Tag Archives: astrophysics

The Lithium problem. Primordial, cosmological or stellar?

Fabio Iocco is one of the Postdoc working within the OKC on Dark Matter. He is also interested in one of the puzzles keeping astronomers and cosmologists busy: the Lythium problem. Fabio has recently organized a conference dedicated to this mystery and he is getting ready to give the next OKC colloquium, this is why I asked him to tell us a bit more about this topic.

Credit: Belèn Roncoroni

Alright: the “Lithium problem”.
The “Cosmological” Lithium Problem.
The “Primordial” Lithium problem.
We all have heard about it since kindergarden, but would you bet 5000 SEK you know exactly what it is? I did not, so had to look it up. And here is what I have learned.
First of all, let’s play it fair: there’s two stable lithium isotopes, lithium-six and lithium-seven. In the last years it seemed both had problems, but we are talking about the bigger brother here, the one who has had problems for a longer time. Since 1982, 30 years ago -when my brother was born- there have been observations of lithium-seven in metal poor stars of the galactic halo. The most metal poor stars, the smallest mass, therefore the oldest stars to be around. Or at least a good approximation of a lot, a lot old. Ancient, pristine maybe. Ay, there’s the rub: “maybe”. Would you bet they were the first stars to be formed? I would not, but that’s another story. What matters here is that the stars were not the first generation, but the stuff in their atmosphere, what you observe when you take spectra of their surface may have been whatever had been produced “as far back in the past” as we could get with chemistry observations in our galaxy.
Continue reading The Lithium problem. Primordial, cosmological or stellar?

Dissipative photospheres in gamma-ray bursts

The initial stage of a gamma-ray burst. The core of the star has collapsed deep inside the star. A black hole has formed within the star, and within a few seconds launches a jet of matter away. (Larger image) (Credit: NASA / SkyWorks Digital)
The origin of the emission during the prompt phase in gamma-ray bursts is still a mystery. One suggestion is that the photosphere of the relativistic jet plays an important role. Indeed, recently the Fermi gamma-ray space telescope has made interesting observations of the gamma-ray spectra of several GRBs which show a clear signature of a photospheric emission component.

Another recent development in the field is the realisation that energy dissipation naturally should occur close to the jet photosphere. Such theoretical predictions for kinetic outflow as well as for Poynting flux dominated outflows are confirmed by numerical jet simulations.

In a recent paper, that has just been accepted for publication in the Monthly Notices of the Royal Astronomical Society, we present observational evidence for the onset of such subphotospheric dissipation. This is clearly seen during the prompt phase in the exceptionally bright burst GRB090902B. Initially the main spectral emission component is close to a Planck function, expected for a photosphere. Later this component broadens into a spectral shape that is typical for GRBs. This illustrates that the photosphere emission can have a variety shapes. This is indeed what is expected if the dissipation pattern in the jet changes and gives rise to subphotospheric heating. This we show through numerical simulation of the dissipation processes and argue that the change in spectral behaviour as being due to a decrease in the outflow Lorentz factor. This leads to a substantial part of the kinetic energy being dissipated at optical depth of approximately 10. This causes the change in spectral shape since the photons do not have time to thermalise into a Planck function.

These observations show that the photosphere emission indeed is important in GRBs and can even be a common feature.
The spectral shape of the photospheric emission can have a variety of shapes and not only a Planck function shape.
The identification of the photosphere as cause of the main emission in GRBs provides us a way to study the physics of the relativistic jet. This allows us to learn more about these enigmatic events that are the largest explosions in the Universe.

If you want to read the original paper, check this out: http://arxiv.org/abs/1103.0708