journal.

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