Gaia: Milky Way may have formed 'inside-out'

This is a figure illustrating latest Gaia-ESO research findings. 

Credit: Amanda Smith/Institute of Astronomy

A breakthrough using data from the Gaia-ESO project has provided evidence backing up theoretically predicted divisions in the chemical composition of the stars that make up the Milky Way's disc – the vast collection of giant gas clouds and billions of stars that give our Galaxy its 'flying saucer' shape.

By tracking the fast-produced elements, specifically magnesium in this study, astronomers can determine how rapidly different parts of the Milky Way were formed.

The research suggests that stars in the inner regions of the Galactic disc were the first to form, supporting ideas that our Galaxy grew from the inside-out.

Using data from the 8-m VLT in Chile, one of the world's largest telescopes, an international team of astronomers took detailed observations of stars with a wide range of ages and locations in the Galactic disc to accurately determine their 'metallicity' (?): the amount of chemical elements in a star other than hydrogen and helium, the two elements most stars are made from.

Immediately after the Big Bang, the Universe consisted almost entirely of hydrogen and helium, with levels of "contaminant metals" growing over time.

Consequently, older stars have fewer elements in their make-up - so have lower 'metallicity'.

Gerry Gilmore

"The different chemical elements of which stars - and we - are made are created at different rates - some in massive stars which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes," said Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.

Massive stars, which have short lives and die as 'core-collapse supernovae', produce huge amounts of magnesium during their explosive death throes.

This catastrophic event can form a neutron star or a black hole, and even trigger the formation of new stars.

The team have shown that older, 'metal-poor' stars inside the Solar Circle – the orbit of our Sun around the centre of the Milky Way, which takes roughly 250 million years to complete – are far more likely to have high levels of magnesium.

The higher level of the element inside the Solar Circle suggests this area contained more stars that "lived fast and die young" in the past.

The stars that lie in the outer regions of the Galactic disc - outside the Solar Circle - are predominantly younger, both 'metal-rich' and 'metal-poor', and have surprisingly low magnesium levels compared to their metallic properties.

During the latest research, the team found that:
Stars in the young, 'thin' disc aged between 0 – 8 billion years all have a similar degree of metallicity, regardless of age in that range, with many of them considered 'metal-rich'.

There is a "steep decline" in metallicity for stars aged over 9 billion years, typical of the 'thick' disc, with no detectable 'metal-rich' stars found at all over this age.

But stars of different ages and metallicity can be found in both discs.

Maria Bergemann

"From what we now know, the Galaxy is not an 'either-or' system. You can find stars of different ages and metal content everywhere!" said Maria Bergemann from Cambridge Institute of Astronomy, who led the study.

"There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs - that's how we know these two discs probably exist – but they could have very different origins."

Added Gilmore: "This study provides exciting new evidence that the inner parts of the Milky Way's thick disc formed much more rapidly than did the thin disc stars, which dominate near our Solar neighbourhood."

ESA VLT: Oldest solar twin identified

This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. 

On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. 

After spending the majority of its life in this stage, the star's core begins to gradually heat up, the star expands and becomes redder until it transforms into a red giant. 

Following this stage, the star will push its outer layers into the surrounding space to form an object known as a planetary nebula, while the core of the star itself will cool into a small, dense remnant called a white dwarf star. 

Marked on the lower timeline are where our Sun and solar twins 18 Sco and HIP 102152 are in this life cycle. 

The Sun is 4.6 billion years old and 18 Sco is 2.9 billion years old, while the oldest solar twin is some 8.2 billion years old -- the oldest solar twin ever identified. 

By studying HIP 102152, we can get a glimpse of what the future holds for our Sun. 

This image is illustrative; the ages, sizes, and colours are approximate (not to scale). The protostar stage, on the far left of this image, can be some 2000 times larger than our Sun. 

The red giant stage, on the far right of this image, can be some 100 times larger than the Sun. 

Credit: ESO/M. Kornmesser

An international team led by astronomers in Brazil has used ESO's Very Large Telescope to identify and study the oldest solar twin known to date.

Located 250 light-years from Earth, the star HIP 102152 is more like the Sun than any other solar twin—except that it is nearly four billion years older.

This older, but almost identical, twin gives us an unprecedented chance to see how the Sun will look when it ages.

The new observations also provide an important first clear link between a star's age and its lithium content, and in addition suggest that HIP 102152 may be host to rocky terrestrial planets.

Astronomers have only been observing the Sun with telescopes for 400 years—a tiny fraction of the Sun's age of 4.6 billion years.

It is very hard to study the history and future evolution of our star, but we can do this by hunting for rare stars that are almost exactly like our own, but at different stages of their lives.

Now astronomers have identified a star that is essentially an identical twin to our Sun, but 4 billion years older—almost like seeing a real version of the twin paradox in action.

Jorge Melendez (Universidade de São Paulo, Brazil), the leader of the team and co-author of the new paper explains: "For decades, astronomers have been searching for solar twins in order to know our own life-giving Sun better.

But very few have been found since the first one was discovered in 1997. We have now obtained superb-quality spectra from the VLT and can scrutinise solar twins with extreme precision, to answer the question of whether the Sun is special."

The team studied two solar twins—one that was thought to be younger than the Sun (18 Scorpii) and one that was expected to be older (HIP 102152).

They used the UVES spectrograph on the Very Large Telescope (VLT) at ESO's Paranal Observatory to split up the light into its component colours so that the chemical composition and other properties of these stars could be studied in great detail.

They found that HIP 102152 in the constellation of Capricornus (The Sea Goat) is the oldest solar twin known to date.

It is estimated to be 8.2 billion years old, compared to 4.6 billion years for our own Sun. On the other hand 18 Scorpii was confirmed to be younger than the Sun—about 2.9 billion years old.

More information: This research was presented in a paper to appear in "High precision abundances of the old solar twin HIP 102152: insights on Li depletion from the oldest Sun", by TalaWanda Monroe et al. in the Astrophysical Journal Letters. Research paper PDF

ESA VLT Discovers Feeding galaxy caught in distant searchlight

This is an artist's impression of a galaxy accreting material from its surroundings. 

Credit: European Southern Observatory

An international group of astronomers that includes UC Santa Barbara astrophysicist Crystal Martin and former UCSB postdoctoral researcher Nicolas Bouché has spotted a distant galaxy hungrily snacking on nearby gas.

The gas is seen to fall inward toward the galaxy, creating a flow that both fuels star formation and drives the galaxy's rotation.

This is the best direct observational evidence so far supporting the theory that galaxies pull in and devour nearby material in order to grow and form stars. The results will appear in the July 5 issue of the journal Science.

Spiral galaxies like our own Milky Way formed billions of years ago in the dark matter concentrations that began to grow shortly after the Big Bang.

As gas cooled and condensed, stars formed, which, over time, synthesized heavy elements and polluted the galaxy with this enriched material upon their death.

But what that model has not been able to explain is the continuous formation of stars in some galaxies, despite the constant rate at which galaxies turn molecular gas into stars.

The simplest model calls for a closed system and predicts star formation should have ceased long ago due to the limited gas supply.

"It's been a problem," said Martin. Galaxies should use up their gas on a time scale that's much shorter than what has been observed, she explained.

In fact our own galaxy should have already run out of gas, but stars continue to form in it.

"Galaxies must have a mechanism for acquiring more gas," she continued, adding that, historically, no means has existed to directly detect the inflow of the cold fuel.

Now, however, thanks to the background light from the quasar HE 2243-60, Martin and her colleagues have been able to observe distinct signatures near a typical star-forming galaxy that indicate the inflow of gas feeding the galaxy.

In this scenario, gas is drawn into a galaxy and then circles around it, rotating with it before falling in.

Although some evidence of such accretion had been observed in galaxies before, the motion of the gas and its other properties had not been fully explored until now.

The background quasar is, by chance, perfectly well positioned for this study. "This kind of alignment is very rare, but was critical for this study," explained first author Bouché, who is now with the Research Institute in Astrophysics and Planetology in Toulouse, France.

The astronomers used two instruments known as SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) and UVES (Ultraviolet and Visual Echelle Spectrograph), both of which are mounted on European Southern Observatory's Very Large Telescope at the Paranal Observatory in northern Chile.

The new observations showed both how the galaxy itself was rotating and revealed the composition and motion of the gas outside the galaxy.

The result is the discovery of how an active star-forming galaxy feeds its prodigious growth, according to co-author Michael Murphy, from the Swinburne University of Technology in Australia.

"[We've] observed, as directly as possible, the feeding process for forming huge numbers of stars very quickly 11 billion years ago," he said.

The observation also strengthens the argument that low-mass galaxies are formed through these cold streams, which also allow galaxies to prolong their star formation process.

"It is impressive to see in the data the telltale signatures of this infalling gas matching those expected in numerical simulations," said Bouché.

More information: This research was presented in a paper entitled "Signatures of Cool Gas Fueling a Star-Forming Galaxy at Redshift 2.3", to appear in the 5 July 2013 issue of the journal Science.