Author: Fraser Cain

Image credit: NRAO/AUI/NSF
Astronomers at Sweet Briar College and the Naval Research Laboratory (NRL) have detected a powerful new bursting radio source whose unique properties suggest the discovery of a new class of astronomical objects. The researchers have monitored the center of the Milky Way Galaxy for several years and reveal their findings in the March 3, 2005 edition of the journal, “Nature”.

Principal investigator, Dr. Scott Hyman, professor of physics at Sweet Briar College, said the discovery came after analyzing some additional observations from 2002 provided by researchers at Northwestern University. “”We hit the jackpot!” Hyman said referring to the observations. “An image of the Galactic center, made by collecting radio waves of about 1-meter in wavelength, revealed multiple bursts from the source during a seven-hour period from Sept. 30 to Oct. 1, 2002 ? five bursts in fact, and repeating at remarkably constant intervals.”

Hyman, four Sweet Briar students, and his NRL collaborators, Drs. Namir Kassim and Joseph Lazio, happened upon transient emission from two radio sources while studying the Galactic center in 1998. This prompted the team to propose an ongoing monitoring program using the National Science Foundation?s Very Large Array (VLA) radio telescope in New Mexico. The National Radio Astronomy Observatory, which operates the VLA, approved the program. The data collected, laid the groundwork for the detection of the new radio source.

“Amazingly, even though the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths,” NRL astronomer Dr. Joseph Lazio pointed out, “very little has been done to look for radio bursts, which are often easier for astronomical objects to produce.”

The team has monitored the Galactic center for new transient sources and for variability in approximately 250 known sources, but the five bursts from the new radio source, named GCRT J1745-3009, were by far the most powerful seen. The five bursts were of equal brightness, with each lasting about 10 minutes, and occurring every 77 minutes.

The source of the bursts is transient Hyman noted. “It has not been detected since 2002 nor is it present on earlier images.”

Although the exact nature of the object remains a mystery, the team members currently believe that GCRT J1745-3009 is either the first member of a new class of objects or an unknown mode of activity of a known source class.

One important clue to understanding the origin of the radio bursts is that the emission appears to be “coherent,” Hyman said. “There are very few classes of coherent emitters in the universe. Natural astronomical masers ? the analog of laser emission at microwave wavelengths ? are one class of coherent sources, but these emit in specific wavelengths. In contrast, the new transient?s bursts were detected over a relatively large bandwidth.”

“In addition to these intriguing properties, NRL astronomer Dr. Paul Ray and colleague, Dr. Craig Markwardt of NASA?s Goddard Space Flight Center, have searched the source for X-ray emission but have not found any convincing evidence. “The non-detection of X-ray emission is intriguing,” Ray said. “Many sources that emit transient X-ray flares, such as black hole binary star systems, also have associated radio emission. If upon further observations, X-ray emission is definitively detected or ruled out, this will be a significant help in understanding the nature of this remarkable source.”

“Needless to say, the discovery of these transients has been very exciting for our students,” Hyman added. Participating in this research program has inspired at least two of Hyman’s students ? Jennifer Neureuther and Mariana Lazarova ? to pursue graduate studies in astronomy.

This project was supported at Sweet Briar College by funding from Research Corporation and the Jeffress Foundation. Basic research in radio astronomy at NRL is supported by the Office of Naval Research.

Hyman and his NRL colleagues plan to continue monitoring the Galactic center and search for the source again with the VLA and other X-ray and radio telescopes. They are also developing (with Dr. Kent Wood of NRL) a model that attempts to account for the radio bursts as a new type of outburst from a class of sources known as “magnetars.”

NRL is also contributing to an effort to build the world?s largest and most sensitive low-frequency telescope, called the Long Wavelength Array (LWA), which may revolutionize future searches for other radio transient sources. Current plans call for the LWA, which is being developed by the University of New Mexico-led Southwest Consortium, to be sited in New Mexico, not far from the VLA.

“One of the key advantages of observing at long radio wavelengths,” explained NRL astronomer, Dr. Namir Kassim, “is that the field-of-view is so large that a single observation can efficiently detect transient phenomena over a large region.”

“When completed, the LWA may uncover hundreds of previously unknown radio transients, some of which may be examples of Jupiter-like planets orbiting other stars,” Kassim added. Jupiter is the most famous example of a nearby radio transient.

Original Source: NRAO News Release

In the March 4th issue of Science, astronomers report that they have measured the slowest ever motion of a galaxy across the plane of the sky. This distant whirlpool of stars appears to creep along despite its actual speed through space because it is located so far from the Earth. Measuring this galaxy’s glacial pace of only 30 micro-arcseconds per year stretched current radio astronomy technology to its limit.

“A snail crawling on Mars would appear to be moving across the surface more than 100 times faster than the motion we measured for this galaxy,” said Mark Reid (Harvard-Smithsonian Center for Astrophysics), a co-author on the paper.

Reid and his colleagues used the National Science Foundation’s Very Long Baseline Array (VLBA) to measure the motion across the sky of a galaxy located nearly 2.4 million light-years from Earth. While scientists have been measuring the motion of galaxies directly toward or away from Earth for decades, this is the first time that the transverse motion (called proper motion by astronomers) has been measured for a galaxy that is not a nearby satellite of the Milky Way.

An international scientific team analyzed VLBA observations made over two and a half years to detect minuscule shifts in the sky position of the spiral galaxy M33. Combined with previous measurements of the galaxy’s motion toward Earth, the new data allowed the astronomers to calculate M33’s movement in three dimensions for the first time.

M33 is a satellite of the larger galaxy M31, the well-known Andromeda Galaxy that is the most distant object visible to the naked eye. Both are part of the Local Group of galaxies that includes the Milky Way.

The astronomers’ task was not simple. Not only did they have to detect an impressively tiny amount of motion across the sky, but they also had to separate the actual motion of M33 from the apparent motion caused by our Solar System’s motion around the center of the Milky Way. The motion of the Solar System and the Earth around the galactic center, some 26,000 light-years away, has been accurately measured using the VLBA over the last decade.

“The VLBA is the only telescope system in the world that could do this work,” Reid said. “Its extraordinary ability to resolve fine detail is unmatched and was the absolute prerequisite to making these measurements.”

In addition to measuring the motion of M33 as a whole, the astronomers also were able to make a direct measurement of the spiral galaxy’s rotation. Both measurements were made by observing the changes in position of giant clouds of molecules inside the galaxy. The water vapor in these clouds acts as a natural maser, strengthening, or amplifying, radio emission the same way that lasers amplify light emission. The natural masers acted as bright radio beacons whose movement could be tracked by the ultra-sharp radio “vision” of the VLBA.

Reid and his colleagues plan to continue measuring M33’s motion and also to make similar measurements of M31’s motion. This will allow them to answer important questions about the composition, history and fates of the two galaxies as well as of the Milky Way.

“We want to determine the orbits of M31 and M33. That will help us learn about their history, specifically, how close have they come in the past?” Reid explained. “If they have passed very closely, then maybe M33’s small size is a result of having material pulled off it by M31 during the close encounter,” he added.

Accurate knowledge of the motions of both galaxies also will help determine if there is a collision in their future. In addition, orbital analysis can give astronomers valuable clues about the amount and distribution of dark matter in the galaxies.

Reid worked with Andreas Brunthaler of the Max Planck Institute for Radioastronomy in Bonn, Germany; Heino Falcke of ASTRON in the Netherlands; Lincoln Greenhill, also of the Harvard-Smithsonian Center for Astrophysics; and Christian Henkel, also of the Max Planck Institute in Bonn.

Original Source: CfA News Release

An international team of astronomers have accurately determined the radius and mass of the smallest core-burning star known until now.

The observations were performed in March 2004 with the FLAMES multi-fibre spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal Observatory (Chile). They are part of a large programme aimed at measuring accurate radial velocities for sixty stars for which a temporary brightness “dip” has been detected during the OGLE survey.

The astronomers find that the dip seen in the light curve of the star known as OGLE-TR-122 is caused by a very small stellar companion, eclipsing this solar-like star once every 7.3 days.

This companion is 96 times heavier than planet Jupiter but only 16% larger. It is the first time that direct observations demonstrate that stars less massive than 1/10th of the solar mass are of nearly the same size as giant planets. This fact will obviously have to be taken into account during the current search for transiting exoplanets.

In addition, the observations with the Very Large Telescope have led to the discovery of seven new eclipsing binaries, that harbour stars with masses below one-third the mass of the Sun, a real bonanza for the astronomers.

The OGLE Survey
When a planet happens to pass in front of its parent star (as seen from the Earth), it blocks a small fraction of the star’s light from our view [1].

These “planetary transits” are of great interest as they allow astronomers to measure in a unique way the mass and the radius of exoplanets. Several surveys are therefore underway which attempt to find these faint signatures of other worlds.

One of these programmes is the OGLE survey which was originally devised to detect microlensing events by monitoring the brightness of a very large number of stars over extended time intervals. During the past years, it has also included a search for periodic, very shallow “dips” in the brightness of stars, caused by the regular transit of small orbiting objects (small stars, brown dwarfs [2] or Jupiter-size planets). The OGLE team has since announced 177 “planetary transit candidates” from their survey of several hundred thousand stars in three southern sky fields, one in the direction of the Galactic Centre, another within the Carina constellation and the third within the Centaurus/Musca constellations.

The nature of the transiting object can however only be established by subsequent radial-velocity observations of the parent star. The size of the velocity variations (the amplitude) is directly related to the mass of the companion object and therefore allows discrimination between stars and planets as the cause of the observed brightness “dip”.

A Bonanza of Low-Mass Stars
An international team of astronomers [3] has made use of the 8.2-m VLT Kueyen telescope for this work. Profiting from the multiplex capacity of the FLAMES/UVES facility that permits to obtain high-resolution spectra of up to 8 objects simultaneously, they have looked at 60 OGLE transit candidate stars, measuring their radial velocities with an accuracy of about 50 m/s [4].

This ambitious programme has so far resulted in the discovery of five new transiting exoplanets (see, e.g., ESO PR 11/04 for the announcement of two of those).

Most of the other transit candidates identified by OGLE have turned out to be eclipsing binaries, that is, in most cases common, small and low-mass stars passing in front of a solar-like star. This additional wealth of data on small and light stars is a real bonanza for the astronomers.

Constraining the Relation Between Mass and Radius
Low-mass stars are exceptionally interesting objects, also because the physical conditions in their interiors have much in common with those of giant planets, like Jupiter in our solar system. Moreover, a determination of the sizes of the smallest stars provides indirect, crucial information about the behaviour of matter under extreme conditions [5].

Until recently, very few observations had been made and little was known about low-mass stars. At this moment, exact values of the radii are known only for four stars with masses less than one-third of the mass of the Sun (cf. ESO PR 22/02 for measurements made with the Very Large Telescope Interferometer) and none at all for masses below one-eighth of a solar mass.

This situation is now changing dramatically. Indeed, observations with the Very Large Telescope have so far led to the discovery of seven new eclipsing binaries, that harbour stars with masses below one-third the mass of the Sun.

This new set of observations thus almost triples the number of low-mass stars for which precise radii and masses are known. And even better – one of these stars now turns out to be the smallest known!

Planet-Sized Stars
The newly found stellar gnome is the companion of OGLE-TR-122, a rather remote star in the Milky Way galaxy, seen in the direction of the southern constellation Carina.

The OGLE programme revealed that OGLE-TR-122 experiences a 1.5 per cent brightness dip once every 7 days 6 hours and 27 minutes, each time lasting just over 3 hours (about 188 min). The FLAMES/UVES measurements, made during 6 nights in March 2004, reveal radial velocity variations of this period with an amplitude of about 20 km/s. This is the clear signature of a very low-mass star, close to the Hydrogen-burning limit, orbiting OGLE-TR-122. This companion received the name OGLE-TR-122b.

As Fran?ois Bouchy of the Observatoire Astronomique Marseille Provence (France) explains: “Combined with the information collected by OGLE, our spectroscopic data now allow us to determine the nature of the more massive star in the system, which appears to be solar-like”.

This information can then be used to determine the mass and radius of the much smaller companion OGLE-TR-122b. Indeed, the depth (brightness decrease) of the transit gives a direct estimate of the ratio between the radii of the two stars, and the spectroscopic orbit provides a unique value of the mass of the companion, once the mass of the larger star is known.

The astronomers find that OGLE-TR-122b weighs one-eleventh of the mass of the Sun and has a diameter that is only one-eighth of the solar one. Thus, although the star is still 96 times as massive as Jupiter, it is only 16% larger than this giant planet!

A Dense Star
“Imagine that you add 95 times its own mass to Jupiter and nevertheless end up with a star that is only slightly larger”, suggests Claudio Melo from ESO and member of the team of astronomers who made the study. “The object just shrinks to make room for the additional matter, becoming more and more dense.”

The density of such a star is more than 50 times the density of the Sun.

“This result shows the existence of stars that look strikingly like planets, even from close by”, emphasizes Frederic Pont of the Geneva Observatory (Switzerland). “Isn’t it strange to imagine that even if we were to receive images from a future space probe approaching such an object at close range, it wouldn’t be easy to discern whether it is a star or a planet?”

As all stars, OGLE-TR-122b produces indeed energy in its interior by means of nuclear reactions. However, because of its low mass, this internal energy production is very small, especially compared to the energy produced by its solar-like companion star.

Not less striking is the fact that exoplanets which are orbiting very close to their host star, the so-called “hot Jupiters”, have radii which may be larger than the newly found star. The radius of exoplanet HD209458b, for example, is about 30% larger than that of Jupiter. It is thus substantially larger than OGLE-TR-122b!

Masqueraders
This discovery also has profound implications for the ongoing search for exoplanets. These observations clearly demonstrate that some stellar objects can produce precisely the same photometric signals (brightness changes) as transiting Jupiter-like planets [6]. What’s more, the present study has shown that such stars are not rare.

Stars like OGLE-TR-122b are thus masqueraders among giant exoplanets and the outermost care is required to differentiate them from their planetary cousins. Uncovering such small stars can only be done with follow-up high-resolution spectral measurements with the largest telescopes. There is more work ahead for the Very Large Telescope!

More information
The information contained in this press release is based on a research article to appear soon as a Letter to the Editor in the leading research journal “Astronomy & Astrophysics” (“A planet-sized transiting star around OGLE-TR-122” by F. Pont et al.). The paper is available in PDF format on the A&A website.

Notes
[1]: Brown dwarfs, or “failed stars”, are objects which are up to 75 times more massive than Jupiter. They are too small for major nuclear fusion processes to have ignited in its interior.

[2]: The radius of a Jupiter-size planet is about 10 times smaller than that of a solar-type star, i.e. it covers about 1/100 of the surface of that star and hence it blocks about 1 % of the stellar light during the transit.

[3]: The team consists of Fr?d?ric Pont, Michel Mayor, Didier Queloz and St?phane Udry of the Geneva Observatory in Switzerland, Claudio Melo of ESO-Chile, Fran?ois Bouchy at Observatoire Astronomique Marseille Provence in France, and Nuno Santos of the Lisbon Astronomical Observatory, Portugal.

[4]: This amounts to measuring a speed of 180 km/h. By comparison, the motion of the Sun induced by Jupiter is about 13 m/s or 47 km/h. This motion is proportional to the mass of the planet and inversely proportional to the square root of its distance from the star.

[5]: For a normal star like the Sun whose matter behaves like a perfect gas, the stellar size is proportional to the mass. However, for low-mass stars, quantum effects become important and the stellar matter becomes “degenerate”, resisting compression much more than does a perfect gas. For objects with a mass below 75 times the mass of Jupiter, i.e. brown dwarfs, the matter is fully degenerate and their size does not depend on the mass.

[6]: Note that a distant transiting object – star or planet – will always produce a brightness “dip”, however bright it is itself. Before and after the transit, the recorded brightness equals the sum of the brightness of the central star and that of the orbiting object. During the transit, the recorded brightness is this sum minus the light emitted by that part of the central star that is obscured.

Original Source: ESO News Release

Scientists now believe that the formation of Jupiter, the heavy-weight champion of the Solar System?s planets, may have spawned some of the tiniest and oldest constituents of our Solar System?millimeter-sized spheres called chondrules, the major component of primitive meteorites. The study, by theorists Dr. Alan Boss of the Carnegie Institution and Prof. Richard H. Durisen of Indiana University, is published in the March 10, 2005, issue of The Astrophysical Journal (Letters).

?Understanding what formed the chondrules has been one of the biggest problems in the field for over a century,? commented Boss. ?Scientists realized several years ago that a shock wave was probably responsible for generating the heat that cooked these meteoritic components. But no one could explain convincingly how the shock front was generated in the solar nebula some 4.6 billion years ago. These latest calculations show how a shock front could have formed as a result of spiral arms roiling the solar nebula at Jupiter?s orbit. The shock front extended into the inner solar nebula, where the compressed gas and radiation heated the dust particles as they struck the shock front at 20,000 mph, thereby creating chondrules,? he explained.

?This calculation has probably removed the last obstacle to acceptance of how chondrules were melted,? remarked theorist Dr. Steven Desch of Arizona State University, who showed several years ago that shock waves could do the job. ?Meteoriticists have recognized that the ways chondrules are melted by shocks are consistent with everything we know about chondrules. But without a proven source of shocks, they have remained mostly unconvinced about how chondrules were melted. The work of Boss and Durisen demonstrates that our early solar nebula experienced the right types of shocks, at the right times, and at the right places in the nebula to melt chondrules. I think for many meteoriticists, this closes the deal. With nebular shocks identified as the culprit, we can finally begin to understand what the chondrules are telling us about the earliest stages of our Solar System’s evolution,? he concluded.

?Our calculation shows how the 3-dimensional gravitational forces associated with spiral arms in a gravitationally unstable disk at Jupiter?s distance from the Sun (5 times the Earth-Sun distance), would produce a shock wave in the inner solar system (2.5 times the Earth-Sun distance, i.e., in the asteroid belt),? Boss continued. ?It would have heated dust aggregates to the temperature required to melt them and form tiny droplets.? Durisen and his research group at Indiana have independently made calculations of gravitationally unstable disks that also support this picture.

While Boss is well known as a proponent of the rapid formation of gas giant planets by the disk instability process, the same argument for chondrule formation works for the slower process of core accretion. In order to make Jupiter in either process, the solar nebula had to have been at least marginally gravitationally unstable, so that it would have developed spiral arms early on and resembled a spiral galaxy. Once Jupiter formed by either mechanism, it would have continued to drive shock fronts at asteroidal distances, at least so long as the solar nebula was still around. In both cases, chondrules would have been formed at the very earliest times, and continued to form for a few million years, until the solar nebula disappeared. Late-forming chondrules are thus the last grin of the Cheshire Cat that formed our planetary system.

Boss?s research is supported in part by the NASA Planetary Geology and Geophysics Program and the NASA Origins of Solar Systems Program. The calculations were performed on the Carnegie Alpha Cluster, the purchase of which was supported in part by the NSF Major Research Instrumentation Program. Durisen?s research was also supported in part by the NASA Origins of Solar Systems Program.

Original Source: Carnegie Institute News Release

What is the biggest planet?

Something weird is happening inside a nearby stellar nursery. An embryonic star is giving off a healthy glow?in X-rays. Like a precocious child, the developing star (protostar) is far too young for that kind of behavior.

New stars are born when a cloud of dust and gas in interstellar space collapses under its own gravity, or so we thought. The strange behavior of this protostar reveals that something else might help gravity turn a bunch of gas and dust into a star.

Scientists have pierced through a dusty stellar nursery to capture the earliest and most detailed view of a collapsing gas cloud turning into a star, analogous to a baby’s first ultrasound.

The observation, made primarily with the European Space Agency’s XMM-Newton observatory, suggests that some unrealized, energetic process — likely related to magnetic fields — is superheating the surface of the cloud core, nudging the cloud ever closer to becoming a star.

The observation marks the first clear detection of X-rays from a nascent yet frigid precursor to a star, called a Class 0 protostar, far earlier in a star’s evolution than most experts in this field thought possible. X-rays are produced in space by processes that release a lot of energy and heat. The surprise detection of X-rays from such a cold object reveals that matter is falling toward the protostar core 10 times faster than expected from gravity alone.

“We are seeing star formation at its embryonic stage,” said Dr. Kenji Hamaguchi, a NASA-funded researcher at NASA’s Goddard Space Flight Center in Greenbelt, Md., lead author on a report in The Astrophysical Journal. “Previous observations have captured the shape of such gas clouds but have never been able to peer inside. The detection of X-rays this early indicates that gravity alone is not the only force shaping young stars.”

Supporting data came from NASA’s Chandra X-ray Observatory, Japan’s Subaru telescope in Hawaii, and the University of Hawaii 88-inch telescope.

Hamaguchi’s team discovered X-rays from a Class 0 protostar in the R Corona Australis star-forming region, about 500 light years from Earth.

Class 0 is the youngest class of protostellar object, about 10,000 to 100,000 years into the assimilation process. The cloud temperature is about 400 degrees below zero Fahrenheit (minus 240 Celsius). After a few million years, nuclear fusion ignites at the center of the collapsing protostellar cloud, and a new star is formed.

The team speculates that magnetic fields in the spinning protostar core accelerate infalling matter to high speeds, producing high temperatures and X-rays in the process. These X- rays can penetrate the dusty region to reveal the core.

“This is no gentle freefall of gas,” said Dr. Michael Corcoran of NASA Goddard, a co-author on the report. “The X-ray emission shows that forces appear to be accelerating matter to high speeds, heating regions of this cold gas cloud to 100 million degrees Fahrenheit. The X-ray emission from the core gives us a window to probe the hidden processes by which cold gas clouds collapse to stars.”

Hamaguchi likened the generation of X-rays in the Class 0 protostar to what happens during solar flares on our Sun. The solar surface has lots of magnetic loops, which sometimes get tangled and release large amounts of energy. This energy can accelerate electrically-charged particles (electrons and ionized atoms) to velocities of 7 million miles an hour. The particles smash against the solar surface and create X-rays. Similarly tangled magnetic fields might be responsible for X-rays observed by Hamaguchi and his collaborators.

The detection of magnetic fields from an extremely young Class 0 protostar provides a crucial link in understanding the star formation process, because magnetic field loops are believed to play a critical role in moderating the cloud collapse. Only electrically-charged particles, called ions, respond to magnetic fields. The scientists are not sure where the magnetic fields or ions come from. However, X-rays will ionize atoms, creating more ions to be accelerated through magnetic activity and create more X-rays.

The team used XMM-Newton for its powerful light-collecting capability, necessary for this type of observation where so few X-rays penetrate the dusty region, and the exquisite resolving power of Chandra to pinpoint the X-ray source position. The team used the infrared Subaru telescope to determine the protostar’s age.

“The age is based on a well-established chart of spectra, or characteristics of the infrared light, as the protostar evolves over the course of a million years,” said Ko Nedachi, a doctoral student at the University of Tokyo who led the Subaru observation.

The science team also includes Drs. Rob Petre and Nicholas White of NASA Goddard, Dr. Beate Stelzer of the Astronomy Observatory in Palermo, Italy, and Dr. Naoto Kobayashi of University of Tokyo. Kenji Hamaguchi is funded through the National Research Council; Michael Corcoran is funded through Universities Space Research Association.

Original Source: NASA News Release