Inferring the Star Formation Rates of Galaxies

The active star-forming region c in our galaxy, as seen in the infrared. Astronomers estimate star formation rates in other galaxies by measuring their gas content and infrared emission. In a new study, astronomers counted up the new stars in RCW106 and five other local regions of active star formation to check the reliability of this method.  Credit: NASA/Spitzer

Our Milky Way galaxy produces on average a few new stars every year across the entire system. Massive young stars emit large amounts of ultraviolet radiation which heats the local dust, and so the star formation process results in infrared emission. The IRAS satellite, launched by NASA in 1983 for a ten-month mission, discovered that some galaxies in the universe are ultra-luminous, radiating a hundred or even a thousand times as much light, mostly in the infrared, as does the Milky Way.

Astronomers today attribute the source of that intense luminosity to massive bursts of star formation, simply scaled-up versions (called the Schmidt relation) of the processes in the Milky Way. The colors and other morphological characteristics of ultra-luminous galaxies are generally consistent with this interpretation. If true, these galaxies are forming stars with surprisingly high efficiencies and perhaps in unusual ways. Astronomers refining their models are therefore investigating the extent to which star formation rates can legitimately be derived from a simple scaling relation, as well as the extent to which other processes like black hole accretion at the nucleus might supplement the radiation from star formation.

CfA astronomers Sarah Willis, Andres Guzman, Howard Smith, and Juan Rafael Martinez-Galarza and their colleagues decided to investigate these issues by examining the star formation activity in six regions of current, massive star formation in our Milky Way. These molecular clouds are thought to be small prototypes of the powerful star formation regions active in luminous galaxies, but because the clouds are much closer to us, it is possible to count directly the number of new stars in them, rather than just infer their numbers from a luminosity as with the Schmidt relation extrapolation. 

Using infrared images from the Spitzer Space Telescope, complemented by ground-based observations, the team identified 2871 newly formed stars in these regions; they then traced the stellar production rates in different zones across the sources, using the visual extinction as a measure of the amount of dust and gas present. Their results were roughly consistent with a conventional Schmidt relation, but the astronomers found significant deviations across the regions, with the most dramatic locations producing stars a thousand times more efficiently than the least active (but still star forming) regions. The scientists conclude that, at least on the local scale, there is no universal relation between the density of molecular gas and the star formation.



References:

"The Schmidt Law in Six Galactic Massive Star-forming Regions," S. Willis, A. Guzman, M. Marengo, H. A. Smith, J. R. Martínez-Galarza, and L. Allen, ApJ 809, 87, 2015. 

Source: Smithsonian Astrophysical Observatory (SAO)

The last waltz

2MASX J16270254+4328340 

Credit: ESA/Hubble & NASA

Acknowledgement: Judy Schmidt (Geckzilla)

This curious galaxy — only known by the seemingly random jumble of letters and numbers 2MASX J16270254+4328340 — has been captured by the NASA/ESA Hubble Space Telescope dancing the crazed dance of a galactic merger. The galaxy has merged with another galaxy leaving a fine mist, made of millions of stars, spewing from it in long trails.

Despite the apparent chaos, this snapshot of the gravitational tango was captured towards the event’s conclusion. This transforming galaxy is heading into old age with its star-forming days coming to an end. The true drama occurred earlier in the process, when the various clouds of gas within the two galaxies were so disturbed by the event that they collapsed, triggering an eruption of star formation. 

This flurry of activity exhausted the vast majority of the galactic gas, leaving the galaxy sterile and unable to produce new stars.

As the violence continues to subside, the newly formed galaxy’s population of stars will redden with age and eventually begin drop off one by one. With no future generations of stars to take their place, the galaxy thus begins a steady descent towards death.

Source: ESA/Hubble -  Space Telescope

Strange Star Likely Swarmed by Comets

This illustration shows a star behind a shattered comet. 

Image credit: NASA/JPL-Caltech
›Full image and caption

A star called KIC 8462852 has been in the news recently for unexplained and bizarre behavior. NASA's Kepler mission had monitored the star for four years, observing two unusual incidents, in 2011 and 2013, when the star's light dimmed in dramatic, never-before-seen ways. Something had passed in front of the star and blocked its light, but what?

Scientists first reported the findings in September, suggesting a family of comets as the most likely explanation. Other cited causes included fragments of planets and asteroids.

A new study using data from NASA's Spitzer Space Telescope addresses the mystery, finding more evidence for the scenario involving a swarm of comets. The study, led by Massimo Marengo of Iowa State University, Ames, is accepted for publication in the Astrophysical Journal Letters.

One way to learn more about the star is to study it in infrared light. Kepler had observed it in visible light. If a planetary impact, or a collision amongst asteroids, were behind the mystery of KIC 8462852, then there should be an excess of infrared light around the star. Dusty, ground-up bits of rock would be at the right temperature to glow at infrared wavelengths.

At first, researchers tried to look for infrared light using NASA's Wide-Field Infrared Survey Explorer, or WISE, and found none. But those observations were taken in 2010, before the strange events seen by Kepler -- and before any collisions would have kicked up dust.

To search for infrared light that might have been generated after the oddball events, researchers turned to Spitzer, which, like WISE, also detects infrared light. Spitzer just happened to observe KIC 8462852 more recently in 2015.

"Spitzer has observed all of the hundreds of thousands of stars where Kepler hunted for planets, in the hope of finding infrared emission from circumstellar dust," said Michael Werner, the Spitzer project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, and the lead investigator of that particular Spitzer/Kepler observing program.

But, like WISE, Spitzer did not find any significant excess of infrared light from warm dust. That makes theories of rocky smashups very unlikely, and favors the idea that cold comets are responsible. 

It's possible that a family of comets is traveling on a very long, eccentric orbit around the star. At the head of the pack would be a very large comet, which would have blocked the star's light in 2011, as noted by Kepler. Later, in 2013, the rest of the comet family, a band of varied fragments lagging behind, would have passed in front of the star and again blocked its light.

By the time Spitzer observed the star in 2015, those comets would be farther away, having continued on their long journey around the star. They would not leave any infrared signatures that could be detected.

According to Marengo, more observations are needed to help settle the case of KIC 8462852.

"This is a very strange star," he said. "It reminds me of when we first discovered pulsars. They were emitting odd signals nobody had ever seen before, and the first one discovered was named LGM-1 after 'Little Green Men.'"

In the end, the LGM-1 signals turned out to be a natural phenomenon.

"We may not know yet what's going on around this star," Marengo observed. "But that's what makes it so interesting."

Ames manages the Kepler and K2 missions for NASA's Science Mission Directorate. JPL managed Kepler mission development. Ball Aerospace & Technologies Corp. operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.

JPL manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech.

Caltech manages JPL for NASA.

For more information about Kepler and Spitzer, respectively, visit:

http://www.nasa.gov/kepler
http://kepler.nasa.gov
http://www.nasa.gov/spitzer
http://www.spitzer.caltech.edu


Media Contact

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

Michele Johnson
Ames Research Center, Moffett Field, Calif.
650-604-6982
michele.johnson@nasa.gov

Source: JPL - Caltech

Aging Star’s Weight Loss Secret Revealed

PR Image eso1546a

VLT image of the surroundings of VY Canis Majoris seen with SPHERE

PR Image eso1546b

The red hypergiant star VY Canis Majoris 

 PR Image eso1546c

Wide-field view of the sky around VY Canis Majoris

Video 

PR Video eso1546a

Zooming in on the red hypergiant star VY Canis Majoris

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Giant star caught in the act of slimming down

 

A team of astronomers using ESO’s Very Large Telescope (VLT) has captured the most detailed images ever of the hypergiant star VY Canis Majoris. These observations show how the unexpectedly large size of the particles of dust surrounding the star enable it to lose an enormous amount of mass as it begins to die. This process, understood now for the first time, is necessary to prepare such gigantic stars to meet explosive demises as supernovae.

VY Canis Majoris is a stellar goliath, a red hypergiant, one of the largest known stars in the Milky Way. It is 30–40 times the mass of the Sun and 300 000 times more luminous. In its current state, the star would encompass the orbit of Jupiter, having expanded tremendously as it enters the final stages of its life.

The new observations of the star used the SPHERE instrument on the VLT. The adaptive optics system of this instrument corrects images to a higher degree than earlier adaptive optics systems. This allows features very close to bright sources of light to be seen in great detail [1]. SPHERE clearly revealed how the brilliant light of VY Canis Majoris was lighting up clouds of material surrounding it.

And by using the ZIMPOL mode of SPHERE, the team could not only peer deeper into the heart of this cloud of gas and dust around the star, but they could also see how the starlight was scattered and polarised by the surrounding material. These measurements were key to discovering the elusive properties of the dust.

Careful analysis of the polarisation results revealed these grains of dust to be comparatively large particles, 0.5 micrometres across, which may seem small, but grains of this size are about 50 times larger than the dust normally found in interstellar space.

Throughout their expansion, massive stars shed large amounts of material — every year, VY Canis Majoris sees 30 times the mass of the Earth expelled from its surface in the form of dust and gas. This cloud of material is pushed outwards before the star explodes, at which point some of the dust is destroyed, and the rest cast out into interstellar space. This material is then used, along with the heavier elements created during the supernova explosion, by the next generation of stars, which may make use of the material for planets.

Until now, it had remained mysterious how the material in these giant stars’ upper atmospheres is pushed away into space before the host explodes. The most likely driver has always seemed to be radiation pressure, the force that starlight exerts. As this pressure is very weak, the process relies on large grains of dust, to ensure a broad enough surface area to have an appreciable effect [2].

“Massive stars live short lives,” says lead author of the paper, Peter Scicluna, of the Academia Sinica Institute for Astronomy and Astrophysics, Taiwan. “When they near their final days, they lose a lot of mass. In the past, we could only theorise about how this happened. But now, with the new SPHERE data, we have found large grains of dust around this hypergiant. These are big enough to be pushed away by the star’s intense radiation pressure, which explains the star’s rapid mass loss.”

The large grains of dust observed so close to the star mean that the cloud can effectively scatter the star’s visible light and be pushed by the radiation pressure from the star. The size of the dust grains also means much of it is likely to survive the radiation produced by VY Canis Majoris’ inevitable dramatic demise as a supernova [3]. This dust then contributes to the surrounding interstellar medium, feeding future generations of stars and encouraging them to form planets.

Notes

[1] SPHERE/ZIMPOL uses extreme adaptive optics to create diffraction-limited images, which come a lot closer than previous adaptive optics instruments to achieving the theoretical limit of the telescope if there were no atmosphere. Extreme adaptive optics also allows much fainter objects to be seen very close to a bright star.

The images in the new study are also taken in visible light — shorter wavelengths than the near-infrared regime, where most earlier adaptive optics imaging was performed. These two factors result in significantly sharper images than earlier VLT images. Even higher spatial resolution has been achieved with the VLTI, but the interferometer does not create images directly.

[2] The dust particles must be large enough to ensure the starlight can push it, but not so large that it simply sinks. Too small and the starlight would effectively pass through the dust; too large and the dust would be too heavy to push. The dust the team observed about VY Canis Majoris was precisely the right size to be most effectively propelled outwards by the starlight.

[3] The explosion will be soon by astronomical standards, but there is no cause for alarm, as this dramatic event is not likely for hundreds of thousands of years. It will be spectacular as seen from Earth — perhaps as bright as the Moon — but not a hazard to life here.

More Information

This research was presented in a paper entitled “Large dust grains in the wind of VY Canis Majoris”, by P. Scicluna et al., to appear in the journal Astronomy & Astrophysics.

The team is composed of P. Scicluna (Academia Sinica Institute for Astronomy and Astrophysics, Taiwan), R. Siebenmorgen (ESO, Garching, Germany), J. Blommaert (Vrije Universiteit, Brussels, Belgium), M. Kasper (ESO, Garching, Germany), N.V. Voshchinnikov (St. Petersburg University, St. Petersburg, Russia), R. Wesson (ESO, Santiago, Chile) and S. Wolf (Kiel University, Kiel, Germany).

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
• Link to ESOcast about polarimetry
• Photos of the VLT

Contacts

Peter Scicluna
Academia Sinica Institute for Astronomy and Astrophysics
Taiwan
Tel: +886 (02) 2366 5420
Email: peterscicluna@asiaa.sinica.edu.tw

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

NOAO: Oodles of Faint Dwarf Galaxies in Fornax Shed Light on a Cosmological Mystery

Image of the inner 3 square degrees of the NGFS survey footprint compared with the size of the Moon. Low surface brightness dwarf galaxies are marked by red circles. Gray circles indicate previously known dwarf galaxies. The dwarf galaxies, which vastly outnumber the bright galaxies, may be the “missing satellites” predicted by cosmological simulations.

An astonishing number of faint low surface brightness dwarf galaxies recently discovered in the Fornax cluster of galaxies may help to solve the long-standing cosmological mystery of “The Missing Satellites”. The discovery, made by an international team of astronomers led by Roberto Muñoz and Thomas Puzia of Pontificia Universidad Católica de Chile, was carried out using the Dark Energy Camera (DECam) on the 4-m Blanco telescope at Cerro Tololo Inter-American Observatory (CTIO). CTIO is operated by the National Optical Astronomy Observatory (NOAO).

Computer simulations of the evolution of the matter distribution in the Universe predict that dwarf galaxies should vastly outnumber galaxies like the Milky Way, with hundreds of low mass dwarf galaxies predicted for every Milky Way-like galaxy. The apparent shortage of dwarf galaxies relative to these predictions, “the missing satellites problem,” could imply that the cosmological simulations are wrong or that the predicted dwarf galaxies have simply not yet been discovered. The discovery of numerous faint dwarf galaxies in Fornax suggests that the “missing satellites” are now being found.

The discovery, recently published in the Astrophysical Journal, comes as one of the first results from the Next Generation Fornax Survey (NGFS), a study of the central 30 square degree region of the Fornax galaxy cluster using optical imaging with DECam and near-infrared imaging with ESO’s VISTA/VIRCam. The Fornax cluster, located at a distance of 62 million light-years, is the second richest galaxy cluster within 100 million light-years after the much richer Virgo cluster.

The deep, high-quality images of the Fornax cluster core obtained with DECam were critical to the recovery of the missing dwarf galaxies. “With the combination of DECam’s huge field of view (3 square degrees) and our novel observing strategy and data reduction algorithms, we were able to detect extremely diffuse low-surface brightness galaxies,” explained Roberto Muñoz, the lead author of the study.

Because the low surface brightness dwarf galaxies are extremely diffuse, stargazers residing in one of these galaxies would see a night sky very different from that seen from Earth. The stellar density of the faint dwarf galaxies (one star per million cubic parsecs) is about a million times lower than that in the neighborhood of the Sun, or almost a billion times lower than in the bulge of the Milky Way.

As a result, “inhabitants of worlds in one of our NGFS ultra-faint dwarfs would find their sky sparsely populated with visible objects and extremely boring. They would perhaps not even realize that they live in a galaxy!” mused coauthor Thomas Puzia.

The large number of dwarf galaxies discovered in the Fornax cluster echoes the emerging census of satellites of our own Galaxy, the Milky Way. More than 20 dwarf galaxy companions have been discovered in the past year, many of which were also discovered with DECam.

Reference: “Unveiling a Rich System of Faint Dwarf Galaxies in the Next Generation Fornax Survey,” Roberto P. Muñoz et al., 2015 November 1, Astrophysical Journal Letters [http://iopscience.iop.org/article/10.1088/2041-8205/813/1/L15, preprint: http://arxiv.org/abs/1510.02475].

Cerro Tololo Inter-American Observatory is managed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy Inc. (AURA) under a cooperative agreement with the National Science Foundation.

Science Contact

Dr. Thomas H. Puzia
Pontificia Universidad Católica de Chile
Tel: +56-9-89010007
Email: tpuzia@astro.puc.cl

Source: National Optical Astronomy Observatory (NRAO)

Gemini Characterizes Cheshire Cat

Figure 1. Image of Cheshire Cat gravitationally lensed galaxies produced by combining Chandra X-ray data with Hubble Space Telescope optical data. Gemini spectroscopy provided critical spectroscopic data for characterizing the foreground lensing cluster and understanding its history and likely fate as a "fossil group." Image Credit: Hubble Space Telescope and Chandra X-ray Observatory

Figure 2. Color–magnitude diagram of all galaxies detected in the GMOS images with r' brighter than 24.5 mag (595 galaxies, black dots). Blue circles and red squares are the spectroscopically confirmed members of the G1 and G2 groups respectively, with filled and open symbols representing which of these galaxies lie inside and outside of 0.5 r₂₀₀ of the group. Red short dash and blue long dash vertical lines represent a two magnitude gap from the E and W eye galaxies, respectively. The black triangles represent spectroscopically confirmed background and foreground galaxies, while black dots represent objects without spectroscopic redshifts. The two central galaxies of the groups, SDSS J103843.58+484917.7 and SDSS J103842.68+484920.2 (the "eyes" of the Cheshire Cat) are represented by a letter (E and W) in the CMD. 

Gemini observations provide a key scientific context for a striking new image of the gravitationally lensed galaxy group popularly known as the Cheshire Cat. The image, released by NASA’s Chandra X-ray Observatory, combines X-ray data with optical images from the Hubble Space Telescope to paint a remarkable likeness to the famously devious smiling cat in Lewis Carrol’s Alice in Wonderland.

Behind the scenes, Gemini played a critical role in the image’s scientific story by taking the spectral fingerprints of many of the galaxies that make up the foreground cluster which bends the distant galactic light. For more details on the image, and how gravitational lenses work, see the Chandra press release here.

Gemini South astronomer Rodrigo Carrasco, a co-author on the paper which spawned the image, adds that Gemini provided the core spectroscopic data that allowed the team to fully characterize the cluster of foreground galaxies which serves as the lens. "With the Gemini data we were able to study the individual members of the cluster which we expect will eventually become fossil groups," says Carrasco. Fossil groups consist of a giant elliptical central galaxy which is the end-product of interactions with other galaxies over billions of years, and surrounded by smaller, less bright galaxies. "Our results indicate that merging of fossil groups – in this case, the Cheshire Cat – could be one of the ways that a large fraction of fossil groups in the nearby Universe formed." 

The paper, published in The Astrophysical Journal, is available at: http://xxx.lanl.gov/abs/1505.05501. 

Paper Abstract:

Abstract: The Cheshire Cat is a relatively poor group of galaxies dominated by two luminous elliptical galaxies surrounded by at least four arcs from gravitationally lensed background galaxies that give the system a humorous appearance. Our combined optical/X-ray study of this system reveals that it is experiencing a line of sight merger between two groups with a roughly equal mass ratio with a relative velocity of ∼1350 km s^-1. One group was most likely a low-mass fossil group, while the other group would have almost fit the classical definition of a fossil group. The collision manifests itself in a bimodal galaxy velocity distribution, an elevated central X-ray temperature and luminosity indicative of a shock, and gravitational arc centers that do not coincide with either large elliptical galaxy. One of the luminous elliptical galaxies has a double nucleus embedded off-center in the stellar halo. The luminous ellipticals should merge in less than a Gyr, after which observers will see a massive 1.2 − 1.5 x 10¹⁴ M_⊙ fossil group with an M_r = −24.0 brightest group galaxy at its center. Thus, the Cheshire Cat offers us the first opportunity to study a fossil group progenitor. We discuss the limitations of the classical definition of a fossil group in terms of magnitude gaps between the member galaxies. We also suggest that if the merging of fossil (or near-fossil) groups is a common avenue for creating present-day fossil groups, the time lag between the final galactic merging of the system and the onset of cooling in the shock-heated core could account for the observed lack of well-developed cool cores in some fossil groups.

Source: Gemini Observatory

Ghostly and beautiful: “planetary nebulae” get more meaningful physical presence

A collage showing 22 individual planetary nebulae artistically arranged in approximate order of physical size. The scale bar represents 4 light years. Each nebula's size is calculated from the authors' new distance scale, which is applicable to all nebulae across all shapes, sizes and brightnesses. The very largest planetary nebula currently known is nearly 20 light years in diameter, and would cover the entire image at this scale. Credit: ESA/Hubble & NASA, ESO, Ivan Bojicic, David Frew, Quentin Parker. Hi-res image

A way of estimating more accurate distances to the thousands of so-called planetary nebulae dispersed across our Galaxy has been announced by a team of three astronomers based at the University of Hong Kong: Dr David Frew, Prof Quentin Parker and Dr Ivan Bojicic. The scientists publish their results in Monthly Notices of the Royal Astronomical Society.

Despite their name, planetary nebulae have nothing to do with planets. They were described as such by early astronomers whose telescopes showed them as glowing disc-like objects.

We now know that planetary nebulae are actually the final stage of activity of stars like our Sun. When they reach the end of their lives, these stars eject most of their atmosphere into space, leaving behind a hot dense core. Light from this core causes the expanding cloud of gas to glow in different colours as it slowly grows, fading away over tens of thousands of years.

There are thousands of planetary nebulae in our Galaxy alone, and they provide targets for professional and amateur astronomers alike, with the latter often taking spectacular images of these beautiful objects. But despite intense study, scientists have struggled to measure one of their key properties – their distance.

Dr Frew, lead author on the paper, said: "For many decades, measuring distances to Galactic planetary nebulae has been a serious, almost intractable problem because of the extremely diverse nature of the nebulae themselves and their central stars. But finding those distances is crucial if we want to understand their true nature and physical properties."

The solution presented by the astronomers is both simple and elegant. Their method requires only an estimate of the dimming toward the object (caused by intervening interstellar gas and dust), the projected size of the object on the sky (taken from the latest high resolution surveys) and a measurement of how bright the object is (as obtained from the best modern imaging).

The resulting so-called 'surface-brightness relation' has been robustly calibrated using more than 300 planetary nebulae whose accurate distances have been determined via independent and reliable means. Prof Parker explained that, "the basic technique is not new but what marks out this work from what has gone before is the use of the most up-to-date and reliable measurements of all three of those crucial properties".

This is combined with the use of the authors' own robust techniques to effectively remove "doppelgangers" and mimics that have seriously contaminated previous planetary nebulae catalogues and added considerable errors to other distance measurements.

The new approach works over a factor of several hundred thousand in surface brightness, and allows astronomers to measure the distances to planetary nebulae up to 5 times more accurately than previous methods. "Our new scale is the first to accurately determine distances for the very faintest planetaries" said Dr Frew. "Since the largest nebulae are the most common, getting their distances right is a crucial step".

A comparison of the distance scales of two highly evolved nebulae, numbered (1) PuWe 1, (2) Abell 21. Previous distance scales were often inaccurate for the largest, most evolved planetary nebulae, which are the most common type in the Galaxy. The left panel shows the physical sizes of two nearby nebulae, presented at a common scale and using the authors' new calculations. The scale bar represents 4 light years. The right panel shows the physical sizes calculated from a commonly used older distance scale, which considerably underestimates the distances and hence sizes of these objects. Credit: NOAO/AURA/NSF, Ivan Bojicic, David Frew, Quentin Parker (HKU)

Planetary nebulae are a fascinating if brief stage in the life of a low- to middle-weight star. Being able to better measure distances and hence the sizes of these objects will give scientists a far better insight into how these objects form and develop, and how stars as a whole evolve and die.

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Media contacts

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
Mob: +44 (0)7802 877 699
rm@ras.org.uk

Dr Sam Lindsay
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
sl@ras.org.uk

Dr Morgan Hollis
Royal Astronomical Society
Tel: +44 (0)20 7734 3307
mh@ras.org.uk

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Science contacts

Dr David Frew
The University of Hong Kong
Mob: +852 9415 2556
djfrew@hku.hk

Prof Quentin Parker
The University of Hong Kong
Mob: +852 5669 9166
quentinp@hku.hk

Dr Ivan Bojicic
The University of Hong Kong
Mob: +852 6682 7976
ibojicic@hku.hk

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Images and captions

A collage showing 22 individual planetary nebulae artistically arranged in approximate order of physical size. The scale bar represents 4 light years. Each nebula's size is calculated from the authors' new distance scale, which is applicable to all nebulae across all shapes, sizes and brightnesses. The very largest planetary nebula currently known is nearly 20 light years in diameter, and would cover the entire image at this scale. Credit: ESA/Hubble & NASA, ESO, Ivan Bojicic, David Frew, Quentin Parker

A comparison of the distance scales of two highly evolved nebulae, numbered (1) PuWe 1, (2) Abell 21. Previous distance scales were often inaccurate for the largest, most evolved planetary nebulae, which are the most common type in the Galaxy. The left panel shows the physical sizes of two nearby nebulae, presented at a common scale and using the authors' new calculations. The scale bar represents 4 light years. The right panel shows the physical sizes calculated from a commonly used older distance scale, which considerably underestimates the distances and hence sizes of these objects. Credit: NOAO/AURA/NSF, Ivan Bojicic, David Frew, Quentin Parker (HKU)

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Further information

The new work appears in "The Hα surface brightness - radius relation: a robust statistical distance indicator for planetary nebulae", David J. Frew, Quentin A. Parker and Ivan S. Bojicic, Monthly Notices of the Royal Astronomical Society, Oxford University Press. A copy of the paper is available from http://mnras.oxfordjournals.org/lookup/doi/10.1093/mnras/stv1516.

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Notes for editors

The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3900 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others. Follow the RAS on Twitter via @royalastrosoc

Source: Royal Astronomical Society (RAS)

Tiny, Ultracool Star is Super Stormy

Artist's impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field (shown by the blue lines), potentially associated with a flurry of solar-flare-like eruptions. Credit: NRAO/AUI/NSF; Dana Berry / SkyWorks. Low Resolution (jpg) - High Resolution (jpg)

Animation of artist's impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field, potentially associated with a flurry of solar-flare-like eruptions.Credit: NRAO/AUI/NSF; Dana Berry / SkyWorks.  Animation (mov) - Animation2 (mov)

Cambridge, MA -Our Sun is a relatively quiet star that only occasionally releases solar flares or blasts of energetic particles that threaten satellites and power grids. You might think that smaller, cooler stars would be even more sedate. However, astronomers have now identified a tiny star with a monstrous temper. It shows evidence of much stronger flares than anything our Sun produces. If similar stars prove to be just as stormy, then potentially habitable planets orbiting them are likely to be much less hospitable than previously thought.

"If we lived around a star like this one, we wouldn't have any satellite communications. In fact, it might be extremely difficult for life to evolve at all in such a stormy environment," says lead author Peter Williams of the Harvard-Smithsonian Center for Astrophysics (CfA).

The research team targeted a well-known red dwarf star located about 35 light-years from Earth in the constellation Boîtes. The object is so small and cool that it's right on the dividing line between stars (which fuse hydrogen) and brown dwarfs (which don't). One of the things that makes this small star remarkable is that it spins rapidly, completing a full rotation about every 2 hours. Compare that with our Sun, which takes nearly a month to spin once on its axis.

Previous data from the Karl G. Jansky Very Large Array in Socorro, NM showed that this star has a magnetic field several hundred times stronger than our Sun. This puzzled astronomers because the physical processes that generate the Sun's magnetic field shouldn't operate in such a small star.

"This star is a very different beast from our Sun, magnetically speaking," states CfA astronomer and co-author Edo Berger.

The researchers examined the star with the new Atacama Large Millimeter/submillimeter Array (ALMA) and detected emission at a frequency of 95 GHz. This is the first time that flare-like emission at such high frequencies has been detected from a red dwarf star. Our Sun generates similar emission from solar flares but only intermittently. What's more, the emission from this star is 10,000 times brighter than what our own Sun produces, even though it has less than one-tenth of the Sun's mass. The fact that ALMA detected this emission in a brief 4-hour observation suggests that the red dwarf is continuously active.

This has important implications for the search for habitable planets outside the Solar system. Red dwarfs are the most common type of star in our galaxy, which makes them promising targets for planet searches. But because a red dwarf is so cool, a planet would have to orbit very close to the star to be warm enough for liquid water to exist. That proximity would put the planet right in the bull's-eye for radiation that could strip its atmosphere or destroy any complex molecules on its surface.

"It's like living in Tornado Alley in the U.S. Your location puts you at greater risk of severe storms," explains Williams. "A planet in the habitable zone of a star like this would be buffeted by storms much stronger than those generated by the Sun."

Astronomers will study similar stars in the future to determine whether this one is an oddball or an example of an entire class of stormy stars.

These findings have been accepted for publication in The Astrophysical Journal and are available online.

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)

Discovery Measures "Heartbeats" of a Distant Galaxy's Stars

The monstrous elliptical galaxy M87, located 53 million light-years from Earth is the dominant galaxy at the center of the neighboring Virgo cluster of galaxies. Astronomers have measured the "heartbeats" of stars within M87 and used that data to determine the galaxy's age in a new way. This photograph was taken with the Hubble Space Telescope's Advanced Camera for Surveys instrument.  Credit:NASA, ESA, and the Hubble Heritage Team. High Resolution (jpg) - Low Resolution (jpg)

Cambridge, MA -I n many ways stars are like living beings. They're born; they live; they die. And they even have a heartbeat. Using a novel technique, astronomers have detected thousands of stellar "pulses" in the galaxy Messier 87 (M87). Their measurements offer a new way of determining a galaxy's age.

We tend to think of stars as stable and unchanging. However, late in life stars like the Sun undergo a significant transformation. They become very bright and swell up to an enormous size, swallowing any planets that are within Earth's distance from the Sun. Near the end of their lifetime they begin to pulsate, increasing and decreasing their brightness by a large amount every few hundred days. In our own Milky Way galaxy many stars are known to be in this stage of life.

No one had considered the effects of these stars on the light coming from more distant galaxies. In distant galaxies the light of each pulsating star is mixed in with the light of many more stars that are not varying in brightness.

"We realized that these stars are so bright and their pulsations so strong, that they are difficult to hide," said Charlie Conroy, an assistant professor at Harvard University and astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA), who led the research. "We decided to see if the pulsations of these stars could be detected even if we couldn’t separate their light from the sea of unchanging stars that are their neighbors."

The astronomers studied the elliptical galaxy M87, located 53 million light-years from Earth in the constellation Virgo. They examined a unique series of images taken with the Hubble Space Telescope over the course of three months in 2006. They quickly found what they were looking for.

"Amazingly, one in four pixels in the image changes with time," said Pieter van Dokkum, a professor and chair of the astronomy department at Yale University. "We tend to think of galaxies as steady beacons in the sky, but they are actually 'shimmering' due to all the giant, pulsating stars in them."

Analysis of the Hubble data showed that the average pixel varies on a timescale of approximately 270 days. The regular up and down changes in brightness are reminiscent of a heartbeat. "It's as if we're taking the pulse of the galaxy," said Conroy.

Their discovery offers a new way of measuring the age of a galaxy, because the strength and speed of a galaxy's heartbeat varies depending on its age. The team finds that M87 is about 10 billion years old, a number that agrees with previous estimates using different techniques.

The discovery of stellar heartbeats should not be specific to M87; every galaxy in the universe likely shows similar distinctive patterns. The next step is to take the pulse of other galaxies.

"Our models suggest that the pulsations will be stronger in younger galaxies, and that's something we'd love to test," said Jieun Choi, a graduate student at Harvard and a co-author of the study.

The galaxies will keep beating for a while longer. Said van Dokkum, "Cardiac arrest is not expected until a trillion years from now - that's a hundred times longer than the age of the universe."

The research is described in Nature's November 16 Advanced Online Publication in a paper authored by Charlie Conroy (CfA), Pieter von Dokkum (Yale University) and Jieun Choi (CfA).

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)

A young elliptical

NGC 3610

Credit: ESA/Hubble & NASA, Acknowledgement: Judy Schmidt (Geckzilla)

At the centre of this amazing image is the elliptical galaxy NGC 3610. Surrounding the galaxy are a wealth of other galaxies of all shapes. There are spiral galaxies, galaxies with a bar in their central regions, distorted galaxies and elliptical galaxies, all visible in the background. In fact, almost every bright dot in this image is a galaxy — the few foreground stars are clearly distinguishable due to the diffraction spikes that overlay their images.

NGC 3610 is of course the most prominent object in this image — and a very interesting one at that! Discovered in 1793 by William Herschel, it was later found that this elliptical galaxy contains a disc. This is very unusual, as discs are one of the main distinguishing features of a spiral galaxy. And NGC 3610 even hosts a memarkable bright disc.

The reason for the peculiar shape of NGC 3610 stems from its formation history. When galaxies form, they usually resemble our galaxy, the Milky Way, with flat discs and spiral arms where star formation rates are high and which are therefore very bright. An elliptical galaxy is a much more disordered object which results from the merging of two or more disc galaxies. During these violent mergers most of the internal structure of the original galaxies is destroyed. The fact that NGC 3610 still shows some structure in the form of a bright disc implies that it formed only a short time ago. The galaxy’s age has been put at around four billion years and it is an important object for studying the early stages of evolution in elliptical galaxies.

Source: ESA/Hubble - Space Telescope

Dark Matter Dominates in Nearby Dwarf Galaxy

Dwarf galaxies have few stars but lots of dark matter. This Caltech FIRE (Feedback in Realistic Environments) simulation from shows the predicted distribution of stars (left) and dark matter (right) around a galaxy like the Milky Way. The red circle shows a dwarf galaxy like Triangulum II. Although it has a lot of dark matter, it has very few stars. Dark matter-dominated galaxies like Triangulum II are excellent prospects for detecting the gamma-ray signal from dark matter self-annihilation. Credit: A. Wetzel and P. Hopkins, Caltech

Dark matter is called "dark" for a good reason. Although they outnumber particles of regular matter by more than a factor of 10, particles of dark matter are elusive. Their existence is inferred by their gravitational influence in galaxies, but no one has ever directly observed signals from dark matter. Now, by measuring the mass of a nearby dwarf galaxy called Triangulum II, Assistant Professor of Astronomy Evan Kirby may have found the highest concentration of dark matter in any known galaxy.

Triangulum II is a small, faint galaxy at the edge of the Milky Way, made up of only about 1,000 stars. Kirby measured the mass of Triangulum II by examining the velocity of six stars whipping around the galaxy's center. "The galaxy is challenging to look at," he says. "Only six of its stars were luminous enough to see with the Keck telescope." By measuring these stars' velocity, Kirby could infer the gravitational force exerted on the stars and thereby determine the mass of the galaxy.

"The total mass I measured was much, much greater than the mass of the total number of stars—implying that there's a ton of densely packed dark matter contributing to the total mass," Kirby says.

"The ratio of dark matter to luminous matter is the highest of any galaxy we know. After I had made my measurements, I was just thinking—wow."

Triangulum II could thus become a leading candidate for efforts to directly detect the signatures of dark matter. Certain particles of dark matter, called supersymmetric WIMPs (weakly interacting massive particles), will annihilate one another upon colliding and produce gamma rays that can then be detected from Earth.

While current theories predict that dark matter is producing gamma rays almost everywhere in the universe, detecting these particular signals among other galactic noises, like gamma rays emitted from pulsars, is a challenge. Triangulum II, on the other hand, is a very quiet galaxy. It lacks the gas and other material necessary to form stars, so it isn't forming new stars—astronomers call it "dead." Any gamma ray signals coming from colliding dark matter particles would theoretically be clearly visible.

It hasn't been definitively confirmed, though, that what Kirby measured is actually the total mass of the galaxy. Another group, led by researchers from the University of Strasbourg in France, measured the velocities of stars just outside Triangulum II and found that they are actually moving faster than the stars closer into the galaxy's center—the opposite of what's expected. This could suggest that the little galaxy is being pulled apart, or "tidally disrupted," by the Milky Way's gravity.

"My next steps are to make measurements to confirm that other group's findings," Kirby says. "If it turns out that those outer stars aren't actually moving faster than the inner ones, then the galaxy could be in what's called dynamic equilibrium. That would make it the most excellent candidate for detecting dark matter with gamma rays."

A paper describing this research appears in the November 17 issue of the Astrophysical Journal Letters. Judith Cohen (PhD '71), the Kate Van Nuys Page Professor of Astronomy, is a Caltech coauthor.

Written by Lori Dajose

Contact: 

Deborah Williams-Hedges

(626) 395-3227

debwms@caltech.edu

Source: Caltech

The Birth of Monsters

PR Image eso1545a

Massive galaxies discovered in the early Universe

 

PR Image eso1545b

Massive galaxies discovered in the early Universe

Video

PR Video eso1545a

Massive galaxies discovered in the early Universe 

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VISTA pinpoints earliest giant galaxies 

ESO’s VISTA survey telescope has spied a horde of previously hidden massive galaxies that existed when the Universe was in its infancy. By discovering and studying more of these galaxies than ever before, astronomers have, for the first time, found out exactly when such monster galaxies first appeared.

Just counting the number of galaxies in a patch of sky provides a way to test astronomers’ theories of galaxy formation and evolution. However, such a simple task becomes increasingly hard as astronomers attempt to count the more distant and fainter galaxies. It is further complicated by the fact that the brightest and easiest galaxies to observe — the most massive galaxies in the Universe — are rarer the further astronomers peer into the Universe’s past, whilst the more numerous less bright galaxies are even more difficult to find.

A team of astronomers, led by Karina Caputi of the Kapteyn Astronomical Institute at the University of Groningen, has now unearthed many distant galaxies that had escaped earlier scrutiny. They used images from the UltraVISTA survey, one of six projects using VISTA to survey the sky at near-infrared wavelengths, and made a census of faint galaxies when the age of the Universe was between just 0.75 and 2.1 billion years old.

UltraVISTA has been imaging the same patch of sky, nearly four times the size of a full Moon, since December 2009. This is the largest patch of sky ever imaged to these depths at infrared wavelengths. The team combined these UltraVISTA observations with those from the NASA Spitzer Space Telescope, which probes the cosmos at even longer, mid-infrared wavelengths [1].

“We uncovered 574 new massive galaxies — the largest sample of such hidden galaxies in the early Universe ever assembled,” explains Karina Caputi. “Studying them allows us to answer a simple but important question: when did the first massive galaxies appear?”

Imaging the cosmos at near-infrared wavelengths allowed the astronomers to see objects that are both obscured by dust, and extremely distant [2], created when the Universe was just an infant.

The team discovered an explosion in the numbers of these galaxies in a very short amount of time. A large fraction of the massive galaxies [3] we now see around us in the nearby Universe were already formed just three billion years after the Big Bang.

“

We found no evidence of these massive galaxies earlier than around one billion years after the Big Bang, so we’re confident that this is when the first massive galaxies must have formed,” concludes 

Henry Joy McCracken, a co-author on the paper [4].

In addition, the astronomers found that massive galaxies were more plentiful than had been thought. Galaxies that were previously hidden make up half of the total number of massive galaxies present when the Universe was between 1.1 and 1.5 billion years old [5]. These new results, however, contradict current models of how galaxies evolved in the early Universe, which do not predict any monster galaxies at these early times.

To complicate things further, if massive galaxies are unexpectedly dustier in the early Universe than astronomers predict then even UltraVISTA wouldn’t be able to detect them. If this is indeed the case, the currently-held picture of how galaxies formed in the early Universe may also require a complete overhaul.

The Atacama Large Millimeter/submillimeter Array (ALMA) will also search for these game-changing dusty galaxies. If they are found they will also serve as targets for ESO’s 39-metre European Extremely Large Telescope (E-ELT), which will enable detailed observations of some of the first ever galaxies.


Notes

[1] ESO’s VISTA telescope observed in the near-infrared wavelength range 0.88–2.15 μm while Spitzer performed observations in the mid-infrared at 3.6 and 4.5 μm.

[2] The expansion of space means that the more distant a galaxy is, the faster it appears to be speeding away from an observer on Earth. This stretching causes the light from these distant objects to be shifted into redder parts of the spectrum, meaning that observations in the near-to-mid infrared are necessary to capture the light from these galaxies.

[3] In this context, "massive" means more than 50 billion times the mass of the Sun. The total mass of the stars in the Milky Way is also close to this figure.

[4] The team found no evidence of massive galaxies beyond a redshift of 6, equivalent to times less than 0.9 billion years after the Big Bang.

[5] This is equivalent to redshifts between z=5 and z=4.


More Information

This research was presented in a paper entitled “Spitzer Bright, UltraVISTA Faint Sources in COSMOS: The Contribution to the Overall Population of Massive Galaxies at z = 3-7”, by K. Caputi et al., which appeared in the Astrophysical Journal.

The team is composed of Karina I. Caputi (Kapteyn Astronomical Institute, University of Groningen, Netherlands), Olivier Ilbert (Laboratoire d'Astrophysique de Marseille, Aix-Marseille University, France), Clotilde Laigle (Institut d'Astrophysique de Paris, France), Henry J. McCracken (Institut d'Astrophysique de Paris, France), Olivier Le Fèvre (Laboratoire d'Astrophysique de Marseille, Aix-Marseille University, France), Johan Fynbo (Dark Cosmology Centre, Niels Bohr Institute, Copenhagen, Denmark), Bo Milvang-Jensen (Dark Cosmology Centre), Peter Capak (NASA/JPL Spitzer Science Centre, California Institute of Technology, Pasadena, California, USA), Mara Salvato (Max-Planck Institute for Extragalactic Physics, Garching, Germany) and Yoshiaki Taniguchi (Research Center for Space and Cosmic Evolution, Ehime University, Japan).

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 in the Astrophysical Journal
• Photos of VISTA
• VISTA Public Surveys

Contacts

Karina I. Caputi
Kapteyn Astronomical Institute – University of Groningen
The Netherlands
Email: karina@astro.rug.nl

Henry J. McCracken
Institut d'Astrophysique de Paris
France
Email: hjmcc@iap.fr

Bo Milvang-Jensen
Dark Cosmology Center – University of Copenhagen
Denmark
Email: milvang@dark-cosmology.dk

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

Another Dimension: 3D visualisation redefines Milky Way's local architecture

Astronomers have used modern techniques to visualise data from ESA's Hipparcos space astrometry mission in three dimensions. The treatment of the data has offered insights into the distribution of nearby stars and uncovered new groupings of stars in the solar neighbourhood, shedding light on the origins of the stars in Orion and calling into question the existence of the Gould Belt – an iconic ring-shaped structure of stars in the Milky Way. The results show the potential of 3D visualisation of the solar neighbourhood, an approach which is of particular relevance to ESA's Gaia mission which will map the Milky Way and Local Group in 3D with unprecedented sensitivity and accuracy.  

Visualising the local solar neighbourhood in 3D. 

Credit: ESA. Acknowledgement: H. Bouy (CSIC-INTA) & J. Alves (U. Vienna)

 Hi-res image

In a new study published in Astronomy & Astrophysics researchers have created a 3D map [1] of massive O and B type stars (sometimes referred to as OB stars) using data from ESA's Hipparcos satellite, launched in 1989 and operated until 1993. These stars, which live for a maximum of only a few tens of millions of years, are important markers of recent star formation and much can be learnt from studying their distribution in the solar neighbourhood.

Previous studies have looked for groupings of these stellar giants by seeking out concentrations of them in 2D projections. Astronomers use these 2D projections to look at the position and velocity of the stars in a given region and pick out stars that are moving together, and are thus most likely members of the same stellar group.

"Mapping data from missions like Hipparcos in two dimensions has allowed us to identify and classify numerous stellar groups and has profoundly changed our knowledge and understanding of the solar vicinity," explains Hervé Bouy from the Center for Astrobiology (CSIC-INTA), Spain, lead author of the study. "But it comes with significant drawbacks. 2D projections are just not capable of describing all the features of 3D space and using them to model distributions can cause artificial structures to appear and important structures to be hidden in the projection and lost."

Among other drawbacks, all 2D projection methods, including those not described here [2], can be affected by the presence of companion stars. Binary stars – two stars which orbit one another – can interfere with measurements of the motion of the stars in a group causing smaller or less tightly bound groups to be missed when searching for them solely on the basis of their common motion. In this study, rather than project the data onto a series of 2D planes, the astronomers used the measured distances to O and B type stars in the data to map the density and position of the stars in three dimensions. The 3D data analysis and interactive visualisation techniques used in this study, combined with a lack of reliance on velocities as a discovery criterion for the stellar groups, led to several discoveries that had been missed in 25 years of 2D analysis of the data.

"Our study has shown just how different the architecture of the solar neighbourhood looks when mapped in three dimensions," explains João Alves from the University of Vienna, Austria, co-author of the paper. "We have produced a 3D visualisation of all of the Hipparcos O and B type stars within around 1500 light years of the Sun and in doing so have found evidence for new structures in the distribution of nearby hot stars, and new and surprising theories of how those stars formed."

The team found that the solar neighbourhood is dominated by three huge stream-like galactic structures made up of dense clusters and loose associations of young, blue, O and B type stars. These contain several tens of O and B type stars, most of the local well-known clusters, and some previously unreported stellar groups. The first structure runs from the constellation Scorpius to the constellation Canis Majoris covering more than 1100 light years and at least 65 million years of star formation history. The second, located in the constellation Vela, covers at least 500 light years and 30 million years of history. Although all three of the newly discovered streams have a story to tell, it is the third structure, located in the constellation Orion, that is perhaps the most significant due to its mystery-solving qualities.

The origin of the blue supergiants that define the body and belt of the Orion constellation has long been a mystery. The five giant O and B type stars are located between around 250 and 800 light years from Earth and as a result it was assumed that their origin was not, despite their name, in the prolific Orion Nebula star-forming region, which lies around 1300 light years from Earth. However, the discovery of the Orion stream offers a simple solution. It implies that these relatively distant populations are in fact linked as part of a large galactic structure, which spans more than 1000 light years and at least 25 million years of star formation history.

The origin of the body and belt are not the only answers this study might hold for the birth of the Hunter.

"One exciting find from this study relates to Betelgeuse, the red giant in the arm of Orion," remarks Bouy. "The origin of this star has always been shrouded in mystery but through this study we have uncovered a new loosely organised group – or OB association – named Taurion which we believe to be Betelgeuse's birthplace and to contain its sibling stars."

By using modern full-3D data analysis, Bouy and Alves have not only uncovered the previously unknown, they may also have identified a significant visual illusion produced by previous 2D methods. In the 19th century, British astronomer John Herschel and then American astronomer Benjamin Gould identified a 3000 light-year-long partial ring of O and B type stars in the Milky Way. This belt, projected on the sky, was thought to be a grouping of stars and has come to be known as the Gould Belt – a famous and prominent structure in the Milky Way. Now, Bouy and Alves have shown that when mapped in three dimensions this model does not in fact provide a good fit for the distribution of O and B type stars, potentially disproving the existence of this galactic icon and calling for a new interpretation of stellar groups in the solar neighbourhood.

"The Gould Belt is the perfect example of how 2D projections can deceive astronomers," argues Alves. "Our results imply that it is just a projection effect produced by the Sun's position between two of the streams of stars, rather than representing the architecture of the solar neighbourhood itself."

The results published in this study include caveats and possible sources of error in part due to the extinction in the Hipparcos data, in other words the amount of light that was absorbed and scattered by dust on its way to the telescope, compromising the quality of the data. The study is also biased towards young stars, due to its focus on O and B type stars, and dense stellar groups. Despite this, the results show that our current models of the solar neighbourhood are not sufficient to uncover the true structure of how its stellar inhabitants are distributed or trace the history of their formation and evolution, there is significantly more to learn about our local environment.

"These results show just what 3D visualisation can deliver, and how much further it can take us," explains Jos de Bruijne, ESA's Gaia system scientist, also acting as ESA liaison scientist for the Hipparcos mission. "It provides an even stronger case for focussing on the local neighbourhood and, in particular, for doing so in three dimensions. This study really raises the expectations for what the Gaia mission will produce."

Gaia was launched in 2013 with the aim of unveiling the origin and evolution of our Galaxy. It will provide measurements of the positions and velocities with respect to Earth of up to one billion stars in our Galaxy and Local Group with unprecedented accuracy and sensitivity. The three-dimensional map produced by Gaia will far outdo any current or foreseen maps of the stars in the Milky Way. It will include non-O and-B type stars and be able to identify clusters and groups not dense enough to register in the Hipparcos map.

The success of the Hipparcos study in highlighting the benefits of visualising 25-year-old data using modern visualisation methods emphasises the potential of stellar mapping in 3D and Gaia will provide the data needed to peer further into the origin, evolution and structure of our Galaxy.

An interactive tool showing the Hipparcos data represented in three dimensions is available online.

Notes

[1] Hervé Bouy and João Alves have created an interactive tool to show the distribution of O and B stars in the solar neighbourhood. A low-resolution version (quicker loading time) is available here; a high-resolution version is available here.  

[2] Astronomers also use what is called the convergent point method, a method which is responsible for identifying most of the O and B type star associations and clusters discovered so far. Whilst unrelated stars will move in random directions, those in a group will move towards a convergence point where the paths of all its members will intersect.

More Information

"Cosmography of OB stars in the solar neighbourhood" by H. Bouy and J. Alves is published in Astronomy & Astrophysics, November 2015.

ESA's Hipparcos space astrometry mission was a pioneering European project which pinpointed the three-dimensional positions of more than one hundred thousand stars with high precision, and more than one million stars with lesser precision. Launched in August 1989, Hipparcos successfully observed the celestial sphere for 3.5 years before operations ceased in March 1993. Calculations from observations by the main instrument generated the Hipparcos Catalogue of 118 218 stars charted with the highest precision. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho Catalogue of 1 058 332 stars. The Tycho 2 Catalogue, completed in 2000, brings the total to 2 539 913 stars, and includes 99 per cent of all stars down to magnitude 11, almost 100 000 times fainter than the brightest star, Sirius.

Gaia is an ESA mission to survey one billion stars in our Galaxy and local galactic neighbourhood in order to build the most precise 3D map of the Milky Way and answer questions about its origin and evolution. Launched in December 2013, Gaia's routine science operations began in July 2014. The mission's primary scientific product will be a catalogue with the positions, motions, brightnesses, and colours of the surveyed stars. An intermediate version of the catalogue will be released in 2016.

Contacts

Herve Bouy
Center for Astrobiology (CSIC-INTA), Spain
Email: hbouy@cab.inta-csic.es
Phone: +34 622 27 1401

João Alves
University of Vienna, Austria
Email: joao.alves@univie.ac.at
Phone: +43 1 4277 53810

Jos de Bruijne
ESA liaison scientist for Hipparcos
Scientific Support Office
Directorate of Science and Robotic Exploration
ESA, The Netherlands
Email: jdbruijne@cosmos.esa.int
Phone: + 31 71 565 5989

Source: ESA/HIPPARCOS