From Manu at IAC: ” New progress in determining the lifetime of massive stars”


Manu Garcia, a friend from IAC.

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IAC

Instituto de Astrofísica de Canarias – IAC

25 October 2016

The size of the core of the stars determines how much fuel is available and, therefore, how long will evolve and the lives of stars.

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Determined the lifetime of massive stars. Credit: IAA.

The stars draw their energy from fusion reactions taking place at its core, a region of extreme density and temperature. And in the nuclei of the most massive stars a phenomenon called “core overflow” which dramatically changes its evolutionary path, especially as regards their lifetime may occur. Now, a study led by the Institute of Astrophysics of Andalusia (IAA-CSIC) has measured the intensity of this effect and established a clear dependence on the mass of the star.

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To determine how long a star lives need to know the type of stellar nuclear boiler and the type of thermonuclear reactions that occur in it. Stars produce energy through the fusion of hydrogen into helium, but the conditions for this reaction only are present in the core so occurs that the size of this will determine how much fuel is available star and, by extension, how live. And the size of the nucleus of stars depends on how energy is transported to the outer regions.

In stars with more than 1.3 times the mass of the sun the energy produced in the core moves outwards by convection, similar to bubbling of boiling water. Are convective cells which carry the energy and traditionally has resorted to estimate the size of the core by a criterion based on the acceleration of the same: when this is zero, the motion ceases, which would set the limit core.

“However, by inertia, these cells can still go a greater way than dictated by this criterion, resulting in increased core. As a result there will be more fuel available, extending the life of the stars, among other details,” said Antonio Claret , IAA-CSIC researcher who has conducted research.

We can find an analogy of this phenomenon, called “overflow core” (English core overshooting ), in a bonfire in the field: in principle, the stake will only take place where firewood is but due to thermal movements or wind, fire can reach the foliage in the vicinity of the fire.

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Eclipsing binary star system. Credit: IAA.

“Since the overflow of the core alters both evolution and the lifetime of massive stars, determining its intensity and its possible dependence on the mass of the star is one of the challenges of modern astrophysics, said Claret (IAA-CSIC). to do this we must have very accurate data we can compare with theoretical calculations. in the past there have been some attempts but no conclusive results have been obtained mainly due to the scarcity of reliable observational data. ”

A recent study by Claret & Torres (2017), published in The Astrophysical Journal, has examined the magnitude of core overshooting (overflow convective core) and was able to determine how depends on the mass of the star. The research focused on the diffusive approximation. In this process the convective elements dissolve by a diffusion process and, therefore, the region is not fully mixed. Article completes the previous study by the same authors examined the prescription ” step-function “. This latest analysis is based on a sample of twenty eclipsing binary stars located in the solar neighborhood and in the Magellanic Clouds. The researchers were able to infer the overflow of the core of the two stars in the binary system.

Claret and Torres found that the magnitude of this effect increases for stars with between 1.2 and 2 times the mass of the sun, and then remains approximately constant for the most massive stars to about 4.5 solar masses, the upper limit of your sample. Behavior beyond 4.5 solar masses is not yet known. The general trend in terms of the mass is qualitatively similar to that found in the previous study using the approach ” step-function “. This allowed them to establish a connection between the two mechanisms.

The study also examined the influence of two different mixtures of chemicals (opacities) in intensity overflow core and its dependence on the mass, and found that lead to essentially identical results. Similarly, Claret and Torres found that the proportion of primordial helium enrichment galactic law, describing how the abundance of helium in stars depends on the content of heavy elements, has little or no impact. Together with the results of the previous study, these new findings indicate that dependence on the mass phenomenon does not appear to be greatly affected by the basic physics adopted in the theoretical models of stellar evolution. Furthermore, through an analytical model, it has also failed to explain the pattern of changes in the size of convective cells due to the ” core overshooting ” establishing a connection with the stellar mass, the type of nuclear reactions, opacities and the state equation.

The study represents an important step in our understanding of the convective core overflow forward and provides a much needed to develop future models of stellar evolution empirical guide. This has potential implications for ages inferred from the stars, and other physical properties depending on the models.

See the full article here.

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The Instituto de Astrofísica de Canarias(IAC) is an international research centre in Spain which comprises:

The Instituto de Astrofísica, the headquarters, which is in La Laguna (Tenerife).
The Centro de Astrofísica en La Palma (CALP)
The Observatorio del Teide (OT), in Izaña (Tenerife).
The Observatorio del Roque de los Muchachos (ORM), in Garafía (La Palma).

These centres, with all the facilities they bring together, make up the European Northern Observatory(ENO).

The IAC is constituted administratively as a Public Consortium, created by statute in 1982, with involvement from the Spanish Government, the Government of the Canary Islands, the University of La Laguna and Spain’s Science Research Council (CSIC).

The International Scientific Committee (CCI) manages participation in the observatories by institutions from other countries. A Time Allocation Committee (CAT) allocates the observing time reserved for Spain at the telescopes in the IAC’s observatories.

The exceptional quality of the sky over the Canaries for astronomical observations is protected by law. The IAC’s Sky Quality Protection Office (OTPC) regulates the application of the law and its Sky Quality Group continuously monitors the parameters that define observing quality at the IAC Observatories.

The IAC’s research programme includes astrophysical research and technological development projects.

The IAC is also involved in researcher training, university teaching and outreachactivities.

The IAC has devoted much energy to developing technology for the design and construction of a large 10.4 metre diameter telescope, the ( Gran Telescopio CANARIAS, GTC), which is sited at the Observatorio del Roque de los Muchachos.


Gran Telescopio Canarias at the Roque de los Muchachos Observatory on the island of La Palma, in the Canaries, SpainGran Telescopio CANARIAS, GTC