Posted on by Fraser Cain

Brown Dwarfs are Heavier Than Previously Thought

Thanks to the powerful new high-contrast camera installed at the Very Large Telescope, photos have been obtained of a low-mass companion very close to a star. This has allowed astronomers to measure directly the mass of a young, very low mass object for the first time.

The object, more than 100 times fainter than its host star, is still 93 times as massive as Jupiter. And it appears to be almost twice as heavy as theory predicts it to be.

This discovery therefore suggests that, due to errors in the models, astronomers may have overestimated the number of young “brown dwarfs” and “free floating” extrasolar planets.

A winning combination
A star can be characterised by many parameters. But one is of uttermost importance: its mass. It is the mass of a star that will decide its fate. It is thus no surprise that astronomers are keen to obtain a precise measure of this parameter.

This is however not an easy task, especially for the least massive ones, those at the border between stars and brown dwarf objects. Brown dwarfs, or “failed stars”, are objects which are up to 75 times more massive than Jupiter, too small for major nuclear fusion processes to have ignited in its interior.

To determine the mass of a star, astronomers generally look at the motion of stars in a binary system. And then apply the same method that allows determining the mass of the Earth, knowing the distance of the Moon and the time it takes for its satellite to complete one full orbit (the so-called “Kepler’s Third Law”). In the same way, they have also measured the mass of the Sun by knowing the Earth-Sun distance and the time – one year – it takes our planet to make a tour around the Sun.

The problem with low-mass objects is that they are very faint and will often be hidden in the glare of the brighter star they orbit, also when viewed in large telescopes.

Astronomers have however found ways to overcome this difficulty. For this, they rely on a combination of a well-considered observational strategy with state-of-the-art instruments.

High contrast camera
First, astronomers searching for very low mass objects look at young nearby stars because low-mass companion objects will be brightest while they are young, before they contract and cool off.

In this particular case, an international team of astronomers [1] led by Laird Close (Steward Observatory, University of Arizona), studied the star AB Doradus A (AB Dor A). This star is located about 48 light-years away and is “only” 50 million years old. Because the position in the sky of AB Dor A “wobbles”, due to the gravitational pull of a star-like object, it was believed since the early 1990s that AB Dor A must have a low-mass companion.

To photograph this companion and obtain a comprehensive set of data about it, Close and his colleagues used a novel instrument on the European Southern Observatory’s Very Large Telescope. This new high-contrast adaptive optics camera, the NACO Simultaneous Differential Imager, or NACO SDI [2], was specifically developed by Laird Close and Rainer Lenzen (Max-Planck-Institute for Astronomy in Heidelberg, Germany) for hunting extrasolar planets. The SDI camera enhances the ability of the VLT and its adaptive optics system to detect faint companions that would normally be lost in the glare of the primary star.

A world premiere
Turning this camera towards AB Dor A in February 2004, they were able for the first time to image a companion so faint – 120 times fainter than its star – and so near its star.

Says Markus Hartung (ESO), member of the team: “This world premiere was only possible because of the unique capabilities of the NACO SDI instrument on the VLT. In fact, the Hubble Space Telescope tried but failed to detect the companion, as it was too faint and too close to the glare of the primary star.”

The tiny distance between the star and the faint companion (0.156 arcsec) is the same as the width of a one Euro coin (2.3 cm) when seen 20 km away. The companion, called AB Dor C, was seen at a distance of 2.3 times the mean distance between the Earth and the Sun. It completes a cycle around its host star in 11.75 years on a rather eccentric orbit.

Using the companion’s exact location, along with the star’s known ‘wobble’, the astronomers could then accurately determine the companion’s mass. The object, more than 100 times fainter than its close primary star, has one tenth of the mass of its host star, i.e., it is 93 times more massive than Jupiter. It is thus slightly above the brown dwarf limit.

Using NACO on the VLT, the astronomers further observed AB Dor C at near infrared wavelengths to measure its temperature and luminosity.

“We were surprised to find that the companion was 400 degrees (Celsius) cooler and 2.5 times fainter than the most recent models predict for an object of this mass,” Close said.

“Theory predicts that this low-mass, cool object would be about 50 Jupiter masses. But theory is incorrect: this object is indeed between 88 to 98 Jupiter masses.”

These new findings therefore challenge current ideas about the brown dwarf population and the possible existence of widely publicized “free-floating” extrasolar planets.

Indeed, if young objects hitherto identified as brown dwarfs are twice as massive as was thought, many must rather be low-mass stars. And objects recently identified as “free-floating” planets are in turn likely to be low-mass brown dwarfs.

For Close and his colleagues, “this discovery will force astronomers to rethink what masses of the smallest objects produced in nature really are.”

More information
The work presented here appears as a Letter in the January 20 issue of Nature (“A dynamical calibration of the mass-luminosity relation at very low stellar masses and young ages” by L. Close et al.).

Notes
[1]: The team is composed of Laird M. Close, Eric Nielsen, Eric E. Mamajek and Beth Biller (Steward Observatory, University of Arizona, Tucson, USA), Rainer Lenzen and Wolfgang Brandner (Max-Planck Institut for Astronomie, Heidelberg, Germany), Jose C. Guirado (University of Valencia, Spain), and Markus Hartung and Chris Lidman (ESO-Chile).

[2]: The NACO SDI camera is a unique type of camera using adaptive optics, which removes the blurring effects of Earth’s atmosphere to produce extremely sharp images. SDI splits light from a single star into four identical images, then passes the resulting beams through four slightly different (methane-sensitive) filters. When the filtered light beams hit the camera’s detector array, astronomers can subtract the images so the bright star disappears, revealing a fainter, cooler object otherwise hidden in the star’s scattered light halo (“glare”). Unique images of Saturn’s satellite Titan obtained earlier with NACO SDI were published in ESO PR 09/04.

Original Source: ESO News Release

Posted on by Fraser Cain

How Far Can You See?

Image credit: Jason Ware
Amateur astronomy isn’t for everyone. But unlike other interests, it could be! After all, there’s plenty of sky to go around. And to enjoy the sky doesn’t take much. To start, just the power of human sight and the ability to “keep looking up”.

Appreciating the night sky and its numerous denizens is akin to enjoying any great work of art. Anyone captive to a painting by Van Gogh, statue by Roden, sonata by Beethoven, play by Shakespeare, or poem by Tennyson, can certainly appreciate a constellation wrought by nature’s sculpting hand. So like such great works of art, a fine appreciation of the night sky can be cultivated. Yet unlike such works, there is something far more primordial and immediately evocative about the heavens – a thing that defies any need for profound study or inculturation by others.

While it is true that some ingenious devices (such as the quadrant) were developed early on in the history of astronomy, it wasn’t until the time of Galileo (the early 17th century) that astronomers began probing the universe in detail. Before that time, the human eye placed such constraints on what could be seen that all we knew of the heavens was limited to two large bright bodies (Sun and Moon), numerous faint lights (the fixed stars and infrequent novae), and an intermediate group (the planets and occasional comets). Using instruments such as the quadrant (for position), and waterclock (for time), it became possible to predict the movements of all such bodies. And it was prediction – not understanding – that drove observation using the human eye alone.

Ultimately it was the telescope that made discovery – rather than measurement – the driving force behind the science of astronomy. For without the telescope, the Universe would be a far smaller place and populated by far, far fewer things. Consider that at 2.3 million light-years, the most distant celestial object visible unaided – the Great Galaxy of Andromeda – could never have been so-named at all. In fact, it might not have even received its older name: The Great Nebula in Andromeda. First noted in the 10th century text “Book of Fixed Stars”, sharp-eyed Abd-al-Rahman Al Sufi described the Great Galaxy as “a little cloud”. And that – without the telescope – is all we would ever have seen of this:

Because of the telescope, we now know far more about Sun, Moon, planets, comets, and stars than simply where they might be found in the sky. We understand that our Sun is a nearby star and that our Earth, the planets, and those “harbingers of doom” – the comets – are all part of a solar system. We have detected other such stellar systems beyond our own. We know we live in a galaxy that – from a distance of two million light-years – would appear much like M31 -1. We have determined that several billion years hence, our galaxy and M31 will embrace spiral arms. And we recognize that the Universe is extraordinary in its vastness, diversity, beauty, and harmony of inter-connectedness.

We know all this because we possess the telescope – and similar instruments – that can sound the depths of the cosmos across numerous octaves of spectral vibrancy.

But it all begins with the human eye…

The working of the human eye is based on three of the four main properties of light. Light may be refracted, reflected, diffracted, or absorbed. Light enters the eye as parallel beams from the distance. Because it is limited in aperture, the eye is only able to collect a very small proportion of the rays coming from any one thing. That collecting area – roughly 38 square millimeters (fully dilated and dark-adapted), allows the eye to normally see stars down to about magnitude 6. Ancient astronomers – free of the effects of modern sources of atmospheric illumination (light pollution) – were able to catalogue about 6000 individual stars (with a sprinkling of other objects). The faintest of these were classed of the “sixth magnitude”, and brightest of the “first”.

But the eye is also limited by the principle of diffraction. This principle prevents us from seeing exceedingly fine details. Because the eye is limited in aperture, parallel beams of light begin to “spread out” or propagate after entering the iris. Such diffusion means that – despite the use of refraction to focus – photons can only come so close together. For this reason, there is an ultimate limit to how much detail may be seen by any aperture – and that includes the eye itself.

The eye, of course, exploits the principle of refraction to organize beams of light. Photons enter the cornea, bend, and pass to the lens behind it. (The cornea does the bulk of the focusing and leaves about a third up to the lens.) The lens itself adjusts ray angles to bring things – near or far – to focus. It does this by changing radius of curvature. In this way, parallel rays from a distance or diverging rays from nearby may project an image on the retina where tiny neurons convert light-energy into signals for interpretation by the brain. And it is the brain – primarily the occipital lobes at the back of the head – that does the “image processing” needed to give coherence to that steady stream of neural signals arriving from the eye.

To detect light, the retina employs the principle of absorption. Photons cause sensory neurons to depolarize. Depolarization projects chemo-electrical signals from axons to dendrites deeper in the brain. Retinal neurons may be rod-shaped or conical. Rods detect light of any color and are more sensitive to light than cones. Cones detect specific colors only and are found in greater concentration along the main axis of the eye. Meanwhile rods dominate off-axis. The averted eye can see stars roughly two and half-times fainter than those held direct.

Beyond aversion, neural signals passing from the retina (via the optical chiasm) are first processed by the superior collicus. The collicus gives us our visual “flinch” response – but more importantly – it does less filtering of the visual field than the occipital lobes. Because of this, the collicus can detect even fainter sources of light – but only when in apparent motion. Thus the discerning observer can detect faint stars – and faintly glowing objects – some 4 times fainter than those seen through ordinary “straight-on” viewing. (This is done by sweeping the eye across the night sky – or across the field of view of the telescope.)

In addition to aversion and eye movement, the eyes increase sensitivity by adapting to low light conditions. This is done in two ways: First, fine muscles retract the iris (located between cornea and lens) to admit as much light as possible. Second, within roughly 30 minutes of exposure to darkness, “visual purple” (rhodopsin) on retinal rods takes on a transmissive rosey-red color. This change increases the sensitivity of rods to the point where even a single photon of visible light may be detected.

Aside from limitations imposed by diffraction, there is a second natural limit to how much detail can be seen by the eye. For neurons can be made only so small and placed only so close together. Meanwhile at about 25mm’s in focal length, the eye can only see “1x”. Add this to the fact that the greatest opening achieved by the eye (the entrance pupil) is 7mms and human eyes become the effective equivalent of a pair of “1x7mm” binoculars.

All these factors limit the eye – even under the best observing conditions (like the vacuum of space) – to seeing stars (using direct vision) of the eighth magnitude (1500 times fainter than the brightest stars) and resolving close pairs to about 2 arc-minutes of angular separation (1/15th the apparent size of the Moon).

Observational astronomy begins with the eyes. But new instrumentation evolved because some eyes have difficulty focusing light. Because of human near- and far- sightedness, the first spectacle lenses were ground. And it was only a matter of experimentation before someone combined one of each type lens together to form the first telescope or “instrument of long seeing”.

Today’s astronomers are able to augment the human eye’s capacity to the point where we can almost peer back to the beginning of time itself. This is done through the use of chemical and solid-state principles embodied in photography and charge-coupled devices (CCDs). Such tools are able to accumulate photons in a way the eye can not. As a result of these “visual aids”, we have discovered things once unimagined about the universe. Many of these discoveries were unknown to us – even as recently as the beginning of the era of the Great Observatories (the early twentieth century). Today’s astronomy has expanded the range of cosmic vision across numerous bands of the electromagnetic spectrum – from radio to X-rays. But we do far more than simply find stuff and measure positions. We seek to grasp more than light – but comprehension as well…

Today’s amateur astronomers – such as the author – use hand- and mass-produced telescopes from all parts of the world to peer billions of light-years into the depths of the Universe.-2 This type long-seeing is possible because the eye and telescope can work together to collect “more and finer light” from on high.

How far can you see?

-1According to NASA the Milky Way galaxy would appear very much like 15.3 MLY distant barred spiral M83 found in the constellation Hydra (as seen at right). A human being in space would just be able to hold the bright central portion of this 8.3 magnitude galaxy as a “fuzzy star” using averted vision. M83 can easily be found using low power binoculars from Earth.

-2 Bearing a variable visual magnitude of 12.8, 2 billion-light year distant quasar 3C273 can just be held direct by the human eye when augmented by a six-inch / 150mm aperture telescope at 150x through night time skies of 5.5 unaided limiting magnitude and 7/10p seeing stability. A pair of 10x50mm binoculars would reveal 3C273 as a faint star from Earth orbit.

Inspired by the early 1900’s masterpiece: “The Sky Through Three, Four, and Five Inch Telescopes”, Jeff got a start in astronomy and space science at the age of seven. Currently he devotes considerable time maintaining the website Astro.Geekjoy.

Posted on by Fraser Cain

Red Dwarfs Destroy Their Dusty Disks

Astronomers announced Jan. 10 that they have a lead in the case of the missing disks. The report was presented by UCLA graduate student and Ph.D. candidate Peter Plavchan; his adviser, Michael Jura; and Sarah Lipscy, now at Ball Aerospace, to the American Astronomical Society meeting in San Diego. This lead may account for the missing evidence of red dwarfs forming planetary systems.

The evidence
Red dwarfs (or M Dwarfs) are stars like our Sun in many respects but smaller, less massive and fainter. Approximately 70 percent of all the stars in our galaxy are red dwarfs.

“We would like to understand whether these stars form planets, as the other stars in our galaxy do,” said Plavchan, who leads this research investigation.

Approximately half of all newborn stars are known to possess the materials to make planets. When stars are born, the leftover materials form what astronomers refer to as a primordial disk surrounding the star. From this primordial disk, composed of gas and small grains of solid material astronomers call “dust,” planets can start to grow. As these “planetesimals” grow by accreting nearby material in the primordial disk, they also collide with one another. These collisions are frequent and violent, producing more dust forming a new disk of debris after the star is about 5?10 million years old. In our own solar system, we see evidence everywhere of these violent collisions that took place more than 4 billion years ago ? such as the craters on the moon.

The debris disk of “dust” left over from these ancient collisions in our own solar system has long since dissipated. Astronomers, however, have discovered many young stars in the local part of our galaxy where these debris disks still can be seen. These stars are caught in the act of forming planets and are of great interest to astronomers who want to understand how this process works. Curiously though, only two of these stars with debris disks were found to be red dwarfs: AU Microscopium (AU Mic) and GJ 182, located 32.4 light-years and approximately 85 light-years from Earth, respectively.

Despite red dwarfs holding a solid majority among the different kinds of stars in our galaxy, only two have been found with evidence of debris disks. If half of all red dwarfs started with the material to form planets, what happened to the rest of them? Where did the material and dust surrounding these stars go? Factors such as the ages, smaller sizes and faintness of red dwarfs do not fully account for these missing disks.

The investigation
In December 2002 and April 2003, Plavchan, Jura and Lipscy observed a sample of nine nearby red dwarfs with the Long Wavelength Spectrometer, an infrared camera on the 10-meter telescope at the Keck Observatory on Mauna Kea, Hawaii. These nine stars all are located within 100 light-years of Earth and were thought potentially to possess debris disks. None, however, showed any evidence for the presence of warm dust produced by the collisions of forming planets.

Backed by the previous research investigations that also came up empty-handed, the researchers considered what makes red dwarfs different from other bigger, brighter stars that have been found with debris disks.

“We have to consider how the dust in these young red dwarfs gets removed and where it goes,” said Jura, Plavchan’s thesis adviser.

In other young, more massive stars ? A-, F- and G-types ? the dust primarily is removed by Poynting-Robertson drag, radiative blowout and collisions.

“These first two processes are simply ineffective for red dwarfs, so something else must be going on to explain the disappearance of the debris disks,” Plavchan said.

Under Poynting-Robertson drag, a consequence of special relativity, the dust slowly spirals in towards the star until it heats up and sublimates.

The new lead in the case
Plavchan, Jura and Lipscy have discovered that there is another process similar to Poynting-Robertson drag that potentially can solve the case of the missing red dwarf debris disks: stellar wind drag.

Stars like our Sun and red dwarfs possess a stellar wind ? protons and other particles that are driven by the magnetic fields in the outer layers of a star to speeds in excess of a few hundred miles per second and expelled out into space. In our own solar system, the solar wind is responsible for shaping comets’ tails and producing the Aurorae Borealis on Earth.

This stellar wind also can produce a drag on dust grains surrounding a star. Astronomers have long known about this drag force, but it is less important than Poynting-Robertson drag for our own Sun. Red dwarfs, however, experience stronger magnetic storms and consequently have stronger stellar winds. Furthermore, X-ray data show that the red dwarf winds are even stronger when the stars are very young and planets are forming.

“Stellar wind drag can ‘erase’ the evidence of forming planets around red dwarfs by removing the dust that is produced in the collisions that are taking place. Without stellar wind drag, the debris disk would still be there and we would be able to see it with current technology,” Plavchan said.

This research potentially solves the case of the missing disks, but more work is needed. Astronomers know little about the strength of stellar winds around young stars and red dwarfs. While further observations of red dwarfs by the Spitzer Infrared Telescope Facility have supported this research, this case will not be closed until we can directly measure the strength of stellar winds around young red dwarfs.

This research has been submitted to The Astrophysical Journal for publication and is supported by funding from NASA.

Original Source: UCLA News Release

Posted on by Fraser Cain

How Do Large Galaxies Form?

Most present-day large galaxies are spirals, presenting a disc surrounding a central bulge. Famous examples are our own Milky Way or the Andromeda Galaxy. When and how did these spiral galaxies form? Why do a great majority of them present a massive central bulge?

An international team of astronomers [1] presents new convincing answers to these fundamental questions. For this, they rely on an extensive dataset of observations of galaxies taken with several space- and ground-based telescopes. In particular, they used over a two-year period, several instruments on ESO’s Very Large Telescope.

Among others, their observations reveal that roughly half of the present-day stars were formed in the period between 8,000 million and 4,000 million years ago, mostly in episodic burst of intense star formation occurring in Luminous Infrared Galaxies.

From this and other evidence, the astronomers devised an innovative scenario, dubbed the “spiral rebuilding”. They claim that most present-day spiral galaxies are the results of one or several merger events. If confirmed, this new scenario could revolutionise the way astronomers think galaxies formed.

A fleet of instruments
How and when did galaxies form? How and when did stars form in these island universes? These questions are still posing a considerable challenge to present-day astronomers.

Front-line observational results obtained with a fleet of ground- and space-based telescopes by an international team of astronomers [1] provide new insights into these fundamental issues.

For this, they embarked on an ambitious long-term study at various wavelengths of 195 galaxies with a redshift [2] greater than 0.4, i.e. located more than 4000 million light-years away. These galaxies were studied using ESO’s Very Large Telescope, as well as the NASA/ESA Hubble Space Telescope, the ESA Infrared Space Observatory (ISO) satellite and the NRAO Very Large Array.

With the Very Large Telescope, observations were performed on Antu and Kueyen over a two-year period using the quasi-twin instruments FORS1 and FORS2 in the visible and ISAAC in the infrared. In both cases, it was essential to rely on the unique capabilities of the VLT to obtain high-quality spectra with the required resolution.

A fleet of results
From their extensive set of data, the astronomers could draw a number of important conclusions.

First, based on the near-infrared luminosities of the galaxies, they infer that most of the galaxies they studied contain between 30,000 million and 300,000 million times the mass of the Sun in the form of stars. This is roughly a factor 0.2 to 2 the amount of mass locked in stars in our own Milky Way.

Second, they discovered that contrary to the local Universe where so-called Luminous Infrared Galaxies (LIRGs; [3]) are very rare objects, at a redshift from 0.4 to 1, that is, 4,000 to 8,000 million years ago, roughly one sixth of bright galaxies were LIRGs.

Because this peculiar class of galaxies is believed to be going through a very active phase of star formation, with a doubling of the stellar mass occurring in less than 1,000 million years, the existence of such a large fraction of these LIRGs in the past Universe has important consequences on the total stellar formation rate.
As Fran?ois Hammer (Paris Observatory, France), leader of the team, states: “We are thus led to the conclusion that during the time span from roughly 8,000 million to 4,000 million years ago, intermediate mass galaxies converted about half of their total mass into stars. Moreover, this star formation must have taken place in very intense bursts when galaxies were emitting huge amount of infrared radiation and appeared as LIRGs.”

Another result could be secured using the spectra obtained with the Very Large Telescope: the astronomers measured the chemical abundances in several of the observed galaxies (PR Photo 02a/05). They find that galaxies with large redshifts show oxygen abundances two times lower than present-day spirals. As it is stars which produce oxygen in a galaxy, this again gives support to the fact that these galaxies have been actively forming stars in the period between 8,000 and 4,000 million years ago.

And because it is believed that galaxy collisions and mergers play an important role in triggering such phases of enhanced star-forming activity, these observations indicate that galaxy merging still occurred frequently less than 8,000 million years ago.

Spiral Rebuilding
The story revealed by these observations is in agreement with the so-called “hierarchical merging of galaxies” scenario, present in the literature since about 20 years. According to this model, small galaxies merge to build larger ones. As Fran?ois Hammer however points out: “In the current scenario, it was usually assumed that galaxy merging almost ceased 8,000 million years ago. Our complete set of observations show that this is far from being the case. In the following 4,000 million years, galaxies still merged to form the large spirals we observe in the local Universe.”

To account for all these properties, the astronomers thus devised a new galaxy formation scenario, comprising three major phases: a merger event, a compact galaxy phase and a “growth of the disc” phase (see PR Photo 02b/05).

Because of the unique aspects of this scenario, where big galaxies get first disrupted by a major collision to be born again later as a present-day spiral galaxy, the astronomers rather logically dubbed their evolutionary sequence, the “spiral galaxy rebuilding”.

Although being at odds with standard views which assert that galaxy mergers produce elliptical galaxies instead of spiral ones, the astronomers stress that their scenario is consistent with the observed fractions of the different types of galaxies and can account for all the observations.

The new scenario can indeed account for the formation of about three quarters of the present-day spiral galaxies, those with massive central bulge. It would apply for example to the Andromeda Galaxy but not to our own Milky way. It seems that our Galaxy somehow escaped major collisions in the last thousands of million years.

Further observations, in particular with the FLAMES instrument on the VLT, will show if spiral galaxies are indeed relatively recent born-again systems created from major merger events.

More information
The research presented in this Press Release has been published in the leading astronomical journal Astronomy and Astrophysics, vol. 430(1). The paper (“Did most present-day spirals form during the last 8 Gyrs? – A formation history with violent episodes revealed by panchromatic observations” by F. Hammer et al.) is available in PDF format from the A&A web site.

Notes
[1]: The team is composed of Fran?ois Hammer and Hector Flores (Observatoire de Paris, Meudon, France), David Elbaz (CEA Saclay, France), Xian-Zhong Zheng (Observatoire de Paris, Meudon, France and Max-Planck Instiut f?r Astronomie, Germany), Yan-Chun Liang (Observatoire de Paris, Meudon, France and National Astronomical Observatories, China) and Catherine Cesarsky (ESO, Garching, Germany).

[2]: In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a remote galaxy provides an estimate of its distance. The distances and ages indicated in the present text are based on an age of the Universe of 13,700 million years.

[3]: Luminous Infrared Galaxies (LIRGs) are a subset of galaxies whose infrared luminosity is larger than 100,000 million time the luminosity of our Sun. They were first discovered as a class by the ESA ISO satellite and are believed to be galaxies undergoing enhanced stellar formation.

Original Source: ESO News Release

Posted on by Fraser Cain

Keck View of the Water Fountain Nebula

New, very high-resolution (false-color) images of a dying star IRAS16342-3814 (hereafter the Water-Fountain Nebula) taken with the Keck II Telescope equipped with adaptive optics, at the W. M. Keck Observatory on Mauna Kea, Hawaii, are helping astronomers understand the extraordinary deaths of ordinary Sun-like stars. These results are being presented today to the 205th American Astronomical Society meeting in San Diego, California, by Raghvendra Sahai of the Jet Propulsion Laboratory (JPL), California Institute of Technology, Pasadena; D. Le Mignant, R.D. Campbell, F.H. Chaffee of W. M. Keck Observatory, Mauna Kea, Hawaii; and C. S?nchez Contreras of the California Institute of Technology.

Sun-like stars shine sedately for billions of years, but die in spectacular fashion, creating intricate and beautiful gaseous shrouds around them in the relatively short period of about a thousand years or less. These shrouds, called planetary nebulae, come in a wide variety of beautiful non-spherical shapes, in striking contrast to the round shapes of their progenitor stars. The answer to the question of how planetary nebulae acquire their diverse shapes has long eluded astronomers.

The images of the Water-Fountain Nebula (which lies at an estimated distance of 6500 light years in the direction of Scorpius) shown here, were acquired using the adaptive optics (AO) technique, at two near-infrared wavelengths (using filters centered at wavelengths of 2.1 and 3.8 microns). The AO technique removes the blurring effect of Earth’s atmosphere and allows astronomers to take full advantage of large ground-based telescopes like the W. M. Keck Telescope, revealing important details which were hidden even to the sharp eyes of the Hubble Space Telescope (HST). The images show two lobes, which are cavities (each of size about 2000 Astronomical Units) in an extended cloud of gas and dust, illuminated by light from a central star which lies between the two lobes, but is hidden from our view behind a dense, dust lane that separates the two lobes. These near-infrared AO images probe much deeper than HST into the two lobes of the Water-Fountain Nebula, showing a remarkable corkscrew-shaped structure (marked by dashed lines) apparently etched into the lobe walls.

According to JPL Research Scientist Dr. Sahai, ” The corkscrew structure seen here is the proverbial writing on the wall signature of an underlying high-speed jet of matter which has changed its direction in a regular fashion (called precession). These images of the Water-Fountain Nebula thus show direct evidence for a jet actively carving out a bipolar nebula, providing unambiguous support for our recently proposed hypothesis that the shaping of most planetary nebulae is carried out by such jets”.

The discovery of the corskcrew pattern resulting from a precessing jet in the Water-Fountain Nebula is an exciting addition to our knowledge of jets in dying stars as well as astrophysical jets in general. The jets in dying stars are thought to operate for a very short period of time (few hundred years). Finding direct evidence for these jet-like outflows has been generally very difficult, because they are compact, not always active, and it is difficult to see them against the bright nebular background. A detailed comparison of the images of the Water-Fountain Nebula taken with filters of different colors allows scientists to determine the physical properties of the nebula. New AO imaging in a few years from now will enable Dr. Sahai and collaborators to measure the physical motion of matter in the corkscrew pattern, and provide strong constraints on the nebular shaping process.

When Sun-like stars get old, they become cooler and redder, increasing their sizes and energy output tremendously: they are called red giants. Most of the carbon (the basis of life) and particulate matter (crucial building blocks of solar systems like ours) in the universe is manufactured and dispersed by red giant stars. Preplanetary nebulae are formed when the red giant star has ejected most of its outer layers. As the very hot core (six or more times hotter than the Sun) gets further exposed, the cloud of ejected material is bathed with ultraviolet light, making it glow; the object is then called a planetary nebula.

Original Source: Keck News Release

Posted on by Fraser Cain

Galaxy Has Leftover Material from the Big Bang

An astronomer studying small irregular galaxies has discovered a remarkable feature in one of them that may provide key clues to understanding how galaxies form and the relationship between the gas and the stars within galaxies.

Liese van Zee of Indiana University Bloomington, using the National Science Foundation’s Very Large Array radio telescope in New Mexico, found that a small galaxy 16 million light-years from Earth is surrounded by a huge disk of hydrogen gas that has not been involved in the galaxy’s star-formation processes and may be primordial material left over from the galaxy’s formation. “If that’s the case, then we may have found a nearby sample similar to the stuff of the early universe,” van Zee said.

“Why the gas in the disk has remained so undisturbed, without stars forming, is somewhat perplexing. When we figure out how this happened, we’ll undoubtedly learn more about how galaxies form,” she said.

She presented her findings on Wednesday (Jan. 12) at the national meeting of the American Astronomical Society in San Diego, Calif.

The galaxy van Zee studied, called UGC 5288, had been regarded as just one ordinary example of a numerous type called dwarf irregular galaxies. As part of a study of such galaxies, she had earlier made a visible-light image of it at Kitt Peak National Observatory in Arizona.

When she observed the galaxy later using the radio telescope, she found that it is embedded in a huge disk of atomic hydrogen gas. In visible light, the elongated galaxy is about 6,000 by 4,000 light-years, but the hydrogen-gas disk, seen with the VLA, is about 41,000 by 28,000 light-years. “The gas disk is more than seven times bigger than the galaxy we see in visible light,” she said.

The hydrogen disk can be seen by radio telescopes because hydrogen atoms emit and absorb radio waves at a frequency of 1420 MHz, a wavelength of about 21 centimeters.

A few other dwarf galaxies have large gas disks, but unlike these, UGC 5288’s disk shows no signs that the gas was either blown out of the galaxy by furious star formation or pulled out by a close encounter with another galaxy. “This gas disk is rotating quite peacefully around the galaxy,” van Zee explained. That means, she said, that the gas around UGC 5288 most likely is pristine material that has never been “polluted” by the heavier elements produced in stars.

What’s surprising, said Martha Haynes, an astronomer at Cornell University in Ithaca, N.Y., is that the huge gas disk seems to be completely uninvolved in the small galaxy’s star-formation processes. “You need the gas to make the stars, so we might have thought the two would be better correlated. This means we really don’t understand how the star-forming gas and the stars themselves are related,” Haynes said.

It’s exciting to find such a large reservoir of apparently unprocessed matter, Haynes said. “This object and others like it could be the targets for studying pristine material in the universe,” she said.

Haynes was amused that a galaxy that looked “boring” to some in visible-light images showed such a remarkable feature when viewed with a radio telescope.

“This shows that you can’t judge an object by its appearance at only one wavelength. What seems boring at one wavelength may be very exciting at another,” Haynes said.

Original Source: Indiana University

Posted on by Fraser Cain

Stellar Incubators in the Trifid Nebula

NASA’s Spitzer Space Telescope has uncovered a hatchery for massive stars.

A new striking image from the infrared telescope shows a vibrant cloud called the Trifid Nebula dotted with glowing stellar “incubators.” Tucked deep inside these incubators are rapidly growing embryonic stars, whose warmth Spitzer was able to see for the first time with its powerful heat-seeking eyes.

The new view offers a rare glimpse at the earliest stages of massive star formation ? a time when developing stars are about to burst into existence.

“Massive stars develop in very dark regions so quickly that is hard to catch them forming,” said Dr. Jeonghee Rho of the Spitzer Science Center, California Institute of Technology, Pasadena, Calif., principal investigator of the recent observations. “With Spitzer, it’s like having an ultrasound for stars. We can see into dust cocoons and visualize how many embryos are in each of them.”

The new false-color image can be found at http://www.spitzer.caltech.edu/Media. It was presented today at the 205th meeting of the American Astronomical Society in San Diego, Calif.

The Trifid Nebula is a giant star-forming cloud of gas and dust located 5,400 light-years away in the constellation Sagittarius. Previous images taken by the Institute for Radioastronomy millimeter telescope in Spain show that the nebula contains four cold knots, or cores, of dust. Such cores are “incubators” where stars are born. Astronomers thought the ones in the Trifid Nebula were not yet ripe for stars. But, when Spitzer set its infrared eyes on all four cores, it found that they had already begun to develop warm stellar embryos.

“Spitzer can see the material from the dark cores falling onto the surfaces of the embryonic stars, because the material gets hotter as gravity draws it in,” said Dr. William T. Reach of the Spitzer Science Center, co-author of this new research. “By measuring the infrared brightness, we can not only see the individual embryos but determine their growth rate.”

The Trifid Nebula is unique in that it is dominated by one massive central star, 300,000 years old. Radiation and winds emanating from the star have sculpted the Trifid cloud into its current cavernous shape. These winds have also acted like shock waves to compress gas and dust into dark cores, whose gravity caused more material to fall inward until embryonic stars were formed. In time, the growing embryos will accumulate enough mass to ignite and explode out of their cores like baby birds busting out of their eggs.

Because the Trifid Nebula is home to just one massive star, it provides astronomers a rare chance to study an isolated family unit. All of the newfound stellar embryos are descended from the nebula’s main star. Said Rho, “Looking at the image, you know exactly where the embryos came from. We use their colors to determine how old they are. It’s like studying the family tree for a generation of stars.”

Spitzer discovered 30 embryonic stars in the Trifid Nebula’s four cores and dark clouds. Multiple embryos were found inside two massive cores, while a sole embryo was seen in each of the other two. This is one of the first times that clusters of embryos have been observed in single cores at this early stage of stellar development.

“In the cores with multiple embryos, we are seeing that the most massive and brightest of the bunch is near the center. This implies that the developing stars are competing for materials, and that the embryo with the most material will grow to be the largest star,” said Dr. Bertrand Lefloch of Observatoire de Grenoble, France, co-author of the new research.

Spitzer also uncovered about 120 small baby stars buried inside the outer clouds of the nebula. These newborns were probably formed around the same time as the main massive star and are its smaller siblings.

Other authors of this work include Dr. Giovanni Fazio, Smithsonian Astrophysical Observatory, Cambridge, Mass.

NASA’s Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA’s Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center, Pasadena, Calif. JPL is a division of Caltech.

The new Spitzer image is a combination of data from the telescope’s infrared array camera and multiband imaging photometer. The infrared array camera was built by NASA Goddard Space Flight Center, Greenbelt, Md.; its development was led by Fazio. The multiband imaging photometer was built by Ball Aerospace Corporation, Boulder, Colo., the University of Arizona, Tucson, and Boeing North American, Canoga Park, Calif. The instrument’s development was led by Dr.George Rieke, University of Arizona.

Additional information about the Spitzer Space Telescope is available at http://www.spitzer.caltech.edu.

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Cluster Filled with Pulsars

A dense globular star cluster near the center of our Milky Way Galaxy holds a buzzing beehive of rapidly-spinning millisecond pulsars, according to astronomers who discovered 21 new pulsars in the cluster using the National Science Foundation’s 100-meter Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. The cluster, called Terzan 5, now holds the record for pulsars, with 24, including three known before the GBT observations.

“We hit the jackpot when we looked at this cluster,” said Scott Ransom, an astronomer at the National Radio Astronomy Observatory in Charlottesville, VA. “Not only does this cluster have a lot of pulsars — and we still expect to find more in it — but the pulsars in it are very interesting. They include at least 13 in binary systems, two of which are eclipsing, and the four fastest-rotating pulsars known in any globular cluster, with the fastest two rotating nearly 600 times per second, roughly as fast as a household blender,” Ransom added. Ransom and his colleagues reported their findings to the American Astronomical Society’s meeting in San Diego, CA, and in the online journal Science Express.

The star cluster’s numerous pulsars are expected to yield a bonanza of new information about not only the pulsars themselves, but also about the dense stellar environment in which they reside and probably even about nuclear physics, according to the scientists. For example, preliminary measurements indicate that two of the pulsars are more massive than some theoretical models would allow. “All these exotic pulsars will keep us busy for years to come,” said Jason Hessels, a Ph.D student at McGill University in Montreal.

Globular clusters are dense agglomerations of up to millions of stars, all of which formed at about the same time. Pulsars are spinning, superdense neutron stars that whirl “lighthouse beams” of radio waves or light around as they spin. A neutron star is what is left after a massive star explodes as a supernova at the end of its life.

The pulsars in Terzan 5 are the product of a complex history. The stars in the cluster formed about 10 billion years ago, the astronomers say. Some of the most massive stars in the cluster exploded and left the neutron stars as their remnants after only a few million years. Normally, these neutron stars would no longer be seen as swiftly-rotating pulsars: their spin would have slowed because of the “drag” of their intense magnetic fields until the “lighthouse” effect is no longer observable.

However, the dense concentration of stars in the cluster gave new life to the pulsars. In the core of a globular cluster, as many as a million stars may be packed into a volume that would fit easily between the Sun and our nearest neighbor star. In such close quarters, stars can pass near enough to form new binary pairs, split apart such pairs, and binary systems even can trade partners, like an elaborate cosmic square dance. When a neutron star pairs up with a “normal” companion star, its strong gravitational pull can draw material off the companion onto the neutron star. This also transfers some of the companion’s spin, or angular momentum, to the neutron star, thereby “recycling” the neutron star into a rapidly-rotating millisecond pulsar. In Terzan 5, all the pulsars discovered are rotating rapidly as a result of this process.

Astronomers previously had discovered three pulsars in Terzan 5, some 28,000 light-years distant in the constellation Sagittarius, but suspected there were more. On July 17, 2004, Ransom and his colleagues used the GBT, and, in a 6-hour observation, found 14 new pulsars, the most ever found in a single observation.

“This was possible because of the great sensitivity of the GBT and the new capabilities of our backend processor,” said Ingrid Stairs, a professor at the University of British Columbia in Vancouver. The processor, named, appropriately, the Pulsar Spigot, was built in a collaboration between the NRAO and the California Institute of Technology. The processor, which generates almost 100 GigaBytes of data per hour, allowed the astronomers to gather and analyze radio waves over a wide range of frequencies (1650-2250 MegaHertz), adding to the sensitivity of their system.

Eight more observations between July and November of 2004 discovered seven additional pulsars in Terzan 5. In addition, the astronomers’ data show evidence for several more pulsars that still need to be confirmed.

Future studies of the pulsars in Terzan 5 will help scientists understand the nature of the cluster and the complex interactions of the stars at its dense core. Also, several of the pulsars offer a rich yield of new scientific information. The scientists suspect that one pulsar, which shows strange eclipses of its radio emission, has recently traded its original binary companion for another, and two others have white-dwarf companions that they believe may have been produced by the collision of a neutron star and a red-giant star. Subtle effects seen in these two systems can be explained by Einstein’s general relativistic theory of gravity, and indicate that the neutron stars are more massive than some theories allow. The material in a neutron star is as dense as that in an atomic nucleus, so that fact has implications for nuclear physics as well as astrophysics.

“Finding all these pulsars has been extremely exciting, but the excitement really has just begun,” Ransom said. “Now we can start to use them as a rich and valuable cosmic laboratory,” he added.

In addition to Ransom, Hessels and Stairs, the research team included Paulo Freire of Arecibo Observatory in Puerto Rico, Fernando Camilo of Columbia University, Victoria Kaspi of McGill University, and David Kaplan of the Massachusetts Institute of Technology.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. The pulsar research also was supported by the Canada Foundation for Innovation, Science and Engineering Research Canada, the Quebec Foundation for Research on Nature and Technology, the Canadian Institute for Advanced Research, Canada Research Chairs Program, and the National Science Foundation.

Original Source: NRAO News Release

Posted on by Fraser Cain

New Stars Forming in Our Closest Neighbour

Hubble astronomers have uncovered, for the first time, a population of infant stars in the Milky Way satellite galaxy, the Small Magellanic Cloud (SMC, visible to the naked eye in the southern constellation Tucana), located 210,000 light-years away.

Hubble’s exquisite sharpness plucked out an underlying population of infant stars embedded in the nebula NGC 346 that are still forming from gravitationally collapsing gas clouds. They have not yet ignited their hydrogen fuel to sustain nuclear fusion. The smallest of these infant stars is only half the mass of our Sun.

Although star birth is common within the disk of our galaxy, this smaller companion galaxy is more primeval in that it lacks a large percentage of the heavier elements that are forged in successive generations of stars through nuclear fusion.

Fragmentary galaxies like the SMC are considered primitive building blocks of larger galaxies. Most of these types of galaxies existed far away, when the universe was much younger. The SMC offers a unique nearby laboratory for understanding how stars arose in the early universe. Nestled among other starburst regions with the small galaxy, the nebula NGC 346 alone contains more than 2,500 infant stars.

The Hubble images, taken with the Advanced Camera for Surveys, identify three stellar populations in the SMC and in the region of the NGC 346 nebula ? a total of 70,000 stars. The oldest population is 4.5 billion years, roughly the age of our Sun. The younger population arose only 5 million years ago (about the time Earth’s first hominids began to walk on two feet). Lower-mass stars take longer to ignite and become full-fledged stars, so the protostellar population is 5 million years old. Curiously, the infant stars are strung along two intersecting lanes in the nebula, resembling a “T” pattern in the Hubble plot.

The observations, by Antonella Nota of the European Space Agency (ESA) and the Space Telescope Science Institute (STScI), Baltimore, Md., are being presented today at the meeting of the American Astronomical Society in San Diego, Calif.

The other science team members are: M. Sirianni (STScI/ESA), E. Sabbi (Univ. of Bologna), M. Tosi (INAF – Bologna Observ.), J.S. Gallagher (Univ. of Wisconsin), M. Meixner (STScI), M. Clampin (GSFC), S. Oey (Univ. of Michigan), A. Pasquali (ETH Zurich), L. Smith (Univ. College London), and R. Walterbos (New Mexico State Univ.).

Original Source: Hubble News Release

Posted on by Fraser Cain

New View of Colliding Galaxies

For the first time, astronomers have been able to combine the deepest optical images of the universe, obtained by the Hubble Space Telescope, with equally sharp images in the near-infrared part of the spectrum using a sophisticated new laser guide star system for adaptive optics at the W. M. Keck Observatory in Hawaii. The new observations, presented at the American Astronomical Society (AAS) meeting in San Diego this week, reveal unprecedented details of colliding galaxies with massive black holes at their cores, seen at a distance of around 5 billion light-years, when the universe was just over half its present age.

Observing distant galaxies in the infrared range reveals older populations of stars than can be seen at optical wavelengths, and infrared light also penetrates clouds of interstellar dust more readily than optical light. The new infrared images of distant galaxies were obtained by a team of researchers from the University of California, Santa Cruz, UCLA, and the W. M. Keck Observatory. Jason Melbourne, a graduate student at UC Santa Cruz and lead author of the study, said the initial findings include some surprises and that researchers will continue to analyze the data in the weeks to come.

“We have never been able to achieve this level of spatial resolution in the infrared before,” Melbourne said.

In addition to Melbourne, the research team, led by David Koo of UCSC and James Larkin of UCLA, includes Jennifer Lotz, Claire Max, and Jerry Nelson at UCSC; Shelley Wright and Matthew Barczys at UCLA; and Antonin H. Bouchez, Jason Chin, Scott Hartman, Erik Johansson, Robert Lafon, David Le Mignant, Paul J. Stomski, Douglas Summers, Marcos A. van Dam, and Peter L. Wizinowich at Keck Observatory.

“For the first time in these deep images of the universe we can cover all wavelengths of light from the optical to the infrared with the same level of spatial resolution. This allows us to observe detailed substructures in distant galaxies and study their constituent stars with a precision we couldn’t possibly obtain otherwise,” said Koo, a professor of astronomy and astrophysics at UCSC.

The images were obtained by Wright and the Keck AO team during testing of the laser guide star adaptive optics system on the 10-meter Keck II Telescope. They are the first science-quality images of distant galaxies obtained with the new system. This marks a major step for the Center for Adaptive Optics Treasury Survey (CATS), which will use adaptive optics to observe a large sample of faint, distant galaxies in the early universe, said UCLA’s Larkin.

” We’ve worked very hard for several years taking data around bright stars. But we have been very restricted in terms of the number and types of objects that we can observe. Only with the laser can we now reach the richest and most exciting targets.” Larkin said.

Adaptive optics (AO) corrects for the blurring effect of the atmosphere, which seriously degrades images seen by ground-based telescopes. An AO system precisely measures this blurring and corrects the image using a deformable mirror, applying corrections hundreds of times per second. To measure the blurring, AO requires a bright point-source of light in the telescope’s field of view, which can be created artificially by using a laser to excite sodium atoms in the upper atmosphere, causing them to glow. Without such a laser guide star, astronomers have had to rely on bright stars (“natural guide stars”), which drastically limits where AO can be used in the sky. Furthermore, natural guide stars are too bright to allow observations of very faint, distant galaxies in the same part of the sky, Koo said.

“The advent of the laser guide star at Keck has opened up the sky for adaptive optics observations, and we can now use Keck to focus on those fields where we already have wonderful, deep optical images from the Hubble Space Telescope,” Koo said.

Because the diameter of the Keck Telescope’s mirror is four times larger than Hubble’s, it can obtain images four times sharper than Hubble in the near infrared now that the laser guide star adaptive optics system is available to overcome the blurring effects of the atmosphere.

The images being presented at the AAS meeting were obtained in an area of the sky known as the GOODS-South field, where deep observations have already been made by Hubble, the Chandra X-ray Observatory, and other telescopes. There are six faint galaxies in the images, including two X-ray sources identified by Chandra. The X-ray emissions, combined with the disordered morphology of these objects, suggested recent merger activity, Melbourne said. Mergers can funnel large amounts of matter into the center of a galaxy, and X-ray emissions from the galactic center indicate the presence of a massive black hole that is actively consuming matter.

” We are now fairly certain that we are seeing galaxies that have undergone recent mergers,” Melbourne said. “One of these systems has a double nucleus, so you can actually see the two nuclei of the merging galaxies. The other system is highly disordered–it looks like a train wreck–and is a much stronger X-ray source.”

In addition to lighting up the galactic nucleus with x-ray emissions, mergers also tend to trigger the formation of new stars by shocking and compressing clouds of gas. So the researchers were surprised to find that the system with a double nucleus is dominated by relatively old stars and does not appear to be producing many young stars.

” If we are right about the merger scenario, then this merger is occuring between two galaxies that had already formed most of their stars billions of years before and did not have a lot of gas left over to make new stars,” Melbourne said.

If additional study shows that such objects are common further back in time, these observations could help explain one of the puzzles of galaxy formation. According to the prevailing theory of hierarchical galaxy formation, large galaxies are built up over billions of years through mergers between smaller galaxies. Since mergers trigger star formation, it has been difficult to explain the existence of very large galaxies that lack significant populations of young stars.

“One idea is that you can have a so-called dry merger, where two galaxies full of old stars but little gas merge without forming many new stars. What we are seeing in this object is consistent with a dry merger,” Melbourne said. “Even in a dry merger, there may still be enough gas to feed the black hole, producing X-ray emissions, but not enough to yield a strong burst of star formation.”

Further observations at mid- to far-infrared wavelengths, expected later this year from the Spitzer Space Telescope, may help confirm this. The Spitzer data will provide a better indication of the dust content of the galaxy, a crucial variable in interpreting these observations, Melbourne said.

The laser guide star adaptive optics system was funded by the W. M. Keck Foundation. The artificial laser guide star system was developed and integrated in a partnership between the Lawrence Livermore National Laboratory and the W. M. Keck Observatory. The laser was integrated at Keck with the help of Dee Pennington, Curtis Brown, and Pam Danforth. The NIRC2 near-infrared camera was developed by the California Institute of Technology, UCLA, and the Keck Observatory. The Keck Observatory is operated as a scientific partnership among CalTech, the University of California, and the National Aeronautics and Space Administration.

This work has been supported by the Center for Adaptive Optics, a National Science Foundation Science and Technology Center managed by UC Santa Cruz.

Original Source: Keck News Release