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

Dino Hsu awarded the Friends of the International Center fellowship

Dino Hsu, a 4th-year graduate student in the Cool Star Lab, has been awarded the Friends of the International Center fellowship. This fellowship is awarded to graduate students and medical students that “in some way promote international friendship, understanding, and cooperation in a meaningful way.” Both international scholars and scholars conducting international research with international ties are eligible to apply. Dino was one of 30 applicants selected for this award across UCSD campus.

The Friends of International Center was founded in 1961 to “support international education; to foster friendship, understanding, and cooperation within the international community; and to create a meeting place on the UC San Diego campus for people who share these aims.” The Friends of the International Center fellowship is funded in partnership with UCSD Graduate Division.

Congratulations to Dino!

Burgasser publishes on issues of inclusion in Nature Astronomy

Cool Star Lab PI Adam Burgasser recently published two Comments in Nature Astronomy, in a special issue focused on diversity, equity and inclusion in physical science.

The first Comment, “Why I Teach Growth Mindset“, led by Adam, discusses the concept of mindset, and how fixed mindset can amplify the struggles of marginalized students, mentees and peers in Astronomy and Physics. He describes how he addresses fixed mindset in workshops and in the classroom, and provides a toolkit for hosting a Growth Mindset workshop.

The second Comment, “Toward inclusive practices with indigenous knowledge“, led by Aparna Venakatesan, describes models of partnership with indigenous communities that integrate collaboration with integrity. Inspired by the 2015 Indigenous Worldviews in Informal Science Education conference, examples featured include Cosmic SerpentA Hua He InoaEnVision MaunakeaNative Universe, and Maunakea Scholars. This comment is based on a more detailed white paper submitted to the Astro2020 Decadal Review.

The Comments and Perspectives contributed to the Nature Astronomy issue are free to read and download until early January 2020; copies of these articles are also available on request to Adam.

Dino Hsu awarded the Lattimer Award for Graduate Excellence

Cool Star Lab graduate student Chihchun “Dino” Hsu

Graduate student Chihchun “Dino” Hsu has been awarded the 2019-2020 Carol and George Lattimer Award for Graduate Excellence.

The Lattimer Award honors outstanding graduate students in UCSD’s Division of Physical Sciences (Chemistry & Biochemistry, Mathematics & Physics) who seek interdisciplinary approaches to problem solving and have a strong commitment to education, mentorship, and service. The award is named after local philanthropists Carol and George Lattimer, who have supported UCSD through charitable giving and advising. Carol volunteers at The Preuss School as a mentor, and George has served on the UC San Diego Foundation Board of Trustees.

Dino was one of two recipients of the Lattimer Award this year, and both recipients will be honored at a luncheon in the Spring term.

Congratulations Dino!

Ryan Low awarded Dean’s Undergraduate Award for Excellence

Cool Star Lab undergraduate researcher Ryan Low

Ryan Low, an undergraduate researcher in the Cool Star Lab, has been awarded the UCSD Division of Physical Sciences Dean’s Undergraduate Award for Excellence for 2019-2010.

Ryan is a Physics major and has been a member of the Cool Star Lab since Winter 2018. He has been leading the analysis of optical spectral of very low mass stars and brown dwarfs obtained with the Lick Observatory KAST instrument, and his work is uncovering some of our nearest (and previously overlooked) stellar neighbors.

The Dean’s Award recognizes undergraduate students for academic excellence throughout their undergraduate period. Students are nominated by faculty and advisors, and evaluated by selection committees in each department. Ryan is one of 33 students across the Division that were selected for this award this year. Ryan and his fellow recipients will be honored at a reception in May.

Congratulations Ryan!

Exploring TRAPPIST-1 Virtually

The cosmOcosm team, lead by CU Boulder graduate student Kevin Sweet and including faculty co-leads Tara Knight and Adam Burgasser, brought the newest development of the Sound Planetarium project to the 2019 TRAPPIST-1 conference in Liege, Belgium.

Kevin Sweet setting up the TRAPPIST-1 Virtual demonstration at the conference venue.

The Sound Planetarium project was originally developed to explore spatialized sound in “sonifying” astronomical data. Kevin has been exploring this process in virtual spaces, with a key innovation in making the sound process two-way – both hearing and speaking to our data!

Concept model of the TRAPPIST-1d surface

In honor of the TRAPPIST-1 conference, the team developed a virtual reality setting for the surface of TRAPPIST-1, one in which we can explore both the planetary system at large (viewing the orbits of the planets around its low-mass host star) and the view from the surface. Movement in the simulation was controlled by both “gaze” (where you looked) and basic voice commands.

Kevin explaining what the viewer sees in the virtual space.
Kevin “suiting up” a virtual explorer

We got a lot of great feedback from the folks who tried out the demonstration, which helps us explore new narratives for exploring this amazing system in virtual space.

Kevin discussing ideas with Artem Burdanov

Jessica Birky awarded NSF Graduate Fellowship

Jessica Birky, a graduating senior at UCSD and student participant in the Cool Star Lab’s SDSS Faculty and Student Team program, has been awarded the 2019 NSF Graduate Research Fellowship. Started in 1952, the NSF Graduate Research Fellowship Program (GRFP) supports outstanding graduate students in NSF-supported science, technology, engineering, and mathematics (STEM) Master’s and doctoral degrees at US institutions. In 2018, 2,000 awards were made among 12,000 applicants (roughly in line with other NSF funding programs), making the NSF GRFP one of the most competitive fellowship programs currently available.

Jessica designed her application around developing data-driven spectroscopic models for M dwarfs to determine detailed chemical abundances (<0.1 dex precision) for these common, low-mass stars. She has already started this work at UCSD and MPIA (collaborating with NYU’s David Hogg and UNC’s Andrew Mann) by analyzing SDSS APOGEE data with The Cannon. Jessica’s proposal aims to extend this work by using the thousands of kinematic pairs identified in Gaia, while separating out binaries and rapid rotators.

Jessica has less than a week to decide which graduate program she will be attending, but the fortunate institution will now have a fully-funded prize-winning scholar for the first 3 years!

Congratulations Jessica!

Citizen Scientist Discovers Dusty Debris Disk Around White Dwarf

Citizen scientist Melina Thévenot of Germany helped the Backyard Worlds/Planet 9 program discover a unique white dwarf with a dusty debris disk, and observations made by Cool Star Lab members with the Keck/NIRES instrument were critical its confirmation. The work, led by STScI astronomer John Debes, was reported in Astrophysical Journal Letters today.

Since 2017, the Backyard Worlds project has been engaging citizen scientists to search through data from NASA’s WISE mission to identify overlooked stars in the vicinity of the Sun. These have mostly been cold brown dwarfs, of which the project has found more than 1,000 too date – more than one a day! But it also picks up other dim, red things, in this case the white dwarf LSPM J0207+3331.

White dwarfs are normally “blue” due to their high surface temperatures (they are after all the cores of spent stars), but this white dwarf is surrounded by a complex disk of dusty debris, likely the result of the tidal disruption of an orbiting planet or asteroid (the same process is likely responsible for the rings around Saturn and other giant planets). This disk, heated by the white dwarf, glows in the infrared, allowing it to show up in WISE. While tidal debris disks aren’t new around white dwarfs (Cool Star Lab’s Carl Melis is specialist in this area), both the structure of this disk—which appears to be made of several distinct ring-like components—and that age of the white dwarf are surprising.

Very little was known about J0207+3331 prior to its identification by Melina in the Backyard Worlds program; only one prior paper had identified it as a high proper motion star. After an initial attempt to measure its spectrum was foiled by bad weather, it was therefore up to Cool Star Lab members Adam Burgasser & Jon Rees to get the necessary spectral data. Using the newly-commissioned NIRES instrument on Keck (during admittedly more not so great weather), Adam & Jon managed to measure the near-infrared spectrum of the source, which was largely consistent with a hot blackbody with slight uptick at the red end. This tiny bit of near-infrared excess, and the much greater mid-infrared excess in WISE photometry, could arise from several things, including an unseen brown dwarf companion (much like the first L dwarf ever discovered, GD 165B). However, we were able to show that the combined NIRES spectrum and WISE photometry were inconsistent with any white dwarf-brown dwarf combination, leaving a debris disk as the best model. (For once, Adam was happy not to find a brown dwarf!)

(Left) Analysis of our NIRES spectrum shows that any brown dwarf companion to J0207+0331 would be too small (blue dots) compared to model predictions to reproduce the observed excess, which rules out the binary model. (Right) instead, a model that includes a single white dwarf (orange line) and a debris ring system (red dashed line) can fit both spectral and photometric data (from Debes et al. 2019)

The NIRES spectrum, which contains several weak Hydrogen lines, allowed our team to determine the temperature and surface gravity of the white dwarf, and in turn its mass (0.69±0.02 solar masses) and age (3.0±0.2 billion years). By combining all of the data together, our team was also able to generate a model for the disk, which requires more than one “ring” of material with a total mass greater than a typical asteroid or comet. Both of these features are surprising: structure in the ring suggests there may be another body clearing a gap in the disk, or perhaps there have been two tidal disruption events that happened sequentially. This dust should also be cleared our “relatively” quickly (“relatively” = few 100 million years), requiring a “relatively” recent disruption.

Overall, the properties of J0207+3331 suggest that planetary systems may be continuously dismantled for billions of years after a star dies, which gives us a lot more time to study the innards of planets after tidal dissection (yech!). Moreover, the discovery of such an interesting, and relatively nearby system (only 45 parsecs, or 150 light-years, away), means that there may be many more such systems out there. Plenty of opportunity for future citizen scientists like Melina Thévenot!

Here are some links to press reports on this result:

NASA: https://www.nasa.gov/feature/goddard/2019/citizen-scientist-finds-ancient-white-dwarf-star-encircled-by-puzzling-rings

NOAO: https://www.noao.edu/news/2019/pr1904.php

UCSD: https://ucsdnews.ucsd.edu/pressrelease/astronomers_invite_citizens_to_crowd_source_new_worlds

ASU: https://asunow.asu.edu/20190219-discoveries-citizen-science-finds-ancient-white-dwarf-star

CNN: https://www.cnn.com/2019/02/19/world/white-dwarf-rings-discovery/index.html

Backyard Worlds Blog, where Melina Thévenot describes her discovery: https://blog.backyardworlds.org/2019/02/19/the-crystal-ball-white-dwarf/

Want to find your own new world? Give Backyard Worlds/Planet 9 a try! https://www.zooniverse.org/projects/marckuchner/backyard-worlds-planet-9

Chris Theissen is Awarded both Sagan and NSF Fellowships

Chris Theissen, former Boston University graduate student and postdoctoral researcher in the Cool Star Lab, has been awarded both the NASA Sagan Postdoctoral Fellowship and the NSF Postdoctoral Fellowship for 2019. The award is based on his proposed program: “Planetary Collisions around Low-Mass Stars: Constraining the Timescale for Collisions and Testing the Origin of the Kepler Dichotomy”, which builds from his graduate research investigating infrared excesses around relatively old M dwarfs. He will conduct this work with one of these fellowships under the mentorship of Prof. Quinn Konopacky here at UCSD.

The NASA Sagan Fellowship is one of the most prestigious postdoctoral awards in Astronomy, with an oversubscription rate of 16:1. Similarly, the NSF Postdoctoral Fellowship (which spans all science fields) is highly coveted. Chris has previously been an NSF Graduate K-12 STEM Fellow and Ford Foundation Fellow.

Congratulations Chris!