Weighing Massive Stars In Nearby Galaxy Reveals Excess Of Heavyweights

An international team of astronomers has revealed an ‘astonishing’ overabundance of massive stars in a neighbouring galaxy.

The discovery, made in the gigantic star-forming region 30 Doradus in the Large Magellanic Cloud galaxy, has ‘far-reaching’ consequences for our understanding of how stars transformed the pristine Universe into the one we live in today.

The results are published in the journal Science.

Lead author Fabian Schneider, a Hintze Research Fellow in the University of Oxford’s Department of Physics, said: ‘We were astonished when we realised that 30 Doradus has formed many more massive stars than expected.’

As part of the VLT-FLAMES Tarantula Survey (VFTS), the team used ESO’s Very Large Telescope to observe nearly 1,000 massive stars in 30 Doradus, a gigantic stellar nursery also known as the Tarantula nebula. The team used detailed analyses of about 250 stars with masses between 15 and 200 times the mass of our Sun to determine the distribution of massive stars born in 30 Doradus — the so-called initial mass function (IMF).

Massive stars are particularly important for astronomers because of their enormous influence on their surroundings (known as their ‘feedback’). They can explode in spectacular supernovae at the end of their lives, forming some of the most exotic objects in the Universe — neutron stars and black holes.

Co-author Hugues Sana from the University of Leuven in Belgium said: ‘We have not only been surprised by the sheer number of massive stars, but also that their IMF is densely sampled up to 200 solar masses.’ Until recently, the existence of stars up to 200 solar masses was highly disputed, and the study shows that a maximum birth mass of stars of 200-300 solar masses appears likely.

In most parts of the Universe studied by astronomers to date, stars become rarer the more massive they are. The IMF predicts that most stellar mass is in low-mass stars and that less than 1% of all stars are born with masses in excess of ten times that of the Sun. Measuring the proportion of massive stars is extremely difficult — primarily because of their scarcity — and there are only a handful of places in the local Universe where this can be done.

The team turned to 30 Doradus, the biggest local star-forming region, which hosts some of the most massive stars ever found, and determined the masses of massive stars with unique observational, theoretical and statistical tools. This large sample allowed the scientists to derive the most accurate high-mass segment of the IMF to date, and to show that massive stars are much more abundant than previously thought. Chris Evans from the Science and Technology Facilities Council’s UK Astronomy Technology Centre, the principal investigator of VFTS and a co-author of the study, said: ‘In fact, our results suggest that most of the stellar mass is actually no longer in low-mass stars, but a significant fraction is in high-mass stars.’

Stars are cosmic engines and have produced most chemical elements heavier than helium, from the oxygen we breathe every day to the iron in our blood. During their lives, massive stars produce copious amounts of ionising radiation and kinetic energy through strong stellar winds. The ionising radiation of massive stars was crucial for the re-brightening of the Universe after the so-called Dark Ages, and their mechanical feedback drives the evolution of galaxies. Philipp Podsiadlowski, a co-author of the study from the University of Oxford, said: ‘To quantitatively understand all these feedback mechanisms, and hence the role of massive stars in the Universe, we need to know how many of these behemoths are born.’

Fabian Schneider added: ‘Our results have far-reaching consequences for the understanding of our cosmos: there might be 70% more supernovae, a tripling of the chemical yields and towards four times the ionising radiation from massive star populations. Also, the formation rate of black holes might be increased by 180%, directly translating into a corresponding increase of binary black hole mergers that have recently been detected via their gravitational wave signals.’

The team’s research leaves many open questions, which they intend to investigate in the future: how universal are the findings, and what are the consequences of this for the evolution of our cosmos and the occurrence of supernovae and gravitational wave events?

Astronomers Discover An M-Dwarf Eclipsing Binary System

Astronomers have found a new eclipsing binary system by analyzing archival survey data and conducting follow-up radial velocity measurements. The newly found binary, designated SDSSJ1156-0207, is composed of two M-dwarf stars orbiting each other at a relatively close distance. The finding is presented in a paper published December 24 on the arXiv pre-print repository.

M-dwarfs, especially in eclipsing binaries, could be crucial for improving our understanding about fundamental stellar parameters of low-mass stars. In eclipsing binaries, the orbit plane of the two stars lies so nearly in the line of sight of the observer that the components undergo mutual eclipses. Such systems can provide direct measurement of the mass, radius and effective temperature of stars.

Now, a group of researchers led by Chien-Hsiu Lee of the National Astronomical Observatory of Japan, has identified a new M-dwarf eclipsing binary system. The binary, which received designation SDSSJ1156-0207, was found in the data available in the Sloan Digital Sky Survey (SDSS) and in the Catalina Sky Survey (CSS). The newly detected object was later characterized by follow-up radial velocity measurements using Gemini Multi-Object Spectrograph onboard the Gemini North telescope in Hawaii.

“In this work we present a double-lined, M dwarf eclipsing binary discovered from cross matching Catalina Sky Surveys and Sloan Digital Sky Survey. The physical properties of this system are further characterized using Gemini telescope,” the astronomers wrote in the paper.

According to the study, SDSSJ1156-0207 is a very faint, double-lined M-dwarf eclipsing binary system with a very short period of approximately 0.3 days. Its primary component is about half the size and mass of our sun – with about 0.46 solar radii and 0.54 solar masses. The secondary star is approximately 30 percent the radius of the sun and has a mass of just 0.19 solar masses. Both stars are separated from each other by 0.0077 AU.

The astronomers noted that the very short period indicates that SDSSJ1156-0207 is tidally locked and therefore its orbit is circularized.

“We thus fix the eccentricity to be zero and only fit a non-eccentric orbit,” the paper reads. Moreover, they assume that the secondary star is inflated. This could be also due to tidal locking, which enhances stellar activity and inhibits convection.

Furthermore, the researchers estimated an effective temperature of the system. They reveal that the primary star has an effective temperature of 3,101 K, while the secondary component – 2,899 K.

In concluding remarks, the researchers underline the necessity of further observations of SDSSJ1156-0207, required to provide more detailed information about parameters of this system and to reveal more insights into the inflation mechanism in the secondary star.

“High resolution spectroscopy in the future will help narrow down the basic properties of this system. Further Hα observations will shed light on the stellar activity, providing constraints on the inflation mechanism due to tidal-locking,” the authors concluded.

Tabby’s Star: Alien Megastructure Not The Cause Of Dimming Of The ‘Most Mysterious Star In The Universe’

A team of more than 200 researchers, including Penn State Department of Astronomy and Astrophysics Assistant Professor Jason Wright and led by Louisiana State University’s Tabetha Boyajian, is one step closer to solving the mystery behind the “most mysterious star in the universe.” KIC 8462852, or “Tabby’s Star,” nicknamed after Boyajian, is otherwise an ordinary star, about 50 percent bigger and 1,000 degrees hotter than the Sun, and about than 1,000 light years away. However, it has been inexplicably dimming and brightening sporadically like no other. Several theories abound to explain the star’s unusual light patterns, including that an alien megastructure is orbiting the star.

The mystery of Tabby’s Star is so compelling that more than 1,700 people donated over $100,000 through a Kickstarter campaign in support of dedicated ground-based telescope time to observe and gather more data on the star through a network of telescopes around the world. As a result, a body of data collected by Boyajian and colleagues in partnership with the Las Cumbres Observatory is now available in a new paper in The Astrophysical Journal Letters.

“We were hoping that once we finally caught a dip happening in real time we could see if the dips were the same depth at all wavelengths. If they were nearly the same, this would suggest that the cause was something opaque, like an orbiting disk, planet, or star, or even large structures in space” said Wright, who is a co-author of the paper, titled “The First Post-Kepler Brightness Dips of KIC 8462852.” Instead, the team found that the star got much dimmer at some wavelengths than at others.

“Dust is most likely the reason why the star’s light appears to dim and brighten. The new data shows that different colors of light are being blocked at different intensities. Therefore, whatever is passing between us and the star is not opaque, as would be expected from a planet or alien megastructure,” Boyajian said.

The scientists closely observed the star through the Las Cumbres Observatory from March 2016 to December 2017. Beginning in May 2017 there were four distinct episodes when the star’s light dipped. Supporters from the crowdfunding campaign nominated and voted to name these episodes. The first two dips were named Elsie and Celeste. The last two were named after ancient lost cities — Scotland’s Scara Brae and Cambodia’s Angkor. The authors write that in many ways what is happening with the star is like these lost cities.

“They’re ancient; we are watching things that happened more than 1,000 years ago,” the authors wrote. “They’re almost certainly caused by something ordinary, at least on a cosmic scale. And yet that makes them more interesting, not less. But most of all, they’re mysterious.”

The method in which this star is being studied — by gathering and analyzing a flood of data from a single target — signals a new era of astronomy. Citizen scientists sifting through massive amounts of data from the NASA Kepler mission were the ones to detect the star’s unusual behavior in the first place. The main objective of the Kepler mission was to find planets, which it does by detecting the periodic dimming made from a planet moving in front of a star, and hence blocking out a tiny bit of starlight. The online citizen science group Planet Hunters was established so that volunteers could help to classify light curves from the Kepler mission and to search for such planets.

“If it wasn’t for people with an unbiased look on our universe, this unusual star would have been overlooked,” Boyajian said. “Again, without the public support for this dedicated observing run, we would not have this large amount of data.”

Now there are more answers to be found. “This latest research rules out alien megastructures, but it raises the plausibility of other phenomena being behind the dimming,” Wright said. “There are models involving circumstellar material — like exocomets, which were Boyajian’s team’s original hypothesis — which seem to be consistent with the data we have.” Wright also points out that “some astronomers favor the idea that nothing is blocking the star — that it just gets dimmer on its own — and this also is consistent with this summer’s data.”

Boyajian said, “It’s exciting. I am so appreciative of all of the people who have contributed to this in the past year — the citizen scientists and professional astronomers. It’s quite humbling to have all of these people contributing in various ways to help figure it out.”

Solar Wind and Earth’s Magnetic Field Transports Charged Particles to the Moon

A team of Japanese planetary researchers led by Osaka University’s Professor Kentaro Terada has discovered solar wind and Earth’s magnetic field can transport high-energy ions of biogenic oxygen from the atmosphere of our planet to the lunar surface.

“The Earth is protected from solar wind and cosmic rays by the planet’s magnetic field,” Professor Terada and colleagues explained.

On Earth’s night side, its magnetic field is extended like a comet tail and makes a space like a streamer (we call it a ‘geotail’). At the center of the geotail, there is an area which exists as a sheet-like structure of hot plasma.”

In a paper published the journal Nature Astronomy, the researchers report observations from the Japanese spacecraft Kaguya of significant numbers of high-energy oxygen ions, seen only when the Moon was in the Earth’s plasma sheet.

“We succeeded in observing that oxygen from the ionosphere of Earth was transported to the Moon 236,000 miles (380,000 km) away,” they said.

Examing plasma data of Kaguya’s Magnetic field and Plasma experiment/Plasma energy Angle and Composition experiment (MAP-PACE) about 62 miles (100 km) above the Moon’s surface, and discovered that high-energy oxygen ions appeared only when the Moon and the spacecraft crossed the plasma sheet.

Oxygen ions detected by the team had a high energy of 1-10 keV. These ions can be implanted into a depth of tens of nanometers of a metal particle. This is a very important finding in understanding the complicated isotopic composition of oxygen on the lunar regolith, which has long been a mystery.

Through observations, we demonstrated the possibility that components that lack 16O, which is a stable isotope of oxygen and is observed in the ozone layer, a region of Earth’s stratosphere, were transported to the Moon surface and implanted into a depth of tens of nanometers on the surface of lunar soils.

Our Kaguya observation of significant Earth wind from the current geomagnetic field strengthens the hypothesis that information on the lost ancient atmosphere of our planet could be preserved on the surface of lunar soils.