The project began with a healthy uncertainty about what we were actually doing. As it turned out, we were to be searching for metal-poor stars in the halo of the Milky Way, with the ultimate goal of finding a low-mass Population III star. A task that seems unlikely to prove successful, given that no Population III star has ever been directly observed. The first step was to prepare a brief presentation summarising the project which we delivered to our peers and project supervisor – the esteemed Dr Sobral. We outlined that we would be using data taken by the CFHT using the CaHK filter, a narrow band filter which encompasses the two wavelengths of the spectral lines important to our search. They are the CaHK absorption lines at 3968Å and 3934Å respectively. These two spectral lines of ionized Calcium are very prominent in stellar spectra, as such, are a useful tool in identifying metal-poor stars – which show extremely shallow H and K absorption lines in their spectra. Below is a plot of flux density per wavelength against wavelength for a star from the Sloan Digital Sky Survey (SDSS) spectral database. The H and K absorption lines can be seen to the left of the plot and are very strong in comparison to other spectral lines.

Figure 1: An example of a stars flux density per wavelength against wavelength plot taken from the SDSS spectral database (1Å = 10e-10m and 1erg = 10e-7J).

A lot of background reading was also done by the group during this first week. A fundamental concept is that the metallicity of a star is commonly quantified by the iron content of the star and a logarithmic comparison of that iron content to Solar abundance, as shown by the equation below. The CaHK lines are preferential to the spectral lines of iron because they are more prominent, especially at the low metallicities we hope to be looking at.

Figure 2: Equation that defines stellar metallicity. For example, a star with [Fe/H] = -6.0 would have a millionth of Solar iron abundance.

We took a particular interest in the Pristine survey (Starkenburg et al. 2017), a survey also conducted on the CFHT using the metallicity-sensitive CaHK lines. The survey was successful, and whilst no Population III stars were found, the HK lines proved effective in identifying ultra metal-poor stars ([Fe/H] < -4.0). We also found out that most of these ultra metal-poor stars have an unusual abundance of Carbon ([C/Fe]), which worryingly suggests a ‘metallicity-floor’ for star formation. However, the eloquently named star Pristine 221.8781+9.7844 (Starkenburg et al. 2018) is both ultra metal-poor and has a low [C/Fe] abundance which is a promising sign in our (rather hopeful) search for Population III stars.

Next week will start to work on Python scripts to apply the CaHK filter profile to the spectral data given and thereby work out CaHK magnitudes for the stars. Also, we will start to consider a plethora of other factors, such as, the removal of galaxies from out data set and stellar lifetimes for different spectral types of stars.


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