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

First SPLAT Publication: An In-depth Analysis of GJ 660.1AB

Screen Shot 2016-01-20 at 10.14.17 PMMorehouse College undergraduate Christian Aganze has led the first result to be published from the SpeX Prism Library Analysis Toolkit (SPLAT) project: an in-depth analysis of the M-dwarf binary system GJ 660.1AB.

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Spectral binaries of brown dwarfs

Screen shot 2013-08-22 at 2.02.36 AMHow do brown dwarfs form? Some theories point to a star-like birth, accreting material from a molecular cloud, while some others point to a planet-like formation from a pre-stellar disk. Either way, the essential mechanisms for brown dwarf formation remain under debate by theorists. Given the astronomical timelines of star formation (1-10 million years), we cannot witness the formation process in action, but we can study its consequences on the statistical properties of the systems created.

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Adam Burgasser Awarded Visiting Professorship at the Instituto de Astrofísica de Canarias

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Cool Star Lab PI will be spending Fall 2014 in the Canary Islands, but it won’t quite be a vacation.  He has been awarded a Visiting Professorship to the Instituto de Astrofísica de Canarias (IAC) in La Laguna, Tenerife, through the Campus Atlántico Tricontinental program at La Laguna University, aimed at fostering collaborations between Europe, Africa and the Americas. Adam will work with Nicolas Lodieu, Zhenghua Zhang and Rafael Rebolo at the IAC, examining the spectral and multiplicative properties of ultracool subdwarfs.

A talk given by Adam at the IAC can be viewed at http://iactalks.iac.es/talks/view/672.

Orbital Artwork Earns Award

We normally think of orbits as the paths of satellites going around the Earth, or planets going around their host star, in both cases caused by the gravitational attraction between the two bodies. But the stars themselves also orbit within and around our collective systems of stars, the Milky Way Galaxy. In this case, the gravitational force is a cumulative attraction distributed among other stars, gas, dust and dark matter in the Galaxy, the last making up about 95% of the mass of our Galaxy. While we don’t have the longevity to observe the roughly quarter-million-year orbits of stars like the Sun, we can predict them using basic laws of physics.

In 2009, Adam was investigating the kinematics and Galactic orbits of several dozen low-temperature subdwarfs (metal-poor stars that likely formed early in our Galaxy’s history), and generated a visualization of these orbits for a press release at the American Astronomical Society meeting in Pasadena.  Here’s one of the images from this release:

And here’s a movie generated for the press conference, tracing the path of one of the “diving” stars LST 1610-0040 (note that slow down as the star passes the region of the Sun and the radio broadcast sphere around Earth, inspired by the opening sequence in the movie Contact):

 

A few years later, Adam decided to look a larger sample, over 500 L-type dwarfs discovered by colleague Sarah Schmidt in the Sloan Digital Sky Survey. Schmidt had measured tangential (proper) and radial motions, and combining these with distance estimates it is possible to predict the orbits of these stars. Adam mapped the million-year motions of these stars as they travelled around the Galaxy to produce the following pictures:

Computed Galactic orbits of 500 L dwarfs as viewed from above the Galactic plane.

Computed Galactic orbits of 500 nearby L dwarfs as viewed from above the Galactic plane. Most are confined to the same annulus that the Sun occupies in its orbit, although there are some far flung stars that happen to be local today.

Computed Galactic orbits of 500 L dwarfs as viewed from along the mid-plane of the Galaxy.

Computed Galactic orbits of 500 L dwarfs as viewed from along the mid-plane of the Galaxy. Again, most are confined to this mid-plane, with a rare set of stars on highly inclined orbits taking them hundreds to thousands of light-years above and below the plane.

 

Computed Galactic orbits of 500 L dwarfs mapped into cylindrical coordinates (radius from the Galactic center and vertically through the Galactic poles).  The Sun resides at the densest concentration of orbit lines.

Computed Galactic orbits of 500 L dwarfs mapped into cylindrical coordinates (radius from the Galactic center and vertically through the Galactic poles). Here we discern distinct patterns of orbits, from “box-type” (constrained to a narrow range of radii and heights) to “comet-type” (almost purely radial) to “halo” (large deflections away from the plane. The Sun resides at the densest concentration of orbit lines.

 

These images earned 2nd prize in the 2011 Art in Science competition at UCSD, and was used as cover artwork for the 6th Annual Artfest 55.