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Water on exoplanet cloud tops could be found with hi-tech instrumentation

University of Warwick astronomers have shown that water vapour can potentially be detected in the atmospheres of exoplanets by peering literally over the tops of their impenetrable clouds.

By applying the technique to models based upon known exoplanets with clouds the team has demonstrated in principle that high resolution spectroscopy can be used to examine the atmospheres of exoplanets that were previously too difficult to characterise due to clouds that are too dense for sufficient light to pass through.

Their technique is described in a paper for the Monthly Notices of the Royal Astronomical Society and provides another method for detecting the presence of water vapour in an exoplanet’s atmosphere — as well as other chemical species that could be used in future to assess potential signs of life. The research received funding from the Science and Technologies Facilities Council (STFC), part of UK Research and Innovation (UKRI).

Astronomers use light from a planet’s host star to learn what its atmosphere is composed of. As the planet passes in front of the star they observe the transmission of the stellar light as it skims through the upper atmosphere and alters its spectrum. They can then analyse this spectrum to look at wavelengths that have spectral signatures for specific chemicals. These chemicals, such as water vapour, methane and ammonia, are only present in trace quantities in these hydrogen and helium rich planets.

However, dense clouds can block that light from passing through the atmosphere, leaving astronomers with a featureless spectrum. High resolution spectroscopy is a relatively recent technique that is being used in ground-based observatories to observe exoplanets in greater detail, and the Warwick researchers wanted to explore whether this technology could be used to detect the trace chemicals present in the thin atmospheric layer right above those clouds.

While astronomers have been able to characterise the atmospheres of many larger and hotter exoplanets that orbit close to their stars, termed ‘hot Jupiters’, smaller exoplanets are now being discovered at cooler temperatures (less than 700°C). Many of these planets, which are the size of Neptune or smaller, have shown much thicker cloud.

They modelled two previously known ‘warm Neptunes’ and simulated how the light from their star would be detected by a high resolution spectrograph. GJ3470b is a cloudy planet that astronomers had previously been able to characterise, while GJ436b has been harder to characterise due to a much thicker cloud layer. Both simulations demonstrated that at high resolution you can detect chemicals such as water vapour, ammonia and methane easily with just a few nights of observations with a ground-based telescope.

The technique works differently from the method recently used to detect phosphine on Venus, but could potentially be used to search for any type of molecule in the clouds of a planet outside of our solar system, including phosphine.

Lead author Dr Siddharth Gandhi of the Department of Physics at the University of Warwick said: “We have been investigating whether ground-based high resolution spectroscopy can help us to constrain the altitude in the atmosphere where we have clouds, and constrain chemical abundances despite those clouds.

“What we are seeing is that a lot of these planets have got water vapour on them, and we’re starting to see other chemicals as well, but the clouds are preventing us from seeing these molecules clearly. We need a way to detect these species and high resolution spectroscopy is a potential way of doing that, even if there is a cloudy atmosphere.

“The chemical abundances can tell you quite a lot about how the planet may have formed because it leaves its chemical fingerprint on the molecules in the atmosphere. Because these are gas giants, detecting the molecules at the top of the atmosphere also offers a window into the internal structure as the gases mix with the deeper layers.”

The majority of observations of exoplanets have been done using space-based telescopes such as Hubble or Spitzer, and their resolution is too low to detect sufficient signal from above the clouds. High resolution spectroscopy’s advantage is that it is capable of probing a wider range of altitudes.

Dr Gandhi adds: “Quite a lot of these cooler planets are far too cloudy to get any meaningful constraints with the current generation of space telescopes. Presumably as we find more and more planets there’s going to be more cloudy planets, so it’s becoming really important to detect what’s on them. Ground based high resolution spectroscopy as well as the next generation of space telescopes will be able to detect these trace species on cloudy planets, offering exciting potential for biosignatures in the future.”

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Enormous planet quickly orbiting a tiny, dying star

Thanks to a bevy of telescopes in space and on Earth — and even a pair of amateur astronomers in Arizona — a University of Wisconsin-Madison astronomer and his colleagues have discovered a Jupiter-sized planet orbiting at breakneck speed around a distant white dwarf star. The system, about 80 light years away, violates all common conventions about stars and planets. The white dwarf is the remnant of a sun-like star, greatly shrunken down to roughly the size of Earth, yet it retains half the sun’s mass. The massive planet looms over its tiny star, which it circles every 34 hours thanks to an incredibly close orbit. In contrast, Mercury takes a comparatively lethargic 90 days to orbit the sun. While there have been hints of large planets orbiting close to white dwarfs in the past, the new findings are the clearest evidence yet that these bizarre pairings exist. That confirmation highlights the diverse ways stellar systems can evolve and may give a glimpse at our own solar system’s fate. Such a white dwarf system could even provide a rare habitable arrangement for life to arise in the light of a dying star.

“We’ve never seen evidence before of a planet coming in so close to a white dwarf and surviving. It’s a pleasant surprise,” says lead researcher Andrew Vanderburg, who recently joined the UW-Madison astronomy department as an assistant professor. Vanderburg completed the work while an independent NASA Sagan Fellow at the University of Texas at Austin.

The researchers published their findings Sept. 16 in the journal Nature. Vanderburg led a large, international collaboration of astronomers who analyzed the data. The contributing telescopes included NASA’s exoplanet-hunting telescope TESS and two large ground-based telescopes in the Canary Islands.

Vanderburg was originally drawn to studying white dwarfs — the remains of sun-sized stars after they exhaust their nuclear fuel — and their planets by accident. While in graduate school, he was reviewing data from TESS’s predecessor, the Kepler space telescope, and noticed a white dwarf with a cloud of debris around it.

“What we ended up finding was that this was a minor planet or asteroid that was being ripped apart as we watched, which was really cool,” says Vanderburg. The planet had been destroyed by the star’s gravity after its transition to a white dwarf caused the planet’s orbit to fall in toward the star.

Ever since, Vanderburg has wondered if planets, especially large ones, could survive the journey in toward an aging star.

By scanning data for thousands of white dwarf systems collected by TESS, the researchers spotted a star whose brightness dimmed by half about every one-and-a-half days, a sign that something big was passing in front of the star on a tight, lightning-fast orbit. But it was hard to interpret the data because the glare from a nearby star was interfering with TESS’s measurements. To overcome this obstacle, the astronomers supplemented the TESS data from higher-resolution ground-based telescopes, including three run by amateur astronomers.

“Once the glare was under control, in one night, they got much nicer and much cleaner data than we got with a month of observations from space,” says Vanderburg. Because white dwarfs are so much smaller than normal stars, large planets passing in front of them block a lot of the star’s light, making detection by ground-based telescopes much simpler.

The data revealed that a planet roughly the size of Jupiter, perhaps a little larger, was orbiting very close to its star. Vanderburg’s team believes the gas giant started off much farther from the star and moved into its current orbit after the star evolved into a white dwarf.

The question became: how did this planet avoid being torn apart during the upheaval? Previous models of white dwarf-planet interactions didn’t seem to line up for this particular star system.

The researchers ran new simulations that provided a potential answer to the mystery. When the star ran out of fuel, it expanded into a red giant, engulfing any nearby planets and destabilizing the Jupiter-sized planet that orbited farther away. That caused the planet to take on an exaggerated, oval orbit that passed very close to the now-shrunken white dwarf but also flung the planet very far away at the orbit’s apex.

Over eons, the gravitational interaction between the white dwarf and its planet slowly dispersed energy, ultimately guiding the planet into a tight, circular orbit that takes just one-and-a-half days to complete. That process takes time — billions of years. This particular white dwarf is one of the oldest observed by the TESS telescope at almost 6 billion years old, plenty of time to slow down its massive planet partner.

While white dwarfs no longer conduct nuclear fusion, they still release light and heat as they cool down. It’s possible that a planet close enough to such a dying star would find itself in the habitable zone, the region near a star where liquid water can exist, presumed to be required for life to arise and survive.

Now that research has confirmed these systems exist, they offer a tantalizing opportunity for searching for other forms of life. The unique structure of white dwarf-planet systems provides an ideal opportunity to study the chemical signatures of orbiting planets’ atmospheres, a potential way to search for signs of life from afar.

“I think the most exciting part of this work is what it means for both habitability in general — can there be hospitable regions in these dead solar systems — and also our ability to find evidence of that habitability,” says Vanderburg.

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Possible marker of life spotted on venus

An international team of astronomers today announced the discovery of a rare molecule — phosphine — in the clouds of Venus. On Earth, this gas is only made industrially or by microbes that thrive in oxygen-free environments. Astronomers have speculated for decades that high clouds on Venus could offer a home for microbes — floating free of the scorching surface but needing to tolerate very high acidity. The detection of phosphine could point to such extra-terrestrial ‘aerial’ life.

“When we got the first hints of phosphine in Venus’s spectrum, it was a shock!,” says team leader Jane Greaves of Cardiff University in the UK, who first spotted signs of phosphine in observations from the James Clerk Maxwell Telescope (JCMT), operated by the East Asian Observatory, in Hawai’i. Confirming their discovery required using 45 antennas of the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a more sensitive telescope in which the European Southern Observatory (ESO) is a partner. Both facilities observed Venus at a wavelength of about 1 millimetre, much longer than the human eye can see — only telescopes at high altitude can detect it effectively.

The international team, which includes researchers from the UK, US and Japan, estimates that phosphine exists in Venus’s clouds at a small concentration, only about twenty molecules in every billion. Following their observations, they ran calculations to see whether these amounts could come from natural non-biological processes on the planet. Some ideas included sunlight, minerals blown upwards from the surface, volcanoes, or lightning, but none of these could make anywhere near enough of it. These non-biological sources were found to make at most one ten thousandth of the amount of phosphine that the telescopes saw.

To create the observed quantity of phosphine (which consists of hydrogen and phosphorus) on Venus, terrestrial organisms would only need to work at about 10% of their maximum productivity, according to the team. Earth bacteria are known to make phosphine: they take up phosphate from minerals or biological material, add hydrogen, and ultimately expel phosphine. Any organisms on Venus will probably be very different to their Earth cousins, but they too could be the source of phosphine in the atmosphere.

While the discovery of phosphine in Venus’s clouds came as a surprise, the researchers are confident in their detection. “To our great relief, the conditions were good at ALMA for follow-up observations while Venus was at a suitable angle to Earth. Processing the data was tricky, though, as ALMA isn’t usually looking for very subtle effects in very bright objects like Venus,” says team member Anita Richards of the UK ALMA Regional Centre and the University of Manchester. “In the end, we found that both observatories had seen the same thing — faint absorption at the right wavelength to be phosphine gas, where the molecules are backlit by the warmer clouds below,” adds Greaves, who led the study published today in Nature Astronomy.

Another team member, Clara Sousa Silva of the Massachusetts Institute of Technology in the US, has investigated phosphine as a “biosignature” gas of non-oxygen-using life on planets around other stars, because normal chemistry makes so little of it. She comments: “Finding phosphine on Venus was an unexpected bonus! The discovery raises many questions, such as how any organisms could survive. On Earth, some microbes can cope with up to about 5% of acid in their environment — but the clouds of Venus are almost entirely made of acid.”

The team believes their discovery is significant because they can rule out many alternative ways to make phosphine, but they acknowledge that confirming the presence of “life” needs a lot more work. Although the high clouds of Venus have temperatures up to a pleasant 30 degrees Celsius, they are incredibly acidic — around 90% sulphuric acid — posing major issues for any microbes trying to survive there.

ESO astronomer and ALMA European Operations Manager Leonardo Testi, who did not participate in the new study, says: “The non-biological production of phosphine on Venus is excluded by our current understanding of phosphine chemistry in rocky planets’ atmospheres. Confirming the existence of life on Venus’s atmosphere would be a major breakthrough for astrobiology; thus, it is essential to follow-up on this exciting result with theoretical and observational studies to exclude the possibility that phosphine on rocky planets may also have a chemical origin different than on Earth.”

More observations of Venus and of rocky planets outside our Solar System, including with ESO’s forthcoming Extremely Large Telescope, may help gather clues on how phosphine can originate on them and contribute to the search for signs of life beyond Earth.

This research was presented in the paper “Phosphine Gas in the Cloud Decks of Venus” to appear in Nature Astronomy.

The team is composed of Jane S. Greaves (School of Physics & Astronomy, Cardiff University, UK [Cardiff]), Anita M. S. Richards (Jodrell Bank Centre for Astrophysics, The University of Manchester, UK), William Bains (Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, USA [MIT]), Paul Rimmer (Department of Earth Sciences and Cavendish Astrophysics, University of Cambridge and MRC Laboratory of Molecular Biology, Cambridge, UK), Hideo Sagawa (Department of Astrophysics and Atmospheric Science, Kyoto Sangyo University, Japan), David L. Clements (Department of Physics, Imperial College London, UK [Imperial]), Sara Seager (MIT), Janusz J. Petkowski (MIT), Clara Sousa-Silva (MIT), Sukrit Ranjan (MIT), Emily Drabek-Maunder (Cardiff and Royal Observatory Greenwich, London, UK), Helen J. Fraser (School of Physical Sciences, The Open University, Milton Keynes, UK), Annabel Cartwright (Cardiff), Ingo Mueller-Wodarg (Imperial), Zhuchang Zhan (MIT), Per Friberg (EAO/JCMT), Iain Coulson (EAO/JCMT), E’lisa Lee (EAO/JCMT) and Jim Hoge (EAO/JCMT).

An accompanying paper by some of team members, titled “The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere,” was published in Astrobiology in August 2020. Another related study by some of the same authors, “Phosphine as a Biosignature Gas in Exoplanet Atmospheres,” was published in Astrobiology in January 2020.

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Can a black hole fire up the cold heart of the Phoenix Galaxy Cluster?

Radio astronomers have detected jets of hot gas blasted out by a black hole in the galaxy at the heart of the Phoenix Galaxy Cluster, located 5.9 billion light-years away in the constellation Phoenix. This is an important result for understanding the coevolution of galaxies, gas, and black holes in galaxy clusters.

Galaxies are not distributed randomly in space. Through mutual gravitational attraction, galaxies gather together to form collections known as clusters. The space between galaxies is not entirely empty. There is very dilute gas throughout a cluster which can be detected by X-ray observations.

If this intra-cluster gas cooled, it would condense under its own gravity to form stars at the center of the cluster. However, cooled gas and stars are not usually observed in the hearts of nearby clusters, indicating that some mechanism must be heating the intra-cluster gas and preventing star formation. One potential candidate for the heat source is jets of high-speed gas accelerated by a super-massive black hole in the central galaxy.

The Phoenix Cluster is unusual in that it does show signs of dense cooled gas and massive star formation around the central galaxy. This raises the question, “does the central galaxy have black hole jets as well?”

A team led by Takaya Akahori at the National Astronomical Observatory of Japan used the Australia Telescope Compact Array (ATCA) to search for black hole jets in the Phoenix Galaxy Cluster with the highest resolution to date. They detected matching structures extending out from opposite sides of the central galaxy. Comparing with observations of the region taken from the Chandra X-ray Observatory archive data shows that the structures detected by ATCA correspond to cavities of less dense gas, indicating that they are a pair of bipolar jets emitted by a black hole in the galaxy. Therefore, the team discovered the first example, in which intra-cluster gas cooling and black hole jets coexist, in the distant Universe.

Further details of the galaxy and jets could be elucidated through higher-resolution observations with next generation observational facilities, such as the Square Kilometre Array scheduled to start observations in the late 2020s.

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Cool new worlds found in our cosmic backyard

How complete is our census of the Sun’s closest neighbors? Astronomers using NSF’s NOIRLab facilities and a team of data-sleuthing volunteers participating in Backyard Worlds: Planet 9, a citizen science project, have discovered roughly 100 cool worlds near the Sun — objects more massive than planets but lighter than stars, known as brown dwarfs. Several of these newly discovered worlds are among the very coolest known, with a few approaching the temperature of Earth — cool enough to harbor water clouds.

Discovering and characterizing astronomical objects near the Sun is fundamental to our understanding of our place in, and the history of, the Universe. Yet astronomers are still unearthing new residents of the Solar neighborhood. A remarkable breakthrough was announced today, with the discovery of roughly 100 cool brown dwarfs near the Sun [1].The new Backyard Worlds discoveries bridge a previously empty gap in the range of low-temperature brown dwarfs, identifying a long-sought missing link within the brown dwarf population.

These cool worlds offer the opportunity for new insights into the formation and atmospheres of planets beyond the Solar System,” said Aaron Meisner from the National Science Foundation’s NOIRLab and the lead author of the research paper. “This collection of cool brown dwarfs also allows us to accurately estimate the number of free-floating worlds roaming interstellar space near the Sun.”

This major advancement was made possible with archival data from the Nicholas U. Mayall 4-meter Telescope at Kitt Peak National Observatory (KPNO) and the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), which were made available through the Community Science and Data Center (CSDC), all programs of NSF’s NOIRLab. Large survey data sets were then made available to the Backyard Worlds volunteers using NOIRLab’s Astro Data Lab science platform. The results, to be published in TheAstrophysical Journal, demonstrate the rapidly growing role of survey and archival data research in astronomy today.

Brown dwarfs lie somewhere between the most massive planets and the smallest stars. Lacking the mass needed to sustain nuclear reactions in their core, brown dwarfs resemble cooling embers. Their low mass, low temperature and lack of internal nuclear reactions make them extremely faint — and therefore extremely difficult to detect. Because of this, when searching for the very coolest brown dwarfs, astronomers can only hope to detect such objects relatively close to the Sun.

To help find our Sun’s coldest and nearest neighbors, the astronomers of the Backyard Worlds project turned to a worldwide network of more than 100,000 citizen scientists [2]. These volunteers diligently inspect trillions of pixels of telescope images to identify the subtle movements of brown dwarfs and planets. Despite the abilities of machine learning and supercomputers, there’s no substitute for the human eye when it comes to scouring telescope images for moving objects.

The keen eyes of the Backyard Worlds volunteers have already discovered more than 1,500 cold worlds near to the Sun, and today’s paper presents roughly 100 of the coldest in that sample. According to Meisner, this is a record for any citizen science program by a factor of about 20, and 20 citizen scientists are listed as co-authors of the study. A handful of these cool worlds — which are among the very coldest brown dwarfs known — approach the temperature of Earth. NASA’s Spitzer Space Telescope provided the brown dwarf temperature estimates [3].

Brown dwarfs are expected to cool as they age, passing from near-stellar temperatures down to planetary temperatures and below, fading all the while and eventually winking out. The new discoveries attest to this picture by uncovering elusive examples of brown dwarfs approaching Earth-temperature.

“This paper is evidence that the solar neighborhood is still uncharted territory and citizen scientists are excellent astronomical cartographers,” said co-author Jackie Faherty of the American Museum of Natural History. “Mapping the coldest brown dwarfs down to the lowest masses gives us key insights into the low-mass star formation process while providing a target list for detailed studies of the atmospheres of Jupiter analogs.”

Citizen scientist, Astro Data Lab user, and paper co-author Jim Walla added, “It’s awesome to know that our discoveries are now counted among the Sun’s neighbors and will be targets of further research.”

Alongside the dedicated efforts of the Backyard Worlds volunteers, NOIRLab’s Astro Data Lab was instrumental in this research. The technical burden of downloading billion-object astronomical catalogs is typically insurmountable for individual investigators — including most professional astronomers. “AstroData Lab’s open and accessible web portal allowed Backyard Worlds citizen scientists to easily query massive catalogs for brown dwarf candidates,” explained NOIRLab astronomer Stephanie Juneau, who helped introduce the citizen scientists to Astro Data Lab. Astro Data Lab also enables convenient matching between data sets from NOIRLab telescopes and external facilities, such as NASA’s WISE satellite, that jointly contributed to these brown dwarf discoveries.

In addition to Astro Data Lab’s making data accessible to the Backyard Worlds collaboration, archival observations by telescopes at two other NOIRLab Programs — CTIO and KPNO — were also key to this discovery. “Wide-area imaging from NOIRLab’s Mayall and Blanco telescopes was also critical,” explained Aaron Meisner. “To select only the very coldest brown dwarfs, we inspected deep images from a variety of sensitive astronomical surveys.”

“It’s great to see such thrilling results from NOIRLab’s efforts to broaden participation in astronomy research,” said Chris Davis of the National Science Foundation, the US agency that supports operations at the Kitt Peak and Cerro Tololo observatories and at CSDC. “By making archival data from NSF’s Mayall and Blanco telescopes publicly available and easily accessible through CSDC, folks with a fascination for astronomy can make a real contribution to science and to our understanding of the Universe.”

The approach of the Backyard Worlds project — searching for rare objects in large data sets — is also one of the goals for the upcoming Vera C. Rubin Observatory [4]. Currently under construction on Cerro Pachón in the Chilean Andes, Rubin Observatory will image the visible sky from the southern hemisphere every three nights over ten years, providing a vast amount of data that will enable new ways of doing astrophysical research.

“Vast modern data sets can unlock landmark discoveries, and it’s exciting that these could be spotted first by a citizen scientist,” concludes Aaron Meisner. “These Backyard Worlds discoveries show that members of the public can play an important role in reshaping our scientific understanding of our solar neighborhood.”

Notes

[1] The closest of these new discoveries is roughly 23 light-years away from the Sun. Many more of these brown dwarfs are in the 30-60 light-year distance range.

[2] Backyard Worlds: Planet 9 is hosted by Zooniverse.

[3] Complementary follow-up observations were also supplied by Keck Observatory, Mont Mégantic Observatory, and Carnegie Institution for Science’s Las Campanas Observatory.

[4] Rubin Observatory and Department of Energy (DOE) Legacy Survey of Space and Time Camera are operated by NSF’s NOIRLab and SLAC National Accelerator Laboratory (SLAC).

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Stellar egg hunt with ALMA

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) took a census of stellar eggs in the constellation Taurus and revealed their evolution state. This census helps researchers understand how and when a stellar embryo transforms to a baby star deep inside a gaseous egg. In addition, the team found a bipolar outflow, a pair of gas streams, that could be telltale evidence of a truly newborn star.

Stars are formed by gravitational contraction of gaseous clouds. The densest parts of the clouds, called molecular cloud cores, are the very sites of star formation and mainly located along the Milky Way. The Taurus Molecular Cloud is one of the active star-forming regions and many telescopes have been pointed at the cloud. Previous observations show that some cores are actually stellar eggs before the birth of stars, but others already have infant stars inside.

A research team led by Kazuki Tokuda, an astronomer at Osaka Prefecture University and the National Astronomical Observatory of Japan (NAOJ), utilized the power of ALMA to investigate the inner structure of the stellar eggs. They observed 32 starless cores and nine cores with baby protostars. They detected radio waves from all of the nine cores with stars, but only 12 out of 32 starless cores showed a signal. The team concluded that these 12 eggs have developed internal structures, which shows they are more evolved than the 20 quite cores.

“Generally speaking, radio interferometers using many antennas, like ALMA, are not good at observing featureless objects like stellar eggs,” says Tokuda. “But in our observations, we purposely used only the 7-m antennas of ALMA. This compact array enables us to see objects with smooth structure, and we got information about the internal structure of the stellar eggs, just as we intended.”

Increasing the spacing between the antennas improves the resolution of a radio interferometer, but makes it difficult to detect extended objects. On the other hand, a compact array has lower resolution but allows us to see extended objects. This is why the team used ALMA’s compact array of 7-m antennas, as known as the Morita Array, not the extended array of 12-m antennas.

They found that there is a difference between the two groups in the gas density at the center of the dense cores. Once the density of the center of a dense core exceeds a certain threshold, about one million hydrogen molecules per cubic centimeter, self-gravity leads the egg to transform into a star.

A census is also useful for finding a rare object. The team noticed that there is a weak but clear bipolar gas stream in one stellar egg. The size of the stream is rather small, and no infrared source has been identified in the dense core. These characteristics match well with the theoretical predictions of a “first hydrostatic core,” a short-lived object formed just before the birth of a baby star. “Several candidates for the first hydrostatic cores have been identified in other regions,” explains Kakeru Fujishiro, a member of the research team. “This is the first identification in the Taurus region. It is a good target for future extensive observation.”

Kengo Tachihara, an associate professor at Nagoya University mentions the role of Japanese researchers in this study. “Japanese astronomers have studied the baby stars and stellar eggs in Taurus using the Nagoya 4-m radio telescope and Nobeyama 45-m radio telescope since the 1990s. And, ALMA’s 7-m array was also developed by Japan. The present result is part of the culmination of these efforts.”

“We have succeeded in illustrating the growth history of stellar eggs up to their birth, and now we have established the method for the research,” summarizes Tokuda. “This is an important step to obtain a comprehensive understanding of star formation.”

 

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Hubble uses Earth as proxy for identifying oxygen on potentially habitable exoplanets

Taking advantage of a total lunar eclipse, astronomers using NASA’s Hubble Space Telescope have detected Earth’s own brand of sunscreen — ozone — in our atmosphere. This method simulates how astronomers and astrobiology researchers will search for evidence of life beyond Earth by observing potential “biosignatures” on exoplanets (planets around other stars).

Hubble did not look at Earth directly. Instead, the astronomers used the Moon as a mirror to reflect sunlight, which had passed through Earth’s atmosphere, and then reflected back towards Hubble. Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of transiting exoplanets. These atmospheres may contain chemicals of interest to astrobiology, the study of and search for life.

Though numerous ground-based observations of this kind have been done previously, this is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope. Hubble detected the strong spectral fingerprint of ozone, which absorbs some of the sunlight. Ozone is important to life because it is the source of the protective shield in Earth’s atmosphere.

On Earth, photosynthesis over billions of years is responsible for our planet’s high oxygen levels and thick ozone layer. That’s one reason why scientists think ozone or oxygen could be a sign of life on another planet, and refer to them as biosignatures.

“Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life,” explained Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, lead researcher of Hubble’s observations.

Although ozone in Earth’s atmosphere had been detected in previous ground-based observations during lunar eclipses, Hubble’s study represents the strongest detection of the molecule to date because ozone — as measured from space with no interference from other chemicals in the Earth’s atmosphere — absorbs ultraviolet light so strongly.

Hubble recorded ozone absorbing some of the Sun’s ultraviolet radiation that passed through the edge of Earth’s atmosphere during a lunar eclipse that occurred on January 20 to 21, 2019. Several other ground-based telescopes also made spectroscopic observations at other wavelengths during the eclipse, searching for more of Earth’s atmospheric ingredients, such as oxygen and methane.

“One of NASA’s major goals is to identify planets that could support life,” Youngblood said. “But how would we know a habitable or an uninhabited planet if we saw one? What would they look like with the techniques that astronomers have at their disposal for characterizing the atmospheres of exoplanets? That’s why it’s important to develop models of Earth’s spectrum as a template for categorizing atmospheres on extrasolar planets.”

Her paper is available online in The Astronomical Journal.

Sniffing Out Planetary Atmospheres

The atmospheres of some extrasolar planets can be probed if the alien world passes across the face of its parent star, an event called a transit. During a transit, starlight filters through the backlit exoplanet’s atmosphere. (If viewed close up, the planet’s silhouette would look like it had a thin, glowing “halo” around it caused by the illuminated atmosphere, just as Earth does when seen from space.)

Chemicals in the atmosphere leave their telltale signature by filtering out certain colors of starlight. Astronomers using Hubble pioneered this technique for probing exoplanets. This is particularly remarkable because extrasolar planets had not yet been discovered when Hubble was launched in 1990 and the space observatory was not initially designed for such experiments.

So far, astronomers have used Hubble to observe the atmospheres of gas giant planets and super-Earths (planets several times Earth’s mass) that transit their stars. But terrestrial planets about the size of Earth are much smaller objects and their atmospheres are thinner, like the skin on an apple. Therefore, teasing out these signatures from Earth-sized exoplanets will be much harder.

That’s why researchers will need space telescopes much larger than Hubble to collect the feeble starlight passing through these small planets’ atmospheres during a transit. These telescopes will need to observe planets for a longer period, many dozens of hours, to build up a strong signal.

To prepare for these bigger telescopes, astronomers decided to conduct experiments on a much closer and only known inhabited terrestrial planet: Earth. Our planet’s perfect alignment with the Sun and Moon during a total lunar eclipse mimics the geometry of a terrestrial planet transiting its star.

But the observations were also challenging because the Moon is very bright, and its surface is not a perfect reflector because it is mottled with bright and dark areas. The Moon is also so close to Earth that Hubble had to try and keep a steady eye on one select region, despite the Moon’s motion relative to the space observatory. So, Youngblood’s team had to account for the Moon’s drift in their analysis.

Where There’s Ozone, There’s Life?

Finding ozone in the skies of a terrestrial extrasolar planet does not guarantee that life exists on the surface. “You would need other spectral signatures in addition to ozone to conclude that there was life on the planet, and these signatures cannot necessarily be seen in ultraviolet light,” Youngblood said.

On Earth, ozone is formed naturally when oxygen in the Earth’s atmosphere is exposed to strong concentrations of ultraviolet light. Ozone forms a blanket around Earth, protecting it from harsh ultraviolet rays.

“Photosynthesis might be the most productive metabolism that can evolve on any planet, because it is fueled by energy from starlight and uses cosmically abundant elements like water and carbon dioxide,” said Giada Arney of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, a co-author of the science paper. “These necessary ingredients should be common on habitable planets.”

Seasonal variability in the ozone signature also could indicate seasonal biological production of oxygen, just as it does with the growth seasons of plants on Earth.

But ozone can also be produced without the presence of life when nitrogen and oxygen are exposed to sunlight. To increase confidence that a given biosignature is truly produced by life, astronomers must search for combinations of biosignatures. A multiwavelength campaign is needed because each of the many biosignatures are more easily detected at wavelengths specific to those signatures.

“Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren’t strong enough,” Arney explained. “We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets.”

NASA has a forthcoming observatory called the James Webb Space Telescope that could make similar kinds of measurements in infrared light, with the potential to detect methane and oxygen in exoplanet atmospheres. Webb is currently scheduled to launch in 2021.

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Strange dismembered star cluster found at Galaxy’s edge

An international team of astronomers has discovered the remnant of an ancient collection of stars that was torn apart by our own galaxy, the Milky Way, more than two billion years ago.

The extraordinary discovery of this shredded ‘globular cluster’ is surprising, as the stars in this galactic archaeological find have much lower quantities of heavier elements than in other such clusters. The evidence strongly suggests the original structure was the last of its kind, a globular cluster whose birth and life were different to those remaining today.

Our Galaxy is home to about 150 globular clusters, each a ball of a million or so stars that orbit in the Galaxy’s tenuous stellar halo. These globular clusters are old and have witnessed the growth of the Milky Way over billions of years.

The study, published in Nature, was led by University of Sydney PhD student, Zhen Wan, and his supervisor, Professor Geraint Lewis, as part of the S5 international collaboration.

Using the Anglo-Australian Telescope in outback New South Wales, this collaboration measured the speeds of a stream of stars in the Phoenix constellation, revealing them to be remnants of a globular cluster that was pulled apart by the gravity of the Milky Way about two billion years ago.

Mr Wan said: “Once we knew which stars belonged to the stream, we measured their abundance of elements heavier than hydrogen and helium; something astronomers refer to as metallicity. We were really surprised to find that the Phoenix Stream has a very low metallicity, making it distinctly different to all of the other globular clusters in the Galaxy.

“Even though the cluster was destroyed billions of years ago, we can still tell it formed in the early Universe from the composition of its stars.”

HEAVY METALS

After the Big Bang, only hydrogen and helium existed in any substantial amount in the Universe. These elements formed the first generation of stars many billions of years ago. It is within these and later stellar generations that heavier elements were formed, such as the calcium, oxygen and phosphorus that in part make up your bones.

Observations of other globular clusters have found that their stars are enriched with heavier elements forged in earlier generations of stars. Current formation theories suggest that this dependence on previous stars means that no globular cluster should be found unenriched and that there is a minimum metallicity ‘floor’ below which no cluster can form.

But the metallicity of the Phoenix Stream progenitor sits well below this minimum, posing a significant problem for our ideas of globular cluster origins.

“This stream comes from a cluster that, by our understanding, shouldn’t have existed,” said co-author Associate Professor Daniel Zucker from Macquarie University.

S5 team leader, Dr Ting Li from Carnegie Observatories, said: “One possible explanation is that the Phoenix Stream represents the last of its kind, the remnant of a population of globular clusters that was born in radically different environments to those we see today.”

While potentially numerous in the past, this population of globular clusters was steadily depleted by the gravitational forces of the Galaxy, which tore them to pieces, absorbing their stars into the main body of the galactic system. This means that the stream is a relatively temporary phenomenon, which will dissipate in time.

“We found the remains of this cluster before it faded forever into the Galaxy’s halo,” Mr Wan said.

As yet, there is no clear explanation for the origins of the Phoenix Stream progenitor cluster and where it sits in the evolution of galaxies remains unclear.

Professor Lewis said: “There is plenty of theoretical work left to do. There are now many new questions for us to explore about how galaxies and globular clusters form, which is incredibly exciting.”

Is the Phoenix Stream unique? “In astronomy, when we find a new kind of object, it suggests that there are more of them out there,” said co-author Dr Jeffrey Simpson from the University of New South Wales. While globular clusters like the progenitor of the Phoenix Stream may no longer exist, their remnants may live on as faint streams.”

Dr Li said: “The next question to ask is whether there are more ancient remnants out there, the leftovers of a population that no longer exists. Finding more such streams will give us a new view of what was going on in the early Universe.”

“This is a regime we have hardly explored. It’s a very exciting time,” she said.

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How galaxies die: New insights into the quenching of star formation

Astronomers studying galaxy evolution have long struggled to understand what causes star formation to shut down in massive galaxies. Although many theories have been proposed to explain this process, known as “quenching,” there is still no consensus on a satisfactory model.

Now, an international team led by Sandra Faber, professor emerita of astronomy and astrophysics at UC Santa Cruz, has proposed a new model that successfully explains a wide range of observations about galaxy structure, supermassive black holes, and the quenching of star formation. The researchers presented their findings in a paper published July 1 in the Astrophysical Journal.

The model supports one of the leading ideas about quenching which attributes it to black hole “feedback,” the energy released into a galaxy and its surroundings from a central supermassive black hole as matter falls into the black hole and feeds its growth. This energetic feedback heats, ejects, or otherwise disrupts the galaxy’s gas supply, preventing the infall of gas from the galaxy’s halo to feed star formation.

“The idea is that in star-forming galaxies, the central black hole is like a parasite that ultimately grows and kills the host,” Faber explained. “That’s been said before, but we haven’t had clear rules to say when a black hole is big enough to shut down star formation in its host galaxy, and now we have quantitative rules that actually work to explain our observations.”

The basic idea involves the relationship between the mass of the stars in a galaxy (stellar mass), how spread out those stars are (the galaxy’s radius), and the mass of the central black hole. For star-forming galaxies with a given stellar mass, the density of stars in the center of the galaxy correlates with the radius of the galaxy so that galaxies with bigger radii have lower central stellar densities. Assuming that the mass of the central black hole scales with the central stellar density, star-forming galaxies with larger radii (at a given stellar mass) will have lower black-hole masses.

What that means, Faber explained, is that larger galaxies (those with larger radii for a given stellar mass) have to evolve further and build up a higher stellar mass before their central black holes can grow large enough to quench star formation. Thus, small-radius galaxies quench at lower masses than large-radius galaxies.

“That is the new insight, that if galaxies with large radii have smaller black holes at a given stellar mass, and if black hole feedback is important for quenching, then large-radius galaxies have to evolve further,” she said. “If you put together all these assumptions, amazingly, you can reproduce a large number of observed trends in the structural properties of galaxies.”

This explains, for example, why more massive quenched galaxies have higher central stellar densities, larger radii, and larger central black holes.

Based on this model, the researchers concluded that quenching begins when the total energy emitted from the black hole is approximately four times the gravitational binding energy of the gas in the galactic halo. The binding energy refers to the gravitational force that holds the gas within the halo of dark matter enveloping the galaxy. Quenching is complete when the total energy emitted from the black hole is twenty times the binding energy of the gas in the galactic halo.

Faber emphasized that the model does not yet explain in detail the physical mechanisms involved in the quenching of star formation. “The key physical processes that this simple theory evokes are not yet understood,” she said. “The virtue of this, though, is that having simple rules for each step in the process challenges theorists to come up with physical mechanisms that explain each step.”

Astronomers are accustomed to thinking in terms of diagrams that plot the relations between different properties of galaxies and show how they change over time. These diagrams reveal the dramatic differences in structure between star-forming and quenched galaxies and the sharp boundaries between them. Because star formation emits a lot of light at the blue end of the color spectrum, astronomers refer to “blue” star-forming galaxies, “red” quiescent galaxies, and the “green valley” as the transition between them. Which stage a galaxy is in is revealed by its star formation rate.

One of the study’s conclusions is that the growth rate of black holes must change as galaxies evolve from one stage to the next. The observational evidence suggests that most of the black hole growth occurs in the green valley when galaxies are beginning to quench.

“The black hole seems to be unleashed just as star formation slows down,” Faber said. “This was a revelation, because it explains why black hole masses in star-forming galaxies follow one scaling law, while black holes in quenched galaxies follow another scaling law. That makes sense if black hole mass grows rapidly while in the green valley.”

Faber and her collaborators have been discussing these issues for many years. Since 2010, Faber has co-led a major Hubble Space Telescope galaxy survey program (CANDELS, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey), which produced the data used in this study. In analyzing the CANDELS data, she has worked closely with a team led by Joel Primack, UCSC professor emeritus of physics, which developed the Bolshoi cosmological simulation of the evolution of the dark matter halos in which galaxies form. These halos provide the scaffolding on which the theory builds the early star-forming phase of galaxy evolution before quenching.

The central ideas in the paper emerged from analyses of CANDELS data and first struck Faber about four years ago. “It suddenly leaped out at me, and I realized if we put all these things together — if galaxies had a simple trajectory in radius versus mass, and if black hole energy needs to overcome halo binding energy — it can explain all these slanted boundaries in the structural diagrams of galaxies,” she said.

At the time, Faber was making frequent trips to China, where she has been involved in research collaborations and other activities. She was a visiting professor at Shanghai Normal University, where she met first author Zhu Chen. Chen came to UC Santa Cruz in 2017 as a visiting researcher and began working with Faber to develop these ideas about galaxy quenching.

“She is mathematically very good, better than me, and she did all of the calculations for this paper,” Faber said.

Faber also credited her longtime collaborator David Koo, UCSC professor emeritus of astronomy and astrophysics, for first focusing attention on the central densities of galaxies as a key to the growth of central black holes.

Among the puzzles explained by this new model is a striking difference between our Milky Way galaxy and its very similar neighbor Andromeda. “The Milky Way and Andromeda have almost the same stellar mass, but Andromeda’s black hole is almost 50 times bigger than the Milky Way’s,” Faber said. “The idea that black holes grow a lot in the green valley goes a long way toward explaining this mystery. The Milky Way is just entering the green valley and its black hole is still small, whereas Andromeda is just exiting so its black hole has grown much bigger, and it is also more quenched than the Milky Way.”

In addition to Faber, Chen, Koo, and Primack, the coauthors of the paper include researchers at some two dozen institutions in seven countries. This work was funded by grants from NASA and the National Science Foundation.

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Stellar fireworks celebrate birth of giant cluster

Astronomers created a stunning new image showing celestial fireworks in star cluster G286.21+0.17.

Most stars in the universe, including our Sun, were born in massive star clusters. These clusters are the building blocks of galaxies, but their formation from dense molecular clouds is still largely a mystery.

The image of cluster G286.21+0.17, caught in the act of formation, is a multi-wavelength mosaic made out of more than 750 individual radio observations with the Atacama Large Millimeter/submillimeter Array (ALMA) and 9 infrared images from the NASA/ESA Hubble Space Telescope. The cluster is located in the Carina region of our galaxy, about 8000 light-years away.

Dense clouds made of molecular gas (purple ‘fireworks streamers’) are revealed by ALMA. The telescope observed the motions of turbulent gas falling into the cluster, forming dense cores that ultimately create individual stars.

The stars in the image are revealed by their infrared light, as seen by Hubble, including a large group of stars bursting out from one side of the cloud. The powerful winds and radiation from the most massive of these stars are blasting away the molecular clouds, leaving faint wisps of glowing, hot dust (shown in yellow and red).

“This image shows stars in various stages of formation within this single cluster,” said Yu Cheng of the University of Virginia in Charlottesville, Virginia, and lead author of two papers published in The Astrophysical Journal.

Hubble revealed about a thousand newly-formed stars with a wide range of masses. Additionally, ALMA showed that there is a lot more mass present in dense gas that still has to undergo collapse. “Overall the process may take at least a million years to complete,” Cheng added.

“This illustrates how dynamic and chaotic the process of star birth is,” said co-author Jonathan Tan of Chalmers University in Sweden and the University of Virginia and principal investigator of the project. “We see competing forces in action: gravity and turbulence from the cloud on one side, and stellar winds and radiation pressure from the young stars on the other. This process sculpts the region. It is amazing to think that our own Sun and planets were once part of such a cosmic dance.”

“The phenomenal resolution and sensitivity of ALMA are evident in this stunning image of star formation,” said Joe Pesce, NSF Program Officer for NRAO/ALMA. “Combined with the Hubble Space Telescope data we can clearly see the power of multiwavelength observations to help us understand these fundamental universal processes.”

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