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.

Cool Star Lab Contributes to Discovery of “Extreme” Metal-poor Brown Dwarfs

Cool Star Lab researchers, including current and former members, have contributed to the discovery of two brown dwarfs with unusual metal abundances, suggesting that they are part of a long-sought, ancient population of brown dwarfs formed early in the history of the Milky Way Galaxy.

Illustration of a brown dwarf with a Galactic backdrop by study co-author and citizen scientist William Pendrill

Most of the stars and brown dwarfs near the Sun contain elements in the same proportion as our host star. Hydrogen and helium making up about 98% of the Sun’s mass, and all other elements – which astronomers collectively refer to as “metals” – comprising a mere 2%. Yet a tiny fraction of stars – about 0.3% – are even more deprived of heavy elements. These are referred to as metal-poor “subdwarfs”, the “sub” relating to their position below the stellar Main Sequence on the Hertzsprung-Russell color-magnitude diagram. Subdwarfs are typically the most ancient stars in the Milky Way Galaxy, formed before massive stars were able to produce heavy elements that seeded later generations of stars. They mostly populate the halo of our Galaxy, an extended, roughly spherical extended distribution of stars (in contrast to the Galaxy’s “young disk”) formed either before the Galaxy had its current shape, or as old stars were flung out by dynamical encounters with giant molecular clouds. Subdwarfs therefore provide a window into the early star formation and dynamical history of our Galaxy.

Since brown dwarfs were first conjectured nearly 60 years ago and first discovered 25 years ago, scientists have wondered whether these objects could have formed in the metal-poor environment of the early Galaxy. The mechanism(s) that create brown dwarfs remains an open question, as these low-mass objects have difficulty forming by the standard model of star formation: the gravitational collapse and fragmentation of giant molecular clouds. A molecular cloud with fewer metals makes this process even more difficult. Nevertheless, metal-poor brown dwarfs exist. The first to be identified, a metal-poor late L dwarf 2MASS J0532+8246, was discovered by our team in 2003. Theoretical analysis of this source indicates it is right on the mass boundary between stars and brown dwarfs. In fact, evolutionary models predict that most brown subdwarfs, being old, should be much colder and be members of the T and Y spectral classes. While a few “modestly” metal-poor T subdwarfs have been identified over the past decade, we had not yet found the unambiguous brown dwarf equivalents of the Galaxy’s halo population.

This has now changed with the discovery of two exceptionally metal-poor T dwarfs by the citizen science 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. These objects are actually discovered by citizen scientists through the Backyard Worlds: Planet 9 Zooniverse platform, while the astronomers on the team confirm the discoveries through follow-up observations and analysis. This discoveries in this paper were made by citizen scientists Paul Beaulieu, Sam Goodman, William Pendrill, Austin Rothermich, and Arttu Sainio, who are all co-authors on the paper.

Sky images of the T subdwarf discovery WISEA J181006.18−101000.5 taken with WISE and NEOWISE in two epochs: mid-2010 and early 2017. The target is the orange source that moves slightly to the right between these epochs, a consequence of its high velocity and proximity to the Sun (from Schneider et al. 2020).

The two discoveries, WISEA J041451.67−585456.7 and WISEA J181006.18−101000.5, were both identified as high-priority sources for follow-up, as they have large proper motions (angular motion across the sky) and unusual colors. The Cool Star Lab team initially targeted WISEA J181006.18−101000.5 with the Keck/NIRES spectrograph in August 2019, but were unable to obtain a spectrum of the source in the crowded field of view. However, imaging data obtained in this run allowed collaborators Eric Mamajek and Federico Marocco at the Jet Propulsion Laboratory to obtain a spectrum on month later using the Palomar/TripleSpec spectrograph. WISEA J041451.67−585456.7 was observed with the Magellan/FIRE spectrograph in February 2020 by the study’s lead author Adam Schneider (Adam Burgasser was Co-PI on the construction of this instrument).

The infrared spectra of WISEA J041451.67−585456.7 (left) and WISEA J181006.18−101000.5 (right) taken with the Magellan/FIRE and Palomar/Triplespec instruments, in black; compared to known normal-metallicity and metal-poor L and T dwarfs. The fact that these known templates provided poor matches to the data suggested that the discoveries were truly unique sources (from Schneider et al. 2020).

The spectra obtained were very unusual. While the infrared spectra of T dwarfs are distinguished by strong absorption bands of methane and water, which can only form in atmospheres cooler than about 1200 ºK, these spectra were mostly smooth, with only weak hints of methane and water, and two strong absorption but unusually shaped features at 1.1 µm and 1.4 µm. 2MASS J0532+8246 had shown similar spectral peculiarities, so Cool Star Lab graduate student Roman Gerasimov generated a new set of low-temperature, metal-poor atmosphere models of using the PHOENIX code with the San Diego Supercomputer Center to see if these could reproduce the data. Indeed, the key features of the observed spectra were confirmed: the strong absorption bands at 1.1 µm and 1.4 µm are indeed caused by methane and water in a low-temperature atmosphere; while the smooth part of the spectrum is caused by enhanced absorption from molecular hydrogen found in unusually high-pressure atmospheres, a consequence of the lack of heavy elements in the gas. The best-fit models have metal abundances 10-100 times less than that of the Sun. Roman’s analysis therefore confirmed these sources as the first “extremely” metal-poor T dwarfs.

A comparison of the same spectra to metal-poor atmosphere models generated by Roman Gerasimov. While these do not provide perfect fits, they can explain the combination of strong absorption features at 1.1 µm and 1.4 µm and the smooth spectrum at longer wavelengths as arising from a depletion of heavy elements (from Schneider et al. 2020).

While these may be the first ancient, metal-starved brown dwarfs to be found, they are likely not the last. Backyard Worlds: Planet 9 citizen scientists continue to search the WISE and NEOWISE data for faint moving objects, and the recent addition of CatWISE data to the sample (containing over 12 years of infrared satellite monitoring) will make it easier to spot out faint moving sources. In addition, future deep survey data obtained with the Vera Rubin Observatory will expand our sensitivity to these intrinsically rare sources. Not only will these sources probe the early history of our Galaxy, they will also help us understand the role of elemental abundances on thermal evolution, gas chemistry, and cloud formation in cool brown dwarfs. More to come!

The discovery of these two metal-poor T dwarfs is reported in Schneider, Burgasser, Gerasimov, et al. 2020 “WISEA J041451.67-585456.7 and WISEA J181006.18-101000.5: The First Extreme T-type Subdwarfs?”, accepted for publication to the Astrophysical Journal. This paper is available on the arXiv at https://ui.adsabs.harvard.edu/abs/2020arXiv200703836S/abstract. Other coauthors on the study not mentioned above include Jonathan Gagne, Sam Goodman, Paul Beaulieu, William Pendrill, Austin Rothermich, Arttu Sainio, Marc J. Kuchner, Dan Caselden, Aaron M. Meisner, Jacqueline K. Faherty*, Chih-Chun Hsu*, Jennifer J. Greco, Michael C. Cushing, J. Davy Kirkpatrick, Daniella Bardalez Gagliuffi*, Sarah E. Logsdon*, Katelyn Allers, and John H. Debes. (* = current/former Cool Star Lab members). This work used the Extreme Science and Engineering Discovery Environment (XSEDE) Comet cluster at the San Diego Supercomputer Center (program AST190045), which is supported by National Science Foundation grant number ACI-1548562. The original NASA press release can be found at https://www.nasa.gov/feature/goddard/2020/two-bizarre-brown-dwarfs-found-with-citizen-scientists-help

Cool Stars Lab shines in 2016 Summer Research Conference

Screen Shot 2016-08-21 at 11.57.35 PM

The Cool Star Lab had a strong showing at this year’s 2016 UCSD Summer Research Conference, held August 11th around campus.  Ten students from the Lab presented results during the full day event, including six in Session #1 alone!

[Read more…]

Cool Star Lab Presents Work at 2016 Physics Education Research Conference

Mike Lopez, Isabela Rodrigues and Adam Burgasser presented their early analysis of an experimental Physics course at the 2016 Physics Education Research Conference in Sacramento, CA. This was the first PERC for all three researchers. In addition, former Cool Star Lab member Dianna Cowern was on hand for the overlapping American Association of Physics Teachers (AAPT) meeting presenting her work on the Physics Girl video series.

Screen Shot 2016-08-22 at 8.21.16 AM

Past and present Cool Star Lab members at the 2016 PERC; from left to right: Isabela Rodrigues, Dianna Cowern, Adam Burgasser and Mike Lopez.

The primary research presentations focused on analysis of an experiment conducted in Fall 2015 to implement Cooperative Problem Solving (CPS) in the large Physics 1A Introductory Mechanics course. The current course format, which is lecture-based, does not specifically build up students’ problem solving skills, skills that students often struggle with and which may benefit them more in their other majors and in their future careers. Inspired by work being done by Thomas Gredig at CSU Long Beach, I implemented a form of CPS as described in Heller & Heller (2010) as a flipped-format course, with online video lectures providing the primary instruction and class time primarily devoted to problem-solving skills and techniques.  To validate the model, Adam taught 9 sections of the CPS course with 1 (large) section of interactive lecture. The work presented at the PERC was a preliminary analysis of student outcomes.

[Read more…]

UCSD Selected as an SDSS FAST Site

The Cool Star Lab has joined the SDSS Faculty and Student Team (FAST) initiative, with a research program focused on analyzing high resolution spectroscopy of ultracool dwarfs with SDSS-APOGEE.

[Read more…]