Friday, February 09, 2024

 SPACE

Researchers discover cosmic dust storms from Type Ia supernova


Peer-Reviewed Publication

CHINESE ACADEMY OF SCIENCES HEADQUARTERS


Cosmic dust—like dust on Earth—comprises groupings of molecules that have condensed and stuck together in a grain. But the exact nature of dust creation in the universe has long been a mystery. Now, however, an international team of astronomers from China, the United States, Chile, the United Kingdom, Spain, etc., has made a significant discovery by identifying a previously unknown source of dust in the universe: a Type Ia supernova interacting with gas from its surroundings.  

The study was published in Nature Astronomy on Feb. 9, and was led by Prof. WANG Lingzhi from the South America Center for Astronomy of the Chinese Academy of Sciences. 

Supernovae have been known to play a role in dust formation, and to date, dust formation has only been seen in core-collapse supernovae—the explosion of massive stars. Since core-collapse supernovae do not occur in elliptical galaxies, the nature of dust creation in such galaxies has remained elusive. These galaxies are not organized into a spiral pattern like our Milky Way but are giant swarms of stars. This study shows that thermonuclear Type Ia supernovae, the explosion of white dwarf stars in binary systems with another star, may account for a significant amount of dust in these galaxies. 

The researchers monitored a supernova, SN 2018evt, for over three years using space-based facilities like NASA’s Spitzer Space Telescope and NEOWISE missions, ground-based facilities like the Las Cumbres Observatory’s global network of telescopes, and other facilities in China, South America, and Australia. They found that the supernova was running into material previously cast off by one or both stars in the binary system before the white dwarf star exploded, and the supernova sent a shock wave into this pre-existing gas.  

During more than a thousand days of monitoring the supernova, the researchers noticed that its light began to dim precipitously in the optical wavelengths that our eyes can see and then started glowing brighter in infrared light. This was a telltale sign that dust was being created in the circumstellar gas after it cooled following the supernova shock wave passing through it. 

"The origins of cosmic dust have long been a mystery. This study marks the first detection of a significant and rapid dust formation process in the thermonuclear supernova interacting with circumstellar gas," said Prof. WANG, first author of the study. 

The study estimated that a large amount of dust must have been created by this one supernova event—an amount equal to more than 1% of the Sun's mass. As the supernova cools, the amount of dust created should increase, perhaps tenfold. While these dust factories are not as numerous or efficient as core-collapse supernovae, there may be enough of these thermonuclear supernovae interacting with their surroundings to be a significant or even dominant source of dust in elliptical galaxies. 

"This study offers insights into the contribution of thermonuclear supernovae to cosmic dust, and more such events may be expected to be found in the era of the James Webb Space Telescope (JWST)," said Prof. WANG Lifan from Texas A&M University, a co-first author of the study. The Webb telescope sees infrared light that is perfect for the detection of dust. 

"The creation of dust is just gas getting cold enough to condense," said Prof. Andy Howell from Las Cumbres Observatory and the University of California Santa Barbara. Howell is the Principal Investigator of the Global Supernova Project whose data was used in the study. "One day that dust will condense into planetesimals and, ultimately, planets. This is creation starting anew in the wake of stellar death. It is exciting to understand another link in the circle of life and death in the universe."

Migration solves exoplanet puzzle


Simulations provide a potential explanation for the mysterious gap in the size distribution of super-Earths.



MAX PLANCK INSTITUTE FOR ASTRONOMY

Artistic representation of an exoplanet whose water ice on the surface is increasingly vaporizing and forming an atmosphere during its approach to the central star of the planetary system 

IMAGE: 

ARTISTIC REPRESENTATION OF AN EXOPLANET WHOSE WATER ICE ON THE SURFACE IS INCREASINGLY VAPORIZING AND FORMING AN ATMOSPHERE DURING ITS APPROACH TO THE CENTRAL STAR OF THE PLANETARY SYSTEM. THIS PROCESS INCREASES THE MEASURED PLANETARY RADIUS COMPARED TO THE VALUE THE PLANET WOULD HAVE AT ITS PLACE OF ORIGIN.

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CREDIT: THOMAS MÜLLER (MPIA)




Ordinarily, planets in evolved planetary systems, such as the Solar System, follow stable orbits around their central star. However, many indications suggest that some planets might depart from their birthplaces during their early evolution by migrating inward or outward. This planetary migration might also explain an observation that has puzzled researchers for several years: the relatively low number of exoplanets with sizes about twice as large as Earth, known as the radius valley or gap. Conversely, there are many exoplanets smaller and larger than this size.

Six years ago, a reanalysis of data from the Kepler space telescope revealed a shortage of exoplanets with sizes around two Earth radii,” Remo Burn explains, an exoplanet researcher at the Max Planck Institute for Astronomy (MPIA) in Heidelberg. He is the lead author of the article reporting the findings outlined in this article, now published in Nature Astronomy.

Where does the radius valley come from?

In fact, we – like other research groups – predicted based on our calculations, even before this observation, that such a gap must exist,” explains co-author Christoph Mordasini, a member of the National Centre of Competence in Research (NCCR) PlanetS. He heads the Division of Space Research and Planetary Sciences at the University of Bern. This prediction originated during his tenure as a scientist at MPIA, which has been jointly researching this field with the University of Bern for many years.

The most commonly suggested mechanism to explain the emergence of such a radius valley is that planets might lose a part of their original atmosphere due to the irradiation from the central star – especially volatile gases like hydrogen and helium. “However, this explanation neglects the influence of planetary migration,” Burn clarifies. It has been established for about 40 years that under certain conditions, planets can move inward and outward through planetary systems over time. How effective this migration is and to what extent it influences the development of planetary systems impacts its contribution to forming the radius valley.

Enigmatic sub-Neptunes

Two different types of exoplanets inhabit the size range surrounding the gap. On one hand, there are rocky planets, which can be more massive than Earth and are hence called super-Earths. On the other hand, astronomers are increasingly discovering so-called sub-Neptunes (also mini-Neptunes) in distant planetary systems, which are, on average, slightly larger than the super-Earths.

However, we do not have this class of exoplanets in the Solar System,” Burn points out. “That’s why, even today, we’re not exactly sure about their structure and composition.

Still, astronomers broadly agree that these planets possess significantly more extended atmospheres than rocky planets. Consequently, understanding how these sub-Neptunes’ characteristics contribute to the radius gap has been uncertain. Could the gap even suggest that these two types of worlds form differently?

Wandering ice planets

Based on simulations we already published in 2020, the latest results indicate and confirm that instead, the evolution of sub-Neptunes after their birth significantly contributes to the observed radius valley,” concludes Julia Venturini from Geneva University. She is a member of the PlanetS collaboration mentioned above and led the 2020 study.

In the icy regions of their birthplaces, where planets receive little warming radiation from the star, the sub-Neptunes should indeed have sizes missing from the observed distribution. As these presumably icy planets migrate closer to the star, the ice thaws, eventually forming a thick water vapour atmosphere.

This process results in a shift in planet radii to larger values. After all, the observations employed to measure planetary radii cannot differentiate whether the determined size is due to the solid part of the planet alone or an additional dense atmosphere.

At the same time, as already suggested in the previous picture, rocky planets ‘shrink’ by losing their atmosphere. Overall, both mechanisms produce a lack of planets with sizes around two Earth radii.

Physical computer models simulating planetary systems

The theoretical research of the Bern-Heidelberg group has already significantly advanced our understanding of the formation and composition of planetary systems in the past,” explains MPIA Director Thomas Henning. “The current study is, therefore, the result of many years of joint preparatory work and constant improvements to the physical models.

The latest results stem from calculations of physical models that trace planet formation and subsequent evolution. They encompass processes in the gas and dust disks surrounding young stars that give rise to new planets. These models include the emergence of atmospheres, the mixing of different gases, and radial migration.

 “Central to this study were the properties of water at pressures and temperatures occurring inside planets and their atmospheres,” explains Burn. Understanding how water behaves over a wide range of pressures and temperatures is crucial for simulations. This knowledge has been of sufficient quality only in recent years. It is this component which permits realistic calculation of the sub-Neptunes’ behaviour, hence explaining the manifestation of extensive atmospheres in warmer regions.

It’s remarkable how, as in this case, physical properties on molecular levels influence large-scale astronomical processes such as the formation of planetary atmospheres,” Henning adds.

If we were to expand our results to cooler regions, where water is liquid, this might suggest the existence of water worlds with deep oceans,” Mordasini says. “Such planets could potentially host life and would be relatively straightforward targets for searching for biomarkers thanks to their size.

Further work ahead

However, the current work is just an important milestone. Although the simulated size distribution closely matches the observed one, and the radius gap is in the right place, the details still have some inconsistencies. For instance, too many ice planets end up too close to the central star in the calculations. Nonetheless, researchers do not perceive this circumstance as a disadvantage but hope to learn more about planetary migration in this way.

Observations with telescopes like the James Webb Space Telescope (JWST) or the under-construction Extremely Large Telescope (ELT) could also assist. They would be capable of determining the composition of planets depending on their size, thus providing a test for the simulations described here.

Background information

The MPIA scientists involved in this study are Remo Burn and Thomas Henning.

Other researchers include Christoph Mordasini (University of Bern, Switzerland [Unibe]), Lokesh Mishra (Université de Genève, Switzerland [Unige], and Unibe), Jonas Haldemann (Unibe), Julia Venturini (Unige), and Alexandre Emsenhuber (Ludwig Maximilian University Munich, Germany, and Unibe).

The NASA Kepler space telescope searched for planets around other stars between 2009 and 2018 and discovered thousands of new exoplanets during its operation. It utilised the transit method: when a planet’s orbit is inclined in a way that the plane lies within the telescope’s line of sight, planets periodically block part of the star’s light during their orbit. This periodic fluctuation in the star’s brightness enables an indirect detection of the planet and determination of its radius.


Size distribution of observed and simulated exoplanets with radii smaller than five Earth radii


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