Searching For Metal Poor Stars – Week 3

It’s been a slow week to say the least.

We decided to make the following plot to help visualise how the filters affect the spectral types and so how colour-colour diagrams really work, i.e. why some combinations of filters are better at showing metallicity than others. This was made by predicting the black body curves for each spectral type using Planck’s Law.

The black body curve of each spectral type and the filters used in order to better visualise how the filters affect each type of star.

Next we downloaded more spectra from POLLUX to gain a larger range of data. The result was the following plot, which is a relief because it looks somewhat similar to the one in the proposal we were given (Figure 1, McGee_metalpoor_INT_2014A). The stars with a metallicity value of 0.0 don’t really follow a trend and we’ve excluded the points for M stars as they are huge outliers. The reason M type stars are so different is because they can be white planet-like and have strong absorption bands because they allow molecules to form, and so the spectra can make them look more red or blue.

box-image copy.png
Our synthetic stars from the POLLUX database with set metallicities.

After this we continued making cuts on out whole catalogue to narrow down where we will find metal poor stars. We decided to start using b-v filters instead of g-i as we found that they showed our metallicities more easily. We created these cuts by plotting our synthetic spectra on top of our catalogue and creating a region around each metallicity. The catalogue was also cut down by filtering out the stars with absolute magnitudes of less than 24, which is a good limit to select bright stars from the sources in our data. We made the figure below to demonstrate how we made our cuts. It first shows all of the stars in the catalogue, then just the ‘bright’ stars (magnitude<24), and then shows our metallicity cuts.

A gif showing our entire catalogue of stars, then just the ones with a magnitude value lower than 24, then we applied our criteria created from the synthetic spectra to show stars with low metallicity. All stars in red, only stars with magnitude < 24 in blue, and then metallicity cuts.

Another task this week was to calculate the search volume of the data from the COSMOS survey. This is so we can obtain a source density and compare it with the stellar density of the halo. Of course, our data will contain distant galaxies that can appear as stars and so these need to be filtered out. The most effective way to do this is to use J and K filters, as they fall within the infrared range and so easily sort galaxies from stars. The problem with this however, is that by appealing this constraint we could be eliminating some dim stars that are still within our own galaxy. Next week we’re going to look for a solution to this problem, but for the mean time the plot below shows the result of applying this constraint. In Vega magnitudes (as opposed to AB magnitudes which is what we have been working in), if the J-K value is less than 1 the source is considered a star and more than 1 it’s considered a galaxy.

Screen Shot 2018-07-05 at 15.49.55
Our catalogue data plotted in J-K to separate which sources are likely to be galaxies (red) and which are stars (blue).

Hopefully next week will bring a lot more progress, and we can reduce our criteria for metal poor stars by making more cuts to the data, as well as finding a better way to distinguish between stars and galaxies in our data.


– Alice

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s