LAMPSS – Last Stages

I’m sure you’ve all been eagerly anticipating the next iteration of the LAMPSS blog, so sorry for the delay. The final pieces of the puzzle have (finally) fallen into place:

1) Spectral Type Conditions

Figure 1: The full (HK-g)-1.5(g-i) against (g-i) plot we created, including all spectral types.

After considering the inverse temperature against (g-i) plot (figure 5 of the week 4 blog post), we decided the above plot was the best way to separate spectral types of stars by their (g-i) index. The results are shown below:

Figure 2: Table showing the (g-i) conditions used to separate spectral types of stars.

2) Distance Calculations

Karolina completed her distance calculating Python script which employs the distance modulus equation (log10(d/pc) = (m-M+5)/5), to calculate distances using magnitudes in the u, g, r, i and v wavelength bands. The average of the results is used as the final distance to a star. A positional plot of all sources in our catalogue defined as stars by our star-galaxy separation conditions is shown below.

Figure 3: 3D plot showing the position of all stars within the LAMPSS catalogue, spectral type is indicated by colour.

We are searching for stars towards the halo of the Milky Way. By plotting the distance histogram for stars within our sample (figure 4), we conclude the halo stretches from ~25kpc to ~150kpc.

Figure 4: Distance histogram for stars within our catalogue. We judge the halo to cutoff at a distance of ~150kpc.

3) Metallicity Conditions

All that remained before we could start identifying our metal-poor stars towards the halo, was to work out if our stars were metal-poor or not. After considerable effort, Ellie used the Polyfit function on Python to derive metallicity conditions from the colour-colour plot shown in figure 1. Degree 2 polynomials were used to fit lines of different [Fe/H], and provide us with metallicity conditions in terms of g, i and CaHK. The dashed lines represent the mid-point between lines defining integer values of [Fe/H]. Sources were given an integer value of [Fe/H], depending on which pair of dashed lines they lay between.

Figure 5: Finalised metallicity conditions

Due to sources with (g-i)<-1 or (g-i)>1.5 greatly affecting the fitting of the metallicity curves, they were disregarded. This is not a problem for G type stars ( 0.3<(g-i)<0.8), however some K type stars (0.8<(g-i)<2.4) will lie outside the (g-i) range for which these conditions are valid. Therefore, some extrapolation is necessary to classify K type stars by [Fe/H]. Limitations aside, we now have all the tools required to find metal-poor stars. Next time, we present our findings.


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