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

Speak Your Mind

This site uses Akismet to reduce spam. Learn how your comment data is processed.