Astrobites at AAS 235: Day 4

Welcome to the winter American Astronomical Society (AAS) meeting in Honolulu, Hawai’i! Astrobites is attending the conference as usual, and we will report highlights from each day here. If you’d like to see more timely updates during the day, we encourage you to search the #aas235 hashtag on twitter. We’ll be posting once a day during the meeting, so be sure to visit the site often to catch all the news!


First light spectrum of 51 Pegasi as captured by NEID on the WIYN telescope. [Guðmundur Kári Stefánsson/Princeton University/Penn State/NSF’s National Optical-Infrared Astronomy Research Laboratory/KPNO/AURA]

Plenary Lecture: Diet of the Stars: Mass Accretion and Mass Loss (by Ellis Avallone)

“A picture is worth a thousand words, but a spectrum is worth a thousand pictures,” says Dr. Andrea Dupree (Center for Astrophysics | Harvard & Smithsonian). The first plenary of the day was on stellar spectroscopy, Dupree’s specialty, and the power it gives us in understanding stellar processes. 

The talk focused on two main topics in stellar astrophysics: mass accretion and mass loss. There are many outstanding questions related to these processes, like their effects on the star, the star’s environment, and the planets around the star. However, answers to these questions remain conjectures until we can look at a star’s spectrum. 

TW Hya

An ALMA view of TW Hya, a young star surrounded by a protoplanetary disk. [S. Andrews (Harvard-Smithsonian CfA)/B. Saxton (NRAO/AUI/NSF)/ALMA (ESO/NAOJ/NRAO)]

Dupree then dove into discussing accretion. After a molecular cloud collapses to form a stellar core, an accretion disk forms around the young star. This accretion disk is threaded with the star’s magnetic field and material from the accretion disk is funneled along the magnetic field lines onto the surface of the star. Although this process sounds simple, showing that this theory is correct requires measurements of accretion in action in young stars. Dupree then discussed a case where we’ve directly measured accretion in the young star TW Hya. This star is unique in that the accretion disk is facing us (i.e. we’re observing the pole of the star). Because of this, astronomers have been able to make magnetic field maps of the pole and determine that the radial magnetic field varies with time. 

To understand the accretion process in this system, Dupree then launched a campaign to observe TW Hya, a young star, in multiple wavelengths. Using spectra obtained by the Chandra X-Ray Observatory, Dupree was able to obtain full plasma diagnostics for the young star’s atmosphere and follow accretion onto TW Hya through time. “Veiling” was also observed — a process by which accretion onto the surface of the star produces emission that “fills in” absorption lines. This phenomenon has also been observed on the most well-studied star in the universe, the Sun. After a solar flare, material falls back to the solar surface, which produces the enhanced emission that causes veiling. Therefore, the Sun can act as a proxy when trying to understand accretion on young stars. 

Parker Solar Probe

Illustration of the Parker Solar Probe spacecraft approaching the Sun. [NASA/Johns Hopkins APL/Steve Gribben]

Dupree then switched gears to discuss mass loss in stars, a complex process that affects everything from stellar evolution to the chemical enrichment of the interstellar medium to exoplanet environments. For this section of the talk, Dupree began with the Sun, whose solar wind carries material away from the Sun in a stream. Comet tails first indicated that there was an outflow of material coming from the Sun. This stream has now been well-observed via in-situ probes like the recently launched Parker Solar Probe and eclipse observations, allowing us to apply our knowledge of the solar wind to other stellar systems. Focusing again on TW Hya, Dupree discussed what had been discovered about its stellar winds (with spectra!). The wind of TW Hya is highly structured and was found to be variable by utilizing a near-infrared transition of helium. Dupree also mentioned that this same helium line can also be used to observe winds from gaseous exoplanets via transmission spectroscopy. 

Dupree concluded her talk with a discussion of Betelgeuse, which has been a hot topic recently. It’s been fading and is the faintest it’s been in nearly 50 years. Does this mean it’s about to go supernova? We’ll just have to wait and see. This, and other unanswered stellar questions may soon be answered by a spectroscopist near you.

Special Session on Mauna Kea (by Mia de los Reyes and Ellis Avallone)

This special session focused on Hawaiian perspectives on Maunakea and the Thirty Meter Telescope. The panel consisted of four kānaka maoli (indigenous Hawaiians) and was moderated by Noelani Kalipi (Kohala Institute, Executive Director). 

The panel began with a presentation from two kia’i, or protectors of the mauna. The first speaker, Pua Case, one of the leaders of the Ku Kia’i Mauna movement, sought to set the tone of the session. Protectors of Maunakea practice kapu aloha, a philosophy of nonviolence that has been heavily present in the movement to protect Maunakea. Case went on to say that “the focus of [the session] was not to debate.” Rather, the focus of this special session was to listen and learn from each other, and its purpose was to transport astronomers to the mauna and invite them to visit and learn. 

Case then played a video that urged the audience to think about what is sacred and to work to understand the relationship between the people and the sky. The short film concluded directly: “We [kia’i] speak for the mountain and the mountain says no.” 

Case then discussed ceremony that takes place at the Mauna, demonstrating the chant done on the Mauna three times a day. Case also outlined the importance of bringing offerings — something meaningful brought for the land or elders. 

Mauna Kea

Mauna Kea, as viewed from Mauna Loa Observatory. [Nula666]

The next kia’i speaker was Lanakila Mangauil, who started his presentation by discussing his upbringing. Mangauil was part of the generation that was not kept from learning ancestral practices and wisdom, and he experienced Hawaiian culture as part of his day-to-day life. This was not the case several generations ago, as Mangauil remarks that his great grandmother was part of the generation that was punished for speaking Hawaiian. Turning to a more somber note, he mentions that he is also part of the first generation that can’t promise their children a future, referring to the current climate crisis. He compares the conflict on Maunakea to the current conflict between the Earth and human-caused climate change. Mangauil ends his presentation by emphasizing that we must do better for future generations. 

The kia’i’s portion of the session was concluded with another ceremony led by Pua Case for acknowledging the information the audience had just received. The purpose was to leave astronomers with “kuleana” (responsibility). Case left the audience with a powerful statement: “We are not a camping trip on the mountain, we are not your luau, this is not a whim.” Case urged the audience to understand the land and know how you will impact it and its people before you come.

The next part of the session was led by two native Hawaiian astronomy graduate students, Makana Silva (The Ohio State University) and Tyler Trent (University of Arizona), who both indicated that they support the Thirty Meter Telescope project. Silva began by stating that as a native Hawaiian, his view and perspective were just as important as that of the kia’i, and that perspectives vary even amongst native Hawaiians. He explained that with the telescope, he can practice his culture today in a modern fashion and carry Hawaiian’s long-standing practice of astronomy into modern times. Silva also quoted the environmental impact statement and cultural impact statement, which claim that the Thirty Meter Telescope will not disturb the summit, but said he would reassess if other studies were to come forward. He concluded his statements by saying that “the Thirty Meter Telescope is something that Hawaiians can leave as a legacy for the next generation.”

Next, Trent outlined his perspective on the Thirty Meter Telescope project, primarily focusing on potential consequences if the project left Hawai`i. He lists the telescope’s scholarship fund for the advancement of children in Hawai`i and the workforce pipeline as notable ventures that would be lost. Trent also remarks that “astronomy gives Hawaiians an opportunity to advance economically” outside the other primary industries of tourism and the military. Additionally, the rejection of the Thirty Meter Telescope sheds a negative light on astronomy and acts as a symbolic rejection of astronomy. 

For more information on this multifaceted issue, see our previous astrobites (here and here) and the links within them.

Press Conference: Cosmology & Exoplanets: Beyond the Nobel Prize (by Susanna Kohler)

This morning’s press conference was moderated by our very own Tarini Konchady, Astrobites author and AAS Media Fellow. Tarini introduced the panel’s topic — Cosmology & Exoplanets — with the explanation that the somewhat odd pairing was inspired by the split of the most recent Nobel Prize.

First up, Daniel Gilman opened with new work on how gravitational lensing is helping us to refine our understanding of dark matter. Different models of dark matter disagree: are the particles primarily “warm” or “cold”? These temperatures refer to the particles’ speed — and since warm particles move faster, they tend to wipe out small-scale structure. To determine which of these models is a better fit to our observations, Daniel Gilman (University of California, Los Angeles) and collaborators used the Hubble-observed gravitational lensing of distant quasars to hunt for evidence of small-scale dark-matter structure in the invisible halos of intervening galaxies. The team’s discovery of small clumps of dark matter has caused them to declare the warm dark matter picture unlikely, instead favoring the cold dark matter model. Press release

Lensed Quasars

Each of these Hubble Space Telescope snapshots reveals four distorted images of a background quasar surrounding the central core of a foreground massive galaxy. [NASA, ESA, S.H. Suyu, and K.C. Wong]

What cosmology topic could be more controversial than dark matter? Geoff Chih-Fan Chen (University of California, Davis) leaps into the fray with a new measurement of the Hubble constant, a value that describes how fast the universe is expanding. There are two main ways of measuring the Hubble constant: one that uses supernovae and the distance ladder to measure the value locally, and one that uses the cosmic microwave background to measure the value in the early universe. The two approaches have found different values for H0, raising the question of whether this is just an error in one of the measurements, or if the tension is real and the value of H0 is actually different locally vs. in the early universe. By measuring the time delays of flickering variability in a gravitationally lensed quasar, Chen and collaborators were able to obtain an independent measurement of the Hubble constant in the local universe — and it matches up nicely with the previous measurements using supernovae. This further confirms the gulf between the two measurements of H0 and suggests a faster expansion rate in the local universe than in the early universe. Press release

We next transition to exoplanets, with Jason Wright (Penn State University) introducing NEID, an extreme-precision Doppler spectrograph that recently saw first light (see the cover image above). NEID (pronounced NOO-id) was installed on the 3.5-meter WIYN telescope at Kitt Peak National Observatory, and it will help us search for radial-velocity wobbles caused by the tug of planets on their host stars. NEID will have a precision roughly three times greater than previous generation state-of-the-art instruments, allowing us to detect wobbles of just 30 cm/s. For reference, the Sun’s wobble induced by Earth is about 10 cm/s, and Jupiter’s influence is around 1,200 cm/s. NEID is expected to start its main science mission this year. Press release

K dwarfs

Ed Guinan says K dwarfs might be the Goldilocks star.

Last up, Edward F. Guinan (Villanova University) used a series of stellar surveys to sell us on K stars as the best target in the search for habitable exoplanets. These dwarfs — which are slightly cooler and less luminous than the Sun — have stable lifetimes of 15 to 50 billions of years (in comparison to our Sun’s ~10 billion years), giving planets plenty of time to evolve life forms. While K dwarfs aren’t as common as M dwarfs, they’re still more frequently found than Sun-like stars. And, importantly, they’re much less volatile than M dwarfs, which notoriously flare in high-energy emission. K-dwarf habitable zones are larger, which is another advantage relative to M dwarfs: for M dwarfs, “If you want to be warm, you have to be close — but you’re getting close to a nuclear power plant,” Guinan points out. K dwarfs might just be the Goldilocks star we want … so let’s go hunt some habitable planets! Press release

Special Session: New Horizons Results at 2014 MU69 (by Briley Lewis)

In this special session, we get a taste of some planetary science, thanks to the New Horizons Kuiper Belt Extended Mission team. They report on the scientific results from the spacecraft’s 2019 flyby of Kuiper Belt Object (KBO) 2014 MU69, now formally known as Arrokoth. 


Alan Stern’s comparison of 2014 MU69 (Arrokoth) to Pluto’s mountains near Sputnik Planitia.

New Horizons PI Alan Stern started off with an overview of the mission. After its flyby of Pluto, the spacecraft was operational and ready to find a new target to further explore the outer solar system. Targeting MU69 was difficult, since it’s much smaller than Pluto (see image) and we were only able to observe about 1% of its orbit before flyby. It’s a cold classical KBO, meaning it was born at its current distance from the Sun and has low eccentricity and inclination. It’s about 33 km long, and it looks like two disks smushed together, known as a contact binary. Learning more about MU69 can tell us about how it formed, which in turn gives information on the conditions of the early solar system.

But before we talk about MU69, Marc Buie tells us about how we got the spacecraft there in the first place. To find a new destination, there was a lot of searching, including massive campaigns with ground-based telescopes to look for occultations, where the KBO passes in front of a distant star, giving us information on its size and shape based on the starlight it blocks out in transit. The team led multiple campaigns — spread through South Africa, Argentina, and even on the SOFIA airplane-based observatory — and got enough observations to narrow down their predictions on the shape and ensure the spacecraft would be in the right place for the flyby.


The bi-lobed object MU69, as captured by NASA’s New Horizons spacecraft during its flyby. [NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute]

The rest of the session focused on specifics on what we’ve learned about MU69 from the New Horizons data, and how that all ties together to paint a picture of how it formed. Alissa Earle tells us that the color of MU69’s two lobes is pretty much the same, and it’s consistent with other cold classical KBOs. Since the two lobes are similar, this implies they formed close to each other and then came together slowly. John Spencer noted how the two flattened lobes are remarkably aligned at their equators — unlikely to be a coincidence! Instead, these two lobes probably formed from a rotating clump due to streaming instabilities (a known mechanism in planet formation) and their alignment is a natural consequence of their orbital motion. (On a slightly related note, Spencer also searched near MU69 for rings or satellites — nothing so far!) Orkan Umurhan gave us some additional information on MU69’s properties: a temperature ranging from 16–55 K (very cold!) and a mean density of around 0.45 g/cm3 (less dense than liquid water). Lastly, Kelsi Singer looked at the craters on MU69 and what they can show us about its past. There are very few small craters on its surface, meaning there aren’t that many small KBOs to create those craters — but what does this mean for the Kuiper Belt? Either not that many small objects are made in the outer solar system, or they’re made and then somehow destroyed. Hierarchical accretion would likely lead to many small objects, and gravitational collapse would lead to few. 

All signs point to a “gentle” merger leading to the contact binary, since if the two lobes were moving faster than 2.5 meters per second, they would have destroyed or significantly altered each other on impact. These slow collision speeds (and other evidence, like Singer’s cratering results) point to cloud collapse instead of hierarchical accretion for how planetesimals form — an important result for planet formation!

Plenary Lecture: The Future of Infrared Astronomy in the Context of Spitzer, SOFIA, and JWST (by Briley Lewis)


Peter Eisenhardt with a temperature map of an exoplanet from Spitzer.

Spitzer is nearing its last days — the famous infrared satellite is shutting down at the end of the month, January 30th, 2020. Peter Eisenhardt (JPL) takes us on a tour of Spitzer’s greatest hits, illustrating the telescope’s lasting legacy. He tackles three main questions through the lens of Spitzer observations: where did we come from, how did the universe evolve, and are we alone?

For almost 17 years, Spitzer has been observing the infrared sky, peering into distant galaxies to answer those first two questions. It has estimated the rate at which stars form over the history of the universe, and also the mass density of those stars, giving us an idea of how the universe has changed over time. Spitzer has also provided great information on exoplanets, trying to answer the fundamental question of our place in the universe; it created the first exoplanet phase curve and temperature map, and was essential to the confirmation of the famous TRAPPIST-1 system.

Now, it’s spending its last days observing distant galaxies as part of a precursor survey, providing important context on where JWST and other future missions should point to see the early universe. It has also already observed many of the planets planned for early JWST observations, paving the way for the success of these future studies. 


James De Buizer discussing SOFIA (and claiming his title of “tallest astronomer”).

Next, James De Buizer (SOFIA Science Center) told us about SOFIA, the only far infrared observatory available for astronomy until the proposed Origins Space Telescope, planned tentatively for the 2030s if selected by the decadal survey. This unique facility — a telescope on an aircraft — flies up above the parts of the atmosphere that block far-IR light. Since SOFIA isn’t in orbit, it’s easily serviceable and upgradable, making it a flexible and powerful resource for infrared astronomy.

It’s already made numerous important contributions to astronomy: observations of the galactic center, mapping of star formation in the Orion Nebula, information on the importance of galactic outflows in the intergalactic medium (all the stuff between galaxies), and signatures of helium hydride, a tracker of how molecular hydrogen formed before there was enough dust in the early universe. De Buizer emphasizes that this is the only far infrared observatory that we’ll have access to for the next few years, making it absolutely integral to other upcoming missions such as JWST. There’s a lot SOFIA will be able to do to complement the science done by other new observatories!


SOFIA, a modified Boeing 747SP carrying a 2.7-m telescope. [NASA]

Press Conference: Astronomy Confronts Satellite Constellations (by Aaron  Tohuvavohu)

[Coming soon!]

Newton Lacy Pierce Prize: Life and Times of the Lowest Mass Galaxies (by Mia de los Reyes)

As #AAS235 began winding down, Daniel R. Weisz (University of California, Berkeley) gave the second-to-last plenary of the day. After discussing the importance of dwarf galaxies and the many astrophysical questions they can be used to answer, Weisz focused on dwarf galaxies as tools for “near-field” cosmology. Nearby dwarf galaxies are analogs to the lowest-mass galaxies at high redshifts, and they can shed light on some of the events that happened in the early universe.

NGC 147

NGC 147, a dwarf spheroidal galaxy in the Local Group. [Ole Nielsen]

Weisz began by describing how resolved stellar populations can be used to measure star formation histories of dwarf galaxies. To do this, we plot where individual stars fall on a color–magnitude diagram; by fitting this plot with models, we can estimate when galaxies formed most of their stars. The Hubble Space Telescope has revolutionized this game with its high angular resolution, which allows us to resolve many more individual stars in nearby dwarf galaxies. In 2014, Weisz’s group published a study using archival HST data to get star formation histories of 40 local dwarfs — the largest sample to date! 

With these data, Weisz’s group aimed to investigate the link between dwarf galaxies and reionization, the epoch in the universe when the neutral universe was ionized by high-energy UV photons. First, Weisz asked how dwarf galaxies were affected by reionization. By seeing what dwarf galaxies’ star formation histories looked like before and after the epoch of reionization (which lasted roughly between redshifts 6–10), his group showed that reionization quenched star formation in the lowest-mass objects, called “ultra faint dwarf” galaxies.

Weisz then asked how dwarf galaxies might contribute to reionization. From the star formation histories of dwarf galaxies, one can estimate how the ultraviolet luminosity of dwarf galaxies changed over time. Weisz’s group used this to figure out how much dwarf galaxies — in particular, the “ultra-faint dwarfs,” which are expected to be extremely common — would have contributed to reionization.

It turns out that ultra-faint dwarf galaxies must have been rarer than expected, otherwise they actually contribute too many high-energy photons to explain our observations of reionization! In the future, the James Webb Space Telescope will allow us to check this directly by actually doing a statistical census of the number of ultra-faint dwarf galaxies.

However, all of these results are based on observations of the dwarf galaxies around the Milky Way. Are these Milky Way satellites truly representative of all dwarf galaxies? “The natural place to answer this question is M31,” according to Weisz, since it’s the nearest system of satellite galaxies. He compared the star formation histories of Milky Way satellites with those of M31’s satellites, and preliminary results seem to show some differences! Around the Milky Way, ultra-faint dwarfs “quench” (finish most of their star formation) very early, while more luminous galaxies quench late. In the M31 system, none of the dwarf galaxy satellites quench quite as early as the Milky Way galaxies; most of them actually quench at intermediate times! These suggest that the Milky Way system is unique, and that studying the Milky Way satellites alone might give us a biased view of dwarf galaxies.

More data are needed to answer these questions. Weisz is leading an ongoing project to obtain more data from HST. In the future, hopefully we’ll be able to study more distant galaxies. JWST — and next-generation larger space telescopes — will enable measurements of individual stars in galaxies at further distances, letting us test if dwarf galaxies in our Local Group (of which M31 and the Milky Way are the two biggest galaxies) are unique.

Lancelot M. Berkeley Prize: The Event Horizon Telescope: Imaging a Black Hole (by Briley Lewis)

One of the biggest science events of this year was the first image of a black hole. Sheperd Doeleman (Center for Astrophysics | Harvard & Smithsonian) represented the Event Horizon Telescope (EHT) team today to talk about the journey to that amazing first image of a black hole, and where EHT is going in the future.

EHT 2017 campaign

Eight stations of the EHT 2017 campaign over six geographic locations. [EHT Collaboration et al 2019]

The size and shape of a black hole’s shadow encodes lots of information, including effects of general relativity. The most obvious choice for a target to look at is Sgr A*, the central black hole of our Milky Way. It’s 4 million times the mass of the Sun, and trying to see its shadow is the same as trying to see a grapefruit on the moon. Scientists figured out that if you’re observing in the radio, you’d need a baseline about the size of Earth itself to resolve this, and thus the EHT was born.

The team took existing radio telescopes all around Earth — even at the South Pole — and added technology to them, such as extremely precise hydrogen maser clocks, to link them together into one powerful instrument and achieve this goal. This Earth-sized telescope uses the idea of very long baseline interferometry (VLBI) and produces so much data that the fastest way to transfer all of it is by flying a bunch of hard drives somewhere in an airplane. Once they had their first data, the EHT team divided into four groups to analyze the data, in an effort to avoid human bias. They all came up with remarkably similar images, showing good agreement on the basic structure of the shadow of the black hole in a different galaxy, M87.


The first image of a black hole, from the plenary by Sheperd Doeleman.

So, what did they find in this first black hole observation? M87’s black hole is huge, around 6 billion times the mass of the Sun. Also, it looks like the spin of the black hole is comparatively unimportant to the features we see, but the metric used for general relativity is very important. 

Looking to the future, adding more telescopes will lead to better images. Doeleman calls this the “ngEHT” (next generation EHT). The near future goals include: adding three new stations (Greenland, Kitt Peak, and NOEMA), looking at the polarization of M87, imaging our very own Sgr A*, and adding even more telescopes beyond the already planned three. By the end of the decade, he hopes to have better angular resolution and dynamic range, with the goal of imaging and understanding the connection between a black hole and its jets. 

Given that Sgr A* is so nearby, and such an interesting target for our studies of the galactic center, why hasn’t the team imaged it yet? Sgr A* is variable on much smaller timescales than M87, which presents difficulties considering the EHT has to wait for the Earth to rotate to get more baseline coverage from the telescopes. Someday telescopes in low Earth orbit could solve this problem, making time-domain movies of black hole variability.

This first image of a black hole has been an international sensation, and Doeleman doesn’t underestimate the effect his team’s work has had; the image has been seen by 4 billion people, and now he’s trying to use their success to bring the public into science. This has been a huge collaborative effort, too, with 250 team members spread across 60 institutes and 20 countries. As Doleman says, “If you want to go far, go together. To work on the biggest questions and succeed is indescribable … and we are not done yet.”

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