LAMPSS – Final Results!

Nevermind Season 8 of Game of Thrones, this is the biggest unveiling of 2019 – the official results of the Lancaster Astrophysics Metal-poor Star Search. So without further ado, LAMPSS present 16 potentially extremely metal-poor ([Fe/H] <-3.0) stars:

Figure 1: Table showing all LAMPSS extremely metal-poor candidates and some of their properties. Distance and age were calculated by us. Note some of the errors on the ages!
Figure 2: LAMPSS_1229, our [Fe/H] = -5.0 candidate, is at the centre of this image from the Hubble Space Telescope.

Once we had applied our conditions for stars, spectral type and metallicity, we had our metal-poor candidates. The nature of candidates was manually checked the source using Hubble Space Telescope imagery (https://irsa.ipac.caltech.edu/data/COSMOS/index_cutouts.html) to ensure the sources were not galaxies. Whilst doing so we found some interesting galaxies:

Figure 3: A jellfyfish galaxy found during our checks of NASA/IPAC archives, characterised by the tail of gas caused by ram pressure stripping. Image on the right is from our own catalogue, viewed on the DS9 software.

We compared our metal-poor candidates with that of WARP. A project running parallel to ours that used INT data, rather than CFHT. We found a definite correlation between two sets of sources:

Figure 4: A plot of WARP’s [Fe/H] value against our [Fe/H] value for a selection of metal-poor sources that appear in both catalogues. There is a definite correlation in the values of metallicity determined by the two groups.


We studied a parameter space that has never been explored before, so, in order to make more meaningful, general comparisons we calculated stellar number densities using the volumes calculated in week 4. By then applying the (g-i) conditions for spectral type, we plotted the number density per spectral type per metallicity for the region of halo surveyed (figure 5). Again, we checked our results against WARPs (figure 6).

Figure 5: Stellar number density per spectral type per metallicity plot for the region of the halo studied. G and K stars are a lot more common than the hotter A and F stars. Also noticeable, is the lack of any [Fe/H] = -4.0 stars.
Figure 6: WARPs results overplotted onto figure 5. They find a lower density for all spectral types and metallicities.

Admittedly, our project has several limitations. All of our predictions are based on the fact that a star lies on the main sequence, however many stars we surveyed will be at different stages in their evolution, thereby making our predictions inaccurate. Additionally, many of the candidates shown in figure 1 are K type stars and, as mentioned in the last blog post, some extrapolation is required to apply our metallicity conditions to K type stars. However, applying Davids old adage – “Within a factor of 10 is fine” – we remain confident that the LAMPSS metal-poor stars are indeed just that.

So that brings our project to a conclusion. We have written a report ( LAMPSS: Discovery of Metal-Poor Stars in the Galactic Halo with the CaHK filter on CFHT MegaCam (Worrell et al. 2019)), which was submitted very comfortably before the deadline. Now all that remains is a presentation at the PLACE conference (4th/5th June). I’d like to thank David Sobral for his considerable help and guidance throughout the project, as well as the rest of LAMPSS for being stellar.

as you were JB x

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.

-Jack