LAMPSS – WEEK 2

To start the week, we began by narrowing our search to G and K type stars. Thanks to 0, B, A and F stars being too short-lived for their own good, we can discard them, as a true low-mass population III must be nearly as old as the universe. Furthermore, by considering the Jeans mass criterion for star formation and using the temperature of the universe at several hundred million years old (when the first stars began forming), it can be shown that first generation M stars are very unlikely.

Figure 1: The Jeans Mass criterion for star formation, if the mass of a gas cloud is greater than MJ then the cloud will collapse (k = Boltzmann constant, T = temperature, µ = mean molecular weight, mH = mass of Hydrogen and Rc = Radius of cloud).

As mentioned last week, we will be using data from the CFHT, with particular interest in the filter including the CaHK lines. CFHT was one of the telescopes used in the Cosmic Evolution Survey (COSMOS), a much larger astronomical survey which covers 2deg2 of the sky and looks to probe the formation and evolution of galaxies. Courtesy of Karolina, the filter profiles of the i, g and CaHK filter of the MegaPrime/MegaCam on the CFHT were plotted. She then proceeded to plot the spectra of a 5000K G type star, opening the doors for the return of everybody’s favourite unit – the erg/cm2/s/Å.

Figure 2: Plot to show the filter profiles of the infrared (i), green (g) and CaHK filter profiles of the CFHT (Information from http://cosmos.astro.caltech.edu/page/filterset).
Figure 3: Raw spectra of a G type star at 5000K.

We are after the magnitude of the stars in different filter bands in order to make a metallicity scale and start to evaluate which stars in our sample are metal-poor. An example is shown below from the Pristine Survey.

Figure 4: An example of how to separate stars by metallicity by creating a colour-colour plot using i, g and the metallicity-sensitive CaHK magnitude. Taken from the Pristine Survey (Starkenburg et al 2017).

The apparent magnitude (m) is related to flux as follows: m = -2.5log10(F) + ZP. Therefore, we must obtain the flux density per wavelength of stars in different filters and then integrate with respect to wavelength to first get the flux, enabling us to calculate the apparent magnitude in that filter. We first need to interpolate and normalise the filter profiles before the convolution of the filter profiles and stellar spectra can be plotted. The first step next week will be to test our method by obtaining the apparent magnitude of the Sun (A G2v star). As an example, a convolved plot of the i, g and CaHK filter profiles from figure 2 and the G type spectra from figure 3 is shown below.

Figure 5:  The flux density per wavelength of a 5000K G type star through the i, g and CaHK filters.

-Jack

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