Friday, March 03, 2023

Ultracool dwarf binary stars break records

Astrophysicists discover the closest and oldest ultracool dwarf binary ever observed


Peer-Reviewed Publication

NORTHWESTERN UNIVERSITY

Illustration of the binary stars 

IMAGE: ILLUSTRATION OF THE OLDEST KNOWN PAIR OF ULTRACOOL DWARF STARS THAT ORBIT EACH OTHER SO CLOSELY, THEY TAKE LESS THAN ONE EARTH DAY TO REVOLVE AROUND EACH OTHER. view more 

CREDIT: ADAM BURGASSER/UC SAN DIEGO

Northwestern University and the University of California San Diego (UC San Diego) astrophysicists have discovered the tightest ultracool dwarf binary system ever observed.

The two stars are so close that it takes them less than one Earth day to revolve around each other. In other words, each star’s “year” lasts just 17 hours.

The newly discovered system, named LP 413-53AB, is composed of a pair of ultracool dwarfs, a class of very low-mass stars that are so cool that they emit their light primarily in the infrared, making them completely invisible to the human eye. They are nonetheless one of the most common types of stars in the universe. 

Previously, astronomers had only detected three short-period ultracool dwarf binary systems, all of which are relatively young — up to 40 million years old. LP 413-53AB is estimated to be billions of years old — similar age to our sun — but has an orbital period that is at least three times shorter than the all ultracool dwarf binaries discovered so far.

The research was published on March 1 in the Astrophysical Journal Letters

“It’s exciting to discover such an extreme system,” said Chih-Chun “Dino” Hsu, a Northwestern astrophysicist who led the study. “In principle, we knew these systems should exist, but no such systems had been identified yet.”

Hsu is a postdoctoral researcher in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics(CIERA). He began this study while a Ph.D. student at UC San Diego, where he was advised by Professor Adam Burgasser.

The team first discovered the strange binary system while exploring archival data. Hsu developed an algorithm that can model a star based on its spectral data. By analyzing the spectrum of light emitted from a star, astrophysicists can determine the star’s chemical composition, temperature, gravity and rotation. This analysis also shows the star’s motion as it moves toward and away from the observer, known as radial velocity. 

When examining the spectral data of LP 413-53AB, Hsu noticed something strange. Early observations caught the system when the stars were roughly aligned and their spectral lines overlapped, leading Hsu to believe it was just one star. But as the stars moved in their orbit, the spectral lines shifted in opposite directions, splitting into pairs in later spectral data. Hsu realized there were actually two stars locked into an incredibly tight binary.

Using powerful telescopes at the W.M. Keck Observatory, Hsu decided to observe the phenomenon for himself. On March 13, 2022, the team turned the telescopes toward the constellation Taurus, where the binary system is located, and observed it for two hours. Then, they followed up with more observations in July, October and December as well as January 2023.

“When we were making this measurement, we could see things changing over a couple of minutes of observation,” Burgasser said. “Most binaries we follow have orbit periods of years. So, you get a measurement every few months. Then, after a while, you can piece together the puzzle. With this system, we could see the spectral lines moving apart in real time. It’s amazing to see something happen in the universe on a human time scale.”

The observations confirmed what Hsu’s model predicted. The distance between the two stars is about 1% of the distance between the Earth and the sun. “This is remarkable, because when they were young, something like 1 million years old, these stars would have been on top of each other,” said Burgasser. 

The team speculates that the stars either migrated toward each other as they evolved, or they could have come together after the ejection of a third — now lost — stellar member. More observations are needed to test these ideas.

Hsu also said that by studying similar star systems researchers can learn more about potentially habitable planets beyond Earth. Ultracool dwarfs are much fainter and dimmer than the sun, so any worlds with liquid water on their surfaces — a crucial ingredient to form and sustain life — would need to be much closer to the star. However, for LP 413-53AB, the habitable zone distance happens to be the same as the stellar orbit, making it impossible to form habitable planets in this system. 

“These ultracool dwarfs are neighbors of our sun,” Hsu said. “To identify potentially habitable hosts, it’s helpful to start with our nearby neighbors. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found.”

To fully explore these scenarios, Hsu, Burgasser and their collaborators hope to pinpoint more ultracool dwarf binary systems to create a full data sample. New observational data could help strengthen theoretical models for binary-star formation and evolution. Until now, however, finding ultracool binary stars has remained a rare feat.

“These systems are rare,” said Chris Theissen, study co-author and a Chancellor’s Postdoctoral Fellow at UC San Diego. “But we don’t know whether they are rare because they rarely exist or because we just don’t find them. That’s an open-ended question. Now we have one data point that we can start building on. This data had been sitting in the archive for a long time. Dino’s tool will enable us to look for more binaries like this.”

Resurrected supernova provides missing-link

Peer-Reviewed Publication

NATIONAL INSTITUTES OF NATURAL SCIENCES

An image of the central region of M77 

IMAGE: AN IMAGE OF THE CENTRAL REGION OF M77 TAKEN BY THE HUBBLE SPACE TELESCOPE (LEFT), IN WHICH THE POSITION OF SN 2018IVC IS MARKED. RIGHT PANELS SHOW THE EXPANDED VIEW AROUND SN 2018IVC BASED ON THE DATA TAKEN BY ALMA, AT ~200 DAYS (UPPER RIGHT) AND ~ 1000 DAYS (LOWER RIGHT), CLEARLY SHOWING THAT THE REBRIGHTENING HAPPENED AT ABOUT ONE YEAR AFTER THE SN EXPLOSION. view more 

CREDIT: CREDIT: (LEFT) BASED ON OBSERVATIONS MADE WITH THE NASA/ESA HUBBLE SPACE TELESCOPE, AND OBTAINED FROM THE HUBBLE LEGACY ARCHIVE, WHICH IS A COLLABORATION BETWEEN THE SPACE TELESCOPE SCIENCE INSTITUTE (STSCI/NASA), THE SPACE TELESCOPE EUROPEAN COORDINATING FACILITY (ST-ECF/ESA) AND THE CANADIAN ASTRONOMY DATA CENTRE (CADC/NRC/CSA). (RIGHT) ALMA (ESO/NAOJ/NRAO), K. MAEDA ET AL.

Astronomers have discovered a supernova exhibiting unprecedented rebrightening at millimeter wavelengths, providing an intermediate case between two types of supernovae: those of solitary stars and those in close-binary systems.

Many massive stars end their lives in a catastrophic explosion known as a supernova (SN). Supernovae increase rapidly in brightness, and then fade over the course of several months.

Astronomers have long known that the presence or absence of a close binary companion can affect the evolution of massive stars. In a close binary system, gravitational interactions with the binary companion will strip large amounts of material from the SN progenitor long before the final explosion. In these cases, the progenitor will be quiet up until the time of the actual SN. On the other hand, in the case of an SN progenitor with no binary companion or a distant companion, leading up to the SN explosion the progenitor will keep most of its initial mass.

Of course smart alecs will ask, “What happens when the binary is not too close and not too distant?” Not just smart alecs, astronomers also wanted to know. The break came when an international research team, led by Keiichi Maeda (Professor at the Graduate School of Science, Kyoto University) and Tomonari Michiyama (ALMA Joint Postdoctoral Fellow at the Graduate School of Science, Osaka University), used ALMA (The Atacama Large Millimeter/submillimeter Array) to monitor a supernova known as SN 2018ivc as it dimmed for about 200 days after the initial explosion. The results showed that SN 2018ivc was an unusual object, so the team decided to check up on it again, at about 1000 days after the explosion. They found that the object was actually rebrightening, the first time this phenomenon had ever been observed in millimeter wavelength radiation.

Comparison to numerical modeling suggests that interaction with an intermediate-distance binary companion about 1500 years before the SN explosion created a large hollow shell of circumstellar medium. At 200 days after the SN, the ejecta flying out from the explosion had yet to reach the shell. Then sometime between 200 and 1000 days, the ejecta collided with the circumstellar medium.

These results appeared as K. Maeda et al. “Resurrection of Type IIL Supernova 2018ivc: Implications for a Binary Evolution Sequence Connecting Hydrogen-rich and Hydrogen-poor Progenitors” in The Astrophysical Journal Letters on March 1, 2023.

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