The most metal-poor stars in the galactic bulge found

The most metal-poor stars in the galactic bulge found

An international team of astrophysicists has identified the most metal-poor stars in the Milky Way bulge. They present their findings in a paper published on Oct. 2. The scientists describe the discovery and analysis of four bulge giants - the lowest metallicity bulge stars studied with high-resolution spectroscopy to date. Using the Fibre Large Array Multi Element Spectrograph (FLAMES) spectra, through the Gaia-ESO Survey they have derived abundances of twelve elements. "Given the uncertainties, we find a chemical similarity between these bulge stars and halo stars of the same metallicity, although the abundance scatter may be larger, with some of the stars showing unusual ratios," the team writes in the paper.

The first in the Universe (referred to as Population III stars) have been extensively searched for, both in the local Universe and at high redshift, but despite massive efforts no true Population III star has yet been found. There is an argument that no such stars should remain today: models of their formation indicate that they would have been massive and shortlived. Recent simulations however have suggested that disc fragmentation could have produced smaller mass stars, some of which may have survived to the present day.

Surveys focusing on the discovery of these old and metal-poor stars have almost exclusively targeted the Galactic halo, although some more recent studies have looked at dwarf galaxies of the Local Group. The halo is known to be on average more metal-poor than other Galactic components, and some of these halo stars pass through the solar neighbourhood, making them relatively uncomplicated to observe.

Few, if any, dedicated attempts have been made to search the Galactic bulge for extremely metal-poor stars. The huge number of stars in the bulge, the distance to the bulge, and the high degree of extinction in the Galactic plane make it practically very difficult to find metal-poor stars there. "This letter is the first in a series of papers exploring the results of the EMBLA (Extremely Metal-poor BuLge stars with AAOmega) survey, which aims to find the most metal-poor stars in the bulge," the paper reads.

The researchers report that from the first 3,600 stars observed in April and July 2012, ten were immediately identified as very metal-poor candidates. Six of these targets were observed with FLAMES/UVES spectrograph on the Very Large Telescope (VLT) as part of the Gaia-ESO Survey in May and August of 2012. Finally, the team has identified stars that are confirmed (based on high-resolution spectroscopy) to have lower metallicities than any previously published metal-poor bulge star. "We have presented an abundance analysis of four of these [stars], observed in high-resolution as part of the Gaia-ESO Survey. These four are all considerably more metal-poor than any previously studied star," they conclude.

Gaia-ESO is a public spectroscopic survey, systematically covering all major components of the MilkyWay, from halo to star forming regions, providing the first homogeneous overview of the distributions of kinematics and elemental abundances.


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Citation: The most metal-poor stars in the galactic bulge found (2014, October 6) retrieved 17 July 2019 from https://phys.org/news/2014-10-metal-poor-stars-galactic-bulge.html
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Oct 06, 2014
A star could meet the physics of a so-called "Population III" star while still being very young, or possibly by being very low mass (sub-solar) compared to the median expectation of "population III". There are a few ways one could imagine such scenarios unfolding.

Young:

It's easy to envision inter-galactic nebulae, far away from the stellar-dense galactic spirals, as having the most pure, most primordial hydrogen and helium. From time to time, you'd expect some of these nebulae to collide and compress, if they manage to be cool enough and far enough away from something heating/ionizing like a black hole jet.

Old w/ low mass:
Conceivable. Maybe 0.8 to 0.9 solar masses so that it burns very slowly.

Problems:
The older a star is the more likely it has interacted with supernova remnants from larger, pop III and Pop 1 stars, which should have contaminated that small/old star's native spectra with...metals.

Metalicity by itself is unreliable for dating due to situational paradoxes.

Oct 06, 2014
Because you don't know the whole history of a star, indeed you know next to nothing of it's history, the metalicity isn't entirely reliable. For all you know, the star could have formed elsewhere, picked up the metals it does have, and then later moved to it's current location via ejection and re-capture.

Also, if you see a very low metal star in an environment surrounded by metal-rich stars, that would be a smoking gun of recapture, as it clearly has no business being there, since it would have been highly contaminated by super-novas and mass ejections from those metal-rich stars, had it been there throughout it's history.

Knowing the star was formed elsewhere, then captured at it's current location won't tell you where it actually came from; You have no secondary method of absolute dating it or "post-dicting" it's prior trajectory.

You could try comparing isotopic ratios to theoretical daughter products given stellar mass, but that only goes so far...

Oct 06, 2014
Not surprising since denser regions of matter near the grey hole — which is periodically ejecting new matter therefrom — spawn accelerated growth from within of those 'young' stars formed therefrom. These young stars should be metal poor as the new matter forms hydrogen, dominating the ratio between those few metals formed.

Oct 06, 2014
Milky Way bulge?

Harlow Shapley, how far we have come.

yyz
Oct 06, 2014
"These young stars should be metal poor..."

So this would constitute another failure of LaViolettes's hypothesis, as stellar populations near the galactic center reveal a population of stars with near-solar metallicity values: http://arxiv.org/.../0407188

Oct 07, 2014
@Returners: There are other ways than metallicity to observe star ages.

But the interesting bit is to find popIII stars that are as pristine as possible at a guess. Those would be among isolated halo stars (after their fragmentation birth) recently migrating into the bulge. Scarce, but with so many stars... possible.

@Tuxford: The SMBH isn't described as "grey". It is a BH with disk and so jets. Happens in cases, but likely not all. The disk is heavily contaminated by nucleosynthesis as the material is accreted from the gas, star and supernova dense surroundings. [ http://en.wikiped...ack_hole ]

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