scholarly journals Making bright giants invisible at the Galactic Centre

2019 ◽  
Vol 492 (1) ◽  
pp. 250-255 ◽  
Author(s):  
Pau Amaro-Seoane ◽  
Xian Chen ◽  
Rainer Schödel ◽  
Jordi Casanellas

ABSTRACT Current observations of the Galactic Centre (GC) seem to display a core-like distribution of bright stars from ∼5 arcsec inwards. On the other hand, we observe young, massive stars at the GC, with roughly 20–50 per cent of them in a disc, mostly in the region where the bright giants appear to be lacking. In a previous publication we put the idea forward that the missing stars are deeply connected to the presence of this disc. The progenitor of the stellar disc is very likely to have been a gaseous disc that at some point fragmented and triggered star formation. This caused the appearance of overdensity regions in the disc that had high enough densities to ensure stripping large giants of their atmospheres and thus rendering them very faint. In this paper, we use a stellar evolution code to derive the properties that a red giant would display in a colour–magnitude diagram, as well as a non-linearity factor required for a correct estimate of the mass loss. We find that in a very short time-scale, the red giants leave their standard evolutionary track. The non-linearity factor has values that not only depend on the properties of the clumps, but also on the physical conditions of the giant stars, as we predicted analytically. According to our results, envelope stripping works, moving stars on a short time-scale from the giant branch to the white dwarf stage, thus rendering them invisible to observations.

2019 ◽  
Vol 82 ◽  
pp. 225-232
Author(s):  
C. Gehan ◽  
B. Mosser ◽  
E. Michel

Red giant stars present mixed modes, which behave as pressure modes in the convective envelope and as gravity modes in the radiative interior. This mixed character allows to probe the physical conditions in their core. With the advent of long-duration time series from space-borne missions such as CoRoT and Kepler, it becomes possible to study the red giant core rotation. As more than 15 000 red giant light curves have been recorded, it is crucial to develop a robust and efficient method to measure this rotation. Such measurements of thousands of mean core rotation would open the way to a deeper understanding of the physical mechanisms that are able to transport angular momentum from the core to the envelope in red giants. In this work, we detail the principle of the method we developed to obtain automatic measurements of the red giant mean core rotation. This method is based on the stretching of the oscillation spectra and on the use of the so-called Hough transform. We finally validate this method for stars on the red giant branch, where overlapping rotational splittings and mixed-mode spacings produce complicated frequency spectra.


1993 ◽  
Vol 21 (2) ◽  
pp. 196-201
Author(s):  
Søren Achim Nielsen ◽  
Thomas Hougaard

An alternative test is presented, in which algal cultures are used for testing toxic substances. This test system is based on variations in the size distribution of cells in test cultures as a measurement of growth. Thus, inhibition of mitotic activity is used as a measurement for toxic effects. The test can be performed on a short time-scale and is very sensitive to even weak toxic doses.


1996 ◽  
Vol 32 (2) ◽  
pp. 212-221 ◽  
Author(s):  
Eglee Gomez Fermin ◽  
Francisco G. Figueiras ◽  
Belen Arbones ◽  
Maria Luisa Villarino

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