New Hypervelocity Binary Star Challenges Dark Matter, Stellar Acceleration Models

W. M. Keck Observatory press release

A team of astronomers at the Friedrich Alexander University led by Péter Németh has discovered a binary star moving nearly at the escape velocity of our galaxy. There are about two dozen so-called hypervelocity stars known to be escaping the galaxy. While all of them are single stars, PB3877 is the first wide binary star found to travel at such a high speed. Additionally, the results of the new study challenge the commonly accepted scenario that hypervelocity stars are accelerated by the supermassive black hole at the galactic center. The findings are being published in the Astrophysical Journal Letters today.

PB3877 Binary
PB3877 is a hyper-velocity wide binary star zooming through the outskirts of the Milky Way galaxy. This image shows its current location as well as our Sun.
The team, in collaboration with researchers from the California Institute of Technology, showed the binary cannot originate from the Galactic Center, and no other mechanism is known that is able to accelerate a wide binary to such a high velocity without disrupting it. They therefore hypothesized there must be a lot of dark matter to keep the star bound to the Milky Way galaxy; or the binary star, PB3877, could be an intruder that has been born in another galaxy and may or may not leave the Milky Way again.

PB3877 was first reported to be a hyper-velocity, hot compact star, when it was discovered form the Sloan Digital Sky-Survey (SDSS) data in 2011. New spectroscopic observations were done with the 10 meter Keck II telescope at W. M. Keck Observatory on Maunakea, Hawaii and with the 8.2 meter Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile.Caltech astronomers Thomas Kupfer and Felix Fürst observed PB3877 with the ESI Instrument fitted on the Keck II telescope.

“When we looked at the new data, much to our surprise, we found weak absorption lines that could not come from the hot star,” Kupfer said. “The cool companion, just like the hot primary, shows a high radial velocity. Hence, the two stars form a binary system, which is the first hyper-velocity wide binary candidate.”

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Fossil Star Clusters Reveal Their Age

W. M. Keck Observatory press release

Using a new age-dating method and the W. M. Keck Observatory on Maunakea, an international team of astronomers have determined that ancient star clusters formed in two distinct epochs – the first 12.5 billion years ago and the second 11.5 billion years ago. These results are being published in Monthly Notices of the Royal Astronomical Society.

Cosmic Timeline of Globular Clusters
A cosmic timeline showing the birth of the Universe in a Big Bang 13.7 billion years ago to the present day. Credit: NASA/CXC/SAO and A. Romanowsky
Although the clusters are almost as old as the Universe itself, these age measurements show the star clusters – called globular clusters – are actually slightly younger than previously thought.

“We now think that globular clusters formed alongside galaxies rather than significantly before them,” research team leader, Professor Duncan Forbes of Swinburne University of Technology said.

The new estimates of the star cluster average ages were made possible using data obtained from the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, which was carried out on Keck Observatory’s 10-meter, Keck II telescope. Observations were carried out over years using the powerful DEIMOS multi-object spectrograph fitted on Keck II, which is capable of obtaining spectra of one hundred globular clusters in a single exposure.

DEIMOS breaks the visible wavelengths of objects into spectra, which the team used to reverse-engineer the ages of the globular clusters by comparing the chemical composition of the globular clusters with the chemical composition of the Universe as it changes with time.

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Scientists Discover Brightest Early Galaxy and Likely First Generation Stars

W. M Keck Observatory press release

Astronomers using several of the largest telescopes on Earth and space have discovered the brightest galaxy yet found in the early Universe and have strong evidence that examples of the first generation of stars lurk within it. The results have been accepted for publication in The Astrophysical Journal.

Early Galaxy
An artist’s impression of a galaxy in the early universe, credit: David Sobral
The team’s expansive study was made using the 10-meter W. M. Keck Observatory, the 8.2-meter ESO Very Large Telescope and Subaru Telescope, as well as the NASA/ESA Hubble Space Telescope. The team discovered — and confirmed — a number of surprisingly bright very young galaxies.

A team — led by David Sobral from the Institute of Astrophysics and Space Sciences, the Faculty of Sciences of the University of Lisbon in Portugal, and Leiden Observatory in the Netherlands — peered back into the ancient Universe, to the reionization period approximately 800 million years after the Big Bang. Instead of conducting a narrow and deep study of a small area of the sky, the team broadened their scope to produce the widest survey of very distant galaxies ever attempted.

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Supernova Ejects Galaxy’s Fastest Star

W. M. Keck Observatory press release

Scientists using the W. M. Keck Observatory and Pan-STARRS1 telescopes on Hawaii have discovered a star that breaks the galactic speed record, traveling with a velocity of about 1,200 kilometers per second or 2.7 million miles per hour. This velocity is so high, the star will escape the gravity of our galaxy. In contrast to the other known unbound stars, the team showed that this compact star was ejected from an extremely tight binary by a thermonuclear supernova explosion. These results will be published in the March 6 issue of Science.

US 708 Supernova Ejected Star
An artist impression of the mass-transfer phase followed by a double-detonation supernova that leads to the ejection of US 708. Credit: ESA/Hubble, NASA, S. Geier
Stars like the Sun are bound to our Galaxy and orbit its center with moderate velocities. Only a few so-called hypervelocity stars are known to travel with velocities so high that they are unbound, meaning they will not orbit the galaxy, but instead will escape its gravity to wander intergalactic space.

A close encounter with the supermassive black hole at the centre of the Milky Way is typically presumed the most plausible mechanism for kicking these stars out of the galaxy.

A team of astronomers led by Stephan Geier (European Southern Observatory, Garching) observed the known high-velocity star know as US 708 with the Echellette Spectrograph and Imager instrument on the 10-meter, Keck II telescope to measure its distance and velocity along our line of sight. By carefully combining position measurements from digital archives with newer positions measured from images taken during the course of the Pan-STARRS1 survey, they were able to derive the tangential component of the star’s velocity (across our line of sight).

Putting the measurements together, the team determined the star is moving at about 1,200 kilometers per second – much higher than the velocities of previously known stars in the Milky Way galaxy. More importantly, the trajectory of US 708 means the supermassive black hole at the galactic center could not be the source of US 708’s extreme velocity.

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Comprehensive Andromeda Study Hints of Violent History

W. M. Keck Observatory press release

Dots show locations of stars in the Keck Observatory spectroscopic survey superimposed on an image of Andromeda. Credit: Claire Dorman/ESA
Dots show locations of stars in the Keck Observatory spectroscopic survey superimposed on an image of Andromeda. Credit: Claire Dorman/ESA
A detailed study of the motions of different stellar populations in Andromeda galaxy by UC Santa Cruz scientists using W. M. Keck Observatory data has found striking differences from our own Milky Way, suggesting a more violent history of mergers with smaller galaxies in Andromeda’s recent past. The findings are being presented on Thursday, January 8, at the winter meeting of the American Astronomical Society in Seattle.

The structure and internal motions of the stellar disk of a spiral galaxy hold important keys to understanding the galaxy’s formation history. The Andromeda galaxy, also called M31, is the closest spiral galaxy to the Milky Way and the largest in the local group of galaxies.

“In the Andromeda galaxy we have the unique combination of a global yet detailed view of a galaxy similar to our own. We have lots of detail in our own Milky Way, but not the global, external perspective,” said Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz.

The new study, led by UC Santa Cruz graduate student Claire Dorman and Guhathakurta, combined data from two large surveys of stars in Andromeda conducted at the Keck Observatory in Hawaii as well as data from the Hubble Space Telescope.

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Star Trail Photography

Taking star trails is one of the easiest forms of nighttime photography. it requires less equipment than full out astrophotography, only a camera that can take a long exposure and a tripod. In a pinch you can do without the tripod.

Three Lasers
Three lasers in operation, Subaru, Keck 1 and Keck 2, 23 x 4min with a Canon 60D
In star trail photography a long exposure is used to reveal the scene. With illumination provided by starlight the needed exposure will be minutes long, during which time the rotation of the Earth will cause the stars to trail. Each star will trace a short streak on the camera detector as it moves through the field of view.

For a number of reasons taking one very long exposure is a problem with digital cameras. Without getting into a technical discussion of noise, dark current and hot pixels we will simply advise taking short exposures. You can always try a twenty minute or half hour exposure and see for yourself. Thus the technique is to take a series of short exposures, usually one to five minutes long, and add these together in processing. By taking a series of short exposures, the final exposure length is limited only by the camera battery or the arrival of dawn.

If the camera is sensitive enough, and you have a fast lens, you might try starscape photography, where the stars are not trailed by the motion of the Earth. In contrast, star trail photography can be done by almost any camera that can take a long exposure. The difference is in the length of the exposures, long versus short, star trail or starscape.

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Watery Asteroid Discovered in Dying Star Points to Habitable Exoplanets

W. M. Keck Observatory press release

Astronomers have found the shattered remains of an asteroid that contained huge amounts of water orbiting an exhausted star, or white dwarf. This suggests that the star GD 61 and its planetary system – located about 150 light years away and at the end of its life – had the potential to contain Earth-like exoplanets.

The new research findings used data collected from NASA’s Hubble Space Telescope, both of W. M. Keck Observatory’s Keck I and Keck II telescopes, as well NASA’s FUSE telescope, and are reported today in the journal Science.

Watery Asteroid
Artist impression of a rocky and water-rich asteroid being torn apart by the strong gravity of the white dwarf star GD 61. Credit: Copyright Mark A. Garlick,, University of Warwick and University of Cambridge.
This is the first time both water and a rocky surface – two key ingredients for habitable planets – have been found together beyond our solar system.

Earth is essentially a “dry” planet, with only 0.02% of its mass as surface water, meaning oceans came long after it had formed; most likely when water-rich asteroids in the solar system crashed into our planet.

The asteroid analyzed is composed of 26% water mass, very similar to Ceres, the largest asteroid in the main belt of our solar system. Both are vastly more water-rich compared with Earth.

The new discovery shows the same water delivery system could have occurred in this distant, dying star’s solar system – as latest evidence points to it containing a similar type of water-rich asteroid that would have first brought water to Earth.

Astronomers at the Universities of Cambridge and Warwick say this is the first “reliable evidence” for water-rich, rocky planetary material in any extrasolar planetary system.

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Throughout the astronomical descriptions and event posts here on Darker View I use the term magnitude to describe the brightness of an object in the sky. Magnitude is a simple scale, but somewhat confusing without a quick introduction.

The origins of our current magnitude scale are as old as the science of astronomy itself. One of the first stellar catalogs, the Almagest, was compiled by Claudius Ptolemy in the 2nd century. To denote the brightness of stars the catalog assigned the brightest as being “stars of the first rank”, with a corresponding second rand, third rank, etc. The dimmest of stars, the faintest visible to the unaided eye, were assigned to the sixth rank. This system was used with little alteration for the next two millennium. Subsequent catalogs and observers used their own versions of the scale, perhaps adding a decimal place to denote finer differences in brightness. As there was no instrumental method of measuring the brightness, magnitude estimates varied widely from source to source.

With the dawn of modern photographic methods and later electronic methods, it became possible to systematize the scale. It was desirable to create a scale that approximated the old system and time honored traditions. Thus the current magnitude scale was developed, understanding the origins allows understanding of the modern system.

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