Stellar Family Portrait Takes Imaging Technique to New Extremes

picture: Zoom in onto Trumpler 14
picture: Trumpler 14 in the Carina Nebula



picture: Widest adaptive optics view of the open star cluster Trumpler 14

The young star cluster Trumpler 14 is revealed in another stunning ESO image. The amount of exquisite detail seen in this portrait, which beautifully reveals the life of a large family of stars, is due to the Multi-conjugate Adaptive optics Demonstrator (MAD) on ESO’s Very Large Telescope. Never before has such a large patch of sky been imaged using adaptive optics [1], a technique by which astronomers are able to remove most of the atmosphere's blurring effects.

Noted for harbouring Eta Carinae — one of the wildest and most massive stars in our galaxy — the impressive Carina Nebula also houses a handful of massive clusters of young stars. The youngest of these stellar families is the Trumpler 14 star cluster, which is less than one million years old — a blink of an eye in the Universe’s history. This large open cluster is located some 8000 light-years away towards the constellation of Carina (the Keel).

A team of astronomers, led by Hugues Sana, acquired astounding images of the central part of Trumpler 14 using the Multi-conjugate Adaptive optics Demonstrator (MAD, [2]) mounted on ESO’s Very Large Telescope (VLT). Thanks to MAD, astronomers were able to remove most of the blurring effects of the atmosphere and thus obtain very sharp images. MAD performs this correction over a much larger patch of the sky than any other current adaptive optics instrument, allowing astronomers to make wider, crystal-clear images.

Thanks to the high quality of the MAD images, the team of astronomers could obtain a very nice family portrait. They found that Trumpler 14 is not only the youngest — with a refined, newly estimated age of just 500 000 years — but also one of the most populous star clusters within the nebula. The astronomers counted about 2000 stars in their image, spanning the whole range from less than one tenth up to a factor of several tens of times the mass of our own Sun. And this in a region which is only about six light-years across, that is, less than twice the distance between the Sun and its closest stellar neighbour!

The most prominent star is the supergiant HD 93129A, one of the most luminous stars in the Galaxy. This titan has an estimated mass of about 80 times that of the Sun and is approximately two and a half million times brighter! It makes a stellar couple — a binary star — with another bright, massive star. The astronomers found that massive stars tend to pair up more often than less massive stars, and preferably with other more massive stars.

The Trumpler 14 cluster is undoubtedly a remarkable sight to observe: this dazzling patch of sky contains several white-blue, hot, massive stars, whose fierce ultraviolet light and stellar winds are blazing and heating up the surrounding dust and gas. Such massive stars rapidly burn their vast hydrogen supplies — the more massive the star, the shorter its lifespan. These giants will end their brief lives dramatically in convulsive explosions called supernovae, just a few million years from now.

A few orange stars are apparently scattered through Trumpler 14, in charming contrast to their bluish neighbours. These orange stars are in fact stars located behind Trumpler 14. Their reddened colour is due to absorption of blue light in the vast veils of dust and gas in the cloud.

The technology used in MAD to correct for the effect of the Earth’s atmosphere over large areas of sky will play a crucial role in the success of the next generation European Extremely Large Telescope (E-ELT).

Notes

[1] Telescopes on the ground suffer from a blurring effect introduced by atmospheric turbulence. This turbulence causes the stars to twinkle in a way that delights poets but frustrates astronomers, since it smears out the fine details of the images. However, with adaptive optics techniques, this major drawback can be overcome so that the telescope produces images that are as sharp as theoretically possible, i.e. approaching conditions in space. Adaptive optics systems work by means of a computer-controlled deformable mirror that counteracts the image distortion introduced by atmospheric turbulence. It is based on real-time optical corrections computed at very high speed (several hundreds of times each second) from image data obtained by a wavefront sensor (a special camera) that monitors light from a reference star.

[2] Present adaptive optics systems can only correct the effect of atmospheric turbulence in a very small region of the sky — typically 15 arcseconds or less — the correction degrading very quickly when moving away from the reference star. Engineers have therefore developed new techniques to overcome this limitation, one of which is multi-conjugate adaptive optics. MAD uses up to three stars instead of one as references to remove the blur caused by atmospheric turbulence over a field of view thirty times larger than that available to existing techniques.

Date: Thursday, December 03, 2009

Key Terms: Adaptive optics,open cluster,young star cluster,ESO, Trumpler 14,Very Large Telescope,MAD,HD 93129A,Binary Star,Stellar Winds,E-ELT,Arcsecond,Carina Constellation(the keel).

Revised and Edited By: Imran Khan.
year: 2009

A Superbright Supernova That’s the First of Its Kind

picture: In this schematic illustration of the material ejected from SN 2007bi, the radioactive nickel core (white) decays to cobalt, emitting gamma rays and positrons that excite surrounding layers (textured yellow) rich in heavy elements like iron. The outer layers (dark shadow) are lighter elements such as oxygen and carbon, where any helium must reside, which remain unilluminated and do not contribute to the visible spectrum.

But not the last, now that astronomers know where to look

Berkeley, CA – An extraordinarily bright, extraordinarily long-lasting supernova named SN 2007bi, snagged in a search by a robotic telescope, turns out to be the first example of the kind of stars that first populated the Universe. The superbright supernova occurred in a nearby dwarf galaxy, a kind of galaxy that’s common but has been little studied until now, and the unusual supernova could be the first of many such events soon to be discovered.

SN 2007bi was found early in 2007 by the international Nearby Supernova Factory (SNfactory) based at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory. The supernova’s spectrum was unusual, and astronomers at the University of California at Berkeley subsequently obtained a more detailed spectrum. Over the next year and a half the Berkeley scientists participated in a collaboration led by Avishay Gal-Yam of Israel’s Weizmann Institute of Science to collect and analyze much more data as the supernova slowly faded away.

The analysis indicated that the supernova’s precursor star could only have been a giant weighing at least 200 times the mass of our Sun and initially containing few elements besides hydrogen and helium – a star like the very first stars in the early Universe.

“Because the core alone was some 100 solar masses, the long-hypothesized phenomenon called pair instability must have occurred,” says astrophysicist Peter Nugent. A member of the SNfactory, Nugent is the co-leader of the Computational Cosmology Center (C3), a collaboration between Berkeley Lab’s Physics Division and Computational Research Division (CRD), where Nugent is a staff scientist. “In the extreme heat of the star’s interior, energetic gamma rays created pairs of electrons and positrons, which bled off the pressure that sustained the core against collapse.”

“SN 2007bi was the explosion of an exceedingly massive star,” says Alex Filippenko, a professor in the Astronomy Department at UC Berkeley whose team helped obtain, analyze, and interpret the data. “But instead of turning into a black hole like many other heavyweight stars, its core went through a nuclear runaway that blew it to shreds. This type of behavior was predicted several decades ago by theorists, but never convincingly observed until now.”

SN 2007bi is the first confirmed observation of a pair-instability supernova. The researchers describe their results in the 3 December 2009 issue of Nature.

On the trail of a strange beast

SN 2007bi was recorded on images taken as part of the Palomar-QUEST Survey, an automated search with the wide-field Oschin Telescope at the California Institute of Technology’s Palomar Observatory, and was quickly detected and categorized as an unusual supernova by the SNfactory. The SNfactory has so far discovered nearly a thousand supernovae of all types and amassed thousands of spectra, but has focused on those designated Type Ia, the “standard candles” used to study the expansion history of the Universe. SN 2007bi, however, turned out not to be a Type Ia. For one thing, it was at least ten times as bright.

“The thermonuclear runaway experienced by the core of SN 2007bi is reminiscent of that seen in the explosions of white dwarfs as Type Ia supernovae,” says Filippenko, “but on a much larger scale and with a far greater amount of power.”

“The discovery is a great example of how we can get all the science, in addition to cosmology, out of the SNfactory search,” says Greg Aldering, SNfactory project leader, who was not an author of the Nature paper. “Berkeley Lab and Caltech’s Astronomy Department agreed that we would split the work, the Lab handling the Type Ia’s and Caltech all the other types.”

Nugent contacted Gal-Yam, then a Caltech postdoctoral fellow, the lead investigator for the all-other category. “I asked, are you interested? He said, sure!” Nugent then contacted Filippenko, who was about to conduct a night of observation with the 10-meter Keck I telescope on the summit of Mauna Kea in Hawaii. Filippenko immediately set out to obtain an optical spectrum of the unusual supernova.

Caltech researchers subsequently acquired additional spectra with the Keck telescope, as did Paolo Mazzali’s team from the Max Planck Institute for Astrophysics in Garching, Germany, using the Very Large Telescope (VLT) in Chile.

Says Mazzali, “The Keck and VLT spectra clearly indicated that an extremely large amount of material was ejected by the explosion, including a record amount of radioactive nickel, which caused the expanding gases to glow very brightly.”

Rollin Thomas of CRD, a member of C3 and the SNfactory, aided the early analysis, using the Franklin supercomputer at the National Energy Research Scientific Computing Center (NERSC) to run a code he developed to generate numerous synthetic spectra for comparison with the real spectrum.

“The code uses hundreds of cores to systematically test a large number of simplified model supernovae, searching through the candidates by adjusting parameters until it finds a good fit,” says Thomas. “This kind of data-driven approach is key to helping us understand new types of transients for which no reliable theoretical predictions yet exist.” The model fit was unambiguous: SN 2007bi was a pair-instability supernova.

“The central part of the huge star had fused to oxygen near the end of its life, and was very hot,” Filippenko explains. “Then the most energetic photons of light turned into electron-positron pairs, robbing the core of pressure and causing it to collapse. This led to a nuclear runaway explosion that created a large amount of radioactive nickel, whose decay energized the ejected gas and kept the supernova visible for a long time.”

Gal-Yam organized a team of collaborators from many institutions to continue to observe SN 2007bi and obtain data as it slowly faded over a span of 555 days. Says Gal-Yam, “As our follow-up observations started to roll in, I immediately realized this must be something new. And indeed it turned out to be a fantastic example of how we are finding new types of stellar explosions.”

Because it had no hydrogen or helium lines, the usual classification scheme would have labeled the supernova a Type Ic. But it was so much brighter than an ordinary Type Ic that it reminded Nugent of only one prior event, a supernova designated SN 1999as, found by the international Supernova Cosmology Project but unfortunately three weeks after its peak brightness.

Understanding a supernova requires a good record of its rise and fall in brightness, or light curve. Although SN 2007bi was detected more than a week after its peak, Nugent delved into years of data compiled by NERSC from the SNfactory and other surveys. He found that the Catalina Sky Survey had recorded SN 2007bi before its peak brightness and could provide enough data to calculate the duration of the rising curve, an extraordinarily long 70 days – more evidence for the pair-instability identification.

A fossil laboratory of the early Universe

“It’s significant that the first unambiguous example of a pair-instability supernova was found in a dwarf galaxy,” says Nugent. “These are incredibly small, very dim galaxies that contain few elements heavier than hydrogen and helium, so they are models of the early Universe.”
Dwarf galaxies are ubiquitous but so faint and dim – “they take only a few pixels on a camera,” says Nugent, “and until recently, with the development of wide-field projects like the SNfactory, astronomers had wanted to fill the chip with galaxies” – that they’ve rarely been studied. SN 2007bi is expected to focus attention on what Gal-Yam and his collaborators call “fossil laboratories to study the early Universe.”

Says Filippenko, “In the future, we might end up detecting the very first generation of stars, early in the history of the Universe, through explosions such as that of SN 2007bi – long before we have the capability of directly seeing the pre-explosion stars.”

With the advent of the multi-institutional Palomar Transient Factory, a fully automated, wide-field survey to find transients, led by Caltech’s Shri Kulkarni, and with the aid of the Deep Sky Survey established by Nugent at NERSC to compile historical data from Palomar-QUEST, the SNfactory, the Near Earth Asteroid Team, and other surveys, the collaborators expect they will soon find many more ultrabright, ultramassive supernovae, revealing the role of these supernovae in creating the Universe as we know it today.


Edited By : Imran Khan
Key Terms: Pair Instability Supernova,SN 2007bi,light curve
Year:2009
Wednesday, December 02, 2009

Blushing Dusty Nebula

This close-up of an area in the northwest region of the large Iris Nebula seems to be clogged with cosmic dust. With bright light from the nearby star HD 200775 illuminating it from above, the dust resembles thick mounds of billowing cotton. It is actually made up of tiny particles of solid matter, with sizes from ten to a hundred times smaller than those of the dust grains we find at home. Both background and foreground stars are dotted throughout the image. Researchers studying the object are particularly interested in the region to the left and slightly above centre in the image, where dusty filaments appear redder than is expected.

North is down, East is right. The field of view is 3.3 arcminutes. The image is a composite of four images obtained through blue, green, near-infrared and H-alpha filters.Credit: NASA & ESA

Tuesday, December 01, 2009

A recent NASA/ESA Hubble Space Telescope image of part of NGC 7023, or the Iris Nebula, highlights a perfect dust laboratory in the sky.

On Earth, we tend to find dust nothing more than a nuisance that blankets our furniture and causes us to sneeze. Cosmic dust can also be a hindrance to astronomers because cameras using visible light cannot see through it. However, studying cosmic dust in detail helps astronomers to pin down the ingredients of the raw mixture that eventually gives birth to stars.

This close-up of an area in the northwest region of the large Iris Nebula seems to be clogged with cosmic dust. With bright light from the nearby star HD 200775 [1] illuminating it from above, the dust resembles thick mounds of billowing cotton. It is actually made up of tiny particles of solid matter, with sizes from ten to a hundred times smaller than those of the dust grains we find at home [2]. Both background and foreground stars are dotted throughout the image.

NGC 7023 is a reflection nebula, which means it scatters light from a massive nearby star, in this case, HD 200775. Reflection nebulae are different from emission nebulae, which are clouds of gas that are hot enough to emit light themselves. Reflection nebulae tend to appear blue because of the way light scatters, but parts of the Iris Nebula appear unusually red.

Researchers studying the object are particularly interested in the region to the left and slightly above centre in the image, where they find dusty filaments to be redder than expected. An unknown chemical compound, most likely based on hydrocarbons, is responsible for the red tinge. The high resolution and sensitivity of Hubble’s instruments allow astronomers to study the area in detail. Images and spectra are only part of the analysis. On Earth, scientists are performing additional laboratory tests to assess better the exact chemical composition of the nebula.

NGC 7023 was discovered by Sir William Herschel in 1794; the nebula is in the constellation of Cepheus, the King, in the northern sky. NGC 7023 is approximately 1400 light–years from Earth and about six light-years across. This aethereal image was taken by Hubble's Advanced Camera for Surveys. Astronomers also used Hubble’s Near Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument to try to determine which chemical elements are present in the nebula.

Notes for you:

North is down, East is right. The field of view is 3.3 arcminutes. The image is a composite of four images obtained through blue, green, near-infrared and H-alpha filters.

[1] HD 200775 is about ten times the mass of the Sun.

[2] The typical sizes of cosmic dust grains range between a few hundredths of a micron and several microns.

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

These observations were obtained by a team led by Karl Gordon from the Space Telescope Science Institute in Baltimore, Maryland, USA.

Vampires and collisions rejuvenate stars

Image credit: NASA, ESA and Francesco Ferraro (University of Bologna)

Wednesday, December 23, 2009

Using the NASA/ESA Hubble Space Telescope, astronomers have uncovered two distinct kinds of "rejuvenated" stars in the globular cluster Messier 30. A new study shows that both stellar collisions and a process sometimes called vampirism are behind this cosmic "face lift". The scientists also uncover evidence that both sorts of blue stragglers were produced during a critical dynamical event (known as "core collapse") that occurred in Messier 30 a few billion years ago.

Stars in globular clusters [1] are generally extremely old, with ages of 12-13 billion years. However, a small fraction of them appear to be significantly younger than the average population and, because they seem to have been left behind by the stars that followed the normal path of stellar evolution and became red giants, have been dubbed blue stragglers [2]. Blue stragglers appear to regress from "old age" back to a hotter and brighter "youth", gaining a new lease on life in the process. A team of astronomers used Hubble to study the blue straggler star content in Messier 30, which formed 13 billion years ago and was discovered in 1764 by Charles Messier. Located about 28 000 light-years away from Earth, this globular cluster — a swarm of several hundred thousand stars — is about 90 light-years across.

Although blue stragglers have been known since the early 1950s, their formation process is still an unsolved puzzle in astrophysics. "It’s like seeing a few kids in the group picture of a rest-home for retired people. It is natural to wonder why they are there," says Francesco Ferraro from the University of Bologna in Italy, lead author of the study that will be published this week in Nature [3]. Researchers have been studying these stars for many years and knew that blue stragglers are indeed old. They were thought to have arisen in a tight binary system [4]. In such a pair, the less massive star acts as a "vampire", siphoning fresh hydrogen from its more massive companion star. The new fuel supply allows the smaller star to heat up, growing bluer and hotter — behaving like a star at an earlier stage in its evolution.

The new study shows that some of the blue stragglers have instead been rejuvenated by a sort of "cosmic facelift", courtesy of cosmic collisions. These stellar encounters are nearly head-on collisions in which the stars might actually merge, mixing their nuclear fuel and re-stoking the fires of nuclear fusion. Merged stars and binary systems would both be about twice the typical mass of individual stars in the cluster.

"Our observations demonstrate that blue stragglers formed by collisions have slightly different properties from those formed by vampirism. This provides a direct demonstration that the two formation scenarios are valid and that they are both operating simultaneously in this cluster," says team member Giacomo Beccari from ESA.
Using data from the now-retired Wide Field Planetary Camera 2 (WFPC2) aboard Hubble, astronomers found that these "straggling" stars are much more concentrated towards the centre of the cluster than the average star. "This indicates that blue stragglers are more massive than the average star in this cluster," says Ferraro. "More massive stars tend to sink deep into the cluster the way a billiard ball would sink in a bucket of honey."

The central regions of high density globular clusters are crowded neighbourhoods where interactions between stars are nearly inevitable. Researchers conjecture that one or two billion years ago, Messier 30 underwent a major "core collapse" that started to throw stars towards the centre of the cluster, leading to a rapid increase in the density of stars. This event significantly increased the number of collisions among stars, and favoured the formation of one of the families of blue stragglers. On the other hand, the increase of stellar crowding due to the collapse of the core also perturbed the twin systems, encouraging the vampirism phenomenon and thus forming the other family of blue stragglers. "Almost ten percent of galactic globular clusters have experienced core collapse, but this is the first time that we see the effect of the core collapse imprinted on a stellar population," says Barbara Lanzoni, University of Bologna.

"The two distinct populations of blue stragglers discovered in Messier 30 are the relics of the collapse of the core that occurred two billion years ago. In a broad context our discovery is direct evidence of the impact of star cluster dynamics on stellar evolution. We should now try to see if other globular clusters present this double population of blue stragglers," concludes Ferraro.


Notes for you:

[1] Globular clusters are dense agglomerations of several hundred thousand stars. Present among the earliest inhabitants of our Milky Way, they formed in the vast halo of our galaxy before it flattened to form a pancake-shaped spiral disc. Star formation essentially stopped in globular clusters 13 billion years ago, so astronomers expect to find only old stars and they use globular cluster ages as a benchmark for estimating the age of the Universe.

[2] In 1953, astronomer Allan Sandage found a puzzling new population of stars that seemed to go against the rules of stellar evolution in globular clusters. Sandage detected hot young blue stars in the globular cluster Messier 3, and subsequently in other globular clusters. He dubbed them stragglers because they looked like they were trailing or left behind by other blue stars in the cluster that had long ago evolved to the red giant stage.

[3] This research was presented in a paper that appears in the 24 December 2009 issue of Nature, “Two distinct sequences of blue straggler stars in the globular cluster M30”, by F. R. Ferraro et al.

[4] In 1964 astronomers Fred Hoyle and W.H. McCrea independently suggested that blue stragglers result when two stars capture each other and form a tight binary system.