Tag Archives: stellar

A second minor planet may possess Saturn-like rings

Researchers detect features around Chiron that may signal rings, jets, or a shell of dust.

By Jennifer Chu


CAMBRIDGE, Mass. – There are only five bodies in our solar system that are known to bear rings. The most obvious is the planet Saturn; to a lesser extent, rings of gas and dust also encircle Jupiter, Uranus, and Neptune. The fifth member of this haloed group is Chariklo, one of a class of minor planets called centaurs: small, rocky bodies that possess qualities of both asteroids and comets.

Scientists only recently detected Chariklo’s ring system — a surprising finding, as it had been thought that centaurs are relatively dormant. Now scientists at MIT and elsewhere have detected a possible ring system around a second centaur, Chiron.

In November 2011, the group observed a stellar occultation in which Chiron passed in front of a bright star, briefly blocking its light. The researchers analyzed the star’s light emissions, and the momentary shadow created by Chiron, and identified optical features that suggest the centaur may possess a circulating disk of debris. The team believes the features may signify a ring system, a circular shell of gas and dust, or symmetric jets of material shooting out from the centaur’s surface.

“It’s interesting, because Chiron is a centaur — part of that middle section of the solar system, between Jupiter and Pluto, where we originally weren’t thinking things would be active, but it’s turning out things are quite active,” says Amanda Bosh, a lecturer in MIT’s Department of Earth, Atmospheric and Planetary Sciences.

Bosh and her colleagues at MIT — Jessica Ruprecht, Michael Person, and Amanda Gulbis — have published their results in the journal Icarus.

Catching a shadow

Chiron, discovered in 1977, was the first planetary body categorized as a centaur, after the mythological Greek creature — a hybrid of man and beast. Like their mythological counterparts, centaurs are hybrids, embodying traits of both asteroids and comets. Today, scientists estimate there are more than 44,000 centaurs in the solar system, concentrated mainly in a band between the orbits of Jupiter and Pluto.

While most centaurs are thought to be dormant, scientists have seen glimmers of activity from Chiron. Starting in the late 1980s, astronomers observed patterns of brightening from the centaur, as well as activity similar to that of a streaking comet.

In 1993 and 1994, James Elliot, then a professor of planetary astronomy and physics at MIT, observed a stellar occultation of Chiron and made the first estimates of its size. Elliot also observed features in the optical data that looked like jets of water and dust spewing from the centaur’s surface.

Now MIT researchers — some of them former members of Elliot’s group — have obtained more precise observations of Chiron, using two large telescopes in Hawaii: NASA’s Infrared Telescope Facility, on Mauna Kea, and the Las Cumbres Observatory Global Telescope Network, at Haleakala.

In 2010, the team started to chart the orbits of Chiron and nearby stars in order to pinpoint exactly when the centaur might pass across a star bright enough to detect. The researchers determined that such a stellar occultation would occur on Nov. 29, 2011, and reserved time on the two large telescopes in hopes of catching Chiron’s shadow.

“There’s an aspect of serendipity to these observations,” Bosh says. “We need a certain amount of luck, waiting for Chiron to pass in front of a star that is bright enough. Chiron itself is small enough that the event is very short; if you blink, you might miss it.”

The team observed the stellar occultation remotely, from MIT’s Building 54. The entire event lasted just a few minutes, and the telescopes recorded the fading light as Chiron cast its shadow over the telescopes.

Rings around a theory

The group analyzed the resulting light, and detected something unexpected. A simple body, with no surrounding material, would create a straightforward pattern, blocking the star’s light entirely. But the researchers observed symmetrical, sharp features near the start and end of the stellar occultation — a sign that material such as dust might be blocking a fraction of the starlight.

The researchers observed two such features, each about 300 kilometers from the center of the centaur. Judging from the optical data, the features are 3 and 7 kilometers wide, respectively.  The features are similar to what Elliot observed in the 1990s.

In light of these new observations, the researchers say that Chiron may still possess symmetrical jets of gas and dust, as Elliot first proposed. However, other interpretations may be equally valid, including the “intriguing possibility,” Bosh says, of a shell or ring of gas and dust.

Ruprecht, who is a researcher at MIT’s Lincoln Laboratory, says it is possible to imagine a scenario in which centaurs may form rings: For example, when a body breaks up, the resulting debris can be captured gravitationally around another body, such as Chiron. Rings can also be leftover material from the formation of Chiron itself.

“Another possibility involves the history of Chiron’s distance from the sun,” Ruprecht says. “Centaurs may have started further out in the solar system and, through gravitational interactions with giant planets, have had their orbits perturbed closer in to the sun. The frozen material that would have been stable out past Pluto is becoming less stable closer in, and can turn into gases that spray dust and material off the surface of a body. ”

An independent group has since combined the MIT group’s occultation data with other light data, and has concluded that the features around Chiron most likely represent a ring system. However, Ruprecht says that researchers will have to observe more stellar occultations of Chiron to truly determine which interpretation — rings, shell, or jets — is the correct one.

“If we want to make a strong case for rings around Chiron, we’ll need observations by multiple observers, distributed over a few hundred kilometers, so that we can map the ring geometry,” Ruprecht says. “But that alone doesn’t tell us if the rings are a temporary feature of Chiron, or a more permanent one. There’s a lot of work that needs to be done.”

Nevertheless, Bosh says the possibility of a second ringed centaur in the solar system is an enticing one.

“Until Chariklo’s rings were found, it was commonly believed that these smaller bodies don’t have ring systems,” Bosh says. “If Chiron has a ring system, it will show it’s more common than previously thought.”

This research was funded in part by NASA and the National Research Foundation of South Africa.

Source: MIT News Office

The SPHERE instrument is shown shortly after it was installed on ESO’s VLT Unit Telescope 3. The instrument itself is the black box, located on the platform to one side of the telescope.

Credit:
ESO/J. Girard

The Strange Case of the Missing Dwarf

New SPHERE instrument shows its power


The new SPHERE instrument on ESO’s Very Large Telescope has been used to search for a brown dwarf expected to be orbiting the unusual double star V471 Tauri. SPHERE has given astronomers the best look so far at the surroundings of this intriguing object and they found — nothing. The surprising absence of this confidently predicted brown dwarf means that the conventional explanation for the odd behaviour of V471 Tauri is wrong. This unexpected result is described in the first science paper based on observations from SPHERE.

Some pairs of stars consist of two normal stars with slightly different masses. When the star of slightly higher mass ages and expands to become a red giant, material is transferred to other star and ends up surrounding both stars in a huge gaseous envelope. When this cloud disperses the two move closer together and form a very tight pair with one white dwarf, and one more normal star [1].

The SPHERE instrument is shown shortly after it was installed on ESO’s VLT Unit Telescope 3. The instrument itself is the black box, located on the platform to one side of the telescope. Credit: ESO/J. Girard
The SPHERE instrument is shown shortly after it was installed on ESO’s VLT Unit Telescope 3. The instrument itself is the black box, located on the platform to one side of the telescope.
Credit:
ESO/J. Girard

One such stellar pair is called V471 Tauri [2]. It is a member of the Hyades star cluster in the constellation of Taurus and is estimated to be around 600 million years old and about 163 light-years from Earth. The two stars are very close and orbit each other every 12 hours. Twice per orbit one star passes in front of the other — which leads to regular changes in the brightness of the pair observed from Earth as they eclipse each other.

A team of astronomers led by Adam Hardy (Universidad Valparaíso, Valparaíso, Chile) first used the ULTRACAM system on ESO’s New Technology Telescope to measure these brightness changes very precisely. The times of the eclipses were measured with an accuracy of better than two seconds — a big improvement on earlier measurements.

The eclipse timings were not regular, but could be explained well by assuming that there was a brown dwarf orbiting both stars whose gravitational pull was disturbing the orbits of the stars. They also found hints that there might be a second small companion object.

Up to now however, it has been impossible to actually image a faint brown dwarf so close to much brighter stars. But the power of the newly installed SPHERE instrument on ESO’s Very Large Telescope allowed the team to look for the first time exactly where the brown dwarf companion was expected to be. But they saw nothing, even though the very high quality images from SPHERE should have easily revealed it [3].

There are many papers suggesting the existence of such circumbinary objects, but the results here provide damaging evidence against this hypothesis,” remarks Adam Hardy.

If there is no orbiting object then what is causing the odd changes to the orbit of the binary? Several theories have been proposed, and, while some of these have already been ruled out, it is possible that the effects are caused by magnetic field variations in the larger of the two stars [4], somewhat similar to the smaller changes seen in the Sun.

A study such as this has been necessary for many years, but has only become possible with the advent of powerful new instruments such as SPHERE. This is how science works: observations with new technology can either confirm, or as in this case disprove, earlier ideas. This is an excellent way to start the observational life of this amazing instrument,” concludes Adam Hardy.

Notes

[1] Such pairs are known as post-common-envelope binaries.

[2] This name means that the object is the 471st variable star (or as closer analysis shows, pair of stars) to be identified in the constellation of Taurus.

[3] The SPHERE images are so accurate that they would have been able to reveal a companion such as a brown dwarf that is 70 000 times fainter than the central star, and only 0.26 arcseconds away from it. The expected brown dwarf companion in this case was predicted to be much brighter.

[4] This effect is called the Applegate mechanism and results in regular changes in the shape of the star, which can lead to changes in the apparent brightness of the double star seen from Earth.

Soure : ESO

Fig. 1: ESO/S. Ramstedt (Uppsala University, Sweden) & W. Vlemmings (Chalmers University of Technology, Sweden)

ALMA reveals Mira’s secret life

Studying red giant stars tells astronomers about the future of the Sun — and about how previous generations of stars spread the elements needed for life across the Universe. One of the most famous red giants in the sky is called Mira A, part of the binary system Mira which lies about 400 light-years from Earth. In this image ALMA reveals Mira’s secret life.

Mira A is an old star, already starting to throw out the products of its life’s work into space for recycling. Mira A’s companion, known as Mira B, orbits it at twice the distance from the Sun to Neptune.

Mira A is known to have a slow wind, which gently molds the surrounding material. ALMA has now confirmed that Mira’s companion is a very different kind of star, with a very different wind. Mira B is a hot, dense white dwarf with a fierce and fast stellar wind.

Fig. 1: ESO/S. Ramstedt (Uppsala University, Sweden) & W. Vlemmings (Chalmers University of Technology, Sweden)
Fig. 1: ESO/S. Ramstedt (Uppsala University, Sweden) & W. Vlemmings (Chalmers University of Technology, Sweden)

New observations show how the winds from the two stars have created a fascinating, beautiful and complex nebula. The remarkable heart-shaped bubble at the center is created by Mira B’s energetic wind inside Mira A’s more relaxed outflow. The heart, which formed some time in the last 400 years or so, and the rest of the gas surrounding the pair show that they have long been building this strange and beautiful environment together.

By looking at stars like Mira A and Mira B scientists hope to discover how our galaxy’s double stars differ from single stars in how they give back what they have created to the Milky Way’s stellar ecosystem. Despite their distance from one another, Mira A and its companion have had a strong effect on one another and demonstrate how double stars can influence their environments and leave clues for scientists to decipher.

Other old and dying stars also have bizarre surroundings, as astronomers have seen using both ALMA and other telescopes. But it’s not always clear whether the stars are single, like the Sun, or double, like Mira. Mira A, its mysterious partner and their heart-shaped bubble are all part of this story.

More information

The new observations of Mira A and its partner are presented in this paper.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

Source: ALMA Observatory

The Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile has taken this beautiful image, dappled with blue stars, of one of the most star-rich open clusters currently known — Messier 11, also known as NGC 6705 or the Wild Duck Cluster. Credit: ESO

Wild Ducks Take Flight in Open Cluster

he Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile has taken this beautiful image, dappled with blue stars, of one of the most star-rich open clusters currently known — Messier 11, also known as NGC 6705 or the Wild Duck Cluster. Credit: ESO
The Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile has taken this beautiful image, dappled with blue stars, of one of the most star-rich open clusters currently known — Messier 11, also known as NGC 6705 or the Wild Duck Cluster. Credit: ESO

Messier 11 is an open cluster, sometimes referred to as a galactic cluster, located around 6000 light-years away in the constellation of Scutum (The Shield). It was first discovered by German astronomer Gottfried Kirch in 1681 at the Berlin Observatory, appearing as nothing more than a fuzzy blob through the telescope. It wasn’t until 1733 that the blob was first resolved into separate stars by the Reverend William Derham in England, and Charles Messier added it to his famous catalogue in 1764.

Messier was a comet hunter and the catalogue came into being as he was frustrated by constantly observing fixed, diffuse objects that looked like comets (for example, objects that we now know to be clusters, galaxies and nebulae). He wanted a record in order to avoid accidentally observing them again and confusing them with possible new comets. This particular stellar cluster was noted down as the eleventh such object — hence the name of Messier 11.

Open clusters are typically found lying in the arms of spiral galaxies or in the denser regions of irregular galaxies, where star formation is still common. Messier 11 is one of the most star-rich and compact of the open clusters, being almost 20 light-years across and home to close to 3000 stars. Open clusters are different to globular clusters, which tend to be very dense, tightly bound by gravity, and contain hundreds of thousands of very old stars — some of which are nearly as old as the Universe itself.

Studying open clusters is great way to test theories of stellar evolution, as the stars form from the same initial cloud of gas and dust and are therefore very similar to one another — they all have roughly the same age, chemical composition, and are all the same distance away from Earth. However, each star in the cluster has a different mass, with the more massive stars evolving much faster than their lower mass counterparts as they use up all of their hydrogen much sooner.

In this way, direct comparisons between the different evolutionary stages can be made within the same cluster: for example, does a 10 million year old star with the same mass as the Sun evolve in a different way to another star that is the same age, but half as massive? In this sense, open clusters are the closest thing astronomers have to “laboratory conditions”.

Because the stars within open clusters are very loosely bound to one another, individuals are very susceptible to being ejected from the main group due to the effect of gravity from neighbouring celestial objects. NGC 6705 is already at least 250 million years old, so in a few more million years it is likely that this Wild Duck formation will disperse, and the cluster will break up and merge into its surroundings [1].

This image was taken by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in northern Chile.

Notes

[1] The alternative and evocative name for NGC 6705, the Wild Duck Cluster, came about in the 19th century. When the cluster was seen through a small telescope it was noticed that the brightest stars formed an open triangle pattern on the sky that resembled ducks flying in formation.

More information

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 15 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Portugal, Spain, Sweden, Switzerland and the United Kingdom. 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 the European partner of a revolutionary astronomical telescope ALMA, the largest astronomical project in existence. ESO is currently planning the 39-metre European Extremely Large optical/near-infrared Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

 Source :ESO