K. Hawkins and colleagues performed a detailed analysis of J01020100-7122208 with the goal of shedding light on its origin. They proposed that, instead of yellow super giant and red giant, it is probably an evolved blue straggler. Their study recently appeared in Arxiv.
In 2018, astronomers using telescopes in northern Chile have discovered a rare runaway star in the Small Magellanic Cloud. The star is designated J01020100-7122208. It’s speeding across its little galaxy at 300,000 miles per hour (500,000 km/hour). They claimed, this star to be a yellow super giant ejected from the Small Magellanic Cloud, but it was more recently claimed to be a red giant accelerated by the Milky Way’s central black hole. Thus, its origin and nature still challenges us.
Now, in order to unveil its nature, K. Hawkins and colleagues, analysed photometric, astrometric and high resolution spectroscopic observations to estimate the orbit, age, and 16 elemental abundances.
Their results showed that this star has a retrograde and highly-eccentric orbit, e=0.914. Correspondingly, it likely crossed the Galactic disk at 550pc from the Galactic centre. They also obtained a spectroscopic mass and age of 1.09 M and 4.51 Gyr respectively.
Moreover, they found that its chemical composition is similar to the abundance of other retrograde halo stars and that the star is enriched in europium, having [Eu/Fe] = 0.93 ± 0.24, and is more metal-poor than reported in the literature, with [Fe/H] = -1.30 ± 0.10.
From this information they concluded that J01020100-7122208 is likely not a star ejected from the central black of the Milky Way or from the Small Magellanic Cloud. Instead, they proposed that it is simply a halo star which was likely accreted by the Milky Way in the distant past but its mass and age suggest it is probably an evolved blue straggler.
Reference: D. Brito-Silva, P. Jofré, D. Bourbert, S. E. Koposov, J. L. Prieto, K. Hawkins, “J01020100-7122208: an accreted evolved blue straggler that wasn’t ejected from a supermassive black hole”, Arxiv, 2021. arXiv:2111.08821
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Exploding stars generate dramatic light shows. Infrared telescopes like Spitzer can see through the haze and to give a better idea of how often these explosions occur.
You’d think that supernovae – the death throes of massive stars and among the brightest, most powerful explosions in the universe – would be hard to miss. Yet the number of these blasts observed in the distant parts of the universe falls way short of astrophysicists’ predictions.
A new study using data from NASA’s recently retired Spitzer Space Telescope reports the detection of five supernovae that, going undetected in optical light, had never been seen before. Spitzer saw the universe in infrared light, which pierces through dust clouds that block optical light – the kind of light our eyes see and that unobscured supernovae radiate most brightly.
To search for hidden supernovae, the researchers looked at Spitzer observations of 40 dusty galaxies. (In space, dust refers to grain-like particles with a consistency similar to smoke.) Based on the number they found in these galaxies, the study confirms that supernovae do indeed occur as frequently as scientists expect them to. This expectation is based on scientists’ current understanding of how stars evolve. Studies like this are necessary to improve that understanding, by either reinforcing or challenging certain aspects of it.
“These results with Spitzer show that the optical surveys we’ve long relied on for detecting supernovae miss up to half of the stellar explosions happening out there in the universe,” said Ori Fox, a scientist at the Space Telescope Science Institute in Baltimore, Maryland, and lead author of the new study, published in the Monthly Notices of the Royal Astronomical Society. “It’s very good news that the number of supernovae we’re seeing with Spitzer is statistically consistent with theoretical predictions.”
The “supernova discrepancy” – that is, the inconsistency between the number of predicted supernovae and the number observed by optical telescopes – is not an issue in the nearby universe. There, galaxies have slowed their pace of star formation and are generally less dusty. In the more distant reaches of the universe, though, galaxies appear younger, produce stars at higher rates, and tend to have higher amounts of dust. This dust absorbs and scatters optical and ultraviolet light, preventing it from reaching telescopes. So researchers have long reasoned that the missing supernovae must exist and are just unseen.
“Because the local universe has calmed down a bit since its early years of star-making, we see the expected numbers of supernovae with typical optical searches,” said Fox. “The observed supernova-detection percentage goes down, however, as you get farther away and back to cosmic epochs where dustier galaxies dominated.”
Detecting supernovae at these far distances can be challenging. To perform a search for supernovae shrouded within murkier galactic realms but at less extreme distances, Fox’s team selected a local set of 40 dust-choked galaxies, known as luminous and ultra-luminous infrared galaxies (LIRGs and ULIRGs, respectively). The dust in LIRGs and ULIRGs absorbs optical light from objects like supernovae but allows infrared light from these same objects to pass through unobstructed for telescopes like Spitzer to detect.
The researchers’ hunch proved correct when the five never-before-seen supernovae came to (infrared) light. “It’s a testament to Spitzer’s discovery potential that the telescope was able to pick up the signal of hidden supernovae from these dusty galaxies,” said Fox.
“It was especially fun for several of our undergraduate students to meaningfully contribute to this exciting research,” added study co-author Alex Filippenko, a professor of astronomy at the University of California, Berkeley. “They helped answer the question, ‘Where have all the supernovae gone?’”
The types of supernovae detected by Spitzer are known as “core-collapse supernovae,” involving giant stars with at least eight times the mass of the Sun. As they grow old and their cores fill with iron, the big stars can no longer produce enough energy to withstand their own gravity, and their cores collapse, suddenly and catastrophically.
The intense pressures and temperatures produced during the rapid cave-in forms new chemical elements via nuclear fusion. The collapsing stars ultimately rebound off their ultra-dense cores, blowing themselves to smithereens and scattering those elements throughout space. Supernovae produce “heavy” elements, such as most metals. Those elements are necessary for building up rocky planets, like Earth, as well as biological beings. Overall, supernova rates serve as an important check on models of star formation and the creation of heavy elements in the universe.
“If you have a handle on how many stars are forming, then you can predict how many stars will explode,” said Fox. “Or, vice versa, if you have a handle on how many stars are exploding, you can predict how many stars are forming. Understanding that relationship is critical for many areas of study in astrophysics.”
“Our study has shown that star formation models are more consistent with supernova rates than previously thought,” said Fox. “And by revealing these hidden supernovae, Spitzer has set the stage for new kinds of discoveries with the Webb and Roman space telescopes.”
More About the Mission
NASA’s Jet Propulsion Laboratory in Southern California conducted mission operations and managed the Spitzer Space Telescope mission for the agency’s Science Mission Directorate in Washington. Science operations were conducted at the Spitzer Science Center at Caltech in Pasadena. Spacecraft operations were based at Lockheed Martin Space in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC at Caltech. Caltech manages JPL for NASA.
Featured image: The image shows galaxy Arp 148, captured by NASA’s Spitzer and Hubble telescopes. Specially processed Spitzer data is shown inside the white circle, revealing infrared light from a supernova hidden by dust. Credit: NASA/JPL-Caltech
Reference: Ori D Fox et al, A Spitzer survey for dust-obscured supernovae, Monthly Notices of the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab1740
New research led by NASA provides a closer look at a nearby star thought to resemble our young Sun. The work allows scientists to better understand what our Sun may have been like when it was young, and how it may have shaped the atmosphere of our planet and the development of life on Earth.
Many people dream of meeting with a younger version of themselves to exchange advice, identify the origins of their defining traits, and share hopes for the future. At 4.65 billion years old, our Sun is a middle-aged star. Scientists are often curious to learn exactly what properties enabled our Sun, in its younger years, to support life on nearby Earth.
Without a time machine to transport scientists back billions of years, retracing our star’s early activity may seem an impossible feat. Luckily, in the Milky Way galaxy – the glimmering, spiraling segment of the universe where our solar system is located – there are more than 100 billion stars. One in ten share characteristics with our Sun, and many are in the early stages of development.
“Imagine I want to reproduce a baby picture of an adult when they were one or two years old, and all of their pictures were erased or lost. I would look at a photo of them now, and their close relatives’ photos from around that age, and from there, reconstruct their baby photos,” said Vladimir Airapetian, senior astrophysicist in the Heliophysics Division at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and first author on the new study. “That’s the sort of process we are following here – looking at characteristics of a young star similar to ours, to better understand what our own star was like in its youth, and what allowed it to foster life on one of its nearby planets.”
Kappa 1 Ceti is one such solar analogue. The star is located about 30 light-years away (in space terms, that’s like a neighbor who lives on the next street over) and is estimated to be between 600 to 750 million years old, around the same age our Sun was when life developed on Earth. It also has a similar mass and surface temperature to our Sun, said the study’s second author, Meng Jin, a heliophysicist with the SETI Institute and the Lockheed Martin Solar and Astrophysics Laboratory in California. All of those factors make Kappa 1 Ceti a “twin” of our young Sun at the time when life arose on Earth, and an important target for study.
Airapetian, Jin, and several colleagues have adapted an existing solar model to predict some of Kappa 1 Ceti’s most important, yet difficult to measure, characteristics. The model relies on data input from a variety of space missions including the NASA/ESA Hubble Space Telescope, NASA’s Transiting Exoplanet Survey Satellite and NICER missions, and ESA’s XMM-Newton. The team published their study today in The Astrophysical Journal.
Like human toddlers, toddler stars are known for their high bursts of energy and activity. For stars, one way this pent-up energy is released is in the form of a stellar wind.
Stellar winds, like stars themselves, are mostly made up of a superhot gas known as plasma, created when particles in a gas have split into positively charged ions and negatively charged electrons. The most energetic plasma, with the help of a star’s magnetic field, can shoot off away from the outermost and hottest part of a star’s atmosphere, the corona, in an eruption, or stream more steadily toward nearby planets as stellar wind. “Stellar wind is continuously flowing out from a star toward its nearby planets, influencing those planets’ environments,” Jin said.
Younger stars tend to generate hotter, more vigorous stellar winds and more powerful plasma eruptions than older stars do. Such outbursts can affect the atmosphere and chemistry of planets nearby, and possibly even catalyze the development of organic material – the building blocks for life – on those planets.
Stellar wind can have a significant impact on planets at any stage of life. But the strong, highly dense stellar winds of young stars can compress the protective magnetic shields of surrounding planets, making them even more susceptible to the effects of the charged particles.
Our Sun is a perfect example. Compared to now, in its toddlerhood, our Sun likely rotated three times faster, had a stronger magnetic field, and shot out more intense high-energy radiation and particles. These days, for lucky spectators, the impact of these particles is sometimes visible near the planet’s poles as aurora, or the Northern and Southern Lights. Airapetian says 4 billion years ago, considering the impact of our Sun’s wind at that time, these tremendous lights were likely often visible from many more places around the globe.
That high level of activity in our Sun’s nascence may have pushed back Earth’s protective magnetosphere, and provided the planet – not close enough to be torched like Venus, nor distant enough to be neglected like Mars – with the right atmospheric chemistry for the formation of biological molecules.
Similar processes could be unfolding in stellar systems across our galaxy and universe.
“It’s my dream to find a rocky exoplanet in the stage that our planet was in more than 4 billion years ago, being shaped by its young, active star and nearly ready to host life,” Airapetian said. “Understanding what our Sun was like just as life was beginning to develop on Earth will help us to refine our search for stars with exoplanets that may eventually host life.”
A Solar Twin
Though solar analogues can help solve one of the challenges of peeking into the Sun’s past, time isn’t the only complicating factor in studying our young Sun. There’s also distance.
We have instruments capable of accurately measuring the stellar wind from our own Sun, called the solar wind. However, it’s not yet possible to directly observe the stellar wind of other stars in our galaxy, like Kappa 1 Ceti, because they are too far away.
When scientists wish to study an event or phenomenon that they cannot directly observe, scientific modeling can help fill in the gaps. Models are representations or predictions of an object of study, built on existing scientific data. While scientists have previously modeled the stellar wind from this star, Airapetian said, they used more simplified assumptions.
The basis for the new model of Kappa 1 Ceti by Airapetian, Jin, and colleagues is the Alfvén Wave Solar Model, which is within the Space Weather Modeling Framework developed by the University of Michigan. The model works by inputting known information about a star, including its magnetic field and ultraviolet emission line data, to predict stellar wind activity. When the model has been tested on our Sun, it has been validated and checked against observed data to verify that its predictions are accurate.
“It’s capable of modeling our star’s winds and corona with high fidelity,” Jin said. “And it’s a model we can use on other stars, too, to predict their stellar wind and thereby investigate habitability. That’s what we did here.”
Previous studies have drawn on data gathered by the Transiting Exoplanet Survey Satellite (TESS) and Hubble Space Telescope (HST) to identify Kappa 1 Ceti as a young solar proxy, and to gather the necessary inputs for the model, such as magnetic field and ultraviolet emission line data.
“Every model needs input to get output,” Airapetian said. “To get useful, accurate output, the input needs to be solid data, ideally from multiple sources across time. We have all that data from Kappa 1 Ceti, but we really synthesized it in this predictive model to move past previous purely observational studies of the star.”
Airapetian likens his team’s model to a doctor’s report. To get a full picture of how a patient is doing, a doctor is likely to talk to the patient, gather markers like heart rate and temperature, and if needed, conduct several more specialized tests, like a blood test or ultrasound. They are likely to formulate an accurate assessment of patient well-being with a combination of these metrics, not just one.
Similarly, by using many pieces of information about Kappa 1 Ceti gathered from different space missions, scientists are better able to predict its corona and the stellar wind. Because stellar wind can affect a nearby planet’s magnetic shield, it plays an important role in habitability. The team is also working on another project, looking more closely at the particles that may have emerged from early solar flares, as well as prebiotic chemistry on Earth.
Our Sun’s Past, Written in the Stars
The researchers hope to use their model to map the environments of other Sun-like stars at various life stages.
Specifically, they have eyes on the infant star EK Dra – 111 light-years away and only 100 million years old – which is likely rotating three times faster and shooting off more flares and plasma than Kappa 1 Ceti. Documenting how these similar stars of various ages differ from one another will help characterize the typical trajectory of a star’s life.
Their work, Airapetian said, is all about “looking at our own Sun, its past and its possible future, through the lens of other stars.”
To learn more about our Sun’s stormy youth, watch this video and see how energy from our young Sun — 4 billion years ago — aided in creating molecules in Earth’s atmosphere, allowing it to warm up enough to incubate life.
Banner image: A view of the Sun from the Extreme ultraviolet Imaging Telescope on ESA/NASA’s Solar and Heliospheric Observatory, or SOHO. Credits:ESA/NASA
Reference: Vladimir S. Airapetian et al, One Year in the Life of Young Suns: Data-constrained Corona-wind Model of κ 1 Ceti, The Astrophysical Journal (2021). DOI: 10.3847/1538-4357/ac081e
Middle-aged stars can experience their own kind of midlife crisis, experiencing dramatic breaks in their activity and rotation rates at about the same age as our Sun, according to new research published today in Monthly Notices of the Royal Astronomical Society: Letters. The study provides a new theoretical underpinning for the unexplained breakdown of established techniques for measuring ages of stars past their middle age, and the transition of solar-like stars to a magnetically inactive future.
Astronomers have long known that stars experience a process known as ‘magnetic braking’: a steady stream of charged particles, known as the solar wind, escapes from the star over time, carrying away small amounts of the star’s angular momentum. This slow drain causes stars like our Sun to gradually slow down their rotation over billions of years.
In turn, the slower rotation leads to altered magnetic fields and less stellar activity – the numbers of sunspots, flares, outbursts, and similar phenomena in the atmospheres of stars, which are intrinsically linked to the strengths of their magnetic fields.
This decrease in activity and rotation rate over time is expected to be smooth and predictable because of the gradual loss of angular momentum. The idea gave birth to the tool known as ‘stellar gyrochronology’, which has been widely used over the past two decades to estimate the age of a star from its rotation period.
However recent observations indicate that this intimate relationship breaks down around middle age. The new work, carried out by Bindesh Tripathi, Prof. Dibyendu Nandy, and Prof. Soumitro Banerjee at the Indian Institute of Science Education and Research (IISER) Kolkata, India, provides a novel explanation for this mysterious ailment.
Using dynamo models of magnetic field generation in stars, the team show that at about the age of the Sun the magnetic field generation mechanism of stars suddenly becomes sub-critical or less efficient. This allows stars to exist in two distinct activity states – a low activity mode and an active mode. A middle aged star like the Sun can often switch to the low activity mode resulting in drastically reduced angular momentum losses by magnetized stellar winds.
Prof. Nandy comments: “This hypothesis of sub-critical magnetic dynamos of solar-like stars provides a self-consistent, unifying physical basis for a diversity of solar-stellar phenomena, such as why stars beyond their midlife do not spin down as fast as in their youth, the breakdown of stellar gyrochronology relations, and recent findings suggesting that the Sun may be transitioning to a magnetically inactive future.”
The new work provides key insights into the existence of low activity episodes in the recent history of the Sun known as grand minima – when hardly any sunspots are seen. The best known of these is perhaps the Maunder Minimum around 1645 to 1715, when very few sunspots were observed.
The team hope that it will also shed light on recent observations indicating that the Sun is comparatively inactive, with crucial implications for the potential long-term future of our own stellar neighbour.
Featured image: Artist’s impression of the spinning interior of a star, generating the stellar magnetic field. This image combines a dynamo simulation of the Sun’s interior with observations of the Sun’s outer atmosphere, where storms and plasma winds are generated.Credit: CESSI / IISER Kolkata / NASA-SVS / ESA / SOHO-LASCO
Reference: Bindesh Tripathi et al, Stellar mid-life crisis: subcritical magnetic dynamos of solar-like stars and the breakdown of gyrochronology, Monthly Notices of the Royal Astronomical Society: Letters (2021). DOI: 10.1093/mnrasl/slab035
An international collaboration of astronomers has identified a curious occurrence of nine stars like objects that appeared and vanished in a small region within half an hour in an old photographic plate.
Astronomers collaborating across counties track vanishing and appearing celestial objects by comparing old images of the night sky with new modern one, register unnatural phenomena, and probe deep into such phenomena to record changes in the Universe.
Scientists from Sweden, Spain, USA, Ukraine, and India, including Dr. Alok C. Gupta, Scientist from ARIES, investigated early form of photography that used glass plates to capture images of the night sky from the 12th of April 1950, exposed at Palomar Observatory in California, USA and detected these transient stars which were not to be found in photographs half an hour later and not traced since then. Such a group of objects appearing and disappearing at the same time have been detected for the first time in the history of astronomy.
The astronomers have not found any explanation in well-established astrophysical phenomena like gravitational lensing, fast radio bursts, or any variable star that could be responsible for this cluster of fast changes in the sky.
Dr. Alok C. Gupta, Scientist from Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, an autonomous institution of the Department of Science and Technology (DST), Government of India, participated in this study which was recently published in Nature’s “Scientific Reports”. The study led by Dr. Beatriz Villarroel of Nordic Institute of Theoretical Physics, Stockholm, Sweden, and Spain’s Instituto de Astrofísica de Canarias, used the 10.4 m Gran Telescopio Canarias (the largest optical telescope around the world) at Canary Islands, Spain, to do deep second epoch observations. The team hoped to find a counterpart at the position of every object that had appeared and vanished on the plate. The counterparts found are not necessarily physically connected to the weird objects.
The scientists are still exploring the reasons behind the observation of these strange transient stars and are still not sure about what triggered their appearance and disappearance. “The only thing we can say with certainty is that these images contain star-like objects that should not be there. We do not know why they are there,” says Dr. Alok C. Gupta.
The astronomers are examining the possibility that the photographic plates were contaminated with radioactive particles causing false stars on the plates. But if the observation is proven to be real, another option is solar reflections from reflective, unnatural objects in orbit around Earth several years before the first human satellite was launched.
The astronomers who belong to the collaboration Vanishing & Appearing Sources during a Century of Observations (VASCO) have still not sorted out the root cause of the “nine simultaneous transients”. They are now eager to look for more signatures of solar reflections in these digitized data from the 1950s in a hope to find aliens.
When a star like our Sun gets to be old, in another seven billion years or so, it will no longer be able to sustain burning its nuclear fuel. With only about half of its mass remaining it will shrink to a fraction of its radius and become a white dwarf star. White dwarf stars are common; over 95% of all stars will become white dwarfs. The most famous one is the companion to the brightest star in the sky, Sirius, but more particularly all stars known to host exoplanets will also end their lives as white dwarfs. Astronomers have established that planets orbiting stars can usually survive the late stages of their host’s evolution. The rocky planets are broken apart and disbursed into dusty debris disks, and so white dwarf stars should retain remnant evidence of their planetary companions. Emission from these dusty disks is seen as excess infrared radiation; when some of this material accretes onto the white dwarf itself, the elements produce features in the star’s spectrum. A small fraction, about 4%, of white dwarfs with dust disks also have gaseous components that have been seen in emission . While very rare – only about a dozen are known – these gaseous disk white dwarfs are thought to provide a particularly useful diagnostic of dynamical instabilities and disruption events in white dwarf disks, and astronomers have been on the lookout for more cases.
CfA astronomer Warren Brown was a member of a team that combined new optical observations from the Gaia space mission’s all sky survey with infrared catalog information to search for white dwarf stars whose infrared excesses signal the presence of a disk. They identified about 110 candidates which they followed up with optical spectroscopy using multiple ground-based telescopes, from which they discovered six new gaseous disk-hosting white dwarfs. Their analysis of the spectra of these objects revealed that the disks are more complex than expected: over 50 emission lines are seen and they differ in their widths, strengths, and shapes. The lines also have strikingly different variability characteristics, with some stars showing lines that barely vary at all over about three years of monitoring while in at least one host the lines vary by 50%. Many of the observed lines have profiles that permit kinematic modeling, for example indicating a flat disk rotating in so-called Keplerian motion (with the faster velocities closer to the star, as in the case of the planets in our solar system). The new results show that white dwarf stars have rich, dynamically active environments that can be used to better understand how a star’s system of planets evolves as the star enters old age.
Nitrogen, most of the time appears as colorless and odorless gas, which makes up 78% of the present atmosphere of the Earth. However, nitrogen is made in shining stars across the Universe, like most elements heavier than helium.
In our Milky Way, a small group of “field stars” (stars that are not located in star clusters) are found to have more nitrogen elements than others. These stars are named as “N-rich field stars”. How do they form and where do they come from are still a mystery.
Using high-resolution spectroscopic data from the Magellan Clay telescope, Dr. YU Jincheng and Prof. TANG Baitian from Sun Yat-sen University provide new observational evidence in revealing the origin of “N-rich field stars”. The study was published in The Astrophysical Journal.
Globular clusters (GCs) had been traditionally considered as simple stellar population systems, i.e., all member stars originate from the same molecular cloud, share the same age and chemical composition. However, an increasing number of studies show that almost all GCs host two or more groups of stars with different chemical abundances, which is the so-called multiple populations.
Stars with enhanced nitrogen, sodium, sometimes helium, aluminum, and silicon, but depleted carbon, oxygen, sometimes magnesium, are called the “second-generation” stars, distinct from the primordial “first-generation” stars.
Most scenarios trying to explain this phenomenon assume that chemically enriched second-generation stars were formed in light-element-polluted environment partially mixed with the ejecta of first-generation stars.
“The unique chemical pattern enables us to identify the second-generation stars from globular clusters, which is crucial to reveal the co-evolution of Milky Way and globular clusters,” said Dr. YU Jincheng, lead author of the study.
Prof. TANG Baitian, the corresponding author of this study, led a project that has found over one hundred N-rich field stars based on the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey, which is one of the largest homogenous sample of N-rich field stars for follow-up studies.
LAMOST is operated by the National Astronomical Observatories of Chinese Academy of Sciences (NAOC).
To fully understand their formation and origin, an international research team, including members from China and Chile, obtained the high-resolution spectra for 15 N-rich field stars over three nights in the Magellan Clay telescope. They measured the precise abundances of over twenty elements, which can be used as “name tags” for stars to find their birth places.
“Their chemical pattern is highly consistent with globular cluster second-generation stars,” said Dr. YU Jincheng. “It is a strong evidence to support their globular cluster origin.”
A very special N-rich field star with extreme low alpha-element (e.g., magnesium, calcium, silicon, titanium) abundances is spotted. “Its low alpha abundances are consistent with those of extragalactic stars, which imply slower chemical enrichment. The extragalactic origin of this star is also confirmed by its dynamics”, said Prof. TANG Baitian.
“Our findings also help us in understanding how N-rich field stars form, and how they leak into Milky Way,” supplemented by Dr. YU.
Featured image: The relation between abundances of alpha-elements and metallicity for different sources (N-rich field stars, globular clusters, Milky Way field stars, extragalactic field stars, etc.) (Image by YU Jincheng)
Any life identified on planets orbiting white dwarf stars almost certainly evolved after the star’s death, says a new study led by the University of Warwick that reveals the consequences of the intense and furious stellar winds that will batter a planet as its star is dying. The research is published in Monthly Notices of the Royal Astronomical Society, and lead author Dr Dimitri Veras will present it today (21 July) at the online National Astronomy Meeting (NAM 2021).
The research provides new insight for astronomers searching for signs of life around these dead stars by examining the impact that their winds will have on orbiting planets during the star’s transition to the white dwarf stage. The study concludes that it is nearly impossible for life to survive cataclysmic stellar evolution unless the planet has an intensely strong magnetic field – or magnetosphere – that can shield it from the worst effects.
In the case of Earth, solar wind particles can erode the protective layers of the atmosphere that shield humans from harmful ultraviolet radiation. The terrestrial magnetosphere acts like a shield to divert those particles away through its magnetic field. Not all planets have a magnetosphere, but Earth’s is generated by its iron core, which rotates like a dynamo to create its magnetic field.
“We know that the solar wind in the past eroded the Martian atmosphere, which, unlike Earth, does not have a large-scale magnetosphere. What we were not expecting to find is that the solar wind in the future could be as damaging even to those planets that are protected by a magnetic field”, says Dr Aline Vidotto of Trinity College Dublin, the co-author of the study.
All stars eventually run out of available hydrogen that fuels the nuclear fusion in their cores. In the Sun the core will then contract and heat up, driving an enormous expansion of the outer atmosphere of the star into a ‘red giant’. The Sun will then stretch to a diameter of tens of millions of kilometres, swallowing the inner planets, possibly including the Earth. At the same time the loss of mass in the star means it has a weaker gravitational pull, so the remaining planets move further away.
During the red giant phase, the solar wind will be far stronger than today, and it will fluctuate dramatically. Veras and Vidotto modelled the winds from 11 different types of stars, with masses ranging from one to seven times the mass of our Sun.
Their model demonstrated how the density and speed of the stellar wind, combined with an expanding planetary orbit, conspires to alternatively shrink and expand the magnetosphere of a planet over time. For any planet to maintain its magnetosphere throughout all stages of stellar evolution, its magnetic field needs to be at least one hundred times stronger than Jupiter’s current magnetic field.
The process of stellar evolution also results in a shift in a star’s habitable zone, which is the distance that would allow a planet to be the right temperature to support liquid water. In our solar system, the habitable zone would move from about 150 million km from the Sun – where Earth is currently positioned – up to 6 billion km, or beyond Neptune. Although an orbiting planet would also change position during the giant branch phases, the scientists found that the habitable zone moves outward more quickly than the planet, posing additional challenges to any existing life hoping to survive the process.
Eventually the red giant sheds its entire outer atmosphere, leaving behind the dense hot white dwarf remnant. These do not emit stellar winds, so once the star reaches this stage the danger to surviving planets has passed.
Dr Veras said: “This study demonstrates the difficulty of a planet maintaining its protective magnetosphere throughout the entirety of the giant branch phases of stellar evolution.”
“One conclusion is that life on a planet in the habitable zone around a white dwarf would almost certainly develop during the white dwarf phase unless that life was able to withstand multiple extreme and sudden changes in its environment.”
Future missions like the James Webb Space Telescope due to be launched later this year should reveal more about planets that orbit white dwarf stars, including whether planets within their habitable zones show biomarkers that indicate the presence of life, so the study provides valuable context to any potential discoveries.
So far no terrestrial planet that could support life around a white dwarf has been found, but two known gas giants are close enough to their star’s habitable zone to suggest that such a planet could exist. These planets likely moved in closer to the white dwarf as a result of interactions with other planets further out.
Dr Veras adds: “These examples show that giant planets can approach very close to the habitable zone. The habitable zone for a white dwarf is very close to the star because they emit much less light than a Sun-like star. However, white dwarfs are also very steady stars as they have no winds. A planet that’s parked in the white dwarf habitable zone could remain there for billions of years, allowing time for life to develop provided that the conditions are suitable.”
Focusing on MKG stars, Pascucci and colleagues suggested that most common planet around a lower mass stars is small in mass. They also suggested that the radius of planet depends on the stellar mass. Meaning, it increases with the stellar mass. Now, Lozovsky and colleagues explored why more massive stars tend to host larger planets. Using exoplanetary data of planets with measured masses and radius, they explored 3 possible explanations for the identified correlation between the planetary radius and stellar mass.
Case-1: Planets are larger due to thermal inflation: since more massive stars are more luminous, larger stellar irradiation could inflate the planetary radius.
Case-2: Planets around more massive stars are more massive and are in consequence larger for any given composition.
Case-3: Planets around more massive stars tend to be more volatile-rich and are therefore larger in size.
They first confirmed that planets surrounding larger stars tend to be larger in mass and radius.
Later, they showed that, larger radii of planets surrounding more massive stars (like G and K) cannot be explained by inflation due to higher irradiation of the star, nor by a higher planetary mass (i.e. Case 1 and 2): the effect of both properties on the radius was found to be significantly smaller than the observed difference. In other words, they showed that the difference in planetary mass, which is given by RV/TTV data, is insufficient to explain the observed difference in planetary radii, and the planetary temperature is even found to anticorrelate with the radius.
Finally, it has been shown that the larger planetary radii can be explained by a larger fraction of volatile material (H-He atmospheres) among planets surrounding more massive stars (i.e. by Case-3). This is because planets forming around more massive stars tend to accrete H-He atmospheres more efficiently. This can be a result of more massive protoplanetary disks that lead to faster core growth; allowing the cores to accrete substantial gaseous envelopes before the gas disk dissipates.
“Thus, we argue that planets around G- and K-stars are different by formation.”, they wrote. “It is desirable to have further constraints available to test our findings. Further information on the planetary composition would come from JWST and Ariel missions with constraints on atmospheric composition, which will greatly shed more light into the diversity of planetary volatile envelopes, their formation efficiencies and evolutions.” they concluded.
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Reference: Michael Lozovsky, Ravit Helled, Illaria Pascucci, Caroline Dorn, Julia Venturini, Robert Feldmann, “Why do more massive stars host larger planets?”, Arxiv, 2021. arXiv:2107.09534
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