LAMPSS – Week 4 – Science!

Hello there, hope you’ve all had a good week and earnt a few quid. This week LAMPSS made progress in several areas.

1) Volume Estimate

We found the minimum apparent magnitude in the HK filter in the COSMOS CaHK catalogue and plotted a histogram (see figure 1) to find the maximum HK apparent magnitude (where it starts to fall off i.e 25.3). The absolute HK magnitude of each type of star (O, A, G etc) was calculated by Karolina’s scripts. The area was calculated using figure 2, giving an area of 1.010 deg^2. With these values, we had everything required for a preliminary volume estimate for each stellar classification. The distance modulus was used to find the maximum possible distance of each type of star for the given apparent and absolute HK magnitudes. The volume we are studying was approximated as a small section of a sphere (approximately 41253 deg^2 in a sphere), with the aforementioned distances being the radius. This resulted in a preliminary volume estimate of 2.86 x 10^9 pc^3 for G-type stars.

Figure 1: Histogram of apparent magnitude in HK filter through 3” aperture. We used the value where the counts begin to drop off (25.3) as the maximum apparent magnitude in HK.
Figure 2: The entirety of the COSMOS CaHK catalogue plotted by position in the sky. This plot was used to estimate the area of the field of view for our study.

2) Numerical Error Script

Chief error analyser Sam completed his numerical error calculator to much excitement. The script can be used in tandem with other scripts, so will be very useful in the weeks to come.

3) Galaxy Removal

We have finalised our criterion for removing galaxies. The cleanest cut was given by the following:

𝐽−𝐾< −0.6143(𝐵−𝑢)−1.6121

𝐵−𝑢< −0.674

This gave 70% completion and 30% contamination, which will have to be taken into consideration later on.

4) Stellar Metallicity Plots

Metallicity affects the HK magnitudes of a star, and as such, a plot of some colour indices including HK against another colour index yields different curves for different metallicities (as well as for different star types). Below is our plot of (HK-g)-1.5(g-i) against g-i (figure 3). Some metal-rich K and M stars appeared off to the right of figure 3, however they were cut out. Comfortingly, it looks rather like the plot from the Pristine Survey included in the week 2 blog post – a comparison is given in figure 4.

Figure 3: Plot of (HK-g)-1.5(g-i) against g-I for stars of different metallicities and types.
Figure 4: Comparison of our own plot with that of the Pristine Survey.

We are also in need of a condition to separate out stars by spectral type. To that end, Karolina started to plot temperature against the g-i colour index (figure 5) to see if there was a relationship between temperature and g-i which cleanly removes other stellar classes. However, there is some overlap between G type stars and the F and K type stars. Further investigation into how to separate G and K type stars from others will be done next week. Both figures 3 and 5 were created using data provided by David from POLLUX, in future we can apply the same methods to obtain results from our actual catalogue.

Figure 5: Plot of inverse temperature against the g-i colour index for F, G and K type stars.

Mainly so we could shoehorn a gif somewhere into this blog, we also plotted normalised flux against wavelength in the region of HK lines for different metallicities, from Solar ([Fe/H] = 0) to hyper metal-poor ([Fe/H] = -5.0). However, this gives an excellent visual representation of the changes in stellar spectra due to metallicity and in particular, the changes in HK magnitude we are looking for in this project. The data for this plot was obtained from POLLUX for a 5000K star (available at

Figure 6: Plot of normalised flux against wavelength for a 5000K star at varying stellar metallicities


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