Foundations of Astronomical Data Science: Making the Isochrone DataFrame

Calculating Isochrone

In fact, we can use MESA Isochrones & Stellar Tracks (MIST) to compute it for us. Using the MIST Version 1.2 web interface, we computed an isochrone with the following parameters:

  • Rotation initial v/v_crit = 0.4
  • Single age, linear scale = 12e9
  • Composition [Fe/H] = -1.35
  • Synthetic Photometry, PanStarrs
  • Extinction av = 0

The following cell downloads the results:

download('' +

To read this file we will download a Python module from this repository.

download('' +

Now we can read the file:

import read_mist_models

filename = 'MIST_iso_5fd2532653c27.iso.cmd'
iso = read_mist_models.ISOCMD(filename)
Reading in: MIST_iso_5fd2532653c27.iso.cmd

The result is an ISOCMD object.


It contains a list of arrays, one for each isochrone.


We only got one isochrone.


So we can select it like this:

iso_array = iso.isocmds[0]

It is a NumPy array:


But it is an unusual NumPy array, because it contains names for the columns.

dtype([('EEP', '<i4'), ('isochrone_age_yr', '<f8'), ('initial_mass', '<f8'), ('star_mass', '<f8'), ('log_Teff', '<f8'), ('log_g', '<f8'), ('log_L', '<f8'), ('[Fe/H]_init', '<f8'), ('[Fe/H]', '<f8'), ('PS_g', '<f8'), ('PS_r', '<f8'), ('PS_i', '<f8'), ('PS_z', '<f8'), ('PS_y', '<f8'), ('PS_w', '<f8'), ('PS_open', '<f8'), ('phase', '<f8')])

Which means we can select columns using the bracket operator:

array([0., 0., 0., ..., 6., 6., 6.])

We can use phase to select the part of the isochrone for stars in the main sequence and red giant phases.

phase_mask = (iso_array['phase'] >= 0) & (iso_array['phase'] < 3)
main_sequence = iso_array[phase_mask]

The other two columns we will use are PS_g and PS_i, which contain simulated photometry data for stars with the given age and metallicity, based on a model of the Pan-STARRS sensors.

We will use these columns to superimpose the isochrone on the color-magnitude diagram, but first we have to use a distance modulus to scale the isochrone based on the estimated distance of GD-1.

We can use the Distance object from Astropy to compute the distance modulus.

import astropy.coordinates as coord
import astropy.units as u

distance = 7.8 * u.kpc
distmod = coord.Distance(distance).distmod.value

Now we can compute the scaled magnitude and color of the isochrone.

mag_g = main_sequence['PS_g'] + distmod
color_g_i = main_sequence['PS_g'] - main_sequence['PS_i']

Now we can plot it on the color-magnitude diagram like this.

plt.plot(color_g_i, mag_g);
<Figure size 432x288 with 1 Axes>

Color magnitude diagram of our selected stars with theoretical isochrone overlaid as blue curve.

The theoretical isochrone passes through the overdense region where we expect to find stars in GD-1.

We will save this result so we can reload it later without repeating the steps in this section.

So we can save the data in an HDF5 file, we will put it in a Pandas DataFrame first:

import pandas as pd

iso_df = pd.DataFrame()
iso_df['mag_g'] = mag_g
iso_df['color_g_i'] = color_g_i

       mag_g  color_g_i
0  28.294743   2.195021
1  28.189718   2.166076
2  28.051761   2.129312
3  27.916194   2.093721
4  27.780024   2.058585

And then save it.

filename = 'gd1_isochrone.hdf5'
iso_df.to_hdf(filename, 'iso_df')