X-Ray Emission from Massive Stars

A composite image of the massive star forming region Cygnus OB2. Astronomers have determined that the X-ray emission from massive stars arises from shocks. The image shows X-ray emission from Chandra (blue), infrared from Spitzer (red), and optical data from the Isaac Newton Telescope (orange). Credit: X-ray: NASA/CXC/SAO/J.Drake et al, Infrared: NASA/JPL-Caltech, Optical: Univ. of Hertfordshire/INT/IPHAS

Massive young stars are known to emit strong X-rays. Unlike the X-ray emission from lower mass stars, however, which arises in stellar photospheres, the X-rays from massive stars are thought to result from powerful shocks. Several kinds of shocks can be responsible, produced either by very strong winds driven by the star’s radiation, by the head-on collision between winds that have been magnetically channeled by the star’s magnetic field, or by wind collisions in a binary stellar system in which each stars has a wind. Sorting out the mechanisms enables astronomers to identify the most active physical processes at work, and thereby decode additional information about the star’s physical makeup and evolutionary status.

CfA astronomers Nick Wright, Jeremy Drake, and Marcio Guarcello and their colleagues used the Chandra X-ray Observatory to study the emission from 106 massive stars in the relatively nearby Cygnus-OB2 cluster. This relatively large sample enabled the scientist to test their models by examining, for example, whether or not there are clear correlations between a star’s X-ray strength and its luminosity.

The astronomers find for their massive stars that there is a well-defined correlation between the X-ray and total stellar luminosity, with the X-ray strength being about sixteen million times less; indeed, their relation is similar to one previously reported for another massive star-forming region, and favors the first (radiatively driven) kind of shocks. For the most massive stars in the sample, however, the team does find evidence for colliding shocks. The new results help to constrain models of X-ray emission from massive stars. Because the relations are about the same as in other massive star clusters but now extended to different clusters and cluster environments, the new work also shows that the mechanisms are not very sensitive to the local conditions.

Reference(s):

"X-Ray Emission from Massive Stars in Cyg OB2," G. Rauw, Y. Nazé, N. J. Wright, J. J. Drake, M. G. Guarcello, R. K. Prinja, L. W. Peck, J. F. Albacete Colombo, A. Herrero, H. A. Kobulnicky, S. Sciortino, and J. S. Vink, ApJS 221, 1, 2015.

Source: Smithsonian Astrophysical Observatory (SAO)

Spirals in Dust Around Young Stars May Betray Presence of Massive Planets

MWC 758

[Right] — Observations taken by the European Southern Observatory's Very Large Telescope show a protoplanetary disk around the young star MWC 758. The disk has two spiral arms that extend over 10 billion miles from the star.

[Left] — A computer model reproduces the two-spiral-arm structure; the "x" is the location of a putative planet. The planet, which cannot be seen directly, probably excites the two spiral arms.

Photo Credit: NASA, ESA, ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z. Zhu (Princeton University).  Release Images

A team of astronomers is proposing that huge spiral patterns seen around some newborn stars, merely a few million years old (about one percent our sun's age), may be evidence for the presence of giant, unseen planets. This idea not only opens the door to a new method of planet detection, but also could offer a look into the early formative years of planet birth.

Though astronomers have cataloged thousands of planets orbiting other stars, the very earliest stages of planet formation are elusive because nascent planets are born and embedded inside vast, pancake-shaped disks of dust and gas encircling newborn stars, known as circumstellar disks.

The conclusion that planets may betray their presence by modifying circumstellar disks on large scales is based on detailed computer modeling of how gas-and-dust disks evolve around newborn stars, which was conducted by two NASA Hubble Fellows, Ruobing Dong of Lawrence Berkeley National Laboratory, and Zhaohuan Zhu of Princeton University. Their research was published in the Aug. 5 edition of The Astrophysical Journal Letters.

"It's difficult to see suspected planets inside a bright disk surrounding a young star. Based on this study, we are convinced that planets can gravitationally excite structures in the disk. So if you can identify features in a disk and convince yourself those features are created by an underlying planet that you cannot see, this would be a smoking gun of forming planets," Dong said.

Identifying large-scale features produced by planets offers another method of planet detection that is quite different from all other techniques presently used. This approach can help astronomers find currently forming planets, and address when, how, and where planets form.

Gaps and rings seen in other circumstellar disks suggest invisible planets embedded in the disk. However gaps, presumably swept clean by a planet's gravity, often do not help show the location of the planet. Also, because multiple planets together may open a single common gap, it’s very challenging to estimate their numbers and masses.

Ground-based telescopes have photographed two large-scale spiral arms around two young stars, SAO 206462 and MWC 758. A few other nearby stars also show smaller spiral-like features. "How they are created has been a big mystery until now. Scientists had a hard time explaining these features," Dong said. If the disks were very massive, they would have enough self-gravity to become unstable and set up wave-like patterns. But the disks around SAO 206462 and MWC 758 are probably just a few percent of the central star's mass and therefore are not gravitationally unstable.

The team generated computer simulations of the dynamics of a disk and how a star's radiation propagates through a disk with embedded planets. This modeling created spiral structures that very closely resemble observations. The mutual gravitational interaction between the disk and the planet creates regions where the density of gas and dust increases, like traffic backing up on a crowded expressway. The differential rotation of the disk around the star smears these over-dense regions into spiral waves. "Although it had been speculated that planets can produce spiral arms, we now think we know how," said Zhu.

"Simulations also suggest that these spiral arms have rich information about the unseen planet, revealing not only its position but also its mass," Zhu said. The simulations show that if there were no planet present, the disk would look smooth. To make the grand-scale spiral arms seen in the SAO 206462 and MWC 758 systems, the unseen planet would have to be bulky, at least 10 times the mass of Jupiter, the largest planet in our solar system.

The first planet orbiting a normal star was identified in 1995. Thanks to ground-based telescopes and NASA's Kepler mission, a few thousand exoplanets have been cataloged to date. But because the planets are in mature systems, many millions or a few billion years old, they offer little direct clues as to how they formed.

"There are many theories about how planets form but very little work based on direct observational evidence confirming these theories," Dong said. "If you see signs of a planet in a disk right now, it tells you when, where, and how planets form."

Astronomers will use the upcoming NASA James Webb Space Telescope to probe circumstellar disks and look for features, as simulated by the modeling, and will then try to directly observe the predicted planet causing the density waves.

Contacts

Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4514
villard@stsci.edu

Ruobing Dong
Lawrence Berkeley National Laboratory, and University of California, Berkeley, California
rdong2013@berkeley.edu

Zhaohuan Zhu
Princeton University, Princeton, New Jersey
zhzhu@astro.princeton.edu

Source: HubbleSite

Planet-hunting SPHERE Images First Circumbinary Planet System with Disc

 HD 106906AB

Credit: ESO, A. M. Lagrange (Université Grenoble Alpes)

Observations by ESO’s planet-finding instrument, SPHERE, a high-contrast adaptive optics system installed on the third Unit Telescope of ESO’s Very Large Telescope, have revealed the edge-on disc of gas and dust present around the binary star system HD 106906AB.

HD 106906AB is a double star located in the constellation of Crux (The Southern Cross). Astronomers had long suspected that this 13 million-year-old stellar duo was encircled by a debris disc, due to the system’s youth and characteristic radiation. However, this disc had remained unseen — until now! The system’s spectacular debris disc can be seen towards the lower left area of this image. It is surrounding both stars, hence its name of circumbinary disc. The stars themselves are hidden behind a mask which prevent their glare from blinding the instrument.

These stars and the disc are also accompanied by an exoplanet, visible in the upper right, named HD 106906 b, which orbits around the binary star and its disc at a distance greater than any other exoplanet discovered to date — 650 times the average Earth–Sun distance, or nearly 97 billion kilometres. HD 106906 b has a mammoth mass of up to 11 times that of Jupiter, and a scorching surface temperature of 1500 degrees Celsius.

Thanks to SPHERE, HD 106906AB has become the first binary star system to have both an exoplanet and a debris disc successfully imaged, providing astronomers with a unique opportunity to study the complex process of circumbinary planet formation.

Source: ESO/Images

VISTA Discovers New Component of Milky Way

PR Image eso1542a

VISTA finds hidden feature of Milky Way

Videos

 

PR Video eso1542a

VISTA finds hidden feature of Milky Way

PR Video eso1542b

VISTA finds hidden feature of Milky Way

Astronomers using the VISTA telescope at ESO’s Paranal Observatory have discovered a previously unknown component of the Milky Way. By mapping out the locations of a class of stars that vary in brightness called Cepheids, a disc of young stars buried behind thick dust clouds in the central bulge has been found.

The Vista Variables in the Vía Láctea Survey (VVV) [1] ESO public survey is using the VISTA telescope at the Paranal Observatory to take multiple images at different times of the central parts of the galaxy at infrared wavelengths [2]. It is discovering huge numbers of new objects, including variable stars, clusters and exploding stars (eso1101, eso1128, eso1141).

A team of astronomers, led by Istvan Dékány of the Pontificia Universidad Católica de Chile, has now used data from this survey, taken between 2010 and 2014, to make a remarkable discovery — a previously unknown component of our home galaxy, the Milky Way.

“The central bulge of the Milky Way is thought to consist of vast numbers of old stars. But the VISTA data has revealed something new — and very young by astronomical standards!” says Istvan Dékány, lead author of the new study.

Analysing data from the survey, the astronomers found 655 candidate variable stars of a type called Cepheids. These stars expand and contract periodically, taking anything from a few days to months to complete a cycle and changing significantly in brightness as they do so.

The time taken for a Cepheid to brighten and fade again is longer for those that are brighter and shorter for the dimmer ones. This remarkably precise relationship, which was discovered in 1908 by American astronomer Henrietta Swan Leavitt, makes the study of Cepheids one of the most effective ways to measure the distances to, and map the positions of, distant objects in the Milky Way and beyond.

But there is a catch — Cepheids are not all the same — they come in two main classes, one much younger than the other. Out of their sample of 655 the team identified 35 stars as belonging to a sub-group called classical Cepheids — young bright stars, very different from the usual, much more elderly, residents of the central bulge of the Milky Way.

The team gathered information on the brightness, pulsation period, and deduced the distances of these 35 classical Cepheids. Their pulsation periods, which are closely linked to their age, revealed their surprising youth.

“All of the 35 classical Cepheids discovered are less than 100 million years old. The youngest Cepheid may even be only around 25 million years old, although we cannot exclude the possible presence of even younger and brighter Cepheids,” explains the study’s second author Dante Minniti, of the Universidad Andres Bello, Santiago, Chile.

The ages of these classical Cepheids provide solid evidence that there has been a previously unconfirmed, continuous supply of newly formed stars into the central region of the Milky Way over the last 100 million years. But, this wasn’t to be the only remarkable discovery from the survey’s dataset.

Mapping the Cepheids that they discovered, the team traced an entirely new feature in the Milky Way — a thin disc of young stars across the galactic bulge. This new component to our home galaxy had remained unknown and invisible to previous surveys as it was buried behind thick clouds of dust. Its discovery demonstrates the unique power of VISTA, which was designed to study the Milky Way’s deep structures by wide-field, high-resolution imaging at infrared wavelengths.

“This study is a powerful demonstration of the unmatched capabilities of the VISTA telescope for probing extremely obscured galactic regions that cannot be reached by any other current or planned surveys,” remarks Dékány.

“This part of the galaxy was completely unknown until our VVV survey found it!” adds Minniti.

Further investigations are now needed to assess whether these Cepheids were born close to where they are now, or whether they originate from further out. Understanding their fundamental properties, interactions, and evolution is key in the quest to understand the evolution of the Milky Way, and the process of galaxy evolution as a whole.

Notes

[1] The VVV survey is observing the central parts of our galaxy in five near-infrared bands. The total area of this survey is 520 square degrees and contains at least 355 open and 33 globular clusters. The VVV is multi-epoch in nature in order to detect a large number of variable objects and will provide more than 100 carefully spaced observations at different times for each part of the sky covered. A catalogue with about a billion point sources including about a million variable objects is expected. These will be used to create a three-dimensional map of the bulge of the Milky Way galaxy.

[2] The dust clouds in interstellar space absorb and scatter visible light very effectively and make them opaque. But at longer wavelengths, such as those observed by VISTA, the clouds are much more transparent, allowing the regions beyond the dust to be probed.

More Information

This research was presented in a paper entitled “The VVV Survey reveals classical Cepheids tracing a young and thin stellar disk across the Galaxy’s bulge”, by I. Dékány et al., in the Astrophysical Journal Letters.

The team is composed of I. Dékány (Instituto Milenio de Astrofísica, Santiago, Chile; Pontificia Universidad Católica de Chile, Santiago, Chile), D. Minniti (Universidad Andres Bello, Santiago, Chile; Instituto Milenio de Astrofísica MAS and Basal CATA, Santiago, Chile; and Vatican Observatory, Vatican City State), D. Majaess (Saint Mary’s University, Halifax, Nova Scotia, Canada; Mount Saint Vincent University, Halifax, Nova Scotia, Canada) , M. Zoccali (Pontificia Universidad Católica de Chile, Santiago, Chile; Instituto Milenio de Astrofísica, Santiago, Chile), G. Hajdu (Pontificia Universidad Católica de Chile, Santiago, Chile; Instituto Milenio de Astrofísica, Santiago, Chile), J. Alonso-García (Universidad de Antofagasta, Antofagasta, Chile; Instituto Milenio de Astrofísica, Santiago, Chile), M. Catelan (Pontificia Universidad Católica de Chile, Santiago, Chile; Instituto Milenio de Astrofísica, Santiago, Chile), W. Gieren (Universidad de Concepción, Concepción, Chile; Instituto Milenio de Astrofísica, Santiago, Chile) and J. Borissova (Universidad de Valparaíso, Valparaíso, Chile; Instituto Milenio de Astrofísica, Santiago, Chile).

ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

Links

• Research paper
• VVV Survey webpages
• Photos of VISTA

Contacts

Istvan Dékány
Instituto Milenio de Astrofí­sica, Pontificia Universidad Católica de Chile
Santiago, Chile
Email: idekany@astro.puc.cl

Dante Minniti
Universidad Andres Bello
Santiago, Chile
Tel: +56 2 2661 8732
Email: dante@astrofisica.cl

Daniel Majaess
Saint Mary’s University, Mount Saint Vincent University
Halifax, Canada
Email: dmajaess@ap.smu.ca

Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Source: ESO

Black Hole Has Major Flare

This diagram shows how a shifting feature, called a corona, can create a flare of X-rays around a black hole
Image credit: NASA/JPL-Caltech.  › Full image and caption

The baffling and strange behaviors of black holes have become somewhat less mysterious recently, with new observations from NASA's Explorer missions Swift and the Nuclear Spectroscopic Telescope Array, or NuSTAR. The two space telescopes caught a supermassive black hole in the midst of a giant eruption of X-ray light, helping astronomers address an ongoing puzzle: How do supermassive black holes flare?

The results suggest that supermassive black holes send out beams of X-rays when their surrounding coronas -- sources of extremely energetic particles -- shoot, or launch, away from the black holes.

"This is the first time we have been able to link the launching of the corona to a flare," said Dan Wilkins of Saint Mary's University in Halifax, Canada, lead author of a new paper on the results appearing in the Monthly Notices of the Royal Astronomical Society. "This will help us understand how supermassive black holes power some of the brightest objects in the universe."

Supermassive black holes don't give off any light themselves, but they are often encircled by disks of hot, glowing material. The gravity of a black hole pulls swirling gas into it, heating this material and causing it to shine with different types of light. Another source of radiation near a black hole is the corona. Coronas are made up of highly energetic particles that generate X-ray light, but details about their appearance, and how they form, are unclear.

Astronomers think coronas have one of two likely configurations. The "lamppost" model says they are compact sources of light, similar to light bulbs, that sit above and below the black hole, along its rotation axis. The other model proposes that the coronas are spread out more diffusely, either as a larger cloud around the black hole, or as a "sandwich" that envelops the surrounding disk of material like slices of bread. In fact, it's possible that coronas switch between both the lamppost and sandwich configurations.

The new data support the "lamppost" model -- and demonstrate, in the finest detail yet, how the light-bulb-like coronas move. The observations began when Swift, which monitors the sky for cosmic outbursts of X-rays and gamma rays, caught a large flare coming from the supermassive black hole called Markarian 335, or Mrk 335, located 324 million light-years away in the direction of the constellation Pegasus. This supermassive black hole, which sits at the center of a galaxy, was once one of the brightest X-ray sources in the sky.

"Something very strange happened in 2007, when Mrk 335 faded by a factor of 30. What we have found is that it continues to erupt in flares but has not reached the brightness levels and stability seen before," said Luigi Gallo, the principal investigator for the project at Saint Mary's University. Another co-author, Dirk Grupe of Morehead State University in Kentucky, has been using Swift to regularly monitor the black hole since 2007.

In September 2014, Swift caught Mrk 335 in a huge flare. Once Gallo found out, he sent a request to the NuSTAR team to quickly follow up on the object as part of a "target of opportunity" program, where the observatory's previously planned observing schedule is interrupted for important events. Eight days later, NuSTAR set its X-ray eyes on the target, witnessing the final half of the flare event.

After careful scrutiny of the data, the astronomers realized they were seeing the ejection, and eventual collapse, of the black hole's corona.

"The corona gathered inward at first and then launched upwards like a jet," said Wilkins. "We still don't know how jets in black holes form, but it's an exciting possibility that this black hole's corona was beginning to form the base of a jet before it collapsed."

How could the researchers tell the corona moved? The corona gives off X-ray light that has a slightly different spectrum -- X-ray "colors" -- than the light coming from the disk around the black hole. By analyzing a spectrum of X-ray light from Mrk 335 across a range of wavelengths observed by both Swift and NuSTAR, the researchers could tell that the corona X-ray light had brightened -- and that this brightening was due to the motion of the corona.

Coronas can move very fast. The corona associated with Mrk 335, according to the scientists, was traveling at about 20 percent the speed of light. When this happens, and the corona launches in our direction, its light is brightened in an effect called relativistic Doppler boosting.

Putting this all together, the results show that the X-ray flare from this black hole was caused by the ejected corona.

"The nature of the energetic source of X-rays we call the corona is mysterious, but now with the ability to see dramatic changes like this we are getting clues about its size and structure," said Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology in Pasadena, who was not affiliated with the study.

Many other black hole brainteasers remain. For example, astronomers want to understand what causes the ejection of the corona in the first place.

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA's Jet Propulsion Laboratory in Pasadena, California, for NASA's Science Mission Directorate in Washington. NuSTAR was developed in partnership with the Danish Technical University and the Italian Space Agency (ASI). The spacecraft was built by Orbital Sciences Corp., Dulles, Virginia. NuSTAR's mission operations center is at UC Berkeley, and the official data archive is at NASA's High Energy Astrophysics Science Archive Research Center. ASI provides the mission's ground station and a mirror archive. JPL is managed by Caltech for NASA.

Media Contact

Whitney Clavin
Jet Propulsion Laboratory, Pasadena, California
818-354-4673
whitney.clavin@jpl.nasa.gov

Source: Jet Propulsion Laboratory/Caltech

Suzaku Finds Common Chemical Makeup at Largest Cosmic Scales

This visible light view shows the central part of the Virgo Cluster. The brightest object is the giant elliptical galaxy M87 (left of center). The image spans approximately 1.2 degrees, or about 2.4 times the apparent diameter of a full moon. Credits: NOAO/AURA/NSF. Download the image in HD at NASA's Scientific Visualization Studio

Suzaku mapped iron, magnesium, silicon and sulfur in four directions all across the Virgo galaxy cluster for the first time. The northern arm of the survey (top) extends 5 million light-years from M87 (center), the massive galaxy at the cluster's heart. Ratios of these elements are constant throughout the cluster, which means they were mixed well early in cosmic history. The dashed circle shows what astronomers call the virial radius, the boundary where gas clouds are just entering the cluster. Some prominent members of the cluster are labeled as well. The background image is part of the all-sky X-ray survey acquired by the German ROSAT satellite. The blue box at center indicates the area shown in the visible light image. Credits: A. Simionescu (JAXA) and Hans Boehringer (MPE).  Download the graphic in HD at NASA's Scientific Visualization Studio

A new survey of hot, X-ray-emitting gas in the Virgo galaxy cluster shows that the elements needed to make stars, planets and people were evenly distributed across millions of light-years early in cosmic history, more than 10 billion years ago.

The Virgo cluster, located about 54 million light-years away, is the nearest galaxy cluster and the second brightest in X-rays. The cluster is home to more than 2,000 galaxies, and the space between them is filled with a diffuse gas so hot it glows in X-rays. 

Using Japan's Suzaku X-ray satellite, a team led by Aurora Simionescu, an astrophysicist at the Japan Aerospace Exploration Agency (JAXA) in Sagamihara, acquired observations of the cluster along four arms extending up to 5 million light-years from its center.

"Heavier chemical elements from carbon on up are produced and distributed into interstellar space by stars that explode as supernovae at the ends of their lifetimes," Simionescu said. This chemical dispersal continues at progressively larger scales through other mechanisms, such as galactic outflows, interactions and mergers with neighboring galaxies, and stripping caused by a galaxy's motion through the hot gas filling galaxy clusters.

Supernovae fall into two broad classes. Stars born with more than about eight times the sun's mass collapse under their own weight and explode as core-collapse supernovae. White dwarf stars may become unstable due to interactions with a nearby star and explode as so-called Type Ia supernovae.

These different classes of supernovae produce different chemical compositions. Core-collapse supernovae mostly scatter elements ranging from oxygen to silicon, while white dwarf explosions release predominantly heavier elements, such as iron and nickel. Surveying the distribution of these elements over a vast volume of space, such as a galaxy cluster, helps astronomers reconstruct how, when, and where they were produced. Once the chemical elements made by supernovae are scattered and mixed into interstellar space, they become incorporated into later generations of stars. 

The overall composition of a large volume of space depends on the mix of supernova types contributing to it. For example, accounting for the overall chemical makeup of the sun and solar system requires a mix of roughly one Type Ia supernova for every five core-collapse explosions. 

"One way to think about this is that we're looking for the supernova recipe that produced the chemical makeup we see on much larger scales, and comparing it with the recipe for our own sun," said co-author Norbert Werner, a researcher at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University in California.

In an earlier study led by Werner, Suzaku data showed that iron was distributed uniformly throughout the Perseus Galaxy Cluster, but information about lighter elements mainly produced by core-collapse supernovae was unavailable. The Virgo Cluster observations supply the missing ingredients. Reporting their findings in the Oct. 1 issue of The Astrophysical Journal, Simionescu and her colleagues show they detect iron, magnesium, silicon and sulfur all the way across a galaxy cluster for the first time. The elemental ratios are constant throughout the entire volume of the cluster and roughly consistent with the composition of the sun and most of the stars in our own galaxy.

Because galaxy clusters cover enormous volumes of space, astronomers can use one example to extrapolate the average chemical content of the universe. The study shows that the chemical elements in the cosmos are well mixed, showing little variation on the largest scales. The same ratio of supernova types -- the same recipe -- thought to be responsible for the solar system's makeup was at work throughout the universe. This likely happened when the universe was between 2 and 4 billion years old, a period when stars were being formed at the fastest rate in cosmic history.

"This means that elements so important to life on Earth are available, on average, in similar relative proportions throughout the bulk of the universe," explained Simionescu. "In other words, the chemical requirements for life are common throughout the cosmos."

Launched on July 10, 2005, Suzaku was developed at the Institute of Space and Astronautical Science (ISAS) in Japan, which is part of JAXA, in collaboration with NASA and other Japanese and U.S. institutions. NASA's Goddard Space Flight Center in Greenbelt, Maryland, supplied Suzaku's X-ray telescopes and data-processing software, and operated a facility supporting U.S. astronomers who used the satellite.

Suzaku operated for 10 years -- five times its target lifespan -- to become the longest-functioning Japanese X-ray observatory. On Aug. 26, JAXA announced the end of the mission due to the deteriorating health of the spacecraft.

"Suzaku provided us with a decade of revolutionary measurements," said Robert Petre, chief of Goddard's X-ray Astrophysics Laboratory. "We're building on that legacy right now with its successor, ASTRO-H, Japan's sixth X-ray astronomy satellite, and we're working toward its launch in 2016."

Francis Reddy
NASA's Goddard Space Flight Center, Greenbelt, Maryland

Source: NASA/Galaxies

Smoke ring for a halo

Credit: ESA/Hubble & NASA
Acknowledgement: Judy Schmidt (geckzilla.org)

Two stars shine through the centre of a ring of cascading dust in this image taken by the NASA/ESA Hubble Space Telescope. The star system is named DI Cha, and while only two stars are apparent, it is actually a quadruple system containing two sets of  binary stars.

As this is a relatively young star system it is surrounded by dust. The young stars are moulding the dust into a wispy wrap.

The host of this alluring interaction between dust and star is the Chamaeleon I dark cloud — one of three such clouds that comprise a large star-forming region known as the Chamaeleon Complex. DI Cha's juvenility is not remarkable within this region. In fact, the entire system is among not only the youngest but also the closest collections of newly formed stars to be found and so provides an ideal target for studies of star formation.

Source: ESA/Hubble/Images

Mysterious Starburst Unshrouded in Nearby Galaxy

Figure 1. Color composite image of the central region of NGC 253, from Flamingos 2 images using the filters J (blue), H (green) and Ks (red). This region of the edge-on viewed galaxy appears completely veiled in optical images due to the presence of large amounts of dust (so dense that it is still obscuring some regions at the near-infrared spectral range). The wavelength range covered by F-2 goes from 1 to 2.5 μm. The field of view is 420 x 144 arcseconds.

Figure 2. Color composite image of the core region of NGC 253, from T-ReCS images using the filters Si-2 (blue), [NeII] (green) and Qa (red). The nucleus candidate IRC appears as the brightest object in the infrared. The wavelength range covered by T-ReCS images goes from 8 to 20 μm. The field of view is 32 x 23 arcseconds.


The nearest spiral galaxy with a nuclear starburst (greatly enhanced star formation near a galaxy’s center) is also the site of a long-standing astronomical mystery. Designated NGC 253, or by amateurs as the Silver Dollar Galaxy, the core of this galaxy is so shrouded by gas and dust that the exact location of its core has remained unresolved for years. Now, thanks to research by Guillermo Günthardt of the National University of Cordoba (Argentina), Gemini South infrared data appear to have unambiguously pinpointed the galaxy’s core. In the process, evidence for a lowish-mass, but rapidly growing, black hole as the starburst’s trigger is painting a new picture of this enigmatic galaxy (Figure 1).

NGC 253 is the nearest spiral galaxy with a nuclear starburst. The nuclear region is so veiled by large amounts of dust associated to the star formation process that it has been unclear, until now, where this galaxy’s true galactic nucleus lies. Guillermo Günthardt, from the National University of Cordoba (Argentina), and an international team suggest that the brightest near- and mid-infrared source in the central region, named the IRC (Infrared Core) by the authors, is the galaxy’s core. Long considered just a large young star cluster, Günthardt et al. present several features leading to the conclusion this source is the genuine galactic nucleus. This contradicts the previous idea that a source called TH2 (a bright fuzzy radio source catalogued by Turner and Ho in 1985) is the best candidate for the nucleus.

In the team’s paper, to appear in the next issue of The Astronomical Journal the team presents kinematic, spectrophotometric, and morphological evidence that support the hypothesis that the IRC is NGC 253's galactic nucleus. This includes the fact that the IRC is the most massive object in NGC 253's central region, the major source of the nuclear starburst outflow, the molecular gas rotation center, and it is almost coincident with the galactic bar symmetry center.

Günthardt’s team obtained near-infrared observations with Flamingos 2 (F2) on the Gemini South telescope, including spectroscopy and images in four bands. In order to penetrate the veil of dust deeper, F2 observations were complemented by mid-infrared images obtained at Gemini South with T-ReCS (Thermal-Region Camera Spectrograph) in 2011, again, using four filters. At near- and mid-infrared wavelengths the IRC, TH7 in the radio source catalogue, appears an order of magnitude brighter than any other objects in the central region of NGC 253, moreover, there is no infrared source detected at the previous candidate position (TH2). The T-ReCS high-spatial-resolution images show a shell-like structure around the IRC, and F2 spectra show the largest turbulence motions in the ionized gas at this location, with expanding velocities over 500 km per second. This observation reveals that the IRC is also the main present source of the galaxy-wide gaseous winds detected in 2013 with ALMA by Bolatto et al.

The innermost radial velocity measures of the molecular gas do not exclude the possible presence of a few million solar masses black hole at the center of the IRC. Considering that NGC 253 is a large spiral galaxy with a mass of more than 7 x 10¹¹ times the mass of our Sun, the IRC is an unexpectedly lightweight core, but which might be growing rapidly as it co-evolves with the violent star-formation process taking place in the galaxy’s nuclear region.

The off-center position of the IRC and of the nuclear disk, with respect to the galaxy’s bulge of stars, also contributed to the historic uncertainty in the nucleus location and implies a decoupling of the central gas and nuclear cluster from the older galactic structure. In 2015 Emsellem et al. theorized just such a decoupling using numerical simulation models in which a small nuclear core oscillates around the center of symmetry of a barred galaxy. In such a scenario, the small black hole in NGC 253 would not only grow rapidly while it accretes the dense nuclear gas, but also would efficiently trigger star formation due to its dance around the galaxy’s geometrical center.

This work is available on Astro-ph at: http://arxiv.org/abs/1509.00330

Paper Abstract:

NGC253 is the nearest spiral galaxy with a nuclear starburst, which becomes the best candidate to study the relationship between starburst and AGN activity. However, this central region is veiled by large amounts of dust, and it has been so far unclear which is the true dynamical nucleus. The near infrared spectroscopy could be advantageous in order to shed light on the true nucleus identity. Using Flamingos 2 at Gemini South we have taken deep K-band spectra along the major axis and through the brightest infrared source. We present evidence showing that the brightest near infrared and mid infrared source in the central region, already known as radio source TH7 and so far considered just a stellar supercluster, in fact, presents various symptoms of a genuine galactic nucleus. Therefore, it should be considered a valid nucleus candidate. It is the most massive compact infrared object in the central region, located at 2.0 arcseconds of the symmetry center of the galactic bar. Moreover, our data indicate that this object is surrounded by a large circumnuclear stellar disk and it is also located at the rotation center of the large molecular gas disk of NGC 253. Furthermore, a kinematic residual appears in the H2 rotation curve with a sinusoidal shape consistent with an outflow centered in the candidate nucleus position. The maximum outflow velocity is located about 14 parsecs from TH7, which is consistent with the radius of a shell detected around the nucleus candidate observed at 18.3 μm (Qa) and 12.8 μm ([NeII]) with T-ReCS. Also, the Brγ emission line profile is blue-shifted and this emission line has also the highest equivalent width at this position. All these evidences point out TH7 as the best candidate to be the galactic nucleus of NGC 253. 

Source: Gemini Observatory

Hubble Frontier Fields view of MACSJ0416.1–2403

PR Image heic1523a

Hubble Frontier Fields view of MACSJ0416.1–2403

 

PR Image heic1523b

Hubble Frontier Fields view of MACSJ0717.5+3745

 

PR Image heic1523c

Hubble Frontier Fields view of Abell 2744

Observations by the NASA/ESA Hubble Space Telescope have taken advantage of gravitational lensing to reveal the largest sample of the faintest and earliest known galaxies in the Universe. Some of these galaxies formed just 600 million years after the Big Bang and are fainter than any other galaxy yet uncovered by Hubble. The team has determined, for the first time with some confidence, that these small galaxies were vital to creating the Universe that we see today.

An international team of astronomers, led by Hakim Atek of the Ecole Polytechnique Fédérale de Lausanne, Switzerland, has discovered over 250 tiny galaxies that existed only 600-900 million years after the Big Bang [1] — one of the largest samples of dwarf galaxies yet to be discovered at these epochs. The light from these galaxies took over 12 billion years to reach the telescope, allowing the astronomers to look back in time when the universe was still very young.

Although impressive, the number of galaxies found at this early epoch is not the team’s only remarkable breakthrough, as Johan Richard from the Observatoire de Lyon, France, points out, “The faintest galaxies detected in these Hubble observations are fainter than any other yet uncovered in the deepest Hubble observations.”

By looking at the light coming from the galaxies the team discovered that the accumulated light emitted by these galaxies could have played a major role in one of the most mysterious periods of the Universe’s early history — the epoch of reionisation. Reionisation started when the thick fog of hydrogen gas that cloaked the early Universe began to clear. Ultraviolet light was now able to travel over larger distances without being blocked and the Universe became transparent to ultraviolet light [2].

By observing the ultraviolet light from the galaxies found in this study the astronomers were able to calculate whether these were in fact some of the galaxies involved in the process. The team determined, for the first time with some confidence, that the smallest and most abundant of the galaxies in the study could be the major actors in keeping the Universe transparent. By doing so, they have established that the epoch of reionisation — which ends at the point when the Universe is fully transparent — came to a close about 700 million years after the Big Bang [3].

Lead author Atek explained, “If we took into account only the contributions from bright and massive galaxies, we found that these were insufficient to reionise the Universe. We also needed to add in the contribution of a more abundant population of faint dwarf galaxies.”

To make these discoveries, the team utilised the deepest images of gravitational lensing made so far in three galaxy clusters, which were taken as part of the Hubble Frontier Fields programme [4]. These clusters generate immense gravitational fields capable of magnifying the light from the faint galaxies that lie far behind the clusters themselves. This makes it possible to search for, and study, the first generation of galaxies in the Universe.

Jean-Paul Kneib, co-author of the study from the Ecole Polytechnique Fédérale de Lausanne, Switzerland, explains, “Clusters in the Frontier Fields act as powerful natural telescopes and unveil these faint dwarf galaxies that would otherwise be invisible.”

Co-author of the study Mathilde Jauzac, from Durham University, UK, and the University of KwaZulu-Natal, South Africa, remarks on the significance of the discovery and Hubble’s role in it,“Hubble remains unrivalled in its ability to observe the most distant galaxies. The sheer depth of the Hubble Frontier Field data guarantees a very precise understanding of the cluster magnification effect, allowing us to make discoveries like these.”

These results highlight the impressive possibilities of the Frontier Fields programme with more galaxies, at even earlier time, likely to be revealed when Hubble peers at three more of these galaxy clusters in the near future.

Notes

[1] The calculated redshift for these objects is between z = 6 and z = 8.

[2] Neutral hydrogen gas absorbs all the high-energy ultraviolet light emitted by hot young stars very efficiently. At the same time, the absorbed ultraviolet light ionises the hydrogen. The very low density ionised hydrogen gas filling the universe became fully transparent.The hot stars carve out transparent bubbles in the gas and once all these bubbles merge to fill all of space, reionisation is said to be complete and the Universe becomes transparent to ultraviolet light.

[3] This corresponds to a redshift of about z = 7.5.

[4] The Hubble Frontier Fields is a three-year, 840-orbit programme which will yield the deepest views of the Universe to date, combining the power of Hubble with the gravitational amplification of light around six different galaxy clusters to explore more distant regions of space than could otherwise be seen.

Notes for Editors

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.


More Information

Image credit: NASA, ESA and the HST Frontier Fields team (STScI)

This research was presented in a paper entitled “Are Ultra-faint Galaxies at z = 6−8 Responsible for Cosmic Reionization? Combined Constraints from the Hubble Frontier Fields Clusters And Parallels”, by H. Atek et al., to appear in the Astrophysical Journal.

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

The international team of astronomers in this study consists of Hakim Atek (Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne, Switzerland ; Department of Astronomy, Yale University, USA), Johan Richard (CRAL, Observatoire de Lyon, France), Mathilde Jauzac (Institute for Computational Cosmology, Durham University, UK; Astrophysics and Cosmology Research Unit, University of KwaZulu-Natal, South Africa), Jean-Paul Kneib (Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne, Switzerland; Aix Marseille Université, CNRS, LAM UMR 7326, France), Priyamvada Natarajan (Department of Astronomy, Yale University, USA), Marceau Limousin (Aix Marseille Université, CNRS, LAM UMR 7326, France), Daniel Schaerer (Observatoire de Genève, Switzerland; CNRS, IRAP, France), Eric Jullo (Aix Marseille Université, CNRS, LAM UMR 7326, France), Harald Ebeling (Institute for Astronomy, University of Hawaii, USA), Eiichi Egami (Steward Observatory, University of Arizona, USA), and Benjamin Clement (CRAL, Observatoire de Lyon, France).

Links

• Images of Hubble
• Frontier Fields programme
• Link to science paper
• Hubblecast 70: Peering around cosmic corners

Contacts

Hakim Atek
Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne, Switzerland
Astronomy Departement, Yale University, USA
Tel: +1 203 645 82 23
Email: hakim.atek@epfl.ch

Johan Richard
Centre de Recherche Astrophysique de Lyon
Observatoire de Lyon, Université Lyon 1, France
Cell: +33 4 78 86 83 7
Email: johan.richard@univ-lyon1.fr

Mathilde Jauzac
Durham University, UK
University of KwaZulu-Natal, South Africa
Tel: +44 7445218614
Email: mathilde.jauzac@durham.ac.uk

Jean-Paul Kneib
Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne, Switzerland
Laboratoire d’Astrophysique de Marseille, France
Tel: +41 22 3792473, +41 21 693 04 63, +33 695 795 392
Email: jean-paul.kneib@epfl.ch

Mathias Jäger
ESA/Hubble, Public Information Officer
Garching bei München, Germany
Cell: +49 176 62397500
Email: mjaeger@partner.eso.org

Source: ESA/Hubble News

Cosmic "Death Star" is Destroying a Planet

In this artist's conception, a Ceres-like asteroid is slowly disintegrating as it orbits a white dwarf star. Astronomers have spotted telltales signs of such an object using data from the Kepler K2 mission. It is the first planetary object detected transiting a white dwarf. Within about a million years the object will be destroyed, leaving a thin dusting of metals on the surface of the white dwarf.  Credit: Mark A. Garlick / markgarlick.com .    High Resolution (jpg)  -  Low Resolution (jpg)

The Death Star of the movie Star Wars may be fictional, but planetary destruction is real. Astronomers announced today that they have spotted a large, rocky object disintegrating in its death spiral around a distant white dwarf star. The discovery also confirms a long-standing theory behind the source of white dwarf "pollution" by metals.

"This is something no human has seen before," says lead author Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics (CfA). "We're watching a solar system get destroyed."

The evidence for this unique system came from NASA's Kepler K2 mission, which monitors stars for a dip in brightness that occurs when an orbiting body crosses the star. The data revealed a regular dip every 4.5 hours, which places the object in an orbit about 520,000 miles from the white dwarf (about twice the distance from the Earth to the Moon). It is the first planetary object to be seen transiting a white dwarf.

Vanderburg and his colleagues made additional observations using a number of ground-based facilities: the 1.2-meter and MINERVA telescopes at Whipple Observatory, the MMT, MEarth-South, and Keck.

Combining all the data, they found signs of several additional chunks of material, all in orbits between 4.5 and 5 hours. The main transit was particularly prominent, dimming the star by 40 percent. The transit signal also showed a comet-like pattern. Both features suggest the presence of an extended cloud of dust surrounding the fragment. The total amount of material is estimated to be about the mass of Ceres, a Texas-sized object that is the largest main-belt asteroid in our solar system.

The white dwarf star is located about 570 light-years from Earth in the constellation Virgo. When a Sun-like star reaches the end of its life, it swells into a red giant and sloughs off its outer layers. The hot, Earth-sized core that remains is a white dwarf star, and generally consists of carbon and oxygen with a thin hydrogen or helium shell.

Sometimes, though, astronomers find a white dwarf that shows signs of heavier elements like silicon and iron in its light spectrum. This is a mystery because a white dwarf's strong gravity should quickly submerge these metals.

"It's like panning for gold - the heavy stuff sinks to the bottom. These metals should sink into the white dwarf's interior where we can't see them," explains Harvard co-author John Johnson (CfA).

Theorists speculated that white dwarfs showing evidence of heavy metals became "polluted" when they consumed rocky planets or asteroids. However, the evidence was often circumstantial. A fraction of polluted white dwarfs showed signs of surrounding debris disks, but the origin of the disks was uncertain. This system shows all three: a polluted white dwarf, a surrounding debris disk, and at least one compact, rocky object.

"We now have a 'smoking gun' linking white dwarf pollution to the destruction of rocky planets," says Vanderburg.

Questions remain about the origin of these rocky objects. The most likely scenario is that an existing planet's orbit became unstable and it was kicked inward.

What is certain is that the remaining objects will not last forever. They are being vaporized by the intense heat of the white dwarf. They also are orbiting very close to the tidal radius, or distance at which gravitational tides from the white dwarf can rip apart a rocky body. Within the next million years or so, all that will remain of these asteroidal bits is a thin metal dusting on top of an innocent-looking white dwarf star.

This discovery will be published in the Oct. 22, 2015, issue of the journal Nature.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

Source: Harvard-Smithsonian Center for Astrophysics (CfA)

Starburst galaxy Messier 94

Messier 94 

Credit: ESA/Hubble & NASA

This image shows the galaxy Messier 94, which lies in the small northern constellation of the Hunting Dogs, about 16 million light-years away.

Within the bright ring around Messier 94 new stars are forming at a high rate and many young, bright stars are present within it – thanks to this, this feature is called a starburst ring.

The cause of this peculiarly shaped star-forming region is likely a pressure wave going outwards from the galactic centre, compressing the gas and dust in the outer region. The compression of material means the gas starts to collapse into denser clouds. Inside these dense clouds, gravity pulls the gas and dust together until temperature and pressure are high enough for stars to be born.

Source: ESA/Hubble -  Space Telescope

ASASSN-14li: Destroyed Star Rains onto Black Hole, Winds Blow it Back

ASASSN-14li

Credit Spectrum: NASA/CXC/U.Michigan/J.Miller et al.; 

Illustration: NASA/CXC/M.Weiss

Astronomers have observed material being blown away from a black hole after it tore a star apart, as reported in our press release. This event, known as a "tidal disruption," is depicted in the artist's illustration.

Astronomers used a trio of X-ray telescopes - NASA's Chandra X-ray Observatory, Swift Gamma Ray Burst Explorer, and ESA's XMM-Newton - to observe a tidal disruption located in the center of a galaxy about 290 million light years away. This makes this tidal disruption, dubbed ASASSN-14li, the closest tidal disruption discovered in ten years. The event was discovered in an optical search by the All-Sky Automated Survey for Supernovae (ASAS-SN) in November 2014. Theory predicts that early in the evolution of a tidal disruption, material from the shredded star (seen as the reddish-orange streak) should be pulled towards the black hole at a high rate, generating a huge amount of light. The amount of light should decline as the disrupted material falls onto the black hole, shown as the small black circle in the upper left of the illustration. In the case of ASASSN-14li, astronomers estimate the mass of the black hole is a few million times that of the Sun.

Gas often falls toward black holes by spiraling inward in a disk. But how this process starts has remained a mystery. In ASASSN-14li, astronomers were able to witness the formation of such a disk by looking at the X-ray light at different wavelengths (known as the "X-ray spectrum") and tracking how that changed over time. The researchers determined that the observed X-rays come from material that is either very close to or is actually in the smallest possible stable orbit around the black hole.

The illustration shows a disk of stellar debris around the black hole in the upper left of the illustration, and a long tail of debris that has been flung away from the black hole. The X-ray spectrum obtained with Chandra (seen in the inset box) and XMM-Newton both show clear evidence for absorption lines, i.e. dips in X-ray intensity over a narrow range of wavelengths. In an X-ray light version of the Doppler Shift, the absorption lines are shifted to bluer wavelengths than expected, giving evidence for a wind blowing towards us and away from the black hole.

The presence of a wind moving away from the black hole is shown as the bluish white lines in the artist's illustration. The wind is not moving fast enough to escape the black hole's gravitational grasp. An alternative explanation for the relatively low speed is that gas from the disrupted star is following an elliptical orbit around the black hole and is observed at the greatest distance from the black hole where it is traveling the slowest. These results confirm recent theoretical predictions for the structure and evolution of tidal disruptions events.

These results appeared in a paper in the October 22nd issue of the journal Nature. The authors of that paper are Jon M. Miller (University of Michigan), Jelle Kastra (SRON Institute for Space Research), Cole Miller (University of Maryland, College Park), Mark Reynolds (Michigan), Gregory Brown (University of Warwick), Bradley Cenko (Maryland), Jeremy Drake (Harvard-Smithsonian Center for Astrophysics), Suvi Gezari (Maryland), James Guillochon (CfA), Kayhan Gultekin (Michigan), Jimmy Irwin (University of Alabama), Andrew Levan (Warwick), Dipankar Maitra (Wheaton College), Peter Maksym (Alabama), Richard Mushotsky (Maryland), Paul O'Brien (University of Leicester), Fritz Paerels (Columbia University), Jelle de Plaa (SRON), Enrico Ramirez-Ruiz (University of California, Santa Cruz), Tod Strohmayer (Maryland), and Nial Tanvir (Leicester).

NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. Swift is managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Fast Facts for ASASSN-14li:

Category: Black Holes
Coordinates (J2000): RA 12h 48m 15.20s | Dec +17° 46' 26.20"
Constellation: Coma Berenices
Observation Date: 08 and 11 Dec 2014
Observation Time: 22 hours.
Obs. ID: 17566, 17567
Instrument: HRC
References: Miller, J. et al, 2015, Nature (accepted)
Distance Estimate: About 290 million light years (z=0.0206)

Source: NASA’s Chandra X-ray Observatory

Final Kiss of Two Stars Heading for Catastrophe

PR Image eso1540a

Artist’s impression of the hottest and most massive touching double star

PR Video eso1540b

Zooming in on VFTS 352

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Videos

 

PR Video eso1540a

Artist’s impression of the hottest and most massive touching double star

PR Video eso1540b

Zooming in on VFTS 352

PR Video eso1540c

Fulldome artist’s impression of the hottest and most massive touching double star

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VLT finds hottest and most massive touching double star

Using ESO’s Very Large Telescope, an international team of astronomers have found the hottest and most massive double star with components so close that they touch each other. The two stars in the extreme system VFTS 352 could be heading for a dramatic end, during which the two stars either coalesce to create a single giant star, or form a binary black hole.

The double star system VFTS 352 is located about 160 000 light-years away in the Tarantula Nebula [1]. This remarkable region is the most active nursery of new stars in the nearby Universe and new observations from ESO’s VLT [2] have revealed that this pair of young stars is among the most extreme and strangest yet found.

VFTS 352 is composed of two very hot, bright and massive stars that orbit each other in little more than a day. The centres of the stars are separated by just 12 million kilometres [3]. In fact, the stars are so close that their surfaces overlap and a bridge has formed between them. VFTS 352 is not only the most massive known in this tiny class of “overcontact binaries” — it has a combined mass of about 57 times that of the Sun — but it also contains the hottest components — with surface temperatures above 40 000 degrees Celsius.

Extreme stars like the two components of VFTS 352, play a key role in the evolution of galaxies and are thought to be the main producers of elements such as oxygen. Such double stars are also linked to exotic behaviour such as that shown by “vampire stars”, where a smaller companion star sucks matter from the surface of its larger neighbour (eso1230).

In the case of VFTS 352, however, both stars in the system are of almost identical size. Material is, therefore, not sucked from one to another, but instead may be shared [4]. The component stars of VFTS 352 are estimated to be sharing about 30 per cent of their material.

Such a system is very rare because this phase in the life of the stars is short, making it difficult to catch them in the act. Because the stars are so close together, astronomers think that strong tidal forces lead to enhanced mixing of the material in the stellar interiors.

“The VFTS 352 is the best case yet found for a hot and massive double star that may show this kind of internal mixing,” explains lead author Leonardo A. Almeida of the University of São Paulo, Brazil. “As such it’s a fascinating and important discovery.”

Astronomers predict that VFTS 352 will face a cataclysmic fate in one of two ways. The first potential outcome is the merging of the two stars, which would likely produce a rapidly rotating, and possibly magnetic, gigantic single star. “If it keeps spinning rapidly it might end its life in one of the most energetic explosions in the Universe, known as a long-duration gamma-ray burst,” says the lead scientist of the project, Hugues Sana, of the University of Leuven in Belgium [5].

The second possibility is explained by the lead theoretical astrophysicist in the team, Selma de Mink of University of Amsterdam: “If the stars are mixed well enough, they both remain compact and the VFTS 352 system may avoid merging. This would lead the objects down a new evolutionary path that is completely different from classic stellar evolution predictions. In the case of VFTS 352, the components would likely end their lives in supernova explosions, forming a close binary system of black holes. Such a remarkable object would be an intense source of gravitational waves.”

Proving the existence of this second evolutionary path [6] would be an observational breakthrough in the field of stellar astrophysics. But, regardless of how VFTS 352 meets its demise, this system has already provided astronomers with valuable new insights into the poorly understood evolutionary processes of massive overcontact binary star systems.


Notes

[1] This star’s name indicates that it was observed as part of the VLT FLAMES Tarantula Survey, which utilised the FLAMES and GIRAFFE instruments on ESO’s Very Large Telescope (VLT) to study over 900 stars in the 30 Doradus region of the Large Magellanic Cloud (LMC). The survey has already led to many exciting and important findings including the fastest rotating star (eso1147), and an extremely massive solitary runaway star (eso1117). It is helping to answer many fundamental questions concerning how massive stars are affected by rotation, binarity and the dynamics in dense star clusters.

[2] This study also used brightness measurements of VFTS 352 over a period of twelve years made as part of the OGLE survey.

[3] Both components are classed as O-type stars. Such stars are typically between 15 and 80 times more massive than the Sun and can be up to a million times brighter. They are so hot that they shine with a brilliant blue-white light and have surface temperatures over 30 000 degrees Celsius.

[4] These regions around the stars are known as Roche lobes. In an overcontact binary such as VFTS 352 both stars overfill their Roche lobes.

[5] Gamma-ray Bursts (GRBs) are bursts of highly energetic gamma rays that are detected by orbiting satellites. They come in two types — short duration (shorter than a few seconds), and long duration (longer than a few seconds). Long-duration GRBs are more common and are thought to mark the deaths of massive stars and be associated with a class of very energetic supernova explosions.

[6] Predicted by Einstein’s theory of general relativity, gravitational waves are ripples in the fabric of space and time. Significant gravitational waves are generated whenever there are extreme variations of strong gravitational fields with time, such as during the merger of two black holes.



More Information

This research was presented in a paper in entitled “Discovery of the massive overcontact binary VFTS 352: Evidence for enhanced internal mixing”, by L. Almeida et al., in the Astrophysical Journal.
The team is composed of L.A. Almeida (Johns Hopkins University, Baltimore, Maryland, USA; Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Brazil), H. Sana (STScI, Baltimore, Maryland, USA; KU Leuven, Belgium), S.E. de Mink (University of Amsterdam, Netherlands), F. Tramper (University of Amsterdam, Netherlands), I. Soszynski (Warsaw University Observatory, Poland), N. Langer (Universität Bonn, Germany), R.H. Barbá (Universidad de La Serena, Chile), M. Cantiello (University of California, Santa Barbara, USA), A. Damineli (Universidade de São Paulo, Brazil), A. de Koter (University of Amsterdam, Netherlands; Universiteit Leuven, Belgium), M. Garcia (Centro de Astrobiologa (INTA-CSIC), Spain), G. Gräfener (Armagh Observatory, UK), A. Herrero (Instituto de Astrofísica de Canarias, Spain; Universidad de La Laguna, Spain), I. Howarth (University College London, UK), J. Maíz Apellániz (Centro de Astrobiologa (INTA-CSIC), Spain), C. Norman (Johns Hopkins University, USA), O.H. Ramírez-Agudelo (University of Amsterdam, Netherlands) and J.S. Vink (Armagh Observatory, UK).
ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.


Links

• Research paper published in the Astrophysical Journal
• Freely accessible preprint of the research paper
• Photos of the VLT

Contacts


Leonardo Almeida
Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG/USP)
São Paulo, Brazil
Tel: +55 011 3091 2818
Email: leonardodealmeida.andrade@gmail.com

Hugues Sana
University of Leuven
Leuven, Belgium
Tel: +32 (0) 16 32 19 36
Email: hugues.sana@kuleuven.be

Selma de Mink
University of Amsterdam
Amsterdam, The Netherlands
Tel: +31 (0) 6 11 12 15 13
Email: S.E.deMink@uva.nl

Richard Hook
ESO Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org

Source: ESO