Hubble ‘cranes’ in for a closer look at a galaxy

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IC 5201 sits over 40 million light-years away from us. As with two thirds of all the spirals we see in the universe — including the Milky Way, the galaxy has a bar of stars slicing through its center. Credit: ESA/Hubble & NASA

In 1900, astronomer Joseph Lunt made a discovery: Peering through a telescope at Cape Town Observatory, the British-South African scientist spotted this beautiful sight in the southern constellation of Grus (The Crane): a barred spiral galaxy now named IC 5201.

Over a century later, the galaxy is still of interest to astronomers. For this image, the NASA/ESA Hubble Space Telescope used its Advanced Camera for Surveys (ACS) to produce a beautiful and intricate image of the galaxy. Hubble’s ACS can resolve individual stars within other galaxies, making it an invaluable tool to explore how various populations of stars sprang to life, evolved, and died throughout the cosmos.

IC 5201 sits over 40 million light-years away from us. As with two thirds of all the spirals we see in the Universe—including the Milky Way—the galaxy has a bar of stars slicing through its center.

Hubble unveils monster stars

Astronomers using the unique ultraviolet capabilities of the NASA/ESA Hubble Space Telescope have identified nine monster stars with masses over 100 times the mass of the Sun in the star cluster R136. This makes it the largest sample of very massive stars identified to date. The results, which will be published in the Monthly Notices of the Royal Astronomical Society, raise many new questions about the formation of massive stars.

An international team of scientists using the NASA/ESA Hubble Space Telescope has combined images taken with the Wide Field Camera 3 (WFC3) with the unprecedented ultraviolet spatial resolution of the Space Telescope Imaging Spectrograph (STIS) to successfully dissect the young star cluster R136 in the ultraviolet for the first time.

R136 is only a few light-years across and is located in the Tarantula Nebula within the Large Magellanic Cloud, about 170 000 light-years away. The young cluster hosts many extremely massive, hot and luminous stars whose energy is mostly radiated in the ultraviolet. This is why the scientists probed the ultraviolet emission of the cluster.

As well as finding dozens of stars exceeding 50solar masses, this new study was able to reveal a total number of nine very massive stars in the cluster, all more than 100 times more massive as the Sun. However, the current record holder R136a1 does keep its place as the most massive star known in the Universe, at over 250 solar masses. The detected stars are not only extremely massive, but also extremely bright. Together these nine stars outshine the Sun by a factor of 30 million.

The scientists were also able to investigate outflows from these behemoths, which are most readily studied in the ultraviolet. They eject up to an Earth mass of material per month at a speed approaching one percent of the speed of light, resulting in extreme weight loss throughout their brief lives.

“The ability to distinguish ultraviolet light from such an exceptionally crowded region into its component parts, resolving the signatures of individual stars, was only made possible with the instruments aboard Hubble,” explains Paul Crowther from the University of Sheffield, UK, and lead author of the study. “Together with my colleagues, I would like to acknowledge the invaluable work done by astronauts during Hubble’s last servicing mission: they restored STIS and put their own lives at risk for the sake of future science!”

In 2010 Crowther and his collaborators showed the existence of four stars within R136, each with over 150 times the mass of the Sun. At that time the extreme properties of these stars came as a surprise as they exceeded the upper-mass limit for stars that was generally accepted at that time. Now, this new census has shown that there are five more stars with more than 100 solar masses in R136. The results gathered from R136 and from other clusters also raise many new questions about the formation of massive stars as the origin of these behemoths remains unclear.

Saida Caballero-Nieves, a co-author of the study, explains: “There have been suggestions that these monsters result from the merger of less extreme stars in close binary systems. From what we know about the frequency of massive mergers, this scenario can’t account for all the really massive stars that we see in R136, so it would appear that such stars can originate from the star formation process.”

In order to find answers about the origin of these stars the team will continue to analyse the gathered datasets. An analysis of new optical STIS observations will also allow them to search for close binary systems in R136, which could producemassive black hole binaries which would ultimately merge, producing gravitational waves.

“Once again, our work demonstrates that, despite being in orbit for over 25 years, there are some areas of science for which Hubble is still uniquely capable,” concludes Crowther.

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Hubble observes calcium-rich supernovae

The NASA/ESA Hubble Space Telescope offers a multitude of spectacular images of celestial objects and a huge amount of scientific data helpful for astronomers. A team of scientists from UK and Sweden has recently made use of Hubble to study the host galaxies and environments of five calcium-rich supernovae that could provide new insights on the evolution of stellar systems. Their research waspublished online on Feb. 25 in the arXiv journal.

Calcium-rich supernovae, also called calcium-rich transients, are a type of supernova that eject a preponderance of calcium into space, less luminous than other supernova types and evolving more rapidly. According to previous studies, a large fraction of them are found at significant distances from the nearest galaxy, well outside the bulk of the stellar light. It is believed that this type of transient may be a major producer of calcium in the universe.

The researchers, led by Joseph Lyman of the University of Warwick, UK, employed Hubble’s Advanced Camera for Surveys/Wide Field Channel (ACS/WFC) and Wide Field Camera 3 (WFC3) to obtain images of five calcium-rich supernovae, among which three exhibit large offsets and two are coincident with the disk of their hosts.

“Our sample consists of five examples of the calcium-rich supernova class, which were targeted with Hubble for two orbits each,” the scientists wrote in a paper published on arXiv.

The supernovae, lying well outside their hosts, are designated SN 2003dr, SN 2005E and SN 2007ke, and were observed using ACS. The scientists found no detected sources underlying the locations of these supernovae, ruling out the presence ofmassive stars, dwarf galaxies and globular clusters at these locations.

SN 2003dr is the most interesting of this group as it is quite complex. It lies offset along the minor axis of the galaxy and thus off the disk light, although it is relatively close in linear distance. Hubble images also show a strong tidal feature that passes through the location of this supernova along the southern and western sides of the galaxy.

The two calcium-rich transients appearing to be in line-of-sight with the disks of late type galaxies, named SN 2001co and SN 2003dg, were imaged by Hubble’s WFC3. The team discovered that they both display strong patchy star formation and significant dust lanes – typical for their morphological types.

“In each case, the transient’s location appears close to regions of star formation. (…) Furthermore, the lack of distinguishable underlying sources at their locations is in agreement with the findings for the remote sample, although in these cases there is clearly an underlying stellar population from the galactic disk,” the paper reads.

Thanks to the new research, almost every calcium-rich supernova located within 300 million light years has been observed in detail. The study also confirmed that the majority of host galaxies of these transients are disturbed or merging systems.

However, how these calcium-rich supernovae form is still debated. It could be due to white dwarf mergers with neutron stars due to the collapse of massive stars. According to Lyman, the mechanism of the supernova explosion could cause the neutron star to be ‘kicked’ to very high velocities.

“This high-velocity system can then escape its galaxy, and if the binary system survives the kick, the white dwarf and neutron star will merge causing the explosive transient,” Lyman said.

The scientists concluded that as new members of the class emerge, it will be prudent to further test this apparent bias of calcium-rich supernovaeproduction in disturbed and merging systems.

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Scientists get first glimpse of black hole eating star, ejecting high-speed flare

An international team of astrophysicists led by a Johns Hopkins University scientist has for the first time witnessed a star being swallowed by a black hole and ejecting a flare of matter moving at nearly the speed of light.

The finding reported Thursday in the journal Science tracks the star—about the size of our sun—as it shifts from its customary path, slips into the gravitational pull of a supermassive black hole and is sucked in, said Sjoert van Velzen, a Hubble fellow at Johns Hopkins.

“These events are extremely rare,” van Velzen said. “It’s the first time we see everything from the stellar destruction followed by the launch of a conical outflow, also called a jet, and we watched it unfold over several months.”

Black holes are areas of space so dense that irresistible gravitational force stops the escape of matter, gas and even light, rendering them invisible and creating the effect of a void in the fabric of space. Astrophysicists had predicted that when a black hole is force-fed a large amount of gas, in this case a whole star, then a fast-moving jet of plasma – elementary particles in a magnetic field – can escape from near the black hole rim, or “event horizon.” This study suggests this prediction was correct, the scientists said.

“Previous efforts to find evidence for these jets, including my own, were late to the game,” said van Velzen, who led the analysis and coordinated the efforts of 13 other scientists in the United States, the Netherlands, Great Britain and Australia.

Supermassive black holes, the largest of black holes, are believed to exist at the center of most massive galaxies. This particular one lies at the lighter end of the supermassive black hole spectrum, at only about a million times the mass of our sun, but still packing the force to gobble a star.

The first observation of the star being destroyed was made by a team at the Ohio State University, using an optical telescope in Hawaii. That team announced its discovery on Twitter in early December 2014.

After reading about the event, van Velzen contacted an astrophysics team led by Rob Fender at the University of Oxford in Great Britain. That group used radio telescopes to follow up as fast as possible. They were just in time to catch the action.

By the time it was done, the international team had data from satellites and ground-based telescopes that gathered X-ray, radio and optical signals, providing a stunning “multi-wavelength” portrait of this event.

It helped that the galaxy in question is closer to Earth than those studied previously in hopes of tracking a jet emerging after the destruction of a star. This galaxy is about 300 million light years away, while the others were at least three times farther away. One light year is 5.88 trillion miles.

The first step for the international team was to rule out the possibility that the light was from a pre-existing expansive swirling mass called an “accretion disk” that forms when a black hole is sucking in matter from space. That helped to confirm that the sudden increase of light from the galaxy was due to a newly trapped star.

“The destruction of a star by a black hole is beautifully complicated, and far from understood,” van Velzen said. “From our observations, we learn the streams of stellar debris can organize and make a jet rather quickly, which is valuable input for constructing a complete theory of these events.”

Van Velzen last year completed his doctoral dissertation at Radboud University in the Netherlands, where he studied jets from supermassive black holes. In the last line of the dissertation, he expressed his hope to discover these events within four years. It turned out to take only a few months after the ceremony for his dissertation defense.

Van Velzen and his team were not the only ones to hunt for radio signals from this particular unlucky star. A group at Harvard observed the same source with radio telescopes in New Mexico and announced its results online. Both teams presented results at a workshop in Jerusalem in early November. It was the first time the two competing teams had met face to face.

“The meeting was an intense, yet very productive exchange of ideas about this source,” van Velzen said. “We still get along very well; I actually went for a long hike near the Dead Sea with the leader of the competing group.”

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