09 Feb 2016   |   Network update from University of Bologna
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University of Bologna: A “cosmic scale” unmasks thin-looking fat stars

A team of researchers of the Bologna University has proposed a “cosmic scale” able to unmask these elusive thin-looking fat stars. A few heavy stars generated by stellar interactions are hidden in the core of ancient stellar systems made of millions lighter stars, with masses lower than that of our Sun.

The new tool has been designed by a team of researchers of the Department of Physics and Astronomy of the University of Bologna (UniBO), led by Prof. Francesco Ferraro, in collaboration with INAF - . This is a new result of the five-year project Cosmic-Lab funded by the European Research Council. By using this innovative “cosmic scale”, the mass of stars (a key parameter driving the entire stellar evolution) can be measured and an evolved Blue Straggler Star has been unmasked in the globular cluster 47 Tucanae.

The research has been recently published in the Volume 816 (10th January 2016) of The Astrophysical Journal.

Globular clusters (GCs) are very old stellar aggregates composed of up to a few million stars formed approximately at the same epoch (about 13 billion years ago). Because of their age and since no recent star formation occurred, all stars more massive than 0.8-0.9 M¤ in GCs should have already concluded their life-cycle, ending as white dwarfs, neutron stars, or black holes. At odds with such a prediction, a peculiar population of heavy stars (1.2-1.6 M¤) still evolving in the very early phases of their life-cycle are observed in GCs: these are the so-called Blue Straggler Stars (BSSs).

To try and explain the existence of these objects, astronomers hypothesise that they formed through mass-enhancement processes (possibly involving stellar collisions and mass transfer in binary systems). However, these mechanisms are not completely understood yet, and essentially nothing is known to date about the advanced stages of BSS evolution. This is because, while BSSs can be easily identified during their core hydrogen-burning phase (when they appear brighter and bluer than the normal cluster stars), they are completely indistinguishable (in terms of luminosity and colour) from their low-mass sisters during the advanced stages of the subsequent evolution. Thanks to the proposed cosmic scale the team of astronomers has been able to distinguish one evolved BSS (with a mass of 1.4 M¤) in the multitude of its low-mass sisters in the globular cluster 47 Tucanae.

“Indeed the mass of stars is the key parameter in stellar Astrophysics: it drives the entire evolution of stars (from birth to death) “ – explains Prof. Ferraro of UniBO, who led the research - “Broadly speaking, the life of massive stars is much shorter than that of low-mass stars. This is the reason why, in GCs, we do not expect to observe stars as massive as BSSs in early stages of their life. Unfortunately, defining methods for measuring stellar masses is quite difficult in general. Our discovery demonstrates that the detailed comparison between chemical abundances derived from neutral and from ionized elements is a powerful cosmic scale, which we used to identify a more massive star (an evolved BSS) in the see of its (otherwise indistinguishable) sisters”.

“The basic idea is quite simple – continues Ferraro - the abundance of a given chemical element obtained from the absorption lines of ionized atoms sensitively depends on stellar mass, while such a dependence is negligible for neutral spectral features of the same element. Since the abundances obtained from the two measurements must agree, the difference between the two values can be used to derive the stellar mass. Indeed, the difference between the two abundances (derived from neutral and from ionized species) can be used as the pointer of a cosmic scale: when the correct stellar mass is assumed, the pointer marks zero”.

"The approach is quite powerful since it allows to accurately estimate mass differences of any star (independently of it being "normal" or "peculiar") - adds Emilio Lapenna from UniBO – thus minimizing the uncertainties and possible zero-point offsets due to the use of different methods”.

“Although this is not the very first identification of an evolved-BSS in a GC – continues Ferraro - the discovery provides the long-sought tool enabling systematic searches for these objects in GCs, thus giving access to still unexplored phases of BSS evolution”.

“Indeed, since evolved-BSSs are 20 times more luminous than their progenitors, detailed spectroscopic follow-up studies are largely facilitated and open the possibility to even go back to the formation channel. In fact, a few characterizing features impressed by the formation process could be still observable in such advanced stages of the evolution’- adds  Alessio Mucciarelli from UniBO.

“Thanks to future measurements by means of the cosmic scale, predictions from both the collisional and the mass transfer formation scenarios can be verified, thus opening a new perspective on our comprehension of BSS evolutionary paths and formation processes” concludes Barbara Lanzoni from UniBO.

Additional information on the discovery can be found at: http://www.cosmic-lab.eu/Cosmic-Lab/cosmic_scale.html

Related images can be downloaded at: http://www.cosmic-lab.eu/Cosmic-Lab/image_gallery_ebss.html


This research is part of the project COSMIC-LAB (web site: http://www.cosmic-lab.eu) funded by the European Research Council (under contract ERC-2010-AdG-267675).

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